The large size- and electronegativity-mismatch between cation and anion in InN results in the conduction band minimum lying far below the universal branch point energy (E_B) [1]. This property is the common origin of the phenomena investigated here: the proclivity of InN towards donor-type impurities and defects; the quantized electron accumulation layer at the surface of n-type InN; and the inversion layer at the surface of p-type InN. High energy (1-2 MeV) particle irradiation of InN and InGaN alloys has recently been shown to stabilize the Fermi level at E_B, in agreement with the amphoteric defect model [2]. Here, it is shown that even low-energy (400 eV) N-ion bombardment and annealing at 550 K results in the formation of donor-type defects that significantly increase the electron density in the top ~15 nm of the InN. This stabilizes the Fermi level at E_B in the near-surface region, resulting in no surface space-charge layer. Conversely, at the surface of undamaged InN, electron accumulation layers exist. Meanwhile, tunnelling spectroscopy of such native electron accumulation at the surfaces of n-type InN reveals that these layers constitute a quantized two-dimensional (2D) electron gas system. The tunnelling spectra show structures reflecting the 2D electronic subbands in the surface quantum well. The tunnelling spectra are compared with calculations of the quantum well potential, the subband energies, and the charge-profile. Lastly, calculations of the properties of inversion layers at the surfaces of p-type InN are compared with experimental data from Mg-doped InN.

[1] I. Mahboob, T. D. Veal, C. F. McConville, Hai Lu, W. J. Schaff, J. Furthmüller, and F. Bechstedt, Phys. Rev. B 69, 201307(R) (2004).

[2] S. X. Li, K. M. Yu, J. Wu, R. E. Jones, W. Walukiewicz et al., Phys. Rev. B 71, 161201(R) (2005).