Pump-probe measurements of thermal transport across metal-metal interfaces and semiconductors

Date
2019
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University of Delaware
Abstract
First, I test a previously proposed model, called the diffuse mismatch model for electrons (DMMe), that mathematically predicts the thermal conductance of metal-metal interfaces. Prior to my work, there were only two experimental works based on a limited materials set (Al-Cu and Pd-Ir) with which to compared experiment and the DMMe theory. It was an open question whether the DMMe should hold in more general cases - especially in cases where at least one material does not behave like a free-electron metal and/or the interface is smooth enough to allow non-diffuse transmission of electrons. To address this, a set of aluminum-X samples (X = Cu, Ag, Fe, Ni) were grown and we used the time domain thermoreflectance (TDTR) technique to measure the metal-metal interface conductance. We then compared the data to two variants of the DMMe - using both a crude theory based on free-electron metals, and using accurate band structures provided by density functional theory. ☐ Second, I develop a version of TDTR using ultraviolet light (UV-TDTR) to directly excite semiconductors. At λ = 400nm, the photon energy is much greater than most semiconducting bandgaps, potentially allowing semiconducting transducers to absorb light within about 10nm of the surface, potentially enabling direct measurements of semiconductor-semiconductor interfaces. I describe measurements of the thermoreflectance coefficient for some direct-bandgap semiconducting materials. We fit thermal transport models to thermorefectance data collected from the system. We have identified some challenges that are not yet resolved: (1) It appears that III-V semiconductors degrade over the course of several minutes to hours under UV light, and this occurs at lower laser fluences than can be explained based on thermal damage thresholds. (2) I found that the UV-TDTR signal from bulk semiconducting samples does not match the expectation of a thermal models if we assume that the heat is both absorbed and probed near the surface. Since both observations were potentially related to electron-hole pair lifetime and/or diffusivity, we studied differences in the signal for semiconductors with different doping and nanostructuring conditions, which should change the recombination rate. However, no significant changes to the signal were observed.
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