Tin monoxide (SnOx) has attracted much attention as a promising p-type oxide material. Its behavior is in contrast to SnO2-x (4+), which is highly degenerated n-type semiconductor with high visible transparency by doping F, Sb, or Ta [1,2]. The p-n junctions can be easily realized between SnOx and SnO2-x by reactive sputtering, resulting in novel optoelectronic applications. Because p-type SnOx films still has an obviously low conductivity compared with n-type SnO2-x films, it is crucial to find new way to improve the conductivity of the p-type SnOx films. However, the origin of p-type conductivity of SnOx is still a subject of debate [3, 4], and lack of the direct experimental demonstration. In this study, we firstly deposited various SnOx film with the highly precise control on the stoichiometric compositions, and then investigated the local structure of SnOx film to determine the possible defect structure using the extend X-ray absorption fine structure (EXAFS).
In this study, p-type SnOx films were deposited by reactive sputtering (RM400, FEP) using a Sn metal target, in which noble gases (Ar or Ne) were used as the sputtering gas and O2 was used as the reactive gas. An impedance control system with plasma control unit was used to precisely adjust the inlet of oxygen, and hence control the oxidation state of target surface. With the impedance control system, each cathode voltage corresponded to only one value of the O2 flow ratio, and the typical transition region in reactive sputtering disappeared. X-ray diffraction patterns showed that the p-type SnO films were successfully fabricated using such system. The best hall mobility and hall density of the film was 3.38 cm2/Vs and 1.12×1018 cm-3, respectively, when Ar gas was used as the sputtering gas. Whereas, the best hall mobility and hall density of the films were 1.9 cm2/Vs and 1.05×1018 cm-3, respectively, when Ne gas was used as the sputtering gas.
It is found that SnOx films have the high hole mobility in a relatively reduced state. Such reduced state is considered to exist a lot of defects. In order to understand the relationship between the defect structure and the conduction mechanism in SnOx, we deposited the p-type SnOx films with the different carrier densities, and studied their defect structure by investigating the local structure around Sn atoms using EXAFS measurements. We discussed the relationship between the defect structure and the conduction mechanism, and determined the orign of p-type conductivity in SnOx films.
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