Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2000-11-24
2003-12-09
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S043010, C372S045013, C372S046012, C372S050121
Reexamination Certificate
active
06661823
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface light emitting semiconductor laser device employed for optical interconnection, optical exchange, or optical information processing, and to a fabricating method thereof.
2. Description of the Related Art
In recent years, research has advanced on optical interconnections, which aim for remarkable improvements in transmission speed, as information transmitting means of logical circuit elements. As a parallel light source thereof, attention has been paid to a surface light emitting laser (vertical cavity surface emitting laser diode, hereinafter, referred to as “VCSEL”) in which light emitting elements can be arranged in a two-dimensional manner at a high density. Iga, et al. have conducted advanced research on VCSELs, as described in IEEE Journal of Quantum Electronics, Volume 27, page 1332 issued in 1988.
The latest VCSEL has the structure shown in
FIGS. 7A and 7B
. That is, a resonator
302
is provided in a vertical direction with respect to a horizontal face of a semiconductor substrate
301
. The resonator
302
is formed by: an active layer
303
that confines a carrier to generate a light; a lower reflection mirror
304
formed by a semiconductor multi-layered film; an upper reflection mirror
305
formed by a semiconductor multi-layered film; and a spacer layer
306
for aligning a phase of the light emitted in the active layer with ends of both of the upper and lower semiconductor multi-layered reflection mirrors. As a constituent element other than the resonator, there are provided an upper contact layer
307
, an upper electrode
308
that also functions as a laser emitting port, an inter-layer insulating film
310
, and a lower electrode
309
.
In order to oscillate a laser, it is required to confine a carrier and light in the horizontal direction of the carrier
301
(that is, in a direction parallel to a plane including the substrate). As a means for fabricating a substrate
301
and a narrow structure in the horizontal direction, there are provided a method of fabricating a thin columnar (post) structure of several tens of microns by dry etching to form the post section itself as a current path (single post type); a method of fabricating such a post structure, and then making a portion of an AlAs oxide layer insulated by steam oxidization, thereby limiting a current path (AlAs oxidation type); and a method of forming an insulation region by a proton impeller, thereby limiting a current path (proton impeller type).
At present, it is known that a VCSEL having a low threshold current value and excellent current-light characteristics can be fabricated by AlAs oxidization (Journal of Applied Electronics Physical Properties Study Group, Volume 5, Issue 1, 1999, page 11). Reference numeral
312
in
FIG. 8
denotes an AlAs layer;
312
A denotes an oxide region; and
312
B denotes a non-oxide region. An opening
313
is provided at the upper electrode
308
to form a laser emitting port.
There are the following problems with the foregoing conventional VCSEL.
That is, first, the contact resistance of the upper electrode
308
is large. Second, the light output of the device is gradually lowered as power is supplied, and reliability is very low. It was found that these two problems are caused by a GaAs contact layer being thin (several nm to several tens of nm).
In an end face light emitting laser or in a VCSEL having a wavelength of 850 nm or longer, the thickness of the GaAs contact layer, which is the top surface layer of a substrate contacted by the upper electrode, is usually 50 nm to several hundreds nm. In a VCSEL having a short wavelength bandwidth such as a VCSEL with a 780 nm bandwidth, if the GaAs contact layer is thick, the light absorption in that layer increases, and the threshold current value is significantly increased. Therefore, in order to restrict light absorption and obtain practical characteristics, the thickness of the GaAs contact layer is reduced to about several nm or several tens of nm.
On the other hand, during the processes of fabricating a VCSEL, the GaAs contact layer that is the top surface layer is exposed to a variety of process environments, and is easily damaged. For example, in VCSEL fabrication, several steps of film-depositing, such as film-depositing an SiO film by a CVD method onto the GaAs contact layer surface, are carried out. At this time, separation of As from the GaAs top layer is promoted by exposure to the CVD plasma gas or by heating. The GaAs located at a depth of several tens of nm from the top layer is transformed into a layer having poor stoichiometry, or crystalline defects arise. In the subsequent contact hole forming step, this GaAs transformed layer is exposed to a buffered fluoric acid. In general, although GaAs is not easily etched by buffered fluoric acid, this GaAs transformed layer is etched. Here, in the case where the GaAs contact layer is as thick as one hundred nm, several tens of nm of the damaged layer is merely eliminated. In the case where a contact layer in an VCSEL of a 780 nm bandwidth is originally as thin as several tens of nm, all of the GaAs contact layer is etched and disappears, and the lower AlGaAs layer is exposed.
When an upper electrode is formed on the surface of the AlGaAs layer that appears when the GaAs contact layer disappears, the contact resistance increases, and the voltage (drive voltage) required for driving the device increases. In addition, although an opening of the upper electrode serves as a light emitting port, AlGaAs is produced on the surface of the emitting port. Thus, this AlGaAs reacts with the oxygen or moisture in air, and AlGaAs is oxidized and blackened (this is referred to as a “AlGaAs blackening phenomenon”). In particular, this blackening phenomenon rapidly progresses by supplying power. As a result, the laser light to be emitted from the emitting port is absorbed by the AlGaAs oxide layer whose top surface has blackened, and making the light output is lowered.
In general, making the GaAs contact layer thicker prevents the GaAs contact layer from disappearing, but on the other hand, light absorption increases. In addition, after the foregoing processes have been completed, the GaAs is eliminated, the GaAs film thickness deviate from the design value therefor, and the reflectance of the reflection mirror is reduced. As a result, the threshold current value of the device is increased.
Along with the above-described two problems, according to the foregoing conventional VCSEL, there is a third problem that the GaAs contact layer is etched by a resist developing liquid employed in the photolithography step when a contact electrode is formed.
That is, as described above, in a VCSEL with a short wavelength bandwidth such as a VCSEL with a 780 nm bandwidth, in the case where the GaAs contact layer is thickened, light absorption in the layer is increased and the threshold current value of the device is significantly increased. Therefore, the GaAs contact layer is reduced to some nm or several tens of nm.
However, in a VCSEL contact layer employing Au for an electrode metal, in general, the layer is often formed by a method in which a negative pattern resist is formed, the electrode metal is film deposited, and then the layer and the negative pattern resist are lifted off. In this case, as shown in
FIGS. 18A
to
18
C, a GaAs contact layer
401
comes into direct contact with the resist developing liquid, and thus, is etched by the developing liquid (reference numeral
404
in the figure). As in the previously described case, in particular, in the case where the GaAs contact layer is several tens of nm, which is very thin, all of the GaAs contact layer
401
may be etched and disappear depending on the developing conditions, such that a lower layer AlGaAs
402
is exposed. Even if the AlGaAs is not fully exposed, the thickness of the GaAs contact layer differs per processed batch, and dispersion in contact resistance value occurs between VCSELs processed in different batches.
Ishii Ryoji
Miyamoto Yasuaki
Otoma Hiromi
Sakurai Jun
Fuji 'Xerox Co., Ltd.
Ip Paul
Oliff & Berridg,e PLC
Rodriguez Armando
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