Study of thin chemical vapour deposited tungsten oxide films by positron annihilation spectroscopy

Thin Solid Films 347 (1999) 302±306

Study of thin chemical vapour deposited tungsten oxide ®lms by positron annihilation spectroscopy N. Djourelov a,*, D. Gogova b, M. Misheva c a

b

Institute of Nuclear Research and Nuclear Energy, 72 Tzarigradsko Shoosse Boulevard, 1784 So®a, Bulgaria Central Laboratory of Solar Energy and New Energy Sources, 72 Tzarigradsko Shoossee Boulevard, 1784 So®a, Bulgaria c Faculty of Physics, So®a University, 5 J. Bourchier Boulevard, 1126 So®a, Bulgaria Received 8 May 1998; received in revised form 15 November 1998; accepted 11 December 1998

Abstract The positron lifetime technique has been used to study the relationship between the structure of chemical vapour deposited tungsten oxide thin ®lms and the deposition parameters. Some of the positron annihilation characteristics have been evaluated for WO3 bulk material. The positron annihilation lifetime results show that there is no saturation trapping and that a considerable part of positrons annihilate from the free state. Pick-off annihilation of orthopositronium localized in two kinds of voids is observed. The growth of deposition temperature decreases the disordered material fraction. The increase in total gas ¯ow rate leads to a decrease of large void sizes. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Tungsten oxide; Positron spectroscopy; Chemical vapour deposition

1. Introduction Solid state electrochromic devices for `smart' windows, non-emissive displays and automobile rear-view mirrors are of considerable technological and commercial interest. Tungsten oxide ®lms (WO3) are the most important layer in such devices and therefore their properties are the subject of numerous investigations [1,2]. The relationship between microscopic and macroscopic properties of materials and deposition parameters provides important guidance when optimizing the material characteristics of a given application. WO3 ®lms exhibit different electrical and optical properties depending on deposition conditions and techniques. Optical properties of WO3 coatings depend on crystalline structure, ®lm porosity, water and oxygen content. Voids play an important role in the electrochromic response of WO3 ®lms. Their concentration is related to the ®lm electrochromic ef®ciency. A number of papers published up to now concern the structural properties of chemical vapour deposited (CVD) polycrystalline WO3 ®lms [3±5]. Positron annihilation techniques (PAT) are widely used to study vacancy type defects in condensed matter. In this paper the information received by the posi* Corresponding author. E-mail address: [email protected] (N. Djourelov)

tron annihilation lifetime (PAL) method on CVD tungsten oxide ®lms is presented.

2. Sample preparation Thin ®lms of WO3 were obtained by pyrolytic decomposition of tungsten hexacarbonyl (W(CO)6) vapours in oxygen stream at atmospheric pressure in a CVD horizontal reactor with cold walls. Tungsten hexacarbonyl powder (Merck type) placed in the chamber was kept at a given temperature tsubl and was carried by argon ¯ow. Oxygen of 99.99% purity was introduced into the CVD reactor. The gas lines made of Galtek type Te¯on were heated at tsubl or slightly above. The substrates (soda lime glass 74% SiO2, 13% Na2O, 11% CaO, 2% Al2O3) were situated on a graphite susceptor and heated by a high frequency generator to tdep. More details can be seen elsewhere [5]. Some of the samples were annealed at temperatures tann ˆ 4008C or 5008C for 2 h in air after preparation. Film thicknesses were measured by a Talystep pro®lometer. By weighing the substrates and the ®lms deposited on them by an analytical balance Mettler AE 163, the ®lm density was calculated. The error was estimated to be about 5% due to the uncertainty of thickness determination, as mass measurement error could be neglected.

