Ionicity effects on compound semiconductor (110) surfaces

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Abstract
Properties of clean and controllably oxidized surfaces of in situ cleaved GaAs, CdTe, ZnTe, ZnSe, and ZnS were probed by synchrotron radiation‐induced photoelectron spectroscopy. We concentrate on variations in submonolayer oxygen adsorption due to changing semiconductor ionicity. A roughly exponential dependence of O2 sticking coefficient on electronegativity difference correlates well with estimates based on other techniques when the molecular state of the adsorbate is considered. Monitoring the O2 interaction by the semiconductor substrate core‐level chemical shift, changes in surface bonding with ionicity are shown by the cation behavior. We also point out that the predominant angular momentum of the intrinsic empty surface states is an important characteristic. Using the dipole selection rules governing photoemission partial‐yield transitions, significant anion s‐like empty surface state density exists on all (110) surfaces studied. Increasing ionicity appears mainly to change the atomic character of cation‐derived empty surface states from p‐ to s‐like. Core‐level transitions to these surface states are strongly influenced by final‐state effects. Our self‐consistent measurements of p‐core exciton‐binding energies show a large increase in surface final‐state effects with increasing ionicity, while bulk conduction‐band‐edge excitons become weaker. The varying bonding requirements and possibly the associated surface‐atom positions provide a unifying concept for understanding these ionicity effects.

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