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Vacuum 83 (2009) 1326–1332

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Vacuum

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Effect of substrate structures on epitaxial growth and electrical properties of WO3

thin films deposited on ð1012Þ and (0001) a-Al2O3 surfaces

Ahmad Al Mohammad*

Nano-Materials Labs, Physics Department, Atomic Energy Commission of Syria, 17th Nissan Street, P.O. Box 6091, Damascus, Syrian Arab Republic

a r t i c l e i n f o

Article history:Received 3 December 2008Received in revised form7 April 2009Accepted 8 April 2009

Keywords:WO3 thin filmRHEEDGas sensorEpitaxial growthElectrical conductivity

* Tel.: þ963 11 2132580; fax: þ963 11 6112289.E-mail addresses: scientific@aec.org.sy, aalmoham

0042-207X/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.vacuum.2009.04.038

a b s t r a c t

The effect of surface structures of annealed ð1012Þ and (0001) a-Al2O3 substrates on epitaxial growth andelectrical properties of electron beam deposited WO3 thin films has been investigated. (0001) and ð1012Þa-Al2O3 surfaces were used in (1� 1) stoichiometric and reconstructed forms. The structure and themorphology of WO3 films were determined by transmission electron microscopy (TEM), selected areaelectron diffraction (SAED) and reflection high energy electron diffraction (RHEED). Generally the filmsconsist of micro-grains of monoclinic WO3 and the (010) planes are parallel to the substrate surfaces.Certain epitaxial relationships between WO3 films and the substrate surfaces were found. Thesephenomena are interpreted by nucleation and growth theories in relation to a variation of the density ofsurface oxygen vacancies of the a-Al2O3 substrates. The electrical conductivity of the WO3 films wasmeasured as a function of annealed temperatures of the substrates. The activation energy for conductiondeduced from the Arrhenius equation is found to be dependent on the grain size and the morphology ofWO3 films.

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1. Introduction

Tungsten oxide (WO3) films have been widely studied and usedin many applications such as electro-optical, electro-chromic,ferroelectric and gas sensors [1–3]. The growth behavior of WO3

films on the a-Al2O3 substrates was studied using several tech-niques such as molecular beam epitaxy, reactive radio-frequency(RF) magnetron sputtering and thermal evaporation [3–6].Knowing the growth behavior of WO3 is helpful for the fabricationof high quality WO3 thin films [7]. In our previous reports, wefocused on the effect of surface structure of substrates on control-ling the growth parameters of the WO3 films [5,8]. We found thatthe surface structure of a-Al2O3 substrates determined the prop-erties of WO3 films such as the crystallographic structures, theelectrical conductivities, gas sensing and optical properties [8–14].It has been shown [14–16] that the increasing of temperatureduring thermal treatments of a-Al2O3 substrates under vacuumreduced their surface structures. It led to the formation of a thinaluminum-rich layer giving the excitation of aluminum surfaceplasmons on a-Al2O3 surfaces. The surface structures are typicallydeduced from low energy electron diffraction (LEED) and reflectionhigh energy electron diffraction (RHEED) experiments. The

mad@aec.org.sy

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reconstruction of clean a-Al2O3 surfaces heated at high tempera-ture has been reported including a ðO31� O31R9�Þ surface struc-ture of the (0001) substrates. (2�1) and (2� 2) surface structureshave been observed by LEED on the ð1012Þ surface. All the obser-vations were performed after annealing in vacuum [14,15].

In this work, we investigated the morphology and the epitaxialgrowth of WO3 films on the ð1012Þ and (0001) a-Al2O3 surfaces bymeans of transmission electron microscope (TEM), selected areaelectron diffraction (SAED) and RHEED. The electrical conductivityof WO3 films prepared on different surface structures of a-Al2O3

substrates was measured to relate the conductivity mechanisms tothe structure and morphology of WO3 films.

