- Email: [email protected]
Study of the surface roughness of CVD-tungsten oxide thin films
Applied Surface Science 218 (2003) 162–168
Study of the surface roughness of CVD-tungsten oxide thin films R.E. Tannera,b, A. Szekeresc,*, D. Gogovad, K. Geshevad a
Department of Physics, University of Warwick, Coventry CV4 7AL, UK Department of Chemical Engineering, Yale University, P.O. Box 208286, New Haven, CT 06520, USA c Institute of Solid State Physics, Tzarigradsko Chaussee 72, 1784 Sofia, Bulgaria d Central Laboratory of Solar Energy and New Energy Sources, Tzarigradsko Chaussee 72, 1784 Sofia, Bulgaria b
Received 8 April 2003; received in revised form 8 April 2003; accepted 8 April 2003
Abstract The surface layer formed during chemical vapour deposition (CVD) of tungsten trioxide thin films was studied by means of atomic force microscopy (AFM), scanning electron microscopy (SEM) and spectroscopic ellipsometry (SE). Films were deposited at atmospheric pressure by pyrolytic decomposition of tungsten hexacarbonyl (W(CO)6) and were annealed at 400– 500 8C. Data from SE experiments and theoretical simulations showed that a layer forms at the surface of the WO3 film that has a different structure and composition from the bulk film. This surface layer becomes thicker with increasing oxygen flow rate during film deposition. This layer was predominantly amorphous for as-deposited films and predominantly crystalline after annealing. Root mean squared (rms) roughness values were calculated from AFM images of the surface layer. A high degree of surface roughness was revealed after deposition and annealing (40 nm), and the roughness value increased after additional annealing to 470 8C in 101 Pa of O2. # 2003 Elsevier Science B.V. All rights reserved. PACS: 68.35.Bs (surface structure and topography); 61.16.Ch (scanning probe microscopy: scanning tunnelling, atomic force, scanning optical, magnetic force, etc.); 68.55.a (thin film structure and morphology); 67.70.þn films (including physical adsorption); 81.15.Kk (vapour phase epitaxy, growth from vapour phase) Keywords: Morphology and growth; Tungsten oxide; Atomic force microscopy; Surface
1. Introduction Electrochromic materials undergo a change in transparency when a small voltage is applied. Electrochromism is exploited in architectural and automotive markets for active regulation of the amount of radiation passing through an electrochromic material. *
Corresponding author. Tel.: þ359-2-7144734; fax: þ359-2-9753632. E-mail address: [email protected] (A. Szekeres).
‘‘Antidazzle’’ electrochromic mirrors, for example, are electronically tintable or darken automatically to reduce headlight glare and are fitted to many cars. Electrochromic windows improve both energy efficiency and comfort within buildings and vehicles by reducing unwanted solar heating and glare. The active electrochromic layer is typically a thin film of tungsten trioxide (WO3) [1,2] that darkens as a result of the injection of ions (Hþ or Liþ) from a storage layer.
WO3 þ xLiþ þ xe $ Lix WO3 colourless
0169-4332/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00575-0
coloured
R.E. Tanner et al. / Applied Surface Science 218 (2003) 162–168
Prohibitive costs mean that smart windows are not being produced on a large scale. Films of WO3 have been deposited by a number of techniques including vacuum evaporation [3], reactive sputtering [4] and two-stage chemical vapour deposition (CVD) [5]. The CVD process is attractive as it enables films to be deposited at atmospheric pressure. Moving the substrate through the CVD reactor allows continuous growth and avoids downtime. In previous work [6], we have shown that direct deposition of tungsten oxide is possible at atmospheric pressure by pyrolytic decomposition of tungsten hexacarbonyl (W(CO)6). This technology yields films with mostly amorphous and highly porous structure, which is desirable as it enhances ion mobility [7]. The film structure is strongly influenced by the deposition temperature (i.e. substrate temperature) and the oxygen content in the gas environment during film growth. The present study is an atomic force microscopy (AFM), scanning electron microscopy (SEM) and spectroscopic ellipsometry (SE) investigation of the layers formed at the surface of WO3 thin films. Such layers differ in structure and composition from the basic volume of the WO3 film [7]. Formation essentially proceeds during the CVD deposition process, but additional changes in thickness and structure were discovered as a result of post-deposition annealing in O2. Annealing in O2 increased the root mean squared (rms) roughness of the film surface by 20% and degree of crystallinity increased. These changes are important due to their influence on ion intercalation in electrochromic devices where the WO3 film acts as a functional layer.
