An investigation of the growth of bismuth whiskers and nanowires during physical vapour deposition

Bismuth thin films of thickness in the region of 500 nm have been prepared by planar magnetron sputtering onto glass, silicon and GaAs substrates. Electron microscopy of these films reveals that bismuth whiskers grow spontaneously when the substrate is heated to temperatures between 110ºC and 140ºC during deposition and the optimum temperature for such growth is largely independent of substrate. Depositing films under similar conditions using thermal evaporation does not, however, produce the whisker growth. X-ray diffraction has been employed to investigate film texture with temperature and it has been shown that the film crystallites are predominantly [110] and [111] oriented. The [110] orientation of the crystallites dominates at deposition temperatures above 110ºC for sputter deposition and the [111] at lower temperatures. The optimum temperature for whisker growth coincides with the temperature for the change between predominant orientations. While sputter deposition appears to favour films with crystallite orientation of [110], thermal evaporation favours [111] and has a higher change-over temperature. The whiskers that grow from the film emerge at off-normal angles between 43.3º and 69.2º with a mean of 54±3º. The projected length of whiskers on a 500 nm film on a GaAs substrate shows a wide distribution to a maximum of more than 100 μm. The mean projected length for this sample was 16±1 μm and the diameter is around 0.5 μm. Measurements of the electrical properties of the whiskers at room temperature reveals ohmic behaviour with an estimated resistivity of 2.2±0.2 μΩm. Detailed examination of scanning electron micrographs, eliminates all growth mechanisms except tip growth by a non-catalysed vapour-solid/vapour-liquid-solid method. By depositing thinner films it is shown that this spontaneous growth of whiskers offers a route to fabricate high quality bismuth nanowires of lengths exceeding 10 μm.