A II-VI compound semiconductor laser diode includes a plurality of layers of II-VI semiconductor forming a pn junction supported by a single crystal GaAs semiconductor substrate. The layers forming the pn junction include a first cladding layer of a first conductivity type, a second cladding layer of a second conductivity type, and at least a first guiding layer between the first and second cladding layers. A CdZnSe or other II-VI semiconductor quantum well active layer is positioned within the pn junction. Electrical energy is coupled to the laser diode by first and second electrodes.

BACKGROUND OF THE INVENTION 
The present invention relates generally to semiconductor laser diodes. In 
particular, the present invention is a laser diode fabricated from Group 
II-VI compound semiconductors which emits coherent radiation in the blue 
and green portion of the spectrum . Semiconductor laser diodes are 
generally known and disclosed, for example, in Chapter 12 of Sze, Physics 
of Semiconductor Devices, 2nd ed. pp. 681-742 (1981). To date, most 
commercially available laser diodes are fabricated from Group III-V 
compound semiconductors and their alloys such as GaAs and AiGaAso These 
devices emit light in the infrared and red portions of the spectrum, e.g., 
at wavelengths between 630 and 1550 nm. Laser diodes of these types are 
used in a wide range of applications such as communications, recording, 
sensing and imaging systems. 
Nonetheless, there are many applications for which the wavelength of light 
generated by infrared and red laser diodes is not suitable. Commercially 
viable laser diodes which emit radiation at shorter wavelengths, for 
example in the green and blue portions of the spectrum (ie., at 
wavelengths between 590 and 430 nm) would have widespread application. 
Shorter wavelength laser diodes would also increase the performance and 
capabilities of many systems which currently use infrared and red laser 
diodes. 
Wide band gap II-VI semiconductors and alloys, and in particular ZnSe, have 
for many years been called promising materials for the fabrication of blue 
and green light emitting devices. In the 1960's, laser action was 
demonstrated in several II-VI semiconductors using electron-beam pumping 
techniques. Colak et al., Electron Beam Pumped II-VI Lasers, J. Crystal 
Growth 72, 504 (1985) includes a review of this work. There have also been 
more recent demonstrations of photopumped and electron-beam pumped lasing 
action from epitaxial II-VI semiconductor materials. See eg., Potts et 
al., Electron Beam Pumped Lasing In ZnSe Grown By Molecular-Beam Epitaxy, 
Appl. Phys. Lett., 50, 7 (1987) and Ding et al., Laser Action In The 
Blue-Green From Optically Pumped (Zn, Cd)Se/ZnSe Single Quantum Well 
Structures, Appl. Phys. Lett. 57, p. 2756 (1990). As research on wide band 
gap II-VI semiconductor devices progressed, several key technological 
difficulties were identified. These difficulties included: 1) the 
inability to produce low-resistivity p-type ZnSe and related alloys; 2) 
the inability to form device-quality ohmic contacts to p-type ZnSe and 
related alloys, and 3) the lack of a suitable lattice-matched 
heterostructure material system. 
Modern epitaxial growth techniques such as molecular beam epitaxy (MBE) and 
metalorganic chemical vapor deposition (MOCVD) are now used to fabricate 
device quality undoped and n-type ZnSe layers, typically on GaAs 
substrates. The growth of low resistivity p-type ZnSe using Li and N 
(NH.sub.3) as dopants has also been reported. For some time it appeared 
that the upper limit of obtainable net acceptor concentrations (N.sub.A 
-N.sub.D) was about 10.sup.17 cm.sup.-3. Recently, however, significantly 
greater net acceptor concentrations have been achieved in ZnSe:N grown by 
MBE using nitrogen free radicals produced by an rf plasma source. See eg., 
Park et al., P-type ZnSe By Nitrogen Atom Beam Doping During Molecular 
Beam Epitaxial Growth, Appl. Phys. Lett. 57, 2127 (1990) and copending 
Park et al. U.S. Pat. application Ser. No. 07/573,428 filed Aug. 24, 1990, 
now U.S. Pat. No. 5,248,631 issued on Sep. 28, 1993, entitled Doping Of 
IIB-VIA Semiconductors During Molecular Beam Epitaxy. The largest net 
acceptor concentration in ZnSe achieved through the use of these 
techniques is 2.times.10.sup.18 cm.sup.-3. Using these technologies, 
rudimentary blue light emitting diodes have been reported by several 
laboratories. See eg., the Park et al. Appl. Phys. Lett. article referred 
to immediately above. 
