Vertical cavity surface emmitting lasers with transparent electrodes

Optically transparent and electrically conductive cadmium tin oxide or indium tin oxide is employed in vertical cavity surface emitting lasers for vertical current injection. Continuous wave lasing at room temperature is achieved in GaAs/AlGaAs quantum well lasers. Devices with a 10 .mu.m optical window which also serves as a vertical current injection inlet give lasing threshold currents as low as 3.8 mA. The differential series resistance is (350-450) .OMEGA. with a diode voltage of (5.1-5.6) V at the lasing threshold. Far field pattern of the laser emission is Gaussian-like with a full width at half maximum of 7.degree..

FIELD OF THE INVENTION 
This invention concerns Vertical Cavity Surface Emitting lasers having 
optically transparent electrodes. 
BACKGROUND OF THE INVENTION 
The Vertical Cavity Surface Emitting Laser diode, hereinafter referred to 
as a VCSEL, is attractive as a device which may be produced by planar 
technology and as a class of devices with a wide range of potential uses 
including optical communications, optical discs, laser printers and light 
sensing systems. In the VCSEL, the lasing cavity is perpendicular to the 
optical surface of a laser chip. Therefore, high packing density, compared 
to the packing density of edge-emitting lasers with the lasing cavity 
parallel to the surface of the laser chip, is obtainable. This leads to a 
promising future in high density laser arrays, high data transmission in 
optical communication systems, high parallel processing in optical 
communication systems, as well as supplying a route for fast and high 
capacity data transmission between electronic chips. Furthermore, the 
radial symmetry of their beams makes them suited for beam-combining with 
cylindrical fibers. 
In the VCSEL the light output is in the film growth direction which is 
usually parallel to the direction of the injection current. Due to this 
feature, the mirror through which laser emission takes place and the 
electrical contact physically occupy the same side of the laser structure, 
i.e. either on the top or on the bottom of the device. Typically, the 
mirror is located approximately in the center of the surface while the 
electrode is located peripherally of the mirror. For example, see Kenichi 
Iga, "Recent Advances of Surface Emitting Semiconductor Lasers," 
Optoelectronics-Devices and Technologies, Vol. 3, No. 2, December 1988, 
pp. 131-142, and L. M. Zinkiewicz et al., "High Power Vertical-Cavity 
Surface-Emitting AlGaAs/GaAs Diode Lasers," Appl. Phys. Letters, Vol. 54, 
No. 20, May 15, 1989, pp. 1959-1961. 
An attempt to simplify the construction of a VCSEL by combining the mirror 
and the electrode into a single unit led to relatively low quantum 
efficiencies. See Deppe D. G. et al., "AlGaAs-GaAs and AlGaAs-GaAs-InGaAs 
vertical cavity surface emitting lasers with Ag mirrors," Journal of 
Applied Physics, Vol. 66, No. 11, Dec. 11, 1989, pp. 5629-5631. The 
mirrors comprised a 0.55 .mu.m thick reflective Ag mirror which also acted 
as the electrode of the laser. The emission took place through the 
.lambda./4 reflector semiconductor stack arranged opposite to the 
mirror/electrode. U.S. Pat. No. 4,949,351 issued Aug. 14, 1990 to Koich 
Imanaka discloses a Ti, Pt and Au layered structure with a total thickness 
of 900 .ANG., which is used as an electrode-mirror. An article by E. F. 
Schubert et al., "Low-threshold vertical cavity surface-emitting lasers 
with metallic reflectors", Applied Physics Letters, 57 (2), Jul. 9, 1990, 
p. 117-119, L. W. Tu et al., "Vertical-Cavity Surface Emitting Lasers With 
Semi-Transparent Metallic Mirrors And High Quantum Efficiencies", Applied 
Physics Letters, 57(20), Nov. 12, 1990, pp. 2045-2047, and U.S. 
application Ser. No. 07/526,204, filed May 21, 1990 (Deppe D. G. disclose 
a VCSEL with a metallic mirror which simultaneously acts as an electrode 
of the device with a thickness sufficient to permit lasing emission 
through the mirror-electrode. However, while the quantum efficiency of the 
latter is improved over the structure of Deppe D. G. or Imanaka, there is 
still a substantial loss in transmission of the lasing emission through 
the metallic mirror-electrode. 
