Chemical vapor deposition of metal chalcogenide films

A process for depositing a film of metal chalcogenide is disclosed. The process comprises providing a single source of a metal chalcogenide and heating said source to a temperature sufficient to sublime the single source under a pressure ranging from 0.0001 to 760 torr so that the sublimate is delivered into a reaction zone. Within this reaction zone, a substrate is deposed upon which deposition may occur. The reaction zone is heated to approximately 200.degree. to 800.degree. C. The sublimate is passed through this reaction zone and over the substrate to produce a thin film of metal chalcogenide which is deposited upon the substrate.

TECHNICAL FIELD 
The present invention relates to the deposition of metal chalcogenide films 
onto a substrate through the use of chemical vapor deposition techniques 
utilizing a single precursor source. 
BACKGROUND ART 
In recent years, thin films of certain species of metal chalcogenides, such 
as titanium disulfide (TiS.sub.2) and other transition metal sulfide 
materials have been studied. Films of TiS.sub.2 may be formed using 
chemical vapor deposition (CVD) methods, by sulfurization of titanium 
metals at elevated temperatures, and sputtering methods. 
A number of CVD processes have been described by the prior art. 
Deficiencies in such methods were discussed in the parent case which is 
herein incorporated by reference. Typically in these prior art processes, 
gaseous streams of the two reactants (i.e. titanium tetrachloride and 
hydrogen sulfide) are mixed in a heated reactor to deposit the desired 
metal chalcogenide as a film on a substrate suspended in the reactor. It 
is often necessary in these processes to use a large excess of the 
chalcogenide source (i.e. H.sub.2 S) in order to achieve reasonable 
deposition rates. Moreover, the chemical yield of the films in such cases 
is often extremely low. This results in the waste of most of the agents 
required to form the film, which in turn, leads to an inefficient process 
with concomitant toxic waste problems. 
It would be highly desirable to have a volatile single source precursor 
capable of being sublimed into the CVD reactor to deposit the film. 
Ideally such precursors would contain the correct stoichiometry of 
elements needed for the metal chalcogenide film and would minimize waste 
materials. Unfortunately, however, the prior art has been unable to 
provide such a material and no single source precursor to CVD titanium 
disulfide films has been reported in the literature. 
The most important commercial application for titanium disulfide films is 
as cathodes in lithium batteries. In such an application it is highly 
desirable that the thin film of TiS.sub.2 have a crystallographic 
orientation such that the c-axis is parallel to the plane of the 
substrate. In such an orientation, pores in the crystals are perpendicular 
to the plane of the substrate and are optimum for the intercalation of 
lithium, which constitutes the primary discharge reaction in the lithium 
battery. Conversely, an orientation in which the pores of the crystals are 
parallel to the plane of the substrate leads to an inefficient cathode 
reaction, due to poor intercalation of lithium ions into the TiS.sub.2. 
It has been demonstrated by Kikkawa et al. (J. Mater. Res. 1990, 5, 2894) 
and Kanehori et al. (J. Electrochem. Soc. 1989, 136, 1265) that TiS.sub.2 
films with predominant (110) crystallographic orientation provide optimum 
cathode performance in a lithium battery. However, such preferred films 
with the highly desirable (110) orientation have only been prepared from 
CVD techniques using two separate gaseous streams of titanium 
tetrachloride and hydrogen sulfide and thus incorporate all of the prior 
art limitations, including the undesirable deposition characteristics 
discussed above. 
SUMMARY OF THE INVENTION 
The problems and limitations of the prior art have been overcome with the 
invention disclosed herein. The invention comprises a process for 
depositing a film of a metal chalcogenide comprising the steps of 
providing a single source of a metal chalcogenide; heating the single 
source to a temperature sufficient to sublime the single source at a 
pressure ranging from 0.0001 to 760 torr so that the sublimate is 
delivered into a reaction zone; affording a substrate within the reaction 
zone to define a surface upon which deposition may occur; heating the 
reaction zone to approximately 200.degree. to 800.degree. C.; and passing 
the sublimate over the substrate to produce a film of metal chalcogenide 
which is deposited on the substrate. The invention further comprises a 
method for preparing films of the desired crystallographic orientation 
using the single source CVD precursors as disclosed herein. 
