Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2000-03-02
2003-10-21
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S043010, C372S046012, C372S050121
Reexamination Certificate
active
06636541
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser device.
2. Description of the Related Art
Optic-fiber communication systems, which have ultra-fast speed and broad band capabilities, have been practically used. An InGaAsP semiconductor laser device has been vigorously developed as a light source for use in such optic-fiber communication systems. In the InGaAsP semiconductor laser device, a substrate is made of InP, and layers are made of InGaAsP mixed crystal material which lattice-matches the InP substrate. The InGaAsP semiconductor laser device has current-light output characteristics which change greatly depending on the operating temperature. To obtain stable operation of the semiconductor laser device over a wide temperature range, a Peltier element is commonly used as a temperature control device. This results in an increase in the cost of a light module including the semiconductor laser device. To avoid the use of the temperature control device, a means for improving the temperature characteristic of the semiconductor laser device itself is desired.
The general features and characteristics of a semiconductor laser device having a multi-quantum-well structure will be briefly described. Among factors which determine the temperature characteristic of a semiconductor laser device is a so-called carrier overflow where electrons which are injected into an active layer are not confined in a well layer and then pass through the active layer. There is a known method for reducing the overflow in which the amount of light confined in the active layer is increased so that threshold-carrier density is lowered. This method has disadvantage such that the output light of the semiconductor laser device is deformed by changing the amount of the confined light.
There is another known method for reducing the carrier overflow in which the amount of confined light is not changed, but the forbidden band width of a barrier layer is increased so that the difference in the forbidden band width between the well layer and the barrier layer is increased. Although this method obtains a large band offset in the conduction band, the band offset in the valence band in also increased, resulting a reduction in hole injection efficiency.
Hereinafter, a conventional semiconductor laser device
200
will be described with reference to
FIGS. 3
,
4
A and
4
B.
FIG. 3
is a cross-sectional view illustrating the conventional semiconductor laser device
200
. In
FIG. 3
, on an n-type InP substrate
201
, an n-type InP cladding layer
202
having a thickness of 400 nm, an InGaAsP waveguide layer
203
having an energy bandgap wavelength of 1.05 &mgr;m and a thickness of 50 nm, an active layer
204
, an InGaAsP waveguide layer
205
having an energy bandgap wavelength of 1.05 &mgr;m and a thickness of 50 nm, a p-type InP cladding layer
206
having a thickness of 400 nm, and a p-type InGaAsP contact layer
207
having a thickness of 200 nm are successively provided. An n-side electrode
210
and a p-side electrode
211
are provided on the lower side of the n-type InP substrate
201
and the upper side of the p-type InGaAsP contact layer
207
, respectively.
The active layer
204
includes five InGaAsP well layers
208
having compressive strain and six InGaAsP barrier layers
209
having tensile strain which are alternately laminated. Here, the strain means an incommensurate structure between an InGaAsP layer and the n-type InP substrate
201
. The degree of the strain is defined as the difference in the lattice constant. The degree of the strain is here specified by a strain factor represented by the following expressions:
(
C
208/209
−C
201
)/C
201
×100(%)
where C
208/209
is the lattice constant of the InGaAsP well layer
208
or the InGaAsP barrier layer
209
, and C
201
is the lattice constant of the n-type InP substrate
201
.
The InGaAsP well layer
208
has a greater lattice constant than that of the n-type InP substrate
201
, so that the strain factor of the InGaAsP well layer
208
has a positive value. The InGaAsP barrier layer
209
has a smaller lattice constant than that of the n-type InP substrate
201
, so that the strain factor of the InGaAsP barrier layer
209
has a negative value.
FIG. 4A
is a diagram illustrating the strain factor of each semiconductor layer in the vicinity of the active layer
204
of the conventional semiconductor laser device
200
. In
FIG. 4A
, the six InGaAsP layers
209
(indicated by intervals A) each have the same thickness of 10 nm and the same strain factor of −0.6%. The five InGaAsP layers
208
(indicated by intervals B) each have the same thickness of 6 nm and the same strain factor of 1.0%.
A strain amount of a layer is defined as a strain factor multiplied by a thickness of the layer. The strain amount of the whole active layer
204
is substantially zero because the positive strain amounts of the InGaAsP well layers
208
and the negative strain amounts of the InGaAsP barrier layers
209
are canceled.
The n-type InP cladding layer
202
, the InGaAsP waveguide layer
203
, the InGaAsP waveguide layer
205
, and the p-type InP cladding layer
206
correspond to intervals C, D, E, and F, respectively, as shown in FIG.
4
A.
FIG. 4B
is a schematic diagram showing energy bands in the vicinity of the active layer
204
. Intervals A to E indicate the respective layers of the semiconductor laser device
200
, each of which corresponds to the same reference numeral in FIG.
4
A. In
FIG. 4B
, each of the barrier layers
209
has the same energy bandgap. Band offsets X in the conduction band between the barrier layers
209
and the well layers
208
have the same value. Band offsets Y in the valence band between the barrier layers
209
and the well layers
208
have the same value. Here, a band offset is defined as the difference in an energy level between a barrier layer
209
and a well layer
208
which are adjacent to each other.
Next, the flow of electrons in the semiconductor laser device will be described. When a voltage is applied between an n-side electrode and a p-side electrode, electrons flow in the conduction band from the n-side electrode
210
to the InP substrate
201
to the n-type InP cladding layer
202
(interval C) to the InGaAsP waveguide layer
203
(interval D) to the active layer
204
(intervals A and B) to the InGaAsP waveguide layer
205
(Interval E) to the p-type InP cladding layer
206
(interval F) to the p-type InGaAsP contact layer
207
to the p-type electrode
211
. At the same time, holes flow in the valence band from the p-type electrode
211
to the p-type InGaAsP contact layer
207
to the p-type InP cladding layer
206
(interval F) to the InGaAsP waveguide layer
205
(interval E) to the active layer
204
(intervals A and B) to the InGaAsP waveguide layer
203
(interval D) to the n-type InP cladding layer
202
(interval C) to the InP substrate
201
to the n-side electrode
210
.
The electrons flowing in the conduction band and the holes flowing in the valence band recombine in the well layers
208
of the active layer
204
, resulting in light emission.
In the conventional semiconductor laser device
200
, however, the small band offset X in the conduction band between the barrier layer
209
and the well layer
208
causes electrons to overflow from the active layer
204
.
Moreover, the great band offset Y in the valence band between the barrier layer
209
and the well layer
208
causes a nonuniform amount of hole injection.
One attempt to solve this problem may be made by increasing the absolute values of the strain factors of the barrier layer
209
and the well layer
208
. In this case, however, the thicknesses of these layers exceed the limit of the critical thickness, so that crystal defects occur. Moreover, the thicknesses of the barrier layer
209
and the well layer
208
fluctuate, resulting in a loss in the flatness of these layers.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, a semiconductor l
Ishino Masato
Kito Masahiro
Matsui Yasushi
Nakayama Hisashi
Ip Paul
RatnerPrestia
Rodriguez Armando
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