Thin films of AlCrFeCoNiCu high-entropy alloy by pulsed laser deposition

Pulsed laser deposition has been used to prepare thin films of the high entropy alloy AlCrFeCoNiCu. The 35 nm films were deposited in ultra-high vacuum onto glass at room temperature and above and analysed using X-ray diffraction and X-ray photoelectron spectroscopy. Films deposited at room temperature exhibit a mix of FCC and BCC reflections, the FCC crystallites having size similar to the film thickness, but the BCC crystallites are larger. The intensity of the reflections from both crystal structures reduce with increasing deposition temperature, the fall in BCC commencing at lower temperature than the FCC associated with a reduction of the content of Al and Cu. X-ray photoelectron spectroscopy shows that the films deposited at room temperature are closer to stoichiometry than those at higher temperatures. An important feature of the X-ray photoelectron spectroscopy depth profiles is surface segregation, the outer 3 nm of the high entropy alloy films has higher concentration of Al and, to a lesser extent, Cr

The ordered Heusler alloy Pd2MnIn investigated by photoelectron spectroscopy

The full, ordered, antiferromagnetic Heusler alloy Pd2MnIn has been investigated using synchrotron radiation photoelectron spectroscopy. The Pd 4d Cooper minimum at 130 eV photon energy and the Mn 3p to 3d resonance at 49.5 eV photon energy have been used to determine the partial spectral weights of the Pd 4d and Mn 3d derived states. These partial spectral weights are consistent with published partial density of states calculated using the augmented spherical wave method. A quantitative fit of the resonant profile yielded an asymmetry parameter of 1.6 ± 0.4, indicating delocalization of the filled Mn 3d states

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

Structure and resistivity of bismuth thin films deposited by pulsed DC sputtering

Bismuth thin films have been deposited using pulsed DC magnetron sputter deposition under four deposition conditions, combining powers of 50 and 100 W and argon gas pressures, 2 and 10 mTorr. Estimated deposition rates were between 0.08 and 3.5 nm s−1. The films were examined using scanning electron microscopy, cross-sectioning using a focussed ion beam (FIB), X-ray diffraction and sheet resistance and Hall effect measurement. Room temperature deposition gave a predominant orientational texture of (111) rhombohedral. However, higher film thickness, low sputtering power, high sputtering gas pressure and deposition onto a heated substrate above 125 °C increase the fraction of (110) orientation. FIB cross-sectioning indicates that films deposited at room temperature have an irregular crystalline structure with voids, but those grown at 160 °C are denser with a better polycrystalline structure. Transport measurements indicate a dominance of conduction by electrons, with films deposited at room temperature having high sheet resistance, low sheet resistance being favoured by low sputtering pressure. Deposition at higher temperatures improves the conductivity but increases the Hall coefficient

Normal incidence standing wave analysis of the Cu(1 1 1)- (√3×√3)R 30°-Mn structure: evidence for stacking faults or large-scale alloying?

The interaction of Mn with the Cu(1 1 1) surface has been investigated using the normal incidence X-ray standing wave (NIXSW) method. Analysis of the NIXSW was complicated by the apparent large shift in position of many layers of the Cu atoms relative to their expected bulk positions indicated by a change in the coherent position. Quantitative analysis of the NIXSW shows that there are at least two but possibly many sites for each element in the altered layer

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

Thin films of bismuth of thickness 100nm have been deposited onto Si(111) substrates held at 120°C using magnetron sputter deposition. The three-inch magnetrons were operated at DC powers between 30W and 50W to give deposition rates between 0.03 and 0.09nms-1. Examination of the surface of the thin films using scanning electron microscopy revealed nanowires of diameter 80-120nm and length between 3 and 16μm emerging from the film. An investigation of the effect of deposition rate found little influence on the nanowire diameter, but indicated that some longer nanowires could be found on the film deposited at the lowest rate. A 20nm film grown on glass at 110°C shows a nanowire with dimension of around 50nm. Using a relatively higher rate for the first 25nm followed by a lower rate to 100nm produced some very straight and regular nanorods

Anomalous Nernst effect in Co2MnSi thin films

Separation of the anomalous Nernst and spin Seebeck voltages in bilayer devices is often problematic when both layers are metallic, and the anomalous Nernst effect (ANE) becomes non-negligible. Co2MnSi, a strong candidate for the spin generator in spin Seebeck devices, is a predicted half-metal with 100% spin polarisation at the Fermi energy, however, typically B2 or L21 order is needed to achieve this. We demonstrate the optimisation of thin film growth of Co2MnSi on glass, where choice of deposition and annealing temperature can promote various ordered states. The contribution from the ANE is then investigated to inform futuremeasurements of the spin Seebeck. A maximum ANE coefficient of 0.662 µV K−1 is found for an A2 disordered polycrystalline Co2MnSi film. This value is comparable to ordered Heuslerthin films deposited onto single crystal substrates but obtained at a far lower fabrication temperature and material cost.</div

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

Reduction of secondary electron yield for E-cloud mitigation laser ablation surface engineering

Developing a surface with low Secondary Electron Yield (SEY) is one of the main ways of mitigating electron cloud and beam-induced electron multipacting in high-energy charged particle accelerators. In our previous publications, a low SEY < 0.9 for as-received metal surfaces modified by a nanosecond pulsed laser was reported. In this paper, the SEY of laser-treated blackened copper has been investigated as a function of different laser irradiation parameters. We explore and study the influence of micro- and nano-structures induced by laser surface treatment in air of copper samples as a function of various laser irradiation parameters such as peak power, laser wavelength (λ = 355 nm and 1064 nm), number of pulses per point (scan speed and repetition rate) and fluence, on the SEY. The surface chemical composition was determined by x-ray photoelectron spectroscopy (XPS) which revealed that heating resulted in diffusion of oxygen into the bulk and induced the transformation of CuO to sub-stoichiometric oxide. The surface topography was examined with high resolution scanning electron microscopy (HRSEM) which showed that the laser-treated surfaces are dominated by microstructure grooves and nanostructure features

Low secondary electron yield of laser treated surfaces of copper, aluminium and stainless steel

Reduction of SEY was achieved by surface engineering through laser ablation with a laser operating at • = 355 nm. It was shown that the SEY can be reduced to near or below 1 on copper, aluminium and 316LN stainless steel. The laser treated surfaces show an increased surface resistance, with a wide variation in resistance found de-pending on the exact treatment details. However, a treated copper surface with similar surface resistance to aluminium was produced