First-principles Study On Thermal Transport Of Gallium Nitride And Its Doping System

As the third-generation wide-bandgap semiconductor,gallium nitride(GaN)has a wide range of applications in semiconductor lighting,laser detectors,radio frequency and microwave devices,power electronics and other fields due to its excellent electrical and optical properties.Since intrinsic semiconductor has a single property,doping is often used to make GaN obtain suitable properties to meet different requirements.The study of the heat transport properties of doped GaN is of great significance for promoting effective heat dissipation and prolonging the life of the device.In this thesis,the effects of carbon(C)and indium(In)atomic substitutions on heat transfer are studied by using first principles calculations under the framework of density functional theory and phonon Boltzmann transport equation.The calculated thermal conductivity of undoped three-dimensional gallium nitride(3D-GaN)and two-dimensional gallium nitride(2D-GaN)are 246W/mK and 65W/mK,respectively,and carbon atoms replace three-dimensional gallium nitride(3D-GaCN)and two-dimensional single-layer gallium nitride(2D-GaCN)have thermal conductivity of 146W/mK and 12W/mK,respectively.Indium atoms replace three-dimensional gallium nitride(3D-InGaN)and two-dimensional single-layer gallium nitride(2D-InGaN)thermal conductivity is 121W/mK and 5 W/mK respectively.Compared with the high thermal conductivity of original gallium nitride,the thermal conductivity of doped three-dimensional gallium nitride and two-dimensional gallium nitride are significantly reduced by half and nearly an order of magnitude,respectively.The results show that there is a large acousto-optic phonon band gap in GaN due to the large mass variation of GaN and Ga atoms,and the low frequency phonon mode plays a leading role in heat transport.Our calculation confirms that the highly suppressed phonon lifetimes together with the reduced phonon group velocity are responsible for the large reduction of thermal conductivity in the doped GaN.We further elucidate that the reduced phonon bandgap and the presence of the localized vibrations expand the scattering phonon space,thus highly suppressing phonon lifetimes in the low-frequency rangeUsing the method of virtual lattice approximation,combined with the first principles,the thermal conductivity of the alloy InxGa1-xN is calculated as a function of the alloy concentration.The study found that when the concentration is small(0<x<0.2,0.8<x<1),the thermal conductivity of the alloy is greatly affected by the concentration,and when the concentration is large(0.2<x<0.8),the thermal conductivity of the alloy is affected by the concentration.The impact is small.When x=0.5,the thermal conductivity of the alloy(Ino.5Ga0.5N)is the smallest,19W/mK,which is much smaller than the thermal conductivity of 3D-InGaN(121 W/mK).This is because the virtual lattice approximately ignores the bonding distance between different elements and the local difference on the charge axisOur findings expound the reasons for the large reduction of thermal conductivity in GaN with substitution and provide intrinsic physical insights into the underlying phonon scattering mechanisms.Moreover,the relationship between the thermal conductivity of InxGal-XN alloy and the alloy concentration is obtained,which provides guidance for the selection of dopant and doping measurement in device design.