Surface emitting semiconductor laser diode and fabricating method of the same

A surface emitting semiconductor laser diode includes a ace emitting laser oscillating portion having a multi-quantum well area and being disposed between a pair of upper and lower reflector stacks for stable light-emitting, a photodiode portion for monitoring a laser beam emitted form the surface emitting laser oscillating portion and electrodes for a laser diode, a photodiode and ground for driving the surface emitting laser oscillating portion and photodiode portion. At least a part of the electrode for the photodiode is directly contacted on the surface of a semiconductor layer of the uppermost part of a deposited structure including the surface emitting laser oscillating portion. Thereby, a fabricating process of the device can be simplified, and the efficiency in monitoring can be maximized, since the photodiode is constituted being adjacent to window area through which a laser beam is directly emitted outward.

BACKGROUND OF THE INVENTION 
The present invention relates to a surface emitting semiconductor laser 
diode where an LD (laser diode) and a PD (photodiode) are integrated on a 
single chip, and more particularly to a surface emitting semiconductor 
laser diode and a fabricating method thereof, which can solve an optical 
output cutting off problem around a window by constructing the PD as a 
Schottky diode structure utilizing a rectifying contact between metal and 
a semiconductor. 
A semiconductor laser diode is widely used not only in an optical 
information processor such as a CDP (compact disc player), a LDP (laser 
disc player) or a VDR (video disc recorder) but also as an optical 
communication device. Wide such a wide use, the semiconductor laser diode 
is assembled into a single module with an additional PD for monitoring 
power of the LD in order to stabilize power in operation. In the above 
method for monitoring the output of the LD using the additional PD, 
assembling the LD and PD is cumbersome and an additional cost for the PD 
is incurred. To correct the above problems, a study of a monolithic 
integration of the LD and PD in fabrication of a surface emitting 
semiconductor laser has been vigorously undertaken. 
FIG. 1 is a cross section showing a conventional surface emitting 
semiconductor laser diode. Referring to the drawing, an n-electrode 11 for 
an LD, which is made of ohmic metal formed by depositing AuGe and Au, is 
disposed in the lowermost portion of the device. An n-GaAs substrate 12 is 
deposited on the n-electrode. Successively deposited on n-GaAs substrate 
12, are: a Bragg reflector stack 13 where n-AlGaAs and n-AlAs are 
deposited, a GaAs (or AlGaAs) active region 14 forming a multi-quantum 
well for oscillating a laser beam, a Bragg reflector stack 15 where 
P-AlGaAs and P-AlAs are deposited, and a p-GaAs contact layer 16. A ridge 
20 is formed on the center of p-GaAs contact layer 16 by depositing a GaAs 
intrinsic semiconductor layer 18 and an n-GaAs layer 19. Beside ridge 20, 
on either portion, of an upper surface of p-GaAs contact layer 16, a 
p-electrode 17 of ohmic metal is formed by depositing Cr (or AuZn) and Au. 
An n-electrode 21 for a PD of ohmic metal is formed on the surface of 
n-GaAs layer 19 by depositing AuGe and Au. Reference numerals 22, 23 and 
24 indicate a power port for the PD, a power port for the LD and a current 
cut-off area, respectively. 
In the conventional surface emitting semiconductor laser diode having such 
a structure, when forward bias is applied by n-electrode 11 for the LD and 
p-electrode 17 of ground electric potential, light is emitted from GaAs 
(or AlGaAs) active region 14. The light is emitted outward after passing 
through p-AlGaAs/p-AlAs Bragg reflector stack 15, p-GaAs contact layer 16, 
GaAs intrinsic semiconductor layer 18 and n-GaAs layer 19. In the above 
process, the uppermost part of pAlGaAs/p-AlAs Bragg reflector stack 15, 
GaAs intrinsic semiconductor layer 18 and n-GaAs layer 19 acts as a 
photodiode formed on the path of the emitted light. When bias is applied 
by n-electrode 21 for the PD and p-electrode 17, the above photodiode can 
be used to either modulate laser beam or monitor laser power. 
FIG. 2 shows a schematic equivalent circuit diagram of the conventional 
surface emitting semiconductor laser diode having the LD and PD of FIG. 1. 
