High temperature semiconductor diode laser

A semiconductor laser device, and method for making such, having higher operating temperatures than previously available. A semiconductor epitaxial layer is bonding to a cleaving assembly which allows the epitaxial layer to be manipulated without use of traditional substrate forms. The resulting semiconductor laser is bonded to a metal portion which serves as a heat sink for dissipating heat from the active lasing region. The resulting semiconductor lasers can be cooled by thermoelectric cooling modules, thus eliminating the necessity of using more bulky cryogenic systems.

BACKGROUND 
The present invention is directed to a semiconductor laser, and a method 
for making such, having a high thermal conductivity heat sink component 
enabling the laser to be operated at higher temperatures. 
Long-wavelength semiconductor lasers, which emit tunable infrared radiation 
in the 3 to 30 micron spectral range, are primarily used in tunable diode 
laser (TDL) spectroscopy systems. TDL spectroscopy, which involves tuning 
the laser around the absorption bands of a particular molecule, can 
readily measure sub parts per billion concentrations of trace gases making 
it a useful tool for detecting and monitoring gaseous pollutants. New 
pollution emission standards dictated by the Clean Air Act of 1990 will 
require the monitoring of thousands of smokestacks and other pollution 
sources throughout the United States. TDL spectroscopy, with its high 
resolution capabilities, is ideally suited for such monitoring. 
Atmospheric chemists are presently designing and building TDL 
spectrometers to measure trace gas concentrations throughout the Earth's 
atmosphere (M. Loewenstein, "Diode Laser Harmonic Spectroscopy Applied to 
In Situ Measurements of Atmospheric Trace Molecules", J. Quant. Spectros. 
Radiat. Transfer, 1988, 40, p. 249). Moreover, long-wavelength tunable 
diode lasers can be used in feedback control systems to actually reduce 
emission of pollutants (J. A. Sell, "Tunable Diode Laser of Carbon 
Monoxide in Engine Exhaust", SPIE, 1983, 438, p.67 ). Advances in laser 
performance may eventually allow extension of this pollution control 
technology to automobiles thus greatly expanding the market for 
long-wavelength lasers. TDL spectroscopy has also been used to study 
sub-monolayer concentrations of adsorbates on substrate surfaces (V. M. 
Bermudez, R. L. Rubinovitz and J. E. Butler, "Study of Vibrational Modes 
of Subsurface Oxygen on A1 (111) Using Diode Laser Infrared 
Reflection-Absorption Spectroscopy", J. Vac. Sci. Tech., 1988, A6, p. 
717). Due to the non-invasive nature of the laser probe, this technique 
can provide useful information on catalytic reactions and chemical 
processes (J. E. Butler, N. Bottka, R. S. Sillman, D. K. Gaskill, "In 
Situ, Real-Time Diagnostics of OMVPE Using IR-Diode Laser Spectroscopy", 
J. Crystal Growth, 1986, 77, p. 163). 
An important TDL spectroscopy feature is its ability to identify and 
differentiate among compounds that contain different isotopes of a 
particular element. TDL spectrometers can therefore be used to monitor the 
motion of isotope-tagged tracer molecules. For example, pollutants tagged 
with .sup.13 C could be released in the atmosphere and their motion 
monitored with airborne TDL spectrometers. This offers the unique 
advantage of observing chemical reaction pathways. 
Another example of isotope tracing is in medical diagnostics. Metabolic 
pathways can be monitored by measuring the .sup.13 CO.sub.2 /.sup.12 
CO.sub.2 ratio in the exhaled breath of a patient who has been 
administered a substance tagged with the non-radioactive isotope .sup.13 
C. The .sup.13 CO.sub.2 production correlates directly with the rate at 
which the particular substance is metabolized. For example, a simple 
diabetes test would involve feeding a patient .sup.13 C-labeled sugar and 
monitoring the .sup.13 CO.sub.2 production rate. Such non-invasive 
analysis of metabolic pathways can form the basis for a whole new field of 
health research and patient diagnosis. (U. Lachish, S. Rotter, E. Adler, 
U. El-Hanany, "Tunable Diode Laser Based Spectroscopic System for Ammonia 
Detection in Human Respiration", Rev. Sci. Instrum., 1987, 58, p. 923, and 
R. M. Scheck and D. L. Wall, "Medical Diagnostics with TDLs", Photonics 
Spectra, January 1991, p. 110). 
Although some near-infrared spectrometers based upon III-V semiconductor 
lasers have been developed (D. E. Cooper and R. U. Martinelli, 
"Near-Infrared Diode Lasers Monitor Molecular Species", Laser Focus World, 
November 1992, 133), the most widely used TDL spectrometers are based upon 
narrow bandgap IV-VI semiconductor (also known as lead salt) lasers. 
Temperature tuned IV-VI semiconductor lasers operate in the 3 to 30 .mu.m 
spectral region, where gas molecules have their strongest absorption 
lines, and continue to exhibit better performance characteristics than 
lasers made from other narrow bandgap semiconductors such as HgCdTe (R. 
Zucca, M. Zandian, J. M. Arias, and R. V. Gil, "HgCdTe Double 
Heterostructure Diode Lasers Grown by Molecular Beam Epitaxy", J. Vac. 
