JP2003151910A - GaN-BASED SEMICONDUCTOR BASE - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a base, where an AlGaN crystal layer having reduced dislocation density is grown, even while an LEPS method is being used. SOLUTION: Irregularities, where the LEPS method can be carried out, are machined on the surface of a crystal substrate 1, and a GaN-based crystal layer 2 is grown from the recess or projection. At this point, AlGaN layers 21, 23, 25, and 27, where an Al composition is small and AlGaN layers 22, 24, 26, and 28 where the Al composition is large are alternately grown as a lamination structure, and at the same time, they are grown, until an upper surface becomes flat. As a result, growth is made preferentially by the LEPS method while being AlGaN, and an AlGaN crystal layer is achieved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a GaN-based semiconductor substrate having at least a surface layer of a GaN-based semiconductor crystal layer, and more particularly, the GaN-based semiconductor crystal layer includes an AlGaN layer having a reduced dislocation density. It is about.

[0002]

2. Description of the Related Art In order to improve the performance (lifetime, initial characteristics) of a semiconductor device or device using a GaN-based semiconductor crystal (hereinafter, also referred to as "GaN-based crystal"), the dislocation density of the crystal is reduced. Is essential. For this purpose, various growth techniques for reducing the dislocation density, such as the ELO method (also referred to as a selective growth method and a lateral epitaxial growth method), have been developed and applied at the time of device formation. A GaN-based semiconductor means In _X Ga _Y Al _Z N (0 ≦ X
≦ 1, 0 ≦ Y ≦ 1, 0 ≦ Z ≦ 1, X + Y + Z = 1), for example, AlN, Ga
N, AlGaN, InGaN and the like are listed as important compounds.

On the other hand, in a semiconductor laser using a GaN-based crystal (the wavelength of generated light is in the green region to the ultraviolet region), an n-type contact layer (n-GaN) and an n-type cladding layer (n) are generally formed on a substrate. -AlGaN), n-type light propagation layer (n-GaN),
Light emitting layer (InGaN-based MQW layer), p-type AlGaN cap layer, p-type light propagation layer, p-type cladding layer (n-AlGa)
N), a p-type contact layer (p-GaN) is formed, and light propagates in the active layer and the light propagation layer in the lateral direction (direction along the stripe formed perpendicular to the stacking direction). When the light propagates in the lateral direction, the light is prevented from seeping out to the clad layer and a layer (for example, a contact layer) outside the clad layer, and a radiation pattern (FFP: Far Field Pattern) is formed.
In order to make n) unimodal and not reduce the optical density of the propagating light, it is necessary that the clad layer be made of AlGaN having a band gap larger than that of the light propagating layer and a small refractive index. Then, the AlGaN layer is preferably a relatively thick layer. Further, the AlGaN-based ultraviolet light receiving element uses AlGaN as a light sensitive layer, but the long wavelength end of detectable light is at the band gap (E) of the light sensitive layer.
g), that is, the AlN composition ratio. At this time, a relatively thick AlGaN layer is also required.

In the conventional ELO method, a mask made of a material (SiO ₂ or the like) in which a GaN-based crystal cannot substantially grow is used, and lateral growth (lateral growth) is performed on the mask to dislocation lines. The dislocation density is reduced by controlling the propagation direction. However, even with such a mask, in AlGaN, crystals are grown starting from the mask due to the presence of the Al component. Therefore, in the conventional ELO using the mask, it is difficult to preferably grow AlGaN in the lateral direction, and AlGaN with high quality and low dislocation density is formed.
No layer was obtained.

On the other hand, a lateral growth method without using a mask has also been proposed. This method is called the LEPS method or the like. For example, as shown in FIG. 5 (a), as shown in FIG. 5 (a), stripe groove processing is performed on a crystal substrate 110 such as sapphire to form unevenness, and as shown in FIG. As shown in the figure, from the upper part of the convex portion of the unevenness, through the low temperature buffer layer (not shown), Ga
An N-based crystal 210 is grown, and as shown in FIG.
In this method, the GaN-based crystal layer 310 is formed by laterally growing on the recess (that is, in the air) and adjoining each other.
Hereinafter, the crystal growth method using the irregularities will be referred to as “LEPS method” for description. Regarding the LEPS method, for example, International Publication WO 00/55893 and the literature (Jpn. J. App)
l. Phys. Vol. 40 (2001) L583. ) Is described in detail.

