Electron beam evaporation of oriented Nb films onto GaAs crystals in ultrahigh vacuum

Thin layers of Nb, 100-400 Å thick, were grown by electron beam evaporation on (100)GaAs substrates in a molecular beam epitaxy system. The crystallographic relationship between deposit and substrate was monitored in situ by reflection high-energy electron diffraction, and after deposition by transmission electron microscopy and grazing-incidence x-ray diffraction. In spite of the large lattice mismatch (17%) and the low deposition temperature (40-400°C), a quite well oriented deposit with the orientation (100)Nb∥(100)GaAs and [001]Nb∥[011]GaAs was obtained for a substrate temperature of ∼170°C. Changing the substrate temperature from the optimum value of ∼170°C in either direction resulted in a gradual deterioration of the epitaxy.