JP2021007142A - Nitride semiconductor substrate - Google Patents

Abstract

To provide a nitride semiconductor substrate that can efficiently reduce a leakage current in the vertical direction.SOLUTION: In a nitride semiconductor substrate according to the present invention in which a buffer layer and an operating layer made of nitride semiconductors are sequentially laminated on a Si single crystal substrate, the buffer layer includes a single first initial layer formed in contact with the Si single crystal substrate and a single second initial layer formed on the first initial layer, and the first initial layer is formed of AlN, the second initial layer is formed of AlzGa1-zN (0.12≤z≤0.65), and in an X-Y graph in which an X-axis is z×100 and a Y-axis is the carbon concentration in the second initial layer, X is 12 or more and 65 or less and Y is within a range of Y=1E+17×exp(-0.05×X) and Y=1E+21×exp(-0.05×X).SELECTED DRAWING: Figure 1

Description

The present invention relates to a nitride semiconductor substrate used for high-speed and high-voltage elements and the like, and more particularly to a nitride semiconductor substrate configured by laminating gallium nitride (GaN) on a dissimilar substrate made of silicon (Si).

Nitride semiconductor substrates in which GaN is laminated on a Si single crystal substrate are usually manufactured by the organic vapor deposition (MOCVD) method. At that time, a technique for forming an initial layer composed of an aluminum nitride (AlN) layer and an aluminum nitride gallium (AlGaN) layer on a Si single crystal is known.

Patent Document 1 describes a highly reliable compound semiconductor device on a Si substrate, which maintains good crystallinity of a compound semiconductor and suppresses the generation of current collapse, but also suppresses an off-leakage current to realize a high withstand voltage. A compound semiconductor laminated structure having a first buffer layer made of AlN as a material and a second buffer layer made of AlGaN as a material formed above the first buffer layer is included, and the second buffer layer There is a disclosure of a compound semiconductor device containing carbon in a higher concentration from the lower surface to the upper surface.

In Patent Document 2, as an example of manufacturing a group 13 nitride semiconductor substrate, first, a silicon single crystal substrate having a diameter of 6 inches as a base substrate 1, a main surface orientation of (111), and a boron-doped specific resistance of 0.004 Ωcm. Is set in a MOCVD apparatus, trimethylaluminum (TMA) and ammonia (NH ₃ ) are used as raw material gases, and an AlN single crystal layer having a carbon concentration of 1 × 10 ¹⁸ atoms / cm ³ and a thickness of 100 nm is vaporized at 1000 ° C. In all subsequent formations of the Group 13 nitride semiconductor layer after growth, the growth temperature is set to 1000 ° C., and fine adjustment is made in the range of 1 to 15 ° C. to obtain a raw material on the AlN single crystal layer. Using trimethylgallium (TMG), TMA, and NH ₃ as gas, a vapor phase of an Al _x Ga _1-x N single crystal layer (x = 0.1) having a carbon concentration of 5 × 10 ¹⁹ atoms / cm ³ and a thickness of 300 nm. There is a description that it was grown to form the initial nucleation layer 2.

Japanese Unexamined Patent Publication No. 2014-17422 Japanese Unexamined Patent Publication No. 2016-219690

The AlN layer first formed on the Si single crystal substrate can obtain high quality crystal quality (high crystallinity, good surface flatness) as the film is formed at a high temperature, but the etching rate of Si also increases. .. Therefore, since the film formation temperature of AlN is usually limited, it cannot be said that the flatness of the surface of the AlN layer is sufficiently secured. If this flatness is poor, the superlattice structure of (Al) GaN and AlN applied in stress control is disturbed, and there arises a problem that stress control becomes ineffective.

Therefore, in order to flatten the rough surface of the AlN layer, it is common to form an AlGaN layer on the AlGaN layer. The AlGaN layer is required to have a high resistance in view of not only flattening the film but also being used in a horizontal electronic device.

