JP2001308464A - Nitride semiconductor element, method for manufacturing nitride semiconductor crystal, and nitride semiconductor substrate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form an AlGaN layer having such a thickness that the layer can sufficiently function as a crystal growing base layer with a low defect density. SOLUTION: A first AlGaN layer 13 is formed on a sapphire substrate 11 through a low-temperature buffer layer 12. Then a second AlGaN layer 14 containing Al at a concentration lower than that of the first AlGaN layer 13 is formed on the layer 13. The layer 14 is grown while a facet structure is formed from the opening of a mask 21. The thickness of the layer 14 is adjusted to >=5 μm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a nitride semiconductor device having a small number of crystal defects, a nitride semiconductor substrate, and a technique for manufacturing the same.

[0002]

2. Description of the Related Art Since nitride semiconductor materials have a sufficiently large forbidden band width and a direct transition type between bands, application to a short wavelength light emitting device has been actively studied. In addition, since the saturation drift speed of electrons is high and a two-dimensional carrier gas can be used by a heterojunction, application to electronic devices is also expected.

[0003] Nitride semiconductor layers constituting these devices are formed by metal organic vapor phase epitaxy (MOVPE), molecular beam epitaxy (MBE), hydride vapor phase epitaxy (HVPE).
It is obtained by performing epitaxial growth on a base substrate using a vapor phase growth method such as that described above. However, since there is no underlying substrate whose lattice constant matches that of the nitride semiconductor layer, it was difficult to obtain a high-quality growth layer, and the obtained nitride semiconductor layer contained many crystal defects. . Since this crystal defect is a factor that hinders the improvement of the device characteristics, studies for reducing the crystal defect in the nitride semiconductor layer have been actively conducted.

As a method for obtaining a group III element nitride crystal having relatively few crystal defects, a low-temperature deposited buffer layer (buffer layer) is formed on a heterogeneous substrate such as sapphire and an epitaxial growth layer is formed thereon. Methods are known. “Applied Physics (Vol. 68, No. 7 (1999), No. 7
68-773), the following process is shown as an example of a crystal growth method using a low-temperature deposited buffer layer. First, AlN or GaN is deposited on a substrate such as sapphire at about 500 ° C. to form an amorphous film or a continuous film partially containing polycrystal. By elevating the temperature to around 1000 ° C., a part of the nucleus is evaporated and crystallized to form a high density crystal nucleus. It is described that a GaN film having relatively good crystallinity can be obtained using this as a growth nucleus. However, even if a method of forming a low-temperature deposited buffer layer is used, as described in the above-mentioned document, crystal defects such as threading dislocations and vacancy pipes exist in the order of 10 ⁸ to 10 ¹¹ cm ^-2 , It was insufficient to obtain a high-performance device as is rare.

Therefore, a method of forming a semiconductor multilayer film constituting an element portion on a thick GaN layer as an underlayer for crystal growth has been actively studied.

As a method of forming a GaN underlayer having few dislocations in a crystal, ELO (Epitaxial Lateral Overgrowth) is used.
th) The technology is known. In Japanese Patent Application Laid-Open No. H11-251253, by using this ELO technique, the crystal defect near the wafer surface is reduced to 1 × 10 ⁵ / cm ² or less.
It is said that an N substrate was obtained. However, ELO
In the case of using, it was difficult to reduce the crystal defect density over the entire surface of the wafer. In ELO, GaN selectively grows laterally from an opening in a mask. In the region where the mask is formed, the dislocation is prevented from proceeding upward, but in the opening, the dislocation is inherited from the underlying layer as it is, and the upper region has a structure including many threading dislocations. Therefore, in the crystal growth by ELO, a portion having few crystal defects is certainly formed, while a region having many threading dislocations is also formed at the same time, and it is difficult to reduce the crystal defect over the entire surface of the wafer. In the examples of the above publication, the crystal defect density near the surface was observed to be 1 × 10 ⁴ or less by TEM. However, while such a region having low crystal defects was formed, A region including a crystal defect is also formed at the same time. Actually, `` Applied Physics Letter Volume
71 to 1997 Pp.2472-2474 "is, on the SiO ₂ mask, most dislocations are not observed, 10 ^{108 to 109} in the mask opening
It is reported that a cm- ² dislocation is observed.

On the other hand, the present inventors have developed a FIELO (Facet-Initiated Epitaxial) which is a further development of the ELO method.
Lateral overgrowth technology is being developed ("Applied Physics" (Vol. 68, No. 7, 1999, pp. 774 to 779)). This technique is similar to ELO in that selective growth is performed using a SiO ₂ mask, but is different in that a facet is formed in a mask opening. By forming facets, it changes the direction of propagation of dislocations,
This is to reduce threading dislocations reaching the upper part of the epitaxial growth layer. By using this method, for example, a thick GaN layer is grown on a base substrate such as sapphire,
Thereafter, by removing the base substrate, a high-quality GaN substrate having relatively few crystal defects can be obtained, and by using this, a light-emitting element having better performance than before can be obtained.

Further, Japanese Patent Application Laid-Open No. 9-83016 discloses that
The composition is gradient on the SiC substrate so that the x value is gradually reduced.
Al_xGa_1-xA configuration for growing an N (0 ≦ x ≦ 1) layer
It has been disclosed. SiC has almost the same lattice constant as AlN
GaN has a larger lattice constant than these
You. The technology described in the above publication takes this lattice constant into account.
Thus, the crystallinity of the nitride semiconductor layer is improved. under
GaN layer is formed on top of SiC as ground substrate
To form GaN directly on SiC
And a large lattice constant difference at the interface between the two
Defects occur. Therefore, in the above publication, from the SiC side
Composition so that Al composition gradually decreases toward GaN side
And thereby gradually reduce the lattice constant difference between SiC and GaN.
It is relaxing all the time. Al_xGa_1-xN (0 ≦ x ≦ 1) layer
As a specific configuration, the composition gradient in a single layer
In addition to the_0.9Ga_0.1N layer 0.2μ
m, Al _0.8Ga_0.2N layer 0.2 μm, Al_0.7Ga_0.3N
Layer 0.2 μm,..., Al_0.2Ga_{0 .8}N layer 0.2 μm, A
l_0.1Ga_0.9N layers starting from 9 layers stacked in order of 0.2 μm
The structure of the multilayer film is shown. Thus the composition gradient
Formed on the AlGaN layer
Crystallinity of the nitride semiconductor layer to be improved
It is described that a new device can be realized.

This technique disperses the strain caused by the difference in lattice constant between the SiC substrate and the GaN layer so that the lattice constant changes almost continuously, and reduces the strain concentration location. The elimination improves the crystallinity. As a specific configuration, utilizing the fact that the lattice constants of SiC and AlGaN are close to each other, the lowermost layer of the composition gradient layer has a high A
1 layer is disposed. Therefore, this technology
This is an effective technique when a SiC substrate having substantially the same lattice constant as AlN is used, and it is difficult to apply it to other dissimilar substrates such as sapphire. When applied to such a heterogeneous substrate, crystal systems having greatly different lattice constants are adjacent to each other at the interface between the lowermost layer of the composition gradient layer and the heterogeneous substrate, and many crystal defects occur. Since this crystal defect is inherited by the upper layer as it is, the uppermost layer of the composition gradient layer has a structure including many crystal defects.

