Structure and optical properties of tin oxide films

Solar Energy Materials 19 (1989) 79-94 North-Holland, Amsterdam

79

STRUCTURE AND OPTICAL PROPERTIES OF TIN OXIDE FILMS H. D E M I R Y O N T and K.E. N I E T E R I N G Ford Motor Company, Glass Division, 15000 Commerce Drive North, Dearborn, 341 48120-1225, USA

In the study, we investigated the relationship between macroscopic and microscopic properties of tin oxide films. Three kinds of tin oxide films were evaluated: (1) amorphous films deposited by thermal evaporation of S n O 2 powder; (2) spray-deposited tin oxide films with poor crystallinity (DBDA films); and (3) spray-deposited tin oxide films with high crystallinity (MBTC films). Optical properties of these films were evaluated in terms of the macroscopic properties of TO. X-ray diffraction studies for morphologic investigation and FTIR spectra for bond properties were also evaluated to define the micro-macro properties relationship.

1. Introduction In recent years, there have been extensive studies on thin films exhibiting high electrical conductivity, as well as high transparency in the visible solar spectrum and high reflectivity in the infrared (IR) region. Transparent metal electrodes of gold, silver, and occasionally copper in very thin film form (approximately 15 nm thick) were used in early optoelectronic devices. Practical applications of transparent conducting coatings increased significantly after the introduction of wide-band-gap semiconductor films. There are three semi-conducting oxides known having transparent electrode properties of practical significance. They are tin oxide, indium oxide, and cadmium stannate. Tin oxide (TO) coatings developed swiftly into commercial products (e.g., Ford low-e glass and N E S A glass), due to their easy manufacturing and excellent properties. The high transparency of the tin oxide conductors, combined with mechanical hardness and good environmental stability, opened up numerous applications for them including electrodes for photovoltaics and photoelectrochemical cells, liquid crystals, electrochromics, electroluminescent cells, as well as heat mirrors for architectural windows, automotive glasses, and for solar collectors, special furnaces, etc. U n d o p e d tin oxide films are semiconductors with a wide band gap (approximately 3.5-4 eV) [1-3], refractive index of approximately 1.9 [3] and a tetragonal rutile structure. M a n y techniques have been developed for the deposition of tin oxide films. A m o n g these chemical vapor deposition [4] (CVD), spray pyrolytic process [5,6], reactive evaporation [7], and reactive sputtering [8] are the commonly used methods. In this study, tin oxide films were deposited by spray deposition and thermal 0165-1633/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

80

H. Demi~ont, K.E. Nietering / Structure and optical properties of tin oxide films

evaporation techniques and the relationships between their optical and structural properties was investigated.

2. Experiment Tin oxide films were deposited by (1) thermal evaporation of SnO 2 (amorphous films) and (2) spray deposition (crystalline films) using monobutyl tin trichloride (MBTC) and dibutyl tin diacetate (DBDA) solutions. Thermal evaporated TO film properties are different than those of sprayed TO films. In this paper, we examined optical properties, structure, and bond properties of TO films. Property dependence on thickness and starting materials are also investigated. The details of the spray-deposited (S-TO) film deposition procedure of T O films are given elsewhere [9]. The experimental apparatus used to deposit S-TO films is shown in fig. 1. Experimental parameters of the spray deposition technique are given in table 1. SnO 2 powder served as the source material for thermal evaporation of TO films (E-TO). A tantalum boat contained the powder during heating. The substrate temperature was less than 80 ° C during deposition. In a residual air pressure of approximately 10 5 Torr the deposition rate was approximately 20 n m / s . Thick films of SnO 2 E-TO exhibit a characteristic yellow color. These films are optically transparent and electrically non-conductive. All samples were simultaneously deposited on (1) Corning 7059 glass for optical investigation and X-ray diffraction studies, and (2) Si wafers for Fourier transform infrared (FTIR) spectra for bond properties. Spectrophotometric transmittance characteristics T versus ~ of spray-deposited TO films measured in the spectral region 0.2 < ~k< 3.2 /~m using a Perkin-Elmer Model Lambda 9 spectrophotometer. F T I R data were taken using a Mattson Instruments Model Cygnus 25 spectrophotometer over the 400-5000 cm-1 region.