Therefore, there is still a need for a VCSEL with an improved quantum 
efficiency and light transmission and which could be also produced in a 
simplified manner utilizing planar technology. 
SUMMARY OF THE INVENTION 
This invention is a semiconductor vertical cavity surface emitting laser 
comprising a lasing cavity with an active layer, a top and bottom 
multilayer DBR mirror and a top and bottom electrode for applying 
excitation current in the direction parallel to the direction of optical 
propagation. The VCSEL is a semiconductor device wherein the semiconductor 
material is a III-V or II-VI compound semiconductor such as GaAs, GaInAs, 
InP, InGaPAs, AlGaInAs, AlGaAs and other related semiconductors. In 
accordance with this invention, the top electrode comprises an optically 
transparent material selected from conductive semiconductors having 
conductivities within a range of from 1.times.10.sup.3 to 1.times.10.sup.5 
.OMEGA..sup.-1 cm.sup.-1, light transmissivity of at least 80 percent and 
absorption of less than 10 percent. Cadmium tin oxide and indium tin 
oxide, deposited in thicknesses ranging from 50 to 500 nm, satisfy these 
requirements. The electrode layer is upon a very thin metal barrier layer 
which forms a non-alloyed barrier between the p-type top mirror and the 
n-type electrode layer. For a VCSEL with a GaAs active layer, the light 
output from the top electrode side yields an external differential quantum 
efficiency as high as 54 percent. This VCSEL is suitable for fabrication 
utilizing planar technology. 
The vertical-injection VCSEL structure using optically transparent and 
electrically conductive cadmium tin oxide or indium tin oxide provides a 
solution to the fundamental difficulty in prior art VCSELs, i.e. here the 
light and current occupy the same path. Room temperature continuous wave 
operation with low threshold current is achieved. The Gaussian-like far 
field pattern indicates a single fundamental transverse mode.

DETAILED DESCRIPTION 
The invention is a VCSEL in which the lasing cavity comprises an active 
layer, a top and bottom mirror each consisting of a stack of a plurality 
of pairs (or periods) of semiconductor layers forming quarter-wave 
multilayer distributed Bragg reflector (DBR) structure, and a top and 
bottom electrode, respectively. The top electrode is of semiconductor 
material which is optically transparent to lasing emission from the active 
layer and permits lasing emission to take place through the top mirror. 
The light output from the mirror and through the optically transparent top 
electrode yields a high differential quantum efficiency which is as high 
as 54 percent. This device may be conveniently produced by planar 
technology. 
FIG. 1 is a general schematic representation of a VCSEL according to this 
invention, denominated generally as 10. For reasons of clarity the 
elements of the VCSEL are not drawn to scale. VCSEL 10 comprises, in an 
ascending order, a bottom electrode, 11; a substrate, 12; a quarter-wave 
stack of a plurality of pairs (or periods) of semiconductor layers which 
forms a bottom mirror, 13; a first confining layer, 14; an active region, 
15; a second confining layer, 16; another quarter-wave stack of a second 
plurality of pairs (or periods) of semiconductor layers which forms a top 
mirror, 17; an ionized region, 18, of ions implanted into peripheral 
annuler region of the top mirror defining a centrally located circular 
window, 19, in the top mirror; a dielectric layer, 20, upon the top 
mirror, said dielectric layer having a centrally located window, 21, 
essentially coextensive vertically with window 19; a barrier metal layer, 
22, upon dielectric layer 20 and that portion of top mirror 17 which is 
exposed in window 21; and an optically transparent, semiconductor layer, 
23, which acts as a top electrode of the device. While not shown, 
additional confining and buffer layers may be included into the laser 
structure. Optionally, if the conductivity of the uppermost layer of top 
mirror 17 is insufficient to form a non-alloyed ohmic contact to barrier 
layer 22, a thin highly doped contacting layer (not shown) may be 
interposed between the top mirror 17 and barrier layer 22. 
