U.S. patent application number 11/038008 was filed with the patent office on 2006-01-26 for domain epitaxy for thin film growth.
This patent application is currently assigned to Kopin Corporation. Invention is credited to Jagdish Narayan.
Application Number | 20060016388 11/038008 |
Document ID | / |
Family ID | 32074336 |
Filed Date | 2006-01-26 |
United States Patent
Application |
20060016388 |
Kind Code |
A1 |
Narayan; Jagdish |
January 26, 2006 |
Domain epitaxy for thin film growth
Abstract
A method of forming an epitaxial film on a substrate includes
growing an initial layer of a film on a substrate at a temperature
T.sub.growth, said initial layer having a thickness h and annealing
the initial layer of the film at a temperature T.sub.anneal,
thereby relaxing the initial layer, wherein said thickness h of the
initial layer of the film is greater than a critical thickness
h.sub.c. The method further includes growing additional layers of
the epitaxial film on the initial layer subsequent to annealing. In
some embodiments, the method further includes growing a layer of
the film that includes at least one amorphous island.
Inventors: |
Narayan; Jagdish; (Raleigh,
NC) |
Correspondence
Address: |
HAMILTON, BROOK, SMITH & REYNOLDS, P.C.
530 VIRGINIA ROAD
P.O. BOX 9133
CONCORD
MA
01742-9133
US
|
Assignee: |
Kopin Corporation
Taunton
MA
|
Family ID: |
32074336 |
Appl. No.: |
11/038008 |
Filed: |
January 18, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10608780 |
Jun 27, 2003 |
6955985 |
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11038008 |
Jan 18, 2005 |
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60393008 |
Jun 28, 2002 |
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60479206 |
Jun 17, 2003 |
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Current U.S.
Class: |
117/89 ;
257/E21.125; 257/E21.461 |
Current CPC
Class: |
H01L 21/02499 20130101;
H01L 21/02491 20130101; C30B 25/18 20130101; C30B 25/105 20130101;
H01L 21/0254 20130101; H01L 21/02472 20130101; H01L 21/0242
20130101; C30B 25/02 20130101; H01L 21/02617 20130101; C30B 29/40
20130101; H01L 21/02458 20130101; C30B 29/403 20130101; H01L
21/02554 20130101; C30B 23/02 20130101; H01L 21/02381 20130101 |
Class at
Publication: |
117/089 |
International
Class: |
C30B 23/00 20060101
C30B023/00; C30B 25/00 20060101 C30B025/00; C30B 28/12 20060101
C30B028/12; C30B 28/14 20060101 C30B028/14 |
Goverment Interests
GOVERNMENT SUPPORT
[0012] The invention was supported, in whole or in part, by a grant
from the National Science Foundation and by the U.S. Department of
Energy under the Contract No. DE-AC05-00OR22725 and Contract No.
W-31-109-ENG-38. The Government has certain rights in the
invention.
Claims
1. A method of forming an epitaxial film on a substrate, comprising
the steps of: (a) growing an initial layer of a film on a substrate
at a temperature T.sub.growth, said initial layer having a
thickness h; (b) annealing the initial layer of the film at a
temperature T.sub.anneal, thereby substantially completely relaxing
the initial layer.
2. The method of claim 1 further including growing additional
layers of the film over the initial layer subsequent to
annealing.
3. The method of claim 1 wherein said thickness h of the initial
layer of the film is greater than a critical thickness h.sub.c.
4. The method of claim 1 wherein h between about 1 and about 5
monolayers.
5. The method of claim 1 wherein T.sub.growth is about equal to
T.sub.anneal.
6. The method of claim 1 wherein T.sub.growth is less than
T.sub.anneal.
7. The method of claim 1 wherein growth of the initial layer
includes two-dimensional growth.
8. The method of claim 1 wherein the substrate includes Si(100) and
the film includes TiN.
9. The method of claim 1 wherein the substrate includes Si(111) and
the film includes at least one 111-nitride selected from the group
consisting of AlN, GaInN, and AlGaInN.
10. The method of claim 9 wherein the film includes AlN.
11. The method of claim 1 wherein the substrate includes
Al.sub.2O.sub.3(0001) and wherein the film includes at least one
member selected from the group consisting of ZnO, AlN, GaInN, and
AlGaInN.
12. The method of claim 11 wherein the film includes ZnO.
13. The method of claim 2 further including the step of growing a
layer of the film that includes at least one amorphous area.
14. The method of claim 14 wherein at least one amorphous area
includes Si.
15. The method of claim 14 wherein at least one area of amorphous
growth includes silicone nitride or silicone oxide.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 10/608,780 (now allowed), filed on Jun. 27, 2003, which claims
the benefit of U.S. Provisional Application No. 60/393,008, filed
on Jun. 28, 2002 and U.S. Provisional Application No. 60/479,206,
filed on Jun. 17, 2003. The entire teachings of the above
applications are incorporated herein by reference.
INCORPORATION BY REFERENCE
[0002] The entire teachings of the following are incorporated
herein by reference: [0003] U.S. application Ser. No. 10/608,780
filed on Jun. 27, 2003; [0004] U.S. application Ser. No. 10/463,219
filed on Jun. 17, 2003; [0005] U.S. Provisional Application No.
