JP2023157999A - nitride semiconductor substrate - Google Patents

Abstract

To improve crystal quality of a nitride semiconductor substrate.SOLUTION: A method for manufacturing a nitride semiconductor substrate comprises: the step of preparing a base substrate; forming a first base layer on the base substrate; forming a metal layer on the first base layer; performing heating to form voids in the first base layer; epitaxially growing a second base layer above the first base layer to form the main surface of the second base layer into a mirror surface; the three-dimensional growth step of directly epitaxially growing a single crystal of a group III nitride semiconductor having a top surface having an exposed (0001)plane on the main surface of the second base layer, forming a plurality of recessed parts constituted of inclination interfaces except the (0001)plane on the top surface, gradually expanding the inclination interfaces as the same go above the second base layer and eliminating the (0001)plane from the top surface to grow a first layer having a surface constituted of only the inclination interfaces; and the flattening step of epitaxially growing the single crystal of the group III nitride semiconductor on the first layer to eliminate the inclination interfaces and growing a second layer having a surface formed into a mirror surface.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for manufacturing a nitride semiconductor substrate, a nitride semiconductor substrate, and a laminated structure.

Various methods have been disclosed for obtaining a free-standing substrate (hereinafter also referred to as "nitride semiconductor substrate") made of a single crystal of a Group III nitride semiconductor (for example, Patent Document 1).

Japanese Patent Application Publication No. 2003-178984

An object of the present invention is to improve the crystal quality of a nitride semiconductor substrate.

According to one aspect of the invention,
a step of preparing a base substrate;
forming a first base layer made of a group III nitride semiconductor on the base substrate;
forming a metal layer on the first base layer;
performing heat treatment to form voids in the first underlayer;
A second base layer made of a single crystal of a Group III nitride semiconductor and having a main surface whose nearest low-index crystal plane is the (0001) plane is epitaxially grown above the first base layer, a step of mirror-finishing the main surface of the stratum;
A single crystal of a group III nitride semiconductor having a top surface with an exposed (0001) plane is epitaxially grown directly on the main surface of the second underlayer, and is composed of an inclined interface other than the (0001) plane. A plurality of recesses are formed on the top surface, and the inclined interface is gradually enlarged as it goes above the second underlayer, so that the (0001) plane disappears from the top surface, so that the surface is only the inclined interface. a three-dimensional growth process of growing a first layer consisting of;
A planarization step of epitaxially growing a group III nitride semiconductor single crystal on the first layer, eliminating the inclined interface, and growing a second layer having a mirrored surface;
A method of manufacturing a nitride semiconductor substrate is provided.

According to another aspect of the invention:
In the method for manufacturing a nitride semiconductor substrate according to the above aspect, a nitride semiconductor substrate obtained by slicing the second layer is provided.

According to yet another aspect of the invention,
A nitride semiconductor substrate having a diameter of 2 inches or more and having a main surface whose nearest low-index crystal plane is a (0001) plane,
When X-rays of Kα1 of Cu are irradiated to the main surface through a Ge (220)-plane two-crystal monochromator and a slit, and an X-ray rocking curve measurement of (0002) plane diffraction is performed, the slit Subtract the half width FWHMb of the (0002) plane diffraction when the width in the ω direction of the slit is 0.1 mm from the half width FWHMa of the (0002) plane diffraction when the width in the ω direction of the slit is 0.1 mm. A nitride semiconductor substrate is provided in which the difference FWHMa-FWHMb is 30% or less of FWHMa.

According to another aspect of the invention:
A nitride semiconductor substrate having a diameter of 2 inches or more,
When the main surface of the nitride semiconductor substrate is observed with a field of view of 250 μm square using a multiphoton excitation microscope and the dislocation density is determined from the dark spot density, the region where the dislocation density exceeds 3×10 ⁶ cm ⁻² is the main surface of the nitride semiconductor substrate. There is provided a nitride semiconductor substrate in which 80% or more of the main surface has a region where the dislocation density is less than 1×10 ⁶ cm ⁻² and is not present on the main surface.

According to yet another aspect of the invention,
A nitride semiconductor substrate having a main surface whose nearest low-index crystal plane is a (0001) plane,
The X-ray rocking curve of the (0002) plane is measured at each position on a straight line passing through the center within the principal plane, and the peak angle ω between the X-ray incident on the principal plane and the principal plane is calculated on the straight line. When the peak angle ω is approximated by a linear function of the position,
The radius of curvature of the (0001) plane determined by the reciprocal of the slope of the linear function is 10 m or more,
A nitride semiconductor substrate is provided in which an error of the measured peak angle ω with respect to the linear function is 0.05° or less.

According to yet another aspect of the invention,
a base layer made of a single crystal of a group III nitride semiconductor, having a mirror-finished main surface, and a low-index crystal plane closest to the main surface being a (0001) plane;
A group III nitride semiconductor single crystal having a top surface with the (0001) plane exposed is directly epitaxially grown on the main surface of the underlayer, and a plurality of slanted interfaces other than the (0001) plane are formed. is formed by creating a recess on the top surface, gradually enlarging the sloped interface as it goes above the base layer, and causing the (0001) plane to disappear from the top surface, so that the surface is formed only by the sloped interface. A first layer consisting of;
a second layer having a mirror-finished surface by epitaxially growing a single crystal of a Group III nitride semiconductor on the first layer and eliminating the inclined interface;
A laminated structure having the following is provided.

According to the present invention, the crystal quality of a nitride semiconductor substrate can be improved.

1 is a flowchart showing a method for manufacturing a nitride semiconductor substrate according to an embodiment of the present invention. 3 is a flowchart showing a base structure manufacturing process. (a) to (e) are schematic cross-sectional views showing a part of a method for manufacturing a nitride semiconductor substrate according to an embodiment of the present invention. (a) to (c) are schematic cross-sectional views showing a part of a method for manufacturing a nitride semiconductor substrate according to an embodiment of the present invention. 1 is a schematic perspective view showing a part of a method for manufacturing a nitride semiconductor substrate according to an embodiment of the present invention. (a) to (b) are schematic cross-sectional views showing a part of a method for manufacturing a nitride semiconductor substrate according to an embodiment of the present invention. (a) to (b) are schematic cross-sectional views showing a part of a method for manufacturing a nitride semiconductor substrate according to an embodiment of the present invention. 1 is a schematic cross-sectional view showing a part of a method for manufacturing a nitride semiconductor substrate according to an embodiment of the present invention. (a) is a schematic cross-sectional view showing the growth process under standard growth conditions in which the inclined interface and the c-plane do not expand or contract, and (b) is a schematic cross-sectional view showing the growth process in which the inclined interface expands and the c-plane contracts. FIG. 1 is a schematic cross-sectional view showing the growth process under one growth condition. FIG. 6 is a schematic cross-sectional view showing the growth process under a second growth condition in which the inclined interface shrinks and the c-plane expands. (a) is a schematic top view showing a nitride semiconductor substrate according to an embodiment of the present invention, and (b) is a schematic cross section along the m-axis of the nitride semiconductor substrate according to an embodiment of the present invention. FIG. 3(c) is a schematic cross-sectional view along the a-axis of a nitride semiconductor substrate according to an embodiment of the present invention. (a) is a schematic cross-sectional view showing X-ray diffraction on a curved c-plane, and (b) and (c) are views showing fluctuations in the diffraction angle of the (0002) plane with respect to the radius of curvature of the c-plane. It is. It is a figure which shows the observation image of the cross section of the laminated structure of an Example observed by the fluorescence microscope. FIG. 2 is a view of the main surface of the nitride semiconductor substrate of the example, observed using a multiphoton excitation microscope. (a) is a diagram showing the results of X-ray rocking curve measurement of (0002) plane diffraction along the m-axis direction for each of the nitride semiconductor substrates of Example and Comparative Example 1, and (b) 1 is a diagram showing the results of X-ray rocking curve measurement of (0002) plane diffraction along the a-axis direction for each of the nitride semiconductor substrates of Example and Comparative Example 1. (a) is a diagram showing a normalized X-ray diffraction pattern when X-ray rocking curve measurements were performed with different slits on the nitride semiconductor substrate of the example, and (b) is a diagram of a comparative example. FIG. 2 is a diagram showing a normalized X-ray diffraction pattern when the same measurement as in Example was performed on the nitride semiconductor substrate No. 1.

<Findings obtained by the inventors>
First, the findings obtained by the inventors will be explained.

The method in Patent Document 1 mentioned above is called VAS (Void-assisted Separation Method). The VAS method is performed, for example, as follows.

First, a first crystal layer made of a group III nitride semiconductor is formed on a predetermined base substrate. After forming the first crystal layer, a metal layer is formed on the first crystal layer. Once the metal layer is formed, heat treatment is performed in an atmosphere containing a predetermined gas to form fine holes in the metal layer and to create voids in the first crystal layer through the mesh (nannet) of the metal layer. form. After forming voids in the first crystal layer, a second crystal layer made of a group III nitride semiconductor is formed above the first crystal layer. At this time, some of the voids described above remain. After forming the second crystal layer, when the temperature is lowered from the growth temperature of the second crystal layer, the second crystal layer is peeled off from the underlying substrate due to the above-mentioned remaining voids. After the second crystal layer is peeled off, a nitride semiconductor substrate having high crystal quality can be obtained by slicing and polishing the second crystal layer.

Here, when a crystal layer consisting of a single crystal of a Group III nitride semiconductor is grown thickly with the c-plane as the growth plane, the dislocation density at the surface of the crystal layer tends to be inversely proportional to the thickness of the crystal layer. be.

Therefore, in the above-mentioned VAS method, in order to reduce the dislocation density of the second crystal layer, it is conceivable to grow the second crystal layer thickly above the first crystal layer.

However, in the above-mentioned VAS method, tensile stress caused by the initial nuclei of the crystals attracting each other is accumulated on the base substrate side of the second crystal layer. On the other hand, the c-plane of the second crystal layer is curved into a concave spherical shape due to the tensile stress generated in the second crystal layer. When the second crystal layer is grown thickly on the concavely curved c-plane, the stress applied to the second crystal layer gradually changes to compressive stress as the second crystal layer becomes thicker. Therefore, the stress difference between the base substrate side and the surface side of the second crystal layer gradually increases. If the stress difference becomes too large, cracks may occur in the second crystal layer.

Thus, in the VAS method, it is difficult to grow the second crystal layer thickly with the c-plane as the growth plane.

The present invention is based on the above findings discovered by the inventor.

<One embodiment of the present invention>
Hereinafter, one embodiment of the present invention will be described with reference to the drawings.

(1) Method for manufacturing a nitride semiconductor substrate A method for manufacturing a nitride semiconductor substrate according to this embodiment will be described using FIGS. 1 to 8.
FIG. 1 is a flowchart showing a method for manufacturing a nitride semiconductor substrate according to this embodiment. FIG. 2 is a flowchart showing the base structure manufacturing process. 3(a)-(e), FIG. 4(a)-(c), and FIG. 6(a)-FIG. 8 are schematic cross-sectional views showing a part of the method for manufacturing a nitride semiconductor substrate according to this embodiment. It is. FIG. 5 is a schematic perspective view showing a part of the method for manufacturing a nitride semiconductor substrate according to this embodiment. Note that in FIGS. 3(a) to 3(e), the lower side of the base substrate 1 is omitted. Moreover, FIG. 5 corresponds to a perspective view at the time of FIG. 4(b), and shows a part of the first layer 30 grown on the base structure 10. In addition, in FIG. 6(b), thin solid lines in the second layer 40 indicate crystal planes in the middle of growth, and in FIG. 4(c) and FIGS. 6(a) to 8, dotted lines indicate dislocations. .

In the following, in a group III nitride semiconductor crystal having a wurtzite structure, the <0001> axis (for example, the [0001] axis) will be referred to as the "c-axis", and the (0001) plane will be referred to as the "c-plane". Note that the (0001) plane is sometimes referred to as the "+c plane (group III element polar plane)" and the (000-1) plane is sometimes referred to as the "-c plane (nitrogen (N) polar plane)." Further, the <1-100> axis (for example, the [1-100] axis) is referred to as the "m-axis", and the {1-100} plane is referred to as the "m-plane". Note that the m-axis may be expressed as the <10-10> axis. Further, the <11-20> axis (for example, the [11-20] axis) is referred to as the "a-axis", and the {11-20} plane is referred to as the "a-plane".

As shown in FIG. 1, the method for manufacturing a nitride semiconductor substrate according to the present embodiment includes, for example, a base structure manufacturing step S100, a three-dimensional growth step S200, a planarization step S300, a slicing step S400, and a polishing step S400. Step S500.

(S100: Base structure fabrication process)
First, the base structure 10 is manufactured by, for example, the above-mentioned VAS method.

Specifically, the base structure manufacturing process S100 includes, for example, a base substrate preparation process S110, a first base layer forming process S120, a metal layer forming process S130, a void forming process S140, and a second base layer forming process. S150.

(S110: Base substrate preparation step)
First, as shown in FIG. 3(a), a base substrate 1 is prepared. The base substrate 1 is made of, for example, a material different from a group III nitride semiconductor. Specifically, the base substrate 1 is, for example, a sapphire substrate. Note that the base substrate 1 may be, for example, a Si substrate or a gallium arsenide (GaAs) substrate.

The diameter of the base substrate 1 is, for example, 2 inches (50.8 mm) or more. Further, the thickness of the base substrate 1 is, for example, 300 μm or more and 1 mm or less.

The base substrate 1 has, for example, a main surface 1s that becomes an epitaxial growth surface. The closest low-index crystal plane to the main surface 1s is, for example, the c-plane.

In this embodiment, the c-plane of the base substrate 1 is inclined with respect to the principal surface 1s. The c-axis of the base substrate 1 is inclined at a predetermined off angle with respect to the normal to the main surface 1s. The off-angle within the main surface 1s of the base substrate 1 is uniform over the entire main surface 1s. The magnitude of the off-angle at the center of the principal surface 1s of the base substrate 1 is, for example, greater than 0° and less than or equal to 1°. The off-angle within the main surface 1s of the base substrate 1 influences the off-angle at the center of the main surface 6s of the second base layer 6, which will be described later.

(S120: First base layer forming step)
Next, as shown in FIG. 3B, a first base layer (first crystal layer) 2 made of a group III nitride is formed on the main surface 1s of the base substrate 1.

Specifically, for example, a group III source gas and a nitrogen source gas are supplied to the base substrate 1 heated to a predetermined growth temperature by metal organic vapor phase epitaxy (MOVPE). For example, by supplying trimethyl gallium (TMG) gas as a group III raw material gas and ammonia gas (NH ₃ ) as a nitrogen raw material gas, a low-temperature grown GaN buffer layer and a GaN and are grown in this order on the main surface 1s of the base substrate 1. Note that during growth of the GaN layer, n-type impurities may be doped into the GaN layer by, for example, supplying monosilane (SiH ₄ ) gas as an n-type dopant gas.

At this time, the growth conditions for the low-temperature grown GaN buffer layer as the first underlayer 2 are adjusted so as to obtain the desired crystal quality of the GaN layer. Specifically, the growth temperature of the low-temperature grown GaN buffer layer is set to, for example, 400° C. or higher and 600° C. or lower.

Further, at this time, the conditions for growing the GaN layer as the first underlayer 2 are adjusted so that desired voids are formed in the void forming step S140, which will be described later. Specifically, the growth temperature of the GaN layer is set to, for example, 1,000° C. or more and 1,200° C. or less.

