LEEM Phase Contrast

Abstract

Contrast in low energy electron microscopy (LEEM) originating in the phase of the imaging electron wave is discussed. A wave-optical model is reviewed in which LEEM step contrast is calculated as the interference of the Fresnel diffracted waves from terrace edges which meet at a step. Model predictions which take into account instrumental resolution and beam coherence effects are compared to experimental observations of steps on the W(110) and Si(111) surfaces. Most importantly, this work allows for the routine identification of the step sense with LEEM by inspection. A quantum-mechanical Kronig–Penney model is also presented to explain the quantum size effect (QSE) in electron reflectivity from thin films, which underlies LEEM quantum size contrast. Model predictions reproduce the non-free electron dispersion which is observed in experiment for Cu films on the W(110)surface. This model also serves to demonstrate the relationship between electron reflectivity and electron band structure at a fundamental level.