60/479,206, filed on Jun. 17, 2003; [0006] U.S. Provisional
Application No. 60/393,008, filed on Jun. 28, 2002; [0007] U.S.
Provisional Application No. 60/389,750, filed on Jun. 17, 2002;
[0008] U.S. patent application Ser. No. 10/187,466, filed on Jun.
28, 2002 [0009] U.S. patent application Ser. No. 10/187,465, filed
on Jun. 28, 2002 [0010] U.S. patent application Ser. No.
10/187,468, filed on Jun. 28, 2002; and [0011] U.S. Pat. No.
5,406,123.
BACKGROUND OF THE INVENTION
[0013] Epitaxial growth of thin films and control of defects in
thin film heterostructures are key considerations for the
next-generation microelectronic, optical and magnetic devices (H.
J. Queisser and E. E. Haller, Science (1998) 281: 945, S. Nakamura,
Science (1998) 281: 956, S. Mahajan, Acta Mater. (2000) 48: 137).
As device feature sizes are getting smaller, a single dislocation
is liable to control the device performance. In the
well-established lattice-matching-epitaxy, where lattice misfit is
small (i.e. less than 7-8%) the film grows pseudomorphically up to
a certain thickness (critical thickness) before it becomes
energetically favorable for the film to contain dislocations (J. W.
Mathews and A. E. Blakeslee, J. Crystal Growth (1974) 27:188, J. W.
Mathews in "Epitaxial Growth", Part B, Materials Science Series
(1975) 560, Academic Press, New York). In this case, the
dislocations are generated at the surface and then they glide to
the interface. The Burgers vectors and planes of the dislocations
are dictated by the slip vectors and glide planes of the crystal
structure of the film (J. Narayan and S. Sharan, Mat. Sci.
Engineering B (1991) 10: 261). On the other hand, if the
dislocations are generated at the edge of islands during
three-dimensional growth, the geometrical constraints determine the
Burgers vectors of the dislocations, which lie in the
film-substrate interface. For example, during three-dimensional
growth of germanium on silicon, it was found that 90.degree.
dislocations with a/2<110> Burgers vectors were created at
the edge of germanium islands lying in the (001) film-substrate
interface (F. K. LeGoues et al. Phys Rev. Lett. (1994) 73: 300). It
is believed that the lattice matching epitaxy during thin film
growth is possible as long as the lattice misfit between the film
and the substrate is less than 7-8%. Smaller lattice misfit leads
to smaller interfacial energy and coherent epitaxy is formed. Above
this misfit the film generally will grow in a textured or largely
polycrystalline manner. Such films contain plane boundaries. Plain
boundaries consist of dislocation, which impede charge carrier
movement and thus deleteriously affect the performance of
semiconductor devices.
[0014] Accordingly, there is a need for electronic and
semiconductor devices where a single crystal thin film layer of
arbitrary crystal structure is epitaxially grown on top of a
pre-selected substrate and where a lattice misfit between the
epitaxial layer and the substrate is arbitrarily large.
SUMMARY OF THE INVENTION
[0015] In general, the instant invention relates to domain-matched
epitaxial growth of a film on top of a substrate, whereby the
integral multiples of lattice planes match across the
film-substrate interface.
[0016] The method of the invention includes forming an epitaxial
film on a substrate by growing an initial layer of a film on a
substrate at a temperature T.sub.growth, said initial layer having
a thickness h. The initial layer of the film is then annealed at a
temperature T.sub.anneal, thereby relaxing the initial layer.
[0017] The advantages of the present invention include, for
example, the ability to design and grow essentially strain-free
epitaxial films on a substrate having arbitrarily large lattice
misfit. Additional strain beyond perfect domain matching is
relieved by a systematic (periodic) variation in domain size
resulting in relieving most of strain within a couple of monolayers
of the epitaxial material, so that the rest of the film can be
grown free from lattice strains and the attending defects.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a plot of strain (lattice misfit) versus
film-substrate planar spacing ratio. The LME region is above about
12/13 ratio or below about 7.7% strain. The plot can be used to
predict the planar spacing ratio and domain periodicity of an
epitaxial layer grown according to the method of the present
invention.
[0019] FIG. 2 is a high-resolution microphotograph of a
cross-section in <110> direction from the TiN/Si(100) system
grown according to the method of the presernt invention. The
microphotograph shows domain matching of TiN with silicon. Here the
frequency factor (.alpha.=0.5) for 4/3 and 4/5 domains. The
a/2<110> misfit dislocations lie in {111} planes in both TiN
and silicon.
[0020] FIG. 3(a) is a microphotograph depicting domain epitaxy in
AlN/Si(111) system, grown according to the method of the invention.
The microphotograph of a high-resolution cross-section of a
(01-10)AlN epitaxial layer and a (11-20)Si substrate is showing the
matching of AlN(2{overscore (11)}0) and Si(220)planes with
.alpha.=0.25 for 4/5 and 5/6 domains, while corresponding (inset)
diffraction pattern shows the alignment AlN and Si planes.
[0021] FIG. 3(b) is a schematic of arrangement of atoms in the
basal plane of AlN and Si(111) system as grown by the method of the
present invention.
