JPH1032370A - Growfh method of nitride compound semiconductor and semiconductor device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To turn a buffer layer into columnar crystal of high density without deteriorating it in surface state by a method in which the buffer layer is turned into columnar crystal after a dielectric film is formed on its surface. SOLUTION: An N-GaN contact layer 4, an N-GaN clad layer 5, a Zn-doped GaInN active layer 6, a P-GaN clad layer 7, and a P-GaN contact layer 8 are successively regrown on a GaN buffer layer 2 of columnar crystal. Thereafter, an etching mask is formed on the P-GaN contact layer 8, and the laminate composed of the layers 4 to 8 is dry-etched as far as a part of the N-GaN contact layer 4. Then, a P-side electrode 9 and an N-side electrode 10 are formed, and the laminate with the electrodes 9 and 10 is divided into light emitting diode element chips of nitride compound semiconductor. A light emitting diode device of this structure is possessed of an epitaxial layer uniform in crystal orientation, so that it is lessened in light scattering loss, and a regrown nitride compound layer is improved in crystallinity.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a crystal growth method for obtaining a high-quality nitride-based compound semiconductor layer. In particular, a method of growing a nitride-based compound semiconductor light emitting device structure suitable for obtaining laser light by spontaneous emission light and stimulated emission light from the ultraviolet region to the visible region, and a semiconductor device such as a light emitting diode and a semiconductor laser device using the same And its manufacturing method.

[0002]

2. Description of the Related Art An (Al _x Ga _{1 -x} ) _{1 -y} In _y N nitride-based compound semiconductor composed of AlN and InN centered on GaN is a direct transition type in all composition regions and has a large band gap. It is a material having energy. Therefore, it is strongly expected as a next-generation short-wavelength light-emitting element material. However, since these compound semiconductors have a hexagonal crystal structure, conventional compound semiconductors such as GaAs, InP, etc.
There is no substrate crystal that is lattice-matched as in a group V compound semiconductor. For this reason, nitride compound semiconductors are currently grown using sapphire having a similar hexagonal structure as a substrate crystal. However, (000
1) Nearly 16% of lattice irregularities exist between a plane sapphire substrate crystal and GaN, which is a typical nitride-based compound semiconductor. Therefore, in growing a nitride-based compound semiconductor layer on a sapphire substrate, it has been very difficult to obtain an epitaxial film having good surface morphology.

On the other hand, a two-step growth method using a buffer layer grown at a low temperature has been proposed. (S. Nakamu
ra, Jpn. J. Appl. Phys. 30, L17
05 (1991)). In this method, a buffer layer is formed on a sapphire substrate at a low temperature (about 600 ° C.), and then heated to a high temperature (about 1000 ° C.) to grow a nitride-based compound semiconductor layer. Here, the buffer layer after low-temperature growth is in an amorphous state. That is, the crystal of the buffer layer is not oriented with respect to the substrate crystal axis. However, due to the process of raising the temperature to the growth temperature of the semiconductor layer that grows at the next higher temperature and the subsequent high temperature, columnar crystals oriented in the crystal axis of the substrate are formed at a high density in the buffer layer that was amorphous. Is done. This high-density columnar crystal acts as a nucleus during the growth of a nitride-based compound semiconductor that is subsequently grown. As a result, it becomes possible to grow a nitride-based compound semiconductor epitaxial film better than the case of direct growth despite the presence of a large lattice mismatch. However, the conventional technique has the following problems.

[0004] In the temperature raising process in which the temperature is higher than that in the growth of the low-temperature buffer layer and before the high-temperature growth, decomposition and desorption of nitrogen, which is a Group V element, occur from the surface of the buffer layer, and the surface flatness is significantly deteriorated. I do. This is due to the high equilibrium vapor pressure of nitrogen. The deterioration of the surface flatness leads to a reduction in the density of the nucleus and an unevenness of the nucleus which are important in the initial stage of the subsequent high-temperature growth of the nitride-based compound semiconductor layer. As a result, not only is the orientation of the epitaxial film disturbed with respect to the substrate crystal axis, but also the surface morphology becomes non-uniform. In addition, such deterioration of the surface state of the buffer layer becomes a problem when manufacturing a quantum well structure that requires steep interface flatness.

