Chemical Vapor Deposition of MoS2 and TiS2

Preparation of M0S2 thin films by chemical vapor deposition Woo Y. Lee, Theodore M. Besmann, and Michael W. Stott Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6063 (Received 16 June 1993; accepted 9 February 1994)

The chemical vapor deposition (CVD) of MoS 2 by reaction of H 2 S with molybdenum halides was determined to be thermodynamically favored over a wide range of temperature, pressure, and precursor concentration conditions as long as excess H 2 S was available. The thermochemical stability of H 2 S, MoF 6 , and M0CI5 was also assessed to address their suitability as precursors for the CVD of MoS 2 . The results from the thermodynamic analysis were used as guidance in the deposition of MoS 2 thin films from MoF6 and H 2 S. The (002) basal planes of MoS 2 films deposited above 700 K were preferentially oriented perpendicular to the substrate surface.

I. INTRODUCTION Lamellar solid lubricants such as MoS 2 have been developed for tribological applications in high temperature and vacuum environments where the use of traditional liquid lubricants becomes ineffective or cannot be tolerated.1 The lattice structure of MoS 2 consists of a hexagonal sheet of Mo atoms, which lies between two hexagonal sheets of S atoms. The Mo and S sheets are held together strongly by covalent bonding whereas the interaction between the MoS 2 layers (i.e., between adjacent S sheets) is through weak van der Waals forces. This layered crystallographic arrangement allows the MoS 2 layers to easily shear between basal planes and is responsible for its excellent lubricity. Also, MoS 2 has been considered as a cathode material for solidstate secondary lithium batteries.2 In this application, the ability to intercalate the MoS 2 layers with lithium metal ions is being utilized. MoS 2 has been prepared as a thin film by a variety of methods including sputter deposition, 34 pulsed laser evaporation,5 sulfurization of electroplated Mo,6-7 drying or heating of chemical solutions containing liquid Mo and S precursors,8"10 chemical vapor deposition (CVD),2 and metal-organic CVD (MOCVD). 1112 Among these techniques, MoS 2 prepared by sputter deposition has been most frequently evaluated for tribological purposes. The processing-microstructureproperty relationships of sputter-deposited MoS 2 are relatively well-documented.3'4'13"16 Interestingly, the preparation of MoS 2 by CVD has not received much attention, although it seems that the technique offers several advantages over sputter deposition. First, the non-line-of-sight nature of CVD should provide superior film uniformity and conformal coverage on substrates having intricate shapes, internal passages, or large dimensions. Second, CVD offers the possibility of incorporating MoS 2 into hard nitride and carbide CVD film matrices as finely dispersed, discrete 1474

grains via codeposition. This type of self-lubricating composite film, if appropriately engineered, might provide the improved wear protection required for some of the high temperature automotive and aircraft applications. It is noted that the versatility of CVD in creating ceramic composite films by codeposition and in controlling their microstructure has been previously demonstrated for many binary combinations including BN + A1N, S13N4 + TiN, and TiSi2 + SiC.17"19 Several investigators have studied the deposition of MoS 2 by MOCVD. Hofmann11 and van Zomeren et al.n used a carbonyl precursor, Mo(CO) 6 , and H 2 S to produce MoS 2 films in the temperature range of 448 to 573 K. In general, the MoS 2 films were botryoidal and contained grains that were estimated to be smaller than 100 nm. In both studies, the MoS 2 films were not sufficiently characterized to assess their potential as a solid lubricant. Hofmann observed that the basal planes of MoS 2 tended to be preferentially oriented along the direction perpendicular to the substrate surface. Similar growth behavior has been observed from MoS 2 prepared by sputter deposition.1314"16 Recently, Imanishi et al.2 deposited MoS 2 from H 2 S and M0CI5 and studied its current discharge and charge characteristics in electrolyte solutions containing Li. According to their x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS) analyses, MoS 2 was the only solid phase deposited in the temperature range of 673 to 723 K. As in the case of Hofmann's work, the MoS 2 basal planes were preferentially oriented perpendicular to the substrate surface. In the present study, the thermodynamics of the MoS 2 deposition by reaction of H 2 S with molybdenum fluoride or chloride were analyzed. It is well recognized in the CVD community that understanding a CVD process requires the comprehensive integration of thermodynamics, kinetics, transport phenomena, and nucleation and growth studies. In the absence of