Of the wide band gap II-VI semiconductor systems that are reasonably well 
developed, ie., ZnSeTe, CdZnSe, ZnSSe and CdZnS, only CdZnS-ZnSe offers a 
lattice-matched system. Unfortunately, this system offers only a very 
small band gap difference (about 0.05 eV), which is far too small for the 
carrier confinement needed for simple double heterostructure laser diodes. 
Therefore, to achieve a band gap difference greater than 0.2 eV, it would 
be necessary to use a strained-layer system (eg., ZnSe-Cd.sub.x Zn.sub.1-x 
Se with x.gtoreq.0.2). To prevent misfit dislocations which degrade the 
luminescence efficiency, the thickness of the strained layer should be 
kept less than the critical thickness. However, a simple double 
heterostructure laser made accordingly would have an active layer 
thickness so thin (due to the large mismatch required for sufficient band 
gap difference) that the optical mode would be very poorly confined. Thus, 
the confinement factor (overlap between the optical mode and the light 
generating region) would be small, and substrate losses would be high, 
causing prohibitively high threshold currents. Therefore, simple double 
heterostructure laser diodes are not practical in these wide band gap 
II-VI materials. 
For these reasons, there have been no known demonstrations of laser diodes 
fabricated from II-VI compound semiconductors. Commercially viable laser 
diodes of this type would be extremely desirable and have widespread 
application. 
SUMMARY OF THE INVENTION 
The present invention is a II-VI compound semiconductor laser diode 
including a plurality of layers of II-VI semiconductor forming a pn 
junction. The layers forming the pn junction include a first cladding 
layer of a first conductivity type, a second cladding layer of a second 
conductivity type, and at least a first guiding layer between the first 
and second cladding layers. A quantum well active layer of II-VI 
semiconductor is positioned within the pn junction. The layers of II-VI 
semiconductor are supported by a single crystal semiconductor substrate. 
Electrical energy is coupled to the device by first and second electrodes 
on opposite sides of the layers of II-VI semiconductor. A preferred 
embodiment of the invention is a laser diode including a CdZnSe quantum 
well active layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The structure of a laser diode 10 in accordance with the present invention 
is illustrated generally in FIG. 1. Laser diode 10 is a wide band gap 
II-VI device fabricated from heteroepitaxial layers of ZnS.sub.x 
Se.sub.1-x, ZnSe and Cd.sub.y Zn.sub.1-y Se grown by molecular beam 
epitaxy (MBE) on a GaAs substrate. Prototypes of this device have 
exhibited laser action, emitting coherent blue-green light near 490 nm 
from a Cd.sub.y Zn.sub.1-y Se quantum well structure under pulsed current 
injection at 77 K. 
Laser diode 10 is fabricated on a GaAs substrate 12, and includes lower 
(first) and upper (second) ZnSe light-guiding layers 14 and 16, 
respectively, separated by a Cd.sub.1-y Zn.sub.y Se quantum well active 
layer 18. The surfaces of light-guiding layers 14 and 16 opposite active 
layer 18 are bounded by lower and upper ZnS.sub.x Se.sub.1-x cladding 
layers 20 and 22, respectively. A lower ZnSe ohmic contact layer 24 is 
positioned on the surface of lower cladding layer 20 opposite 
light-guiding layer 14, while an upper ZnSe ohmic contact layer 26 is 
positioned on the surface of upper cladding layer 22 opposite 
light-guiding layer 16. A GaAs buffer layer 28 separates substrate 12 from 
lower ZnSe contact layer 24 to assure high crystalline quality of the 
contact and subsequently deposited layers. A polyimide insulating layer 34 
covers the surface of upper ohmic contact layer 26 opposite upper cladding 
layer 22. Electrical contact to the ohmic contact layer 26 is made by Au 
electrode 30 which is formed in a window stripe in insulating layer 34. A 
thin Ti layer 31 and subsequently a final Au layer 33 are applied over 
polyimide layer 34 and exposed portions of Au electrode 30 to facilitate 
lead bonding. Electrical contact to the lower side of laser diode 10 is 
made by an In electrode 32 on the surface of substrate 12 opposite the 
lower ohmic contact layer 24. 