When a nitride-based compound semiconductor laser is taken into consideration, the deterioration of crystallinity including the disorder of the orientation of the epitaxial film causes emission light generated in the active layer by recombination of injected carriers. When guiding between resonators,
It becomes an increase factor of light scattering loss. This leads to a decrease in optical gain and an increase in threshold current density.

Further, in the prior art, it was an important factor to control the buffer layer within a certain thickness range. Specifically, the film thickness needs to be around 20 nm. This is because when the thickness of the buffer layer is thin, decomposition of nitrogen,
Desorption occurs, but the recrystallization time is short, so that deterioration of the surface state can be minimized. on the other hand,
When the thickness is increased, the time required for recrystallization is required, so that the deterioration of the surface state of the buffer layer, that is, the decrease in the nucleation density cannot be ignored.

[0007]

SUMMARY OF THE INVENTION It is an object of the present invention to perform high-density and uniform columnar crystallization without deteriorating the surface flatness of an amorphous buffer layer grown at a low temperature.
This is to epitaxially grow a nitride-based compound semiconductor layer uniformly oriented with respect to the substrate crystal axis. It is another object of the present invention to reduce the crystal defect density of the nitride-based compound semiconductor by increasing the thickness of the buffer layer.

[0008]

In order to achieve the above object, the present invention adopts the following means (steps). As a first means (step), a dielectric film is formed on a nitride-based compound semiconductor layer grown at a low temperature, a heat treatment is applied at a temperature higher than the growth temperature, and the film is removed to remove the nitride-based compound semiconductor. The layer is regrown at high temperature. As a second means, these nitride-based compound semiconductor layers that regrow at low and high temperatures are made of (Al _x G
a _1-x ) _1-y In _y N (however, 0 ≦ x ≦ 1, 0 ≦ y ≦ 1)
And As a third means, a semiconductor device such as a light-emitting diode or a laser diode is used as an element structure using a nitride-based compound semiconductor layer grown by these steps.

First, forming a dielectric film on an amorphous nitride-based compound semiconductor layer grown at a low temperature, which is a first means, involves decomposing nitrogen from the buffer layer surface due to heat treatment. This is for suppressing desorption. As a result, columnar crystallization proceeds without deteriorating the surface state of the buffer layer. Further, by using the present invention, the thickness of the buffer layer can be made thicker than in the prior art. Increasing the thickness of the buffer layer also has the effect of reducing the crystal defect density due to large lattice mismatch between the sapphire substrate and the nitride-based compound semiconductor as compared with the prior art.

More specifically, a crystal substrate having a hexagonal structure such as sapphire is placed in an ammonia (NH ₃ )
A semiconductor layer having an amorphous structure made of a nitride-based compound semiconductor is formed at a temperature of 00 to 600 ° C. This semiconductor layer is a buffer layer proposed by Nakamura described above. The present inventor has reported that, in a nitride-based compound semiconductor formed as amorphous, N bonds, that is, nitrogen (N) which is a group V element is II
We noticed that the binding force from the group I elements was weaker than in the crystals. N is equal to or higher than the temperature at which the nitride-based compound semiconductor layer is formed as an amorphous structure (60).
(Higher than 0 ° C.), the N
We focused on a phenomenon in which the desorption of methane becomes remarkable according to the temperature. on the other hand,
The crystal growth temperature of the nitride-based compound semiconductor is about 1000 ° C. That is, when a nitride-based compound semiconductor crystal is grown on the buffer layer in an amorphous state, crystallization of the buffer layer and N desorption therefrom proceed substantially simultaneously.

The inventor of the present invention has found that the composition of the buffer layer deviates from the stoichiometric ratio due to the desorption of N, so that vacancies increase at the sites of the atoms arranged in the crystal, and this increases the regularity of the atoms. It is considered that the alignment is hindered and the crystal structure is impaired, that is, the crystal orientation decreases, the crystal growth surface becomes uneven, and crystal defects are caused. In order to solve this problem, the buffer layer is crystallized before growing the nitride-based compound semiconductor on the buffer layer, and the buffer layer is formed of a dielectric film so that N is not eliminated during the crystallization process of the buffer layer. We considered to cover the surface and block the path of N.

