Thermal Conductivity of GaAs-GaP Superlattices Nanowires

In the last decades, continuous efforts have been made to understand and control phonons with great potential for numerous technological applications [1]. The objective of this work is to investigate phonons interference and different phonon transport regimes, which is crucial for phonons manipulation. Nanowires are promising candidates for studying phonons interference effects because they offer unique possibilities in terms of heterostructuring and also enable the growth of high-quality nanowire junctions. In this respect, a superlattice (a lattice made by different materials periodically alternated) can be used to investigate the behavior of phonons scattered from interfaces. Phonons scattered from single interfaces lose their phase information, leading to diffusive thermal transport. However, scattered phonons can interfere before losing their phase information resulting in a modified phonon dispersion and thermal conductivity in case of periodic repetition of interfaces with a lengthscale comparable to the phonon mean free path [2,3].<br/>In this work, we are investigating the phonon interference effect in superlattice (SL) nanowires through thermal conductivity measurements. Thermal conductivity of SL nanowires is measured using thermal bridge method using a microdevice which consists of two SiNx suspended platforms with platinum coils that act as a heater and sensor. These platforms aresupported by 0.5mm long suspended SiNx beams, onto which gold lines are deposited, to ensure thermal isolation [4]. The nanowire is suspended between the two platforms using a micromanipulator. To measure the thermal conductance, the temperature is raised on one platform while the change in temperature on the second platform is measured as a function of heating power. To assess the temperature, the resistance of meanders is measured by four-point probe technique and the heat flux across the nanowire is calculated by measuring the power dissipated in the meanders [5].<br/><br/>The thermal conductivity of GaAs-GaP superlattice nanowires is measured for various SL periods from 4.8 to 23.3 nm. The nanowire sample consists of a GaAs-GaP superlattice embedded between two GaP segments. It is observed that the thermal conductivity decreases with decreasing period length and there is a minimum around the period of 8 nm, afterwards the thermal conductivity increases for smaller periods. The minimum in thermal conductivity occurs at the transition between particle-like and wave-like behavior of phonons. In the incoherent regime (i.e. particle-like behavior), the thermal conductivity decreases due to scattering from the increasing number of interfaces. When the period length is comparable to the phonons coherence length in the system, phonons scattered from the interfaces start to interfere consequently increasing the thermal conductivity. This work is a promising step toward using SL nanowires as a powerful platform for exploiting coherent phonons in thermal applications.<br/><br/>[1] M. Maldovan, Nature <b>503</b>, 209 (2013).<br/>[2] M. Maldovan, Nature Materials <b>14</b>, 667 (2015)<br/>[3] J. Ravichandran, Nature Materials<b> 13</b>, 168 (2014).<br/>[4] P. Kim, Physical Review Letters <b>87</b>, 19 (2001).<br/>[5] L. Shi, Journal of Heat Transfer <b>125</b>, 881 (2003).