Raman study of WO3 thin films

Solid State Ionics 14 (1984) 217-220 North-Holland, Amsterdam

RAMAN STUDY OF WO3 THIN FILMS M. PHAM THI and G. VELASCO Laboratoire de Physico-Chimie et Dispositifs Ioniques, L.C.R. Thomson CSF, Domaine de Corbeville, 91401 Orsay, France

Received 21 May 1984 Revised manuscript received 18 June 1984

The influence of substrates, thermal treatment and coloration-bleaching cycles on the structure of WO3 thin films used in electrochromic devices has been investigated by Raman microscopy. Films (2000-8000 A) were prepared by RF sputtering from a metallic tungsten target at a constant pressure (5 × 10-3 Tort) of pure oxygen or a mixture of At-20% 02. They are amorphous, transparent and electrochromic. Thermal treatment at 360°C produces crystallization. Modifications of the WO3 framework are also induced by coloration-bleaching cycles.

1. Introduction

2. Experimental

Tungsten trioxide and its derivatives have been studied in detail as ferroelectric, semi-conducting and electrooptic materials. Recently, the electrochromic properties of this compound have received much attention because of its potential application in display devices. Electrochromic devices appear to have some advantages compared with other passive displays, such as good contrast, wide viewing angle, and open-circuit memory. The basic phenomenon of electrochromism may be defined as a color change produced in a material by an ion-insertion mechanism. In the case o f W O 3 , the color change may be induced by electrochemical insertion of different cations (H +, Li+, Ag+, Na ÷ .... ); tungsten-bronze phases are assumed to be formed [ 1 - 3 ] . The physical conditions of this insertion depend on the structure, texture and composition of the WO 3 film [4]. Amorphous layers [5] and the triclinic form, but not the monoclinic form, o f W O 3 thin films produced by W oxidation are good electrochromic materials [6,7]. We have investigated the vibrational spectra of different WO3 thin films deposited by radio-frequency sputtering on various substrates. Raman spectroscopy can give information on the tungsten-oxide lattice, the arrangement of the tungsten coordination, and the influence o f the insertion process.

WO 3 thin f'dms of various thickness were deposited onto different substrates: (i) polished silicon single crystals sliced perpendicular to the [100] orientation; (ii) pyrex slides covered with a 7200 A thin film of indium-tin oxide (ITO). These ITO thin films were then subjected to a 400°C thermal treatment. WO 3 layers were deposited by reactive RF sputtering from a tungsten target (4 inches in diameter). The plasma used was pure oxygen or an A r - O 2 gas mixture containing 20% 0 2 ; deposition rates were about 50 and 100 A per min, respectively. The WO 3-film thickness varied between 2000 A and 8000 A. Experimental electrochromic symmetric cells (ECC) were formed with two 2000 A WO 3 thin films deposited onto ITO layers coated on pyrex substrates. A thick solid-electrolyte layer (0.3 mm of hydrogen uranyl phosphate: HUP) was sandwiched between these two WO 3 films. The coloration-bleaching cycle o f the ECC cells was carried out by applying a square-wave dc voltage of about 3 V at 1 Hz. Raman spectra were recorded with an ISA MOLE microprobe using for excitation the 5145 A line of a Spectra Physics ionized argon laser. The laser beam was focused by the microscope down to about a onesquare-micron spot at the sample surface. Because o f the low intensity of the spectra obtained in that ex-

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citation geometry, the layer homogeneity could be checked by displacing the sample under the microscope. X-ray D e b y e - S c h e r r e r analysis were performed with a Philips PN 1720/1399 diffractometer.

