JP2000277862A - Nitride semiconductor device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stabilize the transversal mode of a nitride semiconductor device in the vertical direction by placing between a substrate and a clad layer a GaN semiconductor layer containing Al equal to or higher than the Al composition of the clad layer. SOLUTION: A buffer layer 2 is deposited on a sapphire substrate 1, and between the sapphire substrate 1 including the deposited buffer layer 2 and a clad layer 4 an AlGaN layer 3 containing Al equal to or higher than the Al composition of the clad layer 4 is formed. A GaN optical guide layer 5, a multiple quantum well active layer 6, a GaN optical guide layer 7, a clad layer 8, and a GaN contact layer 9 are formed thereon in this order. As a result, cracking is suppressed and vertical transversal mode is stabilized at the same time.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a GaN-based semiconductor light-emitting device such as a semiconductor laser expected to be applied to the field of optical information processing and the like, and a method of manufacturing the same.

[0002]

2. Description of the Related Art A nitride semiconductor having nitrogen (N) as a group V element is regarded as a promising material for a short-wavelength light emitting device because of its large band gap. Among them, gallium nitride-based compound semiconductors (GaN-based semiconductors: Al _x Ga _y In _z N (0 ≦ x, y, z
≦ 1, x + y + z = 1)) has been actively studied, and blue light-emitting diodes (LEDs) and green LEDs have been commercialized. In addition, a semiconductor laser having an oscillation wavelength in a 400 nm band has been eagerly desired for increasing the capacity of an optical disk device, and a semiconductor laser using a GaN-based semiconductor as a material has attracted attention and is now reaching a practical level.

FIG. 5 is a structural sectional view of a GaN-based semiconductor laser in which laser oscillation is achieved. Sapphire substrate 50
GaN buffer layer 502, n-GaN layer 503, n-AlGaN cladding layer 50 on the substrate 1 by metal organic chemical vapor deposition (MOVPE)
4, n-GaN optical guide layer _{505, Ga 1-x In x} N / Ga 1-y In y N (0 <
a multiple quantum well (MQW) active layer 506 composed of y <x <1),
p-GaN second optical guide layer 507, p-AlGaN cladding layer 50
8. A p-GaN contact layer 509 is grown. And pG
A ridge stripe having a width of about 3 to 10 microns is formed on the aN contact layer 509, and both sides thereof are formed of SiO25.
11 embedded. Thereafter, a p electrode 51 made of, for example, Ni / Au is formed on the ridge stripe and SiO ₂ 511.
An n-electrode 512 made of, for example, Ti / Al is formed on the surface which is etched until the n-GaN layer 503 is exposed. In this device, when the n-electrode 512 is grounded and a voltage is applied to the p-electrode 510, holes are injected from the p-electrode 510 side toward the MQW active layer 506, and electrons are injected from the n-electrode 512 side. An optical gain is generated within it, and laser oscillation in an oscillation wavelength band of 400 nm is caused.
Ga _1-x In _x N / Ga _1-y In _y N which is the material of the MQW active layer 506
The oscillation wavelength changes depending on the composition and thickness of the thin film. At present, continuous oscillation at room temperature or higher is realized.

By controlling the width and height of the ridge stripe, the laser is designed to oscillate in the fundamental mode in the horizontal transverse mode. That is, by providing a difference in the light confinement coefficient between the fundamental transverse mode and the higher-order mode (first-order or higher mode), oscillation in the fundamental transverse mode is enabled.

[0005]

However, a problem remains in the horizontal mode in the vertical direction (vertical horizontal mode). FIG. 6 shows a vertical refractive index distribution and a light distribution of the constituent materials of the semiconductor laser shown in FIG. In order to give a large light intensity to the active layer and the light guide layer, the sixth mode must be used. This is because the laser of FIG. 5 has two cores (portions having a high refractive index). That is, the first core is an active layer and a light guide layer, and the second core is a GaN layer (a layer between the sapphire substrate and the cladding layer).