J. Mater. Res., Vol. 9, No. 6, Jun 1994

W.Y. Lee, T. M. Besmann, and M.W. Stott: Preparation of M0S2 thin films by chemical vapor deposition

prior analytical work, equilibrium calculations are useful as a first step in determining thermodynamic limitations for the deposition process. The results obtained from the thermodynamic analysis were used as the basis for designing and conducting initial deposition experiments. II. THERMODYNAMIC ANALYSIS A. Thermochemical data A computer program, CHEMSAGE, 20 was used to calculate chemical equilibria for the M o - S - C l - H and M o - S - F - H systems. The program minimizes the total Gibbs free energy of all possible gaseous, liquid, and solid species which might be present in a particular chemical system. Since the accuracy of this type of thermodynamic analysis is sensitive to the possibility of omitting stable chemical species and uncertainties in the thermochemical data, it is important to assess the stability of each chemical species and its thermodynamic properties. In this study, the thermodynamic properties of chemical species considered for the M o - S - C l - H and M o - S - F - H systems were assessed in the temperature range of 400 to 1600 K using several thermodynamic data references.21"23 The chemical species considered for the M o S - C l - H and M o - S - F - H systems are listed in Table I. Most of the thermodynamic data used in this work were directly obtained from the SGTE Database.21 In this database, the thermodynamic data are compiled in terms of A / / ; and S° at 298.15 K, AH" for phase transitions, and Cp, where AHJ is the standard enthalpy of formation, 5° is the entropy, and Cp is the molar specific heat at constant pressure. Cp is expressed as a function of temperature, Cp = a + bT + cT2 + d/T2 where a, b, c, and d are constants and T is temperature in Kelvin. It was necessary to modify the SGTE data used for several species to conform to recent revisions made in the JANAF Tables.22 The temperature range validated for solid and liquid S data was extended from 800 to 2000 K, and a solid phase transition at 374 K was eliminated

due to the lack of experimental evidence. For MoS 2 (s), the values of Cp and, therefore, AGJ were slightly lower in JANAF compared to those compiled in SGTE (e.g., by approximately 13 J/mole/K and 5 kJ/mole, respectively, at 1600 K). Chemical equilibria for the fluoride and chloride systems were, however, found to be relatively insensitive to the above data revisions. The AG; values for 5 3 (g), S 4 (g), 5 5 (g), S 6 (g), S 7 (g), and 5 8 (g) in SGTE were different from those tabulated in JANAF by as much as 11 kJ/mole at 1600 K. However, since these species had relatively low partial pressures in the temperature range studied in this investigation, the data for these species were not modified. The stability of M0S3 as a solid compound appears questionable. In Barin,23 the data for M0S3 are tabulated up to 1000 K although the species is not listed in JANAF. A calculated M o - S binary phase diagram given in Massalski24 does not indicate M0S3 as a possible solid. Barin's reported data were based on his own work and another source.25 If Barin's data for M0S3 are accurate, M0S3 is expected to be relatively stable in sulfur-rich environments since AG° for the reaction, MoS 2 + S = M0S3, is -28.4 kJ/mole at 298.15 K and -13.7 kJ/mole at 800 K. In the present study, equilibrium calculations were performed with and without M0S3 to assess its potential importance in the chloride and fluoride systems. Another potential source of uncertainty in this study was the omission of M0CI3, MoCl 2 , and MoCl in the calculations which might exist as gaseous and condensed species. The species having more than several halogens do, however, become increasingly less stable with fewer halogen bonds. Thus, the calculations performed without the subchlorides, even if they do exist, would still likely be relatively accurate. B. Precursor stability In CVD processes, the reactivity and volatility of reagents should be examined to determine their chemical stability and transportability as precursors at elevated temperatures. For example, one major problem in using

TABLE I. Chemical species considered for the M o - S - C l - H system (total 33 species) and M o - S - F - H system (total 43 species). Mo-S-Cl-H system Gas species Condensable species

Cl, Cl 2 , H, HC1, HS, H 2 , H 2 S, H 2 S 2 , Mo, M0CI4, M0CI5, MoCl 6 , S, S 2 , S 3 , S 4 , S 5 , S 6 , S 7 , S 8 , SCI, S2C1, S2C12 S, SC12, S 2 C1 2 , M0CI4, MoCls, MoCl 6 , Mo, MoS 2 , M0S3, Mo 2 S 3