Layers 24, 20 and 14 are all doped n-type with C1 (ie., are of a first 
conductivity type) in prototypes of laser diode 10. Layers 16, 22 and 26 
are all doped p-type with N (ie., are of a second conductivity type). 
Active layer 18 is an undoped quantum well layer of Cd.sub.0.2 Zn.sub.0.8 
Se semiconductor deposited to a thickness of 0.01 .mu.m. Light-guiding 
layers 14 and 16 are both 0.5 .mu.m thick. Lower light-guiding layer 14 is 
doped to a net donor concentration of 1.times.10.sup.17 cm.sup.-3, while 
upper light-guiding layer 16 is doped to a net acceptor concentration of 
2.times.10.sup.17 cm.sup.-3. Cladding layers 20 and 22 are layers of 
ZnS.sub.0.07 Se.sub.0.93 semiconductor deposited to thicknesses of 2.5 
.mu.m and 1.5 .mu.m, respectively. The net donor concentration of the 
lower cladding layer is 1.times.10.sup.18 cm.sup.-3. The net acceptor 
concentration of the upper cladding layer is 2.times.10.sup.17 cm.sup.-3. 
Ohmic contact layers 24 and 26 are both deposited to a thickness of 0.1 
.mu.m in these prototype devices. The lower contact layer is doped n-type 
to a net donor concentration of 1.times.10.sup.18 cm.sup.-3. The upper 
contact layer is doped p-type to a net acceptor concentration of 
1.times.10.sup.18 cm.sup.-3. 
Other parameters and materials can also be used in the fabrication of laser 
diodes 10 in accordance with the present invention. For example, the 
thicknesses of layers 24, 20, 14, 16, 22 and 26 can be varied as needed 
for given applications. Typical thickness ranges for contact, cladding and 
light-guiding layers are 0.03 to 1.0 .mu.m, 0.5 to 5.0 .mu.m, and 0.1 to 
1.0 .mu.m, respectively. In general, the thicknesses of light-guiding 
layers 14 and 16 should be chosen to minimize the width of the optical 
mode. If the layers 14 and 16 are too thin, the evanescent tails will 
extend far into cladding layers 20 and 22. Cladding layers 20 and 22 must 
be thick enough to make absorption of the optical mode in substrate 12 and 
electrode 32 negligible. The composition of the Cd.sub.x Zn.sub.1-x Se 
(which determines the laser wavelength) with x of approximately 0.2 was 
selected to provide a large enough band gap difference (.DELTA.E.sub.g of 
approximately 0.2 eV) to facilitate effective carrier confinement. Larger 
x will provide deeper quantum wells, but would require a thinner layer due 
to increased lattice mismatch, thereby decreasing the efficiency of the 
collection of carriers into the well. 
The composition of the ZnS.sub.y Se.sub.1-y with of approximately 0.07 was 
selected to provide sufficient difference in refractive index from the 
index of the ZnSe guiding layers to form a low-loss waveguide. This 
composition also provides excellent morphology since it is nearly lattice 
matched to the GaAs substrate at the growth temperature of 300.degree. C. 
Other n-type dopants which may be used include Ga, Al, In, I, F, and Br. 
Oxygen or Li acceptors can also be used for the p-type dopants. Other 
Group V p-type dopants which might be used include arsenic and 
phosphorous. Greater donor and acceptor concentrations can also be used, 
although they should not be so high as to cause excessive free-carrier 
absorption. 
