WO2013038980A1 - Substrate having buffer layer structure for growing nitride semiconductor layer - Google Patents

Abstract

A substrate having a buffer layer structure for growing a nitride semiconductor layer, comprising an Al layer and an AlN crystal layer stacked sequentially on a (111) main surface of an Si single-crystal substrate, the Al layer having a thickness of two atomic layers to ten atomic layers, and the surface of the AlN crystal layer having a surface that is Al polar with the plane orientation of a (0001) surface.

Description

Substrate having a buffer layer structure for growing a nitride semiconductor layer

The present invention relates to improvement of a substrate having a buffer layer structure for growing a nitride semiconductor layer, and more particularly to improvement of a buffer layer structure of the substrate. An epitaxial wafer including a plurality of nitride semiconductor layers stacked on such an improved substrate can be preferably used for manufacturing a nitride semiconductor device such as a heterojunction field effect transistor.

When an epitaxial wafer including a laminated structure of, for example, a GaN channel layer and an AlGaN barrier layer required for a heterojunction field effect transistor is manufactured, a GaN substrate is expensive, so that it is on a substrate of a different material such as sapphire, SiC, or Si Conventionally, these nitride semiconductor layers have been crystal-grown.

When a nitride semiconductor layer is grown by MOCVD (Metal Organic Vapor Deposition) on a substrate of a different material, strain based on the difference in crystal structure, lattice mismatch, thermal expansion coefficient, etc. between the substrate and the semiconductor layer Various buffer layer structures are used for relaxation.

For example, Japanese Patent Laid-Open No. 2-229476 of Patent Document 1 teaches that an AlN layer is deposited as a buffer layer on a sapphire substrate at a relatively low substrate temperature of 400 ° C. or higher and 900 ° C. or lower. Such a buffer layer deposited at a relatively low temperature is also called a low-temperature buffer layer.

However, the low-temperature buffer layer contains microcrystals and polycrystals in the amorphous matrix. Therefore, when the substrate temperature is increased to about 1000 ° C. or higher in order to grow a nitride semiconductor layer for a semiconductor device on the low-temperature buffer layer, the amorphous parent phase in the buffer layer is polycrystallized. It will contain a relatively large amount of dislocations inside. In the nitride semiconductor multilayer structure for devices grown on the buffer layer, a large amount of dislocations are introduced, the crystal quality varies, and cracks tend to occur.

On the other hand, for example, Japanese Patent Application Laid-Open No. 2002-367917 of Patent Document 2 teaches that an AlN crystal layer is deposited on a sapphire substrate as a buffer layer at a relatively high substrate temperature of 1100 ° C. or more and 1250 ° C. or less. Such a buffer layer deposited at a relatively high temperature is also referred to as a high temperature buffer layer.

However, in Japanese Patent Application Laid-Open No. 2007-59850 of Patent Document 3, cracks are less likely to occur in a nitride semiconductor multilayer structure grown on a high-temperature buffer layer, but flatness at the atomic level on the surface of the buffer layer. In order to ensure this, it is stated that the thickness of the buffer layer must be increased. In fact, U.S. Patent No. 6,057,031 teaches that in the embodiment of the invention, a high temperature AlN buffer layer is deposited to a fairly large 2 .mu.m thickness. Patent Document 3 also states that if the thickness of the buffer layer is increased, the substrate is likely to warp due to the difference in lattice constant between the substrate and the buffer layer, and the deposition temperature of the AlN buffer layer is further increased. It also states that if the height is increased, the surface of the buffer layer is more likely to become cloudy.

In view of such a problem, Patent Document 3 discloses that the temperature, pressure, and source gas during the deposition of the high-temperature AlN buffer layer are controlled in order to suppress the occurrence of white turbidity even when the high-temperature AlN buffer layer is formed thin. It teaches changing at least one of MOCVD conditions such as flow rate.

JP-A-2-229476 Japanese Patent Laid-Open No. 2002-367917 JP 2007-59850 A

An AlN crystal having a hexagonal wurtzite structure is a polar crystal in which Al atoms and N atoms are arranged asymmetrically along the c-axis. When the thickness of the AlN crystal layer is grown on the non-polar substrate in the c-axis direction, either the Al polarity where Al atoms exist stably on the surface or the N polarity where N atoms exist stably exists on the surface. grow up. This difference in polarity appears characteristically in the morphology of the crystal growth surface, while the Al polar surface is a highly flat surface, whereas the N polar surface is a remarkably uneven surface having hexagonal facets. Tend to be.

