Direct observation of the band gap shrinkage in amorphous In ₂O₃-ZnO thin films

We investigated the dependence of valence- and core-level photoemission spectra of amorphous In₂O₃-ZnO (a-IZO) films on carrier density by using hard x-ray photoemission spectroscopy (h ν 8000 eV). The valence band edge distinctly shifts toward high binding energy with the increase in carrier density from 0.80 to 3.96 × 10 20 cm - 3, and an abrupt jump for the shift of the valence band edge from high to low binding energy occurs at a carrier density of 4.76 × 10 20 cm - 3. After considering the effect of nonparabolic bandstructure, the shifts are still less than the width of the occupied conduction band, providing direct evidence for the band gap shrinkage. Our calculation results indicate that the contribution of the band gap shrinkage increases as the carrier density increases, which accords with the observations in doped conducting crystal materials, such as Sn doped In₂O ₃. Moreover, it is found that the conduction electrons of a-IZO films are strongly perturbed by the ionization of core levels, which leads to obvious plasmon satellites in core photoemission spectra lines.