JP2015070091A - Group iii nitride semiconductor substrate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a group III nitride semiconductor substrate which can satisfy both of reduction in leakage current and suppression of current collapse.SOLUTION: The group III nitride semiconductor substrate comprises: a substrate 21; a buffer layer 22 formed on the substrate 21; and a nitride laminate 23 formed on the buffer layer 22. The nitride laminate 23 includes a first nitride layer 23a composed of AlGaN(0<x≤1), a second nitride layer 23b composed of AlGaN(0<y≤1) and a third nitride layer 23c composed of AlGaN(0<z≤1) which are formed on the buffer layer 22. A carbide concentration of the first nitride layer 23a is higher than a carbon concentration of the second nitride layer 23b, and a carbon concentration of the second nitride layer 23b is higher than a carbon concentration of the third nitride layer 23c. The carbon concentration of the third nitride layer 23c is 9×10atoms/cmor more and 2.5×10atoms/cmor less and a difference between every two nitride layers are adjusted within an adequate range.

Description

  The present invention relates to a group III nitride semiconductor substrate, and more particularly to a group III nitride semiconductor substrate suitable for a normally-off type semiconductor device that can achieve both reduction of leakage current and suppression of current collapse.

  Nitride semiconductor materials such as gallium nitride (GaN) and aluminum nitride (AlN) have a higher carrier mobility and a wider band gap than silicon (Si), and thus a high electron mobility transistor (High). It is used for an Electricon Mobility Transistor (HEMT), a heterojunction field effect transistor (HJFET), and the like.

  FIG. 1 is a schematic cross-sectional view of an example of a conventional HEMT. The HEMT 100 shown in this figure has a structure in which a buffer layer 12 made of GaN, a channel layer 13 made of GaN, and an electron supply layer 14 made of AlGaN are sequentially stacked on a substrate 11. A source electrode 15, a gate electrode 16, and a drain electrode 17 are provided on the surface.

  In the HEMT 100, a two-dimensional electron gas layer 13a is formed immediately below the interface between the channel layer 13 and the electron supply layer 14, and the two-dimensional electron gas is used as a carrier. When a voltage is applied between the source electrode 15 and the drain electrode 17 to operate the device, electrons supplied to the channel layer 13 move at high speed in the two-dimensional electron gas layer 13 a and move to the drain electrode 17. To do. At this time, the drain current flowing between the source electrode 15 and the drain electrode 17 can be controlled by controlling the voltage applied to the gate electrode 16 to change the thickness of the depletion layer immediately below the gate electrode 16. .

  In recent years, the HEMT is expected to be applied to power-saving elements for power supplies that require high current / high voltage operation. For that purpose, it is indispensable not only that the device has a low loss and a high breakdown voltage but also a so-called normally-off type in which no current flows when the gate voltage is off, from the viewpoint of safety. Therefore, a technique has been proposed in which a leak current generated in the buffer layer is reduced by increasing the resistance by adding impurities to the buffer layer. For example, Patent Document 1 describes a technique for increasing the resistance of a buffer layer by adding a IIB group element, particularly zinc (Zn), to the buffer layer at a high concentration.

  However, in a HEMT having a buffer layer with increased resistance by adding impurities, when a high stress electric field is applied, electrons are trapped in defects formed by the increased resistance impurities, the drain current decreases, and the on-resistance rapidly increases. There is a problem that the occurrence of current collapse, which is a phenomenon that increases rapidly, becomes remarkable. In order to suppress this current collapse, it is necessary to reduce the concentration of the high-resistance impurity that forms a defect that traps electrons, but this causes an increase in leakage current.

Thus, the reduction of leakage current and the suppression of current collapse are in a trade-off relationship, and various techniques have been proposed to achieve both the reduction of leakage current and the suppression of current collapse. For example, in Patent Document 2, Al _x Ga ₁ having a thickness of 200 nm to 1500 nm on the buffer layer and a carbon concentration of 5 × 10 ¹⁶ atoms / cm ^{3 to} 1 × 10 ¹⁸ atoms / cm ^3. _A nitride laminate composed of -xN (0.05 ≦ x ≦ 0.25) is provided, and Al _y Ga _1-y N (0 ≦ y ≦ 0.04) of 5 nm to 200 nm is formed on the nitride laminate. In other words, a technique that achieves both high threshold voltage and suppression of current collapse is described.