Sci. Technol., 1992, B 10, p. 1587; A. Ravid and A. Zussman, "Laser Action 
and Photoluminescence in an Indium-Doped n-type Hg.sub.1-x Cd.sub.x 
Te(x=0.375) Layer Grown by Liquid Phase Epitaxy", J. Appl. Phys., 1993, 
73, p. 3979). However, by comparison with research on III-V and II-VI 
semiconductor materials and devices, research on IV-VI semiconductor 
materials and devices has lagged. Consequently, techniques that may prove 
successful in improving the performance of IV-VI semiconductor lasers have 
yet to be explored. 
Maximum operating temperature is presently considered the most important 
limiting factor for IV-VI semiconductor tunable diode lasers. The 
highest-known operating temperature for devices operated in continuous 
wave (cw) mode, which is preferred over pulsed mode for infrared 
spectroscopy applications, is 203K for a laser emitting in the 3.5 .mu.m 
range (Z. Feit, D. Kostyk, R. J. Woods, and P. Mak, "Single-Mode Molecular 
Beam Epitaxy Grown PbEuSeTe/PbTe Buried Heterostructure Diode Lasers for 
Co.sub.2 High-Resolution Spectroscopy", Appl. Phys. Lett., 1991, 58, p. 
343). Longer wavelength devices have even lower maximum operating 
temperatures. Thus, low operating temperatures necessitate the use of 
cumbersome liquid nitrogen or liquid helium cooling systems in 
spectrometers based upon these laser devices. A low laser operating 
temperature also limits the tuning range of individual devices. If TDL 
operating temperatures can be increased to above 220 to 230 K then 
thermoelectric cooling (TEC) modules could be used enabling a significant 
simplification of TDL spectrometer instrumentation. Any increase in 
maximum operating temperature will also expand TDL tuning range, thus 
further simplifying TDL spectrometer operation. 
Thermal modeling of IV-VI semiconductor lasers (R. Rosman, A. Katzir, P. 
Norton, K. H. Bachem, and H. Preier, "On the Performance of Selenium Rich 
Lead-Salt Heterostructure Laser with Remote p-n Junction", IEEE J. Quantum 
Electronics, 1987, QE-23, p.94) shows that there is a large difference 
under maximum operating temperature conditions, as much as 60 degrees, 
between the heat sink temperature and the active region temperature. This 
thermal gradient is reflected in the maximum operating temperature 
difference between pulsed and cw operation, observed to be as much as 120 
degrees (B. Spanger, U. Schiessl, A. Lambrecht, H. Bottner, and M. Tacke, 
"Near-Room-Temperature Operation of Pb.sub.1-x Sr.sub.x Se Infrared Diode 
Lasers Using Molecular Beam Epitaxy Techniques", Appl. Phys. Lett., 1988, 
53, p. 2582). Improving heat removal from the active region would 
therefore lead to an increase in the maximum operating temperatures of 
IV-VI semiconductor lasers. The major factor limiting heat dissipation 
from the active region is the substrate which is still attached to the 
laser structure. 
A substrate removal procedure has been developed for III-V semiconductor 
laser fabrication by E. Yablonovitch, E. Kapon, T. J. Gmitter, C. P. Yun, 
and R. Bhat, in "Double Heterostructure GaAs/AlGaAs Thin Film Diode Lasers 
on Glass Substrates", IEEE Photonics Tech. Lett., 1989, 1, p. 41. 
According to this technique, an AlGaAs/GaAs/AlGaAs laser structure is 
grown upon a GaAs substrate with a 500 .ANG. AlAs selectively etchable 
release layer interposed between the substrate and laser structure. The 
laser structure is then supported from above by Apiezon W.TM. wax ("black 
wax") while the AlAs layer is etched with dilute HF. This "epitaxial 
lift-off" (ELO) process, which was developed to enable hybrid device 
packaging, does not degrade the performance of the laser device. Other 
examples of methods of epitaxial liftoff are seen in F. Agahi, K. M. Lau, 
A. Baliga, D. Loeber, and N. Anderson, "Photo-Pumped Strained-Barrier 
Quantum Well Lasers Fabricated by Epitaxial Liftoff", ISDRS, University of 
Virginia, Charlottesville, Va. (1993); C. Camperi-Ginestet, M. Hargis, N. 
Jokerst and M. Allen, "Alignable Epitaxial Liftoff of GaAs Materials with 
Selective Deposition Using Polyimide Diaphragms", IEEE Transactions 
Photonic Tech. Lett., Vol. 3, No. 12, pgs. 1123-1126, December 1991; and 
E. Yablonovitch, E. Kapon, T. J. Gmitter, C. P. Yun and R. Bhat, "Double 
Heterostructure GaAs/AlGaAs Thin Film Diode Lasers on Glass Substrates, 
IEEE Photonic Tech. Lett., Vol. 1, No. 2, pgs. 41-42, February 1989. 
However, epitaxial-lift off methods using wax are difficult, 
time-consuming, require extra handling steps and are not readily adaptable 
to mass production techniques. A method for producing a laser not confined 
to the limitations of epitaxial lift-off using the wax method and which 
resulted in the production of a laser operable at higher temperatures 
would be desirable.

