JP2003060314A - Nitride semiconductor element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a GaN-based semiconductor laser having stable vertical and lateral modes by improving optical confinement. SOLUTION: In this GaN-based semiconductor laser, an n-type Al0.07 Ga0.93 N first clad layer, an active layer, and a p-type Al0.07 Ga0.93 N second clad layer are laminated upon a substrate. In addition, the optical confinement to the active layer is improved, and occurrence of cracks in crystals is suppressed by forming an AlGaN layer having an Al composition, which is higher than (or equal to) that of the first clad layer between the first clad layer and substrate.

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, which is expected to be applied to the field of optical information processing, and a manufacturing method.

[0002]

2. Description of the Related Art Nitride semiconductors containing nitrogen (N) as a group V element are regarded as a promising material for short-wavelength light emitting devices because of their 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 researched, and blue light emitting diodes (LEDs) and green LEDs have been put to practical use. Further, a semiconductor laser having an oscillation wavelength in the 400 nm band is eagerly awaited in order to increase the capacity of an optical disk device, and a semiconductor laser made of a GaN-based semiconductor has been drawing 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 has been achieved. Sapphire substrate 50
GaN buffer layer 502, n-GaN layer 503, and n-AlGaN clad layer 50 on the first layer by metalorganic vapor phase epitaxy (MOVPE method).
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. The 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 SiO 25 is formed on both sides of the ridge stripe.
It is embedded by 11. Then, on the ridge stripe and the SiO ₂ 511, a p-electrode 51 made of, for example, Ni / Au
An n-electrode 512 made of, for example, Ti / Al is formed on the surface of the n-GaN layer 503, which is 0 or partially etched until the n-GaN layer 503 is exposed. In this element, 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, so that the MQW active layer 506 is An optical gain is generated inside, and laser oscillation in the oscillation wavelength 400 nm band occurs.
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 above room temperature has been realized.

By controlling the width and height of the ridge stripe, this laser is devised so as to oscillate in the fundamental mode in the horizontal transverse mode. That is, by providing a difference in the optical confinement coefficient between the fundamental transverse mode and the higher-order modes (first-order and higher modes), oscillation in the fundamental transverse mode is possible.

[0005]

However, a problem remains in the horizontal transverse mode (vertical transverse mode). FIG. 6 shows the refractive index distribution and the light distribution in the vertical direction 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 is inevitable. This is because the laser of FIG. 5 has two cores (portions having a high refractive index). That is, the first core is the active layer and the light guide layer, and the second core is the GaN layer (the layer between the sapphire substrate and the cladding layer).

When the laser is oscillated in the sixth mode, a large number of light emitting points are imaged even in a far field image (FFP). Therefore, when the light is focused by a lens or the like, it cannot be focused on a single spot. As a way to solve this,
(1) AlGaN first cladding layer (n-AlGa in FIG. 5)
Method for thickening N clad layer 504), (2) AlGa
A method of improving the Al composition of the N first cladding layer is conceivable. Both are effective in reducing the amount of light that seeps 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 AlGaN layer having a high Al composition is deposited on the GaN layer, cracks will occur during cooling. The cause of this is not clear, but it is considered to be due to the difference in the thermal expansion coefficient of the sapphire substrate, GaN, and AlGaN. If the active layer is deposited on the cracked AlGaN, problems such as deterioration of uniformity and reliability will occur.

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

[0009]

In the nitride semiconductor device of the present invention, the Al composition between the substrate and the clad layer is equal to or higher than the Al composition of the clad layer.
It has a GaN-based semiconductor layer containing Al. However,
The buffer layer is excluded.

Further, the nitride 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. An average Al composition of the thin film multilayer structure is equal to the average Al composition of the clad layer structure, or is higher than the average Al composition of the clad 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 the 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 clad layer structure or the average Al composition of the clad layer structure. A layer having a higher average Al composition and a higher Al composition and a lower A constituting a thin film multilayer structure.
Impurities are added to any of the layers having the l composition.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the drawings. The manufacturing method of the present invention is
The nitride semiconductor growth method is not limited to the MOVPE method, and has been proposed so far for growing a nitride semiconductor layer such as a hydride vapor phase epitaxy method (H-VPE method) and a molecular beam epitaxy method (MBE method). It can be applied to all the methods described.