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3. Results and discussion

3.1. Optical properties The spectrophotometric transmittance spectrum, T0-X, of Corning 7059 glass, was used to evaluate refractive index spectrum, ns-)~, of the substrate. T-)~ plots of the film-substrate system with interference extrema were used to calculate the refractive index, n()~), the extinction coefficient, k()~), and the thickness, d, of the T O films. The details of this method are given elsewhere [10-12]. T versus )~ and R versus )~ plots of the TO films were used simultaneously to evaluate n()~) and k()~) values in the IR region where interference extrema are suppressed due to the high optical absorption, even in the thick film case. Fig. 2 illustrates the transmittance spectra of some of our typical samples. In this figure, curve (a) shows the T-)~ plot of a thermally evaporated TO film, curves (b) and (c) give the T-)~ plots of spray-deposited T O films using D B D A and M B T C solutions respectively. The main differences between T-)~ plots of evaporated (E-TO) and sprayed (S-TO) films are (1) the threshold wavelength )kth of the opaque region (for X < )'th; T = 0) is greater for E-TO films than for S-TO films (0.32 and 0.27/~m respectively) and (2) the decrease in I R transmittance of S-TO samples. Fig. 2 shows T ÷ ()~) and T-()~) (envelope curves passing on interference extrema of the T-)~ curve) and T0(A ) spectra. The T0-)~ curve was used to evaluate the refractive index, ns()~ ) of the substrate. T+()~): T-(k,) couples were used to calculate n()~) and k()~) values. Film thickness was obtained from T ÷ and T values corresponding to the interference extrema in the very weak absorption region where T0()~) and T+()~) curves are close. The equations used to calculate the optical parameters of a weakly absorbing film from spectrophotometric transmittance characteristics are given elsewhere [9-12]. Optical parameters of each T O sample were calculated from their experimental transmittance spectra only for the interference effect dominant region. The results are shown in figs. 3 and 4. Fig. 3 illustrates n versus )~ plots of the samples. Film thickness and type of solution used to deposit S-TO films are also indicated on the figure. The main feature observed on n-)~ plots of S-TO films are given below. There are two regions on n-)~ plots: (1) a non-dispersive region (dn/d)~ = 0) at )~ > 0.6 /~m for thin TO films and ), > 1.0 ttm for thick ones; and (2) a dispersive region (dn/d)~ < 0) at near UV. E-TO films show similar n-)~ plots to those of thin (d < 200 nm) S-TO films, with a refractive index value greater than 2.00 for both type (MBTC and D B D A ) films. For thicker spray-deposited TO films, the refractive index decreases with increasing thickness, eg., n = 1.85 for a 1.2 /~m thick T O film. Increasing of the void phase in the deposits is responsible for this reduction of the refractive index. As can be seen from fig. 3, the refractive index increases sharply for )~ < 0.35 ~m. Between this dispersive and non-dispersive region, a pronounced minimum at 0.5/~m is observed on n versus )~ plots of thicker T O films (d > 350 nm). The change in refractive index, n (see fig. 3), depends on the film thickness. The thicker the film, the lower n. This minimum is believed to be characteristic for spray-deposited T O films and does not exist on n versus )~ plots of homogeneous,

1-1. Demiryont, K.E. Nietering

/ Structure and optical properties of tin oxide films

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amorphous films of T O and some other oxides [11-13]. This anomaly observed on refractive index spectra of our thicker S-TO but not on thick E-TO films may be expected for multi-component films, the components of which exhibit different dispersion spectra. X-ray diffraction studies on our S-TO films indicate that they

H. Demiryont, K.E. Nietermg / Structure and optical properties of tin oxide films

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consist of multi-oriented microcrystallites of cassiterite. The main crystal orientations of TO films are (200), (211), and (110). Thus, the anomaly observed on the refractive index spectra of crystalline TO films may result from the existence of the multi-oriented TO microcrystallites, each orientation having a different refractive index spectra. Fig. 4 shows a k versus X plot of some typical spray-deposited TO

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H. Demiryont, K.E. Nietering / Structure and optical properties of tin oxide films