The prototypes of laser diode 10 are fabricated on Si-doped n.sup.+ -type 
GaAs substrate 12 having a (100) crystal orientation. Substrates 12 of 
this type are commercially available from a number of manufacturers 
including Sumitomo Electric Industries, Ltd. GaAs buffer layer 28 is 
deposited to a thickness of 1 .mu.m in this embodiment, and doped n+ with 
Si to a net donor concentration of 1.times.10.sup.18 cm.sup.-3. Other 
appropriate substrates (eg., ZnSe, GainAs or Ge) and buffer layers such as 
AlGaAs, AlAs, GaInP, AiInP, AlInAs or GainAs can also be used. The 
thickness of buffer layer 28 can also be varied while providing an 
appropriate high-quality surface for growing the II-VI semiconductors. If 
an appropriate high-quality substrate and appropriate surface preparation 
is used, buffer layer 28 may not be needed. 
The lattice constants of the ZnSSe cladding layers 20 and 22 and the 
adjacent ZnSe layers 24, 14 and 16, 26, respectively, are mismatched by 
about 0.3%. Preliminary transmission electron microscopy (TEM) studies 
indicate that the ZnSe of light-guiding layers 14 and 16 is at least 
partially relaxed by dislocations formed at the interfaces of the 
light-guiding layers and the adjacent ZnSSe cladding layers 20 and 22, 
respectively. These preliminary studies also indicate that the thickness 
of the CdZnSe quantum well active layer 18 is less than the critical 
thickness for this material system. Quantum well active layer 18 is 
therefore pseudomorphic, minimizing dislocations in the light-emitting 
region of laser diode 10. The maximum pseudomorphic thicknesses for 
strained epitaxial layers such as 18 depends on the composition and can be 
calculated from formulae described in Matthews et al., Defects In 
Epitaxial Multilayers, J. Crystal Growth, vol. 27, p. 118 (1974). The 
inclusion of quantum well layer 18, which could also be a pseudomorphic 
layer of other semiconductor material such as ZnSeTe, facilitates the low 
threshold current operation of laser diode 10 when positioned within the 
thicker, low-loss II-VI waveguide. The waveguide can be made with higher 
refractive index light-guiding layers 14 and 16 and lower refractive index 
cladding layers 20 and 22 which can have a relatively small difference in 
their band gaps and need not be exactly lattice matched. The composition 
of the light-guiding layers may be graded to minimize dislocations and/or 
to form a graded index waveguide. 
FIG. 2 is an illustration of a molecular beam epitaxy (MBE) system 50 used 
to fabricate the laser diode 10 described above. MBE system 50 includes 
two MBE chambers 52 and 54 interconnected by ultrahigh vacuum (UHV) 
pipeline 56. Each chamber 52 and 54 includes a high energy electron gun 
58, a phosphorus screen 60, a substrate heater 90 and a flux monitor 62. 
MBE chambers such as 52 and 54 are generally known and commercially 
available. A Perkin-Elmer Model 430 MBE system was used to produce the 
prototype laser diodes 10. 
MBE chamber 52 is used to grow the GaAs buffer layer 28 on substrate 12 and 
includes a Ga effusion cell 64 and an As cracking cell 66. A Si effusion 
cell 68 is also provided as a source of n-type dopants. Substrate 12 is 
cleaned and prepared using conventional or otherwise known techniques, and 
mounted to a Molybdenum sample block (not shown in FIG. 2) by In solder 
before being positioned within chamber 52. By way of example, substrate 
preparation techniques described in the Cheng et al. article 
Molecular-Beam Epitaxy Growth Of ZnSe Using A Cracked Selenium Source, J. 
Vac. Sci. Technol., B8, 181 (1990) were used to produce the prototype 
laser diode 10. The Si doped buffer layer 28 can be grown on substrate 12 
by operating MBE chamber 52 in a conventional manner, such as that 
described in Technology and Physics of Molecular Beam Epitaxy, ed. E. H. 
C. Parker, Plenum Press, 1985. The resulting buffer layer 28 has an Asrich 
surface which exhibited a c(4.times.4) reconstruction as observed by 
reflection high energy electron diffraction (RHEED). The sample block 
bearing the GaAs substrate 12 and buffer layer 28 is then transfered to 
MBE chamber 54 through UHV pipeline 56 for further processing. 
