Investigation of thin film thermal transport using micro-Raman thermometry and tip enhanced Raman spectroscopy

In recent years, a steady increase in the need for packing more energy in smaller devices had driven the need for better understanding of thermal transport in thin films and designing methods to manipulate them. This can be attributed as the reason for numerous efforts made in the past decade to achieve temperature resolution at nanoscale. Such techniques can help us in understanding the thermal conductivity of thin films which can result in an improved performance of these devices. The temperature probing technique must be non-invasive to avoid damaging the vulnerable thin films, fast to study the energy transport and should high reliability. In this work, a thermal measurement system based on Raman spectroscopy is described in detail. The micro-Raman thermometer provides a non-contact, optical and reliable method to measure thermal conductivity of thin films. Thermal conductivity of thin films of two-dimensional materials and topological insulators were determined. Bismuth-Antimony-Telluride-Selenide material group was studied in detail because of high figure of merit, ZT value in room temperature and effect of topological surface state at thin films. The bulk electron thermal conductivity of Bi0.1Sb1.9Te 3 alloy was determined. The Lorenz number for the thermoelectric material was extracted by measuring samples with varied electrical conductivity. The change in thermal conductivity of Bi2Te2Se with respect to thickness was evaluated which showed an enhanced thermal conductivity as thickness reduces. To improve the spatial resolution of the micro-Raman thermometer, the Raman spectrometer was combined with an atomic force microscope to provide a side- illumination Tip enhanced Raman spectrometer. The TERS setup established was tested using bismuth telluride and brilliant cresyl blue (BCB) for enhancement. The challenges in achieving a significant enhancement in Raman signal is discussed and efforts were made to overcome them. The enhancement obtained was much pro- nounced for BCB when compared to Bi2Te3. The enhanced Raman signal intensity is calibrated to achieve temperature sensing.