Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2000-12-26
2004-07-27
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S045013, C372S046012, C372S050121, C372S068000, C372S044010
Reexamination Certificate
active
06768755
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser device having a current blocking layer.
2. Description of the Prior Art
A refractive index guided semiconductor laser device supplied with refractive index difference in a direction parallel to an active layer for forming a light guide is developed in general.
FIG. 34
is a typical sectional view showing a conventional semiconductor laser device
120
described in Japanese Patent Laying-Open No. 8-222801 (1996).
In the semiconductor laser device
120
shown in
FIG. 34
, an n-type cladding layer
122
, an active layer
123
, a p-type cladding layer
124
and a p-type contact layer
127
are successively formed on an n-type substrate
121
, and the p-type contact layer
127
and the p-type cladding layer
124
are etched for forming flat portions on a ridge portion and on both sides of the ridge portion.
Further, a first current blocking layer
125
having a low carrier concentration is formed on the flat portions of the p-type cladding layer
124
located on both sides of the ridge portion, and an n-type current blocking layer
126
is formed on the first current blocking layer
125
having a low carrier concentration. A p-type contact layer
128
is formed on the p-type contact layer
127
and the n-type current blocking layer
126
.
When the semiconductor laser device
120
is driven, a reverse bias voltage is applied to a p-n junction between the n-type current blocking layer
126
and the p-type cladding layer
124
. Thus, the n-type current blocking layer
126
cuts off a current so that the current is injected into the ridge portion in a narrowed state.
In general, a p-n junction formed between an n-type current blocking layer and a p-type cladding layer has large electric capacitance, and hence serves as a factor inhibiting high-speed operation of a semiconductor laser device. The electric capacitance of the p-n junction is increased as the carrier concentration in this p-n junction is increased.
Therefore, the semiconductor laser device
120
shown in
FIG. 34
is provided with the current blocking layer
125
having a low carrier concentration, in order to reduce the electric capacitance in the p-n junction between the n-type current blocking layer
126
and the p-type cladding layer
124
.
This current blocking layer
125
has a lower carrier concentration than the n-type current blocking layer
126
. Therefore, the current blocking layer
125
having a low carrier concentration defines a depletion region in the p-n junction between the n-type current blocking layer
126
and the p-type cladding layer
124
, for reducing the electric capacitance. Thus, the semiconductor laser device
120
is enabled to operate at a high frequency.
In the semiconductor laser device
120
having the current blocking layer
125
of a low carrier concentration having a narrower band gap than the p-type cladding layer
124
, however, valence bands of the p-type cladding layer
124
and the current blocking layer
125
of a low carrier concentration have energy band structures shown in FIG.
35
.
FIG. 35
is a model diagram showing the energy band structures of the valence bands of the p-type cladding layer
124
and the current blocking layer
125
having a low carrier concentration. As shown in
FIG. 35
, the band gap of the current blocking layer
125
having a low carrier concentration is sufficiently smaller than the band gap of the p-type cladding layer
124
, and hence carriers are readily injected from the p-type cladding layer
124
into the current blocking layer
125
having a low carrier concentration and stored therein. Consequently, since depletion of the p-n junction between the n-type current blocking layer
126
and the p-type cladding layer
124
is inhibited, electric capacitance between the current blocking layer
125
having a low carrier concentration and the p-type cladding layer
124
is increased. Therefore, the operating speed of the semiconductor laser device
120
cannot be sufficiently increased.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a semiconductor laser device sufficiently increased in operating speed.
A semiconductor laser device according to an aspect of the present invention comprises an active layer, a first cladding layer of a first conduction type provided on the active layer, a current blocking layer of a second conduction type provided on the first cladding layer except a current injection region, a low carrier concentration layer provided on the side of the current blocking layer between the first cladding layer and the current blocking layer and having a lower carrier concentration than the current blocking layer and a depletion enhancement layer provided on the side of the first cladding layer between the first cladding layer and the current blocking layer for inhibiting storage of carriers in the low carrier concentration layer.
In the semiconductor laser device, the depletion enhancement layer inhibits storage of carriers from the first cladding layer into the low carrier concentration layer. Thus, the low carrier concentration layer is kept in a depleted state. Therefore, electric capacitance between the current blocking layer and the first cladding layer is kept small for sufficiently increasing the operating speed of the semiconductor laser device.
The band gaps of the first cladding layer, the depletion enhancement layer and the low carrier concentration layer may be reduced in this order.
Thus, the depletion enhancement layer having an intermediate band gap is provided between the first cladding layer having a large band gap and the low carrier concentration layer having a small band gap.
In this case, the band offset between the first cladding layer and the depletion enhancement layer is smaller than the band offset between the first cladding layer and the low carrier concentration layer, whereby carriers are hardly injected from the first cladding layer into the depletion enhancement layer while carriers are more hardly injected into the low carrier concentration layer. Further, the carriers are injected from the first cladding layer into both of the low carrier concentration layer and the depletion enhancement layer in a divided manner, whereby the quantity of carriers stored in the low carrier concentration layer is reduced. Thus, storage of carriers in the low carrier concentration layer can be inhibited by the simple structure of setting the band gap of the depletion enhancement layer to the intermediate level between the low carrier concentration layer and the first cladding layer.
The first cladding layer may have a flat portion formed on the active layer and a ridge portion formed on the flat portion in the current injection region, the depletion enhancement layer may be formed on the flat portion located on both sides of the ridge portion and on the side surfaces of the ridge portion, and the low carrier concentration layer and the current blocking layer may be successively formed on the depletion enhancement layer.
In this case, the depletion enhancement layer inhibits storage of carriers from the flat portion of the first cladding layer into the low carrier concentration layer. Thus, the low carrier concentration layer is kept in the depleted state, and the electric capacitance between the flat portion of the first cladding layer and the current blocking layer is kept small.
The thickness of the depletion enhancement layer is preferably at least 10 nm. Thus, the semiconductor laser device is more improved in high-frequency characteristic.
The thickness of the depletion enhancement layer is preferably at least 15 nm. Thus, the semiconductor laser device is further improved in high-frequency characteristic.
The semiconductor laser device may further comprise a ridge-shaped second cladding layer of a first conduction type provided on the depletion enhancement layer in the current injection region, the depletion enhancement layer may be formed on the first cladding layer, and the lower car
Hata Masayuki
Hiroyama Ryoji
Inoue Daijiro
Nomura Yasuhiko
Takeuchi Kunio
Ip Paul
Rodriguez Armando
Sanyo Electric Co,. Ltd.
Westerman Hattori Daniels & Adrian LLP
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