DESCRIPTION OF THE INVENTION 
An objective of the present invention is to provide a laser having 
increased heat dissipation from the lasing region, thus allowing higher 
operating temperatures, and thereby optimizing cooling requirements. It is 
projected that by improving heat removal from the active region of the 
laser, the IV-VI semiconductor TDL maximum cw operating temperatures can 
be increased by more than 50 K, thereby placing a considerable portion of 
the TDL operating temperature range within the effective range of 
thermoelectric cooling devices. 
There has been a considerable need for improving heat dissipation from the 
active region of TDL devices due to the fact that IV-VI semiconductors 
have extremely low thermal conductivities, (see Table I). 
TABLE I 
______________________________________ 
Thermal conductivities (watt/cm K.) of various materials at 300 K. 
PbSe PbTe BaF.sub.2 
GaAs Silicon 
Copper 
CVD Diamond 
______________________________________ 
0.018 
0.023 0.329 0.60 1.412 5.0 &gt;10.0 
______________________________________ 
All previous IV-VI semiconductor TDLs have been packaged with relatively 
thick (.about.250 .mu.m) PbTe substrates still attached to the device 
structure. Since PbTe is 217 times less thermally conductive than copper, 
heat is effectively removed from only one side of the device (the 
non-substrate side). By eliminating the substrate entirely and placing a 
second heat sink within microns of the active region, as is shown in the 
present invention, heat dissipation is effectively doubled. Thermal 
modeling results show that maximum TDL operating temperatures increase by 
more than 60 degrees when this second copper heat sink is used, a 
realistic value considering the work of R. Rosman, A. Katzir, P. Norton, 
K. H. Bachem, and H. Preier, "On the Performance of Selenium Rich 
Lead-Salt Heterostructure Laser with Remote p-n Junction", IEEE J. Quantum 
Electronics, 1987, QE-23, p.94. Use of the method of TDL fabrication 
procedure shown herein should increase maximum cw operating temperatures 
to greater than 260 K. 
The present invention, therefore, comprises a method for producing a 
tunable semiconductor diode laser using a IV-VI semiconductor material 
wherein the growth substrate has been removed (III-V and II-VI 
semiconductor materials may also be used). The overall process is 
described briefly in the flow chart shown in FIG. 1. In Step I, a cleaving 
assembly and a semiconductor epitaxial layer (or layers) which has been 
grown on an appropriate substrate by methods well known to one of ordinary 
skill in the art is provided. The exposed surface of the semiconductor 
epitaxial layer is then bonded to a prepared surface of the cleaving 
assembly, which is composed of a plurality of connected plates (Step II). 
The substrate is then removed, leaving only the semiconductor epitaxial 
layers bonded to the cleaving assembly (Step III). The single plates of 
the cleaving assembly are separated individually, causing the epitaxial 
layers to cleave along parallel crystallographic planes such as {100} or 
{110} planes (Step IV). Individual laser bars are then cut, if necessary, 
from each plate (Step V). The laser bar is then mounted on a cold head to 
form a packaged tunable diode laser (Step VI). 
More particularly, the present invention comprises a method of producing a 
semiconductor laser device. The first step comprises providing a 
semiconductor epitaxial layer grown on a substrate. Next, a cleaving 
assembly having at least one cleaving portion is provided. The 
semiconductor epitaxial layer is bonded to the cleaving assembly wherein 
at least a portion of the semiconductor epitaxial layer is bonded to the 
cleaving portion or plate of the cleaving means and an adjacent portion of 
the semiconductor epitaxial layer is bonded to an adjacent portion of the 
cleaving assembly (which may be an adjacent cleaving portion or plate) 
which is adjacent the cleaving portion. Following this, the substrate is 
removed from the semiconductor epitaxial layer. Finally, the cleaving 
portion or plate is separated from the cleaving assembly wherein the 
portion of the semiconductor epitaxial layer bonded to said cleaving 
portion is cleaved from the adjacent portion of the semiconductor 
epitaxial layer. These steps may be repeated for additional cleaving 
portions or plates which may comprise the cleaving assembly. These steps 
form Fabry-Perot laser cavities in the semiconductor epitaxial layer 
resulting in a semiconductor laser device comprising a heat conductive 
portion derived from the cleaving portion and a laser portion derived from 
the epitaxial layer. 
In a preferred version of the invention, the method has an initial step of 
providing a semiconductor assembly having an epitaxial layer grown on a 
substrate. The epitaxial layer has an upper surface with an epitaxial 
bonding layer disposed thereon. A cleaving means, more particularly a 
cleaving jig, comprising a plurality of detachably connected plates is 
provided. Each plate has an end surface, which end surfaces cooperate to 
form a cleaving jig bonding surface which has a cleaving jig bonding layer 
disposed thereon. 
