Observation of extrinsic surface states on (112̄0) CdS

Surface states have been detected by surface photovoltage spectroscopy on (112̄0) CdS surfaces subjected to various treatments in UHV and studied by Auger electron spectroscopy and LEED. All surface electronic features can be related to chemical contamination or lattice nonstoichiometry. Energy level spectra of air-exposed CdS exhibit a set of discrete states due to adsorption of C, O, and Cl. Ion bombardment generates a pair of states 2.35 eV and ~0.8 eV above the valence band edge due to S interstitials and vacancies, respectively. Oxygen adsorption produces a broad continuum of states. Changes in surface atomic order show no direct effect on these electronic features. No intrinsic surface states, filled or empty, are observed by surface photovoltage spectroscopy on clean, stoichiometric (112̄0) faces of CdS.