2. Experimental

The substrates consisted of thin slides of a-Al2O3, cut from analumina single crystal. The slides were mechanically and chemi-cally polished so that the (0001) or ð1012Þ planes were parallel tothe surface. Prior to the evaporation, the a-Al2O3 substrates wereheated at either 1100 K for stoichiometric substrates or 2000 K forreconstructed ones. All substrates were heated for 3 h undera vacuum of about 10�3 Pa. Their surface structures were examinedby RHEED (Elettrorava, Italy), and atomic force microscopy, AFM(Autoprobe CP, Park Scientific Instruments, USA). The tungstenoxide thin films were electron beam deposited (EBD) usinga commercial system (Elettrorava, Italy) simultaneously on all the

A. Al Mohammad / Vacuum 83 (2009) 1326–1332 1327

substrates which were placed near one to other on the sameheating support and under a residual O2 pressure about 5�10�1 Pa.The WO3 powder (Aldrich, w20 m, 99þ% purity) was compressed indiscs of 5 mm and annealed in an oxygen atmosphere at 1000 K for1 h before using in the crucible of electron gun. During the WO3

deposition, the a-Al2O3 substrates were maintained at a depositiontemperature Td¼ 575 K. The vapor flow corresponds to a depositionrate of approximately 2 nm/min and the average thickness depos-ited is approximately 40 nm. The structure and morphology of thinfilms were studied by RHEED. Some films were then detached fromtheir substrates in hydrofluoric acid to be examined ex-situ by TEM(Siemens102, A.V. is 125 KV). The conductivity is calculated fromthe experimentally determined resistance R, which is monitoredduring the three heating and cooling cycles. The heating cycles arecarried out in a residual pressure of oxygen about 10 Pa fortemperature at 623 K.

3. Results

The (0001) and ð1012Þ surfaces of a-Al2O3 have been chosen fortheir completely different crystallographic properties, which can beseen in Fig. 1. The (0001) surface is a basal plane of a hexagonalalumina lattice due to the -Al–O–Al–Al–O- crystal structure. It canbe formed by close-packed hexagonal O planes, if the crystal isending by oxygen layer. If the upper plane is the aluminum one, thesurface is formed by Al atoms lying on trigonal oxygen sites. Theð1012Þ surface is the more stable crystallographic plane andconsists of two atomic layers. The first is centered rectangularstructure cutting the Al and O crystal layers. The second is formedby zigzag lines parallel of only oxygen atoms. Before the depositionof WO3 films the a-Al2O3 surfaces were examined by RHEED andAFM methods, Fig. 2. The azimuth directions of incidence RHEEDelectron beam on surfaces were registered. The SAED and theRHEED patterns of WO3 deposited films on all substrates exhibita monoclinic crystallographic structure (P21/n space group of WO3)with lattice parameters: a¼ 7.311(4) Å, b¼ 7.542(6) Å,c¼ 7.691(6) Å, b(a, c)¼ 90�88. Fig. 3 is an example of the TEMmicrographs and the SAED patterns obtained from deposited WO3

thin films. These micrographs show regular shaped grains onsubstrates while the mean grain size varies from substrate toanother. In all cases, the SAED patterns confirm that the films havea monoclinic structure with (010) planes, which are parallel to thesubstrate surfaces. We have achieved a more accurate estimation ofthe film structure and epitaxial relationships between WO3 filmsand a-Al2O3 substrates by RHEED investigations. In general, thedeposition of the WO3 films results in electron diffraction patternswith well defined elongated reflections perpendicular to theshadow line. The reflection lines are of 00l, �201 and �101 typesdue to large monocrystalline domains with the (010) plane parallel

Fig. 1. (a) The (0001) a-Al2O3 plane with the crystallographic directions and the distancelographic directions and the distances between the surface oxygen atoms.

to surface. These patterns are interpreted from the monoclinicstructure of WO3. Fig. 4 represents the principal RHEED patterns ofthe WO3 films on the different a-Al2O3 substrates and the corre-sponded theoretical interpretation of these patterns. We deducethat the WO3 films on all substrates consist of one or two domainswhose surfaces are parallel to the (010) plane. Hence the WO3 filmsare formed of monocrystalline domains in epitaxy on the a-Al2O3

supports according to the certain relationships. The main results ofthe investigations of the TEM micrographs and the RHEED and theSAED patterns are reported in Table 1.

