PROBING MEAN-FREE-PATHS OF THE DOMINANT HEAT-CARRYING PHONONS IN SEMICONDUCTORS AND DIELECTRICS

Abstract

MEAN-FREE PATHS (MFPS) OF PHONONS ARE CRUCIAL FOR UNDERSTANDING HEAT TRANSPORT AT NANOSCALE. IN THIS THESIS, WE PRESENT EXPERIMENTAL STUDIES ON THE MFPS OF PHONONS RESPONSIBLE FOR HEAT CONDUCTION IN SEMICONDUCTORS AND DIELECTRICS. THE PRIMARY TOOL USED IS TIME-DOMAIN THERMOREFLECTANCE (TDTR). WE DEVELOPED A DUAL-FREQUENCY TDTR APPROACH TO ACCURATELY MEASURE THE CROSS-PLANE THERMAL CONDUCTIVITY OF SILICON FILMS OVER A WIDE RANGE OF FILM THICKNESSES AND TEMPERATURES. WE FIND THAT THE CONTRIBUTION OF LONG-MFP PHONONS TO HEAT CONDUCTION IS LOWER THAN FIRST-PRINCIPLES PREDICTIONS BY LINDSAY ET AL. WE DEMONSTRATE THAT ONE POSSIBLE REASON FOR THIS DISCREPANCY COULD BE THE OMISSION OF AKHIESER?S DAMPING IN THE FIRST-PRINCIPLES CALCULATION. WE ALSO DEVELOPED A NOVEL TECHNIQUE CALLED FOURIER TRANSFORM OF TDTR (FT-TDTR) TO PROBE PHONON MFPS. WE DEMONSTRATE EXTRACTION OF TEMPERATURE RESPONSES AT FREQUENCIES UP TO 500 MHZ, AND SHOW THAT THE FREQUENCY-DEPENDENT EFFECTIVE THERMAL CONDUCTIVITY REVEALS