MoS2 thin film synthesis by soft sulfurization of a molybdenum layer

Materials Chemistry and Physics 42 ( 1995) 291-297

MATERIALS CHEM;&T&‘i&lD

Materials Science Communication

MO& thin film synthesis by soft sulfurization of a molybdenum layer

H. Hadouda a, J. Pouzet a, J.C. Bernede a, A. Barreau b a Luboraroire de Physique des MatPriaux pour I’Electronique, FacultP des Sciences et des Techniques. 2 rue de la HoussiniPre,

44072 Names Cedex 03, France

’ Service Commun de Microsopie Electronique ti Balayage de Nantes, Facultk des Sciences et des Techniques, 2 rue de lu Houssini?re, 44072 Nantes Cedex 03, France

Received 18 May 1994; revised 10 May 1995; accepted 24 May 1995

Abstract

MO& layers synthesized by annealing MO foils and evaporated MO thin films under sulfur pressure have been investigated by scanning electron microscopy, X-ray analysis, X-ray photoelectron spectroscopy and electrical resistance measurements. After appropriate processing (T> 823 K) the films crystallize in the hexagonal structure. When the annealing temperature is lower than the glass melting temperature, the crystallites develop preferentially with the c axis parallel to the plane of the substrate. The mean grain size, estimated by the full-width-at- half-maximum method, is about 8 nm. When the annealing temperature is 1173 K, the crystallites develop preferentially with the c axis perpendicular to the plane of the substrate and the mean grain size is about 20 nm. It has been found that, when they are annealed under dynamic vacuum (4 h, 673 K) after the first annealing, the films are stoichiometric. The bonding energies of the S (2~) and Mo( 3d) lines are in close agreement with those obtained with a reference powder. The surface morphology of the layers obtained from an evaporated MO thin film is more homogeneous than that of the layers obtained directly from a MO foil. The electrical resistance is governed by grain boundary scattering mechanisms.

Keywords: Films; Molybdenum; Synthesis; Sulfurization

1. Introduction

Molybdenum disulfide (MO&) is a layered semiconductor which is very important in industry. It is undoubtedly the

best-known layered transition metal dichalcogenide. It has attracted considerable interest as a solid lubricant [l-4] (very low friction coefficient) and as an electrode or photo- electrode in electrochemical investigation [ 5,6]. MoS, single crystals have been studied by Tributsch [ 51, Bucher [ 61, and Tributsch et al. [7] for photovoltaic applications.

MO&, as the other molybdenum dichalcogenides, can act as efficient electrodes in the realization of photoelectrochem- ical solar cells. The main advantage of these semiconductors is the prevention of electrolyte corrosion, because the pho- totransitions involve no bonding d-d orbitals of MO atoms [7-91.

For economic reasons, it should be interesting to obtain textured MO!!& thin films. This form of metal dichalcogenides has attracted many scientists [ lO-151. In this paper we describe a new technique to obtain MoS, layers on conductive substrates.

2. Experimental techniques

2.1. MoS,$lm preparation

The films were synthesized by soft chalcogenization, with reference to previous works [ 14-181, of the surface of MO substrates ( 10 X 13 X 0.25 mm). Two kinds of samples have been used. Some MoS, layers have been synthesized directly from MO foils, while the others have been synthesized from MO films evaporated on the MO foil. In this case a MO layer was evaporated with an electron beam on the MO foil and then covered with an evaporated sulfur layer. This last film protects the MO films from oxidization during transfer from the deposition apparatus to the annealing tube.

The MO substrates were placed in a vacuum-sealed Pyrex tube with a small amount of sulfur. The vacuum in the tube was up to 10e3 Pa. Then the samples were heated at a temperature below 873 K in the low-temperature process or at 1173 K in the high-temperature process. In the former case Pyrex tubes were used, while silica tubes were used in the latter case.

0254.0584/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved

S.S010254-0584(95)01S81-E

292 H. Hadouda et al. /Materials Chemistry and Physics 42 (1995) 291-297

2.2. Characterization techniques

The structure of the films was examined using an analytical X-ray system-type DIFFRACT AT V3.1 Siemens instrument which uses a graphics program EVA. The wavelength A was 1.5406 A. The grain size D was estimated from the full width at half maximum (F?VHM) of the diffraction peaks [ 191. The FWHM was given directly by the DIFFRACT AT. The degree of preferred (OO.Z)-type orientation, i.e. crystallites with the c axis perpendicular to the plane of the substrate, can be deduced from the X-ray spectra using the formula given

by Janda and Kubovy [ 201:

where F( 0O.Z) is the degree of preferential c axis orientation perpendicular to the plane of the substrate, I( hk.Z) the intensity of the (hkl) line for MoS, thin films and Z,(hk.Z) the intensity of the (hk.Z) line of a randomly oriented powder sample according to ASTM data. In the same way, the degree of preferred (hk.0) -type orientation, i.e. crystallites with the c axis parallel to the plane of the substrate, is given by

~z(hk.O)izOWk.O)

F( h/LO) = ht.’ phi I) f zOU*k.l)

Photoelectron spectroscopy (XPS) measurements were performed with a magnesium X-ray source ( 1253 eV> oper- ating at 10 kV and 10 mA (XPS measurements were performed at Nantes University-CNRS).