Further, the technique described in the above publication is to disperse the strain caused by the lattice constant difference between the substrate and the GaN layer in the layer structure by changing the lattice constant almost continuously by the composition gradient. However, this does not reduce the sum of the strain energies themselves. Therefore, there is a certain limit to the reduction of crystal defects, and it is difficult to reduce crystal defects to a level that is currently desired. Further, when SiC is used, the lattice constant difference from GaN or AlGaN is small and a certain effect can be obtained. However, when sapphire or the like is used, the lattice constant difference is further increased, and the strain energy itself is increased. Even if the strain energy is dispersed in the structure, it is difficult to sufficiently reduce the crystal defects.

In order to obtain a high performance nitride semiconductor device, it is desired to further reduce crystal defects in a substrate or an underlayer serving as an underlayer for crystal growth.
In the prior art, it has been difficult to reduce crystal defects over the entire surface of the underlying substrate.

On the other hand, in a semiconductor laser using a nitride semiconductor, a lower refractive index and a thicker clad layer are desired from the viewpoint of improving the light confinement ratio. This makes it possible to further reduce the threshold value of the semiconductor laser to extend its life, to achieve high output, to shape a laser beam spot, and to improve temperature characteristics.

[0013] Further, for example, in a semiconductor laser for use in a digital video disk (DVD), it is an important technical problem to eliminate multi-spots in a far-field image of a laser beam. This phenomenon is also caused by the insufficient light confinement effect, and here too, it is strongly desired to improve the light confinement rate by lowering the refractive index and increasing the thickness of the cladding layer.

An AlGaN layer is usually employed for the cladding layer of the nitride semiconductor laser. To improve the light confinement effect, the Al composition of the AlGaN layer is increased.
It is effective to increase the thickness to improve the refractive index of the cladding layer. However, in such a case, cracks tend to occur in the cladding layer due to a difference in lattice constant and a difference in thermal expansion coefficient between the crystal growth underlayer and the AlGaN layer. As a result, the light confinement effect is rather reduced. Not only will it be reduced, but the life of the device will be shortened.

For this reason, in designing a nitride semiconductor device, a material having a small lattice constant is used as an underlayer.
That is, it is advantageous to use a material having a high Al composition. In the case where the lattice constant of the underlayer is increased, if a layer having a small lattice constant is disposed above the underlayer, tensile strain is generated in the layer and cracks are easily generated. On the other hand, when the lattice constant of the underlayer is reduced, if a layer having a large lattice constant is disposed above the underlayer, compressive strain occurs in the layer.
In general, structures with compressive-mode residual strain exhibit higher strength than structures with tensile-mode residual strain (this is known by the so-called Matthews equation), and thus the underlying layer If a material having a small lattice constant is used, the strength of the upper layer can be improved.

Therefore, as an underlayer material for crystal growth,
If AlGaN is used instead of conventional GaN, the Al composition difference between the semiconductor layer formed on the upper part and the underlayer is reduced,
As a result, a compressive strain or a relatively small tensile strain occurs in the upper semiconductor layer. Thereby, the occurrence of cracks in the upper semiconductor layer is effectively prevented.

Conventionally, a method for obtaining an AlGaN underlayer is to form a low-temperature buffer layer on a substrate, and then form an AlGaN layer thereon via a GaN layer,
A method of forming an AlGaN layer directly on a low-temperature buffer layer has been used.

First, the above method will be described. GaN
When an AlGaN layer is formed via a layer, an AlGaN layer having a smaller lattice constant is formed on GaN, and cracks are likely to occur when the thickness is increased.
Because the lattice constant of AlGaN is considerably smaller than that of the underlying GaN (for example, Semiconductors and Semimetals Vol.
8 “High Brightness Light Emitting diodes” ed. By
GB Stringfellowand M. George Craford, 1997 by A
This is because AlGaN suffers from tensile strain. This strain increases with the Al composition, and for example, is about 0.17% in the Al _0.07 Ga _0.93 N layer. In addition, the strain increases as the thickness of the AlGaN layer increases. From this, the occurrence of cracks rapidly increases when the product of the strain amount and the layer thickness in the AlGaN layer exceeds a substantially constant value. In the prior art, the thickness of AlGaN that can be grown without causing cracks is at most a few μm,
It was impossible to grow AlGaN having a thickness of 100 μm or more. Further, even if the AlGaN layer can be grown without causing cracks, the AlGaN layer becomes a structure containing many crystal defects in the layer, which hinders the improvement of the device performance, and is also a factor after the crystal growth and in the use of the device. This was also the cause of cracks.

Next, the method described above, that is,
On the method of forming an AlGaN layer directly on a buffer layer
State. Even in this case, cracks can be prevented.
In order to achieve this, the thickness of the AlGaN layer and the Al
About imposed. `` Appl.Phys.Letts.Vol.75, No.19, pp2
960-2962 "has a low temperature buffer layer on the substrate
3μm thick Al_0.03Ga_0.97Form an N layer and then
1μm thick Al _0.06Ga_0.94Form N clad layer
The illustrated nitride semiconductor laser is shown. Such a structure
To prevent multiple spots in the far-field image
Is described.

The technique described in the above publication is to form an AlGaN cladding layer having a high Al composition on a thick AlGaN layer having a low Al composition and increase the light confinement effect by the action of these AlGaN layers. is there. Al _0.03
The magnitude of the strain due to the Al composition difference between the Ga _0.97 N layer and the Al _0.06 Ga _0.94 N cladding layer is about 0.07%, and as a result, it is considered that cracks are less likely to occur. However,
According to experiments performed by the present inventors, it has been confirmed that even if such a two-stage AlGaN structure is employed, it is difficult to grow crack-free AlGaN having a thickness of 100 μm or more. Also,
In this structure, it is very difficult to further increase the aluminum composition and the film thickness of the cladding layer, and there are certain restrictions on the improvement of the refractive index of the cladding layer.

As described above, in the prior art, it was difficult to form an AlGaN layer having a thickness large enough to exhibit a function as an underlayer without generating cracks.

[0022]

As described above, the present invention relates to an underlayer (typically, an undersubstrate) for growing a nitride semiconductor crystal from the viewpoint of improving the performance of an element.
Further reduction in dislocation density is desired.

In a light emitting device such as a semiconductor laser using a nitride semiconductor, a high A
It is required to realize an AlGaN clad layer having a large composition and a large film thickness. In order to produce such a clad layer without generating cracks, an AlG
It is desired to be constituted by aN. Further, if a high-quality AlGaN underlayer can be realized, the range of choice of materials for the semiconductor multilayer film formed thereon can be increased, which is useful for electronic devices.

As described above, if a thick AlGaN underlayer having a low surface defect density and a relatively high Al composition can be obtained, it is possible to manufacture an unprecedented high-performance device. Specifically, a layer structure having a higher Al composition can be grown as a cladding layer of the laser, and as a result, confinement of electrons and light can be improved, and the characteristics of a short-wavelength laser can be improved. For application to electronic devices, improvement of high-frequency characteristics by realizing an AlGaN high-resistance substrate, and drastic improvement of device performance by suppressing electron leakage to the substrate can be expected.

