Dual electrical behavior of multivalent metal cation-based oxide and its application to thin-film transistors with high mobility and excellent photobias stability

The effect of multivalent metal cations, including vanadium(V) and tin (Sn), on the electrical properties of vanadium-doped zinc tin oxide (VZTO) was investigated in the context of the fabrication of thin-film transistors (TFTs) using a single VZTO film and VZTO/ZTO bilayer as channel layers. The single VZTO TFT did not show any response to the gate voltage (insulator-like behavior). On the other hand, the VZTO/ZTO bilayer TFT exhibited a typical TFT transfer characteristic (semiconducting behavior). X-ray photoelectron spectroscopy revealed that, in contrast to what is commonly true in many oxides, oxygen vacancies (VO) in VZTO did not provide a dominant contribution to the total carrier concentration, because the VO peak area in the single VZTO film was 5.4% and reduced to 4.5% in VZTO/ZTO bilayer. Instead, Sn 3d_5/2 and V 2p_3/2 spectra suggest that the significant reduction in Sn and V ions is strongly related to the insulator-like behavior of the VZTO film. In negative-bias illumination tests and illumination tests with various photon energies, the VZTO/ZTO bilayer TFT had much better stability than the ZTO TFT. This result is attributed to the reduction of donor-like states (V_O) that can be positively ionized by blue and green illumination.