At the nanoscale, the phonon transport is heavily suppressed by the scattering at the interface . Examples of such interfaces include grain boundaries within a polycrystal or heterostructure interfaces within a nanocomposite. The phonon scattering by these interfaces can reduce the lattice thermal conductivity. When bulk-like electrical properties can still be conserved, improved thermoelectric performance can be achieved in various nanostructured bulk materials.
In numerous molecular dynamics simulations, the interfacial thermal resistance (RK) of a grain boundary has a strong dependence on the misorientation between two grains [2,3]. Similar crystal-orientation dependence is anticipated for a heterostructure interface but the phonon transport process becomes more complicated. Interfacial atom diffusion and participation of optical phonons are two main concerns in such cases. With film-wafer bonding, high-quality Si-Ge heterostructure interfaces have been obtained with its high electrical conductance . Such interfaces provide an ideal model system for interfacial studies. However, the corresponding thermal study is still lacking.
In this work, a 70-nm-thick Si thin film is hot pressed onto a Ge wafer to represent a Si-Ge heterostructure interface formed in SiGe nanocomposites. The interfacial RK is measured using the 3w method for varied crystal misorientations. Strains at the bonded interface are measured with Raman spectroscopy to reveal its relationship with RK, along with interfacial structure characterization by transmission electron microscopy. These detailed interfacial thermal studies can provide important guidance for interface engineering to tune heat transport inside a material or device.
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