JP2010251738A - Nitride semiconductor epitaxial substrate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nitride semiconductor epitaxial substrate having a buffer structure with excellent manufacturing efficiency, wherein cracking of a device active layer is suppressed and while improvement in crystallinity such as reduction in dislocation density is achieved, warpage accompanying film thickening of a nitride semiconductor is suppressed. <P>SOLUTION: The nitride semiconductor epitaxial substrate includes: an Si substrate 1; a first multilayer buffer region 3, wherein an Al<SB>a</SB>Ga<SB>1-a</SB>N (0.9≤a≤1.0) single-crystal layer 31 of 2 to 10 nm in thickness and an Al<SB>b</SB>Ga<SB>1-b</SB>N (0≤b≤0.1) single-crystal layer 32 of 10 to 30 nm in thickness are alternately and repeatedly laminated; a second multilayer buffer region 4, wherein an Al<SB>c</SB>Ga<SB>1-c</SB>N (0.9≤c≤1.0) single-crystal layer 41 of 2 to 10 nm in thickness and an Al<SB>d</SB>Ga<SB>1-d</SB>N (0≤d≤0.1) single-crystal layer 42 of 200 to 500 nm in thickness are alternately and repeatedly laminated; a GaN single-crystal layer 5; and an Al<SB>x</SB>Ga<SB>1-x</SB>N (0≤x≤1) single-crystal layer 6. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a nitride semiconductor epitaxial substrate having a HEMT (High Electron Mobility Transistor) structure suitably used for a light emitting diode, a laser light emitting element, an electronic element operable at a high speed and a high temperature, and the like. About.

  Nitride semiconductors typified by gallium nitride (GaN) and aluminum nitride (AlN) have a wide band cap, and have excellent characteristics such as high electron mobility and high heat resistance. Applications to devices and electronic devices that can operate at high speed and high temperature are expected.

Since the nitride semiconductor has a high melting point and a very high equilibrium vapor pressure of nitrogen, bulk crystal growth from the melt is not easy. For this reason, a single crystal at a practical level is produced by heteroepitaxial growth on a heterogeneous substrate.
Conventionally, sapphire, 6H—SiC, Si, or the like has been used as a substrate for epitaxially growing a GaN or AlN single crystal layer.

Among these substrates, the Si substrate is excellent in crystallinity as compared with other substrates, has a large area, and can be obtained at a low price. Therefore, the manufacturing cost of the nitride semiconductor can be reduced, which is preferable. . Moreover, high-quality crystals can be stably supplied by mature Si single crystal manufacturing technology.
Furthermore, the formation of the nitride semiconductor single crystal layer on the Si substrate is advantageous in terms of development cost because the subsequent device process can use the current device process as it is, and practical application is required. ing.

  However, when a nitride semiconductor crystal is formed on a Si substrate, warpage or cracking occurs in the nitride semiconductor single crystal film due to the difference in thermal expansion coefficient between Si and nitride semiconductor, and Si and nitride semiconductor. Due to the difference in the crystal lattice constants, a number of dislocations and crystal defects have occurred.

For this, a buffer layer is generally provided between the Si substrate and the nitride semiconductor layer to reduce the cracks and dislocations.
For example, in Non-Patent Document 1, by providing a multilayer structure buffer region composed of an AlN / GaN multilayer film with an AlN layer thickness of 5 nm and a GaN layer thickness of 20 nm or more, warpage with respect to the film thickness is reduced, and cracks are introduced. It is also described that it can be prevented.

In Patent Document 1, as the multilayer buffer region, a first buffer layer made of Al _x Ga _1-x N (0.5 ≦ x ≦ 1) having a thickness of 5 nm and Al _y having a thickness of 20 nm are disclosed. A nitride semiconductor having a smooth surface at the atomic layer level and no cracks by alternately laminating a plurality of second buffer layers made of Ga _1-y N (0.01 ≦ y ≦ 0.2). It is described that can be formed.