Fig. 1, SEM pictures (low magnification) of a MO& layer synthesized at

low temperature (a) from a MO foil and (b) from a MO thin film evaporated

on a MO foil.

Data acquisition and treatment were realized using a com- puter and a standard program. The quantitative studies were based on the determination of the Mo(3d) and S(2p) peak areas with 2.5 and 0.125, respectively, as sensitivity factors. The sensitivity factors of the spectrometer were given by

Leybold, the manufacturer.

layer has grown directly from a MO foil, while in Fig. 1 (b) a MO film has been evaporated on the MO foil before the annealing process.

Electronic microanalyses were performed in order to con- trol the homogeneity of the films. Also, we performed obser- vations of the surface topography using a field-effect scanning electron microscope.

The resistivity of the MoS, films was measured in the temperature range from 100 to 573 K on sandwich samples. Gold dots were evaporated onto the MoS2 films to attach copper wires with silver paste. After mechanical etching, the MO foil substrate works as an electrode.

At the end of the process, after annealing under dynamic

vacuum, it can be seen that the surface of the films appears homogeneous and rough at low magnification but with small heaps randomly arranged at the surface of the layer in the case of the MoS, directly obtained from MO foil (Fig. 1 (a) ) . Higher magnifications show that this surface appears com- paratively granular and inhomogeneous with grains irregu- larly shaped (Fig. 2(a) ) . There is also a poor adherence of this layer to the substrate. Moreover, in the case of a layer

obtained from a MO evaporated film, the photograph (Fig. 2(b) ) shows that the crystallites are quite homogeneous with a lamellar microstructure.

3. Experimental results and discussion

3.1. Chemical and morphological study

The above discussion concerns the low-temperature annealing process (823 I TI 873 K) . At high temperature the crystallites are much larger while, if the lamellar microstructure is preserved, the c axis of the crystallites appears to be essentially perpendicular to the plane of the substrate. The average grain size is estimated to be about 100 nm from Fig. 3.

Photographs showing the surface morphology after syn- These films have also been observed after solid state reac-

thesis of the layers are shown in Fig. 1. In Fig. 1 (a) the MoS, tion in the vacuum-sealed tube but before annealing under

H. Hadouda et al. /Materials Chemistry and Physics 42 (1995) 291-297 293

Fig. 2. SEM pictures (high magnification) of a MO& layer synthesized at low temperature (a) from a MO foil and (b) from a MO thin film evaporated on a MO foil.

Fig. 3. SEM picture of a MoS, layer synthesized at high temperature.

dynamic vacuum (Fig. 4(a)). It can be seen that a lot of small heaps appear at the surface of the layers while after the final annealing (Fig. 2(b)) these heaps have disappeared. We imagine that during the cooling of the tube some sulfur condensation takes place at the surface of the layers, since we can see in the backscattered-electron mode that the composition of the inhomogeneities is different from that of the main part of the film (Fig. 4(b) > . In the backscattered-elec-

tron mode the darker domains correspond to those composed of a lighter element, here the sulfur. This sulfur excess is

sublimated by annealing the samples under dynamic vacuum

for 4 h at 673 K. This study has been corroborated by XPS analysis. At the

end of the process, XPS spectra are identical for all speci- mens. 0( Is), C( Is), Mo(3d) and S(2p) lines are shown in

Fig. 5. Carbon and oxygen detected at the surface of the films correspond to surface pollution since the samples were placed

in room atmosphere before XPS measurements. This oxygen contamination can affect the properties of the layers; however, it is essentially a surface contamination, as shown earlier

in the case of MoS, [ 121. After annealing under chalcogen

pressure the XPS oxidized MO peak has disappeared even if MO is oxidized before the annealing. The disappearance of

the MO doublet tied to the oxygen has been explained by an

expulsion of the oxygen because, during the annealing, the Se partial pressure is far higher than that of OZ. Such a result

has been obtained by many authors who work on silicides of refractory metals. Even WS, thin films have been obtained

by sulfurization of W03 layers [ 211. The positions of the lines for the oxides and the elements are shown on the figure by arrows [ 221. Also, reference spectra have been obtained

by us from MoS, reference powder stuck on a copper substrate.

r‘ig. 4. (a) SEM picture of a MoS, layer before annealing under dynamic

vacuum; (b) backscattered electron mode.