(First Embodiment) 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 is placed on the sapphire substrate 1 at 500.degree.
G and NH ₃ are supplied to deposit the GaN buffer layer 2. After that, the temperature is raised to 1020 ° C., 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 cladding layer 4, n-GaN optical guide layer 5, multiple layers. Quantum well (MQ
W) Active layer 6, p-GaN optical guide layer 7, p-Al _0.07 Ga
_{The 0.93} N cladding layer 8 and the p-GaN contact layer 9 are sequentially stacked. 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 transverse mode. The stripe width is about 3-5 microns. A p-electrode 10 is formed on the p-GaN contact layer 9, and the 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 a part of the insulating film 11. The surface of the n-Al _0.15 Ga _0.85 N layer 3 which was etched until a part of the N-layer 3 was exposed had an n-electrode 1
3 is 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 to the active layer. Gain is generated and laser oscillation occurs 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. This layer is thick enough at 4 microns.
Therefore, light can be sufficiently confined in the vertical direction, and laser oscillation can be generated in a stable vertical transverse mode. However, n-Al _0.07 Ga _0.93 N cladding layer 4 and 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 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 for stabilizing the vertical transverse mode to select a material having a refractive index equal to or smaller than that of the cladding layer 4.

The reason for providing the n-AlGaN layer 3 in this structure is not only for stabilizing the vertical transverse mode. 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
layer having a low l composition (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 cladding layer 4, n-GaN optical guide layer 5
Then, the Al composition decreases stepwise (monotonically). The authors have experimentally found that with such a configuration, cracks that have conventionally occurred do not occur. The reason why the crack generation can be suppressed is not clear, but it is considered that the direction in which the stress is applied due to the difference in the thermal expansion coefficient becomes constant. Since no cracks are generated during cooling, the active layer can have high quality and a highly reliable device can be obtained.

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

In the structure shown in FIG. 2, the GaN buffer layer 201 and the n-Al _0.2 Ga _0.8 N layer 2 are formed on the 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 clad layer 4 is similar to the structure in FIG.

By thus reducing the Al composition in multiple stages, it is possible to further suppress the occurrence of cracks and improve the yield as compared with the structure of FIG.

Further, the structure of FIG. 2 has an Al composition of FIG.
However, it may be linearly and monotonically decreased from the buffer layer toward the active layer as shown in FIG. 3B. In this case, the entire Al-containing layer also serves as the cladding layer. The absence of a GaN layer between the Al-containing layer and the buffer layer is absolutely necessary for stabilizing the vertical transverse mode and suppressing cracks.

(Third Embodiment) In the first and second embodiments, A between the cladding layer 4 and the buffer layer is
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 for crack suppression will be described. The "thin film" referred to here means a thickness at which light is not significantly affected by a change in the refractive index of the film, and means a layer having a film 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 the layer provided between the buffer layer and the n-GaN optical guide layer. In FIG. 4A, the Al composition 8 is added to the n-AlGaN cladding layer.
% (Constant) layer is used, 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 AlGaN (Al composition = 18%) having a high Al composition and a GaN layer, and has a film thickness of 5 n.
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 film thickness of the layers constituting the thin film multilayer structure is 5 nm.
Since it is very thin, the light is not affected by the change in its refractive index. Therefore, light is confined by the average refractive index of the thin film multilayer structure. The total thickness of the thin film multi-layer structure is about 1 micron, and laser oscillation can be performed in a stable vertical transverse mode. In order to effectively confine light, the Al composition of the AlGaN layer is 0.
1 or more is desirable.

In the first and second embodiments, A
It was explained that the absence of a GaN layer between the layer containing 1 and the buffer layer is necessary for crack suppression. In FIG. 4, a GaN layer (5 nm) is formed under the cladding layer.
Will exist. However, it has been experimentally found that the generation of cracks during cooling can be suppressed by making the layer very thin and having a multi-layer structure. Ga
It is considered that the Al0.18Ga0.82N layer sandwiching the N layer relaxes the strain due to thermal contraction. That is, it functions as a strain relaxation layer.