  By the way, in order to improve the breakdown voltage of the semiconductor element, it is desirable to increase the thickness of the nitride semiconductor, but with the increase in the thickness of the nitride semiconductor, there is a problem that the warpage of the nitride semiconductor epitaxial substrate also increases. It was.

  For this, for example, in Patent Document 2, a single layer structure buffer region is provided between an AlN / GaN multilayer buffer region and a thinner AlN / GaN sub multilayer structure buffer region. It is disclosed that the warpage of the nitride semiconductor epitaxial substrate can be reduced if the thickness is increased by changing the thickness of these layers to increase the thickness.

JP 2007-67077 A JP 2008-205117 A

Otsuka Koji, “Applied Physics”, 2007, Vol. 76, No. 5, pp. 489-494

  However, when forming an AlN / GaN multilayer film as described in Non-Patent Document 1 and Patent Document 1, the growth rate of AlN is slower than the growth rate of GaN. The manufacturing efficiency of the nitride semiconductor epitaxial substrate is inferior.

  In addition, forming a plurality of types of multilayer structure buffer regions and single layer structure buffer regions in various film thicknesses as described in Patent Document 2 makes the manufacturing process complicated and increases the film thickness. Therefore, also in this case, the manufacturing efficiency of the nitride semiconductor epitaxial substrate is inferior.

  Therefore, in the manufacture of a nitride semiconductor epitaxial substrate, it is possible to improve the crystallinity such as the generation of cracks and the reduction of dislocation density, and the device activity of the nitride semiconductor while suppressing warpage. There has been a demand for a buffer structure that can increase the thickness of the layer and that can be efficiently formed.

  The present invention has been made to solve the above technical problem, and the occurrence of cracks in the device active layer is suppressed, and while improving crystallinity such as reduction of dislocation density, the nitride semiconductor is improved. An object of the present invention is to provide a nitride semiconductor epitaxial substrate having a buffer structure excellent in manufacturing efficiency, in which warpage associated with thickening is suppressed.

A nitride semiconductor epitaxial substrate according to the present invention includes a Si substrate, an Al _a Ga _1-a N (0.9 ≦ a ≦ 1.0) single crystal layer having a thickness of 2 nm to 10 nm on the Si substrate, and A first multilayer buffer region in which Al _b Ga _1-b N (0 ≦ b ≦ 0.1) single crystal layers having a thickness of 10 nm or more and 30 nm or less are alternately and repeatedly stacked on the first multilayer buffer region; Al _c Ga _1-c N (0.9 ≦ c ≦ 1.0) single crystal layer having a thickness of 2 nm to 10 nm and Al _d Ga _1-d N (0 ≦ d ≦ 0) having a thickness of 200 nm to 500 nm .1) Second multilayer buffer region in which single crystal layers are alternately and repeatedly stacked, GaN single crystal layer formed on the second multilayer buffer region, and Al formed on the GaN single crystal layer _x Ga _1-x N (0 <x <1) single crystal layer It is a sign.
By providing such two types of multilayer buffer regions, it is possible to suppress the occurrence of cracks in the device active layer and to improve the crystallinity such as the reduction of the dislocation density. Warpage associated with film formation can also be suppressed.

Among the nitride semiconductor epitaxial substrates, in particular, a first multilayer buffer in which an Si substrate and an AlN single crystal layer having a thickness of 5 nm and a GaN single crystal layer having a thickness of 20 nm are alternately stacked on the Si substrate. A second multilayer buffer region in which an AlN single crystal layer having a thickness of 5 nm and a GaN single crystal layer having a thickness of 200 nm to 500 nm are alternately and repeatedly stacked on the first multilayer buffer region; It is preferable that a GaN single crystal layer formed on the two multilayer buffer regions and an Al _x Ga _1-x N (0 <x <1) single crystal layer formed on the active layer.

