Bismuth and Germanium Nanoscale Cluster Devices
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Transistors are the fundamental components of computer processors. The dimensions of transistors used in microprocessors are decreasing every year and the challenge of maintaining this trend now requires nanoscale dimensions. A potential method of achieving nanoscale dimensions is using atomic clusters as building blocks. It is therefore desirable to investigate transistor-like behaviour in cluster devices. Traditionally, transistor devices are made from semiconducting materials. It was therefore proposed that gated behaviour would be observable in devices that are fabricated from germanium clusters. A germanium cluster source was designed and built. Field effects were successfully observed in films of germanium clusters. Immediately after deposition, the gate effect of germanium cluster films was insignificant. As the films slowly oxidized in vacuum, a decrease in the overall carrier concentration was observed which lead to an increase in the gate effect, with a maximum change in resistance observed of 12%. When films of germanium clusters were exposed to air, a resistance decrease was observed, attributed to water vapour adsorbing on the surface. The phenomenon was further investigated and the proposed resistance change mechanism involves water vapour creating surface defects which act as donors and cause the electron concentration in the film to increase. Films of germanium clusters were sensitive to hydrogen concentrations above 1% in air, with up to a factor of 25 decrease in resistance observed at room temperature for 5% hydrogen concentration. Thin films were found to be most sensitive. The higher sensitivity was attributed to the larger surface-to-volume ratio. The proposed mechanism for sensing is that defects are created on the surface of the film, which in turn act as donors which cause the electron concentration in the film to increase. Bismuth is a semimetal and gate effects have previously been observed in bismuth nanowires. Parallel bismuth nanowires of 300nm diameter were successfully deposited at a distance of 200nm apart allowing one of the wires to be used as a gate. The gate effects observed in bismuth cluster structures were weak and inconclusive, with a small gate effect (change in resistance of 0.1%) observed at 11K in some devices.