Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2001-10-31
2003-04-08
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S046012, C372S044010, C372S050121
Reexamination Certificate
active
06546033
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser device having an end-facet window structure, and a process for producing such a semiconductor laser device.
2. Description of the Related Art
In conventional semiconductor laser devices, when optical output power is increased, currents generated by optical absorption in vicinities of end facets generate heat, i.e., raise the temperature at the end facets. In addition, the raised temperature reduces the semiconductor bandgaps at the end facets, and therefore the optical absorption is further enhanced. That is, a vicious cycle is formed, and the end facets are damaged. This damage is the so-called catastrophic optical mirror damage (COMD). Thus, the maximum optical output power is limited due to the COMD. In addition, when the optical power reaches the COMD level, the optical output deteriorates with time. Further, the semiconductor laser device is likely to suddenly break down due to the COMD. It is known that high reliability in high output power operation can be achieved when window structures are formed in the vicinities of end facets, i.e., crystals having a greater bandgap than an active layer are formed in the vicinities of the end facets, so as to prevent the light absorption in the vicinities of end facets.
For example, Kazushige Kawasaki et al. (“0.98 &mgr;m band ridge-type window structure semiconductor laser (1),” Digest 29a-PA-19, 1997 Spring JSAP Annual Meeting, The Japan Society of Applied Physics) disclose a semiconductor laser device in the 980 nm band, which has a window structure formed by injecting Si ions into end regions of a ridge structure and disordering an In
0.2
Ga
0.8
As quantum well by thermal diffusion. However, the process for producing the above semiconductor laser device is very complicated and long since the vicinities of end facets are required to be insulated by injection of H ions after the injection of the Si ions in the vicinity of the active layer in order to prevent a current flow in the vicinities of end facets.
In addition, when the active layer contains aluminum, the reliability of the semiconductor laser device is decreased due to oxidation of aluminum. In particular, when a window structure is formed by removing near-edge portions of the active layer and regrowing semiconductor layers in the near-edge portions, aluminum is exposed on the regrowth boundary. Therefore, the reliability of the semiconductor laser device is further decreased.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a semiconductor laser device which does not contain aluminum in an active layer, has a window structure being non-absorbent of light in vicinities of end facets, and is reliable in a wide output power range from low to high output power.
Another object of the present invention is to provide a process which can produce, by a simple process, a semiconductor laser device which does not contain aluminum in an active layer, has a window structure being non-absorbent of light in vicinities of end facets, and is reliable in a wide output power range from low to high output power.
(1) According to the first aspect of the present invention, there is provided a semiconductor laser device comprising: a GaAs substrate of a first conductive type; a lower cladding layer of the first conductive type, formed on the GaAs substrate; a first lower optical waveguide layer made of In
x2
Ga
1−x2
As
1−y2
P
y2
having a first bandgap, and formed on the lower cladding layer, where 0≦x
2
≦0.3 and x
2
=0.49y
2
; an intermediate layer made of In
x5
Ga
1−x5
P and formed on the first lower optical waveguide layer, where 0<x
5
<1; a second lower optical waveguide layer made of In
x2
Ga
1−x2
As
1−y2
P
y2
having the first bandgap, and formed on the intermediate layer except for near-edge areas of the first intermediate layer located in first vicinities of opposite end facets of the semiconductor laser device so as to leave first portions of spaces in the first vicinities of opposite end facets, where 0≦x
2
≦0.3 and x
2
=0.49y
2
; a compressive strain active layer made of In
x1
Ga
1−x1
As
1−y1
P
y1
having a second bandgap smaller than the first bandgap, and formed on the second lower optical waveguide layer so as to leave second portions of the spaces in second vicinities of opposite end facets, where 0<x
1
≦0.4 and 0≦y
1
≦0.1; an upper optical waveguide layer made of In
x2
Ga
1−x2
As
1−y2
P
y2
having the first bandgap, and formed on the compressive strain active layer so as to leave third portions of the spaces in third vicinities of opposite end facets, where 0≦x
2
≦0.3 and x
2
=0.49y
2
; a cap layer made of In
x5
Ga
1−x5
P and formed on the upper optical waveguide layer so as to leave fourth portions of the spaces in fourth vicinities of opposite end facets, where 0<x
5
<1; a non-absorbent layer made of In
x6
Ga
1−x6
As
1−y6
P
y6
having a third bandgap greater than the second bandgap, and formed over the cap layer so that the spaces are filled with the non-absorbent layer, where 0≦x
6
≦0.3 and x
6
=0.49y
6
; an upper cladding layer of a second conductive type, formed on the non-absorbent layer; and a contact layer of the second conductive type, formed on the upper cladding layer.
In addition, each of the lower and upper cladding layers, the first and second lower optical waveguide layers, the upper optical waveguide layer, and the non-absorbent layer are assumed to have such a composition as to lattice-match with the active layer.
In this specification, the lattice matching is defined as follows.
When c
1
and c
2
are lattice constants of first and second layers, respectively, and the absolute value of the amount (c
1
−c
2
)/c
2
is equal to or smaller than 0.001, the first layer is lattice-matched with the second layer. For example, when cs and c are the lattice constants of a substrate and a layer grown above the substrate, respectively, and the absolute value of the amount (c−c
s
)/c
s
is equal to or smaller than 0.001, the layer grown above the substrate is lattice-matched with the substrate.
Further, the first conductive type is different in polarity of carriers from the second conductive type. That is, when the first conductive type is p type, and the second conductive type is n type.
Preferably, the semiconductor laser device according to the first aspect of the present invention may also have one or any possible combination of the following additional features (i) to (v).
(i) The contact layer may be formed on the upper cladding layer except for near-edge areas of the upper cladding layer located in fifth vicinities of the end facets of the semiconductor laser device, and an insulation layer may be formed on the near-edge areas of the upper cladding layer so as to prevent current injection through the near-edge areas of the upper cladding layer.
(ii) Each of the lower and upper cladding layers may be made of one of Al
z1
Ga
1−z1
As and In
x3
(Al
z3
Ga
1−z3
)
1
31 x3
As
1−y3
P
y3
, where 0.2≦z
1
≦0.8, x
3
=0.49y
3
, 0.9<y
3
≦1, and 0≦z
3
≦1.
(iii) Regions of the semiconductor laser device above at least a mid-thickness of the upper cladding layer except for a stripe region of the semiconductor laser device may be removed so as to form a ridge and realize index guidance of light.
(iv) The semiconductor laser device according to the first aspect of the present invention may further comprise a current confinement layer made of one of Al
z2
Ga
1−x2
AS and In
0.49
Ga
0.51
P which lattice-match with GaAs, and formed above the upper optical waveguide layer so as to form an internal current confinement structure realizing index guidance of light, where 0.2<z
2
<1.
(v) In order to compensate for the strain of the active layer, two In
x4
Ga
1−x4
As
1−y4
P
y4
tensile strain barrier layers (0≦x
4
<0.49y
4
, 0<y
4
&lE
Fuji Photo Film Co. , Ltd.
Jackson Cornelius H
Sughrue & Mion, PLLC
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