AlN has a larger forbidden bandgap than GaN and has a very high resistance. However, since the GaN component is mixed in the AlGaN layer, the forbidden band width is somewhat smaller than that of AlN, and the resistance is lowered. Further, since this AlGaN layer is also a layer close to the Si single crystal substrate, there are many dislocations in the crystal, and the dislocations lower the resistance.

To solve the above problems, a method of increasing resistance by doping impurities such as carbon that can compensate for electrons is applied. However, doping with a high concentration of carbon causes deterioration of crystal quality. Therefore, it is necessary to make the AlGaN layer an appropriate Al composition, increase the forbidden band width, and make it unnecessary to dope a high concentration of carbon.

However, it cannot be said that the relationship between the Al composition and the carbon concentration of this AlGaN layer has been technically established. For example, in Patent Document 1, Al _x Ga _1-x N has a thickness of about 200 nm, about 0.8 ≦ x ≦ 0.9 (for example, about x = 0.9), and a C concentration of 5 × 10. ^{It is about 17} / cm ^{3 to} 3 × 10 ¹⁸ / cm ³ (for example, about 1 × 10 ¹⁸ / cm ³ ). However, in this AlGaN layer, the leakage current to the Si single crystal substrate cannot be sufficiently reduced.

Further, in Patent Document 2, an Al _x Ga _1-x N single crystal layer (x = 0.1) having a carbon concentration of 5 × 10 ¹⁹ atoms / cm ³ and a thickness of 300 nm is used. In this case, the reverse is true. In addition, it cannot be said that the crystallinity of the AlGaN layer is sufficiently secured because the carbon concentration is too high.

In view of the above, in view of the above, in particular, in a nitride semiconductor substrate in which GaN is laminated on a Si single crystal substrate, the formation conditions of the initial AlGaN layer are appropriately adjusted, and the Al composition and the impurity carbon concentration are within a certain range. It is an object of the present invention to provide a nitride semiconductor substrate having high crystallinity in which leakage in the vertical direction is suppressed when used as a horizontal device.

The nitride semiconductor substrate of the present invention is a substrate in which a buffer layer and an operating layer made of a nitride semiconductor are sequentially laminated on a Si single crystal substrate, and the buffer layer is in contact with the Si single crystal substrate. The first initial layer of the single layer formed on the first initial layer and the second initial layer of the single layer formed on the first initial layer are included, the first initial layer is formed of AlN, and the second initial layer is formed. An XY graph formed by Al _z Ga _1-z N (0.12 ≦ z ≦ 0.65), with the X-axis as z × 100 and the Y-axis as the carbon concentration in the second initial layer. Above, X is 12 or more and 65 or less, and Y is in the range between Y = 1E + 17 × exp (−0.05 × X) and Y = 1E + 21 × exp (−0.05 × X). It is characterized by that.

By having such a configuration, in particular, in a nitride semiconductor substrate in which GaN is laminated on a Si single crystal substrate, leakage in the vertical direction is suppressed when used as a horizontal device.

It further has a layer in contact with the second initial layer, the layer in contact with the second initial layer is formed of Al _c1 Ga _1-c1 N, the X axis is c1 × 100, and the Y axis is the second initial layer. In the XY graph showing the carbon concentration in the layer in contact with the layer, X is 0 or more and 20 or less, Y is Y = 8E + 18 × exp (−0.03 × X), and Y = 4E + 20 × exp (-0. It is preferably within the range of 03 × X).

The carbon concentration of the layer in contact with the second initial layer is preferably lower than the carbon concentration of the second initial layer.

According to the present invention, when a group III nitride semiconductor film is applied to a horizontal device, the film formation conditions of the second initial layer are appropriately adjusted to keep the Al composition and the impurity carbon concentration within a certain range. When used as a horizontal device, leakage in the vertical direction can be suppressed, resulting in better withstand voltage characteristics. Since the crystallinity of the second initial layer is high, the nitride semiconductor layer formed on the second initial layer is also of high quality.