A more detailed construction of VCSEL 10, in accordance with the invention, 
may be described as being generally as follows: 
Substrate 12 is a heavily doped n.sup.+ -type III-V or II-VI semiconductor, 
such as GaAs, AlGaAs, GaInAs, InP, InGaPAs, AlGaInAs and other related 
group III-V or II-VI compound semiconductors. Typically, the thickness of 
the substrate ranges from 100 to 650 .mu.m and the doping concentration of 
the substrate ranges from 1.times.10.sup.17 to 4.times.10.sup.18 
cm.sup.-3. In some applications, such as opto-electronic integrated 
circuitry, substrate 12 may be first grown on a master substrate of 
silicon, which is in common to a number of devices grown on the master 
substrate. 
An n+ doped quarterwave semiconductor stack upon substrate 12 is composed 
of a plurality of pairs or periods of semiconductor layers forming 
multilayer distributed Bragg reflector (DBR) bottom mirror 13 with a 
number of pairs (or periods) typically ranging from 10 to 40. One 
semiconductor layer in each pair or period has a higher index of 
refraction than the other semiconductor layer of the pair. The thickness 
of each semiconductor in the pair equals .lambda./4.eta., wherein .lambda. 
is the operational optical wavelength of the laser device and .eta. is the 
refractive index of the semiconductor material. For a device with an 
active region lasing at .lambda.=0.87 .mu.m, such as a GaAs laser, a 
quarterwave stack of pairs of such semiconductors as GaAs and AlAs with 
refractive indices of 3.64 and 2.97, respectively, will consist of 62 nm 
thick GaAs layer and 73 nm thick AlAs layer while a stack of Al.sub.0.05 
Ga.sub.0.95 As and AlAs will consist of pairs of layers 60 nm and 73 nm 
thick each, respectively. To reduce series resistance, the n-type bottom 
mirror may be deposited as a one-step graded structure, each period of 
such bottom mirror being with a structure of Al.sub.0.14 Ga.sub.0.86 
As(500 .ANG.)/Al.sub.0.57 Ga.sub.0.43 As(100 .ANG.)/AlAs(500 
.ANG.)/Al.sub.0.57 Ga.sub.0.43 As(100 .ANG.). Alternatively, the mirror 
structure may be provided with linear grading or with a superlattice 
grading of the stack. 
Confining layers 14 and 16 are provided to confine active region 15 and to 
adjust the length (L) of the optical cavity. This optical cavity length 
should be 2 L=N.multidot..lambda., wherein N is an integer and .lambda. is 
an operating optical wavelength of the laser. To obtain constructive 
interference, the thickness of the confining layers should be a multiple 
of .lambda./2 or .lambda./4. Typically, the thickness of each confining 
layer ranges from 0 to 3 .mu.m. The confining regions are Al.sub.x 
Ga.sub.1-x As, with x ranging from 0.1 to 0.4. 
Active region 15 is a region in which electrons (-) and holes (+) recombine 
providing, under proper stimulation, a lasing emission. The active region 
with a thickness ranging from 0.1 to 1 .mu.m, is a multi-quantum well 
(MQW) structure with very thin barriers. Each quantum well includes a 
narrow gap semiconductor 1 to 30 nm thick, confined by wide-gap 
semiconductor about 1 to 20 nm in thickness. 
The second quarter-wave stack of from 2 to 20 pairs or periods of high 
index/low index material layers similar to the pairs or periods in bottom 
mirror 13, but with p.sup.+ -type doping (1.times.10.sup.18 to 
5.times.10.sup.19 cm.sup.-3), forms multilayer DBR top mirror 17 upon 
confining layer 16. The peripheral region of the top mirror includes 
ion-implanted region 18. The ions are implanted into the peripheral region 
of top mirror 17, creating window 19 so that both the current and the 
lasing emission are confined to a narrow centrally located region. The 
ions are selected from ions (protons) of elements which do not affect the 
conductivity type of the material in which they are implanted. Ions, such 
as H.sup.+ or O.sup.+, are implanted in concentrations ranging from 
1.times.10.sup.18 to 5.times.10.sup.19 per cm.sup.3. 
