Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1998-10-06
2003-02-25
Leung, Quyen (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S096000
Reexamination Certificate
active
06526081
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a surface emitting semiconductor laser and a method of fabricating the same, and more particularly, it relates to a surface emitting semiconductor laser including a current confining region formed by ion implantation or selective oxidation and a method of fabricating this surface emitting semiconductor laser.
Vertical cavity surface emitting lasers are advantageous not only in obtaining a light beam with a circular section but also in two-dimensionally integrating plural emitting portions on a single substrate at a high density. Also, the vertical cavity surface emitting lasers can be operated with small power consumption and fabricated at a low cost. Owing to these advantages, the surface emitting semiconductor lasers have been regarded as a promising light source for the optical communications and optical information processing of the next generation, and have been variously examined and developed. Recently, the surface emitting laser diodes are realized in a variety of structures, including one having a very low threshold current of approximately 10 &mgr;A (microampere) on the laboratory level and one commercially available at approximately 3000 yen.
The surface emitting semiconductor lasers are classified, depending upon their current confinement structures, into the following three types: Lasers including a simple mesa structure; lasers including a current confining layer formed by ion implantation; and lasers including a current confining layer formed by selectively oxidizing a semiconductor layer including Al. The lasers including a simple mesa structure have been utilized since the initial stage of the examination until today. The lasers including a current confining layer formed by ion implantation are widely used in commercially available semiconductor lasers. The lasers including a current confining layer formed by selectively oxidizing a semiconductor layer including Al are still now under research in laboratories. In view of electrical resistance and heat resistance, a laser with a planer structure including a current confining layer formed by ion implantation or selective oxidation, or a laser including a mesa structure with a very large area is advantageous to a laser including a simple mesa structure.
The surface emitting semiconductor lasers with a current confinement structure formed by ion implantation can be fabricated in various types of structures.
FIGS. 9A and 9B
are schematic diagrams of conventional surface emitting semiconductor lasers
900
and
910
, respectively described as a first conventional technique.
Each of the surface emitting semiconductor lasers
900
and
910
is formed on an n-type GaAs substrate
901
, and has a multilayer structure for laser oscillation including an n-type lower mirror
902
, an active region
904
and a p-type upper mirror
905
. The lower mirror
902
is formed on the substrate
901
, and the active region
904
is sandwiched between the lower mirror
902
and the upper mirror
905
. The active region
904
comprises an active layer
903
of a strained quantum well including an In
0.2
Ga
0.8
As layer working as a well layer and a GaAs layer working as a barrier layer sandwiched between cladding layers of Al
0.5
Ga
0.5
As, and is designed so as to oscillate light with a wavelength of approximately 980 nm. Furthermore, a p-type electrode
906
is formed on the upper mirror
905
. Also, an n-type electrode
907
is formed on the back surface of the n-type substrate
901
, so that the light output of the laser can be taken out from the back surface of the substrate
901
.
In the surface emitting semiconductor laser
900
, an ion implanted region
908
formed by ion implantation is disposed in an area surrounding a given closed area within the upper mirror
905
. On the other hand, in the surface emitting semiconductor laser
910
, the ion implanted region
908
is formed so as to make the active region
904
be a closed area.
Now, the operation of the conventional surface emitting semiconductor lasers
900
and
910
will be described. Since the ion implanted region
908
is a relatively higher resistance area, a current injected into the active region
904
through the p-type electrode
906
and the n-type electrode
907
is confined by the ion implanted region
908
. Accordingly, the current injected into the laser can be efficiently injected into the small closed area, resulting in largely decreasing a threshold current.
An example of the surface emitting semiconductor lasers including a current confining layer formed by selective oxidation is described in Applied Physics Letter, 66 (1995), pp. 3413-3415.
FIG. 10
is a sectional view for schematically showing the structure of a second conventional surface emitting semiconductor laser
1000
described in this paper.
In the conventional surface emitting semiconductor laser
1000
, an active layer
1020
and a p-type upper mirror
1030
are successively stacked on an n-type lower mirror
1010
, and a mesa is formed through etching to expose the lower mirror
1010
. Furthermore, a ring-shaped p-type electrode
1040
is formed on the top surface of the upper mirror
1030
. The upper mirror
1030
is formed by stacking AlGaAs and GaAs, in which merely the lowermost AlGaAs is formed as an Al
0.98
Ga
0.02
As layer
1032
with a composition ratio of Al of 0.98 and the remaining portion is formed as an Al
0.9
Ga
0.1
As/GaAs mirror
1033
formed by alternately stacking Al
0.9
Ga
0.1
As layers with a composition ratio of Al of 0.9 and GaAs layers. By utilizing a difference (of approximately 15:1) in the oxidation rate between Al
0.98
Ga
0.02
As and Al
0.9
Ga
0.1
As, the Al
0.98
Ga
0.02
As layer
1032
alone is selectively oxidized from the side face of the mesa, thereby forming an Al
x
O
y
region
1031
.
Next, the operation principle of the conventional surface emitting semiconductor laser
1000
will be described. Since the Al
x
O
y
region
1031
serves as an insulator, a current injected into the laser is confined by the Al
x
O
y
region
1031
so as to flow merely through the Al
0.98
Ga
0.02
As layer
1032
, that is, a small area. Accordingly, the current can be efficiently confined, resulting in decreasing a threshold current. Moreover, owing to a difference in the refractive index between the Al
0.98
Ga
0.02
As layer
1032
and the Al
x
O
y
region
1031
, light is confined in the lateral direction to some extent, which can further decrease the threshold current.
However, when the ion implanted region crosses the active layer in the first conventional technique, the threshold current can be increased because the active layer is damaged by the ion implantation. Also, when the ion implanted region is disposed within the upper mirror, although the ion implantation does not damage the active layer, the following problems can be caused: Since the concentration distribution of the implanted ion in the vertical direction has a spread (namely, the change in the ion concentration is not abrupt), the current confining region is unavoidably formed in a position away from the active layer. Accordingly, the current is spread in the lateral direction while flowing between the current confining region and the active layer, and hence, the current cannot be effectively confined. Also, since the current confining region has a large thickness in the vertical direction, the device resistance is inevitably increased.
On the other hand, in the second conventional laser, the semiconductor layer with a thickness of several tens nm is required to be oxidized from the side face of the mesa in the lateral direction by several tens &mgr;m (micrometer). Since it is very difficult to control the oxidation rate and the shape to be oxidized, it is difficult to form the current confining region in a desired shape. When a necessary and minimum number of layers (one layer in
FIG. 10
) are to be oxidized so as not to increase the device resistance, it is necessary to form a hybrid mirror including two types of AlGaAs layers having different co
Leung Quyen
Matsushita Electric - Industrial Co., Ltd.
Nixon & Peabody LLP
Studebaker Donald R.
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