FIG. 1 is a schematic cross-sectional view showing an aspect of the nitride semiconductor substrate of the present invention. FIG. 2 is a graph showing the relationship between the Al composition and the impurity carbon concentration in the second initial layer in the nitride semiconductor substrate of the present invention.

Hereinafter, the nitride semiconductor substrate of the present invention will be described in detail with reference to the drawings. The nitride semiconductor substrate is a substrate in which a buffer layer and an operating layer made of a nitride semiconductor are sequentially laminated on a Si single crystal substrate, and the buffer layer is formed in contact with the Si single crystal substrate. The first initial layer of the single layer and the second initial layer of the single layer formed on the first initial layer are included, the first initial layer is formed of AlN, and the second initial layer is Al _z. In the XY graph formed by Ga _1-z N (0.12 ≦ z ≦ 0.65), the X-axis is z × 100, and the Y-axis is the carbon concentration in the second initial layer. X is 12 or more and 65 or less, and Y is in the range between Y = 1E + 17 × exp (−0.05 × X) and Y = 1E + 21 × exp (−0.05 × X).

FIG. 1 is a schematic cross-sectional view showing an aspect of the nitride semiconductor substrate of the present invention. Here, a HEMT structure will be used for description. That is, as the nitride semiconductor substrate W, the buffer layer B is laminated on one main surface of the base substrate S, and the operating layer G is formed on the buffer layer B.

Here, the schematic diagram shown in the present invention is a schematic simplification and emphasis on the shape for the sake of explanation, and the shape, dimensions, and ratio of details are different from the actual ones. Further, the reference numerals are omitted for the same configurations, and other configurations unnecessary for explanation are not described.

In the present invention, the base substrate S is a Si single crystal. Since the layer first formed on the dissimilar substrate is naturally selected and designed in consideration of the features of the dissimilar substrate, other physical property values of the base substrate S are not particularly limited.

For example, the type and concentration of impurities contained in the Si single crystal, its concentration, carbon concentration, nitrogen concentration, oxygen concentration, defect density, and the method for producing the Si single crystal can be arbitrarily selected according to the required specifications. Further, the surface on which the nitride semiconductor layer is formed may have an off angle in the range of -4 ° to 4 °.

The buffer layer B has a structure in which a plurality of nitride semiconductor layers are laminated, and the structure can be formed by a known method depending on the application and purpose. For example, as described in Patent Document 2, it can be said that it is preferable to first form an appropriate initial layer and then stack one or more nitride semiconductor layers having different compositions and impurity concentrations.

Here, as the nitride semiconductor, a combination of a Group 13 element such as Ga, Al, and indium (In) and a Group 15 element such as nitrogen is exemplified.

The operating layer G is a general term for a layer that functions as a device and various layers that are attached to the layer. In the HEMT shown in FIG. 1, the electron traveling layer 101 and the electron supply layer 102 correspond to the operating layer G. Further, a cap layer made of GaN or the like may be provided on the cap layer.

The nitride semiconductor substrate W is not particularly limited in its application, but can be said to be particularly suitable for power devices capable of increasing high frequency and high withstand voltage.

In the present invention, the buffer layer B includes a single first initial layer formed in contact with the Si single crystal substrate and a single second initial layer formed on the first initial layer. That is, the single-layer first initial layer 11 and the single-layer second initial layer 12 are laminated in this order directly above the base substrate S.

The first initial layer 11 is formed of AlN. The first initial layer 11 in the present invention has a role as in a known technique, that is, a role of preventing a direct reaction between Si and Ga. The first initial layer 11 may have at least a function as a layer directly above the Si single crystal, and not only AlN having an Al ratio of 100% but also AlGaN having an Al ratio of 2 to 3% lower than 100%. It may be formed by.

For the same reasons as described above, the first initial layer 11 is not required to be a strict single layer, and it is permissible to include some compositional inclination. In addition, various elements (Si, Ga, C, etc.) that are inevitably mixed when forming layers continuously by the MOCVD method may be present within a range that does not impair the effects of the present invention.