The present invention has been made in view of the above circumstances, has a low defect density, effectively prevents cracks from occurring,
It is an object of the present invention to provide a technique for removing an undersubstrate and forming an AlGaN layer having a sufficient thickness that can be used as a single film.

[0026]

According to the present invention, according to the present invention, a first single crystal layer made of a nitride semiconductor and having an in-plane average lattice constant a and a first single crystal layer formed thereon or directly via a surface protective layer are formed. Average lattice constant b (b>) made of a nitride semiconductor
a) a second single crystal layer having a layer thickness of 5 μm or more, and a strain relaxation region comprising a nitride semiconductor layer forming an element region formed above the strain relaxation region. A nitride semiconductor device is provided.

The in-plane average lattice constant refers to an average value of lattice constants in a plane perpendicular to the thickness direction of the first or second single crystal layer. For example, A grown on sapphire c-plane
When the 1GaN layer is the first single crystal layer, the lattice constant of the a-axis becomes the in-plane average lattice constant. When the composition of the first single crystal layer changes in the layer thickness direction, the average value of the lattice constant means a value obtained by averaging the in-plane average lattice constant in the layer thickness direction.

According to the present invention, Al_xGa_1-xN (0
<X <1), and a first single crystal layer directly thereon,
Or a layer having a thickness of 5 μm or more formed via a surface protective layer
Al _yGa_1-ySecond single crystal layer made of N (0 <y <x)
And a strain relaxation region consisting of:
Portion, a nitride semiconductor layer constituting an element region was formed
A nitride semiconductor device is provided.

As described in the section of the prior art, as a method of growing an AlGaN layer on a substrate, a low-temperature buffer layer is formed on a substrate, and then an AlGaN layer is formed thereon via a GaN layer.
A method of forming a GaN layer and a method of forming an AlGaN layer directly on a low-temperature buffer layer have been used. In these methods, tensile strain occurs in the AlGaN layer due to the lattice constant, and cracks are likely to occur. According to the method described above, AlG directly formed on the low-temperature buffer layer is used.
Many defects occur in the aN layer, which are inherited by the crystal structure formed thereon, making it difficult to obtain a high-quality crystal growth layer.

The nitride semiconductor device of the present invention solves such a problem by providing a strain relaxation region.

The strain relaxation region in the nitride semiconductor device of the present invention has a structure in which a second single crystal layer having a larger in-plane average lattice constant is laminated on the first single crystal layer. I have. The second single crystal layer has a thickness of 5 μm or more. Therefore, the following operation and effect can be obtained.

As a first effect, since the first single crystal layer functions as a buffer layer, the distortion of the nitride semiconductor layer forming the element region formed on the second single crystal layer can be significantly reduced. .

In a conventional semiconductor laser, a cladding layer, an active layer, and the like constituting an LD structure are subjected to a binding force by a sapphire substrate crystal, and strain is accumulated according to a difference between a lattice constant and a thermal expansion coefficient from the substrate crystal. Is done. This is because the thickness of the sapphire substrate is large, and each semiconductor layer on the substrate is
This is due to being formed so as to follow the crystal structure of the substrate. Here, since the lattice mismatch between the sapphire substrate and the nitride semiconductor is as large as about 14%, the intrinsic strain in the nitride semiconductor has a large value. On the other hand, in the nitride semiconductor device of the present invention, the nitride semiconductor layer constituting the device region is not mainly a substrate, but is mainly subjected to the binding force of the second single crystal layer, and the semiconductor layer constituting the device region has Strain corresponding to the difference between the lattice constant and the coefficient of thermal expansion with the second single crystal layer is accumulated. This is because the first single crystal layer can contain many crystal defects, and misfit dislocations are generated at the interface with the first and second single crystal layers, thereby forming an element region. This is because the binding force that the nitride semiconductor layer receives from the substrate is weakened. Therefore, if the second single crystal layer is appropriately selected so that the lattice constant difference from the nitride semiconductor layer becomes small, the intrinsic strain in the nitride semiconductor can be effectively reduced.

As a second effect, misfit dislocations are generated at the interface between the first and second single crystal layers, so that defects inherent in the first single crystal layer propagate to the second single crystal layer. Can be prevented. Misfit dislocations occur along the interface at the interface due to lattice mismatch.
The misfit dislocation serves to block the progress of the dislocation propagating upward from the first single crystal layer. For this reason, even if the first single crystal layer is configured to include many crystal defects in order to release strain energy, the propagation of the crystal defects to the second single crystal layer is effectively prevented.

As a third effect, the second single crystal layer has a thickness of 5 μm.
Since the film is formed with a thickness of at least m, dislocations disappear in the second single crystal layer, and the surface dislocation density can be reduced. As described above, the propagation of dislocations from the first single crystal layer is blocked by misfit dislocations,
Some dislocations propagate to the second single crystal layer as they are. In order to eliminate the dislocation, it is effective to increase the thickness of the second single crystal layer. Dislocations collide with each other in the thick second single crystal layer and disappear, and the direction of propagation of the dislocation changes in a direction parallel to the substrate, so that dislocations penetrating the second single crystal layer surface are greatly reduced. Because it is done. This effect becomes even more remarkable when the layer growth method adopts FIELO, which performs layer growth while forming a facet structure. As described above, in the present invention, it is important that the thickness of the second single crystal layer is a certain value or more, and it is 5 µm or more, preferably 30 µm or more, more preferably 100 µm or more.

As a fourth effect, the distortion of the second single crystal layer becomes a compression mode, and the crystal defects of the second single crystal layer can be greatly reduced. Therefore, when a semiconductor layer forming an element region is formed thereon, a semiconductor layer with few crystal defects can be obtained, and a high-quality light-emitting element and electronic element which have not been achieved in the past can be realized. In addition, by applying compressive strain, the thickness of the second single crystal layer can be increased without generating cracks, and as described above, dislocations penetrating into the surface of the second single crystal layer are significantly reduced. it can.

Here, regarding the technique for forming an AlGaN layer having a low Al composition on an AlGaN layer having a high Al composition,
As described in the section of the prior art, JP-A-9-83016
There is a disclosure in Japanese Patent Publication No. Hereinafter, in order to clarify the features of the present invention, differences from this conventional technique will be described.

The technique described in the above publication is based on the SiC substrate and the Ga
By disposing a composition gradient layer between the N layer and the lattice constant and changing the lattice constant substantially continuously, the strain generated due to the difference between the two lattice constants is dispersed, and the strain concentration portion is eliminated. It improves the crystallinity. For such a purpose, a composition gradient layer is a layer whose composition continuously changes,
Alternatively, it has a multilayer structure in which thin films having a thickness of about 0.2 μm are stacked. By doing so, the occurrence of the strain concentration portion is prevented. On the other hand, the present invention intentionally introduces a strain concentration portion between the first layer and the second layer, and causes a misfit dislocation therein. This misfit dislocation has a function of suppressing dislocation propagation from the first layer to the second layer and releasing strain energy. In order to introduce such misfit dislocations, the thickness of the second layer is set to 5 μm or more. If it is less than 5 μm, it is difficult to generate misfit dislocations to such an extent that the dislocation propagation suppressing function and the strain energy releasing function are exhibited. Further, when the thickness of the second layer is 5 μm or more, dislocations collide with each other in the second single crystal layer and disappear, or the direction of propagation of dislocations changes in a direction parallel to the substrate. The effect of significantly reducing dislocations penetrating into the surface of the single crystal layer can also be obtained.