2. Experimental details The CVD thin films of WO3 were deposited by pyrolytic decomposition of W(CO)6 vapour in an argon/oxygen stream at atmospheric pressure in a horizontal reactor with cold walls. Glass and silicon substrates were heated at temperatures of 200, 300 and 400 8C during deposition. After growth, the thickness of the films was measured by a Talystep profilometer. The precursor W(CO)6 powder was placed in a sublimator immersed in a silicon oil bath that was heated up to 115 8C. The amount of W(CO)6 vapour was controlled by changing the argon carrier gas flow
163
rate through the sublimator. It has been established [8] that the partial pressure of W(CO)6 is nearly a linear function of the flow rate of the carrier Ar gas through the source in the range of 100–1000 sccm. In the experiments described here, the Ar flow rate was in the range 100–500 sccm and we therefore assume that the sublimation rate of W(CO)6 is proportional to the Ar flow rate. The films were deposited at different oxygen flow rates, keeping the flow rate of the Ar carrier constant. The ratio of Ar to O2 gas flow rates is denoted here as ArWðCOÞ6 =O2 . After deposition some samples were subjected to an annealing in air for 60 min at temperatures of 400, 450 and 500 8C. To find out if the annealing temperature and ambient would further change the surface layer thickness or its structure, some of the samples were subjected to additional extended period of annealing (180 min). In a chamber that was first evacuated by turbo-molecular pump to 105 Pa, these samples were heated at 470 8C in 101 Pa of purified O2. The surface morphology and the roughness were examined by recording AFM images with a Digital Instruments Nanoscope IIIa in tapping mode with etched Si tips (model OTESPA, Digital Instruments). Values of rms roughness were calculated from the height values in the AFM images using the DI software. The SEM experiments were carried out with electron microscope model JSM-5300 in secondaryelectron mode. Ellipsometric measurements and multiple-layer modelling were carried out to acquire more information about the composition and thickness of the rough surface layer. These measurements were performed on a Rudolph Ellipsometer in the spectral region 300– 800 nm. An incidence angle of 458 was used, which allowed the separation of the reflections from the film and from the back surface of the substrate when 2 mm thick glass substrates were used. The accuracy of the polariser, analyser and incidence angles is within 0.018. Our experience [7,9] has shown that the film structure is optically inhomogeneous and multi-layer modelling is necessary for the data analysis. The Bruggeman effective-medium-approximation theory [10] was used for the modelling and each sublayer was considered as a heterogeneous dielectric mixture of amorphous and crystalline phases of tungsten oxide and voids. In the theoretical model, the dielectric
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function data of stoichiometric crystalline WO3 [11] was taken as reference for the crystalline phase. Since the WO3 crystal is optically anisotropic, we averaged the values of the dielectric constants along the three principal crystallographic directions in the biaxial crystal. For the amorphous phase, the dielectric function data of amorphous WO3 from [3] was used. The ellipsometric data was fitted by minimising the difference between the experimental and theoretical curves using an iterative least-squares method. The results from the ellipsometric analysis were compared with those from the AFM measurements.
3. Results and discussion Our observations have shown that a small amount of oxygen in the deposition ambient leads to a smooth, homogeneous, small-grained WO3 surface, while increasing the oxygen flow rate results in the appearance of randomly distributed large crystallites that form a continuous surface film [12]. Annealing of the films promotes the crystallisation process, and at 500 8C the film structure is fully crystalline [7]. AFM measurements of the surface are in agreement with this analysis. An AFM image of the film surface after deposition and annealing in air is presented in Fig. 1a. Irregularly distributed domed crystallites form
surface termination with a somewhat lumpy appearance. Subjecting the samples to 180 min prolonged annealing at 470 8C in 10–1 Pa of O2 caused only minor qualitative modification of the surface (Fig. 1b). The general morphology of the surface was unchanged. This contrasts with the effect of annealing films prepared from a tungstic acid sol complex, where large straight-edged crystallites were created at the surface after annealing under the same conditions [13]. On a smaller length-scale, however, linescans [14] and calculated rms surface roughness values show that the surface morphology has been altered by the annealing treatment: crystallisation in the surface layer is visible as a more jagged appearance in linescans and the AFM image (Fig. 1b). In separate investigations [15], we showed that the surface morphology of WO3 single crystals is dominated by bulk oxidation and reduction. Under oxidising conditions mobile W5þ is oxidised at the surface to form immobile W6þ, while reducing conditions release interstitial W5þ to the bulk. Oxidation occurs at 525 8C in 10–2 Pa of O2; reduction at 500–575 8C in 4 103 Pa of O2 [15]. In the case of the thin film in Fig. 1b, we deduce that net oxidation is in operation: the high O2 partial pressure leads to the nucleation of crystallites at the surface. After 180 min growth, the angular facets of the crystallites are visible at the surface of the film in AFM.
