X-ray diffraction spectra of the TGO films formed by thermal
oxidation at different temperatures between 450 and 850 8C are
presented in Fig. 1. It is found that all the TGO films maintain
rutile structure of SnO2. As the oxidation temperature increases,
diffraction peak intensity enhances, originating the improved
crystalline quality obtained. It is noticed that the relative intensity
of (113) diffraction is apparently higher than that of the standardpowder XRD database, which indicates that the films are (113)
oriented, different from the results of Ji et [17].
The film composition of the DC sputtered films was
determined by EDAX. The Ga/Sn ratio of the TGO film
oxidized at 650 8C (Fig. 2) was 0.26 as determined by EDAX
analysis, which was larger than the Ga/Sn ratio of the target,
which we believe is caused by preferential sputtering rate of Ga
relatively to Sn. The second reason for the higher ratio is the
loss of tin during high temperature oxidation process due to its
high vapor pressure.
3.2. Optical properties
Fig. 3 shows the UV–vis absorption spectra of the TGO films
obtained by thermal oxidation at different temperatures. The
results show that no remarkable changes were found for the
band-gap of the films, except the increase of the absorbance.
The overall transmittance of the films is higher than 85% in the
visible range. The thickness of these films was evaluated by the
absorption spectra and the absorption coefficient of tin oxide
films, which gave an average value of about 200 nm [18]. The
direct optical band-gap was estimated to be 3.8 eV, which was
in the range of reported values (3.6–4.0 eV) [2–6].