As described above, in the technology described in the above publication,
The strain energy generated between the substrate and the nitride semiconductor layer is dispersed in the composition gradient layer. On the other hand, the present invention
The strain is released by the crystal defects and misfit dislocations in the first layer, and the strain energy itself is reduced, so that the crystal defects can be more effectively reduced.

In the nitride semiconductor device of the present invention, when the thickness of the first single crystal layer is d ₁ (μm) and the thickness of the second single crystal layer is d ₂ (μm), the following formula (1) It is preferable to adopt a configuration satisfying (xy) × d ₁ × d ₂ > 0.1 (1). By doing so, misfit dislocations can be reliably generated at the interface between the first and second single crystal layers, and the above-described stress relaxation function becomes more remarkable.

In the nitride semiconductor device of the present invention, the first and second single crystal layers are made of AlGaN,
When the element region is configured to include the AlGaN cladding layer and the active layer, the Al composition and the film thickness of the cladding layer can be increased, and an excellent light confinement effect can be obtained.
In a conventional nitride semiconductor laser, when an LD structure (laser structure) including a cladding layer and an active layer is formed, a low-temperature buffer layer is formed on a heterogeneous substrate, and a low-temperature buffer layer is formed directly on the low-temperature buffer layer, or a submicron to 3 μm layer. An LD structure was formed via a thick GaN layer or AlGaN layer. In such a structure, the LD structure is constrained by the crystal structure of the heterogeneous substrate, and many crystal defects occur due to the difference between the lattice constant and the coefficient of thermal expansion between the two. On the other hand, in the nitride semiconductor device of the present invention, the second single crystal layer having a thickness of 5 μm or more and having a small surface defect density is disposed below the LD structure, and a buffer layer is provided below the second single crystal layer. A first single crystal layer that functions as a layer is formed. For this reason, a layer below the first single crystal layer (for example, a heterogeneous substrate) has a weaker binding force on the LD structure, and the degree of restraint by the second single crystal layer is increased. Here, since the second single crystal layer is made of AlGaN similarly to the cladding layer, the difference in lattice constant between the two is small, and the distortion of the cladding layer is greatly reduced. Further, since the defect density of the second single crystal layer is low, defects that propagate to a cladding layer or the like located above the second single crystal layer are reduced, and from this point, an effect of improving the quality of the LD structure is obtained.

Further, according to the present invention, Al _x G
Step of forming a first single crystal layer made of a _1-x N (0 <x <1), and step of forming a mask having a plurality of openings thereon directly or via a surface protective layer A second single crystal layer having a layer thickness of 5 μm or more is formed by vapor-phase growing an Al _y Ga _1-y N (0 <y <x) crystal while forming a facet structure using the plurality of openings as growth regions. And a step of forming a nitride semiconductor crystal.

The “facet structure” in this crystal growth method refers to a structure having a flat crystal growth surface at a certain angle with respect to the substrate (see “Applied Physics”, Vol.
7, 1999, pp. 774-779)). While forming such a facet structure, Al _y Ga _1-y N
In order to grow the crystal in vapor phase, the Al _y Ga _1-y N crystal grown from the adjacent opening unites and the propagation direction of the transition changes in the direction parallel to the substrate, and the second layer having a low surface transition density A single crystal layer is obtained.

According to the present invention, Al _x Ga is formed on the substrate.
Forming a first single crystal layer made of _1-xN (0 <x <1), and directly or via a surface protective layer on the first single crystal layer;
Forming a seed crystal layer made of Al _y Ga _1-y N (0 <y <x), forming a mask having a plurality of stripe-shaped openings on the seed crystal layer, and performing etching; forming a stripe-shaped Al _y Ga _1-y N layer, Al _y Ga _1-y N the stripe Al _y Ga _1-y N layer as a starting point
(0 <y <x) a step of growing a crystal in a vapor phase to form a second single crystal layer made of Al _y Ga _1-y N having a layer thickness of 5 μm or more. A method for producing a crystal is provided.

According to these nitride semiconductor crystal manufacturing methods, a thick AlGaN layer having a low surface defect density can be formed. Specifically, an AlGaN layer having an average crystal defect density of 1 × 10 ⁷ cm ⁻² or less over the entire element formation surface can be formed. Further, in the present invention, the layer thickness is set to 5 μm or more in order to lower the crystal defect density on the element formation surface of the second single crystal layer, but according to this method, the thickness is set to 30 μm or more, and further to 100 μm or more. Can maintain a low defect density.

Further, according to the present invention, there is provided a nitride semiconductor substrate including an AlGaN layer having a thickness of 40 μm or more, and having an average crystal defect density of 1 × 10 ⁷ cm ⁻² or less over the entire element formation surface. Is provided.

Since the nitride semiconductor substrate has a low average crystal defect density on the entire element formation surface and includes an AlGaN layer having a thickness of 40 μm or more, crystal defects of the nitride semiconductor layer grown thereon are greatly reduced. In addition, the strain energy inherent in these nitride semiconductor layers can be effectively reduced. For this reason, it is possible to manufacture an unprecedented high-performance nitride semiconductor device.

In particular, when the nitride semiconductor substrate of the present invention is applied to a semiconductor laser or the like, an AlGa film having a high Al composition and a large thickness is used.
N-cladding layer can be manufactured without cracks, and the threshold of semiconductor laser is further reduced to extend its life, to achieve higher output, to shape laser beam spot shape, and to improve temperature characteristics. Can be. In addition, when applied to an electronic element, improvement in high-frequency characteristics and suppression of electron leakage to the substrate can be achieved, and device performance can be significantly improved.

The nitride semiconductor substrate of the present invention has an element formation surface
The average crystal defect density of the whole is 1 × 10 ⁷cm^-2Below
You. The average crystal defect density is measured for the entire element formation surface.
This is the average value of the determined crystal defect densities. A crystal defect is a seed
It contains various modes of dislocations and other defects. EL
In the method using O, one of the regions where the mask is provided is used.
In the region of the portion, crystal defects are extremely reduced. Only
However, there are many crystal defects near the mask opening.
In the present invention, the "average of the entire element formation surface
The “crystal defect density” has a relatively large value.

As a specific means for realizing an AlGaN substrate having an average crystal defect density of 1 × 10 ⁷ cm ⁻² or less as described above, for example, Al _x Ga ₁ -xN (0 <x
<1) a first single crystal layer, and an Al _y having a layer thickness of 5 μm or more formed thereon or directly or via a surface protective layer.
A second single crystal layer made of Ga _1-y N (0 <y <x);
Forming a strain relaxation region composed of The function of the strain relaxation region is as described above. In this case, it is preferable that the thickness of the second single crystal layer be 30 μm or more. Further, the thickness of the first single crystal layer is d
₁ (μm) and the thickness of the second single crystal layer is d ₂ (μm), and the following formula (1) is satisfied: (x−y) × d ₁ × d ₂ > 0.1 (1) It is preferable that By doing so, the strain relaxation effect becomes more remarkable, and the surface defect density can be reduced more reliably.