In the next step, a cleaving jig-semiconductor assembly is formed by 
bonding the epitaxial bonding layer of the semiconductor assembly to the 
bonding layer of the cleaving jig wherein is formed a bonding layer 
between the cleaving jig bonding surface and the upper surface of the 
epitaxial layer. Following this, the substrate is removed from the 
cleaving jig-semiconductor assembly. Finally, a plate is disconnected from 
the cleaving jig-semiconductor assembly wherein the epitaxial layer bonded 
to the plate is cleaved from a portion of the epitaxial layer bonded to an 
adjacent plate. This disconnection step forms a semiconductor laser cavity 
(Fabry-Perot) device comprising a plate portion which is heat conductive 
and a semiconductor portion. 
This process may be followed by the additional step of cutting the 
semiconductor laser device into a plurality of laser bars wherein each 
laser bar comprises a plate portion and a semiconductor portion. Further, 
in the process, the step of providing a semiconductor assembly may be 
preceded by the additional step of applying the epitaxial bonding layer to 
the upper surface of the epitaxial layer. Also, the step of providing a 
cleaving jig may be preceded by the additional step of applying the 
cleaving jig bonding layer to the cleaving jig bonding surface. The plates 
may be further defined as being made from metal or from diamond. 
Additionally, an isolating layer for electrically isolating the laser bar 
may be applied to a portion of the plate portion of the laser bar. 
Finally, the laser bar thus electrically isolated may be disposed upon a 
cold finger whereby the isolating layer of the laser bar is disposed upon 
a mounting surface of the cold finger and the semiconductor portion of the 
laser bar is disposed adjacent a laser facing surface of the cold finger. 
A second cold finger, which also may be comprised of metal or diamond, may 
be disposed upon a portion of the laser bar for further enhancing heat 
dissipation. 
The epitaxial layer may be a IV-VI, III-V or II-VI semiconductor. Further, 
the semiconductor assembly may further comprise a release layer positioned 
between the epitaxial layer and the substrate. Further, the laser bar may 
be further defined as having a semiconductor portion tunable within a 
range of from about 3 to about 30 micrometers. Further, the epitaxial 
layer of the semiconductor assembly may further comprise at least one 
single mode waveguide region. The plate of the cleaving jig may be further 
defined as having a thickness in a range of from about 100 to about 1000 
micrometers. 
The semiconductor laser assembly produced by the method of the present 
invention comprises (1) a cold finger having a mounting surface, and (2) a 
semiconductor laser having (a) a heat conductive portion having an end 
surface and a mounting surface, and (b) a semiconductor epitaxial layer 
having a bonding surface and an exposed end surface. In the semiconductor 
laser, the bonding surface of the epitaxial layer is bonded to the end 
surface of the heat conductive portion. The semiconductor laser is mounted 
upon the cold finger by engaging the mounting surface of the semiconductor 
laser with the mounting surface of the cold finger. The heat conductive 
portion of the semiconductor laser in one version of the invention is 
metal. The heat conductive portion of the semiconductor laser in another 
version of the invention is diamond. Further in a preferred version of the 
invention, the cold finger is comprised of metal. 
The semiconductor laser assembly may further comprise an isolating layer 
disposed between the mounting surface of the semiconductor laser and the 
cold finger which serves to electrically isolate the mounting surface of 
the semiconductor laser from the cold finger. The assembly may further 
comprise a first electrical lead attached to a portion of the cold finger 
and a second electrical lead attached to the heat conductive portion of 
the semiconductor laser. The cold finger may further comprise a laser 
facing surface adjacent or against which the exposed end surface of the 
semiconductor epitaxial layer is disposed. 
The semiconductor laser assembly may further comprise a second cold finger 
disposed upon a second contact surface of the heat conductive portion of 
the semiconductor laser. Portions of the second cold finger which are 
adjacent the first cold finger are electrically isolated from the first 
cold finger. This embodiment of the semiconductor laser assembly may 
further comprise a first electrical lead and a second electrical lead. In 
such case, each lead is attached to a separately isolated electrically 
conductive portion of the semiconductor laser assembly. At least one 
version of the semiconductor laser assembly described above may further 
comprise an optical pumping means for inducing emission of laser light. 
Any of the versions of the semiconductor laser device described herein may 
be installed in a spectrometer equipped to receive such a device. 
The cleaving means used in a preferred version of the present invention is 
a cleaving jig comprising a plurality of metal plates. Each metal plate 
has a first facing surface, a second facing surface, a first side surface, 
a second side surface, an upper end surface and a lower end surface. The 
plurality of plates are positioned against each other such that each plate 
has at least one facing surface disposed against a facing surface of an 
adjacent plate. The lower end surfaces of the plurality of plates are all 
oriented in the same plane. The cleaving jig further comprises a 
connecting assembly which detachably connects the plurality of adjacently 
disposed plates into a plate assembly having a first end and a second end 
and wherein the lower end surfaces of the plates cooperate to form a 
bonding surface for bondingly connecting a semiconductor epitaxial layer 
to the plate assembly. 
The connecting assembly may further comprise at least a first screw with 
screw threads and a second screw with screw threads. Each plate further 
comprises at least a first hole and a second hole through which the first 
screw and second screw are driven for connecting the plates and forming 
the plate assembly. 
