High-power semiconductor laser device having current...

Details High-power semiconductor laser device having current... High-power semiconductor laser device having current...

2001-04-20

2003-07-29

Ip, Paul (Department: 2828)

Pulse or digital communications

Repeaters

Testing

C375S213000, C375S213000

Reexamination Certificate

active

06600770

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser device having an internal current confinement structure and an index-guided structure.
2. Description of the Related Art
(1) In many conventional current semiconductor laser devices which emit light in the 0.9 to 1.1 &mgr;m band, a current confinement structure and an index-guided structure are provided in crystal layers which constitute the semiconductor laser devices so that the semiconductor laser device oscillates in a fundamental transverse mode. For example, IEEE Journal of Selected Topics in Quantum Electronics, vol. 1, No. 2, 1995, pp. 102 discloses a semiconductor laser device which emits light in the 0.98 &mgr;m band. This semiconductor laser device is formed as follows.
On an n-type GaAs substrate, an n-type Al
0.48
Ga
0.52
As lower cladding layer, an undoped Al
0.2
Ga
0.8
As optical waveguide layer, an Al
0.2
Ga
0.8
As/In
0.2
Ga
0.8
As double quantum well active layer, an undoped Al
0.2
Ga
0.8
As optical waveguide layer, a p-type AlGaAs first upper cladding layer, a p-type Al
0.67
Ga
0.33
As etching stop layer, a p-type Al
0.48
Ga
0.52
As second upper cladding layer, a p-type GaAs cap layer, and an insulation film are formed in this order. Next, a narrow-stripe ridge structure is formed above the p-type Al
0.67
Ga
0.33
As etching stop layer by the conventional photolithography and selective etching, and an n-type Al
0.7
Ga
0.3
As and n-type GaAs materials are embedded in both sides of the ridge structure by selective MOCVD using Cl gas. Then, the insulation film is removed, and thereafter a p-type GaAs layer is formed. Thus, a current confinement structure and an index-guided structure are built in the semiconductor laser device.
However, the above semiconductor laser device has a drawback that it is very difficult to form the AlGaAs second upper cladding layer on the AlGaAs first upper cladding layer, since the AlGaAs first upper cladding layer contains a high Al content and is prone to oxidation, and selective growth of the AlGaAs second upper cladding layer is difficult.
(2) In addition, IEEE Journal of Selected Topics in Quantum Electronics, vol. 29, No. 6, 1993, pp. 1936 discloses a semiconductor laser device which oscillates in a fundamental transverse mode, and emits light in the 0.98 to 1.02 &mgr;m band. This semiconductor laser device is formed as follows.
On an n-type GaAs substrate, an n-type Al
0.4
Ga
0.6
As lower cladding layer, an undoped Al
0.2
Ga
0.8
As optical waveguide layer, a GaAs/InGaAs double quantum well active layer, an undoped Al
0.2
Ga
0.8
As optical waveguide layer, a p-type Al
0.4
Ga
0.6
As upper cladding layer, a p-type GaAs cap layer, and an insulation film are formed in this order. Next, a narrow-stripe ridge structure is formed above a mid-thickness of the p-type Al
0.4
Ga
0.6
As upper cladding layer by the conventional photolithography and selective etching, and an n-type In
0.5
Ga
0.5
P material and an n-type GaAs material are embedded in both sides of the ridge structure by selective MOCVD. Finally, the insulation film is removed, and then electrodes are formed. Thus, a current confinement structure and an index-guided structure are realized in the layered construction.
However, the above semiconductor laser device also has a drawback that it is very difficult to form the InGaP layer on the AlGaAs upper cladding layer, since the AlGaAs upper cladding layer contains a high Al content and is prone to oxidation, and it is difficult to grow an InGaP layer having different V-group component, on such an upper cladding layer.
(3) Further, IEEE Journal of Selected Topics in Quantum Electronics, vol. 1, No. 2, 1995, pp. 189 discloses an all-layer-Al-free semiconductor laser device which oscillates in a fundamental transverse mode, and emits light in the 0.98 &mgr;m band. This semiconductor laser device is formed as follows.
