Group II- and III-oxides, such as CdO, ZnO, and In₂O₃, have a significant size and electronegativity mismatch between the large cation and the small electronegative anion oxygen: these materials can be described as significantly cation-anion mismatched oxide semiconductors (SCAMOS). Due to this mismatch, their electronic structure is characterised by a particularly low Γ-point conduction band minimum which lies below the charge neutrality level [1,2]. We show that this ultimately determines the charge character of native defects, hydrogen impurities and surface states, unifying bulk and surface electronic properties via a single energy level.

In particular, native defects and hydrogen impurities are both shown to be donors, even in already heavily n-type material [1], in contrast to the behaviour observed in most semiconductors, and explaining the propensity for SCAMOS to exhibit high unintentional conductivity. Further, the formation energy for compensating native defects to form when n-type doping these materials is low compared to in conventional semiconductors and so these materials can be easily doped n-type: SCAMOS can therefore be made highly conducting despite their generally large optical band gaps, allowing them to be used as transparent conducting oxides. These materials are also shown to support a build up of charge (that is, an electron accumulation) at their surfaces [1,2], in contrast to the surface carrier depletion of most semiconductors. This is associated with positively charged donor surface states and a downward bending of the bands at the surface, which further results in a quantization of the conduction band states [3]. This electron accumulation has important implications for metal-SCAMOS contacts.

[1] P. D. C. King et al., Phys. Rev. B 79, 035203 (2009).

[2] P. D. C. King et al., Phys. Rev. Lett. 101, 116808 (2008).

[3] L. F. J. Piper, P. D. C. King et al., Phys. Rev. B 78, 165127 (2008).

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