Thin Solid Films 408 (2002) 302–309 0040-6090/02/$ - see front matter ᮊ 2002 Elsevier Science B.V. All rights reserved. PII: S0040-6090 Ž 02 . 00090-1 Phase transformations in WO thin films during annealing 3 A. Al Mohammad , M. Gillet* 1 UMR-CNRS, Laboratoire Materiaux et Microelectronique de Provence, Faculte des Sciences de Saint-Jerome, Universite D’Aix Marseille III, ´´ ´ ´ˆ´ 52 Ave. Escadrille Normandie Niemen, 13397 Marseille Cedex 20, France Received 30 June 2001; received in revised form 20 November 2001; accepted 2 February 2002 Abstract WO thin films have been annealed in air in the temperature range 20–450 8C and the changes in grain size and crystallographic 3 structure have been investigated as a function of the annealing conditions. During annealing, important changes in grain size and structure occur and have been characterised by electron microscopy and electron diffraction. As the annealing temperature increases, the monoclinic structure successively transforms to WO Ø y H O phase, hexagonal WO , WO (Magneli phases) and 1 332 3 3yx monoclinic WO . The formation of WO was only observed when the thin film was annealed on its substrate. This non- 33yx stoichiometric WO phase exhibits specific fringe contrast imaging of well-ordered crystallographic shear planes. The most 3yx frequent Magneli phase we have observed corresponds to the W O structure. ᮊ 2002 Elsevier Science B.V. All rights reserved. 40 118 Keywords: Atomic force microscopy (AFM); Oxides; Phase transitions; Transmission electron microscopy (TEM) 1. Introduction Transition metal oxides have interesting physical and chemical properties, which make them useful for a number of applications. For example, WO oxide is 3 known for its electrochromic w 1 x and interesting catalytic properties w 2,3 x , and in recent years, WO appears to be 3 a good candidate for gas sensors w 4,5 x . In particular, WO thin films were reported to have excellent sensitiv- 3 ity to various gases, including NO w 6–8 x ,HS w 9 x and x 2 NH w 10 x . 3 WO films have been prepared by various methods: 3 vacuum thermal evaporation of WO powder w 7,11 x , 3 chemical vapour deposition w 12 x or reactive sputtering of metallic tungsten by ArqO plasma w 8,13–15 x . The 2 degree of structural order of these thin oxide films depends on the preparation method and annealing treat- ment. As-deposited WO thin films are usually poly- 3 crystalline with short-range order (often called ‘amorphous’); however, for applications such as sensors, *Corresponding author. Tel.: q33-4-91-28-83-71; fax: q33-4-91- 28-87-72. E-mail address: marcel.gillet@sermec.u-3mrs.fr (M. Gillet), ahmadalmohammad@yahoo.fr (A. Al Mohammad). Permanent address: AECS, Damacus, Syria. Fax: q963-11- 1 6112289. they have to undergo annealing in order to complete the oxidation process and stabilisation of the structural and electrical properties. Several studies have demonstrated that the film structure and sensing properties w 11,15,16 x depend on the stabilising treatments. In most cases, the annealing and sensing property tests are performed between 200 and 500 8C. In this temperature range, WO crystals undergo structural changes w 17 x . The most 3 stable WO phase at room temperature has a monoclinic 3 structure. This phase is transformed to orthorhombic at 330 8C and is stable up to 740 8C. However, other phases have been observed in specific conditions: a hexagonal WO structure has been obtained w 18 x by 3 dehydration of the tungsten oxide hydrate WO Ø y HO, 1 332 which is formed by hydrothermal treatment at 120 8C of an aqueous suspension of either a tungsten acid gel or a crystallised dihydrate (WO Ø2H O). The hexagonal 32 WO structure consists of a network built up of WO 36 octahedra sharing all their corners and arranged in (001) layers normal to the -axis. The stacking of identical ™ c (001) layers along the -axis produces large tunnels. If ™ c the monoclinic WO is reduced, oxygen loss induces 3 new structural features, giving rise to WO phases. 3yx Electron microscopy has been intensively used to deter- mine the structure of a number of non-stoichiometric 303A. Al Mohammad, M. Gillet / Thin Solid Films 408 (2002) 302–309 Fig. 1. (a) Electron micrograph of an as-deposited WO thin film; (b) corresponding TED pattern; and (c) intensity profile: Is f (D)(Isdiffracted 3 intensity, Dsdiameter of the diffraction rings). phases of tungsten oxides: WO (0-x-0.3) w 19– 3yx 25 x . These non-stoichiometric phases are derived from the WO structure and have been interpreted in terms 3 of crystallographic shear planes w 28 x . This structure consists of blocks of WO octahedra sharing corners 6 regularly separated by defect planes (crystallographic shear planes). In these defect planes, WO octahedra 6 share their edges. In this paper, we have investigated the changes in granulometry and structure in thin tungsten oxide films during annealing in air over a temperature range of 20– 450 8C. The WO films were annealed step by step, 3 with the structure observed after each step by transmis- sion electron microscopy (TEM) and transmission elec- tron diffraction (TED). 