Michael D. Cropper

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 110 and 140 °C 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 [1 1 0] and [1 1 1] oriented. The [1 1 0] orientation of the crystallites dominates at deposition temperatures above 110 °C for sputter deposition and the [1 1 1] 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 [1 1 0], thermal evaporation favours [1 1 1] and has a higher change-over temperature. The whiskers that grow from the film emerge at off-normal angles between 43.3° and 69.2° with a mean of 54 ± 3°. 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 µΩ m. 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.

Co2MnSi:Pt multilayers for giant spin Seebeck devices

The spin Seebeck effect (SSE) has been widely studied as a potential mechanism for energy harvesting. However, the efficiency of such devices, utilizing the spin thermoelectric effect in thin film form, has not yet reached a sufficient value to make them economically viable. It is therefore imperative that advances are made to investigate means by which the thermoelectric signal can be enhanced. Multilayers of Co2MnSi and Pt are fabricated and characterized in an attempt to observe enhanced voltages. We report that bilayers of ferromagnetic conductor/normal metal (FM/NM) exhibit a Longitudinal SSE response and that repetitive stacking of such bilayers results in an increased thermoelectric voltage that is highly dependent upon the quality of CMS/Pt and Pt/CMS interfaces.

Orientationally textured thin films of WOx deposited by pulsed laser deposition

Pulsed laser deposition from a compound target in an oxygen atmosphere has been used to produce sub-stoichiometric WOx films of 30 nm thickness on Si(100) and SrTiO3(100) substrates. The growth temperature was 500 °C and the pressure of the O2 background was 2.5 × 10−2 mbar. The films have been assessed using x-ray photoelectron spectroscopy, x-ray reflectivity (XRR), x-ray diffraction (XRD) and scanning electron microscopy. The chemical shift of the tungsten 4f states showed that the tungsten was close to fully oxidized. XRR measurements and scanning electron micrographs showed the films on SrTiO3(100) to be much smoother than those on Si(100) which were granular. XRD in the Bragg–Brentano geometry combined with texture analysis showed that the films were textured with the [001], [010], [100] directions normal to the surface. The films on SrTiO3(100) were found to be biaxially textured with the film directions aligning with those in the substrate. The nature of the texture was sensitive to the laser fluence used. Higher fluence promoted [001] texture whereas lower fluence promoted [010] and [100]. Intermediate fluences produced smooth, highly ordered films with biaxial texture. Investigations using the laser repetition rate indicate that the mechanism for the difference is the overall deposition rate, which is affected by fluence. On Si(100) the films were rougher and exhibited only uniaxial texture.

Optimisation of Co 2 MnSi thin films and multilayers for spin Seebeck devices

The spin Seebeck effect is defined as the generation of a pure spin current (J<inf>s</inf>) when a magnetised material is subjected to a temperature gradient (ΔT).