Deliberate modification of the behavior of n-type cadmium telluride/electrolyte interfaces by surface etching. Removal of fermi level pinning

Single-crystal, n-type CdTe (E_g = 1.4 eV) has been studied with respect to barrier height, E_B, when contacting a liquid electrolyte solution containing a fast, one-electron, outer-sphere redox reagent. We approximate E_B as equal to the photovoltage measured by cyclic voltammetry of various redox couples at illuminated n-CdTe vs. a reversible electrode. n-CdTe surfaces pretreated with an oxidizing etch give an E_B of ∼0.5 V ± 0.1 V in H₂O/0.1 M NaClO₄ or CH₃CN/0.1 M [n-Bu₄N]ClO₄ that is independent of the E_1/2 of the added redox couple. A reducing etch pretreatment gives an E_B in either of the electrolyte solutions that depends on E_1/2 of the redox couple in a manner consistent with a nearly ideal semiconductor. The reduced CdTe exhibits an E_B of up to 0.9 V for a redox couple having E_1/2 near 0.0 V vs. SCE, whereas couples having E_1/2 negative of ∼-1.0 V vs. SCE give zero photovoltage. Auger spectroscopy and X-ray photoelectron spectroscopy (XPS) of the reduced and oxidized surfaces are qualitatively different. The reduced surface exhibits signals for Cd and Te in relative intensitives that are consistent with a close to stoichiometric (1/1) surface. The oxidized surface exhibits little or no detectable Cd signal, and the Te signal is consistent with a thick overlayer of elemental Te. The data are consistent with the conclusion that the CdTe/Te interface is Fermi level pinned (E_B independent of contacting medium); the semimetallic Te overlayer behaves as a metal contacting CdTe, and the CdTe/Te interface energetics are therefore not influenced by changes in the contacting medium.