Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1999-01-28
2001-08-14
Arroyo, Teresa M. (Department: 2881)
Coherent light generators
Particular active media
Semiconductor
C372S043010, C438S478000
Reexamination Certificate
active
06275515
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser device having a BTlGaAs active layer on a GaAs substrate and operating in a long wavelength region for communication purposes.
2. Description of the Related Art
Optical communication generally employs signal light having wavelengths of 1.3 &mgr;m and 1.55 &mgr;m, and an InGaAsP/InP semiconductor laser device on an InP substrate is used as the light source.
However, the InGaAsP/InP semiconductor laser has a problem that the discontinuity in the conduction band between the InGaAsP active layer and InP clad layers which sandwich the active layer is as small as about 100 meV and, as a consequence, overflow of electrons from the active layer into the clad layer becomes conspicuous as the temperature rises, resulting in rapidly increasing current.
A GaAs/AlGaAs laser and a strained quantum well laser based on InGaAs/AlGaAs on a GaAs substrates, on the other hand, can provide better temperature characteristics because AlGaAs having a larger band gap energy can be used as the clad layer which produces a discontinuity in the conduction band as large as 300 meV or greater.
When producing a surface emitting laser with a perpendicular resonance, a GaAs/AlAs semiconductor multi-layer film can be used as a reflector layer, and therefore high reflectivity can be obtained.
It is difficult to make practical use of an active layer material which can be grown on a GaAs substrate while achieving lattice matching with the GaAs substrate and is capable of oscillating with a wavelength of 1.3 &mgr;m or 1.55 &mgr;m.
Researches on growing a semiconductor layer made of a GaInNAs material on a GaAs substrate have been done (for example, Kondo & Uomi; Applied Physics vol.65 (1996), p148), but commercial applications have not been achieved due to a problem in crystalline characteristics and other reasons.
SUMMARY OF THE INVENTION
That is, an object of the present invention is to provide a semiconductor laser device on a GaAs substrate and oscillating with a wavelength of 1.3 &mgr;m or 1.55 &mgr;m, and a method of producing the same.
The present inventors have studied intensively. Found that a semiconductor laser device having a BTlGaAs layer used an active layer is capable of lattice matching with a GaAs substrate and oscillating with a wavelength of 1.3 &mgr;m or 1.55 &mgr;m, and particularly that a BTlGaAs layer of better crystalline characteristics can be obtained by employing an organometallic vapor phase deposition process wherein cyclopentadienyl thallium is used as a source for supplying Tl when growing the BTlGaAs layer. Thus, the present invention has been completed.
That is, the present invention provides a semiconductor laser device comprising at least a first clad layer of a first conductivity type, an active layer, a second clad layer of a second conductivity type and a contact layer of the second conductivity type formed successively on a GaAs substrate of the first conductivity type, with current flowing across the GaAs substrate and the contact layer thereby to carry out laser oscillation in the active layer, wherein the active layer is made of a GaAs based mixed crystal semiconductor of III and V groups of zincblende structure with part of the Ga atoms being substituted with at least B and Tl.
As the active layer is made of the GaAs based mixed crystal semiconductor of III and V groups of zincblende structure with the group III elements consisting of at least Ga, and B and Tl substituting Ga, and with the group V element consisting of As, thereby to optimize the composition of the active layer, then it is made possible to produce a semiconductor laser device which has a BTlGaAs layer used an active layer and is capable of lattice matching with a GaAs substrate and oscillating with a wavelength of 1.3 &mgr;m or 1.55 &mgr;m.
The active layer made of GaAs based mixed crystal semiconductor preferably has the following composition.
(B
1−x
Tl
x
)
1−y
Ga
y
As(0<x<1, 0<y<1)
The active layer may also have quantum well structure of BTlGaAs/BTlAlGaAs.
It is because threshold value of the semiconductor laser device can be decreased by employing the structure described above.
The BTlGaAs preferably has a band gap which corresponds to a wavelength of 1.3 &mgr;m or 1.55 &mgr;m.
The BTlGaAs preferably has a composition of (B
0.38
Tl
0.62
)
0.27
Ga
0.73
As or (B
0.3
Tl
0.7
)
0.36
Ga
0.64
As.
It is because these compositions enable the semiconductor laser device to oscillate at a wavelength of 1.3 &mgr;m or 1.55 &mgr;m, respectively.
Peak wavelength of the laser output light is preferably 1.3 &mgr;m or 1.55 &mgr;m, to meet the wavelength requirement in the optical communications.
The active layer is preferably made of a GaAs based mixed crystal semiconductor of III and V groups of zincblende structure with a part of the Ga atoms being substituted with at least B and Tl, in order to achieve matching or quasi-matching with the lattice constant of the GaAs substrate.
Lattice matching between the GaAs substrate and the active layer makes it possible to improve the performance characteristics of the semiconductor laser device, particularly to decrease the threshold current by employing the quasi-matched strained quantum well structure.
The present invention also provides a semiconductor laser device which has a reflecting layer in each of the lower portion of the first clad layer and the upper portion of the second clad layer, and causes laser oscillates to occur between the reflecting layers as a surface emitting laser of perpendicular resonance type.
The reflecting layers are preferably AlAs/GaAs reflecting layers.
The semiconductor laser device of the present invention is formed on the GaAs substrate, and therefore the AlAs/GaAs reflecting layers of high reflectivity can be formed while maintaining lattice matching. The reflecting layers may also be made in multi-layer structure.
The first clad layer and the second clad layer are preferably made of AlGaAs layers.
By using the AlGaAs layer for the clad layers, it is made possible to make the discontinuity in the conduction band between the active layer and the clad layers as large as 300 meV or higher. As a result, overflow of electrons from the active layer into the clad layer does not occur even at high temperatures and good temperature characteristics can be obtained.
The present invention also provides a method of producing a semiconductor laser device, which comprises forming at least a first clad layer, an active layer made of BTlGaAs, a second clad layer and a contact layer, successively, on a GaAs substrate by a crystal growing process, wherein said crystal growing process is an organometallic vapor phase deposition process.
The active layer made of BTlGaAs can be formed with good crystalline characteristics by employing the organometallic vapor phase deposition process.
It is preferable that cyclopentadienyl thallium be used for the source of supplying Tl to the active layer.
It is preferable that triethylboron be used for the source of supplying Tl to the active layer.
As will be clear from the above description, in the semiconductor laser device of the present invention, long-wavelength laser having oscillation wavelength of 1.3 &mgr;m or 1.55 &mgr;m can be formed on the GaAs substrate by forming the active layer made of BAlGaAs on the GaAs substrate.
As a result, it is made possible to use the AlGaAs layer as the clad layer, and the discontinuity in the conduction band between the active layer and the clad layers can be made as large as 300 meV or higher, thereby achieving good temperature characteristics,
In case the semiconductor laser device is made as a surface emitting laser, the AlAs/GaAs layer having high reflectivity can be used as the reflector film.
Since the GaAs substrate which has greater size than the InP substrate and is hard to crack is used, it is made possible to improve the yield of production and reduce the production cost.
Also according to the method of producing the semiconductor laser device
Kajikawa Yasutomo
Mihashi Yutaka
Miyashita Motoharu
Nagai Yutaka
Arroyo Teresa M.
Leydig , Voit & Mayer, Ltd.
Menefee James
Mitsubishi Denki & Kabushiki Kaisha
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