The photoelectron spectral features and corresponding energy band diagrams of amorphous indium gallium zinc oxide (a-IGZO) thin films were investigated for different surface chemistries. Cleaned-IGZO surface had a deep subgap state (DSS), the binding energy (BE) of which expanded to similar to 1.5 eV. When stored in air, IGZO surface became contaminant passivated and DSS became negligible. Sputtering resulted in phase separation of surface into metallic In and lesser In and Zn containing IGZO. Compared with IGZO, the air-passivated surface and phase-separated surface, respectively, had a more weakly conducting environment and a higher BE spectral shift. (C) 2011 Elsevier B. V. All rights reserved.