Patent Document 3 discloses a first compound semiconductor having a buffer layer of GaN and not normally-off type, but having a carbon concentration of 1 × 10 ¹⁸ / cm ³ or more and 1 × 10 ²¹ / cm ³ or less on the substrate. And a barrier layer made of a compound semiconductor having a carbon concentration of 5 × 10 ¹⁷ / cm ³ or less and a band gap energy larger than that of the first semiconductor layer, and the carbon concentration on the barrier layer 5 × 10 ¹⁷ / cm ³ or less, a compound semiconductor channel layer having a band gap energy smaller than that of the barrier layer, and a compound semiconductor electron supply layer having a band gap energy larger than that of the electron supply layer on the channel layer A technique for achieving both reduction of leakage current and suppression of current collapse is described.

JP 2002-57158 A JP 2013-8938 A JP 2013-38157 A

However, even with the techniques described in Patent Documents 2 and 3, it has been found that the reduction of leakage current and the suppression of current collapse are still insufficient.
Therefore, an object of the present invention is to provide a group III nitride semiconductor substrate suitable for normally-off type semiconductor devices, which can achieve both reduction of leakage current and suppression of current collapse.

The inventors of the present invention have intensively studied how to solve the above problems. As a result, a nitride laminated body having a laminated structure including three nitride layers is provided on the buffer layer, and the carbon concentration of the nitride layer on the channel side is 9 × 10 ¹⁵ atoms / cm ³ or more and 2.5 ×. It was found that it was effective to adjust the carbon concentration difference between the nitride layers to an appropriate range after setting it to 10 ¹⁶ atoms / cm ³ or less, and the present invention was completed.

That is, the gist of the present invention is as follows.
(1) A substrate, a buffer layer made of a group III nitride formed on the substrate, and a nitride laminate formed on the buffer layer, the nitride laminate being the buffer layer A first nitride layer formed of Al _x Ga _1-x N (0 <x ≦ 1) and Al _y Ga _1-y N formed on the first nitride layer. A second nitride layer made of (0 <y ≦ 1) and a third nitride made of Al _z Ga _1-z N (0 <z ≦ 1) formed on the second nitride layer Each of the first, second, and third nitride layers contains carbon, and the carbon concentration of the first nitride layer is the second nitride layer. The carbon concentration of the second nitride layer is higher than the carbon concentration of the third nitride layer, and the carbon concentration of the third nitride layer is higher than the carbon concentration of the third nitride layer. 9 × is at ¹⁰ 15 atoms / cm ³ or more 2.5 × ¹⁰ 16 atoms / cm ³ or less, the difference between the carbon concentration of the third nitride layer of a carbon concentration and the second nitride layer of 4. 5 × 10 ¹⁵ atoms / cm ³ or more and 3.5 × 10 ¹⁶ atoms / cm ³ or less, and the difference between the carbon concentration of the second nitride layer and the carbon concentration of the first nitride layer is 1. A group III nitride semiconductor substrate, wherein the group is 2 × 10 ¹⁷ atoms / cm ³ or more and 3.5 × 10 ¹⁸ atoms / cm ³ or less.

(2) The difference between the carbon concentration of the third nitride layer and the carbon concentration of the second nitride layer is 4.5 × 10 ¹⁵ atoms / cm ³ or more and 1.7 × 10 ¹⁶ atoms / cm ³ or less. And the difference between the carbon concentration of the second nitride layer and the carbon concentration of the first nitride layer is 1.3 × 10 ¹⁷ atoms / cm ³ or more and 3.5 × 10 ¹⁸ atoms / cm ³ or less. The group III nitride semiconductor substrate according to (1) above.

(3) The composition of Al in the first nitride layer, the second nitride layer, and the third nitride layer is 0.03 or more and 0.10 or less, described in (1) or (2) Group III nitride semiconductor substrate.

(4) The group III according to any one of (1) to (3), wherein the first nitride layer, the second nitride layer, and the third nitride layer have the same Al composition. Nitride semiconductor substrate.

(5) The group III nitride according to any one of (1) to (4), wherein the buffer layer includes carbon, and the concentration of the carbon is 1.0 × 10 ¹⁹ atoms / cm ³ or more. Semiconductor substrate.

(6) Any one of (1) to (5), wherein the buffer layer has a superlattice structure in which Al _1-α Ga _α N (0 ≦ α <0.5) and AlN are alternately stacked. The group III nitride semiconductor substrate as described in 2.

