JPH08228042A - Semiconductor laser element - Google Patents

Abstract

PURPOSE: To enable the low threshold value current and the low noise to be realized by a method wherein the second lower clad layer, the first lower clad layer, a lower photoguide layer, an upper photoguide layer, the first upper clad layer, the second upper clad layer, etc., are successively formed while respective refractive indexes are set up to meet the relational expressions. CONSTITUTION: Within this semiconductor laser element 10, a buffer layer 12 on an n-GaAs substrate 11, the second clad layer 13b having refractive index of nc12 , the first clad layer 13a having another refractive index of nc11 , a lower photoguide layer 14 having the other refractive index ng1 , an active layer 31 having the other refractive index na , an upper photoguide layer 16 having the same refractive index na , the first upper clad layer 17a having the other refractive index ncu1 , the second upper clad layer 17b, etc., having the other refractive index ncu2 are successively laminated on the n-GaAs substrate 11. In such a constitution, respective refractive indexes have to meet the expressions thereof. Furthermore, most of the light is confined in the active layer 31, the upper and lower photoguide layers 14, 16 by the second upper and lower clad layers 13a, 13b having higher refractive indexes than those of the first upper and lower clad layers 17a, 17b while widely distributing the remaining light.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structure of a semiconductor laser device, and more particularly to a semiconductor laser device having a low threshold current, low noise, and good ellipticity of a far-field pattern of laser radiation.

[0002]

2. Description of the Related Art In recent years, the practical application of semiconductor laser devices, which have the advantages of small size, high output, and low price, has led to the application of lasers to general industrial machines and consumer machines where it has been difficult to use laser light sources. There is. Above all, the progress in the optical disk device and optical communication fields is remarkable. from now on,
Semiconductor laser devices are expected to be applied to many fields.

FIG. 3 shows a change in refractive index and a light intensity distribution in a light emitting region of a semiconductor laser device which has been conventionally used as a light source for an optical disk. In this semiconductor laser device, a lower clad layer, an active layer, and an upper clad layer are sequentially formed on a semiconductor substrate. This structure is DH (Double
-Heterostructure) structure. Further, in order to reduce the threshold current, an optical guide layer having a refractive index larger than that of the cladding layer and smaller than that of the active layer is formed between the cladding layer and the active layer.
separate Confinement Heter
GRIN-SCH (Grad) in which the refractive index of the optical structure is continuously changed.
ed Index Separate Confine
A mentHeterostructure structure has been proposed. Hereinafter, the light guide layer in the GRIN-SCH structure is particularly called a GRIN layer.

FIG. 4 shows a change in refractive index and a light intensity distribution in a light emitting region of a semiconductor laser device having a GRIN-SCH structure. According to these structures, most of the light is confined in the active layer and the light guide layer (GRIN layer), and the optical confinement coefficient, which is the ratio of the amount of light confined in the active layer to the total amount of light, increases. Therefore, the threshold current density can be reduced.

Further, in an optical disk device or the like, various external resonators are formed in the optical system from the semiconductor laser element to the reflection surface of the optical disk, and noise is generated in the laser light emitted from the semiconductor laser element. In order to reduce this noise, it has been studied to cause the semiconductor laser device to self-oscillate. It is said that increasing the optical confinement factor in the active layer is effective for self-sustained pulsation. SCH structure, GRI
The introduction of the N-SCH structure is also effective in this respect.

On the other hand, as a semiconductor laser device such as a light source for an optical disk, there is an increasing demand for improving the ellipticity of the far-field pattern of the laser emission light as well as the low threshold current and the low noise. The semiconductor laser device having the SCH structure and the GRIN-SCH structure confine most of the light in the active layer and the optical guide layer in order to realize low threshold current density and self-sustained pulsation by increasing the optical confinement coefficient in the active layer. There is. However, this increases the far-field pattern in the direction perpendicular to the growth interface between the active layer and the light guide layer, which causes deterioration of the ellipticity.

As one means for improving the ellipticity of the far-field pattern, Chen et al., Applied Physic
s Letters 56, 1409 (1990), there is a structure in which external waveguides are provided on both sides of the active layer.

FIG. 5 shows the change in the refractive index and the light intensity distribution in the light emitting region of the semiconductor laser device having this structure.
According to this structure, a part of the excited light is emitted from the external waveguide 6
Waves 1 and 62 are guided, and a small side peak appears in the near-field image in the vertical direction at the growth interface between the active layer and the cladding layer. Therefore, the far-field image in the vertical direction becomes very small, and the ellipticity of the far-field image is significantly improved.

[0009]

The structure shown by Chen et al. Is based on the principle that a large amount of light is guided by an external waveguide. However, this brings about a large decrease in the light confinement factor in the active layer, and the SCH structure, GRIN-S
This is contrary to the original purpose of the CH structure, which is to reduce the threshold current density.

The present invention solves the above problems, and an object thereof is to provide a semiconductor laser device which has a low threshold current, low noise, and good ellipticity of laser radiation.