0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(99)0001 4-0

N. Djourelov et al. / Thin Solid Films 347 (1999) 302±306

303

Table 1 Sample preparation conditions, sample types, thickness d, density r , gas ¯ow rates, deposition rate Vd and fraction f®lm of emitted positrons that annihilate in the ®lms No

tdep ( oC)

Ia Ib Ic IIa IIb IIc IIIa IIIb IIIc IVa IVb Va Vb VIa VIb

200

70

300

110

300

110

400

110

400

110

400

110

a

tsubl ( oC)

tann ( oC)

Flow W(CO)6 £ 10 6 (m 3/s)

Flow O2 £ 10 6 (m 3/s)

Vd (nm/s)

d (mm)

r (g/cm 3)

f®lm (%)

AS 1 400 500 AS 400 500 AS 400 500 AS 500 AS 500 AS 500

3.33

46.67

0.033

2.2

4.2

11.8

3.33

46.67

0.550

2.5

4.5

13.8

1.67

23.33

0.483

2.3

4.5

12.9

3.33

46.67

6.667

3

5

17.0

1.67

20.0

6.167

2

5

13.6

10.0

30.0

6.333

1.9

6

14.0

As-obtained (not heat-treated) samples.

3. Apparatus and measurements The positron lifetime spectrometer used was a standard fast-fast coincidence apparatus based on PilotU scintillators, RCA31024 PMT and Ortec electronics. It provided a time resolution ca. 270 ps FWHM. The positron source, prepared by 22NaCl solution evaporated on and covered by 1 mg/cm 2 Kapton foils, was sandwiched between two identical samples. Four lifetime spectra were recorded for each pair of the samples. The analyses of lifetime spectra with PATFIT program were performed [6]. Each spectrum contained more than 1:5 £ 106 coincidences. Corrections for positron annihilation in the positron source were made [7]. To get the net effect of positron annihilation in the ®lms, two kinds of lifetime measurements have been carried out: for positron incidences on the ®lm side and on the substrate side of samples (`face-to-face' and `back-toback' experiments, correspondingly). In the `back-to-back' experiment, information about positron characteristics in the substrate was received. The spectra from the `backto-back' experiment were ®tted well (x2 , 1:1) with two exponents (t1 ˆ 297…3† ps, t2 ˆ 934…5† ps, I1 ˆ 64:0…4†%). No particular attention was paid to this information but it was used for a correct interpretation of results from the `face-to-face' experiment. By the layer computational program (Pascal 6.0) based on the Monte Carlo algorithm [8] that part of the emitted positrons that annihilate in the substrate was calculated using data for thicknesses and densities. It was treated as an addition to the source correction. In Table 1, the fractions f®lm of positrons annihilated in the ®lms during the `face-to-face' experiment are presented. Positron annihilation lifetime spectroscopy (PALS) gives Ê to 20 A Ê) the possibility to evaluate the sizes (radii from 2 A of pores in the studied samples. The triplet positron and electron bound state, ortho-positronium (o-Ps), created in

amorphous regions can be trapped at open-volume holes. O-Ps remains there [9] until its positron annihilates with a medium electron mainly (pick-off process). There is a relationship between the o-Ps lifetime t and the hole radius R. The speci®c form of this relationship depends on the accepted theoretical model. Usually, the following formula [10] ÿ
ÿ
 t ˆ 1=l ˆ …1=2† 1 2 R=R0 1 …1=2p†sin 2pR=R0 21 …1† is used. It is derived for the case of an in®nite spherical potential with a uniform electron layer. In the formula t Ê , respectively, and and R are expressed in ns and A Ê is the empirically R0 ˆ R 1 DR, where DR ˆ 1:656 A found thickness of the electron layer; l ˆ 1=t is the rate of o-Ps pick-off annihilation. 4. Results and discussion The values of ®lm density, thickness and sample preparation conditions are presented in Table 1. The studied samples are classi®ed in six types according to the preparation conditions. The polycrystalline WO3 coatings have densities higher than the amorphous ones, but in all cases their densities are lower in comparison to those of crystalline monoclinic WO3 r ˆ 7:16 g/cm 3 [11], i.e. the ®lms are highly porous. Film densities are affected mostly by the deposition temperature: high deposition temperature results in higher ®lm densities (see Table 1). High porosity is essential for the electrochromic process because the ionic transport and coloration ef®ciency increase with increasing ®lm porosity [12,13]. The stoichiometric ratio is also very important for the electrochromic behaviour of the material. It can be changed by varying reaction gas ¯ows. The oxygen ¯ow changes have a small

304

N. Djourelov et al. / Thin Solid Films 347 (1999) 302±306

Fig. 1. Schematic two-dimensional arrangement of crystallites in polycrystalline material.