Therefore, if the AlN crystal layer is grown as a buffer layer without considering the polar face as in Patent Document 3, the surface of the AlN crystal layer in which the Al polar face and the N polar face are mixed is generated, and high surface flatness is obtained. I can't get it. The mixture of the Al polar face and the N polar face is inherited into the nitride semiconductor multilayer structure for devices grown on the AlN buffer layer, and further deteriorates the surface flatness of the semiconductor multilayer structure.

In view of the problems as described above, the main object of the present invention is to provide a substrate having an improved buffer layer structure for growing a semiconductor laminated structure for a nitride semiconductor device.

As a result of intensive studies, the present inventors have not formed a high-temperature AlN buffer layer directly on the silicon substrate, but have a surface flatness compared to a conventional high-temperature AlN buffer layer by interposing an Al layer. As a result, it has been found that a novel buffer layer structure with significantly improved can be obtained.

According to the present invention, a substrate having a buffer layer structure for growing a nitride semiconductor layer has an Al layer and an AlN crystal layer sequentially stacked on a (111) main surface of a Si single crystal substrate, The Al layer has a thickness of 2 atomic layers or more and 10 atomic layers or less, and the surface of the AlN crystal layer has a (0001) plane orientation and an Al polar surface.

The Al layer preferably has a thickness of 2 atomic layers or more and 4 atomic layers or less. The substrate according to the present invention may further have an AlGaN crystal layer on the AlN crystal layer. The AlGaN crystal layer can also include a plurality of sub-layers in which the Al composition ratio is sequentially reduced.

According to the present invention as described above, a good Al polar surface can be obtained on the surface of the AlN crystal buffer layer by uniformly forming the Al layer on the Si substrate surface before growing the AlN crystal buffer layer. That is, since the surface of the AlN crystal buffer layer substantially includes only the Al polar face, it has high flatness.

It is typical sectional drawing which shows an example of the laminated structure of the heterojunction field effect transistor which can be produced using the board | substrate by this invention. It is an optical dark-field micrograph of the surface of the board | substrate produced as the comparative example 1 using a prior art. It is an optical dark-field micrograph of the surface of the board | substrate produced as the comparative example 2 using a prior art. It is an optical dark-field micrograph of the board | substrate produced as the reference example 1 closely related to this invention. It is an optical dark field microscope photograph of the board | substrate produced as Example 1 of this invention. It is an optical dark field micrograph of the board | substrate produced as Example 2 of this invention. It is an AFM (atomic force microscope) image of the surface of the board | substrate in the comparative example 2 by a prior art. It is an AFM image of the surface of the board | substrate in Example 1 by this invention. It is a graph which shows the surface uneven | corrugated shape on one scanning line regarding the AFM image in the comparative example 2 of FIG. It is a graph which shows the surface uneven | corrugated shape on one scanning line regarding the AFM image in Example 1 of FIG.

(Embodiment)
FIG. 1 is a schematic cross-sectional view showing an example of a laminated structure of heterojunction field effect transistors that can be manufactured using a substrate according to the present invention. An example of a method for manufacturing such a stacked structure of heterojunction field effect transistors will be described below. As the substrate 1, a Si substrate having a (111) main surface is used. First, after removing the surface oxide film of the Si substrate 1 with a hydrofluoric acid-based etchant, the substrate is set in a chamber of an MOCVD (metal organic chemical vapor deposition) apparatus.

In the MOCVD apparatus, the Si substrate 1 is heated to 1050 ° C., and the substrate surface is cleaned for 300 seconds in a hydrogen atmosphere with a chamber internal pressure of 13.3 kPa. Thereafter, an Al layer 2a and an AlN crystal buffer layer 2b are laminated on the Si substrate 1 under the conditions detailed in Examples described later.

Thereafter, the substrate temperature is increased to 1150 ° C., and Al _{0.7 is} flown under the conditions of TMA (trimethylaluminum) flow rate = 90.0 sccm, TMG (trimethylgallium) flow rate = 12.7 sccm, and NH ₃ flow rate = 12.5 slm. A Ga _0.3 N layer 3 is deposited to a thickness of 400 nm. Subsequently, an Al _0.4 Ga _0.6 N layer 4 is deposited to a thickness of 400 nm under the conditions of TMA flow rate = 50.9 sccm, TMG flow rate = 22.1 sccm, and NH ₃ flow rate = 12.5 slm, Further, an Al _0.1 Ga _0.9 N layer 5 is deposited to a thickness of 400 nm under the conditions of TMA flow rate = 16.4 sccm, TMG flow rate = 30.4, and NH ₃ flow rate = 12.5 slm. Thereby, the composition gradient buffer layer structure 3-5 is formed.