In FIG. 4B, the clad layer also has a thin film multi-layer structure (described as a second thin film multi-layer structure in the drawing). 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, Al _{0.18 is formed} as a layer having a high Al composition.
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 Al composition.
May be included.

Further, in order to suppress cracks, it is also effective to reduce the doping amount of impurities.

Undoped Al _x Ga _1-x N (0≤x≤1)
In comparison with n-type Al _x Ga _{1 -x} N, cracks tend to occur more easily. In FIG. 4, the generation of cracks can be further suppressed by adding impurities to either one of the layers forming the thin film multilayer structure. In particular, it is preferable to dope the AlGaN layer having a large band gap, because it does not hinder the injection of majority carriers (electrons) into the active layer. Si or Se is preferable as the n-type impurity.

Although the present invention has been described by taking the GaN semiconductor laser as an example, it is needless to say that the present invention has a great effect also in growing an active region of a light emitting diode, an electronic device or the like. In the light emitting diode, generation of cracks can be suppressed and a high quality active layer can be obtained, so that the light emitting efficiency can be improved. In addition, the mobility of carriers is greatly improved in electronic devices.

Although the first to third embodiments have been individually described, 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 of FIG. 4 may be gradually decreased toward the active layer.

[0034]

As described above, in the GaN-based semiconductor device of the present invention, Al between the substrate and the clad layer is equal to or higher than the Al composition of the clad layer. By including the GaN-based semiconductor layer (excluding the buffer layer), cracks can be suppressed, a high-quality active layer can be stacked on the clad layer, and the vertical transverse mode can be stabilized at the same time. As a result, it is possible to obtain a device that is highly reliable and has no problems when used for an optical disc.

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. An average Al composition of the thin film multilayer structure is equal to the average Al composition of the clad layer structure, or is higher than the average Al composition of the clad layer structure. Has an Al composition,
Therefore, it is possible to stack a high-quality active layer by suppressing cracks and stabilize the vertical transverse mode at the same time, and as a result, it is possible to obtain a device that is highly reliable and has no problems when used in an optical disk. it can.

In addition, the high Al that constitutes the thin film multilayer structure
By adding impurities to either the composition layer or the low Al composition layer, the occurrence of cracks can be further prevented and a high-quality active layer can be obtained.

[Brief description of drawings]

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

FIG. 2 is a device sectional view of a GaN semiconductor laser showing a second embodiment of the present invention.

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

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

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

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]

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 cladding layer 5 n-GaN optical guide layer 6 MQW active layer 7 p-GaN optical 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

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor, Yuzaburo Ban             1006 Kadoma, Kadoma-shi, Osaka Matsushita Electric             Sangyo Co., Ltd. F-term (reference) 5F073 AA11 AA13 AA45 AA51 AA74                       AA89 CA07 CB05 CB07 DA05                       DA35 EA18

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, Furthermore, between the first clad layer and the substrate,
A nitride semiconductor device having a nitride semiconductor layer (excluding a buffer layer) containing Al equal to or higher than the Al composition of the first cladding layer. 2. A nitride semiconductor device comprising an active layer and a cladding layer structure sandwiching the active layer, wherein the nitride semiconductor device has a thin film multilayer structure between the cladding 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 clad layer structure or the average Al composition of the clad layer structure. A nitride semiconductor device having a higher average Al composition. 3. The thickness of the layers constituting the thin film multilayer structure is λ.
/ (4n) or less, The nitride semiconductor element of Claim 2. 4. A nitride semiconductor device comprising an active layer and a clad layer structure sandwiching the active layer, the nitride semiconductor device having 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 clad layer structure or the average Al composition of the clad layer structure. A nitride semiconductor device having a higher average Al composition and having impurities added to either a high Al composition layer or a low Al composition layer forming a thin film multilayer structure. 5. The nitride semiconductor device according to claim 4, wherein impurities are added to a layer having a high Al composition that constitutes a thin film multilayer structure. 6. A layer of high Al composition is Al _x Ga _1-x N (0.1 <x ≦ 1)
The nitride semiconductor device according to any one of claims 2 to 5, wherein