JP2975473B2 - Semiconductor laser device - Google Patents

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, a low noise, and a good ellipticity of a far-field image of laser radiation.

[0002]

2. Description of the Related Art In recent years, with the practical use of semiconductor laser devices having the advantages of small size, high output, and low cost, the application of lasers to general industrial machines and consumer machines, for which use of laser light sources has been difficult in the past, has been advanced. I have. Above all, there has been remarkable progress in fields such as optical disc devices and optical communication. In the future, semiconductor laser devices are expected to be applied to more fields.

FIG. 7 shows a change in a refractive index and a light intensity distribution in a light emitting region of a semiconductor laser device conventionally used as a light source for an optical disk or the like. 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 called DH (Double-Het)
erostructure) structure. Further, in response to a demand for lowering the threshold current, a SCH (Separate Confinement H) having an optical guide layer having a higher refractive index than the cladding layer and a lower refractive index than the active layer is formed between the cladding layer and the active layer.
GRIN-SCH (Graded Index) in which the refractive index of the light guide layer is continuously changed.
Separate Confinement Heterostructure) structure has been proposed. Hereinafter, the light guide layer in the GRIN-SCH structure is particularly called a GRIN layer.

FIG. 8 shows a refractive index change 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 light 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.

In an optical disk device and 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 laser light emitted from the semiconductor laser element. In order to reduce this noise, it has been studied to cause self-excited oscillation in a semiconductor laser device. It is said that increasing the light confinement coefficient in the active layer is effective for self-sustained pulsation.
Introducing the N-SCH structure is also effective in this respect.

On the other hand, as for a semiconductor laser device such as a light source for an optical disk, there is an increasing demand for a low current threshold value and low noise as well as an improvement in the ellipticity of a far-field image of laser radiation. In the semiconductor laser device having the SCH structure and the GRIN-SCH structure, most of the light is confined in the active layer and the light guide layer in order to realize low threshold current density and self-pulsation by increasing the light confinement coefficient in the active layer. . However, this increases the far-field pattern in the direction perpendicular to the growth interface between the active layer and the light guide layer, and causes deterioration of the ellipticity. One way to improve the ellipticity of this far-field image is by Chen et al. In Applied Physics Letters.
56, 1409 (1990), there is a structure in which active layers are provided on both sides of a waveguide.

FIG. 9 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 transmitted to the external waveguide 6.
Waves 1 and 62 are guided, and a small side peak stands in the near-field image perpendicular to the growth interface between the active layer and the cladding layer. Therefore, the far-field image in the vertical direction is very small,
The ellipticity of the far-field image is also greatly improved.

[0008]

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 results in a significant decrease in the optical confinement coefficient in the active layer, and the SCH structure, GRIN-S
This is contrary to the original purpose of the CH structure, that is, the lower threshold current density.

An object of the present invention is to provide a semiconductor laser device having a low threshold current, a low noise, and a good ellipticity of laser radiation light.

[0010]

According to a semiconductor laser device of the present invention, a lower clad layer, a lower light guide layer, an active layer, an upper light guide layer, and an upper clad layer are sequentially laminated on a semiconductor substrate. At least one of the cladding layer and the upper cladding layer comprises at least two layers,
The refractive index of the layer close to the active layer is smaller , and the relationship of the refractive index of the lower refractive index layer of the cladding layer <the refractive index of the light guide layer <the refractive index of the active layer is satisfied, and The guide layer includes a portion having a higher refractive index than the layer having a higher refractive index of the cladding layer, thereby achieving the above object. Further, in the semiconductor laser device of the present invention, a portion of the optical guide layer having a higher refractive index than that of the clad layer having a higher refractive index may be formed of a direct transition semiconductor. Furthermore, the semiconductor laser device of the present invention
It is preferable that a portion of the optical guide layer, which is formed of a GaAs-based semiconductor material and has a higher refractive index than that of the clad layer having a higher refractive index, has an Al composition ratio of 0.42 or less. Furthermore, in the semiconductor laser device of the present invention, it is preferable that the active layer is made of AlGaAs having an Al composition ratio of 0.14. Further, in the semiconductor laser device of the present invention, a cap layer is further laminated on the upper clad layer, and a layer having a larger refractive index of the clad layer has a thickness of 1.0 μm.
It is preferable that the thickness be 1.5 μm or less.