Device layers 24, 20, 14, 18, 16, 22, and 26 are all grown on the buffer 
layer 28 and GaAs substrate 12 within MBE chamber 54. To this end, chamber 
54 includes a Zn effusion cell 70, cracked-Se effusion cell 72, ZnS 
effusion cell 74 (as a source of S), Cd effusion cell 76 and a standard Se 
(ie., primarily Se.sub.6) effusion cell 79. As shown, cracked-Se effusion 
cell 72 includes a bulk evaporator 84 and high temperature cracking zone 
82, and provides a source of cracked Se (including Se.sub.2 and other Se 
molecules with less than 6 atoms). The bulk evaporator 84 and high 
temperature cracking zone 82 used to produce the prototype laser diodes 10 
are of a custom design, the details and capabilities of which are 
described in the Cheng et al. J. Vac. Sci. Technol. article referenced 
above. C1 effusion cell 78 which utilizes ZnCl.sub.2 source material 
provides the C1 n-type dopant. The p-type dopant is provided by N 
free-radical source 80. Free-radical source 80 is connected to a source 86 
of ultra-pure N.sub.2 through leak-valve 88. The free-radical source 80 
used in the fabrication of laser diodes 10 is commercially available from 
Oxford Applied Research Ltd. of Oxfordshire, England (Model No. MPD21). 
This source has a length of 390 mm. The beam exit plate at the end of the 
source is made of pyrolytic boron nitride (PBN) and has nine 0.2 mm 
diameter holes through it. This source is mounted on a standard port for 
an effusion cell through a 10" extension tube. N.sub.2 source 86 used to 
fabricate laser diodes 10 is of research purity grade, produced by 
Matheson Gas Products. The pressure at the inlet of the leak-valve of 
source 86 is 5 psi. 
MBE chamber 54 is operated in a manner described in the Cheng et al. 
article Growth Of p- and n- Type ZnSe By Molecular Beam Epitaxy, J. 
Crystal Growth 95, 512 (1989) using the Se.sub.6 source 79 as the source 
of Se to grow the n-type contact, cladding and light-guiding layers 24, 20 
and 14, respectively, of the prototype laser diode 10. Quantum well active 
layer 18 is grown in a manner described in the Samarth et al. article, 
Molecular Beam Epitaxy of CdSe and the Derivative Alloys Zn.sub.1-x 
Cd.sub.x Se and Cd.sub.1-x Mn.sub.x Se, J. Electronic Materials, vol. 19. 
No. 6, p. 543 (1990). 
MBE chamber 54 is operated in a manner described in the Park et al. U.S. 
Pat. No. 5,248,631, issued Sep. 28, 1993 and entitled Doping Of IIB-VIA 
Semiconductors During Molecular Beam Epitaxy, using the Se.sub.6 source 79 
to grow the p-type light-guiding layer 16 and cladding layer 22. The 
disclosure contained in the above-referenced Park et al. U.S. Pat. No. 
5,248,631 is incorporated herein by reference. Lower ZnSSe cladding layer 
20 is doped n-type using the ZnCl.sub.2 source. Other aspects of the 
techniques used to grow cladding layers 20 and 22 are described in the 
Matsumura et al. article, Optimum Composition In MBE-ZnS.sub.x Se.sub.1-x 
/ZnSe For High Quality Heteroepitaxial Growth, J. Crys. Growth, vol. 99, 
p. 446 (1990). 
A low resistivity p-type ZnSe ohmic contact layer 26 has been achieved by 
growing the contact layer at low temperature within MBE chamber 54 
utilizing the cracked Se source 72 (ie., cracking zone 82 and evaporator 
84), while at the same time doping the semiconductor material of the 
contact layer p-type in accordance with the above-referenced Park U.S. 
Pat. No. 5,248,631. The low temperature growth technique used to produce 
the contact layer 26 of the prototype laser diode 10 is described 
generally in the Cheng et al. article Low Temperature Growth Of ZnSe By 
Molecular Beam Epitaxy Using Cracked Selenium, Appl. Phys. Lett. (Feb. 
