A high energy beam (3-100keV) is directed at the sample surface at a grazing angle. The electrons are diffracted by the crystal structure of the sample and then impinge on a phosphor screen mounted opposite to the electron gun (compare with LEED). The resulting pattern is a series of streaks. The distance between the streaks being an indication of the surface lattice unit cell size. The grazing incidence angle ensures surface specificity despite the high energy of the incident electrons. If a surface is atomically flat, then sharp RHEED patterns are seen. If the surface has a rougher surface, the RHEED pattern is more diffuse. This behaviour can lead to 'RHEED oscillations' as a material is evaporated onto a surface. RHEED is therefore of particular use with MBE.