[0022] FIG. 4(a) is a high resolution TEM cross-section with
(01-10) foil plane of sapphire and (2-1-10) plane of ZnO showing
domain epitaxy in ZnO/.alpha.Al.sub.2O.sub.3(sapphire) system,
grown according to the method of the present invention.
[0023] FIG. 4(b) is a Fourier-filtered image of a matching of
(-2110) ZnO and (30{overscore (3)} 0) sapphire planes of the system
of FIG. 4(a) with the frequency factor (.alpha.=0.5) for 5/6 and
6/7 domains.
[0024] FIG. 4(c) is a photograph of an electron diffraction pattern
corresponding to FIG. 4(a), showing the alignment of planes in ZnO
and sapphire.
[0025] FIG. 4(d) is a schematic of arrangement of atoms in the
basal plane of ZnO and sapphire of the system of FIG. 4(a).
[0026] FIGS. 5(a) and (b) show X-ray surface diffraction
measurements along the (H, 0, --H, 0.3) direction showing the
growth according to the method of the present invention of ZnO
films on sapphire with sapphire in-plane lattice parameter
approaching H=0.845 corresponding to a fully relaxed position after
a few monolayers.
[0027] FIGS. 6(a) and (b) are cross-section TEM micrographs from
ZnO/Sapphire specimens grown according to the method of the present
invention under two different diffraction conditions (g vectors )
showing low density of threading dislocations, stacking faults and
domain boundaries. Most of the dislocations are confined to the
interface.
DETAILED DESCRIPTION OF THE INVENTION
[0028] The foregoing and other objects, features and advantages of
the invention will be apparent from the following more particular
description of preferred embodiments of the invention, as
illustrated in the accompanying drawings in which like reference
characters refer to the same parts throughout the different views.
The drawings are not necessarily to scale, emphasis instead being
placed upon illustrating the principles of the invention.
[0029] It has now been discovered that a new technique allows
epitaxial growth of films with any lattice misfit on a given
substrates with atomically clean surfaces.
[0030] As used herein the term "lattice misfit," also referred to
as "unrelaxed misfit strain," is defined as
.epsilon..sub.c=a.sub.f/a.sub.s-1, where a.sub.f and a.sub.s are
lattice constants of the film and the substrate, respectively.
[0031] As used herein, the term "epitaxy" refers to a process in
which a thin layer, referred to as a "film," of a single crystal
material is deposited on a single crystal substrate in such a
manner that the crystallographic structure of the substrate is
reproduced in the growing film. The term "heteroepitaxy" refers to
an epitaxial growth wherein chemical composition of an "epi"
material, i.e. a film, is different from the chemical composition
of the substrate. Epitaxy can be achieved by any of the methods
known in the art. Such methods include pulsed laser deposition
(PLD), chemical vapor deposition (CVD), metal-organic vapor
deposition (MOCVD), and molecular beam epitaxy (MBE).
[0032] In PLD, high-power pulsed laser beams are used to ablate the
target in a vacuum (less than about 10.sup.-6 torr) or a controlled
atmosphere. This produces a film of the same composition as the
target.
[0033] In CVD, epitaxial growth is implemented by means of chemical
reaction of a gas-phase epi material with an exposed solid
substrate. The MOCVD is a similar procedure commonly used when the
epi material is a III-V semiconductor. MBE, is a method of
epitaxial growth wherein physical deposition of the epi material is
carried out in ultra-high vacuum (at about 10-8 torr) and at the
substrate temperatures not exceeding about 800.degree. C. In MBE,
an stream (a beam) of the molecules of the epi material is directed
at a chemically clean substrate surface.
[0034] As used herein, the term "domain" refers to a portion of an
interface between a substrate and an epitaxially grown layer
comprising a whole number "n" of lattice planes on the substrate
side of the interface, said planes having a separation distance
"d.sub.s" and a whole number "m" of lattice planes on the epitaxial
layer side of the interface, said planes having a separation
distance "d.sub.f" and wherein the distances between the maximally
separated planes within the portion of the interface on either side
is about equal. As used herein, the term "domain-matching" refers
to a step of forming at least one domain as defined above in a
process of growing an epitaxial layer on a substrate.
[0035] As used herein, the term "critical thickness" refers to a
thickness of a film, such that it becomes energetically favorable
for the film to contain dislocations if the thickness of the film
is above the critical thickness.
[0036] As used herein, the term "relaxation" refers to orienting
the components (asymmetric units such as atoms or molecules) of a
material of a film so that said orientation is energetically most
favorable. Relaxation is measured by a degree of difference between
the experimentally measured lattice constant of a material and the
equilibrium value of the lattice constant. As used herein, the term
"complete relaxation" refers to a state of a material wherein the
lattice constant of the material is equal to the equilibrium
lattice constant of this material.
[0037] According to the method of the present invention, the
integral multiples of lattice planes match across the
film-substrate interface, and the size of the domain equals the
integral multiples of planar spacing. Furthermore, the size of the
domain can be varied periodically to accommodate the lattice misfit
that is not accommodated by perfect matching.
[0038] Without being limited to any particular theory, it is
believed that the theoretical basis of the invention is as
follows.
