Physical properties of pyrolytically sprayed tin-doped indium oxide coatings

Thin Solid Films, 205 (1991) 93-100

93

Physical properties of pyrolytically sprayed tin-doped indium oxide coatings H. Haitjema* and J. J. Ph. Elich Delft University of Technology, Faculty of Applied Physics, P.O. Box 5046, 2600 GA Delft (The Netherlands) (Received February l l, 1991; accepted May 14, 1991)

Abstract The optical and electrical properties of tin-doped indium oxide coatings obviously depend on a number of production parameters. This dependence has been studied to obtain a more general insight into the relationships between the various coating properties. The coatings have been produced by spray pyrolysis using a solution of indium chloride in butylacetate with tin chloride as a dopant. The influence of the tin concentration, the carrier gas for the spraying~ the coating thickness, the substrate temperature and annealing has been investigated using various techniques. The best result is obtained when producing a coating of about 300nm thickness, using nitrogen as the carrier gas to prevent an excess of oxygen in the coating. When the tin doping is about 5%, the maximum density and mobility of the free electrons are obtained (about 5 x 10~° cm ~ and 45 cm 2 V- ~s- ~ respectively). The substrate temperature has to be high; about 580 °C in the case of glass substrates. The electrical and optical properties are satisfactorily described using the theory of ionized impurity scattering. The study of the relation between the structural and the electrical-optical properties shows that the structural properties, i.e. the crystallite size and orientation, influence the electrical and optical properties only weakly.

1. Introduction Tin-doped indium oxide coatings are well known as electrically conducting and IR-reflecting coatings. They are also transparent in the optical region. Together with tin oxide, zinc oxide and cadmium-tin oxide they are one of the wide band gap semiconductor transparent IR reflectors. The applications of transparent conducting coatings range from heat-reflecting oven windows, antifrost coatings on windows and conducting antireflection coatings on solar cells to spectrally selective solar absorber coatings. Extensive investigations into the properties of indium oxide coatings have been reported in the literature; we especially mention the work of Frank et al. [1], which is more or less extended in this paper, and the work of H a m b e r g and Granqvist [2], which gives an extensive treatment of the fundamental band gap absorption, I R lattice oscillations and a model for the ionized impurity scattering. In this paper, results are described obtained by preparing coatings by spray pyrolysis while varying process parameters such as the tin concentration, the * Present address: Netherlands Measurements Institute, Van Swinden Laboratory, P.O. Box 654, 2600 AR Delft, The Netherlands.

0040-6090/91/$3.50

carrier gas for the spraying, the coating thickness and the substrate temperature. Also, the effect of annealing is investigated. An analysis of the optical, electrical and structural properties is given to obtain information about the physical principles which govern the coating properties.

2. Preparation technique The coatings have been prepared by the spray pyrolysis method which has been described extensively elsewhere [3]. The coatings that will be discussed in this paper have been deposited on Pyrex and window pane substrates. Quartz has not been used as a substrate material because we observed no difference between coatings deposited on Pyrex and window panes on the one hand and because quartz is not suitable for the production of coatings on a larger scale on the other hand. In most cases the substrate has been heated and sprayed several times to achieve a sufficient coating thickness without too large a fall in the temperature of the substrate during the spraying process. As the spraying solution, a solution of indium chloride in butylacetate with tin chloride as a dopant has been used. The solutions contained tin:indium atomic concentration fractions of 0%, 1%, 2%, 4%, 6%, 8% and

© 1991 -- ElsevierSequoia, Lausanne

94

H. Hai(/ema, J. J. Ph. Elich / Physical properties o/pyrolytic lnzO/Sn

12!~g. Out of the 10 c m x 10 cm substrates, 28 m m circles have been sawn which have been used as the samples on which various measurements have been carried out.

