Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Including integrally formed optical element
Reexamination Certificate
2001-08-09
2003-07-08
Pham, Long (Department: 2823)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Including integrally formed optical element
C438S022000, C438S029000, C438S030000, C438S039000, C438S045000, C438S046000, C438S133000
Reexamination Certificate
active
06589806
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a semiconductor laser comprised of a pnpn thyrister and including a current strangulation structure, and also to a method of fabricating the same.
2. Description of the Related Art
A semiconductor laser including an active layer formed by selective growth and having a pnpn thyrister block structure formed by selective growth can be fabricated without carrying out a step of etching a semiconductor layer. Hence, a width of an active layer can be accurately controlled, ensuring uniformity in characteristic and reproducibility of a semiconductor laser.
FIG. 1
illustrates one of conventional semiconductor lasers having such a structure as mentioned above.
The illustrated semiconductor laser is comprised of an n-InP substrate
701
, an electrode
711
formed on a lower surface of the n-InP substrate
701
, a stripe including an n-InP clad layer
703
, an MQW active layer
704
, and a p-InP clad layer
705
, a p-InP block layer
707
covering the n-InP substrate
701
and the stripe therewith, an n-InP block layer
708
formed on the p-InP block layer
707
, a p-InP clad layer
709
formed on the n-InP block layer
708
, a p-InGaAs cap layer
710
formed on the p-InP clad layer
709
, and an electrode
712
formed on the p-InGaAs cap layer
710
.
In the illustrated semiconductor laser, since a current is strangulated in the stripe including the MQW active layer
704
, around the stripe is formed a pnpn thyrister block structure comprised of the n-InP substrate
701
, the p-InP block layer
707
, the n-InP block layer
708
, and the p-InP clad layer
709
. The pnpn thyrister block structure prevents a current from running outside the stripe.
Japanese Unexamined Patent Publication No. 5-67849 has suggested a semiconductor light-emitting device including a p-InP substrate formed with a mesa-stripe, an n-InP block layer, a p-InP buffer layer, an InGaAsP active layer, and n-InP clad layer all deposited on the p-InP substrate, a p-InP buried layer, an n-InP current-blocking layer, and a p-InP current-blocking layer all deposited in a recess formed in the InGaAsP active layer and the n-InP clad layer, an n-InP buried layer covering the n-InP clad layer and the p-InP current-blocking layer therewith, and a pair of electrodes.
Japanese Unexamined Patent Publication No. 8-330676 has suggested a semiconductor laser including a p-InP substrate, a pair of SiO
2
stripe masks formed on the p-InP substrate in a [011] direction and spaced away from each other by 1.5 &mgr;m, and a multi-layered structure including an active layer, formed in the 1.5 &mgr;m-space by MOVPE selective growth.
Japanese Unexamined Patent Publication No. 9-266349 has suggested a semiconductor laser including a p-InP substrate including a buffer layer, a trapezoidal selective growth portion formed on the p-InP substrate. The selective growth portion includes a p-clad layer, SCH strain MQW layer, and n-clad layer, and acts as a waveguide for a laser having a wavelength of 1.3 &mgr;m. Around the selective growth portion are deposited p-InP buried layer, n-InP layer, p-InP layer, and SCH-MQW carrier recombination layer. An n-InP clad buried layer covers the above-mentioned structure, and an n-InGaAsP contact layer is formed on the n-InP clad buried layer.
However, the above-mentioned conventional semiconductor lasers are accompanied with a problem that turn-on occurs in the pnpn thyrister at a high temperature or when much current is applied to the semiconductor lasers, and hence, it is not always ensured to provide a sufficient block breakdown voltage.
It is most effective to design a block layer to have a greater thickness in order to enhance a breakdown voltage of a thyrister. However, a thickness of a current-blocking layer has upper limitation in the above-mentioned conventional semiconductor lasers, because of a demand in a waveguide layer to have a small height. As a result, it is quite difficult or almost impossible in the above-mentioned conventional semiconductor lasers to make a thickness of a current-blocking layer greater in order to enhance a breakdown voltage of a thyrister.
SUMMARY OF THE INVENTION
In view of the above-mentioned problem, it is an object of the present invention to provide a semiconductor laser and a method of fabricating the same both of which is capable of preventing occurrence of turn-on in a pnpn thyrister, and providing a sufficient block breakdown voltage even at a high temperature or even when much current is applied to a pnpn thyrister.
In one aspect of the present invention, there is provided a semiconductor laser including (a) an n-type semiconductor substrate, (b) an active layer formed on the n-type semiconductor substrate, (c) a first p-type semiconductor layer formed adjacent to the active layer, (d) an n-type semiconductor layer formed adjacent to the first p-type semiconductor layer, (e) a second p-type semiconductor layer formed adjacent to the n-type semiconductor layer, and (f) a lightly doped n-type semiconductor layer formed between the n-type substrate and the first p-type semiconductor layer.
In accordance with the above-mentioned semiconductor laser, the lightly doped n-type semiconductor layer formed between the n-type substrate and the first p-type semiconductor layer suppresses electron-ejection into the first p-type semiconductor layer. As a result, it is possible to reduce the number of electrons passing through the first p-type semiconductor layer and charging up in the n-type semiconductor layer, ensuring enhancement in a breakdown voltage of a thyrister even at a high temperature or even when much current is applied to the thyrister.
The lightly doped n-type semiconductor layer contains an impurity at preferably 3×10
17
cm
−3
or smaller, and more preferably at 1×10
17
cm
−3
or smaller. Such concentration of an impurity more effectively suppresses electron-ejection into the adjacent p-type semiconductor layer, ensuring an increase in a breakdown voltage of a thyrister even at a high temperature or even when much current is applied to a thyrister.
There is no lower limitation in the concentration of an impurity. However, it is preferable that the lightly doped n-type semiconductor layer contains an impurity at such a concentration that there does not occur auto-dope caused by a p-type impurity. For instance, the lightly doped n-type semiconductor layer contains an impurity at preferably 1×10
15
cm
−3
or greater.
The lightly doped n-type semiconductor layer has a thickness of preferably 0.5 &mgr;m or greater, and more preferably 1.0 &mgr;m or greater.
If the lightly doped n-type semiconductor layer is too thin, there might occur the tunneling effect in which electrons pass through the lightly doped n-type semiconductor layer. There is no upper limitation in a thickness of the lightly doped n-type semiconductor layer, unless the thickness does not deteriorate a structure of a semiconductor laser.
For instance, the lightly doped n-type semiconductor layer is designed to contain Si, S or Se as an n-type impurity.
It is preferable that the lightly doped n-type semiconductor layer extends entirely over p-n junction plane, which would effectively suppress electron-ejection into the adjacent p-type semiconductor layer.
For instance, the lightly doped n-type semiconductor layer may be formed in the n-type semiconductor substrate, in which case, it is preferable that the lightly doped n-type semiconductor layer has a depth equal to a thickness of the n-type semiconductor substrate.
For instance, the lightly doped n-type semiconductor layer may be formed on the n-type semiconductor substrate.
There is further provided a semiconductor laser including (a) a p-type semiconductor substrate, (b) an active layer formed on the p-type semiconductor substrate, (c) a first n-type semiconductor layer formed adjacent to the active layer, (d) a p-type semiconductor layer formed adjacent to the first n-type semiconductor layer, (e) a second n-type semiconductor
McGinn & Gibb PLLC
NEC Electronics Corporation
Pham Long
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