Nanoscale Laser Flash Measurements of Ballistic Heat Currents in AlN Heterostructures

This study aims to understand the interplay of ballistic heat currents, interfacial phonon scattering, and mean-free-path distributions on heat conduction in AlN heterostructures. On submicron length-scales, theory predicts that heat transfer will be carried across an AlN heterostructure by a combination of ballistic and diffusive heat-currents [1,2]. The fraction of ballistic vs. diffusive heat currents is expected to depend the mean-free-path distribution of AlN and the frequency dependence of interfacial phonon scattering rates. Experimental metrologies like time-domain thermoreflectance and frequency-domain thermoreflectance cannot test such theoretical predictions because they struggle to differentiate ballistic vs. diffusive heat-currents. In this talk, we describe a new experimental approach that overcomes this challenge. Our approach relies on integrating an epitaxial buried heater layer into an AlN heterostructure during sample growth. We used MBE to grow an epitaxial 60nm-TiN/650nm-AlN/60nm-NbN tri-layer on sapphire. We then performed nanoscale laser flash measurements of heat conduction [3,4] across the AlN between 80 and 600 K. We monitor the time-evolution of the surface TiN layer’s temperature after ultrafast heating of the buried NbN layer. Energy carried ballistically traverses the AlN layer at the speed of sound, while energy transported diffusively arrives more slowly. By comparing our experimental data to theoretical simulations, we were able to quantify the importance of sub-continuum ballistic transport effects. At room temperature, we observe that approx. one quarter of the heat-current across the 650nm AlN layer is ballistic. Our experimental study improves fundamental understanding of how heat evolves as a function of time and space in sub-continuum heat transfer problems, and improves understanding of the frequency-dependence of interfacial phonon scattering.<br/><br/>Acknowledgement: This work was supported as part of ULTRA, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0021230.<br/><br/><i>References</i><br/>[1] Chen, G. Ballistic-Diffusive Heat-Conduction Equations. <i>Phys Rev Lett</i> <b>2001</b>, <i>86</i> (11), 2297–2300. https://doi.org/10.1103/PhysRevLett.86.2297.<br/>[2] Joshi, A. A.; Majumdar, A. Transient Ballistic and Diffusive Phonon Heat Transport in Thin Films. <i>J Appl Phys</i> <b>1993</b>, <i>74</i> (1), 31–39. https://doi.org/10.1063/1.354111.<br/>[3] Peng, W.; Wilson, R. B. Nanoscale Laser Flash Measurements of Diffuson Transport in Amorphous Ge and Si. <i>APL Mater</i> <b>2022</b>, <i>10</i> (4), 041111. https://doi.org/10.1063/5.0082627.<br/>[4] Peng, W.; Wilson, R. B. Thermal Model for Time-Domain Thermoreflectance Experiments in a Laser-Flash Geometry. <i>J Appl Phys</i> <b>2022</b>, <i>131</i> (13), 134301. https://doi.org/10.1063/5.0082549.