In MBE, which technique observes growth in situ by directing an electron beam at the surface at a grazing angle and examining the diffraction pattern?

Answer: B. RHEED (reflection high-energy electron diffraction)

RHEED directs a high-energy electron beam at the growing surface at a grazing angle and observes the diffraction pattern on a fluorescent screen. It reveals surface flatness and crystal structure, and because its intensity oscillates with each atomic layer grown, the number of layers can be counted in situ. It is an observation method made possible by the ultra-high vacuum of MBE.