Further, at this time, for example, the surface of the first base layer 2 is mirror-finished. Note that the "mirror surface" here refers to a surface where the difference in surface unevenness is less than the wavelength of visible light, and the surface of the first underlayer 2 has hillocks with exposed facets other than the c-plane. may have occurred. Specifically, the root mean square roughness RMS of the surface of the first underlayer 2 is, for example, less than 10 nm, preferably less than 1 nm. By mirror-finishing the surface of the first base layer 2 in this manner, it is possible to make the appearance of voids substantially uniform over the entire principal surface 1s of the base substrate 1 in the void forming step S140 described below.

(S130: Metal layer forming step)
Next, as shown in FIG. 3(c), a metal layer 3 is formed on the first base layer 2. For example, the metal layer 3 is formed by vacuum evaporation or sputtering.

The metal layer 3 is configured to form fine holes therein through heat treatment in the void forming step S140 described below, promote decomposition of the first base layer 2, and form voids in the first base layer 2. It is preferable that the material is made of a material that Specifically, the metal layer 3 that satisfies this condition includes, for example, titanium (Ti), scandium (Sc), yttrium (Y), zirconium (Zr), hafnium (Hf), vanadium (V), and niobium (Nb). ), tantalum (Ta), chromium (Cr), molybdenum (Mo), tungsten (W), rhenium (Re), iron (Fe), ruthenium (Ru), osmium (Os), cobalt (Co), rhodium (Rh ), iridium (Ir), nickel (Ni), palladium (Pd), manganese (Mn), copper (Cu), platinum (Pt), or gold (Au). In this embodiment, the metal layer 3 is, for example, a Ti layer.

At this time, the thickness of the metal layer 3 is, for example, 1 μm or less, preferably 300 nm or less, and more preferably 100 nm or less. By setting the thickness of the metal layer 3 to 1 μm or less, preferably 300 nm or less, and more preferably 100 nm or less, the decrease in flatness of the metal layer 3 (metal nitride layer 5) can be suppressed in the void forming step S140 described below. I can do it. Thereby, deterioration in crystal quality of the second base layer 6 formed on the metal nitride layer 5 can be suppressed. Note that the lower limit of the thickness of the metal layer 3 is not particularly limited. However, from the viewpoint of stably forming the metal nitride layer 5 in the void forming step S140 described later, it is preferable that the thickness of the metal layer 3 is, for example, 0.5 nm or more.

(S140: void forming step)
After forming the metal layer 3, a predetermined heat treatment is performed to form voids in the first base layer 2.

Specifically, the base substrate 1 described above is put into an electric furnace, and the base substrate 1 is placed on a susceptor having a predetermined heater. After the base substrate 1 is placed on the susceptor, the base substrate 1 is heated by a heater and heat-treated in an atmosphere containing a predetermined gas.

At this time, the heat treatment is performed, for example, in an atmosphere containing at least one of nitrogen (N ₂ ) gas and nitrogen-containing gas. Examples of the nitrogen-containing gas include at least one of NH ₃ gas and hydrazine (N ₂ H ₄ ) gas. As a result, the metal layer 3 is nitrided while being aggregated, and a mesh-like metal nitride layer 5 (also referred to as a nanonet of the metal nitride layer 5) having a high density of fine holes (through holes) on the surface is formed. can.

Furthermore, at this time, heat treatment is performed in an atmosphere containing at least one of hydrogen (H ₂ ) gas and hydrogen-containing gas. Examples of the hydrogen-containing gas include at least one of the above-mentioned NH ₃ gas and N ₂ H ₄ gas. Thereby, a part of the first base layer 2 can be etched through the mesh of the metal nitride layer 5, and voids with high density can be formed in the first base layer 2.

Note that at this time, the step of forming nanonets in the metal nitride layer 5 and the step of forming voids in the first base layer 2 may be performed simultaneously, or may be performed separately in different atmospheres. good.

Further, at this time, the heat treatment conditions are adjusted so that a predetermined void ratio of the first underlayer 2 is obtained. Specifically, the partial pressure ratio of the hydride gas in the atmosphere is set to, for example, 10% or more and 95% or less, preferably 50% or less. Further, the heat treatment temperature is, for example, 1,000°C or more and 1,100°C or less. Further, the heat treatment time is, for example, 1 minute or more and 100 minutes or less.

From the above void forming step S140, the void-containing first base layer 4 is formed as shown in FIG. 3(d).

(S150: Second base layer formation step)
After forming the void-containing first base layer 4, as shown in FIG. 3(e), a second base layer (second crystal Layer) 6 is grown epitaxially.

Specifically, for example, the base substrate 1 is carried into a predetermined vapor phase growth apparatus. Next, a group III source gas and a nitrogen source gas are supplied to the base substrate 1 heated to a predetermined growth temperature by hydride vapor phase epitaxy (HVPE). For example, a GaN layer as the second base layer 6 is epitaxially grown on the void-containing first base layer 4 and the metal nitride layer 5 by supplying gallium chloride (GaCl) gas and NH ₃ gas. Note that when growing the GaN layer as the second underlayer 6, for example, by supplying at least one of dichlorosilane (SiH ₂ Cl ₂ ) gas and tetrachlorogermane (GeCl ₄ ) gas as an n-type dopant gas. , n-type impurities may be doped into the GaN layer.

In the initial stage of growth of the second underlayer 6, initial nuclei of island-like crystals are generated. At this time, the frequency of island crystals in the second base layer 6 depends on the degree of supersaturation when growing on the nanonets in the metal nitride layer 5 and the variation in the opening width of the nanonets. As the second underlayer 6 grows, initial nuclei of island-like crystals grow laterally. After that, when the distance between adjacent initial nuclei becomes closer, it becomes more energetically stable if the surfaces of the initial nuclei are combined into one, rather than if there are two surfaces of the initial nuclei. Therefore, adjacent initial nuclei are forcibly attracted to each other (associate). The main surface 6s of the second base layer 6 is formed by the initial nuclei coming together over the entire second base layer 6.

At this time, for example, the main surface 6s of the second base layer 6 is mirror-finished. Note that the "mirror surface" here refers to a surface where the difference in surface unevenness is less than the wavelength of visible light, as described above, and the main surface 6s of the second base layer 6 has no surface other than the c-plane. Hillocks with exposed facets may be formed. Specifically, the root mean square roughness RMS of the principal surface 6s of the second base layer 6 is, for example, less than 10 nm, preferably less than 1 nm. Although the details will be described later in (4), by mirror-finishing the main surface 6s of the second base layer 6, the growth form of the first layer 30, which will be described later, can be changed to an island-like shape that occurred in the early stage of growth of the second base layer 6. It can be changed depending on the growth form of the crystal. Thereby, the average distance between the closest apexes of the first layer 30, which will be described later, can be controlled based on the first growth conditions in the three-dimensional growth step S200, and the distance between the nearest apexes can be increased.

At this time, by setting the low-index crystal plane closest to the main surface 1s of the base substrate 1 to be the c-plane, the low-index crystal plane closest to the mirrored main surface 6s of the second base layer 6 is also c-plane. It becomes a surface.

Further, at this time, the growth conditions of the second underlayer 6 are adjusted so that initial nuclei of crystals that grow in an island shape are generated at a predetermined frequency. Specifically, the growth temperature of the second base layer 6 is set to, for example, 1,000° C. or more and 1,100° C. or less. In addition, the ratio of the partial pressure of the flow rate of NH ₃ gas as a nitrogen source gas to the partial pressure of GaCl gas as a group III source gas during the growth of the second underlayer 6 (hereinafter also referred to as "V/III ratio") is, for example, 1 or more and 50 or less.

Further, at this time, the thickness of the second base layer 6 is, for example, 100 μm or more and 1.5 mm or less, preferably 200 μm or more and 500 μm or less. If the thickness of the second base layer 6 is less than 100 μm, it becomes difficult to mirror-finish the main surface 6s of the second base layer 6. On the other hand, by setting the thickness of the second base layer 6 to 100 μm or more, the main surface 6s of the second base layer 6 can be easily mirror-finished. Thereby, the growth form of the first layer 30 is reliably changed from the growth form of island-like crystals that occurred at the initial stage of growth of the second base layer 6, and the first layer grows three-dimensionally in the three-dimensional growth step S200 described later. The distance between the 30 closest peaks can be easily increased. Further, by setting the thickness of the second base layer 6 to 200 μm or more, the main surface 6s of the second base layer 6 can be stably mirror-finished. On the other hand, if the thickness of the second base layer 6 is more than 1.5 mm, cracks may occur in the second base layer 6. On the other hand, by setting the thickness of the second base layer 6 to 1.5 mm or less, the occurrence of cracks in the second base layer 6 can be suppressed. Furthermore, by setting the thickness of the second base layer 6 to 500 μm or less, generation of cracks in the second base layer 6 can be stably suppressed.

Moreover, at this time, the second base layer 6 grows from the void-containing first base layer 4 onto the void-containing first base layer 4 and the metal nitride layer 5 through the holes in the metal nitride layer 5 . Some of the voids in the void-containing first base layer 4 are filled with the second base layer 6, but other parts of the voids in the void-containing first base layer 4 remain. A flat void is formed between the second base layer 6 and the metal nitride layer 5 due to the voids remaining in the void-containing first base layer 4 . This void causes the second base layer 6 to peel off in the peeling step S380, which will be described later.

Further, at this time, tensile stress is introduced into the second base layer 6 by the initial nuclei generated during the growth process attracting each other. Therefore, due to the tensile stress generated in the second base layer 6, internal stress acts so that the c-plane of the second base layer 6 is depressed. Furthermore, the dislocation density on the main surface 6s side of the second underlayer 6 is low, while the dislocation density on the undersubstrate 1 side of the second underlayer 6 is high. Therefore, even due to the difference in dislocation density in the thickness direction of the second base layer 6, internal stress acts so that the c-plane of the second base layer 6 is depressed. Therefore, the c-plane of the second base layer 6 is curved into a concave spherical shape with respect to the main surface 6s. The term "spherical shape" used herein means a curved surface that is approximated to a spherical surface. Moreover, "spherical surface approximation" here means that a perfect circular spherical surface or an ellipsoidal spherical surface is approximated within a predetermined error range. Since the c-plane is curved as described above, the off-angle that the c-axis makes with respect to the normal to the center of the main surface 6s of the second base layer 6 has a predetermined distribution.

The base structure 10 is obtained through the base structure manufacturing step S100 described above.

(S200: Three-dimensional growth process (first layer growth process))
Thereafter, as shown in FIGS. 4(b), 4(c), and 5, a group III nitride semiconductor single crystal having a top surface 30u with an exposed c-plane 30c is Epitaxial growth is performed directly on the surface 6s. As a result, the first layer (three-dimensional growth layer) 30 is grown.

At this time, a plurality of recesses 30p surrounded by inclined interfaces 30i other than the c-plane are generated in the top surface 30u of the single crystal, and as the second base layer 6 goes above the inclined interfaces 30i, the inclined interfaces 30i are gradually , and the c-plane 30c is gradually reduced. This causes the c-plane 30c to disappear from the top surface 30u. As a result, the first layer 30 whose surface is composed of only the inclined interface 30i is grown.

That is, in the three-dimensional growth step S200, the first layer 30 is three-dimensionally grown so as to intentionally roughen the main surface 6s of the mirror-finished second base layer 6. Note that even if such a growth form is formed, the first layer 30 is grown as a single crystal as described above. In this respect, the first layer 30 is different from a so-called low-temperature growth buffer layer that is formed as an amorphous or polycrystalline layer on a foreign substrate such as sapphire before a group III nitride semiconductor is epitaxially grown on the substrate. be.

In this embodiment, as the first layer 30, for example, a layer made of the same group III nitride semiconductor as the group III nitride semiconductor constituting the base substrate 1 is epitaxially grown. Specifically, for example, a GaN layer is epitaxially grown as the first layer 30 by heating the base substrate 1 using the HVPE method and supplying GaCl gas and NH ₃ gas to the heated base substrate 1. .

Here, in the three-dimensional growth step S200, the first layer 30 is grown under, for example, predetermined first growth conditions in order to bring about the above-described growth process.

First, reference growth conditions in which the inclined interface 30i and the c-plane 30c neither expand nor contract will be described using FIG. 9(a). FIG. 9A is a schematic cross-sectional view showing the growth process under standard growth conditions in which the inclined interface and the c-plane neither expand nor contract.

In FIG. 9(a), the thick solid line indicates the surface of the first layer 30 for each unit time. The inclined interface 30i shown in FIG. 9(a) is the inclined interface that is the most inclined with respect to the c-plane 30c. In addition, in FIG. 9A, the growth rate of the c-plane 30c in the first layer 30 is G _c0 , the growth rate of the inclined interface 30i in the first layer 30 is G _i , and in the first layer 30 Let α be the angle formed by the c-plane 30c and the inclined interface 30i. Further, in FIG. 9A, it is assumed that the first layer 30 is grown while maintaining the angle α between the c-plane 30c and the inclined interface 30i. Note that it is assumed that the off-angle of the c-plane 30c of the first layer 30 is negligible compared to the angle α between the c-plane 30c and the inclined interface 30i.

As shown in FIG. 9(a), when the inclined interface 30i and the c-plane 30c do not expand or contract, the locus of the intersection between the inclined interface 30i and the c-plane 30c is perpendicular to the c-plane 30c. . From this, the standard growth condition in which the inclined interface 30i and the c-plane 30c neither expand nor contract satisfies the following formula (a).
G _c0 =G _i /cosα...(a)

Next, a first growth condition in which the inclined interface 30i expands and the c-plane 30c contracts will be described using FIG. 9(b). FIG. 9(b) is a schematic cross-sectional view showing the growth process under the first growth condition in which the inclined interface expands and the c-plane contracts.

Also in FIG. 9(b), similarly to FIG. 9(a), the thick solid line indicates the surface of the first layer 30 for each unit time. Further, the inclined interface 30i shown in FIG. 9(b) is also the inclined interface that is most inclined with respect to the c-plane 30c. In addition, in FIG. 9(b), the growth rate of the c-plane 30c of the first layer 30 is G _c1 , and the progress rate of the trajectory of the intersection of the inclined interface 30i and the c-plane 30c of the first layer 30 is Let it be _R1 . Furthermore, the narrower angle between the locus of the intersection of the inclined interface 30i and the c-plane 30c and the angle formed by the c-plane 30c is α _R1 . When the angle formed by the R ₁ direction and the G _i direction is α', α'=α+90−α _R1 . Note that it is assumed that the off-angle of the c-plane 30c of the first layer 30 is negligible compared to the angle α between the c-plane 30c and the inclined interface 30i.

As shown in FIG. 9(b), the progress rate _R1 of the trajectory of the intersection of the inclined interface 30i and the c-plane 30c is expressed by the following equation (b).
R ₁ =G _i /cosα'...(b)

Further, the growth rate G _c1 of the c-plane 30c of the first layer 30 is expressed by the following equation (c).
G _c1 = R ₁ sin α _R1 ...(c)

By substituting equation (b) into equation (c), G _c1 is expressed by the following equation (d) using G _i .
G _c1 = G _i sin α _R1 /cos (α+90-α _R1 ) ... (d)

In order for the inclined interface 30i to expand and the c-plane 30c to contract, it is preferable that α _R1 <90°. Therefore, it is preferable that the first growth condition in which the inclined interface 30i expands and the c-plane 30c contracts satisfies the following formula (1) using formula (d) and α _R1 <90°.
G _c1 >G _i /cosα...(1)
However, as mentioned above, G _i is the growth rate of the inclined interface 30i that is the most inclined with respect to the c-plane 30c, and α is the growth rate of the inclined interface 30i that is the most inclined with respect to the c-plane 30c, and the growth rate of the inclined interface 30i that is the most inclined with respect to the c-plane 30c. It is the angle formed by

Alternatively, it can be considered that it is preferable that G _c1 under the first growth conditions is larger than G _c0 under the reference growth conditions. Also from this, equation (1) can be derived by substituting equation (a) into G _c1 >G _c0 .

