  The transport properties and optical transmittance and absorption spectra for
the nostoichiometric amorphous Indium Gallium Zinc Oxide (a-IGZO) films with
Gallium and Zinc deficiencies are investigated. The resistivity and carrier
concentration variation with temperature both reveal that the films possess
degenerate semiconductor (or metal) characteristics. The thermopower is
negative and decreases linearly with decreasing temperature, indicating the
electron diffusion thermopower governs the thermal transport process in each
film. Using free-electron-like model, we extracted the electron effective mass,
which is about three times as large as that of the stoichiometric one and
increases with increasing carrier (electron) concentration. Neglecting the
variation in the energy with the wavevector near the valence band maximum and
using the free-electron-like model, we also obtained the electron effective
mass via the optical absorption spectra measurement. The magnitude of the
effective mass obtained via optical spectra measurement is comparable to that
obtained via thermopower measurement for each film. Our results strongly
suggest that the nostoichiometric a-IGZO films possess free-electron-like
pseudo-energy-bandstructure.
