Local coordination structure and electronic structure of the large electron mobility amorphous oxide semiconductor In-Ga-Zn-O: Experiment andab initiocalculations

Ionic amorphous oxide semiconductors (IAOSs) are new materials for flexible thin film transistors that exhibit field-effect mobilities of ~100.3em0excm²0.2em0exV^-10.2em0exs^-1 [K. Nomura et al., Nature 488, 432 (2004)]. The local coordination structure in an IAOS, In-Ga-Zn-O (a-IGZO), was examined using extended x-ray absorption fine structure analysis combined with ab initio calculations. The short-range ordering and coordination structures in a-IGZO are similar to those in the corresponding crystalline phase, InGaZnO₄, and edge-sharing structures consisting of In-O polyhedra remain in the amorphous structure. The In³⁺ $5s$ orbitals form an extended state with a band effective mass of ~0.2mₑ at the conduction band bottom.