[0006]

However, according to the research conducted by the present inventors, when an AlGaN is grown from a convex portion by the LEPS method, for example, the problem that the lateral growth rate is slow and the lateral growth rate is low. The problem that coalescence of grown crystals becomes very slow or does not occur, and even when the crystals are coalesced, a large number of cracks are generated due to voids that are easily formed in the coalesced part. It became clear that it was difficult to grow as an AlGaN crystal layer.

The object of the present invention is to solve the above-mentioned problems,
AlGa with reduced dislocation density while using PS method
It is to provide a GaN-based semiconductor substrate having an N crystal layer on the upper surface.

[0008]

The GaN-based crystal substrate of the present invention has the following features. (1) A GaN-based semiconductor substrate having a crystal substrate and a GaN-based semiconductor crystal layer grown on the substrate, wherein the upper surface of the crystal substrate is processed to have irregularities, and the GaN-based semiconductor crystal layer is A layer grown from the concave and / or convex portions of the unevenness so as to cover the unevenness, and the layer further includes AlGa.
A laminated structure composed of N crystal layers is included, and the laminated structure is characterized in that AlGaN crystal layers are laminated so that at least adjacent layers in the laminating direction have different Al composition ratios. GaN-based semiconductor substrate.

(2) The unevenness is formed as a stripe pattern, and the longitudinal direction of the stripe is the <11-20> direction or the <1-100> direction of the GaN-based semiconductor crystal grown thereon. (1) The GaN-based semiconductor substrate as described above.

(3) The unevenness is formed as a stripe pattern having a rectangular wave-shaped cross section, and the groove depth of the concave groove portion is d,
The GaN-based semiconductor substrate according to (1) above, wherein the groove width is W1 and the aspect ratio defined by d / W1 is 0.01 or more.

(4) In the laminated structure, the AlGaN crystal layer having the larger Al composition ratio and the smaller Al composition ratio A are used.
The GaN-based semiconductor substrate according to (1) above, wherein the lGaN crystal layers are alternately laminated.

(5) The larger Al composition ratio is 0.01 to
The GaN-based semiconductor substrate according to (4) above, wherein the Al composition ratio of the smaller one is 0.7 to 0.1.

(6) The GaN-based semiconductor substrate according to any one of (1) to (5), wherein the laminated structure is a superlattice structure.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION A GaN-based crystal base material (the base material) according to the present invention is a crystal substrate 1 as illustrated in FIG.
And a GaN-based crystal layer 2 grown on the substrate. Concavities and convexities (protrusions 1a and recesses 1b in the same figure) are processed on the surface of the crystal substrate 1 so that the LEPS method can be carried out, and the GaN-based crystal layer 2 is provided with recesses and / or depressions of the concavities and convexities according to the LEPS method. It grows from the convex portion so as to cover the irregularity. Here, the difference from the conventional LEPS method is that the GaN-based crystal layer 2 grown on the concavities and convexities is different from the conventional LEPS method.
The point is that it does not include a layer grown from only a single material as seen in the S method, but includes a laminated structure composed of AlGaN crystal layers. The laminated structure has an AlGaN crystal layer (21 to 2) so that adjacent layers have different Al composition ratios.
8) is laminated. In other words, the GaN-based crystal layer 2 is laminated by the AlGaN crystal layer,
This is a layer obtained by performing crystal growth specific to the LEPS method.

With the structure including the above-mentioned laminated structure, the AlGaN crystal can be grown as a layer having a reduced dislocation density while using the LEPS method.
The uppermost layer of this laminated structure may be made as thick as necessary and used as a clad layer of the device or the like.

In the above laminated structure, the Al composition ratios between the layers adjacent to each other in the laminating direction may be different from each other.
The method of stacking (combining) composition ratios can be roughly classified into the following modes. (A) A laminated structure in which an AlGaN crystal layer having a larger Al composition ratio and an AlGaN crystal layer having a smaller Al composition ratio are alternately grown. (B) A laminated structure in which the Al composition ratio changes stepwise. (C) A structure other than the above (a) and (b), in which AlGaN layers having different Al composition ratios are stacked regularly or irregularly.

