InN exhibits a high tendency for unintentional n-type conductivity, both in the bulk and on the surface.  We have used first-principles calculations based on density functional theory to investigate various possible causes for the doping.  Our results indicate that native point defects are unlikely to be the source of conductivity.  Instead, attentions should be focused on unintentional incorporation of impurities.   In particular, hydrogen has a low formation energy in InN and acts as a shallow donor [1].  Interstitial hydrogen is stable in the bond-center configuration, with a H-N stretching vibration at 3050 cm^-1.   Hydrogen can also substitute for nitrogen in InN, bonding equally to the four In nearest neighbors in a multicenter-bond configuration (a highly unusual type of chemical bond).  Substitutional hydrogen, somewhat counterintuitively, is a double donor.  In addition to the bulk conductivity, accumulation of electrons has been almost universally observed on InN surfaces.  While donor impurities adsorbed on the surface could of course contribute to this conductivity, we have proposed that the accumulation layers are an intrinsic property of the material that can be attributed to the fact that on polar surfaces occupied surface states are located above the conduction-band minimum (CBM).  Fermi-level pinning occurs due to occupied surface states above the CBM, for all In/N ratios, thus explaining the observed electron accumulation [2]. Interestingly, we have found an absence of electron accumulation on nonpolar surfaces of InN at moderate In/N ratios, a prediction that has been experimentally confirmed.  Consequences for growth as well as for devices will be discussed.

This work was performed in collaboration with A. Janotti, J. L. Lyons, and D. Segev.

[1] A. Janotti and C. G. Van de Walle, Appl. Phys. Lett. 92, 032104 (2008).

[2] D. Segev and C. G. Van de Walle, EuroPhys. Lett. 76, 306 (2006).

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1. Multiscale modelling of group-III nitride growth