Surface effects on metal/CdTe junctions and CdTe heterojunctions

Abstract

X-ray photoelectron spectroscopy analysis indicates that four different surface compositions result for CdTe single crystals from five different treatments: cleaving (stoichiometric), bromine-in-methanol etch (Te-rich), chromate etch (Te-rich and TeO₂), oxidation in air (TeO₂), hydrogen heat treatment of etched surface (stoichiometric). Major differences of surface treatment are found on Schottky barriers subsequently formed by vacuum evaporation of the metal under practical laboratory conditions. The diode current J₀ is reduced for an oxidised surface, and much increased for an etched surface, compared to that for a cleaved or heat-treated surface. The barrier height of metal/CdTe junctions formed on cleaved or heat-treated surfaces depends on the metal work-function, reaching 0.99 V for an Al/CdTe junction, but no dependence of barrier height on work-function is found for etched surfaces, all showing a barrier height of about 0.6 V. Secondary effects on J₀ due to crystal orientation have also been observed. Effects of surface treatment were explored with ITO/CdTe and CdS/CdTe heterojunction structures, with the indium-tin oxide (ITO) and CdS being deposited by electron beam evaporation. The CdS/CdTe junctions show the same trends as the metal/CdTe junctions. The ITO/CdTe junctions require a low temperature heat treatment in air; the etched surfaces are more resistant to degradation under this heat treatment than the cleaved or heat-treated surfaces, and solar cells were prepared with V_oc=0.81 V, J_sc=20 mA cm^-2, and solar efficiency of 10.5%.