Fundamental Considerations in Domain-Matched Epitaxy
[0039] In domain matching epitaxy (DME), matching of lattice planes
is considered, which could be different in different directions of
the film-substrate interface. In the DME framework, the crystal
structure of the film can have either the same or different
orientation relationship with the substrate, depending on the
nature of the misfit. The misfit is accommodated by matching of
integral multiples of lattice planes, and there is one extra
half-plane (dislocation) corresponding to each domain. The misfit
can range from being very small to very large. In a case of
relatively small misfit, the DME reduces to the conventional
lattice matching epitaxy (LME), where matching of the same planes
or lattice constants is considered with a misfit typically less
that about 7-8%. If the misfit falls in between the perfect
matching ratios of planes, then the size of the domain can vary in
a systematic (periodic) way to accommodate the additional misfit.
In LME, the unrelaxed misfit strain EC is less than 7 to 8% and is
relaxed by the introduction of dislocations beyond the critical
thickness during thin film growth. In DME, the matching of lattice
planes of the film (separated by distance d.sub.f) with the those
of the substrate (separated by distance d.sub.s) with similar
crystal symmetry is considered. In DME, the film and the substrate
planes could be quite different as long as they maintain the
crystal symmetry. The LME, on the other hand, involves the matching
of the same planes between the film and the substrate. In DME, the
initial misfit strain (.epsilon.=d.sub.f/d.sub.s-1 ) could be very
large, but can be relaxed by matching of m planes of the film with
n of the substrate. This matching of integral multiples of lattice
planes leaves a residual strain of .epsilon..sub.r given by
.epsilon..sub.r=(md.sub.f/nd.sub.s-1) (1), here m and n are
integers. In the case of a perfect matching md.sub.f=nd.sub.s, and
the residual strain .epsilon..sub.r is zero. If .epsilon..sub.r is
finite, then two domains may alternate with a certain frequency to
provide for a perfect matching according to,
(m+.alpha.)d.sub.f=(n+.alpha.)d.sub.s (2), where .alpha. is a
frequency factor. For example, if .alpha.=0.5, then m/n and
(m+1)/(n+1) domains alternate with equal frequency.
[0040] Assuming d.sub.f>ds, we have n>m. Therefore, n-m=1 or
f(m) (3). The difference between n and m could be 1 or some
function of m, corresponding to the lowest energy of the
system.
[0041] From equations (1) through (3), thye following equation can
be derived, (m+.alpha.).epsilon.=1 or f(m) (4). Equation (4)
governs the domain epitaxy. FIG. 1 shows a general plot of misfit
percent strain as a function of ratio of film/substrate lattice
constants of major planes matching across the interface. In FIG. 1,
n-m=1 for .epsilon.=0-50% and n-m=f(m) for .epsilon.=50 to
100%.
[0042] Table I provides a summary of different systems, which have
been grown with various misfit strains. Table I also includes the
systems, which fall in between the two domains, where two domains
alternate with periodicity needed for a essentially complete
relaxation. TABLE-US-00001 TABLE I Domain Epitaxy for Thin Film
Growth Planar spac- m/n ing ratio Experimental Examples Strain
.epsilon. % 1/10 0.1 90.0% 1/9 0.11 88.8% 1/8 0.125 87.5% 1/7 0.143
85.7% 1/6 0.166 83.3% 1/5 0.20 80.0% 1/4 0.25 75.0% 1/3 0.33 66.7%
1/2 and 0.33-0.50 Mo, Nb, Ta, W/Si; Ni.sub.3Al/Si(100) 50.0 % 1/3
1/2 0.50 Fe/Si, Cr/Si, NiAl//Si(100) 2/3 0.666 Cu/Si(100) 33.33 %
1/ 2 0.707 SrTiO3/Ge(100) 29.28 % 3/4 0.750 TiN/Si(100) 25.00% 4/5
0.80 AlN/Si(111) 5/6 0.83 .alpha.-Al.sub.2O.sub.3/ZnO(0001), 6/7
0.857 .alpha.-Al.sub.2O.sub.3/ZnO(0001), Cu/TiN(100) 14.29 % 7/8
0.8750 .alpha.-Al.sub.2O.sub.3/GaN(0001) 8/9 0.888
.alpha.-Al.sub.2O.sub.3/AlN(0001) 11.11 % 9/10 0.90
Y.sub.123/MgO(001) 10.0 % 11/12 0.9166 Y.sub.123/MgO(001) 8.33%
12/13 0.9230 STO/MgO(001) 7.69% 13/14 0.3286 7.14% 14/15 0.9333
6.67% 16/17 0.9412 5.88% 17/18 0.9444 5.55% 18/19 0.9474 5.26%
19/20 0.9500 5.00% 20/21 0.9524 4.76% 22/23 0.9556 4.35% 24/25 0.96
Ge/Si(100) 4.0% 31/32 0.9687 3.13% 49/50 0.98 Ge-Si/Si(100)
2.0%
[0043] It should be noted that a 45.degree. rotation in some cubic
systems and a 30.degree. rotation in certain hexagonal systems are
part of the domain-matching concept involving the matching of major
planes between the film and the substrate. The plot in FIG. 1
provides a unified view of lattice-matching and domain-matching
epitaxy with misfit strain ranging from 2-90% (50% corresponding to
1/2 matching). If the domain matching is not perfect, epitaxy
occurs by accommodating the additional misfit by changing the
domain size, controlled by the parameter .alpha.. In this
framework, it is important to realize that the nature of
dislocations remains the same, only their periodicity changes.