85

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Fig. 5. Refractive index versus film thickness plot of TO samples. Refractive index values are determined in the minimum absorption region (X--1.0 ~.m); E-TO: thermally evaporated tin oxide films; S-TO: spray-deposited tin oxide films. films. The main feature of these figures is that k values of MBTC films are smaller than that of D B D A films. A k-X plot of an E-TO film is also shown in fig. 4a. A k-), plot of E-TO film is different than that of S-TO films in the IR region. The abrupt increase in n and k values in the shorter wavelengths is associated with fundamental band gap absorption in these films. The increase in k values at higher wavelengths (), > 1/~m) may be due to free electron contribution to the absorption. This contribution is possible only for crystalline films (S-TO) but cannot exist on amorphous TO (E-TO) films. Fig. 5 illustrates an n versus d plot of TO films. The refractive indices shown in the figure correspond to n values determined in the absorption minimum (k = kmi. at ~ = 1 /~m) region. E-TO and thin S-TO films (d < 200 nm) have n = 2.0. The refractive index of S-TO films decreases with increasing film thickness for thicker films. By assuming spray-deposited tin oxide films are a two-phase system consisting of solid and void phases, according to the Bruggeman model [14] of the effective medium theory, optical dielectric functions, c, provide the volume fractions of inclusions of the medium. In the case of a two-phase system, effective dielectric function % of the medium is related to the dielectric functions of the void phase, ~v, and dielectric phase, %, with volume fractions of qv and 1 - qv, respectively. This model assumes that the embedded particles are spheres. Bruggeman's effective medium model is given by: (v -- (e

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In this equation, ~v = 1, and % ( X ) = [ n ( X ) - i k ( X ) ] 2 is the calculated dielectric function from optical data. The inset of fig. 6 shows the unit cell of the effective medium used to calculate void concentration of TO films. A film thickness versus void concentration plot of spray-deposited TO films is shown in fig. 6. An ~d = 4 value (from n = 2 for X > 500 nm) is used to calculate q~ values from eq. (1). From

86

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Fig. 6. T h i c k n e s s d e p e n d e n c e of void c o n c e n t r a t i o n in s p r a y - d e p o s i t e d tin oxide films. Void c o n c e n t r a tion was c a l c u l a t e d from B r u g g e m a n effective m e d i u m m o d e l a s s u m i n g spherical inclusion of void and dielectric m a t e r i a l phases. U n i t cell of the model is s h o w n in the inset. ~v = dielectric c o n s t a n t of void phase; c d = dielectric c o n s t a n t of oxide; % = dielectric c o n s t a n t of effective m e d i u m ; qv = v o l u m e c o n c e n t r a t i o n of void; 1 - qv v o l u m e c o n c e n t r a t i o n of oxide. Inflection p o i n t of qv versus d plot is a p p r o x i m a t e l y 0.7 V m.

this figure, it is concluded that S-TO films are void free below a thickness of 200 nm. The void concentration increases linearly with increasing thickness for 200 < d < 700 nm region and following an inflection point at 700 nm it saturates below 20%. The absorption coefficient a ( ~ ) ( = 4 ~ r k(~)/~) can be calculated from the spectrophotometrically determined k ( h ) values. Fig. 7 shows the absorption coefficient versus photon energy, E, plots of some T O samples. The spectral dependence of a exhibits three regions: (1) a power law region at high photon energies; (2) an exponential absorption region at intermediate energies; and (3) an inverse power law region at low photon energies. The last region does not exist on E-TO films. If a parabolic density of states is assumed for valence and conduction bands (see fig. 9), one would expect, for photon energy, E, greater than the energy gap Eg, the absorption coefficient to vary as: ~ ( e ) = C ( e - Eg) m E '

(2)

where m is a power factor generally being around 2. This empirical relationship was first derived by Tauc [15] to determine the energy gap, Eg, of indirect band gap materials. For direct transition materials, a(E) is given by: = A(e-

(3)

To decide whether a material has a direct or indirect band gap, (aE) I/m versus E, and ~2 versus E plots of a given film are traced. The optical energy gap is defined by extrapolating the linear portion of the absorption spectrum to ~E = 0.