Note that the growth conditions for enlarging the inclined interface 30i that is the most inclined with respect to the c-plane 30c are the most severe, so if the first growth condition satisfies formula (1), the other inclined interfaces 30i can also be enlarged. becomes possible.

Specifically, for example, when the inclined interface 30i that is most inclined with respect to the c-plane 30c is a {10-11} plane, α=61.95°. Therefore, it is preferable that the first growth condition satisfies the following formula (1'), for example.
G _c1 >2.13G _i ...(1')

Alternatively, as described later, for example, if the inclined interface 30i is a {11-2m} plane with m≧3, the inclined interface 30i that is most inclined with respect to the c-plane 30c is a {11-23} plane. Therefore, α=47.3°. Therefore, it is preferable that the first growth condition satisfies the following formula (1''), for example.
G _c1 >1.47G _i ...(1")

As the first growth condition of this embodiment, for example, the growth temperature in the three-dimensional growth step S200 is lower than the growth temperature in the planarization step S300, which will be described later. Specifically, the growth temperature in the three-dimensional growth step S200 is, for example, 980°C or more and 1,020°C or less, preferably 1,000°C or more and 1,020°C or less.

Further, as the first growth condition of this embodiment, for example, the V/III ratio in the three-dimensional growth step S200 may be made larger than the V/III ratio in the planarization step S300, which will be described later. Specifically, the V/III ratio in the three-dimensional growth step S200 is, for example, 2 or more and 20 or less, preferably 2 or more and 15 or less.

Actually, as the first growth condition, at least one of the growth temperature and the V/III ratio is adjusted within the above ranges so as to satisfy equation (1).

Note that other conditions among the first growth conditions of this embodiment are, for example, as follows.
Growth pressure: 90 to 105 kPa, preferably 90 to 95 kPa
Partial pressure of GaCl gas: 1.5-15kPa
_N2 gas flow rate/ _H2 gas flow rate: 0 to 1

Here, the three-dimensional growth step S200 of this embodiment is classified into two steps based on, for example, the form of the first layer 30 during growth. Specifically, the three-dimensional growth step S200 of this embodiment includes, for example, an inclined interface expanding step S220 and an inclined interface maintaining step S240. Through these steps, the first layer 30 includes, for example, the inclined interface expanding layer 32 and the inclined interface maintaining layer 34.

(S220: Inclined interface expansion step)
First, as shown in FIG. 4(b), FIG. 4(c), and FIG. Then, epitaxial growth is performed on the second base layer 6.

At the initial stage of growth of the inclined interface enlarging layer 32, the inclined interface enlarging layer 32 grows in the direction normal to the main surface 1s of the base substrate 1 (direction along the c-axis) with the c-plane 30c as a growth plane.

By gradually growing the inclined interface enlarging layer 32 under the first growth condition, as shown in FIGS. , a plurality of recesses 30p are formed by inclined interfaces 30i other than the c-plane. A plurality of recesses 30p constituted by inclined interfaces 30i other than the c-plane are randomly formed on the top surface 30u. As a result, the inclined interface expansion layer 32 is formed in which the c-plane 30c and the inclined interface 30i other than the c-plane coexist on the surface.

Note that the "tilted interface 30i" herein means a growth interface that is tilted with respect to the c-plane 30c, and includes a facet with a low index other than the c-plane, a facet with a high index other than the c-plane, or a facet with a plane index. Contains slopes that cannot be represented by . Note that facets other than the c-plane are, for example, {11-2m}, {1-10n}, and the like. However, m and n are integers other than 0.

In this embodiment, the inclined interface enlarging layer 32 is grown on the mirror-finished second base layer 6 described above, and the first growth conditions are adjusted so as to satisfy formula (1), so that the inclined interface 30i is grown. , for example, a {11-2m} plane with m≧3 can be generated. Thereby, the inclination angle of the {11-2m} plane with respect to the c-plane 30c can be made gentle. Specifically, the inclination angle can be 47.3° or less.

By further growing the inclined interface enlarging layer 32 under the first growth condition, as shown in FIG. The inclined interface 30i is gradually enlarged, and the c-plane 30c is gradually reduced. Note that at this time, as one goes above the second base layer 6, the angle of inclination that the inclined interface 30i makes with respect to the main surface 1s of the base substrate 1 gradually becomes smaller. As a result, most of the inclined interface 30i finally becomes the {11-2m} plane with m≧3 as described above.

As the inclined interface enlarging layer 32 is further grown, the c-plane 30c of the inclined interface enlarging layer 32 disappears from the top surface 30u, and the surface of the inclined interface enlarging layer 32 is composed only of the inclined interface 30i. As a result, the inclined interface enlarging layer 32 in which cones are continuously combined is formed.

In this way, by creating a plurality of recesses 30p composed of inclined interfaces 30i other than the c-plane on the top surface 30u of the inclined interface enlarging layer 32 and eliminating the c-plane 30c, as shown in FIG. 4(c), Then, a plurality of valleys 30v and a plurality of tops 30t are formed on the surface of the inclined interface enlarging layer 32. Each of the plurality of valleys 30v is a downwardly convex inflection point on the surface of the inclined interface enlarging layer 320, and is formed above the position where each of the inclined interfaces 30i other than the c-plane occurs. On the other hand, each of the plurality of peaks 30t is an upwardly convex inflection point on the surface of the inclined interface enlarging layer 320, and is a c-plane with a pair of inclined interfaces 30i enlarged in opposite directions. 30c is formed at or above the position where it (last) disappeared. The valley portions 30v and the top portions 30t are formed alternately in the direction along the main surface 1s of the base substrate 1.

In the present embodiment, in the second base layer forming step S150, the second base layer 6 having a mirror-finished main surface 6s is grown on the main surface 1s of the base substrate 1, and then in the inclined interface enlarging step S220. , a sloped interface 30i other than the c-plane is generated on the surface of the sloped interface expansion layer 32. Thereby, the plurality of valleys 30v are formed at positions upwardly away from the main surface 6s of the second base layer 6.

Through the growth process of the inclined interface expansion layer 32 as described above, dislocations propagate in a bent manner as described below. Specifically, as shown in FIG. 4(c), the plurality of dislocations extending in the direction along the c-axis within the second underlayer 6 are transferred from the second underlayer 6 to the inclined interface expansion layer 32. propagates in the direction along the c-axis of In the region of the inclined interface expansion layer 32 that is grown using the c-plane 30c as a growth plane, dislocations propagate in the direction along the c-axis of the inclined interface expansion layer 32. However, when a dislocation that has propagated in the direction along the c-axis of the inclined interface expansion layer 32 is exposed to the inclined interface 30i, the dislocation is located approximately perpendicular to the inclined interface 30i at the exposed position of the inclined interface 30i. It propagates by bending in the direction. That is, the dislocation propagates in a bent direction with respect to the c-axis. As a result, in the steps after the inclined interface expansion step S220, dislocations are locally collected above approximately the center between the pair of top portions 30t. As a result, the dislocation density on the surface of the second layer 40, which will be described later, can be reduced.

At this time, in this embodiment, when looking at an arbitrary cross section perpendicular to the main surface 1s of the base substrate 1, the closest of the plurality of peaks 30t across one of the plurality of valleys 30v. The average distance L between the pair of top portions 30t in the direction along the principal surface 1s of the base substrate 1 (also referred to as the “average distance between nearest adjacent top portions”) is, for example, more than 100 μm. When the average distance L between the nearest apexes is 100 μm or less, as in the case where fine hexagonal pyramidal crystal nuclei are generated on the second base layer 6 from the initial stage of the inclined interface expanding step S220, the inclined interface is enlarged. In the steps after step S220, the distance that the dislocations bend and propagate becomes shorter. For this reason, dislocations are not sufficiently collected above the approximate center between the pair of top portions 30t of the inclined interface expansion layer 32. As a result, the dislocation density on the surface of the second layer 40, which will be described later, may not be sufficiently reduced. On the other hand, in this embodiment, by setting the average distance L between the closest vertices to more than 100 μm, the distance over which dislocations bend and propagate is ensured at least more than 50 μm in the steps after the inclined interface expansion step S220. be able to. Thereby, dislocations can be sufficiently collected above the approximate center between the pair of top portions 30t of the inclined interface expansion layer 32. As a result, the dislocation density on the surface of the second layer 40, which will be described later, can be sufficiently reduced.

In the present embodiment, for example, it is preferable that there is no portion in the first layer 30 where the distance between the closest apexes is 100 μm or less. In other words, when looking at an arbitrary cross section perpendicular to the main surface 1s of the base substrate 1, it is preferable that the distance between the closest apexes is more than 100 μm over the entire surface of the first layer 30. Thereby, the dislocation density can be reduced substantially uniformly over the entire surface of the second layer 40, which will be described later.

On the other hand, in this embodiment, the average distance L between the closest apexes is less than 800 μm. If the average distance L between the closest peaks is 800 μm or more, the height from the valley 30v to the top 30t of the inclined interface enlarging layer 32 becomes excessively high. Therefore, in the planarization step S300, which will be described later, the thickness of the second layer 40 becomes thick until it becomes mirror-finished. On the other hand, in the present embodiment, by setting the average distance L between the closest apexes to less than 800 μm, the height from the valley portion 30v to the apex portion 30t of the inclined interface enlarging layer 32 can be reduced. Thereby, the second layer 40 can be mirror-finished quickly in the planarization step S300, which will be described later.

At this time, the inclined interface expansion layer 320 includes a first c-plane growth region 60 grown with the c-plane 30c as a growth plane and an inclined interface 30i other than the c-plane, based on the difference in growth planes during the growth process. An inclined interface growth region 70 (gray part in the figure) is formed as a growth surface.

Furthermore, at this time, in the first c-plane growth region 60, a valley portion 60a is formed at a position where the inclined interface 30i occurs, and a peak portion 60b is formed at a position where the c-plane 30c disappears. Furthermore, in the first c-plane growth region 60, a pair of inclined portions 60i are formed on both sides of the peak portion 60b as a locus of the intersection of the c-plane 30c and the inclined interface 30i.

Further, at this time, the first growth condition satisfies equation (1), so that the angle β formed by the pair of inclined portions 60i is, for example, 70° or less.

Details of these areas will be described later.

(S240: Inclined interface maintenance step)
After the c-plane 30c disappears from the surface of the inclined interface expansion layer 32, as shown in FIG. The growth of the first layer 30 is continued over the period of time. As a result, a sloped interface maintenance layer 34 having a surface where the sloped interface 30i occupies more than the c-plane 30c is formed on the sloped interface expansion layer 32. By forming the inclined interface maintaining layer 34, it is possible to reliably eliminate the c-plane 30c over the entire surface of the first layer 30.

At this time, the c-plane 30c may reappear on a part of the surface of the inclined interface maintaining layer 34, but the area ratio occupied by the inclined interface growth region 70 in the creeping cross section along the main surface 1s of the base substrate 1 is 80%. % or more, it is preferable to mainly expose the inclined interface 30i on the surface of the inclined interface maintaining layer 34. Note that the higher the area ratio occupied by the inclined interface growth region 70 in the creeping cross section is, the better, and is preferably 100%.

Moreover, at this time, the growth conditions in the inclined interface maintenance step S240 are maintained at the above-mentioned first growth conditions, similarly to the inclined interface expanding step S220. Thereby, the inclined interface maintaining layer 34 can be grown using the inclined interface 30i as a growth surface.

In addition, at this time, by growing the inclined interface maintaining layer 34 using the inclined interface 30i as a growth surface under the first growth condition, as described above, at the position where the inclined interface 30i is exposed in the inclined interface expanding layer 32, The dislocations that have propagated by bending in a direction oblique to the c-axis continue to propagate in the same direction in the inclined interface maintenance layer 34 as well.

Through the above three-dimensional growth step S200, the first layer 30 having the inclined interface expanding layer 32 and the inclined interface maintaining layer 34 is formed.

In the three-dimensional growth step S200 of the present embodiment, the height from the main surface 6s of the second base layer 6 to the top 30t of the first layer 30 (maximum height in the thickness direction of the first layer 30) is, for example, More than 100 μm and less than 1.5 mm.

(S300: Planarization process (second layer growth process))
After growing the first layer 30 in which the c-plane 30c has disappeared, a single crystal of a group III nitride semiconductor is further epitaxially grown on the first layer 30, as shown in FIGS. 6(b) and 7(a). let

At this time, as one goes above the first layer 30, the inclined interface 40i is gradually reduced and the c-plane 40c is gradually enlarged. As a result, the inclined interface 30i formed on the surface of the first layer 30 disappears. As a result, a second layer (flattening layer) 40 having a mirror-finished surface is grown.

In this embodiment, as the second layer 40, for example, a layer whose main component is the same group III nitride semiconductor as the group III nitride semiconductor constituting the first layer 30 is epitaxially grown. In the planarization step S300, GaCl gas, NH ₃ gas, and SiH ₂ Cl ₂ gas as an n-type dopant gas are supplied to the base substrate 1 heated to a predetermined growth temperature, thereby forming the second layer. 40, a silicon (Si) doped GaN layer is epitaxially grown. Note that GeCl ₄ gas or the like may be supplied as the n-type dopant gas instead of SiH ₂ Cl ₂ gas.

Here, in the planarization step S300, the second layer 40 is grown under, for example, predetermined second growth conditions in order to bring about the above-described growth process.

A second growth condition in which the inclined interface 40i shrinks and the c-plane 40c expands will be described using FIG. 10. FIG. 10 is a schematic cross-sectional view showing the growth process under the second growth condition in which the inclined interface shrinks and the c-plane expands. FIG. 10 shows a process in which the second layer 40 is grown on the first layer 30 in which the inclined interface 30i that is most inclined with respect to the c-plane 30c is exposed.

In FIG. 10, similarly to FIG. 9(a), the thick solid line indicates the surface of the second layer 40 for each unit time. In addition, in FIG. 10, the growth rate of the c-plane 40c of the second layer 40 is G _c2 , the growth rate of the inclined interface 40i of the second layer 40 is G _i , and the growth rate of the inclined interface 40i of the second layer 40 is G c2. The progress rate of the locus of the intersection between the interface 40i and the c-plane 40c is assumed to be _R2 . Furthermore, the narrower angle between the locus of the intersection of the inclined interface 40i and the c-plane 40c and the angle formed by the c-plane 30c is α _R2 . When the angle formed by the _R2 direction and the G _i direction is α'', α''=α−(90−α _R2 ). Further, in FIG. 10, it is assumed that the second layer 40 is grown while maintaining the angle α between the c-plane 30c and the inclined interface 30i in the first layer 30. It is assumed that the off-angle of the c-plane 40c of the second layer 40 is negligible compared to the angle α between the c-plane 30c and the inclined interface 30i.

As shown in FIG. 10, the progress rate _R2 of the trajectory of the intersection between the inclined interface 40i and the c-plane 40c is expressed by the following equation (e).
R ₂ =G _i /cos α”...(e)

Further, the growth rate G _c2 of the c-plane 40c of the second layer 40 is expressed by the following equation (f).
G _c2 = R ₂ sin α _R2 ...(f)

By substituting equation (e) into equation (f), G _c2 is expressed by the following equation (g) using G _i .
G _c2 = G _i sin α _R2 /cos (α + α _R2 -90) ... (g)

In order for the inclined interface 40i to shrink and the c-plane 40c to expand, it is preferable that α _R2 <90°. Therefore, the second growth condition under which the inclined interface 40i shrinks and the c-plane 40c expands preferably satisfies the following equation (2) using equation (g) and α _R2 <90°.
G _c2 <G _i /cosα...(2)
However, as mentioned above, G _i is the growth rate of the inclined interface 40i that is the most inclined with respect to the c-plane 40c, and α is the growth rate of the inclined interface 40i that is the most inclined with respect to the c-plane 40c, and the growth rate of the inclined interface 40i that is the most inclined with respect to the c-plane 40c. It is the angle formed by

Alternatively, when the growth rate of the c-plane 30c of the second layer 40 under the standard growth conditions is _Gc0 , _Gc2 under the second growth conditions is smaller than _Gc0 under the standard growth conditions. This can also be considered preferable. From this, equation (2) can be derived by substituting equation (a) into G _c2 <G _c0 .