In the boundary portion of each layer in the above laminated structure, the Al composition ratio is continuous (gradient shape) even if the Al composition ratio changes stepwise and the boundary surface is clear.
The boundary surface may become unclear by changing to. In the present invention, if the different Al composition ratios are changed in layers even if the boundary surface is not clear, it is regarded as a laminated structure. The continuity and discontinuity of the Al composition ratio change can be freely controlled in vapor phase growth when the LEPS method is performed.

The laminated structure of (a) above is not limited to a structure in which only two kinds of large and small AlGaN are alternately grown, but AlGaN crystal layers having a larger (or smaller) Al composition ratio are formed. The Al composition ratios may be different from each other. For example, the Al composition may be increased from the lower layer side to the upper layer side as a whole while alternately repeating large and small Al compositions. Further, after alternately growing only two kinds of large and small types of AlGaN, the last one layer may have a desired Al composition ratio.
Hereinafter, the laminated structure of (a) above will be described by representatively taking an aspect of alternately growing only two kinds of large and small types of AlGaN.

According to the research conducted by the present inventors, GaN is a material for which the LEPS method is effective because it has a sufficient lateral growth rate, but AlGaN has a slow lateral growth due to the Al component. It is difficult to obtain a preferable crystal layer. Therefore,
Like the laminated structure of (a) above, an Al _y Ga _{1 -y} N layer having a larger Al composition ratio (0 ≦ y ≦ 1) and an Al _x Ga _{1 -x} N layer having a smaller Al composition ratio (0 ≦ x ≦ 1, x <y) layers are alternately grown to form one layer. As a result, as shown in FIG. 1, Al having a small Al composition ratio
_{The x} Ga _1-x N layers (21, 23, 25, 27) serve as layers for promoting lateral growth, and have a large Al composition ratio of Al _y G.
Although including the a _{1 -y} N layer, the layer 2 as a whole preferably grows laterally from the upper portion of the convex portion 1 a of the crystal substrate and adjoins each other to form a GaN-based crystal layer 2.

In the laminated structure (a), the Al composition ratio _x of the Al _x Ga _1-x N layer having the smaller Al composition ratio is 0.
1 or less is preferable, and x = 0 (that is, GaN) is most preferable in terms of lateral growth. On the other hand, the Al composition ratio _y of the Al _y Ga _1-y N layer having the larger Al composition ratio is the same as the Al composition ratio y required for the clad layer or the light receiving layer of the element formed using the substrate.
The composition ratio of GaN is not limited, and is 0.0.
1 ≦ y ≦ 0.7, particularly 0.02 ≦ y ≦ 0.5 is mentioned as a preferable range for practical use.

The thickness of each of the Al _x Ga _1-x N layer and the Al _y Ga _1-y N layer is preferably about 50 nm or less, and particularly about 10 nm or less. A superlattice structure is preferable. With the superlattice structure, the next layer structure is formed before dislocations are introduced and the strain is relaxed, so that the generation of cracks can be prevented, and the Al _x Ga _{1 -x} N layer and the Al layer can be prevented. An AlGaN layer having a property as an AlGaN mixed crystal having an average composition in which the thickness of the _y Ga _1-y N layer is weighted can be produced.

As shown in FIG. 3A, the alternating laminated structure of the Al _x Ga _1-x N layers and the Al _y Ga _1-y N layers has at least the crystals grown from the adjacent growth starting points. It is preferred to repeat the alternating lamination until the are bonded together. Even after being bonded to each other, as shown in FIG. 1, alternate lamination may be continued until the upper surfaces of the layers become flat, and as shown in FIG. 3B, an Al _y Ga _1-y N layer is formed. It is also possible to grow only 2a into a thick film until the upper surface of the layer 2a is flat.

The total layer thickness of the GaN-based crystal layer 2 is not limited,
Although it depends on the growth conditions and the cycle of unevenness,
The thickness of the AlGaN layer required in a light receiving element or the like is approximately 1 μm to 10 μm. Also, the surface AlG
The aN layer may be grown thick if necessary.