The layer thickness of the first initial layer 11 is not particularly limited, but is generally in the range of 40 to 150 nm, preferably 80 to 120 nm.

The second initial layer 12 is formed of Al _z Ga _1-z N (0.12 ≦ z ≦ 0.65). The purpose of this layer is to flatten the rough surface of the first initial layer 11. By forming the first initial layer 11 and the second initial layer 12, it is possible to obtain the effects of improving crystallinity by reducing the dislocation density and suppressing warpage due to thickening of the film.

If the Al ratio z of the second initial layer 12 is larger than 0.65, it becomes difficult to secure good surface flatness of the second initial layer 12. On the other hand, if z is less than 0.12, the difference in lattice constant from the first initial layer 11 may be too wide, causing frequent dislocations and cracks.

The layer thickness of the second initial layer 12 is also not particularly limited, but is generally in the range of 50 to 450 nm, preferably 200 to 350 nm.

Like the first initial layer 11, the second initial layer 12 is not required to be a strict single layer, and it is permissible to include some compositional inclination.

Then, in the second initial layer 12, X is 12 or more and 65 or less, and Y is Y in the XY graph in which the X axis is z × 100 and the Y axis is the carbon concentration in the second initial layer. It is within the range between = 1E + 17 × exp (−0.05 × X) and Y = 1E + 21 × exp (−0.05 × X).

FIG. 2 is a graph showing the relationship between the Al composition and the impurity carbon concentration in the second initial layer 12 in the nitride semiconductor substrate W of the present invention. The relationship between _z in Al _z Ga _1-z N (0.12 ≦ z ≦ 0.65) in the second initial layer 12 and the carbon concentration is X = 12, X = 65, Y = 1E + 17 × exp ( The exceptional effect of the present invention is obtained when it is within the region surrounded by −0.05 × X) and Y = 1E + 21 × exp (−0.05 × X).

In the present invention, the relationship between the Al ratio and the carbon concentration is based on the grounds that it is optimal to make the second initial layer 12 an appropriate Al composition, increase the forbidden band width, and do not dope a high concentration of carbon. It is clear. When this relationship is within the above range, the second initial layer 12 has both crystallinity and high resistance.

Such a carbon concentration can be set to a desired value by, for example, adjusting the growth temperature or the growth pressure, which is a known technique in the metalorganic vapor phase growth method (MOCVD). Further, the Al ratio z can also be adjusted by adjusting the flow rate of the raw material gas and the supply time.

Hereinafter, more preferable aspects of the present invention will be described. Further, regarding the relationship between the Al ratio z in the second initial layer 12 and the carbon concentration, in the XY graph, X is 26 or more and 45 or less, and Y is Y = 7E + 17 × exp (−0.05 × X). ) And Y = 1E + 19 × exp (−0.05 × X). Since the above range is a region where the carbon concentration is suppressed to a lower level, it can be said that the design emphasizes crystal quality.

In forming the second initial layer 12, when the Al ratio z is high at the initial stage of formation and the Al ratio z is reduced as the layer grows, the crystallinity is emphasized in the first half region of the layer and the pressure resistance is increased in the second half region. The design is emphasized, and the effect of the present invention can be obtained more efficiently without unnecessarily increasing the layer thickness of the second initial layer 12, which is more preferable.

The buffer layer B may have a multilayer buffer layer m in addition to the first initial layer 11 and the second initial layer 12. In an example of the multilayer buffer layer m, an Al _c Ga _1-c N (0 ≦ c ≦ 0.8) single crystal layer having a thickness of 15 to 50 nm and an Al N layer having a thickness of 3 to 10 nm are alternately and repeatedly laminated. , It is a multi-layer structure having an overall layer thickness of about 500 to 2000 nm. By further providing such a multilayer buffer layer m, the stress relaxation effect in the buffer layer B can be effectively exhibited.

The operating layer G has a laminated structure including an electron traveling layer 101 and an electron supply layer 102. On the operation layer G, another layer such as a cap layer or a passivation layer may be provided depending on the purpose and application at the time of manufacturing the device. The layer thicknesses of the electron traveling layer 101 and the electron supply layer 102 are known values.

