Multi-fields Coupling Effects On Phonon And Thermal Properties Of GaN-based Hetero-nanofilms

As a typical representative of the third generation of semiconductor materials,gallium nitride(GaN)has excellent physical and chemical properties,which made it widely used in high-power and high-frequency electronic devices,short-wavelength optoelectronic devices,high-temperature devices and irradiation devices.In practical applications,semiconductor devices suffer from a multi-physical environment,such as electromagnetic field,temperature field,stress field or irradiation field.Multi-physical fields changes the properties of GaN-based heterostructures,and the modification of material characteristics would changes the distribution of physical fields such as internal temperature field,electric field and stress field,resulting in a complex thermo-mechanical-electro coupling behavior.There are few theoretical studies on mechanical and thermal properties of GaN-based heterostructures under multi-fields.In our thesis,a model of AlN(Aluminum nitride)/GaN/AlN-nanofilm is conducted,we analyze the influences of prestress and surface/interface stress on phonon and thermal properties based on the continuum elasticity theory and Boltzmann transport equation.Furthermore,we investigate the effects of thermo-mechanical coupling on heat conduction of the nanofilm using finite element method(FEM).Firstly,in a framework of continuum elastic model,the influence of prestress and film thickness on phonon properties such as phonon dispersion relations in GaN-based heterogenous nanofilms are calculated based on the acoustoelatic theory,.Results show that positive prestress reduces the phonon energy while negative prestress increases it.The thinner the GaN layer is,the more obvious the influence is.Boltzmann transport equation is further used to analyze the variety of phonon thermal conductivity as prestress and temperature.We find that positive prestress reduces the thermal conductivity in shear(SH)and flexural(AS)modes,which is opposite in dilatational(SA)mode.Secondly,we analyzed the phonon and thermal properties under surface/interface stress considering the effect of surface/interface stress on elastic modulus.The elastic model was used to quantitatively analyze the quantum confined phonon in nanofilms under surface/interface stress,and the relationship between surface/interface stress and phonon thermal conductivity was further calculated.Results show that the positive(negative)surface/interface stress increases(decreases)the phonon energy.In SH mode,phonon thermal conducitivty of nano films increases with the surface/interface stress varying from positive to negative,but it is reversed in AS and SA modes.Finally,we investigated the effects of prestress on the heat conduction of GaN-based nanofilm considering the thermo-mechanical coupling,based on the results above.FEM is used to solve the temperature distribution of the nanofilm under heating pulses.It is found that compressive prestress reduces the peak temperature of the film and fasten the heat response through increasing the thermal conductivity.As the pulse number increases,the stress coupling effect becomes more and more significant.In this paper,the influence of prestress and surface/interface stress on the acoustic phonon and thermal properties of GaN-based hetero-nanofilms were quantitatively analyzed using AlN/GaN/AlN-nanofilm as an object.Our results show that the magnitude and direction of the prestress and surface stress,temperature and thickness modify the phonon characteristics jointly,thus adjust the thermal conductivity.On this basis,the influence of thermo-mechanical coupling on heat conduction of semiconductor nanofilms is discussed.This work provides a theoretical reference for the safety and stability of semiconductor devices under multi-physical fields,and also provides a guidance for the precise design and effective applications of semiconductor devices.