The dielectric film is made of, for example, SiO ₂
Some can be formed by a thermal CVD method at 0 ° C. or lower (500 to 600 ° C.). The dielectric material used in the present invention is SiO ₂
However, the temperature is not limited to 600 ° C., and the temperature at which an amorphous nitride-based compound semiconductor can be crystallized (about 100 ° C.).
(0 ° C.) is a condition that the dielectric film is not damaged. The formation temperature of the dielectric film is preferably 5
A temperature as low as 00 ° C. or lower, more preferably 400 to 300 ° C., is preferable from the viewpoint of making the probability of N desorption at the time of forming a dielectric film close to zero. Further, the dielectric film is desirably a material having chemical or physical properties such that the crystallized nitride-based compound semiconductor layer can be completely removed from the nitride-based compound semiconductor layer without damaging it. .

Next, the composition of the nitride-based compound semiconductor layer formed (laminated) on the buffer layer, which is the second and third means, is changed to (Al _x Ga _{1 -x} ) _{1 -y} In _y N (however, , 0 ≦ x ≦ 1, 0
≦ y ≦ 1) and forming a light emitting diode or a laser element structure using the semiconductor layer (laminated structure)
It is preferable in realizing light emission from the ultraviolet region to the visible region.

As described above, by forming a dielectric film on the buffer layer grown at a low temperature and recrystallizing it, the nitride-based compound semiconductor layer is regrown on the buffer layer from which the dielectric film has been removed. A nitride-based compound semiconductor epitaxial film having crystallinity can be obtained.

According to the method for growing a nitride-based compound semiconductor of the present invention described above, a semiconductor device (particularly, a semiconductor optical device such as a light emitting diode or a semiconductor laser device) having the following structural characteristics can be manufactured. Can be formed. Its composition is (1)
It is configured by stacking a semiconductor layer made of a III-V compound semiconductor containing nitrogen on a substrate crystal having a hexagonal structure,
(2) The laminated semiconductor layer has a hexagonal structure, and (3) the composition of the semiconductor layer at the bonding surface with the substrate crystal is defined by the requirement that the composition substantially satisfies the stoichiometric ratio. . Looking at the completed semiconductor device, (1) and (2) are macroscopic views of the buffer layer formed on the substrate crystal having a hexagonal structure such as sapphire and the nitride-based compound semiconductor layer laminated on this layer. This is the basic structure of the semiconductor device as viewed from the perspective. On the other hand, (3) is a feature based on the method for growing a nitride-based compound semiconductor described above, and is based on the result that N desorption does not occur in the semiconductor layer, that is, the buffer layer bonded to the substrate crystal. Here, the definition of “substantially” is given when the buffer layer is formed including p-type or n-type impurities (for example, when a very small part of the group V element is used).
This is because the case of substitution with a group IV element) was considered. Even if the buffer layer has a stoichiometric composition, a defect is introduced into the buffer layer due to a lattice mismatch between the substrate crystal and the nitride-based compound semiconductor crystal. Based on the configuration of such a semiconductor device, a nitride-based compound semiconductor layer is
A light emitting diode structure having a junction may be formed, or a laser diode structure (laser resonator structure) having at least a pn junction and a resonator may be formed.

In the above description and in the following description, a nitride-based compound semiconductor is defined as a group III-V compound semiconductor containing at least N (nitrogen) as a group V element. Al, Ga, and I
It includes any element selected from the group of n, and may include any element selected from the group of P, As, and Sb in addition to N as the group V element. In some cases, the group V element is only N.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a light emitting diode and a semiconductor laser device manufactured by column-crystallization of a buffer layer made of a nitride-based compound semiconductor and then growing a nitride-based compound semiconductor layer on the buffer layer will be described. Embodiments of the present invention will be specifically described with reference to examples.

Embodiment 1 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.

As raw materials, trimethylgallium, trimethylaluminum and ammonia gas, which are organic metal raw materials, were used. A GaN low-temperature buffer layer (thickness: 20 nm) 2 was deposited on a (0001) plane sapphire substrate 1 at a growth temperature of 600 ° C. by metal organic chemical vapor deposition. Thereafter, the sample was taken out from the growth reaction chamber, and was subjected to normal pressure (about 760 Torr).
r) thermal CVD method in SiH ₄ + O ₂ atmosphere (temperature 500
To 600 ° C.) using an SiO ₂ film (200 nm thick) 3
Was formed. Here, the method of forming the dielectric film formed on the buffer layer and the type of film may be any other.

