Synthesis and characterization of flash-evaporated MoO3 thin films

......... CRYSTAL

GROWTH

ELSEVIER

Journal of Crystal Growth 156 (1995) 235-244

Synthesis and characterization of flash-evaporated MoO 3 thin films C. Julien

a,*

A. Khelfa

a

O.M. Hussain

a,l

G.A. Nazri b

a Laboratoire de Physique des Solides, associ~ au CNRS, Unic,ersit~ Pierre et Marie Curie, 4 place Jussieu, F-75252 Paris Cedex 05, France b Physical Chemistry Department, General Motors Research Laboratories, Warren, Michigan 48090, USA

Received 26 September 1994; manuscript received in final form 12 May 1995

Abstract

Molybdenum trioxide with an orthorhombic symmetry is one of the most interesting layered intercalation materials because of its use in solid state batteries and display systems. In the present investigation, thin films of M o O 3 w e r e prepared by flash-evaporation technique on silica glass and silicon substrates maintained at different temperatures, TS, in the range of 30-300°C. The films were systematically characterized by studying their structural, optical and electrical properties. X-ray diffraction and SEM analysis showed that films have an orthorhombic layered structure. The Raman scattering and infrared absorption were studied to establish the Ts dependence of the film properties. The energy gap of M o O 3 films is located between 2.8 and 3.2 eV depending on thc substrate and annealing temperature. AC and DC conductivities were measured as a function of T~. The effect of annealing treatment was also investigated.

I. I n t r o d u c t i o n

Transition-metal oxides that can u n d e r g o reversible lithium i n t e r c a l a t i o n / e x t r a c t i o n at ambient t e m p e r a t u r e are of great technical interest as c a t h o d e materials for secondary lithium batteries [i]. A m o n g these, M o O 3 is one of the most interesting intercalation materials for ambient temperature solid-state lithium batteries [2,3]. T h e interest in these materials arises from: (i) the or-

* Corresponding author. Fax: +33 144 274 541; E-mail: [email protected]. 1Present address: Thin Film Laboratory, Department of Physics, Sri Venkateswara University, Tirupati 517 502, India.

thorhombic phase ( a - M o O 3) has the unique two-dimensional layered structure [4]; (ii) oxide c o m p o u n d s always exhibit higher electrochemical activity than chalcogenides for instance; (iii) thin films of M o O 3 can be easily p r e p a r e d using num e r o u s evaporation techniques; (iv) M o O 3 is one of the oxide c o m p o u n d s with highest stability. M u c h attention has b e e n given to the p h e n o m e n a of p h o t o c h r o m i s m and electrochromism of M o O 3 thin films because of their applications in display devices [5-7]. T h e ability of ionic insertion leads to their use in microbatteries [8]. M o l y b d e n u m trioxide possesses an intriguing layered structure consisting of double layers of M o O 6 o c t a h e d r a held together by covalent forces in the (100) and (001) directions but by strong

0022-0248/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 0 2 2 - 0 2 4 8 ( 9 5 ) 0 0 2 6 9 - 3

236

C. Julien et al. /Journal of Crystal Growth 156 (1995) 235-244

Van der Waals forces in the (010) direction. The important consequence of this is that large MoO 3 crystallites have elongated slab-like habits, preferentially exposing the (010) direction. The M o - O distances along the O - M o - O vertex (sharing the axis of the MoO 6 octahedra) are distorted to produce alternate long and short M o - O bonds [9,10]. MoO 3 thin films were usually prepared by vacuum evaporation [11-18], and sometimes by RF sputtering [19,20], electron beam technique [21] and chemical vapour deposition [22]. However, as far as we know, there is no investigation on flash-evaporated films. A number of papers have been published on various aspects of MoO 3 films. The structural and optical properties of MoO 3 thin films grown by thermal evaporation and sputtering techniques were studied by several investigators [5,13-15]. The electrical properties of thin amorphous films of MoO 3 have been given in many papers. Nadkarni and Simmons reported an energy band model for the M / M o O 3 / M sandwich structure and they made use the model to interpret the electrical properties of MoO 3 films. The AC and DC properties of MoO 3 films were described in their papers [12,23,24]. Anwar and Hogarth [18] reported the AC electrical conductivity properties of thin films of amorphous MoO 3 as a function of the substrate temperature and film thickness, and discussed the results in terms of the theory put forward by Elliott [25]. In the present investigation, thin films of MoO 3 are grown by the flash-evaporation technique and systematically characterized b y studying their compositional, structural, optical and electrical properties. The Raman scattering and infrared absorption studies are also carried out to establish the film properties.

