The Role of Crystalline Imperfections on the Thermal Conductivity of Functional Oxide Thin Films

Understanding nanoscale thermal transport in functional oxide thin films is critical for a wide variety of applications. In particular, these materials are frequently used as gate dielectrics and insulating buffers in electronic and thermoelectric devices. Given the imperfect structural nature of most functional materials, it is critical to understand how defects, dislocations, and varying degrees of crystalline disorder impact thermal transport in oxide thin films. This work attempts to contribute to this body of knowledge by focusing on two main studies.
First, we consider the effect of defects on thermal conductivity. Phonon scattering in crystalline systems can be strongly dictated by a wide array of defects, many of which can be difficult to observe via standard microscopy techniques. We experimentally demonstrate that the phonon thermal conductivity of MgO thin films is proportional to the crystal coherence length, a property of a solid that quantifies the length scale associated with crystalline imperfections. Sputter deposited films were prepared on (100) silicon and then annealed to vary the crystalline coherence, as characterized using x-ray diffraction line broadening. We find that the measured thermal conductivity of the MgO films varies proportionally with crystal coherence length, which is ultimately limited by the grain size. The microstructural length scales associated with crystalline defects, such as small angle tilt boundaries, dictate this crystalline coherence length and our results demonstrate the role that this length scale plays on the phonon thermal conductivity of thin films. Our results suggest that this crystalline coherence length scale provides a measure of the limiting phonon mean free path in crystalline solids, a quantity that is often difficult to measure and observe with more traditional imaging techniques.
Second, we study density and length scale effects in amorphous thin films. We measure the room temperature thermal conductivity of atomic layer deposition-grown amorphous Al2O3 and TiO2 thin films as a function of film thickness and atomic density. For films thinner than ~50 nm, we measure an effective thermal conductivity that is reduced with decreasing film thickness. This dependence is attributed to the increased influence of thermal boundary resistances as film thickness is reduced. In addition, we fit for a thickness-independent intrinsic thermal conductivity using a series-resistor model. For films thicker than ~50 nm, there is no significant dependence on thickness or substrate. We observe a strong density dependence of the thermal conductivity, which agrees well with a differential effective-medium approximation modified with a minimum limit model.