Thin Solid Films, 158 (1988) 285 298 PREPARATION AND CHARACTERIZATION 285

C H A R A C T E R I Z A T I O N OF MoSe 2 THIN FILMS

A. MALLOUKY AND J. C. BERNEDE Laboratoire de Physique des Matkriaux et Composants de l'Electronique, 2, rue de la Houssinikre, 44072 Nantes Ckdex (France)

(Received October 5, 1987; accepted December 7, 1987)

The influence of sputtering and annealing on the properties of MoSe 2 films was investigated by scanning electron microscopy, electron microprobe analysis, X-ray photoelectron spectroscopy (XPS), X-ray analysis, optical absorption and electrical resistivity measurements. It was found that stoichiometric thin films are obtained after appropriate annealing. The c axis orientation depends strongly on the thin film composition before annealing~ The crystallite size varies between 10 and 200 nm. The optical gaps were determined to be almost equal to that of MoSe2 single crystals. The chemical shifts of the XPS lines were found to be in good agreement with those of a stoichiometric powder reference.

1. INTRODUCTION

Interest in the application of MoSe 2 for reducing friction 1 or as a photo- electrode in photoelectrochemical photovoltaic cells 2 has increased in the last few years. Thus papers concerning the properties of MoSe2 films prepared by r.f. magnetron sputtering have been published recently by Bichsel et al. 3 5. They have shown that magnetron-sputtered films can be obtained under carefully selected conditions with the correct stoichiometry and the basal plane of the MoSe2 crystallites perpendicular to the substrate. The longitudinal resistivity of their films was insensitive to the stoichiometry while the MoSe2 was p type.

In this paper another process for obtaining stoichiometric thin films is described. In addition, the structure, the chemical composition, the morphology, the optical gap and the variation in the resistivity with the temperature of the MoSe2 thin films are presented.

2. THIN FILM PREPARATION

The layers were prepared by d.c. diode sputtering while their chemical composition was determined by electron microprobe analysis and X-ray photo- electron spectroscopy (XPS). The d.c. diode system used in this investigation has been described elsewhere 6.

0040-6090/88/$3.50 © Elsevier Sequoia/Printed in The Netherlands

286 A. MALLOUKY, J. (3. BERNEDE

The MoSe 2 water-cooled target was made of pressed MoSe2 powder. The substrate holder was grounded and the substrate temperature could be monitored between 150 and 600 K. The sputtering argon pressure, the target-to-substrate distance and the target voltage were respectively 3.5 Pa, 4 cm and 1.2 kV. The films were deposited onto chemically cleaned slides of silica, glass, mica or molybdenum.

The composition of the deposited films was investigated as a function of the substrate temperature. As shown in Table I the composition depends strongly on the substrate temperature Ts. It can be seen that layers with a marked selenium deficiency are obtained. The best Mo:Se ratio was obtained between 325 and 425 K.

T A B L E 1 THIN FILM ('OMPOSITION IL";. SUBSTRA'IE TEMPERAFURI (THE SUBSTRATE BEING (;LASSI OB~IAINEI) BY

ELECTRON MICROPROBE ANALYSIS

M o (",3 Se (";3

325 K 43

425 K 59

545 K 96

675 K 98.5

A n n e a l i n g wi th Se p o w d e r for 24 h at 775 K (T s - 425 KI 28.5 A n n e a l i n g wi th Se p o w d e r for 24 h at 775 K (T s = 425 K l" 20"

The s ame sample af ter a n n e a l i n g u n d e r v a c u u m for 8 h at 535 K 34

57

41

4

1.5

71.5

20

66

X P S q u a n t i t a t i v e analysis .

The film deposition processes require a post-deposition treatment to achieve the correct stoichiometry. After sputter deposition, the film was placed with a small amount of selenium in a vacuum-sealed silica tube and heated at T 1 = 780 K for 24 h. Films with excess selenium were obtained (Table I). Finally, the film was heated under dynamic vacuum at T 2 = 573 K for 10 h and the correct stoichiometry was obtained (Table I).

Scanning electron micrographs showing the variation in morphology for the films before and after annealing are shown Fig. 1. In Fig. l(b) we can see small heaps of selenium on the surface of MoSe 2 films. This selenium is condensed on the film during the cooling of the silica tube. In Fig. l(c), obtained after annealing under vacuum, the selenium heaps have disappeared.