According to the present invention, a nitride laminate composed of three nitride layers is provided on the buffer layer, and the carbon concentration of the nitride layer on the channel layer side is set to 9 × 10 ¹⁵ atoms / cm ³ or more and 2.5 × 10 6. ¹⁶ atoms / cm ³ or less, and the difference in carbon concentration between the nitride layers is adjusted to an appropriate range, so that the group III suitable for normally-off type semiconductor devices that can achieve both current collapse and leakage current reduction and current collapse suppression. A nitride semiconductor substrate can be obtained.

It is a schematic cross section of an example of a HEMT according to the prior art. 1 is a schematic cross-sectional view of a nitride semiconductor substrate according to the present invention. 4 is a SIMS profile of carbon and aluminum for Experimental Example 1.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. A group III nitride semiconductor substrate according to the present invention includes a substrate, a buffer layer made of group III nitride formed on the substrate, and a nitride stack formed on the buffer layer. Here, the nitride stacked body includes a first nitride layer made of Al _x Ga _1-x N (0 <x ≦ 1) formed on the buffer layer, and the first nitride layer. A formed second nitride layer made of Al _y Ga _1-y N (0 <y ≦ 1), and Al _z Ga _1-z N (0) formed on the first nitride layer. <Z ≦ 1) and a third nitride layer, and each of the first, second and third nitride layers contains carbon, and carbon of the first nitride layer The concentration is higher than the carbon concentration of the second nitride layer, the carbon concentration of the second nitride layer is higher than the carbon concentration of the third nitride layer, and the carbon concentration of the third nitride layer is 9 ×. 10 ¹⁵ atoms / cm ³ or more and 2.5 × 10 ¹⁶ atoms / cm ³ or less, and the difference between the carbon concentration of the third nitride layer and the carbon concentration of the second nitride layer Is 4.5 × 10 ¹⁵ atoms / cm ³ or more and 3.5 × 10 ¹⁶ atoms / cm ³ or less, and the difference between the carbon concentration of the second nitride layer and the carbon concentration of the first nitride layer is It is important that it is 1.2 × 10 ¹⁷ atoms / cm ³ or more and 3.5 × 10 ¹⁸ atoms / cm ³ or less.

  First, the knowledge that led to the completion of the present invention will be described. The present inventors have investigated in detail the cause of the insufficient compatibility between reduction of leakage current and suppression of current collapse by the techniques described in Patent Documents 2 and 3. As described above, to reduce the leakage current, it is effective to increase the resistance by adding impurities to the buffer layer. On the other hand, to suppress current collapse, it is effective to reduce the carbon concentration of the buffer layer to reduce defects that trap carriers.

  With respect to this point, it is clear that a carbon layer-added buffer layer is provided as in Patent Document 2 and the carbon concentration of the buffer layer is adjusted so as to achieve both reduction of leakage current and suppression of current collapse. It is difficult to.

  In the normally-off type, the leakage current is measured after applying an electric field stress. That is, the required fracture resistance is higher than that of the normally-off type. As a result of detailed evaluations by the present inventors, it has been found that the technique described in Patent Document 3 is still inadequate for reducing the leakage current and suppressing the current collapse as a normally-off type.

  As a result of investigating in detail the cause of the inability to achieve both reduction of leakage current and suppression of current collapse by the technique described in Patent Document 3, the inventors have found that the difference in carbon concentration between the two semiconductor layers indicates leakage current and current. It was found to have a big impact on collapse. That is, it was found that when the difference in carbon concentration between the two semiconductor layers is large, the current collapse deteriorates, and when the difference in carbon concentration is small, the leakage current increases.

  The reason for this is not necessarily clear, but is presumed as follows. That is, when the difference in carbon concentration between the nitride layers is increased, the current collapse is thought to be because deep levels due to carbon acting as an electron trap increase in the nitride layer and at the interface between the layers. On the other hand, the leakage current increases when the carbon concentration difference is small because the residual carriers in the nitride layer cannot be compensated.

  Thus, the present inventors have found that it is important to set the difference in carbon concentration between semiconductor layers within an appropriate range in order to achieve both reduction of leakage current and suppression of current collapse. Furthermore, when the semiconductor layer has a two-layer structure, the inventors reduce the carbon concentration of the semiconductor layer on the channel layer side, increase the carbon concentration of the semiconductor layer on the substrate side, and further increase the carbon concentration between the semiconductor layers. It was determined that it was difficult to achieve both reduction of leakage current and suppression of current collapse by setting the difference between the two to an appropriate concentration range.