The cleaving jig may further comprise a first end block plate positioned at 
the first end of the plate assembly and a second end block plate 
positioned at the second end of the plate assembly. The second end block 
plate has a pair of holes with screw threads for matingly engaging the 
screw threads of the first screw and the second screw when the first screw 
and second screw are inserted through the first holes and second holes of 
the plates. The cleaving jig may further comprise a metallic bonding layer 
disposed upon the bonding surface. 
A more detailed description of the process of fabricating the apparatus is 
provided below and in FIGS. 2-20. 
Laser Fabrication 
Shown in FIGS. 2 and 3 is a plurality of metal plates 10 which comprise 
subunits of a cleaving jig. Each metal plate 10, or cleaving portion, 
comprises an upper end 12, a lower end 14, a front surface 16, a rear 
surface 18, an upper side 20, a lower side 22, a first lateral side 24 and 
a second lateral side 26. Each plate 10 further has a height 28 which 
extends from the upper end 12 to the lower end 14, a width 30 which 
extends from the first lateral side 24 to the second lateral side 26 and a 
thickness 32 which extends from the front surface 16 to the rear surface 
18. Each plate 10 further comprises a first hole 34a extending from the 
front surface 16 to the rear surface 18 and a second hole 34b in a 
different position in the plate 10 parallel to the first hole 34a. A first 
connector 36a and a second connector 36b are inserted through the holes 
34a and 34b, respectively, of the plurality of plates 10, thereby forming 
the plates 10 into a cleaving jig 38. Together the upper sides 20 of the 
plates 10 of the jig 38 form an upper surface 40 of the jig 38. Each plate 
10 is made from a high conductivity metal, preferably an oxygen-free high 
conductivity copper. As noted above, the plates 10 could be made from high 
thermal conductivity diamond, therefore, it will be understood that where 
used herein, the term metal can be substituted for by the word diamond. 
The thickness 32 of the plate 10 ultimately determines the length of the 
cavity of the laser. The thickness 32 is generally from about 100 to 1000 
micrometers and more preferably from 300 to 1000 micrometers. 
Shown in FIGS. 4A and 4B and represented by the general reference numeral 
38a is an alternative version of the cleaving jig of the present 
invention. The jig 38a is the same as the jig 38 except that the plates 10 
are sandwiched between a first end block 42a and a second end block 42b. 
First end block 42a has holes 34a and 34b which are counter bored and 
second end block 42b has holes 34a and 34b which are threaded for threadly 
engaging the threads of the connecting screws 36a and 36b. The heads of 
the connecting screws 36a and 36b can be recessed within the counter bored 
holes 34a and 34b of the first end block 42a. Shown in FIG. 4B is the 
completely assembled cleaving jig 38a. The jig 38a can be held in a 
milling device to allow milling of the edges of the plates to create a 
flush and uniform assembly of plates 10. 
Shown in FIG. 5A is a semiconductor-substrate assembly 46 comprising a 
semiconductor epitaxial layer 48, which may further comprise a plurality 
of layers such as layers 48a, 48b and 48c as shown in FIG. 5B and as is 
well known to one of ordinary skill in the art. The layer 48 has a 
thickness 50 which preferably is from about 3 to 5 micrometers. The 
semiconductor-substrate assembly 46 further comprises a substrate 52 which 
in one version is silicon and which has a thickness 54, and a release 
layer 56 which in one embodiment is BaF.sub.2 and which is disposed 
between the epitaxial layer 48 and the substrate 52 and which has a 
thickness 58. The semiconductor-substrate assembly 46 also has an upper 
exposed surface 60 which is the outermost layer of the epitaxial layer 48. 
Shown in FIG. 6 is a cleaving jig 38 to which a bonding layer 62 has been 
applied by standard thermal evaporation techniques over the jig surface 
layer 40. The jig bonding layer 62 has a thickness 64 and an upper surface 
layer 66. Shown in FIG. 7 is a semiconductor-substrate assembly 46 to 
which a semiconductor bonding layer 68 has been applied by standard 
thermal evaporation techniques over the upper surface layer 60. The 
bonding layer 68 has a thickness 70 and an upper surface layer 72. Shown 
in FIG. 8 is a jig-semiconductor-substrate assembly 74 comprising a jig 
assembly 38 which is bonded to a semiconductor-substrate assembly 46 via a 
jig-semiconductor bonding layer 76 located between the upper surface 42 of 
the cleaving jig 38 and the upper surface 60 of the 
semiconductor-substrate 46. The jig 38 and semiconductor substrate 
assembly 46 are bonded to one another at a temperature of generally 
greater than about 235.degree. C. The resulting jig-semiconductor bonding 
layer 76 has a thickness 78. 
In the process of the present invention, the bonding layers 66 and 72 will 
preferably use a gold/tin eutectic bonding medium each having a thickness 
of about 1/2 micrometer, such as that described in: G. S. Matijasevic, C. 
C. Lee, C. Y. Lee, "Au-Sn Alloy Phase Diagram and Properties Related to 
Its Use as a Bonding Medium", Thin Solid Films, 1993, 223, p. 276 which is 
hereby incorporated herein by reference. Preferably, the thickness 78 of 
the bonding layer 76 is about one micrometer or less to allow cleavage of 
the bonding layer 76. It will be understood by one of ordinary skill in 
the art that the bonding layers 66 and 72 may each be comprised of a 
plurality of layers of bonding material. 
