Metallic surface states induced perturbation in electron affinity of epitaxial AlN films

We report the influence of metallic surface states induced alteration in the electron affinity of epitaxial AlN films. A systematic and in-depth photoemission analysis of epitaxial AlN films grown under different conditions by plasma-assisted molecular beam epitaxy system was performed. The presence of remnant metallic aluminium and native oxide (with different contribution) on the surface (and sub-surface) perturbed the surface chemistry and electronic structure of the grown films. However, no significant change in the morphology of the films was witnessed. It was observed that these metallic states pin the Fermi Level (FL) near valence band edge and lead to the reduction of electron affinity (EA). The metallic states initiate charge transfer from surface to bulk which induces change in the surface and interface dipoles strength. Therefore, the EA of the films varied between 0.6 - 1.0 eV due to the variation in contribution of metallic states and native oxide. However, the surface barrier height (SBH) increased (4.2 - 3.5 eV) adversely due to the availability of donor-like surface states in metallic aluminium rich films.