3. Experimental

~' = e/meffp

techniques

3.1. Determination o f optical properties The reflectance in the 0.3 40pro region and the transmittance in the 0.3 2.5 lain region have been determined with Perkin Elmer Lambda 9 and type 883 spectrophotometers. From these measurements, the optical constants (the refractive index n and the extinction coefficient k) are derived using various methods: in the 0.3 0.8 pm spectral region using the "envelope" [4] and the "'reflectance-transmittance" R T method [5], in the 0.8-2.5 pm spectral region using the R - T method and in the 2.5 4 0 pm region using a modified Kramers Kronig analysis [6]. 3.2. Determination o f electrical properties The resistivity and the Hall coefficient are determined from measurements according to the van de Pauw method [7]. A sample holder is used which enables the sample temperature to be varied between 77 and 600 K [8]. From the resistivity and the Hall coefficient, the free electron mobility # and the free electron density n are derived. 3.3. Combining optical and electrical properties To enable a comparison between electrical and optical parameters, the complex resistivity p is calculated from the optical constants n and k at a wavelength 2 (frequency (~ = 2xc/2) using the definition i e,((o) = ( n - i k ) 2 = G~, - ....... ' ~:o(op((o)

(1)

where 6~,, is the relative permittivity for (n ---, ~c, which can be taken equal to 4 for an indium oxide coating. As a first estimate, the coating can be modelled according to the Drude model, which does not incorporate the cause of the collisions of the free electrons, but uses a general relaxation time. This yields the equation ,, + i(,~ p((o) = pA(o) + ipA(n) - ~:°~:~(OP z

(2)

where p~ and p~ are the real and complex parts respectively of the resistivity, eo is the vacuum permittivity and Ogp is the plasma frequency. This frequency is defined in terms of the electron density n_, which is determined by electrical measurements, and the effective electron mass meff as H e2

(o~ -

(3) 80Er xmeff

with e being the elementary electron charge. The relaxation frequency 7, which is assumed to be real and frequency independent in the Drude model, is related to the electrical mobility/~ by (4)

The real part of p((o) can be compared directly with the d.c. value Pd.¢. which is determined electrically: Pd.c. = p((o = 0) = 07 e/t) t

(5)

When comparing optical and electrical properties, the effective electron mass is derived from the complex part of p((o) (determined from optical measurements) and using eqns. (2) and (3), and the electron density (which is measured electrically), it is found that in most cases the real part of p((o), which is called pr((o), is constant for ~o < (%. This enables the definition of an "optical" mobility by ~op, = In et~r(;. ~ 5 ~tm) ~, ~

(6)

When the mobility is determined by scattering by ionized impurities, which are always present in a doped coating, the relaxation coefficient in eqn. (2) can be onsidered to be both complex and frequency dependent. According to the theory of Gerlach and Grosse [9], which was applied to indium oxide coatings by Hamberg and Granqvist [10], this leads to the following expression t\~r p((o): P(<") =

iT-7on;o-,: j/o k2[{e(k'')} dk+i

(0 2 ~-;0E,~ (gp

l

',~:(k,O)} ']

(7)

where N~ is the density of the ionized impurities, Z is the charge of the impurities and e(k,(o) is the dielectric function of the free electron gas which depends on the frequency (o and on the electron wavevector k. The full equations for e(k,~o) have been given by Hamberg and Granqvist [10], Application ofeqn. (7) leads to a complex part of p(~o) which differs little from the Drude model as given by eqn. (2); the real part of p((o) is constant for 2 >2p and is roughly proportional to 217 for )~ < ),p. A limiting value for the d.c. mobility #o .... called # . . . . can be derived from eqn. (7) using the electron density and the effective mass and taking the limit for (o ~ 0: #max = {n_ep(oJ ~ 0)} 1

(8)

3.4. Determination o f grain size and orientation The X-ray diffraction measurements are carried out using an Enraf Nonius Diffractis 583 stabilized generator and a Philips PW 1050 powder diffractometer. Cu K s radiation is used and diffractograms are taken using continuous scanning.

H. Haitjema, J. J. Ph. Elich / Physical properties of pyrolytic lneO3.'Sn

From a diffractogram the preferential orientation is derived by comparing the peak intensity of different lines with the distribution of randomly oriented crystallites. An estimate of the grain size is obtained from the peak width using the Debye-Scherrer equation. This "grain size" should be considered as an average distance between lattice imperfections rather than the size of the polycrystalline grains.