Referring to the drawing, when photodiode current I.sub.p is applied to a 
feedback circuit for controlling laser diode current I.sub.L, the 
integrated structure of FIG. 1 behaves as a self-monitoring laser. On the 
other hand, if modulated bias voltage is applied between photodiode 
voltage V.sub.PD and the ground, variation in the bias will change 
absorption coefficient and refractive index of the photodiode, thereby 
modulating the amplitude and/or phase of the laser output. In this mode, 
the integrated structure acts as an integrated laser and a modulator. 
Such a conventional surface emitting semiconductor laser diode has a merit 
of monitoring directly the laser beam emitted upward by a P-I-N type 
photodiode formed through additional crystal growth above an emitting 
region. However, since the light being emitted outward is considerably 
absorbed while passing the P-I-N type photodiode, the quantum efficiency 
of the device is sharply reduced. The additional crystal growth is also 
cumbersome in fabrication process of the device. 
SUMMARY OF THE INVENTION 
To solve the above problems, it is an object of the present invention to 
provide a surface emitting semiconductor laser diode and a fabricating 
method thereof, by which a fabricating process can be simplified and a 
drop in the quantum efficiency of the device can be prevented. 
Accordingly, to achieve the above object, there is provided a surface 
emitting semiconductor laser diode, comprising a surface emitting laser 
oscillating portion having a multi-quantum well area and being disposed 
between a pair of upper and lower reflector stacks for stabilizing 
light-emission, a photodiode portion for monitoring a laser beam emitted 
from the surface emitting laser oscillating portion, and electrodes for a 
laser diode, a photodiode and ground for driving the surface emitting 
laser oscillating portion and photodiode portion, wherein at least a part 
of the electrode for the photodiode is directly contacted on the surface 
of a semiconductor layer of the uppermost part of a deposited structure 
including the surface emitting laser oscillating portion. 
Also, to achieve the above object, there is provided a method of 
fabricating a surface emitting semiconductor laser diode comprising the 
steps of: depositing in order and growing on a substrate an n-reflector 
stack, a multi-quantum well active region, a p-reflector stack and a 
p-contact layer; forming a current cut off area in a predetermined section 
of a structure formed by the depositing growth; forming a p-ohmic metal 
layer of a predetermined pattern for constituting an electrode for ground, 
on the upper surface of the p-contact layer of the structure having the 
current cut off area; forming an insulation layer on the upper surface and 
one side of said p-ohmic metal layer; and forming rectifying contact metal 
layers each for constituting an electrode for a laser diode and an 
electrode for a photodiode on the upper surface of said insulation layer 
and a part of the upper surface of the uppermost part of the deposited 
structure and beneath the substrate.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 3, in order to fabricate a surface emitting semiconductor 
laser diode, according to a first embodiment of the present invention, an 
n-GaAs substrate 31 is primarily provided. The following are successively 
deposited and grown on substrate 31: a Bragg reflector stack 32 where 
n-AlGaAs and n-AlAs are deposited and for reflecting a laser beam in a 
predetermined direction, an active region 33 constituting a multi-quantum 
well as an area of oscillating the laser beam, a Bragg reflector stack 34 
where p-AlGaAs and p-AlAs are deposited and for reflecting the laser beam 
in the predetermined direction with n-AlGaAs/n-AlAs Bragg reflector stack 
34; and a p-GaAs contact layer 35 for facilitating the combination of 
metal and a semiconductor. Here, MBE (molecular beam epitaxy), LPE (liquid 
phase epitaxy) or MOCVD (metal organic chemical vapor deposition) methods 
are adopted in such a growth. 
When a first growth is completed as described above, a current cut-off area 
36 is formed through the ion implantation process in a predetermined 
section of a structure formed by the above deposition growth, as shown in 
FIG. 4. In such an ion implantation process, protons are accelerated by an 
ion implantation apparatus to destruct semiconductor crystal in the 
predetermined section of the structure, so that the section has a high 
resistance value. Current cut-off area 36 is for concentrating the current 
flowing dispersively through the device into its center portion, thereby 
reducing the consumption of electric power supplied for the oscillation of 
a laser. 
On the other hand, when the formation of current cut-off area 36 is 
completed, a p-ohmic metal layer 37 of a predetermined pattern for 
constituting an electrode for ground is formed on p-GaAs contact layer 35. 
Here, p-ohmic metal layer 37 is formed by fixing a mask of a predetermined 
pattern (not shown) such as an SiO.sub.2 mask on contact layer 35 and then 
depositing Cr (or AuZn) and Au. 