Lattice Relaxation and Defect Reduction
[0044] The rapid relaxation process of DME is consistent with the
fact that the critical thickness under relatively large misfits is
less than 1 monolayer. As a result, dislocations can nucleate
during initial stages of growth and confine most of the defects
near the interface, leaving fewer defects near an active region of
a device.
[0045] Since the critical thickness at which it becomes
energetically feasible for the film to contain dislocations is less
than one monolayer, the dislocations nucleate at free-surface steps
within 1 monolayer and locate at the interface where there is an
energy minimum. An important consideration here is a large number
density of surface steps within the monolayer, which can provide
easy nucleation sites for dislocations. If the initial growth is
two-dimensional, the dislocations can propagate throughout the
entire length of the film and confine themselves near the interface
without creating threading dislocations. However, if the initial
growth is a mixture of two-dimensional (2D) and three-dimensional
(3D) growth, then dislocation segments may not propagate throughout
the entire length and threading segments may form. Depending upon
the nature of growth characteristics and the number density of
surface steps, this consequence of DME can be used to reduce the
number density of threading dislocations and confine most of the
misfit dislocations near the interface.
[0046] However, if the critical thickness is relatively large in a
low-misfit system, then the dislocations nucleate at the
free-surface steps and then glide to the interface. The process
creates a half-loop configuration with two threading segments and a
straight segment along the interface. Since there is a nucleation
barrier for the dislocation, misfit is not fully relaxed. In
addition, threading segments do not expand to the edges due to the
presence of other dislocations and obstacles, and as a result, a
high density of these dislocations is retained within the film.
Since these dislocations are purely glide or slip dislocations,
their planes and Burgers vectors are controlled by the slip systems
of the film. The relaxation process in low-misfit systems is
gradual due to this nucleation barrier, leading to a large number
of threading dislocations.
Preferred Embodiments of the Invention
[0047] Using the method of the present invention, films having
larger misfits can be grown with fewer defects in the active region
than generally occur by employment of conventional techniques.
[0048] In one embodiment, the present invention is a method of
forming an epitaxial film on a substrate. The surface of the
substrate preferably is atomically clean. Materials of a film and a
substrate preferably have interatomic potentials that are not
substantially different. Most preferably, the interatomic
potentials do not differ by more than a factor of two. Materials of
the film and of the substrate are further selected so that the
symmetry of the crystal structures of two materials is similar.
Examples of crystal structures with similar symmetries are two
structures that have at least one symmetry element in common.
[0049] The method comprises the steps of growing an initial layer
of a film on a substrate at a temperature T.sub.growth, said
initial layer having a thickness h, and annealing the initial layer
of the film at a temperature T.sub.anneal, thereby relaxing the
initial layer. Preferably, the method further includes growing
additional layers of the film. Most preferably, said thickness h of
the initial layer of the film is greater than a critical thickness
h.sub.c.
[0050] As used herein, "T.sub.growth" is a temperature sufficient
to form an initial layer. As used herein, the term "annealing"
refers to holding the initial layer without further deposition of
film material at a temperature T.sub.anneal for a non-zero period
of time. In one embodiment, the period of time during which the
formed initial layer is annealed causes substantially complete
relaxation of the crystal structure of the material of the initial
layer. Preferably, the period of time during which the formed
initial layer is annealed causes essentially complete relaxation of
the crystal structure of the material of the initial layer. As used
herein, "essentially complete relaxation" means that there is no
detectable difference between an experimentally measured lattice
constant and an equilibrium lattice constant.
[0051] As used herein, "T.sub.anneal" is a temperature at which the
formed initial layer is held without further depositing film
material for a period of time sufficient to permit relaxation. In
one embodiment, the period of time is sufficient to permit
substantially complete relaxation. In another embodiment, the
period of time is sufficient to permit essentially complete
relaxation.
[0052] As used herein, "h" is a height of an initial layer formed
during deposition of film material prior to annealing. The value of
h can be measured either in Angstroms or in monolayers of atoms of
the film.
[0053] Preferably, h is greater than h.sub.c. Generally, h is
between about 1 and about 10 monolayers. Preferably, h is between
about 1 and about 5 monolayers.
[0054] Temperature T.sub.growth can be about equal to T.sub.anneal
or T.sub.growth can be less than T.sub.anneal The time required for
annealing is dependent on temperature T.sub.anneal: the higher the
annealing temperature, the less time is required for annealing.
Preferably, T.sub.growth is between about 500.degree. C. and
1000.degree. C. More preferably, T.sub.growth is between about 500
.degree. C. and about 750.degree. C. T.sub.anneal is preferably
between about 500.degree. C. and about 1000.degree. C. More
preferably, T.sub.anneal is between about 700.degree. C. and
900.degree. C.
[0055] Growth and annealing can independently be performed for a
period of time from about 5 seconds to about 5 minutes. Preferably,
growth and annealing can independently be performed for a period of
time from about 1 minute to about 2 minutes.
[0056] The growth of the initial layer can be three-dimensional or
two-dimensional. Two-dimensional growth is preferred.