Note that, since the growth conditions that reduce the inclined interface 40i that is the most inclined with respect to the c-plane 40c are the most severe conditions, if the second growth condition satisfies formula (2), the other inclined interfaces 40i can also be reduced. becomes possible.

Specifically, when the inclined interface 40i that is most inclined with respect to the c-plane 40c is a {10-11} plane, the second growth condition preferably satisfies the following formula (2').
G _c2 <2.13G _i ...(2')

Alternatively, for example, if the inclined interface 30i is a {11-2m} plane with m≧3, the inclined interface 30i that is most inclined with respect to the c-plane 30c is the {11-23} plane, so that the second growth It is preferable that the condition satisfies the following formula (2''), for example.
G _c2 <1.47G _i ...(2")

As the second growth condition of this embodiment, the growth temperature in the planarization step S300 is set higher than the growth temperature in the three-dimensional growth step S200, for example. Specifically, the growth temperature in the planarization step S300 is, for example, 990° C. or more and 1,120° C. or less, preferably 1,020° C. or more and 1,100° C. or less.

Further, as the second growth condition of this embodiment, the V/III ratio in the planarization step S300 may be adjusted. For example, the V/III ratio in the planarization step S300 may be smaller than the V/III ratio in the three-dimensional growth step S200. Specifically, the V/III ratio in the planarization step S300 is, for example, 1 or more and 10 or less, preferably 1 or more and 5 or less.

Actually, as the second growth condition, at least one of the growth temperature and the V/III ratio is adjusted within the above ranges so as to satisfy equation (2).

Note that other conditions among the second growth conditions of this embodiment are, for example, as follows.
Growth pressure: 90 to 105 kPa, preferably 90 to 95 kPa
Partial pressure of GaCl gas: 1.5-15kPa
_N2 gas flow rate/ _H2 gas flow rate: 1 to 20

Here, the planarization step S300 of this embodiment is classified into two steps based on, for example, the form of the second layer 40 during growth. Specifically, the planarization step S300 of this embodiment includes, for example, a c-plane enlargement step S320 and a main growth step S340. Through these steps, the second layer 40 includes, for example, the c-plane enlarged layer 42 and the main growth layer 44.

(S320: c-plane enlargement step)
As shown in FIG. 6(b), a c-plane enlarged layer 42 of a second layer 40 made of a single crystal of a group III nitride semiconductor is epitaxially grown on the first layer 30 under the second growth conditions described above.

At this time, as one goes above the first layer 30, the c-plane 40c is enlarged while the inclined interface 40i other than the c-plane is reduced.

Specifically, by growing under the second growth condition, the c-plane expansion layer 42 grows from the inclined interface 30i of the inclined interface maintaining layer 34 in a direction perpendicular to the c-axis with the inclined interface 40i as the growth plane. (i.e., creeping or lateral). As the c-plane enlarged layer 42 grows laterally, the c-plane 40c of the c-plane enlarged layer 42 begins to be exposed again above the top 30t of the inclined interface maintenance layer 34. As a result, the c-plane enlarged layer 42 is formed in which the c-plane 40c and the inclined interface 40i other than the c-plane coexist on the surface.

When the c-plane enlarged layer 42 is further grown laterally, the c-plane 40c gradually expands, and the inclined interface 40i of the c-plane enlarged layer 42 gradually shrinks. As a result, the recesses 30p formed by the plurality of inclined interfaces 30i on the surface of the first layer 30 are gradually filled.

After that, when the c-plane enlarged layer 42 is further grown, the inclined interface 40i of the c-plane enlarged layer 42 completely disappears, and the recess 30p formed by the plurality of inclined interfaces 30i on the surface of the first layer 30 is completely filled. It will be done. As a result, the surface of the c-plane enlarging layer 42 becomes a mirror surface (flat surface) composed only of the c-plane 40c.

At this time, dislocation density can be reduced by locally collecting dislocations during the growth process of the first layer 30 and the c-plane enlarged layer 42. Specifically, the dislocations that have propagated in the first layer 30 by bending in a direction oblique to the c-axis continue to propagate in the same direction in the c-plane expanded layer 42 as well. As a result, dislocations are locally collected in the c-plane enlarged layer 42 at the meeting portion of the adjacent inclined interfaces 40i above the approximate center between the pair of top portions 30t. Among the plurality of dislocations gathered at the meeting portion of the adjacent inclined interfaces 40i in the c-plane enlarged layer 42, dislocations having mutually opposing Burgers vectors disappear when they come together. Further, some of the plurality of dislocations gathered at the meeting point of the adjacent inclined interfaces 40i form a loop and propagate in the direction along the c-axis (that is, toward the surface side of the c-plane expanded layer 42). suppressed. Note that the other portions of the plurality of dislocations gathered at the meeting part of the adjacent inclined interfaces 40i in the c-plane enlarged layer 42 change their propagation direction from the direction inclined with respect to the c-axis to the direction along the c-axis. It is changed again and propagated to the surface side of the second layer 40. By eliminating some of the plurality of dislocations or suppressing the propagation of some of the plurality of dislocations to the surface side of the c-plane expanded layer 42 in this way, the dislocation density on the surface of the second layer 40 is reduced. can be reduced. Furthermore, by locally collecting dislocations, a low dislocation density region can be formed above the portion of the second layer 40 where the dislocations have propagated in a direction oblique to the c-axis.

At this time, in the c-plane enlarged layer 42, as the c-plane 40c gradually expands, a second c-plane growth region 80, which will be described later and has grown with the c-plane 40c as a growth plane, increases upward in the thickness direction. Therefore, it is formed while gradually expanding.

On the other hand, in the c-plane enlarged layer 42, the inclined interface 40i gradually shrinks, so that the inclined interface growth region 70 gradually shrinks upward in the thickness direction, and ends at a predetermined position in the thickness direction. . Through this growth process of the c-plane enlarged layer 42, the valley portion 70a of the inclined interface growth region 70 is formed at the position where the c-plane 40c has regenerated in cross-sectional view. In addition, in the process of gradually filling in the recess formed by the inclined interface 40i, the peak 70b of the inclined interface growth region 70 is formed at the position where the inclined interface 40i disappears in a cross-sectional view.

In the c-plane enlarging step S320, the surface of the c-plane enlarging layer 42 becomes a mirror surface composed of only the c-plane 40c, so the height in the thickness direction (maximum height in the thickness direction) of the c-plane enlarging layer 42 is , for example, is equal to or higher than the height from the valley portion 30v to the top portion 30t of the inclined interface maintenance layer 34.

(S340: Main growth step (c-plane growth step))
When the inclined interface 40i disappears in the c-plane enlarged layer 42 and the surface becomes mirror-finished, as shown in FIG. A main growth layer 44 is formed over this area. This forms the main growth layer 44 which does not have the inclined interface 40i and has only the c-plane 40c on its surface.

At this time, the growth conditions in the main growth step S340 are maintained at the above-mentioned second growth conditions, similarly to the c-plane enlargement step S320. Thereby, the main growth layer 44 can be grown in a step-flow manner using the c-plane 40c as a growth plane.

Also, at this time, by growing the main growth layer 44 using only the c-plane 40c as a growth surface without exposing the inclined interface 40i, the entire main growth layer 44 forms a second c-plane growth region 80, which will be described later. Become.

In the main growth step S340, the thickness of the main growth layer 44 is, for example, 300 μm or more and 10 mm or less. By setting the thickness of the main growth layer 44 to 300 μm or more, at least one substrate 50 can be sliced from the main growth layer 44 in the slicing step S400 described later. On the other hand, by setting the thickness of the main growth layer 44 to 10 mm, the final thickness is 650 μm, and when slicing a 700 μm thick substrate 50 from the main growth layer 44, even if a kerf loss of about 200 μm is taken into consideration. , at least 10 substrates 50 can be obtained.

Through the above steps, the second layer 40 having the c-plane enlarged layer 42 and the main growth layer 44 is formed.

(S380: Peeling process)
After the growth of the second layer 40 is completed, as shown in FIG. 7(b), the laminated structure 90 having the second base layer 6, the first layer 30, and the second layer 40 is peeled off from the base substrate 1. .

Specifically, in the process of cooling the inside of the chamber of the vapor phase growth apparatus, stress is generated between the above-described laminated structure 90 and base substrate 1 due to the difference in their linear expansion coefficients. For example, the base substrate 1 made of sapphire has a larger coefficient of linear expansion than the laminated structure 90 mainly made of GaN, and thus generates stress that causes the base substrate 1 to contract with respect to the laminated structure 90.

As a result, the above-described laminated structure 90 is naturally peeled off from the base substrate 1 due to the flat gap formed between the metal nitride layer 5 and the second base layer 6. Thereby, the laminated structure 90 can be peeled off from the base substrate 1 without causing cracks in the laminated structure 90.

Through the above steps, the laminated structure 90 of this embodiment is obtained.

Note that the above steps from the second base layer forming step S150 to the planarization step S300 are performed continuously in the same vapor phase growth apparatus without exposing the base substrate 1 to the atmosphere. As a result, an unintended high oxygen concentration region (more than the inclined interface growth region 70 The formation of a region with an excessively high oxygen concentration can be suppressed.

(S400: slicing process)
Next, as shown in FIG. 8, the main growth layer 44 is sliced, for example, along a cutting plane substantially parallel to the surface of the main growth layer 44 using a wire saw. As a result, at least one nitride semiconductor substrate 50 (also referred to as substrate 50) as an as-sliced substrate is formed. At this time, the thickness of the substrate 50 is, for example, 300 μm or more and 700 μm or less.

At this time, the radius of curvature of the c-plane 50c of the substrate 50 is set when the second base layer is grown to the same thickness as the second layer and the second base layer is sliced without performing the three-dimensional growth process and the planarization process. can be made larger than the radius of curvature of the c-plane of a nitride semiconductor substrate (that is, a nitride semiconductor substrate obtained by the conventional VAS method). Thereby, the variation in the off-angle θ of the c-axis 50ca with respect to the normal to the main surface 50s of the substrate 50 can be made smaller than the variation in the c-axis off-angle of the nitride semiconductor substrate obtained by the conventional VAS method.

(S500: Polishing process)
Next, both sides of the substrate 50 are polished using a polishing device. Note that at this time, the final thickness of the substrate 50 is, for example, 250 μm or more and 650 μm or less.

Through the above steps S100 to S500, the substrate 50 according to this embodiment is manufactured.

(Manufacturing process of semiconductor laminate and manufacturing process of semiconductor device)
Once the substrate 50 is manufactured, for example, a semiconductor functional layer made of a group III nitride semiconductor is epitaxially grown on the substrate 50 to produce a semiconductor laminate. After producing the semiconductor laminate, electrodes and the like are formed using the semiconductor laminate, and the semiconductor laminate is diced to cut out chips of a predetermined size. In this way, a semiconductor device is manufactured.

(2) Laminated structure Next, the laminated structure 90 according to this embodiment will be explained using FIG. 7(b).

The laminated structure 90 of this embodiment includes, for example, the second base layer 6, the first layer 30, and the second layer 40. Note that the second base layer 6 can be simply referred to as a "base layer."

The second base layer 6 is made of, for example, a group III nitride single crystal. The second base layer 6 has, for example, (a trace of) a mirror-finished main surface 6s. The closest low-index crystal plane to the main surface 6s of the second underlayer is, for example, the c-plane.

The first layer 30 is grown on the second base layer 6, for example.

The first layer 30 is formed, for example, by creating a plurality of recesses 30p composed of inclined interfaces 30i other than the c-plane in the top surface 30u of a single crystal of a group III nitride semiconductor, and by eliminating the c-plane 30c. It has a plurality of valleys 30v and a plurality of tops 30t. When looking at an arbitrary cross section perpendicular to the main surface 6s of the second base layer 6, the average distance between the closest apexes is, for example, more than 100 μm.

In addition, the first layer 30 is divided into a first c-plane growth region (first low oxygen concentration region) 60 and an inclined interface growth region (high oxygen concentration region) 70, for example, based on the difference in the growth plane during the growth process. ,have.

The first c-plane growth region 60 is a region grown using the c-plane 30c as a growth plane. The first c-plane growth region 60 has, for example, a plurality of valleys 60a and a plurality of peaks 60b in a cross-sectional view.
Note that each of the troughs 60a and the peaks 60b herein means a part of the shape observed based on the difference in emission intensity when the cross section of the laminated structure 90 is observed with a fluorescence microscope, etc. It does not mean a part of the outermost surface shape that occurs during the growth of 30. Each of the plurality of valleys 60a is a downwardly convex inflection point of the first c-plane growth region 60 in cross-sectional view, and is formed at a position where the inclined interface 30i occurs. At least one of the plurality of valleys 60a is provided at a position upwardly away from the main surface 6s of the second base layer 6. On the other hand, each of the plurality of peaks 60b is an upwardly convex inflection point in the first c-plane growth region 60 in cross-sectional view, and forms a pair of inclined interfaces 30i enlarged in opposite directions. It is formed at the position where the c-plane 30c (last) disappears. The valley portions 60a and the peak portions 60b are formed alternately in the direction along the main surface 6s of the second base layer 6.

In cross-sectional view, the first c-plane growth region 60 has a pair of inclined portions 60i provided as a locus of the intersection of the c-plane 30c and the inclined interface 30i on both sides of one of the plurality of peaks 60b. are doing. Note that the sloped portion 60i herein refers to a portion of the shape observed based on the difference in luminescence intensity when the cross section of the stacked structure 90 is observed using a fluorescence microscope, etc., and is a portion of the shape that is observed during the growth of the first layer 30. This does not mean the outermost inclined interface 30i.

In cross-sectional view, the angle β formed by the pair of inclined portions 60i is, for example, 70° or less, preferably 20° or more and 65° or less. The fact that the angle β between the pair of inclined portions 60i is 70° or less means that the growth rate G _i of the inclined interface 30i that is the most inclined with respect to the c-plane 30c of the first layer 30 under the first growth condition is This means that the ratio G _c1 /G _i of the growth rate G _c1 of the c-plane 30c in the first layer 30 was high. Thereby, an inclined interface 30i other than the c-plane can be easily generated. As a result, it becomes possible to easily bend the dislocation at the position where the inclined interface 30i is exposed. Further, by setting the angle β between the pair of inclined portions 60i to 70° or less, a plurality of valleys 30v and a plurality of peaks 30t can be easily formed above the second base layer 6. Furthermore, by setting the angle β formed by the pair of inclined portions 60i to 65° or less, the inclined interface 30i other than the c-plane can be more easily generated, and a plurality of valleys are formed above the second base layer 6. 30v and multiple peaks 30t can be generated more easily. In addition, by setting the angle β between the pair of inclined parts 60i to 20 degrees or more, the height from the valley part 30v to the top part 30t of the first layer 30 is suppressed from increasing, and the second layer 40 becomes mirror-finished. It is possible to suppress the thickness from increasing.