On the Al _0.1 Ga _0.9 N layer 5, 50 cycles of AlN layer (5 nm thickness) / Al _0.1 Ga _0.9 N (20 nm thickness) layer are included under the same substrate temperature. A superlattice multilayer buffer layer structure 6 is deposited. At this time, the AlN layer is deposited under the conditions of TMA flow rate = 102 μmol / min and NH ₃ flow rate = 12.5 slm, and the Al _0.1 Ga _0.9 N layer is TMG flow rate = 720 μmol / min, TMA flow rate = 80 μmol / min. It can be deposited under conditions of min and NH ₃ flow rate = 12.5 slm. Note that the superlattice multilayer buffer layer structure 6 may be omitted from the viewpoint of the manufacturing cost and manufacturing time of the heterojunction field effect transistor.

Thereafter, the substrate temperature is lowered to 1100 ° C., and under the conditions of TMG flow rate = 224 μmol / min and NH ₃ flow rate = 12.5 slm, the GaN layer 7 is deposited to a thickness of 1.0 μm under a pressure of 13.3 kPa. The GaN layer 8 is deposited to a thickness of 0.5 μm under a pressure of 90 kPa. Here, when the deposition pressure is low, carbon contained in TMG is easily doped into the GaN layer, and when the deposition pressure is high, carbon tends to be hardly doped from TMG into the GaN layer.

On the GaN layer 8, an AlN characteristic improving layer 9 (1 nm thickness), an Al _0.2 Ga _0.8 N barrier layer 10 (20 nm thickness), and a GaN cap layer 11 (1 nm) under a pressure of 13.3 kPa. An electron supply layer is deposited, including (thickness). At this time, the AlN layer 9 is deposited under the conditions of TMA flow rate = 51 μmol / min and NH ₃ flow rate = 12.5 slm, and the AlGaN layer 10 is TMG flow rate = 46 μmol / min, TMA flow rate = 7 μmol / min, and NH ₃ flow rate. = 12.5 slm, and the GaN layer 11 can be deposited under conditions of TMG flow rate = 58 μmol / min and NH ₃ flow rate = 12.5 slm.

In the above embodiment, the Al composition ratio of the AlGaN layers 3, 4 and 5 was changed in the order of 0.7, 0.4 and 0.1. The combination of composition ratios is not limited to this combination. Also, the number of AlGaN layers included in the composition gradient buffer layer structure and having different Al composition ratios is not limited to three, and can be any number. What is important is that the Al composition ratio gradually decreases from the lower surface to the upper surface of the composition gradient buffer layer structure. Furthermore, the super lattice multi-layer buffer layer structure 6 is not limited to the repetition of the AlN layer _/ the Al 0.1 _{Ga 0.9} N _layer,, for example _{the Al} 0.1 _{Ga 0.9} N _layer, has another composition ratio AlGaN It is also possible to replace it with a layer.

(Comparative Example 1)
In order to investigate the improvement effect of the substrate of the present invention, a substrate as Comparative Example 1 was produced using conventional technology. The substrate of Comparative Example 1 has the Si substrate 1, the AlN crystal buffer layer 2b, and the composition gradient buffer layer structure 3-5 in FIG. 1, but does not have the Al layer 2a.

That is, in Comparative Example 1, the AlN crystal buffer layer 2b is formed on the Si substrate 1 that has been surface-cleaned in the same manner as in the above-described embodiment. The substrate temperature is 1050 ° C., the pressure is 13.3 kPa, and the TMA is 108.5 sccm. The film was grown to a thickness of 200 nm under MOCVD conditions with a flow rate and NH ₃ flow rate of 12.5 slm. Thereafter, a composition gradient buffer layer structure 3-5 was formed on the AlN crystal buffer layer 2b in the same manner as in the above-described embodiment.