When the laser oscillates in the sixth mode, a large number of light emitting points are formed even in the far-field image (FFP). Therefore, when light is condensed by a lens or the like, it cannot be narrowed down to a single spot. To solve this,
(1) AlGaN first cladding layer (n-AlGa in FIG. 5)
A method of thickening the N cladding layer 504), (2) AlGa
A method of improving the Al composition of the N first cladding layer is considered. Both are effective in reducing the amount of light seeping out of the active layer and the light guide layer.

However, even if the above methods (1) and (2) are tried, new problems arise. When a thick or high Al composition AlGaN layer is deposited on the GaN layer, cracks (cracks) occur during cooling. Although the cause is not clear, it is thought to be due to the difference in the thermal expansion coefficient between the sapphire substrate, GaN, and AlGaN. When an active layer is deposited on cracked AlGaN, problems such as a decrease in uniformity and a decrease in reliability occur.

The present invention has been made in view of the above circumstances, and provides a nitride semiconductor device having a stable transverse mode in the vertical direction. It is particularly effective in application to lasers for optical disks.

[0009]

According to the nitride semiconductor device of the present invention, the distance between the substrate and the cladding layer is equal to or higher than the Al composition of the cladding layer.
It has a GaN-based semiconductor layer containing Al. However,
Excludes buffer layer.

Further, the nitride semiconductor device of the present invention has a clad layer structure sandwiching an active layer, and has a thin film multilayer structure between the clad layer structure and a substrate. It is composed of a layer with a high Al composition and a layer with a low Al composition, and the average Al composition of the thin-film multilayer structure is equal to or higher than the average Al composition of the cladding layer structure. It has an Al composition.

A nitride semiconductor device having an active layer and a clad layer structure sandwiching the active layer, wherein the nitride semiconductor device has a thin film multilayer structure between the clad layer structure and a substrate, The multilayer structure is composed of a layer having a high Al composition and a layer having a low Al composition, and the average Al composition of the thin film multilayer structure is equal to the average Al composition of the cladding layer structure, or the average Al composition of the cladding layer structure. A layer with a higher Al composition and a lower A
An impurity is added to one of the layers having the 1 composition.

[0012]

Embodiments of the present invention will be described below in detail with reference to the drawings. The production method of the present invention
The growth method of nitride semiconductors is not limited to MOVPE, but has been proposed to grow nitride semiconductor layers such as hydride vapor phase epitaxy (H-VPE) and molecular beam epitaxy (MBE). The method can be applied to all methods.

(Embodiment 1) FIG. 1 is a structural sectional view of a GaN-based semiconductor laser showing a first embodiment. The method for manufacturing the laser shown in FIG. 1 is as follows.

First, TM on a sapphire substrate 1 at 500 ° C.
G and NH ₃ are supplied to deposit the GaN buffer layer 2. Thereafter, the temperature is raised to 1020 ° C., and TMG, SiH ₄ , TMA, etc. are supplied to supply n-Al _0.15 Ga _0.85 N layer 3, n-Al _0.07 Ga _0.93 N clad layer 4, n-GaN optical guide layer 5, and multiplexing. Quantum well (MQ
W) Active layer 6, p-GaN optical guide layer 7, p-Al _0.07 Ga
_{A 0.93} N cladding layer 8 and a p-GaN contact layer 9 are sequentially laminated. p-GaN contact layer 9 and p-Al
_{The 0.07} Ga _0.93 N cladding layer 8 is processed into a ridge stripe shape for controlling the horizontal and transverse modes. The stripe width is about 3-5 microns. A p-electrode 10 is formed on the p-GaN contact layer 9, and a sidewall of the ridge is covered with an insulating film 11. A wiring electrode 12 is provided on the surface of the p-electrode 10 in the opening of the insulating film 11 and on a part of the insulating film 11. On the surface etched until a part of the n-Al _0.15 Ga _0.85 N layer 3 is exposed, the n-electrode 1
3 are formed.

In this device, when a voltage is applied between the n-electrode 13 and the p-electrode 10, holes are injected from the p-electrode 10 toward the MQW active layer 6, and electrons are injected from the n-electrode 13. Generates gain and causes laser oscillation at a wavelength of 405 nm. The MQW active layer 6 is composed of a Ga _0.8 In _0.2 N well layer having a thickness of 2.5 nm and a GaN barrier layer having a thickness of 6.0 nm.