On the other hand, the inclined interface growth region 70 is a region grown using the inclined interface 30i other than the c-plane as a growth plane. The lower surface of the inclined interface growth region 70 is formed, for example, following the shape of the first c-plane growth region 60. The inclined interface growth region 70 is continuously provided along the main surface 6s of the second base layer 6.

The inclined interface growth region 70 can more easily incorporate oxygen than the first c-plane growth region 60. Therefore, the oxygen concentration in the inclined interface growth region 70 is higher than the oxygen concentration in the first c-plane growth region 60. Note that the oxygen taken into the inclined interface growth region 70 is, for example, oxygen that is unintentionally mixed into the vapor phase growth apparatus, or oxygen that is released from members (quartz members, etc.) that constitute the vapor phase growth apparatus. It is.

Note that the oxygen concentration in the first c-plane growth region 60 is, for example, 5×10 ¹⁶ cm ⁻³ or less, preferably 3×10 ¹⁶ cm ⁻³ or less. On the other hand, the oxygen concentration in the inclined interface growth region 70 is, for example, 3×10 ¹⁸ cm ⁻³ or more and 5×10 ¹⁹ cm ⁻³ or less.

The second layer 40 has, for example, an inclined interface growth region 70 and a second c-plane growth region (second low oxygen concentration region) 80 based on the difference in the growth plane during the growth process.

The upper surface of the inclined interface growth region 70 in the second layer 40 has, for example, a plurality of valleys 70a and a plurality of peaks 70b in a cross-sectional view. Note that the troughs 70a and the peaks 70b herein each mean a part of the shape observed based on the difference in emission intensity when the cross section of the laminated structure 90 is observed with a fluorescence microscope, etc. It does not mean a part of the outermost surface shape that occurs during the growth of 40. As described above, the plurality of valleys 70a of the inclined interface growth region 70 are formed at the positions where the c-plane 40c occurs again in cross-sectional view. Moreover, the plurality of valleys 70a of the inclined interface growth region 70 are each formed above the plurality of peaks 60b of the first c-plane growth region 60 in cross-sectional view. On the other hand, as described above, the plurality of peaks 70b of the inclined interface growth region 70 are formed at the positions where the inclined interface 40i disappears in cross-sectional view. Moreover, the plurality of peaks 70b of the inclined interface growth region 70 are each formed above the plurality of valleys 60a of the first c-plane growth region 60 in cross-sectional view.

In addition, a surface of the second layer 40 that is substantially parallel to the main surface 6s of the second base layer 6 at the upper end of the inclined interface growth region 70 becomes the boundary surface 40b at the position where the inclined interface 40i disappears in the second layer 40. .

The second c-plane growth region 80 is a region grown using the c-plane 40c as a growth plane. In the second c-plane growth region 80, oxygen uptake is suppressed compared to the inclined interface growth region 70. Therefore, the oxygen concentration in the second c-plane growth region 80 is lower than the oxygen concentration in the inclined interface growth region 70. The oxygen concentration in the second c-plane growth region 80 is, for example, 5×10 ¹⁶ cm ⁻³ or less, preferably 3×10 ¹⁶ cm ⁻³ or less.

In this embodiment, during the growth process of the first layer 30, dislocations bend and propagate in a direction substantially perpendicular to the inclined interface 30i at a position where the inclined interface 30i other than the c-plane is exposed. In the second layer 40, some of the plurality of dislocations disappear, and some of the plurality of dislocations are suppressed from propagating to the surface side of the c-plane enlarged layer 42. Thereby, the dislocation density on the surface of the second layer 40 is lower than the dislocation density on the main surface 6s of the second underlayer 6.

In addition, in this embodiment, the entire surface of the second layer 40 is constituted by a +c plane, and the first layer 30 and the second layer 40 each do not include a polarity inversion domain. In this respect, the laminated structure 90 of the present embodiment differs from a laminated structure formed by the so-called DEEP (Dislocation Elimination by the Epitaxial-growth with inverse-pyramidal pits) method. This is different from a laminated structure that includes a polarity reversal area in the core.

(3) Nitride semiconductor substrate (nitride semiconductor free-standing substrate, nitride crystal substrate)
Next, the nitride semiconductor substrate 50 according to this embodiment will be described using FIG. 11. FIG. 11(a) is a schematic top view showing the nitride semiconductor substrate according to the present embodiment, and FIG. 11(b) is a schematic cross-sectional view along the m-axis of the nitride semiconductor substrate according to the present embodiment, (c) is a schematic cross-sectional view along the a-axis of the nitride semiconductor substrate according to the present embodiment.

In this embodiment, the substrate 50 obtained by slicing the second layer 40 by the above-described manufacturing method is, for example, a free-standing substrate made of a single crystal of a group III nitride semiconductor. In this embodiment, the substrate 50 is, for example, a GaN free-standing substrate.

The diameter of the substrate 50 is, for example, 2 inches or more. Further, the thickness of the substrate 50 is, for example, 300 μm or more and 1 mm or less.

The conductivity of the substrate 50 is not particularly limited, but when manufacturing a semiconductor device as a vertical Schottky barrier diode (SBD) using the substrate 50, the substrate 50 is, for example, n-type, The n-type impurity in the substrate 50 is, for example, Si or germanium (Ge), and the n-type impurity concentration in the substrate 50 is, for example, 1.0×10 ¹⁸ cm ⁻³ or more and 1.0×10 ²⁰ cm ⁻³ or less. .

The substrate 50 has, for example, a main surface 50s that serves as an epitaxial growth surface. In this embodiment, the closest low-index crystal plane to the main surface 50s is, for example, the c-plane 50c.

Note that the main surface 50s of the substrate 50 is, for example, mirror-finished, and the root mean square roughness RMS of the main surface 50s of the substrate 50 is, for example, less than 1 nm.

Further, in this embodiment, the impurity concentration in the substrate 50 obtained by the above-described manufacturing method is lower than that of a substrate obtained by a flux method, an ammonothermal method, or the like.

Specifically, the hydrogen concentration in the substrate 50 is, for example, less than 1×10 ¹⁷ cm ⁻³ , preferably 5×10 ¹⁶ cm ⁻³ or less.

Further, in this embodiment, since the substrate 50 is formed by slicing the main growth layer 44 grown using the c-plane 40c as a growth surface, the inclined interface growth layer grown using the inclined interface 30i or the inclined interface 40i as the growth surface It does not include area 70. That is, the entire substrate 50 is constituted by a low oxygen concentration region.

Specifically, the oxygen concentration in the substrate 50 is, for example, 5×10 ¹⁶ cm ⁻³ or less, preferably 3×10 ¹⁶ cm ⁻³ or less.

(c-plane curvature and off-angle variation)
As shown in FIGS. 11(b) and 11(c), in this embodiment, the c-plane 50c as a low-index crystal plane closest to the main surface 50s of the substrate 50 is, for example, Due to the method, it is curved into a concave spherical shape with respect to the main surface 50s.

In this embodiment, the c-plane 50c of the substrate 50 has a curved shape that is approximated to a spherical surface in each of the cross-section along the m-axis and the cross-section along the a-axis, for example.

In this embodiment, since the c-plane 50f of the substrate 50 is curved into a concave spherical shape as described above, at least a part of the c-axis 50ca is inclined with respect to the normal to the main surface 50s. . The off angle θ, which is the angle that the c-axis 50ca makes with the normal to the main surface 50s, has a predetermined distribution within the main surface 50s.

Note that, of the off angle θ of the c-axis 50ca with respect to the normal to the main surface 50s, the directional component along the m-axis is “θ _m ” and the directional component along the a-axis is “θ _a ”. Note that θ ² =θ _m ² +θ _a ² .

In this embodiment, since the c-plane 50c of the substrate 50 is curved in a concave spherical shape as described above, the off-angle m-axis component θ _m and the off-angle a-axis component θ _a are each 1 of x. It can be approximately expressed by the following function and a linear function of y.

Specifically, for example, the X-ray rocking curve of the (0002) plane is measured at each position on a straight line passing through the center within the main surface 50s, and the peak formed by the X-rays incident on the main surface 50s and the main surface 50s is measured. When the angle ω is plotted against the position on the straight line, the peak angle ω can be approximated by a linear function of the position. Note that the "peak angle ω" herein refers to the angle formed between the X-rays incident on the main surface 50s and the main surface 50s, and is the angle at which the diffraction intensity is maximum. The radius of curvature of the c-plane 50c can be determined by the reciprocal of the slope of the linear function approximated as described above.

In this embodiment, the radius of curvature of the c-plane 50c of the substrate 50 is, for example, larger than the radius of curvature of the c-plane of the nitride semiconductor substrate obtained by the conventional VAS method described above.

Specifically, when the peak angle ω is approximated by a linear function of position in the X-ray rocking curve measurement of the c-plane 50c, the radius of curvature of the c-plane 50c determined by the reciprocal of the slope of the linear function is, for example, , 10 m or more, preferably 15 m or more, more preferably 19 m or more, even more preferably 40 m or more.

In this embodiment, the upper limit of the radius of curvature of the c-plane 50c of the substrate 50 is not particularly limited, since the larger the better. When the c-plane 50c of the substrate 50 is substantially flat, it may be considered that the radius of curvature of the c-plane 50c is infinite.

In this embodiment, since the radius of curvature of the c-plane 50c of the substrate 50 is large, the variation in the off-angle θ of the c-axis 50ca with respect to the normal to the main surface 50s of the substrate 50 can be reduced compared to that of a nitride semiconductor substrate using the conventional VAS method. This can be made smaller than the variation in the off-angle of the c-axis.

Furthermore, in this embodiment, when the peak angle ω is approximated by a linear function of position in the X-ray rocking curve measurement of the c-plane 50c, the error of ω with respect to the linear function of position is small. The error of ω in this embodiment can be applied to a substrate obtained from a crystal layer grown on a base substrate processed by the ELO method using a mask layer, or when the c-plane does not disappear in the three-dimensional growth process. It can be made smaller than the substrate etc. obtained from the second layer.

Specifically, the error of the measured peak angle ω with respect to the linear function approximated as described above is, for example, 0.05° or less, preferably 0.02° or less, more preferably 0.01° or less. It is. Note that since at least some of the peak angles ω may match a linear function, the minimum value of the error is 0°.

(scotoma)
Next, a dark spot on the principal surface 50s of the substrate 50 of this embodiment will be described. Note that the "dark spot" here refers to a point where the luminescence intensity is low observed in an observed image of the main surface 50s using a multiphoton excitation microscope or a cathodoluminescence image of the main surface 50s, and only dislocations are observed. It also includes non-emissive centers caused by foreign matter or point defects. Note that the "multiphoton excitation microscope" is also sometimes called a two-photon excitation fluorescence microscope.

In this embodiment, since the substrate 50 is manufactured using the base structure 10 manufactured by the VAS method, there are few non-emissive centers caused by foreign matter or point defects in the substrate 50. Therefore, 95% or more, preferably 99% or more of the dark spots when observing the main surface of the substrate 50 using a multiphoton excitation microscope or the like are not non-emissive centers caused by foreign matter or point defects, but are dislocations.

Moreover, in this embodiment, the dislocation density on the surface of the second layer 40 is lowered by the above-described manufacturing method than the dislocation density in the nitride semiconductor substrate obtained by the conventional VAS method.
Thereby, dislocations are also reduced in the main surface 50s of the substrate 50 formed by slicing the second layer 40.

Further, in this embodiment, the second layer 40 is sliced by performing the three-dimensional growth step S200 and the planarization step S300 using the base structure 10 in an unprocessed state by the above-described manufacturing method. In the principal surface 50s of the substrate 50 formed by the above method, regions with high dislocation density due to concentration of dislocations are not formed, and regions with low dislocation density are uniformly formed.

Specifically, in this embodiment, when the main surface 50s of the substrate 50 is observed with a field of view of 250 μm square using a multiphoton excitation microscope and the dislocation density is determined from the dark spot density, the dislocation density is 3×10 ⁶ cm ⁻ 80% or more, preferably 90% or more, more preferably 95% or more of the main surface 50s has no regions exceeding ² and has a dislocation density of less than 1×10 ⁶ cm ⁻² .

In other words, in the present embodiment, the average dislocation density over the entire principal surface 50s of the substrate 50 is, for example, less than 1×10 ⁶ cm ⁻² , preferably less than 5.5×10 ⁵ cm ⁻² . , more preferably 3×10 ⁵ cm ⁻² or less.

Further, the principal surface 50s of the substrate 50 of this embodiment includes a dislocation-free region of at least 50 μm square, for example, based on the average distance L between the closest apexes in the above-mentioned three-dimensional growth step S200. Further, the principal surface 50s of the substrate 50 of this embodiment has, for example, dislocation-free regions of 50 μm square that do not overlap at a density of 100 pieces/cm ² or more.

Next, the Burgers vector of dislocations in the substrate 50 of this embodiment will be explained.

In this embodiment, since the dislocation density on the main surface 6s of the second base layer 6 used in the above-described manufacturing method is low, when growing the first layer 30 and the second layer 40 on the second base layer 6, Multiple dislocations rarely combine (mix). This makes it possible to suppress the generation of dislocations having large Burgers vectors within the substrate 50 obtained from the second layer 40.

Specifically, in the substrate 50 of this embodiment, for example, there are many dislocations whose Burgers vector is one of <11-20>/3, <0001>, or <11-23>/3. Note that the "Burgers vector" here can be measured by, for example, a large angle convergent electron diffraction method (LACBED method) using a transmission electron microscope (TEM). Furthermore, a dislocation with a Burgers vector of <11-20>/3 is an edge dislocation, and a dislocation with a Burgers vector of <0001> is a screw dislocation, with a Burgers vector of <11-23>/3. Some dislocations are mixed dislocations in which edge dislocations and screw dislocations are mixed.

In this embodiment, when 100 dislocations are randomly extracted on the main surface 50s of the substrate 50, the Burgers vector is either <11-20>/3, <0001>, or <11-23>/3. The ratio of the number of dislocations is, for example, 50% or more, preferably 70% or more, and more preferably 90% or more. Note that dislocations having a Burgers vector of 2<11-20>/3 or <11-20> may exist in at least a portion of the principal surface 50s of the substrate 50.

(About X-ray rocking curve measurement with different slit widths)
Here, the inventor measured the mosaicity of the crystal constituting the substrate 50 of this embodiment and the curvature (warp) of the c-plane 50c by varying the width of the slit on the incident side and measuring the X-ray rocking curve. We have found that both can be evaluated simultaneously.

First, the influence of crystal mosaicity on X-ray rocking curve measurement will be explained.

The term "crystal mosaicity" used herein refers to variations in crystal plane orientation. In a crystal, the more dislocations there are, the more randomly the crystal plane orientation tends to tilt, and the mosaicity of the crystal tends to increase. In particular, when a plurality of dislocations are arranged linearly to form a lineage, the crystal plane orientations of subgrains adjacent to each other via the lineage are likely to shift, resulting in a high mosaicity of the crystal. When the mosaicity of the crystal is high as described above, when an X-ray rocking curve is measured, the fluctuation (fluctuation, distribution width) of the diffraction angle of the crystal plane becomes large due to the mosaicity.

Next, the influence of the curvature of the c-plane 50c in X-ray rocking curve measurement will be explained using FIG. 12(a). FIG. 12(a) is a schematic cross-sectional view showing X-ray diffraction on a curved c-plane.

