Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1999-08-10
2001-12-25
Davie, James W. (Department: 2881)
Coherent light generators
Particular active media
Semiconductor
C372S045013
Reexamination Certificate
active
06333946
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser device and a process for manufacturing the same. More particularly, it relates to a forward embedded mesa ridge semiconductor laser device provided with a forward mesa ridge on an upper cladding layer and a second upper cladding layer on the surface of the forward mesa ridge to reduce the resistance of the device and stabilize the transverse mode characteristic of a laser beam produced by the laser, and a process for manufacturing the laser.
2. Description of the Relate Art
As the storage capacities of information recording media used for personal computers and multimedia appliances increase, demands have been increasing for MO (magneto-optical disk) and CD-R (field progranmmable compact disk) that are information recording media wherein information can be written by the use of light. For laser devices used in writing information on such programmable optical disks, an infrared emitting laser beam having a wavelength of 780 nm and red laser beams having wavelengths in a range from 650 nm to 685 nm are used, which require increased output power while ensuring a stable transverse mode oscillation laser beam.
FIG. 8
is a perspective view of a forward mesa ridge-embedding type semiconductor laser of the prior art, and
FIG. 9
is a cross sectional view taken along lines IX—IX in FIG.
8
.
In FIG.
8
and
FIG. 9
, numeral
100
denotes the forward mesa ridge-embedding type semiconductor laser device,
102
denotes an n-type GaAs substrate having a (
100
) principal plane,
104
denotes a lower cladding layer made of n-type Al
0.5
Ga
0.5
As disposed on the substrate
102
, and
106
denotes an active layer having a multiple quantum-well (hereinafter referred to as MQW) structure disposed on the lower cladding layer
104
.
The MQW structure comprises wells or well layers made of undoped Al
0.1
Ga
0.9
As, guide layers, and barrier layers (outermost layers) which are made of undoped Al
0.3
Ga
0.7
As (hereinafter the MQW of this constitution will be referred to as an undoped Al
0.3
Ga
0.7
As/Al
0.1
Ga
0.9
As MQW. MQWs made of materials of other constitutions will also be referred to similarly.)
Numeral
108
denotes an upper cladding layer made of p-type Al
0.5
Ga
0.5
As,
108
a
denotes a forward mesa ridge of the upper cladding layer
108
,
108
b
denotes a parallel portion of the upper cladding layer
108
,
110
denotes a cap layer made of p-type GaAs disposed on the forward mesa ridge
108
a
, while the forward mesa ridge
108
a
and the cap layer
110
constitute the forward mesa ridge portion
109
.
Numeral
112
denotes a current blocking layer made of n-type GaAs disposed on the parallel portion
108
b
of the upper cladding layer
108
, embedding the cap layer
110
and the forward mesa ridge
180
a
. Numeral
114
denotes a contact layer made of p-type GaAs disposed on the cap layer
110
and on the current blocking layer
112
in contact therewith. Numeral
116
denotes a p-electrode and
118
denotes an n-electrode.
Numeral
120
denotes a beam emimtting end face,
122
denotes a back end face of the semiconductor laser
100
, and arrow
124
represents a laser beam.
FIG. 9
is a cross sectional view of the forward mesa ridge-embedding type semiconductor laser without the p-electrode
116
and the n-electrode
118
.
The semiconductor laser
100
of such a configuration as described above operates as follows.
When a forward bias voltage is applied between the p-electrode
116
and the n-electrode
118
, holes and electrons are supplied from the p-electrode
116
and from the n-electrode
118
respectively, to the active layer
106
through a path which is limited by the current blocking layer
112
. The carriers (electrons and holes) are confined in the active layer
106
by the lower cladding layer
104
and the upper cladding layer
108
, so that spontaneous emission of light occurs through recombination of electrons and holes in the active layer
106
. The spontaneously emitted light is confined in the forward mesa ridge portion
108
a
that serves as a waveguide between the lower cladding layer
104
and the upper cladding layer
108
, being reflected by the beam emitting end face
120
and the back end face
122
, thereby inducing stimulated emission and providing an output of the laser beam
124
.
In the semiconductor laser
100
, the active layer is an undoped Al
0.3
Ga
0.7
As/Al
0.1
Ga
0.9
As MQW structure while the lower cladding layer
104
and the upper cladding layer
108
have compositions of Al
0.5
Ga
0.5
As, and therefore the lower cladding layer
104
and the upper cladding layer
108
have a larger bandgap and a lower refractive index than the active layer
106
. The current blocking layer
112
is made of GaAs, and therefore has a smaller bandgap and higher refractive index than the upper cladding layer
108
.
Consequently, the light generated in the active layer
106
beneath the forward mesa ridge portion
108
a
of the upper cladding layer
108
is confined in a region defined between the forward mesa ridge portion
108
a
of the upper cladding layer
108
and the lower cladding layer
104
in the vertical direction.
Transverse light is confined by forming the parallel portion
108
b
of the upper cladding layer
108
with a predetermined thickness so that the active layer
106
is adjacent to the current blocking layer
112
, and by having a certain part of light absorbed by the current blocking layer
112
, making use of the difference in the bandgap between the active layer
106
and the current blocking layer
112
via the parallel portion
108
b
. This type of semiconductor laser device is called a gain waveguide semiconductor laser device.
Now a process for manufacturing the semiconductor laser device of the prior art will be described below.
The lower cladding layer
104
of n-type Al
0.5
Ga
0.5
As, the active layer
106
of an undoped Al
0.3
Ga
0.7
As/ Al
0.1
Ga
0.9
As MQW structure, the upper cladding layer
108
of p-type Al
0.5
Ga
0.5
As, and the cap layer
110
of p-type GaAs are formed successively on the n-type GaAs substrate
102
by an epitaxial growth process such as MOCVD.
Then an SiON film is grown on the cap layer
110
by CVD or the like to form a striped mask pattern through photolithography and etching steps, and top portions of the cap layer
110
and of the upper cladding layer
108
are removed by wet etching with the mask pattern used as a mask, thereby to form the forward mesa ridge portion
109
comprising the forward mesa ridge
108
a
and the cap layer
110
.
Side faces of the forward mesa ridge
108
a
are defined by etching in a (
111
) A plane. Formed on the side faces of the forward mesa ridge portion
109
are current blocking layers
112
of n-type GaAs grown in the epitaxial growth process.
The mask pattern of SiON film is removed by wet etching or the like, and the contact layer
114
of p-type GaAs is formed by the MOCVD process on the cap layer
110
and on the current blocking layer
112
.
Finally, the p-electrode
116
is formed on the contact layer
114
by vapor deposition or the like and, after polishing the back surface of the substrate
102
to a thickness of about 100 &mgr;m, the n-electrode
118
is formed by vapor deposition or the like, thereby completing the semiconductor laser
100
.
The reason for providing the cap layer
110
on the upper cladding layer
108
of the semiconductor laser device
100
is because, without the cap layer
110
, the upper cladding layer
108
would be exposed to the atmosphere and oxidized when the etching mask is removed after forming the forward mesa ridge
108
a
, because the upper cladding layer
108
is made of Al
0.5
Ga
0.5
As.
When an oxide film of Al is formed, electrical resistance of the device increases since this portion lies in the current path, and deterioration occurs in the crystal after regrowth. In order to prevent such a problem, the cap layer
110
of p-type GaAs is formed in the first epitaxial growth thereby to prevent an oxide film from bei
Miyashita Motoharu
Nishiguchi Harumi
Ohkura Yuji
Shima Akihiro
Davie James W.
Leydig , Voit & Mayer, Ltd.
Mitsubishi Denki & Kabushiki Kaisha
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