Each layer of the nitride semiconductor substrate W of the present invention is usually formed by deposition by epitaxial growth. The deposition method may be a generally used method, for example, a CVD method such as MOCVD or plasma CVD (PECVD), a vapor deposition method using a laser beam, a sputtering method using an atmospheric gas, or a molecular beam in a high vacuum. The molecular beam epitaxy method (MBE) using the above method, or the organic metal molecular beam epitaxy method (MOMBE) which is a composite of MOCVD and MBE. Further, the raw material for epitaxial growth is not limited to that used in Examples, and for example, the raw material gas for adding carbon is triethylaluminum in addition to trimethylaluminum (TMAl) and trimethylgallium (TMGa). It may be (TEAl) or triethylgallium (TEGa).

As described above, the nitride semiconductor substrate of the present invention can suppress leakage in the vertical direction when used as a horizontal device, and thus has more excellent withstand voltage characteristics. Since the crystal quality of the second initial layer is high, the nitride semiconductor layer formed on the second initial layer is also of high quality.

Hereinafter, the present invention will be specifically described based on examples, but the present invention is not limited to the following examples.

[Example 1]
A 6-inch Si single crystal is put into the MOCVD apparatus as the base substrate S, and annealed at a substrate temperature of 950 ° C. in a hydrogen atmosphere at a pressure of 135 hPa to remove the natural oxide film on the silicon surface and express the atomic step of silicon. I let you. Subsequently, the substrate temperature was set to 1020 ° C., TMAl and ammonia (NH ₃ ) were supplied, and AlN to be the first initial layer 11 was formed to a thickness of 100 nm. Then, TMGa was added to form a 200 nm film of Al _z Ga _1-z N (z = 0.3) as the second initial layer 12. Next, as a stress control layer, a multilayer buffer layer m in which 80 layers of Al _0.1 Ga _0.9 N and Al N were alternately laminated was formed. Further, as the operating layer G, a non-doped GaN layer serving as the electron traveling layer 101 was laminated with a thickness of 1400 nm, and an Al _0.25 Ga _0.75 N layer serving as the electron supply layer 102 was laminated in this order with a thickness of 20 nm to prepare a sample.

[Examples 2 to 5]
The carbon concentration is adjusted mainly by controlling the film formation pressure, and the Al ratio z is adjusted by controlling the flow rates of TMAl and TMGa. Other than that, the same as in Example 1 is performed in Example. 2-5 samples were prepared.

[Comparative Example 1]
A sample was prepared in the same manner as in Example 1 except that the substrate temperature was set to 920 ° C. in Example 1.
The impurity carbon concentration in the obtained sample was 5E + 20 cm ^-3 . It is considered that the crystal quality deteriorated because the amount of impurity carbon was too large.

[Comparative Example 2]
A sample was prepared in the same manner as in Example 1 except that the substrate temperature was set to 1020 ° C. in Example 1.
The impurity carbon concentration contained in the obtained sample was 1E + 16 cm ^-3 . It is probable that in the sample of Comparative Example 2, a sufficiently high resistance value could not be obtained because the amount of impurity carbon was too small.

[Comparative Example 3]
A sample was prepared in the same manner as in Example 1 except that the _{z of} Al _z Ga _1-z N was set to z = 0.11 instead of z = 0.3 in Example 1. The impurity carbon concentration contained in the obtained sample was 5E + 17 cm ^-3 . It is probable that the resistance value did not increase because the forbidden band width could not be sufficiently secured and the impurity carbon concentration was not sufficient because the Al composition was low.

[Comparative Example 4]
A sample was prepared in the same manner as in Example 1 except that the z of Al _z Ga _1-z N was set to z = 0.7 instead of z = 0.3 in Example 1.
The impurity carbon concentration in the obtained sample was 1E + 20 cm ^-3 . The sample of Comparative Example 4 has a high Al composition and a large forbidden bandgap, but it is considered that the impurity carbon concentration is too high and the crystal quality is deteriorated.