A thin layer 20 of dielectric material, such as SiO.sub.2, Si.sub.3 
N.sub.4, borosilicate glass, such as Vycor.RTM., etc., is formed upon top 
mirror 17 with a thickness ranging from 0.01 to 0.1 .mu.m. Layer 20 has a 
centrally located window 21 which is substantially coextensive with window 
19 formed in the ion-implanted area 18. Windows 19 and 21 cooperate in 
restricting the laser emission to the centrally located region. Layer 20 
also permits passage of operating current only through window 21, thus 
restricting the current flow to a narrow area of the active region. Each 
of the windows is from 5 to 50 .mu.m, preferably 10 to 20 .mu.m, in 
diameter. 
Optically transparent top electrode 23 is deposited as a layer on top of 
the structure with lasing emission from active layer 15 taking place 
through the top mirror and through the top electrode. In accordance with 
this invention, the electrode material is selected from optically 
transmissive semiconductor materials having conductivities ranging from 
1.times.10.sup.3 to 1.times.10.sup.5 .OMEGA..sup.-1 cm.sup.-1, light 
transmissivity greater than 80 percent and light absorption of less than 
10 percent. The optically transmissive semiconductor layer is applied in 
the laser structure onto the top mirror to serve as the vertical-injection 
contact without substantially interfering with the light output. Cadmium 
tin oxide (CTO) and indium tin oxide (ITO), with the respective nominal 
formula Cd.sub.2-x Sn.sub.x O.sub.4 with x ranging from 0.01 to 0.5, 
preferably from 0.3 to 0.4, and In.sub.2-y Sn.sub.y O.sub.3, with y 
ranging from 0.01 to 0.2, are especially suitable for this purpose. The 
cadmium tin oxide is optically transparent (greater than 80 percent) with 
a negligible absorption (&lt;1%) and is electrically conductive with a 
conductivity of 2.times.10.sup.3 .OMEGA..sup.-1 cm.sup.-1 and resistivity 
of 5.times.10.sup.-4 .OMEGA.cm at room temperature. The indium tin oxide 
is also optically transparent (greater than 90 percent) with a very small 
absorption (&lt;5%) and is electrically conductive with a conductivity of 
2.5.times.10.sup.3 .OMEGA..sup.-1 cm.sup.-1 and resistivity of 
4.times.10.sup.-4 .OMEGA.cm at room temperature. These materials, when 
deposited in thickness ranging from 50 nm to 500 nm, preferably from 200 
to 300 nm, provide electrical conductivity sufficient for use as an 
electrode of the laser and yet exhibit transparent properties with 
transmissivity (T) greater than 80 percent and absorption &lt;10 percent. 
Prior to the deposition of top electrode 23, a thin metal barrier layer 22 
is deposited on top of the dielectric layer 20 and on that portion of top 
mirror 17 which is exposed in window 21. This barrier layer is deposited 
in a thickness of up to 300 .ANG., preferably from 10 to 50 .ANG.. The 
barrier layer is used to avoid formation of another p-n junction between 
the p-type conductivity top mirror 17 and n-type conductivity 
semiconductor top electrode 23, which could interfere with the lasing 
emission. The barrier layer is selected from metals or alloys which do not 
cause contamination of the materials of the device, are capable of forming 
a non-alloyed ohmic contact with the surface of top mirror 17 and when 
deposited within the above thickness range do not substantially interfere 
with the transmission of the lasing emission. 