H. Demiryont, K.E. Nietering / Structure and opticalproperties of tin oxidefilms Free Electron

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Fig. 8 shows the T a u c plot of T O films. As can be seen in this figure, for each curve a obeys a power law giving two straight lines with different slopes in the E >/2.5 eV region. The power factor, m, is 2 for M B T C films and 1.9 for D B D A films. For the value of p h o t o n energy E below approximately 3.5 eV, the data are well fitted to the straight line. For E > 3.5 eV, the values of ( a E ) increase at a faster rate with p h o t o n energy, suggesting the existence o f a second absorption process at the transition energies of approximately Eg = 3.52 eV for M B T C films, Eg = 3.22 eV for D B D A films and Eg = 2.9 eV for E - T O films. Since the a p p a r e n t absorption edge is sensitive to defects and impurities, we choose the highest energy limit observed for the energy gap. The origin of the absorption starting from 2.5 eV to an energy of Eg is not clear at this time. Indirect transitions or the existence of gap states are usually responsible for the multiple slopes in the absorption spectra [11]. F o r an allowed direct transition, a is given by eq. (3). Since the plot of a 2 versus E does not give a straight line (see inset of the fig. 8), d o m i n a n c e of a direct transition was not concluded. Thus, indirect transitions or transitions via low level impurities such as c a r b o n are responsible for this lower energy absorption region. Impurities in our T O

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films are responsible for the low energy Tauc tail, called an impurity gap, E i m = 2.0 eV for S-TO films. The TO films exhibit an exponential dependence of the absorption coefficient on photon energy (see fig. 7, exponential region). This exponential region, called an Urbach region, is associated with the perturbation of band edges (see figs. 7 and 9). The absorption edge, shown in fig. 7, exhibits the characteristic Urbach form [16] a ( E ) = a o exp

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1-1. Demiryont, K.E. Nietering / Structure and optical properties of tin oxide films

89

from the exponential band-tail absorption. The excess optical absorption is given by [17,181

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- E

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where the upper energy limit E~xp corresponds to Urbach region lower edge. The preintegration constant B in eq. (6) is: B = c[ (1 - 2n)2] 2/18n3h2fi)( e l m ) Z ,

(7)

where c is the speed of light, n is the index of refraction of the T O films in the excess absorption region, e and m are the charge and mass of an electron, respectively, h is the Planck constant, and f~j is the oscillator strength of the absorption transition. B is obtained as 8.8 × 1015 cm -2 eV -1 .for TO films, by assuming f,j = 1. The photon energy dependence of the density of states of TO samples was calculated from eq. (6) using aex(E ) data. A value of N s -- 1018 cm -3 was obtained for amorphous thermally evaporated (E-TO) films in the excess absorption region. S-TO films exhibit a photon energy-dependent density of state curve in the excess absorption region: the density of states decreases with increasing photon energy with a power law of N s approximate E -t. The value of t is approximately 7 for MBTC films and approximately 4 for D B D A films (approximately 0 for E-TO films). According to the Drude free electron model, material becomes more conductive, more absorbing above the plasma wavelength, ~p. Thus, in the )~p < A region metallic properties of the material become dominant. The plasma frequency given by:

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(8)

where N¢ is the free-electron density, m* is the effective electron mass, e is the electronic charge, c is the velocity of light, and c o is the dielectric constant of the free space. The h p < h region provides an a - Am relationship, where the free-electron contribution to a is dominant. The density of states (from eq. (6)) and free-electron density (from eq. (8)) are equal to each other at approximately 3/~m where Ne = N s = 5.6 × 10 2o cm -3. In this calculation, we assumed that the electron effective mass is equal to electron rest mass ( m * = m = 9.1 × 10 -31 kg). The h > 3.0 /zm region can be considered as Drude-free electron region where a - h 2 relationship is observed. The free electron model may be applied to S-TO films for h > Xp (hp approximately 1.0 # m or Ep approximately 1.2 eV). In this region, Ns is proportional to approximately E - t with t approximately 7 for MBTC films and t approximately 4 for D B D A films.