[0011]

A semiconductor laser device according to claim 1 has a refractive index n _a formed on a semiconductor substrate.
An active layer having a refractive index n _g , a light guide layer having a refractive index n _g , a first cladding layer having a refractive index n _c1, and a second cladding having a refractive index n _c2 opposite to the active layer of the first cladding layer. layer, a semiconductor laser device which is constituted by a laminate including, but which is a _{n c1 <n g <n c2} <n a.

[0012] The semiconductor laser device according to claim 2, lower with on a semiconductor substrate, a lower second clad layer having a refractive index n _cl2, first cladding layer lower portion having a refractive index n _cl1, a refractive index n _g1 optical guide layer, the active layer having a refractive index n _a, the upper optical guide layer having a refractive index n _gu, refractive index n _cu1
Upper first cladding layer having a second upper cladding layer having a refractive index n _cu2, the semiconductor laser device but formed by sequentially _{forming, n cl1 <n gl <n} cl2 <n a n cu1 <n gu <n cu2 <N _a .

The operation of the present invention will be described below.

The semiconductor laser device of the present invention has a second refractive index higher than that of the first cladding layer adjacent to at least one of the lower first cladding layer and the upper first cladding layer. The clad layer is formed.
Therefore, most of the light is confined in the active layer and the light guide layer, while the rest of the light is widely distributed.

Therefore, the far-field pattern in the direction perpendicular to the growth interface between the active layer and the light guide layer is reduced with almost no change in the high optical confinement coefficient in the active layer obtained by the SCH structure or GRIN-SCH structure. Thereby, the ellipticity of the far-field pattern of the laser radiation is improved, and it is possible to provide a semiconductor laser device having a low threshold current, low noise, and a good ellipticity of the far-field pattern of the laser radiation.

The structure according to the present invention is a little inferior to the structure shown by Chen et al. In improving the ellipticity, but there is almost no decrease in the low threshold current density.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of a semiconductor laser device of the present invention will be described below with reference to the drawings. FIG. 1 shows a first embodiment of the semiconductor laser device of the present invention. FIG. 2 shows the first shown in FIG.
The refractive index change of the AA 'cross section of an Example and the light intensity distribution are shown.

The semiconductor laser device 10 of the present invention is n-Ga.
On the As substrate 11, an n-GaAs buffer layer (0.5 μm thick) 12, n-Al _w Ga _1-w As is formed by the MBE method.
(0 ≦ w ≦ 1) Lower second cladding layer (w = 0.40,
1.5 μm thick) 13b, n-Al _z Ga _1-z As (0 ≦ z
≦ 1) Lower first cladding layer (z = 0.65, 0.2 μm
_{Thickness) 13a, n-Al y Ga} 1-y As (0 ≦ y ≦ 1) lower GRIN layer (y = z → 0.42,0.1μm thickness) 14,
Multiple quantum well active layer _{31, p-Al y Ga 1} -y As upper G
RIN layer (y = 0.42 → z, 0.1 μm thickness) 16, p
-Al _z Ga _1-z As upper first cladding layer (0.2 μm
Thickness 17a, p-Al _w Ga _1-w As upper second cladding layer (0.1 μm thickness) 17b, p-GaAs protective layer (0.0
3 μm thickness) 18, p-Al _u Ga _1-u As (0 ≦ u ≦ 1)
Etch stop layer (u = 0.55, 0.03 μm thickness) 1
9, n-Al _v Ga _1-v As (0 ≦ v ≦ 1) current block layer (v = 0.1, 0.8 μm thickness) 20, n-GaAs current block layer protective layer (0.1 μm thickness) 21 Are stacked in this order. After each of these layers is continuously grown, the current blocking layer protective layer 21 and the current blocking layer 2 are formed into a stripe groove having a width of 4 μm by photolithography or the like.
0, the etch stop layer 19 is removed, and a stripe groove 24 is formed in the center.

Next, by liquid phase epitaxy, about 0.05 μm thick of the surface of the protective layer 18 and the current blocking layer protective layer 21 at the bottom of the stripe groove 24 is melted back, and p-Al _w Ga _{1-w is formed.} As regrown upper second cladding layer (w = 0.40, thickness of 1.5 μm at groove center) 22, p-G
An aAs cap layer (1 μm thick) 23 is formed. The upper electrode 25 is laminated on the upper surface of the p-GaAs cap layer 23, and the lower electrode 26 is laminated on the lower surface of the n-GaAs substrate 11.

The multiple quantum well active layer 31 is made of Al _x Ga _1-x.
As quantum well layer (x = 0.14, 0.01 μm thick) 32
5 layers, and an Al _t Ga _{1 -t} As barrier layer (t = 0.4,
(0.005 μm thickness) 33 is formed by alternately stacking four layers.

According to this embodiment, the active layer 31 and the GRI are
Although the light confinement in the N layers 14 and 16 is slightly weakened, the spread of light is greatly expanded. According to the semiconductor laser device 10 having this structure, the light confinement coefficient in the active layer 31 is reduced by about 10% as compared with the GRIN-SCH structure, and the growth interface between the active layer 31 and the cladding layers 13a and 17a is perpendicular to the growth interface. The far-field image in the direction can be reduced by about 33%.