When the width of the slit on the incident side of the X-ray is a, the irradiation width (footprint) of the X-ray irradiated onto the main surface of the substrate is b, and the Bragg angle of the crystal is θ _B , the width of the slit on the main surface of the substrate is The X-ray irradiation width b is determined by the following equation (h).
b=a/sinθ _B ...(h)

As shown in FIG. 12(a), when the c-plane of the substrate is curved, the radius of curvature of the c-plane is R, and the central angle formed by the curved c-plane in the range of the X-ray irradiation width b is When half is γ, the radius of curvature R of the c-plane is extremely large relative to the irradiation width b of the X-rays. From this, the angle γ is determined by the following equation (i).
γ=sin ^-1 (b/2R)≒b/2R...(i)

At this time, at the end of the incident side of the region irradiated with X-rays on the c-plane of the substrate (the right end in the figure), the diffraction angle with respect to the main surface of the substrate is θ _B +γ=θ _B +b/2R. Become.

On the other hand, at the light-receiving end of the region irradiated with X-rays on the c-plane of the substrate (the left end in the figure), the diffraction angle with respect to the main surface of the substrate is θ _B - γ = θ _B - b/2R becomes.

Therefore, due to the difference between the diffraction angle with respect to the principal surface of the substrate at the end of the incident side of the c-plane of the substrate and the diffraction angle with respect to the principal surface of the substrate at the end of the light-receiving side of the c-plane of the substrate, the curvature The fluctuation of the diffraction angle of X-rays with respect to the c-plane is b/R.

FIGS. 12(b) and 12(c) are diagrams showing fluctuations in the diffraction angle of the (0002) plane with respect to the radius of curvature of the c-plane. Note that the vertical axis in FIG. 12(b) is a logarithmic scale, and the vertical axis in FIG. 12(c) is a linear scale.

As shown in FIGS. 12(b) and (c), when the width a of the slit on the X-ray incident side is increased, that is, when the X-ray irradiation width b is increased, the , the fluctuation of the diffraction angle of the (0002) plane increases. Further, as the radius of curvature R of the c-plane becomes smaller, the fluctuation of the diffraction angle of the (0002) plane gradually increases. Further, the difference in the fluctuation of the diffraction angle of the (0002) plane when the X-ray irradiation width b is varied becomes larger as the radius of curvature R of the c-plane becomes smaller.

In fact, when measuring the X-ray rocking curve of a substrate with low crystal mosaicity, when the width a of the slit on the entrance side is narrow, the component due to the curvature of the c-plane out of the fluctuation of the diffraction angle of the (0002) plane is small, and components due to crystal mosaicity become dominant. However, when the width a of the slit on the incident side is wide, both a component due to the mosaicity of the crystal and a component due to the curvature of the c-plane are superimposed in the fluctuation of the diffraction angle of the (0002) plane. Therefore, by performing X-ray rocking curve measurements with different widths a of the slits on the incident side, it becomes possible to evaluate both the mosaicity of the crystal and the curvature (warp) of the c-plane at the same time.

Here, the characteristics when performing X-ray rocking curve measurement on the substrate 50 of this embodiment will be described.

In the following, X-rays of Kα1 of Cu are irradiated onto the main surface 50s of the substrate 50 through a Ge (220)-plane two-crystal monochromator and a slit, and an X-ray rocking curve of (0002) plane diffraction is measured. In this case, the half width of the (0002) plane diffraction when the width of the slit in the ω direction is 1 mm, and the half width of the (0002) plane diffraction when the width of the slit in the ω direction is 0.1 mm. Let the half width be "FWHMb". Note that the "ω direction" refers to the direction of rotation (circumferential direction) when the substrate 50 is rotated about an axis passing through the center of the substrate 50 and parallel to the main surface of the substrate 50 as the central axis in X-ray rocking curve measurement. Say something.

As described above, in the substrate 50 of this embodiment, there are few dislocations and low crystal mosaicity over a wide range of the main surface 50s.

As a result, X-ray rocking curve measurements of (0002) plane diffraction were performed at a plurality of measurement points set at 5 mm intervals within the main surface 50s of the substrate 50 of this embodiment, with the width of the slit in the ω direction being 0.1 mm. For example, the half width FWHMb of (0002) plane diffraction is 80 arcsec or less, preferably 50 arcsec or less at 95% or more of all measurement points, preferably 100%.

In addition, in the substrate 50 of this embodiment, the diffraction spectrum of the (0002) plane when X-ray rocking curve measurement is performed with the slit width on the entrance side widened is different from that when the slit width on the entrance side is narrowed. The diffraction spectrum tends to be less narrow than that of the (0002) plane when curve measurement is performed.

As a result, in the substrate 50 of this embodiment, the half-value width FWHMa of (0002) plane diffraction when the width of the slit in the ω direction is 1 mm is, for example, the half width FWHMa of the slit when the width in the ω direction is 0.1 mm. The half-width of (0002) plane diffraction can be equal to or greater than FWHMb.

Further, in the substrate 50 of this embodiment, as described above, there are few dislocations and low crystal mosaicity over a wide range of the main surface 50s. Furthermore, the curvature of the c-plane 50c of the substrate 50 is small, and the radius of curvature of the c-plane 50c is large. Due to these, in the substrate 50 of this embodiment, even if the slit width on the incident side is widened and X-ray rocking curve measurement is performed, the fluctuation of the diffraction angle of the (0002) plane does not become so large. Even if X-ray rocking curve measurements are performed with different slit widths, the difference in the fluctuation of the diffraction angle of the (0002) plane will be small.

As a result, at a predetermined measurement point (for example, the center of the main surface) of the substrate 50 of this embodiment, the half width FWHMa of the (0002) plane diffraction when the width of the slit in the ω direction is 1 mm, The difference FWHMa−FWHMb obtained by subtracting the half-width FWHMb of (0002) plane diffraction when the width of is 0.1 mm is, for example, 30% or less of FWHMa, preferably 22% or less.

Furthermore, when measuring the X-ray rocking curve of (0002) plane diffraction at a plurality of measurement points set at 5 mm intervals within the main surface 50s of the substrate 50 of this embodiment, with different widths of the slits in the ω direction. For example, at 95% or more of all measurement points, preferably 100%, FWHMa-FWHMb is, for example, 30% or less of FWHMa, preferably 22% or less.

Note that in the substrate 50 of this embodiment, even if FWHMa<FWHMb, |FWHMa−FWHMb| is 30% or less.

For reference, in the conventional VAS method nitride semiconductor substrate described above, the curvature of the c-plane 10c is relatively large and the radius of curvature of the c-plane 10c is small. Therefore, the difference FWHMa-FWHMb in a nitride semiconductor substrate obtained by the conventional VAS method is, for example, 50% or more of FWHMa.

(4) Effects obtained by this embodiment According to this embodiment, one or more of the following effects can be obtained.

(a) By growing the first layer 30 three-dimensionally on the base structure 10, a stress canceling effect (stress relaxation effect) can be obtained.

One of the reasons why the stress offsetting effect of the first layer 30 can be obtained is, for example, as follows.

As described above, in the three-dimensional growth step S200, the first layer 30 is three-dimensionally grown using the inclined interface 30i other than the c-plane as a growth surface, thereby forming the inclined interface growth region 70. The inclined interface growth region 70 can more easily incorporate oxygen than the first c-plane growth region 60. Therefore, the oxygen concentration in the inclined interface growth region 70 is higher than the oxygen concentration in the first c-plane growth region 60. In other words, the inclined interface growth region 70 can be considered as a high oxygen concentration region.

In this way, by introducing oxygen into the high oxygen concentration region, the lattice constant of the high oxygen concentration region can be made larger than the lattice constant of other regions other than the high oxygen concentration region (Reference: Chris G. Van de Walle, Physical Review B vol. 68, 165209 (2003)). The first c-plane growth region 60 of the second base layer 6 or the first layer 30 that has grown with the c-plane 30c as a growth plane has a tendency toward the center of curvature of the c-plane due to the curvature of the c-plane of the second base layer 6. A concentrated stress is applied. On the other hand, by making the lattice constant of the high oxygen concentration region relatively large, it is possible to generate stress in the high oxygen concentration region that spreads the c-plane 30c outward in the creeping direction. Thereby, the stress that concentrates toward the center of curvature of the c-plane 30c below the high oxygen concentration region and the stress that spreads the c-plane 30c in the high oxygen concentration region outward in the creeping direction can be offset.

By obtaining the stress canceling effect of the first layer 30 in this way, even if the second layer 40 is grown thickly with tensile stress accumulated in the second base layer 6, the base of the second layer 400 It is possible to suppress the stress difference from occurring between the structure 10 side and the surface side. Thereby, generation of cracks and the like in the second layer 40 can be suppressed.

(b) In this embodiment, by obtaining the stress canceling effect of the first layer 30 described above, the radius of curvature of the c-plane 50c of the substrate 50 obtained from the second layer 40 can be changed from that of the nitride obtained by the conventional VAS method. The radius of curvature can be made larger than the radius of curvature of the semiconductor substrate. Thereby, the variation in the off-angle θ of the c-axis 50ca with respect to the normal to the main surface 50s of the substrate 50 can be made smaller than the variation in the off-angle of the c-axis of the nitride semiconductor substrate obtained by the conventional VAS method. .

(c) In the three-dimensional growth step S200, an inclined interface 30i other than the c-plane is generated on the surface of the single crystal constituting the first layer 30, so that at a position where the inclined interface 30i is exposed, The dislocations can be bent and propagated in a substantially perpendicular direction. This allows dislocations to be locally collected. By locally gathering dislocations, dislocations having mutually opposing Burgers vectors can be eliminated. Alternatively, locally gathered dislocations form a loop, thereby suppressing dislocations from propagating to the surface side of the second layer 40. In this way, the dislocation density on the surface of the second layer 40 can be reduced. As a result, it is possible to obtain a substrate 50 with a lower dislocation density than a nitride semiconductor substrate obtained by the conventional VAS method.

(d) As described above, during the growth process of the second layer 40, some of the plurality of dislocations are eliminated or some of the plurality of dislocations are suppressed from propagating to the surface side of the second layer 40. By doing so, the dislocation density can be reduced more rapidly than when a crystal layer made of a single crystal of a Group III nitride semiconductor is grown using only the c-plane as a growth plane. As a result, the substrate 50 with reduced dislocation density can be efficiently obtained, and its productivity can be improved.

(e) In the three-dimensional growth step S200, the c-plane 30c disappears from the top surface 30u of the first layer 30. Thereby, a plurality of valleys 30v and a plurality of tops 30t can be formed on the surface of the first layer 30. As a result, dislocations propagating from the second underlayer 6 can be reliably bent at the position where the inclined interface 30i in the first layer 30 is exposed.

Here, a case will be considered in which the c-plane remains in the three-dimensional growth process. In this case, in the portion where the c-plane remains, dislocations propagated from the base substrate are propagated substantially vertically upward without being bent, and reach the surface of the second layer. Therefore, dislocations are not reduced above the portion where the c-plane remains, and a high dislocation density region is formed.

On the other hand, according to the present embodiment, in the three-dimensional growth step S200, the c-plane 30c disappears from the top surface 30u of the first layer 30, thereby changing the surface of the first layer 30 to the inclined interface 30i other than the c-plane. A plurality of valleys 30v and a plurality of tops 30t can be formed on the surface of the first layer 30. Thereby, dislocations propagating from the second underlayer 6 can be reliably bent over the entire surface of the first layer 30. By reliably bending the dislocations, it is possible to make it easier to eliminate some of the plurality of dislocations, or to make it difficult for some of the plurality of dislocations to propagate to the surface side of the second layer 40. As a result, it becomes possible to reduce the dislocation density over the entire main surface 1s of the substrate 50 obtained from the second layer 40.

(f) In this embodiment, a second base layer forming step S150 is performed between the void forming step S140 and the three-dimensional growth step S200, and the main surface 6s of the second base layer 6 is mirror-finished. Thereby, the growth form of the first layer 30 can be changed from the growth form of island crystals that occurred at the initial stage of growth of the second underlayer 6.

Here, if the three-dimensional growth step S200 is performed without performing the second base layer forming step S150 immediately after the void forming step S140, a metal nitride layer is formed above the void-containing first base layer from the early stage of growth of the first layer. The first layer grows as island-shaped crystals through the crystals. In this case, the frequency of island crystals in the first layer depends on the degree of supersaturation when growing on the nanonets of the metal nitride layer and the variation in the opening width of the nanonets, as described above. Therefore, the tops of the first layer are formed densely, and the average distance between the closest tops of the first layer becomes short. When the average distance between the closest apexes of the first layer becomes short, dislocations cannot be sufficiently collected. As a result, the dislocation density on the surface of the second layer may not be sufficiently reduced.

On the other hand, in the present embodiment, by mirror-finishing the main surface 6s of the second base layer 6 after the void forming step S140, the growth form of the first layer 30 is changed to the second base layer 6 as described above. can be changed from the growth form of island-shaped crystals that occur at the initial stage of growth.

That is, the first layer 30 grown on the mirror-finished second base layer 6 is not grown as an island crystal from the initial stage of growth, but is grown depending on the first growth conditions described above in the three-dimensional growth step S200. It can be grown in three dimensions. At this time, the average distance between the closest apexes of the first layer 30 is the growth rate G _c0 in the c-axis direction of the c-plane 30c and the growth rate G _i in the inclined direction of the inclined interface 30i as the first growth condition. Depends on the difference. Thereby, the average distance between the closest apexes of the first layer 30 can be controlled based on the first growth condition, and the distance between the closest apexes can be increased.

In addition, in the present embodiment, by mirror-finishing the main surface 6s of the second base layer 6 after the void forming step S140, the second base layer 6 is The morphology of the main surface 6s of the stratum 6 can be made substantially uniform throughout. By making the morphology of the main surface 6s of the second underlayer 6 substantially uniform, the state of occurrence of the inclined interface 30i can be made substantially uniform over the entire surface of the first layer 30 in the three-dimensional growth step S200. . This suppresses the formation of a region where the distance between the closest apexes is short on a part of the surface of the first layer 30, and makes the distance between the closest apexes substantially uniform over the entire surface of the first layer 30. can be made longer.

As a result, in this embodiment, the formation of a region with high dislocation density on a part of the surface of the second layer 40 is suppressed, and the dislocation density is reduced over the entire surface of the second layer 40. be able to.

(g) The number of steps in this embodiment is higher than in the case where a three-dimensional growth process and a planarization process are performed using a free-standing substrate obtained by slicing and polishing a crystal layer made of a single crystal of a group III nitride semiconductor. can be reduced. That is, the slicing process and polishing process for obtaining a free-standing substrate can be omitted. Thereby, the manufacturing cost can be reduced while improving the yield of this embodiment.

(h) In this embodiment, the base structure 10 is not subjected to either of the formation of a mask layer on the main surface 6s of the second base layer 6 and the formation of an uneven pattern on the main surface 6s. A three-dimensional growth step S200 is performed on the target. Note that the term "mask layer" used herein refers to a mask layer made of silicon oxide or the like and having a predetermined opening, which is used in, for example, the so-called ELO (Epitaxial Lateral Overgrowth) method. Moreover, the "uneven pattern" here means at least one of a trench and a ridge that are used in the so-called pendeo epitaxy method and are directly patterned on the main surface. The height difference of the concavo-convex pattern here is, for example, 100 nm or more.

By performing the three-dimensional growth step S200 and the planarization step S300 without having the above-described structure, the main surface 50s of the substrate 50, which is formed by slicing the second layer 40, is free from dislocations caused by concentration of dislocations. The formation of regions with high dislocation density can be suppressed, and regions with low dislocation density can be uniformly formed.

Further, by performing the three-dimensional growth step S200 and the planarization step S300 without having the above-described structure, certain processing steps (mask layer forming step, photolithography step, etc.) can be made unnecessary. , the number of steps in this embodiment can be reduced. Thereby, the manufacturing cost can be reduced while improving the yield of this embodiment.

