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

Abstract

Ionic amorphous oxide semiconductors (IAOSs) are new materials for flexible thin film transistors that exhibit field-effect mobilities of $\sim 10{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ [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, $\mathrm{In}\mathrm{Ga}\mathrm{Zn}{\mathrm{O}}_4$, and edge-sharing structures consisting of In-O polyhedra remain in the amorphous structure. The ${\mathrm{In}}^{3+}$ $5s$ orbitals form an extended state with a band effective mass of $\sim 0.2m_e$ at the conduction band bottom.