JPH10256666A - Crystal growth method of nitride based compound semiconductor and semiconductor light emitting element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve flatness of the surface of a buffer layer, by forming a multilayered film structure of an amorphous buffer layer on a substrate, and epitaxially growing nitride based compound semiconductor crystal on the buffer layer surface. SOLUTION: An N-GaN contact layer 4, N-GaN 5, an undoped multiple quantum well active layer 11, and a P-GaN clad layer 7 are grown again in order on N-GaN buffer layers 2, 3. After an etching mask is formed on the P-GaN clad layer 7, a waveguide structure 12 is formed. The etching mask is used as a mask for selective growth, and an N-GaN current constriction layer 13 is selectively formed. The etching mask is eliminated, and buried growth is performed by the P-GaN contact layer 8. After etching is performed as far as a part of the N-GaN contact layer 4, and a P side electrode 9 and an N side electrode 10 are formed, a resonator is formed and made a chip. Finally a low reflection film 14 and a high reflection film 15 are formed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a method for manufacturing an Al, Ga, In, etc.
Semiconductor materials containing at least nitrogen (N) as a constituent element among III-V compound semiconductors comprising a group V element and a group V element such as N, P, As, Sb (hereinafter referred to as " In particular, the present invention relates to a method for growing a high-quality epitaxial layer suitable for manufacturing a semiconductor light emitting device and a method for manufacturing a semiconductor light emitting device using the same. The present invention also provides a semiconductor light emitting device (nitride-based compound) suitable for a light emitting diode using spontaneous emission light of any wavelength in a range from the ultraviolet region to the visible region, a semiconductor laser device structure using stimulated emission light, and the like. Semiconductor light-emitting device).

[0002]

2. Description of the Related Art A quaternary mixed crystal of (Al _x Ga _{1 -x} ) _{1 -y} In _y N nitride-based compound semiconductor composed of AlN and InN crystals centering on GaN is about 3.4 eV to 6. Since it has a band gap energy of 2 eV and is a direct transition type in all composition regions, it is promising as a next-generation short-wavelength light emitting device material. However, these nitride-based compound semiconductors do not have high-quality substrate crystals that are lattice-matched as compared to similar III-V compound semiconductors such as GaAs and InP.

[0003] Nitride-based compound semiconductors differ from zinc-blende (cubic) crystal structures such as GaAs and InP in that they have a wurtzite (hexagonal) crystal structure.
At present, in crystal growth of nitride-based compound semiconductors, sapphire crystals having a similar hexagonal structure are mainly used as substrates. However, there is a lattice mismatch of about 16% between the (0001) plane sapphire substrate and the GaN crystal. For this reason, even if a nitride-based compound semiconductor layer is grown on a sapphire substrate, it has been very difficult to obtain a high-quality epitaxial film.

On the other hand, a two-step method has been proposed in which a crystal serving as a buffer layer is formed on a substrate at a low growth temperature and then a nitride-based compound semiconductor is grown at a high temperature (S. Nakamu).
ra, Jpn. J. Appl. Phys., Vol. 30, pp. L1705 (1991
Year) and JP-A-4-297023). The latter publication discloses the above-mentioned buffer layer made of a nitride-based compound semiconductor.
It teaches that it must be formed at a growth temperature of 200 ° C. to 900 ° C., but also discloses 600 ° C. as a preferred growth temperature, and discloses that the buffer layer is formed in a polycrystalline state under these conditions. Based on such knowledge, after forming an amorphous or polycrystalline GaN buffer layer on a sapphire substrate at a low temperature of about 600 ° C., the temperature is raised to a high temperature of about 1000 ° C. and a nitride-based compound semiconductor is formed on the buffer layer. A method of growing a layer is employed.

[0005]

However, the technique for forming the buffer layer has the following problems. The GaN buffer layer formed at a low temperature has an amorphous structure, a polycrystalline structure, or a structure in which both are mixed (in other words, a structure other than a single crystal structure). At the stage of growing the nitride-based compound semiconductor layer on the buffer layer, the buffer layer is also heated to 900 ° C. or higher as described above, so that recrystallization starts in the layer. As described above, the lattice constant of the nitride-based compound semiconductor crystal is
Since the lattice constant is different from that of the crystal of the substrate, a large interstitial distortion is generated between the buffer layer and the substrate even when the buffer layer is single-crystallized. For this reason, the recrystallized buffer layer has a low orientation with respect to the substrate crystal, ie, is in an amorphous state (not a proper single crystal), and the flatness of the surface is significantly reduced.

