Stability of sulfur induced reconstructions on InP(0 0 1) surfaces

We have performed quantum chemical calculations based on density functional theory on different surface structures modeling the sulfur treated InP(0 0 1) surface. The models were selected to explain new experimental results from electron diffraction and photoelectron emission, where a phase transition from a low temperature sulfur-rich InP(0 0 1):S (2×1)A-phase to a high temperature sulfur-poor InP(0 0 1):S (2×1)B-phase was observed. The energetic stability of models with different stoichiometry is compared by calculation of thermodynamic phase diagrams.The most probable candidate for the low temperature sulfur-rich structure seems to be a model which involves 0.5 monolayer (ML) sulfur in a PS-mixed-dimer on the surface, 1 ML sulfur inserted on phosphorus sites in the second layer and 0.5 ML sulfur inserted on phosphorus sites in the fourth layer. Additionally, one In-vacancy per (2×1) unit cell in the third layer has to be created to fulfil the electron counting rule. The sulfur-poor surface structure has only one sulfur component (0.5 ML). From the energetic point of view a model with InS-mixed-dimers on the surface is slightly favored in comparison with a model, where the surface sulfur in the InS-dimer is exchanged with phosphorus in the second layer.