Executive Summary : | The present work is a continuation of the DST start-up grant project titled ‘Thermal Transport Beyond Fourier’s Law of Heat Conduction,’ where we have developed a parallelized Monte Carlo solver for the Boltzmann transport equation (BTE) in three dimensions (https://github.com/abhipath90/MCBTE). To the best of our knowledge (also echoed by external referee reports), our code is the first to simulate the ballistic, quasi-ballistic, and diffusive thermal transport at device-level geometries, i.e., nanoscale to the 100 microns. We aim to extend our simulation work to measure thermal transport in a non-Fourier’s regime with the proposed work. In the next three years, our focus is to measure and simulate nanoscale heat generation sources instead of nanoscale samples, i.e., thin films. Nano-heaters are abundant in the semiconductor industry as the transistor’s gate size is aggressively scaled down to 1nm. A combined experimental and simulation approach is necessary to develop robust methods and validate them. These methods will solve grand thermal energy management challenges as actively pursued by the semiconductor, solar photovoltaic and thermoelectric, and bioengineering communities. In particular, our solution will provide direct input to the semiconductor industry to further the Government of India’s impetus for the in-house development of semiconductor chips under the Atmanirbhar Bharat mission. The experimental probing of heat transfer from nano heaters requires measurements at ultrafast timescale (100’s of fs to 10’s of ns) and short wavelengths (10’s of nm). Consequently, we can not use ultrafast visible or near-infrared lasers to probe the heat dissipation from diffraction geometry. Instead, we require soft x-rays at an ultrafast time scale for measurements. The generation of soft x-rays (wavelength ∼20 to 35 nm) using high-harmonic generation from ultrafast Ti:Sapphire near-infrared lasers have been recently demonstrated at RRCAT, Indore. We are presently modifying their setup to achieve desired experimental parameters. Besides the experimental front, we plan to model nanoscale heat transport using two complementary techniques – Monte Carlo simulations of BTE and coupled finite element- Fresnel diffraction, assisted by direct measurements of phonons. Monte Carlo simulations of BTE will simulate the origin of thermal resistance from boundaries and scattering events of individual phonon bundles, while coupled finite element-Fresnel diffraction approach will simulate the experimental geometry and calculate the time-dependent diffraction from surface deformations. Combining both simulation techniques will enable us to analyze experimental geometry and data in detail and rationalize the governing physics of nanoscale heat sources. Moreover, our laboratory’s existing data on phonons and their mean free path and additional vibrational spectroscopy experiments will allow for an experimental feedback loop to simulations to improve modeling fidelity. |