JP3557894B2 - Nitride semiconductor substrate and nitride semiconductor device - Google Patents

Description

[0001]
[Industrial applications]
The present invention relates to a nitride semiconductor (for example, In _X Al _Y Ga _1-XY N, 0 ≦ X, 0 ≦ Y, X + Y ≦ 1) and used for electronic devices such as LEDs, LDs, solar cells, light receiving elements, etc. obtained by laminating a nitride semiconductor on the substrate. To a nitride semiconductor device to be manufactured.
[0002]
[Prior art]
It is generally known that when a semiconductor is grown on a substrate, the use of a substrate lattice-matched with the semiconductor to be grown reduces the number of crystal defects in the semiconductor and improves the crystallinity. However, nitride semiconductors are generally grown on heterogeneous substrates that do not lattice match with nitride semiconductors, such as sapphire, spinel, and silicon carbide, because there is no substrate that lattice-matches in the world at present.
[0003]
On the other hand, various research institutes have attempted to fabricate a GaN bulk crystal to obtain a nitride semiconductor substrate that is completely lattice-matched, but only reports of a few millimeters have been obtained. No, it is far from practical use.
[0004]
As a technique for manufacturing a nitride semiconductor substrate, for example, JP-A-7-202265 and JP-A-7-165498 disclose that a buffer layer made of ZnO is formed on a sapphire substrate, and a nitride semiconductor is formed on the buffer layer. A technique for dissolving and removing the buffer layer after growing the GaN is described. However, the crystallinity of a ZnO buffer layer grown on a sapphire substrate is poor, and even if a nitride semiconductor is grown as a thick film on the buffer layer, crystal defects of the entire crystal are reduced by 10%. ⁸ Pieces / cm ² As described above, it is almost impossible to obtain a high-quality nitride semiconductor substrate. Further, it is also very difficult to continuously grow a thick nitride semiconductor as a substrate on a thin buffer layer made of ZnO because ZnO is decomposed during the growth.
[0005]
The applicant of the present invention has formed a stripe-like protective film having a property that nitride semiconductors are unlikely to grow on a sapphire substrate, and has developed a laser device using undoped GaN as a substrate grown laterally on the protective film. Announced (ICNS'97 Proceedings, October 27-31, 1997, P444-446, and Jpn. J. Appl. Phys. Vol. 36 (1997) pp. L1568-1571, Part 2, No. 12A, 1 December 1997. ). According to the growth of the nitride semiconductor in the lateral direction, a GaN substrate having less crystal defects tends to be easily obtained as compared with a nitride semiconductor substrate formed by a conventional growth method. This laser device has a structure in which an n-side contact layer made of Si-doped GaN is stacked on an undoped GaN substrate, and a separated and confined laser device structure is formed on the contact layer.
[0006]
[Problems to be solved by the invention]
However, although the undoped GaN substrate has the advantage of having very few crystal defects, it has the drawback that the resistivity is high and a sufficient carrier concentration cannot be obtained. Therefore, a high carrier concentration GaN layer doped with Si is provided as a contact layer on an undoped GaN substrate like the laser element.
[0007]
When a nitride semiconductor is used as a substrate, crystal defects of the substrate are reduced, a sufficient carrier concentration is provided for providing an electrode on the substrate, and when a nitride semiconductor substrate is used, the substrate is stacked thereon. It is very important to reduce the crystal defects of the nitride semiconductor. Accordingly, the present invention has been made in view of such circumstances, and an object thereof is to provide a nitride semiconductor substrate having few crystal defects and having a sufficient carrier concentration to form an electrode. Another object of the present invention is to provide a novel nitride semiconductor device structure having the substrate and capable of stacking nitride semiconductors with few crystal defects.
[0008]
[Means for Solving the Problems]
A nitride semiconductor substrate according to the present invention has a first main surface and a second main surface, has a nitride semiconductor containing an n-type impurity, and has a nitride semiconductor on at least one main surface. A nitride semiconductor substrate in which a semiconductor is exposed, wherein the nitride semiconductor substrate has an n-type impurity concentration as approaching either the first main surface or the second main surface. At least three layers in steps A crystal defect on the main surface side having a small concentration gradient region and a low n-type impurity concentration is 1 × 10 ^Five Pieces / cm ^Two It is characterized by the following. It should be noted that both the first main surface and the second main surface do not need to have the nitride semiconductor exposed, and the nitride semiconductor is within the scope of claim 1 even when one of the surfaces is exposed. . For example, in a nitride semiconductor substrate in which a nitride semiconductor is grown as a thick film on a heterogeneous substrate such as sapphire and the heterogeneous substrate remains without being removed, one main surface is on the sapphire side and the other is on the other side. The main surface of. Preferably, a nitride semiconductor in which the main surface of the nitride semiconductor is exposed to both sides in a state where the heterogeneous substrate is removed is used as the substrate.