2. Experimental procedure Films of MoO 3 were prepared by flash evaporation of molybdenum oxide powder (Alpha Products) which was pre-baked prior to evaporation. The films were grown on either silica glass substrates or (lll)-oriented silicon wafers main-

tained in the temperature range 30-300°C in a vacuum of below 10 -3 Pa. For the flash-evaporation technique the source materials were powdered and evaporated from a home-made system described elsewhere [26], in which the controlled temperature of the molybdenum boat was maintained at 1000°C. The rate of deposition was about 4 nm s-1. The thickness of the films was in the range 0.5-0.6/zm. Compositional and structural studies have been performed with photoelectron (PE) spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM). PE spectra were recorded with a Riber Mac-2 ESCA spectrometer using an A l K a (1486.6 eV) X-ray source. XRD patterns were obtained using a Siemens diffractometer equipped with a rotating copper anode X-ray (Cu K a ) source (A = 0.15406 nm) and a positionsensitive detector. Surface morphology was studied using a SEM (model SI-4000) under 10 -s Tort vacuum. Optical properties of the films were carried out using a UV-visible-near infrared double beam spectrometer (Hitachi U-3200) in the spectral range 200-1500 nm. Raman spectra were recorded at room temperature in the frequency range 20-1200 cm -1 using a Jobin-Yvon (model U-1000) double monochromator equipped with two holographic gratings. The scattered light was detected with an IT-F-FWl30 cooled photomultiplier coupled with a computerized photon counting system. The excitation source was the 514.5 nm line of an Ar + ion laser at a power level of 40 mW. The infrared spectra were obtained at room temperature on a Bruker (model IFS 113) vacuum Fourier transform infrared (FTIR) spectrometer equipped with MCT and Ge-bolometer detectors. The MoO 3 films coated on intrinsic silicon substrates were employed for these measurements. The pure silicon spectrum was subtracted from the total spectrum and recorded to avoid the substrate effect. The DC electrical conductivity of the samples was measured using a four probe technique. Ohmic contacts were made with a silver coating. Temperature dependence measurements were carried out using a variable temperature cryostat (model TBT-SMC) where the samples are placed

C. Julien et al. / Journal of Crystal Growth 156 (1995) 235-244

237

Table 1 Photoelectron ionization energies (in eV) from molybdenum oxides Sample

Mo 3d5/2

Mo 3d 3/2

O ls

Ref.

Sputtered MoO 2 Crystalline MoO 3 Flash-evaporated MoO 3

229.1 232.5 232.3

232.3

530.4 530.2 530.0

[6] This work This work

235.5 235.7

on a copper finger in the gas exchanger chamber. The temperature was regulated by a computerized controller with an accuracy of _+0.5 K. The AC electrical conductivity was determined by a bridge technique, recording the real part of the equivalent parallel conductance given by an impedance analyzer HP4192A in the frequency range 5 Hz-13 MHz.

