U.S. patent application number 10/463067 was filed with the patent office on 2003-11-20 for alternative substrates for epitaxial growth.
This patent application is currently assigned to APPLIED OPTOELECTRONICS, INC.. Invention is credited to Hwang, Wen-Yen.
Application Number | 20030213950 10/463067 |
Document ID | / |
Family ID | 29423157 |
Filed Date | 2003-11-20 |
United States Patent
Application |
20030213950 |
Kind Code |
A1 |
Hwang, Wen-Yen |
November 20, 2003 |
Alternative substrates for epitaxial growth
Abstract
A substrate including a base substrate, an interfacial bonding
layer disposed on the base substrate, and a thin film adaptive
crystalline layer disposed on the interfacial bonding layer. The
interfacial bonding layer is solid at room temperature, and is in
liquid-like form when heated to a temperature above room
temperature. The interfacial bonding layer may be heated during
epitaxial growth of a target material system grown on the thin film
layer to provide the thin film layer with lattice flexibility to
adapt to the different lattice constant of the target material
system. Alternatively, the thin film layer is originally a strained
layer having a strained lattice constant different from that of the
target material system but with a relaxed lattice constant very
close to that of the target material system, which lattice constant
is relaxed to its relaxed value by heating the interfacial bonding
layer after the thin film layer is removed from the first
semiconductor substrate, so that the thin film layer has an
adjusted lattice constant equal to its unstrained, relaxed value
and very close to the lattice constant of the target material
system.
Inventors: |
Hwang, Wen-Yen; (Sugar Land,
TX) |
Correspondence
Address: |
APPLIED OPTOELECTRONICS, INC.
13111 JESS PIRTLE BLVD.
SUGAR LAND
TX
77478
US
|
Assignee: |
APPLIED OPTOELECTRONICS,
INC.
|
Family ID: |
29423157 |
Appl. No.: |
10/463067 |
Filed: |
June 17, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10463067 |
Jun 17, 2003 |
|
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09820072 |
Mar 28, 2001 |
|
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60208115 |
May 31, 2000 |
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Current U.S.
Class: |
257/18 ;
257/E21.119; 257/E21.569; 257/E33.069 |
Current CPC
Class: |
Y10T 428/12528 20150115;
Y10T 428/12674 20150115; H01L 33/0093 20200501; H01S 5/183
20130101; H01L 33/105 20130101; Y10T 428/12493 20150115; H01S
2301/173 20130101; H01L 21/76256 20130101; H01S 5/0216 20130101;
Y10T 428/31678 20150401; H01S 5/0234 20210101; H01S 5/0217
20130101 |
Class at
Publication: |
257/18 |
International
Class: |
H01L 029/06 |
Claims
What is claimed is:
1. A substrate comprising: a base substrate layer; and a
relaxed-strained thin film adaptive crystalline layer bonded to the
base substrate layer and having a surface in-plane lattice constant
different from that of the base substrate layer and close to that
of a target material system.
2. The substrate of claim 1, wherein the in-plane lattice constant
is in the same range as that of
In.sub.x(Al.sub.yGa.sub.1-y).sub.1-xAs wherein x is approximately
15% to approximately 45%.
3. The substrate of claim 1, wherein the substrate comprises a
substrate for formation of a vertical cavity surface emitting laser
based on In.sub.x(Al.sub.yGa.sub.1-y).sub.1-xAs.
4. The substrate of claim 3, wherein x is approximately 15% to
approximately 45%.
5. The substrate of claim 1, wherein the thin film adaptive
crystalline layer comprises InGaAs having an In composition between
approximately 15% and approximately 45%.
6. The substrate of claim 1, wherein the base substrate comprises
GaAs, and the thin film adaptive crystalline layer comprises
In.sub.x(Al.sub.yGa.sub.1-y).sub.1-xAs.
7. The substrate of claim 6, wherein x is approximately 15% to
approximately 45%.
8. The substrate of claim 1, wherein the thin film adaptive
crystalline layer comprises a semiconductor.
9. The substrate of claim 8, wherein the semiconductor comprises
InGaAsP, GaSb, InGaAs, InGaP, AlGaP, InSb, InP, AlSb, or InAs.
10. The substrate of claim 1, wherein the base substrate layer
comprises semiconductor, an inorganic material, a metal, or a
combination thereof.
11. The substrate of claim 10, wherein the semiconductor comprises
GaAs, InP, GaP, Si, or Ge.
12. The substrate of claim 10, wherein the inorganic material
comprises sapphire, poly-crystalline boron nitride, or
ceramics.
13. The substrate of claim 10, wherein the relaxed-strained thin
film adaptive crystalline layer is fabricated having a strained
lattice constant equal to a lattice constant of a first
semiconductor substrate on which it is grown, which strained
lattice constant has been adjusted by bonding the thin film
adaptive layer to a carrier substrate via an interfacial bonding
layer and removing the first substrate, and then heating the
interfacial bonding layer to liquidize the interfacial bonding
layer to allow the strained thin film adaptive layer to relax to
its unstrained lattice structure to form the relaxed-strained thin
film adaptive layer, and then bonding the thin film layer to the
base substrate and removing the carrier substrate to expose an
epitaxial growth surface of the thin film adaptive layer.
14. A method of forming a substrate for formation of semiconductor
devices, comprising: forming a strained pseudomorphic thin film
adaptive layer on a first substrate; bonding a first surface of the
thin film adaptive layer to a carrier substrate with an interfacial
bonding layer; removing the first substrate by selective etching or
lift off and leaving the thin film adaptive layer; and liquidizing
the interfacial bonding layer to allow the strained pseudomorphic
thin film adaptive layer to relax its unstrained lattice
structure.
15. The method of claim 14, further comprising the steps of:
bonding the surface of the thin film adaptive layer to a second
substrate; and removing the carrier substrate to expose a second
surface of the thin film adaptive layer.
16. The method of claim 15, further comprising treating the surface
of the second substrate prior to the bonding.
17. The method of claim 14, wherein the liquidizing comprises
heating the interfacial bonding layer.
18. A substrate produced in accordance with the method of claim
14.