[0009]
Further, the nitride semiconductor is exposed on the first main surface and the second main surface, and the concentration gradient region is formed from one main surface to the other main surface. It is characterized by.
[0010]
Further, in the nitride semiconductor substrate of the present invention, the main surface side where the n-type impurity concentration is low is a growth surface of the next semiconductor. The next semiconductor refers to a semiconductor layer having a structure of an electronic device such as a light emitting device such as a laser element or an LED element, a light receiving device such as a solar cell or a light receiving element, using this substrate.
[0011]
The n-type impurity concentration in the region where the n-type impurity concentration is low is undoped to 5 × 10 ¹⁸ /cm ^Three In the range. Further, preferably, the n-type impurity concentration in a region within 5 μm from the surface on the main surface side where the n-type impurity concentration is small is undoped to 5 × 10 ¹⁸ /cm ^Three In the range. This is because, when the main surface on the side where the n-type impurity concentration is reduced is used as the next semiconductor growth surface and the n-type impurity concentration on the main surface is adjusted within the above range, crystal defects are reduced. This is because crystal defects are less likely to be dislocated in the semiconductor layer grown thereon. The n-type impurity is at least one selected from the group consisting of Si, Ge, Sn, S, and O.
[0012]
A nitride semiconductor device according to the present invention has a first main surface and a second main surface, has a nitride semiconductor containing an n-type impurity, and has a nitride semiconductor on at least one main surface. A nitride semiconductor device in which a nitride semiconductor layer having an element structure is grown on a nitride semiconductor substrate on which a semiconductor is exposed, wherein the nitride semiconductor substrate has a first main surface or a 2 has a concentration gradient region in which the n-type impurity concentration decreases as approaching any one of the main surfaces. At least the main surface having a low n-type impurity concentration is a low crystal defect region. On the side of the nitride semiconductor substrate With the InGaN layer interposed, A nitride semiconductor layer is stacked and grown, and a main surface having a high n-type impurity concentration has an ohmic contact n-electrode. Note that an element structure refers to a structure in which at least an n-layer and a p-layer are stacked to have an active region. The crystal defect on the main surface side of the nitride semiconductor substrate having a low n-type impurity concentration is 1 × 10 ^Five Pieces / cm ^Two The following is preferred. The n-type impurity concentration in the region where the n-type impurity concentration is low is undoped to 5 × 10 ¹⁸ /cm ^Three Is preferably within the range.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
In the nitride semiconductor substrate of the present invention, the composition of the nitride semiconductor is not particularly limited, but when GaN substantially does not contain Al and In, the substrate has the least crystal defects. However, interposing an InGaN layer as a buffer layer in the nitride semiconductor substrate is also included in the category of the nitride semiconductor substrate of the present invention. InGaN has an effect of absorbing crystal defects. As the n-type impurity to be doped into the nitride semiconductor substrate, a group IV or group VI element such as Si, Ge, Sn, S, or O is used. Among them, Si and Sn are activated in the nitride semiconductor layer. High efficiency and high carrier concentration.
[0014]
The nitride semiconductor substrate of the present invention has a concentration gradient region adjusted so that the n-type impurity concentration decreases as approaching either one of the first or second main surface. The concentration gradient region may be continuous (gradation) or step-shaped as long as the impurity concentration decreases as it approaches the main surface. For example, it is within the scope of the present invention to have a concentration distribution in which a substantially constant impurity concentration is maintained up to a certain film thickness, and the n-type impurity concentration suddenly decreases when a certain boundary is exceeded. It is preferable that the impurity concentration be reduced continuously or in steps, for example, in at least three layers. This is because, when an electrode is taken out from the nitride semiconductor substrate, when the substrate is provided with an n-electrode with respect to the semiconductor layer laminated on the main surface side, no matter which substrate surface is exposed in the concentration gradient region, The surface becomes n +, and the carrier concentration of the substrate on the element structure side from the substrate surface becomes n−. Therefore, when an electrode is formed on the n + side, ohmic contact with the n electrode is easily obtained, and the carrier is easily diffused uniformly. This is because the efficiency of the element is improved. Furthermore, if the n-type impurity concentration is adjusted so as to become smaller as approaching the main surface, the crystallinity of the nitride semiconductor is improved, and the crystal defects tend to be reduced. Further, the concentration gradient region of the present invention is not necessarily provided over the entire nitride semiconductor substrate, but may be formed partially.
