Structural and electronic properties of AlN(0001) surface under partial N coverage as determined by ab initio approach

Properties of bare and nitrogen-covered Al-terminated AlN(0001) surface were determined using density functional theory (DFT) calculations. At a low nitrogen coverage, the Fermi level is pinned by Al broken bond states located below conduction band minimum. Adsorption of nitrogen is dissociative with an energy gain of 6.05 eV/molecule at a H3 site creating an overlap with states of three neighboring Al surface atoms. During this adsorption, electrons are transferred from Al broken bond to topmost N adatom states. Accompanying charge transfer depends on the Fermi level. In accordance with electron counting rule (ECR), the DFT results confirm the Fermi level is not pinned at the critical value of nitrogen coverage θ_N(1) = 1/4 monolayer (ML), but it is shifted from an Al-broken bond state to Np_z state. The equilibrium thermodynamic potential of nitrogen in vapor depends drastically on the Fermi level pinning being shifted by about 4 eV for an ECR state at 1/4 ML coverage. For coverage above 1/4 ML, adsorption is molecular with an energy gain of 1.5 eV at a skewed on-top position above an Al surface atom. Electronic states of the admolecule are occupied as in the free molecule, no electron transfer occurs and adsorption of a N₂ molecule does not depend on the Fermi level. The equilibrium pressure of molecular nitrogen above an AlN(0001) surface depends critically on the Fermi level position, being very low and very high for low and high coverage, respectively. From this fact, one can conclude that at typical growth conditions, the Fermi level is not pinned, and the adsorption and incorporation of impurities depend on the position of Fermi level in the bulk.