[0011]

The semiconductor laser element of the action of the present invention is the second having a lower or upper Buk upper in at least one of the Rudd layer, close to refractive index higher than that of the torque Rudd layer below Buk Rad layer A cladding layer is formed. Therefore, the remaining light is widely distributed while most of the light is confined in the active layer and the light guide layer.

Therefore, the far-field image in the direction perpendicular to the growth interface between the active layer and the light guide layer is reduced without substantially changing the light confinement coefficient in the high active layer obtained by the SCH structure or GRIN-SCH structure. Thereby, the ellipticity of the far-field image of the laser radiation can be improved, and a semiconductor laser device having a low threshold current, low noise, and good ellipticity of the far-field image of the laser radiation can be provided.

Although the structure according to the present invention is slightly inferior in improving ellipticity as compared with the structure shown by Chen et al., The decrease in low threshold current density is hardly observed.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments 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 is a sectional view of the first embodiment shown in FIG.
The change in the refractive index and the light intensity distribution in the section A ′ are shown.

The semiconductor laser device 10 of this structure has n-Ga
An n-GaAs buffer layer (0.5 μm thick) 12 and n-Al _z Ga _{1 -z} As are formed on an As substrate 11 by MBE.
(0 ≦ z ≦ 1) Lower first cladding layer (z = 0.55,
1.5μm _{thick) 13a, n-Al y Ga} 1-y As (0 ≦ y
≦ 1) Lower GRIN layer (y = 0.4, 0.15 μm thickness)
14. Al _x Ga _{1 -x} As (0 ≦ x ≦ 1) active layer (x =
0.14,0.05μm _{thickness) 15, p-Al y Ga} 1-y A
s Upper GRIN layer (y = z → 0.4, 0.15 μm thickness)
_{16, p-Al z Ga 1} -z As upper first cladding layer (0.
17a, p-GaAs protective layer (0.03 μm)
_{Thick) 18, n-Al u Ga} 1-u As (0 ≦ u ≦ 1) etch stop layer (u = 0.55,0.03μm thickness) 19, n
-Al _v Ga _{1 -v} As (0 ≦ v ≦ 1) current blocking layer (v
= 0.1, 0.8 μm thickness) 20 and an n-GaAs current blocking layer protection layer (0.1 μm thickness) 21 are stacked in this order. After these layers are continuously grown, the current blocking layer protective layer 21, the current blocking layer 20, and the etch stop layer 19 are removed by a photolithography method or the like into a stripe groove having a width of 4 μm, and a stripe groove 24 is formed at the center. Form.

Next, the surface of the protective layer 18 at the bottom of the stripe groove 24 and the current blocking layer protective layer 21 is melted back by a thickness of about 0.05 μm by a liquid phase growth method. And
_{p-Al w Ga 1-w} As (0 ≦ w ≦ 1) regrown second upper cladding layer (w = 0.45, _i.e., directly遷通semiconductor,
A p-GaAs cap layer (1 μm thick) 23 is formed at the center of the groove (1.5 μm thick) 22. The upper electrode 25 is p-Ga
The lower electrode 26 is stacked on the lower surface of the n-GaAs substrate 11 and the lower electrode 26 is stacked on the upper surface of the As cap layer 23.

According to this structure, as compared with the so-called GRIN-SCH structure in which the cladding layer is a single layer, the active layer 1
The far-field image in the vertical direction at the growth interface between the active layer 15 and the GRIN layers 14 and 16 can be reduced by about 20% while the decrease in the light confinement coefficient to 5 is suppressed to about 5%.

This is because, as shown in FIG. 2, most of the light is confined in the active layer 15 and the GRIN layers 14 and 16, while the remaining light is widely distributed. Compared with the light intensity distribution of the light emitting region of the semiconductor laser device having the GRIN-SCH structure shown in FIG.
The difference is clear.

According to an experiment, in the semiconductor laser device 10 of the present embodiment, self-excited oscillation was performed up to 3 mA at a threshold current of 25 mA, and an ellipticity of laser radiation of 3: 1 was obtained. The far-field image was 33 ° vertically and 11 ° horizontally.

