U.S. patent application number 12/168435 was filed with the patent office on 2009-01-08 for ceramic thin film on various substrates, and process for producing same.
This patent application is currently assigned to Sixtron Advanced Materials, Inc. Invention is credited to Cetin Aktik, Mihai Scarlete.
Application Number | 20090008752 12/168435 |
Document ID | / |
Family ID | 29555369 |
Filed Date | 2009-01-08 |
United States Patent
Application |
20090008752 |
Kind Code |
A1 |
Scarlete; Mihai ; et
al. |
January 8, 2009 |
Ceramic Thin Film On Various Substrates, and Process for Producing
Same
Abstract
The process of Polymer Assisted Chemical Vapor Deposition
(PACVD) and the semiconductor, dielectric, passivating or
protecting thin films produced by the process are described. A
semiconductor thin film of amorphous silicon carbide is obtained
through vapor deposition following desublimation of pyrolysis
products of polymeric precursors in inert or active atmosphere.
PA-CVD allows one or multi-layers compositions, microstructures and
thicknesses to be deposited on a wide variety of substrates. The
deposited thin film from desublimation is an n-type semiconductor
with a low donor concentration in the range of 10.sup.14-10.sup.17
cm.sup.-3. Many devices can be fabricated by the PA-CVD method of
the invention such as; solar cells; light-emitting diodes;
transistors; photothyristors, as well as integrated monolithic
devices on a single chip. Using this novel technique, high
deposition rates can be obtained from chemically synchronized Si--C
bonds redistribution in organo-polysilanes in the temperature range
of about 200-450.degree. C.
Inventors: |
Scarlete; Mihai; (Roxboro,
CA) ; Aktik; Cetin; (Sherbrooke, CA) |
Correspondence
Address: |
RATNERPRESTIA
P.O. BOX 980
VALLEY FORGE
PA
19482
US
|
Assignee: |
Sixtron Advanced Materials,
Inc
Varennes
CA
|
Family ID: |
29555369 |
Appl. No.: |
12/168435 |
Filed: |
July 7, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10515450 |
Jun 24, 2005 |
7396563 |
|
|
PCT/CA03/00763 |
May 23, 2003 |
|
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12168435 |
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Current U.S.
Class: |
257/632 ;
257/E23.002; 428/336; 428/411.1; 428/446; 428/698 |
Current CPC
Class: |
C23C 16/325 20130101;
Y10T 428/265 20150115; C23C 16/46 20130101; Y10T 428/31504
20150401 |
Class at
Publication: |
257/632 ;
428/411.1; 428/446; 428/698; 428/336; 257/E23.002 |
International
Class: |
H01L 23/58 20060101
H01L023/58; B32B 9/04 20060101 B32B009/04; B32B 27/00 20060101
B32B027/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 23, 2002 |
CA |
2387274 |
Claims
1-17. (canceled)
18. A ceramic thin film deposited on a substrate comprising chains
from a polymeric source that have been deposited on the substrate
and chemically rearranged and annealed wherein the film is
essentially free of defects.
19. The film according to claim 18, wherein the polymeric source is
a silicon-based polymer.
20. The film according to claim 18, wherein the polymeric source is
a boron nitride polymer.
21. The film according to claim 18, wherein the polymeric source is
a carbon nitride polymer.
22. The film according to claim 18, wherein the polymeric source
comprises Si, C, N or O substituents.
23. The film according to claim 18, wherein the film has a
thickness of at least 100 .ANG..
24. The film according to claim 18, wherein the film in a singular
or multiple layer includes a donor concentration of less than
10.sup.15 cm.sup.-3 in a depletion zone next to the substrate prior
to doping.
25. A semiconductor device comprising the film according to claim
18.
26. The semiconductor device according to claim 25, wherein the
device is a p-type or an n-type.
27. The semiconductor device according to claim 25, wherein the
device is selected from the group consisting of a solar cell,
light-emitting diode, Schottky diode, a transistor, a
photothyristor and an integrated monolithic device on a single
chip.
28. (canceled)
29. The film according to claim 18, wherein the polymeric source is
transported through a gas inlet by the gaseous atmosphere.
30-31. (canceled)
Description
[0001] This application is a divisional of U.S. application Ser.
No. 10/515,450, filed Jun. 24, 2005, now U.S. Pat. No. 7,396,563
issued Jul. 8, 2008, which is a National Phase of International
Application No. PCT/CA03/000763, filed May 23, 2003.
FIELD OF THE INVENTION
[0002] The present invention relates to a ceramic thin film which
may be an amorphous silicon carbide (a-SIC) semiconductor thin film
deposited on various substrates suitable for photovoltaic cells and
a variety of relatively high performance low-cost electronic
devices which can be easily and economically mass-manufactured. By
means of Polymer-Assisted Chemical Vapor Deposition (PA-CVD), new
semiconductor materials with high conversion yields can be produced
at low cost. The process involves gaseous precursors from polymeric
sources, which lead to chemically synchronized construction of a
desirable amorphous silicon carbide structural framework with
special electronic and photonic properties.
BACKGROUND OF THE INVENTION
[0003] Kitabatake et al., disclose in U.S. Pat. No. 6,270,573 B1,
CVD and CVD-related methods of producing silicon carbide
substrates, including the growing of silicon carbide film by
supplying separate silicon atoms and carbon atoms on a surface. The
silicon-carbon bond formation occurs mainly on the surface of the
substrate, a step that usually requires high temperatures, in this
particular case the required temperature being 1300.degree. C. MBE
and MO-CVD may use species that contain a limited number of
pre-existing Si--C bonds in the precursor, this number being
usually related to precursor synthesis requirements.
[0004] Kong et al. (European Patent No. EP 0,970,267) describe a
susceptor design for silicon carbide resulting in minimizing or
eliminating thermal gradients between the two surfaces of a
substrate wafer. The CVD and CVD-related deposition procedures of
Kong et al., require strict control of the temperature field and
the gas flow at the surface of the substrate, where the Si--C bond
formation is occurring.
[0005] Grigoriev et al. (Grigoriev, D. A., Edirisinghe, M. J., Bao,
X., Evans, J. R. G. and Luklinska, Z. B. (2001) "Preparation of
silicon carbide by electrospraying of a polymeric precursor,"
Philosophical Magazine Letters (UK), 81, 4, 2001 by Dept. of
Mater., Queen Mary Univ. of London, UK) present silicon carbide
coatings and films prepared for the first time by electrostatic
atomization of a solution of a polymeric precursor and deposition
onto alumina and zirconia substrates. In the method of Grigoriev et
al., the polymeric source already contains most of the Si--C bonds
required for the formation of the SiC film; however, the molecular
source is carried to the surface inside cages of solvent molecules,
implicitly leading to contamination of the film, shrinking and
outgassing phenomena, due to solvent evaporation and polymer
cracking. These effects will be present in any polymer-assisted
method (spin-coating, spraying, laser ablation . . . ).