The structural characteristic that a shown in FIG. 1, is that the n-Al _0.15 Ga _{0. 85} N layer 3 is present immediately above the buffer layer 2. The thickness of this layer is sufficiently thick as 4 microns.
Therefore, light can be sufficiently confined also in the vertical direction, and laser oscillation can be generated in a stable vertical / transverse mode. However, the n-Al _0.07 Ga _0.93 N clad layer 4 and n-Al _0.07 Ga _0.93 N
If the total thickness of the Al _0.15 Ga _0.85 N layer 3 is 1 μm or more, a stable vertical / transverse mode can be obtained.

Although the Al composition of the n-AlGaN layer 3 is set to 0.13 in FIG. 1, it may be the same as or higher than the Al composition of the cladding layer 4. That is, it is important to select a material having a refractive index equal to or smaller than the refractive index of the cladding layer 4 in order to stabilize the vertical and transverse modes.

The reason why the n-AlGaN layer 3 is provided in this structure is not only to stabilize the vertical and transverse modes. This is because the presence of this layer can greatly suppress the occurrence of cracks.

In FIG. 1, between the n-Al _0.07 Ga _0.93 N cladding layer 4 and the substrate, except for the buffer layer, A
l low composition layer (the conventional n-GaN layer 50 shown in FIG. 5)
3) does not exist. From the substrate (or buffer layer) toward the active _{layer, n-Al 0. 15 Ga 0.85} N layer 3, n-Al
_0.07 Ga _0.93 N clad layer 4, n-GaN optical guide layer 5
And the Al composition decreases stepwise (monotonically). The inventors have experimentally found that such a configuration does not cause a crack which has conventionally occurred. Although the cause of the suppression of the crack generation is not clearly understood, it is considered that the direction in which the stress due to the difference in the thermal expansion coefficient is applied becomes constant. Since cracks do not occur during cooling, the quality of the active layer can be increased, and a highly reliable device can be obtained.

(Embodiment 2) In Embodiment 1, a single AlGaN layer 3 is added to stabilize the vertical and horizontal modes and suppress cracks. Here, a case where the Al composition decreases stepwise or linearly from the substrate 1 toward the active layer will be described.

In the structure shown in FIG. 2, a GaN buffer layer 201 and an n-Al _0.2 Ga _0.8 N layer 2 are formed on a sapphire substrate 1.
02, n-Al _0.16 Ga _0.84 N layer 203, n-Al _0.12 Ga
_0.88 N layer 204, n-Al _0.1 Ga _0.9 N layer 205, n-Al
_{A 0.07} Ga _0.93 N cladding layer 4 is laminated. The structure on the cladding layer 4 is the same as the structure in FIG.

As described above, by reducing the Al composition in multiple stages, the occurrence of cracks can be further suppressed as compared with the structure of FIG. 1, and the yield can be improved.

The structure of FIG. 2 has an Al composition of FIG.
However, as shown in FIG. 3B, it may be linearly and monotonically decreased from the buffer layer toward the active layer as shown in FIG. In this case, the entire layer containing Al also serves as the cladding layer. It is necessary that there is no GaN layer between the Al-containing layer and the buffer layer in order to stabilize the vertical and transverse modes and suppress cracks.

(Embodiment 3) In Embodiments 1 and 2, the distance between the cladding layer 4 and the buffer layer
The case where the layer containing 1 is a thick film has been described. Here, a case where a special thin film multilayer structure is used to suppress cracks will be described. The term “thin film” used herein refers to a layer having such a thickness that light is not greatly affected by a change in the refractive index of the film, and refers to a layer having a thickness of λ / (4n) or less. Here, λ is the oscillation wavelength of the laser, and n is the refractive index of the layer.