When Al _x Ga _1-x N layers and Al _y Ga _1-y N layers are alternately grown on the unevenness of the crystal substrate, either layer may be grown first. For example, in the case of using a GaN low temperature growth buffer layer, the Al _x Ga _1-x N layer having a smaller Al composition is grown first, so that the crystal disorder is reduced and the AlGaN layer is smoothly transferred. Can be expected. Further, when the AlN low temperature growth buffer layer is used, Al _y Ga having a large Al composition is used.
The Al composition of the first layer may be appropriately selected according to the buffer layer at that time, for example, by growing the _1-yN layer first.

The increasing / decreasing pattern of the Al composition ratio in the above-mentioned laminated structure (b) is not limited, but a pattern in which the Al composition ratio is monotonically increased is effective as a method for increasing the Al composition of the uppermost layer. . In addition, depending on the use or purpose, after the Al composition ratio monotonically increases, the Al composition may monotonically decrease above a specific layer, or may increase / decrease in a specific cycle.

The crystal substrate used as the base material is GaN.
As long as the system crystal can grow, even if it is a base crystal substrate (a substrate made of only the same material) to start crystal growth first, a buffer layer for lattice matching on its surface, Further, a GaN-based crystal film may be formed.

As the base crystal substrate, sapphire (C plane, A plane, R plane), SiC (6H, 4H, 3)
C), Si, spinel, ZnO, GaAs, NGO, G
aN, AlN (or a substrate having AlN as a surface layer)
Etc. can be used, but other materials may be used as long as they meet the purpose of the invention. The plane orientation of the substrate is not particularly limited, and may be a just substrate or a substrate having an off angle. Since the LEPS method is a crystal growth method for the purpose of reducing the dislocation density, the usefulness of the present invention becomes more remarkable when the crystal substrate is a substrate made of a material other than GaN, such as sapphire or SiC. Become.

The buffer layer may be formed on the above-mentioned base crystal substrate, if necessary. Particularly, when a sapphire substrate is used, a GaN-based low temperature growth buffer layer of GaN, AlN, AlGaN or the like is preferable. .

The pattern of the unevenness to be processed on the surface of the crystal substrate may be any unevenness that allows the LEPS method. For example, a pattern in which dot-shaped concave portions (or convex portions) are arranged, a stripe-shaped uneven pattern in which linear or curved concave grooves (or convex ridges) are arranged at regular intervals, and the like can be given. The pattern in which the ridges are arranged in a grid pattern is a kind of pattern in which the square hole dot-shaped recesses are regularly arranged. Basically, the cross-sectional shape of the irregularities is preferably rectangular (including trapezoidal) wavy. From the viewpoint of flattening the upper surface of the GaN-based crystal layer to be finally obtained, it is preferable that the pitch of the irregularities is constant.

Among the various irregularities described above, the stripe-shaped irregular pattern in which linear or curved concave grooves (or convex stripes) are arranged at regular intervals can simplify the manufacturing process, It is preferable because it is easy to manufacture.

When the concavo-convex pattern is formed in a stripe shape, the longitudinal direction of the stripe has an important influence on the growth method of the GaN-based crystal. When the longitudinal direction of the stripe is defined as the <1-100> direction of the GaN-based crystal grown on the stripe, the {11-20} plane is a plane that grows laterally from the convex portion depending on the growth conditions. The {11-20} plane is {1
It has the property of growing laterally faster than the −100} plane. This is a mode in which the lateral growth property is more significantly improved by alternately growing layers having different Al composition ratios. Further, in this case, as shown in FIG.
It tends to remain as a cavity.

On the contrary, when the longitudinal direction of the stripe is the <11-20> direction of the GaN-based crystal grown thereon,
The {1-100} plane becomes a surface that grows laterally from the convex portion to the concave portion, and the growth rate in the lateral direction becomes slow. As a result, the growth rate in the C-axis direction becomes faster, and {1-10
Oblique facets such as the 1} plane are easily formed. In this case, the GaN-based crystal can be facet-grown also from the bottom surface in the recess, depending on how the dimensions of the recess and protrusion are taken, and the recess can be filled with the crystal. In this case, as shown in FIG. 4A, the AlGaN crystal layer grows in a triangular convex shape on each upper surface of the concave portion and the convex portion, and then the upper surface is flattened to 1
The tendency to become one layer becomes stronger. In this aspect, FIG.
As shown in (b), layers having different Al composition ratios are alternately grown in part or all of the process from facet growth to flattening, such as in the initial and middle stages.

