Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is
Reexamination Certificate
2002-01-15
2002-12-31
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
C257S079000
Reexamination Certificate
active
06501090
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser and a method of manufacturing the same and, more particularly, a self-aligned stepped substrate (S
3
) type semiconductor laser and a method of manufacturing the same.
2. Description of the Prior Art
The structure of the S
3
type semiconductor laser in the prior art is shown in FIG.
1
.
In
FIG. 1
, an n-type cladding layer
102
formed of n-AlGaInP, a strained quantum well active layer
103
, a first p-type cladding layer
104
formed of p-AlGaInP, a pn alternatively-doped (current) blocking layer
105
formed of AlGaInP, a second p-type cladding layer
106
formed of p-AlGaInP, and a contact layer
107
formed of p-GaAs are formed in sequence on an n-GaAs substrate
101
on which a step
101
a
having a inclined plane is formed. Respective layers
102
to
107
on the n-GaAs substrate
101
have a inclined plane that is almost parallel with the inclined plane of the step
101
a
respectively. Also, the alternatively-doped blocking layer
105
is formed due to the property that the n-type impurity is incorporated readily into the flat portion when the p-type impurity and the n-type impurity are supplied alternatively in growing. In contrast, the p-type impurity is incorporated preferentially into the portion that is parallel with the inclined plane of the step
101
a
of the AlGaInP layer constituting the alternatively-doped blocking layer
105
to thus form the p-type cladding layer
104
.
In
FIG. 1
, the n-type cladding layer
102
and the p-type cladding layers
104
,
106
are divided into first to fourth layer regions
111
to
114
by broken lines. The first to fourth layer regions
111
to
114
are portions in which a ratio of the flow rate of the group V material gas to the flow rate of the group III material gas (referred to as a “V/III ratio” hereinafter) is changed respectively or portions in which the growth temperature is changed respectively.
More particularly, the first and fourth layer regions
111
,
114
are the portions that are formed by the high V/III ratio or at the low growth temperature, and the second and third layer regions
112
,
113
are the portions that are formed by the low V/III ratio or at the high growth temperature. Explanation will be made hereunder by taking the steps of changing the V/III ratio of the material gas as an example, but the similar advantages and structure can be achieved by making a change of the growth temperature.
To differentiate the V/III ratio acts to change lines that define the flat portions and the step portions of the cladding layers
102
,
104
,
106
, i.e., respective profiles of boundary lines between the flat surfaces and the inclined planes of the first to fourth layer regions
111
to
114
(referred to as “growth profiles” hereinafter). In
FIG. 1
, dot-dash lines denote growth profile lines indicating the change of the growth profile.
By the way, in the cladding layers
102
,
104
,
106
, an angle &thgr; between the flat portion and the growth profile line is small in the portions which are grown by the high V/III ratio, and this angle &thgr; exhibits a tendency to increase as the V/III ratio is lowered. For example, an angle &thgr;
01
of the growth profile line of the first layer region
111
is smaller than an angle &thgr;
02
of the growth profile line of the second layer region
112
, and an angle &thgr;
03
of the growth profile line of the third layer region
113
is larger than an angle &thgr;
04
of the growth profile line of the fourth layer region
114
.
It has been found that, in the first to fourth layer regions
111
to
114
, the angle &thgr; between the flat portion and the growth profile line affects the polarization plane of the laser beam in the laser oscillation and that the polarization plane becomes substantially perpendicular to the growth line. In the relationship between the beam shape of the laser beam and the direction of the polarization plane, based on the request to maintain the compatibility with the lasers having other structures, it is requested that the polarization plane should be set in the parallel direction with the inclined plane of the active layer
103
. That is, the angle &thgr; must be set to about 90 degree to the inclined plane of the active layer
103
.
In the structure in the prior art, in the cladding layers
102
,
104
,
106
, the portions that have large influence on the polarization plane and are close to the active layer
103
are grown at the low V/III ratio to form their growth profile lines substantially perpendicularly to the inclined plane of the active layer
103
, and also the polarization planes are set in parallel with the inclined plane portion of the active layer
103
by growing the portions that have small influence on the polarization plane and are far from the active layer
103
at the high V/III ratio. The inclined plane (step portion) of the active layer
103
is referred to as a stripe portion hereinafter.
The reason for that all the cladding layers
102
,
104
,
106
are not grown at the low V/III ratio is that, if the layer growth is carried out at the boundary portion between the n-GaAs substrate
101
and the GaAs contact layer
107
at the low V/III ratio, the crystal defect is ready to generate at the boundary portion between them and therefore such defect should be prevented.
Meanwhile, with the higher speed of the optical disk as the laser beam irradiation object, the optical output required for the semiconductor laser is increased year by year. As one factor to limit the higher output of the semiconductor laser, there is the kink in the current-optical output characteristic of the semiconductor laser.
As one factor to generate the kink, there is the event that normally the growth profile lines are not perfectly parallel with each other at the right and left portions of the stripe portion. If the components that modify the polarization plane differently are present at the right and left portions of the stripe portion, the transverse mode of the laser becomes unstable.
As the method of stabilizing such transverse mode, in Patent Application Publication (KOKAI) Hei 11-26884, it is disclosed that the transition region which appears when the low V/III ratio cladding layer is grown on the high V/III ratio cladding layer should be employed. The transition region has such a property that causes the right and left growth lines of the stripe portion to be formed in parallel to improve the kink level.
In
FIG. 1
, both the second and third layer regions
112
,
113
are grown at the low V/III ratio. In this case, the second layer region
112
corresponds to the transition region that causes the growth profile lines to be formed in parallel, and the third layer region
113
corresponds to the stable region that appears after the transition region is completed. The active layer
103
is formed in the second layer region
112
serving as the transition region.
In order to get the characteristic in the 100 mW class by improving further the kink level, the further optimization of the layer structure is needed.
Major approaches are to narrow the stripe portion further and to strengthen the symmetrization of the growth profiles on both sides of the active layer
103
.
FIG. 2
shows a schematic sectional view obtained when the narrower stripe formation and the growth profile symmetrization strengthening of the active layer are carried out by employing the technology in the prior art. In
FIG. 2
, in order to strengthen the symmetrization of the active layer growth profiles, the active layer
103
is provided in the center portion of the transition region that is grown at the low V/III ratio. Accordingly, two growth profile lines on both sides of the stripe portion of the active layer
103
become parallel.
However, in the structure shown in
FIG. 2
, following problems are caused in the device characteristics.
The first problem is that the polarization plane of the laser beam output from the semiconductor laser is ready to rotate. N
Anayama Chikashi
Furuya Akira
Hasegawa Taro
Nakao Kensei
Sugiura Katsumi
Armstrong Westerman & Hattori, LLP
Fujitsu Quantum Devices Limited
Nelms David
Nhu David
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