Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2000-01-28
2004-09-28
Leung, Quyen (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S046012
Reexamination Certificate
active
06798808
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser device to be used for optical disks, optical communications, and the like, and a method of manufacturing such a device. Particularly, the present invention relates to a semiconductor laser provided with a quantum well active layer which device has good characteristics and high reliability.
2. Description of the Related Art
In recent years, to improve speed in writing information to optical disks typified by CD-R/RW and DVD-R/RW, development of a semiconductor laser having an output power of as high as 100 mW has been expected.
To realize a semiconductor laser having a high power and reliability, it is necessary to prevent deterioration of end faces thereof and operate it at low electric current. To do so, it is more advantageous to provide the semiconductor laser device with a structure having a quantum well active layer than with a structure having a bulk active layer because the former is superior to the latter in the gain characteristics thereof.
FIG. 7
is a sectional view showing the structure of a conventional semiconductor laser device having a quantum well active layer. The device is formed as follows. On an n-type substrate
701
are formed a Si doped n-type buffer layer
702
, a Si doped n-type cladding layer
703
, an undoped optical guide layer
704
, an undoped quantum well active layer
705
, an undoped optical guide layer
706
, a Zn doped p-type cladding layer
707
, and a Zn doped p-type cap layer
708
. Thereafter, the Zn doped p-type cap layer
708
and the Zn doped p-type cladding layer
707
are processed into the shape of a striped ridge, and a Si doped p-type block layer
709
is formed such that the striped ridge is embedded in the block layer
709
. Further, a Zn doped p-type contact layer
710
is formed. In this manner, the semiconductor laser device is constructed. Reference numeral
711
denotes an optical distribution in an optical waveguide.
The optical guide layers
704
and
706
are formed at the lower and upper sides of the quantum well active layer
705
to entrap light in the quantum well active layer
705
and prevent diffusion of impurities into the quantum well active layer
705
from the p-type cladding layer
707
and the n-type cladding layer
703
.
In the case of the semiconductor laser device having the bulk active layer, the diffusion of impurities into the active layer causes formation of a crystal defect acting as the center of recombination of optical carriers in the active layer. Consequently, the characteristic of the semiconductor laser device deteriorates. Further, the impurities diffuse easily into the active layer during the operation of the semiconductor laser device. Consequently, the life thereof deteriorates. In particular, Zn that is used as a p-type doping material diffuses at a high speed in a film. As a result, frequently, the active layer has a p-type electrical conductivity and causes a remote junction. In order to solve such a problem, trials of preventing the diffusion of the Zn have been made by forming, between the active layer and the p-type cladding layer, an undoped layer, a layer having an opposite (n-type) electrical conductivity, or a layer having a different composition in which the diffusion speed is low.
On the other hand, in the semiconductor laser device having the active layer composed of quantum well, the undoped optical guide layers formed at both sides of the quantum well layer serve to prevent the impurities from diffusing into the quantum well layer besides its essential roll.
The present inventors have confirmed that in the semiconductor laser device having the active layer composed of the quantum well, the dopant, or impurities diffuse even into the optical guide layer, which is provided to prevent the diffusion of the dopant into the quantum well layer, and that this results in deterioration of the characteristic of the semiconductor laser device. In particular, it has been found that with the diffusion of the dopant at a carrier concentration of more than 5×10
17
cm
−3
from the cladding layer into the optical guide layer, threshold current rises and the reliability of the device deteriorates. The phenomenon occurs for the following reason: In the case where the active layer is composed of the quantum well as shown in
FIG. 7
, the optical distribution
711
in the optical waveguide is large in the optical guide layers. Therefore, the defect formed by the dopant that has diffused in the optical guide layer acts as the center of recombination of optical carriers that have been distributed in the optical guide layer. The present inventors have also found that the amount of the diffusion of the dopant into the optical guide layer from the cladding layer and the diffusion length thereof depend on the concentration of the dopant of the cladding layer and a manufacturing condition.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a semiconductor laser device with an quantum well active layer which is allowed to offer a high power and a preferable reliability, by preventing impurities from diffusing into an undoped optical guide layer. It is also an object of the present invention to provide a method of manufacturing such a semiconductor laser device.
To solve the above problem, according to an aspect of the present invention, in a semiconductor laser device having a quantum well active layer disposed between a pair of cladding layers, and an optical guide layer disposed between at least one of the cladding layers and the quantum well active layer, an undoped thin spacer layer is provided between the optical guide layer and the at least one of the cladding layers so that the spacer layer absorbs impurities diffusing thereinto from the cladding layer to thereby prevent them from diffusing into the optical guide layer. Therefore, a semiconductor laser device having high reliability can be obtained.
If the thickness of the spacer layer is smaller than 5 nm, the diffusion of impurities, or dopant into the optical guide layer will not be able to be sufficiently prevented. Consequently, the characteristic and reliability of the semiconductor laser device will deteriorate. On the other hand, if the thickness of the spacer layer is 10 nm or larger, an carrier concentration will become low. As a result, an electronic barrier will also be lowered. Consequently, the temperature characteristic of the semiconductor laser device will deteriorate. Accordingly, in order to securely obtain a semiconductor laser device having preferable characteristics and high reliability, the spacer layer may preferably have a thickness of 5 nm or more but less than 10 cm.
If the carrier concentration of a p-type cladding layer is higher than 5×10
18
cm
−3
, a large quantity of impurities will diffuse into the optical guide layer. Consequently, the characteristic of the semiconductor laser device deteriorates. On the other hand, if the carrier concentration of the p-type cladding layer is lower than 8×10
17
cm
−3
disadvantageously, the temperature characteristic of the semiconductor laser device will be lowered and an operational voltage will increase. Therefore, it is preferable that the p-type cladding layer has a carrier concentration in a range of from 8×10
17
cm
−3
to 5×10
18
cm
−3
.
In an embodiment, the spacer layer has a p-type electrical conductivity, and a carrier concentration at an interface between the spacer layer and the optical guide layer is between 5×10
18
cm
−3
and 5×10
17
cm
−3
.
If the spacer layer has an n-type electrical conductivity, this tends to result in formation of a remote junction. If the carrier concentration at the interface between the spacer layer and the optical guide layer is less than 5×10
16
cm
−3
, the temperature characteristic of the semiconductor laser device tends to be lowered and the operational voltage will increase. If the dopant
Kawato Shinichi
Konushi Fumihiro
Ohkubo Nobuhiro
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