[0051]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, when the first and second single crystal layers are AlGaN layers, in order to make the function of the first single crystal layer as a buffer layer more remarkable, the following is required. Such a configuration is preferable.

For example, it is effective to adopt a configuration in which the first single crystal layer is formed on a low-temperature buffer layer. The low-temperature buffer layer is a nitride semiconductor layer grown at 400 to 700 ° C., and includes, for example, a GaN layer and an AlN layer. With this configuration, AlGaN
A large number of crystal defects are contained in the first single crystal layer made of, and as a result, the function of the first single crystal layer as a buffer layer is further improved.

When the Al composition of the first single crystal layer is preferably at least 0.04, more preferably at least 0.07, many crystal defects are formed in the first single crystal layer made of AlGaN. As a result, the function of the first single crystal layer as a buffer layer is further improved. Such a layer having a high Al composition tends to include many defects during growth, and in particular, when the underlying layer is a low-temperature buffer layer, the generation of defects becomes remarkable.

It is also effective to make the thickness of the first single crystal not less than a certain value. For example, preferably 0.3 μm
The thickness is more preferably 0.7 μm or more. AlGa
When the first single crystal layer made of N is formed with such a thick film, many defects are likely to be included during the growth, and particularly when the underlying layer is a low-temperature buffer layer, the generation of defects becomes remarkable.

With the above configuration, the function as a buffer layer becomes more prominent, and the binding force of the nitride semiconductor layer constituting the element region received from a heterogeneous substrate or the like is weakened. Intrinsic distortion can be further reduced.

In the present invention, the second single crystal layer is formed of a FIE
It is preferably formed by LO or pendio epitaxy.

As described above, FIELO is a method in which a mask having a plurality of openings is formed, and an Al _y Ga _1-y N crystal is vapor-phase grown while forming a facet structure using the openings as growth regions. . The Al _y Ga _1-y N crystal grown from the adjacent opening unites, and the direction of propagation of the transition changes in a direction parallel to the substrate, so that a second single crystal layer having a low surface transition density is obtained. When a second single crystal layer having a film thickness of 50 μm or more is grown by using FIELO, the extreme distribution of dislocations observed in the ELO growth between the mask opening and the mask disappears, and the entire surface is covered with the entire surface. Dislocations are distributed uniformly,
Moreover, a crystal having a low dislocation density can be obtained. The simplest method of measuring the distribution of dislocation density is to selectively etch the parts where dislocations protrude from the surface with a chemical solution and measure the density of the formed pits (this is called etch pits) with an optical microscope or Counting using a scanning electron microscope. Thereby, the dislocation density and the distribution can be easily measured.

In addition to the observation of the etch pits, the half-width of the X-ray rocking curve by the double crystal method can be used for evaluating the crystallinity. This half-value width is as wide as about 5 to 6 minutes for an AlGaN crystal grown on a sapphire substrate by A using GaN as a buffer layer by a usual method. This half width is often used for evaluation of crystallinity because it is reduced by improving crystallinity, particularly by reducing dislocations.

With respect to pendioepitaxy, see the document “Tsvetankas, Zheleva, et.a.
l. , MRS Internet J .; Nitride
Semicond. Res. 4S1, G3, 38 (1
999) ". When this method is applied to the formation of the layer structure according to the present invention, for example, the following steps are performed. First, Al _x Ga ₁ -xN (0 <x <
A first single crystal layer consisting of 1) is formed. Then forming a seed crystal layer made of _{Al y Ga 1-y N (} 0 <y <x) thereon. After forming a mask having a plurality of striped openings on the seed crystal layer, etching is performed to form a striped Al _y Ga _1-y N layer. Using this as a starting point, an Al _y Ga _1-y N (0 <y <x) crystal is vapor-phase grown to form a second single crystal layer of Al _y Ga _1-y N having a thickness of 5 μm or more. .

Since it is important for the second single crystal layer in the present invention to have a structure that does not cause new crystal defects in the layer, it is preferable that the layer does not have a composition discontinuity plane in the layer. That is, it is preferable that the second single crystal layer has a substantially single composition or a composition whose composition changes continuously. It is not preferable that an extreme composition discontinuity plane in which the Al composition ratio difference is 0.1 or more is formed in the second single crystal layer. This is because the introduction of dislocations increases and crystallinity deteriorates. When the second single crystal layer is made of AlGaN and the composition is continuously changed in the layer, the difference in Al composition between the lower surface and the upper surface of the second single crystal layer is, for example, It is preferred to be 0.05 or less.

In the present invention, it is preferable to form a discontinuous plane having a lattice constant at the interface between the first single crystal layer and the second single crystal layer. For example, it is preferable to form a discontinuous plane of the Al composition at the interface between the first single crystal layer and the second single crystal layer. By doing so, the crystal defects of the second AlGaN layer can be more stably reduced. The Al composition difference depends on the thickness of the first and second single crystal layers, but is, for example, 0.0
It is preferably at least 2, more preferably at least 0.04.

In the present invention, the first single crystal layer is made of AlG
In the case of using aN, a surface protective layer having a small thickness may be provided on the surface. In this manner, when growing the second single crystal layer by forming a mask or the like, the surface oxidation of the first single crystal layer can be prevented. GaN as surface protection layer
A layer or the like is used, and the thickness is, for example, 0.3 μm or less, preferably 0.2 μm or less. In order to prevent such surface oxidation, it is also effective to make the surface Al composition of the first single crystal layer lower than the average Al composition of the entire first single crystal layer. In the case where the present invention is applied to a semiconductor laser, if the GaN layer is used as a contact layer, there is an advantage that the contact resistance can be reduced.

In the present invention, the in-plane average lattice constant of the nitride semiconductor layer forming the element region is preferably substantially equal to or larger than the in-plane average lattice constant of the first single crystal layer. By doing so, the strain in the nitride semiconductor layer becomes close to zero or becomes a compression mode strain, so that cracks hardly occur. As a result, the element performance is improved, for example, the element life can be greatly improved.

Next, a preferred embodiment of the present invention will be described.
This will be described with reference to FIG.

FIG. 1 is a sectional view of a nitride semiconductor substrate according to the present invention. A sapphire c-plane ((0001) plane) is used as a substrate for crystal growth, and an AlGaN layer is grown thereon. A method for manufacturing this substrate will be described. First, on a sapphire substrate 11 at 500.degree.
A low-temperature buffer layer 12 made of GaN having a thickness of 0 nm is formed. Thereafter, the temperature was raised to 1070 ° C., and Al _0.07 Ga containing Al 7%
After growing an AlGaN layer 13 of _0.93 N for about 1 m,
_An AlGaN layer 14 of _0.05 Ga _0.95 N is grown to 100 μm.
Thereby, a high quality AlGaN layer 14 without cracks can be obtained.

In the AlGaN layer 13 (first single crystal layer) made of Al _0.07 Ga _0.93 N, the lattice mismatch with sapphire is alleviated, and an AlGaN layer with almost no distortion is grown. Relatively high and high defect density. For this reason, it has been confirmed that cracks occur when this layer is grown to a thickness of 1 μm or more.

Therefore, in this embodiment, an AlGaN layer 14 (second single crystal layer) having a low Al concentration is grown thereon. 100 μm without cracks
The above thick film growth is possible. Compressive strain is applied to the first AlGaN layer and the second AlGaN layer, and it becomes possible to grow a thick film crystal without generating cracks.