FIG. 3 shows a second embodiment of the semiconductor laser device of the present invention. The method of manufacturing the semiconductor laser device 10 is basically the same as that of the first embodiment, except that the lower clad layer has a double structure. Specifically, in the first embodiment, n-
The Al _z Ga _1-z As lower first clad layer 13a in place of the step of forming only one layer, in the second embodiment n-Al
_w Ga _1-w As (0 ≦ w ≦ 1) Lower second cladding layer (w = 0.45, that is, a direct transition semiconductor, 1.5 μm
Thickness) 13b, are sequentially formed _{Nn-Al z Ga 1-z} As lower first clad layer (z = 0.55,0.2μm thickness) 13a.

FIG. 4 shows the change in the refractive index and the light intensity distribution in the cross section BB 'of the second embodiment shown in FIG. Although the confinement of light in the active layer 15 and the GRIN layers 14 and 16 is somewhat weakened, the spread of light is greatly increased.
According to the semiconductor laser device 10 having this structure, GRIN-S
Although the decrease in the light confinement coefficient in the active layer 15 is about 9% as compared with the CH structure, the active layer 15 and the cladding layer 13
The far-field image perpendicular to the growth interface of a and 17a is about 33%
Can also be reduced.

According to the results of the experiment, in the semiconductor laser device 10 of this embodiment, self-excited oscillation was performed up to 3 mW at a threshold current of 26 mA, and an ellipticity of laser radiation of 2.5: 1 was obtained. The far-field image is 28 ° vertically and 1 horizontally.
It was 1 ゜.

FIG. 5 shows a reference example of the semiconductor laser device of the present invention. The method of manufacturing the semiconductor laser device 10 is basically the same as that of the second embodiment. The difference is that the active layer 21 has a multiple quantum well structure, and the thickness and the mixed crystal ratio of some of the other layers are changed to reduce the threshold current.

Specifically, the semiconductor laser device 1 of this reference example
Numeral 0 denotes an n-GaAs buffer layer (0.5 μm thick) 12, n-A
l _w Ga _{1 -w} As lower second cladding layer (w = 0.40,
1.5 μm thickness) 13b, n-Al _z Ga ₁ -z As lower first cladding layer (z = 0.65, 0.2 m thickness) 13a, n−
Al _y Ga _1-y As lower GRIN layer (y = z → 0.42,
0.1 μm thick) 14, multiple quantum well active layer 31, p-A
l _y Ga _1-y As upper GRIN 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) 17 a, p-Al _w Ga _{1 -w} A
s upper second cladding layer (0.1 μm thick) 17b, p-G
aAs protective layer (0.03 .mu.m thick) 18, p-Al _u Ga
_1-u As etch stop layer (u = 0.55, 0.03μ)
m _{thick) 19, n-Al v Ga} 1-v As current blocking layer (v
= 0.1, 0.8 μm thickness) 20 and an n-GaAs current blocking layer protection layer (0.1 μm thickness) 21 are stacked in this order.

After these layers are continuously grown, the current blocking layer protective layer 21, the current blocking layer 20, and the current blocking layer 20 are formed into a stripe groove having a width of 4 μm by photolithography or the like.
The etch stop layer 19 is removed. Next, the surface of the protection layer 18 at the bottom of the stripe groove 24 and the surface of the current blocking layer protection layer 21 are melted back by a thickness of about 0.05 μm by a liquid phase growth method, and p-Al _w Ga _1-w As is regrown. Two cladding layers (w = 0.40), ie, direct transition semiconductor
A p-GaAs cap layer (1 μm thick) 23 is formed at the center of the body and groove (1.5 μm thick) 22. The upper electrode 25 is p-
The lower electrode 2 is laminated on the upper surface of the GaAs cap layer 23.
6 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 thickness) 32
And an Al _t Ga _1-t As barrier layer (t = 0.4,
(Thickness: 0.005 μm) 33 is formed by alternately stacking four layers.

[0027] According to the results of the experiment, in the semiconductor laser device 10 of the present embodiment is to self-oscillation at the threshold current 20mA to 3 mW, and the ellipticity of the laser emitted light 2.5: 1 was obtained. The far-field image shows the active layer 31 and the GRIN layers 14, 1
The growth interface of No. 6 was 28 ° in the vertical direction and 11 ° in the horizontal direction.