[0006] Lau et al. (Lau, S. P., Xu, X. L., Shi, J. R., Ding, X. Z.,
Sun, Z. and Tay, B. K. (2001) "Dependences of amorphous structure
on bias voltage and annealing in silicon-carbon alloys," Materials
Science & Engineering, B85 (16), Sch. of Electr. &
Electron. Eng., Nanyang Technol. Inst., Singapore) report on
amorphous silicon-carbon alloy films that have been obtained by
filtered cathodic vacuum arc (FCVA) technique. They have observed
that the disorder of the Si--C network increased with using the
high bias voltages during the deposition. This high is disorder in
the film with high bias voltages induces the smaller nanometer
crystallites after annealing at 1000.degree. C. rather than low
bias. The Raman peaks shift to the high frequency with increasing
the annealing temperature up to 750.degree. C. due to the increase
of nanometer grain size at the same bias. A sharp transition from
nanocrystalline to polycrystalline can be observed when the films
are annealed under 1000.degree. C.
[0007] Jana of al. (Jana, T., Dasgupta, A. and Ray, S. (2001)
"Doping of p-type microcrystalline silicon carbon alloy films by
the very high frequency plasma-enhanced chemical vapor deposition
technique" Journal of Materials Research, 16(7) 2001, 2130-5,
Energy Res. Unit, Indian Assoc. for the Cultivation of Sci.,
Calcutta, India) present the synthesis of p-type silicon-carbon
alloy thin films by very high frequency plasma-enhanced chemical
vapor deposition technique using a SiH.sub.4, H2, CH.sub.4, and
B.sub.2H.sub.6 gas mixture at low power (55 mW/cm.sup.2) and low
substrate temperatures (150-250.degree. C.). Effects of substrate
temperature and plasma excitation frequency on the optoelectronic
and structural properties of the films were studied. A film with
conductivity 5.75 Scm.sup.-1 and 1.93 eV optical gap E.sub.04 was
obtained at a low substrate temperature of 200.degree. C. using
63.75 MHz plasma frequency. The crystalline volume fractions of the
films were estimated from the Raman spectra. They observed that
crystallinity in silicon carbon alloy films depends critically on
plasma excitation frequency. When higher power (117 mW/cm.sup.2) at
180.degree. C. with 66 MHz frequency was applied, the deposition
rate of the film increased to 5.07 nm/min without any significant
change in optoelectronic properties.
[0008] Yamamoto et al. (Yamamoto et al., Diam. Relat. Mater., vol.
10 (no. 9-10), 2001, pp. 1921-6) present a doping procedure whereby
amorphous SiCN films were prepared on Si (100) substrates by
nitrogen ion-assisted pulsed-laser ablation of a SiC target. The
dependence of the formed chemical bonds in the films on nitrogen
ion energy and the substrate temperature was investigated by X-ray
photoelectron spectroscopy (XPS). The fractions of sp.sup.2 C--C,
sp.sup.3 C--C and sp.sup.2 C--N bonds decreased, and that of N--Si
bonds increased when the nitrogen ion energy was increased without
heating during the film preparation. The fraction of sp C--N bonds
was not changed by the nitrogen ion irradiation below 200 eV. Si
atoms displaced carbon atoms in the films and the sp.sup.3 bonding
network was made between carbon and silicon through nitrogen. This
tendency was remarkable in the films prepared under substrate
heating, and the fraction of sp.sup.3 C--N bonds also decreased
when the nitrogen ion energy was increased. Under the impact of
high-energy ions or substrate heating the films consisted of
Sp.sup.2 C--C bonds and Si--N bonds, and the formation of sp.sup.3
C--N bonds was difficult. The Yamamoto procedure proposes a doping
step separate from the synthesis step.
[0009] Budaguan et al. (Budaguan, B. G.; Sherchenkov, A. A.;
Gorbulin, G. L.; Chemomordic, V. D. (2001) "The development of a
high rate technology for wide-bandgap photosensitive a-SiC:H
alloys," Journal of Alloys and Compounds, 327(30) August, 146-50,
Inst. of Electron Technol., Moscow, Russia) discuss in their paper
the deposition process and the properties of a-SiC:H alloy
fabricated for the first time by 55 kHz PECVD. It was found that 55
kHz PECVD allows an increase in the deposition rate of a-SiC:H
films.
[0010] Modiano et al. (Japanese patent No. 145138195) present a
process for producing silicon carbide fibers having a C/Si molar
ratio from 0.85 to 1.39, comprising the steps of rendering
infusible the precursory fibers made from an organosilicon polymer
compound, then primarily baking the infusible fibers in a hydrogen
gas-containing atmosphere. This process for producing silicon
carbide thin films comprises the steps of imparting semiconductor
properties to passivating or dielectric thin films from volatile
precursory species produced from organosilicon polymer
compounds.
[0011] Yang et al. (Yang, Lixin; Chen, Changqing; Ren, Congxin;
Yan, Jinlong; Chen, Xueliang, "Synthesis of SiC Using Ion Beam and
PECVD", International Conference on Solid-State and Integrated
Circuit Technology Proceedings, pp. 811-814) present a process for
producing silicon carbide thin films comprising the steps of
conferring semiconductor properties to passivating or dielectric
thin films from volatile precursory species produced from
organosilicon polymeric compounds.
DISCLOSURE OF THE INVENTION
[0012] An object of the present invention is therefore to provide a
method of depositing a thin ceramic film on an appropriate
substrate.
[0013] Another object of the present invention is to provide a
ceramic thin film deposition on a substrate.
[0014] It is a further object is to provide a semiconductor device
comprising the film according to invention.
[0015] In accordance with one embodiment of the invention there is
provided a method of depositing a thin ceramic film onto a
substrate comprising: providing a polymeric source; providing the
substrate; heating the polymeric source under a gaseous atmosphere
having a pressure and a flowrate, whereby pyrolyzing the polymeric
source to produce a gaseous precursor at a first temperature
comprising chains of the polymeric source; positioning the
substrate to receive the gaseous precursor carried by the gaseous
atmosphere; and cooling the substrate at a second temperature below
the first temperature whereby desublimating the gaseous precursor
onto the substrate, the precursor chemically rearranging and
annealing to produce the film on the substrate, the film having
properties that include an amorphous nature, a crystallinity and a
degree of reticulation.
[0016] In accordance with another embodiment of the invention there
is provided a ceramic thin film deposited on a substrate comprising
chains from a polymeric source that have been deposited on the
substrate and chemically rearranged and annealed wherein the film
is essentially free of defects.
