Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1998-08-20
2001-07-10
Leung, Quyen P. (Department: 2881)
Coherent light generators
Particular active media
Semiconductor
C372S096000
Reexamination Certificate
active
06259715
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an improved surface emitting semiconductor laser array, and more particularly relates to a matrix driving type surface emitting semiconductor laser array improved in power consumption by reducing the resistance of the current passage.
2. Description of Related Art
For using in the fields of optical exchange and optical information processing, surface emitting laser arrays having a two-dimensional matrix array of surface emitting lasers (VCSEL namely Vertical Cavity Surface Emitting Laser) are required, particularly large scale surface emitting semiconductor laser arrays having a large number of surface emitting semiconductor lasers are required.
To use a large scale surface emitting semiconductor array, it is required to drive respective component surface emitting lasers independently, the respective elements should be connected independently in the connection arrangement, therefore, in the case of an independent driving type surface emitting semiconductor laser array of M rows and N columns, M×N connection wiring is required.
A matrix type driving system has been developed in which a plurality of row direction lines for connecting the respective surface emitting lasers in the row direction in parallel are provided on the front surface of the surface emitting semiconductor laser array, and on the other hand a plurality of column direction lines for connecting the respective surface emitting lasers in the column direction are provided on the back surface of the surface emitting semiconductor laser array, then one of the row direction lines and one of the column direction lines are selected to select and light the surface emitting laser located at the intersection position.
In this matrix driving system, a surface emitting semiconductor laser array of M rows and N columns may have M+N electrodes.
One column of the matrix driving type surface emitting semiconductor laser array is described with reference to
FIGS. 15
to
17
.
FIG. 15
is a perspective view,
FIG. 16
is a plain view, and
FIG. 17
is a side view viewing from the right in
FIG. 15
of a matrix driving type surface emitting semiconductor laser array.
In manufacturing the surface emitting semiconductor laser array, by use of molecular beam epitaxy, a bottom contact layer
52
comprising an n-type GaAs layer is formed on a semi-insulating GaAs substrate
51
, and an n-side multilayer reflection film
53
with the total thickness of several &mgr;m having alternately laminated AlAs layers and GaAs layers being ¼ of in-medium wavelength in respective film thickness is formed on the bottom contact layer
52
. Next, an undoped active layer
54
having the same film thickness as an in-medium wavelength and having a laminate structure comprising two Al
0.4
Ga
0.6
As layers with interposition of a laminate quantum well layer comprising three layers of two GaAs layers with a thickness of 10 nm and one In
0.2
Ga
0.8
As layer inserted between the GaAs layers is formed, and on the undoped active layer
54
, a p-side multilayer reflection film
55
with the total thickness of several &mgr;m having alternately laminated AlAs layers and GaAs layers being ¼ of in-medium wavelength in respective film thickness is formed on the undoped active layer
54
. In these processes, Si is used as an n-type dopant and Be is used as a p-type dopant respectively. Next, by applying reactive ion etching, grooves
56
for wiring separation are formed to provide M rows in vertical direction (top-bottom direction in FIG.
16
). The depth of the grooves
56
extends through the bottom contact layer
52
to the semi-insulating GaAs substrate
51
for electrical separation of rows. The grooves
56
are filled with polyimide. Next, by applying vacuum evaporation, an Au layer is formed on the p-side multilayer reflection film
55
, and the Au layer is removed from the boundary areas of the respective columns to form N p-side electrode wiring
57
extending in the column direction (right-left direction in FIG.
16
). Proton is injected on areas between p-side electrode wires
57
, which define the column direction lines, so as to penetrate from the p-side multilayer reflection film
55
at least to the active area
54
, and the groove area is insulated to separate the rows. Only on the ends (near side end in the drawing), etching is continued until the bottom contact layer
52
comprising an n
+
GaAs layer is exposed. On the exposed surface, electrode pads (n-side electrode pads)
58
for respective rows are formed. On the ends of p-side electrodes
57
which define respective column direction lines, electrode pads (p-side electrode pads)
59
for respective columns are formed.
To select an arbitrary point (i, j), row i and column j may be selected. In a surface emitting semiconductor laser array having the above-mentioned structure, because the p-side electrode wires are of gold, the resistance of the p-side electrode wires
57
is as low as several &OHgr; and does not cause any problem, however, because the bottom contact layer
52
is served as the contact line to the n-side electrodes, the line resistance of a surface emitting semiconductor laser located far from an n-type electrode pad
58
is significantly large. Therefore, the power consumption of the matrix driving type surface emitting semiconductor laser array having the above-mentioned structure is significantly large.
The above-mentioned matter is described further in detail with reference to
FIG. 15
again and FIG.
18
.
In the surface emitting semiconductor laser array shown in
FIG. 15
, the bottom contact layer
52
consisting of n
+
GaAs layer is used as the bottom side wiring, the contact resistance between an electrode and semiconductor is at largest several &OHgr; or lower. Herein the contact resistance is confirmed analytically using Transmission Line Model method. In the example shown in
FIG. 15
, the contact between an electrode and a semiconductor is an electrode pad
58
.
FIG. 18
shows a cross sectional view of
FIG. 15
, and the effective contact distance between the electrode pad
58
and the bottom contact layer
52
comprising an n
+
GaAs layer is operated arithmetically. The contact resistance is determined from the product of contact specific resistance and contact area where a current flows through. In this case the electrode material is metal, and the resistance and contact specific resistance are small. However, because the resistance of the n
+
GaAs layer is comparatively large, the electrical effective contact area with the bottom contact layer comprising an n
+
GaAs layer extends to the position shown with Lt in FIG.
18
. Assuming that the contact specific resistance &rgr;c is 10
−6
cm
2
, the specific resistance rs of the n
+
GaAs layer is 3×10
−4
cm &OHgr; (determined under the assumption that the carrier concentration n is 10
19
cm
−3
and carrier mobility &mgr; is 5000 cm
2
/V sec using 1/e n&mgr;. Wherein e is an elementary charge of 6×10
−19
coulomb), and the length of an electrode pad d (horizontal direction of the paper plane in
FIG. 18
) is 1 mm, Lt is calculated from the equation &rgr;c=(Lt×(&rgr;crs))÷coth (d/Lt) and then 10 &mgr;m is obtained as the result. Therefore, in the case of the electrode pad with a width of 10 &mgr;m, the effective contact area is 10 &mgr;m×10 &mgr;m. The contact specific resistance of 10
−6
cm
2
gives the contact resistance between an electrode and semiconductor of 1 &OHgr;, this value is sufficiently smaller than the resistance of the surface emitting laser body.
However, in the surface emitting semiconductor laser array shown in
FIG. 15
, because the n
+
GaAs layer is used for the bottom side wiring, the wiring resistance calculated based on the specific resistance of n
+
GaAs×wiring length×cross section of the wiring (assuming wiring length of 1 mm, width of 20 &mgr;m, and thickn
Fuji 'Xerox Co., Ltd.
Leung Quyen P.
Oliff & Berridg,e PLC
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