Thermoelectric devices (TEDs) are useful in a variety of niche applications, but low efficiencies limit their broader application. Semiconductor nanowires (NWs) could be the key to efficient thermoelectrics, through the benefits of one-dimensional band structures and a greatly reduced thermal conductivity. This thesis explores the transport fundamentals, experimental characterization, and computational approaches relevant to prospective III-V NW TEDs.
Predictive electronic transport models are outlined for NWs and bulk III-Vs. These models are used to determine the optimum carrier concentration for maximizing the thermoelectric figure of merit (ππ) in the bulk and in NWs of arbitrary size. We demonstrate the physical mechanisms underlying electronic thermoelectric improvements in NWs and confirm the superior performance of InSb and InAs, among other III-Vs.
Next, thermal conductivity reduction in structurally complex NWs is investigated as a means of improving ππ. We compare polytypic and twinning superlattice (TSL) GaAs NWs in measurements obtained by a novel application of the 3π method. We find thermal conductivities of 8.4 Β± 1.6 W/m-K and 5.2 Β± 1.0 W/m-K for the polytypic and TSL NWs, respectively, demonstrating a significant difference and an almost ten-fold reduction compared to 50 W/m-K of bulk GaAs.
We employ molecular dynamics simulations and the atomistic Greenβs function method to address phonon engineering in generalized GaAs NW structures. In comparing twinning NWs, we find that a TSL period of 50 Γ
minimizes the lattice thermal conductivity across all the diameters considered. Our results also illustrate the importance of NW surfaces versus the internal crystal structure. Phonon coherence lengths are obtained by analyzing thermal conductivity trends in periodic and aperiodic structures. Transmission spectra are calculated to reveal the phonon frequencies targeted by structural engineering in NWs. These findings explain the range of thermal conductivities obtained for GaAs NWs with various crystal phases.
Finally, to inform future growths of TSL NWs, we study the influence of the substrate temperature and V/III flux ratio on TSL formation in Te-doped GaAs NWs. The crystal structure of several NWs is investigated using transmission electron microscopy, revealing a range of polytypic and TSL morphologies. We find that periodic TSLs form only at low V/III flux ratios of 0.5 and substrate temperatures of 492 to 537 Β°C. To explain these trends, we derive a phase diagram for TSL NWs based on a kinetic growth model. / Thesis / Doctor of Philosophy (PhD) / In a circuit of dissimilar conductors, temperature differences create voltage differences that can drive electrical currents. Similarly, electrical currents in such circuits inherently lead to heating and cooling. These phenomena are known as thermoelectric effects because they couple heat and charge transport (electricity) in a symmetric and reversible way. The goal of some thermoelectric devices (TEDs) is to exploit these effects to generate electrical power or to provide controlled cooling. However, greater conversion efficiencies are required to compete against other existing technologies.
With the advent of nanofabrication, semiconductor nanowires (NWs) have emerged as an attractive material system for efficient TEDs. In this thesis, we explore their thermal and electronic properties. We demonstrate a novel way to measure the NW thermal conductivity and employ computational methods to examine heat transport in NWs with various crystal structures. Finally, we examine how synthesis conditions can determine the morphology of NWs.
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/27657 |
| Date | January 2022 |
| Creators | Ghukasyan, Ara Arayik |
| Contributors | LaPierre, Ray Robert, Engineering Physics |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
Page generated in 0.0022 seconds