[0017] Other objects, advantages and features of the present
invention will become more apparent upon reading of the following
non-restrictive description of preferred embodiments thereof, given
by way of example only with reference to the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1a is a schema of the Polymer-Assisted Chemical Vapor
Deposition reactor;
[0019] FIG. 1b is a temperature-profile in the reactor versus the
distance from the gas input;
[0020] FIG. 2 is a representation of the set of chemical reactions
leading to n-type a-SiC from a generic polysilane precursor
(polymethylsilane, e.g.) within the reactor;
[0021] FIG. 3 is a series of various substrates supporting PA-CVD
films;
[0022] FIG. 4 illustrates an infrared spectrum of a gaseous
precursor formed during deposition of a-SiC PA-CVD films;
[0023] FIG. 5 is a design of the FT-IR cell based on a silicon
single crystal wafer, designed for the monitoring of a film
produced by the PA-CVD process;
[0024] FIG. 6 illustrates the carrier properties of the n-type
PA-CVD deposited a-SiC film;
[0025] FIG. 7 illustrates the semiconductor properties of the
n-type PA-CVD deposited a-SiC film;
[0026] FIG. 8 illustrates the FT-IR spectrum a silicon nitride
PA-CVD deposited film;
[0027] FIG. 9 illustrates the FT-IR spectrum a silicon oxycarbide
PA-CVD synthesized film;
[0028] FIG. 10 illustrates a simple Schottky solar cell;
[0029] FIG. 11 is a p-n junction barrier photovoltaic cell based on
a-SiC;
[0030] FIG. 12 illustrates a stacked p-n junction solar cell;
[0031] FIG. 13a illustrates an energy band diagram of Schottky
structure before intimate contact between metal and
semiconductor;
[0032] FIG. 13b illustrates an energy band diagram of Schottky
structure after intimate contact between metal and semiconductor;
and
[0033] FIG. 14 illustrates the energy band of a p-n junction solar
cell.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0034] The present description refers to a number of documents, the
content of which is herein incorporated by reference in their
entirety.
[0035] An aspect of the present invention is concerned with an
innovative method of forming volatile pyrolysis products from
Si-based polymeric compounds or sources. These sources can be used
to form gaseous precursors to thin films of ceramic materials in
silicon-carbon-nitrogen-oxygen systems that can be deposited on
commonly available substrates as a completely to partially
amorphous semiconductor, at a desired micron-range thickness, onto
a variety of commonly available and resistant substrates of varying
composition, degree of stiffness, shape, density, color, etc., over
a small to an exceptionally large surface. This method is herein
called Polymer-Assisted Chemical Vapor Deposition (PA-CVD). The
thin films produced by the method will be durable, may be flexible
(due to their thinness), and can be used for high-performance
semi-conductors. The film may be deposited on a small as well as a
large surface substrates. The substrates can be rigid or highly
flexible and of various composition, shape, thickness and
color.
[0036] According to the present invention, during PA-CVD (Polymer
Assisted-Chemical Vapor Deposition), most of the Si--C bonds
required for the formation of the silicon carbide structure
pre-exist in the polymeric source. Because most of the Si--C bonds
pre-exist in the polymeric source and gaseous precursor produced
therefrom, the temperature requirements become more flexible,
allowing lower temperatures and expanded operational range. For
this reason the nature of the substrate is less important, in terms
of thermal stability, size or shape. More than 50% of the SiC bonds
pre-exist in the polymeric precursor. Furthermore, the gaseous
precursor is deposited in chemical chains, similar to physical
chains, that then rearrange and bind to one another. The bond
formed between the precursor and the substrate is a van der Waals
physical bond.
[0037] The PA-CVD method of the present invention further
distinguishes itself from other forms of CVD because it does not
require solvent molecules to dissolve the precursor, the
evaporation of these solvents in other CVD methods produces defects
on the surface of the coating they produce, this is one reason why
the PA-CVD method produces films with fewer and essentially no
surface defects.
[0038] PA-CVD further allows the polymeric source to self-adjust to
the temperature field because each polymeric source will develop a
set of gaseous precursors adapted to the particular thermal
conditions (various gradients and temperature-cycles produce
different gaseous precursors, thermodynamically stable under the
specific conditions). This self adjustment is different for
different polymeric sources. The net result is that PA-CVD produces
pure gaseous precursors, containing a maximum number of
pre-existing Si--C bonds thermodynamically stable in the local
existing conditions, thus minimizing the extent of the chemical
reaction required for completion of the silicon carbide structure
at the surface of the substrate. In this process, the polymer
pyrolysis mechanisms naturally leave in the solid residue the
usually larger molecular weight species containing adventitious
oxygen (such as siloxane species), while the gaseous precursors
reaching the substrate are less contaminated.
[0039] The PA-CVD of the present invention allows reduced
deposition temperatures because a majority of the Si--C bonds
pre-exist in the precursor. PA-CVD also uses in-situ doping (N, P,
As, B, . . . containing gas-phase species that react chemically
with the precursor before deposition) allowing in-situ
determination of the dopant concentration and monitoring of the
formation of dopant-containing species (aminosilane formation via
IR spectroscopy could be performed directly in the quartz reactor).
PA-CVD locks the carbon atoms in a sp.sup.3 hybridization state
process of the present invention. PA-CVD allows significantly
higher deposition rates when used on very large substrates as
compared with the a-SiC films produced by any other CVD-related
method, including that taught by the aforementioned Budaguan et
al.
The PA-CVD Process According to the Present Invention:
[0040] 1) allows for the hydrogenation, heat treatment and makes
full use of many different types of gaseous reactants such as,
B.sub.2H.sub.6, NH.sub.3, PH.sub.3, AsH.sub.3, BCl.sub.3,
B.sub.2Cl.sub.6, NCl.sub.3, PCl.sub.3, AsCl.sub.3, CO, O.sub.2,
O.sub.3, CO, CO.sub.2, as well as H.sub.2 pure or inert carried
gases such as Ar or N.sub.2, or similar mixtures, with the inert
gases varying from 0.1 to 99.0% in volume;
2) can accommodate a wide variety of heat sources and treatment
lengths for the polymeric precursors or for the deposited film
under the gaseous atmospheres, for as little as 1 second and up to,
but not limited to, tens of hours, and leading to a wide variety of
passivating, semiconductor, and dielectric thin film materials; 3)
allows secondary annealing under BH.sub.3, B.sub.2H.sub.6,
NH.sub.3, PH.sub.3, AsH.sub.3, BCl.sub.3, B.sub.2Cl.sub.6,
NCl.sub.3, PCl.sub.3, AsCl.sub.3, CO, O.sub.2, O.sub.3, CO,
CO.sub.2 or H.sub.2 gases for several seconds, thereby increasing
the crystallinity and/or the degree of reticulation of the
deposited film; 4) the PA-CVD process can make good use of standard
heating methods as well electronic beams, X-rays, UV and IR
radiation microwave power, laser beams and other energy transfer
mechanisms to produce objects and substrates in flat, or tubular or
complex shapes including, but not restricted to, rods, cylinders,
spheres, ceramic boats, etc.; and 5) produces substrates with
varying conductor, semiconductor or dielectric properties,
including, but not restricted to polycrystalline or amorphous
silicon; quartz; graphite; metals; electronic-grade or refractive
ceramic materials, such as alumina or sintered oxides, nitrides,
phosphides; as well as A.sub.2B.sub.6, A.sub.3B.sub.5, ternary and
quaternary compounds in this class.