Mo-S-F-H system Gas species

Condensable species

F, F 2 , FS, F 2 S, F 3 S, F 4 S, F 5 S, F 6 S, F 1 0 S 2 , H, HF, H 2 F 2 , H 3 F 3 , H4F4, H 5 F 5 , H 6 F 6 , H 7 F 7 , HS, H 2 , H 2 S, H 2 S 2 , Mo, MoF, MoF 2 , MoF 3 , MoF 4 , M0F5, MoF 6 , Mo 2 Fio, Mo 3 F 15 , S, S 2 , S 3 , S 4 , S5, S6, S 7 , Ss S, Mo, MoS 2 , MoS 3 , Mo 2 S 3


1475


organometallic precursors to deposit superconducting oxides containing yttrium, copper, and barium has been that the precursors become chemically unstable at the temperatures required to volatilize the precursors.26 In another example, Lee et al.21 recently demonstrated that understanding the thermal decomposition characteristics of NH 3 was critical in describing the overall thermodynamics of the CVD of Si 3 N 4 from SiF 4 and NH 3 . In this study, the thermochemical stability of MoF 6 , M0CI5, and H 2 S was considered as a function of temperature. Both fluoride and chloride precursors have been used for the CVD of Mo and MoSi 2 . 28 33 MoF6 is a gas above 308 K with sufficient vapor pressure (e.g., 207 kPa at 333 K) so it can be directly metered using a mass flow device. On the other hand, M0CI5 is a hygroscopic solid near room temperature and needs to be handled in an inert environment. A more convenient way to feed MoCl5 into a CVD chamber is to directly chlorinate Mo with Cl2 in the upstream of the deposition region.28 Figure 1 shows that MoCl5 is a condensable phase which is stable up to its boiling point (541 K). The melting point of MoCl5 is 467 K. Powell28 recommended that M0CI5 could be best prepared by reacting Mo with Cl2 in the temperature range of 573 to 773 K. However, Fig. 1 suggests that, at temperatures above 700 K, the formation of M0CI4 is energetically more favorable, and M0CI4, not M0CI5, would be the precursor for the MoS 2 deposition. In contrast, equilibrium calculations indicate that MoF 6 is thermally stable up to 1600 K, and only a small fraction of MoF 6 (e.g., 10~6 at 1000 K and 10' 3 at 1600 K) is expected to be decomposed mainly into M0F5 and F. Thus, other subfluorides such as M0F4, M0F3, etc. have extremely low equilibrium partial pressures.

c o

u n

400

6 0 0 800 1000 1200 1400 1600

T(K) FIG. 1. Equilibrium composition expected upon thermal decomposition of 1 mole of M0CI5 at 101 kPa. All the species shown are gaseous unless indicated as a condensable species (c). 1476

The decomposition of H 2 S into H 2 and S 2 was calculated to be less than 5% at 900 K and approximately 50% at 1600 K. The thermal and catalytic decomposition of H 2 S has been extensively studied in the petrochemical industry as a means of recovering H2 from H 2 S. As reviewed by Raymont,34 the kinetics of the H 2 S decomposition reactions are known to be rapid above 1250 K. Below this temperature, the reaction rates are basically dependent on catalyst type and residence time. The CVD of Mo from MoF 6 and M0CI5 has been accomplished by reduction with H2 (Refs. 28-30): MoF 6 (g) + 3H 2 (g) = Mo(s) + 6HF(g)

(1)

MoCl 5 (g) + f H 2 (g) = Mo(s) + 5HCl(g)

(2)

The present study confirmed that these reactions were thermodynamically favored to proceed. In the context of the present investigation, the results implied that (i) molybdenum halides could not be generated by reacting Mo with HC1 or HF, and (ii) the presence of H2 during the halogenation of Mo would not be desirable. The reaction represented by Eq. (2) was experimentally known to occur in the temperature range of 673 to 1673 K.28'29 According to Stinton,30 Mo could be deposited from MoF 6 and H2 at 1373 K, but no deposit was observed at 1123 K.