19. An optoelectronic apparatus, comprising: a substrate
comprising: a thin film adaptive crystalline layer; and a base
substrate layer, the thin film adaptive crystalline layer bonded to
the base substrate layer and having a surface in-plane lattice
constant, and wherein the in-plane lattice constant is different
from that of the base substrate layer, wherein the thin film
adaptive crystalline layer comprises a strained pseudomorphic thin
film grown on a first semiconductor substrate that is bonded to and
transferred to a second carrier substrate, wherein the first
semiconductor substrate is removed and the in-plane lattice
constant relaxes from an original strained value to a new value
close to an unstrained lattice constant, and wherein the thin film
adaptive crystalline layer is physically or chemically bonded to
the base substrate layer with or without an interfacial bonding
layer; and an optoelectronic device epitaxially grown on the thin
film adaptive crystalline layer.
20. The optoelectronic apparatus of claim 19, wherein the
optoelectronic device is a semiconductor laser.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 09/820,072, filed Mar. 28, 2001, which claims,
under 37 C.F.R. .sctn. 1.78(a)(3), the benefit of the filing date
of provisional U.S. national application No. 60/208,115, entitled
"Fabrication of Vertical Cavity Surface Emitting Lasers Using
Alternative Substrates," by Wen-Yen Hwang, filed May 31, 2000 under
35 U.S.C. .sctn. 111(b), which are both incorporated herein in
their entireties by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention is related to substrates for epilayer
epitaxial growth in which the epilayers are lattice mismatched to
the substrate and, in particular, to alternative substrates for
fabrication of electronic and optoelectronic devices, such as
semiconductor diode lasers, for example vertical-cavity
surface-emitting lasers (VCSELs).
[0004] 2. Description of the Related Art
[0005] The following descriptions and examples are not admitted to
be prior art by virtue of their inclusion within this section.
[0006] Lasers, such as semiconductor diode lasers, have a wide
range of industrial and scientific uses. The use of semiconductor
diode lasers as sources of optical energy is attractive for a
number of reasons. For example, diode lasers have a relatively
small volume and consume a small amount of power as compared to
conventional laser devices. Further, as monolithic devices, they do
not require a combination of a resonant cavity with external
mirrors and other structures to generate a coherent output laser
beam. One disadvantage of the semiconductor diode laser, however,
is the relatively low power of the output beam, as compared to
other types of laser devices.
[0007] Group III-V ("III-V") semiconductor materials have been used
to construct semiconductor lasers. Processing of III-V
semiconductor devices includes vital steps for depositing III-V
materials on a semiconductor substrate. For the deposition of a
thick III-V layer, the lattice constant of the substrate material
has to be very close to that of the deposited III-V layers (epi
layers) with the same crystalline structure. Otherwise, crystalline
defects, especially threading dislocations, will form during
material deposition. When the defect density in the deposited
material is high, it will significantly degrade device performance.
These threading dislocation defects can create leakage paths for
current, provide undesired carrier recombination centers and reduce
device lifetime.
[0008] It is thus very difficult to grow high quality thin film
materials on conventional prior art substrates with a large lattice
mismatch. This lattice-matching requirement for compound
semiconductor material deposition severely limits the possible
choice of compound semiconductor material compositions and device
material structure designs due to the limited choice of available
substrates with the appropriate crystalline structures and lattice
constants. Such substrates include Si, GaAs, InP, GaSb, InAs, and
sapphire, inter alia.
[0009] For material systems for which there are no lattice-matched
prior art substrates, however, some alternative approaches have
been used. E.g., either a thick buffer layer is grown on the
substrate, as proposed in U.S. Pat. No. 5,285,086, or a special
technique, such as the lateral growth method proposed by Parillaud
et al., Appl. Phys. Lett. vol. 68 (1996), p. 2654, is employed
before the growth of the device structure layers. It is known that
defects, in particular threading dislocations, induced by lattice
mismatch can be reduced from b 10.sup.11/cm.sup.2 to
10.sup.5/cm.sup.2 by using the lateral growth method, for example.
However, lattice-mismatched material growth techniques that result
in defects, especially threading dislocation defects, often cause
undesirable performance or characteristics of optoelectronic or
electronic devices grown with such techniques.
[0010] It is desirable to epitaxially fabricate a variety of types
of structures or devices, using a given epi material system, grown
on a given substrate. Such epitaxially fabricated devices include
electronic devices, such as transistors and integrated circuits,
and optoelectronic devices, such as semiconductor lasers,
light-emitting diodes, and photodetectors.
[0011] One such optoelectronic device in which there has recently
been an increased interest is the vertical-cavity surface-emitting
laser (VCSEL). The conventional VCSEL has several advantages, such
as emitting light perpendicular to the surface of the die, and the
possibility of fabrication of two dimensional arrays. VCSELs
typically have a circular laser beam and a smaller divergence
angle, and are therefore more attractive than edge-emitting lasers
in some applications. Long infra-red spectrum wavelength (e.g., the
range from approximately 1.2 .mu.m to approximately 1.8 .mu.m,
including closely-spaced wavelengths around 1.3 .mu.m or
closely-spaced ITU grid wavelengths around 1.55 .mu.m) VCSELs are
also of great interest in the optical telecommunications industry
because of the minimum fiber dispersion at 1.32 .mu.m and the
minimum fiber loss at 1.55 .mu.m. The dispersion shifted fiber will
have both minimum dispersion and minimum loss at 1.55 .mu.m. The
long wavelength VCSEL is typically based on an
In.sub.xGa.sub.1-xAs.sub.yP.sub.1-y active layer lattice matched to
InP cladding layers.
[0012] The structure of a typical VCSEL usually consists of an
active region sandwiched between two distributed Bragg reflector
(DBR) mirrors, as shown schematically in FIG. 1. For the
fabrication of long wavelength (e.g., 1.3 or 1.55 .mu.m) VCSELs, it
is very difficult to form the desired materials in one single
growth step on a substrate. For instance, it is difficult to grow
either the desired 1.3 .mu.m active region on a GaAs substrate or
to grow proper DBR mirrors on an InP substrate, despite the
maturity of the technology for growing the DBR structure on GaAs
substrates. Likewise, it is difficult to grow a 1.3 .mu.m
wavelength DBR structure on an InP substrate, despite the maturity
of the technology for growing the active region. Recently, some
alternative material systems, such as InGaNAs, GaAsSb and InGaAs
quantum dots, have been developed to grow directly on a GaAs
substrate using an Al.sub.xGa.sub.1-xAs/Al.sub.yG- a.sub.1-yAs DBR
for a 1.3 .mu.m wavelength active region. However, these material
systems are very difficult to grow and not easy to reproduce.