Fig. 5 displays the conductivity s of the WO3 films deposited onstoichiometric and reconstructed a-Al2O3 substrates as a functionof the reciprocal temperature (in a Log s versus 103/T plot). We haveobserved for these conditions that during the heating or coolingcycles, the values of conductivity have very small variations whichare attributed to experimental errors. Generally the observedconductivity of the WO3 films on different a-Al2O3 substratessuggests polycrystalline n-type semiconductor behavior.

4. Discussion

The results concern the growth of WO3 films on (0001) andð1012Þ a-Al2O3 surfaces carried out under the same depositionconditions exhibit different crystallographic structure resultingfrom thermal annealing of the a-Al2O3 substrates. In case of ð1012Þsurfaces, the annealing produces reconstructed (1�2) surface withlarge number of oxygen vacancies, which are surface defects.Generally these oxygen vacancies can be considered as nucleationsites. This fact supports the hypothesis that the difference innucleation density is due to defects created by annealing. While theannealing of (0001) surface gives one or more (111) aluminumlayers on the surface, which is responsible for changes in growthmode of WO3 films. The crystallographic and morphologic proper-ties of WO3 films, which are grown on reconstructed (0001) surface,are resulting from Frank-Van der Merwe (layer-by-layer) growthmode. The WO3 films, which grown on other kinds of substrates, areresulting from Volmer–Weber (islands or 3D growth) mode.

It is possible by the RHEED and the SAED methods to explore thevarious orientations of the WO3 film by rotating the sample in itsplane or tilting the substrates’ holders which change the azimuthdirections of the electron beam. These changes could provide animportant information about the reciprocal lattice of the WO3 films.The most important crystallographic properties are crystalsymmetry and in-plane lattice constant. Although the two-dimensional (2D) symmetry of the substrate surface is the mostimportant, the three-dimensional (3D) symmetry can also beinfluential in determining the structure of the interface layer,especially for oxide-on-oxide epitaxial growth. It has been foundthat the minimization of electrostatic repulsion within a few

between the surface aluminum atoms, (b) the ð1012Þ a-Al2O3 plane with the crystal-

Fig. 2. AFM images of a-Al2O3 surface morphologies and corresponding RHEED patterns of: ð1012Þ stoichiometric (a, b), ð1012Þ reconstructed (c, d), (0001) stoichiometric (e, f),(0001) reconstructed (g, h).

A. Al Mohammad / Vacuum 83 (2009) 1326–13321328

interlayer spacings of the interface is a powerful driving force indetermining how perovskites nucleate on oxide substrates [17].Lattice match refers to the quantitative comparison of in-planelattice parameters of the substrate and film. The lattice mismatchcan be defined as:

f ¼ Da=a ¼�

afilm � asub

�=asub (1)

where both lattice parameters are in the growth plane. Thisquantity should be as small as possible in order to reduce strain in

the film. If afilm < (>)asub the film will be in tension (compression)prior to relaxation. Strain energy accumulates rapidly with filmthickness due to the inherent stiffness of most oxides, resulting inmisfit dislocation generation, film buckling, morphological trans-formation from (2D) layer-by-layer to (3D) island growth, or coin-cidence lattice formation. Which of these mechanisms is operativedepends on both the kinetics of film growth and the relativeenergetics of the different processes. Generally, the values of themismatches remained nearly constant giving short interfacedeformations [17].

Fig. 3. TEM images and their SAED patterns of WO3 thin films deposited on a-Al2O3 substrates: (a) and (c) are WO3(010)//(1�1) of ð1012Þ a-Al2O3 and corresponding SAED patternswith twins between grains. (b) and (d) WO3(010)//(1�2) of ð1012Þ a-Al2O3 and corresponding SAED patterns: selected area contains parts of three grains. The angles betweenepitaxial directions of 90� or 45� . (e) and (g) WO3(010)//(1�1) of (0001) a-Al2O3 and SAED patterns. Selected area contains parts of two grains. The angles between epitaxialdirections are 120� or 60� . (f) and (h) WO3(010)//(111)Al//(0001) a-Al2O3 and corresponding SAED patterns.