Optionally, if the conductivity of an uppermost layer of top mirror 17 is 
insufficient to form a non-alloyed ohmic contact to barrier layer 22, a 
thin highly doped contacting layer (not shown) may be provided 
intermediate top mirror 17 and barrier layer 22 with a thickness ranging 
from 0.01 to 0.1 .mu.m, preferably about 0.0625 .mu.m, to facilitate 
establishment of the non-alloyed ohmic contact between the top mirror and 
the barrier metal layer. Typically, the doping concentration in the 
optional contacting layer would range from 1.times.10.sup.19 to 
1.times.10.sup.20 cm.sup.-3, preferably about 5.times.10.sup.19 cm.sup.-3. 
Metal electrode 11 ranging from 1 to 10 .mu.m in thickness is formed on the 
bottom surface of substrate 12 to provide for current flow perpendicularly 
through the active region to cause lasing emission. Indium is a metal 
which may be deposited as a thin layer without causing undue heating of 
the structure. The laser may be mounted with electrode 11 in contact with 
a heat-sink plate, e.g. of copper or some other heat-conductive material 
which does not contaminate the laser materials. 
Semiconductor layers 13 through 17 can be grown upon substrate 12 by such 
known methods as metal organic vapor phase epitaxy (MOVPE) or by molecular 
beam epitaxy (MBE) or by hydride vapor phase epitaxy (VPE). In the 
preferred embodiment, the VCSEL structures are grown by Molecular Beam 
Epitaxy (MBE) technology in a Varian Gen II or Riber MBE system on heavily 
doped (1.times.10.sup.17 -4.times.10.sup.18 cm.sup.-3) substrates 12. 
After layers 13 through 17 are grown, windows 19 and 21 are defined by 
photolithographic technique by depositing a suitable resist on the 
centrally located upper surface of top mirror 17. The partially formed 
structure with resist thereon is transferred to a separate high vacuum 
chamber where the structure is subjected to ion implantation, e.g. H.sup.+ 
or O.sup.+, to form ion-implanted region 18. After dielectric layer 20 is 
formed on the upper surface of top mirror 17 and the resist is removed, 
thin metal barrier layer 22 is deposited on top of the dielectric layer 
and on the upper surface of top mirror 17 exposed in window 21. Such a 
metal layer is conveniently deposited by evaporation at temperatures 
ranging from 100.degree. to 500.degree. C., preferably from 100.degree. to 
250.degree. C., by sputtering or by electron-beam deposition. The latter 
process is preferable since the higher temperatures needed for evaporation 
could result in undesirable alloying of the metal into the semiconductor 
leading to a rough interface morphology which degrades the reflection 
properties of the mirror. After the deposition of the barrier layer, 
optically transparent semiconductor top electrode 23 is deposited on top 
of the structure. The top electrode is deposited with a thickness 
sufficient to enable the semiconductor material of the electrode to act as 
a terminal conductor for the device and yet insufficient to effectively 
reduce lasing emission from the top mirror. For such semiconductor 
materials as cadmium tin oxide and indium tin oxide, an effective 
thickness falls within a range of from 50 nm to 500 nm. Thin bottom 
electrode layer 11, e.g., of In, may then be formed on the bottom surface 
of substrate 12. Finally, the bottom or substrate side of the laser may be 
mounted via the In electrode or by means of a conductive adhesive, such as 
epoxy, on a copper slab which serves as a heat sink in common to other 
devices. 
