Composition dependence of intrinsic surface states and Fermi-level pinning at ternary AlxGa1−xN m-plane surfaces.

Growth on nonpolar group III-nitride semiconductor surfaces has been suggested to be a remedy for avoiding detrimental polarization effects. However, the presence of intrinsic surface states within the fundamental bandgap at nonpolar surfaces leads to a Fermi-level pinning during growth, affecting the incorporation of dopants and impurities. This is further complicated by the use of ternary, e.g., Al x Ga 1 − x N layers in device structures. In order to quantify the Fermi-level pinning on ternary group III nitride nonpolar growth surface, the energy position of the group III-derived empty dangling bond surface state at nonpolar Al x Ga 1 − x N (10 1 ¯ 0) surfaces is determined as a function of the Al concentration using cross-sectional scanning tunneling microscopy and spectroscopy. The measurements show that the minimum energy of the empty dangling bond state shifts linearly toward midgap for increasing Al concentration with a slope of ≈ 5 meV/%. These experimental findings are supported by complementary density functional theory calculations. [ABSTRACT FROM AUTHOR]