[0013] Another alternative approach to fabricate a long wavelength
VCSEL is by using the so-called wafer bonding technique. However,
this approach requires at least two to three wafer growth and one
to two wafer-to-wafer bonding processes, which leads to very high
fabrication cost and very low device yield. Therefore, a single
wafer growth approach would be preferable to the wafer bonding
approach, other considerations being equal.
[0014] One alternative approach to fabricate a long wavelength
VCSEL with a single wafer growth step is to use the (In,Ga,Al)As
material system lattice matched to
In.sub.x(Al.sub.yGa.sub.1-y).sub.1-xAs, where, e.g.,
0.15<.times.<0.45, and growth of an InAlGaAs/InAlAs DBR
structure and a moderately strained InGaAs quantum well (QW)
structure active region. (Depending on the value of x, y is
selected such that the material utilized has a bandgap absorption
edge less than the lasing wavelength, e.g. less than 1.3 .mu.m for
a 1.3 .mu.m VCSEL.) However, there is no commercially available
substrate that is lattice matched to this material system. It is
very difficult to control the composition precisely of a ternary
In.sub.xGa.sub.1-xAs substrate uniformly over a whole wafer.
Therefore, a high quality alternative substrate needs to be
developed for this application.
[0015] One approach is to create a substrate that has the same
crystalline structure and the same surface lattice constant as
those of non-strained In.sub.x(Al.sub.yGa.sub.1-y).sub.1-xAs, where
0.15<.times.<0.45. Another approach is to make a substrate
that has a thin layer that is physically attached to the substrate,
but can freely expand in a direction parallel to the substrate
surface during material growth. This thin surface layer must have
the same crystalline structure and a similar lattice constant as
those of non-strained In.sub.x(Al.sub.yGa.sub.1-y).su- b.1-xAs,
where 0.15<.times.<0.45.
[0016] For lattice-mismatched epitaxial layers, it is widely
accepted that there exists a critical thickness beyond which misfit
dislocations are introduced causing the breakdown of coherence
between the substrate and epitaxial layers. The relaxation
mechanism for lattice-mismatched epilayers known as the
Matthews-Blakeslee model, and other aspects of epitaxial layer
lattice mismatching problems are discussed in J. W. Matthews, S.
Mader & T. B. Light, J. Appl. Phys. 41 (1970): 3800; J. W.
Matthews & A. E. Blakeslee, "Defects in Epitaxial Multilayers
I," J. Qryst. Growth 27 (1974): 118-125; J. W. Matthews & A. E.
Blakeslee, "Defects in Epitaxial Multilayers II," J. Cryst. Growth
29 (1975): 273-280; J. W. Matthews & A. E. Blakeslee, "Defects
in Epitaxial Multilayers III," J. Cryst. Growth 32 (1976): 265-273;
and J. W. Matthews, J. Vac. Sci. Technol. 12 (1975): 126.
[0017] U.S. Pat. No. 5,294,808 for "Pseudomorphic and Dislocation
Free Heteroepitaxial Structures" proposes to use a thin substrate
having a thickness on the order of the "critical" thickness, which
is the thickness at which defects form when growing one lattice
mismatched material on another. The critical thickness is only a
few hundred angstroms, and it is difficult to sustain the
mechanical and chemical processes required for epitaxial (epi)
growth and device fabrication on a substrate having a thickness of
only a few hundred angstroms. However, in practical situations,
after the thin substrate is bonded to the supporting substrate, the
bonding strength between the interface is so strong that this thin
substrate can no longer freely change its lattice constant in the
in-plane direction. Therefore, threading dislocations will still be
generated due to the very limited strain accommodation in the thin
substrate.
[0018] There is, therefore, a need for improved substrates and
fabrication techniques that address the foregoing problems. In
general, there is a need for alternative substrates that can be
used for a variety of epi material systems without giving rise to
conventional problems caused by lattice mismatch between the epi
layers and the substrate. For example, there is a need for
alternative substrates that address the problems associated with
lattice mismatching between a substrate and the (In,Al,Ga)As
material intended to be used for long (e.g., 1.3 or 1.55 .mu.m)
wavelength VCSELs or other special material systems. Such
alternative substrates could be advantageously used for other
material systems and device structures as well, and in general for
any material system for which other substrates cannot satisfy the
lattice-matching requirement for device applications.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] Other objects and advantages of the invention will become
apparent upon reading the following detailed description and upon
reference to the accompanying drawings in which:
[0020] FIG. 1 is a cross-sectional view of a typical layer
structure of a vertical cavity surface emitting laser (VCSEL)
device;
[0021] FIG. 2 is a schematic diagram illustrating a generic layered
structure employed to form defect-free epitaxial layers on an
alternative substrate, in accordance with an embodiment of the
present invention;
[0022] FIGS. 3A, B, C, D, and E are schematic illustrations of a
process employed to fabricate an alternative substrate in
accordance with an embodiment of the present invention;
[0023] FIGS. 4A, B, C, and D are schematic illustrations of another
process employed to fabricate an alternative substrate in
accordance with an embodiment of the present invention;
[0024] FIG. 5A is a schematic illustration of the epi-up
configuration for laser mounting in output power measurement,
according to the invention; and
[0025] FIG. 5B is a schematic illustration of the prior art
epi-down configuration for laser mounting in output power
measurement.
DETAILED DESCRIPTION OF THE INVENTION
[0026] Commonly-owned U.S. patent application Ser. No. 09/426,273,
filed Oct. 25, 1999, for "Compliant Universal Substrates for
Optoelectronic and Electronic Devices," now issued as U.S. Pat. No.
6,406,795, is incorporated by reference herein in its entirety.