The triangular convex crystal at the initial stage of facet growth as shown in FIG. 4A is grown not only by a single material but also by growing AlGaN layers alternately to a triangular convex shape. Good.

When the concavo-convex pattern has a stripe shape and the concavo-convex cross-sectional shape has a rectangular wave shape, the preferred dimension of each part of the concavo-convex shown in FIG. 2 is that the width W1 of the concave groove is 0.5 μm to 20 μm.
μm, the width W2 of the ridge is about 0.5 μm to 20 μm,
The amplitude of the unevenness (depth of the groove) d is about 0.05 μm to 5 μm, and the aspect ratio defined by d / W1 is 0.
01 or more is preferable. Of these ranges, in order to grow only from the upper part of the convex part, the width W1 of the concave groove is 0.5 μm.
It is particularly preferable that m to 20 μm, the width W2 of the ridges are 0.5 μm to 5 μm, and the amplitude d of the irregularities is 0.2 μm to 5 μm. On the other hand, in order to grow facets from both the concave portion and the convex portion, the width W1 of the concave groove is 0.5 μm to 20 μm, the width W2 of the convex stripe is 0.5 μm to 20 μm, and the amplitude d of the concave and convex is 0.0.
It is particularly preferable that the thickness is 5 μm to 3 μm.

As a method of processing the unevenness, for example, a method of forming a pattern according to the mode of the desired unevenness by using an ordinary photolithography technique and performing an etching process by using the RIE technique or the like to obtain the desired unevenness. Are exemplified.

The method for growing a GaN crystal is HVP.
E, MOVPE, MBE method and the like are preferable. The HVPE method is preferable when forming a thick film, but the MOVPE method or MBE method is preferable when forming a thin film.

It is possible to control whether or not to grow while forming a facet surface, depending on the growth conditions (gas species, growth pressure, growth temperature, etc.) when growing a GaN-based crystal. When the NH ₃ partial pressure is low in reduced pressure growth {1
Facets on the −101} surface are likely to appear, and facet surfaces are more likely to appear in atmospheric pressure growth than in depressurization. Further, when the growth temperature is raised, the lateral growth is promoted, but when the growth is performed at a low temperature, the growth in the C-axis direction becomes faster than the lateral growth, and the facet surface is easily formed.

Although the surface layer of the base material is formed by the LEPS method, the surface layer is a high-quality AlGaN layer having a reduced dislocation density. This AlGa
The N layer is useful as a light receiving layer of a light receiving element having sensitivity in a short wavelength region, and contributes to improvement of sensitivity and the like. Further, it is also useful for a light emitting layer of an LED or a semiconductor laser, a clad layer, etc., and improves the characteristics of the device. In addition, the base material is useful for forming a device that requires a high-quality AlGaN layer.

The GaN-based crystal layer of the base material is characterized by having an AlGaN layer with a reduced dislocation density.
The AlGaN layer does not necessarily have to be the surface layer of the base material,
A necessary crystal layer such as a GaN crystal layer may be further provided thereon.

The base material does not necessarily have to be treated and distributed as a single member and may be contained in the base material portion of the laminate formed in one growth step. That is, in the process of forming various GaN-based elements, even if the substrate is formed in the vapor phase growth apparatus and then the element structure is grown thereon without interruption of crystal growth,
The substrate is present in the device.

[0041]

Example 1 In this example, a stripe-shaped concavo-convex pattern was formed on a sapphire substrate, and the protrusions (GaN layer / Al _0.3 Ga) were formed.
Alternating growth of _0.7 N layer), each layer thickness is 100 nm
As a result, the crystal layer covers the concave portion and is alternately grown until the upper surface becomes flat to obtain the base material (the final surface layer is AlGaN.
As a layer), the crystal quality of the surface layer was evaluated.

A C-plane sapphire substrate is subjected to stripe patterning with a photoresist (width of concave groove 3 μm, period 6 μm, longitudinal direction of stripe is <1-100> direction for growing GaN-based crystal), and RIE apparatus is performed. Was etched to a depth of 1.5 μm so as to have a rectangular cross section, to obtain a sapphire substrate having a striped pattern on the surface.

