Crystalline coherence length effects on the thermal conductivity of MgO thin films

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's coherence length, a property of a solid that quantifies the length scale associated with crystalline imperfections. Sputter-deposited films were prepared on (100)-oriented 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 crystalline 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 has 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 imagining techniques.