Impact of Precursor Material on the Structural, Chemical and Gas Chemoresistive Properties of β-Ga₂O₃ Nanowires

Gallium oxide (β-Ga₂O₃) is a wide band gap semiconductor material used in the fabrication of high power, high temperature and gas sensing devices. For its use as gas sensor, Ga₂O₃in form of thin films has been demonstrated to be sensitive towards oxygen and reducing gases at temperatures above 600°C. To decrease the working temperature to bring sensors based on this material towards their compatibility with the Internet of Things (IoT), different innovations have been proposed, mainly based on surface functionalization, material doping or the increase of the surface-to-volume ratio using the nanowire (NW) morphology.<br/>In this work we report the synthesis, characterisation and gas sensing behaviour of β-Ga₂O₃ NWs. These nanowires are fabricated via a metal-assisted vapour liquid solid (VLS) mechanism using either gallium metal evaporation or carbothermal reduction of Ga₂O₃ nanopowders and employing Au as seed for the growth. The NWs have been grown on oxidised silicon or on fused silica substrates. For their use as gas sensors, the NWs have been removed from the substrates where they grew and were transferred to substrates containing microelectrodes. In some cases, these are on placed on suspended microhotplates containing a buried heater. In this way, individual NWs have been contacted using Focused Electron (FEB) and Focused Ion Beam (FIB) Induced Deposition techniques, giving rise to resistors whose value changes in the presence of different gases, i.e., they are chemiresistors.<br/>The grown NWs are monocrystalline and almost defect free, with a diameter of few tens of nm and lengths in the range of several tens of micrometre, which increases with evaporation time. The NWs grown by carbothermal reduction present, in addition, a carbon-containing shell, which is the result of the use of graphite. From the electrical point of view, this shell gives raise to an important reduction of the resistance of the resistor. The electrical properties of these chemoresistive gas sensors have been measured in the presence of different gases, which will be described.<br/>The structural and chemical properties grown materials, their sensing behaviour and the differences among them will be discussed as a function of the fabrication routes employed.