C. Chemical equilibria for Mo-S-CI-H system Figure 2 is a CVD phase diagram constructed for the M o - S - C l - H system as a function of temperature and H 2 S/(MoCl 4 + H2S) molar ratio, ij/, at a pressure of 1 kPa. For this diagram, M0CI4 rather than MoCl5 was assumed to be the precursor. Also, M0S3 was not included in the calculations as a possible solid species. As indicated, MoS 2 is thermodynamically favored to deposit as a single solid phase under excess H 2 S conditions (i.e., if/ > 0.67) in the entire temperature range of 400 to 1600 K. In this region, the thermodynamic yield of MoS 2 is near 100% (Fig. 3). Thus, the resultant MoCl4 and M0CI5 partial pressures are very small under these conditions. When ip is less than 0.67, the yield of MoS 2 from MoCl4 is limited by the availability of H 2 S. As shown in Fig. 2, Mo 2 S 3 is expected to be codeposited with M0S2 with increasing temperature and decreasing H 2 S concentration. When the temperature is further increased, deposition of Mo 2 S 3 as a single phase and, subsequently, codeposition of Mo 2 S 3 and Mo are predicted. On the other hand, if the H 2 S concentration is lowered at temperatures below 500 K, MoCl4 as a condensable phase (either liquid or solid depending on the temperature) is predicted to be codeposited with MoS 2 . An increase in pressure to 101 kPa extends th stability of the MoS 2 single phase to higher temperatures.


W. Y. Lee, T. M. Besmann, and M.W. Stott: Preparation of M0S2 thin films by chemical vapor deposition

1600

correct, the contamination of MoS 2 films due to the presence of M0S3 is expected to be significant in the lower temperature range. In Hofmann's MOCVD work, which was performed at temperatures below 573 K, the presence of M0S3 in his films was not detected by XRD. However, it was difficult to ascertain critically the stoichiometry of the films based on the XRD data since a significant fraction of the films appeared to be amorphous.

Mo 2 S 3 M0+M02S3

1400

MoS2+Mo2S3

r

1200

p- 1000

D. Chemical equilibria for Mo-S-F-H system

800

600

MoCU+MoSg I 400 0.2

0.4

0.6

0.8

1.0

H2S MoCI4+H2S

FIG. 2. CVD solid phase diagram for the Mo-S-Cl-H system calculated at 1 kPa as a function of temperature and reagent molar ratio, H2S/(MoCl4 + H2S).

With the addition of MoS 3 to the equilibrium calculations, M0S3 was predicted to be codeposited with MoS 2 below 650 K and at i// > 0.67. Otherwise, the CVD phase diagram was basically unchanged. Therefore, if the thermodynamic data of M0S3 in Barin are

As in the case of the chloride system, MoS 2 is also predicted to be deposited from MoF 6 and H 2 S over a wide range of conditions (Fig. 4). One of the major differences observed for the fluoride system is that, at high temperatures and low H 2 S concentrations, no solid phase is expected to form whereas Mo 2 S 3 and Mo are predicted for the chloride system. Figure 5 shows that the conversion from MoF6 to MoS 2 is almost 100% in the presence of excess H 2 S. As shown in Fig. 4, sulfur is expected to be codeposited with MoS 2 below 425 K at 1 kPa. Note that the melting point of sulfur is 388.36 K. An increase in pressure from 1 to 101 kPa shifted the MoS 2 single phase zone slightly toward high temperatures. At 101 kPa, the codeposition of MoS 2 and S is expected up to itsuu NO SOLID 1400

•

J

1200

MoS2

1000

800

600 MoS2+S 1

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0

600

800

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1

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1

1

1

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1 ^

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MoF6+H2S

FIG. 3. Equilibrium composition of a mixture containing 1 mole M0CI4 and 5 moles of H2S at 1 kPa. All the species shown are gaseous unless indicated as a solid species (s).

FIG. 4. CVD solid phase diagram for the M o - S - F - H system calculated at 1 kPa as a function of temperature and reagent molar ratio, H2S/(MoF6 + H2S).


1477


The equilibrium content of M0S3 in the MoS 2 + M0S3 deposit is calculated to be approximately 10% at 750 K and 90% at 700 K. The equilibrium analysis suggests that the deposition of MoS 2 by reaction of H2S with MoF 6 or M0CI4 (or M0CI5) is thermodynamically feasible. Since Imanishi et al. previously demonstrated that MoS 2 thin films could be prepared from the M0CI5 + H 2 S mixture, the experimental portion of this work concentrated on the MoF 6 + H 2 S mixture. For the purpose of designing deposition experiments, the calculations also suggested that the H2S/MoFg molar ratio should be higher than 2 to maximize the thermodynamic yield of MoS 2 .