3. T H I N FILM C H A R A C T E R I Z A T I O N

3.1. Determination o f preferred orientation 3.1.1. Experimental procedure The X-ray diffraction technique was used to determine the preferential

orientation of crystallites in MoSe2 films. X-ray spectra of the samples were taken under identical instrumental settings using an X-ray goniometer v. The degree of preferred (00/)-type orientation, i.e. crystallites with the c axis perpendicular to the plane of the substrate, was

~_. lloot) i ~ l(hki, F(ooi) _ (o0t) I(hktt

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CHARACTERIZATION OF MoSe2 FILMS 287

(a) (b)

(c)

Fig. 1. Scanning electron micrographs showing the variation in morphology for the sputtered MoSe 2 thin films: (a) after sputtering (T s = 425 K); (b) after annealing with selenium (T 1 = 780 K; 24 h); (c) after annealing under vacuum (T 2 = 573 K; 8 h).

2 8 8 A. MALI~OUKY, J. ( ' . BERNEI)E

where Floo~) is the degree of preferential c axis orientation, l~hkt ~ the intensity of the (hkl) line for MoSe2 thin films and lo~hk~, the intensity of the (hkl) line of a randomly oriented powder sample according to ASTM data.

The degree of crystallite order G (the ratio of crystallite phase content to the phase providing the non-coherent scattering of X-ray radiation) is described by

[(hkll/ lo~hkl) G = ~ d ,h~t)

where d is the film thickness as measured by a stylus. The grain size of the film was estimated from the full width at half-maximum

(FWH M) of the diffraction peak ~. The instrument factor correction was determined with MoSe 2 powder.

3.1.2. Experimental results Before annealing, the diffraction patterns were either diffuse or presented small

peak intensities. After annealing, the peak intensity of the films deposited on heated substrates (425 K) is higher than the peak intensity of those deposited at 325 K. Therefore the films characterized are the former films (some films deposited at Ts = 545 K are also studied). The quantitative analysis (Table I) and vacuum annealing before recharging which increase the diffraction peaks intensity show the existence of three phases in sputtered films: selenium, molybdenum and MoSe 2, which shows MoSe2 decomposition during sputtering. The X-ray signal has been studied as a function of the substrate, the substrate temperature and the annealing process as shown in Table II. The results are not modified by annealing under vacuum (% < T1). Table II shows that the film orientation is very sensitive to the substrate temperature Ts during deposition on the glass.

We can see that the films are relatively well oriented at T s = 425 K (generally 60?0-80% of the crystallites exhibit a (001) orientation), while the growth of crystallites with (hkO) orientation occurs at Ts = 545 K or when the substrate is a molybdenum foil. The degree of crystallite order increases with the preferential orientation.

For specimens deposited on molybdenum foil, the grain size is found to be about 10nm for different values of the substrate temperature, while for glass substrates the grain size varies between 15 and 30 nm at T s = 425 K. For T s = 545 K the grain size is so large that it cannot be precisely measured by the F W H M method. Diffraction peaks at an angle 20 - 31.4' are shown in Fig. 2. This figure indicates the strong grain size difference between thin films deposited at Ts = 425 K and T s = 545 K. There is no strong correlation between F, G, the grain size and the post- deposition annealing temperature in the experimental range studied (Table II).

3.2. Scanning eh, ctron microscopy studies The scanning electron micrographs of MoSe2 for different steps during the thin

film preparation were discussed in Section 2 (Fig. 11. We can see that there is better crystallization after annealing (Fig. l(c)) than before (Fig. l(a)).

Figure 3 shows the surface morphology of MoS% films deposited on a glass substrate at T s = 545 K. We can see by comparison with Fig. l(c) (Ts = 425 K) that the substrate temperature introduces a change in film morphology. The lamellar

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microstructure of MoS% thin films has disappeared for layers deposited at 7 s = 545 K {Fig. 3). On the glass substrate the grain size increases considerably for deposition at a high substrate temperature: from Fig. 3, we can estimate the grain size of the films to be 100 200 nm.

lqg. 3. S c a n n i n g e lec t ron m i c r o g r a p h s h o w i n g a n M o b % thin film depos i t ed at 7;; = 545 K on a glass

subs t r a t e t annea l i ng : Tl = 780 K, 24 h; T 2 = 573 K, 8 hi.