[0027] To address the problem of lattice mismatch between a prior
art substrate and (In,Al,Ga)As material intended to be used for a
long wavelength (e.g., 1.3 or 1.55 .mu.m) VCSEL or other special
material systems, an alternative substrate is provided for growth
of high-quality pseudomorphic epitaxial (epi) thin films without
generating high-density threading dislocations. The alternative
substrate of the present invention preferably allows high quality
compound semiconductor thin film growth and also endures all the
material epitaxy and device fabrication processing steps. The
present invention provides alternative substrates using wafer
fusion or wafer bonding techniques that facilitate the formation of
high quality devices on these alternative substrates.
[0028] According to the present invention, an alternative substrate
has a base layer and a thin film layer physically bonded to the
substrate. Two basic approaches to providing an alternative
substrate for defect-free (or reduced defect) epitaxial, growth are
disclosed herein: the floating substrate approach and the relaxed
substrate approach, which are described in further detail below.
Depending on the approach employed, the adaptive thin film layer
either (a) has a lattice constant different from that of the target
epi material system, but with a sufficient degree of lattice
flexibility during epitaxial growth of the target material system,
due to the presence of a floating interfacial bonding layer, to
permit the lattice constant of the adaptive thin film layer to
adjust to that of the target system, thereby providing lattice
match and reducing lattice mismatch threading dislocations; or (b)
the thin film layer, which is initially strained and has lattice
mismatch with the target material system, has its in-surface
lattice constant adjusted by relaxation before epi growth so that
it has a lattice constant very close to that of the target material
system. In the second approach, the thin film layer may be bonded
to a base layer with or without an interfacial bonding layer, in
alternative embodiments.
[0029] The present invention thus provides an alternative substrate
for the formation of various devices with special target epi
material systems, such as approximately 1.2 .mu.m to approximately
1.8 .mu.m wavelength VCSELs. The alternative substrate includes a
base layer and a thin film crystalline layer on and bonded to the
base layer, with or without an interfacial bonding layer, depending
on the embodiment. The thin film layer's lattice constant is
adjusted either during epi growth to accommodate the different
lattice constant of the target epi layers; or is adjusted prior to
the epi growth to create a thin film layer lattice-matched to the
target material system. In either case, an interfacial bonding
layer is employed to adjust the thin film layer's lattice constant,
whether before or during epi growth. This approach can also be used
for other material systems for different device applications, as
will be appreciated by those skilled in the art.
[0030] FIG. 2 schematically illustrates an alternative substrate 20
in accordance with an embodiment of the present invention.
Alternative substrate 20 is a multilayer structure having a thick
bulk material base layer or substrate 21 and a thin film adaptive
layer 22, which is bonded to the bulk material base layer 21 with
an interfacial bonding layer 23. The thin film adaptive layer 22
serves as the actual substrate that supports growth of an epi layer
24 in a growth chamber. The thin film adaptive layer 22 could have
a thickness from approximately 5 .mu.m or less to approximately a
few microns (.mu.m), depending on application and design. The thin
film adaptive layer 22 is mechanically robust when it is bonded to
the base layer 21. The interfacial bonding layer 23 may be a thin
metal(s) layer, an inorganic layer, or a combination of any
materials listed below. Alternatively, interfacial bonding layer 23
may simply be the interface formed between the treated (i.e.,
cleaned) surface layers substrate 21 and thin adaptive layer 22, in
alternative embodiments.
[0031] When bonding layer 23 is a metal layer, for example, it may
be formed of one or more metal layers. For example, it may be a
single-layer of Bizmuth (Bi), or other metals such as Pb, In, Sn,
Sb, Al, or the like. Some alloys with low melting temperatures
(e.g., <600.degree. C. or 500.degree. C.) can also be used, such
as In:Sn, Pb:Sn, In:Pb, In:Ag, or other element or alloy. Bonding
layer 23 may have a single layer, or two, three, or more layers of
metals and/or alloys. As described below, the layer structure and
materials of the metal bonding layer 23 are preferably selected to
avoid undesired chemical reaction with the thin adaptive layer 22
and supporting substrate 21.
[0032] The layer 23 is preferably in solid form at temperatures
under 100.degree. C. and provides sufficient bonding strength
between the layers 21 and 22 to hold them together for material
processing and fabrication purposes, i.e., for mechanical
stability. The interfacial bonding layer 23 may actually comprise a
thin interfacial layer that could vanish after bonding layer 22 to
layer 21, or could be as thick as approximately a few micrometers,
depending, in part, on the embodiment.
[0033] In accordance with an embodiment of the invention, there are
two different basic techniques to make an alternative substrate for
VCSELs (e.g., for 1.3 or 1.55 .mu.m wavelength light output) based
on In.sub.x(Al.sub.yGa.sub.1-y).sub.1-xAs, where, for example,
0.15<.times.<0.45. Other ranges for x may also be employed in
alternative embodiments. It is understood that all mole fractions
hereinafter are exactly or approximately the value at the indicated
extremes previously given unless otherwise indicated, as will be
appreciated by those skilled in the art.
[0034] According to a first embodiment or approach, the thin
adaptive layer 22 is made to accommodate the difference between its
lattice constant and that of the epitaxial layer during epitaxial
growth of layer 24, which will be referred to as the "floating
substrate approach." According to a second embodiment or approach,
the in-plane lattice constant of the thin adaptive layer 22 is
modified before epitaxial growth, such that it has the same
(in-plane) lattice constant as that of the target material during
epitaxial growth of layer 24. This Will be referred to as the
"relaxed substrate approach." For the case of a 1.3 or 1.55 .mu.m
VCSEL, the target material can be, for example,
In.sub.x(Al.sub.yGa.sub.1-y).sub.1-xAs, where
0.15<.times.<0.45. Other target material systems may be
employed as well.
[0035] Floating Substrate Approach
[0036] According to one embodiment, the floating substrate approach
is achieved using the interfacial bonding layer 23 between the thin
adaptive layer 22 and the supporting substrate 21, as indicated
above. In this approach, thin adaptive layer 22 has a different
lattice constant than that of the target material system, but has a
lattice flexibility during epi growth. During epi growth of epi
layer 24, the interfacial bonding layer 23 becomes liquid,
partially liquid, or liquid-like (hereinafter referred to as
liquid-like) after the substrate 21 is heated up to a temperature
higher than room temperature (i.e., "liquidizes"), for example,
higher than 100.degree. C. When the interfacial layer 23 becomes
liquid-like, the thin adaptive layer 22 physically floats freely on
the interfacial layer 23 and has the degrees of freedom to expand
or contract its lattice constant to adapt to the lattice constant
of the epitaxial layer being grown thereon. The liquid-like state
of the interfacial bonding layer 23, after substrate heating,
allows the layer 22 to change its lattice constant without
generating (or reduces the occurrence of) threading dislocations,
and/or channels any threading disclocations into the adaptive layer
instead of upward, into the growing epi layer 24.
