phase transformations in wo3 thin films during annealing

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

Thin Solid Films 408 (2002) 302–309

0040-6090/02/$ - see front matter 䊚 2002 Elsevier Science B.V All rights reserved.

PII: S 0 0 4 0 - 6 0 9 0 Ž 0 2 0 0 0 9 0 - 1

Phase transformations in WO thin films during annealing3

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 crystallographic3

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 Ø1y H O phase, hexagonal WO , WO (Magneli phases) and

monoclinic WO The formation of WO3 3yx was only observed when the thin film was annealed on its substrate.This non-stoichiometric WO3yx phase exhibits specific fringe contrast imaging of well-ordered crystallographic shear planes.The most frequent Magneli phase we have observed corresponds to the W O40 118structure.䊚 2002 Elsevier Science B.V All rights reserved

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, WO3 oxide is

known for its electrochromic w1x and interesting catalytic

properties w2,3x, and in recent years, WO appears to be3

a good candidate for gas sensors w4,5x.In particular,

WO thin films were reported to have excellent sensitiv-3

ity to various gases, including NO w6–8x, H S w9x andx 2

NH w10x.3

WO films have been prepared by various methods:3

vacuum thermal evaporation of WO3 powder w7,11x,

chemical vapour deposition w12x or reactive sputtering

of metallic tungsten by ArqO plasma w8,13–15x.The2

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.: 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 w11,15,16x 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 w17x.The most3 stable WO phase at room temperature has a monoclinic3 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 WO3 structure has been obtained w18x by dehydration of the tungsten oxide hydrate WO Ø1y H O,

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 hexagonal3 2

WO structure consists of a network built up of WO3 6 octahedra sharing all their corners and arranged in(001)

layers normal to the ™c-axis.The stacking of identical

(001) layers along the -axis produces large tunnels.If™c

the monoclinic WO is reduced, oxygen loss induces3 new structural features, giving rise to WO3yx phases Electron microscopy has been intensively used to deter-mine the structure of a number of non-stoichiometric

Fig.1.(a) Electron micrograph of an as-deposited WO thin film; (b) corresponding TED pattern; and (c) intensity profile: Is f (D) (Isdiffracted3

intensity,Dsdiameter of the diffraction rings).

phases of tungsten oxides: WO3yx (0-x-0.3) w19–

25x.These non-stoichiometric phases are derived from

the WO structure and have been interpreted in terms3

of crystallographic shear planes w28x.This structure

consists of blocks of WO octahedra sharing corners6

regularly separated by defect planes (crystallographic

shear planes).In these defect planes, WO octahedra6

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 under3

vacuum of a WO powder3 (purity 99.99%).The residual

pressure was approximately 10y 5 torr, the deposition

rate approximately 15 A˚ymin and the film thickness

approximately 300 A.WO was evaporated on˚ 3 (0001)

a-Al O substrates maintained at 300 8C during depo-2 3

sition.For TEM observations, the WO thin films can3

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 w20–100, 100–250 and 250–

450 8Cx for which the WO thin film exhibits one or3 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 undergo3 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 thin3 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

5, (201) monoclinic WO q(220) WO Ø y H O; and Peak 6, (220) monoclinic WO q(222) WO Ø y H Ox; (d) TED of a WO film annealed 1 1

at 180 8C; (e) intensity profile Is f (D) (Peaks 1,2,3,4 and 5, hexagonal WO ; Peaks 19,29,39 and 49, WO Ø y H O); (f) TED of a WO film1

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 Ø1y H O.The hydrate

WO Ø1y H O crystallises in an orthorhombic structure,

with lattice parameters as7.35, bs12.51 and cs7.70

A w26x.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 the3

two phases WO Ø1y H O and hexagonal WO coexist

Fig.2d,e shows the TED pattern of a WO film annealed3

at 180 8C, showing the coexistence of two phases

WO Ø1y H Oqhexagonal WO During this step, the

hydrated oxide WO Ø1y H O phase is transformed to a

hexagonal WO structure, with lattice parameters3 as

7.29 andcs7.66 A w18x.˚

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 thin3 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

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 2 3 substrate at 350 8C wThe imaged structure corresponds to

