Electronic properties of post transition metal oxide semiconductor surfaces

Metal oxides such as ZnO, Ga2O3, CdO, In2O3, and SnO2 exhibit high degree of transparency to visible light while supporting high levels electrical conductivity. The causes of the conductivity and the role played by the surface are current topics of research. This chapter presents a systematic study of the electronic structure and electrical properties of these post-transition metal oxides (PTMO) using a combination of X-ray photoelectron spectroscopy, angle-resolved photoelectron spectroscopy, Hall effect, infrared reflectivity, and optical absorption spectroscopy measurements. Evidence of surface electron accumulation in these PTMO is presented. It is found that for CdO and In2O3, electron accumulation is observed even in the absence of extremely high doping levels. The results also indicate that despite the strong tendency to exhibit surface electron accumulation, these materials can also exhibit an electron depletion layer under the appropriate surface stoichiometry conditions or when certain anions are adsorbed. The proclivity towards surface electron accumulation shown by the PTMOs is discussed in terms of bulk band structure, surface states, and the position of their band edges in an absolute energy scale. The electronic properties of thin films and bulk crystals of the PTMO surfaces also provide information vital for the interpretation of conductivity measurements of PTMO nanostructures, which are often dominated by surface effects.