It is an object of the invention to provide volatile single source 
precursors, which contain the required stoichiometric ratio of elements, 
for the deposition of metal chalcogenide films. 
It is also an object of the invention to provide a CVD process for metal 
chalcogenide films which produces a reduced amount of effluent waste. 
A further object of the invention is to provide a process that produces 
films at pressures lower than ambient atmospheric pressure. 
Finally, it is an object of the invention to provide metal chalcogenide 
films which possess a predominant (110) crystallographic orientation 
through the use of a volatile single source precursor, thereby maximizing 
the films' utility as cathodes for lithium battery applications. 
In satisfying the above-mentioned objects, an organothiol (ER.sub.3 SH) is 
reacted with a titanium tetrahalide in an organic solvent to provide 
compounds of the formulation [TiX.sub.4 (HSER.sub.3).sub.2 ]. This 
material is sublimed and is delivered in the gaseous phase to a heated 
substrate which is supported within a reaction zone. Upon heating to a 
temperature of 200.degree. to 800.degree. C. the substrate is coated with 
a TiS.sub.2 film. Any material not deposited as a film on the desired 
substrate passes through as exhaust which is subsequently entrapped to 
minimize adverse environmental consequences. 
Films prepared with the single source precursors of the invention and which 
are less than two microns thick, prepared at temperatures above 
400.degree. C., exhibit nearly exclusive (001) crystallographic 
orientations. However, such films which are ten microns or more thick and 
grown at temperatures above 400.degree. C. exhibit a predominantly (110) 
crystallographic orientation.

BEST MODE FOR CARRYING OUT THE INVENTION 
To prepare the single source precursors of the invention, titanium 
tetrahalides (TiX.sub.4) are reacted with an organothiol (ER.sub.3 SH) in 
an organic solvent to provide an initial product of the formula TiX.sub.4 
(HSER.sub.3).sub.2. 
X may be a halogen, i.e. any member of the group consisting of fluorine, 
chlorine, bromine, and iodine. It is most preferred that X be comprised of 
chlorine. Accordingly, it is most preferred that titanium tetrachloride 
(TiCl.sub.4) be reacted with an organothiol (HSER.sub.3) in an organic 
solvent. 
Suitable organic solvents can include alkane solvents such as hexane, 
aromatic solvents such as benzene and toluene, and halogenated solvents 
such as dichloromethane. It is most preferable that hexane be used. 
E may be any member of the group consisting of carbon, silicon, germanium, 
or tin. It is preferred that E be carbon. R may be selected from the group 
consisting of hydrogen, an alkyl group, an aryl group and mixtures 
thereof. Most preferably, E and R.sub.3 form a single organic species. 
Most preferably, E will be carbon and R.sub.3 will be comprised of a 
hydrogen and a cyclic pentamethylene chain, the combination thereof 
comprising a cyclohexyl group. 
Accordingly, it is most preferred that the single source precursors be 
prepared by reacting titanium tetrachloride with cyclohexylthiol in 
hexane. However, it will be further appreciated that R may also be 
selected from the alkyl group consisting of a methyl group, a primary 
alkyl group, a secondary alkyl group, a tertiary alkyl group, and mixtures 
thereof. 
Although not wishing to be bound by any particular theory, it is believed 
that the choice of R determines the stability of the complex TiX.sub.4 
(HSER.sub.3).sub.2. 
In particular, when ER.sub.3 is a tert-butyl group, the initial product 
will decompose at temperatures up to and above 25.degree. C. to produce a 
reddish-brown polymeric material. It is believed that TiCl.sub.4 (.sup.t 
BuSH).sub.2 decomposes to produce (TiCl.sub.2 S).sub.n. This material has 
been used in the instant invention to produce TiS.sub.2 films. However, it 
is believed that this material in and of itself is not a single source 
precursor. Rather, the ability of (TiCl.sub.2 S).sub.n to produce 
TiS.sub.2 films relies solely on the presence of residual nondecomposed 
TiCl.sub.4 (.sup.t BuSH).sub.2. 
