Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2000-11-22
2004-11-02
Wong, Don (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S044010, C372S045013, C372S023000, C372S050121
Reexamination Certificate
active
06813290
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor laser device mainly used as a light source in a pickup for an optical disk, and also relates to a method for fabricating the same.
A red-light-emitting semiconductor laser device, which is used as a light source for a digital video disc (which will be herein called a DVD), has an oscillation wavelength in the band from 630 nm to 690 nm. This is shorter than the oscillation wavelength (in the band of 780 nm) of an infrared semiconductor laser device used for a known compact disc (which will be herein called a CD). Therefore, data can be read not only from DVDs but also from CDs by using the red-light-emitting semiconductor laser device.
However, an optical disc like a write once CD (which will be herein called a CD-R), in which an organic compound is used as its recording medium, has an optical reflectance that depends strongly on wavelength. For this and other reasons, it is impossible to read data from CD-Rs with the red-light-emitting semiconductor laser device, and the infrared semiconductor laser device is needed to read data from CD-Rs.
Therefore, to read data from both DVDs and CD-Rs alike, an optical pickup should be provided with two light sources, i.e., a red-light-emitting semiconductor laser device and an infrared semiconductor laser device.
As techniques of providing two light sources for an optical pickup, hybrid integration technique, by which red-light-emitting and infrared semiconductor laser devices are provided independently, and monolithic integration technique, by which red-light-emitting and infrared semiconductor lasers are integrated on a single substrate, are known.
In view of technical difficulty and productivity, the hybrid integration is prevailing at present. To further reduce the size and costs of the optical pickups, however, the monolithic integration could be advantageous in future.
For that reason, a semiconductor laser device, implemented by forming a red-light-emitting semiconductor laser structure for a wavelength band of 650 nm and an infrared semiconductor laser structure for a wavelength band of 780 nm side by side on a single GaAs substrate, was proposed at the 60th Japan Society of Applied Physics Autumn Technical Meeting 3a-ZC-10 (1999).
A red-light-emitting semiconductor laser structure is usually a multilayer structure made up of quaternary mixed crystals (Al
x
Ga
1-x
)
y
In
1-y
P (where 0≦x≦1 and 0≦y≦1), which will be herein simply labeled as AlGaIP), which consist essentially of Al, Ga, In and P. And an infrared semiconductor laser structure is usually a multilayer structure made up of ternary mixed crystals (Al
z
Ga
1-x
As (where 0≦z≦1), which will be herein simply labeled as AlGaAs), which consist essentially of Al, Ga and As.
Also, to form two semiconductor laser structures on a single substrate, a first multilayer structure, made up of multiple semiconductor layers including a first active layer, is defined on the substrate. Then, the first multilayer structure is patterned to form a first semiconductor laser structure. Next, a second multilayer structure, made up of multiple semiconductor layers including a second active layer, is defined on the substrate, on which the first semiconductor laser structure has been formed. Then, the second multilayer structure is patterned, thereby defining a second semiconductor laser structure.
Hereinafter, a known monolithic semiconductor laser device, implemented by forming a red-light-emitting semiconductor laser structure on a first region of a substrate and an infrared semiconductor laser structure on a second region of the same substrate, will be described with reference to FIG.
5
.
As shown in
FIG. 5
, a buffer layer
102
is formed out of an n-type GaAs layer on an n-type GaAs substrate
101
.
A first multilayer structure, consisting of first n-type cladding layer
103
A formed out of an n-type AlGaAs layer, first active layer
104
A formed out of a GaAs layer and first p-type cladding layer
105
A formed out of a p-type AlGaAs layer, is defined on a first region of the buffer layer
102
, and constitutes a red-light-emitting semiconductor laser structure.
A second multilayer structure, consisting of second n-type cladding layer
106
A formed out of an n-type AlGaInP layer, second active layer
107
A formed out of a GaInP layer and second p-type cladding layer
108
A formed out of a p-type AlGaInP layer, is defined on a second region of the buffer layer
102
, and constitutes an infrared semiconductor laser structure.
A lower electrode
109
, which becomes a common electrode, is formed on the lower surface of the n-type GaAs substrate
101
. A first upper electrode
110
is formed on the first p-type cladding layer
105
A and a second upper electrode
111
is formed on the second p-type cladding layer
108
A.
Hereinafter, a method for fabricating the known semiconductor laser device will be described with reference to FIGS.
6
(
a
) through
6
(
d
).
First, a buffer layer
102
formed out of an n-type GaAs layer is deposited on an n-type GaAs substrate
101
and then n-type AlGaAs layer
103
, GaAs layer
104
and p-type AlGaAs layer
105
are deposited in this order over the buffer layer
102
, thereby defining a first multilayer structure as shown in FIG.
6
(
a
).
Next, the first multilayer structure is etched and patterned into a predetermined shape, thereby forming first n-type cladding layer
103
A out of the n-type AlGaAs layer
103
, first active layer
104
A out of the GaAs layer
104
and first p-type cladding layer
105
A out of the p-type AlGaAs layer
104
on a first region of the buffer layer
102
as shown in FIG.
6
(
b
). In this manner, a red-light-emitting semiconductor laser structure is formed out of the first multilayer structure.
Thereafter, n-type AlGaInP layer
106
, GaInP layer
107
and p-type AlGaInP layer
108
are deposited in this order over the entire surface of the buffer layer
102
, on which the red-light-emitting semiconductor laser structure has been defined, thereby forming a second multilayer structure as shown in FIG.
6
(
c
).
Next, the second multilayer structure is etched and patterned into a predetermined shape, thereby forming second n-type cladding layer
106
A out of the n-type AlGaInP layer
106
, second active layer
107
A out of the GaInP layer
107
and second p-type cladding layer
108
A out of the p-type AlGaInP layer
108
on a second region of the buffer layer
102
as shown in FIG.
6
(
d
). In this manner, an infrared semiconductor laser structure is formed out of the second multilayer structure.
Then, a lower electrode
109
is formed on the lower surface of the n-type GaAs substrate
101
. And a first upper electrode
110
is formed on the first p-type cladding layer
105
A and a second upper electrode
111
is formed on the second p-type cladding layer
108
A. Then, the known semiconductor laser device as shown in
FIG. 5
can be obtained.
In the known method for fabricating a semiconductor laser device, the first multilayer structure is etched and patterned into a predetermined shape, thereby exposing the buffer layer
102
. Then, the n-type AlGaInP layer
106
, GaInP layer
107
and p-type AlGaInP layer
108
are deposited in this order over the buffer layer
102
to form the infrared semiconductor laser structure.
In the step of exposing the buffer layer
102
formed out of the n-type GaAs layer, however, over etching should be performed on the lowermost n-type AlGaAs layer
103
and the buffer layer
102
is etched excessively because the compositions of the GaAs and AlGaAs layers are similar. For this reason, the surface of the buffer layer
102
might become rugged considerably or a hole might be made in the buffer layer
102
, and the surface of the buffer layer
102
becomes adversely uneven.
If the n-type AlGaInP layer
106
, GaInP layer
107
and p-type AlGaInP layer
108
are deposited in this order over the buffer layer
102
with the uneven surface, the resultant semiconductor layers have disordered crystal structures
Nguyen Dung
Nixon & Peabody LLP
Studebaker Donald R.
Wong Don
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