Ph.D Thesis

Ph.D StudentKelrich Alexander
SubjectGrowth and Characterization of InP Nanowires and
Nanostructures Using the Selective Area
Vapor-Liquid-Solid Method
DepartmentDepartment of Nanoscience and Nanotechnology
Supervisor PROF. Dan Ritter
Full Thesis textFull thesis text - English Version


Epitaxial growth of semiconductor nanowires is intensively explored for various applications and fundamental research. The successful implementation of nanowire technology requires an understanding of nanowire growth mechanisms in order to achieve control of morphology, composition and crystal structure.

This thesis explores epitaxial growth of indium phosphide nanowires and more complex dimensional heterostructures based on nanowire template. Growth was performed in metalorganic molecular beam epitaxy system on (111) oriented substrates using selective-area vapor-liquid-solid method. In this technique, seed particle, mostly gold, is positioned inside inert mask opening. The particle forms liquid alloy with growth precursors, leading to crystal formation by precipitation from a supersaturated solution. Precise nanowire positioning and diameter control were achieved by electron beam lithography patterning and Au deposition parameters.

The discrepancy between experimental growth rate and theoretical modeling was resolved by incorporating contribution of the annular opening area as an interplay of reduced precursor reemission area and additional decomposition surface. Additionally, the calculation of shading of scattered precursors was found to account for reduced growth rate in dense two-dimensional arrays.

Independent control over axial and radial growth rates along with defect free crystal phase are major challenges in the field of bottom-up nanowire synthesis. A perfect crystal structure is difficult to achieve due to polytypism between the hexagonal (wurtzite) and cubic (zinc blend) crystal phases formed. We demonstrate how a combination of basic growth parameters leads to non-tapered and stacking faults-free InP nanowires. The accompanying theoretical modeling explained well the prevalence of the wurtzite phase at high group V fluxes and low temperatures.

The acquired growth expertise enabled incorporation of InAsP quantum emitter into InP nanowire in accordance with the original goals of this research. We have investigated growth techniques of nanowire-based waveguides which combine vapor-liquid-solid nanowire core epitaxy followed by vapor-solid radial cladding. The target waveguide dimensions and morphology were achieved using robust cladding protocol without sacrificing crystal purity of the waveguide. We have shown strong PL signal at characteristic wavelength for InAsP quantum dot and demonstrated the impact of the core and the waveguide diameter on the emission lines.

Concurrently with the progress in nanowire research, III-V semiconductor nanomembrane synthesis recently draws significant attention for realization of multiterminal devices and novel photonic sources. The second part of this thesis presents a method for synthesis of dimensional heterostructure where a flag-like nanomembrane is grown on a nanowire template. In the heart of the technique lies the ability to unpin the Au particle from the nanowire pedestal to one of its side facets. Methods to achieve controlled structure and morphology of the nanoflag along with its optical characterization are presented. The next logical step in evolution of nanoflags towards their implementation as active optical nanoantennas is controlling the flag direction. The experimental feasibility of directing the Au particle displacement by electric field applied in situ is described in the last part of this thesis. The ability to manipulate nanoparticles at the macro level opens up a whole new area for synthesis of complex nanowire based structures. The theoretical modeling and various applications of this intriguing phenomenon is a matter of future research.