[0037] Thus, in this embodiment, interfacial bonding layer 23 must
not be merely an interface that is vanishingly small; it must be an
actual layer having a finite thickness sufficient to perform the
floating function to give thin adaptive layer 22 lattice
flexibility during epi growth of epi layer 24. The interfacial
layer 23 is preferably thin enough that its surface tension will
not destroy the thin adaptive layer 22 when it is in the
liquid-like state. The interfacial layer 23 also preferably has a
viscosity such that the thin adaptive layer 22 and the epitaxial
layer 24 will not fall off or shift (e.g., slide laterally) during
material epitaxy or device processing. The interfacial layer 23
further does not react or alloy (or minimally reacts or alloys)
chemically with the thin film adaptive layer 22, which reacting
could otherwise destroy layer 22 (e.g., when it is heated to become
liquid-like during epi growth of epi layer 24).
[0038] Substrate or base layer 21 can be formed from any
commercially available high quality substrate material, such as Si,
GaAs, InP, GaP, or the like. However, the thin film adaptive layer
22 can be made from either the same material or a variety of other
materials as the base layer 21. Thin adaptive layer 22 is
preferably a semiconductor layer to facilitate growth thereon of
semiconductor based epi layers and devices in epi layer 24.
[0039] FIGS. 3A-E illustrate a method for fabricating an
alternative (sometimes referred to as a compliant universal (CU))
substrate such as illustrated in FIG. 2, in accordance with the
floating substrate approach, according to an embodiment of the
present invention. In FIGS. 3A and 3B, first and second wafers 300
and 301 are provided, each of which is formed from a suitable bulk
substrate material, for example, Si, GaAs, InP, GaSb, GaP, InAs, or
the like. It should be emphasized that any suitable material may be
employed as the substrate material, including both semiconductor
and non-semiconductor materials. The wafer 300 may undergo a
process (such as e-beam evaporation, thermal evaporation, or
sputtering) to deposit a thin top bonding layer 302 on the
substrate 300, as shown in FIG. 3A.
[0040] In one aspect of the invention, as shown in FIG. 3C, an
etch-stop layer 333 is formed on an alternative second wafer
substrate 331 having the thin adaptive layer 332 disposed thereon.
The etch stop layer 333 may be AlGaAs, InGaP, InAlP, or the like.
The thin adaptive film layer 332 is formed on the etch stop layer
333 using any suitable conventional technique, such as molecular
beam epitaxy (MBE); liquid phase epitaxy (LPE); or a vapor phase
epitaxy (VPE) process such as or metalorganic chemical vapor
deposition (MOCVD, also known as MOVPE). A thin bonding layer also
could be formed on top of the thin adaptive layer 332.
Alternatively, in another embodiment, thin film adaptive layer 303
is formed directly on the second wafer 301, as shown in FIG.
3B.
[0041] Next, as illustrated in FIG. 3D, the second wafer 301 is
inverted relative to the first wafer 300, and the thin film layer
303 is bonded to the top surface bonding layer 302 of the first
wafer 300. The joining of the two wafers 300 and 301 can be the
result of Van der Waals forces, hydrogen bonding, covalent bonding,
ionic bonding, or the like, or any other mechanism, and results in
the bonding layer 302, as shown in FIG. 3E, becoming a finite
thickness interfacial layer. In some embodiments, pressure is
applied during the wafer bonding process. Depending on the detailed
process conditions and the bonding mechanisms, the applied pressure
can vary from approximately zero to over approximately 10 MPascal
or higher. Finally, as illustrated in FIG. 3E, the conventional
substrate second wafer 301 is removed by a selective etching (or
lift off) technique, as will be appreciated by those skilled in the
art. The etch stop layer 333 may be used to prevent removal of any
of the thin film layer 332 if the wafer 331 is used. The exposed
thin film layer 332 (or 303) can be used as a CU substrate platform
for epitaxial growth, while the wafer 300 now becomes the
supporting bulk material base layer (effective substrate).
[0042] Alternatively, instead of depositing bonding layer 302 on
substrate 300, the thin bonding layer 302 could be deposited on top
of the surface of thin adaptive layer 303 or 332, for wafer
bonding, depending on the process design. Or, bonding layer 302 may
be deposited partially on substrate 300, and partially on adaptive
layer 303 or 332, so that after the step shown in FIG. 3D, a
bonding layer is disposed between thin adaptive layer 303 and the
bulk material 300, as shown in FIG. 3E.
[0043] Whether bonding layer 302 is deposited on substrate 300, or
layer 302/332, or partially on both, the bonding layer may be a
single or multilayer metal, as described above with reference to
bonding layer 23.
[0044] Relaxed Substrate Approach
[0045] In the relaxed substrate approach of the present invention,
the lattice constant of the thin adaptive layer 22 (FIG. 2) is
modified before bonding it to the supporting substrate 21.
Preferably, the lattice constant of thin adaptive layer 22 is
modified to match the lattice constant of the material system of
epi layer 24 which is to be grown thereon. This is done by
selecting the material for adaptive layer 22 so that its relaxed
(i.e., unstrained, or "natural," or "original") lattice constant is
very close to, or the same as, that of a target material system to
be grown as epi layer 24. Then, thin adaptive layer 22 is grown on
a first substrate having a different material and thus a different
lattice constant than the thin adaptive layer, giving rise to a
strained thin adaptive layer having a strained lattice structure
different than the non-strained (relaxed) lattice constant.
However, the thin adaptive layer 22 is thin enough so that,
although it is strained, there are no threading dislocations.
