Sulphur-induced electron accumulation on InAs: a comparison of the (0 0 1) and (1 1 1)B surfaces

The clean and sulphur-treated (0 0 1) and (1 1 1)B surfaces of InAs have been studied using high-resolution electron-energy-loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS). The diffuse (2 × 4) low energy electron diffraction pattern observed on the atomic hydrogen cleaned InAs(0 0 1) surface disappeared after S-dosing from an electrochemical cell. Upon annealing at 350 °C, a sharp (2 × 1) reconstruction is observed which changes to a disordered (4 × 2) pattern following sulphur desorption at 400 °C. The clean InAs(1 1 1)B surface exhibits a (1 × 1) structure which also disappears upon S-dosing and returns after annealing at 450 °C. XPS from the (1 1 1)B surface indicates that sulphur is bonded to both indium and arsenic on the as-dosed surfaces, whilst it was only bonded to indium on the (0 0 1) surface. Annealing the (1 1 1)B surface to 200 °C removes all the S–As bonding, and annealing above 300 °C decreases the In:As ratio on both surfaces as a result of sulphur substitution for arsenic atoms. Semi-classical dielectric theory simulations of the HREELS spectra, together with solutions of the Poisson equation, have been used to determine the electron accumulation and band bending profiles as a function of surface treatment. Both the (0 0 1) and (1 1 1)B surfaces exhibit a dramatic increase in the observed downwards band bending and the surface state density after annealing the sulphur-terminated surfaces. The downwards band bending reaches 600 meV, with the surface state density increased by as much as an order of magnitude compared with the clean surfaces. The surface donor density corresponds to approximately 1% of the sulphur atoms at the surface, indicating that sulphur-induced ionised surface states are associated with surface defects.