Fig. 1. Pseudo-3D rendered AFM scans of the surface of a WO3 film deposited on glass at 200 8C and annealed. The images correspond to a 5 mm 5 mm area. (a) Image of the film annealed to 400 8C for 60 min. (b) AFM image of the surface following further annealing to 470 8C in 10–1 Pa of O2 for 180 min.
R.E. Tanner et al. / Applied Surface Science 218 (2003) 162–168
Digital processing of the AFM images was used to provide a quantitative guide to the roughness of the surface. Absolute values should be treated with caution due to the inherent effects of tip convolution with the surface topography, and of the dependence on the scan size. Despite this, it was possible to identify a clear trend in rms roughness by comparing images taken at the same magnification. The rough surface texture was confirmed by the relatively large value of the root mean squared roughness, calculated from Fig. 1a as 40 nm. Following long annealing the rms roughness increased by 20–30%, and in Fig. 1b corresponds to about 50 nm. These values correspond well with the qualitative observations of linescan data [14]. Our previous SEM investigation of the surface of CVD WO3 films has shown that the oxygen flow rate has a strong influence on the surface morphology [12]. Lower oxygen flow rates lead to smooth, homogeneous, small-grained film surfaces, while increased oxygen flow rates lead to the appearance of large crystallites on the smooth film surface. In the study reported here, where we concentrated on the effect of annealing, we reconfirmed that low oxygen flow rates produce a smooth, small-grained surface (Fig. 2). The WO3 film in Fig. 2 was deposited on glass at 200 8C and at a gas flow rate ratio of ArWðCOÞ6 =O2 ¼ 1=12. The surface of a similar film after annealing is shown in the AFM images of Fig. 1.
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Increasing the deposition temperature also increases the ‘‘grainy’’, crystalline appearance of the film, and for a deposition temperature of 400 8C well-pronounced, randomly distributed crystallites develop on the film surface (Fig. 3a). Annealing at 500 8C further enhances the crystalline structure of the as-deposited films. As an illustration, Fig. 3b shows a SEM micrograph of the WO3 film after annealing at 500 8C in air for 60 min. In addition to the larger average size of the crystallites, other changes in film morphology are visible, such as the appearance of elongated crystallites. Additional information about this surface layer was obtained from the analysis of spectral ellipsometric data. In the SE data analysis, the rough surface of WO3 films is considered as a surface layer that is optically different from the bulk volume of the film. The SE data were therefore interpreted using a twolayer model, in which the first is the surface layer and the second is the bulk film volume. Each layer was modelled as a mixture of amorphous and crystalline phases of WO3 and voids. A detailed consideration of the bulk layers is beyond the scope of this paper, and the results of the SE analysis are given elsewhere [7,9]. From the SE modelling of the bulk we conclude that there is a large volume fraction of voids (40–60%), indicating that the as-deposited films are rather porous. This may be due to the high deposition rate
Fig. 2. SEM micrographs of as-deposited WO3 film on glass. Deposition was carried out at 200 8C and at a gas flow rate ratio of ArWðCOÞ6 =O2 ¼ 1=12.
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Fig. 3. SEM micrographs of WO3 film: (a) as-deposited on glass at 400 8C and at a gas flow rate ratio of ArWðCOÞ6 =O2 ¼ 1=12, and (b) following subsequent annealing at 500 8C in air for 60 min.