3. Results and discussion

3.1. Composition Flash-evaporated MoO 3 films are uniformly thick with a blue colour. The colour of the samples suggests that the films contain a number of oxygen vacancies [11]. The colour of these samples varied from light blue to deep blue with increasing substrate temperature (Ts). The photoelectron (PE) spectrum of a flashevaporated MoO 3 film grown at Ts = 120°C has been recorded. This spectrum displays the characteristic 3d5/2 and 3d3/2 doublet caused by the spin-orbital coupling in MoO 3 [27]. The binding energy and spin-orbit splitting (AEMo) are in good agreement with the values reported for MoO 3 by other workers [6,28]. The line shape of the core level O(ls) is Gaussian-like. The binding energy is found to be 530 eV. The Mo(3d) lines are separated by 3.4 eV and their intensity ratio is about 2:3 indicating that these lines are those expected by Mo 6+ ions as in MoO 3 having a d o electron configuration. A comparison of the Mo(3d) and O(ls) binding energies in MoO 2 and MoO 3 crystal is shown in Table 1. It can be seen from these data and the above spectrum that flash-evaporated MoO 3 films most closely resemble stoichiometric MoO 3.

3.2. Structure The structure and the surface morphology of MoO 3 films were studied by X-ray diffraction and scanning electron microscopy (SEM). All the films deposited are observed to be uniformly thick. Fig. 1 shows X-ray diffraction patterns of MoO 3 films deposited at different substrate temperatures, T~. The MoO 3 films deposited at room temperature are amorphous (a-MoO 3) in nature (curve a). Upon increasing substrate temperature, the crystallite size increases and the typical crystalline peaks of MoO 3 appear. The X-ray diffraction spectra of the films exhibit both (0k0) and (Okk) orientations representing a-mixed phase films. The fl-MoO 3 phase, which has a monoclinic structure, is related to the three-dimensional ReO 3 structure; thus the (011) and (022)

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Anqte 2e (deqree) Fig. 1. X-ray diffraction diagrams of MoO 3 films deposited by flash evaporation on a silica substrate maintained at different temperatures, Ts: (a) 30°C, (b) 120°C, and (c) 250°C.

238

C. Julien et al. /Journal of Crystal Growth 156 (1995) 235-244

lines are the signature of this phase. The films grown at Ts = 250°C show predominantly (0k0) orientation (Fig. 1, curve c). Lines at (020), (040) and (060) are clearly observed. It seems that these films grow with their basal planes parallel to the surface of the substrate. It is worth noting that the best crystallinity is obtained for films grown at moderate temperature, and that the increase of Ts favours formation of an as-deposited single-phase. It is apparent from X-ray diffraction diagrams that for films becoming crystalline, the ratio between intensities of the (020) and (060) lines increases with increasing substrate temperature. Their intensities are almost identical for Ts = 250°C. The conclusion is that, as the adatom mobility increases, the phase having the greater thermodynamic stability is favoured as the hierarchy a-MoO 3
Fig. 2. Scanning electron micrographs of M o O 3 films deposited by flash evaporation on a silica substrate maintained at different substrate temperatures, T~: (a) 30°C, (b) 120°C, and (c) 250°C.

C. Julien et al. /Journal of Crystal Growth 156 (1995) 235-244

at 120°C (Fig. 2b) is a mixture of amorphous and crystalline phases. The surface morphological studies show that films prepared in the range 200-300°C exhibit a crystalline part including two types of crystal geometry, i.e., elongated and short (may be monoclinic) crystals. Films prepared at 250°C (Fig. 2c) clearly showed the layered nature of this compound. Elongated micro-crystallites have typical dimensions of 250-1000 nm 2. The layered nature of the film is shown on each crystallite. In all cases the film deposited was uniform in thickness. Heating the films grown at Ts = 120°C above 350°C in air for 4 h converts them to thermodynamically stable a-phase. It is interesting to note that this transformation of mixed phases to aphase occurs in such a way that the resulting a-phase displays a very strong (0k0) preferred orientation. This implies that the/3 to a transformation is topotactic [13]. Some cracks remains in films deposited at moderate temperature. The basic unit of MoO 3 is a distorted MO 6 octahedron. Since, the as-deposited films have a microcrystalline structure, the crystal mainly grows by coalescence with neighbouring crystallites driven by the heat treatment process. Although rearrangement of the atoms is needed for the crystal growth, the rearrangement is very difficult in the film because of the low temperature. 3.3. Vibrational studies Fig. 3 shows the room temperature Raman scattering (RS) spectra of the MoO 3 films grown on ( l l l ) - o r i e n t e d silicon wafers. The polymorphism of the films was corroborated by comparison with the Raman spectra of a crystalline sample, c-MoO 3 [29]. The RS spectra of thin films agree well with the c-MoO 3 counterparts, although a lower signal strength results from thin films. The peak located at 520 cm-1 is due to the vibrational mode of the silicon substrate. Vibrational analysis shows that the M o - O stretching and bending modes occur in the 900-600 and 400-200 cm-1 regions, respectively. These bands are also Raman-active in the case of MoO 3 films. The peak associated with the unique molybdenyl bond (the shortest Mo = O of the structure),

