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Open Access Publications from the University of California

Scholarly Works (2 results)

  • Thesis
  • Peer Reviewed
UC Riverside Electronic Theses and Dissertations (2024)

Nanoscale devices and advanced materials have introduced significant challenges in thermal management due to the increasing importance of both phonon transport within materials and across interface. As devices continue to shrink, the efficient dissipation of heat becomes a critical factor in the performance and reliability of high-power electronics. In materials such as ultra-wide bandgap semiconductors and doped thin films, phonons serve as the primary heat carriers. However, their transport is hindered by scattering mechanisms that reduce thermal conductivity, especially in doped and defected materials. Additionally, interfaces between dissimilar materials play a major role, as thermal boundary resistance at these interfaces can significantly limit thermal transport.As a result, understanding nanoscale thermal transport across interfaces and within novel materials is essential for expanding fundamental knowledge, accurately modeling and predicting nanoscale heat transfer, and ultimately improving the thermal management of modern electronics. This dissertation focuses on phonon transport across interfaces and through novel ultra-wide bandgap materials. By using ultrafast pump-probe laser techniques, including time-domain thermoreflectance (TDTR) and picosecond magneto-optic thermometry, thermal transport is probed in various materials and across interfaces to understand the dynamics of energy carriers and how these carriers interact with defects, doping, and interfacial boundaries. This research first investigates thermal boundary conductance, which measures the efficiency of heat transfer across interfaces between dissimilar materials, particularly between metals and insulators commonly used in spin caloritronic systems. Using time-domain thermoreflectance, we quantify the thermal boundary conductance at metal-insulator interfaces and reveal how interfacial quality, such as roughness or contamination, impacts heat transfer. Additionally, we explore methods to improve interfacial thermal conductivity and enhance heat flow across metal bi-layers through in situ and ex situ synthesis processes for metal/metal interfaces. Based on these findings, we implement a new methodological approach to measure sub-micron high thermal conductivity films, particularly at length scales that challenge the current limits of pump-probe techniques. Finally, we examine thermal transport in ultra-wide bandgap materials such as boron-doped diamond, phosphorus-doped diamond, and silicon-doped aluminum nitride, demonstrating how doping affects phonon interactions and can significantly reduce the thermal conductivity of these materials by introducing new scattering mechanisms

    Cover page: Probing Thermal Transport Across Interfaces and Novel Materials Using Ultrafast Pump-Probe Laser Techniques
    • Article
    • Peer Reviewed
    UC Riverside Previously Published Works (2022)
    The goal of this study is to determine how bulk vibrational properties and interfacial structure affect thermal transport at interfaces in wide band gap semiconductor systems. Time-domain thermoreflectance measurements of thermal conductance G are reported for interfaces between nitride metals and group IV (diamond, SiC, Si, and Ge) and group III-V (AlN, GaN, and cubic BN) materials. Group IV and group III-V semiconductors have systematic differences in vibrational properties. Similarly, HfN and TiN are also vibrationally distinct from each other. Therefore, comparing G of interfaces formed from these materials provides a systematic test of how vibrational similarity between two materials affects interfacial transport. For HfN interfaces, we observe conductances between 140 and 300 MW m-2 K-1, whereas conductances between 200 and 800 MW m-2 K-1 are observed for TiN interfaces. TiN forms exceptionally conductive interfaces with GaN, AlN, and diamond, that is, G > 400 MW m-2 K-1. Surprisingly, interfaces formed between vibrationally similar and dissimilar materials are similarly conductive. Thus, vibrational similarity between two materials is not a necessary requirement for high G. Instead, the time-domain thermoreflectance experiment (TDTR) data, an analysis of bulk vibrational properties, and transmission electron microscopy (TEM) suggest that G depends on two other material properties, namely, the bulk phonon properties of the vibrationally softer of the two materials and the interfacial structure. To determine how G depends on interfacial structure, TDTR and TEM measurements were conducted on a series of TiN/AlN samples prepared in different ways. Interfacial disorder at a TiN/AlN interface adds a thermal resistance equivalent to ∼1 nm of amorphous material. Our findings improve fundamental understanding of what material properties are most important for thermally conductive interfaces. They also provide benchmarks for the thermal conductance of interfaces with wide band gap semiconductors.
      Cover page: Properties for Thermally Conductive Interfaces with Wide Band Gap MaterialsCreative Commons 'BY' version 4.0 license
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