WO 2.95 (W O ).The black lines correspond to the shear planes 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 and3

monoclinic WO3 phases coexist and the observation

demonstrates that the WO hexagonal structure is trans-3

formed to monoclinic WO3 phase.For an annealing

temperature above 400 8C, only the monoclinic WO is3

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 w100x direction, with a

twinning(001) plane and a surface plane parallel to the

(010) plane w27x.Part B of the film corresponds to the

hexagonal WO structure, which often presents holes3

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 the3

(0001) plane.Fig.3d is a TEM micrograph

correspond-ing to the thin hexagonal WO part with a3 (0001) plane

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-2 3

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

exhibits fringe contrast resulting from ordered plane defects perpendicular to the film plane.Such defects occur in slightly reduced crystals (WO3yx), where the

oxygen loss is accounted for by a shearing process

(crystallographic shear structures or Magneli structure) w28,29x.Several electron microscopy studies have

dem-onstrated that such structures are relatively common in non-stoichiometric oxide with a ReO -type structure3

w19,24,25,31,32x.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 depends3

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 structure3 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 WO3yx phase observed is W O40 118 (WO2.95) with

an orthorhombic cell, the lattice parameters of which areas7.3, bs33.9 and cs7.7 A.˚

On various WO thin films annealed at a temperature3

of approximately 350 8C, we observed several other

WO3yxstructures, which are imaged with fringes sepa-rated by a specific distance (for example, the W O20 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 observed2 3 transformation of the WO3yx phases to monoclinic

WO Fig.5 is an electron micrograph that illustrates3 this transformation: part B exhibits a fringe contrast identical to the contrast in Fig.4a and corresponds to a non-stoichiometric WO3yx phase with a CS structure

(W O ).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

WO3yx CS structures.This observation emphasises the role of the substrate during recrystallisation.The

sub-Fig.5.Electron micrograph of a WO thin film annealed at 420 8C,3

showing transformation of the WO3yx structure to the monoclinic

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) theA

oxide films are composed of small crystallites, which

can react with the moisture of the air and give a hydrate

During annealing at temperaturesT )200 8C, the waterA

is desorbed and the hexagonal WO phase is formed by3

dehydration w18x.The hexagonal phase occurs because

of the similarity between the atomic arrangement of

WO Ø1y H O and the hexagonal WO structure.In the

same way, the hexagonal WO is easily transformed to3

monoclinic WO due to the crystallographic similarity3

of the two lattices

A W–O phase diagram has been proposed by Shunk

w30x.This diagram exhibits only two stable

non-stoichi-ometric phases, W O20 58and W O18 49at 420 and 550 8C,

respectively.The WO3yx structures we observed have

already been observed by TEM of non-stoichiometric

oxides of R O structures w30x.We observed W Oe 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 O40 118 (WO2.95) phase frequently observed

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 films3 annealed in air at 450 8C is the monoclinic structure, with WO3yx in variable proportions.This monoclinic structure always presents extensive micro-twinning, which is a characteristic feature of such structures w33– 35x.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 w17x

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 their3 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 WO3yx oxygen-deficient structure with characteristic fringe contrast(Magneli phase) with

well-defined stoichiometry, in which oxygen vacancies are concentrated in well-ordered defect planes.WO has3 large flexibility to accommodate oxygen deficiencies through the formation of CS planes.Thus, various

WO3yxphases with 0)x)0.3 can occur, giving

differ-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 planes3 (or

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 their6 edges and the W6qspecies are reduced to W5q

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 a3 (0001)

Al O2 3 substrate.The as-deposited films are so-called 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

308 A Al Mohammad, M Gillet / Thin Solid Films 408 (2002) 302–309

8C, the hydrate WO Ø1y H O occurs.When the oxide

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 Magneli3

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.Thus, we

plan to carry out further investigations into the surface

structure of WO3 thin films and the corresponding

conductivity to clarify the role of defect formation in

sensing properties

References

w1x J.S.E.M Svensson, C.G Grancqvist, Sol Energy Mater 11

(1984) 29.

w2x F.A.Cotton, G.Wilkinson, Advances in Organic Chemistry,

5th ed., Wiley, 1988, p 829.

w3x G.Sberveglieri, Recent developments in semiconducting thin

film gas sensors, Proceedings of ESS ’94, World Scientific,

Singapore, 1995, pp.37–48.

w4x G.Sberveglieri, L.Depero, S.Groppelli, P.Nelli, WO sput-3

tered thin films for NO monitoring, Sensors Actuators B 26x y

27 (1995) 89.

w5x N.Yamazoe, N.Miura, New approaches in the design of gas

sensors, in: G.Sberveglieri (Ed.), Gas Sensors, Kluwer,

Dor-drecht, 1992, pp.1–42.

w6x C.Cantaalini, H.T.Sun, M.Faccio, M.Pelino, S.Santucci, L.