90

H. Derni(vont, K.E. Nietering / Structure and opticalproperties of tin oxidefilms

Comparative absorption spectra of various materials and a suggested density of states model are shown in figs. 9a and 9b, respectively. A defect-free, single crystal of a pure ideal insulator exhibits a sharp absorption edge at energy gap Eg, below which the material is transparent (c~ = 0 at E < Eg), and above which is opaque. This abrupt energy gap extends to lower energies, due to the existence of structural defects and impurities within the material. Thus the material is transparent up to the Urbach absorption edge, E,. The existence of deep-level impurities and localized states leads to an excess absorption. The slope and width of Urbach and extended excess absorption regions depends on the corresponding density and location of defect states (see fig. 9b). The dependence of the photon energy on a obeys a power law (see eq. (2)) in the Tauc region. The power factor of this region is experimentally found to be 2 for our MBTC TO films and 1.9 for D B D A films. The density of states of valence and conduction bands, N~ and Nc respectively, exhibit a parabolic dependence (see fig. 9b) in the E > Eg region. The exponential absorption dependence in the Urbach region, E < Eg, is produced by the perturbation of the parabolic density of states at the band edge. The extension of either valence band, or conduction band, or both, into the energy gap in an exponential form is responsible for this Urbach region. High quality optical materials such as thin films of SiO 2, Ta205, TiO2, and AI203 exhibit a sharp Urbach edge. Non-stoichiometric films of these materials exhibit an extended Urbach edge with lower slope parameter and greater width. The bond concentration of a metal oxide system exhibiting different valence states controls the absorption character of the material. As an example, Si in Si0~ exhibits five possible valence states, Si °, Si l+, Si 2+, Si 3+, Si 4-+. The bond concentration of reduced valence states is responsible for excess absorption corresponding to deep level transitions. Amorphous TO films (E-TO) exhibit this type of extended absorption (dashed curve at E < Err region in fig. 9a) spectrum. Decreasing x values in metal oxides, MO~, for x < X~toi~hiom~tri~ leads to: (1) an increase in k values for a given wavelength; (2) expansion of the Urbach region into lower photon energies; and (3) the disappearance of the Urbach edge at E a. Thus, the optical properties of the materials are controlled by their microscopic structural, compositional, and chemical properties. The transition from bounded electron model to free electron model is expected for a transition photon energy, Ep, for a given bond concentration case of a multicomponent material. In the proposed density of states model (see fig. 9b), this transition occurs where the extended density of valence and conduction bands overlap. Tin oxide is one of the few single-component materials of which Ep is located in the near IR region. Above this region, E > Ep, dielectric-like optical properties are dominant; below this region the optical properties are metal-like. 3.2. Morphology and bond properties of tin oxide films

X-ray diffraction measurements were carried out on TO films deposited on Corning glass using a Siemens D500 diffractometer, with Cu K a radiation at 1.54 ,~. The results obtained on type A and type B S-TO films as well as E-TO films are shown in fig. 10. Diffracted intensity versus 20 plots of E-TO film (curve a) shows

H. Demiryont, K.E. Nietering / Structure and optical properties of tin oxide films

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PHOTON ENERGY (ARBITRARY UNITS) Fig. 9. (a) Modifications of the absorption coefficient spectra of various materials. An ideal insulator (single crystal of a high purity material) exhibits an abrupt absorption edge at E = Es. a = 0 in E < Eg region. This absorption edge rolls off into energy gap as an exponential form for actual film case. E a is the new absorption edge. Semiconductors exhibit an absorption tail for E < E a. Metallic impurities in an a m o r p h o u s insulator lead to the same long-wavelength region absorption created by localized impurity levels. A m o r p h o u s T O films (E-TO) exhibit tiffs extended tall absorption region at E < E,. The existence of free electrons (metallic case) leads to IR absorption; E < E t r region. Metals and metal-insulator composites exhibit for this type of absorption spectrum. Our crystalline T O sample also exhibits this kind of free electron absorption region. The slope of this region is related with the crystallinity of the sample. A highly crystalline sample exhibits higher slope than that of poorly crystalline one. For an amorphous film, the free electron region disappears. (b) Density of state model to explain observed absorption spectra mentioned above. Parabolic density of states of valence N v and conduction Nc bands. The transition from valence b a n d to conduction band is responsible for Tauc region. Exponential tailing of density of states produces extension of the abrupt absorption edge. Thus, Urbach region is observed. Slope and energy width of the Urbach region is related with the purity and structural properties of the material. Overlapped density of states creates the free electron region where the energy loss occurs due to the scattering rather than electronic transitions.