Thus, as a result of intensive studies on ways to achieve both reduction of leakage current and suppression of current collapse, the present inventors have determined that a semiconductor layer having a stacked structure in which three nitride layers are stacked on a buffer layer. The carbon concentration of the nitride layer on the channel layer side is set to 9 × 10 ¹⁵ atoms / cm ³ or more and 2.5 × 10 ¹⁶ atoms / cm ³ or less, and the difference in carbon concentration between the nitride layers The present inventors have found that it is effective to adjust to an appropriate range so that both current reduction and current collapse can be suppressed, and the present invention has been completed. The group III nitride semiconductor substrate according to the present invention will be described below.

  FIG. 2 shows a schematic diagram of a group III nitride semiconductor according to the present invention. The group III nitride semiconductor substrate 1 shown in this figure includes a substrate 21, a buffer layer 22 formed on the substrate 21, and a nitride laminate 23 formed on the buffer layer 22. Hereinafter, each layer will be described.

  The substrate 21 is a base substrate for forming a semiconductor device, and a single crystal material such as silicon, sapphire, SiC, AlN, or GaN can be used as the material thereof. Further, specifications such as a wafer growth method, plane orientation, oxygen concentration, impurity concentration, conductivity type, resistivity, and thickness can be appropriately set according to the requirements of the nitride semiconductor substrate to be designed.

  The substrate 21 is preferably a Si single crystal substrate. At that time, the plane orientation of the Si single crystal substrate is not particularly specified, and (110), (100), (111) planes, etc. can be used, but in order to grow the (0001) plane of group III nitride. The (110) or (111) plane is preferable for the surface, and it is desirable to use the (111) plane for growth with good surface flatness. The off-angle is appropriately set at 5 ° or less, but preferably 1 ° or less so as not to impair the single crystal growth.

  Moreover, it may be p-type or n-type conductivity type, and can be applied to various resistivities from 0.001 to 100,000 Ω · cm. Further, the resistivity does not necessarily have to be uniform over the entire Si single crystal substrate. Furthermore, impurities for purposes other than controlling conductivity, such as oxygen (O) and nitrogen (N), may be included in the Si substrate. The thickness of the substrate is appropriately set in consideration of the amount of warp after the single crystal growth.

  The buffer layer 22 is made of a group III nitride, relieves lattice distortion caused by lattice mismatch between the substrate 21 and the nitride laminate formed on the buffer layer 22, and suppresses the formation of dislocations. Reduce warpage of semiconductor substrate.

The buffer layer 22 can be a low-temperature buffer layer, a buffer layer made of a single crystal, or a buffer layer having a superlattice structure. In particular, the first layer having a film thickness of about the de Broglie wavelength, It is preferable to have a superlattice structure in which a plurality of second layers having a composition different from that of the first layer and having a film thickness of about the de Broglie wavelength are alternately stacked. For example, a buffer layer 22 having a superlattice structure may be formed by alternately laminating a plurality of layers (for example, 100 pairs) of AlN layers and Al _1-α Ga _α N (0 ≦ α <0.5) layers. it can. Note that α = 0 (that is, GaN) is most preferable because the buffering effect of strain is maximized.

The buffer layer 22 preferably contains carbon, and in this case, the carbon concentration is preferably 1 × 10 ¹⁹ atoms / cm ³ or more and 5 × 10 ¹⁹ atoms / cm ³ or less. As a result, it is possible to obtain a vertical breakdown voltage necessary for a normally-off type device. The carbon concentration can be adjusted by adjusting the raw material gas flow rate of the buffer layer 22 and the substrate temperature.

  In the present specification, the carbon concentration is a value measured by secondary ion mass spectrometry (SIMS). In SIMS, quantitative analysis of each element in the depth direction can be performed while etching from the surface. At that time, the measurement position of the carbon concentration adopted as the carbon concentration of the measurement target layer is the interface between the measurement target layer and the layer adjacent to the surface side of the measurement target layer (device manufacturing side) (measurement target Is the depth position of 50 nm from the device production side surface of the measurement target layer. Here, the position of the interface between the measurement target layer and the adjacent layer is determined from the carbon concentration profile and the aluminum concentration profile in the SIMS analysis profile indicating the concentration of each element with respect to the etching depth, and either one of carbon and aluminum is determined. Alternatively, the central position in the depth direction of the region where both density profiles change sharply can be determined as the position where the interface exists. For example, the carbon concentration of the second nitride layer 23b is a value measured at a depth of 50 nm from the interface between the second nitride layer 23b and the third nitride layer 23a.