95

10 2

4. Results

4.1. Effects of tin doping The coatings have been sprayed with air as the carrier gas on glass substrates at a substrate temperature of 550 °C in 4 cycles of 2 s each. The resulting coating thickness and the electrical properties are summarized in Table 1. In this table it is shown that the electron density and the mobility increase with the tin:indium fraction up to 4%, after which they decrease somewhat. The values of d and p of one sample can be determined with an uncertainty of less than 2%; the values of n_ and/~ with an uncertainty of less than 5%. The standard deviation of d, p, n_ and p determined from different samples produced under equal spraying conditions is about 5%. The optical and the temperature-dependent electrical properties are more conveniently discussed together with the properties after annealing. This is done in the next section.

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Fig. 1. Mobility and electron density of coatings before and after annealing.

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4.2. Annealing effects Annealing the coatings in vacuum or in a hydrogen atmosphere proved to have a substantial influence on the electrical properties of the coatings. In general, for coatings having a low electron density the mobility in particular increases when they are annealed, while for coatings having a high electron density the electron density in particular increases. This has also been observed by Frank and coworkers [11]. This behaviour can be seen in Fig. 1, where the mobility and the electron density of the samples listed in Table 1 are shown before and after annealing. Annealing has the largest effect on the undoped sample, where the conductivity increases by

Fig. 2. Reflectance R and transmittance T o f c o a t i n g s doped with 1% Sn before ( ) and after ( ) annealing and doped with 12% Sn before ( - - - - - ) and after (...) annealing. d

a factor of 1000. Also, in the heavily doped samples having 6%, 8% and 12% tin doping, where the conductivity increases by a factor of 5, the effect of annealing is considerable.

4.2.1. Electrical and optical properties The optical properties also change when the coatings are annealed. This is illustrated in Fig. 2, where the reflectance and the transmittance of the coatings with 1%

T A B L E 1. Thickness and electrical properties of coatings with different tin:indium fractions Sn:ln fraction in solution (%)

Thickness d (nm)

n_ ( × 1026 m - 3)

~ (cm 2 V - 1 s - 1)

p (ll m)

0 1 2 4 6 8 12

142 142 143 147 138 166 175

0.99 1.60 1.96 1.78 1.84 2.14

27.4 29.5 32.0 25.8 26.4 28.1

9.6 × 10 -2 2.90 × 10-s 1.32 × I0 s 9.90 x 10 -6 1.35 × 10-5 1.28 × 10 -5 1.03 × 10 _5

H. Haitjema, J. J. Ph. Elich / Physical properties •~pyrolytic IneO/Sn

96

and 12~o tin doping are given. In this figure it is shown that the IR reflectance increases according to the increase in mobility for the coating with 1~0 tin doping• For the coating with 12~o tin doping the IR reflectance increases and the reflection edge shifts to a lower wavelength, according to the increase in the electron density in this coating. The real and complex part of the resistivity fi(2) have been determined according to the method given in Section 3.3. As indicated in Section 3.3, this enables a comparison between the optical and electrical properties. Figures 3 and 4 give the values of ~(2) for the unannealed and annealed case respectively for the cases of doping with 1~o, 2~o and 12~o Sn. Theoretical curves are drawn on the

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Fig. 3. Real and c o m p l e x parts o f the resistivity o f coatings doped with 1°/, 20/oand 12~o Sn before annealing. F r o m m e a s u r e m e n t s as presented in Fig. 2 the spectral c o m p l e x refractive index was calculated. W i t h eqn. (1) the spectral values ofp~ and p¢ were obtained. C u r v e s calculated from the modified G e r l a c h - G r o s s e theory using eqn. (7) are also s h o w n .

basis of the ionized impurity scattering theory outlined in Section 3.3. The plasma wavelengths and the d.c. resistivities are also indicated in the figures. In the figures it is shown that in the measured curves a similar power law dependence of pr(2) is observed for 2 < 2p to that in the calculated curves. The measured pr(2) at long wavelengths accords well with the measured d.c. values in all cases. In the case of the 2 ~ and 12~o tin-doped annealed coatings the measured and calculated curves of p~(2) almost overlap, which means that for these coatings the maximum attainable mobility due to ionized impurity scattering has almost been achieved. The power law dependence of p,(2) for 2 < 2p has also been observed by Frank and coworkers [1 1]. A similar agreement between the measured and calculated curves of pr(2) as in the case of the 20/o and 12~o tin-doped annealed