After forming p-ohmic metal layer 37, an SiO.sub.2 or Al.sub.2 O.sub.3 
insulation layer 38 for insulation between metal layers for electrodes is 
formed on the upper surface and one side of p-ohmic metal layer 37, as 
shown in FIG. 5. 
When the formation is completed up to insulation layer 38, a rectifying 
contact metal layer 39 for constituting an electrode for a photodiode is 
formed on insulation layer 38 and on a part of the uppermost of the 
resultant structure, as shown in FIG. 6. An n-ohmic metal layer 40 for 
constituting an electrode for a laser diode is formed beneath n-GaAs 
substrate 31, to complete the device. Here, Rectifying contact metal layer 
39 and 40 are formed by depositing AuGe and Au. Reference numerals 41 and 
42 indicate a depletion area when the device is driven and a window 
through which the laser beam is directly emitted, respectively. Reference 
numerals 43 and 44 indicate ports for power supply and a reference numeral 
45 indicates a port for ground. 
FIGS. 7 and 8 show sectional views for illustrating the fabricating process 
of the surface emitting laser diode according to a second embodiment of 
the present invention. 
In the description of the fabricating process of the second embodiment, the 
same portion as that of the first embodiment will be omitted. 
Referring to FIG. 7, after the formation of a current cut-off area 76 and a 
p-ohmic metal layer 77 for constituting an electrode for ground as in the 
first embodiment described in FIG. 4, an SiO.sub.2 or A1.sub.2 O.sub.3 
insulation layer 78 for insulation between metal layers for electrodes is 
formed on the upper surface and one side of p-ohmic metal layer 77. 
After the formation is completed up to insulation layer 78, a transparent 
metal layer 79 of CTO (cadmium tin oxide) or ITO (indium tin oxide) for 
constituting an electrode for a photodiode is formed using 
photolithography both on the surface of insulation layer 78 and on the 
surface of uppermost part of the deposited structure, as shown in FIG. 8. 
Beneath n-GaAs substrate 71, an n-ohmic metal layer 80 is formed by 
depositing AuGe and Au in order to constitute an electrode for a laser 
diode. Reference numerals 81 and 82 indicate a depletion area in driving 
the device and a window through which a laser beam is directly emitted, 
respectively. Reference numerals 83 and 84 indicate ports for power supply 
and 85 indicates a port for ground. 
Referring to FIGS. 6 and 8, the operation of the surface emitting 
semiconductor laser diode having the above structure according to the 
present invention, will be schematically described. 
In FIGS. 6 and 8, Cr/Au (or AuZn/Au) of electrodes 37 and 77 for ground and 
GaAs of p-contacts 35 and 75 make resistive contacts, respectively. 
Between AuGe/Au (CTO or ITO) of electrodes 39 and 79 for photodiodes and 
GaAs of p-contacts 35 and 75, Schottky barriers each are formed to make 
rectifying contacts. AuGe/Au of electrodes 40 and 80 for laser diodes and 
GaAs of n-substrates 31 and 71 make resistive contacts, respectively. 
Thus, when a forward bias is applied between electrode ports 44 and 84 for 
the laser diodes and electrode ports 43 and 83 for photodiodes, current is 
stably provided to the laser diode structure so that laser actively 
oscillates from active regions 33 and 73. 
On the other hand, when current is applied between electrode ports 44 and 
84 for the laser diodes and electrode ports 45 and 85 for ground, a laser 
beam over a threshold current is emitted outward. At this time, since 
depletion areas 41 and 81, the size of which varies according to the size 
of a reverse voltage value, are formed between electrode ports 45 and 85 
for ground and electrode ports 43 and 83 for photodiodes, a part of the 
laser beam absorbed in the areas increases a reverse saturation current 
between electrode ports 45 and 85 for ground and electrode ports 43 and 83 
for photodiodes. Since the size of the saturation current is in direct 
proportion to an output of the laser beam emitted outward, the output of 
the laser diode can be effectively controlled by varying the current 
between electrode ports 45 and 85 for ground and electrode ports 43 and 83 
for photodiodes. 
As described above, the surface emitting semiconductor laser diode and a 
fabricating method thereof according to the present invention can simplify 
the fabrication process of the device by forming a photodiode, without 
additional crystal growing, directly on the surface of the deposited 
structure using a metal-Schottky barrier caused by the combination of a 
semiconductor and metal. Also, since the photodiode is constituted 
adjacent to window areas 42 and 82, through which a laser beam is directly 
emitted outward, the efficiency in monitoring can be maximized.