(i) In this embodiment, the first layer 30 is grown on the mirror-finished second base layer 6 described above, and the first growth conditions are adjusted so as to satisfy formula (1) in the three-dimensional growth step S200. By doing so, in the three-dimensional growth step S200, a {11-2m} plane where m≧3 can be generated as the inclined interface 30i. Thereby, the inclination angle of the {11-2m} plane with respect to the c-plane 30c can be made gentle. Specifically, the inclination angle can be 47.3° or less. By making the inclination angle of the {11-2m} plane gentle with respect to the c-plane 30c, the period of the plurality of top portions 30t can be lengthened. Specifically, when looking at an arbitrary cross section perpendicular to the main surface 1s of the base substrate 1, the average distance L between the closest apexes can be set to exceed 100 μm.

For reference, when etch pits are created in a nitride semiconductor substrate using a predetermined etchant, etch pits constituted by {1-10n} planes are usually formed on the surface of the substrate. On the other hand, in the present embodiment, the surface of the first layer 30 grown under predetermined conditions can produce a {11-2m} plane where m≧3. Therefore, compared to a normal etch pit, it is considered that in this embodiment, an inclined interface 30i unique to the manufacturing method is formed.

(j) In the present embodiment, when looking at an arbitrary cross section perpendicular to the main surface 1s of the base substrate 1, by setting the average distance L between the nearest vertices to more than 100 μm, the distance over which dislocations propagate by bending can be reduced. can be ensured at least over 50 μm. Thereby, dislocations can be sufficiently collected above the approximate center between the pair of top portions 30t of the first layer 30. As a result, the dislocation density on the surface of the second layer 40 can be sufficiently reduced.

(k) In the three-dimensional growth step S200, after the c-plane 30c disappears from the surface of the first layer 30, a predetermined thickness is maintained while maintaining a state in which the inclined interface 30i occupies more than the c-plane 30c on the surface. The growth of the first layer 30 is continued over the period of time. Thereby, after the c-plane 30c disappears, sufficient time can be secured for bending the dislocation at the position where the inclined interface 30i is exposed. Here, if the c-plane is grown immediately after the c-plane disappears, the dislocations may not be sufficiently bent and may propagate substantially vertically toward the surface of the second layer. In contrast, in this embodiment, by ensuring sufficient time for bending the dislocations at the exposed position of the inclined interface 30i other than the c-plane, the dislocations particularly near the top 30t of the first layer 30 are reliably bent. Therefore, propagation of dislocations from the second base layer 6 toward the surface of the second layer 40 in a substantially vertical direction can be suppressed. Thereby, concentration of dislocations above the top portion 30t of the first layer 30 can be suppressed.

<Other embodiments>
The embodiments of the present invention have been specifically described above. However, the present invention is not limited to the above-described embodiments, and various changes can be made without departing from the spirit thereof.

In the above-described embodiment, the case where the substrate 50 is a GaN free-standing substrate has been described, but the substrate 50 is not limited to a GaN free-standing substrate. ), Group III nitride semiconductors such as indium gallium nitride (InGaN), aluminum indium gallium nitride (AlInGaN), that is, Al _x In _y Ga _1-x-y N (0≦x≦1, 0≦y≦1, It may be a free-standing substrate made of a group III nitride semiconductor represented by a composition formula of 0≦x+y≦1).

In the above-described embodiment, the case where the substrate 50 is n-type has been described, but the substrate 50 may be p-type or semi-insulating. For example, when manufacturing a semiconductor device as a high electron mobility transistor (HEMT) using the substrate 50, the substrate 50 preferably has semi-insulating properties.

In the above embodiment, the case where the growth temperature is mainly adjusted as the first growth condition in the three-dimensional growth step S200 has been described, but if the first growth condition satisfies formula (1), the first growth condition is adjusted. , a growth condition other than the growth temperature may be adjusted, or a combination of the growth temperature and a growth condition other than the growth temperature may be adjusted.

In the above-described embodiment, the case where the growth temperature is mainly adjusted as the second growth condition in the planarization step S300 has been described, but if the second growth condition satisfies formula (2), then as the second growth condition, Growth conditions other than the growth temperature may be adjusted, or the growth temperature and growth conditions other than the growth temperature may be combined and adjusted.

In the above-described embodiment, a case has been described in which the growth conditions in the inclined interface maintenance step S240 are maintained at the above-mentioned first growth conditions similarly to the inclined interface expansion step S220. However, the growth conditions in the inclined interface maintenance step S240 satisfies the first growth condition, the growth conditions in the inclined interface maintaining step S240 may be different from the growth conditions in the inclined interface expanding step S220.

In the above-described embodiment, a case has been described in which the growth conditions in the main growth step S340 are maintained at the above-mentioned second growth conditions as in the c-plane enlargement step S320, but the growth conditions in the main growth step S340 are If two growth conditions are satisfied, the growth conditions in the main growth step S340 may be different from the growth conditions in the c-plane expansion step S320.

In the above embodiment, a case has been described in which the main growth layer 44 is sliced using a wire saw in the slicing step S400, but for example, an outer peripheral blade slicer, an inner peripheral blade slicer, an electric discharge machine, etc. may be used.

In the above-described embodiment, a case has been described in which the substrate 50 is obtained by slicing the main growth layer 44 of the stacked structure 90, but the present invention is not limited to this case. For example, the stacked structure 90 may be used as is to produce a semiconductor stack for producing a semiconductor device. Specifically, after the laminated structure 90 is produced, a semiconductor functional layer is epitaxially grown on the laminated structure 90 in a semiconductor laminate production process to produce a semiconductor laminate. After producing the semiconductor laminate, the back side of the laminate structure 90 is polished to remove the second base layer 6, first layer 30, and c-plane enlarged layer 42 from the laminate structure 90. As a result, a semiconductor laminate having the main growth layer 44 and a semiconductor functional layer is obtained, similar to the above-described embodiment. According to this case, the slicing step S400 and the polishing step S500 for obtaining the substrate 50 can be omitted.

Below, various experimental results that support the effects of the present invention will be explained.

(1) Experiment 1
(1-1) Preparation of nitride semiconductor substrate Nitride semiconductor substrates of Examples and Comparative Example 1, and laminated structures of Comparative Example 2 were prepared as follows. In addition, regarding the example, a stacked structure before slicing the nitride semiconductor substrate was also produced. Furthermore, hereinafter, "nitride semiconductor substrate" may be abbreviated as "substrate".

[Production conditions of nitride semiconductor substrate of Example]
(base board)
Material: Sapphire Diameter: 2 inches Thickness: 400μm
Low-index crystal plane closest to the main surface: c-plane No processing such as a mask layer on the main surface.
(First base layer)
Low temperature growth buffer layer:
Material: GaN
Growth temperature: 550℃
Thickness: 50nm
GaN layer:
Material: Undoped (unintentionally doped) GaN
Growth temperature: 1,050℃
Thickness: 350nm
Note that the surface of the GaN layer was mirror-finished.
(metal layer)
Material: Ti
Thickness: 20nm
(Heat treatment conditions)
A two-step heat treatment was performed.
First heat treatment:
Atmosphere: 80% _N2 gas, 20% _NH3 gas
Temperature: 1,050℃
Time: 10 minutes Second heat treatment:
Atmosphere: _H2 gas 80%, _NH3 gas 20%
Temperature: 1,050℃
Time: 20 minutes (second base layer)
Material: Undoped (unintentionally doped) GaN
Growth temperature: 1,050℃
Thickness: approx. 200μm
Note that the main surface of the second base layer was mirror-finished.
(1st layer)
Material: GaN
Growth method: HVPE method First growth conditions:
The growth temperature was set to 980° C. or more and 1,020° C. or less, and the V/III ratio was set to 2 or more and 20 or less. At this time, at least one of the growth temperature and the V/III ratio was adjusted within the above ranges so that the first growth conditions satisfied formula (1).
Height from the main surface of the second base layer to the top of the first layer: approximately 450 μm
(Second layer)
Material: GaN
Growth method: HVPE method Growth temperature: 1,050°C
V/III ratio: 2
Note that the second growth condition satisfies formula (2).
Thickness from the top of the first layer to the surface of the second layer: approximately 800 μm
(Peeling conditions)
Natural peeling occurs when the temperature drops from the second layer growth temperature.
(slicing and polishing conditions)
Slice position: 2nd layer main growth layer Kerf loss during slicing: 200 μm
Double-sided polishing Final thickness of nitride semiconductor substrate: 400μm

In addition, in the example, a plurality of nitride semiconductor substrates were manufactured under the same conditions.

[Production conditions of nitride semiconductor substrate of Comparative Example 1]
In Comparative Example 1, a substrate was produced under the same conditions as the conventional VAS method.
(base board)
Same as the example.
(First crystal layer, metal layer, and heat treatment conditions)
Same as the first base layer, metal layer, and heat treatment conditions of Example.
(Second crystal layer)
The conditions were the same as for the second base layer of the example except that the thickness was 600 μm.
(1st layer and 2nd layer)
none.
(Peeling conditions)
Natural peeling occurs when the temperature drops from the second crystal layer growth temperature.
(slicing and polishing conditions)
The conditions were the same as in the example except that the slice position was the second crystal layer.

[Production conditions of nitride semiconductor substrate of Comparative Example 2]
(base board)
Same as the example.
(First crystal layer, metal layer, and heat treatment conditions)
Same as the first base layer, metal layer, and heat treatment conditions of Example.
(Second crystal layer)
The conditions were the same as for the second base layer of the example except that the thickness was 1700 μm.
(1st layer and 2nd layer)
none.

(1-2) Evaluation (observation using a fluorescence microscope)
In Examples, the cross section of the stacked structure before slicing the nitride semiconductor substrate was observed using a fluorescence microscope.

(Observation using multiphoton excitation microscope)
The main surfaces of each of the nitride semiconductor substrates of Example and Comparative Example 1 were observed using a multiphoton excitation microscope. At this time, the dislocation density was measured by measuring the dark spot density over the entire main surface every 250 μm of visual field. It was confirmed that all of the dark spots in these substrates were dislocations by measuring with the focus shifted in the thickness direction. Further, at this time, the ratio of the number of regions having a dislocation density of less than 1×10 ⁶ cm ⁻² (low dislocation density regions) to the total number of measurement regions in a field of view of 250 μm was determined.

(X-ray rocking curve measurement)
The following two types of X-ray rocking curve measurements were performed on each of the nitride semiconductor substrates of Example and Comparative Example 1.

For X-ray rocking curve measurement, "X'Pert-PRO MRD" manufactured by Spectris was used, and as a monochromator on the incident side, a "Hybrid Monochromator" manufactured by the same company was used. The hybrid monochromator includes, in order from the X-ray light source side, an X-ray mirror and two Ge (220)-plane crystals. In this measurement, first, X-rays emitted from an X-ray light source are made into parallel light by an X-ray mirror. Thereby, the number of photons of the X-rays used (ie, the X-ray intensity) can be increased. Next, the parallel light from the X-ray mirror is converted into monochromatic light of Kα1 of Cu by two crystals of Ge (220) plane. Next, the monochromatic light from the two crystals on the Ge (220) plane is narrowed to a predetermined width through a slit and made incident on the substrate. Note that the resolution when using the hybrid monochromator is approximately 24 arcsec.

Note that the X-rays incident on the substrate in this measurement are considered to be parallel light heading toward the substrate side in the cross section along the ω direction, but in the cross section along the direction perpendicular to the ω direction (direction of the rotation axis of the substrate). The light is not parallel. Therefore, until the X-ray reaches the substrate from the slit, the width of the X-ray in the ω direction is approximately constant, but the width of the X-ray in the direction orthogonal to the ω direction increases. Therefore, in X-ray rocking curve measurement, the half-width of X-rays diffracted by a predetermined crystal plane depends on the width of the slit on the incident side in the ω direction where the X-rays become parallel light.

(X-ray rocking curve measurement 1)
The width of the incident side slit in the ω direction was set to 0.1 mm, and the X-ray rocking curves of the (0002) plane of each of the nitride semiconductor substrates of Example and Comparative Example 1 were measured. At this time, the measurements were performed at a plurality of measurement points set at 5 mm intervals on a straight line passing through the center of each substrate and along each of the m-axis direction and the a-axis direction. As a result of the measurement, the peak angle ω between the X-rays incident on the main surface and the main surface was determined. Thereafter, the peak angle ω was plotted against the position on the straight line, and the peak angle ω was approximated by a linear function of the position. The radius of curvature of the c-plane was determined by the reciprocal of the slope of the linear function.

Further, at each measurement point, the half width FWHMb of the (0002) plane diffraction was determined when the width of the incident side slit in the ω direction was 0.1 mm.

(X-ray rocking curve measurement 2)
The width of the incident side slit in the ω direction was set to 1 mm, and X-ray rocking curve measurements were performed on each of the nitride semiconductor substrates of Example and Comparative Example 1. Note that the measurement was performed at the center of the main surface of each substrate. As a result of the measurement, the half width FWHMa of the (0002) plane diffraction was determined when the width of the incident side slit in the ω direction was 1 mm. Furthermore, the ratio of FWHMa-FWHMb to FWHMa was determined at the center of the main surface of each substrate.

In addition, in X-ray rocking curve measurements 1 and 2, when X-rays are incident on the main surface of each substrate at a Bragg angle of 17.28° of the (0002) plane, the width of the slit in the ω direction is 0.1 mm. In this case, the X-ray footprint is approximately 0.337 mm, and when the width of the slit in the ω direction is 1 mm, the X-ray footprint is approximately 3.37 mm.

(1-3) Results The results are shown in Table 1.

(Comparison between Example and Comparative Example 2)
First, Example and Comparative Example 2 will be compared.

In Comparative Example 2, when the temperature was lowered to room temperature after crystal layer growth, the second crystal layer was found to be finely cracked. Further, crystals were abnormally growing from the broken cross section of the second crystal layer. From this, it is considered that the second crystal layer was cracked during the growth of the second crystal layer. In Comparative Example 2, a nitride semiconductor substrate was not manufactured because it was not possible to obtain a second crystal layer having sufficient thickness and diameter.

On the other hand, in the example, when the temperature was lowered to room temperature after the growth of the second layer was completed, the laminated structure was separated from the base substrate at the boundary between the metal nitride layer and the second base layer. . No cracks were observed in the laminated structure after peeling.

(Comparison between Example and Comparative Example 1)
Example and Comparative Example 1 will be compared using Table 1 and FIGS. 13 to 16. FIG. 13 is a diagram showing an observed image of a cross section of the laminated structure of the example observed with a fluorescence microscope. FIG. 14 is a view of the main surface of the nitride semiconductor substrate of the example observed using a multiphoton excitation microscope. FIG. 15(a) is a diagram showing the results of X-ray rocking curve measurement of (0002) plane diffraction along the m-axis direction for each of the nitride semiconductor substrates of Example and Comparative Example 1. b) is a diagram showing the results of X-ray rocking curve measurement of (0002) plane diffraction along the a-axis direction for each of the nitride semiconductor substrates of Example and Comparative Example 1. In addition, in FIGS. 15(a) and 15(b), the black circles indicate Examples, and the open circles indicate Comparative Example 1. FIG. 16(a) is a diagram showing normalized X-ray diffraction patterns when X-ray rocking curve measurements were performed using different slits for the nitride semiconductor substrate of the example, and FIG. FIG. 3 is a diagram showing a normalized X-ray diffraction pattern when the same measurement as in Example was performed on a nitride semiconductor substrate of Comparative Example 1. Note that FIGS. 16(a) and 16(b) show the measurement results in the direction along the m-axis. Furthermore, in the figure, "Line width" means the footprint of the above-mentioned X-rays.