CHARACTERIZATION OF MoSe2 FILMS 291

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3.3. Optical absorption properties The absorption coefficient 3¢ of MoSe2 was calculated from the transmission

spectra of two samples of different thicknesses as described by Bichsel and Levy 4. The variation in the optical absorption (~ is the absorption coefficient) with the photon energy hv for a silica substrate is shown in Fig. 4 (:~1" ts. hvI.

Goldberg et al. ~ have reported that the energy gap of MoSe2 monocrystals varies between 1.09 and 1.17 eV from ~ 2 cs. hv plots and from :~ t:s. hv n can bc anywhere in the range 1 2 within the experimental error. MoSe2 being a layered indirect band gap semiconductor, the value of n depends on the shape of the density of states. The two extreme cases of the density of states are isotropic and bidimensional for n = 2 and n = 1 respectively. In real situations, generally ~° 1 ~< n ~< 2. The values of Eg have been estimated for the two extreme cases. The optical band gap of the MoS% films was determined to bc 1.15 eV and 1.26 eV by' extrapolating the straight lines of :~,e vs. E and ~. t's. E respectively. The value obtained for the band gap of the MoSe2 films is almost equal to that of MoSe2 monocrystals, which confirms that the films have a stoichiometric composition and the structural properties of MoSe2.

3.4. X-rt O' l?hotoe/ectron ,V~ectroscop)' Electron spectroscopy for chemical analysis tESCAt measurements were

performed with a magnesium X-ray source (1253.6eV) operating at 10kV and 20 mA. Data acquisition and treatment were performed by a computer. The energy resolution was l eV at a pass energy of 50eV. When the substrates ( 10 mm x 11 m m x 1 ram) were insulating, the MoSe2 thin films were grounded with silver paste. Reference spectra were obtained from MoSe2 powder (Table Ill, Figs. 5 and 6) stuck on a copper substrate. ESCA investigations were performed after sputtering and annealing. The quantitative studies were based on the determination of the Mo 3d and Se 3d peak areas with respectively 2.5 and 0.57 as sensitivity factors (Table II1).

The chemical shifts of these lines and the energy difference between the Se 3d and Mo 3d52 peak spectra were compared with those of the MoSe n powder. For stoichiometric thin films the chemical shifts were close to those of the powder (Fig. 6). For thin films with excess selenium there was a modification of the selenium chemical shift (1 eVj and the energy difference between the Se 3d and Mo 3d52 peak

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C M 4 8 G l a s s 425 25 75 0.35 x I I) :~

M i c a 425 34 66

M i c a 425 34.2 65.7

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CHARACTERIZATION OF MoSez FILMS 2 9 3

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= 425 K ; T1 = 780 K , 24h ) .

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(s toichiometr ic) ; - - - , MoSe z thin film with se lenium excess (88°/,, Se, 11% Mo).

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CHARACTERIZATION OF M o S e 2 FILMS 295

spectra decreased to 174.4eV for the stoichiometric thin films or powder and 173.4 eV for thin films with excess selenium.

This correlation between thin film composition and the energy difference between the Se 3d and Mo 3d5/2 peak spectra allows us to check whether the films are MoSe2 or not. The quantitative analysis (Table III) shows that almost stoichiometric films can be obtained after annealing whatever the substrate (glass, mica, molybdenum foil) and the substrate temperature T s are.

3.5. Resist ivi ty studies The room temperature resistivity of the films varied from 0.1 × 1 0 3 to

15 × 103 f~cm: it increased when the grain size of the films decreased, and/or the crystallite order decreased (Table I). The variation in the thin films' resistivity does not seem to be strongly related to the stoichiometry (Table III).

The temperature dependence of the electrical resistivity between 80 and 550 K is shown in Fig. 7(a) for two typical samples deposited on a glass substrate, the ohmic contact being evaporated gold thin film. Figure 7(b) shows the curve of lnR vs. 103/T for an Mo-MoSez -Au sandwich structure.

In the range 80-500 K, the resistivity does not follow an Arrhenius dependence, the samples exhibiting marked variations in O logp/c~T with temperature. The observed slopes are always increasing with temperature. In the range 500-550 K the resistivity follows the Arrhenius dependence with temperature: the activation energy AE is 0.57 eV.

