(Invited) Phonon Transport in Nanoporous Si Films — from Periodic Nanopores to Nanoslots
Authors: Hao Qing 
Published: February 2020

Thermal transport within periodic nanoporous thin films has been widely studied for their potential applications in thermoelectrics and other phonon-related applications (1-4). A good thermoelectric material should have a high electrical conductivity σ, a high Seebeck coefficient S, and a low thermal conductivity k. The combination of these properties leads to a high dimensionless thermoelectric figure of merit (ZT), defined as ZT=S2σT/k, where T represents the absolute temperature. In nanoporous Si thin films, phonons can be diffusively scattered by nanopore edges to reduce the lattice part of the thermal conductivity (kL
), as the classical phonon size effects. On the other hand, electrons with usually much shorter mean free paths (MFPs) are slightly affected, leading to maintained S2σ and improved ZTs. With ultrafine periodic porous features, the phonon dispersion can also be modified due to coherent phonon transport within a periodic structure. This phenomenon involves the wave nature of lattice vibrations and is known as phononic effects. In this situation, kL
can be largely reduced to enhance ZTs.

Among existing studies on periodic nanoporous Si films, inconsistency can often be found among experimental and theoretical studies of the reduced lattice thermal conductivity for varied nanoporous patterns. Such divergence can be partially attributed to measurement errors and pore-edge damage introduced by varied nanofabrication techniques, namely reactive ion etching (RIE), deep RIE (DRIE), and a focused ion beam (FIB). To evaluate the impact of phononic effects, the thermal conductivities of periodic and aperiodic nanoporous Si films are compared in previous studies (5, 6). Along another line, the specific heat C, solely depending on the phonon dispersion, can also be measured to justify the phonon dispersion variation (7). In this work, the thermal conductivity of the same Si thin film is continuously measured with added rows of nanopores drilled by a FIB. When phononic effects exist, it is anticipated that the thermal resistance can be largely increased from single to multiple rows of nanopores. Without phononic effects, the thermal resistance of a patterned Si film should linearly increase with the number of nanopore rows.

Beyond Si films with periodic nanopores, other nanoporous patterns are also investigated for their impact on the thermal transport, e.g., equally spaced nanoslots patterned on a Si thin film. When the neck width between adjacent nanoslots is smaller than the dominant phonon MFPs but longer than the electron MFPs, largely improved thermoelectric performance is anticipated (8). An analytical model has been developed to compute the transport properties of such structures, which yields identical results as complicated phonon and electron MC simulations. To compare with theoretical predictions, thermal measurements are also carried out on a Si thin film with nanofabricated neck region in its middle.

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