1990). The semiconductor body with layers 28, 24, 20, 14, 18, 16 and 22 on 
substrate 12 is heated to a temperature less than 250.degree. C. but high 
enough to promote crystalline growth of the ZnSe doped with the N p-type 
dopants to a net acceptor concentration of at least 1.times.10.sup.17 
Cm.sup.-3. A net acceptor concentration of 1.times.10.sup.18 cm.sup.-3 was 
achieved in the ohmic contact layer 26 of prototype laser diodes 10, when 
grown at a substrate temperature of about 150.degree. C. However, it is 
anticipated that ohmic contact layers 26 with acceptable characteristics 
can be achieved at other growth temperatures down to at least 130.degree. 
C. Other operating parameters of MBE chamber 54 used to produce the ohmic 
contact layer 26 of the prototype laser diodes 10 are as follows: 
Zn beam equivalent pressure: 1.0.times.10.sup.-7 Torr* 
Se cracking zone temperature: 600.degree. C.* 
Se bulk evaporator temperature: 250.degree. C.* 
Growth rate: 0.3-0.6 .mu.m/hr 
Surface reconstruction: Zn-stabilized 
Nitrogen pressure in chamber: &gt;3.5.times.10.sup.-7 Torr* 
rf power: 150-250 W* 
* parameters dependant upon specific MBE system configuration 
FIG. 3 is the current-voltage characteristic of a sample with two coplanar 
Au metal electrodes on a p-type ZnSe contact layer produced for test 
purposes in a manner substantially similar to that described above. The 
ohmic nature of this contact is indicated by the substantially linear 
nature of the curve over the -6 to +6 volt range. 
The mechanisms believed to enable the ohmic nature of contact layer 26 can 
be described with reference to FIG. 4 which is an energy band diagram of 
the Au--p-type ZnSe contact layer interface. In addition to the expected 
shallow impurities 100 utilized by conventional ohmic contacts, additional 
electronic energy states 102 are formed in the contact layer. These 
additional energy states 102 are relatively deep (within the forbidden 
gap) with respect to the valence band maximum, compared to the depth of 
the shallow impurity level 100. Energy states 102 are in effect 
intermediate energy states located at an energy less than the Au Fermi 
level and greater than the shallow impurity level 100. Since the 
probability of charge carriers tunneling between two given energy states 
increases exponentially with decreasing distance between the two states, 
additional energy states 102 greatly increase the tunneling probability by 
providing a temporary residence for the carriers and facilitate multi-step 
or cascade tunneling. The optimum condition is illustrated in FIG. 4 where 
E.sub.F is the Fermi energy and E.sub.A is the acceptor energy. A 
diagramatic depiction of an electron making a multi-step tunneling 
transfer between the ZnSe and Au layers through the additional energy 
states 102 is also shown in FIG. 4. Even better contacts are attainable 
with electronic states at more than one energy level, such that tunneling 
can occur from state to state across the barrier. 
It is anticipated that the introduction of additional energy states 102 can 
be achieved by a number of methods. Doping during growth, diffusion, ion 
implantation or other known techniques can be used to incorporate 
impurities which produce deep levels. One important type of deep level 
impurity is the iso-electronic trap. By way of example, Te is thought to 
form a hole trap in ZnSe. The additional energy states 102 can also be 
achieved by introducing proper native crystal defects such as, but not 
limited to, dislocations, vacancies, interstitials or complexes into 
contact layer 26. This can be done during the deposition of the contact 
layer by choosing the molecular species of the precursors, and/or by other 
appropriate growth conditions. Native defects can also be generated by 
post-growth treatments such as irradiation by electron beams, ion beams, 
radical beams or electromagnetic radiation. However, these techniques must 
be implemented without detrimentally degrading the conductivity of the 
ZnSe or other semiconductor material used for the contact layer. 
It therefore appears that the useful p-type contact layer 26 has a number 
of properties. The net acceptor density N.sub.A -N.sub.D is large, 
preferrably at least 1.times.10.sup.18 cm.sup.-3. This serves to reduce 
the width of the barrier through which the charge carriers must tunnel. 