The nitride laminate 23 includes a first nitride layer 23a made of Al _x Ga _1-x N (0 <x ≦ 1) formed on the buffer layer 22, and the first nitride layer 23a. A second nitride layer 23b made of Al _y Ga _1-y N (0 <y ≦ 1) and Al _z Ga _1-z formed on the first nitride layer 23b. And a third nitride layer 23c made of N (0 <z ≦ 1). Here, by including Al in the first, second, and third nitride layers, the band gap is increased, and a breakdown voltage suitable for a normally-off type can be obtained.

  Each of these nitride layers 23a-23c contains carbon, and the carbon concentration is increased from the third nitride layer 23c toward the first nitride layer 23a. That is, the carbon concentration of the first nitride layer 23a is higher than the carbon concentration of the second nitride layer 23b, and the carbon concentration of the second nitride layer 23b is higher than the carbon concentration of the third nitride layer 23c. Make it high. As a result, the carbon concentration of the nitride layer 23a on the substrate 21 side is increased to reduce the leakage current, and the carbon concentration of the nitride layer 23c, which is a layer immediately below the channel layer, causes carrier trapping. The current collapse can be improved.

  However, as described above, in order to achieve both reduction of leakage current and suppression of current collapse, the difference in carbon concentration between the nitride layers greatly affects the leakage current and current collapse. It is important to set the difference in carbon concentration within an appropriate concentration range. In the nitride laminated body having a two-layer structure as in the nitride semiconductor substrate described in Patent Document 3, the inventors reduce the carbon concentration of the nitride layer on the channel side and the nitride layer on the substrate side. Various adjustments were attempted so as to increase the carbon concentration of the carbon and to make the difference in carbon concentration between the nitride layers within an appropriate concentration range. However, it is judged that it is difficult to use a nitride laminate having a two-layer structure, and in the present invention, the nitride laminate 23 has a three-layer structure. As a result of adjusting the difference between the carbon concentration in the three-layer structure nitride laminate 23 and the carbon concentration between the layers, the inventors succeeded in finding a condition that can achieve both a reduction in leakage current and a suppression of current collapse.

Specifically, after the carbon concentration of the third nitride layer is set to 9 × 10 ¹⁵ atoms / cm ³ or more and 2.5 × 10 ¹⁶ atoms / cm ³ or less, the carbon concentration of the third nitride layer 23c is set. And the carbon concentration of the second nitride layer 23b is 4.5 × 10 ¹⁵ atoms / cm ³ or more and 3.5 × 10 ¹⁶ atoms / cm ³ or less, and the carbon concentration of the second nitride layer 23b is The difference from the carbon concentration of the first nitride layer 23a is set to 1.2 × 10 ¹⁷ atoms / cm ³ or more and 3.5 × 10 ¹⁸ atoms / cm ³ or less.

  Further, the thicknesses of the first, second and third nitride layers are not particularly limited as long as they have appropriate thicknesses from the viewpoint of the warpage and crystallinity of the wafer that does not hinder the device process. For example, the thickness of each nitride layer can be 100 nm or more and 1000 nm or less. Further, it is not necessary that all the nitride layers have the same thickness, and the nitride layers may be set appropriately according to the carbon concentration of each nitride layer. It is preferable to make the second nitride layer 23b thicker than the first and third nitride layers because it is easy to achieve both current collapse reduction and leakage suppression.

By setting such a carbon concentration, leakage current can be suppressed. Specifically, when a HEMT is formed using the group III nitride semiconductor substrate according to the present invention and a voltage of 1000 V is applied between the source electrode and the drain electrode with the gate voltage turned off, the source-drain When the current flowing between them is measured, the current value is 1 × 10 ⁻⁶ A / cm or less. Thus, the lateral leakage current through the buffer layer 22 and the nitride stack 23 can be sufficiently reduced.

  The current collapse is a source-drain current of a source-drain current after applying an electric field stress of 200 V or more between the source electrode and the drain electrode (in this case, the source-drain electrode in this case). Is a ratio to a region where the output current characteristic is linear (for example, 1 V), and the closer the value is to 1, the more reproducible the output current characteristic of the HEMT is. Means good. With HEMT using the group III nitride semiconductor substrate according to the present invention, the current coplus is 0.79 or more.