FIG. 2 shows an optical dark field photomicrograph of the surface of the Al _0.1 Ga _0.9 N layer 5 on the substrate of Comparative Example 1 obtained in this way. In addition, the white line segment in this micrograph has shown the scale of 50 micrometers. As can be seen from the photograph in FIG. 2, the surface of the substrate of Comparative Example 1 contained many fine convex defects, and the defect density was measured to be 4.4 × 10 ⁷ pieces / cm ² .

(Comparative Example 2)
In order to more reliably investigate the improvement effect of the substrate of the present invention, a substrate as Comparative Example 2 was further fabricated using the conventional technique. The substrate of Comparative Example 2 has the Si substrate 1, the AlN crystal buffer layer 2b, and the composition gradient buffer layer structure 3-5 in FIG. 1, but has a silicon nitride layer instead of the Al layer 2a. .

That is, in the comparative example 2, the surface of the Si substrate 1 subjected to the surface cleaning process in the same manner as in the above-described embodiment is performed under the conditions of the substrate temperature of 1050 ° C., the pressure of 13.3 kPa, and the NH ₃ flow rate of 12.5 slm. Nitrided for 40 seconds.

On this nitrided surface, as in Comparative Example 1, the AlN crystal buffer layer 2b has a substrate temperature of 1050 ° C., a pressure of 13.3 kPa, a TMA flow rate of 108.5 sccm, and 12.5 slm. The film was grown to a thickness of 200 nm under MOCVD conditions with NH ₃ flow rate, after which a compositionally graded buffer layer structure 3-5 was formed.

FIG. 3 shows an optical dark field photomicrograph of the surface of the Al _0.1 Ga _0.9 N layer 5 on the substrate of Comparative Example 2 obtained in this way. In addition, the white line segment in this micrograph also shows the scale of 50 micrometers. As can be seen from the photograph in FIG. 3, the surface of the substrate of Comparative Example 2 also contains many fine convex defects, and the defect density was measured to be 2.4 × 10 ⁷ pieces / cm ² . That is, in the substrate of Comparative Example 2, it can be seen that the defect density is reduced to about ½ compared to Comparative Example 1 due to the effect of nitriding the surface of the Si substrate 1.

(Reference Example 1)
In order to investigate the effective range of the present invention, a substrate as Reference Example 1 closely related to the present invention was also produced. The substrate of Reference Example 1 includes the Si substrate 1, the Al layer 2a, the AlN crystal buffer layer 2b, and the composition gradient buffer layer structure 3-5 shown in FIG.

The Al layer 2a was deposited for 6 seconds under the conditions of a substrate temperature of 1050 ° C., a pressure of 13.3 kPa, and a TMA flow rate of 27 sccm. This deposition condition corresponds to the condition for depositing the Al layer 2a having an average thickness of one atomic layer on the surface of the Si substrate 1.

On this Al layer 2a, as in Comparative Example 1, the AlN crystal buffer layer 2b has a substrate temperature of 1050 ° C., a pressure of 13.3 kPa, a TMA flow rate of 108.5 sccm, and NH _{3 of} 12.5 slm. The film was grown to a thickness of 200 nm under flow rate MOCVD conditions, after which a compositionally graded buffer layer structure 3-5 was formed.

FIG. 4 shows an optical dark field photomicrograph of the surface of the Al _0.1 Ga _0.9 N layer 5 on the substrate of Reference Example 1 thus obtained. The white line segment in this micrograph also shows a scale of 50 μm. As can be seen from the photograph in FIG. 4, the surface of the substrate of Reference Example 1 also contains many fine convex defects, and the defect density measured was 1.2 × 10 ⁸ / cm ² . That is, in the substrate of Reference Example 1, the Al layer 2a corresponding to the average thickness of one atomic layer was deposited, but it can be seen that the defect density is increased rather than reduced as compared with Comparative Example 1. This is because, on average, the Al layer 2a corresponding to the thickness of one atomic layer is non-uniform, and the surface of the Si substrate is partially exposed. This non-uniformity rather increases the defect density. It is thought that I let you.

Example 1
A substrate according to Example 1 according to the present invention was prepared in a manner similar to Reference Example 1. The substrate of Example 1 is different from that of Reference Example 1 only in that the deposition conditions for the Al layer 2a are changed.

That is, under the deposition conditions of the Al layer 2a in Example 1, the flow rate of TMA is increased from 27 sccm in Reference Example 1 to 54 sccm. That is, the TMA flow rate of 54 sccm corresponds to the condition for depositing the Al layer 2 a having an average thickness of two atomic layers on the surface of the Si substrate 1.