In the nitride semiconductor epitaxial substrate, an AlN single crystal layer and Al _y Ga _1-y N (0 <y <1) are formed on the Si substrate between the Si substrate and the first multilayer buffer region. It is preferable to provide an initial buffer region in which single crystal layers are stacked.
As an initial growth buffer, AlN single crystal that has high affinity with Si and GaN and does not cause a meltback etching reaction is suitable, and in order to flatten the step on the surface of the AlN single crystal layer, GaN and It is preferable to insert an AlGaN single crystal layer which is a mixed crystal of AlN to form the initial buffer region as described above.

According to the present invention, the occurrence of cracks in the device active layer is suppressed, and the nitride semiconductor in which the warpage accompanying the increase in the thickness of the nitride semiconductor is suppressed while improving the crystallinity such as the reduction of the dislocation density. An epitaxial substrate can be provided.
Furthermore, the nitride semiconductor epitaxial substrate according to the present invention has an advantage that it is excellent in production efficiency, and can also contribute to an improvement in yield in the device process.

It is a schematic sectional drawing which shows the layer structure of the nitride semiconductor epitaxial substrate which concerns on this invention. FIG. 3 is an enlarged view of a first multilayer buffer region 3 and a second multilayer buffer region 4 in FIG. 1.

Hereinafter, the present invention will be described in more detail with reference to the drawings.
FIG. 1 shows an outline of the layer structure of a nitride semiconductor epitaxial substrate according to the present invention. The nitride semiconductor epitaxial substrate shown in FIG. 1 has an initial buffer region 2, a first multilayer buffer region 3, a second multilayer buffer region 4, a GaN single crystal layer 5, an Al _x Ga _1-x on a Si substrate 1. The N single crystal layer 6 has a structure in which the N single crystal layers 6 are sequentially stacked.
The GaN single crystal layer 5 is formed as a device active layer, and an Al _x Ga _1-x N (0 <x <1) single crystal layer 6 is laminated thereon to form a HEMT structure. Is formed.

FIG. 2 shows an enlarged view of the first multilayer buffer region 3 and the second multilayer buffer region 4 of the nitride semiconductor epitaxial substrate of FIG. The first multilayer buffer region 3 includes Al _a Ga _1-a N (0.9 ≦ a ≦ 1.0) single crystal layer 31 having a thickness of 2 nm to 10 nm and Al _b Ga _{1 having a} thickness of 10 nm to 30 nm. _{The -bN} (0 ≦ b ≦ 0.1) single crystal layer 32 has a multi-layer structure in which layers are alternately and repeatedly stacked from the Si substrate 1 side. The second multi-layer buffer area 4 has a thickness of 2nm or more 10nm less _{Al c Ga 1-c N (} 0.9 ≦ c ≦ 1.0) single crystal layer 41 and the thickness of 200nm or more 500nm following Al _d The Ga _1-d N (0 ≦ d ≦ 0.1) single crystal layer 42 has a multilayer structure in which layers are alternately and repeatedly stacked.

In the first multilayer buffer region 3, the thickness of the Al _a Ga _1-a N single crystal layer 31 constituting this is the same as that of the Al _c Ga _1-c N single crystal constituting the second multilayer buffer region 4. Although it is equivalent to the layer 41, the thickness of the Al _b Ga _1-b N single crystal layer 32 is smaller than the thickness of the Al _d Ga _1-d N single crystal layer 42 constituting the second multilayer buffer region 4. . For this reason, the first multilayer buffer region 3 has a larger number of stacks per unit thickness than the second multilayer buffer region 4, that is, has a larger number of interfaces between different layers, and therefore has an effect of reducing the dislocation density. Although large, it is inferior in suppressing the warpage.

The thicknesses of the Al _a Ga _1-a N single crystal layer 31 and the Al _c Ga _1-c N single crystal layer 41 are preferably 2 nm or more and 10 nm or less, and more preferably around 5 nm.
When the thickness is less than 2 nm, it is difficult to form a flat film, flatness is impaired, and stress controllability is reduced. On the other hand, if the thickness exceeds 10 nm, cracks occur during epitaxial growth.