2. Experimental procedure Thin WO films were prepared by evaporation under 3 vacuum of a WO powder (purity 99.99%). The residual 3 pressure was approximately 10 torr, the deposition y5 rate approximately 15 Aymin and the film thickness ˚ approximately 300 A. WO was evaporated on (0001) 3 ˚ a-Al O substrates maintained at 300 8C during depo- 23 sition. For TEM observations, the WO thin films can 3 be detached from their substrate in hydrofluoric acid. After deposition, the thin tungsten oxide films were annealed (either on or without substrate) in air with a mean humidity of 68% in steps of 50 8C from room temperature up to 450 8C. For each step, the annealing time was 15 min. After each step, the structure was observed by TEM. When annealed without substrate, the thin films were taken off the substrate before annealing and placed on a gold grid used for TEM observations. We also performed experiments with con- tinuous annealing and we found that, for a given annealing temperature, the results are identical for a cumulative step-by-step annealing and after continuous annealing. 3. Results During annealing, the grain size and structure of thin oxide films considerably change. We have defined three main temperature domains w 20–100, 100–250 and 250– 450 8C x for which the WO thin film exhibits one or 3 two phases, according to the annealing temperature. For an annealing temperature higher than 250 8C, the phases observed depend on the presence of the substrate during annealing. 3.1. 20–100 8C temperature range During annealing at a temperature between RT and 200 8C, the structure and morphology do not change significantly. The as-deposited film (Fig. 1a) exhibits small crystallites with poor contrast and with a mean diameter f2.5 nm. The TED pattern (Fig. 1b,c) shows that the structure is monoclinic, with lattice parameters as7.29, bs7.53 and cs7.68 A, bs90891, and most ˚ of the crystallites have a (001) plane parallel to the substrate surface 3.2. 100–250 8C temperature range In this temperature range, the WO thin films undergo 3 important changes with or without the substrate. A recrystallisation takes place and the mean crystallite size considerably increases, with a mean diameter of the order of 100 nm. Fig. 2a is a typical electron micrograph showing a common aspect of the annealed WO thin 3 film in the 100–250 8C temperature range. The TED patterns indicate that various phases occur, according to 304 A. Al Mohammad, M. Gillet / Thin Solid Films 408 (2002) 302–309 Fig. 2. (a) Electron micrograph of a WO thin film annealed at 150 8C; (b) corresponding TED pattern; (c) intensity profile Is f (D) wPeak 1, 3 (002) monoclinic WO q(002) WO Ø y H O; Peak 2, (200) monoclinic WO ; Peak 3, (120) monoclinic WO ; Peak 4, (020) WO Ø y H O; Peak 1 1 3332 3 3 332 5, (201) monoclinic WO q(220) WO Ø y H O; and Peak 6, (220) monoclinic WO q(222) WO Ø y HOx; (d) TED of a WO film annealed 1 1 3332 3332 3 at 180 8C; (e) intensity profile Is f (D)(Peaks 1,2,3,4 and 5, hexagonal WO ; Peaks 19,29,39 and 49,WOØ y HO); (f) TED of a WO film 1 33323 annealed at 230 8C; and (g) intensity profile Is f (D)(the plane indices correspond to the hexagonal WO structure). 3 the annealing temperature; we define three temperature steps where one or two phases are predominant. 3.2.1. Temperature step 100–150 8C The TED pattern (Fig. 2b,c) shows that two phases co-exist: monoclinic WO qWO Ø y H O. The hydrate 1 3332 WO Ø y H O crystallises in an orthorhombic structure, 1 332 with lattice parameters as7.35, bs12.51 and cs7.70 A w 26 x . Interpretation of the TED pattern agrees with ˚ this structure. 3.2.2. Temperature step 150–200 8C A new hexagonal WO phase is observed and the 3 two phases WO Ø y H O and hexagonal WO coexist. 1 332 3 Fig. 2d,e shows the TED pattern of a WO film annealed 3 at 180 8C, showing the coexistence of two phases WO Ø y HOqhexagonal WO . During this step, the 1 332 3 hydrated oxide WO Ø y H O phase is transformed to a 1 332 hexagonal WO structure, with lattice parameters as 3 7.29 and cs7.66 A w 18 x . ˚ 3.2.3. Temperature step 200–250 8C During this step, the mean grain size slightly increases and the TED pattern becomes spotty and shows that the oxide structure is hexagonal (Fig. 2f,g). 3.3. 250–400 8C temperature range In this temperature range, we have observed different structures, depending on the annealing procedure. 3.3.1. Films annealed without substrate Fig. 3a presents a TEM micrograph of a WO thin 3 film annealed at 350 8C without substrate: two parts, A and B, with different contrast can be observed and the TED patterns show that they have different structures. 