Most preferably, 100 milligrams of the single source precursor will be 
utilized to provide a 1 to 2 micron thick film on a substrate 2 
centimeters .times.5 centimeters in size. 
Turning to the Figure, the single source precursor 2 will be deposited in 
the closed end 4 of sealed quartz tube 6. Glass tubes have insufficient 
strength at the higher temperatures used herein. 
Using heat source 8, single source precursor 2 is sublimed. Heat source 8 
may provide temperatures from 25.degree. to 300.degree. C. at pressures 
ranging from 0.0001 to 760 torr. Most preferably, heat source 8 will 
supply a temperature of between 25.degree. to 75.degree. C. at a pressure 
of between 0.0001 to 0.1 torr. This typically provides a deposition rate 
of 0.1 microns per minute. 
The sublimate of single source precursor 2 is delivered to a reaction zone 
10. The reaction zone 10 is comprised of that section of the quartz tube 6 
arrayed within a furnace 12. 
Furnace 12 heats reaction zone 10 to temperatures of between 150.degree. to 
1000.degree. C. Most preferably, the reaction zone will be heated to a 
temperature between 400.degree. and 600.degree. C. Disposed within 
reaction zone 10 of quartz tube 6 is a ceramic stage 14 upon which the 
desired substrate 16 has been placed. 
Substrate 16 may be comprised of glass, steel or individual sheets of 
monocrystalline silicon. Low sodium glass is particularly suitable. 
Typical substrates are normally in the range of 2 centimeters by 5 
centimeters, although larger pieces are possible. 
Although not necessary, spraying of the substrate with freon is permissible 
in order to remove any surface particles such as dust and the like. 
Corning 7059 is a suitable glass. Suitable silicon may be obtained from 
Mattheson Scientific of Detroit, Mich. 
Upon the passing over of sublimed precursor 2, substrate 16 is coated with 
a thin film of metal chalcogenide. 
If desired, a stream of inert gas such as nitrogen may be introduced at 
port 18 to assist in the flow of sublimed precursor 2 through the quartz 
tube 6. 
After exiting reaction zone 10, sublimed precursor 2 exits quartz tube 6 
via tubing 20 to exit the system through vacuum pump 22. 
The following examples are provided to illustrate the invention but are not 
intended to limit the invention. It is anticipated that those skilled in 
the art will understand that other reactor configurations are possible. 
All degrees are in centigrade and all parts are by weight percent unless 
otherwise indicated. 
EXAMPLE 1 
A reaction vessel containing titanium tetrachloride (1 mmol) dissolved in 
hexane (20 mL), was cooled to 0.degree. C. in an ice/water bath. 
Tertbutylthiol (2 mmol) was added to this solution and was allowed to stir 
for 0.25 hours. During this time, a yellow crystalline solid of the 
formula TiCl.sub.4 (.sup.t BuSH).sub.2 precipitated from the medium 
(80-90% yield). Spectroscopic analysis revealed: .sup.1 H NMR (CDCl.sub.3, 
.delta.) 1.95 (s, 2SH), 1.42 (s, 2 C.sub.4 H.sub.9); .sup.13 C NMR 
(CDCl.sub.3, ppm) 42.08 (s, C(CH.sub.3).sub.3), 34.72 (s, 
C(CH.sub.3).sub.3). 
EXAMPLE 2 
Complex TiCl.sub.4 (.sup.t BuSH).sub.2 (1 mmol) was allowed to stand at 
25.degree. C. for 1 hr, either in the solid state or dissolved in 
dichloromethane (20 mL), during which time it decomposed to an insoluble 
red-brown polymeric product of the formula (TiCl.sub.2 S).sub.n (54% 
yield). Microanalytical data revealed that: analysis calculated for 
TiCl.sub.2 S predicted Cl=47.00 Intact, Cl=46.43 was found. 
EXAMPLE 3 
A reaction vessel containing titanium tetrachloride (1 mmol) dissolved in 
hexane (20 mL), was cooled to 0.degree. C. in an ice/water bath. 
