Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1999-06-09
2001-05-08
Healy, Brian (Department: 2874)
Coherent light generators
Particular active media
Semiconductor
C372S043010, C372S045013, C372S046012, C372S050121, 43, 43, 43
Reexamination Certificate
active
06229836
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a buried type of semiconductor laser and in particular to a semiconductor laser having excellent elevated-temperature and high-power properties.
2. Description of the Related Art
Recently, a high-power semiconductor laser has been needed for an optical CATV, a light source for EDFA excitation or an OTDR.
For meeting such needs, for example, ELECTRONICS LETTERS, VOL.18, NO.22, 1982 has suggested a DC-PBH (double channel Planar buried hetero-structure) laser, where a leakage current is recombined in recombination layers
2
having the same composition as an active layer
1
, formed on both sides of the active layer as shown in
FIG. 11
, to minimize turn-on of a block layer due to a p-n junction for attempting to improve its elevated-temperature and high-power properties.
However, there are still needs for higher level of elevated-temperature and high-power properties, and thus extensive studies have been conducted for meeting the needs.
For example, JP-A 6-283800 has disclosed a technique that a concentration of a block layer and a channel width are designed to optimize distribution between a leakage current flowing a p-n junction and a current flowing a recombination layer for improving elevated-temperature and high-power properties of a DC-PBH (double channel Planar buried hetero-structure) laser. In the laser, a concentration of a p-type dopant in a p-type current block layer
4
is elevated and a channel width is narrowed to increase a current flowing a recombination layer
2
as shown in
FIG. 2
, while relatively reducing a current flowing a channel
3
. Thus, a semiconductor laser has been suggested, in which a total leakage current at a high-power operation may be reduced to improve its high-power properties.
The technique described in the above publication, however, has the following problems.
First, it may not provide adequate high-power properties. In general, Zn is used as a dopant for a p-type current block layer. As the concentration of the dopant Zn increases, more Zn atoms are diffused into an active layer from a p-type current block layer during forming the p-type current block layer, resulting in increase of internal loss in the active layer. It may cause reduction in an efficiency and deterioration in high-power properties of the laser.
Secondly, an effect of increase of the dopant concentration in the p-type current block layer may be substantially saturated about at 1×10
18
cm
−3
because increase of the dopant concentration in the block layer may increase a leakage current flowing the recombination layer while saturating a carrier-recombination rate in the recombination layer. In other words, there is a certain limit for improvement by increasing the dopant concentration.
Thirdly, a laser may become less reliable. As described above, an elevated dopant concentration in the p-type current block layer may increase contamination of the active layer with the dopant, leading to making the laser less reliable.
SUMMARY OF THE INVENTION
A semiconductor laser of this invention has been developed for solving the above problems. Specifically, an object of this invention is to provide a higher-power semiconductor laser by improving recombination of carrier in a recombination layer in a DC-PBH type of semiconductor laser. Another object of this invention is to provide a higher-power and highly reliable semiconductor laser without increasing the dopant concentration in the p-type current block layer.
This invention provides a DC-PBH (double channel Planar buried hetero-structure) type of semiconductor laser comprising current block layers and recombination layers on both sides of an active layer, the recombination layers containing a dopant.
This invention also provides a DC-PBH type of semiconductor laser comprising current block layers and recombination layers on both sides of an active layer, the recombination layers containing lattice defects.
The configurations and effects of the DC-PBH types of semiconductor lasers will be described with reference to the drawings.
A semiconductor laser of this invention is characterized in that a dopant or lattice defects (damages in a crystal) are introduced to recombination layers in a DC-PBH type of semiconductor laser.
FIG. 1
shows an example of a semiconductor laser according to this invention. The semiconductor laser comprises a pair of channels
3
on both sides of an active layer
1
and, outside of the pair of channels
3
, recombination layers
2
, which comprise a dopant and/or lattice defects.
FIG. 2
schematically shows a leakage current flowing a block layer in a DC-PBH type of semiconductor laser. For the DC-PBH type of semiconductor laser, the leakage current may be divided into two types, depending on their paths. One is a leakage current flowing the channel
3
via an n-InP type current block layer
5
and a p-InP type current block layer
4
, and the other is a leakage current flowing the recombination layer
2
from the p-InP type current block layer
4
. The former current may cause potential reduction in the p-InP type current block layer
4
and thus may induce turn-on of a pnpn thyristor configured in the block layer, which is a particularly significant problem,
FIG. 3
shows calculation results of optical-output dependency of these leakage-current components, with two-dimensional optical-device simulator. As the optical output increases, the leakage currents increase. In particular, a usual DC-PBH type of semiconductor laser exhibits a feature that the leakage current flowing the channel sharply increases while the leakage current flowing the recombination layer more slowly increases. Therefore, it may be possible to improve high-power properties for a laser by designing the block layers so that the leakage current flowing the recombination layer is relatively increased while relatively reducing the leakage current flowing the channel.
On the basis of such a concept, the semiconductor laser described in JP-A 6-283800 is provided, in which the dopant concentration in the p-type current block layer is increased and the channel is narrower to relatively increase the leakage current flowing the recombination layer while relatively reducing the leakage current flowing the channel. As described above, the leakage current flowing the channel may be reduced and high-power properties may be improved to some degree, but there are various problems such as a certain limit in the improvement.
On the other hand, the semiconductor laser of this invention increases a carrier-recombination rate in the recombination layer to increase the leakage current flowing the recombination layer while reducing the leakage current flowing the channel. In a conventional semiconductor laser, the recombination layer has the same composition as that of the active layer, and thus a carrier lifetime in the recombination layer is relatively longer, resulting in a lower recombination rate. Therefore, even if the dopant concentration in the p-type current block layer is increased, it does not contribute to significant increase in the leakage current flowing the recombination layer. In the light of the situation, in this invention, a dopant or lattice defects are introduced to provide different properties from the active layer. The dopant or lattice defects may play a role of recombination centers to reduce a carrier lifetime in the recombination layer, leading to increase in a recombination rate and thus increase in the leakage current flowing the recombination layer.
Thus, in this invention, a dopant or lattice defects are introduced to a recombination layer to which they have not been conventionally introduced, to permit increase of the leakage current flowing the recombination layer which has been limited with a carrier-recombination rate in the recombination layer. As a result, it may reduce the leakage current flowing a channel and even the total leakage current at a high power.
A semiconductor laser according to this invention may eliminate nece
Healy Brian
NEC Corporation
Sughrue Mion Zinn Macpeak & Seas, PLLC
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