On the other hand, the second multilayer buffer region 4 is less effective in reducing the dislocation density than the multilayer buffer region 3, but the Al _d Ga _1-d N single crystal layer 42 is thick. The GaN single crystal layer 5 and the Al _x Ga _1-x N single crystal layer 6 which are upper layers than the second multilayer buffer region 4 are excellent in the warp suppressing effect.

The thickness of Al _b Ga _1-b N single crystal layer 32 is preferably 10nm or more 30nm or less, and more preferably is around 20 nm.
It is difficult to form a single crystal layer 32 having a high Ga composition flat with a thickness of less than 10 nm. When the thickness is less than 10 nm, the flatness is impaired and stress controllability is reduced.
On the other hand, when the thickness exceeds 30 nm, there is no problem in terms of flatness and stress controllability of the single crystal layer 32. However, in order to efficiently bend and eliminate dislocations, a larger number of interfaces (per unit thickness) of single crystal layer 31 having a high Al composition / single crystal layer 32 having a high Ga composition (AlN / GaN) is formed. From the viewpoint of being preferable, a large film thickness is disadvantageous.

The thickness of the Al _d Ga _1-d N single crystal layer 42 is preferably 200 nm or more and 500 nm or less, and more preferably around 250 nm.
The GaN single crystal layer 5 is stressed whether it is too close or too far from the Al _c Ga _1-c N single crystal layer 41 / Al _d Ga _1-d N single crystal layer 42 (AlN / GaN) interface. Controllability is reduced. Since the thickness of the Al _d Ga _1-d N single crystal layer 42 in contact with the GaN single crystal layer 5 corresponds to the distance from the AlN / GaN interface to GaN single crystal layer 5, Al _d Ga _1-d N single The thickness of the crystal layer 42 is preferably within the above range.

Therefore, if the dislocation density is reduced early in the epitaxial growth of the nitride semiconductor, an increase in the dislocation density in each layer formed thereon can be suppressed.From the viewpoint of reducing the dislocation density, It is preferable that the first buffer region is formed first, and the second buffer region is formed thereon.
On the contrary, when the second buffer layer is formed first and the first buffer layer is formed thereon, the warping suppression effect can be sufficiently obtained, but the dislocation density is sufficiently reduced during the film formation process. The crystallinity is poor.

In the multi-layer buffer region, it is only necessary that a large number of interfaces of different layers exist. Therefore, the Al _a Ga _1-a N single crystal layer 31 and the Al _b Ga _1-b N single layer in the first multi-layer buffer region 3 are used. crystal layer 32, if it is laminated alternately, the first layer of the first multilayer buffer region 3 located in the Si substrate 1 side may be a Al _b Ga _1-b N single crystal layer 32. Similarly, the Al _c Ga _1-c N single crystal layer 41 and the Al _d Ga _1-d N single crystal layer 42 in the second multilayer buffer region 4 are also located on the Si substrate 1 side. The first layer of 4 may be an Al _d Ga _1-d N single crystal layer 42.

  As described above, in the nitride semiconductor epitaxial substrate according to the present invention, by providing two types of multilayer buffer regions 3 and 4, suppression of crack generation in the device active layer and reduction of dislocation density are achieved, In addition, warpage associated with the increase in the thickness of the nitride semiconductor is also suppressed.

The Al _a Ga _1-a N single crystal layer 31 and the Al _c Ga _1-c N single crystal layer 41 preferably satisfy 0.9 ≦ a ≦ 1.0 and 0.9 ≦ c ≦ 1.0. In particular, an AlN single crystal layer in which a = c = 1 is preferable.
The Al _b Ga _1-b N single crystal layer 32 and the Al _d Ga _1-d N single crystal layer 42 preferably satisfy 0 ≦ b ≦ 0.1 and 0 ≦ d ≦ 0.1. A GaN single crystal layer in which b = d = 0 is preferable.
As described above, by alternately and repeatedly laminating two types of AlGaN single crystal layers having greatly different composition ratios of Al and Ga, the stress relaxation effect in the first and second buffer regions can be more effectively exhibited. .

