Date of Award

Spring 1-1-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Advisor

Victor M. Bright

Second Advisor

Pavel Kabos

Third Advisor

Kris A. Bertness

Fourth Advisor

Y.C. Lee

Fifth Advisor

Yifu Ding

Abstract

In this thesis, we address the design and application of a microscope and probes for near-field scanning microwave microscopy. We provide an introduction to the development of microwave microscopy and its contributions to material metrology. In particular, we focus on its application to the study of photovoltaics. We then expand beyond these studies to the fabrication of nanowire-based probes for microwave microscopy. These probes provide avenues for advancing an array of scanning probe techniques, including continued measurements on photovoltaics with improved resolution.

To begin, we present a near-field scanning microwave microscope that has been configured for imaging photovoltaic samples. Our system incorporates a Pt-Ir tip inserted into an open-ended coaxial cable allowing the microwave reflection S11 signal to be measured across a sample. A phase-tuning circuit increased impedance-measurement sensitivity by allowing for tuning of the S11 minimum down to -78 dBm. A bias-T and preamplifier enabled simultaneous, non-contact measurement of the DC tip-sample current and a tuning fork feedback system provided simultaneous topographic data. Light-free tuning fork feedback provided characterization of photovoltaic samples both in the dark and under illumination. In addition to single-point measurements on Si and GaAs samples, microwave measurements were obtained on a Cu(In,Ga)Se2 (CIGS) sample. The S11 and DC features were found to spatially broaden around grain boundaries with the sample under illumination. The broadening is attributed to optically-generated charge that becomes trapped and changes the local depletion of the grain boundaries.

Next, we report on the fabrication of a GaN nanowire probe for near-field scanning microwave microscopy. A single nanowire was Pt-bonded to a commercial Si cantilever prior to either an evaporated Ti/Al or an ALD W coating, providing a microwave signal pathway. Testing over a calibration sample shows the probe to have capacitance resolution down to ~0.03 fF with improved sensitivity and reduced uncertainty compared with a commercial microwave probe. Imaging of MoS2 sheets found the probe to be immune to surface contamination, owing to its flexible, high-aspect ratio morphology. By improving microwave and topographical sensitivity in a mechanically robust architecture, this probe serves as an ideal platform for additional complimentary scanning probe techniques.

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