[0006] The decrease in the surface flatness of the buffer layer lowers the density of crystal nuclei important for epitaxial growth and makes the crystal nuclei non-uniform in an initial step of growing a nitride-based compound semiconductor epitaxial layer on the surface. Further, the diffusion of the source species supplied for epitaxial growth on the surface of the buffer layer is inhibited. As a result, there is a problem that the orientation of the epitaxial layer is disturbed with respect to the crystal axis of the substrate, and the surface morphology becomes non-uniform. In addition, such deterioration of the surface state of the buffer layer becomes a serious problem even when a quantum well structure requiring steep interface flatness is manufactured.

The problem to be solved by the present invention is to eliminate the problem that the surface flatness of the buffer layer is deteriorated due to the recrystallization of the buffer layer, and to further improve the surface flatness. Its purpose is to realize epitaxial growth of a high-quality nitride-based compound semiconductor crystal.

[0008]

In order to solve the above-mentioned problems, the present invention provides an amorphous buffer layer which grows on a substrate at a low temperature in a multilayer structure, and improves the surface flatness of the buffer layer. Then, a good-quality nitride-based compound semiconductor crystal is epitaxially grown on the surface.

Here, "amorphous" in the description of the present invention
The definition of the term is made clear in advance. The region in the amorphous state in this description is not limited to a state where regularity is not found in the atomic arrangement like glass over the entire region,
For example, a state in which fine crystal grains (lumps of single crystals) exist in a region, and a state in which a region is formed as an aggregate of the crystal grains (a so-called polycrystalline state) are included. This definition is, for example, that a diffraction pattern of a region in a polycrystalline state by electron diffraction or the like is very similar to an amorphous structure when the size of crystal grains existing in the region is small and the orientation of each crystal axis is random. It is in view of the fact that it does. For example, in the following description, “a more amorphous state” is to be understood as “a state in which the average size of crystal grains is smaller (finer)”. Also, in the following description, as described above, Al, Ga, In, etc.
III- consisting of Group III elements and Group V elements such as N, P, As, Sb
Of the group V compound semiconductors, a semiconductor material containing at least a nitrogen (N) element as a constituent element is conveniently referred to as a “nitride-based compound semiconductor”.

Returning again to the description of the present invention. In the present invention, the means specifically taken are as follows. As a first means, a buffer layer formed on a substrate has a multilayer structure including at least first and second buffer layers. As a second means, the growth temperature T2 of the second buffer layer is set lower than the growth temperature T1 of the first buffer layer (T1> T2). Third, the buffer layer and the nitride-based compound semiconductor layer are composed of (Al _x Ga _1-x )
_1-y In _y N (where 0 ≦ x ≦ 1, 0 ≦ y ≦ 1).
As a fourth and fifth means, a light emitting diode or a laser diode element structure is constituted by using the nitride-based compound semiconductor layers formed on these buffer layers.

First, when the buffer layer, which is the first means, has a multilayer structure, the flatness can be improved as compared with the surface state of the conventional single-layer buffer layer. This can be achieved by performing the second means of growing the second buffer layer at a lower temperature than the first buffer layer. Since the first buffer layer has a high growth temperature, it has a strong orientation to the substrate crystal, and its surface state is deteriorated by reflecting a large lattice mismatch with the substrate. On the other hand, the orientation of the second buffer layer can be weakened by setting the growth temperature lower than that of the first buffer layer. That is, the crystallinity of the second buffer layer is reduced to make the second buffer layer more amorphous. Such a second buffer layer has an effect of improving the surface condition of the first buffer layer having low surface flatness. As a result, the crystal nucleus density in the buffer layer after the temperature rise can be increased and uniform, and a high-quality nitride-based compound semiconductor layer can be epitaxially grown. This uniform nucleation of crystal nuclei
There is also an effect of reducing the density of crystal defects as compared with the prior art.

In contrast to the above, when the growth temperature of the second buffer layer is higher than that of the first buffer layer, the orientation on the surface of the buffer layer is higher than the interface between the substrate and the buffer layer. Becomes stronger. That is, even if the buffer layer has a multilayer structure, the surface state is deteriorated, and the formation of crystal nuclei becomes uneven. As a result, a high-quality nitride-based compound semiconductor layer cannot be epitaxially grown.

With respect to the grounds of the present invention described above, a specific description will be given below with consideration of a phenomenon relating to a manufacturing process of a semiconductor optical device from a limited aspect by the inventor.