As shown in FIG. 13, in the laminated structure of the example, the first layer is a first layer grown with the c-plane as the growth plane based on the difference in the growth plane (that is, the difference in oxygen concentration) during the growth process. It had a planar growth region and an inclined interface growth region grown using an inclined interface as a growth surface. The first c-plane growth region had a plurality of concave portions and a plurality of convex portions. The average value of the angle between the pair of inclined parts in the first c-plane growth region was approximately 45.1°. Further, the average distance between the closest apexes was approximately 109 μm. Further, the inclined interface growth region was continuously formed along the main surface of the base substrate.

As shown in Table 1, the average dislocation density on the main surface of the substrate of Example was significantly reduced compared to the substrate of Comparative Example 1, and was less than 5.5×10 ⁵ cm ⁻² .

Further, in the substrate of Example, there was no region where the dislocation density exceeded 3×10 ⁶ cm ⁻² . Note that even in the region with the highest dislocation density, the dislocation density was less than 1.6×10 ⁶ cm ⁻² . Further, in the substrate of the example, regions in which the dislocation density was less than 1×10 ⁶ cm ⁻² (low dislocation density regions) existed in 90% or more of the main surface 50s. The dislocation density in the low dislocation density region was 1.3×10 ⁵ to 7.6×10 ⁵ cm ⁻² .

Further, as shown by the square frame in FIG. 14, the main surface of the substrate of the example included a dislocation-free region of at least 50 μm square. Further, the main surface of the substrate of the example had non-overlapping dislocation-free regions of 50 μm square at a density of 100 pieces/cm ² or more.

Further, as shown in Table 1 and FIGS. 15(a) and (b), in the substrate of the example, the radius of curvature of the c-plane was larger than that of the substrate of comparative example 1, and was 19 m or more.

In addition, in the substrate of the example, when the peak angle ω was approximated by a linear function of the position in the X-ray rocking curve measurement of the c-plane in order to obtain the radius of curvature of the c-plane, the error with respect to the linear function was small. Specifically, the error of the measured peak angle ω with respect to the linear function approximated as described above was 0.01° or less.

In addition, as shown in Table 1 and FIGS. 15(a) and (b), in the substrate of the example, when the width of the slit in the ω direction was 0.1 mm at all measurement points (i.e., 100%), ( 0002) The half width FWHMb of surface diffraction was 50 arcsec or less.

As shown in FIG. 16(b), in the substrate of Comparative Example 1, when the width of the slit in the ω direction was set to 0.1 mm, the X-ray diffraction spectrum was narrow, but when the width of the slit in the ω direction was set to 1 mm, When this happened, the X-ray diffraction spectrum was broadened.

Therefore, as shown in Table 1, in the substrate of Comparative Example 1, FWHMa-FWHMb was 50% or more of FWHMa.

On the other hand, as shown in FIG. 16(a), in the substrate of the example, even when the width of the slit in the ω direction was increased from 0.1 mm to 1 mm, the X-ray diffraction spectrum was slightly broadened. , its spread was small.

As a result, as shown in Table 1, in the substrate of the example, FWHMa-FWHMb was 0% or more and 30% or less of FWHMa.

(summary)
According to the above examples, by growing the first layer three-dimensionally on the underlying structure, it was possible to obtain the stress offsetting effect of the first layer. By obtaining the stress offsetting effect of the first layer, even if the second layer is grown thickly in a state where tensile stress is accumulated in the second base layer, generation of cracks etc. in the second layer can be suppressed. I confirmed that it was done.

Further, according to the example, in the three-dimensional growth process, the first growth conditions were adjusted so as to satisfy equation (1). This made it possible to reliably eliminate the c-plane during the growth process of the first layer. By reliably eliminating the c-plane, it was possible to reliably bend dislocations at positions where the inclined interface in the first layer was exposed. As a result, it was confirmed that the dislocation density on the main surface of the nitride semiconductor substrate could be efficiently reduced.

Further, according to the example, in the second base layer forming step, by mirror-finishing the main surface of the second base layer, the growth form of the first layer is changed to an island formed at the initial stage of growth of the second base layer. By changing the growth form of crystals, it was possible to increase the distance between the closest apexes of the first layer that grows three-dimensionally in the three-dimensional growth process. It was confirmed that this suppressed the formation of a high dislocation density region on a part of the main surface of the substrate and made it possible to lower the dislocation density over the entire main surface of the substrate.

Further, according to the example, by growing the first layer on the mirror-finished second underlayer described above and adjusting the first growth conditions so as to satisfy formula (1), it is possible to It was possible to make the average distance more than 100 μm. It was thus confirmed that the dislocation density on the main surface of the substrate could be sufficiently reduced. Furthermore, it was confirmed that by setting the average distance between the closest apexes to more than 100 μm, it was possible to form a dislocation-free region of at least 50 μm square.

Further, according to the example, by obtaining the stress canceling effect by the first layer described above, the radius of curvature of the c-plane of the substrate of the example was changed to the c-plane of the substrate of comparative example 1, which corresponds to the conventional VAS method. It was confirmed that the radius of curvature could be made larger than the radius of curvature of

Further, according to the example, as described above, there were few dislocations over a wide range of the main surface of the substrate, and the mosaicity of the crystal in the substrate was low. As a result, it was confirmed that in the substrate of the example, the FWHMb was 50 arcsec or less over a wide range of the main surface.

Further, according to the example, as described above, the mosaicity of the crystal was low and the radius of curvature of the c-plane of the substrate was large. Based on these results, in the example, it was confirmed that the difference in half-width, FWHMa-FWHMb, was 30% or less of FWHMa when X-ray rocking curve measurements were performed with different widths of the slits on the entrance side.

<Preferred embodiments of the present invention>
Hereinafter, preferred embodiments of the present invention will be additionally described.

(Additional note 1)
a step of preparing a base substrate;
forming a first base layer made of a group III nitride semiconductor on the base substrate;
forming a metal layer on the first base layer;
performing heat treatment to form voids in the first underlayer;
A second base layer made of a single crystal of a Group III nitride semiconductor and having a main surface whose nearest low-index crystal plane is the (0001) plane is epitaxially grown above the first base layer, a step of mirror-finishing the main surface of the stratum;
A single crystal of a group III nitride semiconductor having a top surface with an exposed (0001) plane is epitaxially grown directly on the main surface of the second underlayer, and is composed of an inclined interface other than the (0001) plane. A plurality of recesses are formed on the top surface, and the inclined interface is gradually enlarged as it goes above the second underlayer, so that the (0001) plane disappears from the top surface, so that the surface is only the inclined interface. a three-dimensional growth process of growing a first layer consisting of;
A planarization step of epitaxially growing a group III nitride semiconductor single crystal on the first layer, eliminating the inclined interface, and growing a second layer having a mirrored surface;
A method for manufacturing a nitride semiconductor substrate having the following.

(Additional note 2)
In the three-dimensional growth step,
forming a plurality of valleys and a plurality of peaks on the surface of the first layer by creating the plurality of recesses in the top surface of the single crystal and eliminating the (0001) plane;
When looking at an arbitrary cross section perpendicular to the main surface, a pair of peaks that are closest to each other among the plurality of peaks across one of the plurality of valleys are in a direction along the main surface. The method for manufacturing a nitride semiconductor substrate according to Supplementary Note 1, wherein the average distance between the substrates is more than 100 μm.

(Additional note 3)
In the three-dimensional growth step,
The method for manufacturing a nitride semiconductor substrate according to Supplementary Note 2, wherein the average distance between the pair of tops that are closest to each other is less than 800 μm.

(Additional note 4)
In the three-dimensional growth step,
After the (0001) plane disappears from the surface, the first layer is grown to a predetermined thickness while maintaining a state where the inclined interface occupies more than the (0001) plane on the surface. The method for manufacturing a nitride semiconductor substrate according to any one of Supplementary Notes 1 to 3, which is continued.

(Appendix 5)
After the planarization step, slicing at least one nitride semiconductor substrate from the second layer,
In the step of slicing the nitride semiconductor substrate,
The radius of curvature of the (0001) plane of the nitride semiconductor substrate is determined by growing the second base layer to the same thickness as the second layer and not performing the three-dimensional growth step and the planarization step. The method for manufacturing a nitride semiconductor substrate according to any one of appendices 1 to 4, wherein the radius of curvature of the (0001) plane of the nitride semiconductor substrate is larger than the radius of curvature of the (0001) plane of the nitride semiconductor substrate when two underlying layers are sliced.

(Appendix 6)
The method for manufacturing a nitride semiconductor substrate according to appendix 5, wherein the (0001) plane of the nitride semiconductor substrate has a radius of curvature of 10 m or more.

(Appendix 7)
According to any one of Supplementary Notes 1 to 6, further comprising a peeling step of peeling off the laminated structure having the second base layer, the first layer, and the second layer from the base substrate after the planarization step. The method for manufacturing the nitride semiconductor substrate described above.

(Appendix 8)
In the three-dimensional growth step,
The method for manufacturing a nitride semiconductor substrate according to any one of Supplementary Notes 1 to 7, wherein the inclined interface is a {11-2m} plane where m≧3.

(Appendix 9)
In the three-dimensional growth step, the first layer is grown under a first growth condition that satisfies formula (1),
The method for manufacturing a nitride semiconductor substrate according to any one of appendices 1 to 8, wherein in the planarization step, the second layer is grown under a second growth condition that satisfies formula (2).
G _c1 >G _i /cosα...(1)
G _c2 <G _i /cosα...(2)
(However, the growth rate of the (0001) plane of the first layer is G _c1 , the growth rate of the (0001) plane of the second layer is G _c2 , and the growth rate of the (0001) plane of the first layer and the Let G _i be the growth rate of the inclined interface that is most inclined with respect to the (0001) plane in each of the two layers, and The angle between the inclined interface and the (0001) plane is α.)

(Appendix 10)
The method for manufacturing a nitride semiconductor substrate according to any one of Supplementary Notes 1 to 9, wherein the steps from forming the second base layer to the planarizing step are performed continuously in the same vapor phase growth apparatus.

(Appendix 11)
In the method for manufacturing a nitride semiconductor substrate according to any one of Supplementary Notes 1 to 10, a nitride semiconductor substrate obtained by slicing the second layer.

(Appendix 12)
A nitride semiconductor substrate having a diameter of 2 inches or more and having a main surface whose nearest low-index crystal plane is a (0001) plane,
When X-rays of Kα1 of Cu are irradiated to the main surface through a Ge (220)-plane two-crystal monochromator and a slit, and an X-ray rocking curve measurement of (0002) plane diffraction is performed, the slit Subtract the half width FWHMb of the (0002) plane diffraction when the width in the ω direction of the slit is 0.1 mm from the half width FWHMa of the (0002) plane diffraction when the width in the ω direction of the slit is 0.1 mm. The difference FWHMa-FWHMb is 30% or less of FWHMa in the nitride semiconductor substrate.

(Appendix 13)
When the main surface is observed with a multiphoton excitation microscope with a field of view of 250 μm square and the dislocation density is determined from the dark spot density, there is no region on the main surface where the dislocation density exceeds 3 × 10 ⁶ cm ^-2 . , the nitride semiconductor substrate according to appendix 12, wherein the region in which the dislocation density is less than 1×10 ⁶ cm ⁻² is present in 80% or more of the main surface.

(Appendix 14)
A nitride semiconductor substrate having a diameter of 2 inches or more,
When the main surface of the nitride semiconductor substrate is observed with a field of view of 250 μm square using a multiphoton excitation microscope and the dislocation density is determined from the dark spot density, the region where the dislocation density exceeds 3×10 ⁶ cm ⁻² is the main surface of the nitride semiconductor substrate. A nitride semiconductor substrate in which a region having a dislocation density of less than 1×10 ⁶ cm ⁻² is present in 80% or more of the main surface.

(Appendix 15)
15. The nitride semiconductor substrate according to any one of appendices 12 to 14, wherein the main surface includes a dislocation-free region of at least 50 μm square.

(Appendix 16)
The X-ray rocking curve of the (0002) plane is measured at each position on a straight line passing through the center within the principal plane, and the peak angle ω between the X-ray incident on the principal plane and the principal plane is calculated on the straight line. When the peak angle ω is approximated by a linear function of the position,
The radius of curvature of the (0001) plane determined by the reciprocal of the slope of the linear function is 10 m or more,
The nitride semiconductor substrate according to any one of appendices 12 to 15, wherein an error of the measured peak angle ω with respect to the linear function is 0.05° or less.

(Appendix 17)
A nitride semiconductor substrate having a main surface whose nearest low-index crystal plane is a (0001) plane,
The X-ray rocking curve of the (0002) plane is measured at each position on a straight line passing through the center within the principal plane, and the peak angle ω between the X-ray incident on the principal plane and the principal plane is calculated on the straight line. When the peak angle ω is approximated by a linear function of the position,
The radius of curvature of the (0001) plane determined by the reciprocal of the slope of the linear function is 10 m or more,
A nitride semiconductor substrate in which an error of the measured peak angle ω with respect to the linear function is 0.05° or less.

(Appendix 18)
a base layer made of a single crystal of a group III nitride semiconductor, having a mirror-finished main surface, and a low-index crystal plane closest to the main surface being a (0001) plane;
A group III nitride semiconductor single crystal having a top surface with the (0001) plane exposed is directly epitaxially grown on the main surface of the underlayer, and a plurality of slanted interfaces other than the (0001) plane are formed. is formed by creating a recess on the top surface, gradually enlarging the sloped interface as it goes above the base layer, and causing the (0001) plane to disappear from the top surface, so that the surface is formed only by the sloped interface. A first layer consisting of;
a second layer having a mirror-finished surface by epitaxially growing a single crystal of a Group III nitride semiconductor on the first layer and eliminating the inclined interface;
A laminated structure with

(Appendix 19)
The first layer is
a first c-plane growth region grown with the (0001) plane as a growth plane;
a sloped interface growth region grown using the sloped interface as a growth surface;
has
The laminated structure according to appendix 18, wherein the second layer has a second c-plane growth region grown with the (0001) plane as a growth plane.

(Additional note 20)
The laminated structure according to appendix 19, wherein the inclined interface growth region is continuously provided along the main surface of the underlayer.

(Additional note 21)
The first c-plane growth region is
a convex portion provided at a position where the (0001) plane disappears;
a pair of inclined parts provided on both sides of the convex part as a locus of an intersection between the (0001) plane and the inclined interface;
has
21. The laminated structure according to appendix 19 or 20, wherein the angle formed by the pair of inclined portions is 70° or less.

1 Base substrate 2 First base layer 3 Metal layer 4 Void-containing first base layer 5 Metal nitride layer 6 Second base layer 10 Base structure 30 First layer 40 Second layer 50 Nitride semiconductor substrate (substrate)

Claims (3)

A nitride semiconductor substrate having a diameter of 2 inches or more and having a main surface whose nearest low-index crystal plane is a (0001) plane,
The main surface has non-overlapping 50 μm square dislocation-free regions at a density of 100 pieces/cm ² or more,
The X-ray rocking curve of the (0002) plane is measured at each position on a straight line passing through the center within the main surface, and the angle between the X-ray incident on the main surface and the main surface is the maximum diffraction intensity. When the peak angle ω is plotted against the position on the straight line and the peak angle ω is approximated by a linear function of the position,
The radius of curvature of the (0001) plane determined by the reciprocal of the slope of the linear function is 10 m or more,
The error of the peak angle ω measured at each position on the straight line with respect to the linear function is 0.05° or less. Nitride semiconductor substrate. The nitride semiconductor substrate according to claim 1, wherein an error of the peak angle ω measured at each position on the straight line with respect to the linear function is 0.01° or less. The nitride semiconductor substrate according to claim 1 or 2, wherein the oxygen concentration is 5×10 ¹⁶ cm ⁻³ or less.

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