Shown in FIG. 9 is a jig-semiconductor assembly 80 which is obtained when 
the substrate 52 is removed from the jig-semiconductor substrate assembly 
74 by etching away or dissolving the release layer 56 in a manner known to 
one of ordinary skill in the art. The jig-semiconductor assembly 80 
comprises the cleaving jig 38 which is bonded via the bonding layer 76 to 
the epitaxial layer 48. Also represented in FIG. 9 by dashed lines 
extending below the cleaving plates are a plurality of cleavage planes 82 
which extend substantially perpendicularly through the epitaxial layer 48. 
After removal of the substrate 52 to form the jig-semiconductor assembly 
80, the connectors 36a and 36b are removed, then each plate 10 can be 
detached from the adjacent plate 10 of the assembly 80 by sliding the 
plate 10 such that the epitaxial layer 48 is cleaved along {100} or {110} 
cleavage planes 82 to form a plate-semiconductor assembly 84 represented 
in FIG. 10. The plate-semiconductor assembly 84 has a height 86 which 
extends from the exposed lower surface 87 of the epitaxial layer 48, 
through the bonding layer 76 to the lower side 22 of the plate 10. 
Shown in FIGS. 11 and 12 is a laser bar 88 which has been cut from the 
plate-semiconductor assembly 84 by cutting from the lower side 22 of the 
plate 10 through the plate 10 through the epitaxial layer 48. The laser 
bar 88 thus has a plate portion or metal finger portion 89 and a 
semiconductor laser portion 90. The semiconductor laser portion 90 may be 
comprised of a plurality of layers shown in FIGS. 10-11 as layers 90a, 90b 
and 90c. It will be understood by one of ordinary skill in the art that 
the semiconductor laser portion 90 may comprise more or fewer than the 
three layers 90a, 90b and 90c shown in FIGS. 11 and 12. The laser bar 88 
has a cavity length 91 which is preferably in the range of from 100 to 
1000 micrometers, more preferably between 300 and 1000 micrometers, a 
height 92 which may be from about 500 micrometers to as much as 1 
centimeter, and an overall length 93. The metal finger portion 89 of the 
laser bar 88 has an end 94 with an end surface 96. The semiconductor laser 
portion 90 has an end 98. The laser bar 88 has a front surface 100, a rear 
surface (not shown), a first side surface 102 and a second side surface 
104. In FIG. 12, the laser bar 88 is shown with an isolating layer 106 
which has been disposed upon a portion thereof, preferably the first side 
surface 102. The isolating layer 106 isolates the metal finger portion 89 
of the laser bar 88 enabling it to serve as an electrode. The isolating 
layer 106 could be either a thin plate of thermally conductive sapphire or 
a wafer of chemically vapor deposited (CVD) diamond or other suitable 
electrically isolating material. CVD diamond wafers are commercially 
available, for example, from Harris Diamond Corp., Arlington, N. J. or 
Norton Diamond Film, Northboro, Mass. (see G. Lu and E. F. Borchelt, "CVD 
Diamond Boosts Performance of Laser Diodes", Photonics Spectra, September 
1993, p. 88). With thermal conductivities more than twice that of copper, 
CVD diamond will assist in the dissipation of heat from the laser active 
region. 
After the isolating layer 106 has been applied, the laser bar 88 is 
disposed upon a metallic electrode 108 of a metal cold finger 110 as shown 
in FIG. 13 in a manner well known in the art resulting in a laser assembly 
generally referred to by reference numeral 112. The laser assembly 112 may 
have electrical leads 114 and 116 connected to the metal finger 89 and to 
the metal electrode 108. Moreover, the metal electrode 108 and the cold 
finger 110 may be integral to each other forming in essence a unitary 
electrode. The laser assembly 112 can then be installed in an instrument 
for use in spectroscopic applications known generally in the art such as 
those cited above and in the article by R. S. Eng, J. F. Butler and K. J. 
Linden; "Tunable Diode Laser Spectroscopy: An Invited Review", Optical 
Engineering, Vol. 19, No. 6, pgs. 945-960, 1980. For example, the laser 
assembly 112 could be installed in a tunable diode laser system for high 
resolution spectroscopy (FIG. 1.6 in Eng et al. cited above). 
An additional metal heat sink may be disposed over the second side surface 
104 to further increase the heat dissipation from the lasing region as 
further explained below. 
Shown in FIG. 14, and referred to generally by the reference numeral 120 is 
a dual heat sink laser assembly. The dual heat sink laser assembly 120 is 
exactly the same as the laser-cold finger assembly 112 described above 
except that it further comprises a second isolating layer 122, a second 
metallic electrode 124, and a second cold finger 126. The second isolating 
layer 122 is disposed between the semiconductor laser portion 90 and the 
first metallic electrode 108. The second metallic electrode 124 is 
disposed upon a portion of the laser bar 88 to form an electrical contact 
therewith. The second cold finger 126 is in turn disposed upon the second 
metallic electrode 124 generally as indicated in FIG. 14. The arrow 128 
generally indicates the direction of light emission from the laser diode 
120. 
