Date of Award
Doctor of Philosophy in Electrical Engineering - (Ph.D.)
Electrical and Computer Engineering
Hieu Pham Trung Nguyen
Dong Kyun Ko
Semiconductor nanowires are quasi-one-dimensional objects with unique physical properties and strong potential in nanophotonics, nanoelectronics, biosensing, and solar cell devices. The next challenge in the development of nanowire functional structures is the nanowire axial heterojunctions, especially lattice mismatched heterojunctions. Si and Ge have a considerable lattice mismatch of ~ 4.2% as well as a mismatch in the coefficient of thermal expansion, and the formation of a Si1-xGex transition layer at the heterointerface creates a non-uniform strain and modifies the band structures of the adjacent Si and Ge nanowire segments. These nanostructures are produced by catalytic chemical vapor deposition employing vapor-liquid-solid mechanism on (111) oriented p-type Si substrate, and they exhibit unique structural properties including highly localized strain, and short-range interdiffusion/intermixing revealed by transmission electron microscopy, scanning electron microscopy and energy dispersive x-ray spectroscopy. Our studies of the structural properties of axial Si-Ge nanowire heterojunctions show that despite the 4.2% lattice mismatch between Si and Ge they can be grown without a significant density of structural defects. The lattice mismatch induced strain is partially relieved due to spontaneous SiGe intermixing at the heterointerface during growth and lateral expansion of the Ge segment of the nanowire, which is in part due to a higher solubility of Ge in metal precursors. The mismatch in Ge and Si coefficients of thermal expansion and low thermal conductivity of Si/Ge nanowire heterojunctions are proposed to be responsible for the thermally induced mechanical stress detected under intense laser radiation.
The performed electrical measurements include current-voltage, conductance-voltage, transient electrical measurements under various applied voltages at temperatures ranging from 20 to 400K. We find that Si-Ge nanowire heterojunctions exhibit strong current instabilities associated with flicker noise and damped oscillations with frequencies close to 10-30 MHz. Flicker (or 1/f ) noise is characterized and analyzed on carrier number fluctuation model and mobility fluctuation model noise mechanism, respectively. The proposed explanation is based on a carrier transport mechanism involving electron transitions from Ge to Si segments of the NWs, which requires momentum scattering, causes electron deceleration at the Ge-Si heterointerface and disrupts current flow. Both Si/Ge heterojunctions and NW surface states are demonstrated to be the two dominant elements that strongly influence the electrical characteristics of nanowires.
Wang, Xiaolu, "Silicon-germanium nanowire heterojunctions: Optical and electrical properties" (2017). Dissertations. 3.