Semiconductor laser

Details Semiconductor laser Semiconductor laser

1999-02-09

2001-09-04

Leung, Quyen P. (Department: 2881)

Coherent light generators

Particular active media

Semiconductor

C372S046012

Reexamination Certificate

active

06285695

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a semiconductor laser, and more particularly to compositions of semiconductor layers of a semiconductor laser.
2. Description of the Related Art
As-a-1060 nm-band semiconductor laser completely free from aluminum, there has been reported a semiconductor laser of a strain compensation structure comprising an n-InGaAsP clad layer, an undoped InGaAsP optical waveguide layer, an InGaAsP barrier layer which is 0.7% in tensile strain, an InGaAs quantum-well active layer which is 2.1% in compression strain, an InGaAsP barrier layer which is 0.7% in tensile strain, an undoped InGaAsP optical waveguide layer, a p-InGaAsP clad layer and a p-GaAs capping layer formed on an n-GaAs substrate in this order. See, for instance, “Applied Physics Letters, 99(1996)” pp248. However the Al-free semiconductor laser has reliability of only about 250 mW class and practically cannot be used as a higher output semiconductor laser.
Further as a 680 nm-band semiconductor laser whose active layer is free from aluminum, there has been reported a semiconductor laser in which a GaInP active layer is imparted with compressive strain, a side barrier layer is provided with an AlGaInP layer having tensile strain sufficient to cancel the compressive strain of the active layer, and the end face band gap is increased by relaxation in crystal structure in the vicinity of the radiating end face of the laser, thereby reducing absorption of light during oscillation and suppressing deterioration of the end faces due to absorption of light. However in order to use the tensile-strained side barrier layer in a 1000 nm-band semiconductor laser, an InGaAs active layer which is high in proportion of In must be used and the thickness of the active layer must be as small as the critical film thickness, which makes the crystal unstable. Further it is difficult to obtain a high quality crystal due to diffusion of In and accordingly it is difficult to obtain a semiconductor laser of high reliability.
SUMMARY OF THE INVENTION
In view of the foregoing observations and description, the primary object of the present invention is to provide a 1.0 &mgr;m-band semiconductor laser which is good in durability and is highly reliable even during oscillation at a high power.
In accordance with the present invention, there is provided a semiconductor laser comprising a first clad layer having one of p-type conductivity and n-type conductivity, a first optical waveguide layer, a first barrier layer of GaAs
1-y2
P
y2
, a quantum-well active layer of In
x3
Ga
1-x3
As
1-y3
P
y3
, a second barrier layer of GaAs
1-y2
P
y2
, a second optical waveguide layer and a second clad layer having the of p-type conductivity and n-type conductivity formed in this order on a GaAs substrate,
wherein each of the first and second clad layers is of a composition which matches with the GaAs substrate in lattice,
each of the first and second optical waveguide layers is of a InGaAsP composition which matches with the GaAs substrate in lattice,
each of the first and second barrier layers is 10 to 30 nm in thickness and is of a composition which has tensile strain relative to the GaAs substrate, the product of the tensile strain and the thickness of each of the first and second barrier layers being 5 to 20% nm,
the quantum-well active layer of In
x3
Ga
1-x3
As
1-y3
P
y3
is 6 to 10 nm in thickness and is of a composition which has compressive strain of not smaller than 1.0% relative to the GaAs substrate, and
the sum of the product of the tensile strain and the thickness of the first barrier layer and that of the second barrier layer is larger than the product of the compressive strain and the thickness of the quantum-well active layer.
It is preferred that the sum of the product of the tensile strain and the thickness of the first barrier layer and that of the second barrier layer be larger than the product of the compressive strain and the thickness of the quantum-well active layer at least by 3% nm.
Generally the first and second barrier layers are the same in composition, strain and thickness and accordingly the product of the tensile strain and the thickness of the first barrier layer is generally equal to that of the second barrier layer.
The tensile strain &Dgr;
1
of each of the first and second barrier layers relative to the GaAs substrate is expressed as follow.
&Dgr;
1
=(|
a
aGaAs
−a
1
|/a
GaAS
)×100(%)
wherein a
GaAs
represents the lattice constant of the GaAs substrate and a
1
represents the lattice constant of the barrier layer.
Similarly the compressive strain &Dgr;
2
of the quantum well active layer relative to the GaAs substrate is expressed as follow.
&Dgr;
2
=(|
a
GaAs
−a
2
|/a
GaAs
)×100(%)
wherein a
GaAs
represents the lattice constant of the GaAs substrate and a
2
represents the lattice constant of the active layer.
In the semiconductor laser in accordance with the present invention, since the active layer contains no Al, durability is increased. Further by virtue of the GaAsP tensile-strained barrier layers, the band gap is increased by lattice relaxation in the vicinity of the active layer, whereby absorption of light at the radiating end face of the laser can be reduced. Further by virtue of the first and second barrier layers whose tensile strain compensate for a part of compressive strain of the active layer, which is close to the critical thickness, during crystal growth, an active layer of high quality can be obtained. Further by virtue of a GaAsP layer, diffusion of In during crystal growth can be suppressed, whereby a crystal of high quality can be obtained. Further by virtue of the GaAsP tensile-strained barrier layers, the height of the barrier between the active layer and the barrier layers is enlarged, whereby leakage of electrons and positive holes from the active layer to optical waveguide layers can be reduced, whereby the drive current can be reduced and generation of heat at the end faces of the laser can be reduced. Accordingly, a laser which is highly reliable even during oscillation at a high power can be provided.


REFERENCES:
Zhang et al, “Strain-Compensated InGaAs/GaAsP/GaInAsP/GaInP Quantum Well Lasers (lambda~0.98 um) Grown By Gas-source Molecular Beam Epitaxy”, Appl. Phys. Lett., vol. 62, No. 14, pp.1644-1646, Apr. 1993.*
Japanese Abstract No. 09270558 dated Oct. 14, 1997.
Japanese Abstract No. 08056045 dated Feb. 27, 1996.
Toshiaki Fukunaga et al. Reliable operation of strain-compensated 1.06 &mgr;m InGaAs/InGaAsP/GaAs single quantum well lasers, Jul. 1996, vol. 69.
Japanese Abstract No. 09148682 dated Jun. 6, 1997.
“Reliable operation of strain-compensated 1.06 &mgr;m In GaAs/InGaAsP/GaAs single quantum well lasers”, Toshiaki Fukunaga et al., Appl. Phys. Letters vol. 69 No. 2, (1996), pp. 248-250.

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