Hydroxyl Group Adsorption on GaN (0001) Surface: First Principles and XPS Studies

In this work, density functional theory (DFT) calculations and x-ray photoelectron spectroscopy (XPS) were carried out to investigate the hydroxyl groups on a wurtzite GaN (0001) surface. Surface treatments of GaN with piranha and HCl-based solutions were studied via XPS, and peak shifts in the Ga 2p and O 1s XPS spectra were caused by the signal change resulting from surface hydroxyl groups. Further DFT study revealed that the adsorption of hydroxyl groups is more favourable near the centre location than near gallium atoms. To investigate the thermodynamic stability of hydroxyl groups under different coverages, a surface phase diagram of hydroxyl group adsorption on the GaN (0001) surface was constructed over a coverage range of 1/6-1 monolayer (ML). The results showed that a high hydroxyl group coverage is more likely to be present on the GaN surface. The energy barrier for split hydroxyl groups is 1.41eV. Therefore, the hydroxyl groups can be stable at room temperature. These results provide a systematic explanation of the adsorption between the hydroxyl groups and the GaN (0001) surface.