10"

400

6 0 0 800 1000 1200 1400 1600 III. EXPERIMENTAL

T(K) FIG. 5. Equilibrium composition of a mixture containing 1 mole of MoF6 and 5 moles of H2S at 1 kPa. All the species shown are gaseous unless indicated as a solid species (s).

500 to 550 K. When MoS 3 is added to the equilibrium calculations, the phase stability zone for the single phase MoS 2 deposition becomes smaller as shown in Fig. 6. At the H 2 S/(MoF 6 + H2S) molar ratio of 0.8, MoS 3 is predicted to be codeposited with MoS 2 below 750 K and deposited as a single phase below 650 K. IDUU

NO SOLID 1400 1200

r

M0S2

1000

800 —'

-

M0S2+M0S3

[

600

1

400

'

M0S3 I

1

0.2

I

I

0.4

0.6

0.8

1.0

H2S MoF 6 +H 2 S

FIG. 6. CVD solid phase diagram for the M o - S - F - H system calculated at 1 kPa with the addition of M0S3 as a possible solid species. 1478

A hot-wall quartz tube reactor was used for deposition experiments as shown in Fig. 7. The flow rate of MoF 6 (unknown purity, but made from 99.95% pure Mo; Johnson Matthey), H 2 S (99.5%; Matheson), and Ar (99.999%; Matheson) was measured and controlled using mass flow meters (MKS Instruments). MoF 6 and H 2 S + Ar were introduced into the reactor as separate gas streams. The diameter of the stainless steel extension tube used for feeding MoFg was 0.64 cm. The diameter and length of the quartz tube were 5 cm and 50 cm, respectively. An electrical resistance heater was used to heat undoped Si(100), stainless steel, and graphite substrates. As illustrated in Fig. 7, the silicon (1 cm X 1 cm) and stainless steel (1.27 cm X 1.27 cm) substrates were placed using an alumina holder. The graphite disk (2.54 cm diameter and 0.64 cm thickness) was placed perpendicular to the direction of gas flow using a graphite substrate holder. A chromel-alumel thermocouple that was positioned at 31 cm from the reactor inlet was used to measure temperature. The same thermocouple was also used to measure temperature variations in the axial direction under inert gas flow. An MKS pressure transducer was used to measure pressure in the reactor. An MKS pressure controller, which was interfaced with a ballast valve located between the reactor and a mechanical vacuum pump, was used to control the reactor pressure. The surface morphology and coating crystallinity were characterized using a Hitachi (S-800) field emission scanning electron microscope (SEM) and a Scintag x-ray diffractometer (XRD) using Cu K a radiation. The low pressures (1 kPa) and flow rates (110 cm3/ min at STP) used in this study resulted in laminar and diffusive gas flow. The Reynolds (Re) number based on the reactor tube diameter was estimated to be less than 5 at typical deposition conditions. Also, the Peclet (Pe) number was approximated to be less than 2 in the region where MoF 6 and H 2 S were mixed. The Pe number is defined here as vd/DUaVf>-^x where v is the velocity of



Resistance Heater Thermocouple

Quartz Tube

AI 2 O 3

Stainless Steel

^ ^

-Graphite Silicon

Mass Flow Meters

Scrubber

Pump

H2S Ar MoF6 FIG. 7. Experimental apparatus used to deposit M0S2 from MoF 6 and H 2 S.

