Origins of Fermi-level pinning on and polar and nonpolar surfacesD. Segev and C. G. Van de Walle
Materials Department, University of California - Santa Barbara, CA 93106-5050, USA
received 12 July 2006; accepted in final form 18 August 2006
published online 8 September 2006
Using band structure and total energy methods, we study the atomic and electronic structures of the polar (+c and -c plane) and nonpolar (a and m plane) surfaces of GaN and InN. We identify two distinct microscopic origins for Fermi-level pinning on GaN and InN, depending on surface stoichiometry and surface polarity. At moderate Ga/N ratios unoccupied gallium dangling bonds pin the Fermi level on n-type GaN at 0.5-0.7 below the conduction-band minimum. Under highly Ga-rich conditions metallic Ga adlayers lead to Fermi-level pinning at 1.8 above the valence-band maximum. We also explain the source of the intrinsic electron accumulation that has been universally observed on polar InN surfaces. It is caused by In-In bonds leading to occupied surface states above the conduction-band minimum. We predict that such a charge accumulation will be absent on the nonpolar surfaces of InN, when prepared under specific conditions.
73.20.At - Surface states, band structure, electron density of states.
68.35.Bs - Structure of clean surfaces (reconstruction).
© EDP Sciences 2006