Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2000-12-28
2003-10-28
Davie, James (Department: 2812)
Coherent light generators
Particular active media
Semiconductor
C372S036000, C372S050121, C372S096000
Reexamination Certificate
active
06639927
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a surface emitting semiconductor laser with a current confinement structure for confining a current into a restricted active region, which can be suitably used in optical information communications, optical information processing apparatuses, and recording apparatuses such as laser beam printers. This invention also relates to its fabrication method.
2. Related Background Art
A surface emitting semiconductor laser has been marked with keen interest as a light source in optical information communications and processing, and its development has been energetically advanced. The surface emitting semiconductor laser can be characterized by low electric power consumption, low threshold current, two-dimensional dense integration capability, and dynamic single mode operation.
A conventional surface emitting semiconductor laser is illustrated in FIG.
1
. The laser is fabricated as follows. A p-type GaAs/AlGaAs multi-layer mirror
11
is etched into a mesa shape, and sides of the mesa mirror
11
are filled with insulating material such as polyimide
4
. An anode electrode
21
is then formed as illustrated in FIG.
1
. In
FIG. 1
, there are further formed an n-type GaAs/AlGaAs multi-layer mirror
12
, an InGaAs/GaAs active region
13
, a layer
14
which is selectively oxidized except for its central portion which is not oxidized, and a cathode electrode
22
formed on the bottom face of an n-type GaAs substrate
1
.
In the device of the above configuration, however, its thermal resistance increases since the sides of the mesa are surrounded by the insulating material
4
having high thermal resistance. Further, the area of the electrode
21
is very small due to a very small device size, and its contact resistance is large. Thus, thermal characteristics of this device are not preferable, and its oscillation wavelength is therefore likely to shift when the device is driven. In addition, its serial electrical resistance is large, and its consumption electric power hence increases.
The following surface emitting semiconductor laser is proposed in Japanese Patent Application Laid-Open No. 7(1995)-38196 to solve the above problem. In this laser, sides of an active layer and other layers exposed by etching are covered with a material having high electrical resistance and high thermal conductivity, and those sides are surrounded by a metal to reduce its thermal resistance. Thus, an additional process is needed in this fabrication process to form a region with high electric resistance and large thermal conductivity on the side wall.
As a technique for improving the performance of a surface emitting semiconductor laser, there also exists the technique in which an AlAs layer inserted in a semiconductor multi-layer mirror is partially oxidized to form the current confinement structure. In this technique, sides of the semiconductor multi-layer mirror are exposed by dry etching, and the exposed sides are then heat-treated in water vapor. The AlAs layer is thus oxidized to a desired width in an in-plane direction (a direction perpendicular to a layering direction) with its central portion being left as is, and an insulation layer is partially formed in the AlAs layer to fabricate the current confinement structure.
Further, there exists the technique for utilizing an AlAs layer as an etch stop layer, which is disclosed in Japanese Patent Application Laid-Open No. 10(1998)-294528. In this technique for fabricating a surface emitting semiconductor laser, dry etching is conducted using an etching gas whose etching rate is low for AlAs. When the AlAs layer is used as an etch stop layer, over-etching of the AlAs layer can be prevented, and exposure of an active layer can be hence prevented. The exposure of the active layer is likely to lead to a decrease in radiation efficiency due to damage to the active layer. Accordingly, this technique is very effective and valuable.
In this technique wherein the AlAs layer is used as an etch stop layer, a mixed gas with a high etching rate for a GaAs layer and a low etching rate for an AlAs layer is used, but a ratio of etching rates between GaAs and AlAs can be sufficiently controlled by changing conditions, such as a vacuum degree, even with a conventional etching which uses a fluorine gas. Further, there is no disclosure that an oxidized AlAs layer is used as an insulating layer between p-type and n-type layers, in Japanese Patent Application Laid-Open No. 10(1998)-294528.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a surface emitting semiconductor laser whose thermal characteristic can be readily improved, whose electrical resistance can be readily reduced, whose electric power consumption for driving can be readily lowered, whose threshold current can be readily reduced, which can be fabricated by a relatively simple process, which can be readily arrayed, and which can be readily provided with a current confinement structure; and its fabrication method. This object is achieved by effectively utilizing a selective oxidization layer. (As used herein, selective oxidization layer refers to either a selective oxidizable layer that is not yet oxidized or a selective oxidized layer that is already oxidized, or both, according to contex.)
It is another object of the present invention to provide a surface emitting semiconductor laser with a built-in current confinement structure, whose fabrication yield can be readily improved by effectively utilizing a selective oxidization layer (e.g., Al
x
Ga
1−x
As layer (typically 0.8≦x≦1)), that is already oxidized, as an etch stop layer; and its fabrication method.
The present invention is generally directed to a surface emitting semiconductor laser which includes an active region formed on a growth substrate; upper and lower mirror layers that sandwich the active region to construct a vertical cavity; a selective oxidization layer that is selectively oxidized and insulated and that is provided on the side of the active region opposite to the side of the substrate; and a current injecting unit for injecting a current into the active region. In this structure, a post portion is formed by removing semiconductor material formed on the substrate down to an uppermost or halfway level of the selective oxidization layer while the selective oxidization layer is used as an etch stop layer, and the selective oxidization layer is formed to act as both a current confinement layer for current injection and an insulating layer for the current injecting unit.
In this structure, p-type semiconductor is insulated from n-type semiconductor by the selective oxidization layer, except for a portion of a current, or devices are insulated from each other by the selective oxidization layer when a plurality of surface emitting semiconductor lasers are arrayed. Accordingly, there is no need to separately form an insulating layer, and the fabrication process can be hence simplified. Further, the current confinement structure can be formed by the selective oxidization layer, and a contact area between the semiconductor layer and an electrode metal of the current injecting unit can be enlarged. Therefore, thermal characteristic can be improved, series electrical resistance can be reduced, electric power consumption for driving can be lowered, and threshold current can be reduced.
The selective oxidization layer is typically composed of semiconductor including aluminum which is selectively or partially oxidized. The Al mole fraction of the selective oxidization layer can be changed with a desired distribution along a direction perpendicular to the substrate. The oxidization rate increases as the Al mole fraction increases. Thereby, a slope of the refractive index can be created in the selective oxidization layer, and the selective oxidization layer can act as a kind of lens. The light density in the active region is thus increased, and the threshold current is further reduced.
When the post portion is formed by removing the semicondu
Sakata Hajime
Sato Takahiro
Canon Kabushiki Kaisha
Davie James
Fitzpatrick ,Cella, Harper & Scinto
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