Therefore, among the nitride semiconductor epitaxial substrates, as a particularly preferable layer structure, an SiN substrate, and an AlN single crystal layer having a thickness of 5 nm and a GaN single crystal layer having a thickness of 20 nm are alternately repeated on the Si substrate. A first multilayer buffer region that is laminated, and a second multilayer in which an AlN single crystal layer having a thickness of 5 nm and a GaN single crystal layer having a thickness of 200 nm to 500 nm are alternately and repeatedly laminated on the first multilayer buffer region. A multilayer buffer region, a GaN single crystal layer formed on the second multilayer buffer region, and an Al _x Ga _1-x N (0 <x <1) single crystal layer formed on the active layer, The structure provided with.

Since the AlN single crystal layer has a slower epitaxial growth rate than the GaN single crystal layer, the ratio of the thickness of the AlN single crystal layer per unit thickness is larger than that of the second multilayer buffer region 4. The region 3 requires a longer time to form than the second multilayer buffer region 4, but not only the first multilayer buffer region 3 but also the second multilayer buffer region 4 is formed by the layer configuration as described above. As a result, the film formation time of the entire buffer region can be shortened.
In addition, the composition of each layer and each film thickness of such a multilayer buffer region do not need to be changed in various ways, and thus can be easily controlled.
Therefore, after forming the first buffer region 3 superior in the effect of reducing the dislocation density, and then forming the second buffer region 4 with a faster growth rate on the first buffer region 3, even when the film thickness is increased, The manufacturing efficiency of the nitride semiconductor epitaxial substrate can be improved.

In the first multilayer buffer region 3, one layer of Al _a Ga _1-a N single crystal layer 31 and one layer of Al _b Ga _1-b N single crystal layer 32 are repeatedly stacked 10 to 100 times. The number of stacked layers is preferably 10 to 100.
If each of the Al _a Ga _1-a N single crystal layer 31 and the Al _b Ga _1-b N single crystal layer 32 is too thin or the number of stacked layers is too small, stress relaxation by the multilayer buffer region occurs during the film growth process. However, the effect of suppressing cracks and warpage cannot be sufficiently obtained, and the crystallinity (low dislocation density) is also lowered.
On the other hand, when each layer is too thick or the number of laminated groups is too large, the cost is high and the manufacturing efficiency is poor, which is not preferable.
The same applies to the lamination of the Al _c Ga _1-c N single crystal layer 41 and the Al _d Ga _1-d N single crystal layer 42 constituting the second multilayer buffer region 4.

Si single crystal is used for the Si substrate 1, and the manufacturing method thereof is not particularly limited. Those produced by the Czochralski (CZ) method or those produced by the floating zone (FZ) method may be used. An epitaxially grown crystal layer (Si epi substrate) may be used.
By epitaxially growing a nitride semiconductor on a Si single crystal substrate, it is possible to utilize devices and techniques used in a conventional Si semiconductor manufacturing process, and manufacturing with a large diameter and low cost is possible.

In the initial buffer region 2 formed on the Si substrate 1, an AlN single crystal layer 21 and an Al _y Ga _1-y N (0 <y <1) single crystal layer 22 are laminated in this order from the Si substrate 1 side. However, this serves as a protection and base for the surface of the Si substrate before the first multilayer buffer region 3 is formed.
Si and Ga are very reactive, and when Ga adheres to the surface of the Si substrate in the initial stage of growth, the surface of the Si substrate becomes rough due to the meltback etching reaction.
For this reason, it is preferable to form an AlN single crystal layer having a high affinity with Si and GaN and not causing the meltback etching reaction as an initial growth buffer.
However, since the growth of the AlN single crystal takes a temperature lower than the original optimum growth temperature in consideration of the melting point of Si, the surface of the AlN single crystal layer has a shape including a large number of steps. In addition, even when a nitride semiconductor crystal such as GaN or AlN is grown, it is difficult to obtain a good quality crystal.
Therefore, in order to flatten the level difference on the surface of the AlN single crystal layer, an Al _y Ga _1-y N (0 <y <1) single crystal layer, which is a mixed crystal of GaN and AlN, is further inserted. It is preferable to form the buffer region 2.
In the case where the initial buffer area is not provided, for example, a special substrate cleaning pretreatment is preferably performed.