[0057] In another embodiment, the method of the present invention
further includes the step of growing a layer of the film that
includes at least one area of amorphous growth. Areas, or
"islands," of amorphous growth can generally be created by using
either impurities (dopants) that exist in film material or by
introduction of new or additional impurities. Many dislocations
that would otherwise propagate beyond the initial layer of film
subsequent to relaxation terminate at the islands. As used herein,
"introduction of additional impurity" refers to increasing
concentration of a dopant that is already present in the material
of the film.
[0058] Preferably, suitable impurities exist in the material of the
layer and, upon introduction of additional impurity, either alone
or in combination with the material deposited as the film can form
amorphous phase. Examples of suitable impurities are silicon and
germanium. Silicone or germanium oxides and nitrides can form an
amorphous phase. One of ordinary skill in the art can readily
identify other suitable impurities. As a non-limiting example, a
layer of a GaN film including at least one area of amorphous growth
that includes silicon nitride or silicon oxide can be produced.
After the initial layer is deposited and relaxed, the growth
continues using MOCVD or pulsed laser deposition techniques,
described below, until the film is about 1000 .ANG. thick. Then,
silane and ammonia, if MOCVD is employed, or silicon and nitrogen
gas, if the pulsed laser deposition technique is employed, are used
for SiN.sub.x formation, where x is between about 0.5 and about
1.33. The temperature for this process is from about 800.degree. C.
to about 1000.degree. C. Alternatively, to form SiO.sub.y, wherey
is between about 0.5 and about 2, silane and nitrous oxide
(N.sub.2O) at a temperature of from about 400.degree. C. to about
500.degree. C. are used.
[0059] The methods of the present invention generally can
incorporate any of the epitaxy techniques known in the art. In a
preferred embodiment, an apparatus as described in U.S. Pat. No.
5,406,123 is used. While, according to the present invention,
overlayers, such as TiN, AlN on Si(100) and ZnO on
.alpha.-Al.sub.2O.sub.3(0001), are preferably formed using pulsed
laser deposition processes as described in J. Narayan et al., Appl.
Phys. Lett. (1992) 61:1290, the entire teachings of which are
herein incorporated by reference, it is to be understood that
layers may be formed using various known processes including, but
not limited to, molecular beam epitaxy (MBE), hydride vapor phase
epitaxy (HVPE), magnetron sputtering techniques, and chemical vapor
deposition (CVD).
[0060] The method of the present invention can be employed to
produce components of various devices such as light-emitting
diodes, laser diodes, UV detectors, and/or broad spectrum (e.g.
white) light sources used, for example, in light bulbs.
Microelectronic devices manufactured according to embodiments of
the present invention can be transistors such as CMOS transistors,
field-effect transistors, and/or bipolar transistors, diodes, field
emitters, and/or power devices as well as other integrated circuit
devices. Optoelectronic devices manufactured according to the
method of the present invention can provide junctions between a
substrate and a overlayer to provide LEDs, laser diodes, UV
detectors, broad band light sources, and/or other optoelectronic
devices. Junctions can be provided by doping and/or other
techniques as will be understood by those skilled in the art.
EXEMPLIFICATION
Example 1
TiN Film on Si(100) Substrate Accommodates a 22% Misfit
[0061] Epitaxial growth of TiN on silicon substrate represents a
major milestone for next-generation semiconductor devices for
direct ohmic contacts as well as for discussion barriers in copper
metallization. However, with a misfit of over 22% for cube-on-cube
TiN (a=0.424 nm) epitaxy over silicon (a=0.543 nm), it is beyond
the critical strain (approximately 7-8%) of conventional lattice
matching. However, epitaxial growth of TiN on silicon substrate was
demonstrated by the concept of domain matching epitaxy. The films
were grown using a standard pulsed laser deposition method known in
the art. Briefly, the initial layer of TiN of thickness h about 1-2
monolayers was deposited at T.sub.growth of between about
600.degree. C. and 650.degree. C. and then annealed at T.sub.anneal
of about 750.degree. C. for about 1 to about 2 minutes. After
annealing, the growth was resumed at T.sub.growth.
[0062] FIG. 2 shows a detailed high-resolution cross-section TEM
micrograph, where the 3/4 and 4/5 domains alternate. The micrograph
was taken in the <110> zone axis of Si and TiN, it is
interesting to note the matching of {111} extra half planes in
silicon as well as TiN. From FIG. 1, the lattice misfit of 22% lies
in the middle 3/4 and 4/5 matching, which explains the alternating
of domains. In fact with .alpha.=0.5 (see equation 2),
3.5a.sub.Si=19.01 matches quite well with
4.5.times.a.sub.TiN=19.08, which also represents the size of domain
for the system with virtually no residual misfit.
[0063] The nature of dislocations can be established directly from
the high-resolution TEM micrographs. The Burgers vector of the
dislocations is determined to be a/2 <110> lying in {111}
planes. The two sets of a/2<110> dislocations combine at the
interface to produce a/2<110> dislocations lying in the {001}
interface. This dislocation reaction can be described as:
a/2[101](11-1)+a/2[01-1](111).fwdarw.a/2[110](001). In some cases,
the dislocations do not combine and create an extended core
structure associated with the pair of dislocations. The formation
of a/2<110> dislocations in {111} plane in TiN with a sodium
chloride structure represents a significant finding. The TiN having
a sodium chloride structure has {110} slip planes with
a/2<110> Burgers vectors. Only under certain extreme
nonequilibrium conditions such as high field, a/2<110> lying
in {001} planes have been observed (18). However, this is first for
a/2<110> dislocation in {111} planes of sodium chloride
structure. These new dislocations or slip systems may impact
mechanical and physical properties of TiN films or materials of
sodium chloride structure, in general, in a significant way.