Further, the difference between the carbon concentration of the third nitride layer 23c and the carbon concentration of the second nitride layer 23b is set to 4.5 × 10 ¹⁵ atoms / cm ³ or more and 3.5 × 10 ¹⁶ atoms / cm ³ or less. The difference between the carbon concentration of the second nitride layer 23b and the carbon concentration of the first nitride layer 23a is set to 1.2 × 10 ¹⁷ atoms / cm ³ or more and 3.5 × 10 ¹⁸ atoms / cm ³ or less. As a result, current collapse can be further suppressed. Specifically, the current collapse value can be set to 0.85 or more without increasing the leakage current.

  The Al composition in the first nitride layer 23a, the second nitride layer 23b, and the third nitride layer 23c constituting the nitride laminate 23 is not particularly limited, but is 0.03 or more and 0.10 or less. It is preferable that Thereby, the lateral leakage current can be reduced while maintaining the crystallinity.

  The composition of Al in the first nitride layer 23a, the second nitride layer 23b, and the third nitride layer 23c can be different from each other within the range specified above. It is preferable to have an equal Al composition. Thereby, generation | occurrence | production of the two-dimensional electron gas resulting from a composition difference can be suppressed, and a leak current can be reduced.

  Thus, the group III nitride semiconductor substrate according to the present invention is suitable for a normally-off type semiconductor device that can achieve both reduction of leakage current and suppression of current collapse.

A HEMT is manufactured by growing a channel layer and an electron supply layer on the group III nitride semiconductor substrate according to the present invention, and forming a source electrode, a gate electrode, and a drain electrode on the electron supply layer. Can do. Here, the channel layer is made of, for example, B _a1 Al _b1 Ga _c1 In _d1 N (0 ≦ a1 ≦ 1, 0 ≦ b1 ≦ 1, 0 ≦ c1 ≦ 1, 0 ≦ d1 ≦ 1, a1 + b1 + c1 + d1 = 1) material. be able to. In addition, the electron supply layer has, for example, B _a2 Al _b2 Ga _c2 In _d2 N (0 ≦ a2 ≦ 1, 0 ≦ b2 ≦ 1, 0 ≦ c2 ≦ 1, 0 ≦ d2 ≦ 1, a2 + b2 + c2 + d2) having a larger band gap than the channel layer = 1) It can be made of a material. In this case, both layers can be composed of a single composition or a plurality of compositions. In particular, in order to avoid alloy scattering and lower the specific resistance of the current conducting portion, at least a portion of the channel layer that is in contact with the electron supply layer is preferably made of a GaN material.

  The group III nitride semiconductor substrate shown in FIG. 2 shows an example of a typical embodiment, and the present invention is not limited to this embodiment. For example, an intermediate layer or other superlattice layer that does not adversely affect the effect of the present invention can be inserted between the layers, or the composition can be inclined.

(Experimental Examples 1-8)
On a 6-inch p-type Si single crystal substrate (plane orientation: (111), dopant: boron (B), specific resistance: 0.005 Ω · cm, thickness: 625 μm), as a buffer layer, AlN (thickness: 5 sets) and 100 sets of GaN (thickness: 25 nm) were alternately stacked. Here, the buffer layer contains 2 × 10 ¹⁹ atoms / cm ³ of carbon. Next, as a nitride stack, a first nitride layer (Al _0.08 Ga _0.92 N, thickness: 200 nm), a second nitride layer (Al _0.08 Ga _0.92 N, thickness) : 550 nm) and a third nitride layer (Al _0.08 Ga _0.92 N, thickness: 200 nm) were sequentially laminated to obtain a group III nitride semiconductor substrate according to the present invention. At that time, the first nitride layer and the second nitride layer are changed by changing the flow rate of the source gas and the substrate temperature for growing the first nitride layer, the second nitride layer, and the third nitride layer. And the 3rd nitride layer was made to contain the carbon concentration shown in Table 1 which is a result of having measured by SIMS. As an example of the SIMS analysis result, the SIMS profile of Experimental Example 1 is shown in FIG. This SIMS profile is obtained by performing SIMS measurement at the center of the wafer.
Subsequently, after a channel layer (GaN, thickness: 50 nm) was grown, an electron supply layer (AlGaN, thickness: 20 nm) was grown. Table 2 shows the growth conditions (pressure and flow rate of the source gas, and substrate temperature) of each layer in Experimental Example 1. As a growth method of each layer, the MOCVD method was adopted, and TMA (trimethylaluminum), TMG (trimethylgallium), and ammonia were used as the source gas. Moreover, nitrogen and hydrogen were used as carrier gas. Thereafter, the central portion of the substrate was cut out at 20 mm × 30 mm, and a φ76 μm pattern electrode using Ti / Al / Ni / Au as an electrode material was prepared to prepare a HEMT sample.
As a result of SIMS analysis, the carbon concentration of the channel layer was 1.5 × 10 ¹⁶ atoms / cm ³ in all of Experimental Examples 1 to 8.

