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

Abstract

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 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.2

Spontaneous growth of bismuth nanowires on a sputter-deposited thin bismuth film

“…For films grown on glass substrates it has also been pointed out that the thermal expansion coefficient of bismuth greatly exceeds that of glass so that cooling the sample from growth temperature to room temperature will not stress the film, making a stress-driven growth mechanism less likely [25]. The intriguing possibility that the whiskers grow in situ during film growth would be consistent with the observation that they grow on several different substrates [23].…”

“…Recently there have been several reports of the spontaneous growth of bismuth nanowires and whiskers from sputter-deposited thin films without the requirement for post-deposition heating [22,23,24,25,26]. This PVD method requires the deposition of bismuth using magnetron sputtering at a low-rate onto a substrate that is maintained at an elevated temperature.…”

“…With low growth rates they emerge at angles around 54° off-normal and appear to be promoted by the existence of [110] R , [012] H orientational texture in the film [23]. It has been suggested [25] that the growth direction precludes growth from the more dominant [001] H ,…”

“…The growth mechanism of these whiskers is not known, but it has been proposed that they could grow around a spiral dislocation [22], or be the result of a catalyst free diffusion driven mechanism [23] such as asymmetric corner-crossing [26]. It has been reported that the film growth time [25] or growth rate [24] affects the diameter of the whiskers.…”

“…We have recently reported the growth of 500 nm diameter whiskers on 500 nm bismuth thin films [23] and indicated that nanowires may also be produced given a suitably thin film. Here we report on this phenomenon further, this time focussing on the production of nanowire diameter of order 100 nm or less.…”