By forming a nitride semiconductor layer constituting an element region on this AlGaN layer 14, the nitride semiconductor element of the present invention can be manufactured. In this nitride semiconductor device, the crystal of the nitride semiconductor layer constituting the device region is mainly subjected to the binding force not by the sapphire substrate 11 but by the AlGaN layer 14, and the difference between the lattice constant and the thermal expansion coefficient of the AlGaN layer 14 is reduced. The corresponding distortion is accumulated. This is because the AlGaN layer 13 (first single crystal layer) has a structure including many crystal defects, and
This is because the generation of misfit dislocations at the interface between the AlGaN layer 13 and the AlGaN layer 14 reduces the binding force of the substrate in the AlGaN layer 13.

The reason why the AlGaN layer 13 contains many crystal defects is due to the difference in lattice constant between the AlGaN layer 13 and the low-temperature buffer layer 12 made of GaN.
The occurrence of misfit dislocation at the interface of the layer 14 is due to the difference in lattice constant between the two layers and the fact that the thickness of both layers is equal to or more than a certain value.

The nitride semiconductor substrate according to this embodiment is made of Al
The Al composition of the GaN layer 13 (first single crystal layer) is x, the thickness is d ₁ (μm), and the A composition of the AlGaN layer 14 (second single crystal layer) is A
When the composition is y and the thickness is d ₂ (μm), (xy) × d ₁ × d ₂ = 2 (x = 0.07, y = 0.05, d ₁ = 1 (μm) , D ₂
= 100 (μm)). Therefore, misfit dislocations can be generated at the interface between the first single crystal layer and the second single crystal layer to such an extent that the dislocation propagation suppressing function and the strain energy releasing function are exhibited.

[0071]

Example 1 In this example, a nitride semiconductor substrate having the structure shown in FIG. 1 was manufactured. As the substrate crystal, (00
Metalorganic chemical vapor deposition (MOVPE) using an organic metal as a group III raw material on a sapphire substrate
A hydride VPE method using gallium chloride (GaCl) (HVP
An AlGaN layer was formed by E).

The sapphire substrate 11 ((0001) plane) is
Set in the VPE device and supply H ₂ gas and N ₂ gas while 108
The temperature is raised to 0 ° C. and the surface is heat-treated. Then 500 ℃
Temperature, and trimethylgallium (TMG) and ammonia (NH ₃ ) gas were respectively 10 μmol / min and 5000 cc / min.
Low-temperature buffer layer 1 made of GaN and supplied with a thickness of 40 nm
Form 2

Next, the temperature of the above crystal is raised to 1050 ° C. while supplying NH ₃ gas and H ₂ gas. After the temperature stabilizes, trimethylaluminum (TMA) and TMG
μmol / min, supplied at 80 μmol / min, and 1 μm thick Al _0.1 Ga
_A first AlGaN 13 made of _0.9 N is formed. Next, TMA,
And, the supply of TMG is stopped and NH ₃ gas, H ₂ gas and
The temperature is lowered while supplying N ₂ gas.

Next, the above crystals were subjected to hydrogen chloride (HCl) / gallium (Ga), aluminum chloride (AlCl ₃ ), ammonia (N
H ₃ ) and hydrogen (H ₂ ) as raw materials. The temperature is raised in an H ₂ carrier gas, and NH ₃ gas is supplied at around 600 ° C. to raise the temperature to 1070 ° C. The H ₂ gas and NH ₃ gas flow rates are 4000 cc / min and 1000 cc / min, respectively. After the temperature stabilizes, HCl is added on Ga at 20 cc /
min, and further, AlCl ₃ is volatilized as a raw material of Al and supplied at 1 cc / min to grow the second AlGaN layer 14. An AlGaN layer having a thickness of 300 μm grows after 5 hours of growth. The Al composition was 0.04 from the X-ray diffraction measurement. After growth, the etch pits corresponding to the defects were examined using a mixture of phosphoric acid and sulfuric acid at 240 ° C., and a low value of 7 × 10 ⁶ / cm ² was obtained. The distribution of the etch pits was uniform over the entire surface. No crack was observed on the surface of the second AlGaN layer grown by this method.

In the present embodiment, the first AlGa
0.1 as Al composition of N layer, as Al composition of second AlGaN layer
Although an example of 0.04 is shown, similar effects can be obtained if the Al composition of the second AlGaN layer is smaller than the Al composition of the first AlGaN layer. Further, the same effect can be obtained by adding an n-type or p-type impurity.

Example 2 In this example, a nitride semiconductor substrate having the structure shown in FIG. 2 was manufactured. Metal organic chemical vapor deposition (MOVPE) using an organic metal as a group III raw material and hydride VPE using gallium chloride (GaCl) using a sapphire substrate as a substrate crystal as in Example 1.
An AlGaN layer was formed by a method (HVPE). In this example, the second AlGaN layer was formed by lateral overgrowth.

The sapphire substrate (0001) surface 11 is
Set to the device and supply H ₂ gas and N ₂ gas at 1080 ° C
And heat-treat the surface. Thereafter, the temperature was lowered to 500 ° C., and trimethylgallium (TMG) and ammonia (NH ₃ ) gas were supplied at 10 μmol / min and 5000 cc / min, respectively, to form a low-temperature buffer layer 12 of GaN having a thickness of 40 nm.
To form

Next, the temperature of the above crystal is raised to 1050 ° C. while supplying NH ₃ gas and H ₂ gas. After the temperature stabilizes, trimethylaluminum (TMA) and TMG
The first AlGaN layer 13 is supplied at a rate of 80 μmol / min and 80 μmol / min, and is formed of Al _0.1 Ga _0.9 N and has a thickness of 1 μm. Next, TM
The supply of A and TMG is stopped, and the temperature is lowered while supplying NH ₃ gas, H ₂ gas and N ₂ gas.

The grown crystal is taken out of the apparatus and
On the AlGaN layer 13, a 0.5 μm thick SiO _TwoForm a film and
Stratification by photolithography and wet etching
An ip-shaped mask 21 is formed. Mask width and aperture
The part widths are 2 μm and 1 μm, respectively.

The above crystals were treated with hydrogen chloride (HCl) / gallium (G
a), aluminum chloride (AlCl ₃ ), ammonia (NH ₃ ),
It is set in an HVPE apparatus using hydrogen (H ₂ ) as a raw material. The temperature is raised in an H ₂ carrier gas, and NH ₃ gas is supplied at around 600 ° C. to raise the temperature of the crystal to 1070 ° C. The H ₂ gas and NH ₃ gas flow rates are 4000 cc / min and 1000 cc / min, respectively. After the temperature stabilizes, HCl is added on Ga at 20 cc /
min, and further, AlCl ₃ is volatilized as a raw material of Al and supplied at 1 cc / min to grow the second AlGaN layer 22. In this growth, the growth starts from the opening and
An AlGaN facet is formed. With growing time,
Lateral growth proceeds on the mask 21, and the AlGaN facets grown from the adjacent openings unite. Thereafter, an AlGaN layer 22 having a flat surface is formed. After 5 hours of growth, an AlGaN layer 22 having a thickness of 250 μm was grown. The Al composition was found to be 0.04 from X-ray diffraction measurement and photoluminescence peak measurement on the surface. After growth, 24
When the etch pit corresponding to the defect was examined using a mixed solution of phosphoric acid and sulfuric acid at 0 ° C., a low value of 5 × 10 ⁶ / cm ² was obtained.
The distribution of the etch pits was uniform over the entire surface, and no distribution due to the mask pattern observed in conventional ELO growth was observed. No crack was observed on the surface of the second AlGaN layer grown by this method.