Lozzi, M.Passacantando, Sensors Actuators B 31 (1996) 81.

w7x M.Akiyama, Z.Zhang, J.Tamaki, N.Miura, N.Yamazoe, T.

Harada, Sensors Actuators B 13-14 (1993) 619.

w8x M.Penza, M.A.Tagliente, L.Mirenghi, G.Gerardi,

C.Mar-tucci, G.Cassano, Sensors Actuators B 50 (1998) 9.

w9x A.Agrawal, H.Habibi, Thin Solid Films 169 (1989) 257 w10x H.Meixner, J.Gerblinger, U.Lampe, M.Fleischer, Sensors

Actuators B 23 (1995) 119.

w11x D.Manno, A.Serra, M.Di Giulio, G.Micocci, A.Tepore,

Thin Solid Films 324 (1998) 44.

w12x P.Tagtstrom, U.Janson, Thin Solid Films 352 ¨ ¨ (1999) 107 w13x H.M Lin, C.M Hsu, H.Y Yang, P.Y Lee, C.C Yang, Sensors

Actuators B 22 (1994) 63.

w14x Z.Xu, J.F.Vetelino, R.Lee, D.C.Parker, J.Vac.Sci.Technol.

A 4 (1986) 2377.

w15x X.G Wang, Y.S Jiang, N.H Yang, L Yuan, S.J Pang, Appl.

Phys.Sci.143 (1999) 135.

w16x L E.Depero, S.Groppeli, I.Natali-Sora, L.Sangaletti, G Sberveglieri, E.Tondello, J.Solid-State Chem.121 (1996)

379.

w17x E.Salje, K.Viswanathan, Acta Cryst.A 31 (1975) 356 w18x B.Gerand, G.Nowogroski, J.Guenot, M.Figlarte,

J.Solid-State Chem.29 (1979) 429.

w19x R.J.D Tilley, Mater Res Bull 5 (1970) 813.

w20x J.G Allpress, R.J.D Tilley, M.J Sienko, J Solid-State Chem.

3 (1971) 440.

w21x J.G Allpress, G Gado, Cryst Lattice Defects 1 (1970) 331 w22x L.A Bursill, B.G Hyde, J Solid-State Chem 4 (1972) 430 w23x M Sundberg, T.J.D Tilley, J Solid-State Chem 11 (1974)

150.

w24x S.Ijima, J.Solid-State Chem.14 (1975) 52–65.

w25x R Pickering, R.D.J Tilley, J Solid-State Chem 16 (1976)

247.

w26x B.Gerand, G.Nowogroski, M.Figlarz, J.Solid-State Chem.

38 (1981) 312–320.

w27x M.Gillet, A.Al Mohammad, C.Lemire, Thin Solid Films,

submitted for publication.

w28x J.S Anderson, B.G Hyde, J Phys Chem Solids 28 (1967)

1393.

w29x S.Anderson, A.Magneli, Naturwiss 43 (1956) 495.

w30x F.A Shunk, Constitution of Binary Alloys, McGraw Hill, 1969,

Suppl.2, p.586

w31x K.Kossuge, H.Okinaka, S.Kachi, K.Kagasawa, Y.Bando,

T.Takada, Jpn.J.Appl.Phys.9 (1970) 1004.

w32x J.Van Landuyt, S.Amelinks, Mater.Res.Bull.5 (1970) 267.

w33x N.R.Averty, Surf.Sci.33 (1972) 107.

w34x A E.Lee, K E.Singer, Proc.R.Soc.Lond.323 (1971)

523–539.

w35x R.A.Dixon, J.J.Williams, D.Doris, J.Rebane, F.H.Jones,

R.G.Egdell, S.W.Downes, Surf.Sci.399 (1998) 199.