(Experimental example 9)
HEMTs were produced as in Experimental Examples 1-8. However, the thickness of the first nitride layer was set to 800 nm without growing the second nitride layer. That is, the nitride laminate has a two-layer structure. The other conditions are the same as in Experimental Examples 1-8. The growth conditions (pressure and flow rate of the source gas, and substrate temperature) of each layer are as shown in Table 3.

(Experimental example 10)
HEMTs were produced as in Experimental Examples 1-8. However, the thickness of the first nitride layer was 1000 nm without growing the second and third nitride layers. That is, the nitride laminate has a single layer structure. The other conditions are the same as in Experimental Examples 1-8. Table 4 shows the growth conditions (pressure and flow rate of the source gas, and substrate temperature) of each layer.

(Experimental example 11)
A HEMT of a conventional example which is different from Experimental Examples 1 to 8 and is not normally-off type was produced. However, the second nitride layer has a two-layer structure that does not grow, and the first and third nitride layers are made of GaN. Each layer was formed under the growth conditions (pressure and flow rate of source gas and substrate temperature) shown in Table 5 with the thickness of the first nitride layer being 1400 nm and the thickness of the second nitride layer being 200 nm. . The other conditions are the same as in Experimental Examples 1-8.

<Evaluation of leakage current>
About Experimental Examples 1-11, the leakage current was measured and evaluated using the leakage current measuring apparatus (Agilent Technologies company make, model number: 82357A). This evaluation was performed by applying a voltage of 1000 V between the source electrode and the drain electrode with the gate voltage turned off, and measuring the current flowing between the source and drain electrodes. The obtained results are shown in Table 1. The evaluation criteria for leakage current are as follows.
: 1 × 10 ⁻⁸ A / cm or less ○: 1 × 10 ⁻⁶ A / cm or more and less than 1 × 10 ⁻⁸ A / cm Δ: 1 × 10 ⁻⁴ A / cm or more and 1 × 10 ⁻⁶ A / cm Less than ×: Less than 1 × 10 ⁻⁴ A / cm or HEMT destruction

<Evaluation of current collapse>
About Experimental Examples 1-11, the value of the current collapse was evaluated using the semiconductor parameter analyzer (made by KEITHLEY). In this evaluation, first, the voltage between the source electrode and the drain electrode was swept at 0 to 10 V, the source-drain current I ₁ at 10 V was measured, and then an electric field stress of 200 V was applied between the source and drain electrodes. Thereafter, the source-drain current I ₂ was measured and I ₂ / I ₁ was obtained in the same manner as before application of the electric field stress. The obtained results are shown in Table 1. Note that the closer to 1 the value of I 2 _/ I _1, it means that the reproducibility of the output current characteristics of the HEMT are good. The evaluation criteria for current collapse are as follows.
◎: 0.88 or more ○: 0.78 or more and less than 0.88 Δ: 0.68 or more and less than 0.78 ×: Less than 0.68

<Comprehensive evaluation>
It was evaluated whether or not both reduction of leakage current and suppression of current collapse were realized. Therefore, in both the evaluation of the leakage current and the evaluation of the current collapse, the case of ○ or ○ was evaluated as ◯, the one evaluated as △ was evaluated as △, and the one evaluated as × was evaluated as ×. As can be seen from Table 1, in Experimental Examples 3, 5, 7, and 8, HEMTs in which both reduction of leakage current and suppression of current collapse were realized were obtained. On the other hand, in Experimental Examples 1, 2, 4, 6, and 9 to 11, it was impossible to achieve both reduction of leakage current and suppression of current collapse. Among these, in Experimental Examples 1, 4 and 6, although the carbon concentration difference between the nitride layers was large and the leakage current was sufficiently reduced, the current collapse was not sufficiently suppressed. In Experimental Example 2, the difference in carbon concentration between the nitride layers was small, and the current coplus was sufficiently suppressed, but the leakage current was not sufficiently suppressed. Further, in Experimental Example 9, the nitride laminate had a two-layer structure, the carbon concentration difference between the layers was very large, and the leakage current was sufficiently reduced, but the suppression of current coplus was significantly reduced. It was enough. Furthermore, in Experimental Example 10, the nitride laminate has a single-layer structure, and as in Experimental Example 9, the leakage current was sufficiently reduced, but the suppression of current coplus was extremely insufficient. there were. Furthermore, in Experimental Example 11, although the nitride laminate has a two-layer structure and the first and third nitride layers are made of GaN, the current coplus was greatly suppressed. The suppression of leakage current was extremely insufficient.
Thus, it can be seen that in the HEMT fabricated using the group III nitride semiconductor substrate of the present invention, both reduction of leakage current and current coplus can be achieved.

