Thermal transport evolution due to nanostructural transformations in Ga-doped indium-tin-oxide thin films

TL;DR

This study combines theoretical and experimental methods to analyze how Ga doping affects the thermal conductivity and phonon transport in ITO thin films, revealing a structural transition and hybridized vibrations that reduce thermal conductivity.

Contribution

It provides new insights into the structural transformation and phonon behavior in Ga-doped ITO films, linking nanostructural changes to thermal transport properties.

Findings

Thermal conductivity decreases nonlinearly with Ga doping.

Structural transformation from nanocrystalline to amorphous occurs at 15-20 at. % Ga.

Hybridized phonon vibrations cause reduced mean free paths and lower thermal conductivity.

Abstract

We report on a comprehensive theoretical and experimental investigation of thermal conductivity in indium-tin-oxide (ITO) thin films with various Ga concentrations (0-30 at. %) deposited by spray pyrolysis technique. X-Ray diffraction (XRD) and scanning electron microscopy have shown a structural transformation in the range 15-20 at. % Ga from the nanocrystalline to the amorphous phase. Room temperature femtosecond time domain thermoreflectance measurements showed nonlinear decrease of thermal conductivity in the range 2.0-0.5 W/(m K) depending on Ga doping level. Comparing density functional theory calculations with XRD data it was found that Ga atoms substitute In atoms in the ITO nanocrystals retaining Ia-3 space group symmetry. The calculated phonon dispersion relations revealed that Ga doping leads to the appearance of hybridized metal atom vibrations with avoided-crossing behavior. These hybridized vibrations possess shortened mean free paths and are the main reason behind the thermal conductivity drop in nanocrystalline phase. An evolution from propagative to diffusive phonon thermal transport in ITO:Ga with 15-20 at. % of Ga was established. The suppressed thermal conductivity of ITO:Ga thin films deposited by spray pyrolysis may be crucial for their thermoelectric applications.