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Fig. 3. Raman scattering spectra of (a)-(c) MoO 3 films deposited on (lll)-oriented silicon wafers and (d) crystalline MoO 3. Films were grown using different conditions: (a) as-deposited at Ts = 30°C, (b) as-deposited at Ts = 120°C, and (c) deposited at T~ = 120°C and annealed in 0 2 atmosphere. The peak at 995 cm -1 attributed to the molybdenyl bond is characteristic of the layered structure of a - M o O 3.

which is responsible for the layered structure of a-MoO3, is clearly observed in the spectra at about 995 c m - t. The narrow shape of this peak is attributed to the terminal oxygen [30]. The characteristic intense line at 820 cm-1 is due to the alternating bond lengths in the MoO 6 units. The RS spectrum of an as-deposited film grown at Ts = 30°C (Fig. 3a) shows that these films have a rather amorphous structure, i.e, we mainly observe the phonon density of states. However, the presence of weak peaks indicate that the a - M o O 3 phase is formed and further studies will show the dependence of heat treatment on the crystallographic structure of such a film. In the RS spectrum of a film deposited at Ts = 120°C, we observed the appearance of broad bands in the high-frequency regions, which are due to the stretching modes (Fig. 3b). After an annealing

240

c. Julien et al. /Journal of Crystal Growth 156 (1995) 235-244

treatment in 0 2 atmosphere for 4 h, the RS spectrum resembles that of c-MoO 3. The R a m a n peaks are sharp and the bands located at 288 and 337 cm -1, which are due to the Raman-active bending modes, are well resolved. It is interesting to note that the RS spectrum of a film deposited at Ts = 120°C (Fig. 3b) displays a band located at 750 cm -1, which is attributed to the stretching mode of fl-MoO 3 [31]. This band is absent in either the spectrum of a film deposited at higher t e m p e r a t u r e (T= > 200°C) or the spectrum of annealed film (Fig. 3c). We may conclude that all the structural investigations on M o O 3 thin films converge towards similar results, describing the layered nature of films grown at T= = 250°C. F T I R spectra of M o O 3 films deposited at different substrate temperatures have been investigated in the frequency range 500-1200 cm -1. A broad band between 500 and 1000 cm -1 was observed to be resolved into strong absorption peaks at around 570, 625, 700, 840 and 985 cm-1. Analysis is based on the fact that the structure of M o O 3 is formed by M o O 6 octahedra for which their stretching and bending vibrational infraredactive modes invariably occur in the 500-1000 cm -1 and 200-400 cm -1 regions, respectively. Generally, additional sub-classifications are used for the short terminal M o - O bonds and the

bridging M o - O - M o or three-coordinated oxyg e n - m o l y b d e n u m bonds. The terminal oxygen atoms (OMo) have three translational degrees of freedom. The observed high frequency bonds, the stretching modes of the (OMo) terminal, lie in the range 885-1007 cm -1. The frequency of the band due to the stretching mode of the (OMo) is observed at 985 cm -1 for M o O 3 thin films. The infrared-active stretching modes due to the alternating bond lengths in M o O 6 octahedron are observed at 570 and 840 cm-1 for M o O 3 films. These results are in good agreement with those recorded for M o O 3 crystal and can be calculated using the XY 6 molecular model [29]. The strong band at 700 cm-1, which is attributed to a stretching mode of the ( O M o 2) units, may be weakened. This may cause a shift to lower frequencies. This result is not understandable in terms of the bond strength. Other effects should possibly be considered such as the disordered structure of the films. The substrate t e m p e r a t u r e effect is clearly observed in the absorption infrared spectra, which supports the structural data. The F T I R peaks situated at 570 and 985 cm - t are well resolved with increasing substrate t e m p e r a t u r e (from ambient to 300°C). When M o O 3 films become polycrystalline, the M o O 6 octahedra are progressively

