Theory and Observation of Intrinsic Surface States on Ionic Crystals

A comprehensive theory of surface states on ionic crystals has been derived using Seitz's approach to bulk crystal energy states as a starting point. Surface ions are considered equivalent to bulk ions except for their reduced Madelung constant. The relationship of surface levels to bulk bandgap is expressed as a function of surface geometry and bulk material properties, and it is calculated using classical electrostatics for many surfaces of 46 halides, oxides, and sulfides. Symmetry arguments show that only for a checkerboard-like surface are the surface states symmetrically disposed about midgap. Numerical calculations show that one electron trap is formed from each surface cation and one hole trap for each surface anion; trap depths should be deepest for HgS, CdS, and ZnS; and for a $\left(11\mathrm{}\overline{2}0\right)$ CdS surface the trap depths should be 0.2 to 0.4 eV, depending on the effective ionic charge assumed of 0.5 to 2.0. Intrinsic surface states were detected on the $\left(11\mathrm{}\overline{2}0\right)$ surfaces of vapor phase grown insulating CdS single crystal platelets. Photoconductivity experiments (response time and thermally stimulated currents) indicate that these surface states function as traps for the photocarriers (electrons and holes) from the bulk.