Semiconductor laser device having InGaAs compressive-strain...

Details Semiconductor laser device having InGaAs compressive-strain... Semiconductor laser device having InGaAs compressive-strain...

2001-12-11

2003-05-06

Ip, Paul (Department: 2828)

Coherent light generators

Particular active media

Semiconductor

C372S043010, C372S045013

Reexamination Certificate

active

06560261

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser device having a compressive-strain active layer.
2. Description of the Related Art
M. Sagawa et al. (“High-power highly-reliable operation of 0.98-micrometer InGaAs-InGaP strain-compensated single-quantum-well lasers with tensile-strained InGaAsP barriers,” IEEE Journal of Selected Topics in Quantum Electronics, Vol. 1, No. 2, 1995, pp.189) disclose semiconductor laser device which emits laser light in the 0.98-micrometer band without employing Al in any of its constituent layers. This semiconductor laser device is formed as follows.
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 on an n-type GaAs substrate in this order. Next, a narrow-stripe ridge structure is formed above the p-type InGaP first upper cladding layer by 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, an index-guided semiconductor laser device having a current confinement structure is realized.
The above semiconductor laser device oscillates in a fundamental transverse mode. It is reported that the reliability of the above semiconductor laser device is improved since the strain in the active layer can be compensated for. However, in the above semiconductor laser device, the characteristic temperature, which is obtained from the temperature dependence of the threshold current, is as low as 156K. Therefore, in practice, temperature control is necessary, and thus cost reduction is difficult.
In the case that a semiconductor laser device having a layering structure as described above is driven under a high output condition, dark line failure due to the optical absorption by the laser facet, which has a plurality of surface states, and a phenomenon called facet destruction, become more likely to occur. Both of these defects hinder the improvement of reliability in semiconductor laser devices, and also act as an obstacle to high output operation thereof.
A method that is commonly employed to solve the problem mentioned above and to stably operate a semiconductor laser device at high output is to reduce the optical power density at the active layer. Commonly, d/&Ggr;, which is the thickness of the active layer d divided by the coefficient of optical confinement &Ggr;, is utilized as a parameter that represents the optical power density. It can be said that the larger the value of d/&Ggr;, the smaller the optical power density.
If the thickness of active layer d is increased, by the increase of threshold current density, the characteristics of the semiconductor laser device drastically deteriorate. Therefore, the range within which control is possible becomes extremely narrow, thus it is not preferable as a parameter to be altered. On the other hand, it is possible to regulate the coefficient of optical confinement &Ggr; by altering as parameters the thickness of the optical waveguide layer as well as the composition of the AlGaAs, which is the cladding layer, without a large deterioration in the characteristics of the semiconductor laser. Although some change occurs in the shape of a far field pattern by altering &Ggr;, it is known that it is possible to obtain a desired far field pattern by selecting an appropriate thickness for the optical waveguide layer and an appropriate cladding composition, at a low optical power density.
SUMMARY OF THE INVENTION
The present invention has been developed in view of the above circumstances, and an object of the present invention is to provide a semiconductor laser device in which the temperature dependence of the threshold current is improved, and which has improved reliability even at high output.
The first semiconductor laser device according to the present invention comprises: a compressive-strain active layer made of In
x3
Ga
1−x3
As
1−y3
P
y3
(0<x3≦0.4 and 0≦y3≦0.1); and an optical waveguide layer made of In
0.49
Ga
0.51
P, and that lattice-matches with GaAs on both the upper and lower sides of the active layer; wherein tensile-strain barrier layers made of In
x2
Ga
1−x2
As
1−y2
P
1−y2
(0≦x2<0.49y2 and 0<y2≦0.4) are provided between the active layer and the optical waveguide layers
In the first semiconductor laser device according to the present invention, an absolute value of a sum of a first product and a second product may be equal to or smaller than 0.25 nm, where the first product is a product of a strain and a thickness of the compressive-strain active layer, and the second product is a product of a strain of the lower and upper tensile-strain barrier layers and a total thickness of the lower and upper tensile-strain barrier layers.
The second semiconductor laser device according to the present invention comprises: 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 and made of In
0.49
Ga
0.51
P which is undoped or of the first conductive type; a lower tensile-strain barrier layer made of In
x2
Ga
1−x2
As
1−y2
P
1−y2
(0≦x2<0.49y2 and 0<y2≦0.4) and formed above the lower optical waveguide layer; a compressive-strain active layer made of In
x3
Ga
1−x3
As
1−y3
P
y3
(0<x3≦0.4, 0<y3≦0.1) and formed on the lower tensile-strain barrier layer; an upper and formed above the compressive-strain active layer; an upper optical waveguide layer formed above the upper tensile-strain barrier layer and made of In
0.49
Ga
0.51
P which is undoped or of a second conductive type; an etching stop layer made of GaAs of the second conductive type and formed on the upper optical waveguide layer; a current confinement layer made of In
0.49
(Al
z2
Ga
1−z2
)
0.51
P (0.15≦z2≦1) of the first conductive type and formed on the etching stop layer; a cap layer made of In
0.49
Ga
0.51
P of the first conductive type or the second conductive type and formed above the current confinement layer; an upper cladding layer of the second conductive type, formed over the cap layer; and a contact layer made of GaAs of the second conductive type and formed above the upper cladding layer. A portion of the semiconductor layer formed by the cap layer, the current confinement layer, as well as the etching stop layer are removed from one resonator facet to the other that faces it from the resonator formed by the layers described above, to a depth at which the optical waveguide layer formed of In
0.49
Ga
0.51
P which is undoped or of a second conductive type is exposed. The groove formed thereby is filled by the cladding layer of the second conductive type formed above the cap layer to form a refractive index waveguide structure. In the above semiconductor laser device, an absolute value of a sum of a first product and a second product is less than or equal to 0.25 nm, where the first product is a product of a strain and a thickness of the compressive-strain active layer, and the second product is a product of a strain of the lower and upper tensile-strain barrier layers and a total thickness of the lower and upper tensile-strain barrier layers.
With regard to the second semiconductor laser of the present invention, it is preferable that the cladding layer of the second conductive type be composed of either Al
z1
Ga
1−z1
As (0.6≦z1≦0.8) or In
0.49
(Ga
1−z3
Al
z3
)
0.51
P (0.1≦z3<z2).
The third semiconductor laser device according to the present i

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