According to the results of the experiment, in the semiconductor laser device 10 of the present embodiment, self-excited oscillation up to 3 mW with a threshold current of 20 mA and an ellipticity of 2.5: 1 of the laser emission light were obtained. The far-field image is the active layer 31 and GRIN layer 14, 1
28 ° vertically and 11 ° horizontally at the growth interface of 6
Met.

Although the above embodiments have been described only for the AlGaAs system, since the principle of the present invention is based only on the magnitude relation of the refractive index between the semiconductor layers of the semiconductor laser device, In (AlGa) P system, AlGaAsP system. Other such as
It goes without saying that the present invention can be applied to a semiconductor laser device using any material such as III-V group, further II-VI group, and chalcopyrite type.

[0024]

The semiconductor laser device of the present invention has the first lower portion.
A second cladding layer having a refractive index higher than that of the adjacent first cladding layer is formed on at least one of the lower side of the cladding layer and the upper side of the upper first cladding layer.
Therefore, most of the light can be confined in the active layer and the light guide layer, while the remaining light can be widely distributed. And
It is possible to reduce the far-field image in the direction perpendicular to the growth interface between the active layer and the light guide layer, while hardly changing the high optical confinement coefficient in the active layer obtained by the SCH structure and the GRIN-SCH structure. This gives a low threshold current,
It is possible to provide a semiconductor laser device having low noise and a good ellipticity of the far-field pattern of laser radiation.

If this element is used in, for example, an optical disk device, the current consumption is reduced, the reading accuracy of recorded information is improved,
The size of the optical pickup unit is reduced by simplifying the optical system, and the performance of the entire optical disc device is greatly improved.

[Brief description of drawings]

FIG. 1 is a sectional view schematically showing a semiconductor laser device according to an embodiment of the present invention.

FIG. 2 is a diagram showing a change in refractive index and a light intensity distribution of the semiconductor laser device of the example of FIG.

FIG. 3 is a diagram showing a refractive index change and a light intensity distribution in a light emitting region of a semiconductor laser device having a conventional DH structure.

FIG. 4 is a diagram showing a refractive index change and a light intensity distribution in a light emitting region of a semiconductor laser device having a conventional GRIN-SCH structure.

FIG. 5 is a diagram showing a change in refractive index and a light intensity distribution in a light emitting region of a conventional semiconductor laser device having a structure shown by Chen et al.

[Explanation of symbols]

 3, 13, 43 Lower clad layer 3a, 13a, 43a Lower first clad layer 3b, 13b, 43b Lower second clad layer 4, 14, 44 Lower optical guide layer (GRIN layer) 5, 45 Active layer 6, 16, 46 upper optical guide layer (GRIN layer) 7, 17, 47 upper clad layer 10, 40 semiconductor laser device 11, 41 substrate 12, 42 buffer layer 17a, 47a upper first clad layer 17b, 47b upper second clad layer 18 protection Layer 19 Etch stop layer 20, 50 Current blocking layer 21 Current blocking layer Protection layer 22 Re-growth upper second cladding layer 23, 51 Cap layer 24 Stripe groove 25 Upper electrode 26 Lower electrode 31 Multiple quantum well active layer 32 Quantum well layer 33 Barrier layer 48 Contact layer 49 Mesa region 61, 62 External waveguide

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Masafumi Kondo, 22-22 Nagaike-cho, Abeno-ku, Osaka-shi, Osaka, Japan Sharp Corporation (72) Hiroyuki Hosoba, 22-22 Nagaike-cho, Abeno-ku, Osaka, Osaka Co., Ltd. (72) Inventor Shinji Kaneiwa 22-22 Nagaike-cho, Abeno-ku, Osaka-shi, Osaka Inside Sharp Corporation (72) Toshio Hata 22-22 Nagaike-cho, Abeno-ku, Osaka-shi, Osaka Sharp Corporation Within

Claims (2)

[Claims] 1. A refractive index n _a formed on a semiconductor substrate.
An active layer having a refractive index n _g , a light guide layer having a refractive index n _g , a first cladding layer having a refractive index n _c1, and a second cladding having a refractive index n _c2 opposite to the active layer of the first cladding layer. layer, a semiconductor laser device which is constituted by a laminate comprising a semiconductor laser element which is a _{n c1 <n g <n c2} <n a. To 2. A semiconductor substrate, a lower second clad layer having a refractive index n _cl2, a lower first clad layer having a refractive index n _cl1, a lower optical guide layer having a refractive index n _g1, the refractive index n _a active layer having an upper optical guide layer having a refractive index n _gu, upper first cladding layer having a refractive index n _cu1, the second upper cladding layer having a refractive index n _cu2, the semiconductor laser device but formed by successively forming, _{n cl1 <n gl <n cl2} <n a n cu1 <n gu <n cu2 < semiconductor laser device according to claim 1, characterized in that a n _a.