LEEM phase contrast

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