Date of Award

Spring 1-1-2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Advisor

Victor M. Bright

Second Advisor

Kris Bertness

Third Advisor

Norman Sanford

Fourth Advisor

Scott Bunch

Fifth Advisor

Bart Van Zeghbroeck

Abstract

Gallium nitride (GaN) nanowires have potential as nanoscale optoelectronic building blocks that can be functionally integrated with silicon MEMS and IC devices. This dissertation presents an overview of the synthesis, characterization, and application of GaN nanowire light-emitting-diodes (LEDs) grown by plasma-assisted molecular beam epitaxy (MBE). Specifically, this research demonstrates discrete axial p-n junction nanowires that produce ultra-violet (UV) electroluminescence at ~40 nW optical power. It further demonstrates that a two-nanowire optical interconnect device can be fabricated from axial p-n junction nanowires with light-emitting and photoconductive capabilities. The nanowire structures obtained from MBE growth were found to depend sensitively on the morphology and crystallographic polarity of the underlying Aluminum Nitride (AlN) nucleation layer. These observations were enabled by piezoresponse force microscopy, which was developed and validated against polarity sensitive etching using uniform and mixed polarity AlN layers. The polarity and overall morphology of the AlN layers could be controlled by the V/III flux ratio and substrate temperature during MBE growth. GaN nanowires were observed to propagate the structural characteristics and crystallographic polarity of the underlying AlN layer, and in some cases a differential growth rate with respect to polarity was observed. Band-edge electroluminescence was obtained in axial p-n junction nanowires that incorporated a thin AlGaN electron blocking layer in the p-region of the device. Electroluminescence was below detection limits for p-n junction nanowires with no blocking layer, despite diode-like I-V characteristics and optically measured internal quantum efficiencies (IQEs) of ~1 %. I-V measurements of the p-regions in p-n junction nanowires, as well as nanowires doped with Mg only, indicate low p-type conductivity and asymmetric Schottky-like p-contacts. These observations, in conjunction with device models, suggest that imbalanced carrier injection from the junction and p-contact can produce significant non-radiative losses. The role of the blocking layer in these devices is attributed to a reduction in the electron overflow current, which permits larger biasing and increased hole injection at the p-contact. This dissertation also discusses the fabrication, device characteristics, and optical coupling of a two-nanowire device comprising GaN nanowires with light-emitting and photoconductive capabilities. Axial p-n junctions were transferred to a non-native substrate, and selectively contacted to form discrete optical source or detector nanowire components. The performance of the individual nanowires was characterized by electroluminescence measurements and spectrally-resolved photoconductivity measurements, which were then compared with the coupled behavior.

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