4. DISCUSSION AND CONCLUSION

In Table IV the characterization parameters of our MoSe 2 thin films are compared with those ofBischel et al. a 5?

We can see that many results are comparable but the differences in the thin film realization processes introduce some striking variations. While d.c. diode sputtered films require post-deposition annealing to be stoichiometric, r.f. planar magnetron sputtered films can be obtained immediately with correct composition. MoSe 2 being a semiconductor, d.c. sputtering may introduce a charge effect near the target which may increase the resputtering of the more weakly bonded species of the growing layer 16, After the decomposition of MoSe/dur ing sputtering, part of the deposited selenium on the substrate evaporates because of the high vapour pressure, and layers with selenium deficiency are then obtained. When the substrate temperature increases, the evaporation of selenium from the growing film increases the selenium deficiency. Careful selection of the annealing conditions gives samples with the correct stoichiometry and stable electrical properties. This is not the case for the thin films of Bischel and Levy before annealing where, whatever the method used, stable samples need an annealing post-deposition treatment.

The preferential orientation of the r.f. magnetron sputtered films is always the same, i.e. the c axis is parallel to the plane of the substrate. Very different results are obtained with the d.c. diode sputtered films after annealing. For a high substrate temperature Ts or with molybdenum foil as the substrate, i.e. with large excess of molybdenum, the post-annealing process leads to thin films with the c axis parallel

C H A R A C T E R I Z A T I O N OF M o S e 2 FILMS 297

The grain size increases with substrate temperature and is smaller for the molybdenum foil substrate than for other substrates. In the same way, scanning electron micrographs show that the films exhibit a lamellar microstructure except for the film with poor (00/) preferential orientation.

Good agreement exists for the optical band gap and the XPS spectrum between MoSe2 thin films, powder and single crystal. The shift in binding energy (about 1.5 eV) for Mo 3d and Se 3d between the results of Bischel and Levy and our results may be related to charging effects during measurements or to the calibration of the instruments.

Whatever the value of the resistivity the shape of the curve of lnp vs. IO00/T is always the same (Fig. 7) even with th e Mo-MoS ez -A u sandwich structure.

Several general trends are immediately apparent from examination of the data. (a) The room temperature resistivity of the thin film is higher by two or three

decades than that of single crystals (Table IV). (b) As have Bischel and Levy, we have shown that the influence of stoichio-

metry is not responsible for the strong variations in film resistivities. (c) The room temperature resistivity decreases when the grain size and/or the

crystalline order increases. (d) 0 logp/OT always increases with increasing temperature, stabilizing only

when T > 500 K (AE = 0.57 eV). It has been shown 17 that the resistivity of MoSe2 is strongly related to the

stoichiometry of the compound but this is not the case for the thin films. Therefore results (a)-(d) could be well interpreted in terms of grain boundary theory 18'19 or by postulating that the growth ofmicrocrystallites during annealing forms one phase of a certain degree of order, while the material which unites the grown crystallites forms a second phase of presumably substantial disorder. The data could then be discussed in terms of a quasi-amorphous semiconductor model 2° but X-ray study shows us that the films are crystallized. Therefore the latter assumption is less probable than the former. During the formation of MoSe 2 by annealing under selenium pressure grain boundaries form and the film might become less continuous, which would account for the higher resistivity values observed.

At high temperature ( T > 500 K) the activation energy corresponds to a band gap of 1.14 eV which is very close to the optical band gap. The transition to intrinsic conduction is observed at 500 K, which is lower than the 700 K obtained by Evans and Hazelwood2L In conclusion there is much experimental evidence that the thin films obtained are MoSe2 thin films. Hall and thermoelectric power measurements are being undertaken in our laboratory to improve the transport model in MoSe2 thin films.

A C K N O W L E D G M E N T S

The authors wish to thank Messrs. Bohn, Barreau and Rzepka for performed measurements, Mr. Spiesser for helpful discussions and Mr. Latefffor his help in the English translation.

ESCA analysis has been carried out at Nantes with a Leybold spectrometer (University of Nantes, Centre Nationale de la Recherche Scientifique).

2 9 8 A. MALLOUKY, J. ('. BERNEDE

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