First, the "dislocation problem" recognized by the present inventors.
Is described. 200 ° C to 900 ° C on a crystal substrate (single crystal substrate or single crystal film epitaxially grown on it)
The nitride-based compound semiconductor film (buffer layer) formed at a growth temperature in the range of ° C. is formed in an amorphous state. The average size (eg, particle size) of the crystal grains contained in the amorphous semiconductor film increases as the growth temperature increases. And, as the crystal grains become larger, the time required for forming a single crystal film by the recrystallization step becomes shorter. In the recrystallization step, the amorphous semiconductor layer is considered to be single-crystallized by the above-mentioned crystal grains being restricted by the orientation of the crystal axis of the crystal base on which the amorphous semiconductor layer is formed, and by combining the crystal grains. However, if there is a gap between the lattice constant of the crystal substrate and the “original lattice constant” of the material constituting the buffer layer when the material constituting the buffer layer is crystallized, many dislocations (misfit dislocations) are present in the single crystallized buffer layer. Etc.) occur. Dislocations start from single crystallization in a region in contact with the interface between the crystal substrate and the buffer layer, and extend from the interface toward the upper surface of the buffer layer as the single crystallization progresses toward the upper surface of the buffer layer. Therefore, in order to prevent dislocation of the nitride-based compound semiconductor crystal grown on the upper surface of the buffer layer, it is important to start crystal growth of the nitride-based compound semiconductor before the dislocation reaches the upper surface of the buffer layer.

Next, the "problem of interface stability" recognized by the present inventors will be described. When lowering the growth temperature of the buffer layer made of a nitride-based compound semiconductor, there is a problem whether the buffer layer is formed stably on the crystal substrate. Although this problem has been pointed out in the above-mentioned publication, the present inventor further requires a growth temperature of 200 ° C.
Investigations were also made under the above conditions. That is, a material having a hexagonal crystal structure such as sapphire (Al ₂ O ₃ ), zinc oxide (ZnO), or silicon carbide (SiC) is currently used as a crystal substrate used for crystal growth of a nitride-based compound semiconductor. Has been adopted. As is apparent from these constituent elements and compositions, the crystal substrate and the nitride-based compound semiconductor are different materials. Therefore, as compared with the case where the nitride-based compound semiconductors are heterojunction, it is necessary to pay more attention to the stability of bonding between the constituent atoms of the crystal substrate and the constituent atoms of the nitride-based compound semiconductor at the interface. The inventor has noted that the growth temperature of the buffer layer is one of the parameters to be considered,
It was also recognized that the robustness (hardness) of the amorphous layer depending on the size of the crystal grains was one of the parameters to be considered.

Furthermore, the "problem of the upper surface of the buffer layer" recognized by the present inventors will be described. The inventor has found that when the crystal grains contained in the buffer layer have a certain size, the irregularities formed on the upper surface of the buffer layer adversely affect the growth of the nitride-based compound semiconductor crystal on the upper surface. . That is, since the atoms of the group III element and the group V element supplied to the upper surface of the buffer layer to form the nitride-based compound semiconductor crystal are more likely to be localized in the concave portions, the crystal nuclei formed by these atoms (nitride Of the compound-based compound semiconductor) is not uniformly distributed on the upper surface of the buffer layer, and as a result, the crystal growth becomes non-uniform.
Under the crystal growth conditions of a nitride-based compound semiconductor requiring a high temperature of 900 ° C. or more, the progress of single crystallization with the crystal grains of the buffer layer as nuclei further promotes the unevenness on the upper surface of the buffer layer, and thus the nitride-based compound semiconductor The problem of non-uniform crystal growth is exacerbated.

In consideration of the above problems, the present inventor has proposed that a buffer layer be formed on a crystal substrate by forming at least two amorphous nitride compound semiconductor layers having different average sizes of crystal grains (however, (Formation before the recrystallization step).
Then, the average size of the crystal grains included in the first buffer layer formed on the crystal substrate is equal to the average size of the crystal grains included in the second buffer layer formed on the first buffer layer. It has been found that when the condition larger than the size is satisfied, the following action appears.

First, as a first effect, when the size of crystal grains of the second buffer layer is appropriately suppressed, the degree of unevenness on the upper surface of the second buffer layer is considerably reduced, and the nitride-based compound semiconductor on the upper surface is reduced. It has been found that crystal growth occurs substantially uniformly. This phenomenon was observed even when another buffer layer was provided on the second buffer layer. this is,
Since the crystal grains contained in the second buffer layer are sufficiently small,
The concave portion on the upper surface of the first buffer layer is buried to form a smooth surface, and even during heating, since the progress of single crystallization in the second buffer layer is slow, the unevenness on the upper surface of the first buffer layer is increased. This is probably due to sufficient absorption.