of 70 nm/min. At this value, growth cannot be considered to be a thermodynamically controlled process and kinetic limitations mean that migration of the atoms at the hot surface is very low. As a result, disordered and highly porous material is obtained. This is supported by the values of the film density (3.6–5.5 g cm3) obtained from the weight measurements [16], which are much lower than those of crystalline WO3 material (7.16 g cm3) [2]. Deposition at 200 8C results in amorphous films, while films deposited at 300 8C are a mixture of amorphous and crystalline phases of WO3. The structure of films
deposited at 400 8C is predominantly crystalline with large number of voids still present. For films deposited on crystalline substrates (such as Si), annealing at 450 8C promotes conversion to a crystalline film structure [7], while films deposited on amorphous substrates (such as glass) require temperatures above 500 8C [9]. In Fig. 4 we present the results of simulated surface layer compositions for films deposited at different temperatures and at a gas flow rate ratio of ArWðCOÞ6 =O2 ¼ 1=6. With increasing temperature, the volume fraction of crystalline phase increases, indicating that higher deposition
R.E. Tanner et al. / Applied Surface Science 218 (2003) 162–168
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temperature promotes the crystallisation even during the film growth. After annealing, the crystalline WO3 and void fractions are dominant in the surface region. A typical dependence is demonstrated in Fig. 5, which shows
results for films deposited at 300 8C and annealed at 500 8C in air. Each phase varies in an approximately linear fashion with gas flow rate ratio, with the proportion of crystalline material showing the strongest dependence on the ArWðCOÞ6 =O2 ratio. At low O2 partial pressures the void fraction in the surface layer was about 43%, with a high proportion of crystalline WO3 and a negligible amount of amorphous material. However, the crystalline fraction decreased to 18% for higher O2 pressures, with a concomitant increase in both the amount of amorphous material and void fraction. The large void fraction at high O2 flow rate is confirmed by the high values of roughness calculated from the AFM images (Fig. 1a). The observed increase of the void fraction in the surface layer after annealing the films is an indirect indication that crystallisation leads to surface roughening, as observed in the AFM image of Fig. 1b. The SE results and modelling revealed how the thickness of the surface layer of the WO3 films changed as a function of gas flow rate ratio and deposition temperature. Fig. 6 shows that the higher deposition temperature leads to a thinner surface layer in the WO3 film. For both temperatures, however, the thickness dependence on oxygen flow rate is almost linear. The surface layer thickness decreased with
Fig. 5. Volume fraction of the amorphous and crystalline phases of WO3 and voids versus ratio of gas flow rates ArWðCOÞ6 =O2 for the surface layer of WO3 films deposited on glass at 300 8C and annealed at 500 8C in air for 60 min.
Fig. 6. Thickness of the surface layer determined by SE as a function of oxygen content in the deposition ambient defined as ArWðCOÞ6 =O2 for WO3 films deposited at 200 8C and 300 8C and annealed at 400 8C and above (450 or 500 8C). The point with star illustrates the AFM rms roughness value for the corresponding film.
Fig. 4. Volume fraction of amorphous and crystalline phases of WO3 and voids in the surface layer of WO3 films as a function of deposition temperature. The films were deposited on Si substrates at a gas flow rate ratio of ArWðCOÞ6 =O2 ¼ 1=12.
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increasing flow rate ratio of the reactive gases, i.e. with decreasing oxygen content. There is good agreement with the surface roughness values obtained from AFM measurements, which is highlighted in Fig. 2 with a ‘‘star’’ datapoint. In general, the annealing time (60 min in this case) had a weak influence on the surface layer thickness, but changes the proportion of the amorphous and crystalline phases of WO3.
4. Conclusions From the results of AFM, SEM and SE investigations, we conclude that the CVD process conditions described in the paper lead to the formation of disordered and highly porous WO3 films with a rough surface. Annealing at 400–500 8C promotes the crystallisation process but causes further surface roughening. The SE measurements show how the rough surface layer varies in thickness with the deposition conditions. The AFM images confirm this analysis and reveal a rms roughness of 40–50 nm, with higher values resulting from prolonged heat treatment. Acknowledgements The authors thank John Cronin and Lori Crawford of Schott–Donnelly Advanced Technology Centre in Tucson. We are grateful for financial support from US Department of Energy ‘‘Basic Energy Sciences Program’’ (grant DE-FG02-98ER14882). References [1] S.K. Deb, A novel electrophotographic system, Appl. Opt. Suppl. 3 (1969) 192–195.