the MoF6 stream, d is the diameter of the quartz tube, and Z)MoF6-Ar is the binary diffusion coefficient between MoF 6 and Ar. The viscosity for the gas mixture was calculated using the Chapman-Enskog relationship and Wilke's mixture approximation.35 The binary diffusion coefficient was also calculated using a correlation based on the Chapman-Enskog theory.35 In the context of the present work, the Pe number could be interpreted as a ratio of the rate of convective mass transfer in the axial direction to the rate of diffusive mass transfer in the radial direction. The low Pe number indicated that molecular diffusion in the radial direction was rapid and, therefore, the MoF 6 and H 2 S + Ar streams were relatively well-mixed in the deposition region. IV. EXPERIMENTAL RESULTS AND DISCUSSION In initial deposition experiments, the formation of powder in the gas phase was observed at temperatures of 700 to 1000 K, a pressure of 1 kPa, a H 2 S flow rate of 10 cm3/min, and a MoF6 flow rate of 2.5 cm3/min. In subsequent experiments, an Ar flow rate of 100 cm3/min was added to the H 2 S stream to lower reagent concentrations in the gas phase. The Ar dilution substantially reduced the degree of powder generation in the gas phase and helped produce smooth thin films at 603 to 808 K. The films deposited on Si(100) at 808 and 703 K had highly faceted surface features as shown in Figs. 8(a) and 8(b), respectively. Other conditions used for these experiments were a H 2 S flow rate of 10 cm3/min, a MoF6 flow rate of 2.5 cm3/min, and a pressure of 0.3 to 0.6 kPa. An XRD pattern of the film deposited at 703 K, shown in Fig. 9, indicates that the deposit is pure crystalline MoS 2 . In addition, intense (100) and (110) diffrac-

tion peaks and a weak (002) peak suggested that the (002) basal planes of this film were preferentially oriented perpendicular to the substrate surface. Note that the unidentified minor peaks were from the Si substrate. A similar XRD pattern was obtained from the film deposited at 808 K. This type of growth behavior was previously observed in other CVD and PVD studies,2'3'11'14"16 as discussed earlier. In general, the crystalline MoS 2 films deposited on Si adhered well as deposited. The growth rate of MoS 2 was dependent on the location of substrates in the reactor, and was typically in the order of several microns per hour. The color of the films was dark metallic gray. The films were soft, becoming lubricious and very shiny upon rubbing. The films could also be easily scratched and removed. In Fig. 10, the surface morphology of MoS 2 deposited on stainless steel and graphite is shown. These films were deposited in the same experiment from which the film shown in Fig. 8(b) was obtained. As illustrated in Fig. 7, the substrates were placed about 2 cm apart in this experiment. Although there was some temperature variation along the axial direction in the deposition zone, the variation was measured to be less than 10 °C between the stainless steel and graphite substrates. In comparing Figs. 10(a) and 10(b) with Fig. 8(b), the films deposited on the stainless steel and graphite substrates appeared much less faceted than the deposit on the Si substrate. Particularly, the film on the graphite substrate appeared much more botryoidal. At the lower temperatures of 658 and 603 K, films deposited on Si had nodular surface features as shown in Figs. 11 (a) and l l ( b ) . An XRD pattern of the film deposited at 603 K showed a broad, but weak (002)


1479

W.Y. Lee, T. M. Besmann, and M.W. Stott: Preparation of MoS2 thin films by chemical vapor deposition

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(b) FIG. 8. SEM micrographs of MoS 2 films deposited on Si(100) at (a) 808 K and (b) 703 K.

reflection with no (100) peak, suggesting that (i) the film was mostly amorphous and (ii) some portion of the film probably had some degree of an ordered structure in which the basal planes were oriented parallel to the substrate surface.

The films deposited on Si at the low temperatures tended to undergo extensive microcracking as shown in Fig. ll(a). Also, some areas of the films spalled off from the Si substrates as soon as the substrates were cooled to room temperature, as compared to the fact that the

(100)

c

26 (Degree) FIG. 9. XRD pattern of MoS 2 film deposited on Si(100) at 703 K. 1480


(110)


(a)

(b)

FIG. 10. SEM micrographs of MoS2 films deposited on (a) stainless steel and (b) graphite at temperatures of 703 ± 10 K.