As a second effect, it has been found that dislocation of a nitride-based compound semiconductor crystal grown on the second buffer layer or on another buffer layer formed thereon can be reduced. This is also considered to be due to the delay in single crystallization of the second buffer layer due to the fact that the crystal grains contained in the second buffer layer were made smaller than those of the first buffer layer. The delay in the single crystallization of the second buffer layer described herein refers to the single crystallization that proceeds toward the upper surface of the second buffer layer with the crystal grains in the region in contact with the interface with the first buffer layer as nuclei. Delay.

The single crystallization of both buffer layers made of a nitride-based compound semiconductor, that is, the recrystallization step at the time of raising the temperature,
Both proceed with crystal grains existing in the buffer layer as nuclei. In the first buffer layer, single crystallization is fast because the crystal grains are large. For this reason, a single crystal region having crystal grains existing in contact with the crystal substrate as nuclei, that is, influenced by the orientation and lattice constant of the crystal substrate, easily reaches the upper surface of the first buffer layer. It is expected that the impact when the region and the region which is single crystallized with the crystal grains existing in the vicinity of the upper surface as a nucleus will be large. Therefore, dislocations in the first buffer layer are likely to extend from the interface with the crystal substrate toward the upper surface thereof.

On the other hand, in the second buffer layer, single crystallization is slow because of small crystal grains. In addition, even in the case of single crystallization with crystal grains as nuclei existing in contact with the interface with the first buffer layer in the early stage of the recrystallization step, the nitride is not affected by the orientation of the crystal substrate and the lattice constant. It proceeds with the intrinsic crystal structure of the base compound semiconductor. Further, the coalescence of the single crystal regions formed for each crystal grain also becomes gentle. Therefore, the first
Inherit dislocations in the area near the interface with the buffer layer of
This can significantly reduce the probability of reaching the upper surface of the second buffer layer. Therefore, the influence of the crystal base on the single crystallization near the upper surface of the second buffer layer is reduced, and as a result, the dislocation of the nitride-based compound semiconductor crystal growing on the upper surface of the second buffer layer is reduced (see the above-described second embodiment). 2).

One of the means for realizing the above two functions is to change the growth temperatures of the first buffer layer and the second buffer layer. The specific value of the growth temperature is
Although a slight variation is caused by other growth conditions, as a desirable example, the growth temperature T1 of the first buffer layer is set to 550.
C. ≦ T1 ≦ 650 ° C., and the growth temperature T2 of the second buffer layer is 4
After setting the first buffer layer in the process by setting 50 ° C. ≦ T2 ≦ 550 ° C., it is recommended that the growth temperature be lowered to grow the second buffer layer. When the growth temperature of the nitride-based compound semiconductor is lowered to about 500 ° C., cubic crystal grains are generated together with hexagonal crystal grains. For this reason, in the above-mentioned recrystallization step of the second buffer layer, a single crystallization phenomenon with cubic crystal grains as nuclei causes a new crystal in the process of single crystallization (hexagonal) of the nitride-based compound semiconductor. The problem of forming defects is feared. However, as far as the present inventor investigated, the appearance of this problem was not recognized. This is considered to be because the cubic crystal grains are absorbed by single crystals having hexagonal crystal grains as nuclei and disappear in the gentle recrystallization step of the second buffer layer.

Return to the description of the outline of the present invention.

Next, a light emitting diode or a laser device structure is formed by using the (Al _x Ga _{1 -x} ) _{1 -y} In _y N nitride compound semiconductor which is the third, fourth and fifth means. Is for realizing light emission from the ultraviolet region to the visible region. That is, as described above, at least two buffer layers are formed on a crystal substrate, and a multilayer film of nitride compound semiconductor crystals (at least two nitride compound semiconductor layers) is grown thereon. 10 ⁸ even when using SiC substrate
The dislocation density of the nitride-based compound semiconductor crystal, which cannot be reduced to less than cm ⁻³ , can be reduced to 10 ⁶ cm ⁻³ or less. Therefore, by appropriately setting the band gap profile (increase / decrease of the band gap in the laminating direction) of the multilayer film, a semiconductor light emitting device such as a light emitting diode or a laser diode composed of a nitride compound semiconductor multilayer film is formed. Can be.