[2] C.G. Granqvist, Handbook of Inorganic Electrochromic Materials, Elsevier, Amsterdam, 1995 (Chapters 3, 8, 9). [3] S.K. Deb, Optical and photoelectric properties and colour centres in thin films of tungsten oxide, Phil. Mag. 27 (1973) 801–822. [4] D. Green, Optical constants of sputtered WO3, Appl. Opt. 29 (1990) 4547–4549. [5] D. Davazoglou, A. Donnadieu, Structure and optical properties of WO3 thin-films prepared by chemical vapour deposition, Thin Solid Films 147 (1987) 131–142. [6] K. Gesheva, G. Stoyanov, D. Gogova, APCVD in-situ growing and investigation of electrochromic WO3 films, Mat. Res. Soc. Symp. Proc. 415 (1996) 155–166. [7] A. Szekeres, D. Gogova, K. Gesheva, Optical properties of thin CVD-tungsten oxide films by spectroscopic ellipsometry, J. Cryst. Growth 198–199 (1999) 1235–1239. [8] L.K. Thomas, A. Berghaus, M.R. Jacobson, Preparation of W–Wox–cermets for solar selective absorbers, Proc. SPIE 2255 (1994) 119–126. [9] A. Szekeres, D. Gogova, Optical constants of thin CVDtungsten oxide films, Proc. SPIE 3573 (1998) 200–203. [10] D.E. Aspnes, Optical properties of thin films, Thin Solid Films 89 (1982) 249–262. [11] J.F. Owen, K.J. Teegarden, H.R. Shanks, Optical properties of the sodium–tungsten bronzes and tungsten trioxide, Phys. Rev B 18 (1978) 3827–3837. [12] A. Szekeres, K. Gesheva, D. Gogova, T. Ivanova, in: J.M. Marshall, A.G. Petrov, A. Vavrek, D. Nesheva, D. DimovaMalinovska, J.M. Maud (Eds.), Proceedings of 11th International School on Condensed Matter Physics, Bookcraft, Bath, 2001, p. 264. [13] R.E. Tanner, unpublished results. [14] R.E. Tanner, A. Szekeres, D. Gogova, K.A. Gesheva, Studies of CVD-WO3 thin film surfaces by atomic force microscopy and spectroscopic ellipsometry, J. Mater. Sci. Mater. Electron., in press. [15] R.E. Tanner, E.I. Altman, Effect of surface treatment on the g-WO3(0 0 1) surface: a comprehensive study of oxidation and reduction by scanning tunnelling microscopy and lowenergy electron diffraction, J. Vac. Sci. Technol. A 19 (2001) 1502–1509. [16] D. Gogova, K.A. Gesheva, A. Szekeres, M. SendovaVassileva, Structural and optical properties of CVD thin tungsten oxide films, Phys. Stat. Sol. (a) 176 (1999) 969–984.
Study of the surface roughness of CVD-tungsten oxide thin films R.E. Tannera,b, A. Szekeresc,*, D. Gogovad, K. Geshevad a
Department of Physics, University of Warwick, Coventry CV4 7AL, UK Department of Chemical Engineering, Yale University, P.O. Box 208286, New Haven, CT 06520, USA c Institute of Solid State Physics, Tzarigradsko Chaussee 72, 1784 Sofia, Bulgaria d Central Laboratory of Solar Energy and New Energy Sources, Tzarigradsko Chaussee 72, 1784 Sofia, Bulgaria b
Received 8 April 2003; received in revised form 8 April 2003; accepted 8 April 2003
Abstract The surface layer formed during chemical vapour deposition (CVD) of tungsten trioxide thin films was studied by means of atomic force microscopy (AFM), scanning electron microscopy (SEM) and spectroscopic ellipsometry (SE). Films were deposited at atmospheric pressure by pyrolytic decomposition of tungsten hexacarbonyl (W(CO)6) and were annealed at 400– 500 8C. Data from SE experiments and theoretical simulations showed that a layer forms at the surface of the WO3 film that has a different structure and composition from the bulk film. This surface layer becomes thicker with increasing oxygen flow rate during film deposition. This layer was predominantly amorphous for as-deposited films and predominantly crystalline after annealing. Root mean squared (rms) roughness values were calculated from AFM images of the surface layer. A high degree of surface roughness was revealed after deposition and annealing (40 nm), and the roughness value increased after additional annealing to 470 8C in 101 Pa of O2. # 2003 Elsevier Science B.V. All rights reserved. PACS: 68.35.Bs (surface structure and topography); 61.16.Ch (scanning probe microscopy: scanning tunnelling, atomic force, scanning optical, magnetic force, etc.); 68.55.a (thin film structure and morphology); 67.70.þn films (including physical adsorption); 81.15.Kk (vapour phase epitaxy, growth from vapour phase) Keywords: Morphology and growth; Tungsten oxide; Atomic force microscopy; Surface
1. Introduction Electrochromic materials undergo a change in transparency when a small voltage is applied. Electrochromism is exploited in architectural and automotive markets for active regulation of the amount of radiation passing through an electrochromic material. *
Corresponding author. Tel.: þ359-2-7144734; fax: þ359-2-9753632. E-mail address: [email protected] (A. Szekeres).
‘‘Antidazzle’’ electrochromic mirrors, for example, are electronically tintable or darken automatically to reduce headlight glare and are fitted to many cars. Electrochromic windows improve both energy efficiency and comfort within buildings and vehicles by reducing unwanted solar heating and glare. The active electrochromic layer is typically a thin film of tungsten trioxide (WO3) [1,2] that darkens as a result of the injection of ions (Hþ or Liþ) from a storage layer.