92

H. Demirvont K.E. Nietering / Structure and optwal properfles of tin oxide films (x20) (a)

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o "E

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A

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20 Angle (degrees)

Fig. 10. X-ray diffraction pattern obtained on TO samples. (a) Intensity versus 20 plot of amorphous (E-TO) sample. No well-defined peak observed. T - h plot of this sample is shown in fig. 2a. (b) Poorly crystalline S-TO sample exhibits some diffraction peaks with broad full half-widths. T - A plot of this sample is shown in fig. 2b. (c) S-TO sample (MBTC) with high crystallinity. Narrow, intense (200) peak is the characteristic of this sample. T - X plot of this sample is shown in fig. 2c.

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Fig. 11. Fourier transfer infrared (F'TIR) absorption spectra of samples. (a) FTIR spectrum obtained from bare Si wafer. (b) FTIR absorption spectra of Si + TO film system. (c) FTIR difference absorption spectrum of curves b and a for MBTC film. (d) Difference absorption spectrum of a DBDA film. Three characteristic absorption peaks of SnO 2 molecular vibrations are observed in the 550-450 c m - 1 window.

H. Demiryont, K.E. Nietering / Structure and optical properties of tin oxide films

93

that thermally evaporated TO films are amorphous (diffraction lines do not exist). S-TO films ( d = 650 nm for DBDA sample and d = 1200 nm for MBTC sample, respectively) are shown in fig. 10, curves b and c respectively. (Spectrophotometric transmission characteristics of these films are shown in figs. 2a-2c, respectively.) From curves b and c it may be clearly seen that (1) S-TO films are crystalline, (2) they exhibit different crystalline orientations, (3) well-established diffraction lines are dominant in MBTC films, and (4) crystallinity of DBDA films is generally poor. On the other hand, crystallinity of S-TO films was found to be deposition temperature dependent. If the films are deposited at a lower substrate temperature (Ts < 565°C), the TO films obtained had poor crystallinity. (200) and (211) types of crystalline orientations are dominant in the MBTC and DBDA S-TO samples. F T I R spectroscopy was used to examine TO bond properties of samples with films deposited on polished silicon wafers. Fig. 11 illustrates F T I R absorbance spectra of samples for the characteristic window (1000-400 cm-1). Curve a in the figure shows the absorbance of the substrate. Curve b corresponds to F T I R spectra of the film-substrate system. Curves c and d illustrate characteristic vibration modes of MBTC and DBDA films. From the figure it may be clearly seen that the vibration modes of S n - O molecules are within the 450-550 cm -1 window. The degree of crystallinity of the sample seems to affect the molecular vibration of SnO 2.

4. Conclusions

In this study, we reported the relationship between macroscopic and microscopic properties of tin oxide films. Three kinds of tin oxide films were investigated: (1) amorphous films deposited by thermal evaporation of SnO 2 powder; (2) spray-deposited tin oxide films with poor crystallinity (DBDA films); and (3) spray-deposited tin oxide films with high crystallinity (MBTC films). Optical properties of these films were investigated in terms of the macroscopic properties of TO. X-ray diffraction studies for morphologic investigation and F T I R spectra for bond properties were also evaluated to define the micro-macro properties relationship. The physical properties of TO films were found to be structure dependent. Amorphous TO films exhibit dielectric-like properties, i.e., high resistivity (approximately 10 6 ~ cm), high transparency in visible and IR region. Crystalline films exhibit high conductivity or low resistivity (approximately 30 f~/rq) as well as high optical absorption in the IR region. Amorphous samples exhibit a flat T-)~ curve with very low absorption, giving a flat Urbach tail. The energy gap of these films is Eg = 2.9 eV. Crytalline TO exhibits higher energy gap (Eg = 3.55 eV) and an impurity gap Eim at approximately 2.0 eV. The characteristic feature of crystalline films is that they have a Drude free electron region responsible for the IR spectrophotometric characteristics and high conductivity of the samples. In the Drude region, free electron density Ne and density of states Ns provides Ne = Ns = 5.6 x 1020 cm -3 at 3.0/~m.

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H. Demiryont, 1(.E. Nietering / Structure and optical properties of tin oxide films

Acknowledgement The authors wish to thank Dr. James N. Lingscheit of Ford Motor Company, Glass Division for his critical review of this paper.

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