Adsorption of Hydrogen at the GaN(000-1) Surface: An Ab Initio Study

Abstract

ABSTRACT: Ab initio calculations are used to determine the basic physical properties of hydrogen adsorption at the N-terminated GaN(0001̅) surface. It was shown that the Fermi level is pinned at the valence band maximum (VBM) for hydrogen coverage θH = 0.75 ML above which the Fermi level is shifted to the conduction band minimum (CBM). Application of the electron counting rule (ECR) indicates that for θH = 0.75 ML, the Fermi level is not pinned, located in the bandgap. Ab initio calculations confirmed that hydrogen adsorption energy depends on the Fermi level at the surface as predicted and explained earlier (Krukowski et al. J. Appl. Phys. 2013, 114, 143705 and Krukowski et al. J. Appl. Phys. 2013, 115, 043529). The adsorption energy contains electron transfer contribution, below θH = 0.75 ML to VBM and above that coverage to CBM; thus it is ΔEads(H) ≅ 5.8 eV and ΔEads(H) ≅ 3.1 eV for a single H atom for these coverage ranges, respectively. Accordingly, the H2 equilibrium pressure at the GaN(0001̅) surface varies from very low for θH < 0.75 ML, about 10−20 bar, to a fraction of a bar for θH > 0.75 ML.