The initial time of providing a buffer region 2, to the first of the first layer of the multi-layer buffer area 3 which is formed continuously with the Al _b Ga _1-b N single crystal layer 32, Al _y Ga ₁ initial buffer area 2 _-y N single crystal layer 22 is in contact with a layer having the same or relatively close composition.
Originally, from the viewpoint of dislocation reduction and stress control, it is preferable that the interface has a greatly different composition ratio. However, at the stage where the Al _y Ga _1-y N single crystal layer 22 is formed, sufficient crystallinity is still obtained. Is not obtained. For this reason, from the viewpoint of increasing the crystallinity at the early stage of growth faster, providing an interface having a close composition ratio as described above works advantageously for reducing dislocations and controlling stress on the layer formed thereon. Therefore, it is preferable.

The thickness of the AlN single crystal layer 21 and the Al _y Ga _1-y N single crystal layer 22 is preferably as thin as possible from the viewpoint of manufacturing cost. However, the AlN single crystal layer 21 has crystallinity necessary as a buffer. In order to obtain this, a corresponding film thickness is required, and the thickness is preferably about 100 to 150 nm.
Also, the Al _y Ga _1-y N single crystal layer 22 needs to have a corresponding film thickness in order to smooth the steps existing in the AlN single crystal layer 21, and is preferably about 100 to 200 nm in thickness. .

Further, as described above, the GaN single crystal layer 5 formed on the multilayer buffer region is formed as a device active layer, and Al _x Ga _1-x N (0 <X <1) The HEMT structure is formed by the single crystal layer 6.
Therefore, the GaN single crystal layer 5 is preferably formed with a thickness of about 500 to 3000 nm from the viewpoint of ensuring good crystallinity and flatness.
The Al _x Ga _1-x N single crystal layer 6 is preferably formed with a thickness of about 20 to 50 nm in order to avoid generation of cracks.

  The epitaxial growth method according to the present invention is not particularly limited, and may be a generally used method such as MOCVD (Metal Organic Chemical Vapor Deposition) or PECVD (Plasma Enhanced Chemical Vapor Deposition). A CVD method, a vapor deposition method using a laser beam, a sputtering method using an atmospheric gas, or the like can be used.

EXAMPLES Hereinafter, although this invention is demonstrated further more concretely based on an Example, this invention is not restrict | limited by the following Example.
[Example 1]
A nitride semiconductor epitaxial substrate having a layer structure as shown in FIG. 1 was produced by the following steps.
First, a Si substrate 1 having a diameter of 4 inches is set in an MOCVD apparatus, and an AlN single crystal layer 21 having a thickness of 100 nm is formed by vapor phase growth at 1100 ° C. using trimethylaluminum (TMA) and NH ₃ as raw materials. Further, on that, trimethylgallium (TMG), TMA and NH ₃ are used as raw materials, and a 200 nm thick Al _y Ga _1-y N (y = 0.1) single crystal is formed by vapor phase growth at 1000 ° C. A layer (Al _0.1 Ga _0.9 N single crystal layer) 22 was laminated to form the initial buffer region 2.

Next, TMA and NH ₃ are used as raw materials, and an Al _a Ga _1-a N (a = 1) single crystal layer (AlN) having a thickness of 5 nm is formed on the initial buffer region 2 by vapor phase growth at 1000 ° C. Single crystal layer) 31 was laminated.
An Al _b Ga _1-b N (b = 0) single crystal layer (GaN single crystal) having a thickness of 20 nm is formed on the AlN single crystal layer 31 by vapor phase growth at 1000 ° C. using TMG and NH ₃ as raw materials. Layer) 32 was laminated.
The AlN single crystal layer 31 and the GaN single crystal layer 32 were alternately and repeatedly stacked in the same process, and the number of stacked layers was set to 50 to form the first multilayer buffer region.
Further, the above first multilayer buffer region 3, an _{Al c Ga 1-c N (} c = 1) single crystal layer (AlN single crystal layer) 41 and the thickness of the thickness of 5 nm 250 nm of Al _d Ga _{1- d} N (d = 0) single crystal layers (GaN single crystal layers) 42 are alternately and repeatedly stacked, and the number of stacked layers is 24. The other steps are the same as in the formation of the first multilayer buffer region 3. A second multilayer buffer region 4 was formed.

