Structure and preferred orientations in ion-plated niobium films and correlation of the substrate bias voltage with calculated strain energies

Abstract Ion-plated niobium films deposited onto stainless steel substrates were investigated as a function of film thickness and bias voltage. The resulting structures and preferred orientations were determined by reflection high energy electron diffraction. The patterns indicated the presence of Nb 6 O and possibly β-Nb 2 O 5 and α-Nb 2 O 5 diffraction rings superimposed on the well-defined b.c.c. pattern for niobium at 1.1 Pa sputtering pressure, 0 to -600 V substrate bias voltage, and 1–19 μm film thickness. The development of preferred orientations in the surface regions bore a systematic relationship to the bias voltage but was virtually independent of film thickness in the range investigated. The strong [222] one-degree orientation at zero bias voltage changed to a predominantly [211] orientation at about -300 V and finally to a predominantly [200] orientation at -600 V. It is known that in annealed metallic films, grain growth is at a maximum in those grains with orientations requiring the minimum strain energy increase. In the results reported here, a systematic relationship has been established between the substrate bias voltage (and thus the kinetic energy of the depositing species) and the calculated strain energies in niobium crystals showing that incident ions or atoms with low kinetic energies give rise to film crystal structures with a preferred orientation having a lower strain energy.