[0015]
As a preferred embodiment, in the nitride semiconductor substrate in which the nitride semiconductor is exposed on the first main surface and the second main surface, a concentration gradient region is formed from one main surface to another main surface. . That is, the adjustment is performed so that the n-type impurity concentration on one main surface side is high and the n-type impurity concentration on the other main surface is low. In the case of this nitride semiconductor substrate, if a nitride semiconductor having an element structure is grown on a side having a lower n-type impurity concentration and an electrode is taken out from the substrate side, an n-electrode and a p-electrode are provided on the same surface side. In the exposed structure and the structure in which the n-electrode and the p-electrode are opposed to each other, the exposed surface has a high carrier concentration and makes it easy to form ohmic contact with the n-electrode, regardless of which substrate surface is exposed. Further, since the resistivity has changed from low resistance to high resistance, carriers are easily diffused to the element side, and the efficiency of the element is improved.
[0016]
The main surface on the side where the n-type impurity concentration is low has an undoped to 5 × 10 5 n-type impurity concentration in a region within 5 μm from the main surface. ¹⁸ / Cm ³ It is desirable to be within the range. By adjusting to this range, for example, crystal defects within 5 μm from the main surface are 1 × 10 ⁵ / Cm ² The following is very preferable. Preferably 1 × 10 ¹⁸ / Cm ³ Below, most preferably 8 × 10 ¹⁷ / Cm ³ Do the following. 5 × 10 ¹⁸ / Cm ³ This is because if the number exceeds the limit, the number of crystal defects increases, and when an element structure is formed thereon, the life of the element tends to be shortened. When undoped, the undoped region is 1 μm or less, more preferably 0.5 μm or less, and most preferably 0.2 μm or less. If the undoped region exceeds 1 μm, the operating voltage in the series direction tends to increase, and the operating voltage tends to increase. Even if the device life is shortened, it is needless to say that the present invention is much longer than a device in which a device structure is directly grown on a conventional sapphire substrate.
[0017]
【Example】
[Example 1]
FIGS. 1A to 1E are cross-sectional views schematically showing the structure of each wafer obtained in each step of the method for manufacturing a nitride semiconductor substrate. FIG. 2 is a schematic sectional view showing the structure of a laser device according to one embodiment of the present invention. Hereinafter, the present invention will be described in detail with reference to these drawings. Note that the method for manufacturing a nitride semiconductor substrate shows the most preferred embodiment, and the nitride semiconductor substrate of the present invention is not necessarily limited to this method.
[0018]
A wafer of a heterogeneous substrate 1 made of sapphire having a diameter of 2 inches is set in a reaction vessel of a MOVPE (metal organic chemical vapor deposition) apparatus, and 500 ° C. is placed on the heterogeneous substrate 1 as shown in FIG. Then, a buffer layer (not shown) made of GaN is grown at 200 Å, and an underlayer 2 made of undoped GaN is grown thereon at a higher temperature (for example, 1050 ° C.) with a thickness of 6 μm.
[0019]
The heterogeneous substrate 1 may be any substrate as long as the substrate is made of a material different from that of the nitride semiconductor. For example, in addition to the sapphire C surface, sapphire having an R surface and an A surface as main surfaces, spinel (MgA1 ₂ O ₄ ), An insulating substrate such as SiC (including 6H, 4H, and 3C), ZnS, ZnO, GaAs, Si, and an oxide substrate lattice-matched with a nitride semiconductor. Substrate materials can be used. Further, a substrate in which the main surface of the substrate material is turned off may be used. The method of growing the underlayer 2 is not particularly limited, and conventional methods known for growing nitride semiconductors such as MOVPE, MBE (molecular beam vapor deposition), and HVPE (hydride vapor phase epitaxy) are known. You can grow in the way. Since the material of the underlayer 2 is different from that of the heterogeneous substrate 1, the underlayer 2 has a very large number of crystal defects. ⁸ Pieces / cm ² As described above, no matter how thick a film is grown, crystal defects due to lattice mismatch between a heterogeneous substrate and a nitride semiconductor, a coefficient of thermal expansion, etc., extend vertically from an interface between the nitride semiconductor and the heterogeneous substrate. , Does not become a nitride semiconductor substrate. Most preferably, undoped GaN is grown on the underlayer. Note that the number of crystal defects in the present invention indicates the number of crystal defects that can be measured on a photograph in TEM observation.
