Ultra-wide bandgap amorphous oxide semiconductors for NBIS-free thin-film transistors

The transparency of oxide semiconductors is a significant feature that enables the fabrication of fully transparent electronics. Unfortunately, practical transparent electronics using amorphous oxide semiconductors (AOSs) have not yet been realized, owing to significant photo-instabilities of these materials. Previous studies have revealed that the photo-instability can be attributed to sub-gap states (SGSs) near the valence-band maximum (VBM). Thus, it is inferred that the energy difference between the SGSs and the conduction-band minimum must be widened sufficiently in order to make it fully transparent over the entire visible-light region. In this work, we examined the electronic structures of a variety of AOSs and found that their ionization potentials vary greatly, depending upon the specific metal cations. This finding enabled us to increase the optical bandgap by modifying the VBM levels, resulting in a high mobility of 9 cm2/Vs and an ultra-wide bandgap of 3.8 eV for amorphous Zn–Ga–O (a-ZGO). We show that a-ZGO thin-film transistors exhibit no negative-bias illumination-stress instability with no passivation and no light-shielding layer.