Date of Award

4-2016

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

First Advisor

Chen Yang

Committee Member 1

Luis M. Kruczenski

Committee Member 2

Oana Malis

Committee Member 3

Xiulin Ruan

Abstract

III-V semiconductor nanowires (NWs) have shown great potential to be building blocks for optical, optoelectronic, and electronic devices due to their special transverse confinement of electrons and photons along the nanowire axis. In addition, semiconductor nanowires with subwavelength structures exhibit strong optical Mie resonance, making them ideal platforms for realizing novel optical devices, such as extreme solar energy absorbers and broadband light trapping devices. This special 1D optical Mie resonance can be enhanced by using semiconductor-core dielectric-shell (CS) and metal-core semiconductor-shell dielectric-outer shell (CSS) nanowire heterostructures. Those advantages can be even leveraged up by utilizing nanowire arrays, attributing to the increasing optical inter-wire interaction between incident light and nanostructures.

However, to form a very thin, vertical IIIV nanowire array is challenging for both conventional top-down and bottom-up approaches due to the limitation of the resolution of lithographically defined masks and thermodynamic limits of growth direction and diameter of nanowires, respectively. By employing nanoscale self-mask effects, those limitations can be circumvented. In this dissertation, we presented a novel top-down etching method to fabricate very thin, high aspect ratio and vertical III-V nanowire arrays without lithographically defined masks. The mechanism of the formation of nanowire arrays was proposed and verified by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) in this work. Optical characterizations, such as optical reflectance and Raman spectroscopy, were also performed on those nanowire arrays. By employing those nanowire arrays, broadband light trapping can be achieved.

Besides, the effects of contact electrodes, such as indium tin oxide (ITO), silver, and copper, on semiconductor nanowire solar cell devices with different bandgaps were also investigated with a focus on optical absorption. Although traditional conductive oxide materials, such as indium tin oxide (ITO) and aluminum zinc oxide (AZO), have been successfully used in solar cell thin film devices, those conductive oxide contact electrodes will have different optical behavior applied in 1D nanowire devices due to 1D optical Mie resonance in nanowires. We found metal contact electrodes, such as silver and copper, will have comparable optical performance with conventional ITO contact electrodes while the semiconductor nanowire devices approaching to 1D limit. Our results also show the contact electrodes will affect the semiconductor materials with different bandgaps through different ways, which can be considered as a guideline for the future device applications.

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