97

H. Haitjema, J. J. Ph. Elich / Physical properties of pyrolytic IneO3:Sn TABLE 2. Electrical and optical properties determined at room temperature Sn:In (~o)

Before or after annealing

n_ t/ ~.p (×1026 m -a) (cm2 V-] s-l) (i.tm)

meff (reel)

/'/opt

//max

1

Before After

0.99 1.29

27.4 38.0

3.10 3.04

0.21 0.27

29 33

82 57

2

Before After

1.60 2.06

29.5 43.6

2.44 2.45

0.21 0.28

34 40

75 49

12

Before After

2.14 6.56

28. I 33.6

2.09 1.52

0.21 0.30

32 32

72 36

coatings has been obtained by Hamberg and Granqvist [2, 12] for heavily doped (n_ > 5 x 10 26 m -3) coatings prepared by reactive electron beam evaporation. The optical and electrical properties of the coatings doped with !~o, 2~o and 12~ Sn before and after annealing are summarized in Table 2. In the table some interesting effects can be observed. The effective mass meff increases as a result of annealing, which causes the maximum mobility #max to decrease. A further effect is that the optical mobility/~ov,, as defined by eqn. (6), is generally slightly larger than the d.c. value for unannealed coatings and smaller for the annealed coatings. The effects of annealing can be explained by presuming that the excess oxygen present in the coating is removed by the annealing process. This excess oxygen, which is originally present in the coating, reduces the conductivity in two ways. (a) It reduces the free electron density by acting as a trapping centre for the electrons originally induced by the dopant. (b) It reduces the mobility by acting as a scattering centre for the remaining electrons. This hypothesis, proposed by Frank et al. [11], explains the effects of annealing on the mobility and the electron density as shown in Fig. 1 and in Table 2. As the optical and the d.c. mobility do not differ much, the grain boundaries do not play an important role. However, from the change in the difference between optical and d.c. mobility it can be assumed that part of the excess oxygen is present at the grain boundaries. The effective masses listed in Table 2 are of the same magnitude as found in the literature. Clanget [13] has found a value of 0.28 when the electron density is low. This value increases with increasing n_ to 0.32 for n_ ~,~ 5.5 X 1026 m - 3 . This trend is also observed for the annealed coatings in Table 2, although the absolute values are somewhat smaller. This supports the suggestion of Weiher and Ley [14] that the conduction band is not parabolic. Other values found in the literature are 0.24 [15], 0.30 [16] and 0.35 [17]. 4.2.2. Tin concentration and free electron density The tin content of some coatings has been determined

by electron microprobe analysis. It proved that the tin content in the coatings was about 1/2 to I/3 of the tin content in the solution. This is not in agreement with the results of some researchers who find that the tin content of the coating is about the same as in the spray solution [17, 18]. The free electron density is plotted against the tin density in the coating for some samples in Fig. 5. In this

1o

8 o

6

I

~expectod

o

+ before Mwle&|lnll o tYger unnotltng

4

o

4 ------>

Fig. 5. Free electron density

8 [¢:n'1 vs.

(.

12 16 lO=DB/rn 3 )

20

the tin concentration.

figure also a theoretical curve, based on the assumption that each tin atom gives rise to one free electron, has been drawn. The figure shows that, in one case, the annealed coating with a 4 ~ Sn:In fraction in the spraying solution, the expected electron density has been achieved. In the other cases the free electron density is less, especially in the unannealed cases. These results are quite different from the results for fluorine-doped SnO 2 coatings [6], where a maximum electron density of about one-third of the donor concentration is found. According to Frank and K6stlin [l 8], a smaller electron density than the tin concentration can be attributed to additional oxygen which is still present in the coating. Frank claims that the free electron density equals the tin concentration when the coating is annealed at 500 °C at a partial oxygen pressure of less than l 0 - 2 ° bar. The maximum electron density that Frank and coworkers