effect on the ®lm deposition rate, but they affect the ®lm stoichiometry. A low oxygen ¯ow during coating deposition leads to oxygen de®cient oxides (substoichiometric WO32y ®lms). Unfortunately, anion vacancies (missed oxygen atoms) are assumed to exhibit an effective positive charge and, therefore, to repel positrons. High W(CO)6:O2 ratios produce ®lms exhibiting dark blue coloration. W(CO)6:O2 gas ¯ow ratio lower than 1/3 is needed to obtain a transparent ®lm. W(CO)6 conditionally stands for the ¯ow through the sublimator (Ar 1 W(CO)6), hereafter. Tungsten oxide ®lm microstructure comprises crystalline material, voids and disordered material at the grain boundaries [5]. The reported grain size values for polycrystalline in situ CVD WO3 ®lms are 40±60 nm [14]. The grain surfaces are suf®ciently rough to allow the existence of voids between the grain boundaries. Following the ideas presented in [15±17] for explanation of PAL results in the so called nanocrystalline materials, polycrystalline materials of very small crystallite size (5/10 nm), a structural model of WO3 coating has been developed. A schematic two-dimensional picture of this model is presented in Fig. 1. According to the model, the ®lm structure consists of crystalline material (site 1), large voids (site 3) and disordered material (site 2a) with small voids (site 2b) at the grain boundaries. The ®lm high porosity is explained by the exis-

tence of a great number of the above mentioned two types of voids. We have not found published papers describing investigations of tungsten oxide ®lms or bulk material by the PAL method. Dryzek et al. have investigated tungsten oxide by angular correlation of annihilation radiation [18]. The method suggested in [19] has been used to evaluate the lifetime t bulk of free positrons in the bulk of a perfect WO3 crystal. Using the unit-cell volume V ˆ 91 a.u. the value tbulk ˆ 180 ps has been obtained. The corresponding value of the lifetime of positrons trapped at W monovacancies has been found to be tv ˆ 240 ps. There is no theoretical or experimental evidence for the value of the positron lifetime t a in the bulk of amorphous WO3. The following experimental observations, however, can help to determine a reasonable t a value. In amorphous metal alloys, the lifetime values are always longer than those expected in the bulk of annealed constituents, but shorter than those of metal vacancies [20,21]. The situation for amorphous Si is similar. From the data given in [22], the value of ta ˆ 241 ps for the bulk positron lifetime in amorphous Si can be obtained. It is between 256 ps (lifetime of a positron trapped at Si monovacancy) and 219 ps (lifetime of free positron annihilation in Si) [19]. Since the average distance between the molecules in the amorphous state is larger than in the crystalline state, it is reasonable to expect that t a in amorphous WO3, as in amorphous alloys and amorphous Si, will be between t bulk and t v, i.e. 180 , ta , 240 ps. The lifetime spectra were ®tted by three negative exponential terms without any constraints. The results showed that the heat-treatment of the studied samples does not change considerably their structure, as seen by the present study. In order to simplify the following considerations, only the weighted average values of the lifetimes and their relative intensities are presented in Table 2. The presence of a lifetime in the order of 2±3 ns is due to pick-off annihilation of o-Ps and shows that Ps is formed in the disordered regions and annihilates in large voids (site 3 in Fig. 1). The corresponding values of pore radii are calculated by Eq. (1) (Table 2). The assignment of the second component is more complicated. In our opinion, it is an unresolved mixture of several components: annihilation of positrons in disordered regions (amorphous state), site 2a in Fig. 1, 180 , t2 a , 240 ps

Table 2 Average values of: lifetimes t i, intensities Ii, calculated t bulk and large void radii R Type

t 1 (ps)

I1 (%)

t 2 (ps)

I2 (%)

t 3 (ps)

I3 (%)

t bulk (ps)

R(nm)

I II III IV V VI

87(5) 83(10) 98(10) 93(3) 91(4) 80(7)

32(2) 31(2) 33(1) 30(1) 32(1) 33(1)

451(10) 530(9) 557(2) 555(4) 596(3) 559(7)

64(2) 62(1) 62(1) 63(1) 63(1) 60(1)

3221(27) 2406(149) 2622(78) 2022(54) 2703(23) 2212(12)

3.5(4) 6.1(5) 5.2(3) 6.4(1) 5.8(2) 6.8(1)

196(12) 204(21) 223(18) 229(8) 218(9) 192(14)

0.377(2) 0.32(1) 0.336(6) 0.287(5) 0.342(3) 0.304(1)

N. Djourelov et al. / Thin Solid Films 347 (1999) 302±306

Fig. 2. t 2 versus total gas ¯ow rate.

