Adsorption of gallium on GaN(0001) surface in ammonia-rich conditions: A new effect associated with the Fermi level position

• Simulation of adsorption at GaN(0001) surface requires 4×4 slabs.
• Adsorption of gallium depends whether Fermi level is pinned at surface.
• At nonpinned Fermi level energy of adsorption depends on doping in the bulk.
• Energy of Ga adsoprtion is by 2 eV higher for p-type GaN(0001) surface.

Density functional theory (DFT) calculations were used to study GaN(0001) surface covered with NH3 admolecules and NH2 radicals, corresponding to physical conditions during GaN growth by hydride vapor phase epitaxy (HVPE) and metalorganic vapor phase epitaxy (MOVPE). Using larger representation of the surface i.e. slabs of lateral size 4×4, the effect of the doping was examined. It is shown that for specific surface coverage the electron counting (EC) rule is fulfilled so that the pinning of the Fermi level by surface states and band bending disappears. In this case, according to Krukowski et al. (2013) [14], the doping of the semiconductor (n- or p-type) and the related Fermi level are extremely important for stability of the surface and the adsorption/desorption processes. The difference in adsorption energies of gallium atoms at n- and p-type GaN(0001) surface exceeds the energy gap. This effect is observed in a narrow range of surface coverage, therefore cannot be detected in the calculations using small systems i.e. 2×2 slabs. This new phenomenon may be crucial for the growth of GaN and the incorporation of dopants and impurities into semiconductor crystals.