According to the present invention, a nitride laminate composed of three nitride layers is provided on the buffer layer, and the carbon concentration of the nitride layer on the channel layer side is set to 9 × 10 ¹⁵ atoms / cm ³ or more and 2.5 × 10 6. ¹⁶ atoms / cm ³ or less, and the difference in carbon concentration between the nitride layers is adjusted to an appropriate range, so that the group III suitable for normally-off type semiconductor devices that can achieve both current collapse and leakage current reduction and current collapse suppression. Since a nitride semiconductor substrate can be obtained, it is useful in the semiconductor manufacturing industry.

1 Group III nitride semiconductor substrate 11, 21 Substrate 12, 22 Buffer layer 13 Channel layer 14 Electron supply layer 15 Source electrode 16 Gate electrode 17 Drain electrode 23 Nitride stack 23 a First nitride layer 23 b Second nitride Layer 23c Third nitride layer 100 HEMT

Claims (6)

A substrate,
A buffer layer formed on the substrate;
A nitride stack formed on the buffer layer;
With
The nitride laminate is formed on the first nitride layer made of Al _x Ga _1-x N (0 <x ≦ 1) and formed on the buffer layer. A second nitride layer made of Al _y Ga _1-y N (0 <y ≦ 1) and Al _z Ga _1-z N (0 <y) formed on the second nitride layer. and a third nitride layer composed of z ≦ 1),
Each of the first, second and third nitride layers contains carbon, and the carbon concentration of the first nitride layer is higher than the carbon concentration of the second nitride layer, The carbon concentration of the nitride layer is higher than the carbon concentration of the third nitride layer,
The carbon concentration of the third nitride layer is 9 × 10 ¹⁵ atoms / cm ³ or more and 2.5 × 10 ¹⁶ atoms / cm ³ or less,
The difference between the carbon concentration of the third nitride layer and the carbon concentration of the second nitride layer is 4.5 × 10 ¹⁵ atoms / cm ³ or more and 3.5 × 10 ¹⁶ atoms / cm ³ or less,
The difference between the carbon concentration of the second nitride layer and the carbon concentration of the first nitride layer is 1.2 × 10 ¹⁷ atoms / cm ³ or more and 3.5 × 10 ¹⁸ atoms / cm ³ or less. A group III nitride semiconductor substrate characterized by: The difference between the carbon concentration of the third nitride layer and the carbon concentration of the second nitride layer is 4.5 × 10 ¹⁵ atoms / cm ³ or more and 1.7 × 10 ¹⁶ atoms / cm ³ or less,
The difference between the carbon concentration of the second nitride layer and the carbon concentration of the first nitride layer is 1.3 × 10 ¹⁷ atoms / cm ³ or more and 3.5 × 10 ¹⁸ atoms / cm ³ or less. The group III nitride semiconductor substrate according to claim 1.   The group III nitride semiconductor according to claim 1 or 2, wherein a composition of Al in the first nitride layer, the second nitride layer, and the third nitride layer is 0.03 or more and 0.10 or less. substrate.   The group III nitride semiconductor substrate according to claim 1, wherein the first nitride layer, the second nitride layer, and the third nitride layer have the same Al composition. The group III nitride semiconductor substrate according to any one of claims 1 to 4, wherein the buffer layer contains carbon, and the concentration of the carbon is 1.0 × 10 ¹⁹ atoms / cm ³ or more. The buffer layer has a _{Al 1-α Ga α N (} 0 ≦ α <0.5) and superlattice structure AlN are alternately stacked, III nitride according to any one of claims 1 to 5 Semiconductor substrate.

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