(I2a); positrons trapped at defects in the crystalline regions, site 1, 240 , t2 b , 500 ps (I2b); and pick-off annihilation of o-Ps localized in small voids in disordered regions, site 2b, 500ps , t2 c , t3 (I2c). At this I2 a 1 I2 b 1 I2 c ˆ I2 . Unfortunately, because of the small proportion of positrons that annihilates in the ®lm (,15%) and the low resolution of the used spectrometer, the suggested components cannot be separated. As we have no possibility to evaluate t 2c, the sizes of the small voids cannot be determined. The lower value of t 2 for type I can be associated with the larger value of I2a due to the lower deposition temperature [14]. For the other types, the fact that t 2 values are in the order of 500±600 ps, leads to the conclusion that the main part of this component is due to pick-off annihilation of o-Ps

Fig. 3. Large void radius versus total gas ¯ow rate.

305

localized in the small voids (site 2b). The relatively high I2 values con®rm the high porosity of the studied WO3 ®lms. The ®rst component is due to the annihilation of free positrons in crystalline regions (site 1 in Fig. 1). There is also a contribution I1p due to self annihilation of p-Ps. Due to spin statistics, without ortho-para conversion, 1/4 of the formed Ps is in singlet state with lifetime of 125 ps. Obviously, I1 p ˆ …I3 1 I2 c †=4. We have not tried to evaluate this contribution to t 1(I1) because it is impossible to make a reasonable suggestion about the value of I2c. The value of t 1 is lower than the theoretical t bulk and consequently there is no saturation trapping, so the trapping model is valid [23]. The values of t bulk calculated from the experimental results are shown in Table 2. They exceed the theoretically estimated t bulk value. This could follow from not taking into account p-Ps self annihilation contribution to t 1 and/or uncertainties in f®lm calculation coming from the corresponding errors in the densities and the thicknesses determination. As was mentioned above, in the present case PAL spectroscopy cannot detect the in¯uence of sample annealing. This observation, however, is not in contradiction with the results obtained by spectroscopic ellipsometry [24] and Raman spectroscopy [14] according to which the degree of crystallinity increases with annealing. We consider the observed discrepancy as connected with the ®lm microstructure changes during the growth process. X-ray diffraction shows that the ®lms are amorphous at small thicknesses but if the deposition process lasts longer under the same conditions, the ®lm continues to grow as polycrystalline with ®ne grained microstructure [1,3]. The studied ®lms in [14,24] have a thickness of 0.1±0.2 mm, while the ®lms studied in this work have thicknesses of ,2 mm. So, we consider the present ®lms as consisting of two sublayers, a thin predominately amorphous layer, close to the glass substrate, and, over it, a thicker polycrystalline layer. The annealing converts the amorphous layer to a polycrystalline layer, but because of the comparatively low annealing temperature it does not in¯uence the crystalline perfection and void structure in the rest of the ®lm. Since the fraction of positrons, annihilating in the amorphous layer, is less than 10%, the structural modi®cation of this layer does not change visibly the positron lifetime results. An investigation with slow positrons could give a correct answer on this subject. A comparison of data for types II and IV and for types III and V (see Table 2) prepared at different deposition temperatures, shows that the values of the lifetime t 2 are higher for higher tdep. This can be explained supposing the fraction I2a of positrons annihilating with lifetime t 2a in the disordered material to be lower at higher tdep which can be connected with a higher deposition rate at tdep ˆ 4008C. The samples of types II and III show lower values of I3 in comparison to those for samples of types IV and V. The