In the preferred embodiment, the VCSEL is an Al.sub.x Ga.sub.1-x As/GaAs 
with x being defined appropriately hereinbelow for each semiconductor 
layer of the structure. The laser structure comprises in an ascending 
sequence a 1 to 2 .mu.m thick In electrode 11, a 500 .mu.m thick 
(001)-oriented heavily doped (2.times.10.sup.18 cm.sup.-3) n.sup.+ -GaAs 
substrate 12, bottom mirror 13 consisting of a quarter-wave stack of 30 
periods of n+-type semiconductor layers forming the multilayer distributed 
Bragg reflector (DBR) bottom mirror. Each period of the bottom mirror is 
of an one-step graded structure of Al.sub.0.14 Ga.sub.0.86 As(500 
.ANG.)/Al.sub.0.57 Ga.sub.0.43 As(100 .ANG.)/AlAs(580 .ANG.)/Al.sub.0.57 
Ga.sub.0.43 As(100 .ANG.). It is Si-doped with a doping concentration of 
3.times.10.sup.18 cm.sup.-3 near the substrate, which is then reduced to 
1.times.10.sup.18 cm.sup.-3 in the last 6-10 periods near the active 
layer. The reflectivity spectrum of the DBR structure (bottom mirror 13) 
with the one-step graded structure, as measured with a Perkin-Elmer Lambda 
9 UV/VIS/NIR Spectrophotometer, showed a broad high reflectivity band 
centered at .about.0.87 .mu.m with a reflectivity&gt;99 percent, which 
matches a calculated reflectivity curve very well. 
The bottom mirror is followed by n.sup.+ -confinement layer 14, a p.sup.- 
-active region layer 15 of four GaAs/AlGaAs quantum well structures about 
0.1 .mu.m total thickness and n.sup.+ --confinement layer 16. Each of the 
confinement layers is about 820 .ANG. thick. The GaAs/AlGaAs four quantum 
well structures of the active region are grown in a Riber MBE system. The 
active region is undoped, and consists of four 100 .ANG. GaAs quantum 
wells with 70 .ANG. Al.sub.0.3 Ga.sub.0.7 As barriers. The active region 
is clad on the top and bottom by respective confinement layers 14 and 16. 
One-third of each confinement layer near the active region is undoped, and 
the rest is lightly doped (1.times.10.sup.16 -1.times.10.sup.17). Each 
confinement layer is linearly graded Al.sub.x Ga.sub.1-x As with x graded 
from 0.3 to 0.57 near the mirrors. This graded-index, separate-confinement 
heterostructure helps the carrier confinement and reduces the lasing 
threshold current. 
P-type top mirror 17 is a 20-period semiconductor mirror, which is also 
one-step graded to reduce the series resistance. Each period of the top 
mirror has a structure of Al.sub.0.14 Ga.sub.0.86 As (500 
.ANG.)/Al.sub.0.57 Ga.sub.0.43 As (100 .ANG.)/AlAs (580 .ANG.)/Al.sub.0.57 
Ga.sub.0.43 As (100 .ANG.). It is Be-doped with a doping concentration of 
5.times.10.sup.18 cm.sup.-3 in the first 16 periods. Then, the dopant 
concentration is increased to 2.times.10.sup.19 cm.sup.-3 near its upper 
surface to facilitate contacting. 
At this fabrication stage the incomplete laser structure may be examined 
with the reflectivity measurement using an Anritsu MS9001B optical 
spectrum analyzer. The reflectivity measurements show Fabry-Perot 
resonance as a clear dip in the stop band. Then, ion-implanted region 18 
is formed by implanting 300 ke V H.sup.+ ions (or 2000 ke V O.sup.+ ions) 
with a dose of 1.times.10.sup.15 cm.sup.-2 into top mirror 17 with a 10-20 
.mu.m diameter window protected by 6.2 .mu.m thick photoresist (e.g. 
Shipley AZ Photoresist 4200). Upon formation of the ion-implanted region 
18, and with the photoresist still in place, a 500 to 5000 .ANG., 
preferably 1000 to 2000 .ANG., thick SiO.sub.2 layer 20 is grown on the 
surface of the top mirror layer at 100.degree. C. in a high vacuum chamber 
by electron beam evaporation. Thereafter the photoresist is stripped with 
acetone, and after the SiO.sub.2 layer and that surface of the top mirror 
which is exposed in window 21 are plasma cleaned, a silver layer, from 10 
to 50 .ANG. thick, is deposited (preferably by evaporation) upon the 
SiO.sub.2 layer and also on the exposed surface of the top mirror. 
