Density Functional Study of the First Wetting Layer on the GaN (0001) Surface

The first wetting layer on the GaN (0001) surface has been investigated at the level of density. functional theory. Many water adsorption models. have been analyzed and it is observed that the number of water molecules that can be dissociated is limited to 0.375 ML of adsorption sites; further water dissociation will cost energy penalty. The coverage of hydroxyl groups on surface could be up to 0.75 ML instead. It is also observed that the additional charge on the surface will totally transfer to water adsorbates when the water dissociation number is 0.375 ML. Meanwhile, the surface states will disappear when all the adsorption sites are occupied by dissociated or intact water. All of these phenomena can be attributed to the electron counting rule of III-V semiconductor growth theory. We suggest that the electron counting rule, could be generally applied to the water adsorption on polar III-V and II-VI semiconductor surfaces.