Thermal studies of nanoporous thin films with added periodic nanopores—a new approach to evaluate the importance of phononic effects

Phonon transport within periodic nanoporous materials has been intensively studied for its potential applications in thermoelectrics, heat waveguides, thermal diodes, and heat imaging. It is now acknowledged that wave effects of lattice vibrations, i.e., phononic effects, are only important for ultrafine nanoporous structures and/or at cryogenic temperatures. Experimental evidence is usually found by comparing the thermal conductivities of periodic and aperiodic nanoporous Si films, or by measuring the specific heat to check the phonon dispersion variation. In this work, a new approach is used to evaluate the impact of possible wave effects, simply by comparing the thermal conductivity of the same Si thin film with increased rows of nanopores as drilled with a focused ion beam. In the temperature range of 85–300 K, it is found that the total thermal resistance of the thin film increases almost linearly with the number of rows, indicating dominant incoherent phonon transport or negligible wave effects. For nanopores with small spacing, the amorphous regions around pore edges can largely overlap so that the whole region can be treated as a homogeneous amorphous material with nanoporosity. When the pore spacing is larger, the amorphous pore edges expand the effective pore diameter that is used in the data analysis based on frequency-dependent phonon Monte Carlo simulations. A better understanding of these nanoporous films can benefit their applications in thermoelectrics and thermal management of thin-film-based electronic devices.