On an n-type GaAs substrate, an n-type InGaP cladding layer, an undoped InGaAsP optical waveguide layer, an InGaAsP tensile strain barrier layer, an InGaAs double quantum well active layer, an InGaAsP tensile strain barrier layer, an undoped InGaAsP optical waveguide layer, a p-type InGaP first upper cladding layer, a p-type GaAs optical waveguide layer, a p-type InGaP second upper cladding layer, a p-type GaAs cap layer, and an insulation film are formed in this order. Next, a narrow-stripe ridge structure is formed above the p-type InGaP first upper cladding layer by the conventional photolithography and selective etching, and an n-type In
0.5
Ga
0.5
P material is embedded in both sides of the ridge structure by selective MOCVD. Finally, the insulation film is removed, and a p-type GaAs contact layer is formed. Thus, a current confinement structure and an index-guided structure are realized.
The reliability of the above semiconductor laser device is improved since the strain in the active layer can be compensated for. However, the above semiconductor laser device also has a drawback that the kink level is low (about 150 mW) due to poor controllability of the ridge width.
(4) Furthermore, IEEE Journal of Quantum Electronics, vol. 29, No. 6, 1993, pp. 1889-1894 discloses an internal striped structure semiconductor laser device which oscillates in a fundamental transverse mode, and emits light in the 0.8 &mgr;m band. This semiconductor laser device is formed as follows.
On an n-type GaAs substrate, an n-type AlGaAs lower cladding layer, an AlGaAs/GaAs triple quantum well active layer, a p-type AlGaAs first upper cladding layer, an n-type AlGaAs current confinement layer, and an n-type AlGaAs protection layer are formed in this order. Next, a narrow-stripe groove is formed, by the conventional photolithography and selective etching, to such a depth that the groove penetrates the n-type AlGaAs current confinement layer. Next, over the above structure, a p-type AlGaAs second upper cladding layer and a p-type GaAs contact layer are formed.
In the above semiconductor laser device, the stripe width can be controlled accurately, and high-output-power oscillation in a fundamental transverse mode can be realized by the difference in the refractive index between the n-type AlGaAs current confinement layer and the p-type AlGaAs second upper cladding layer. However, the above semiconductor laser device also has a drawback that it is difficult to form an AlGaAs layer on another AlGaAs layer since the AlGaAs layers are prone to oxidation.
As explained above, the conventional current semiconductor laser devices which contain an internal current confinement structure, and oscillate at a wavelength of 0.9 to 1.1 micrometers are not suitable for manufacturing and difficult to form a stripe structure with high accuracy. In addition, it is difficult to regrow upper layers after a current confinement layer is formed, when aluminum exists at the regrowth interface, since aluminum is prone to oxidation. Further, even when the upper layers are regrown, the regrowth interface is prone to defect formation. Therefore, the above conventional current semiconductor laser devices are not reliable.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a reliable semiconductor laser device which can stably oscillate in an oscillation mode even when output power is high.
According to the present invention, there is provided a semiconductor laser device comprising: a GaAs substrate of a first conductive type; a lower cladding layer of the first conductive type, formed on the GaAs substrate; a lower optical waveguide layer formed on the lower cladding layer; a compressive strain quantum well active layer made of In
x3
Ga
1−x3
As
1−y3
P
y3
, and formed on the lower optical waveguide layer (where 0<x3≦0.4, and 0≦y3≦0.1); an upper optical waveguide layer formed on the compressive strain quantum well active layer; a first etching stop layer made of In
x6
Ga
1−x6
P of a second conductive type, and formed on the upper optical waveguide layer (where 0≦x6≦1); a second etching stop layer made of In
x1

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