Fundamental Study on Si Nanowires for Advanced MOSFETs and Light-Emitting Devices

Since nanotechnology made it possible to fabricate and control structures with dimensions much smaller than 1 μm, physical phenomena peculiar to the nanoscale structures have attracted much attention and have been expected to be utilized in novel devices. Si nanowires (NWs) are one of the structures: quantum confinement effect (QCE) becomes conspicuous in Si NWs, and Si NWs are potential structures for applications to advanced metal-oxide-semiconductor field effect transistors (MOSFETs) and light-emitting devices. Although Si MOSFETs have made progress with miniaturization according to the scaling law so far, further scaling will not be easy in the future because of problems such as short channel effects. To overcome the problems, surrounding-gate structures such as NW MOSFETs have been proposed. The structures can control the short channel effects effectively. Unlike bulk Si, Si nanostructures including Si NWs are promising candidates of light-emitting devices, because the QCE in nanostructures increases the uncertainty of the crystal momentum, by which no-phonon radiative recombination may increase, and with which improvement in light-emitting efficiency is expected. In this thesis, first, a fabrication process of Si NWs with a cross-sectional size below 10 nm is established, and then carrier transport characteristics in Si NW MOSFETs were studied. The size dependence of the mobility was investigated, from which the electronic states and the carrier scattering mechanism peculiar to Si NWs were inferred. Moreover, to clarify the origin of the current oscillation with gate-voltage sweep in NW MOSFETs, the author proposed an analytical transport model, taking account of acoustic phonon scattering in Si NWs. In addition to the transport characteristics, the optical properties of Si NWs were also studied, aiming at light-emitting diodes. The light-emitting properties of Si NWs were estimated by calculating the absorption coefficients for band-to-band transition. Si-NW pn-diodes were fabricated, and their electroluminescence was investigated. In Chapter 2, a fabrication process of Si NWs by a top-down approach using silicon-on-insulator, electron beam lithography and sacrificial oxidation was established. The NW shapes were investigated by cross-sectional and top views using TEM and SEM. Formation of NWs as narrow as 13 nm was accomplished by electron beam lithography. After downsizing by sacrificial oxidation, formation of an NW with a minimum cross-sectional size W (square root of the area) of 4 nm (height: 2 nm, width: 7 nm) was achieved. In Chapter 3, Si-NW MOSFETs, whose NW size (W) was changed from 18 nm to 4 nm, were characterized. Both nand p-channel MOSFETs showed good gate control with a nearly ideal subthreshold swing of 63 mV/decade. The shift of subthreshold voltage with decreasing the NW size was observed and can be explained by the band-edge shift by the QCE. Electron and hole mobilities were compared and characterized in one Si NW using a special device structure. The W dependence of the ratio of the electron and hole mobilities indicated two types of