98

H. Hai(jema, J. J. Ph. Elich

Phrsical properties qtp.t'rolytic hTeO3.Sn

found is 1.5 × 1027 m 3 which corresponds to a 500 tin doping of the coating. This tin doping corresponds to the solubility limit of about 51~i, Sn in I n 2 0 3 [19]. We have annealed our coatings at a lower temperature (350 ~C ), whereas first the partial oxygen pressure was reduced by evacuation to about 10 0 bar and then H 2 gas was led in the vacuum chamber. Probably in our case this annealing procedure was not in all cases sufficient to achieve the highest possible electron density.

ments are given. The intensity measurements are given relative to the (400) peak in the 300 nm sample. In the table a strong preferential orientation in the (400) direction is observed for the 160 nm and 300 nm coatings• The intensity and the grain size of the (222) peak slightly decrease with the thickness.,~This indicates that a reorientation in the first 80 nm of~he coating takes place when subsequent layers are deposited during the spraying process• In the literature, a preferential (400) orientation

4.2.3. X-ray diffraction measurements The X-ray diffractograms of the different samples were all similar: all showed a preferential orientation in the (400) direction. Also, there were no differences between the diffractograms of the as-grown and the annealed samples. This means that the changes in optical and electrical properties due to annealing are not caused by any change in the crystal structure of the coating.

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4.3. Coating thickness effects 4.3.1. Preparation conditions

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A series of coatings with different thicknesses has been produced. A solution with a 43/o tin:indium ratio has been used and has been sprayed on Pyrex substrates using nitrogen as the carrier gas. The substrate temperature was 600 °C and the spraying time was 5 s per cycle and the number of spraying cycles was 1,2 and 4 for the different samples. This resulted in coatings of about 80, 160 and 300 nm thickness.

4.3.2. Optical and electrical properties The reflectance and transmittance of the three coatings are shown in Fig. 6. The transmittance decreases and the IR reflectance increases with increasing coating thickness, as is to be expected. The characteristic electrical and optical properties are listed in Table 3. The table indicates that the optical and electrical properties tend to improve with the coating thickness. Further, it is observed that the electron density and the mobility which are achieved without annealing are higher than in the coatings prepared with a different tin doping described in Section 4.1. The difference is that in this case the coatings have been prepared using nitrogen as the carrier gas instead of air in the case discussed in Section 4.1. This will cause less excess oxygen to be incorporated in the coating.

4.3.3. X-ray diffraction measurements In Table 4 the results of X-ray diffraction measure-

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Fig. 6. Reflectance and t r a n s m i n a n c e of coatings of 80 nm (" - .), 160 nm ( ) and 300 nm ( ) thicknesses.

T A B L E 4. Results of X-ray diffraction measurements on coatings with different thicknesses Crystal plane

Relative intensities and grain sizes for the following thicknesses ...........

Relative intensities (211) (222) (400) (431) (440) (622) Grain sizes (nm) (211) (222) (400) (622)

80 nm

160 nm

300 nm

< 2.8 9.0 8.0 < 2.8 < 2.8 < 2.8

<2.8 7.6 28 ~ 2.8 < 2.8 5.8

3.1 6,2 100 < 2.8 < 2.8 9.()

23 ~l

23 31 25

23 18 37 25

Randomly oriented In203

14 100 30 I[t 35 25

T A B L E 3. Electrical and optical properties determined at r o o m temperature (the same samples as in Fig. 6} Thickness (nm)

80 160 300

n (xlO2~,m 3.65 3.21 4.22

3)

/td.¢. (cm2V-Is-l)

Pd.c (xlO

37.6 38.3 47.7

4.54 5.07 3.22

~m)

";.p (tam)

meff (too)

]/opt (cm2V-ls

1.87 1.89 1.72

0.28 0.26 0.28

37 37 45

1)

,//max (cm2V 48 55 47

*s *)

H. Haitjema, J. J. Ph. Elich / Physical properties of pyrolytic lneO3.'Sn is observed by nearly all researchers [17, 20-22] except Mammana [23], who finds a (222) preferential orientation. The latter result might be related to the small grain size of 3-10 nm in the coatings. The conclusion of Fan and Foley [24], that the electrical and optical properties appear to be independent of the orientation effects, is confirmed by our results. The crystal orientations in the 80 and the 300 nm coating are very different, but the difference in optical and electrical properties of these coatings (Table 3) is much less dramatic.