[0020]
After the underlayer 2 is grown, the wafer is taken out of the reaction vessel, and the underlayer 2 is etched into a shape having stripe-shaped irregularities as shown in FIG. 1B using an RIE (reactive ion etching) apparatus. The width of the concave portion of the stripe is 5 μm, and the width of the convex portion is 5 μm. In this etching, the concave portion exposes the nitride semiconductor surface, but the concave portion may be formed by etching until the surface of the heterogeneous substrate is exposed. FIG. 1B shows a sectional view in a direction perpendicular to the stripe.
[0021]
After the formation of the concavities and convexities, as shown in FIG. ₂ The protective film 3 is formed to a thickness of 0.1 μm so that the nitride semiconductor layer on the side surface of the unevenness is exposed. The protective film is made of a material having a property such that the nitride semiconductor does not grow or hardly grows on the surface thereof. _X ), Silicon nitride (Si _X N _Y ), Titanium oxide (TiO) _X ), Zirconium oxide (ZrO) _X ), A metal having a melting point of 1200 ° C. or higher, and the like, in addition to these multilayer films. These protective film materials are resistant to the growth temperature of the nitride semiconductor of 600 ° C. to 1100 ° C., and have such a property that the nitride semiconductor does not grow or hardly grows on the surface thereof. When a protective film is formed partially on the surface of the nitride semiconductor and the nitride semiconductor is grown in the lateral direction on the protective film, crystal defects of the nitride semiconductor do not extend in the vertical direction. As a result, crystal defects of the nitride semiconductor grown on the protective film are drastically reduced, and a nitride semiconductor substrate can be grown. In particular, by forming a protective film on the upper surface of the nitride semiconductor layer grown on the heterogeneous substrate as in this embodiment, exposing the side surface, and growing from the side surface, the crystal defects are reduced to 1 × 10 ⁵ Pieces / cm ² The following nitride semiconductor substrate is obtained.
[0022]
After the formation of the protective film 3, the wafer is set again in the MOVPE reaction vessel, the temperature is set to 1050 ° C., and the first nitride semiconductor layer 4 made of undoped GaN is set to 20 μm as shown in FIG. It grows with the film thickness. The first nitride semiconductor layer 4 grows from the side surface of the underlayer 2 provided with the unevenness, and grows in the vertical direction while growing in the horizontal direction on the protective film. Although a large number of crystal defects are generated in the underlayer 2 due to lattice mismatch with the heterogeneous substrate, the crystal defects penetrate to the surface of the first nitride semiconductor grown on the protective film 3 in the lateral direction. Difficult, for example, 1 × 10 crystal defects ⁵ Pieces / cm ² Hereinafter, under preferable conditions, 1 × 10 ⁴ Pieces / cm ² The following nitride semiconductor substrate is obtained.
[0023]
Further, after the growth of the first nitride semiconductor 4, the wafer is transferred to an HVPE apparatus, and gallium chloride, ammonia is used as a source gas, and silane gas (SiH ₄ ), And first, 1 × 10 ²⁰ / Cm ³ The doped GaN is grown for 50 Å. When the n-side impurity is first highly doped, the n-type impurity concentration is 1 × 10 ¹⁷ / Cm ³ 5 × 10 or more ²¹ / Cm ³ Below, preferably 1 × 10 ¹⁸ / Cm ³ ~ 1 × 10 ²¹ / Cm ³ It is desirable to adjust to. 5 × 10 ²¹ / Cm ³ If the number is larger than the above, crystallinity deteriorates, and conversely, crystal defects increase due to n-type impurities, and the crystal defects tend to be easily dislocated. Also 1 × 10 ¹⁷ / Cm ² If it is smaller than this, a sufficient carrier concentration cannot be obtained and the ohmic property with the n-electrode tends to deteriorate. After growing the Si at a high concentration, the flow rate of the silane gas is gradually decreased, and the second nitride of Si-doped GaN having a concentration gradient such that the Si concentration decreases as the Si concentration moves away from the heterogeneous substrate. The semiconductor layer 4 ′ is grown to a thickness of 300 μm, and the Si concentration of at least the last 5 μm of the 300 μm is set to 1 × 10 ¹⁸ / Cm ³ Hereinafter, the final 0.1 μm portion is undoped.
[0024]
After the growth of the second nitride semiconductor layer, a portion of the foreign substrate 1, the underlayer 2, the protective film 3, the first nitride semiconductor layer 4, and the second nitride semiconductor layer 4 'are polished from the foreign substrate side. Then, a nitride semiconductor substrate having a thickness of 200 μm is obtained.
