Growth and characterization of silicon and germanium nanowires and heterostructures

One dimensional (1D) nanostructures have attracted considerable interest in recent years, owing to their unique electrical, optical and quantum properties. In particular, semiconducting nanowires (NW) are of technological significance, because of their applications in electronics, chemical and biological sensing, and energy conversion. However, the field is fraught with unanswered questions pertaining to the growth fundamentals. In this research, Silicon and Germanium nanowires (NW) and their heterostructures have been systematically characterized using in situ transmission electron microscopy and post growth characterization techniques. For those nanowires that were characterized post-growth, the nanowire growths were carried out under ultra high vacuum (UHV) conditions using Au-catalyzed vapor-liquid-solid (VLS) growth mechanism by a collaborator, Dr. Eric Dailey. Post growth characterization was performed using electron microscopy techniques. In situ growth experiments were carried out in a Tecnai F20 environmental transmission electron microscope.;Silicon nanowire growth orientation control and the stability of the gold catalyst atop the NW were studied in detail. Epitaxial NWs begin growing normal to the substrate atop a tapered base. After a kinetically determined distance NWs kinked to the energetically favorable growth orientation. Interestingly, the Au coverage on the sidewalls was determined to be diameter and growth condition-dependent. In situ growth experiments revealed that a liquid AuSi spreads from the seed along the sidewalls of growing Si NWs for some growth conditions. The spreading behavior of the liquid AuSi film was explained by considering the interface thermodynamics of liquid droplets on cylinders.;Furthermore, core/shell and axial heterostructures based on NWs were investigated. Axial heterostructure growths along various orientations were characterized to explore the ability to obtain straight and compositionally abrupt heterojunctions. Straight heterostructure growth with good yield was observed only in the case of <110>-oriented Si-Ge axial NWs. The interface widths were found to be dependent on NW diameter, growth temperature of the Ge segment and quench rates between growths. Core/shell heterostructures along various orientations and coaxial dimensions were characterized to determine the limits for obtaining highly strained defect free structures. Finally, growth of Si1-x NWs using VLS was investigated to explore composition modulation by varying the growth parameters and the resulting growth morphologies.