A. Al Mohammad / Vacuum 83 (2009) 1326–1332 1329

Fig. 4. a, c and e main RHEED patterns of WO3 thin films deposited on different a-Al2O3 substrates obtained with rotation the substrates in their planes. b, d and f theoreticalpatterns which interpret a, c and e RHEED patterns, with WO3 zone axes.

A. Al Mohammad / Vacuum 83 (2009) 1326–13321330

We consider that these changes in properties of WO3 films aremainly related to the changes in the WO3–a-Al2O3 bonding.Different kinds of bonding can exist between an oxide and another

Table 1Main epitaxial relationships between WO3 thin films and a-Al2O3 substratesdeduced from TEM, SAED and RHEED patterns.

Substratestructure

Meandimensions[nm]

Form of WO3 grainswith diffractingdomains (DD)

Epitaxial relationships

(1� 1) ofð1012Þa-Al2O3

30� 350 Needles with only oneDD

(010)WO3//ð1012Þ a-Al2O3 and[100]WO3//ð2131Þ a-Al2O3

(1� 2) ofð1012Þa-Al2O3

28� 45 Rectangular with DD1 (010)WO3//ð1012Þ a-Al2O3 and[100]WO3//ð2131Þ a-Al2O3

Rectangular with DD2 Or (010)WO3//ð1012Þ a-Al2O3 and[100]WO3//ð2131Þ a-Al2O3

(1� 1) of(0001)a-Al2O3

150 Small hexagonal withDD1

(010)WO3//(0001)a-Al2O3 and[100]WO3//ð2131Þ a-Al2O3

Small hexagonal withDD2

Or (010)WO3//(0001) a-Al2O3 and[100]WO3//ð1012Þ a-Al2O3

(111)Al//(0001)a-Al2O3

600 Hexagonal with onlyone DD

(010)WO3//(111)Al//(0001) a-Al2O3 and [100]WO3//ð1012Þ Al//ð2131Þ a-Al2O3

resulting either in chemical bonds or in Van Der Waals forces. Thekinds of these forces are according to the reactivity of the depositedoxide and substrates. Fig. 6 illustrates the structural models for the

1.5 2.0 2.5 3.0 3.5

-5.5

-5.0

-4.5

-4.0 WO3/(-1012)α-Al2O3 reconstructed WO3/(0001)α-Al2O3 reconstructed WO3/(-1012)α-Al2O3 stoichiometric WO3/(0001)α-Al2O3 stoichiometric

Lo

g σ

103/T [K

-1]

Fig. 5. Mean values of conductivity s of the stable WO3 films as a function of thereciprocal temperature (in a Log s versus 103/T plot) during heating and cooling cycles.

Table 2Activation energy of WO3 thin films deposited on various a-Al2O3 substrates.

Substrate structure Mean dimensions ofWO3 grains [nm]

Activationenergy (eV)

(1� 1) of ð1012Þa-Al2O3

30� 350 0.29 (2)

(1� 2) of ð1012Þa-Al2O3

28� 45 0.34 (2)

(1� 1) of (0001)a-Al2O3

150 0.41 (2)

(111)Al//(0001)a-Al2O3

600 0.49 (3)

Fig. 6. The models of (010) WO3 epitaxy on a-Al2O3 substrate surfaces: (a) (010)WO3//(0001) a-Al2O3 and [101]WO3//[11–20] a-Al2O3 [101]WO3//[01–10] a-Al2O3

[101]WO3//[10–10] a-Al2O3 (b) (010)WO3//(111)Al//(0001) a-Al2O3 and [101]WO3//[01–1]Al//[12–30] a-Al2O3 (c) (010)WO3//(�1012) a-Al2O3 and [101]WO3//[01–1]Al//[01-10] a-Al2O3 (d) (010)WO3//(�1012) a-Al2O3 and [101]WO3//[01–1]Al//[01-10] a-Al2O3 [101]WO3//[01–1]Al//[12–30] a-Al2O3.

