Detecting coherent phonon wave effects in superlattices using time-domain thermoreflectance

Abstract

Superlattices (SLs), structures consisting of periodic layers of thin films of several angstroms to tens of nanometers thick, have unique electrical and thermal properties that make them well suited for applications in optoelectronics and as fundamental learning tools in the realm of thermoelectrics. One unique characteristic of SLs is their low thermal conductivity compared to a bulk material with the same molecular composition. This property has given rise to extensive theoretical and experimental investigations regarding thermal transport through SLs. The different thermal transport characteristics have been studied in the context of various transport regimes. In this thesis, an experimental investigation of thermal transport in the coherent regime through a SL is presented. The trend in thermal conductivity that can be expected if such coherent wave effects exist is derived from the Landauer-Biittiker formalism, which treats energy transport as a transmission process. The frequency-dependent transmission probability for phonons through the SL is found via an application of the transfer matrix method (TMM). The calculations show that the integral effect of the buildup of phonon stopbands in the SL is minimal. Thus, if coherent wave effects are present, the conductance of the SL is nearly constant as the number of periods is increased, and the thermal conductivity, which is the product of the conductance and the total thickness of the SL, increases linearly with number of periods. To test the predictions, five GaAs/AlAs SLs with one, three, five, seven, and nine periods of one layer of GaAs of 12 nm thickness, and one layer of AlAs of 12 nm thickness are grown using MOCVD. The thermal conductivities of the SLs are measured using a transient thermoreflectance (TTR) technique at temperatures ranging from 30K to 300K. The results are the first-ever experimental evidence for the presence of coherent wave effects in heat transport through SLs.