HEXAGONAL SURFACE-STRUCTURE DURING THE FIRST STAGES OF NIOBIUM GROWTHON SAPPHIRE (11(2)OVER-BAR0)

A detailed reflection high-energy electron diffraction study of the fi rst stages of the niobium growth on (11 (2) over bar 0)(s) sapphire is presented for several substrate temperatures. It is shown that the ni obium film exhibits an hexagonal surface structure when the deposited thickness is smaller than a critical value, which, depending on the su bstrate temperature, varies between 5 and 15 Angstrom. For thicknesses larger than this critical thickness, the surface hexagonal structure relaxes to the (110) bcc niobium structure. The hexagonal surface stru cture is observed for high substrate temperatures (820-410 degrees C) but does not appear when the substrate temperature is 270 degrees C. T he epitaxial relationships between the substrate, the surface hexagona l structure of niobium and the cubic niobium phase are presented.