Adsorption of Hydrogen at the GaN(0001̅) Surface: An Ab Initio Study

Ab initio calculations are used to determine the basic\udphysical properties of hydrogen adsorption at the N-terminated\udGaN(0001̅) surface. It was shown that the Fermi level is pinned at the\udvalence band maximum (VBM) for hydrogen coverage θH = 0.75 ML\udabove which the Fermi level is shifted to the conduction band\udminimum (CBM). Application of the electron counting rule (ECR)\udindicates that for θH = 0.75 ML, the Fermi level is not pinned, located\udin the bandgap. Ab initio calculations confirmed that hydrogen\udadsorption energy depends on the Fermi level at the surface as\udpredicted and explained earlier (Krukowski et al. J. Appl. Phys. 2013,\ud114, 143705 and Krukowski et al. J. Appl. Phys. 2013, 115, 043529).\udThe adsorption energy contains electron transfer contribution, below\udθH = 0.75 ML to VBM and above that coverage to CBM; thus it is\udΔEads(H) ≅ 5.8 eV and ΔEads(H) ≅ 3.1 eV for a single H atom for these coverage ranges, respectively. Accordingly, the H2\udequilibrium 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\udθH > 0.75 ML.This work has been partially supported by the Polish Ministry of Science and Higher Education within the SICMAT Project financed under the European Founds for Regional Development (Contract No. UDA-POIG.01.03.01-14-155/09)Jacek Piechot