Largely reduced cross-plane thermal conductivity of nanoporous In_0.1Ga_0.9N thin films directly grown by metal organic chemical vapor deposition

Abstract

In recent year, nanoporous Si thin films have been widely studied for their potential applications in thermoelectrics, in which high thermoelectric performance can be obtained by combining both the dramatically reduced lattice thermal conductivity and bulk-like electrical properties. Along this line, a high thermoelectric figure of merit (ZT) is also anticipated for other nanoporous thin films, whose bulk counterparts possess superior electrical properties but also high lattice thermal conductivities. Numerous thermoelectric studies have been carried out on Si-based nanoporous thin films, whereas cost-effective nitrides and oxides are not systematically studied for similar thermoelectric benefits. In this work, the cross-plane thermal conductivities of nanoporous In_0.1Ga_0.9N thin films with varied porous patterns were measured with the time-domain thermoreflectance technique. These alloys are suggested to have better electrical properties than conventional Si_xGe_1−x alloys; however, a high ZT is hindered by their intrinsically high lattice thermal conductivity, which can be addressed by introducing nanopores to scatter phonons. In contrast to previous studies using dry-etched nanopores with amorphous pore edges, the measured nanoporous thin films of this work are directly grown on a patterned sapphire substrate to minimize the structural damage by dry etching. This removes the uncertainty in the phonon transport analysis due to amorphous pore edges. Based on the measurement results, remarkable phonon size effects can be found for a thin film with periodic 300-nm-diameter pores of different patterns. This indicates that a significant amount of heat inside these alloys is still carried by phonons with ~300 nm or longer mean free paths. Our studies provide important guidance for ZT enhancement in alloys of nitrides and similar oxides.

Keywords

nanoporous film / thermoelectrics / phonon / mean free path / diffusive scattering

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Acknowledgments

This material is based on research sponsored by Defense Advanced Research Agency (DARPA) under agreement number FA8650-15-1-7523 and US Air Force Office of Scientific Research under award number FA9550-16-1-0025. The US Government is authorized to reproduce and distribute reprints for Governmental purposes notwithstanding any copyright notation thereon. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of Air Force Research Laboratory (AFRL) and the DARPA or the US Government. X.W.W., J.Z., and X.J.W. would like to thank the supports from the National Science Foundation (NSF) through the University of Minnesota MRSEC under Award Number DMR-1420013 and from the Legislative-Citizen Commission on Minnesota Resources (LCCMR).

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