The p-type dopant concentration (nitrogen in laser diode 10) must also be 
large, preferrably at least 1.times.10.sup.19 cm.sup.-3. In addition to 
forming the shallow acceptor levels, the nitrogen impurities also appear 
to participate in the formation of the deep energy states. At a minimum, 
the amount of nitrogen required is that which will provide adequate 
concentrations of both types of levels. The growth conditions must also be 
appropriate to form the defects at the energy levels described above. The 
low temperature growth technique described above has been shown to produce 
these material properties (contact resistances less than 0.4 ohm-cm.sup.2 
have been achieved). 
The low-temperature photoluminescence (PL) spectrum from a good ohmic 
contact layer such as 26 is shown in FIG. 8. The observed characteristics 
include: 1) the very weak near band edge PL; 2) the appearance of the 
defect band at 2.3 eV (18,500 cm.sup.-1); and 3) the presence of a band 
(presumably associated with donor-acceptor-pair recombination) at about 
2.5 eV (20,400 cm.sup.-1). The band edge PL is expected to be weak for 
materials which have significant concentrations of deep levels since the 
deep levels provide long wavelength and nonradiative channels which 
compete with the near band edge processes. The emission band at 
approximately 2.3 eV is associated with a transition from the conduction 
band to a deep (acceptor) level about 0.5 eV above the valence band 
maximum. This is near the energy position that is believed to be the most 
effective for cascade tunneling. The emission band at 2.5 eV is believed 
to be related to transitions from donor to acceptor states. No or minimal 
donor states would be preferrable, eliminating this transition, or 
shifting its occurance to slightly higher energies. 
In general, and other than the differences described below, conventional 
processes (ie., those used for Si and III-V semiconductor devices) are 
used to complete the fabrication of prototype laser diode 10. Following 
the deposition of contact layer 26, the as yet incomplete laser diode 10 
is removed from MBE chamber 54. Electrode 30 includes Au which is vacuum 
evaporated onto contact layer 26 and patterned into a stripe (typically 
about 20 .mu.m wide) using conventional photolithography and lift-off. An 
insulating layer 34 of is then applied over electrode 30 and the exposed 
surface of contact layer 26. For an insulator that can be applied at low 
temperatures, polyimide photoresist is preferred. Probimide 408 from 
Ciba-Geigy Corp. was used to produce laser diode 10. A stripe (about 20 
.mu.m wide) of the polyimide layer 34 directly above electrode 30 is 
removed by UV exposure through a photomask and development using the 
manufacturer's recommended processing recipe, except for the 
post-development cure. To cure the developed polyimide, the device was 
flood exposed to 1 J/cm.sup.2 of UV light from a mask aligner, and baked 
at 125.degree. C. on a hot plate in air for 3 minutes. Ti-Au layer 31 is 
then evaporated on the exposed surface of the Au electrode 30 and 
polyimide layer 34 to facilitate lead-bonding. The In used for MBE 
substrate bonding also served as electrode 32 on substrate 12. Opposite 
ends of the device were cleaved along (110) planes to form facet mirrors. 
Cavity length of the prototype devices 10 is about 1000 .mu.m. Laser 
diodes 10 were then bonded p-side up to ceramic sample holders with 
silver-filled epoxy. 
Improved performance of these laser devices can be gained by providing 
better lateral confinement of the optical mode. This can be achieved by 
forming an index-guided laser 10' such as that shown in FIG. 7. 
Index-guided laser 10' is similar to laser 10 and can be fabricated with 
the same II-VI semiconductor layers. Portions of laser 10' which 
correspond to those of laser 10 are indicated with identical but primed 
(ie., "X'") reference numerals. In the embodiment shown, laser 10' 
includes a waveguide or rib 35 in the cladding layer 22' and contact layer 
26'. Rib 35 can be formed to a width of about 5 .mu.m by ion beam etching 
with a Xe or Ar ion beam or by wet-chemical etching. Conventional 
photoresist can be used as a mask for this process. Other known and 
conventional techniques can also be used to provide lateral waveguiding. 
These techniques include using substrates in which grooves have been 
etched (ie., channelled-substrate lasers), or etching a rib and re-growing 
a top cladding layer (ie., a "buried heterostructure" laser). Improvements 
in the threshold current or the differential quantum efficiency may be 
achieved by dielectric coatings of the facets to adjust the 
reflectivities. 