305A. Al Mohammad, M. Gillet / Thin Solid Films 408 (2002) 302–309 Fig. 3. (a) Electron micrograph of a WO thin film annealed at 350 8C without substrate; (b,c) TED patterns of parts A and B, respectively; and 3 (d) high-resolution micrograph of part B. 306 A. Al Mohammad, M. Gillet / Thin Solid Films 408 (2002) 302–309 Fig. 4. (a) Electron micrograph of a WO thin film annealed on a- 3 Al O substrate at 350 8C. wThe imaged structure corresponds to 23 WO (WO ). The black lines correspond to the shear planes 2.95 40 118 parallel to (100) planes.x (b) Corresponding TED pattern. Part A (Fig. 3b) has a monoclinic structure and part B (Fig. 3c) corresponds to the hexagonal WO structure. 3 Thus, in this temperature range, the hexagonal WO and 3 monoclinic WO phases coexist and the observation 3 demonstrates that the WO hexagonal structure is trans- 3 formed to monoclinic WO phase. For an annealing 3 temperature above 400 8C, only the monoclinic WO is 3 observed. The thin film with monoclinic structure (part A in Fig. 3b) exhibits a typical contrast with parallel strips. This contrast has been interpreted as being twinned microdomains elongated in the w 100 x direction, with a twinning (001) plane and a surface plane parallel to the (010) plane w 27 x . Part B of the film corresponds to the hexagonal WO structure, which often presents holes 3 with a hexagonal shape and seems to be formed by the stacking of thin layers with poor contrast. These hex- agonal WO layers have their surface parallel to the 3 (0001) plane. Fig. 3d is a TEM micrograph correspond- ing to the thin hexagonal WO part with a (0001) plane 3 parallel to the surface. The contrast can be considered as a projection of the hexagonal structure on a plane perpendicular to the incident electron beam. This con- trast is in good agreement with the existence of large tunnels separated by 7.7 A and arranged in hexagonal ˚ rings. 3.3.2. Films annealed with their substrate When annealed on their Al O substrate in the tem- 23 perature range 250–400 8C, the thin tungsten oxide films exhibit a typical contrast observed by electron microscopy. Fig. 4a is an example of the electron micrographs observed for tungsten oxide annealed at 350 8C on its Al O substrate. This electron micrograph 23 exhibits fringe contrast resulting from ordered plane defects perpendicular to the film plane. Such defects occur in slightly reduced crystals (WO ), where the 3yx oxygen loss is accounted for by a shearing process (crystallographic shear structures or Magneli structure) w 28,29 x . Several electron microscopy studies have dem- onstrated that such structures are relatively common in non-stoichiometric oxide with a ReO -type structure 3 w 19,24,25,31,32 x . The possible crystallographic shear planes ( CS planes) have low indices of type (1k0)(with ks1,2,3,«). For a phase with a given structure, the CS planes are parallel to and separated by slabs of the WO mother structure, the thickness of which depends 3 on the phase composition. In the micrograph of Fig. 4a, the fringe equidistance measured is ds17"1 A. The ˚ corresponding TED pattern is shown in Fig. 4b. It exhibits two kinds of diffraction spots: spots with a strong intensity provided by the WO mother structure 3 and super spots with a lower intensity located between the main spots. These super spots given by the ordered defects (CS planes) parallel to the (100) planes of the mother structure correspond to the super lattice of the CS structure. From the TED pattern, we deduced that the WO phase observed is W O (WO ) with 3yx 40 118 2.95 an orthorhombic cell, the lattice parameters of which are as7.3, bs33.9 and cs7.7 A. ˚ On various WO thin films annealed at a temperature 3 of approximately 350 8C, we observed several other WO structures, which are imaged with fringes sepa- 3yx rated by a specific distance (for example, the W O 20 58 CS structure with fringes separated by 23 A). On the ˚ same specimen, it is possible to observe two or three different CS structures corresponding to different compositions. 3.4. 400–450 8C temperature range As previously mentioned, tungsten oxide films annealed without substrate at a temperature above 400 8C exhibit only the monoclinic structure. For oxide film annealed on an Al O substrate at 400 8C, we observed 23 transformation of the WO phases to monoclinic 3yx WO . Fig. 5 is an electron micrograph that illustrates 3 this transformation: part B exhibits a fringe contrast identical to the contrast in Fig. 4a and corresponds to a non-stoichiometric WO phase with a CS structure 3yx (WO ). Part A, which lies on both sides of part B, 40 118 has a different contrast and the TED pattern indicates that the structure is monoclinic. 4. Discussion When annealed on their substrates at a temperature higher than 250 8C, the thin tungsten oxide films exhibit WO CS structures. This observation emphasises the 3yx role of the substrate during recrystallisation. The sub- 307A. Al Mohammad, M. Gillet / Thin Solid Films 408 (2002) 302–309 Fig. 5. Electron micrograph of a WO thin film annealed at 420 8C, 3 showing transformation of the WO structure to the monoclinic 3yx WO structure. 