Theoretical modelling of nano-structured (Hf,Zr)N/(Sc,Y)N metal/semiconductor superlattices for thermoelectric energy conversion

Abstract

Thermoelectric materials that convert heat flux into electrical energy are a topic of great scientific interest because of their potential to deal with the present day energy challenges. Wide range applications and commercialization of these materials though critically depends on our ability to improve their efficiency by at least two to three folds from the present day laboratory value. The present thesis work deals with modelling how electrons and phonons behave in nano-structured metal/semiconductor superlattices based on (Hf,Zr)N/(Sc,Y)N system, using first-principles density functional theory based calculations. Electronic structure, vibrational spectra, and thermal properties of these nitrides in bulk form are determined. The superlattices, made of these nitrides are simulated to understand their potential and suitability for thermoelectric applications. The first chapter of the thesis provides a brief introduction to thermoelectric materials, and related research. Starting from basic concepts like Seebeck and Peltier effect, we have invoked modern concepts like Mahan & Sofo theory, and Boltzmann transport theory to present the theoretical basis of advanced thermoelectric materials. The conflicting design challenges that prevent us from developing highly efficient thermoelectric materials are also discussed. Insightful pictures about the metal/semiconductor superlattices are given, and their working principle is discussed. In the second chapter, we present the electronic structure, vibrational spectra, and thermal properties of bulk ScN, ZrN, and HfN. Hubbard U correction is used to fit the band gap with the experimental values and to connect it with the transport related effective mass calculations. Vibrational spectra, lattice specific heat, and lattice thermal conductivity are also estimated to understand their potential to be used as component materials for metal/semiconductor superlattices. In the third chapter, we have used a combination of (1) generalised gradient approximation (GGA), (2) A Hubbard U correction along with GGA, and (3) GW approximation to estimate band gaps, effective masses, and volume deformation potentials of YN. Vibrational spectra, and thermal properties are estimated to understand its potential and suitability for alloying with other nitrides for thermoelectric applications. The last chapter of this thesis contains extensive theoretical analysis of ZrN/ScN and HfN/ScN metal/semiconductor superlattices for thermoelectric applications. The crystal structure, interface energy density, and inter planer distances are discussed. Electronic structures of these superlattices are calculated, and enhancement of Seebeck coefficient along the growth direction is discussed. Vibrational spectrum, lattice thermal conductivity, and its dependence on temperature, and contribution of phonons as a function of frequency is analysed. Finally we comment on the relative superiority of HfN based systeM.S. over ZrN ones.