4.3.4. Scanning electron micrographs Scanning electron micrographs have been taken of the sample surface. Figure 7 gives the pictures of the 80 nm

(a)

(b)

Fig. 7. Scanning electron micrographs of(a) an 80 nm and (b) a 300 n m tin-doped I n 2 0 3 coating.

and the 300 nm coatings. Just as in the case of tin oxide coatings the grain size observed at the surface increases with the coating thickness. When comparing the grain size with the 100 nm bar on the pictures, it can be observed that the grains are about 30 nm when the coating thickness is 80 nm and about 100 nm when the coating thickness is 300 nm. When comparing these dimensions with values obtained by X-ray diffraction given in Table 4, it must be considered that the latter value should be interpreted as a "mean distance between lattice distortions". In general, the latter is smaller than the grain size which is observed in scanning electron micrographs.

4.4. Influence of the substrate temperature A series of coatings has been produced at different substrate temperatures during the spraying process. A solution with a tin:indium atomic ratio of 0.02 has been used as the spraying solution. The coatings were produced on a window pane using air as the carrier gas in 4 cycles of 2 s each. The thickness and the electrical properties of the coatings are given in Table 5. In this table it is shown that the thickness, electron density and mobility increase with the substrate temperature. The increase in the electron density and the mobility may be a combined effect of the increasing coating thickness and the increasing substrate temperature. In the 500-600 °C range the electrical properties do not change much,

99

T A B L E 5. Thickness and electrical properties of coatings prepared at different substrate temperatures Values for the following 400 substrate temperatures (°C)

450

500

550

600

Thickness d (nm) n_ (×1026 m -3) ,Ud.¢. (cm 2 V -1 s -1) Pd.¢. (× 10-5 f~ m)

52 1.9 12 2.7

70 2.1 25 1.2

88 2.0 29 1.1

96 2.1 32 0.93

30 1.1 11 5.2

particularly not when compared with the preparation of tin oxide coatings [6]. For a spraying solution that is the same as ours, Frank and K6stlin [18] use a substrate temperature of 500 °C. Siefert [25] concludes that the In20 3 coatings are polycrystalline when the substrate temperature is higher than 350 °C. Compared with these results, it seems that the substrate temperature where we obtain an optimum is rather high. Most researchers use an alcoholic solution of InC13. Of these workers, Kulaszewicz [26] finds that amorphous films are formed when the substrate temperature is 400 °C or lower. The optimum spraying temperatures obtained with this solution are from 450 °C [27] to 500 °C [22, 28]. A more extensive analysis of the influence of the preparation conditions on the coating properties has not been given in the literature, probably because the coating properties do not change much when the preparation conditions are about optimal.

5. Conclusions

The coating properties are slightly dependent on the spraying conditions. Apart from the substrate temperature, which must be high enough to obtain a polycrystalline coating (with our equipment 550-600 °C), the composition of the atmosphere in which the coating is prepared seems to be most critical. An excess of oxygen in the coating influences the free electron density and the mobility in a negative way. With our equipment, this can be prevented by using nitrogen as the carrier gas. When the maximum obtainable mobility of about 40-50 cm 2 V - l s- 1 due to ionized impurity scattering is not achieved in a sprayed coating, it will probably be achieved after annealing the coating in a reducing gas at a temperature of 350 °C or higher. The electron density also increases as a result of the annealing procedure. A maximum free electron density of about the tin concentration can be achieved. A relation between the structural and electrical-optical properties could not be established. This relation, if present, is apparently much less strong than in the case of tin oxide coatings.

H. Haitjema, J. J. Ph. Elich

100

Physical properties ~/pyrolvtic In:O3.Sn

Acknowledgments The authors gratefully acknowledge E. Sonneveld and the Solid State Group for measuring the X-ray diffraction patterns and J. Toth for making the scanning electron micrographs. This investigation in the program of the Foundation for Fundamental Research on Matter (FOM) has been supported by the Netherland Technology Foundation. References I. 2 3 4 5 6 7 8

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