On this Al layer 2a, as in the case of Reference Example 1, the AlN crystal buffer layer 2b has a substrate temperature of 1050 ° C., a pressure of 13.3 kPa, a TMA flow rate of 108.5 sccm, and NH _{3 of} 12.5 slm. The film was grown to a thickness of 200 nm under flow rate MOCVD conditions, after which a compositionally graded buffer layer structure 3-5 was formed.

FIG. 5 shows an optical dark field photomicrograph of the surface of the Al _0.1 Ga _0.9 N layer 5 on the substrate of Example 1 obtained in this way. The white line segment in this micrograph also shows a scale of 50 μm. As can be seen from the photograph in FIG. 5, the fine convex defects were remarkably reduced on the surface of the substrate of Example 1, and the defect density measured was 1.1 × 10 ⁵ / cm ^2. It was. That is, in the substrate of Example 1, the defect density is drastically reduced to about 1/400 compared with Comparative Example 1 as an effect of depositing the Al layer 2a corresponding to the average thickness of the two atomic layers. .

This is because the surface of the Si substrate 1 is covered with the Al layer 2a corresponding to the thickness of the two atomic layers on the average without being exposed, and the AlN crystal has a smooth surface of Al polarity on the Al layer 2a. This is presumably because the buffer layer 2b has grown. That is, since the AlN crystal buffer layer 2b has an Al-polar smooth surface, it is considered that the defect density is drastically reduced also on the surface of the composition gradient buffer layer structure 3-5 grown thereon.

(Example 2)
A substrate according to Example 2 according to the present invention was further fabricated similar to Example 1. The substrate of Example 2 is different from that of Example only in that the average deposition thickness of the Al layer 2a is changed.

That is, under the deposition conditions of the Al layer 2a in the second embodiment, the flow rate of TMA is further increased from 54 sccm in the first embodiment to 108 sccm. That is, the TMA flow rate of 108 sccm corresponds to the condition for depositing the Al layer 2 a having an average thickness of four atomic layers on the surface of the Si substrate 1.

On this Al layer 2a, as in the case of Example 1, the AlN crystal buffer layer 2b has a substrate temperature of 1050 ° C., a pressure of 13.3 kPa, a TMA flow rate of 108.5 sccm, and NH _{3 of} 12.5 slm. The film was grown to a thickness of 200 nm under flow rate MOCVD conditions, after which a compositionally graded buffer layer structure 3-5 was formed.

FIG. 6 shows an optical dark field photomicrograph of the surface of the Al _0.1 Ga _0.9 N layer 5 on the substrate of Example 2 obtained in this way. The white line segment in this micrograph also shows a scale of 50 μm. As can be seen from the photograph in FIG. 6, fine convex defects were significantly reduced on the surface of the substrate of Example 2 as compared with Comparative Example 1, and the defect density was measured to find 1.9 × 10 ^5. Pieces / cm ² . However, it can be seen that the defect density in the substrate of Example 2 does not change significantly as compared to Example 1, but rather increases slightly.

The reason for this is that the Al layer 2a corresponding to the thickness of the four-atom layer on the average does not completely cover the surface of the Si substrate 1 and does not leak, but as the thickness of the Al layer increases, It is believed that Al microprotrusions begin to form on the surface, which can have an adverse effect. Therefore, the thickness of the Al layer 2a is preferably a thickness of 10 atomic layers or less in order to avoid the adverse effects of the fine protrusions generated on the surface.

(Comparison between Comparative Example 2 and Example 1)
7 and 8 show AFM (atomic force microscope) images of the surface of the Al _0.1 Ga _0.9 N layer 5 on the substrates of Comparative Example 2 and Example 1, respectively. Also in these AFM images of FIGS. 7 and 8, it can be confirmed that fine convex defects on the surface of the substrate of Example 1 are significantly reduced as compared with Comparative Example 2.

9 and 10 show graphs showing the cross-sectional shape of the surface irregularities along one scanning line in the AFM images of FIGS. 7 and 8, respectively. That is, the horizontal axis of these graphs represents the distance (μm) parallel to the surface, and the vertical axis represents the distance (nm) in the direction perpendicular to the plane parallel to the surface. The surface roughness can be measured from such a surface cross-sectional shape. When the surface roughness was measured for the AFM images of FIGS. 7 and 8, the RMS roughness (average square root roughness) was 6.93 nm and 3.89 nm, and Ra (arithmetic average roughness) was 5. They were 32 nm and 3.06 nm. That is, it can be seen that the surface roughness of the substrate of Example 1 is significantly improved as compared with Comparative Example 2.