By reducing the dislocation density of the nitride-based compound semiconductor crystal constituting the light emitting region, electrons and holes injected as an operating current into this light emitting region are not lost at the lattice defect portion due to the dislocation, and are surely recombined. To emit light. The light-emitting region may be formed of any of p-type, n-type, and undoped, and a pn or pin junction may be formed with a multilayer structure. In addition, a resonator structure (a Fabry-Perot type, a distributed feedback type, a distributed Bragg reflection type, etc.) for oscillating laser light is grown on the multilayer buffer layer of the present invention. (In a laminated structure composed of nitride compound semiconductor layers), the guided region in the resonator is composed of a nitride compound semiconductor crystal having a dislocation density of 10 ⁶ cm ⁻³ or less, so that stimulated emission occurs. Efficiency is also greatly improved.

With respect to the buffer layer configuration for the first means of the present invention described above, in this section, the characteristics of the amorphous layer in the manufacturing process stage of the semiconductor light emitting device are mainly described. At the stage when the device is completed, the above-mentioned buffer layer having a multilayer structure has a substantially single crystal layer appearance (that is, it is not possible to find the characteristics of the crystal grain size and the growth temperature present in each buffer layer). Therefore, the structural features of the multi-layered buffer layer at the stage when the semiconductor light emitting devices of the above-described third to fifth means are completed based on the first means will be described as a summary of examples in the embodiments of the invention described later. I do.

[0027]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of a nitride-based compound semiconductor light-emitting device manufactured using a buffer layer having a multilayer structure according to the present invention will be described below with reference to examples of a light-emitting diode and a semiconductor laser device. This will be specifically described with reference to the related drawings.

First Embodiment First, a nitride-based compound semiconductor light emitting diode according to a first embodiment of the present invention will be described with reference to FIGS.

In this embodiment, trimethylgallium and trimethylaluminum, which are organic metals, are used as raw materials for group III elements, and ammonia gas is used as raw materials for group V elements. The first Ga is grown on the (0001) plane sapphire substrate 1 at a growth temperature of 600 ° C. by using a metal organic chemical vapor deposition method.
N buffer layer (thickness 10 nm) 2, then growth temperature 500
The second buffer layer 3 (thickness: 10 nm) was deposited to form a multilayer structure. Here, if the total thickness of the multilayer buffer layer is 20 nm, three, four, five, or the like may be used.

Next, the temperature was increased to 1000 ° C. in a mixed gas atmosphere of ammonia gas and hydrogen,
3 was recrystallized. After the recrystallization of the multilayer buffer layer, a nitride-based compound semiconductor multilayer structure shown below was grown at a temperature of 1000 ° C. First, recrystallized GaN
An n-GaN contact layer (thickness 4 μm, n = 3 × 10 ¹⁸ cm ⁻³ ) 4 on the multilayer buffer layers 2 and ³ , n-GaN
Cladding layer (2.5 μm thick, n = 1 × 10 ^{18 18} cm
^-3 ) 5, Zn-doped Ga _0.9 In _0.1 N active layer (thickness:
05 μm, p = 1 × 10 ¹⁸ cm ⁻³ ) 6, p-GaN cladding layer (1.5 μm in thickness, p = 8 × 10 ¹⁷ cm ⁻³ ) 7,
And p-GaN contact layer (0.5 μm thick, n =
3 × 10 ¹⁸ cm ⁻³ ) 8 were sequentially grown. After that, heat CV
After an etching mask was formed on the p-GaN contact layer 8 by the D method and the photolithography technique, etching was performed to a part of the n-GaN contact layer 4 using the dry etching technique. Etching at this time may be performed by any method such as wet, RIE, RIBE, and ion milling. Thereafter, using an oxide film and a photolithography technique, a p-side electrode 9 and an n-side electrode 10 were formed by electron beam evaporation and lift-off, and then formed into chips.

FIG. 1 shows the structure of a light-emitting diode device using the nitride-based compound semiconductor fabricated. When current injection was performed on this light-emitting diode element, strong spontaneous emission was confirmed. Its emission spectrum has a current value of 20 m.
In A, the wavelength was 440 nm. For comparison, when compared with a light-emitting diode device having a similar structure manufactured by a two-stage growth method using a single-layer buffer layer according to the prior art, the light-emitting intensity of the device manufactured according to the present invention is one order of magnitude higher. Was something.

FIG. 2 shows a film having a half-width of X-ray diffraction in a GaN epitaxial film grown at a temperature of 1000 ° C. on a multi-layer buffer layer 16 according to the present embodiment and a single-layer buffer layer 17 according to the prior art. Shows thickness dependence. It can be seen that the half width of the multilayer buffer layer 16 is smaller than that of the single film buffer layer 17. This indicates that the uniformity of the lattice spacing of the grown nitride-based compound semiconductor layer was improved because the surface flatness of the multilayer buffer layer 16 was improved. That is, in the nitride-based compound semiconductor epitaxial film manufactured in Example 1, light emission loss is reduced as compared with the related art by improving the crystal orientation, so that strong light emission can be obtained.

