Thermopower Modulation Clarification of the Operating Mechanism in Wide Bandgap BaSnO3 -SrSnO3 Solid-Solution Based Thin Film Transistors

The transparent oxide semiconductor (TOS) with large bandgap (E_g ≈ 4 eV) based thin-film transistors (TFTs) showing both high carrier mobility and UV-visible transparency has attracted increasing attention as a promising component for next generation optoelectronics. Among TOSs, BaSnO₃ -SrSnO₃ solid-solutions (E_g = 3.5-4.2 eV) are good candidates because the single crystal shows very high mobility. However, the TFT performance has not been optimized due to the lack of fundamental knowledge especially the effective thickness (t_eff ) and the carrier effective mass (m*). Here, it is demonstrated that the electric field thermopower (S) modulation method addresses this problem by combining with the standard volume carrier concentration (n_3D ) dependence of S measurements. By comparing the electric field accumulated sheet carrier concentration (n_2D ) and n_3D at same S, it is clarified that the t_eff (n_2D /n_3D ) of the conducting channel becomes thicker with increasing Sr concentration, whereas the m* becomes lighter. The former would be due to the increase of E_g and latter would be due to the enhancement of overlap population of neighboring Sn 5s orbitals. The present analyses technique is useful to experimentally clarify the t_eff and m*, and essentially important to realize advanced TOS-based TFTs showing both high optical transparency and high mobility.