FIG. 4 shows the Al composition distribution of a layer provided between the buffer layer and the n-GaN light guide layer. In FIG. 4A, the n-AlGaN cladding layer has an Al composition of 8
% (Constant) layer, and a special thin-film multilayer structure is provided between the cladding layer and the buffer layer. This thin film multilayer structure is composed of a high Al composition AlGaN (Al composition = 18%) and a GaN layer, and has a film thickness of 5n.
m. The average Al composition of this thin film multilayer structure is 9%, which is higher than the Al composition of the n-AlGaN cladding layer.

The thickness of the layers constituting the thin film multilayer structure is 5 nm
Light is not affected by its refractive index change. Therefore, light will be confined by the average refractive index of the thin film multilayer structure. The total thickness of the thin film multilayer structure is about 1 micron, and laser oscillation can be performed in a stable vertical and transverse mode. In order to effectively confine light, the Al composition of the AlGaN layer is set to 0.1.
One or more is desirable.

In Embodiments 1 and 2, A
It has been described that the absence of a GaN layer between the layer containing 1 and the buffer layer is necessary for suppressing cracks. In FIG. 4, a GaN layer (5 nm) is formed under the cladding layer.
Will exist. However, it has been experimentally found that a very thin layer and a multilayer structure can suppress the occurrence of cracks during cooling. Ga
This is probably because the Al0.18Ga0.82N layer sandwiching the N layer alleviates the strain due to thermal shrinkage. That is, it functions as a strain relaxation layer.

In FIG. 4B, the clad layer also has a thin film multilayer structure (in the figure, described as a second thin film multilayer structure). Here, the average Al composition of the clad layer is lower than the average Al composition of the first thin-film multilayer structure below the clad layer, which is important for crack suppression.

In FIG. 4, the layer having a high Al composition has Al _0.18
Although Ga _0.82 N has been described using GaN as a layer having a low Al composition, it is sufficient if there is a difference in the Al composition.
May be contained.

In order to further suppress cracks, it is effective to reduce the doping amount of impurities.

Undoped Al _x Ga ₁ -xN (0 ≦ x ≦ 1)
Cracks tend to occur more easily in n-type Al _x Ga ₁ -xN than in n. In FIG. 4, cracks can be further suppressed by adding an impurity to one of the layers constituting the thin film multilayer structure. In particular, it is preferable to dope the AlGaN layer having a large band gap because the doping of the majority carrier (electrons) into the active layer is not hindered. As the n-type impurity, Si or Se is desirable.

In the present invention, a GaN-based semiconductor laser has been described as an example. However, it is needless to say that the present invention has a great effect when growing an active region such as a light emitting diode or an electronic device. In the light-emitting diode, the occurrence of cracks can be suppressed and a high-quality active layer can be obtained, so that the luminous efficiency can be improved. In the electronic device, the carrier mobility is greatly improved.

Although the first to third embodiments have been described individually, it goes without saying that the effects of the present invention are great even if they are combined. For example, the Al composition of the thin film multilayer structure in FIG. 4 may be gradually reduced toward the active layer.

[0034]

As described above, the GaN-based semiconductor device of the present invention has a structure in which, between the substrate and the cladding layer, Al that is equal to or higher than the Al composition of the cladding layer is formed. By having a GaN-based semiconductor layer (excluding the buffer layer) contained, cracks can be suppressed and a high-quality active layer can be laminated on the clad layer, and at the same time, the vertical and transverse modes are stabilized. As a result, it is possible to obtain a device having high reliability and having no trouble when used for an optical disk.

Further, the GaN-based semiconductor device of the present invention has a clad layer structure sandwiching the active layer, and has a thin film multilayer structure between the clad layer structure and the substrate. It is composed of a layer with a high Al composition and a layer with a low Al composition, and the average Al composition of the thin-film multilayer structure is equal to or higher than the average Al composition of the cladding layer structure. Has an Al composition,
Therefore, a high-quality active layer can be laminated by suppressing cracks, and at the same time, the vertical and transverse modes can be stabilized. As a result, a device having high reliability and free from defects when used in an optical disc can be obtained. it can.

Further, high Al having a thin film multilayer structure is formed.
When impurities are added to either the composition layer or the low Al composition layer, cracks can be further prevented from occurring, and a high-quality active layer can be obtained.