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C. Julien et al. /Journal of Crystal Growth 156 (1995) 235-244

connected and the structural reorganization is mainly observed for the M o - O stretching mode, which is associated with the unique character of the layered structure of oz-MoO 3. The layered structure is observed with the appearance of the molybdyl mode located at 985 cm - 1

3.4. Optical properties The optical transmission spectra have been recorded on MoO 3 films (700 nm thick) deposited at different substrate temperatures. We observe an absorption edge in the frequency range below 390 nm and a large absorption band situated at 800 nm. The optical transmission above the absorption edge is decreasing and the absorption edge shifts towards the high-wavelength side with increasing T~. The variation of optical absorption coefficient with wavelength for the films deposited at different substrate temperatures is shown in Fig. 4a. The fundamental absorption edge occurs at about 4 eV with a high value of the absorption coefficient 3 x 104 cm-1. A broad absorption band is observed in the red region of the absorption spectra. As a result, various degrees of coloration occurred. It is worth noting that the coloration of the films is associated with the absorption edge shift. For example, films grown at room temperature are white and transparent, while films grown at high substrate temperature ( TS _> 120°C) are deeply colored. The intrinsic absorption edge of the films was evaluated in terms of the direct transition. Thus, a plot of [(a - c~0)hog]2 as a function of the photon energy yields linear behaviour in the region of strong absorption near the absorption edge, as shown in Fig. 4b. Extrapolating the linear portion of this straight portion to zero absorption gives a value of the optical gap. The evaluated optical bandgap of MoO 3 films deposited at room temperature is 3.37 eV, which is in good agreement with previous works [9,14]. This value is very close to that of the bandgap, E g = 3.05 eV, in crystalline MoO 3 [32]. The bandgap of MoO 3 films decreases with increasing T~ and reaches a value of 2.80 eV for films deposited at 300°C. This result can be explained as follows. When the substrate temperature of the samples is in-

241

creased, the colour of the films is changed from white to blue. The lowering of the gap is attributed to the centres formed by capturing an electron in double-charged oxygen vacancies whose level lies close to the valence band. The maximun intensity of the broad absorption band located at about 1.5 eV increases with increasing the substrate temperature. This effect is attributed to the excitation of trapped electrons into the conduction band. The optical studies of MoO 3 films suggest that the films are sub-stoichiometric MoO 3_x where x is the small fraction of the stoichiometry deviation. Samples thus contain a number of oxygen vacancies, which are truly charged structural defects capable of capturing one or two electrons. The oxygen vacancies occupied by electrons act as donors. These donor centres are in the forbidden gap and form a narrow donor band at about 0.3 eV below the conduction band [12]. The effect of heat treatment on optical properties of MoO 3 films deposited at TS = 120°C has been studied in different ambients, i.e., air and vacuum. The results showed that the bandgap increases to 3.3 eV by heat treating the films in air at 350°C for 4 h, whereas the films heat treated in a vacuum below than 10 -3 Pa at 350°C during 2 h exhibited a bandgap of 2.83 eV. The increase in bandgap for the films heat treated in air is attributed to the partial filling of oxygen vacancies. The decrease in bandgap and increase of absorption in the long-wavelength region is due to the formation of more oxygen-ion vacancies in the films.

