Abstract

The electron affinity of a semiconductor relates the band structure of the semiconductor at the surface to the vacuum ground state near the surface. Wide bandgap semiconductors have the possibility of exhibiting a negative electron affinity (NEA) meaning that electrons in the conduction band are not bound by the surface. The surface conditions are shown to be of critical importance in obtaining a negative electron affinity. In this study, angle resolved UV-photoemission spectroscopy (ARUPS) is used to detect the effect. Surface terminations ranging from adsorbates, metals and insulators are shown to induce an NEA on diamond. In particular, it is demonstrated that hydrogen termination can induce a NEA on (111), (100), (110) and (311) surfaces. Theoretical studies have indicated a basis for the observed NEA effects. In contrast, as-prepared surfaces with oxygen termination often exhibit a positive electron affinity. It is shown that thin metal layers of Ti, Ni, Cu, and Co can induce a NEA on the (111) surface, and new evidence indicates NEA on the (100) surface. NEA materials could prove useful in cold cathode applications, and potential and limitations are discussed.