WO3 þ xLiþ þ xe $ Lix WO3 colourless
0169-4332/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00575-0
coloured
R.E. Tanner et al. / Applied Surface Science 218 (2003) 162–168
Prohibitive costs mean that smart windows are not being produced on a large scale. Films of WO3 have been deposited by a number of techniques including vacuum evaporation [3], reactive sputtering [4] and two-stage chemical vapour deposition (CVD) [5]. The CVD process is attractive as it enables films to be deposited at atmospheric pressure. Moving the substrate through the CVD reactor allows continuous growth and avoids downtime. In previous work [6], we have shown that direct deposition of tungsten oxide is possible at atmospheric pressure by pyrolytic decomposition of tungsten hexacarbonyl (W(CO)6). This technology yields films with mostly amorphous and highly porous structure, which is desirable as it enhances ion mobility [7]. The film structure is strongly influenced by the deposition temperature (i.e. substrate temperature) and the oxygen content in the gas environment during film growth. The present study is an atomic force microscopy (AFM), scanning electron microscopy (SEM) and spectroscopic ellipsometry (SE) investigation of the layers formed at the surface of WO3 thin films. Such layers differ in structure and composition from the basic volume of the WO3 film [7]. Formation essentially proceeds during the CVD deposition process, but additional changes in thickness and structure were discovered as a result of post-deposition annealing in O2. Annealing in O2 increased the root mean squared (rms) roughness of the film surface by 20% and degree of crystallinity increased. These changes are important due to their influence on ion intercalation in electrochromic devices where the WO3 film acts as a functional layer.
2. Experimental details The CVD thin films of WO3 were deposited by pyrolytic decomposition of W(CO)6 vapour in an argon/oxygen stream at atmospheric pressure in a horizontal reactor with cold walls. Glass and silicon substrates were heated at temperatures of 200, 300 and 400 8C during deposition. After growth, the thickness of the films was measured by a Talystep profilometer. The precursor W(CO)6 powder was placed in a sublimator immersed in a silicon oil bath that was heated up to 115 8C. The amount of W(CO)6 vapour was controlled by changing the argon carrier gas flow
163
rate through the sublimator. It has been established [8] that the partial pressure of W(CO)6 is nearly a linear function of the flow rate of the carrier Ar gas through the source in the range of 100–1000 sccm. In the experiments described here, the Ar flow rate was in the range 100–500 sccm and we therefore assume that the sublimation rate of W(CO)6 is proportional to the Ar flow rate. The films were deposited at different oxygen flow rates, keeping the flow rate of the Ar carrier constant. The ratio of Ar to O2 gas flow rates is denoted here as ArWðCOÞ6 =O2 . After deposition some samples were subjected to an annealing in air for 60 min at temperatures of 400, 450 and 500 8C. To find out if the annealing temperature and ambient would further change the surface layer thickness or its structure, some of the samples were subjected to additional extended period of annealing (180 min). In a chamber that was first evacuated by turbo-molecular pump to 105 Pa, these samples were heated at 470 8C in 101 Pa of purified O2. The surface morphology and the roughness were examined by recording AFM images with a Digital Instruments Nanoscope IIIa in tapping mode with etched Si tips (model OTESPA, Digital Instruments). Values of rms roughness were calculated from the height values in the AFM images using the DI software. The SEM experiments were carried out with electron microscope model JSM-5300 in secondaryelectron mode. Ellipsometric measurements and multiple-layer modelling were carried out to acquire more information about the composition and thickness of the rough surface layer. These measurements were performed on a Rudolph Ellipsometer in the spectral region 300– 800 nm. An incidence angle of 458 was used, which allowed the separation of the reflections from the film and from the back surface of the substrate when 2 mm thick glass substrates were used. The accuracy of the polariser, analyser and incidence angles is within 0.018. Our experience [7,9] has shown that the film structure is optically inhomogeneous and multi-layer modelling is necessary for the data analysis. The Bruggeman effective-medium-approximation theory [10] was used for the modelling and each sublayer was considered as a heterogeneous dielectric mixture of amorphous and crystalline phases of tungsten oxide and voids. In the theoretical model, the dielectric
164
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function data of stoichiometric crystalline WO3 [11] was taken as reference for the crystalline phase. Since the WO3 crystal is optically anisotropic, we averaged the values of the dielectric constants along the three principal crystallographic directions in the biaxial crystal. For the amorphous phase, the dielectric function data of amorphous WO3 from [3] was used. The ellipsometric data was fitted by minimising the difference between the experimental and theoretical curves using an iterative least-squares method. The results from the ellipsometric analysis were compared with those from the AFM measurements.