[0041] The PA-CVD process does not require massive silicon-carbon
bond formation on the surface of the substrate, since most of the
existing Si--C network in the amorphous phase is obtained via
massive Si--C bond redistribution during processes preceding the
deposition of the thin film.
[0042] The PA-CVD design allows the use of a broad series of
polymeric precursors for use in the synthesis of silicon-based thin
films including, but not limited to, oxides, nitrides, carbides and
variously weighted combinations in homogeneous phases or
multi-layered structures. The resulting films are of considerable
interest as electronic and optoelectronic materials as well as for
protective coatings. A large variety of appropriate silicon-based
polymeric sources can be used, such as polysilanes,
polycarbosilanes, polycarbosilazanes, polysiloxanes and
polysiloxazanes. Other appropriate polymeric sources include:
carbon nitride polymeric sources and boron nitride polymeric
sources which too have been found to produce useful ceramic
products. The possible carbon nitride polymeric sources (CxNy)
include: polyepoxy-, polyamides, polyamines, polyimides, polyureas
and polyurethanes. It is noted that polyamides, polyamines,
polyimides and polyureas can be used in mixtures with the other
polymeric sources and as a possible source of nitrogen in the
reaction. Polymers sources with other backbones can be envisaged
comprising: Al, B, Ge, Ga, P. As, N, In, Sb, S, Se, Te, In and
Sb.
[0043] By means of p-n homo- or heterojunctions and using a variety
of flexible or rigid substrates, it is possible to fabricate solar
cells, light-emitting diodes, transistors, photothyristors, and
similar devices. Using high breakdown electrical field and high
electron saturation velocity, it is further possible to produce
high frequency, high power and high temperature electronic and
optoelectronic devices. By combining optical and electronic
properties, the materials may also serve to fabricate integrated
monolithic devices on a single chip.
[0044] The PA-CVD process is relatively simple requiring only a
driving force generated in the furnace (2), being the
supersaturation ratio obtained in a temperature gradient field.
Furthermore, PA-CVD distinguishes itself from other forms of
chemical vapor deposition because it does not require the following
more sophisticated driving forces:
a) ion implantation, ion beam enhanced deposition, reactive ion
beam sputtering and plasma enhanced chemical vapor deposition
(PECVD); b) RF power (as described in "High temperature annealing
of hydrogenated amorphous silicon carbide thin films" INS 01-17
6910830 A2001-11-6855-060 (PHA) NDN-174-0691-0829-5 Yihua Wang;
Jianyi Lin; Cheng Hon Alfred Huan; Zhe Chuan Feng; Soo Jin Chua);
c) IR and UV laser photolysis (as described in "Laser gas-phase
photolysis of organosilicon compounds: approach to formation of
hydrogenated Si/C, Si/C/F, Si/C/O and Si/O phases" INS 00-50
6791677 A2001-03-8250F-001 (PHA) NDN-174-0679-1676-1 Pola, J.
JOURNAL NAME-PINSA-A (Proceedings of the Indian National Science
Academy) Part A (Physical Sciences)); and d) electron cyclotron
resonance (as described in "Application of electron cyclotron
resonance chemical vapor deposition in the preparation of
hydrogenated SiC films. A comparison of phosphorus and boron
doping" INS 98-04 5814339 A98058115H-021 (PHA); B9803-0520F-017
(EEA) NDN-174-0581-4338-2 Yoon, S. F.; Ji, R.).
[0045] An reactor (2), for PA-CVD is illustrated in FIG. 1a which
shows the principal characteristics of a reactor that can be used
for PA-CVD, while FIG. 1b illustrates the varying temperature
profile within the PA-CVD reactor.
[0046] Referring to FIG. 1A, there is a quartz reactor (2), into
which one or several polymer derived precursors (19), enter through
gas inlet (1). The quartz reactor (2) is also referred to as the
furnace or the PA-CVD reactor. Furthermore, a gaseous atmosphere
(60), either inert or active, also enters through the gas inlet
(1). The inert atmosphere includes argon, nitrogen or other inert
gases. While the active atmosphere includes gases such as ammonia,
carbon monoxide or similar gases. Before operation the reactor (2)
is purged with a selected atmosphere (60).
[0047] The gas inlet (1), has a high-vacuum seal to minimize the
ingress of oxygen impurities from the surrounding air drawn into
the reactor (2). The total pressure in the reactor is measured with
a pressure controller (3) which also controls the flowrate into the
reactor (2). The outside of the reactor is heated with electric
heating elements (4), which surround the PA-CVD reactor (2) to
produce the temperature gradient illustrated in FIG. 1b. There are
more heating elements (4) near the inlet of the reactor (2), while
there are fewer surrounding the deposition area of the reactor. The
PID (Proportional-Integral-Derivative) temperature controller (5),
ensures that the temperature within the reactor (2) is in the
appropriate range for the polymer-derived precursor (19) used, the
type of gaseous atmosphere (60) and substrate (6) to be coated. The
substrate (6), is placed in the deposition area of the reactor (2).
Typically the substrate (6), is a piece or part made of silicon,
quartz, metal, ceramics, etc. The gas phase near the gas outlet
(9), of the reactor (2), is analyzed by an FT-IR (Fourier Transform
InfraRed) spectrometer (7). The FT-IR spectrometer (7), allows the
in situ verification of the deposition process and the oxygen
impurities present. A SiC film (8) is deposited on the substrate
(6) through the decomposition of the silicon based polymeric
sources (19) and their chemical rearrangement to SiC on the surface
of the substrate (6). The deposited film (8) can be a single or
multiple layered film.