3 strate has two main effects: (1) the film is epitaxially oriented and the grain size increases; and (2) in large epitaxial grains, the CS planes are easily observed by TEM and TED. This does not mean that such defects resulting from oxygen deficiency do not exist in smaller, misoriented grains (annealed without substrate); how- ever, they are probably only embryonic, with no specific contrast in TEM and no well-defined diffraction spots in TED. During annealing, we observed successive phases: With low annealing temperatures (T -200 8C) the A oxide films are composed of small crystallites, which can react with the moisture of the air and give a hydrate. During annealing at temperatures T )200 8C, the water A is desorbed and the hexagonal WO phase is formed by 3 dehydration w 18 x . The hexagonal phase occurs because of the similarity between the atomic arrangement of WO Ø y H O and the hexagonal WO structure. In the 1 332 3 same way, the hexagonal WO is easily transformed to 3 monoclinic WO due to the crystallographic similarity 3 of the two lattices. A W–O phase diagram has been proposed by Shunk w 30 x . This diagram exhibits only two stable non-stoichi- ometric phases, W O and W O at 420 and 550 8C, 20 58 18 49 respectively. The WO structures we observed have 3yx already been observed by TEM of non-stoichiometric oxides of R O structures w 30 x . We observed W O e 3 20 58 (WO ) formed at an annealing temperature of 400 8C, 29 in agreement with the phase diagram of Shunk. How- ever, the W O (WO ) phase frequently observed 40 118 2.95 is not reported in the phase diagram of Shunk. This result concerns thin oxide films, which can exhibit some specific metastable structures. The final structure of WO observed in thin films 3 annealed in air at 450 8C is the monoclinic structure, with WO in variable proportions. This monoclinic 3yx structure always presents extensive micro-twinning, which is a characteristic feature of such structures w 33– 35 x . However, it is well known that the monoclinic phase, which is stable between RT and 330 8C, trans- forms to an orthorhombic structure above 330 8C w 17 x . To explain the monoclinic structure observed in this work, there are two hypotheses: either (1) the thin films (our films have of a thickness -500 A) have a specific ˚ structure and do not experience the phase transition at 330 8C; or (2) the orthorhombic structure formed during annealing above 330 8C is reversibly transformed to monoclinic phase when the temperature decreases down to RT, which is the usual temperature for TEM observation. Recently, a number of experiments have demonstrated that WO thin films are particularly interesting for their 3 sensing properties and that non-stoichiometric structures are involved in the mechanism of film conductivity because of the free electrons originating from oxygen vacancies. Annealing in the 300–450 8C temperature range leads to WO films with variable stoichiometry. 3 We observed a WO oxygen-deficient structure with 3yx characteristic fringe contrast (Magneli phase) with well- defined stoichiometry, in which oxygen vacancies are concentrated in well-ordered defect planes. WO has 3 large flexibility to accommodate oxygen deficiencies through the formation of CS planes. Thus, various WO phases with 0)x)0.3 can occur, giving differ- 3yx ent CS structures, as we have observed. When the shear vector of the CS planes has a component normal to the surface plane, which is the case for the CS structures that we have observed, a small step is created along the CS plane–surface intersection. We can expect that these types of defects (CS planes) contribute to the surface reactivity. It is possible that, for slightly reduced films, the WO structure includes only some CS planes (or 3 embryos of shear planes) randomly arranged. In all cases, the formation of the CS planes gives new atomic co-ordination, in which the WO octahedra share their 6 edges and the W species are reduced to W . 6q 5q 5. Conclusion Table 1 summarises the main results for the various structures observed during annealing of WO thin films. 3 The thin films were obtained by evaporation under vacuum of WO powder and deposition on a (0001) 3 Al O substrate. The as-deposited films are so-called 23 amorphous, formed of very small crystallites of mono- clinic structure, which react with moisture in the atmos- phere. During annealing at low temperature T -200 A 308 A. Al Mohammad, M. Gillet / Thin Solid Films 408 (2002) 302–309 8C, the hydrate WO Ø y H O occurs. When the oxide 1 332 film is annealed at a temperature )200 8C, the grain size considerably increases. During recrystallisation, the hydrate is transformed to hexagonal and monoclinic WO . The final monoclinic structure exhibits Magneli 3 phases, which are characteristic of non-stoichiometric structures and give specific defects on the film surface. We think that these intrinsic defects, which depend on the annealing procedure (temperature and substrate), play an important role in the electronic conductivity, and consequently in the sensing behaviour. 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