3. Results and discussion Our observations have shown that a small amount of oxygen in the deposition ambient leads to a smooth, homogeneous, small-grained WO3 surface, while increasing the oxygen flow rate results in the appearance of randomly distributed large crystallites that form a continuous surface film [12]. Annealing of the films promotes the crystallisation process, and at 500 8C the film structure is fully crystalline [7]. AFM measurements of the surface are in agreement with this analysis. An AFM image of the film surface after deposition and annealing in air is presented in Fig. 1a. Irregularly distributed domed crystallites form
surface termination with a somewhat lumpy appearance. Subjecting the samples to 180 min prolonged annealing at 470 8C in 10–1 Pa of O2 caused only minor qualitative modification of the surface (Fig. 1b). The general morphology of the surface was unchanged. This contrasts with the effect of annealing films prepared from a tungstic acid sol complex, where large straight-edged crystallites were created at the surface after annealing under the same conditions [13]. On a smaller length-scale, however, linescans [14] and calculated rms surface roughness values show that the surface morphology has been altered by the annealing treatment: crystallisation in the surface layer is visible as a more jagged appearance in linescans and the AFM image (Fig. 1b). In separate investigations [15], we showed that the surface morphology of WO3 single crystals is dominated by bulk oxidation and reduction. Under oxidising conditions mobile W5þ is oxidised at the surface to form immobile W6þ, while reducing conditions release interstitial W5þ to the bulk. Oxidation occurs at 525 8C in 10–2 Pa of O2; reduction at 500–575 8C in 4 103 Pa of O2 [15]. In the case of the thin film in Fig. 1b, we deduce that net oxidation is in operation: the high O2 partial pressure leads to the nucleation of crystallites at the surface. After 180 min growth, the angular facets of the crystallites are visible at the surface of the film in AFM.
Fig. 1. Pseudo-3D rendered AFM scans of the surface of a WO3 film deposited on glass at 200 8C and annealed. The images correspond to a 5 mm 5 mm area. (a) Image of the film annealed to 400 8C for 60 min. (b) AFM image of the surface following further annealing to 470 8C in 10–1 Pa of O2 for 180 min.
R.E. Tanner et al. / Applied Surface Science 218 (2003) 162–168
Digital processing of the AFM images was used to provide a quantitative guide to the roughness of the surface. Absolute values should be treated with caution due to the inherent effects of tip convolution with the surface topography, and of the dependence on the scan size. Despite this, it was possible to identify a clear trend in rms roughness by comparing images taken at the same magnification. The rough surface texture was confirmed by the relatively large value of the root mean squared roughness, calculated from Fig. 1a as 40 nm. Following long annealing the rms roughness increased by 20–30%, and in Fig. 1b corresponds to about 50 nm. These values correspond well with the qualitative observations of linescan data [14]. Our previous SEM investigation of the surface of CVD WO3 films has shown that the oxygen flow rate has a strong influence on the surface morphology [12]. Lower oxygen flow rates lead to smooth, homogeneous, small-grained film surfaces, while increased oxygen flow rates lead to the appearance of large crystallites on the smooth film surface. In the study reported here, where we concentrated on the effect of annealing, we reconfirmed that low oxygen flow rates produce a smooth, small-grained surface (Fig. 2). The WO3 film in Fig. 2 was deposited on glass at 200 8C and at a gas flow rate ratio of ArWðCOÞ6 =O2 ¼ 1=12. The surface of a similar film after annealing is shown in the AFM images of Fig. 1.
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Increasing the deposition temperature also increases the ‘‘grainy’’, crystalline appearance of the film, and for a deposition temperature of 400 8C well-pronounced, randomly distributed crystallites develop on the film surface (Fig. 3a). Annealing at 500 8C further enhances the crystalline structure of the as-deposited films. As an illustration, Fig. 3b shows a SEM micrograph of the WO3 film after annealing at 500 8C in air for 60 min. In addition to the larger average size of the crystallites, other changes in film morphology are visible, such as the appearance of elongated crystallites. Additional information about this surface layer was obtained from the analysis of spectral ellipsometric data. In the SE data analysis, the rough surface of WO3 films is considered as a surface layer that is optically different from the bulk volume of the film. The SE data were therefore interpreted using a twolayer model, in which the first is the surface layer and the second is the bulk film volume. Each layer was modelled as a mixture of amorphous and crystalline phases of WO3 and voids. A detailed consideration of the bulk layers is beyond the scope of this paper, and the results of the SE analysis are given elsewhere [7,9]. From the SE modelling of the bulk we conclude that there is a large volume fraction of voids (40–60%), indicating that the as-deposited films are rather porous. This may be due to the high deposition rate
Fig. 2. SEM micrographs of as-deposited WO3 film on glass. Deposition was carried out at 200 8C and at a gas flow rate ratio of ArWðCOÞ6 =O2 ¼ 1=12.