Initial tests of the prototype laser diodes 10 were conducted at 77 K by 
pulsing the devices, typically with 500 nsec pulses and a 500 .mu.sec 
period. Current measurements were made with a box-car averager, while a 
large Si photodetector was used to collect and monitor the output light 
intensity from one end facet of the device. The measured light output as a 
function of current (ie., L-I) characteristics from one of the devices is 
illustrated in FIG. 5. The threshold current is 74 mA, which corresponds 
to a threshold current density of 320 A/cm.sup.2. Differential quantum 
efficiencies in excess of 20% per facet have been measured, as have pulsed 
output powers of over 100 mW per facet. The coherent light is strongly TE 
polarized and a "speckle pattern" is clearly visible. The output laser 
beam has an elliptical far-field pattern, with a divergence of roughly 
40.degree..times.4.degree. for the central lobe. Side lobes are visible, 
indicating higher order transverse modes. The measured L-I 
characteristics, such as that shown in FIG. 5, do indicate some dependence 
on pulse length. At high current densities, the gain in the single quantum 
well prototype devices tends to saturate. At the same time, the index of 
refraction is reduced due to the injection of excess carriers, which tends 
to make the region under the stripe of electrode 30 anti-guiding. Thermal 
effects become important at these current densities, as thermal gradients 
and the temperature dependence of the index provide lateral optical 
confinement. It is expected that these characteristics will be alleviated 
by index-guided versions such as laser diode 10'. 
FIG. 6 is a graph of the optical spectra that are characteristic of the 
prototype laser diodes 10 at 77 K. The spectra illustrated in FIG. 6 were 
acquired using a SPEX 1403 double monochromator. At currents below 
threshold, the spontaneous emission occurs at 490 nm and has a FWHM of 
about 3 nm. Above threshold, the 1060 .mu.m long device operates in many 
longitudinal modes centered at 489.6 .mu.m, and which are separated by 
0.03 nm. 
Laser operation has been observed in the prototype laser diodes 10 for 
short periods of time at temperatures as high as 200 K. At room 
temperature the devices emit at 502 nm, but do not lase. 
The operating voltage of the prototype laser diodes 10 at the threshold 
current is approximately 15 V. This characteristic indicates that there is 
still room for improvement in the ohmic contact between electrode 30 and 
contact layer 26 and/or improvement in the conductivity of p-type layers 
16, 22, and 26. Reducing this series resistance and improving the 
heat-sinking of the device (ie., by solder-bonding the p-type type side 
down) are expected to facilitate CW operation at higher temperatures. 
It is expected that the inventive concepts disclosed herein and used to 
fabricate the prototype laser diode 10 are equally well suited to the 
fabrication of laser diodes from a wide variety of other compound II-VI 
semiconductor alloys, especially from other ZnSe alloys. For example, 
improved operating characterisics will be achieved by using lattice 
matched materials such as Cd.sub.x Zn.sub.1-x S (with x of approximately 
0.61) and ZnSe to form the waveguide. The quantum well in such a device 
may include CdZnSe. This semiconductor system will not suffer from misfit 
dislocations which can decrease efficiency and the useful lifetime of the 
devices. Also, a multiple quantum well active layer made of a 
strained-layer superlattice could replace the single pseudomorphic quantum 
well layer 18. 
Ohmic contact layer 26 might also be improved by using thin layers of 
smaller band gap II-VI alloys such as ZnSe.sub.1-x Te.sub.x, Cd.sub.x 
Zn.sub.1-x Se and Hg.sub.x Zn.sub.1-x Se. Group VI sources other than 
Se.sub.2 can also be used to produce ohmic contacts in accordance with the 
present invention. Other Group VI species X.sub.m where m&lt;6, as well as 
other sources of these species, should be suitable substitutes. Other 
metals (eg., Pt) or other electrically conductive materials having a large 
work function (eg., &gt;5 eV) and suitable for a stable semiconductor 
interface can also be used as electrodes. 
In conclusion, although the present invention has been described with 
reference to preferred embodiments, changes can be made in form and detail 
without departing from the spirit and scope of the invention.