[0046] A first surface of the thin film layer is then bonded to a
surface of a second, supporting substrate (incidentally having a
lattice constant different from that of the thin film layer), with
a flexible interfacial bonding layer. The first substrate is
removed, and an interfacial bonding layer is used, similar to the
relaxation approach described above with respect to the floating
substrate approach, to relax the thin adaptive layer, to adjust its
lattice constant to its non-strained, relaxed value. Thereafter,
the now-non-strained thin adaptive layer is preferably mounted on
another substrate or support, e.g. bulk material (with our without
a second interfacial bonding layer), and the first bonding layer
and its supporting substrate is removed from the thin adaptive
layer. The thin adaptive layer 22, supported by a bulk material
substrate, may then be utilized to grow an epi layer thereon
lattice matched to the non-strained lattice constant of the thin
adaptive layer.
[0047] This process results in an alternative substrate for the
formation of semiconductor devices, which substrate has a
crystalline base layer and/or bulk material, and, on the base
layer, a thin film layer having a lattice constant very close to
that of the target material system. In this approach, interfacial
bonding layer 23 may be merely an interface between the base and
the adjusted thin film layer, because it need not provide the
floating function during epitaxial growth of epi layer 24 that it
provides in the floating substrate approach. Alternatively,
interfacial bonding layer 23 may be a real layer having finite
thickness.
[0048] Referring now to FIG. 4, there is illustrated a method of
fabricating an alternative. substrate in accordance with the
relaxed substrate approach of the present invention. In FIG. 4, the
foregoing steps are illustrated in further detail with respect to
carrier (support) substrate 400 and conventional substrate 402.
First, a high-quality thin adaptive layer 403 (i.e., corresponding
to layer 22 of FIG. 2) is grown on a conventional (first) substrate
402. The thin adaptive layer 403 needs to have a relaxed lattice
constant which is very close to that of a target material system.
For example, the target material system may be Ga.sub.xIn.sub.1-xAs
where, e.g., x=20%, i.e. Ga.sub.0.20In.sub.0.80As. In this case,
Ga.sub.0.20In.sub.0.80As may also be selected for the material for
thin adaptive layer 403, so that its relaxed lattice constant and
crystalline lattice structure is identical to that of the target
material system for the epi layers to be grown thereon. To
fabricate a thin layer 403 of Ga.sub.0.20In.sub.0.80As, a
lattice-mismatched substrate such as GaAs or InP may be used. This
will give rise to a strained-lattice layer, having a different
lattice constant than non-strained (relaxed)
Ga.sub.0.20In.sub.0.80As.
[0049] The thickness of the thin adaptive layer 403 is preferably
smaller than its critical thickness (e.g., <100 .ANG.) such that
no dislocation generation or lattice relaxation will likely occur.
Quality and thickness control of the strained thin adaptive layer
403 is preferable for this process. As will be understood by those
skilled in the art, after its fabrication on substrate 402, the
lattice constant of the strained adaptive layer 403 is the same as
that of the substrate 402 in the direction parallel to the wafer
surface. However, as described below, this lattice constant of
layer 403 is relaxed to its non-strained value through a special
process in accordance with the present invention.
[0050] Thus, after forming strained adaptive layer 403 on substrate
402, thin bonding layers 404 and 405 are deposited on the thin
adaptive layer 403 and the carrier (second) substrate 400,
respectively, as shown in FIG. 4A. (Alternatively, only one of
bonding layers 404, 405 may be employed.) Then, the two substrates
are put together face-to-face and bonded together, as illustrated
in FIG. 4B, to form a bonding layer 406 from the thin bonding
layers 404 and 405. This bonding process may be accomplished by
applying heat or pressure, or a combination of both, to the two
wafers, as will be appreciated by those skilled in the art. Bonding
layers 404, 405 may comprise suitable materials, such as the
materials utilized for forming interfacial bonding layer 23, or
other suitable metals or other materials.
[0051] After the two wafers are bonded together, the conventional
(first) substrate 402 is etched away, leaving only the thin
adaptive layer 403 on the bonding layer 406, mounted on second,
carrier substrate 400 (FIG. 4C). Then, heat is applied to the
carrier substrate 400, as in FIG. 4C, until the interfacial bonding
layer 406 becomes liquid-like, such that the thin adaptive layer
403 can freely change its lattice constant to its relaxed value, to
relieve internal strain. After the thin adaptive layer 403 is
relaxed, then the carrier substrate 400 can be cooled down and the
bonding layer 406 is re-solidified. Thus, after cooling, this
results in a now-relaxed thin adaptive layer 403 which has a
relaxed lattice constant, which is identical to, or at least closer
to, that of the target material system. This layer 403 may then be
used as the alternative substrate, when mounted on a proper
support, to permit epitaxial growth of the target material system,
with reduced threading dislocation defects.
[0052] In one embodiment, the result shown in FIG. 4C may be used
as an alternative substrate, after appropriate treating of the top
(exposed) surface of thin adaptive layer 403. In this case, carrier
substrate 400 serves as the support. However, preferably, thin
adaptive layer 403 is bonded to a new (third), bulk material
support substrate 410, as shown in FIG. 4D. In this embodiment,
thin adaptive layer 403 is bonded to a new bulk substrate 410,
which can be a prior art semiconductor substrate or a dielectric
crystal substrate that has a thermal expansion coefficient very
close to that of the thin adaptive layer 403 (to reduce fracturing
or damage during heating). The final step is to remove the carrier
substrate 400 and bonding layer 406. This may be done by chemical
etching, or by mechanical removal by melting the bonding layer 406.
The exposed surface of thin adaptive layer 403, after layers 400
and 406 are removed, may then be suitably treated to permit
epitaxial growth thereon of the target material system.
[0053] Bonding layer 406 may consist of any suitable bonding layer
material. The bonding layers 406 and 302 (FIG. 3) preferably have
the proper chemical and physical properties, including: (1) no or
insignificant chemical reaction with the thin adaptive layer 403 or
303 (or 332); (2) solid at room temperature, e.g., up to
approximately 100.degree. C.; (3) liquid-like form when heated to
an elevated temperature (e.g., approximately 100-500.degree. C.);
(4) physically strong hold of the carrier substrate 400 (or the
bulk material 300) and the thin adaptive layer 403 (or 303)
together; and (5) no or insignificant physical or chemical damage
to the thin adaptive layer 403 (or 303) upon heating. The bonding
layers 406 and 302 can be, for example, metals (e.g., as described
above with reference to bonding layer 23), inorganic materials, or
organic materials.
