THERMOELECTRICITY AND HEAT CONDUCTION IN III-V NANOWIRES

Abstract

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 (𝑍𝑇) 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 𝑍𝑇. We compare polytypic and twinning superlattice (TSL) GaAs NWs in measurements obtained by a novel application of the 3𝜔 method. We find thermal conductivities of 8.4 ± 1.6 W/m-K and 5.2 ± 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 Green’s function method to address phonon engineering in generalized GaAs NW structures. In comparing twinning NWs, we find that a TSL period of 50 Å 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 °C. To explain these trends, we derive a phase diagram for TSL NWs based on a kinetic growth model.