Phase Diagram for Twinning Superlattice Te-Doped GaAs Nanowires.

Twinning superlattices (TSLs) are a growing class of semiconductor structures proposed as a means of phonon and optical engineering in nanowires (NWs). In this work, we examine TSL formation in Te-doped GaAs NWs grown by a self-assisted vapor-liquid-solid mechanism (with a Ga droplet as the seed particle), using selective-area molecular beam epitaxy. In these NWs, the TSL structure is comprised of alternating zincblende twins, whose formation is promoted by the introduction of Te dopants. Using transmission electron microscopy, we investigated the crystal structure of NWs across various growth conditions (V/III flux ratio, temperature), finding periodic TSLs only at the low V/III flux ratio of 0.5 and intermediate growth temperatures of 492 to 537 °C. These results are explained by a kinetic growth model based on the diffusion flux feeding the Ga droplet.

Modelling thermoelectric transport in III-V nanowires using a Boltzmann transport approach: a review.

A review of models for determining the thermoelectric transport coefficients [Formula: see text] (Seebeck coefficient), [Formula: see text] (electrical conductivity), and [Formula: see text] (electronic thermal conductivity) is presented, for the cases of bulk and nanowire structures, along with derivations and a discussion of calculation methods. Results for the transport coefficients in GaAs, InAs, InP and InSb are used to determine the thermoelectric figure of merit, where an enhancement by two orders of magnitude is found for the nanowire case as compared with the bulk. The optimal electron concentration is determined as a function of nanowire diameter for both background and modulation doped nanowires.

Thermal transport in twinning superlattice and mixed-phase GaAs nanowires.

With continuing advances in semiconductor nanowire (NW) growth technologies, synthesis of tailored crystal structures is gradually becoming a reality. Mixtures of the bulk zinc blende (ZB) and wurtzite (WZ) phase can be achieved in III-V NWs under various growth conditions. Among the possible crystal structures, the twinning superlattice (TSL) has attracted particular interest for tuning photonic and electronic properties. In this work, we investigated the mechanisms underlying thermal transport in pristine, TSL, and disordered polytypic GaAs NWs, using non-equilibrium molecular dynamics and transmission spectra obtained from the atomistic Green's function method. We found that a TSL period of 50 Å minimizes the thermal conductivity and determine a phonon coherence length of about 20 to 50 nm, depending on the NW diameter. Our findings indicate strong dependence of the thermal conductivity on the NW surface and internal structure at a given diameter, critical for thermoelectric optimization.

Thermal Conductivity of GaAs Nanowire Arrays Measured by the 3ω Method.

Vertical nanowire (NW) arrays are the basis for a variety of nanoscale devices. Understanding heat transport in these devices is an important concern, especially for prospective thermoelectric applications. To facilitate thermal conductivity measurements on as-grown NW arrays, a common NW-composite device architecture was adapted for use with the 3ω method. We describe the application of this technique to obtain thermal conductivity measurements on two GaAs NW arrays featuring ~130 nm diameter NWs with a twinning superlattice (TSL) and a polytypic (zincblende/wurtzite) crystal structure, respectively. Our results indicate NW thermal conductivities of 5.2 ± 1.0 W/m-K and 8.4 ± 1.6 W/m-K in the two samples, respectively, showing a significant reduction in the former, which is the first such measurements on TSL NWs. Nearly an order of magnitude difference from the bulk thermal conductivity (~50 W/m-K) is observed for the TSL NW sample, one of the lowest values measured to date for GaAs NWs.