Characterization of Thermal Properties of Materials And Thermal Transport Phenomena Using Thermoreflectance Method And First Principles Study

Abstract

Understanding heat transport phenomena at the nanoscale has gained more traction in the recentdecade, especially in the last couple of years, due to unprecedented innovation in the field of Artificial Intelligence and Machine Learning tools. Continuous progression of Moore’s Law has led to the incorporation of billions of transistors into a smaller package as length scales decrease, but computational needs increase. This has accelerated our need to understand heat transport at the nanoscale, which can aid in designing efficient nano-structured and nano-engineered devices for effective thermal management or energy conversion applications. Therefore, there is a pressing need to evaluate novel materials that can be potential candidates to meet the approaching demands of the future. Improvements in the capabilities of pump-probe Measurement techniques such as Frequency Domain Thermoreflectance (FDTR) can allow accurate experimental determination of the thermal properties, such as volumetric heat capacities, thermal conductivities, anisotropy, and thermal boundary conductance of novel 2D thin film and bulk materials. Additionally, the application of computational tools like Quantum ESPRESSO and ShengBTE has aided in developing insights on the effectiveness of a potential material system by evaluating its thermal transport properties. The work in this thesis will demonstrate that by establishing a high degree of agreement in the experimental and computational results, one can quantify the factors that contribute to the material’s thermal properties. Using FDTR, we measured the bulk properties of ZnSe and ZnTe materials and demonstrated a good agreement between the results from the experimental and computational methods. Our experimentally measured thermal conductivities ( 17Wm-1K-1 for ZnSe and 14Wm-1K-1 for ZnTe) were consistent with first-principles predictions (23.2 Wm-1K-1 and 13.72 Wm-1K-1at 300 K). Computational results provided us with insights examined the length-dependent thermal conductivity of both materials, further showing the effect of phonon–boundary scattering being present at modern device dimensions. We then examined STO thin films at varying thicknesses to show FDTR’s sensitivity to nanoscale thermal transport, where we observed a significant reduction of 93% from the bulk value for the 12 nm thin film. This indicated the consequence of boundary scattering and substrate effects. Sensitivity analysis showed the measurement to be sensitive to cross-plane thermal conductivity and volumetric heat capacity properties, which made their evaluation possible. The study illustrated the underlying mechanisms of thermal transport in the oxide perovskite thin films. We need to understand the thermal properties of oxide thin-film systems, especially those with silicon substrates, as they serve as an important platform for integration of these materials in microelectronic devices. Through the computational study of intermetallic materials like ReSi1.75, ReAlSi, and ReGaSi, we are the first ones to report the thermal properties of the materials. The first-principles calculations allow us to understand the thermal transport phenomena in bulk and nanoscale, as well as understand their temperature-dependent properties. This study was essential in proving the potential of the materials for thermoelectric applications and provides insights into how the material can be tuned for its application to developing efficient thermoelectric devices. The final experimental chapter examined the effects of annealing of Strontium Chromate thin films films under different environments—a process with limited literature due to the difficulty of computational modeling because of the strong electron correlations exhibited by the materials of this family. Measurements performed by FDTR showed that after annealing, there were observable changes in thermal conductivity and heat capacity, which were consistent with observed electronic measurements that show a transition towards metallic behavior. Further study is necessary to determine the microscopic origin of these changes, but this work establishes a foundational background With regard to SCO thin films and the ability of FDTR to characterize thermally complex oxide systems grown on silicon, adding to the barely available literature. The successful experimental analysis of STO enabled us to experimentally characterize the thermal properties of Strontium Chromate (SCO) thin films on the SI substrate. Here, we attempted to characterize the effects of annealing environments on the thermal properties of the SCO thin films. We were able to quantify the effects of annealing environments, along with validating their phase transition from semiconducting to metallic behavior, which was observed in the literature through electrical and structural characterizations.