Fig. 2(b) shows the XRD patterns of the SnO2 thin films depending on the growth temperatures. According to the JCPDS card (No. #41-1445), SnO2 thin film exhibited a rutile structure with crystalline planes of (110), (200), and (211). Also, the peak intensity gradually increased with increasing growth temperatures. Thus, an increase in film crystallinity with increases in the growth temperature contributed to the higher film density of the films. It is noted that the weak peak located at 24.62° appeared at higher growth temperatures, and can’t be indexed using JCPDS cards, because there is no information for this peak in any JCPDS cards. However, it is certain that the peak at 24.62° was generated from a rutile SnO2 structure, as shown in previous studies[8] and [15].
The electrical properties as well as corrosion behavior are one of the most important properties for SnO2 thin films in practical applications. First, therefore, the electrical properties of the film deposited at various temperatures was measured by Hall Effect measurement. From a negative sign of the Hall coefficient, it was confirmed that the majority carriers were electrons in the PEALD-SnO2 thin films, regardless of the growth temperatures. Fig. 3(a) shows the resistivity variation of the SnO2 films as a function of growth temperatures. At a lower growth temperature of 150 °C, the film showed a very high electrical resistivity of 5.6 Ω cm. However, a rapid decrease in the electrical resistivity (0.08 Ω cm) of the film was observed as the growth temperature increased to 350 °C. It was contributed by the improvement of the Hall mobility as well as carrier concentrations with increasing growth temperatures. As already shown in previous studies [2], the excess oxygen within the SnO2 thin films can partially fill the oxygen vacancies, which acts as majority electron donors, to reduce the carrier concentration. Therefore, the lower carrier mobility at low growth temperature could be explained by the decreased crystallinity and by the increased grain boundary scattering due to the excess oxygen. However, both carrier mobility and carrier concentration increased to decrease the electrical resistivity of the film, because excess oxygen could be readily removed, and the degree of crystallinity also gradually increased at higher growth temperatures. Second, we evaluated the corrosion behavior of a bare SS316L and SnO2 coated SS316L samples, as shown in Fig. 3(b). The potentiodynamic polarization curves indicated that the corrosion resistance of SS316L was greatly improved by PEALD-SnO2thin films. Also, both Ecorr and icorr gradually improved with the growth temperature. For a quantitative analysis, the Ecorr and icorr values were obtained from the Tafel plot by extrapolating the linear portion of the curve in Fig. 3(b). Those values are also summarized in Table 1. Thus, the best corrosion resistance was obtained when the SnO2thin film was deposited at 350 °C, which resulted from the low Cl impurity and the high film density, as well as improved crystallinity of the SnO2 thin film, which was caused by the high growth temperature [12].