[0048] Referring to FIG. 1B, the temperature profile within and
along the length of the reactor (2) is represented. The input zone
(10) shows a constant lower temperature associated with the gas
inlet (1). In the heating zone (11), there is a increase in the
temperature due to the large amount of heating elements (4) at the
inlet. In the heating zone (11), the rearrangement of the silicon
based polymeric sources occurs which leads to the formation of the
poly(carbosilane) species. The next temperature is that of the
pyrolysis zone (12), where there is direct precursor formation and
doping occurring. This is followed by a reduction in the number of
heating elements (4) in the deposition zone (13) which consequently
cools the particularly zone of the reactor (2) thus lowering the
temperature. The deposition zone (13) is represented by a constant
temperature while the SiC is being deposited on the substrate (6).
Finally there is the gas exit zone (14), where the temperature
falls in the gas outlet (9) of the reactor (2) and approaches that
of the ambient temperature outside the reactor (2). The
temperatures in the reactor (2) and represented in FIG. 1B, vary
from 100 to 1000.degree. C. In a preferred embodiment the PA-CVD
temperature zone varies from 200 to 450.degree. C.
[0049] The PA-CVD process and resulting materials produced are
based on the innovative design and directed behaviour of volatile,
relatively large molecular mass, gaseous precursors derived from
silicon-based polymeric sources. This PA-CVD synthesis of inorganic
thin silicon-based films includes silicon carbide, silicon nitride,
silicon oxide, silicon oxycarbide, silicon oxynitride, silicon
oxycarbonitride and other such materials, in pure and doped
forms.
[0050] The tolerance of the polymeric source to depolymerization
processes is related directly to silicon-carbon, silicon-nitrogen
and silicon-oxygen relative bond stability in the above-mentioned
precursors (19), in a given thermal environment resulting from
given inert or active pyrolysis conditions. This PA-CVD process was
tested by subjecting the classes of above-mentioned various
silicon-based polymers to various thermal budgets controlling the
depolymerization conditions (thermal cracking, chemical
decomposition and polymeric disproportionation). The deposited
films can be used as active, passivation, dielectric or protective
coatings for semiconductor discrete or integrated devices or
implantable materials. Any combination of these categories of pure
or doped materials can be deposited on metals, ceramics, glasses or
plastics, in single- or multiple-layered structures, at
temperatures above 300.degree. C.
[0051] The resulting semiconductor thin film possesses highly
desirable electronic, optoelectronic and photonic properties that
make it highly suitable for standard, cost-effective fabrication of
a variety of electronic and optoelectronic devices, including
photovoltaic cells. The a-SiC thin film is an n-type and/or p-type
mono or heterojunction with a donor concentration of
10.sup.14-10.sup.17 cm.sup.-3.
[0052] An interesting feature of the present invention is the role
played by the gaseous precursors (19) from polymeric sources, in
the deposition of the thin films of silicon carbide. This process
is based on the polymeric source being first cracked to produce
large molecular weight gaseous precursor with pre-existing
silicon-carbon bonds. These provide the building blocks for the
silicon carbide thin film to be deposited on the desired
substrate.
[0053] Another principle of the proposed method is to create at the
outset, in the gas phase, the majority of the required bonds that
will constitute the solid silicon carbide. Consequently, the role
of the chemical reactions occurring on the substrate is drastically
limited to the completion of the remaining small number of bonds
required for the silicon carbide structure. This technique permits
much lower operating temperature during growth of the silicon
carbide thin film than standard industrial practices. This
technique facilitates a high rate of mass transfer during
desublimation of the large precursor molecules, thereby increasing
the growth rate of silicon carbide on the substrate. The lower
operating temperature provides an environment for lowering the
amount of unintentional impurities in the deposited film.
Desublimation is herein defined as a change of phase from a gaseous
species directly to a solid species.
[0054] The PA-CVD process takes full advantage of the Si--C and
Si--N bonds pre-existing in the primary polymeric source. This
invention incorporates the theoretical concept of "anticeramic
yield" of the gaseous precursors: the traditional method for
producing silicon carbide from polymeric sources is through
rearrangement of the solid residue left after pyrolysis of the
precursor sources, with a ceramic yield amounting to 80% solid;
recent efforts are towards maximization of the amount of polymer
remaining solid, thereby increasing the yield to higher values. The
theory applying to the new concept generally involves the opposite:
to maximize the fraction of polymer that is vaporized for the
formation of the desired new gaseous precursor leading to the
amorphous silicon carbide deposit, almost the entire polymer source
is vaporized, with the net result that the ceramic yield is almost
nil while the polymer transforms itself into a new gaseous source
resulting in almost 100% anticeramic yield.
[0055] In the PA-CVD process as represented in FIG. 2, the first
phase of the deposition involves significantly higher a-SiC
deposition rates compared to the CVD method because of
chemically-synchronized Si--C bond redistribution in
organo-polysilanes. Still referring to FIG. 2, in the first
chemical reaction (15), a thermally activated methylene insertion
into silicon-silicon bonds takes place to produce poly(carbosilane)
precursor. This intramolecular reaction, known as the Kumada
rearrangement, (Shiina, K.; Kumada, M. (1958) in J. Org. Chem., 23,
139), provides the structural framework of silicon carbide,
(Scarlete, M.; Brienne, S.; Butter, S. S, and Harrod, J. F. (1994),
Chem. Mater., 6, 977). By simply heating the polymer precursor, a
very large number of Si--C bonds are appropriately redistributed at
a very fast rate.
[0056] The second reaction (16), also represented in FIG. 2, leads
to the introduction of nitrogen atoms as donor impurities into the
silicon carbon precursors. The formation of the aminocarbosilane
precursor in reaction (16), occurs via a reaction with ammonia,
found either in the atmosphere or in the polymer derived
source.
[0057] While still referring to FIG. 2, the third reaction (17),
results in high molecular weight species through the formation of
secondary amine species, leading to increased desublimation
capacity. The formation of the secondary amine species is via the
Si--H/N--H dehydrogenation. The fourth reaction (18), governs the
formation of the film derived from the third reaction (17) onto the
substrate (6). The formation of high molecular mass ternary amine
species, which are direct precursors of the n-type a-SiC-- film on
the substrate via desublimation, with the reaction (18) presented
in FIG. 2 being a transamination reaction, and this reaction being
illustrative of one possible chemical mechanisms.
[0058] The temperature in the furnace varies between 100.degree. C.
and 1000.degree. C. depending on the stage and the specific local
requirements of the process steps in the aforementioned reactions.
The gaseous species are monitored by FT-IR spectroscopy (7) of
samples extracted near the reactor outlet (9). The a-SiC thin films
can also be characterized by IR spectroscopy while the
concentration of adventitious oxygen in the thin film can be
measured by using a Czochralski (Scarlete, M., J. Electrochem.
Soc., (1992), 139 (4), 1207), silicon window as a standard.
