Properties and device applications of silicon and silicon-germanium nanostructures with different dimensions
Department of Electrical and Computer Engineering
Doctor of Philosophy
Krasnoperov, Lev N.
Defect-free crystalline Si/SiGe(Ge) nanostructures are demonstrated despite the 4% lattice mismatch between Si and Ge. The lattice mismatch-induced strain is sufficiently relaxed through the designed, cluster morphology, or nanowire (NW) structures. Future device applications of these nano structures require complete understanding of their structural, optical, electrical and thermal properties. This study explores these properties in two-dimensional (2D) Si/Si:B delta-doped multilayers, 2D Si/Si1-xGex planar multilayers, three-dimensional (3D) Si/Si1-xGex cluster multilayers, one-dimensional (1D) Si NWs and 1D Si/Ge NW heterojunctions (HJs).
Raman scattering and photoluminescence measurements show that by alternating heavily boron-doped layers with layers of undoped Si in Si/Si:B multilayers, dopant segregation and strain can be avoided. Current-voltage and capacitance-voltage measurements show Schottky-barrier-like characteristics in these nano structures. The studied samples exhibit significant dependence of optical reflection on temperature and applied electric field, and hence have a potential to be used as electrically controllable mirrors.
High Ge content 2D (planar) and 3D (cluster) Si/SiGe multilayers are studied thoroughly using Raman spectroscopy. Low frequency Raman measurements show formation of strong folded longitudinal acoustic (FLA) phonons in the 2D sample, indicating abrupt interfaces and good superlattice structure. By utilizing the multi-modal feature of Raman scattering, local temperatures are found by comparing the intensities of Stokes and anti-Stokes signals at specific wavenumbers, and the thermal conductivity of each sample is estimated. A strong correlation between FLA and thermal conductivity is found: in samples with high intensity FLA, thermal conductivity is almost twice increases, when compare to samples without FLA.
Fabrications of Si NWs and Si/Ge NW HJs are explored, including interference-lithography-based photoresist patterning for Au catalysts. Raman spectroscopy shows significant strain in Si NWs and Si/Ge NW HJs. In the HJs, the temperature dependence in PL peak positions suggested a preferential composition at the hetero-junction. Raman-spectroscopy-based temperature measurements show significant decrease in the thermal conductivity of NW HJs: more than one order of magnitude less than that in Si NWs and two orders of magnitude less than that in c-Si. The high mobility and good carrier transport, combined with the substantially decreased thermal conductivity gives these Si/Ge and Si/SiGe nanostructures great potential in CMOS compatible, integrated thermoelectric device applications.
njit-etd2010-085 (127 pages ~ 7,971 KB pdf)
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Created October 21, 2011