[Brief description of the drawings]

FIG. 1 is a sectional view of an element of a GaN-based semiconductor laser showing a first embodiment of the present invention.

FIG. 2 is a cross-sectional view of a GaN-based semiconductor laser according to a second embodiment of the present invention;

FIG. 3 is a diagram showing a profile of an Al composition of an AlGaN layer.

FIG. 4 is a view showing a profile of an Al composition of an AlGaN layer.

FIG. 5 is a sectional view of a conventional GaN-based quantum well semiconductor laser device;

FIG. 6 is a diagram for explaining a conventional problem, showing a refractive index distribution and a light distribution of constituent materials of a semiconductor laser.

FIG. 7 is a diagram for explaining a conventional problem.

[Explanation of symbols]

_Reference Signs _List 1 sapphire substrate 2 buffer layer 3 n-Al _0.15 Ga _0.85 N layer 4 n-Al _0.07 Ga _0.93 N clad layer 5 n-GaN light guide layer 6 MQW active layer 7 p-GaN light guide layer 8 p-Al _0.07 Ga _0.93 N cladding layer 9 p-GaN contact layer 10 p electrode 11 insulating film 12 wiring electrode 13 n electrode 201 GaN buffer layer 202 n-Al _0.2 Ga _0.8 N layer 203 n-Al _0.16 Ga _0.84 N layer 204 n-Al _0.12 Ga _0.88 N layer 205 n-Al _0.1 Ga _0.9 N layer 301 Sapphire substrate 302 Buffer layer 303 n-GaN layer 304 n-AlGaN cladding layer 305 n-GaN light guide layer 306 MQW active layer 307 p-GaN light guide layer 308 p-AlGaN Cladding layer 309 p-GaN contact layer 310 p electrode 311 SiO ₂ 312 n electrode

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[Procedure amendment]

[Submission date] April 19, 2000 (2004.1.
9)

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Claims

[Correction method] Change

[Correction contents]

[Claims]

Continued on the front page (72) Inventor Yusaburo Ban 1006 Kazuma Kadoma, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. F term (reference) 5F041 AA40 CA04 CA05 CA34 CA40 CA46 5F073 AA13 AA51 AA74 BA06 CA07 CB06 EA29

Claims (6)

[Claims] 1. A nitride semiconductor device comprising an active layer and a clad layer sandwiching the active layer, wherein the first clad layer between the active layer and the substrate contains Al, Further, between the first cladding layer and the substrate, the first cladding layer
A nitride semiconductor device having a nitride semiconductor layer containing Al equal to or higher than the Al composition of the first cladding layer (excluding a buffer layer). 2. A nitride semiconductor device comprising an active layer and a clad layer structure sandwiching said active layer, wherein said nitride semiconductor device has a thin film multilayer structure between said clad layer structure and a substrate. The multilayer structure is composed of a layer having a high Al composition and a layer having a low Al composition, and the average Al composition of the thin film multilayer structure is equal to the average Al composition of the cladding layer structure, or the average Al composition of the cladding layer structure. A nitride semiconductor device having a higher average Al composition. 3. The thickness of a layer constituting a thin film multilayer structure is λ.
The nitride semiconductor device according to claim 2, wherein the ratio is not more than / (4n). 4. A nitride semiconductor device having an active layer and a clad layer structure sandwiching the active layer, wherein the nitride semiconductor device has a thin film multilayer structure between the clad layer structure and a substrate. The multilayer structure is composed of a layer having a high Al composition and a layer having a low Al composition, and the average Al composition of the thin film multilayer structure is equal to the average Al composition of the cladding layer structure, or the average Al composition of the cladding layer structure. A nitride semiconductor device having a higher average Al composition, wherein one of a high-Al composition layer and a low-Al composition layer constituting a thin-film multilayer structure is doped with an impurity. 5. The nitride semiconductor device according to claim 4, wherein an impurity is added to a layer having a high Al composition constituting the thin film multilayer structure. 6. A layer having a high Al composition comprising Al _x Ga ₁ -xN (0.1 <x ≦ 1)
The nitride semiconductor device according to any one of claims 2 to 5, wherein