RHEED

Reflection High-Energy Electron Diffraction (RHEED) is an in situ surface characterization technique used in ultrahigh vacuum environments to monitor the growth and structure of crystalline surfaces, especially during epitaxial thin-film deposition such as molecular beam epitaxy (MBE).

An electron beam with keV energies is directed at a surface at a grazing incidence, typically a few tenths to a few degrees above the surface plane. The elastic scattering from the two-dimensional surface lattice produces a diffraction pattern that is observed on a nearby phosphor screen or detector. Because the incidence is grazing, the scattering is effectively two-dimensional, and the resulting patterns are combinations of streaks for flat surfaces and spots for rough or three-dimensional features. The specular beam provides a direct measure of surface order.

RHEED patterns yield information about surface periodicity, reconstruction, and cleanliness. In real time, one can monitor growth rate through oscillations in the specular (RHEED) intensity during layer-by-layer growth; the oscillation period corresponds to the time to complete a monolayer. By analyzing pattern changes, researchers infer lattice constants, surface symmetry, and roughness. RHEED is widely used to control and optimize epitaxial growth, film thickness, and interface quality.

Requirements include ultrahigh vacuum and a robust electron source; care is needed to interpret patterns, which can be ambiguous for complex surfaces. Electron exposure may alter sensitive materials and high-energy electrons can cause damage; patterns depend on incidence angle, surface temperature, and instrumental geometry.