[0054] Preferably, thin adaptive layers 303, 403 are semiconductor
materials, so that semiconductor devices may be grown thereon.
Carrier substrate 400 need not be a semiconductor, but need only be
strong enough to act as a support during the process illustrated in
FIG. 4. Bulk material 410, on the other hand, while it need not be
a semiconductor, preferably has a thermal expansion coefficient
close to that of thin adaptive layer 403, to prevent cracking or
other undesirable effects during changes in heat. Bulk material 300
need not be a semiconductor, but should be strong enough to support
layer 302. Preferably, the thermal expansion coefficient for
substrate 300 is not very different from that of thin adaptive
layer 303, although in an embodiment it need not be as closely
matched as for substrate 410 and thin adaptive layer 403, because
bonding layer 302 is between layer 303 and substrate 300 during
epitaxial growth on layer 303.
[0055] Thus, the present invention provides, in the relaxed
substrate approach, for the formation of an alternative substrate
for epitaxial growth of a target material system. The alternative
substrate comprises a thin film adaptive crystalline layer bonded
to a base substrate layer. The thin film layer may be said to have
an unstrained lattice constant equal or very close to the lattice
constant of the target material system, where the thin film layer
originally (in formation) had a strained lattice constant equal to
the lattice constant of a first semiconductor substrate on which it
is grown, which strained lattice constant has been adjusted by
bonding the thin film adaptive layer to a carrier substrate with an
interfacial bonding layer and removing the first substrate, and
then heating the interfacial bonding layer to liquidize the
interfacial bonding layer to allow the strained pseudomorphic thin
film adaptive layer to relax to its unstrained lattice structure,
and then bonding the thin film layer to the base substrate and
removing the carrier substrate to expose an epitaxial growth
surface of the thin film adaptive layer. The thin film adaptive
layer may be referred to herein as a relaxed-strained or
unstrained-strained thin film layer to indicate that it was
fabricated a strained layer with a first (strained) lattice
constant and then relaxed, via the interfacial bonding layer
technique, to adjust its lattice constant to the natural,
unstrained lattice constant for the material of the thin film
layer.
[0056] As mentioned above, these alternative substrate fabrication
techniques of the disclosed embodiments, for example, for a 1.3 or
1.55 .mu.m wavelength VCSEL, also can be applied to other material
systems for different device applications. With reference to FIGS.
3A-3E and 4A-4E, three different exemplary material systems for the
method described above will now be discussed.
[0057] Exemplary Material Systems
[0058] I. GaInAs/InP
[0059] The thin adaptive film 303 or 332 (or 403) can be, for
example, an InGaAs thin film with an In composition approximately
15% to approximately 45%. Its thickness can be, for example,
approximately 3 nm to approximately 30 nm. The second wafer or
second substrate 301 or 331 (or 402) can be, for example, InP. As
the lattice constant of Ga.sub.xIn.sub.1-xAs,
0.20<.times.<0.45, is smaller than that of InP, if a layer is
grown whose thickness is less than its critical thickness, then no
or little additional threading dislocation defects will or will
likely be formed during the growth of this thin epitaxial layer. In
this embodiment, the thin film 303 or 332 (or 403) is a
pseudomorphic tensile strained epitaxial layer. The thin film 303
or 332 (or 403) can be bonded in any orientation relative to the
bulk material substrate 300 (or the substrate 410), which may be
GaAs, InP, Si, sapphire, or other suitable materials. Thin adaptive
layer 303 or 403 may be composed of InGaAsP, GaSb, InGaAs, InGaP,
AlGaP, InSb, InP, AlSb, or InAs, SiC, Ge, GaP, InAs, GaSb, or the
like. However, a relative orientation of approximately 0.degree. or
approximately 90.degree. is usually preferred when considering
device processing control of the sample.
[0060] II. GaInAs/GaAs
[0061] The thin adaptive film 303 or 332 (or 403) can be, for
example, an InGaAs thin film with an In composition between
approximately 15% and approximately 40%. Its thickness can be, for
example, approximately 3 nm to approximately 50 nm. The second
wafer or second substrate 301 or 402 can be, for example, GaAs. As
the lattice constant of Ga.sub.xIn.sub.1-xAs, where
0.15<.times.<0.40, is larger than that of GaAs, if a layer is
grown whose thickness is less than its critical thickness, then no
or little additional threading dislocation defects will or will
likely be formed during the growth of this thin epitaxial layer. In
this embodiment, the thin adaptive film 303 or 332 (or 403) is a
pseudo-morphic compressive strained epitaxial layer, which can be
bonded in any crystalline orientation relative to the substrate 300
(or 400). The substrate 300 (or 400) can be, for example, GaAs,
InP, Si, sapphire, SiC, Ge, GaP, InAs, GaSb, or the like. However,
a relative orientation of approximately 0.degree. or approximately
90.degree. is usually preferred when considering device processing
control of the sample.
[0062] III. GaSb/InAs or InAs/GaSb
[0063] The substrate 301 (or 402) can be, for example, either InAs
or GaSb. Whichever material is chosen, i.e., InAs or GaSb, the thin
adaptive film layer 303 or 332 (or 403) can be, for example, the
other of these two materials, and have a thickness approximately 3
nm to approximately 30 nm. As the lattice mismatch between InAs and
GaSb is less than 0.7%, the GaSb or InAs thin film is not relaxed
and no or little threading defects should be formed in the epi
layer. The thin adaptive film 303 or 332 (or 403) can be bonded in
any orientation relative to substrate 300 (or 400), which can be,
for example, GaAs, InP, Si, sapphire, SiC, Ge, GaP, InAs, GaSb, or
the like. However, a relative orientation of approximately
0.degree. or approximately 90.degree. is usually preferred when
considering device processing control of the sample.
[0064] These are just a few examples of possible ways to fabricate
CU substrates according to the present invention. There are many
other ways to achieve a CU substrate using different III-V
materials, as will be appreciated by those skilled in the art.
