A few experimentally observed properties of wurtzite InN initially appear to be unusual and controversial. However, InN merely represents a material lying at the extreme of the band-edge endpoints and thus can be explained within existing chemical trends. Both the In and N act to pull the conduction band minimum (CBM) and valence band maximum down with respect to the universal Branch-point energy (E_B), which lies close to the `average' mid-gap position of the semiconductor [1]. In fact the CBM of InN lies ~ 0.9 eV below E_B, resulting in electron accumulation [2], unlike for almost all III-V semiconductors where E_B lies within the Γ-point band gap, except for InAs which also accumulates.

Since the bulk Fermi level of InN generally lies far below E_B, even for degenerate n-type material, donor-like defects are energetically favourable, as described by the amphoteric defect model [3]. Here, it is shown that even low-energy N ion bombardment and annealing can result in the formation of amphoteric defects that stabilise the Fermi level at E_B. In contrast, at surfaces prepared by atomic hydrogen cleaning, the Fermi level is pinned slightly below E_B, allowing the electron accumulation to be facilitated by unoccupied donor-like surface states [4].

Finally, the effect of the electron accumulation on the optoelectronic properties of InN is considered. The electron accumulation results in an overestimation of the bulk electron density. By correcting this overestimation, the Moss-Burstein shift can be described using a band gap of 0.64 eV and a corresponding band-edge effective mass of 0.045 m₀ based on chemical trends [5].

[1] J. Tersoff, Phys. Rev. B, 32, 6968 (1985).
[2] I. Mahboob et al., Phys. Rev. Lett. 92, 036804 (2004).
[3] W. Walukiewicz, Appl. Phys. Lett. 54, 2094 (1989).
[4] L. F. J. Piper et al., submitted to ICNS-6 Conference Proceedings (2005).
[5] L. F. J. Piper et al., Phys. Rev. B, submitted (2005).