Knowledge of the different bulk transformations that occur on sintering can assist interpretation of certain film properties. However, one should note that the TG/DTA results should not be directly extrapolated to the case of a film, because they were recorded on gel powders and at a slow heating rate, whereas the films prepared in this study were rapidly calcined in a preheated furnace [26] . The weight loss of the dried gel as a function of temperature indicates there are distinct temperature regions for decomposition. The entire thermal effect accompanied by a total weight loss of 70% in the TG curve (Figure 1). DTA curve indicates that the metal complexes decompose exothermally between 300˚C and 500˚C. The crystallization onset temperature is about 430˚C and the peak temperature of crystallization is 470˚C. The exothermic decomposition of the precursor mass resulted in in-situ temperatures which facilitated the formation/crystallization of ZnO phase at relatively lower external temperature.

Figure 2 shows a representative X-ray diffraction spectrum for zinc oxide film with well defined peaks corresponding to the hexagonal crystalline structure. The film is randomly oriented. The indium dopant does not have significant influence on the X-ray diffraction patterns. The doped films give the same diffraction patterns as the undoped samples. We have previously shown [27] that the crystallite orientation can simply and intentionally be tailored by choosing an adequate solvent-chelating ligand combination. We also demonstrated that the film electronic conductivity is independent of the degree of c-axis orientation. Therefore, in the present work we did not use methoxyethanol-monoethanolamine system which yields highly oriented films to avoid the toxicity of methoxyethanol solvent.

Three-dimentional AFM pictures of three ZnO samples and their two-dimentional projections are given in Figure 3. The crystallites are slightly elongated, with a tendency for the In-doped material to show larger grains.

The presence of intergranular voids partly explains the relatively low film densities. The increase in grain size for the doped specimens may be ascribed to the lesser number of nucleation sites in the amorphous oxide layer. This result is in contrast to what is generally observed for tin-doped indium oxide where the grain growth is inhibited by the addition of the dopant.

The measured changes in electrical resistivity, carrier concentration, and Hall mobility for the as-deposited and vacuum annealed IZO thin films are presented in Table 1. The resistivity decreases sharply with increasing In content up to 1 at.% and then increases with further addition of In. The lowest resistivity is, obtainable only under reduced condition. The corresponding carrier concentration and Hall mobility are 1.2 × 10²⁰ cm⁻³ and 11 cm²/V∙s, respectively. Very low mobility values are obtained for the as-deposited films regardless of the doping concentration. Vacuum annealing results in a remarkable increase in both the electronic mobility and the electron concentration which remains far below that calculated assuming that every dissolved indium atom supplies one free electron. In fact, with increasing doping concentration, less than 40% of In incorporated contributes one excess electron to the conduction band. The same trend is observed for the mobility values which are smaller than that of single ZnO crystals (180 cm²/V∙s) [28] . Essentially, the same behavior was observed for zinc oxide films doped with aluminum [26] [27] or boron [29] . Scattering at grain boundaries is sometimes postulated in polycrystalline films with small crystallite size. This mechanism

^aRatio of measured carrier concentration to that calculated under the assumption that every dissolved In atom supplies one free electron: n × 100/4.2 × 10²⁰ C_In.

seems to be less important in our samples since the free path length of electrons was much smaller than the crystallite size. Excess indium above certain critical concentration seems to form neutral defects rather than generating carriers. Therefore, the source of the problem is likely to be connected with the appearance of additional phases. Lee et al. [2] have studied In₂O₃-ZnO system in the form of thin films. They have observed the presence of In₂O₃ and Zn_kIn₂O_k+3 (k = 2 - 5) phases. They attributed the modest Hall mobility values measured for their films to the scattering by these structures. Goyal et al. [30] have studied IZO sprayed films using X-ray photoelectron spectroscopy. They suggested that indium atoms are probably forming an unknown complex. A quantitative analysis [31] of the electronic mobility has shown that the scattering at neutral and ionized defects prevails in the intermediate temperature range as acoustic and optical phonons are not excited. Jin et al. [32] have found that the free electron is predominantly scattered by ionized impurities in ZnO:Al films made by reactive sputtering. On the basis of anion interstitial model, Frank and Köstlin [33] have successfully demonstrated the formation of the following neutral defects for the sprayed ITO films: ionizable (Sn₂O_i") complex, non ionizable (Sn₂O₄)^x complex and an (Sn₂O_i")(Sn₂O₄)^x associate.

