title: Dielectrophoresis Control of Semiconductor Nanowires for Sensing Technology
Creators: name: Laumier, Siriny
Publisher: name: University of Liverpool Repository
Year: 2023

description: Semiconductor nanowires (NWs) synthesis successes have given keys to unprecedented nano-scale sensitivity opening up new opportunities in device applications. NWs’ potential relies upon the possibility of engineering and modifying properties such as sensitivity and carrier transport, by tailoring the NWs’ morphology and conductivity. New challenges have taken place with the downscaling of electronics for NWs integration and assembly techniques. Amongst a large variety of integration techniques, dielectrophoresis (DEP) is a powerful tool for the precise manipulation of NWs of different compositions and sizes. However, experimental implementation and analysis with DEP often lack depth regarding the optimisation of the technique and the effects of the parameters on the performances of the final devices, which is crucial for the understanding of NWs’ electric transport properties and technology improvement. Consequently, this thesis presents a comprehensive study of the experimental implementation of DEP which is of paramount importance to obtaining optimum conditions for NWs alignment. With this aim in mind, the presented work demonstrates a detailed investigation of the electrical and optical properties of germanium (Ge) and gallium-arsenide-bismuth (GaAsBi) NWs-based devices by DEP as a function of the collection frequency. The Ge and GaAsBi NWs were obtained by MOVPE and MBE respectively as a collaborative work with the Institute of Material for Electronic and Magnet (Italy) for the Ge NWs, and with the research team of Dr Robert Richard at the University of Sheffield (Department of Electrical Engineering and Electronics) for the GaAsBi NWs. Firstly, to maximise NWs alignment precision, optimum DEP parameters are for the first time thoroughly extracted by testing the effects of different mediums (chemical inertia, volatility and contact angle) and electrode designs (gradients and electric field). Secondly, fabricated with a DEP frequency range of 500 kHz to 10 MHz the devices were electrically characterised using voltage-current response. An asymmetric diode-like behaviour was found to be originating from heterostructured Ge NWs specifically orientated by electrophoresis combined with DEP. This result is particularly promising for orientation control demonstrated for the first time, tuning and altering current response from chemically heterostructured nanowires. A particular focus was given to the effect of increasing frequency on the device performances such as carrier transport. Despite a decrease of aligned NWs corroborating theoretical analysis, increasing frequency collected higher conductivity Ge NWs with carrier mobility improving from 2θ at 500 kHz to 4.38θ at 10 MHz demonstrated using Mott-Gurney, and GaAsBi NWs with carrier mobility increasing from 5.29 ± 0.027 to 100 ± 0.70 cm2 V−1.s−1 at 500 kHz and 10 MHz demonstrated using the Fermi-velocity law. Such selectivity is of great potential to improve sensing technology transduction. Using optimum parameters previously found, a low-cost and simple voltage divider system joined to DEP is demonstrated for the first time to improve the alignment technique of a single Ge nanowire. The resulting spectral response was consistent with optical characterisations found in the literature for a single Ge nanowire and demonstrated high sensitivity near-infrared and communication wavelength as confirmed with a high responsivity of 6.2 x 105 A/W at 1550 nm. Such high resistivity is amongst the highest ever obtained for NWs. Furthermore, a NWs-based biosensor for the spike protein of the SARS-CoV-2 was fabricated by multilayered surface functionalisation evidenced by Raman spectroscopy. Upon exposure to increasing concentration of the protein, the biosensors transduced increasing current response with a working range of at least 1 aM to 100 fM. Selectivity to the spike protein was testified using bovine serum albumin as a negative control reference. The GaAsBi NWs were for the first time fully characterised and implemented as devices by DEP. The NWs surface roughness showcased the importance of surface properties that influenced DEP collection and carrier transport. Spectral responses from the devices brought to light the different bismuth content at the origin of reduced band-gap energy shown by the cut-off energies of the spectrum. With a Bi content increase of roughly 1% in GaAs the photodetectors presented high responsivity from 1.3 x 104 A/W to 5.6 x 104 A/W. Effective NWs-based biosensors and photodetectors were proof of concept devices that corroborate NWs and dielectrophoresis functionality paving the way to future nanotechnology improvement.