When the Al composition was larger than 70%, that is, when z> 0.7, the surface smoothness of the second initial layer 12 could not be ensured.

[Example 6]
In Example 6, when forming the second initial layer 12, the Al ratio z is initially set to 0.35, the ratio is gradually lowered as the layer is formed, and finally the Al ratio z is formed. Was set to 0.25. The layer thickness was 50 nm. Other than that, it was the same as in Example 1.

[Evaluation 1-Carbon concentration]
The carbon concentration in the second initial layer 12 of each sample was measured using secondary ion mass spectrometry (SIMS).

[Evaluation 2-Vertical leak current]
From each sample, a strip-shaped test piece having a width of 20 mm was cleaved and cut out from the central portion of the main surface of the substrate to the end portion of the substrate. Next, a part of the electron supply layer 102 and the electron traveling layer 101 of this test piece was removed by dry etching. In this state, a 10 mm ² Au electrode is vacuum-deposited on the surface exposed by dry etching to form a Schottky electrode, and a commercially available curve tracer is used to energize the Si single crystal substrate side to obtain IV characteristics. The measurements were made and the current values at 600 V were compared. Then, 1E-8 (A) or less was regarded as a pass.

[Evaluation 3-Crystallinity]
For each sample, the half width of the locking curve in the X-ray diffraction of the (002) plane of the second initial layer 12 was measured. Then, 2000 arcsec or less was regarded as a pass.

Table 1 summarizes the production conditions and evaluation results of each sample.

From the results in Table 1, those within the range of the present invention had good withstand voltage characteristics (leakage current in the vertical direction) and crystallinity (half width of the second initial layer 12). Further, the sample of Example 6 had both crystallinity and pressure resistance at a higher level than those of Examples 1 to 5.

Here, there is also a more preferable range for the layer m ₀ (Al _c1 Ga _1-c1 N, not shown) formed in contact with the second initial layer. That is, in the XY graph, X (c × 100) is 0 or more and 20 or less, and Y (carbon concentration in the layer m ₀ formed in contact with the second initial layer) is Y = 8E + 18 × exp (−0). It is within the range between .03 × X) and Y = 4E + 20 × exp (−0.03 × X).

By doing so, it has been found that the balance between the crystallinity and the withstand voltage of the nitride semiconductor substrate of the present invention can be compatible at a higher level. Hereinafter, as Examples 7 to 9, c1 of the layer m ₀ formed in contact with the second initial layer and a sample in which the carbon concentration contained therein was adjusted were prepared. Then, the leak current and the half width (crystallinity) were evaluated in the same manner as in Example 1.

[Example 7]
In Example 7, c1 = 0 and the carbon concentration was 7E + 19 cm ^-3 .
The leak current was slightly lower than that of Example 2, which was excellent in this respect. On the other hand, the full width at half maximum was 1860 arcsec, and the crystallinity was slightly inferior to that of 1850 arcsec of Example 2 due to the relatively high carbon concentration. However, considering both crystallinity and pressure resistance, it can be said that the characteristics are better than those of Example 2.

[Example 8]
In Example 8, c1 = 0.1 and the carbon concentration was 5E + 19 cm ^-3 .
The leak current was slightly lower than that of Example 1, which was excellent in this respect. On the other hand, the full width at half maximum was 1480 arcsec, which was slightly better than 1500 arcsec in Example 1. It can be said that both the crystallinity and the pressure resistance are better than those of Example 1.

[Example 9]
In Example 9, c1 = 0.2 and the carbon concentration was 1E + 20 cm ^-3 .
The leak current was slightly lower than that of Example 4, which was excellent in this respect. On the other hand, the full width at half maximum was 1450 arcsec, which was slightly better than 1600 arcsec in Example 4 and 1480 arcsec in Example 8. It can be said that both the crystallinity and the pressure resistance are better than those of Example 4.