[0059] In this method, gaseous precursors from polymeric sources
are produced first, contrary to the classical polymeric route. A
definite advantage of this process is a purification that involves
the polymer source during the sublimation process.
[0060] This purification occurs during PA-CVD, the effect of
adventitious oxidation of the initial solid polymeric source is
reduced by the decreased capacity of oxidized backbones to produce
volatile material (e.g., at the limit, a high degree of oxidation
produces SiO.sub.2 with negligible volatility). The oxidized
material is therefore concentrated in the solid residue, while the
precursors reaching the substrate are purified this way. This
purification helps to produce films that have very few chemical
impurities and consequently fewer surface defects.
[0061] FIG. 3 shows the various types of substrates that can be
coated, as well as, their nature and complexity with a-SiC film
deposited by the process of this invention. The a-SiC thin film can
be deposited on a regular ceramic material of a complicated shape,
quartz, electronic-grade sintered alumina, polished alumina,
silicon single crystal wafer, graphite, and other commonly
available and relatively inexpensive materials. Several of the
materials have been coated with the PA-CVD method of the invention
on one side (the dark surface) while the other side was masked
during deposition and the mask removed leaving the pale surface
which can also be seen in FIG. 3. Therefore, the PA-CVD method is
also compatible with conventional techniques such as masking
understood by those skilled in the art.
[0062] Referring to FIG. 4, the in situ FT-IR spectrum analysis of
a gaseous precursor at the outlet (9), of the reactor (2), shows
the numerous peaks that correspond to the SiH bonds formed when
chemical change in the structure of the solid polymeric source
produces a polymer derived precursor (19). The increasing
temperature near the inlet of the reactor (2), breaks down the
polymeric source into various subunits to produce the gaseous
precursor (19).
[0063] Referring to FIG. 5, where there is represented a schematic
of FT-IR cell based on a silicon single crystal wafer (68), which
is designed for the monitoring of a PA-CVD film (8) deposited on a
substrate (6) by the process of the invention. The substrate (6)
with a coating (8) is held in place, the thickness of the deposited
layer has been exaggerated so that the upper half and lower half of
the FT-IR cell actually sit one on top of the other and are sealed
by the represented O-ring (67). There is a protective gas swept
through the FT-IR cell from an inlet (64) to an outlet (65), which
maintains the appropriate inert atmosphere. The IR beam (61) is
projected onto the substrate and it is bent and reflected through
the deposited film (8), and then collected through a microscope
objective (62) and detected by a MCT (mercury-cadmium-tellurium,
Hg, Cd--Te) detector (63). The Fr-IR cell is mounted on an
adjustable 2D micrometric stand (66) which allows the Fr-IR to be
adjusted appropriately with respect to the IR beam (61).
[0064] Referring to FIG. 6, the carrier properties of an n-type
PA-CVD film produced are represented. The graph represents the
donor concentration, n (cm.sup.-3) versus the width of the
depletion zone, w (.mu.m). We observe that the sample tested has a
low donor concentration which can be below 10.sup.13 but range to
10.sup.15 cm.sup.-3 in the of the graph. A preferred range that is
less than 10.sup.14 cm.sup.-3. These low donor level values are
before any doping. The width of the depletion zone (W) which is
measured in (.mu.m) is a function of the material connected at the
junction, in the case of FIG. 6, that of an n-type film with the
substrate. W must not be confused with the film thickness. Thicker
films (above 20 .mu.m) were required for the method of detection
used to quantify the carrier concentration in the depletion zone,
and the thin films obtained by this method (100 .ANG. to 0.1 .mu.m)
will have the same type of curve as found in FIG. 6, at the far
lower thicknesses.
[0065] FIG. 7 represents the semiconductor properties of an n-SiC
PA-CVD film which is a qualitative indication of the quality of the
film, indicated by the curve of current versus voltage.
[0066] Referring to FIG. 8, there is represented an FT-IR spectrum
of a PA-CVD film of deposited silicon nitride. The main peak around
800 cm.sup.-1 being that of Si--N.
[0067] Referring to FIG. 9, which represents the FT-IR spectrum of
PA-CVD films of a synthesized silicon oxycarbide with each of the
three samples (a), (b) and (c) exposed to a temperature maximum of
1100.degree. C. but annealed for the progressively increasing time
periods of 8, 16 and 24 hours respectively. It must be noted, that
the interstitial oxygen peak found in sample (a) at approximately
1100 cm-1, increases as the film is annealed for longer periods.
This indicates the conversion limited resistance of the film to
oxidation.
[0068] Referring to FIG. 10, there is represented a simple Schottky
solar cell. The cell comprises one layer only of a-SiC (22), a
metallic substrate (20) acting as anode (when layer (22) is n-type)
or cathode (when layer (22) is p-type). The suitable metal is an
inexpensive conductive material and its thickness or uniformity of
thickness is not critical (viz. aluminum foil). The ohmic contact
layer (21) deposited by physical evaporation or other
physico-chemical means, providing effective contact with the
overlying semiconductor layer as well as the underlying metallic
substrate, consisting of aluminum or similar conductor (200 nm) if
n-type, or aluminum/nickel (100 nm/100 nm) if p-type; the surface
of which must be cleaned by chemical etching or mechanical means to
avoid oxidation with respect to layer (22). Alternatively, layers
(21) and (22) could be made or fabricated as one composite layer
over which layer (22) could be deposited. The semiconductor layer
of a-SiC of n- or p-type (22), with free carrier density between
10.sup.14 and 10.sup.17 cm.sup.-3, of 0.2 to 1 .mu.m thickness,
being the critical element resulting from the PA-CVD process and
acting as the heart of the cell, with the upper free carrier
capacity being a critical factor. The top layer gold (Au) layer
(23) of 5 to 10 nm thickness, acting as cathode if the
semiconductor is n-type or as anode if it is p-type. The gold
deposited mechanically or chemically onto n-type semiconductor. The
gold layer is sufficiently thin as to allow light to reach the
semiconductor.
[0069] FIG. 11 is a p-n junction barrier photovoltaic cell based on
a-SiC with the multiple layers produced by the process of the
invention. In this photovoltaic cell, a metallic substrate is
acting as an anode (24). Layer (25) is a metallised (aluminum)
ohmic contact layer (.about.100 nm). These layers are followed by
an n-type a-SiC (.about.750 nm) layer (26), p-type a-SiC
(.about.250 nm) layer (27), a nickel ohmic layer (28) and an
aluminum top contact layer (29) serving as cathode which covers
approximately 10 percent of illuminated surface.