<Embodiment 2> Next, the manufacture of a nitride-based compound semiconductor laser device according to a second embodiment of the present invention will be described with reference to FIG. In the same manner as in Example 1, a double hetero multilayer structure for laser was sequentially grown by metal organic chemical vapor deposition. First, an n-GaN contact layer (thickness of 4 nm) is formed on GaN buffer layers 2 and 3 (each having a thickness of 10 nm) composed of first and second buffer layers on a sapphire substrate 1.
μm, n = 3 × 10 ¹⁸ cm ⁻³ ) 4, n-GaN cladding layer (2.5 μm, n = 1 × 10 ¹⁸ cm ⁻³ ) 5, undoped multiple quantum well structure active layer (well layer Ga _0.8 In _0.2
N, thickness 2 nm, barrier layer GaN, thickness 4 nm) 11, p
-GaN cladding layer (thickness: 1.5 μm, p = 8 × 10 ¹⁷⁾
cm ⁻³ ) 7 were sequentially regrown. After that, a stripe etching mask having a width of 5 μm is formed on the p-GaN cladding layer by a thermal CVD method and a photolithography technique.
P-GaN cladding layer 7 using dry etching technology
Is etched until 0.3 μm is left to form a waveguide structure 12.
Was formed. Further, the n-GaN current confinement layer (thickness 1 μm, n
= 3 × 10 ¹⁸ cm ⁻³ ) 13 was selectively grown. Thereafter, the etching mask was removed, and buried growth was performed on the p-GaN contact layer (thickness 0.5 μm, n = 3 × 10 ¹⁸ cm ⁻³ ) 8. Then, as in Example 1, etching was performed up to a part of the n-GaN contact layer 4 to form the p-side electrode 9 and the n-side electrode 10, and then a resonator was formed by a cleavage method to obtain a chip. Finally, a low reflection film 1 is formed on the cleavage plane of the device.
4 and the highly reflective film 15 were formed by sputtering. FIG. 3 shows a laser diode element structure according to a second embodiment of the present invention.

When current injection was performed on this laser element, strong spontaneous emission light having a peak wavelength of 430 nm was observed at a current value of 20 mA. Further, as the amount of injected current was increased, the spontaneous emission light intensity was further increased, and the half-value width was also narrowed. Then, stimulated emission light began to be seen at a current value of 50 mA, and the oscillation wavelength at that time was 420 nm. On the other hand, when current injection was performed in a device structure in which a similar device structure was grown by a conventional two-stage growth method, stimulated emission light was not observed. This is because the crystal orientation of the grown nitride-based compound semiconductor layer was disturbed and the light scattering loss inside the crystal increased because the surface flatness of the single-film buffer layer grown at a low temperature was low. Further, when the defect density in these crystals was evaluated by observation with a transmission electron microscope, dislocations of about 10 ⁹ cm ^-3 were observed in a sample grown by a two-step method using a conventional single-film buffer layer. On the other hand, in the sample using the multilayer buffer layer grown in the present example, it was reduced to about 10 ⁵ cm ⁻³ . As described above, in the present invention, by forming the buffer layer into a multilayer film, the crystal defect density of the grown nitride-based compound semiconductor layer can be reduced.

<Summary of Embodiments> Embodiments 1 and 2 described above
FIG. 4 schematically shows an example of an image when the semiconductor light emitting device is completed with respect to the buffer layer described in the above section. FIG. 4 shows a first buffer layer G on a sapphire substrate (crystal base) 1.
aN layer 2, GaN layer 3 as a second buffer layer, n-Ga
The N contact layers 4 are stacked in this order. Also, the lines extending in the vertical direction shown in each of the GaN layers 2 to 4 are:
Indicates dislocation. In particular, as the main surface of the hexagonal crystal substrate,
When a nitride-based compound semiconductor crystal is grown on the (0001) plane, dislocations occur in the c-axis direction of the crystal substrate and in a direction perpendicular to the (0001) main plane.