According to von Mises criterion, five independent slip systems are
needed for a crystal to undergo a plastic deformation by slip. In
TiN having a sodium chloride structure, there are only two
independent a/2<110>{110} slip systems available, which
restricts a general deformation, resulting in twinning and
fracture. However, with a/2<110>{111} slip systems, there are
384 ways of choosing five independent slip systems, which can lead
to a general deformation of TiN (J. P. Hirth and J. Lothe, Theory
of Dislocations, P. (1998) John Wiley, New York).
Example 2
Variations in Domain Size Accommodate a 20 % Strain in III-nitride
Epitaxy on Si(111) System
[0064] Epitaxial growth of III-nitrides having a wurtzite structure
on silicon (111) substrates are needed as a template to grow GaInN
and AlGaInN alloys as well as to integrate III-nitride based
optoelectronic devices with microelectronic devices. Additionally,
AlN has high thermal conductivity (320 w/MK), high thermal
stability (up to 2200.degree. C.), high resistivity (10.sup.13
.OMEGA.-cm), high dielectric strength (14 kV/cm), and high chemical
inertness. The hardness and thermal expansion coefficient
(2.56.times.10.sup.-6/K are comparable to that of silicon. The
above properties make AlN an ideal candidate for application in
microelectronic to optoelectronics including high-temperature
devices and electronics packaging.
[0065] Epitaxial growth of AlN (0001) with hexagonal wurtzite
structure (a=3.11A, c=4.982A) on silicon (111) substrate occurs via
matching of four silicon (220) planes with five (2{overscore
(1)}{overscore (1)}0) planes of AlN. The spacing of (2 {overscore
(1)} {overscore (1)} 0)AlN planes (a/2=1.556A) result is close to
19% strain with (220) planes of silicon. Using this strain, we
found by using the plot in FIG. 1 that, 5 AlN (2 {overscore (1)}
{overscore (1)} 0)/4 (220) matching results with less than 1%
residual strain.
[0066] Using the standard pulsed laser deposition technique known
in the art, we have grown an epitaxial film of AlN (0001) having a
hexagonal wurtzite structure on silicon (111) substrate. Briefly,
the initial layer of AlN of thickness h about 1-2 monolayers was
deposited at T.sub.growth of about 650.degree. C. and then annealed
at T.sub.anneal of about 800.degree. C. for about 1 to about 2
minutes. After annealing, the growth was resumed at
T.sub.growth.
[0067] FIG. 3(a) shows a cross-section TEM micrograph where the
alignment of (2 {overscore (1)} {overscore (1)} 0 ) planes of AlN
with (220) planes of silicon is clearly delineated. The (111)
planes of silicon substrate are shown schematically in FIG. 3(b) on
which basal planes of AlN {0001 } grow with a-axis of AlN [2
{overscore (1)} {overscore (1)} 0] aligned with [220] direction of
silicon. In this field of view, five planes of AlN clearly match
with four planes of silicon with one exception where six planes of
AlN match with 5 planes of silicon. This is predicted from our
master diagram in FIG. 1 for a 19% strain. The perfect matching is
predicted from equation (3) for .alpha.=0.25. Thus, the deviations
from the ideal 5/4 matching (corresponding to 20% strain) are
accommodated by variation in domain size, rather than an additional
set of secondary dislocations to relieve the difference in the
strain from the ideal 5/4 matching.
Example 3
Domain Epitaxy of Wurtzite Hexagonal ZnO on .alpha.-Al.sub.20.sub.3
(0001)
[0068] There is a growing interest in growing high quality thin
films of ZnO and its alloys for light emitting diodes (LEDs) and
laser diodes (LEDs) applications. The bandgap of ZnO can be tuned
by alloying with MgO (8.0 eV, upshift) or with CdO (1.9 eV,
downshift). The ZnO can be also used as a template for III-nitride
growth separately as well as on sapphire substrates. Therefore, the
growth of high quality ZnO (having wurtzite hexagonal structure,
a=3.252 A, c=5.213 A) on a practical substrate such as sapphire
(a=4.758 A, c=12.991 A) presents a major challenge. The growth of
systems with such a relatively large misfit is possible only with
domain matching epitaxy, where the misfit can be accommodated the
matching of planes.
[0069] Using the standard pulsed laser deposition technique known
in the art, we have grown an epitaxial film of ZnO having a
hexagonal wurtzite structure on silicon (111) substrate. Briefly,
the initial layer of ZnO of thickness h about 1-2 monolayers was
deposited at T.sub.growth of about 650.degree. C. and then annealed
at T.sub.anneal of about 700.degree. C. for about 1 to about 2
minutes. After annealing, the growth was resumed at
T.sub.growth.