[0025]
As described above, irregularities are formed on the surface of the underlying layer 2 made of the nitride semiconductor directly grown on the heterogeneous substrate 1, the side surfaces of the underlying layer are exposed, and the protective film 3 is formed on the surface of the convex portion. Then, the first nitride semiconductor layer 4 is grown from the side surface to the upper portion of the protective film, and the second nitride semiconductor layer 4 ′ doped with an n-type impurity is grown thereon to reduce the concentration gradient. Obtaining a nitride semiconductor substrate having the same is very important for obtaining a nitride semiconductor substrate having few crystal defects. Furthermore, the first nitride semiconductor layer is grown by MOVPE, the second nitride semiconductor layer grown thereon is grown by HVPE, and the second nitride semiconductor layer is grown to a second nitride semiconductor layer by MOVPE. By growing the thickness of the layer 4 'to a large thickness, a nitride semiconductor substrate with very few crystal defects can be obtained. In addition, when a crystal defect within 5 μm from the surface of the second nitride semiconductor layer obtained by the above method was observed by TEM, 1 × 10 ⁴ Pieces / cm ² In the following, it was found that a very good substrate was obtained.
[0026]
The nitride semiconductor substrate 4 'is transferred to an MOVPE apparatus, and a laser device having a structure shown in FIG. 2 is formed on an undoped GaN (a small amount of Si is contained by diffusion) layer as follows. I do.
[0027]
(Crack prevention layer 22)
Using TMG, TMI (trimethylindium), and ammonia at a temperature of 800 ° C., 1 × 10 ¹⁸ / Cm ³ Doped In _0.06 Ga _0.94 A crack preventing layer 22 made of N is grown to a thickness of 0.15 μm. The crack prevention layer can be formed of an InGaN or GaN layer, and it is preferable that this layer is doped with an n-type impurity to have a thickness of 0.1 to 3 μm. Note that this crack prevention layer can be omitted.
[0028]
(N-side cladding layer 23)
Then, at 1050 ° C., using TMA, TMG and ammonia, undoped Al _0.16 Ga _0.84 A layer of N is grown to a thickness of 25 angstroms, followed by stopping TMA, flowing silane gas, and ¹⁸ / Cm ³ A layer made of doped n-type GaN is grown to a thickness of 25 Å. These layers are alternately stacked to form a superlattice layer, and an n-side cladding layer 23 made of a superlattice having a total film thickness of 1.2 μm is grown. The n-type impurity of the superlattice layer can be doped into either GaN or AlGaN, or both.
[0029]
(N-side light guide layer 24)
Subsequently, the silane gas is stopped, and an n-side optical guide layer 8 made of undoped GaN is grown at 1050 ° C. to a thickness of 0.1 μm. The n-side light guide layer 24 may be doped with an n-type impurity.
[0030]
(Active layer 25)
Next, the temperature was raised to 800 ° C., and ¹⁸ / Cm ³ Doped In _0.01 Ga _0.95 A barrier layer of N is grown to a thickness of 100 Å, followed by undoped In at the same temperature. _0.2 Ga _0.8 A well layer made of N is grown to a thickness of 40 Å. A barrier layer and a well layer are alternately laminated twice, and finally an active layer having a multiple quantum well structure (MQW) having a total film thickness of 380 Å is formed, ending with the barrier layer. The active layer may be undoped as in this embodiment, or may be doped with an n-type impurity and / or a p-type impurity. The impurity may be doped into both the well layer and the barrier layer, or may be doped into either one. Further, as the stacking order, the layers may be stacked from the well layer and terminated with the well layer, the layers may be stacked from the well layer and terminated with the barrier layer, or the layers may be stacked from the barrier layer and terminated with the well layer.
[0031]
(P-side cap layer 26)
Next, the temperature was increased to 1050 ° C., and TMG, TMA, ammonia, Cp2Mg (cyclopentadienyl magnesium) was used, and Mg was reduced to 1 × 10 5 ²⁰ / Cm ³ Doped p-type Al _0.3 Ga _0.7 A p-side cap layer 26 of N is grown to a thickness of 300 Å.
[0032]
(P-side light guide layer 27)
Stop Cp2Mg, TMA, Mg 5 × 10 ¹⁶ / Cm ³ A p-side light guide layer 27 made of doped GaN is grown to a thickness of 0.1 μm.
[0033]
(P-side cladding layer 28)
Then, undoped Al _0.16 Ga _0.84 A layer of N is grown to a thickness of 25 Å, followed by 1 × 10 ¹⁹ / Cm ³ Layers made of doped GaN are grown to a thickness of 25 Å, and they are alternately laminated to grow a p-side cladding layer 28 of a superlattice layer having a total thickness of 0.6 μm.