(Effect of Al layer on the electronic characteristics of the device)
The influence of the Al layer 2a interposed between the Si substrate 1 and the AlN buffer layer 2b on the electronic characteristics of the nitride semiconductor device formed on the AlN buffer layer 2b was examined.

First, as a nitride semiconductor device according to the prior art, a nitride semiconductor device in which the Al layer 2a, the composition gradient buffer layer 3-5, and the superlattice multilayer buffer layer structure 6 are omitted from the stacked structure shown in FIG. It was made. That is, in this nitride semiconductor device, the AlN crystal buffer layer 2b was deposited on the Si substrate 1 under the conditions described in Comparative Example 1. On the AlN crystal buffer layer 2b, the carbon-doped GaN layer 7, the undoped GaN channel layer 8, the AlN characteristic improving layer 9, the Al _0.2 Ga _0.8 N barrier layer 10, and the GaN cap layer 11 are described above. Deposited sequentially under the conditions described in the embodiment.

The electronic characteristics of the GaN channel layer 8 in the nitride semiconductor device according to the related art obtained in this way were obtained by using well-known hole measurement. As a result, in this channel layer 8, the sheet resistance Rs was 1240Ω / □, the sheet carrier concentration Ns was 4.6 × 10 ¹² cm ⁻² , and the carrier mobility μ was 1090 cm ² / Vs.

Next, a nitride semiconductor device using the present invention was fabricated in a manner similar to the above-described conventional nitride semiconductor device. This nitride semiconductor device utilizing the present invention is nitrided according to the above-described prior art only in that the diatomic Al layer 2a according to the first embodiment is interposed between the Si substrate 1 and the AlN crystal buffer layer 2b. It was different from physical semiconductor devices.

The electronic properties of the GaN channel layer 8 in this nitride semiconductor device using the present invention were also determined using well-known hole measurements. As a result, in this channel layer 8, the sheet resistance Rs was 748Ω / □, the sheet carrier concentration Ns was 5.03 × 10 ¹² cm ⁻² , and the carrier mobility μ was 1660 cm ² / Vs.

As is apparent from the above, the sheet resistance Rs is reduced, the carrier concentration Ns is increased, and the carrier concentration is increased in the channel layer in the nitride semiconductor device using the present invention as compared with the nitride semiconductor device according to the prior art. It can be seen that the mobility μ is increased and all the electronic characteristics are improved. In particular, it is preferable that the carrier mobility μ is remarkably improved.

As is clear from the above, according to the present invention, the AlN crystal buffer layer surface is smoothened by uniformly forming an Al layer having a predetermined thickness on the substrate surface before the AlN crystal buffer layer is grown. As a result, the smoothness of the surface of the nitride semiconductor layer grown on the AlN crystal buffer layer can be improved.

Then, by using the substrate having such improved surface smoothness, various nitride semiconductor devices having improved electronic properties can be produced on the substrate.

1 Si substrate, 2a Al layer, 2b AlN crystal layer, 3 Al _0.7 Ga _0.3 N layer, 4 Al _0.4 Ga _0.6 N layer, 5 Al _0.1 Ga _0.9 N layer, 6 AlN / Al _0.1 Ga _0.9 N multilayer buffer structure, 7 carbon-doped GaN layer, 8 undoped GaN channel layer, 9 AlN characteristic improving layer, 10 Al _0.2 Ga _0.8 N barrier layer, 11 GaN cap layer .

Claims (4)

A substrate having a buffer layer structure for growing a nitride semiconductor layer,
An Al layer and an AlN crystal layer sequentially stacked on the (111) main surface of the Si single crystal substrate;
The Al layer has a thickness of 2 atomic layers or more and 10 atomic layers or less,
The surface of the AlN crystal layer has a (0001) plane orientation and an Al polar surface. The substrate according to claim 1, wherein the Al layer has a thickness of 2 atomic layers or more and 4 atomic layers or less. The substrate according to claim 1, further comprising an AlGaN crystal layer stacked on the AlN crystal layer. The substrate according to claim 3, wherein the AlGaN crystal layer includes a plurality of sub-layers in which an Al composition ratio is sequentially reduced.

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