Electronically Controlled Chemical Stability of Compound Semiconductor Surfaces

Effects of a humid environment on the degradation of semiconductors were studied to understand the role of the surface charge on material stability. Two distinctly different semiconductors with the Fermi level stabilization energy E_FS located inside the conduction band (CdO) and valence band (SnTe) were selected, and effects of an exposure to 85 °C and 85% relative humidity conditions on their electrical properties were investigated. Undoped CdO films with bulk Fermi level E_F below E_FS and positively charged surface are very unstable. The stability greatly improves with doping when E_F shifts above E_FS, and the surface becomes negatively charged. This charge-controlled reactivity is further confirmed by the superior stability of undoped p-type SnTe with EF above EFS. These distinct reactivities are explained by the surface attracting either the reactive OH^- or passivating H⁺ ions. The present results have important implications for understanding the interaction of semiconductor surfaces with water or, in general, ionic solutions.