294 H. Hadouda et al. /Materials Chemistry and Physics 42 (1995) 291-297

I . 7.95 265 275

Binding energy E(cV)

240 230

.

Bindin~tkergy E(EV)

536 532 *

526 Binding energy E(eV)

Fig. 5. XPS spectra of (a) C( Is), (b) O(k), (c) Mo(3d) and (d) S(2p).

The chemical shift of the lines and the energy difference between the S( 2p) and the Mo( 3d& peak spectra are com- pared with those of the MoS, powder and of the elements in Table I. The adventitious carbon peak at the surface of the film has been taken as reference. This peak is situated at 284.6 eV. There is a good correlation between the energies meas-

ured on the layers and those obtained from reference samples. The shifts in energy of the Mo(3d& and S(2p) bound in the MoS, structure are + 1.2 and - 2.25 eV, respectively. It has been shown [ 231 that the bonds in the MO& compounds

are ion-covalent. The electron affinity of the MO being 1.8 and that of S being 2.5, there is a small charge exchange between MO and S. The MO atoms are positively charged as shown by the increase of the binding energy of the Mo( 3d) peaks, while the S atoms are negatively charged which explains the small decrease of the binding energy of the S peaks. This correlation allows us to check whether MO and S are bound in the MoS, structure or not.

It can be seen in Fig. 5 (c) that the Mo( 3d) signal is a well- defined doublet with the S (2s) signal observed as a shoulder

Table 1

XPS analysis

Sample

Reference S

Reference MO before etching (MOO,) Reference MO after etching Reference powder Thin film not post-annealed under vacuum,

annealing conditions, T= 850 K, t= 24 h Thin film annealed under dynamic vacuum,

T=680K,t=4h, annealing conditions,

T=850K,t=24h Thin film annealed under dynamic vacuum,

T=680 K, t=4 h, annealing conditions,

T=820K, t=24 h

Binding energy (eV)

S(2P) Mo(3d&

164.25

232.6 227.9

161.9 229 162 229.1

162 229.1

162 229.2

Mo(3d,,,)

235.8 231.1 232.15 232.25

232.25

232.35

Energy difference AE (eV)

between S(2p)

and Mo( 3d,,,) peaks

67.1 67.1

67.1

67.2

Composition (atom%)

MO S

33 61

31 69

36 64

33 67


Table 2

Quantitative electron microprobe analysis

Sample Annealing temperature (K) r(h) Analysed point MO (atom%) S (atom%)

MO foil

MO thin film on MO foil

MO foil




713 24 no.1 33.4 66.6

no. 2 34.6 65.4

823 42 no. 3 33.3 66.1

no. 4 33.3 66.1

823 42 no. 5 32.5 61.5

no. 6 34 66

823 24 no. I 33.2 66.8

no. 8 35.8 64.2

853 24 no. 9 33 67 no. 10 33.4 66.6

1173 24 no. 11 33.3 66.1 no. 12 33.3 66.7

on the lower binding energy side of the Mo( 3d,,,) peak. The decomposition of the XPS peaks into different components and the quantitative interpretation were made after subtrac- tion of the background using the Shirley method 1241. The developed curve-fitting programs permit the variation of parameters such as the Gaussian/Lorentzian ratio, the FWHM, the position and the intensity of the contributions. These parameters were optimized by the curve-fitting programs in order to obtain the best fit. After decomposition of the Mo( 3d) and S (2s) peaks (Fig. 6) XPS quantitative analysis (Table 1) shows that stoichiometric films are obtained at the end of the process, whatever the annealing temperature (8231Ts 1123 K).

The result has been checked by microprobe analysis (Table 2). Here also there is some difficulty because there is an overlap of the SK and the MO, rays. Therefore, we have to use a long acquisition time and then decompose the signal. However, the homogeneity of the films has been checked with success by this technique.

Before annealing under dynamic vacuum a small excess of sulfur is detected by XPS analysis which is a surface technique (Table 1) . After the annealing under dynamic vacuum this small sulfur excess disappears, which is in good agreement with the hypothesis of sulfur-heap condensation at the surface of the layer during the cooling of the tube.