FIG. 1 is a sectional view of a substrate on which an SiO ₂ film 3 is formed. The buffer layer 2 was recrystallized again by using a metalorganic vapor phase epitaxy apparatus in a 1000 ° C. atmosphere in which only ammonia flowed. Here, since the surface of the buffer layer is covered with the SiO ₂ film 3, columnar crystals oriented with respect to the substrate crystal axis can be formed with high density and uniformity without decomposition and desorption of nitrogen. After the recrystallization of the buffer layer, the SiO ₂ film 3 is removed and 1
At a high temperature of 000 ° C., a nitride-based compound semiconductor multilayer structure as shown below was regrown.

First, the columnar crystallized GaN buffer layer 2
An n-GaN contact layer (thickness 4 μm, n = 3 ×
10 ¹⁸ cm ⁻³ ) 4, n-GaN cladding layer (thickness 2.5)
μm, n = 1 × 10 ¹⁸ cm ⁻³ ) 5, Zn-doped Ga _0.9
In _0.1 N active layer (0.1 μm thick, p = 1 × 10 ¹⁸ cm)
^-3 ) 6, p-GaN cladding layer (1.5 μm thick, p =
8 × 10 ¹⁷ cm ⁻³ ) 7 and a p-GaN contact layer (0.5 μm in thickness, n = 3 × 10 ¹⁸ cm ⁻³ ) 8 were sequentially regrown. After that, an etching mask is formed on the p-GaN contact layer 8 by a thermal CVD method and a photolithography technique, and then n-Ga is etched by a dry etching technique.
Etching was performed up to a part of the N contact layer 4. Etching at this time may be performed by any method such as wet, RIE, RIBE, and ion milling. Thereafter, a p-side electrode 9 and an n-side electrode 10 are formed by an electron beam evaporation method and a lift-off method using an oxide film and a photolithography technique, and then formed into a chip, and a light emitting diode element using a nitride-based compound semiconductor as shown in FIG. Was prepared.

In the light emitting diode device, when current was injected, strong spontaneous emission was confirmed. Its emission spectrum shows a wavelength of 440 n at a current value of 20 mA.
m. For comparison, when compared with a light emitting diode device having a similar structure manufactured by two-stage growth, which is a conventional technique, the light emitting intensity of the device manufactured according to the present invention is 1 unit.
It was an order of magnitude stronger. This is a result showing that the crystallinity of the regrown nitride-based compound semiconductor layer was improved.
That is, it is considered that the light scattering loss was reduced because the crystal orientation of the epitaxial film was uniform.

Embodiment 2 Next, a method for manufacturing a nitride-based compound semiconductor laser device according to a second embodiment of the present invention will be described with reference to FIG. The columnar crystallization of the buffer layer grown at a low temperature was performed in the same manner as in the first embodiment of the present invention. However, in the present embodiment, the thickness of the GaN buffer layer 2 was 0.5 μm. Thereafter, the double hetero multilayer structure for laser was sequentially regrown by the metal organic chemical vapor deposition method in the same manner as in Example 1. First, columnar crystallized G at 1000 degrees
On the aN buffer layer 2 (0.5 μm thickness), n-GaN
Contact layer (4 μm thickness, n = 3 × 10 ¹⁸ cm ⁻³ )
4. n-GaN cladding layer (2.5 μm in thickness, n = 1 ×
10 ¹⁸ cm ⁻³ ) 5, undoped multiple quantum well structure active layer (well layer Ga _0.9 In _0.1 N, thickness 3 nm, barrier layer Ga
N, thickness 3 nm) 11 and a p-GaN cladding layer (thickness 1.5 μm, p = 8 × 10 ¹⁷ cm ⁻³ ) 7 were sequentially regrown. Thereafter, 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, and etching is performed using a dry etching technique until the p-GaN cladding layer 7 remains at 0.3 μm. Structure 12 was formed. Further, an n-GaN current confinement layer (thickness 1 μm, n = 3 × 10 ¹⁸ cm ⁻³ ) 13 is selectively grown using this etching mask as a selective growth mask, and after removing the etching mask, the p-GaN contact layer (thickness) is removed. Embedding growth was performed at 0.5 μm, n = 3 × 10 ¹⁸ cm ⁻³ ) 8.