[0034]
(P-side contact layer 29)
Finally, add 1 × 10 Mg ²⁰ / Cm ³ A p-side contact layer 29 made of doped p-type GaN is grown to a thickness of 150 Å.
[0035]
The wafer on which the nitride semiconductor layer for forming the laser element structure is stacked and grown on the nitride semiconductor substrate as described above is taken out of the reaction vessel, and the surface of the uppermost p-side contact layer 29 is covered with SiO 2. ₂ A p-side contact layer 29 and a p-side cladding layer 28 are etched to form a ridge stripe having a width of 1.5 μm. ₂ An insulating film 30 is formed.
[0036]
Next, as shown in FIG. 2, a p-electrode 31 is formed in ohmic contact with the p-side contact layer 29 via the insulating film 30, and a p-pad electrode 32 for bonding is formed on almost the entire surface of the p-electrode 31. I do. Then, the n-electrode 33 is formed on the side of the nitride semiconductor substrate facing the p-electrode.
[0037]
Finally, the nitride semiconductor substrate is cleaved to form a resonance surface of the laser device on the cleaved surface. The cleavage plane of the laser element is a part corresponding to the side surface (M plane) when the nitride semiconductor substrate is represented by a (11-00) plane, that is, when the crystal structure of the nitride semiconductor is represented by a hexagonal prism. After the cleavage, a dielectric multilayer film serving as a mirror is formed only on one of the cleavage planes, the reflectance of the resonance surface is changed, and the side without the mirror is used as the laser light emission surface. Finally, the substrate was separated into chips, and the nitride semiconductor substrate side was placed on a heat sink to form a laser device. The device showed laser oscillation at room temperature and had a threshold current density of 1.5 kA / cm. ² Showed continuous oscillation at room temperature and a life of 1000 hours or more at an output of 20 mW.
[0038]
When a laser element having a stripe-shaped ridge on the p-layer side is manufactured using a nitride semiconductor substrate, it is necessary to provide an ohmic p-electrode in the uppermost p-side contact layer. As shown in FIG. 2, by forming a p-pad electrode for bonding on almost the entire surface of the p-electrode, the heat dissipation is improved. Further, when wire bonding is performed on the p pad electrode, the reliability of the laser element is improved by preventing the bonding position from overlapping the ridge stripe. This is presumed that stress is not directly applied to the ridge portion during bonding, so that an impact is not applied to the active layer below the ridge, and the device is less likely to be painful and reliability is improved.
[0039]
As described above, when an element structure is manufactured using the nitride semiconductor substrate of the present invention, the surface always has a high carrier concentration regardless of which surface is exposed when the dissimilar substrate is removed. Therefore, when an n-electrode is provided on the exposed surface, carriers easily spread over the entire substrate, the threshold value is reduced, and the output is improved. Furthermore, an n-type impurity can be implanted from the substrate surface side on which the n-electrode is to be formed by ion implantation, so that the carrier concentration can be as high as the implantation depth.
[0040]
[Example 2]
In the first embodiment, when growing the second nitride semiconductor layer 4 ′, undoped GaN is grown to a thickness of 30 μm. Subsequently, 1 × 10 ¹⁹ / Cm ³ A doped GaN layer is grown to a thickness of 80 μm, and then ¹⁸ / Cm ³ A doped GaN layer is grown to a thickness of 20 μm, and then ¹⁸ / Cm ³ A doped GaN layer is grown to a thickness of 20 μm, and finally, 5 × 10 ¹⁷ / Cm ³ A doped GaN layer is grown to a thickness of 2 μm. After the growth, when polishing a heterogeneous substrate, the undoped GaN layer of the second nitride semiconductor 4 ′ is polished and removed, and Si is reduced to 1 × 10 ¹⁹ / Cm ³ The doped GaN surface is exposed to form a nitride semiconductor substrate. Otherwise, a laser device was manufactured in the same manner as in Example 1. As a result, a laser device having substantially the same characteristics as in Example 1 was obtained.
[0041]
[Example 3]
FIG. 3 is a schematic sectional view showing the structure of a laser device according to another embodiment of the present invention. Hereinafter, the third embodiment will be described in detail with reference to FIG.
[0042]
In the first embodiment, after growing the first nitride semiconductor layer 4 to a thickness of 20 μm, 1 × 10 ¹⁹ / Cm ³ The doped GaN layer is grown for 150 Å. Then, the amount of Si gas was gradually reduced, and a Si-doped GaN layer was grown to a thickness of 150 μm. ¹⁷ / Cm ³ A second nitride semiconductor layer 4 ′ having a thickness of 300 μm is grown as doped GaN.