3. 5. Electrical properties The DC conductivity, o-0, of MoO 3 films has been studied as a function of the substrate temperature. Fig. 5 shows the temperature dependence of ~r0. We observed that the plot of o-0 versus 1 / T follows an Arrhenius-type behaviour. Conductivities are in the range 10-3-10 -l° S cm -~ with an activation energy in the range 0.6-1.5 eV (Table 2) for the investigated domain of temperature (30-200°C). At room temperature, the electrical conductivity of films deposited at 30°C, which is lower than 10 -1° S cm -~, in-

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Fig. 5. The DC electrical conductivity as a function of the inverse absolute temperature of flash-evaporated M o O 3 films deposited on a silica substrate at: (a) Ts = 30°C, (b) Ts = 120°C, (c) TS= 200°C, and (d) T~= 250°C. The insert shows the activation e n e r g y , Ea, as a function of the substrate temperature.

c r e a s e s with i n c r e a s i n g s u b s t r a t e t e m p e r a t u r e . This m a y b e d u e to t h e i n c r e a s e o f d o n o r c e n t r e s in M o O 3 films. T h e activation e n e r g y o f t h e conductivity, given by t h e A r r h e n i u s plot, d e c r e a s e s with i n c r e a s i n g Ts, as shown in t h e insert of Fig. 5. It is w o r t h n o t i n g t h a t t h e intrinsic conductivity o f M o O 3 films grown at 30°C has an activation e n e r g y E a = 1.48 eV, which c o r r e s p o n d s to a b o u t Eg/2. Fig. 6 shows t h e e l e c t r i c a l conductivity o f f l a s h - e v a p o r a t e d M o O 3 films g r o w n at 120°C a n d h e a t t r e a t e d at 350°C for 3 h e i t h e r in a m b i e n t a t m o s p h e r e (curve b) or in v a c u u m (curve c). T h e M o O 3 films h e a t t r e a t e d in air exhibit a c o n d u c tivity which is two o r d e r s of m a g n i t u d e lower

Table 2 Electrical parameters (obtained from DC data) for the flashd e p o s i t e d M o O 3 films grown at various Ts Ts (°C) O'RT (S cm- 1) E a (eV) 30 < 10-10 1.48 120 1.5 x 10 -1° 1.01 200 2 . 0 x 10 - 9 0.85 250 3.0 X 10-8 0.66

t h a n t h a t o f an a s - d e p o s i t e d film. This effect is a s s o c i a t e d with t h e c h a n g e in t h e film c o l o u r f r o m b l u e to t r a n s p a r e n t . T h e e l e c t r i c a l c o n d u c tivity of t h e s e films is a b o u t 10 -13 S c m - t at r o o m t e m p e r a t u r e . T h e A r r h e n i u s plot o f t h e conductivity shows an activation e n e r g y o f 1.19 eW. T h e c u r r e n t l y a d o p t e d m o d e l for e l e c t r i c a l c o n d u c t i o n in t r a n s i t i o n - m e t a l oxides is t h a t of a hopping-electron mechanism between oxidation states o f t h e m e t a l atoms. If s o m e of the M o 6+ s t a t e is r e d u c e d to s o m e lower o x i d a t i o n state, i.e., M o 5+, we c a n e x p e c t such a c o n d u c t i o n m e c h a n i s m in M o O 3 films grown at m o d e r a t e s u b s t r a t e t e m p e r a t u r e . D i f f e r e n t w o r k e r s [17,27] have o b s e r v e d by X - r a y p h o t o e l e c t r o n spect r o s c o p y t h e f o r m a t i o n o f t h e M o 5+ o x i d a t i o n s t a t e in the b l u e s a m p l e s of M o O 3 films. This was a t t r i b u t e d to an i n t e r n a l e l e c t r o n t r a n s f e r f r o m oxygen to m e t a l l i c o r b i t a l s by t h e r m a l i o n i z a t i o n c r e a t i n g an M o 5 + o x i d a t i o n state. T h e conductivity o f o x y g e n - d e f i c i e n t films grown at high subs t r a t e t e m p e r a t u r e s h o u l d b e h i g h e r b e c a u s e in g e n e r a l m o l y b d e n u m oxides with s m a l l e r O / M o r a t i o have h i g h e r conductivity. This b e h a v i o u r has b e e n i d e n t i f i e d in V 2 0 5 films as t h e c o n s e q u e n c e o f t h e q u e n c h i n g rate, which is t h e d i f f e r e n c e in temperature between the melt and the substrate [33]. A d e c r e a s e of t h e q u e n c h i n g r a t e p r o d u c e s