Thereafter, similarly to the first embodiment, etching is performed up to a part of the n-GaN contact layer 4, and the p-side electrode 9 is etched.
After forming the n-side electrode 10, a resonator was formed by a cleavage method to obtain a chip. Thereafter, a low reflection film 14 and a high reflection film 15 were formed on the cleavage plane of the device by a sputtering method. FIG.
2 shows a laser diode element structure diagram 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 365 nm was observed at a current value of 20 mA. Further, as the amount of injected current was increased, the intensity of spontaneous emission light was further increased, and the half-value width was also narrowed. Then, stimulated emission light began to be seen at a current value of 60 mA. The oscillation wavelength at that time was 360 nm. On the other hand, when a current was injected in a device structure in which a similar structure was grown by a conventional two-step growth method, no stimulated emission light was observed. This is because the surface flatness is deteriorated due to the decomposition and desorption of nitrogen from the surface of the buffer layer grown at a low temperature, so that the crystal orientation of the laser structure by the grown nitride-based compound semiconductor layer is disturbed and light scattering loss inside the crystal is caused. This is because the outflow has increased.

When the defect density in these crystals was evaluated by observation with a transmission electron microscope, threading dislocations of about 10 ⁹ cm ^-3 were observed in the sample grown by the conventional two-step growth method. In the sample using the thick buffer layer grown in the present example, it was reduced to about 10 ⁵ cm ⁻³ . Thus, in the present invention, the crystal defect density can be reduced by increasing the thickness of the buffer layer.

[0027]

The columnar crystallization of the buffer layer according to the present invention is performed by forming a dielectric film on the surface of the layer, whereby the decomposition and desorption of nitrogen from the buffer layer surface during columnar crystallization can be suppressed. Therefore, high-density columnar crystallization can be performed without deteriorating the surface state of the buffer layer. Also, regardless of the thickness of the buffer layer, it is possible to grow a nitride-based compound semiconductor epitaxial layer that is uniformly oriented to the crystal axis of the substrate and has low defects.

Therefore, the thickness of the buffer layer can be made thicker than in the prior art, thereby reducing the crystal defect density caused by the large lattice mismatch existing between the sapphire substrate and the nitride-based compound semiconductor as compared with the prior art. Can also be reduced. That is, the buffer layer can be formed to a thickness that can sufficiently absorb crystal defects from the sapphire substrate bonding interface.
It is possible to reduce the spread of crystal defects to each semiconductor layer that is stacked on the buffer layer and configures the semiconductor device.

Further, the buffer layer and the semiconductor layer formed thereon are formed of (Al _x Ga _{1 -x} ) _{1 -y} In _y N nitride compound semiconductor to form a light emitting diode or a laser device structure. Thereby, a light source capable of emitting light in the ultraviolet region to the visible region can be realized. As a result, a nitride semiconductor light emitting diode having a high light intensity and a semiconductor laser oscillation having a low threshold can be easily realized.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a substrate on which an SiO ₂ film is formed.

FIG. 2 is a structural view of a light emitting diode element according to a first embodiment of the present invention.

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

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Sapphire substrate, 2 ... Buffer layer, 3 ... SiO
₂ film, 4 ... n-GaN contact layer, 5 ... n-GaN 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.

Claims (5)

[Claims] 1. A step of forming a dielectric film on a nitride-based compound semiconductor layer grown on a substrate and then performing a heat treatment, and removing the nitride-based compound semiconductor on the semiconductor layer after removing the dielectric film. A method for growing a nitride-based compound semiconductor, comprising a step of growing. 2. The method according to claim 1, wherein the nitride-based compound semiconductor layer comprises (Al _x G
2. The method for growing a nitride-based compound semiconductor according to claim 1, wherein a _1-x ) In _y N (where 0 ≦ x ≦ 1, 0 ≦ y ≦ 1). 3. The light-emitting diode structure having at least a pn junction in the step of regrowing the nitride-based compound semiconductor layer.
3. The method for growing a nitride-based compound semiconductor according to item 1. 4. The nitride-based semiconductor device according to claim 1, wherein the regrowth step of the nitride-based compound semiconductor layer has a laser diode structure having at least a pn junction and a resonator. A method for growing a compound semiconductor. 5. A semiconductor crystal comprising a group III-V compound semiconductor containing nitrogen, which is laminated on a substrate crystal having a hexagonal structure, wherein the laminated semiconductor layer has a hexagonal structure, and A semiconductor device, wherein a composition of the semiconductor layer at a bonding surface with a substrate crystal substantially satisfies a stoichiometric ratio.