[0043]
After completion of the reaction, the heterogeneous substrate, the underlayer, the protective film, and a part of the first nitride semiconductor layer are removed to obtain a nitride semiconductor substrate. Thereafter, the wafer is set in the MOVPE apparatus in the same manner as in the first embodiment, and the layers from the crack prevention layer 22 to the p-side contact layer 29 are similarly stacked to form a laser element structure.
[0044]
After the growth, the wafer is taken out of the reaction vessel, a mask is formed on the surface of the uppermost p-side contact layer 29, and the Si of the second nitride semiconductor layer 4 ′ is reduced to 1 × 10 2 as shown in FIG. ¹⁹ / Cm ³ The doped GaN layer is exposed, and this layer is used as a contact layer.
[0045]
Next, a mask having a predetermined shape is applied to the p-side contact layer 29, and the p-side contact layer 29 and the p-side cladding layer 28 are etched to form a ridge stripe having a width of 1 μm. ₂ A p-electrode 31 in ohmic contact with the p-side contact layer 29 and a p-pad electrode 32 electrically connected to the p-electrode are formed via the insulating film 30 and exposed. An ohmic contact n-electrode 33 is formed on the surface of the n-side contact layer.
[0046]
After forming the n-electrode and the p-electrode as described above, the semiconductor device is cleaved in the same manner as in the first embodiment to form a resonance surface of the laser device on the cleavage surface of the nitride semiconductor. After cleavage, the resultant was separated into chips to obtain a laser device as shown in FIG. 3, and a laser device having substantially the same characteristics as in Example 1 was obtained.
[0047]
In the case of a laser device having an n-electrode and a p-electrode formed on the same surface as shown in FIG. 3, a p-pad electrode is formed and the bonding position is shifted from the ridge. Further, in a laser device having a ridge on the p-layer side and taking out the n-electrode and the p-electrode from the same surface side, as shown in FIG. 3, the ridge should be located closer to the n-electrode from the center. However, the threshold value tends to decrease.
[0048]
[Example 4]
FIG. 4 is a schematic sectional view showing the structure of an LED element according to another embodiment of the present invention. Hereinafter, a fourth embodiment will be described with reference to FIG.
[0049]
In Example 1, after growing the first nitride semiconductor layer 4 to a thickness of 20 μm, the same MOVPE apparatus was used to remove 1 × 10 ¹⁹ / Cm ³ The doped GaN layer is grown for 20 Å. Then, the amount of Si gas was gradually reduced, and a Si-doped GaN layer was grown to a thickness of 20 μm. ¹⁷ / Cm ³ As a doped GaN layer, a second nitride semiconductor layer 4 ′ having a thickness of 40 μm was formed, and crystal defects within 5 μm from the growth surface main surface were 1 × 10 5 ⁵ / Cm ² A nitride semiconductor substrate having the following nitride semiconductor layer is obtained.
[0050]
(Buffer layer 41)
Subsequently, while the wafer was kept in the reaction vessel, 1 × 10 ¹⁷ / Cm ³ Doped In _0.01 Ga _0.99 A buffer layer 41 made of N is grown to a thickness of 0.1 μm. This buffer layer acts in the same way as the crack prevention layer, makes it difficult for the AlGaN layer grown on the active layer to crack, grows the crystal with good crystallinity, and improves the output of the device. The buffer layer may be made of GaN or undoped. When undoped, the film thickness is set to 1 μm or less, preferably 0.5 μm or less.
[0051]
(Active layer 42)
Next, a barrier layer made of 150 Å of GaN and 30 Å of In _0.4 Ga _0.6 An active layer having a multiple quantum well structure in which three well layers made of N are stacked, and finally a barrier layer is stacked, is grown.
[0052]
(P-side cladding layer 43)
Next, 5 × 10 ¹⁹ / Cm ³ Doped p-type Al _0.1 Ga _0.9 A p-side cladding layer made of N is grown to a thickness of 500 Å.
[0053]
(P-side contact layer 44)
Finally Mg1 × 10 ²⁰ / Cm ³ A p-side contact layer made of doped GaN is grown to a thickness of 0.1 μm.
[0054]
After the completion of the reaction, a mask having a predetermined shape is formed on the surface of the uppermost p-side contact layer 44, and as shown in FIG. ¹⁹ / Cm ³ The doped GaN layer is exposed, and this layer is used as a contact layer.
