A New Look at the Growth of Thin Films of Nb-Sn

Abstract

A15 films grown by vapor deposition onto heated substrates have been generally observed to form in the equilibrium phase. However, vapor deposition is not in fact an equilibrium process and effects of the layer-by-layer growth may be seen. Two such effects will be described here. Both are related to the presence of grains in the growing film, itself a non-equilibrium effect. The first effect to be described is a grain overgrowth process resulting in cone-shaped rather than columnar grains in polycrystalline films grown at 800°C, and single-crystal films grown from a polycrystalline region near the substrate at higher deposition temperatures. The second is a segregation effect believed to be previously unobserved. It is related to surface segregation but dependent on the low bulk mobility and high surface mobility characteristic of materials grown below half their melting temperature. This effect we believe causes certain polycrystalline films to become compositionally inhomogeneous.