However the principle of the present invention is the same as for
the above examples, and these other ways are included within the
scope and spirit of the present invention.
[0065] Non-limiting exemplary metal materials having low melting
temperatures (e.g., below 600.degree. C. or 500.degree. C., typical
temperatures used for MBE) that can be used as the bonding layer
302 (or 406) include, for example, Bi, Pb, In, Sn, Sb, Al, or the
like. Some alloys with low melting temperatures (e.g.,
<600.degree. C. or 500.degree. C.) can also be used, such as
In:Sn, Pb:Sn, In:Pb, In:Ag, and the like. The bonding layer 302 (or
406) also can comprise multiple metal layers, such as layer
combinations of these metals for which some of the layers have a
much higher melting temperature.
[0066] In summary, the present invention provides alternative
substrates that may be formed from conventional semiconductor and
other bulk materials that facilitate growth of lattice-mismatched
threading dislocation defect-free epitaxial layers. This is
accomplished through provision of a thin adaptive film layer, which
is highly flexible due to an interfacial bonding layer that becomes
liquid-like upon heating. The present invention thereby facilitates
the formation of a wide range of devices that were previously not
feasible to construct due to lattice mismatch constraints. A list
follows of exemplary potential applications of this alternative
substrate technology.
[0067] Exemplary Potential Applications
[0068] A variety of devices can be fabricated using the CU
substrate provided in the present invention, including electronic
devices such as transistors and integrated circuits; and
optoelectronics devices, such as lasers (including diode lasers,
VCSELs, and the like), LEDs and photodetectors.
[0069] I. High-Power Mid-Infrared Lasers
[0070] Referring now to FIGS. 5A and 5B, there are shown,
respectively, schematic illustrations of the epi-up configuration
for laser mounting in output power measurement, according to the
invention, and a prior art epi-down configuration for laser
mounting in output power measurement. Pump laser 130 provides
optical pumping light for the laser of epi layer 125. In FIG. 5A,
layer 120 may comprise layers 403 and 410 of FIG. 4, or layers 303,
302, 300 of FIG. 3.
[0071] Sb-base type-II quantum well (QW) or superlattice (SL)
lasers emitting from approximately 2 to approximately 10 .mu.m can
be grown and fabricated according to the invention on the GaAs CU
substrates. The laser active region is composed of but not limited
to either InAs/InGaAlSb/InAs/InAlSb type-II QWs or InAs/InGaAlSb
type-II SLs. The advantage of growing such lasers on the GaAs CU
substrate is that the laser can be bonded on a submount 112 in the
epi-down configuration, as schematically shown in FIG. 5A, instead
of the prior art epi-up configuration, as schematically shown in
FIG. 5B. This is because, in the prior art, a substrate had to be
utilized which is lattice matched to the epi layers, which often
limited the choice to materials that were opaque to the pumping
light from pump laser 130. By utilizing the CU substrate of the
present invention, a substrate which is transparent to the pumping
light may be utilized, even though it is not lattice matched to the
epi layers 125. By using the epi-down configuration, the maximum
laser output power can be dramatically improved with better heat
removal capability from the laser active region.
[0072] II. Mid-Infrared (IR) and IR Photodetectors
[0073] High performance IR photodetectors for detecting wavelengths
approximately 2 to approximately 25 .mu.m can be composed of
InAlGaAs/InAlGaSb type-II SLs lattice matched to GaSb or InAs
substrates. However, both InAs and GaSb substrates highly absorb
radiation at wavelengths longer than approximately 5 .mu.m.
Therefore, epi-side down mounting to the read out circuits is very
difficult to use. IR photodetectors can be grown and fabricated
according to the invention on large bandgap CU substrates, and
hence such photodetectors can be integrated with read out circuits
using the epi-down configuration to allow light to pass from the CU
substrate. An alternative substrate, such as a thin GaSb layer
bonded on top of a GaAs or Si substrate, can be used as a filter to
filter out the visible and ultraviolet (UV) spectra.
[0074] III. Visible and UV Laser Diodes
[0075] Red, orange, and yellow/green diode lasers having InGaAlP
heterostructures can be grown and fabricated according to the
invention on GaAs-based CU substrates without being restricted by
available lattice-matched substrates. High-quality InGaN/AlGaN
ultraviolet, blue, and green lasers with a long lifetime and low
defect density can be grown and fabricated according to the
invention on CU substrates. In the prior art, these devices are
grown on sapphire or SiC substrates with a large lattice mismatch.
This produces a very high defect density and strongly limits device
lifetime. These devices according to the present invention can be
used, for example, in displays, DVDs for optical data storage,
medical applications, and chemical sensors to monitor band-to-band
transitions of gas species.
[0076] IV. High-Temperature, High-Power, High Voltage Electronic
Devices
[0077] Transistors composed of InGaAlN and SiC heterostructures can
sustain high voltage, high temperature, and deliver high power.
These are attractive features for the power industry and the
microwave communications industry. The electrical qualities of
InGaAlN and SiC compounds grown and fabricated according to the
invention on CU substrates are or likely are superior to those
grown on other mismatched substrates for both carrier mobility and
breakdown voltage.
[0078] V. High-Efficiency Visible LEDs
[0079] Red, orange, and yellow/green LEDs having InGaAIP
heterostructures can be grown and fabricated according to the
invention on GaAs-based CU substrates. LEDs emitting from red to UV
wavelengths can be constructed with InGaN/InGaAlN heterostructures
grown and fabricated according to the invention on Si or other CU
substrates. The CU substrates are more attractive than sapphire or
SiC substrates currently being used from both cost and electrical
property standpoints.
[0080] VI. Optoelectronic Integrated Circuits and Electronic
Circuits with Mixed Materials
[0081] III-V compound lasers can be integrated with Si circuits,
according to the invention, likely more easily than by using the
existing integration techniques, such as flip chip bonding and
epitaxial lift off. It is also possible to work on the whole wafer
instead of a fraction of the wafer, as in the flip chip bonding and
epitaxial lift off techniques.
[0082] It will be understood that various changes in the details,
materials, and arrangements of the parts which have been described
and illustrated above in order to explain the nature of the present
invention may be made by those skilled in the art without departing
from the principle and scope of the present invention, as recited
in the following claims.
* * * * *