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N. Djourelov et al. / Thin Solid Films 347 (1999) 302±306

samples of type I contain the largest voids (site 3) of a lowest concentration. A comparison between the samples of type II and III prepared at different total gas ¯ows but at the same gas ¯ow ratio (W(CO)6:O2) shows that the annihilation parameters are different. This is an indication that the signi®cant parameter for the ®lm structure is not gas ¯ow ratio but the total gas ¯ow rate. From Table 1, it can be seen that the lifetimes t 2 and t 3 for samples prepared at the same tdep decrease as a function of the total gas ¯ow rate, Figs. 2 and 3. We should mention that it is more convenient to use slow positrons to investigate thin WO3 ®lms, because they penetrate to a determined depth of the ®lm and positron annihilation in the substrate does not occur. Such experiments are to be done. 5. Conclusion CVD WO3 thin ®lms with substantial porosity have been obtained. The presence of a great number of voids was detected by PAL measurements. Some of the positron annihilation characteristics in WO3 bulk material have been evaluated. The PAL results for CVD WO3 ®lms show that there is no saturation trapping and a considerable proportion of positrons annihilates from free state. The preparation conditions have an important effect on the ®lm microstructure. The increasing tdep decreases the fraction of the disordered material. The decrease of the total gas ¯ow rate leads to an increase of the large void sizes. Additional annealing of the WO3 ®lms at 4008C and 5008C has no in¯uence on the positron characteristics. Acknowledgements This work was supported by the Bulgarian Ministry of

Science and Education under the contract F-456. We thank Assistant Professor Dr K. Gesheva for helpful discussion. References [1] C.G. Granqvist, Handbook of Inorganic Electrochromic Materials, Elsevier, 1995 [2] P.M.S. Monk, R.J. Mortimer, D.R. Rosseinsky, Electrochromism: Fundamentals and Applications, VCH, Weinheim, 1995. [3] D.S. Gogova, K.A. Gesheva, Electrochem. Soc. Proc. 97±25 (1997) 1482. [4] D. Davazoglou, A. Donnadieu, Thin Solid Films 147 (1987) 131. [5] D. Davazoglou, A. Donnadieu, J. Appl. Phys. 72 (1992) 1502. [6] P. Kirkegaard, N.J. Pedersen, M. Eldrup, PATFIT-88, Riso-M-2740, 1989. [7] N. Djourelov, M. Misheva, J. Phys.: Condens. Matter 8 (1996) 2081. [8] R.B. Gregory, W. Su, Mater. Sci. Forum (1992) 105±110. [9] Y.C. Jean, Macromolecules 29 (1996) 5756. [10] M. Eldrup, D. Lightbody, J.N. Sherood, Chem. Phys. 63 (1981) 51. [11] J.P. Randin, J. Electron. Mater. 7 (1978) 47. [12] C.G. Granqvist, Appl. Phys. A 57 (1993) 3. [13] J-G. Zhang, D.K. Benson, C.E. Tracy, S.K. Deb, J. Mater. Res. 8 (1993) 2657. [14] D. Gogova, M. Sendova-Vassileva, A. Veneva, (1999) in press [15] H.E. Schaefer, R. WuÈrshum, M. Scheytt, R. Birringer, H. Gleiter, Phys. Rev. B 38 (1988) 9545. [16] R. WuÈrshum, G. Soyez, H.E. Schaefer, NanoStructured Mater. 3 (1993) 225. [17] L.Y. Xiong, W. Deng, J. Zhu, A. Dupasquier, X.J. Wu, X. Ji, Mater. Sci. Forum 175±178 (1995) 577. [18] J. Dryzek, A. Polaczek, M. Pekala, R. Rataj, J. Phys. Condens. Matter 4 (1992) 1399. [19] M.J. Puska, S. MaÈkinen, M. Manninen, R.M. Nieminen, Phys. Rev. B 39 (1989) 7666. [20] P. Hautojarvi, J. Yli-Kauppila, Report P9/1981, Helsinki University of Technology, Laboratory of Physics. [21] Z. Michno, A. Baranowski, Acta Phys. Polonica A76 (1989) 111. [22] Jia-Jiong Xiong, Bi-Song Cao, Wei-Zhong Yu, Ali-Lien, Yi-Hua Wang, Gang Liu, D.Adler, in: P.C. Jain, R.M. Singru, K.P. Gopinathan (Eds.), Positron Annihilation, World Scienti®c, Singapore, 1985. [23] R.N. West, Adv. Phys. 22 (1973) 263. [24] A. Szekeres, D. Gogova, K. Gesheva, J. Cryst. Growth (1999) in press.