Electrode 23 of cadmium tin oxide (or indium tin oxide) is formed on top of 
barrier electrode 22 with a thickness from about 50 to 500 nm, preferably 
200 to 300 nm. In this thickness range, electrode 23 is sufficiently 
optically transparent to the lasing emission enabling its use as a top 
electrode of the VCSEL. Thicknesses higher than 500 nm may result in an 
increased series resistance without any improvement in the lasing 
efficiency. 
The growth of the optically transparent semiconductor layer 23 uses an RF 
magnetron sputtering system (Anelva Corp., Model SPF-332H). In an 
exemplary embodiment, the target was a sintered disk (3 inches in 
diameter, 1/4 inch in thickness) of a mixture of about 67% CdO and about 
33% SnO.sub.2 (Haselden, San Jose, Calif.). The target was mounted 5 cm 
above the samples. The plate voltage was 1.5K V and the plate current 
approximately 110 mA. A deposition rate of 3 .ANG./sec was maintained 
during the growth. The sputtering gas was a mixture of argon and oxygen at 
a total pressure of from 3 to 4 Pa. The resistivity of cadmium tin oxide 
(CTO) film depends strongly on the partial pressure of oxygen. Minimum 
resistivity is obtained for an oxygen partial pressure P.sub.o.sbsb.2 of 
about 2-4.times.10.sup.-2 Pa in 2-4 Pa argon. The cadmium tin oxide film 
was deposited in a thickness of .about.2000 .ANG.. It has negligible 
absorption (less than 1%, which is limited by the capability of the 
setup) at 0.85 .mu.m. The resistivity of this film is 
.about.5.times.10.sup.-4 .OMEGA.-cm. For the above target, voltage, 
current and argon pressure it was found that the resistivity increases 
rapidly for P.sub.o.sbsb.2 of 1.times.10.sup.-2 and higher. A standard 
buffered oxide etchant was used to etch the deposited cadmium tin oxide 
layer in the process of device isolation. A top electrode of indium tin 
oxide (ITO) material may be produced in a similar manner. 
Before characterizing the lasing properties, the substrate side of the 
sample is bonded with conductive epoxy on a copper slab which serves as a 
heat sink. No other cooling is used. All the experiments are done at room 
temperature. A fine probe is used to electrically contact and pump the 
lasers. Current is vertically injected through the window area, as shown 
by straight arrows in FIG. 1, and the remaining area of the device surface 
is electrically isolated from the top electrode with the SiO.sub.2 layer 
20. 
FIG. 2 is the continuous wave light output power versus direct current. The 
light output power is measured with an ANDO AQ-1125 optical power meter 
calibrated at 0.85 .mu.m. The lasing threshold current is 4.2 mA with 
.about.35% external differential quantum efficiency at a lasing wavelength 
of .about.0.85 .mu.m. Threshold currents as low as 3.8 mA are obtainable. 
50 .ANG. thick barrier layer 22 causes a small reduction in the laser 
output power (about 10%). FIG. 3 is the current-voltage curve which shows 
a voltage of 5.4 V and a differential series resistance of 430.OMEGA. at 
the lasing threshold of 4.2 mA. FIG. 4 shows the far field light intensity 
distribution measured at a detector-sample distance of 8.3 cm. The 
distribution is Gaussian-like, indicating a single fundamental transverse 
mode operation, with a full width at half maximum of 7.0.degree.. 
Measurement is performed at stepped intervals of .about.0.35.degree. each, 
with a resolution of better than 0.2.degree.. 
It is also possible to fabricate a structure without the ion implantation 
region 18. In such a case, top mirror 17 is mesa-etched, and probed 
directly at the top window area. A device with a 40 .mu.m diameter mesa 
gives a pulsed (100 ns, 1 kHz) threshold current of 40 mA, which yields a 
threshold current density of 3 kA/cm.sup.2 (see FIG. 5). More than 30% 
reduction in the light output power results from 300 .ANG. thick Ag 
barrier layer in this sample, and the blocking by a probe itself. The 
light output power at 140 mA is 6.5 mW.