In the present embodiment, the first AlGa
0.1 as Al composition of N layer, Al composition of first AlGaN layer
Although an example of 0.04 is shown, similar effects can be obtained if the Al composition of the second AlGaN layer is smaller than the Al composition of the first AlGaN layer. Further, the same effect can be obtained by adding an n-type or p-type impurity.

Example 3 In this example, a nitride semiconductor substrate having the structure shown in FIG. 3 was manufactured. As in Example 2,
Metal organic chemical vapor deposition (MOVPE) using organic metal as group III raw material and hydride VPE using gallium chloride (GaCl) using sapphire substrate as substrate crystal
An AlGaN layer was formed by a method (HVPE). In this embodiment, a thin GaN layer is formed on a first AlGaN layer, and a second AlGaN layer is formed thereon by lateral overgrowth.

The sapphire substrate 11 ((0001) plane) was
Set in the VPE device and supply H ₂ gas and N ₂ gas while 108
The temperature is raised to 0 ° C. and the surface is heat-treated. Then 500 ℃
Temperature, and trimethylgallium (TMG) and ammonia (NH ₃ ) gas were respectively 10 μmol / min and 5000 cc / min.
Low-temperature buffer layer 1 made of GaN and supplied with a thickness of 40 nm
Form 2

Next, the above crystal is heated to 1050 ° C. while supplying NH ₃ gas and H ₂ gas. After the temperature stabilizes, trimethylaluminum (TMA) and TMG
The first AlGaN layer 13 is supplied at a rate of 80 μmol / min and 80 μmol / min, and is formed of Al _0.1 Ga _0.9 N and has a thickness of 1 μm. After the completion of this growth, the supply of TMA was stopped, and only the GaN layer 31 was grown to a thickness of 50 nm. Thereafter, the supply of TMG is stopped, and the temperature is lowered while supplying NH ₃ gas, H ₂ gas and N ₂ gas.

The grown crystal is taken out of the apparatus and
On the AlGaN layer 13, a 0.5 μm thick SiO _TwoForm a film and
Stratification by photolithography and wet etching
An ip-shaped mask 21 is formed. Mask width and aperture
The part widths are 2 μm and 1 μm, respectively.

The above crystals were treated with hydrogen chloride (HCl) / gallium (G
a), aluminum chloride (AlCl ₃ ), ammonia (NH ₃ ),
It is set in an HVPE apparatus using hydrogen (H ₂ ) as a raw material. The temperature is raised in an H ₂ carrier gas, and NH ₃ gas is supplied at around 600 ° C. to raise the temperature of the crystal to 1070 ° C. The H ₂ gas and NH ₃ gas flow rates are 4000 cc / min and 1000 cc / min, respectively. After the temperature stabilizes, HCl is added on Ga at 20 cc /
min, and further, AlCl ₃ is volatilized as an Al material and supplied at 1 cc / min to grow the second AlGaN layer 32. In this growth, the growth starts from the opening and
An AlGaN facet is formed. With growing time,
Lateral growth proceeds on the mask 21, and the AlGaN facets grown from the adjacent openings unite. Thereafter, an AlGaN layer 32 having a flat surface is formed. After 5 hours of growth, an AlGaN layer 32 having a thickness of 250 μm was grown. The Al composition was found to be 0.04 from X-ray diffraction measurement and photoluminescence peak measurement on the surface.

After the growth, the etch pits corresponding to the defects were examined with a mixture of phosphoric acid and sulfuric acid at 240 ° C., and were found to be 1 × 10 ⁶ / c
A low value of m ² was obtained. The distribution of the etch pits was uniform over the entire surface, and no distribution due to the mask pattern observed in conventional ELO growth was observed. In addition, when the half width of the X-ray rocking curve by the double crystal method was examined to evaluate the crystallinity, it was found that the crystallinity was extremely narrow at 1.2 minutes and was extremely excellent in crystallinity. Also,
No cracks were observed on the surface of the second AlGaN layer grown by this method.

In the present embodiment, the first AlGa
0.1 as Al composition of N layer, Al composition of first AlGaN layer
Although an example of 0.04 is shown, similar effects can be obtained if the Al composition of the second AlGaN layer is smaller than the Al composition of the first AlGaN layer. Further, the same effect can be obtained by adding an n-type or p-type impurity.

<Embodiment 6> This embodiment shows an example in which an AlGaN layer is formed on a sapphire substrate, and then each semiconductor layer constituting a semiconductor laser is formed thereon.

FIG. 4 shows a sapphire substrate 11 ((0001) plane).
In the same manner as in Example 1, a first Al _0.1 Ga _0.9 N layer (thickness: 1 m) to which silicon (Si) is added as an n-type impurity is interposed via a low-temperature buffer layer 12 made of GaN. When,
Similarly, the second Al _0.04 Ga to which Si is added as an n-type impurity
A schematic cross-sectional view of a gallium nitride-based laser formed by forming a _0.96 N layer (thickness: 300 m) 82 and stacking a semiconductor layer on the substrate using metal organic chemical vapor deposition (MOVPE) It is.

In the GaN-based semiconductor laser structure, the substrate grown up to the second AlGaN layer shown in FIG. 4 is set in an MOCVD apparatus, and the temperature is raised to 1050 ° C. in a hydrogen atmosphere. The temperature is changed from 650 ° C. to an NH ₃ gas atmosphere. N-type having a thickness of 0.4m was added Si Al _{0. 15} Ga _0.85 N cladding layer 8
3. 0.1 μm thick n-type GaN optical guide layer 84 doped with Si, undoped In _0.2 Ga _0.8 N 2.5 nm thick
Quantum well layer and 5 nm thick undoped In _0.05 Ga _0.95 N
A three-period multi-quantum well structure active layer 85 composed of a barrier layer, a 20 nm-thick p-type Al doped with magnesium (Mg).
_0.2 Ga _0.8 N layer 86, Mg-added 0.1 μm thick p
-Type GaN optical guide layer 87, a 0.4 μm thick p-type Al _0.15 Ga _0.85 N cladding layer 88 with Mg added, and a 0.5 μm thick p-type GaN contact layer 89 with Mg added sequentially. A laser structure was fabricated. p-type GaN contact layer
After the formation of 89, it is cooled to room temperature in a HN ₃ gas atmosphere and taken out from the growth apparatus. A multiple quantum well structure active layer 85 comprising an undoped In _0.2 Ga _0.8 N quantum well layer having a thickness of 2.5 nm and an undoped In _0.05 Ga _0.95 N barrier layer having a thickness of 5 nm was formed at a temperature of 780 ° C.