A GaN single crystal layer 5 having a thickness of 2000 nm was stacked on the second multilayer buffer region 4 by vapor phase growth at 1000 ° C. using TMG and NH ₃ as raw materials.
Furthermore, TMG, TMA, and NH ₃ are used as raw materials on the GaN single crystal layer 5, and Al _x Ga _1-x N (x = 0.25) having a thickness of 20 nm is formed by vapor phase growth at 1000 ° C. A single crystal layer (Al _0.25 Ga _0.75 N single crystal layer) 6 was laminated to obtain a nitride semiconductor epitaxial substrate.
The thickness of each layer formed by vapor phase growth was adjusted by adjusting the flow rate and supply time of each raw material.

[Example 2]
A nitride semiconductor epitaxial substrate was fabricated in the same process as in Example 1 except that the initial buffer region 2 was not formed.

[Comparative Example 1]
The second multilayer buffer region 4 is not formed, but only the first multilayer buffer region 3 is formed with the number of repeated stacks of the AlN single crystal layer 31 and the GaN single crystal layer 32 as 150. A nitride semiconductor epitaxial substrate was fabricated in the same process as in Example 1.

[Comparative Example 2]
Without forming the second multilayer buffer region 4, only the first multilayer buffer region 3 is formed with the number of repeated stacks of the AlN single crystal layer 31 and the GaN single crystal layer 32 as 290, and the first multilayer buffer region 3 is formed. A GaN single crystal layer 5 having a thickness of 1000 nm was stacked on the buffer region 3. Except for this, a nitride semiconductor epitaxial substrate was fabricated in the same process as in Example 1.

[Comparative Example 3]
Without forming the first multilayer buffer region 3, only the second multilayer buffer region 4 is formed with the number of repeated stacks of the AlN single crystal layer 41 and the GaN single crystal layer 42 as 28, and the second multilayer buffer region 4 is formed. A GaN single crystal layer 5 having a thickness of 1000 nm was stacked on the buffer region 4. Except for this, a nitride semiconductor epitaxial substrate was fabricated in the same process as in Example 1.

[Examples 3-28, Comparative Examples 4-27]
Each thickness of the Al _a Ga _1-a N single crystal layer 31 and the Al _b Ga _1-b N single crystal layer 32 in the first multilayer buffer region 3, and Al _c Ga _{1-c in} the second multilayer buffer region 4 The composition ratios and the number of laminations of the N single crystal layer 41 and the Al _d Ga _1-d N single crystal layer 42 were changed as shown in Table 1 (Example) or Table 2 (Comparative Example). In addition, the Al _b Ga _1-b N single crystal layer 32 is first stacked in the first multilayer buffer region 3, and the Al _d Ga _1-d N single crystal layer 42 is first stacked in the second multilayer buffer region 4. Except for this, a nitride semiconductor epitaxial substrate was fabricated in the same process as in Example 1.
In Examples 3 to 28 and Comparative Examples 2 to 27, the thickness of each layer and the number of layers were adjusted so that the total film thickness was about 5 μm.