FIG. 6 shows a third embodiment of the semiconductor laser device of the present invention. Specifically, the semiconductor laser device 40 of this embodiment
Is formed on the n-GaAs substrate 41 by MOCVD.
n-GaAs buffer layer (0.5 μm thickness) 42, n-I
n _0.5 (Al _{w ′} Ga _{1−w ′} ) _0.5 P (0 ≦ w ′ ≦ 1) lower second cladding layer (w ′ = 0.8, 1.0 μm thickness) 43b;
n-In _0.5 Al _0.5 P lower first cladding layer (0.20 μm
m _{thick) 43a, n-In 0.5 (} Al y 'Ga 1-y') 0.5 P
(0 ≦ y ′ ≦ 1) Lower light guide layer (y ′ = 0.7, 0.1)
5 μm thick) 44, In _0.5 Ga _0.5 P active layer (0.07 μm)
m-thickness) 45, p-In _0.5 (Al _{y ′} Ga _{1-y ′} ) _0.5 P upper light guide layer (0.15 μm thick) 46, p-In _0.5 Al
_0.5P upper first cladding layer (0.20 μm thickness) 47a,
A p-In _0.5 (Al _{w ′} Ga _{1-w ′} ) _0.5 P upper second cladding layer (1.0 μm thickness) 47b and a p-In _0.5 Ga _0.5 P contact layer (0.2 μm thickness) 48 are sequentially formed.

Next, SiO2 is deposited on the contact layer 48, and a stripe-shaped mesa region 49 having a width of 3.5 μm is formed by photolithography or the like. Then this is again M
The n-GaAs current blocking layer (0.8 μm thick) 50 is grown in an OCVD apparatus.

Thereafter, the SiO 2 on the mesa region 49 is removed, and a p-GaAs cap layer (1 μm thick on the mesa surface) 51 is grown again by the MOCVD method.

With this structure, the cladding layers 47a, 47
b is a single layer, compared to the GRIN-SCH structure,
5, the far-field image in the direction perpendicular to the growth interface between the active layer 45 and the SCH layers 44 and 46 can be reduced by about 30%.

According to the results of the experiment, in the semiconductor laser device 40 of the present embodiment, self-excited oscillation was performed up to 3 mW at a threshold current of 25 mA, and an ellipticity of laser radiation of 3.5: 1 was obtained. The far-field image is 28 ° vertically and 8 horizontally.
°.

In the above embodiment, AlGaAs
Although only the system and the In (AlGa) P system are shown, the principle of the present invention depends only on the magnitude relation of the refractive index between the respective semiconductor layers of the semiconductor laser device.
It goes without saying that the present invention can be applied to a semiconductor laser device using any material such as a III-V group, a II-VI group, and a chalcopyrite-based material.

[0034]

According to the semiconductor laser device of the present invention, a second cladding layer having a refractive index higher than that of an adjacent cladding layer is formed on at least one of the lower side of the lower cladding layer and the upper side of the upper cladding layer. Is done. For this reason, the remaining light can be widely distributed while most of the light is confined in the active layer and the light guide layer. The far-field image in the direction perpendicular to the growth interface between the active layer and the light guide layer can be reduced without substantially changing the light confinement coefficient to the high active layer obtained by the SCH structure and the GRIN-SCH structure. Becomes This makes it possible to provide a semiconductor laser device having low threshold current, low noise, and good ellipticity of a far-field image of laser radiation. And the light guide layer,
Higher refractive index than the higher refractive index layer of the cladding layer
By including a portion, the light distribution
That is more than necessary for the higher refractive index layer of the cladding layer.
It is too long, causing side peaks in the far-field image,
Light enters the plate or cap layer too much, causing light absorption,
The value current can be prevented from rising. In addition, the present invention
In this structure, the confinement coefficient of light in the active layer is kept high.
To maintain the light density of the active layer and the light guide layer.
This is very high compared to the
The temperature rise near the conductive layer is severe, and the burden on the device life is large.
Good. Because of this, the refraction of the cladding layer of the light guide layer
Direct transition semiconductor with higher refractive index than higher refractive index layer
Or AlGaAs having an Al composition ratio of 0.42 or less
s to cause deterioration of the element life.
Optical transition near the active layer due to the tangential transition semiconductor carrier
Can be prevented from being used in highly
Life can be secured. Furthermore, the cladding layer
The thickness of the layer having a higher refractive index should be 1.5 μm or less.
With, from active layer and light guide layer to substrate and cap layer
The temperature rise near the active layer by improving heat dissipation
And a longer element life can be reliably obtained.
You. At the same time, the thickness of the higher refractive index layer of the cladding layer
Is set to 1.0 μm or more, so that light
The amount of light absorption generated by the
It can be suppressed to a level that has almost no effect.