1 (L1I11 (a)

(b)

FIG. 11. SEM micrographs of MoS2 films deposited on Si(100) at (a) 658 K and (b) 603 K. J. Mater. Res., Vol. 9, No. 6, Jun 1994

1481


highly oriented M0S2 films deposited on Si at 703 and 808 K adhered well. A similar behavior was observed from sputter-deposited MoS 2 . 16 It was suggested that the improved film adherence of highly oriented MoS 2 films over amorphous (or microcrystalline) films was due to the stronger chemical bonding between the oriented films and substrate. This hypothesis was based on the fact that the surface of the MoS 2 edge planes was much more energetically reactive than that of the basal planes. The observed effects of MoS 2 microstructure on film adherence might also be explained by the anisotropy of the elastic modulus of MoS 2 . The linear coefficients of thermal expansion (CTE) of MoS 2 were estimated to be 6.9 X 1 0 6 K"1 along the a-axis and 11.4 X 10"6 K 1 along the c-axis from 293 to 700 K,36 whereas the CTE of Si(100) was about 3.9 X 10"6 K"1 for the same temperature range.37 Thus, the perpendicularly oriented crystalline films should experience more tensile residual stress upon cooling than the less crystalline deposits having some degree of parallel orientation. However, it was expected that the elastic modulus of MoS 2 was lower in the c-direction than in the a-direction on the basis of the relative inter- and intralayer bond strengths, allowing the perpendicularly oriented films to tolerate a higher level of strain without microcracking or spalling. The perpendicular orientation of the MoS 2 basal planes has been recognized to be undesirable for tribological applications, as the lubricity and oxidative stability of the MoS 2 films are adversely affected.3'4 However, since the wear life of the films is also strongly affected by film adherence,16 the perpendicular orientation may still be preferred. In an ideal sense, the microstructure of the MoS 2 film could be functionally graded to optimize adherence and lubricity: perpendicular orientation near the substrate interface and parallel (or microcrystalline) orientation toward the top portion of the film.

V. CONCLUSIONS The equilibrium calculations suggested that the CVD of MoS 2 by the reaction of H 2 S with molybdenum halides was thermodynamically favored in the presence of excess H 2 S. In accordance with the results from the thermodynamic analysis, the deposition experiments demonstrated that the CVD of MoS 2 was possible using MoF 6 and H 2 S as precursors. The XRD data suggested that M0S3 was not a stable solid phase at 703 K and a MoF 6 /(H 2 S + MoF6) molar ratio of 0.75, whereas the calculations performed with the M0S3 data from Barin indicated that roughly 10% M0S3 and 90% MoS 2 would be codeposited under these conditions. Based on this comparison, and assuming that there were no major kinetic complications, it was likely that the M0S3 data from Barin were probably inaccurate. 1482

The homogeneous kinetics of the MoS2-CVD process appeared to be fairly rapid based on the observation of extensive powder formation in the gas phase at 1 kPa and at temperatures as low as 703 K. Also, the surface kinetics of the deposition process was observed to be sufficiently activated at moderate temperatures of about 700 K to produce crystalline MoS 2 films on silicon and stainless steel substrates. It appeared that the nucleation and growth behavior of MoS 2 was affected considerably by the nature of initial substrate surface. The effects of temperature on the microstructure and adherence of MoS 2 deposited on Si were documented. ACKNOWLEDGMENTS This research was sponsored by the Division of Advanced Energy Projects, United States Department of Energy, under Contract DE-AC05-84OR21400 with Martin Marietta Energy Systems, Inc. The authors thank J. H. DeVan, K. L. More, and T. R. Watkins for reviewing the manuscript. REFERENCES 1. P. Sutor, MRS Bulletin, 24 (May 1991). 2. N. Imanishi, K. Kanamura, and Z. Takehara, J. Electrochem. Soc. 139, 2082 (1992). 3. M. R. Hilton and P.D. Fleischauer, in New Materials Approaches to Tribology: Theory and Applications, edited by L. E. Pope, L. Fehrenbacher, and W. O. Winer (Mater. Res. Soc. Symp. Proc. 140, Pittsburgh, PA, 1989), pp. 227-238. 4. I. L. Singer, in New Materials Approaches to Tribology: Theory and Applications, edited by L. E. Pope, L. Fehrenbacher, and W. O. Winer (Mater. Res. Soc. Symp. Proc. 140, Pittsburgh, PA, 1989), pp. 215-226. 5. M. S. Donley, N. T. McDevitt, T. W. Hass, P. T. Murray, and J. T. Grant, Thin Solid Films 168, 335 (1989). 6. R. G. Bayer and A. K. Trivedi, Metal Finishing, 47 (November 1977). 7. A. N. Zelikman, B.P. Lobashev, Y.V. Makarov, and G.I. Sevost'yanova, Inorg. Mater. 12, 1367 (1976). 8. G. Chatzitheodorou, S. Fiechter, M. Kunst, J. Luck, and H. Tributsch, Mater. Res. Bull. XXIII, 1261 (1988). 9. P. Pramanik and S. Bhattacharya, Mater. Res. Bull. XXV, 15 (1990). 10. K. C. Mandal and A. Mondal, J. Solid State Chem. 85, 176 (1990). 11. W. K. Hofmann, J. Mater. Sci. 23, 3981 (1988). 12. A. A. van Zomeren, J. H. Koegler, P. J. van der Put, and J. Schoonman, Solid State Ionics, 521 (1992). 13. R. I. Christy and H. R. Ludwig, Thin Solid Films 64, 223 (1979). 14. J. Moser and F. Levy, J. Mater. Res. 7, 734 (1992). 15. J. Moser and F. Levy, J. Mater. Res. 8, 206 (1993). 16. P. A. Bertrand, J. Mater. Res. 4, 180 (1989). 17. W. Y. Lee, W.J. Lackey, P. K. Agrawal, and G.B. Freeman, J. Am. Ceram. Soc. 74, 2649 (1991). 18. T. Hirai and S. Hayashi, J. Mater. Sci. 17, 1320 (1982). 19. D.P. Stinton, W.J. Lackey, R.J. Rauf, and T.M. Besmann, Ceram. Eng. Sci. Proc, 668 (July-August 1984). 20. G. Eriksson and K. Hack, Metall. Trans. B 21B, 1013 (1990). 21. SGTE (Scientific Group Thermodata Europe) Solution and Pure Substance Database was supplied by the developers of the ChemSage program, GTT mbH, Aachen, Germany.


