Single-crystal – oriented-crystal – and epitaxy growth processes; – Processes of growth with a subsequent step of heat treating...
Reexamination Certificate
1999-07-14
2001-03-13
Utech, Benjamin L. (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Processes of growth with a subsequent step of heat treating...
C117S002000, C117S084000, C117S104000, C117S904000
Reexamination Certificate
active
06200382
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a semiconductor laser, and in particular, to an AlGaInP based visible light semiconductor laser which is used as a light source for an optical disk, such as, a digital versatile disk (DVD) and a magneto-optical (MO) disk.
A ridge waveguide type laser illustrated in
FIG. 1
is generally used in the conventional AlGaInP based visible semiconductor laser. The laser having such a structure is fabricated by performing the known MOVPE (Metalorganic Vapor Phase Epitaxy) method three times. For example, this method is disclosed in Japanese Unexamined Patent Publications No. Hei. 2-58883 and No. Sho. 62-200786.
A double hetero structure having an active layer is formed in a first crystal growth. Subsequently, the ridge structure of a mesa-stripe shape is formed by etching with a dielectric mask.
In a second crystal growth, a GaAs current block layer is formed on a clad layer outside the mesa-stripe by the use of the selective growth method using the same dielectric mask.
Finally, in a third crystal growth, a p-GaAs contact layer is formed on an entire surface, and the laser structure is formed after forming an electrode.
As illustrated in
FIG. 1
, the semiconductor laser has a n-GaAs buffer layer
170
, a n-AlGaInP clad layer
130
, a MQW active layer
110
, a p-AlGaInP clad layer
120
, a p-GaInP etching stopper layer
140
, a n-GaAs block layer
180
, a p-AlGaInP clad layer
150
, a p-GaInP hetero buffer layer
160
, a-pGaAs cap layer
190
, a p-GaAs contact layer
195
on a n-GaAs substrate
200
. Further, electrodes
210
and
220
are arranged at upper and lower sides of the semiconductor laser.
Referring to
FIG. 2
, description will be made about the conventional crystal growth apparatus for manufacturing such a semiconductor laser device by the use of the MOVPE method.
A susceptor
320
, which mounts a GaAs substrate
330
, is contained in a reactive tube
310
. Further, a V-group raw material gas inlet
340
, a hydrogen gas inlet
360
and a III-group three gas inlet
370
are arranged in the reactive tube
310
.
In this event, organic metal (TMA1, TEGa, TMIn) is supplied as the III-group raw material of the semiconductor laser while hydrogenated V-group gas (PH
3
, AsH
3
) is supplied as the V-group raw material. Herein, hydrogen is supplied to the reactive tube
310
as a carrier gas.
In this case, attention is paid for atmosphere gas and growth temperature in a temperature dropping process after each crystal growth.
As illustrated in
FIG. 3
, mixed atmosphere between hydrogenated V-group gas, which is used for the growth of the final growth layer, and hydrogen gas as the carrier gas is generally used as the atmosphere gas. Herein, AsH
3
gas is, for example, used as the hydrogenated V-group gas when the GaAs is the final layer.
However, the mixed atmosphere is decomposed during the temperature dropping process. Consequently, much active hydrogen exists in the atmosphere.
On the other hand, p-type impurities are inactivated by combining with hydrogen molecules in the crystal growth of the p-type semiconductor crystal of the AlGaInP based compound semiconductor. In consequence, p-carrier concentration is lowered.
The hydrogen is supplied from the atmosphere gas into the semiconductor crystal during the crystal growth and the temperature dropping process in the conventional method. Thereby, the crystal growth is completed on the condition that much hydrogen is left in the p-type semiconductor crystal (p-type impurity passivation region
260
).
As a result, carrier-over increase, which causes reduction of the p-carrier concentration, occurs in the semiconductor laser manufactured by the conventional method. Consequently, deterioration of high temperature operation characteristic becomes remarkable.
Further, hydrogen, which is trapped into the crystal, is readily changed in the state with time. In consequence, it is difficult to realize stable laser characteristic.
Meanwhile, disclosure has been made about the atmosphere gas during the growth of the semiconductor in Japanese Unexamined Patent Publication No. Hei.8-32113 (hereinafter, referred to as a first reference). The first reference discloses a method of manufacturing a p-type GaN based semiconductor device.
More specifically, description has been made at page 1, ninth paragraph in the first reference about a technique featured by cooling at slower speed than natural cooling, preferably under an inactive gas atmosphere after a GaN based semiconductor, which is doped with a p-type impurity, is grown.
However, no description has been made about manufacture of the semiconductor laser in the first reference. Further, the first reference is limited to the GaN based semiconductor. Therefore, the first reference can not be applied for the semiconductor laser, such as, the AlGaInP based semiconductor laser.
SUMMARY OF THE INVENTION
It is therefore an object of this invention to provide a method of manufacturing a semiconductor laser which is capable of avoiding passivation of a p-type semiconductor crystal caused by hydrogen trapped during crystal growth and which has excellent high temperature operation characteristic and stable operation characteristic with time.
According to this invention, a semiconductor laser device has a double hetero structure which is structured by at least a first clad layer, an active layer and a second clad layer on a semiconductor substrate by the use of the organic metal vapor growth method.
With such a structure, crystal is grown in first atmosphere gas containing hydrogen in a temperature rising process.
Subsequently, the first atmosphere gas is switched into second atmosphere gas in a temperature dropping process after the crystal growth. In this event, the second atmosphere gas contains no hydrogen.
In this case, the grown crystal is retained at a predetermined temperature during a preselected duration in the temperature dropping process after switching into the second gas atmosphere.
The predetermined temperature and the preselected duration are selected so that the hydrogen is removed from the grown crystal.
For example, the grown crystal is a p-type semiconductor having p-type impurity, and passivation of the p-type impurity due to the hydrogen is prevented by removing the hydrogen from the grown crystal.
Preferably, the predetermined temperature is 450° C. or more, and the preselected duration is one minute or more.
The second atmosphere gas is at least one selected from group consisting of nitrogen gas, argon gas and helium gas.
Further, the double hetero structure is formed using organic metal as III-group gas, hydrogenated V-group gas as V-group gas and the hydrogen as carrier gas in the organic metal vapor growth method.
In this event, the organic metal is at least one selected from the group consisting of TMA1, TEGa, and TMIn while the hydrogenation V-group gas is either one of PH
3
and AsH
3
.
For example, each of the first and second clad layers comprises either one of AlGaInP and AlInP while the active layer comprises either one of GaInP and AlGaInP.
More specifically, the atmosphere gas is changed from the hydrogen gas+hydrogenation V-group gas into the gas containing no hydrogen, such as, the nitrogen gas in the dropping temperature process in the method of manufacturing the semiconductor laser according to this invention.
Thereby, the trap of the hydrogen into the p-type semiconductor crystal can be effectively prevented. Further, the elimination of the hydrogen, which has been trapped, from the semiconductor can be enhanced.
Consequently, the high p-carrier concentration can be realized. As a result, both the excellent high temperature operation characteristic and the stable laser characteristic with time can be realized.
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patent: 4835117 (1989-05-01), Ohba et al.
patent: 4987097 (1991-01-01), Nitta et al.
patent: 5153889 (1992-10-01), Sugawara et al.
patent: 5432808 (1995-07-01), Hatano et al.
patent: 5621748 (1997-04-01), Kondo et al.
patent: 5740192 (1998-04-0
Endo Kenji
Fujii Hiroaki
Anderson Matthew
NEC Corporation
Sughrue Mion Zinn Macpeak & Seas, PLLC
Utech Benjamin L.
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