Oriented growth of niobium and molybdenum on GaAs crystals

Thin layers of Mo and Nb, 100--400 A thick, were deposited onto clean (100) and (1-bar1-bar1-bar)GaAs substrates under ultrahigh vacuum conditions in a molecular-beam epitaxy system, at slow rates and at relatively low temperatures. The microstructure of the films and the orientation relationship with the substrates were determined by in situ reflection high-energy electron diffraction, by transmission electron microscopy, and by grazing-incidence x-ray diffraction. In spite of the large lattice mismatch to GaAs (11% for Mo and 17% for Nb) and the low deposition temperatures(<400 0C), oriented deposits were obtained on both substrates for both metals. Although both metals are body-centered cubic with similar lattice parameters, and are both of similar chemical behavior, they grow differently on GaAs. Molybdenum grows epitaxially in the (111) orientation on both (100) and (1-bar1-bar1-bar)GaAs substrates, whereas niobium grows with the (100) orientation on (100)GaAs, and with no simple orientation on (1-bar1-bar1-bar)GaAs. In both cases, the orientational spread (deduced from the diffraction patterns) is smallest when the lattice planes parallel to the interface have the same symmetry in film and substrate