[0070] FIG. 12 represents a stacked p-n junction solar cell, the
multiple layers produced by the method of the invention. The layers
of FIG. 12 (with reference numbers followed by the layer
thicknesses, listed from bottom to top) are: (30) metallic
substrate acting as cathode; (31) aluminum-nickel ohmic contact
layer (.about.100 nm/.about.100 nm); (32) p-type a-Ge layer
(.about.200 nm); (33) n-type a-Ge layer (.about.200 nm); (34)
p-type a-Si (.about.200 nm); (35) n-type a-Si (.about.200 nm); (36)
p-type a-SiC (.about.200 nm); (37) n-type a-SiC (.about.200 nm);
and (38) top aluminum anode contact, covering about 10 percent of
the surface.
[0071] In any semiconductor junction, such as in a Schottky
junction shown FIG. 13b or a p-n junction such as shown in FIG.
14B, there is an internal electrical field, E.sub.bi, called
"built-in electrical field", that prevents the charge carriers
(electrons and holes) to stay in the a region of the material
called the "depleted zone." If the depletion region of thickness W,
is illuminated by photons with energies greater than
(E.sub.c-E.sub.v), this region develops pairs of "electron-hole"
which are separated by the internal electrical field: the electrons
are attracted towards the semiconductor while holes are directed
towards the metal, creating a photocurrent when the device is
connected to an external load. The structure is called a
photovoltaic cell or solar cell.
[0072] The current generated in an amorphous semiconductor is
mainly due to a drift component, because the diffusion component is
not significant due to low mobilities of the charge carriers. In
order to collect efficiently the photon energies in an amorphous
semiconductor junction, the depleted region width must be as large
as possible. The depleted zone width, W, is given by:
W=(.di-elect cons.V.sub.bi/qN.sub.D).sup.1/2
where .di-elect cons. is the dielectric constant of the
semiconductor, q is the electron charge, N.sub.D is the electron
concentration, and V.sub.bi is the built-in voltage given by:
V.sub.bi=(.phi..sub.B-(E.sub.c-E.sub.F))
[0073] The width of the depletion region can be increased by
lowering the free carrier concentration of the material.
[0074] If a p-n junction (FIG. 14) is used instead of a Schottky
one, the depletion region width can be increased (in this case,
each type of material has its own depleted zone), therefore
increasing the efficiency of the photovoltaic cell.
[0075] In the Schottky structure (FIG. 11), the energy band diagram
is shown in FIG. 13B before the intimate contact between the metal
and the semiconductor. The work function, .phi..sub.s (41s) is the
energy difference between the vacuum level (39) and the Fermi
level, E.sub.F (40). The vacuum level (39) is the zone where the
electron is free from the semiconductor atoms and has no kinetic
energy. In elemental solids such as a metal, represented in FIG.
13A, the values of the work function .phi..sub.m (41m) are well
established, (see for example Weast, R. C. (1990), CRC Handbook of
Chemistry and Physics, 70th Edition, CRC Press, E-93).
[0076] FIG. 13B illustrates the work function of a semiconductor
(41s) normally denoted by .phi..sub.s. The energy difference
between the vacuum level (39) and the bottom of the conduction band
(42), denoting electron affinity (.chi.), is used as reference
since the Fermi level depends on the carrier concentration in the
semiconductor. However, .phi..sub.s still represents the energy
required to remove an electron from the semiconductor. Referring to
FIGS. 10 and 13B, the conduction level (42) E.sub.c, the valence
level (43) E.sub.v; the affinity (44) .chi.; the work function (45)
.phi..sub.s; and the energy gap (46) E.sub.G are of the
semiconductor layer (22).
[0077] The Fermi level represents the energy for which the
probability to find a free electron, in equilibrium and near zero
Kelvin equals 0.5. The probability of finding an electron at a
given energy level is obtained according to the Fermi-Dirac
function:
F(E)=1/[1+exp(E-E.sub.F)/kT]
[0078] The Fermi level in a semiconductor depends on the free
carrier concentration, and it is closer to the conduction band than
the valence band in n-type semiconductor. Assuming that
(.phi..sub.m>.phi..sub.s, and that the metal-semiconductor
system at equilibrium of FIG. 14A, the Fermi level is at the same
both in the metal and the semiconductor. Therefore, an internally
built-in electric field (E.sub.bi) develops between the metal and
the semiconductor. The field is oriented from the positive charges
to the negative charges, that is, towards the metal. The resulting
built-in voltage is equal to [(.phi..sub.m-.phi..sub.s)/q]. A
depletion layer, of thickness W (53b), is formed where there are no
free charges. The potential energy barrier for electrons moving
from the metal to the semi-conduction is known as the Schottky
barrier height, .phi..sub.B, and is given by:
(.phi..sub.B=(.phi..sub.m-.chi.). Under reverse bias or zero bias
electrical conditions, there is no net current flowing through the
metal-semiconductor junction.
[0079] For photon energies greater than E.sub.G; the electron-hole
pairs are generated in the depleted zone.
[0080] In the p-n structure of FIG. 14B, where the p-type a-SiC
(47) and the n-type a-SiC (48) are represented. The principal
electronic phenomena takes place in the depleted zones (52) and
(53). In this case, the built-in voltage V.sub.bi depends on the
carrier concentration in the semiconductor:
V.sub.bi=(kT/q)Ln(N.sub.AN.sub.D/n.sub.i.sup.2)
where k is the Boltzmann constant, T the temperature, N.sub.A the
hole concentration and n.sub.i the intrinsic carrier
concentration.
[0081] The depletion region widths are given by:
W.sub.n=(.di-elect cons.V.sub.bi/qN.sub.D).sup.1/2
W.sub.p=(.di-elect cons.V.sub.bi/qN.sub.A).sup.1/2
With the conduction level (49) E.sub.c, the Fermi Level (50)
E.sub.F and the valence level (51) also represented in FIG.
14B.
[0082] The examples selected below present tailored PACVD processes
for the use of some of the mentioned polymers in the synthesis of
the previously mentioned materials.
Example 1
Use of Polysilanes as PA-CVD Precursors to N-Type Semiconductor
a-SiC
[0083] The anticeramic yield is optimized with respect to the
average molecular weight and the polydispersity of the polysilane
raw material. In the first step, an appropriate time-dependent
temperature gradient is programmed in the furnace, so that
quantitative polycarbosilane formation is promoted. Possible ranges
for the gradients are 1-10 Kmin.sup.-1 and 3-50 Kcm.sup.-1 in a 2
inch/150 cm horizontal quartz reactor (11). A second step involves
polycarbosilane pyrolysis that may be coupled with a
nitrogen-doping process, via a carefully monitored (flow, pressure
(3), and FT-IR (7)) reaction with electronic-grade ammonia (12), at
a partial pressure level of 10.sup.-6-10.sup.-1 torr in a UHP-Ar
(or N.sub.2) carrier flow. The resulting polymeric gaseous species
(17,18) are transported in the deposition zone (13), where they are
desublimed onto the substrate (6) that can be placed in a
horizontal, vertical or a tilted position, and can be mobile or
immobile during the deposition. The resulted material is an n-type
a-SiC with a donor concentration in the range of
10.sup.14-10.sup.17 cm.sup.-3. The thickness of the resulted film
can be adjusted in the 100 .ANG.-1 .mu.m range, via single/multiple
layered deposition.
