Electric Field Thermopower Modulation Analyses of Silicon-Doped Tin Oxide-Based Thin-Film Transistors

Solution-processed SnO2-based thin-film transistors (TFTs) have attracted significant attention due to their low cost, low toxicity, and the high abundance of SnO2. However, SnO2-TFTs suffer from large negative threshold voltage as well as low on/off ratios because of high residual carrier electrons from oxygen vacancies. It is known that doping elements, such as Si, that form strong chemical bonding with oxygen suppresses oxygen vacancy formation. Here, we fabricated and characterized Si-doped SnO2 (SixSnyO) TFTs. In SixSnyO-TFTs, we found that increasing the Sn molarity leads to an enhancement in field-effect mobility while inducing a negative threshold voltage shift. To clarify the origin of this mechanism, we measured the effective channel thickness of SixSnyO-TFTs by electric field thermopower modulation analyses. The effective mass (m*) of the SixSnyO films slightly decreased with increasing Sn molarity and was approximately 0.15 m0, suggesting that an increase in Sn molarity improves the metal–oxygen network in the film. The effective channel thickness, which corresponds to the accumulation layer thickness, increased with rising sheet carrier concentration at low Sn molarity. On the other hand, it decreased with increasing sheet carrier concentration at high Sn molarity. This suggests that the carrier transport mechanism varies depending on Sn molarity. These findings provide guidance in developing high-mobility and reliable SixSnyO-TFTs through appropriate control of the Sn molarity in SixSnyO.