About the nitride semiconductor epitaxial substrate produced in the said Example and comparative example, the dislocation density in the substrate surface center part of the GaN single-crystal layer used as a device active layer was evaluated with the transmission electron microscope. Further, the occurrence of warpage and cracks (excluding the outer periphery of the substrate) was also evaluated by a dark field image of a laser displacement meter and an optical microscope. The warpage was evaluated based on the difference between the maximum value and the minimum value of the distance that the back surface of the substrate before epitaxial growth was displaced in the substrate thickness direction.
These results are collectively shown in Table 3 (Examples) and Table 4 (Comparative Examples).
In Tables 3 and 4, the warpage was evaluated as follows: ○: 31 μm or less, Δ: more than 31 μm and 50 μm or less, and x: more than 50 μm. The evaluation of the dislocation density was ○: 5 × 10 ⁹ / cm ² or less, and x: more than 5 × 10 ⁹ / cm ² .
The film formation process time does not particularly affect the substrate characteristics, but is preferably shorter in terms of manufacturing efficiency and cost, and is described as a reference index.

As can be seen from the evaluation results shown in Tables 3 and 4, all of the nitride semiconductor epitaxial substrates produced under the conditions of each example had no cracks and had a dislocation density of 5 × 10 ⁹ / cm ² or less. In addition, it was recognized that the warpage tends to be suppressed as compared with the comparative example. In particular, Examples 1 and 3 had the lowest dislocation density and were excellent in warpage suppressing effect. In the case where the initial buffer region was not formed (Example 2), efficiency higher than that in Example 1 was obtained in terms of process time, but the characteristics were slightly lower.

On the other hand, the nitride semiconductor epitaxial substrate fabricated under the conditions of each comparative example has a large dislocation density as a whole and many cracks and warpages are generated.
In the case where the second multilayer buffer region 4 was not formed (Comparative Examples 1 and 2), it was the most excellent from the viewpoint of dislocation density, but the occurrence of cracks and an increase in warpage occurred, and the process time Had the disadvantage of being very long.
In the case where the first multilayer buffer region was not formed (Comparative Example 3), cracks were not generated and it was most excellent from the viewpoint of process time, but the effect of reducing the dislocation density was lowered.

1 Si substrate 2 initial buffer region 21 AlN single crystal layer 22 Al _y Ga _1-y N single crystal layer 3 first multilayer buffer area 31 Al _a Ga _1-a N single crystal layer 32 Al _b Ga _1-b N single Crystal layer 4 Second multilayer buffer region 41 Al _c Ga _1-c N single crystal layer 42 Al _d Ga _1-d N single crystal layer 5 GaN single crystal layer 6 Al _x Ga _1-x N single crystal layer

Claims (3)

A Si substrate, an Al _a Ga _1-a N (0.9 ≦ a ≦ 1.0) single crystal layer having a thickness of 2 nm to 10 nm and an Al _b Ga _{1 having a} thickness of 10 nm to 30 nm on the Si substrate. _-b N (0 ≦ b ≦ 0.1) first multilayer buffer regions in which single crystal layers are alternately and repeatedly stacked, and Al _c Ga having a thickness of 2 nm to 10 nm on the first multilayer buffer region _1-c N (0.9 ≦ c ≦ 1.0) single crystal layers and Al _d Ga _1-d N (0 ≦ d ≦ 0.1) single crystal layers having a thickness of 200 nm to 500 nm are alternately and repeatedly stacked. Second multilayer buffer region formed, a GaN single crystal layer formed on the second multilayer buffer region, and Al _x Ga _1-x N (0 <x < 1) A nitride semiconductor epitaxial substrate comprising a single crystal layer. A Si substrate, a first multilayer buffer region in which an AlN single crystal layer having a thickness of 5 nm and a GaN single crystal layer having a thickness of 20 nm are alternately stacked on the Si substrate, and the first multilayer buffer region A second multilayer buffer region in which an AlN single crystal layer having a thickness of 5 nm and a GaN single crystal layer having a thickness of 200 nm to 500 nm are alternately stacked, and a GaN formed on the second multilayer buffer region. A nitride semiconductor epitaxial substrate comprising a single crystal layer and an Al _x Ga _1-x N (0 <x <1) single crystal layer formed on the active layer. An initial buffer in which an AlN single crystal layer and an Al _y Ga _1-y N (0 <y <1) single crystal layer are sequentially stacked on the Si substrate between the Si substrate and the first multilayer buffer region. The nitride semiconductor epitaxial substrate according to claim 1, further comprising a region.

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