Measurement of In-Plane Thermal Conductivity of Ultrathin Films Using Micro-Raman Spectroscopy

We report a micro-Raman-based optical method to measure in-plane thermal conductivity of ultrathin films. With the use of 20-nm-thick SiO2 substrates that assure in-plane heat transfer, sub-100-nm Bi films and Al2O3 films as thin as 5 nm were successfully measured. The results of Bi films reveal that phonon boundary scattering, both at the surface/interface and at the grain boundaries, reduces in-plane lattice thermal conductivity. The measurements of amorphous Al2O3 films were accomplished using thin Bi film as a Raman temperature sensor, and the results agree with the minimum thermal conductivity models for dielectrics. Our work demonstrates that the micro-Raman method is promising for characterization of in-plane thermal conductivity and phonon behaviors of thin-film structures if the Raman temperature sensor material and substrate material are carefully selected.