Example 2
Use of Polysilanes as PA-CVD Precursors for the Synthesis of Thin
Films of Passivating SiO.sub.xC.sub.y-Glasses
[0084] Silicon oxycarbide (SiO.sub.xC.sub.y) is an amorphous
metastable phase wherein the silicon atoms are bonded to oxygen and
carbon simultaneously. In silicon oxycarbides, high temperature
properties and chemical stability have been reported, exceeding
those of ordinary vitreous silica. Silicon oxycarbide materials
have also the potential for use in a variety of protective
applications within the semiconductor industry. Using PA-CVD
technique, silicon oxycarbides of various compositions have been
deposited on highly resistive single crystal silicon wafers, using
different conditions to vary the oxygen content in the films. The
anticeramic yield is optimized with respect to the average
molecular weight and the polydispersity of the polysilane raw
material.
[0085] In the first step, an appropriate time-dependent temperature
gradient is programmed in the furnace to enhance quantitative
polycarbosilane formation. Possible ranges for the gradients are
1-10 Kmin.sup.-1 and 3-50 Kcm.sup.-1 in a 2 inch/150 cm horizontal
quartz reactor (11). The resulted polymeric gaseous species (17,18)
are transported in the deposition zone (13).
[0086] A third step involves polycarbosilane controlled oxidation
process, via a carefully monitored [(flow, pressure (3), and FT-IR
(7)] reaction with oxygen carrying species including, but not
limited to, O.sub.2, O.sub.3, CO (zone 12, FIG. 1), at a partial
pressure level of 10.sup.-14-10.sup.-1 torr in a UHP-Ar (or
N.sub.2) carrier flow. The oxygen carrying species are introduced
directly in the deposition zone (13).
[0087] The controlled oxidation products are desublimed onto the
substrate (6), that can be placed in a horizontal, vertical or a
tilted position, can be mobile or immobile during the deposition.
The resulted material is a SiO.sub.xC.sub.y glass with an oxygen
content in the range from x=10.sup.-3 to x=1.3 (measured using an
external standard of Cz-silicon single crystal via ASTM F-1188).
The thickness of the resulted film can be adjusted in the 100
.ANG.-1 .mu.m range, via single/multiple layered deposition.
Example 3
Use of Polysilanes as PA-CVD Precursors to Dielectric or
Passivating a-Si.sub.xN.sub.y Thin Films
[0088] The anticeramic yield is optimized with respect to the
average molecular weight and the polydispersity of the polysilane
raw material. An appropriate time-dependent temperature gradient is
programmed in the furnace, so that quantitative polysilazane
formation is promoted. Possible ranges for the gradients are
1-5.degree. K. min.sup.-1 and 3-50.degree. K. cm.sup.-1 where the
temperature increases (11) in a 2 inch/150 cm horizontal quartz
reactor (2). A second step involves pyrolysis of the polysilazane
in the reaction zone (12) in FIG. 1 where the temperature is
relatively constant. This step may be optionally followed by a
transamination processes induced directly in the deposition zone
(13), via a carefully monitored (flow, pressure-parameters (3) and
PID temperature parameters (5) where P=1-25, I=10-250, and
D=0.1-10) reaction under pure electronic-grade gaseous ammonia
introduced in the temperature zone (11), FIG. 1b, at a pressure
level of 1-50 torr over the atmospheric pressure. The resulted
precursor gaseous species (17,18) are transported in the deposition
zone (13), where they are desublimed onto the substrate (6), that
can be placed in a horizontal, vertical or a tilted position, can
be mobile or immobile during the deposition. Function of the
parameters mentioned above, the resulted material is
a-Si.sub.xN.sub.y, with a x/y ratio in the range of 0.75 to 1. The
thickness of the resulted film can be adjusted in the 100 .ANG.-1
.mu.m, via single/multiple layered deposition.
Example 4
Measurement of Free-Charge Carrier Concentration of N-Type a-SiC
Films
[0089] The free-charge carriers in n-type semiconductor a-SiC films
are measured by the capacitance-voltage (CV) method (Schroder. D.
K. (1990), Semiconductor Materials and Device Characterization,
Wiley Interscience p. 41) using the Schumberger impedance analyzer
Solartron 3200. The voltage is varied between -6 and 0 V, and the
resulting capacitance is observed to increase with increasing
voltages. On sample 01-S0001, six Schottky diodes are fabricated
using mercury as anode metal: the mercury probe used provides a
diode area of 0.453 mm.sup.2. The capacitance is given by:
C=.di-elect cons.A/w
where A is the diode area. In the presence of an applied voltage V,
the depletion region width is given by:
W=((.di-elect cons.(V.sub.bi-V))/qN.sub.D).sup.1/2
[0090] The derived values for N.sub.D (electron concentration) in
the diodes are 9 10.sup.17.+-.0.2 10.sup.17 cm.sup.-3.
Example 5
Measurement of Electron Mobilities in N-Type a-SiC Film
[0091] The electron mobilities are measured in three samples
21/NO/2001/05, 06 and 07, using the Van der Pauw method (Van der
Pauw, L. J. (1958), A method of measuring specific resistivity and
Hall coefficient on lamellae of arbitrary shape, Phil. Tech. Rev.,
vol. 20, p. 220). The measured mobilities are defined as Hall
mobilities because the technique is based on the Hall effect. The
measurements are carried out with a current of 1 mA, and a magnetic
field of 5 kG. The correction factors f derived for the three
samples are 0.99, 0.67, and 0.86, respectively. The derived
resistivities are 29.75, 22.32 and 17.24 .OMEGA.cm.sup.-1, and the
derived mobilities are 6.72, 4.48 and 23.19
cm.sup.2V.sup.-1s.sup.-1, and the electron concentrations in the
samples are calculated using the resistivity and mobility as:
3.12.times.10.sup.16, 6.25.times.10.sup.16, and
1.56.times.10.sup.16 cm.sup.3, respectively.
[0092] The above description and drawings are only illustrative of
preferred embodiments which achieve the objects, features and
advantages of the present invention, and it is not intended that
the present invention be limited thereto. Any modification of the
present invention that comes within the spirit and scope of the
following claims is considered part of the present invention.
* * * * *