28.4: Characterization of Negative Electron Affinity for MgO Film by Ultraviolet Photoelectron Spectroscopy

Abstract

Many attempts to investigate electronic band structures of various metal oxides such as magnesium oxide (MgO) and calcium oxide (CaO) have been made for a long time, but exact electronic band structures of metal oxides have not been achieved successfully so far due to surface charging during measurement analysis except for ultrathin film. In this study, ultraviolet photoelectron spectroscopy (UPS) is introduced and electronic band structure for MgO film has been obtained by using a unique way of heating the film, which suppresses the surface charging of MgO film through improvement of electrical conductivity during measurement. Negative electron affinity has been characterized for MgO film in the measured UPS spectrum. The characteristics of negative electron affinity can be understood as originating from image-potential states on MgO surface.

Negative electron affinity opens quantum well in MgO layers on Ag(100)

“…. -1.0 eV) [2]. However, from the other peaks and the band structure calculation in figure 3 we would estimate an electron affinity of about −1.0 eV.…”

“…In this work we studied MgO films on an Ag(100) substrate which grows in well-ordered epitaxial layers [7][8][9][10]. In photoemission spectra an intense peak just above the low-energy cutoff is a characteristic feature for surfaces with negative electron affinity [2,11,12]. For MgO films on Ag(100) we found several peaks which shifted with increasing film thickness to * Author to whom any correspondence should be addressed.…”

“…Magnesium oxide is a widely studied material with many applications. Recently it has attracted interest for photocathodes due to its high quantum efficiency [1] and in display technology [2]. While the occupied valence bands are well studied by photoemission [3] the unoccupied conduction bands are largely unexplored.…”

“…. -1.0 eV) [2] for a 100 nm thick film which was not characterized by surfacescience techniques and was charging during the measurements. Theoretical calculations obtained values of −0.4 eV [5] and −0.2 eV [6] for the electron affinity.…”