As described above, one more preferable aspect of the present invention can be said to be a relatively excellent nitride semiconductor substrate when aiming to achieve both crystallinity and withstand voltage at a higher level.

Here, it is particularly preferable that the carbon concentration of the layer m ₀ formed in contact with the second initial layer is lower than the carbon concentration of the second initial layer 12. Since carbon in a nitride semiconductor exists as an impurity, the crystallinity of the nitride semiconductor tends to decrease as its concentration increases. Since the carbon concentration of the second initial layer 12 is set relatively high, if the carbon concentration of the layer m ₀ formed in contact with the subsequent second initial layer is about the same, the entire multilayer buffer layer m is also crystalline. Will grow without rising too much.

Therefore, in the present invention, as described above, the carbon concentration of the layer m ₀ formed in contact with the second initial layer is made lower than the carbon concentration of the second initial layer 12, which is a feature of the present invention. It is possible to further improve the crystallinity of the multilayer buffer layer m in a form that fits the structure of the initial layer 12 and is easy to manufacture.

[Example 10]
In the sample of Example 4, the carbon concentrations of the layer m ₀ formed in contact with the second initial layer and the second initial layer 12 were substantially equal to 1E + 19 cm ^-3 . On the other hand, in the sample of Example 10, the carbon concentration of the layer m ₀ formed in contact with the second initial layer was set to 9E + 18cm ^-3 by optimizing the production conditions, and the carbon concentration of the second initial layer 12 was 1E + 19cm. ^Lower than ^-3 . A sample was prepared in the same manner as in Example 4 except for this.

As a result, the leak current of Example 10 was equivalent to that of Example 4. Further, as an additional evaluation, when the crystallinity of the multilayer buffer layer m of the samples of Example 4 and Example 10 was compared, in Example 10, the half-value width was about 10% lower than that of Example 4, and the crystallinity was excellent. It was a thing. This is because the crystallinity of the layer m ₀ formed in contact with the second initial layer becomes relatively high by lowering the concentration of carbon which is an impurity, and the crystallinity of the multilayer buffer layer m laminated on the layer m ₀ becomes relatively high. Is also considered to be due to the same improvement.

W Nitride semiconductor substrate S Underlay substrate B Buffer layer 11 First initial layer 12 Second initial layer m Multilayer buffer layer m ₀ Of the multilayer buffer layers, the layer formed in contact with the second initial layer G Operating layer 101 Electronic traveling Layer 102 Electronic supply layer

Claims (3)

A nitride semiconductor substrate in which a buffer layer and an operating layer made of a nitride semiconductor are sequentially laminated on a Si single crystal substrate.
The buffer layer includes a single first initial layer formed in contact with the Si single crystal substrate and a single second initial layer formed on the first initial layer.
The first initial layer is formed of AlN and is made of AlN.
The second initial layer is formed of Al _z Ga _1-z N (0.12 ≦ z ≦ 0.65), the X-axis is z × 100, and the Y-axis is the carbon concentration in the second initial layer. In the XY graph, X is 12 or more and 65 or less, and Y is between Y = 1E + 17 × exp (−0.05 × X) and Y = 1E + 21 × exp (−0.05 × X). Nitride semiconductor substrate characterized by being within the range of. The buffer layer further includes a layer formed in contact with the second initial layer.
Among the layers formed in contact with the second initial layer, the layer formed in contact with the second initial layer is formed of Al _c1 Ga _1-c1 N, the X axis is c1 × 100, and the Y axis is in contact with the second initial layer. In the XY graph with the carbon concentration of, X is 0 or more and 20 or less, Y is Y = 8E + 18 × exp (−0.03 × X), and Y = 4E + 20 × exp (−0.03 × X). The nitride semiconductor substrate according to claim 1, wherein the nitride semiconductor substrate is within the range between the two. The nitride semiconductor substrate according to claim 2, wherein the carbon concentration of the layer formed in contact with the second initial layer is lower than the carbon concentration of the second initial layer.

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