3. Results Tungsten trioxide exhibits several phases that can all be considered as distorted forms o f the ideal ReO 3 structure. Five transitions between - 1 8 0 ° C and 900°C have been identified by Tanisaki [8]. The high-temperature phase is tetragonal, and on cooling the symmetry changes to orthorhombic at 740°C, monoclinic at 330°C, triclinic at 17°C, and to another monoclinic form at - 4 0 ° C . The Raman spectrum o f W O 3 at room temperature has been obtained by Krasser [9], and the normal coordinates of tetragonal WO 3 were calculated by Salje [10,11]. The W - O stretching and bending vibrations have been located between 7 0 0 - 8 0 0 cm -1 and 2 5 0 400 cm -1 , respectively. 3.1. Influence o f substrates The Raman spectra o f WO 3 thin layers sputtered onto a silicon single crystal and a pyrex slide are shown on fig. 1. The films sputtered onto a silicon single crystal (fig. 1a) are more structured than those deposited onto pyrex glass (fig. lb). The presence o f low-frequency modes (e.g. the 140 cm -1 mode on fig. la) indicates the polycrystalline state o f the film. This was confirmed by Xray analysis. All WO 3 thin fdms sputtered onto ITO-coated pyrex substrates exhibited the same spectrum in which broad bands near 300 cm -1 and 800 cm -1 indicate their amorphous character (fig. lb). These films do not show X-ray diffraction patterns and have a good electrochromic behaviour. Their coulometric titration curve, obtained by Schnell and Velasco [12], are sire. ilar to those obtained by Crandall et al. [13] for evaporated amorphous WO 3 thin films. In the Raman spectrum (fig. l b ) , one can observe the W - O stretching vibrations at 800 cm -1 and 940 cm -1 . A similar feature is observed by Mercier et al. [14] on 5200 A WO 3 layers deposited on ITO-coated glass. They as-

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Fig. 1. Raman spectra of WO3 thin films sputtered: (a) onto silicon single-crystal in 02 plasma (8000 A); (b), (c) onto pyrex-glass substrate in 02 plasma, initial film and annealed film at 360"C, respectively (8000 A); (d) onto pyrex-glass substrate in Ar + 20% 02 plasma: annealed film at 360°C (5000 h).

sign the 940 cm - I mode to a W = O terminal stretching vibration o f a hexagonal WO 3 . The Raman spectra that we have recorded on hexagonal WO 3 powder * show a band at 940 cm - I , but its intensity is very weak. On the other hand, recent recorded Raman spectra o f W O 3 hydrates [15], which have a layered structure, also show a band at 950 cm -1 . Both Raman (fig. l c - d ) and X-ray data show that heating o f the WO 3 layers at 360°C for 30 min in air induces their crystallization;furthermore the 950 cm - I band disappears. It is yet difficult to distinguish whether the 950 c m - 1 band must be related to the terminal (W = O) vibration o f the layered structure or to the presence o f water.

* The WO3 hexagonal have been prepared by B. Gerand, G. Nowogrocki, J. Guenot and M. Figlarz. They ate kindly acknowledged.

M. Pham Thi, G. Velasco/Ramanstudy of WOa thin films 3.2. Influence of plasma Before thermal treatment, very weak differences were observed on WO3 films sputtered onto pyrex glass in several plasma compositions; after heating at 360°C for 30 min in air, an evolution of the WO3 structure was observed. Figs. lc and ld compare crystallized films deposited in pure oxygen plasma and in an A r - O 2 gas mixture containing 20% 02, but with different layer thickness: 8000 and 5000 A, respectively. Raman and X-ray analysis reveal that the symmetry of the (lc)-WO 3 films looks like the tetragonal structure with an important local disorder (both X-ray and Raman bands are broad). The (ld)WO 3 film leads to a more ordered orthorhombic film. These different behaviours may be related either to the plasma composition or to the different thickness of the samples.

3.3. Influence of the coloration-bleaching process Fig. 2 compares the Raman spectra before coloration and after coloration-bleaching cycles of a 2000 A WO3 film sputtered on 02 plasma onto ITO-coated glass. Coloration of an ECC cell was carried out at room temperature: the symmetric electrochemical solid cell used was: glass IITO IWO3 Isolid electrolyte IWO3 IITO Iglass. The solid electrolyte was a dense, yellow, transparent,

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Fig. 2. Raman spectra of 2000 A WOa thin film sputtered onto ITO coated glass substrate in O2 plasma: (a) initial film, (b) bleached film after 107 coloration-bleaching cycles.

