High mobility field effect transistors of SnOx by reactive sputtering of Sn target on glass

Abstract

Transparent oxide semiconductors (TOS) have attracted increasing interest recently. TOS can achieved high optical transparent and flexible electronics, such as large area panel display with ultra high definition and a high frame rate. Typical transparent oxide semiconductors such as ZnO, SnO2, In2O3-ZnO, In2O3?Ga2O3, In2O3?Ga2O3-ZnO (IGZO), ZnO?SnO2 (ZTO) have been widely researched. Among them, the TOS based on Sn have received increasing attention because of the high thermal and chemical stability. The tin oxide have two stable phases : SnO and SnO2. In the case of SnO, the valence of Sn is +2, the crystal structure is based on corner shared SnO4 square pyramid, the bandgap is 2.8 eV, and SnO1+x appears to be a p-type semiconductor with extra oxygens. Ou et al. reported their tin oxide films grown by reactive evaporation method were p-type semiconductors with mobilities of ~0.01 cm2 V-1 s-1, which is likely due to formation of the SnO phase. Moreover, oxide semiconductor-based complementary metal-oxide-semiconductor (CMOS) inverters have been demonstrated by using IGZO as the n-type and SnOx as the p-type channel layers. The other phase of the tin oxides is SnO2, where the valence of Sn is +4, the crystal structure is a rutile based on corner and edge shared SnO6 octahedra, the bandgap is 3.6 eV, and SnO2-x becomes an n-type semiconductor with oxygen vacancies. More importantly, the SnO2 films are reported to exhibit the best thermal and chemical stability. In addition, they are inexpensive to make and have high mechanical durability. There have been several attempts to use SnO2 as the channel layer for thin film transistor (TFT), although the field effect mobility values were rather low in the range of 0.1~2.0 cm2 V-1 s-1. For example, Presley et al. reported a fully transparent SnO2-x TFT with a field effect mobility of 0.8~2.0 cm2 V-1 s-1. However, surprisingly, Sun et al. reported a field effect mobility of 158 cm2V-1s-1 in a Sb-doped SnO2 TFT. No such high field effect mobility have been reported ever since. In this paper, we report on the electrical and structural properties of SnOx thin films. We show our investigation that the electrical properties of SnOx films are very sensitive to changes in a total mixing gas (Ar + O2) pressure. Moreover fully transparent field-effect transistors of SnOx channel made by reactive sputtering of a Sn metal target with various gate insulator on glass substrates are investigated. Then we demonstrate FETs in an MIS heterostructure using Indium tin oxide (ITO) electrodes, Al2O3, HfOx and SiO2 as the gate dielectrics, and SnOx as the channel layer on insulating SnOx buffer layer. Key performance metrics including mobility, current on/off ratio, and subthreshold swing of the SnOx TFT devices fabricated on glass substrate with Sn metal target are better than those of conventional metal oxide TFTs.