Shown in FIG. 15 and referred to generally by the reference numeral 120a is 
an alternate version of a dual heat sink laser assembly. The laser 
assembly 120a is exactly the same as the laser assembly 120 except for the 
orientation of the isolating layers which separate the two electrode 
portions of the laser assembly. In this version, a laser bar 88a having a 
semiconductor portion 90a has an isolating layer 106a disposed upon an 
upper surface thereof. The lower surface of the laser bar 88a is 
contactingly disposed upon a mounting surface of a metallic electrode 108a 
which is itself mounted upon a metal cold finger 110a. A second isolating 
layer 122a separates the metallic electrode 108a from a second metallic 
electrode 124a which is disposed upon the laser bar 88a in a position such 
that the isolating layer 106a separates the metallic portion of the laser 
bar 88a from the second metallic electrode 124a. A second metal cold 
finger 126a may be disposed upon the second metallic electrode 124a. 
It will be understood by one of ordinary skill in the art that the cold 
fingers, metallic electrodes, and isolating layers may be arranged in a 
variety of configurations which differ from the configuration shown in 
FIGS. 14 and 15. It will be understood by one of ordinary skill in the art 
that the metallic electrodes 108 and 108a and the cold fingers 110 and 
110a may comprise a single functional unit such that each cold finger 110 
or 110a itself may comprise the electrode 108 or 108a, respectively. This 
is also true for the second cold fingers 126 and 126a and second metallic 
electrodes 124 and 124a. In such cases the terms metallic electrode and 
cold finger may be used interchangeably and either may be generally 
referred to as a "metal electrode". 
The semiconductor diode lasers may also be driven by optical pumping in a 
manner well known to one of ordinary skill in the art. Optically pumped 
devices have the advantage of needing neither p-n junctions nor electrical 
contacts. Since electric current is not driven through the device, lasing 
can be achieved at higher operating temperatures. High power near-infrared 
III-V semiconductor diode lasers or even LEDs may be used as pump sources 
for IV-VI semiconductor lasers, for example. 
Semiconductor/Substrate Assemblies 
By way of further explanation, an example of a IV-VI 
semiconductor/substrate assembly 46 is the Pb.sub.1-x Sn.sub.x Se.sub.1-y 
Te.sub.y system. This pseudobinary alloy can be used as the active region 
in lasers designed to operate in the 6 to 30 .mu.m spectral range. Most 
IV-VI semiconductor DH and BH lasers have been fabricated with 
PbSe.sub.1-y Te.sub.y ternary and Pb.sub.1-x Sn.sub.x Se.sub.1-y Te.sub.y 
quaternary layers lattice matched with Pb.sub.1-x Sn.sub.x Te substrates 
(Y. Horikoshi, M. Kawashima, and H. Saito, "PbSnSeTe-PbSeTe 
Lattice-Matched Double Heterostructure Lasers", Japanese J. Appl. Phys., 
1982, 21, p. 77; and A. Shahar and A. Zussman, "PbSnTe-PbTeSe Lattice 
Matched Single Heterostructure Diode Lasers Grown by LPE on a (111) 
Oriented PbSnTe Substrates", Infrared Physics, 1987, 27, p. 45). 
Work has also been done with Pb.sub.1-x Sn.sub.x Se.sub.1-y Te.sub.y 
quaternary layers lattice matched with PbSe substrates (P. J. McCann, J. 
Fuchs, Z Feit, and C. G. Fonstad, "Phase Equilibria and Liquid Phase 
Epitaxy Growth of PbSnSeTe Lattice Matched to PbSe", J. Appl. Phys., 1987, 
62, p. 2994; and H. Preier, A. Feit, J. Fuchs, D. Kostyk, W. Jalenak, and 
J. Sproul, "Status of Lead Salt Laser Development at Spectra-Physics", 
presented at the Second International Symposium on Monitoring of Gaseous 
Pollutants by Tunable Diode Lasers, November, 1988). 
Such epitaxial structures, when grown on Pb.sub.1-x Sn.sub.x Te or PbSe 
substrates, are usually fabricated into laser devices by cleaving the 
entire layer/substrate structure along {100} planes to form Fabry-Perot 
cavities. 
Another example of a semiconductor substrate assembly 46 consists of three 
LPE layers grown on a BaF.sub.2 substrate. An example of a method of 
growing an epitaxial layer on BaF.sub.2 is provided in U.S. Ser. No. 
07/367,459, entitled "A Chemical Method for the Modification of Substrate 
Surface to Accomplish Heteroepitaxial Crystal Growth", by P. J. McCann and 
C. G. Fonstad, filed Jun. 16, 1989, and which is hereby incorporated 
herein by reference. 
Cleavage of the entire epitaxial layer/substrate structure along {100} 
planes would be difficult with a IV-VI semiconductor/BaF.sub.2 structure 
since BaF.sub.2 tends to cleave along {111} planes. In the present 
invention, this problem is solved for BaF.sub.2 substrate-grown epitaxial 
layers by removing the BaF.sub.2 substrate before cleaving the laser 
structure. Removing the BaF.sub.2 substrate without chemically attacking 
the laser structure is possible since BaF.sub.2 is soluble in water while 
IV-VI semiconductor epitaxial layers are not. An acid solution, preferably 
a 1:3 HCl:H.sub.2 O solution, which dissolves BaF.sub.2 30 times faster 
than water alone, could also be used to accelerate substrate removal. 