Although there is a difference depending on the process conditions, in the semiconductor optical device employing the multilayer buffer layer of the present invention, the first buffer layer 2 formed on the crystal substrate and the second buffer layer 3 This is characterized by a low density of existing dislocations. This feature can be confirmed by, for example, observation with a transmission electron microscope, and the dislocation density generally changes around the interface 18 in many cases. The interface 18 may be unclear depending on the process conditions,
A semiconductor layer composed of two regions having different dislocation densities is still formed on the crystal substrate 1. As an example of the difference in dislocation density between the first buffer layer 2 and the nitride-based compound semiconductor layer 4 formed on the second buffer layer, the former is 1 × 10 ⁸ cm ⁻³ or more, and the latter is 1 × 10 ⁸ cm ⁻³ or more. The dislocation density of the second buffer layer 3 is an intermediate value between 10 ⁷ cm ⁻³ (preferably 1 × 10 ⁶ cm ⁻³ ) or less. Although the interface 18 is shown with emphasis on the formation of unevenness on the upper surface of the first buffer layer as described above, the actual unevenness may be different from that of FIG.

The thickness t of the entire buffer layer composed of the first buffer layer 2 and the second buffer layer 3 is determined as follows because the buffer layer is formed in an amorphous state and needs to be single-crystallized. It must be set in consideration of the points. First,
If it is too thin, a problem arises in that the buffer layer itself is decomposed in the recrystallization step. Therefore, the nitride-based compound semiconductor layer 4
The buffer layer serving as a base on which the semiconductor is grown disappears, and it becomes impossible to form a desired semiconductor light emitting device. On the other hand, if it is too thick, the recrystallization step takes too much time. In the above description, the advantage of extending the time required for the recrystallization step of the second buffer layer relative to the first buffer layer has been described. However, when the degree of the delay time increases, the delay time is formed on the second buffer layer. The thickness of the nitride-based compound semiconductor layer 4 is also increased, and the remaining amorphous region in the buffer layer has a negative effect in terms of strength for supporting the same. In consideration of such a problem, it is necessary to set the thickness t of the entire buffer layer.

One example of setting the thickness is to make t smaller than the thickness t _{4 of} the nitride-based compound semiconductor layer 4 to be formed on the second buffer layer. That is, t>
At t ₄ , even if the growth of the nitride-based compound semiconductor layer 4 ends, there is a high possibility that an amorphous region will remain in the second buffer layer. As another example, 20 nm ≦ t
≦ 30 nm. This range is a desired value experimentally obtained. However, since the setting of t is essentially to avoid the above-described problem, if the requirements are satisfied by improving the process conditions, etc., the present invention can be implemented even if t deviates slightly from the above-described conditions. It does not prevent. When the buffer layer is composed of n layers (n> 2), t is set as the total thickness of the n layers. Also,
With respect to the first and second film thickness distributions with respect to the entire thickness of the buffer layer, an example of uniform distribution has been described in the above embodiment, but the thickness distribution is not limited to being equal, and it is more desirable to carry out the present invention. There is no problem even if the distribution is appropriately changed according to the effect.

Based on the above description, the features of the semiconductor light emitting device realized by employing the multilayer buffer layer of the present invention are as follows.
In terms of an image of a completed device, a first nitride-based compound semiconductor layer 2 (dislocation density D ₁ )
Each of the nitride-based compound semiconductor layers includes a region where the second nitride-based compound semiconductor layer 3 (dislocation density D ₂ ) and the third nitride-based compound semiconductor layer 4 (dislocation density D ₃ ) are stacked in this order. the dislocation density D _n that exists in the, D _1> D _2> structure having a D ₃ the relationship (so to speak, a laminated structure in which the dislocation density is reduced stepwise) becomes. Further, the features captured from another viewpoint include the semiconductor regions 2 and 3 made of the nitride-based compound semiconductor formed on the crystal substrate 1 and the nitride-based compound semiconductor layer 4 formed on the semiconductor region. The dislocation density is higher at a portion of the semiconductor region in contact with the bonding interface with the crystal substrate than at a portion of the semiconductor region in contact with the nitride-based compound semiconductor layer. In the former configuration, one or more additional nitride-based compound semiconductor layers (dislocation density D _F ) may be interposed between the second and third nitride-based compound semiconductor layers 3 and 4, and in this case, D ₂ > D
It is desirable to _F> D _3. Note that the above-described semiconductor region made of a nitride-based compound semiconductor includes an embodiment in which a further nitride-based compound semiconductor layer (buffer layer) exists in the region.