If this element is used in, for example, an optical disk device, the current consumption can be reduced, the reading accuracy of recorded information can be improved,
The miniaturization of the optical pickup unit and the like by the simplification of the optical system are achieved, and the performance of the entire optical disk device is greatly improved.

[Brief description of the drawings]

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

FIG. 2 is a diagram showing a change in a refractive index and a light intensity distribution of the semiconductor laser device of the embodiment of FIG. 1;

FIG. 3 is a sectional view schematically showing a second embodiment of the semiconductor laser device of the present invention.

FIG. 4 is a diagram showing a change in refractive index and a light intensity distribution of the semiconductor laser device of the embodiment in FIG. 3;

FIG. 5 is a sectional view schematically showing a reference example of the semiconductor laser device of the present invention.

FIG. 6 is a sectional view schematically showing a third embodiment of the semiconductor laser device of the present invention.

FIG. 7 is a diagram showing a change in refractive index and light intensity distribution in a light emitting region of a conventional semiconductor laser device having a DH structure.

FIG. 8 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 GRIN-SCH structure.

FIG. 9 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 cladding layer, 3a, 13a, 43a, lower first cladding layer, 3b, 13b, 43b, lower second cladding layer, 4, 14, 44, lower light guide layer (GRIN
5, 15, 45, active layer, 6, 16, 46, upper light guide layer (GRIN
7, 17, 47, upper cladding layer, 10, 40, semiconductor laser element, 11, 41, substrate, 12, 42, buffer layer, 17a, 47a, upper first cladding layer, 17b, 47b, upper first layer 2, cladding layer, 18, protective layer, 19, etch stop layer, 20, 50, current blocking layer, 21, current blocking layer protecting layer, 22, regrowth 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,

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Masafumi Kondo 22-22, Nagaike-cho, Abeno-ku, Osaka-shi, Osaka Inside Sharp Corporation (72) Inventor Hiroyuki Hoso 22-22, Nagaike-cho, Abeno-ku, Osaka-shi, Osaka Sharp Corporation (72) Inventor, Shinji Kaneiwa, 22-22, Nagaike-cho, Abeno-ku, Osaka-shi, Osaka, Japan Inside Sharp Corporation (72) Inventor Toshio Hata 22-22, Nagaike-cho, Abeno-ku, Osaka-shi, Osaka, Japan Sharp Corporation (56) References JP-A-5-243669 (JP, A) JP-A-2-12885 (JP, A) JP-A-2-78290 (JP, A) JP-A-5-235470 (JP, A) JP-A-56 -40294 (JP, A) JP-A-55-96695 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) H01S 3/18

Claims (5)

(57) [Claims] To 1. A semiconductor substrate, a lower cladding layer, the lower optical guide layer, the active layer, an upper optical guide layer, and the upper clad layer are sequentially stacked, the lower clad layer, at least one of the upper cladding layer, at least Two layers, close to the active layer
And the refractive index of the layer having a smaller refractive index of the cladding layer <the refractive index of the light guide layer <the refractive index of the active layer. A semiconductor laser device comprising a portion of the cladding layer having a higher refractive index than a layer having a higher refractive index. 2. The semiconductor according to claim 1, wherein a portion of the light guide layer having a higher refractive index than that of the clad layer having a higher refractive index is formed of a direct transition semiconductor. Laser element. 3. The semiconductor laser device according to claim 1, wherein the semiconductor laser device is mainly formed of an AlGaAs-based semiconductor material, and has a higher refractive index than the optical guide layer having a higher refractive index than the cladding layer. A semiconductor laser device wherein a portion has an Al composition ratio of 0.42 or less. 4. The active layer according to claim 1, wherein the Al composition ratio is 0.14.
4. The semiconductor laser device according to claim 1, comprising GaAs. 5. The semiconductor laser device according to claim 1, wherein a cap layer is further laminated on the upper clad layer, and the layer having a larger refractive index of the clad layer has a thickness greater than that of the upper clad layer. Is 1.0 μm or more and 1.5 μm or less.