Density Functional Theory (DFT) calculations were used to determine adsorption of oxygen at GaN(0001), i.e. Ga-terminated surface. It was shown that at low coverage the oxygen molecule dissociates during adsorption so that the two separate O adatoms are located in H3 sites. Oxygen adatom saturates three Ga broken bonds, modifying their energy by overlap with Op states, so that the three states are degenerate with valence band (VB). The electron counting rule (ECR) indicate on the electron surplus, the excess electrons are donated to other Ga broken bond states, the adsorption energy is equal to 3.74 eV/atom for clean surface. At the first critical coverage θO=[Formula presented]ML, the Fermi level is shifted to conduction band while at the second critical coverage θO=[Formula presented]ML it is shifted down to VBM. The adsorption energy is ΔE[Formula presented]O2 (N)=−3.67eV for θO≤[Formula presented]ML, for θO=[Formula presented]ML and θO=[Formula presented]ML decreases ΔE[Formula presented]O2 (N)=−3.51eV and ΔE[Formula presented]O2 (N)=−3.31eV, respectively, for θO=[Formula presented] the energy jumps to ΔE[Formula presented]O2 (N)=−3.60eV, and for higher coverage θO≥[Formula presented] the energy rapidly decreases to zero and becomes negative The singular point at θO=[Formula presented]ML is essential for stability of oxygen coverage of the surface. The equilibrium pressure at low coverage is 10−5 bar for 1500 K and 10−12 bar 1000 K. It is reduced for higher coverage, due to reduction of the energy and configurational entropy contributions. At the coverage θO=[Formula presented]ML the pressure is reduced by several orders of magnitude, indicating extremely high thermodynamic stability of such coverage, which is responsible for chemical inactivity of GaN(0001) surface observed in experiments, the critical factor for mechano-chemical polishing of the substrates for electronic applications.
All Science Journal Classification (ASJC) codes
- Materials Science(all)
- Condensed Matter Physics
- Mechanics of Materials
- Mechanical Engineering