Next, the crystal having the laser structure formed is set in a polishing machine, and the sapphire substrate 11 is polished from the sapphire substrate 11 so as to include a part of the first AlGaN layer 81 and the second AlGaN layer 82 by about 50 μm. On the exposed AlGaN layer 82 surface, titanium (Ti)
An aluminum (Al) n-type electrode 92 is formed, and a SiO ₂ film 90 is formed on a p-type GaN layer 89 for current constriction, and a nickel (Ni) / gold (Au) p-type The electrode 91 was produced (FIG. 5).

Each of the semiconductor layers constituting the semiconductor laser having the above-mentioned structure had good crystallinity and few defects. In addition, the yield was good, the manufacturing stability was excellent, and continuous oscillation at room temperature was obtained at a threshold current density of 3 kA / cm2 and a threshold voltage of 5 V.

In this embodiment, after the laser structure is formed on the AlGaN layer 82, the first AlGaN
Although the layer 81 and a part of the second AlGaN layer 82 are polished, the first AlGaN layer 81 and a part of the second AlGaN layer 82 may be polished from the sapphire substrate 11 before manufacturing a laser structure. Similar effects can be obtained.

[0095]

As described above, the nitride semiconductor device of the present invention comprises a first single crystal layer having an in-plane average lattice constant a and an in-plane average lattice constant comprising a nitride semiconductor formed thereon. b (b> a) a second single crystal layer having a layer thickness of 5 μm or more;
Is provided, and a device region is provided thereon. For this reason, the strain of the nitride semiconductor layer forming the element region can be significantly reduced. Further, since the surface transition density of the second single crystal layer is low, an element region with few defects can be realized.

For this reason, the light emitting device has the advantage that the device life can be greatly improved. Further, when applied to a semiconductor laser, the Al composition of the cladding layer can be increased and the film thickness can be increased. The light confinement rate can be improved. Further, in an electronic element, improvement of high-frequency characteristics and suppression of electron leakage to a substrate are achieved.

Further, according to the method for manufacturing a nitride semiconductor crystal of the present invention, a strain relaxation region including the first and second single crystal layers can be suitably formed.

Further, the nitride semiconductor substrate of the present invention has a low surface defect density and a structure including at least a certain amount of AlGaN layer. Therefore, when an element region is formed on this substrate, distortion and defects are remarkable. , And it is possible to realize an element with a higher performance than ever before.

[Brief description of the drawings]

FIG. 1 is a sectional view of a nitride semiconductor substrate according to the present invention.

FIG. 2 is a sectional view of a nitride semiconductor substrate according to the present invention.

FIG. 3 is a sectional view of a nitride semiconductor substrate according to the present invention.

FIG. 4 is a sectional view of a nitride semiconductor laser according to the present invention.

FIG. 5 is a sectional view of a nitride semiconductor laser according to the present invention.

[Explanation of symbols]

Reference Signs List 11 sapphire substrate 12 low-temperature buffer layer 13 AlGaN layer 14 AlGaN layer 21 mask 22 AlGaN layer 23 Al _0.06 Ga _0.94 N cladding layer 31 GaN layer 32 AlGaN layer 81 Al _0.1 Ga _0.9 N layer 82 Al _0.04 Ga _0.96 N layer 83 n-type Al _0.15 Ga _0.85 N cladding layer 84 n-type GaN optical guiding layer 85 multiple quantum well structure active layer 86 p-type Al _0.2 Ga _0.8 N layer 87 p-type GaN optical guiding layer 88 p-type Al _0.15 Ga _0.85 N cladding layer 89 p-type GaN Contact layer 90 SiO ₂ film 91 p-type electrode 92 n-type electrode

Continued on the front page (72) Inventor Yoshinari Matsumoto 5-7-1 Shiba, Minato-ku, Tokyo F-term within NEC Corporation 4G077 AA03 BE11 DB08 EF03 5F045 AA04 AB14 AB17 AC03 AC08 AC12 AC13 AF09 BB12 CA09 DA53 DA57 DB02 5F073 AA45 AA55 AA74 CA07 CB07 CB10 CB22 DA04 DA05 EA28 EA29

Claims (12)

[Claims] An in-plane average lattice made of a nitride semiconductor formed directly or via a surface protective layer on a first single crystal layer made of a nitride semiconductor and having an in-plane average lattice constant a. A second single crystal layer having a constant b (b> a) and a layer thickness of 5 μm or more, and a strain relaxation region including a nitride semiconductor layer forming an element region above the strain relaxation region. A nitride semiconductor device characterized by the above-mentioned. 2. A first single crystal layer made of Al _x Ga ₁ -xN (0 <x <1), and an Al layer having a thickness of 5 μm or more formed thereon directly or via a surface protection layer. _y Ga _1-y N (0
A second single crystal layer made of <y <x) and a strain relaxation region made of a nitride semiconductor layer forming an element region formed above the strain relaxation region. Semiconductor device. 3. The nitride semiconductor device according to claim 2, wherein said device region includes an AlGaN cladding layer and an active layer. 4. The thickness of the first single crystal layer is d ₁ (μm),
When the thickness of the second single crystal layer is d ₂ (μm), the following formula (1) is satisfied: (x−y) × d ₁ × d ₂ > 0.1 (1) Item 4. The nitride semiconductor device according to item 2 or 3. 5. The nitride semiconductor device according to claim 1, wherein the thickness of the second single crystal layer is 30 μm or more. 6. An Al _x Ga ₁ -xN (0 <x <1) on a substrate.
Forming a first single crystal layer consisting of, and directly or through a surface protective layer, a step of forming a mask having a plurality of openings, and the plurality of openings as a growth region, Al _y Ga _1-y N while forming a facet structure
(0 <y <x) a vapor phase growth of a crystal to form a second single crystal layer having a layer thickness of 5 μm or more. 7. An Al _x Ga ₁ -xN (0 <x <1) on a substrate.
Forming a first single-crystal layer made of Al _y Ga _{1 -y} N (0 <
y <x), and after forming a mask having a plurality of striped openings on the seed crystal layer, etching is performed to form a striped Al _y Ga
Forming a _1-y N layer, and forming the striped Al _y Ga
Al _y Ga _1-y N (0 <y <x) crystal is vapor-phase grown from the _1-y N layer as a starting point, and a second single crystal layer made of Al _y Ga _1-y N having a layer thickness of 5 μm or more is formed. Forming a nitride semiconductor crystal. 8. An element having an average crystal defect density of 1 × 10 ⁷ c including an AlGaN layer having a thickness of 40 μm or more over the entire element formation surface.
A nitride semiconductor substrate having a size of m ⁻² or less. 9. A first single crystal layer made of Al _x Ga ₁ -xN (0 <x <1) and an Al layer having a thickness of 5 μm or more formed thereon directly or via a surface protective layer. _y Ga _1-y N (0
9. The nitride semiconductor substrate according to claim 8, comprising a second single crystal layer made of <y <x) and a strain relaxation region made of the second single crystal layer. 10. The nitride semiconductor substrate according to claim 9, wherein the thickness of the second single crystal layer is 30 μm or more. 11. The thickness of the first single crystal layer is d ₁ (μ
m), when the thickness of the second single crystal layer is d ₂ (μm), the following formula (1) is satisfied: (x−y) × d ₁ × d ₂ > 0.1 (1) The nitride semiconductor substrate according to claim 9 or 10, wherein 12. The nitride semiconductor substrate according to claim 9, wherein a surface opposite to the device formation surface is made of a nitride semiconductor.