The amount of oxygen present while depositing the films by physical methods can be easily controlled by adjusting the oxygen mass-flow rate which is not the case for chemically prepared layers. Therefore, for the latter techniques, the post treatment is rather essential as in the present case. Oxygen rich atmosphere may damage the films by introducing defects because In³⁺ ion pair may attract an additional oxygen atom which plays the role of electron trap. The oxygen content is expected to increase with increasing C_In, which is consistent with In substituting Zn. Thus, the success of the subsequent vacuum annealing step in reactivating the [In] derived carriers will depend on how tightly oxygen is bonded to the dopant. Ku et al. [34] have shown that oxygen content in sputter gas is a critical parameter that decides the electrical properties of the IZO films prepared by radio frequency magnetron sputtering. Many groups [7] [20] [35] [36] reported that the resistivity of IZO films increases as a function of heat treatment time and temperature in oxygen atmosphere. Another important cause for the relatively low carrier densities is the porous nature of the sol-gel processed layers (low refractive indices, Table 2) which facilitates oxygen indiffusion [37] . Although denser films are obtained after post-deposition treatment, their refractive indices remain below that reported for zinc oxide single crystal (2.02) [38] [39] . The reduction in film thickness by annealing is a result of film densification (relatively higher refractive indices).

In view of the preceding discussion, it is clear that the enhanced electrical conductivity after vacuum annealing is due to the cumulative effect of film densification and decrease in the concentration of neutral defects. Since scattering at ionized defects cannot be avoided (high values of n are required), research in the field of transparent conductors should be directed towards exploring new materials with high electronic mobility. Binary, ternary and quaternary oxides based on the association of those currently in use represent a possible alternative. In the last few years, many groups [40] [41] have been studying various oxide combinations.

Figure 4 shows the transmittance of a representative ZnO film doped 0.8 at.% In before and after annealing. The as deposited films has a transmittance in the visible higher than 90% which slightly decreases following

annealing. Here, it is worth to note that the transmission spectra were recorded on both-side coated samples using air as reference. The high transmittance in the luminous range makes ZnO:In of interest as a window coating. The wavelength at which full absorbance is observed for the annealed films has shifted to a shorter-l region, partly because of the Moss-Burstein effect. The degree of the blue shift depended on both indium content and vacuum annealing. A recent report [22] on spin-coated (IZO) films has shown that the values of direct band gap decreased with increase in the In content.

The optical band gap of ZnO can be estimated from the absorption data. At the absorption edge, the film transmittance is, where t is the film thickness [42] . For direct band-to-band transition, the energy dependence of the absorption coefficient can be described by the relation for parabolic bands [43] , where hn is the photon energy and Eg is the energy band gap. The value of for allowed direct transition and 3/2 for forbidden direct transition. Figure 5 shows the photon energy dependence of a² for the as-deposited and the annealed 1 at.% In-doped film. The straight line in this figure was extrapolated to, and the energy axis intercept was taken as the band value. This figure clearly shows a positive shift, which can be expected from the substantial increase in the free carrier concentration as a result of the annealing. Band gap widening has been reported for nonstoichiometric ZnO (Refs. [44] and [45] ) and IZO thin films [46] [47] .

The band gap widening is usually interpreted in terms of Burstein-Moss (BM) shift [48] resulting from the filling up of lower levels in the conduction band. Assuming parabolic bands, this shift can be described for doped material as:

.

With the reduced effective mass

.

The (BM) model predicts an energy-gap shift proportional to.

However, the (BM) band-filling effect that shifts positively the band-edge energy might be partly counteracted by band gap shrinkage due to electron-electron and electron-impurity scattering. The band gap narrowing has been studied in heavily doped ZnO [44] [49] and ITO [50] . According to Wolff [51] , in heavily n-doped semiconductors the conduction band is shifted downward by the energy.

where is the dielectric constant.

The band gap magnitude of polycrystalline zinc oxide depends on the detailed preparation conditions

[50] . Its value is reported to range from 3.28 eV for insulating ZnO to 3.38 eV for highly conductive films. Pure zinc oxide films prepared in this work have Therefore, the absorption edge shift in indium-doped ZnO layers was taken as the difference between the optical gap and this value. Taking, [52] , and [53] , the absorption edge shift can then be evaluated by

.

Figure 6 shows comparison between theory and experimental data for 13 samples which were prepared at the same conditions but with different doping levels. For the samples prepared in this work, the net effect of the above competing mechanisms is to slightly increase the gap. However, the band gap shift remains below that theoretically predicted (dashed line) for all of the prepared thin films. The difference in the absorption shifts between the calculated and measured values may be attributed to other many body effects which lead to further band gap narrowing. Sernelius [49] [54] reported that in polar semiconductor, there is a displacement of charge from one type of atoms to the other. Thus, the host atoms become weakly charged. A moving, charged particle

causes a displacement polarization, which follows the particle in the crystal. An electron attracts the positively charged atoms and repels the negatively charged ones. Effectively the electron is surrounded by a charge cloud, of positive charge. Similarly, a valence and hole is surrounded by a charge cloud of negative charge. The particle and its charge cloud can be considered as a new particle, a polaron. The displacement causes negative energy contributions.

Theoretical models [55] include band gaps shifts due to the blocking of the lowest states in the conduction band, electron interaction, and impurity scattering (the In³⁺ ions, which replace lattice site Zn²⁺, behave as singly charged scatters). For the thin films that were analyzed in the present work, and since more than 60% of the dopant remains electrically inactive, a major part of indium is likely to form neutral impurities especially for non-annealed samples (see Table 1). Therefore, these impurities among other possibly ignored factors which contribute to should not be neglected.