dc.description.abstract | In this thesis, fabrication and characterization of both Si and Ge nanostructures (nanowire (NW) and quantum dot (QD)) as well as the associated transistors were investigated. Si NWs of various width ranging from 7 to 60 nm were generated using a combination of electron-beam lithographic, C4F8/SF6 plasma etching, and thermal oxidation processes. For one-dimensional Si NW metal-oxide-semiconductor field-effect-transistors (MOSFETs), there appears to be strong channel and width size dependence on the carrier transport wherein. For a NW with a given wire width (W) of 24 nm, clear current plateau/oscillation features were observed around the threshold regime once the channel length (Lg) satisfied the relation of mle/2 = Lg, where le is the electron wavelength, for instance, Lg = 52 and 34 nm. Whereas the NWFETs behaved like a conventional MOSFET when Lg = 42 nm. On the other hand, as the wire width was reduced less than 10 nm, NWFETs exhibited much strong current oscillation behaviors. Temperature-dependent analysis suggests the interplay of quantum interference and intersubband scattering effects being the plausible mechanisms for the observed current behaviors. It also implies there should be dramatic different carrier transports properties when the channel is scaled from three-dimensional planar structures into zero-dimensional QDs.
As for the generation of Ge QDs, thanks to Ge segregation and condensation effects during high temperature thermal oxidation of SiGe/Si-on-insulator (SGOI) structures, Ge QDs are ultimately formed by a progressive concentration of the Ge content within the remaining unoxidized SiGe until the entire Si is used up. As a consequence, tiny and dense (size and density) Ge QDs were formed after thermal oxidation of SGOI planar structures, whereas a single or a few Ge QDs line up along the core of an oxidized SGOI NW which self-aligns with adjacent unconsumed SiGe/Si pads.
The blue shift of the cathodeluminescence (CL) peak energy with a reduction of the QD size revealed the quasi-direct bandgap properties of Ge QDs because of quantum confinement effects. SiGe-QD/Si-pillar heterostructured photodetectors were experimentally demonstrated. Thanks to the effective confinement of holes within the QD by the valence band offset between SiGe and Si, a current enhancement up to 10^4 was achieved under light illumination, suggesting potentials of Ge QDs for fiber-optical communications or optoelectronic applications. On the other hand, incorporating the Ge QDs into the gate stacks of poly-Si thin-film-transistors, preliminary simulation results pointed to a possible promising enhancement in the light absorption efficiency and photoresponsivity by the concentration of the electric field between adjacent Ge QDs. Meanwhile, the calculated quality factor (Q) up to almost 10^5 is obtained if Ge QDs are embedded in L-type Si3N4 photonic crystal cavities, giving an opportunity for the realization of Ge-QD light emitting devices.
The electronic structures of the Ge QDs and the charge transportation wherein were investigated in terms of Ge-QD resonant tunneling diodes (RTDs) and single-electron transistors (SETs). In a designed n+-Si/SiO2/Ge-QD/SiO2/n+-Si RTD, we are able to resolve the one-particle electronic structure of the Ge QD directly from the steady-state tunneling current spectroscopy for the sake that the electron bandwidth of n+-Si electrodes could not cover more than one energy levels of the Ge QD simultaneously. Additionally we observed a salient photogenerated fine structures under suitable light pumping owing to excess holes dwell in the QD. We gained the insight of the transient carrier transport through the Ge QD by applying different voltage trains to a Ge-QD RTD, and found out that the displacement current played a major role in the pulse transient region whereas the tunneling current dominated when the device reached to the steady state. The ultimate goal of this thesis is the experimental demonstration of high performance Ge-QD single-hole transistors with self-aligned gate and source/drain electrodes. As a result of effective suppression of gate-induced tunneling barrier lowing, clear Coulomb blockade oscillations with a large peak to valley current ratio up to 750 is achievable at room temperature.
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