A. Al Mohammad / Vacuum 83 (2009) 1326–1332 1331

(010) WO3 films prepared on the stoichiometric and reconstructeda-Al2O3 substrates.

The activation energy of conduction, EA, frequently was inter-preted using the Arrhenius equation between 300 and 900 K, seerefs. [18–20]. It is obtained from the following formula:

dðLogsÞ=dð1=TÞ ¼ �ðEA=kÞ (2)

where k is Boltzmann’s constant, T is an absolute temperature and s isthe variation of conductivity. The activation energy EA of the WO3

films corresponds obviously to n-type semiconductor like behavior.The overall results of conductivity properties of the four type WO3

films are grouped in Table 2. The observed conductivity of the WO3

films on different a-Al2O3 substrates have activation energiesbetween 0.29 and 0.41 eV. Exclusively the conductivity of WO3 filmson reconstructed (0001) a-Al2O3 surface becomes the largest valueand activation energy becomes also the largest value, EA¼ 0.49(3) eV.The conductivity curves indicate that the samples keep the samesurface structure characterized by a high density of oxygen vacancies[5,17–19]. The surface structure destroys the bonds between theoxygen and the tungsten atoms at grain boundary limits, whichincreases the oxygen vacancies concentration and increases theconductivity [18,19] at WO3 on (0001) reconstructed surfaces. Wesuppose that the non-stoichiometry at the surface of WO3 consideredas n-type semiconductor originates only from oxygen vacancies ofWO6 octahedra. This is discussed more fully in refs. [5,17–19].

Finally, we can deduce the grain sizes, morphology and epitaxialrelationships of the WO3 films which are strongly dependant on thesurface structure of a-Al2O3 substrates. TEM, SAED and RHEEDanalyses show the structure to be very stable with large grainswhich exhibit ripples in many areas due to stresses generatedduring the growth processes. The conductivity behavior of the WO3

thin films with respect to annealing temperature and its interpre-tation as being caused by non-stoichiometry is believed to originatefrom surface oxygen vacancies at grain boundary edges and iscompatible with many similar reports in the literature [8,12,18–20]typically concerned with the use of WO3 thin films as a sensingmaterial for the detection of atmosphere pollutants [18–22].

5. Conclusion

WO3 thin films were deposited simultaneously under identicalconditions on stoichiometric and reconstructed ð1012Þ and (0001)a-Al2O3 substrates, which exhibited different surface structuresafter annealing. The surfaces annealed at 1100 K are stoichiometricand have (1�1) structure for both ð1012 and (0001) surfaces. Thesurfaces annealed at 2000 K are oxygen vacancies and exhibita (1�2) structure for ð1012Þ surfaces and one or more (111)aluminum layers for (0001) ones. TEM, SAED and RHEED observa-tions show that the grain size, grain form and epitaxial relationshipsof WO3 films are different. On stoichiometric substrates the surfacestructure plays an important role in nucleation and growth of WO3

films. Whereas oxygen vacancies on the ð1012Þ reconstructed

A. Al Mohammad / Vacuum 83 (2009) 1326–13321332

surfaces of substrates may result in an increase of defects onsurfaces which create WO3 films with small grain size. In the case of(0001) reconstructed substrates, the surface oxygen vacanciesincrease until the surface consists of only one type of atomic speciesand the surface aluminum layers improve the crystallographic andmorphologic properties of WO3 films. The observed changes in theconductivity and activation energy EA of the WO3 films were seen tocorrespond to n-type semiconductor like behavior. The changes inelectrical properties of the films were believed to be related to thechanges in surface structure of a-Al2O3 substrates.

Acknowledgements

I would like to thank Prof. I. Othman, Director General of theAtomic Energy Commission of Syria for his support and encourage-ment, as well as Dr. M. Kh. Rukiah and Prof. M. Kh. Sabra for valuablediscussions.

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