[0055]
After etching, almost 200 nm of a translucent p-electrode 45 containing Ni and Au is formed on almost the entire surface of the uppermost p-side contact layer, and a p-pad electrode made of Au for bonding is formed on the p-electrode 45. 46 is formed with a thickness of 0.5 μm. On the other hand, an n-electrode 47 containing W and Al is formed on the surface of the n-side contact layer 4 to form an LED element.
[0056]
This LED element emitted pure green light of 520 nm at a forward voltage of 20 mA, Vf was 3.2 V, which was lower than that of the conventional LED, and the withstand voltage in the reverse direction was more than doubled. The improvement in the reverse breakdown voltage is due to the growth of the LED element on the nitride semiconductor substrate having few crystal defects.
[0057]
【The invention's effect】
As described above, when the nitride semiconductor substrate of the present invention is used, an n-electrode can be formed in any layer. Therefore, when an element structure such as an LED element and an LD element is formed on the nitride semiconductor substrate, This is very useful as a substrate for forming an n-electrode. In addition, by removing a heterogeneous substrate and using only a nitride semiconductor substrate, for example, in a device that generates heat, such as a light-emitting device, heat dissipation from the substrate is improved, the life is extended, and the reliability is improved.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view for sequentially explaining the structure of a wafer obtained in each step in a manufacturing method for obtaining a nitride semiconductor substrate of the present invention.
FIG. 2 is a schematic sectional view showing the structure of a laser device according to one embodiment of the present invention.
FIG. 3 is a schematic sectional view showing the structure of a laser device according to another embodiment of the present invention.
FIG. 4 is a schematic sectional view showing the structure of an LED element according to another embodiment of the present invention.
[Explanation of symbols]
1 ... heterogeneous substrate
2 ... Underlayer
3 ... Protective film
4... First nitride semiconductor layer
4 ′ ··· Second nitride semiconductor layer
22, 41: crack prevention layer (buffer layer)
23 ... n-side cladding layer
24 ... n-side light guide layer
25, 42... Active layer
26 ... p-side cap layer
27 ... p-side light guide layer
28, 43 ... p-side cladding layer
29, 44... P-side contact layer
30 ... insulating film
31, 45 ... p electrode
32, 46 ... p pad electrode
33, 47 ... n electrode

Claims (8)

A nitride semiconductor having a first main surface and a second main surface, including a nitride semiconductor containing an n-type impurity, and having the nitride semiconductor exposed on at least one of the main surfaces. A nitride semiconductor substrate, wherein the n-type impurity concentration decreases stepwise to at least three or more layers as approaching either the first main surface or the second main surface. A nitride semiconductor substrate having a concentration gradient region and a crystal defect on the main surface side having a low n-type impurity concentration of 1 × 10 ⁵ / cm ² or less. 2. The nitride semiconductor substrate according to claim 1, wherein the main surface having the lower n-type impurity concentration is a growth surface of a next semiconductor. 3. ^3. The nitride semiconductor substrate according to claim 1, wherein the n-type impurity concentration in the region where the n-type impurity concentration is low is in the range of undoped to 5 × 10 ¹⁸ / cm ^{3. 4} . 4. The nitride semiconductor substrate according to claim 1, wherein the n-type impurity concentration in a region within 5 μm from the surface on the main surface side where the n-type impurity concentration is low is in the range of undoped to 5 × 10 ¹⁸ / cm ^3. 5. The nitride semiconductor substrate according to claim 1, wherein the n-type impurity is at least one selected from the group consisting of Si, Ge, Sn, S, and O. 6. A nitride semiconductor having a first main surface and a second main surface, including a nitride semiconductor containing an n-type impurity, and having the nitride semiconductor exposed on at least one of the main surfaces. A nitride semiconductor element in which a nitride semiconductor layer serving as an element structure is stacked and grown on a substrate, wherein the nitride semiconductor substrate is provided on one of a first main surface and a second main surface. As it approaches, it has a concentration gradient region where the n-type impurity concentration decreases, and at least the main surface having a low n-type impurity concentration is a low crystal defect region, and is located on the nitride semiconductor substrate on the main surface side. A nitride semiconductor device, wherein the nitride semiconductor layer is stacked and grown with an InGaN layer interposed, and a main surface having a high n-type impurity concentration has an n-electrode in ohmic contact. 7. The nitride semiconductor device according to claim 6, wherein crystal defects on the main surface side of the nitride semiconductor substrate having a low n-type impurity concentration are 1 × 10 ⁵ / cm ² or less. 7. The nitride semiconductor device according to claim 6, wherein the n-type impurity concentration in the region where the n-type impurity concentration is low is in the range of undoped to 5 × 10 ¹⁸ / cm ³ .

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