Date of Completion

9-4-2012

Embargo Period

4-17-2018

Keywords

Thermoelectricity, Silicon microwires, Self-heating, Joule heating, Thomson effect, Peltier effect, Nano-fabrication

Major Advisor

Ali Gokirmak

Co-Major Advisor

Helena Silva

Associate Advisor

Brian Willis

Associate Advisor

Rajeev Bansal

Associate Advisor

Mehdi Anwar

Field of Study

Electrical Engineering

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

Self-heating mechanisms of small-scale structures have been an important subject where electrical and thermal transports are coupled, such as many electronic and optoelectronic devices, micro-electro-mechanical systems (MEMS), thermoelectric energy conversion devices and phase-change memory devices. In this work, nanocrystalline silicon microwires (L: 1 – 30 μm, Width: 0.1 – 1 μm, Thickness: 50 – 130 nm) are self-heated either through a single, short duration (< 1 μs) or longer DC/AC signal. The motivation behind this work is to melt the structures using electrical stress and to achieve larger crystalline domains (ultimately a single crystal domain) on the wires upon resolidification. Scanning electron micrographs show very smooth wire surfaces after the voltage pulse compared to as-fabricated nanocrystalline texture. Voltage-pulse induced self-heating leads to significant conductance improvement, suggesting crystallization of the wires. The minimum resistivity during the pulse extracted from wires of different dimensions matches previously reported values for liquid silicon. Hence, nanocrystalline silicon microwires melt through self-heating during the voltage pulse and resolidify upon termination of the pulse, resulting in very smooth and less-resistive crystalline structures.

Strong thermoelectric effects are observed on self-heated wires as asymmetric self-heating and melting of the wires when a microsecond voltage pulse is applied to melt only some portion (~ 50 %) of the wires. This observation has shifted the focus of the work towards understanding of self-heating mechanisms and thermoelectric effects on these wires. The thermoelectric effects are also characterized through capture and analysis of light emission from the self-heated wires biased with DC/AC signals. The hottest spot on the wires consistently appears closer to the lower potential end for n-type, and the higher potential end for p-type microwires, in agreement with previous reports on asymmetric self-heating of microstructures.

Numerical modeling of electrical and thermal transport is performed on the wires using 3-D finite element modeling. Modeling results suggest that elevated temperatures (> 1300 K) give rise to significant electron-hole pair generation and strong thermal gradients (~1 K/nm) lead to substantial gradients in the generation-recombination balance. The modeled results are good agreement with the both microsecond voltage pulse and long duration AC signal experiments.

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