Imaging the Thermalization of Hot Carriers After Thermionic Emission Over a Polytype Barrier

TL;DR

This study uses high-resolution thermal imaging to investigate how hot carriers thermalize in nanowire devices with engineered barriers, revealing detailed thermal and electronic properties relevant for thermoelectric applications.

Contribution

It provides the first direct spatially resolved measurements of electron thermalization and thermoelectric effects in phase-engineered InAs nanowires, including key thermal and electronic parameters.

Findings

Electron thermalization length of 223 nm identified.

Measured Peltier and Seebeck coefficients for the barrier.

Thermal conductivity along the nanowire axis determined.

Abstract

The thermalization of non-equilibrium charge carriers is at the heart of thermoelectric energy conversion. In nanoscale systems, the equilibration length can be on the order of the system size, leading to a situation where thermoelectric effects need to be considered as spatially distributed, rather than localized at junctions. The energy exchange between charge carriers and phonons is of fundamental scientific and technological interest, but their assessment poses significant experimental challenges. We addressed these challenges by imaging the temperature change induced by Peltier effects in crystal phase engineered InAs nanowire (NW) devices. Using high-resolution scanning thermal microscopy (SThM), we have studied current-carrying InAs NWs, which feature a barrier segment of wurtzite (WZ) of varying length in a NW of otherwise zincblende (ZB) crystal phase. The energy barrier acts as a filter for electron transport around the Fermi energy, giving rise to a thermoelectric effect. We find that thermalization through electron-phonon heat exchange extends over the entire device. We analyze the temperature profile along a nanowire by comparing it to spatially dependent heat diffusion and electron thermalization models. We are able to extract the governing properties of the system, including the electron thermalization length of $223 \pm 9$\,nm, Peltier coefficient and Seebeck coefficient introduced by the barrier of $39 \pm 7$\,mV and $89 \pm 21$\,$\mu$V/K, respectively, and a thermal conductivity along the wire axis of $8.9 \pm 0.5$\,W/m/K. Finally, we compare two ways to extract the elusive thermal boundary conductance between NW and underlying substrate.