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polycrystalline thick film of HUP (HUO2PO 4 .4H20 ). HUP is a protonic conductor (a = 10 -3 I2-1 cm - 1 ) that has a two-dimensional planar network of water molecules and protons alternating with layers of UO~ and PO43- ions [16]. After several coloration-bleaching cycles (I 07 cycles), the cell was disassembled and the WO3 layer examined. Raman spectra show a clear modification of the original Film. The width of the 800 cm -1 mode decreases and a band appears around 640 cm -1 . This feature suggests local reorganization, as under thermal treatment, induced by the coloration-bleaching process. A similar modification was observed after a first coloration cycle in liquid electrolyte (N.H 2 SO4). As reported by Green et al. [17], all as-deposited WO3 films, either sputtered or evaporated, characterized by transmission electron microscopy and diffraction were polycrystalline. Films evaporated at room temperature were found to be very fine grained (<30 A), whereas sputtered filrns have a larger grain size (30-300 A). In the case of f'dms deposited by RF reactive sputtering, Raman spectra show that the texture of the films depends on the nature of the substrate: (i) Films deposited on a single crystal show poor electrochromic behavior. X-ray diffraction and Raman scattering show that these f'rims are crystalline. (ii) Films sputtered onto a glass substrate were good electrochromics and Raman spectra showed them to be amorphous. The amorphous molecular solid consisted o f W - O octahedra, and Raman spectroscopy gave no precise information about the stoichiometry of the oxide or the water content. Furthermore, the presence of a 950 cm -1 band in the Raman spectrum of initial WO 3 film, which disappears on heating the film in air at 360°C, is related either to water content inserted in the Film during preparation, as suggested by Raman et al. [15], or to the presence of WO2(W4÷), W2Os(WS÷ ) due to a substoichiometry in the Film. Raman scattering reveals a local reorganization of the as-sputtered WO 3 fdrns after the first colorationbleaching cycle in a solution of H2SO4(N), or after many cycles (107) in the case of a display using a solid electrolyte (HUP transparent disc). The local modification may be induced by various factors: (i) ordering of the WO 3 Film without modification of oxygen stoichiometry by surface polarisation, (i.i) ordering by formation of a stable hydrogen bronze during the coloration process, or (iii) deposition of either crys-

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tallized WO 3 or a hydrated species on the surface o f the film by dissolution o f the substoichiometric oxide (WO2, W 2 0 5). Recently, a modification o f an evaporated WO 3 film by its immersion in a liquid electrolyte ( 3 , 6 N H2SO4) at room temperature was observed by Sun and Holloway [18]. The authors found that amorphous WO 3 films become crystalline after 18 d storage in acid, and interracial attack results in delamination. In contrast, oxygen-enriched films (WO x, x > 2.9), which contain a small quantity o f W O 2 and W 2 0 5 , do not exhibit interfacial attack, and dissolution of WO 2 , W205 could rearrange the surface portion o f the film to give crystalline peaks of WO 3 . x ( H 2 0 ( x = 0, 1,2), Amorphous, sputtered WO 3 films have been reported to contain virtually no water or hydrogen and to show poor electrochromic behaviour [4], but recently Kamko et al. [19] have reported that oxygenenriched films, prepared by RF sputtering under a constant total operating pressure of 4 × 102 Torr, exhibit good electrochromic properties. Films sputtered from a tungsten target in pure oxygen are possibly very enriched in oxygen. When the WO 3 film is immersed in a liquid electrolyte (N-H2SO4) , modification o f the film is observed just after the first-coloration cycle; furthermore a similar modification is revealed after a long time (107 coloration-bleaching cycles) when WO 3 trim is in contact with the solid electrolyte HUP, which contains only four water molecules per HUP molecule. The reorganization o f a WO 3 film may be induced by partial dissolution o f WO 2 and W 2 0 5 contained in the film, and the dissolution mechanism may involve the formation o f hydrates in which water plays a crucial role [20].

4. Conclusion This prehminary work shows that vibrational spectroscopy, and particularly Raman microscopy, is well adapted to non-destructive thin-film characterization and to interface-reaction studies. This is illustrated by examination o f tungsten-trioxide thin films prepared by RF sputtering, which are transparent, amorphous and electrochromic. Thermal treatment at 360°C in air induces their crystallization. Work is in progress with Raman microscopy (Multicanal) and infra-red spectroscopy to study the mechanism o f proton inser-

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tion and its modification o f W O 3 fihns during this process as well as the degradation o f devices.

Acknowledgements We wish greatly to acknowledge Mr. A. Novak (LASIR, CNRS) for his critical reading of this letter, Mr. M. Guyery for his technical assistance in the preparation of the fihns, the Groupe de Chimie du Solide (Ecole Polytechnique) for their X-ray-analysis facility.

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