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Fig. 3. SEM micrographs of WO3 film: (a) as-deposited on glass at 400 8C and at a gas flow rate ratio of ArWðCOÞ6 =O2 ¼ 1=12, and (b) following subsequent annealing at 500 8C in air for 60 min.
of 70 nm/min. At this value, growth cannot be considered to be a thermodynamically controlled process and kinetic limitations mean that migration of the atoms at the hot surface is very low. As a result, disordered and highly porous material is obtained. This is supported by the values of the film density (3.6–5.5 g cm3) obtained from the weight measurements [16], which are much lower than those of crystalline WO3 material (7.16 g cm3) [2]. Deposition at 200 8C results in amorphous films, while films deposited at 300 8C are a mixture of amorphous and crystalline phases of WO3. The structure of films
deposited at 400 8C is predominantly crystalline with large number of voids still present. For films deposited on crystalline substrates (such as Si), annealing at 450 8C promotes conversion to a crystalline film structure [7], while films deposited on amorphous substrates (such as glass) require temperatures above 500 8C [9]. In Fig. 4 we present the results of simulated surface layer compositions for films deposited at different temperatures and at a gas flow rate ratio of ArWðCOÞ6 =O2 ¼ 1=6. With increasing temperature, the volume fraction of crystalline phase increases, indicating that higher deposition
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temperature promotes the crystallisation even during the film growth. After annealing, the crystalline WO3 and void fractions are dominant in the surface region. A typical dependence is demonstrated in Fig. 5, which shows
results for films deposited at 300 8C and annealed at 500 8C in air. Each phase varies in an approximately linear fashion with gas flow rate ratio, with the proportion of crystalline material showing the strongest dependence on the ArWðCOÞ6 =O2 ratio. At low O2 partial pressures the void fraction in the surface layer was about 43%, with a high proportion of crystalline WO3 and a negligible amount of amorphous material. However, the crystalline fraction decreased to 18% for higher O2 pressures, with a concomitant increase in both the amount of amorphous material and void fraction. The large void fraction at high O2 flow rate is confirmed by the high values of roughness calculated from the AFM images (Fig. 1a). The observed increase of the void fraction in the surface layer after annealing the films is an indirect indication that crystallisation leads to surface roughening, as observed in the AFM image of Fig. 1b. The SE results and modelling revealed how the thickness of the surface layer of the WO3 films changed as a function of gas flow rate ratio and deposition temperature. Fig. 6 shows that the higher deposition temperature leads to a thinner surface layer in the WO3 film. For both temperatures, however, the thickness dependence on oxygen flow rate is almost linear. The surface layer thickness decreased with
Fig. 5. Volume fraction of the amorphous and crystalline phases of WO3 and voids versus ratio of gas flow rates ArWðCOÞ6 =O2 for the surface layer of WO3 films deposited on glass at 300 8C and annealed at 500 8C in air for 60 min.
Fig. 6. Thickness of the surface layer determined by SE as a function of oxygen content in the deposition ambient defined as ArWðCOÞ6 =O2 for WO3 films deposited at 200 8C and 300 8C and annealed at 400 8C and above (450 or 500 8C). The point with star illustrates the AFM rms roughness value for the corresponding film.
Fig. 4. Volume fraction of amorphous and crystalline phases of WO3 and voids in the surface layer of WO3 films as a function of deposition temperature. The films were deposited on Si substrates at a gas flow rate ratio of ArWðCOÞ6 =O2 ¼ 1=12.
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increasing flow rate ratio of the reactive gases, i.e. with decreasing oxygen content. There is good agreement with the surface roughness values obtained from AFM measurements, which is highlighted in Fig. 2 with a ‘‘star’’ datapoint. In general, the annealing time (60 min in this case) had a weak influence on the surface layer thickness, but changes the proportion of the amorphous and crystalline phases of WO3.
4. Conclusions From the results of AFM, SEM and SE investigations, we conclude that the CVD process conditions described in the paper lead to the formation of disordered and highly porous WO3 films with a rough surface. Annealing at 400–500 8C promotes the crystallisation process but causes further surface roughening. The SE measurements show how the rough surface layer varies in thickness with the deposition conditions. The AFM images confirm this analysis and reveal a rms roughness of 40–50 nm, with higher values resulting from prolonged heat treatment. Acknowledgements The authors thank John Cronin and Lori Crawford of Schott–Donnelly Advanced Technology Centre in Tucson. We are grateful for financial support from US Department of Energy ‘‘Basic Energy Sciences Program’’ (grant DE-FG02-98ER14882). References [1] S.K. Deb, A novel electrophotographic system, Appl. Opt. Suppl. 3 (1969) 192–195.
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