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C. Julien et aL /Journal of Crystal Growth 156 (1995) 235-244

an increasing conductivity and a decreasing activation energy. The decrease of the A C conductivity with the heat treatment is attributed to the partial filling of oxygen vacancies by heating the material in ambient atmosphere. It is well known that the conduction is mainly due to the presence of the low oxidation state of Mo which results in indirect conduction. The annealing treatment in O2 atmosphere decreases the number of vacancies, i.e., the film becomes more stoichiometric. The increasing O / M o ratio induces a decrease of the conductivity. Experimental results shown in Fig. 6b are in good agreement with this hypothesis. On the other hand, the change in the coloration of the blue sample over the transparent one is due to the mobility of electrons in the defect band as reported by Rabalais et al. [34]. The effect of an annealing treatment at 350°C during 2 h in vacuum is an enhancement of the conductivity (Fig. 6c). We have measured a value 1 × 10 7 S cm 1 at room temperature with an activation energy of 0.61 eV. These results come from an opposite mechanism that have been described before. The vacuum treated films exhibit a smaller I

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electrical

8 conductiv-

o-(w) of flash-evaporated MoO3 films grown at Ts = 250°C as a function of the temperature: (1) 300 K, (2) 180 K, (3) 130 K, and (4) 77 K. ity

Table 3 Electrical parameters (obtained from AC data) for the flashdeposited MoO 3 films grown at ~ = 120°C T (K) o-0 (S cm x) s 300 1.6 × 10- l0 0.62 190 9.0 × 10-12 0.78 130 4.2 × 10- ~2 0.89 77 2.5× 10 ~2 1.08

4. Conclusion

(:3

Fig.

I*D • o

243

Thin films of MoO 3 were prepared by the flash-evaporation technique on glass and silicon substrates maintained in the temperature range 30-300°C. The structural studies revealed that the films formed at 250°C are predominantly aphase MoO 3 with a (0k0) orientation. The surface morphological studies show that films prepared in the range 200-300°C are crystalline with elongated crystal geometry. The layered nature of the film is shown on each crystallite. In all cases, the film deposited was uniform in thickness.

244

C. Julien et al. /Journal of Crystal Growth 156 (1995) 235-244

Vibrational spectra show that for polycrystalline MoO 3 films the MoO 6 octahedra are progressively connected and the structural reorganization is mainly observed for the M o - O stretching mode that is associated with the unique character of the layered structure of or-MoO 3. The optical properties of MoO 3 films suggest that the films are sub-stoichiometric MoO3_x, where x is a small fraction. The evaluated optical bandgap of MoO 3 films grown at room temperature is 3.15 eV and the bandgap decreases with increasing substrate temperature. The electrical conductivity has been investigated as a function of substrate temperature and annealing treatments. The increase of the conductivity with the increasing substrate temperature is attributed to the formation oxygen-deficient films grown at high substrate temperature which induces a smaller O / M o ratio associated with the presence of Mo 5+ oxidation state. This behaviour can be identified as the consequence of the quenching rate, which is the difference in temperature between the melt and the substrate. A decrease of the quenching rate produces an increasing conductivity and a decreasing activation energy. The decreases of the conductivity of films annealed in air is attributed to the partial filling of oxygen vacancies by heating the material whereas opposite behaviour is observed for annealing treatment in vacuum which enhances the loss of oxygen atoms.

Acknowledgements The authors are very grateful to Mr. M. Lemal for the XRD measurements. They wish to thank Dr. J.Y. Emery for his technical assistance in carrying out the XPS experiments.

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