This sapphire substrate is subjected to normal atmospheric pressure MOV.
It is loaded into a PE device and 1100 under a nitrogen gas main component atmosphere.
The temperature was raised to ° C and thermal cleaning was performed. The temperature is lowered to 500 ° C., trimethylgallium (hereinafter referred to as TMG) as a III group raw material, and ammonia as a N raw material are flown,
A GaN low temperature buffer layer having a thickness of 30 nm was formed on the upper surface of the convex portion and the bottom surface of the concave portion.

Then, the temperature is raised to 1000 ° C. and TM
G, GaN layer formed by flowing ammonia (layer thickness 100 n
m) and the growth of an Al _0.3 Ga _0.7 N layer (layer thickness 100 nm) performed by flowing TMG, trimethylaluminum (hereinafter TMA), and ammonia are alternately repeated to form a laminated structure, and laterally. After growing, the crystals grown from the adjacent convex portions become the main crystals, are bonded to each other, and alternate growth is continued until the upper surfaces of the layers are substantially flat, and then the Al _0.15 Ga _0.85 N layer is formed on the uppermost layer as a thickness. Fifty
The substrate was grown to 0 nm to obtain a substrate according to the present invention. Mature G
Total thickness of aN-based crystal layer (from upper surface of convex part of substrate to upper surface of layer)
Was 3 μm.

G added with Si on the substrate
When an aN crystal layer was grown to 1 μm and the upper surface of the GaN crystal layer was observed by cathode luminescence,
The density of the dark spots (= dislocation density) is extremely low at about 1 × 10 ⁴ pieces / cm ² in the region corresponding to the upper part of the recess (the laterally grown portion), and the dislocation density is partially reduced. It was found to be a reduced quality AlGaN crystal layer. However, some cracks were observed.

Example 2 This example is the same as Example 1 except that the laminated structure in which the GaN layer and the Al _0.3 Ga _0.7 N layer are grown alternately is a superlattice structure. The base material was obtained. Both layers of the superlattice structure have a layer thickness of 10 nm.

Further, as in the first embodiment, the crystals grown from the adjacent protrusions are bonded to each other, and alternate growth is continued until the upper surface of the layer becomes substantially flat, and then Al is formed on the uppermost layer.
_{A 0.15} Ga _0.85 N layer was grown to a thickness of 1 μm. Mature G
Total thickness of aN-based crystal layer (from upper surface of convex part of substrate to upper surface of layer)
Was 3 μm.

G on which Si was further added on the substrate
When an aN crystal layer was grown to 1 μm and the upper surface of the GaN crystal layer was observed by cathode luminescence,
The density of dark spots was about 1 × 10 ⁴ pieces / cm ² in the region corresponding to the upper part of the recess as in Example 1, but unlike Example 1, no cracks were generated.

Example 3 In this example, a stripe-shaped concavo-convex pattern was formed on a sapphire substrate, and the (GaN layer / A
l _0.3 Ga _0.7 N layers), and each layer thickness is set to 1
The thickness was set to 0 nm, and the crystals grown from the concave portions and the convex portions were alternately grown until the upper surfaces became flat and integrated to obtain the base material (the final surface layer was an AlGaN layer), and the crystal quality of the surface layer was evaluated.

A C-plane sapphire substrate is subjected to stripe patterning with a photoresist (width of concave groove 3 μm, period 6 μm, longitudinal direction of stripe is <11-20> direction for growing GaN-based crystal), and RIE apparatus is performed. Was etched to a depth of 1.5 μm so as to have a rectangular cross section, to obtain a sapphire substrate having a striped pattern on the surface.

On this sapphire substrate, a GaN layer (layer thickness 10 nm) was grown in the same manner as in Example 1 and Al _0.3
A stacked structure is formed by alternately repeating the growth of a Ga _0.7 N layer (layer thickness 10 nm), and the crystals facet-grown from the protrusions and recesses are integrated with each other until the top surface of the layer becomes substantially flat. After continuing the growth of the above, an Al _0.15 Ga _0.85 N layer was grown to a thickness of 500 nm on the uppermost layer to obtain a substrate according to the present invention. The total thickness of the grown GaN-based crystal layer (from the upper surface of the convex portion of the substrate to the upper surface of the layer) is 3 μm
Met.

