Semiconductor laser device which includes AlGaAs optical...

Details Semiconductor laser device which includes AlGaAs optical... Semiconductor laser device which includes AlGaAs optical...

2001-10-18

2003-09-16

Ip, Paul (Department: 2828)

Coherent light generators

Particular active media

Semiconductor

C372S045013

Reexamination Certificate

active

06621845

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser device having a compressive strain quantum well active layer above a GaAs substrate.
2. Description of the Related Art
Fujimoto et al. (“High Power InGaAs/AlGaAs laser diodes with decoupled confinement heterostructure,” Proceedings of SPIE, Vol. 3628 (1999) pp. 38-45) discloses an internal striped structure semiconductor laser device which emits light in the 0.98 Mm band. This semiconductor laser device is formed as follows.
On an n-type GaAs substrate, an n-type Al
x
Ga
1−x
As lower cladding layer, an n-type GaAs optical waveguide layer, an InGaAs quantum well active layer, a p-type GaAs first upper optical waveguide layer, and an n-type Al
y
Ga
1−y
As current confinement layer are formed in this order. Next, a narrow-stripe groove is formed, by conventional photolithography and selective etching, to such a depth that the groove penetrates the n-type AlGaAs current confinement layer. Thereafter, over the above structure, a GaAs second optical waveguide layer, a p-type AlGaAs upper cladding layer, and a p-type GaAs contact layer are formed. Thus, an internal striped structure is formed, and the semiconductor laser device oscillates in a fundamental transverse mode.
In the above semiconductor laser device, the stripe width can be controlled accurately, and high-output-power oscillation in the 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 GaAs second optical waveguide layer. However, the above semiconductor laser device has a drawback that it is difficult to form a GaAs layer on another AlGaAs layer, since the AlGaAs layers are prone to oxidation. In addition, since the optical waveguide layers are made of GaAs, current leakage is likely to occur. Therefore, AlGaAs leak-current protection layers are provided on both sides of the active layer. Nevertheless, the leakage current is still great, and thus the threshold current is high.
On the other hand, in order to prevent degradation of characteristics of the semiconductor laser device due to oxidation of aluminum included in an exposed regrowth boundary, T. Fukunaga (the inventor of the present patent application) and M. Wada have proposed a semiconductor laser device and a method of producing the semiconductor laser device in a coassigned and copending U.S. Ser. No. 09/634,703, filed on Aug. 7, 2000 and entitled “HIGH-POWER SEMICONDUCTOR LASER DEVICE HAVING CURRENT CONFINEMENT STRUCTURE AND INDEX-GUIDED STRUCTURE,” corresponding to Japanese patent application No. 11(1999)-222169, which is disclosed in Japanese Unexamined Patent Publication No. 2001-053383. In the above semiconductor laser device, the optical waveguide layers are made of InGaAsP, which has a greater bandgap than GaAs and does not contain aluminum. In addition, the current confinement layer is made of InGaP. Thus, the semiconductor laser device has a structure in which aluminum is not exposed on the regrowth layer. However, even in this structure, the leakage current is still great, and therefore the threshold current is high, since the band offset between the conduction bands of the InGaAsP and InGaP layers is small.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a reliable semiconductor laser device which includes an internal stripe groove and a regrown layer over an internal stripe groove, and has the following features:
(a) Aluminum, which is prone to oxidation, does not exist on a regrowth boundary.
(b) The leakage current is suppressed by an index-guided structure formed with high precision.
(c) The semiconductor laser device oscillates in a fundamental transverse mode when the stripe width is small.
(d) The semiconductor laser device produces low noise when the stripe width is great.
According to the present invention, there is provided a semiconductor laser device including: a GaAs substrate of a first conductive type; a lower cladding layer of the first conductive type formed above the GaAs substrate; a lower optical waveguide layer formed above 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 above the lower optical waveguide layer, where 0<x
3
≦0.4 and 0≦y
3
≦0.1; a first upper optical waveguide layer made of In
x2
Ga
1−x2
As
1−y2
P
y2
and formed above the compressive strain quantum well active layer, where x
2
=(0.49±0.01)y
2
, and 0≦x
2
≦0.3; a first etching stop layer made of In
x9
Ga
1−x9
P of a second conductive type and formed above the first upper optical waveguide layer, where 0≦x
9
≦1; a second etching stop layer made of In
x1
Ga
1−x1
As
1−y1
P
y1
and formed on the first etching stop layer other than a stripe area of the first etching stop layer so as to form a first portion of a stripe groove realizing a current injection window, where x
1
=(0.49±0.01)y
1
and 0≦x
1
≦0.3; a current confinement layer made of In
0.49
Ga
0.51
P of the first conductive type and formed above the second etching stop layer so as to form a second portion of the stripe groove; a second upper optical waveguide layer made of A GaAs formed so as to cover the current confinement layer and the stripe groove; an upper cladding layer of the second conductive type, made of one of AlGaAs and In
x4
Ga
1−x4
As
1−y4
P
y4
and formed over the second upper optical waveguide layer, where x
4
=(0.49±0.01)y
4
, and 0.9≦y
4
≦1; a contact layer of the second conductive type; a first electrode formed on an exposed surface of the GaAs substrate; and a second electrode formed on the contact layer. In the semiconductor laser device, the first and second upper optical waveguide layers have an approximately identical refractive index, the upper and lower cladding layers have an approximately identical refractive index, the absolute value of a first product of the strain and the thickness of the compressive strain quantum well active layer is equal to or smaller than 0.25 nm, and each of the lower cladding layer, the lower optical waveguide layer, the first and second upper optical waveguide layers, the first and second etching stop layers, the current confinement layer, the upper cladding layer, and the contact layer has such a composition as to lattice-match with GaAs.
Preferably, the semiconductor laser device according to the present invention may also have one or a combination of the following additional features (i) and (ii).
(i) The semiconductor laser device according to the present invention may further include first and second tensile strain barrier layers both made of In
x5
Ga
1−x5
As
1−y5
P
y5
and respectively formed above and below the compressive strain quantum well active layer, where 0≦x
5
≦0.3 and 0<y
5
≦0.6, and the absolute value of the sum of the first product and a second product of the strain of the first and second tensile strain barrier layers and the total thickness of the first and second tensile strain barrier layers is equal to or smaller than 0.25 nm.
(ii) The second etching stop layer may be one of the first and second conductive types.
The strain &Dgr;a of the compressive strain quantum well active layer is defined as &Dgr;a=(ca−cs)/cs, and the strain &Dgr;b of the first and second tensile strain barrier layers is defined as &Dgr;b=(cb−cs)/cs, where cs, ca and cb are the lattice constants of the GaAs substrate, the compressive strain quantum well active layer, and the first and second tensile strain barrier layers, respectively.
When a layer grown over the substrate has a lattice constant c, and the absolute value of the amount &Dgr;=(c−cs)/cs is equal to or smaller than 0.003, the layer is lattice-matched with the (GaAs) substrate.
When the thickness of the compressive strain quantum well active layer is denoted by da, according to the present inven

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