[0070] FIG. 4(a) shows a high-resolution cross-section TEM
micrograph where the ZnO film plane is (2-1-10) and the sapphire
substrate is (01-10). The epitaxial growth of ZnO film with
atomically sharp interface is clearly demonstrated. The
Fourier-filtered image in FIG. 4(b) clearly delineates the matching
of 5 or 6 (-2110) planes of ZnO with (30{overscore (3)} 0) 6 or 7
planes of sapphire. The corresponding diffraction pattern, which
confirms this alignment of planes, is shown in FIG. 4(c). The
c-plane of ZnO rotates by 30.degree. in the basal c-plane of
sapphire as shown in FIG. 4(d), which leads to alignment of 1/2
(30{overscore (3)} 0) planes of sapphire with (2{overscore (1 1)}
0) planes or `a` planes of the ZnO film. Thus, we are looking at
domain matching of sapphire planes (having a.sub.sap/ {square root
over (3)}. spacing) with `a` planes of ZnO. By alternating the
domains, there is almost a perfect matching as 5.5.times.a.sub.ZnO
(3.2536 ).apprxeq.6.5 .alpha.-Al.sub.2O.sub.3 (2.7512), as
predicted for .alpha.=0.5 from equation (3). These numbers include
planar spacings at the growth temperature, taking into account the
respective coefficients of thermal expansion. From the planar
spacing, we calculate the strain of 15.44%, which falls in between
5/6 and 6/7 matching in the master plot of FIG. 1. This is in
complete agreement with experimental observation of FIG. 4(a) and
FIG. 4(b).
Example 4
In situ X-Ray Diffraction Measurements Show Complete Relaxation
After Deposition of Two Monolayers
[0071] The critical thickness for this system is less than one
monolayer. Two monolayers of ZnO were grown at 585 to 600.degree.
C. Annealing was performed either at 700.degree. C. for about 10
minutes or 800.degree. C. for about 1 minute. After annealing, the
growth was resumed at 585 to 600.degree. C.
[0072] The details of lattice relaxation process during initial
stages of ZnO growth on sapphire (.alpha.-Al.sub.2O.sub.3), (0001))
substrates have been studied by in-situ x-ray diffraction study
using the UNI-CAT undulator beam line at the Advanced Photon
Source. In these experiments, the laser-ablation, filmgrowth
chamber is mounted on a so-called 2+2 x-raydiffractometer where
surface scattering measurements in specular and off-specular
directions were made to investigate the details of initial stages
of thin film growth. Time-slice x-ray crystal truncation rod (CTR)
measurements made after each excimer laser ablation pulse revealed
the surface structure transients associated with ZnO clustering and
crystallization to last about 2 s following the abrupt .about.5
.mu.s duration of laser deposition.
[0073] Specular CTR anti-Bragg measurements at the sapphire (0 0
5/2) position showed only one well-defined growth oscillation,
indicating three-dimensional (3D) growth rather than layer by laser
growth. Off-specular CTR measurements along the (H, 0, --H, 0.3)
direction showed thermally activated relaxation of the 15.44%
lattice mismatch between ZnO and Al.sub.2O.sub.3 along with a
30.degree. in-plane rotation around the c axis. As shown in FIG. 5,
a broad, nearly relaxed ZnO in-plane diffraction peak appears after
the deposition of 3 monolayers at 400.degree. C. (25
pulses/monolayer), while a sharper and more fully relaxed ZnO peak
appears after only 2 monolayers at 585.degree. C. The peak after
150 pulses at 585.degree. C. occurs at H=0.845 corresponding to the
fully relaxed ZnO film. Subsequent measurements (not plotted here)
showed that incommensuration occurs within the first layer of the
deposition, and the nature of the strain is compressive as expected
for matching of a planes of ZnO (spacing 3.2536 .ANG.) with
underlying sapphire planes (2.7512 .ANG.). These results clearly
established a rapid relaxation of ZnO films on sapphire. The
relaxation process was found to be thermally activated because the
ZnO thickness corresponding to complete relaxation decreased as the
deposition temperature increased. The relaxation process requires
the creation of dislocations, which involves nucleation and
propagation of dislocations. Both of these steps are thermally
activated. The nucleation barrier can be partially overcome by the
surface steps, and the propagation is very small in DME due to the
proximity of the interface.
Example 5
Dislocations are Confined to the Inteface in ZnO/Sapphire
Specimens
[0074] FIG. 6(a) and FIG. 6(b) show TEM cross-section of
ZnO/Saphhire specimens, grown as described in Example 4, under two
different diffraction conditions to image dislocations.
[0075] From these micrographs, the density of threading
dislocations with Burgers vector b=1/3[11-20] was estimated to be
10.sup.7cm.sup.-2, which is three orders of magnitude lower than
normally observed for the misfit of this magnitude (15.44%). The
density of stacking faults (planar defects) was estimated to be
10.sup.5 cm-1. It is interesting to note that most of the
dislocations and other defects (stacking faults and domain
boundaries) are confined to the ZnO/Sapphire interface as expected
from domain epitaxy.
EQUIVALENTS
[0076] While this invention has been particularly shown and
described with references to preferred embodiments thereof, it will
be understood by those skilled in the art that various changes in
form and details may be made therein without departing from the
scope of the invention encompassed by the appended claims.
* * * * *