Investigation of nanostructured thin film based Schottky diodes for gas sensing applications
Creators: name: Shafiei, Mahnaz
Publisher: name: RMIT University
Year: 2024

description: In this research, novel nanostructured thin film based Schottky diodes for gas sensing applications were developed and investigated. The author explored the sensing potential of these devices and comprehensively studied a large group of nanostructured thin films, as the gas sensing layers, with aspirations of enhancing sensor’ performance compared to the conventional based sensors. To the best of author’s knowledge, the author developed several nanostructured metal-oxide based Schottky diodes with morphologies that have not been reported previously in literature and presented the first scientific investigation of Pt/graphene-like nano-sheets/SiC based gas sensors. <br><br>The nanostructured materials were deposited on SiC and GaN substrates and characterised. These materials were then applied in the fabrications of Schottky diodes and their gas sensing performance was investigated towards different gaseous species. Hydrogen gas was assessed below explosive level. Propene and NO2 gases were also sensed at above hazardous concentrations for human’s health. Two different types of sensors were investigated: Pt/nanostructured semiconducting metal-oxide and Pt/graphene-like nano-sheets devices. The operational principles of the devices as well as their gas sensing mechanisms are discussed. <br><br>These sensors comprised of thin Pt layers (as both Schottky contact and catalyst) and nanostructured thin films (as the gas sensing layers), which are deposited on substrates with ohmic contacts. SiC and GaN were the selected substrates, as they are wide band semiconducting materials, making them suitable for high temperature applications. To enhance these devices’ sensitivity, author used nanostructured forms of the semiconducting metal-oxides (TiO2, SnO2, ZnO, RuO2, MoO3, WO3) and graphene-like nano-sheets. <br><br>The electrical characteristics and gas sensing performance of the sensors were obtained in the presence and absence of target gas at different temperatures ranging from 25 to 620°C. The Pt/nanostructured metal-oxide based Schottky diodes exhibited a significantly lower breakdown voltage and larger lateral voltage shift upon exposure to the target gas in reverse bias mode of operation than that in forward bias as compared to conventional Schottky diodes. This was associated to the enhanced localized electric fields originating from the sharp edges and corners of the high aspect ratio nanostructures. To the best of author’s knowledge, for the first time it was shown that these enhanced localized electric fields lowered the Schottky barrier height energy at the reverse bias condition, allowing greater charge carrier flow and hence enhanced sensitivity. It was found that amongst the sensors investigated, the devices based on MoO3 nanostructures exhibited the highest sensitivity (a voltage shift of 5.7V towards 1% hydrogen) with excellent baseline stability and a fast response (t90%) of 40s. <br><br>For the Pt/graphene-like nano-sheet/SiC devices, a unique theoretical approach was developed. To the best of author’s knowledge, this theoretical explanation was the first description of the behaviour of the Pt/graphene-like nano-sheet/SiC junctions. The author attributed the unusual behaviour of such junction to the semi-metal like properties of the graphene-like layer, which was placed between Pt and SiC substrate. The sensors showed response to hydrogen gas; while the response towards NO2 was not significant. Therefore, these devices could sense selectively hydrogen at the presence of NO2.