Electronic structure and transport properties in the transparent amorphous oxide semiconductor2CdO⋅GeO2

An amorphous $2\mathrm{CdO}\ensuremath{\cdot}{\mathrm{GeO}}_{2}$ thin film with a band gap of 3.4 eV can be converted from an insulator (conductivity $\ensuremath{\sim}{10}^{\ensuremath{-}9}{\mathrm{S}\mathrm{}\mathrm{cm}}^{\ensuremath{-}1})$ into a degenerate semiconductor $(\ensuremath{\sim}{10}^{2}{\mathrm{S}\mathrm{}\mathrm{cm}}^{\ensuremath{-}1})$ by carrier doping with ion implantation without significant loss in visible transparency. An interesting feature of this material's transport property is its Hall mobility: a pn sign anomaly, which is commonly observed for amorphous semiconductors, is not seen. The estimated Hall mobility is \ensuremath{\sim}10 ${\mathrm{cm}}^{2}$ ${\mathrm{V}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$, which is larger by several orders of magnitude than that of conventional amorphous semiconductors, and is comparable to that in the polycrystalline form. The electronic structure was investigated to understand these features through direct observation of the density of states (DOS) of the conduction band by inverse-photoelectron spectroscopy and molecular orbital calculation of the DOS for a model cluster justified by x-ray structural analysis combined with molecular dynamics and reverse Monte Carlo simulations. Although x-ray structural analysis revealed that disorder in the correlation between Cd-Cd ions was distinctly seen in the amorphous state, the DOS of the conduction band bottom was almost the same in the crystalline and amorphous states. Cluster calculations demonstrated that the bottom of the conduction band is primarily composed of Cd $5s$ orbitals. The characteristic transport properties in this material, such as large electron mobility, may be understood by considering that the magnitude of overlap between the $5s$ orbitals of neighboring ${\mathrm{Cd}}^{2+}$ is large and comparable to that in the crystal and insensitive to the structural randomness inherent to amorphous states.