Besides having a compatible crystal structure and lattice matching 
capabilities, BaF.sub.2 is an attractive substrate material for IV-VI 
semiconductor epitaxy and device development because its thermal expansion 
coefficient is nearly the same as that of the IV-VI semiconductors. 
Although BaF.sub.2 substrates have been used for IV-VI semiconductor vapor 
phase epitaxy since 1970 [19](H. Holloway and E. M. Logothesis, "Epitaxial 
Growth of Lead Tin Telluride", J. Appl. Phys., 1970, 41, p. 3543), LPE 
growth of IV-VI semiconductors on BaF.sub.2 substrates has only recently 
been accomplished. 
Heat dissipation could be also be enhanced by placing a thermoelectric 
cooler near the device in a manner well known in the art. The thermal 
properties of such a device are considered optimal for enabling high 
temperature TDL operation. Thermoelectric coolers are commercially 
available, for example, the "Frigichip" is available from Melcor, Inc., 
Trenton, N.J. Another type of TEC is available from Marlow Industries, 
Inc., Dallas, Tex. 
The fabrication procedure outlined in FIGS. 2-15 yields broad area lasers, 
but can be modified to yield single mode lasers, desired for most infrared 
spectroscopy applications. 
FIGS. 16-20 show the production of a semiconductor laser device having an 
array of waveguides for high power single-mode lasers. FIG. 16 shows a 
IV-VI semiconductor epitaxial layer 130 bonded via a BaF.sub.2 buffer 
layer 132 to a substrate (e.g., silicon) 134. An array of photoresist 
material 136 is disposed upon the upper exposed surface of the epitaxial 
layers 130. The epitaxial-substrate-photoresist assembly 138 is subjected 
to a treatment such as proton bombardment which induces in the epitaxial 
layers regions 140 each having lower indexes of refraction than the 
unaffected portions of the epitaxial layer 130. Areas of the epitaxial 
layer 130 protected from proton bombardment by the photoresist material 
136 serve as the single-mode waveguide regions 142 of the laser in a 
manner well known to one of ordinary skill in the art. 
FIG. 17 shows an epitaxial-substrate-photoresist assembly 138 upon which a 
layer 144 of BaF.sub.2 has been evaporated. The BaF.sub.2 is applied to 
the assembly 138 to cover the lower index of refraction regions 140, as 
indicated in FIG. 18 wherein is shown an epitaxial-substrate assembly 146 
after the photoresist material 136 has been removed. Shown in FIG. 19 is 
an epitaxial-substrate assembly 146 upon which a bonding layer 148 has 
been applied. The bonding layer 148 may be exactly the same as the bonding 
layer 68 described above in FIG. 7. 
The epitaxial layer-substrate assembly 146 having the bonding layer 148 is 
then ready to be bonded to a cleaving jig for producing a semiconductor 
laser in a manner exactly as described above in the steps corresponding to 
FIGS. 8-13. 
The layers 144 which remain on the epitaxial substrate assembly 146 shown 
in FIG. 17 can be protected from dissolution during the substrate removal 
step by bordering the exposed BaF.sub.2 regions with a layer of 
photoresist material (not shown) such that the BaF.sub.2 stripes do not 
extend all the way to the edge of the epitaxial layer. 
Shown in FIG. 20 is a laser bar 150 comprising a metal finger portion 152 
and a single-mode waveguide semiconductor laser portion 154. The laser bar 
150 can then be used in exactly the same way as the laser bar 88 described 
above. 
Laser fabrication from molecular beam epitaxy (MBE) -grown material on 
silicon substrates can be accomplished using the same epitaxial layer 
lift-off procedures that have been developed for III-V laser fabrication. 
Instead of AlAs, though, the BaF.sub.2 buffer layer will function as the 
release layer. After eliminating the silicon substrate, the same procedure 
as outlined in above FIGS. 8-12 can then be used to fabricate laser 
devices. Since there is no thick PbTe substrate, such devices will exhibit 
the same superior active region heat dissipation as LPE-grown lasers 
fabricated using the BaF.sub.2 substrate removal technique. Because MBE 
technology can produce larger bandgap Pb.sub.1-x Eu.sub.x Se.sub.1-y 
Te.sub.y alloys and has greater growth flexibility than LPE it offers the 
best promise for fabrication of TDLs that operate near room temperature. 
MBE growth combined with focused ion beam etching, through the use of a 
portable load-lock vacuum chamber, will also allow the fabrication of 
advanced laser devices such as high power buried heterostructure arrays. 
Each of the patents, pending patent applications, and publications cited 
herein is hereby incorporated herein by reference. 
Changes may be made in the construction and the operation of the various 
components, elements and assemblies described herein or in the steps or 
the sequence of steps of the methods described herein without departing 
from the spirit and scope of the invention as defined in the following 
claims.