W.Y. Lee, T. M. Besmann, and M.W. Stott: Preparation of MoS2 thin films by chemical vapor deposition

22. JANAF Thermochemical Tables: Parts I and II, 3rd ed., J. Physical and Chemical Reference Data 14 (1985). 23. I. Barin, Thermochemical Data of Pure Substances: Parts I and II (VCH mbH, Weinheim, Fed. Rep. Ger., 1989). 24. T. B. Massalski, Binary Alloy Phase Diagrams (American Society for Metals, Metals Park, OH, 1986). 25. K. C. Mills, Thermodynamic Data for Inorganic Data for Sulphides, Selenides, and Tellurides (Butterworths, London, UK, 1974). 26. W.J. Lackey, J.A. Hanigofsky, M.J. Shapiro, W.B. Carter, D.N. Hill, E.K. Barefield, E.A. Judson, D.F. O'Brien, Y.S. Chung, T. S. Moss, and K. L. More, in Proceedings of the 11th International Conference on CVD, edited by K. E. Spear and G. W. Cullen (The Electrochemical Society, Pennington, NJ, 1990), pp. 195-210. 27. W.Y. Lee, J.R. Strife, and R.D. Veltri, J. Am. Ceram. Soc. 75, 2200 (1992). 28. C. F. Powell, in Vapor Deposition, edited by C. F. Powell, J. H. Oxley, and J. M. Blocher (John Wiley & Sons, Inc., New York, 1966), pp. 302-305.

29. M. L. Green and R. A. Levy, J. Metals, 63 (June 1985). 30. D.P. Stinton, Oak Ridge National Laboratory, Oak Ridge, TN (unpublished data). 31. S. Inoue, N. Toyokura, T. Nakamura, M. Maeda, and M. Takagi, J. Electrochem. Soc. 130, 1603 (1983). 32. G.A. West and K.W. Beeson, J. Electrochem. Soc. 135, 1752 (1988). 33. R. Madar and C. Bernard, J. Phys. C5, 479 (1989). 34. M. E. D. Raymont, Hydrocarbon Processing, 139 (July 1975). 35. R. C. Reid, J. M. Prausnitz, and T. K. Sherwood, The Properties of Gases and Liquids, 3rd ed. (McGraw-Hill Book Company, New York, 1977). 36. S. H. El-Mahalawy and B. L. Evans, J. Appl. Crystallogr. 9, 403 (1976). 37. T. K. Touloukian, R. K. Kirby, R. E. Taylor, and T. Y. R. Lee, in Thermal Expansion (Plenum, New York), Vol. 13, Part 1, p. 154.


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Chemical Vapor Deposition of MoS2 and TiS2

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