In this section, the sapphire substrate 1 is a crystal substrate, the GaN layer 2 as a first buffer layer is a first nitride-based compound semiconductor layer, and the GaN layer 2 is a G buffer as a second buffer layer.
The aN layer 3 is formed on the second nitride-based compound semiconductor layer (or on the first
And the n-GaN contact layer 4 is referred to as a third nitride-based compound semiconductor layer (or a nitride-based compound semiconductor layer). The basis is as follows. This is because the crystal substrate is not limited to the sapphire substrate, and the use of a substrate material such as zinc oxide or silicon carbide or the use of a crystal layer epitaxially grown on these substrate materials does not prevent the present invention from being carried out. Further, any of the buffer layers and the nitride-based compound semiconductor layers formed thereon are not limited to GaN, but may be, for example, AlN, Ga
This is because the use of a nitride-based compound semiconductor having a hexagonal crystal structure as an original (intrinsic) crystal structure such as NAs does not prevent implementation of the present invention. Even if these materials are replaced, the dislocation density distribution of the multilayer buffer layer does not change in its essential characteristics, although the degree is different.

[0041]

By using the multilayer buffer layer of the present invention to improve the surface condition, crystal nuclei after recrystallization can be formed with high density and uniformity. As a result, a low-defect nitride-based compound semiconductor epitaxial film uniformly oriented on the substrate crystal can be grown. Therefore, it is possible to easily realize a nitride semiconductor light emitting diode having a high emission intensity and a semiconductor laser oscillation having a low threshold.

[Brief description of the drawings]

FIG. 1 is a structural view of a light emitting diode element according to a first embodiment of the present invention.

FIG. 2 is a diagram showing the film thickness dependence of the X-ray diffraction half width of a GaN film grown on a multilayer buffer layer according to the present invention and a single film buffer layer according to the prior art.

FIG. 3 is a structural view of a laser diode element according to a second embodiment of the present invention.

FIG. 4 is a schematic diagram showing a dislocation density distribution of a multilayer buffer layer according to an embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Sapphire substrate, 2 ... 1st buffer layer, 3 ... 2nd buffer layer, 4 ... n-GaN contact layer, 5 ... n-Ga
N cladding layer, 6 ... Zn doped GaInN active layer, 7 ...
p-GaN cladding layer, 8 ... p-GaN contact layer,
9: p-side electrode, 10: n-side electrode, 11: multiple quantum well active layer, 12: waveguide structure, 13: n-GaN current confinement layer, 14: front low reflection film, 15: rear high reflection film, 16 …
17 is a graph showing the film thickness dependence of the X-ray half width of the GaN film grown on the multilayer buffer layer, and 17 is a graph showing the film thickness dependence of the X-ray half width of the GaN film grown on the single film buffer layer.

Claims (7)

[Claims] 1. A method according to claim 1, wherein at least a first and a second are sequentially formed on the substrate.
Forming a buffer layer having a multilayer structure composed of the buffer layer described above, and then crystal-growing the nitride-based compound semiconductor on the buffer layer. 2. The crystal growth of a nitride-based compound semiconductor according to claim 1, wherein the respective growth temperatures T1 and T2 of the first and second buffer layers are set so that T1> T2. Method. 3. The composition of the buffer layer and the semiconductor layer is Al _x Ga _{1 -x} ) _{1 -y} In _y N (where 0 ≦ x ≦ 1, 0
3. The method for growing a nitride-based compound semiconductor according to claim 1, wherein ≦ y ≦ 1). 4. 4. A crystal substrate, a first nitride-based compound semiconductor layer, a second nitride-based compound semiconductor layer, and a third
Wherein the first nitride-based compound semiconductor layer has a dislocation density higher than that of the second nitride-based compound semiconductor layer and the second nitride-based compound semiconductor layer has a higher dislocation density than the second nitride-based compound semiconductor layer. A dislocation density existing in the nitride-based compound semiconductor layer is higher than that of the third nitride-based compound semiconductor layer. 5. A semiconductor device comprising: a crystal substrate; a semiconductor region composed of a nitride compound semiconductor formed on the crystal substrate; and a nitride compound semiconductor layer formed on the semiconductor region. A semiconductor light emitting device wherein a dislocation density at a portion in contact with the interface with the crystal base is higher than a dislocation density at a portion of the semiconductor region in contact with the nitride-based compound semiconductor layer. 6. A light-emitting diode structure comprising a pn junction together with at least one nitride-based compound semiconductor layer formed on said nitride-based compound semiconductor layer. The semiconductor light-emitting device according to claim 5, wherein: 7. A laminated structure in which a nitride-based compound semiconductor layer is further grown on the nitride-based compound semiconductor layer,
6. The semiconductor light emitting device according to claim 5, wherein a resonator for laser light oscillation is formed in the laminated structure.