3.2. X-ray diffraction (XRD)

The results of the XRD study are summarized in Table 3. It can be seen in Figs. 7 and 8 that all the films are crystallized in the ZH-MoS, structure. The orientation of the crystallites depends strongly on the annealing temperature. The crystallites develop preferentially with their c axis parallel to the plane of the substrate at the low-temperature annealing process and with their c axis perpendicular to the plane of the substrate at high temperature. In this case all the crystallites detected by XRD have their basal plane either perpendicular ( 10%) or parallel (90%) to the plane of the substrate.

The mean grain size of the crystallites has been estimated by the FWHM of the XRD peaks. It can be seen in Table 3

rMa3d 7

12.

4-

-- T- . 236 230 224

Binding energy E(U)

Fig. 6. Results of the Mo( 3d) and S( 2s) curve fitting: - - -, experimental

curve; * * * * * , fitted curve; -, different components,

Fig. 7. XRD spectrum of a MoS, layer synthesized at low temperature

Fig. 8. XRD spectrum of a MO& layer synthesized at high temperature

296

Table 3

XRD analysis

H. Hadouda et al. /Materials Chemistry and Physics 42 (1995) 291-297

Sample F(hk.0)

Low temperature 0.65

High temperature 0.10

F(OO.l)

0.90

Grain size Doo.n (nm)

35

Grain size D, , o (nm)

10

20

&., (nm)

0.6149

0.615

dllo (nm)

0.1575

0.158

-i-” 6_

l

7_ /

**

l

l

J l

T 6_

5, l ?

m 4-

I I

3_ f

ki

*- I l

1 I I III1

12 3

+q$; ;'

Fig. 9. Results of the curve fitting of the resistance of a MoSz layer: * * * * * , experimental points; -, fitted curve.

that the calculated grain size for the films obtained by the high-temperature process is far higher than the grain size for the films obtained by the low-temperature process. However, the grain size measured in the former case is smaller than the value statistically estimated from scanning electron microg- raphy. The discrepancy can be attributed to the fact that, while all the crystallite sizes are taken into account in the averaged FWHM method, only the larger crystallites are concerned by the second technique. The averaged measured values of the interplanar spacing are reported in Table 3. They are in good agreement with the data given in the ASTM X-ray powder diffraction file 38-1388.

The c axis lattice constant was determined from the expres- sion obtained for hexagonal crystals:

_&=;(“‘+%~+“‘)+$

The c axis length determined from the (00.2) spacing for the best films is 12.204 A, which is in good agreement with previously reported values ( 12.2985 A). Having determined c, a can be also calculated from l/g:,,, yielding a value of 3.1488 A which is also in good agreement with previously reportedvalues (3.1611 A).

3.3. Electrical properties

The typical temperature dependence of the resistance of the Mo/MoS,lAu structure between 100 to 500 K is shown in Fig. 9.

It can be seen that, in this temperature range, the resistance variation with the reciprocal temperature does not follow an Arrhenius dependence. The observed slopes a In R/dT

always increase with temperature. The MoS, films are polycrystalline and their resistivity is controlled by the grain

boundaries. Recently, Werner [ 251 has shown that the Arrhe- nius plots must be curved due to potential fluctuaticns at the grain boundaries and that the curve must be fitted by a parab- ola. It can be seen in Fig. 9 that there is a good agreement between the experiment and the fitted curve, which confirms the validity of Werner’s theory for these polycrystallineMoS, thin films.

4. Conclusions

It has been shown that stoichiometric MoS, thin films, crystallized in the 2H-MoS, hexagonal structure, can be synthesized by annealing MO under sulfur pressure.

A morphological study of the MoS, films obtained by synthesis has shown that the quality of the films depends on the MO used. The quality of the homogeneity of the layers is far better when the MO has been evaporated. When the MO foil is not covered by an evaporated layer, the surface of the films is rough and the adherence of the films to the substrate is very poor. These differences can be explained by the rough- ness of the polycrystalline MO while the surface of the evaporated MO is very smooth. In the former case, the poor adherence of the MoS, films may be tied to the formation of MoS, at grain boundaries before any reaction of the MO microcrystallites in the bulk.

The orientation and the size of the crystallites are governed by the annealing process. The XRD analysis shows that the crystallites have their c axis preferentially perpendicular to the plane of the substrate only when a high-temperature process is used (T= 1173 K).

The conductivity process of the films is well described by the grain boundary theory recently developed by Werner [ 251. In order to obtain more homogeneous layers, conductive electrodes pre-evaporated on glass substrates are now under study.

Acknowledgements

This work was supported by a contract between the EEC and the LPME (Jou II CT93 0352).


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MoS2 thin film synthesis by soft sulfurization of a molybdenum layer