Cyclohexylthiol (2 mmol) was then added to this solution and stirred for 
0.25 hours. During this time, a yellow crystalline solid of the formula 
TiCl.sub.4 (C.sub.6 H.sub.11 SH).sub.2 precipitated from the medium (84% 
yield). This compound was thermally stable at 25.degree. C. 
Spectroscopic and analytical data revealed: mp 75.degree.-76.degree. C. 
(sublimes with some decomposition); IR (nujol, cm.sup.-1) v.sub.SH 2492 
(m); .sup.1 H NMR (C.sub.6 D.sub.6, .delta.) 2.51 (m, C.sub.6 H.sub.11 
SH), 1.72-0.82 (m, C.sub.6 H.sub.11 SH); .sup.13 C{.sup.1 H} (C.sub.6 
D.sub.6, ppm) 39.55 (s, CHSH), 37.07 (s, 2 CH.sub.2), 25.89 (s, 2 
CH.sub.2), 25.01 (s, CH.sub.2). Analysis calculated for C.sub.12 H.sub.24 
Cl.sub.4 S.sub.2 Ti: C, 34.14; H, 5.73. Found: C, 33.82; H, 5.68. 
EXAMPLE 4 
Complex TiCl.sub.4 (C.sub.6 H.sub.11 SH).sub.2 (2 mmol) was placed in a 
glass vessel, which was then connected to a quartz tube. A glass 
substrate, supported on a ceramic stage, was placed in the quartz tube. 
Approximately half of the quartz tube was held in a furnace set at 
500.degree. C. The glass vessel, which was held outside the heated section 
of the quartz tube, was heated to a temperature between 50.degree. and 
100.degree. C. at ambient atmospheric pressure. This resulted in 
sublimation of the single source precursor into the heated section of the 
quartz tube which held the heated substrate. Passage of the gaseous 
precursor through the heated zone resulted in the substrate being coated 
with a film of TiS.sub.2. The deposition rate was about 1 micron per 
minute. The film obtained was of high-quality and was bronze colored. 
X-ray diffraction of a 2 micron thick film indicated an exclusive (001) 
crystallographic orientation. 
EXAMPLE 5 
Crystalline TiS.sub.2 film was also deposited using the experimental method 
described in Example 4, except that the reactor was held at a pressure of 
0.1 torr. This resulted in a film of similar quality and identical 
crystallographic orientation obtained in Example 4, except that the 
deposition rate was about 0.05 microns per minute. 
EXAMPLE 6 
TiS.sub.2 films of about 2 microns thickness were prepared on glass 
substrates using the material from Example 3 at 500.degree. C. and 0.1 
torr. X-ray diffraction analysis revealed nearly exclusive (001) 
crystallographic orientations in all of these films. 
EXAMPLE 7 
TiS.sub.2 films of about 10 microns thickness were prepared on glass 
substrates using the material from Example 3 at 500.degree. C. and 0.1 
torr. X-ray diffraction analysis revealed predominant (101) and (110) 
crystallographic orientations. These orientations are suitable for use in 
lithium batteries. 
As is apparent from the foregoing descriptions, the process for depositing 
thin films of metal chalcogenide utilizing single source precursors 
according to the present invention has various advantages, including: 
(1) High-quality films of titanium disulfide are produced at moderate 
temperatures from the single source precursor TiX.sub.4 
(HSER.sub.3).sub.2. 
(2) Films produced using the single source precursors are of high quality 
and are of similar quality to the films prepared using the atmospheric 
pressure CVD reaction of titanium tetrachloride and organothiols. 
(3) Film deposition rates using the single source precursors range from 
0.01 to 2.0 microns per minute, which is faster than prior art processes. 
(4) The use of the single source precursors reduces toxic and odiferous 
reactor effluent. 
(5) The single source precursors provide highly crystallographically 
oriented films. 
(6) Atmospheric pressure and low pressure CVD reactions using single source 
precursors of the formula TiX.sub.4 (HSER.sub.3).sub.2 produce metal 
chalcogenide films with the correct crystallographic orientation for use 
as cathode materials in lithium batteries. 
While the best modes for carrying out the invention have been described in 
detail, those familiar with the art to which the invention relates will 
recognize various alternative designs and embodiments for practicing the 
invention as defined by the following claims.