On the substrate, G added with Si was added.
When an aN crystal layer was grown to 1 μm and the upper surface of the GaN crystal layer was observed by cathode luminescence,
The density of dark spots (= dislocation density) is about 4 overall.
It was about 10 ⁶ pieces / cm ² , and it was found that the AlGaN crystal layer had no cracks and had a reduced dislocation density.

Comparative example Using a sapphire substrate processed in the same manner as in Example 1
I, Al on the unevenness _0.15Ga_0.75Continuous N only 3μm
Grown up. This sample is completely filled with irregularities
It was not a flat surface. Also, the whole surface
There are innumerable cracks running on it and it can not be used for devices
It was.

[0054]

According to the present invention, while using the LEPS method capable of effectively reducing the dislocation density, AlGaN, which has been difficult to grow by this growth method, is grown, and a high quality AlGaN crystal layer is formed on the surface layer. It has become possible to provide a substrate having Further, by alternately growing the AlGaN layers while forming the superlattice structure, cracks can be suppressed more preferably.

[Brief description of drawings]

FIG. 1 is a cross-sectional view schematically showing the structure of a GaN-based crystal substrate of the present invention when a recess is left as a cavity.

FIG. 2 is a diagram showing dimensions of irregularities of a crystal substrate in a GaN-based crystal substrate of the present invention.

FIG. 3 is a cross-sectional view showing a structural example of a laminated structure portion in the GaN-based crystal substrate of the present invention.

FIG. 4 is a cross-sectional view schematically showing the structure of an GaN-based crystal substrate of the present invention in which an AlGaN crystal layer is facet-grown from a concave portion and a convex portion.

FIG. 5 is a schematic view showing a crystal growth process of a conventional LEPS method.

[Description of Reference Signs] 1 crystal substrate 1a convex portion 1b concave portion 2 GaN-based crystal layers 21, 23, 25, 27 AlGa having a small Al composition ratio
N crystal layers 22, 24, 26, 28 AlGa having a large Al composition ratio
N crystal layer

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Yoichiro Ouchi             4-3 Ikejiri, Itami City, Hyogo Prefecture Mitsubishi Electric Cable             Industrial Co., Ltd. Itami Works (72) Inventor Takashi Tsunekawa             4-3 Ikejiri, Itami City, Hyogo Prefecture Mitsubishi Electric Cable             Industrial Co., Ltd. Itami Works F-term (reference) 5F041 AA40 CA34 CA40 CA65 CA74                       CA75                 5F045 AA04 AB17 AC08 AC12 AF09                       BB12 CA12 CA13 DA52

Claims (6)

[Claims] 1. A crystal substrate and GaN grown on the substrate
And a GaN-based semiconductor crystal layer, wherein the upper surface of the crystal substrate is processed to have irregularities, and the GaN-based semiconductor crystal layer covers the irregularities from the concave and / or convex portions of the irregularities. Which is a layer grown by the above method, and the layer further includes a laminated structure composed of an AlGaN crystal layer, and the laminated structure is such that at least adjacent layers in the laminating direction have Al composition ratios different from each other. A GaN-based semiconductor substrate, wherein the layers are laminated. 2. The unevenness is formed as a stripe pattern, and the longitudinal direction of the stripe is the <11-20> direction or the <1-100> direction of the GaN-based semiconductor crystal grown thereon. 1. The GaN-based semiconductor substrate according to 1. 3. The concavo-convex pattern is formed as a stripe pattern having a rectangular wave-shaped cross section, and an aspect ratio defined by d / W1 is 0.01 or more, where d is the groove depth of the concave groove portion and W1 is the groove width. The GaN-based semiconductor substrate according to claim 1. 4. The laminated structure comprises an AlGaN crystal layer having a larger Al composition ratio and an AlG crystal layer having a smaller Al composition ratio.
The aN crystal layer and the aN crystal layer are alternately laminated.
The GaN-based semiconductor substrate described. 5. The larger Al composition ratio is 0.01 to 0.
7. The GaN-based semiconductor substrate according to claim 4, wherein the smaller Al composition ratio is 0 to 0.1. 6. The GaN-based semiconductor substrate according to claim 1, wherein the laminated structure is a superlattice structure.