Phonon Transport In Wurtzite Nitrides And Their Alloys From First-principles

Wurtzite AIN, GaN, InN and their alloys have been widely used in optoelectronic devices, solar cells and transistors and so on. In these applications, the thermal conductivities of these materials are crucial for the device performance, reliability and lifetime. The thermal conductivity determines the efficiency of heat dissipation, it is then an important part of the thermal management. Even the thermal conductivity can be experimentally measured, due to the limitation of crystal growth, the intrinsic thermal conductivities of wurtzite AIN, GaN, InN and their alloys are still lacking, especially for wurtzite InN. In microscale, the heat in crystal is dominantly carried by phonons which can be described by the phonon Boltzmann Transport Equation (BTE). In the process of drift, the phonon can be scattered by intrinsic scattering and sample-dependent scattering. The intrinsic scattering, determining the intrinsic thermal conductivity, is always challenging but can be calculated from first-principles recently. In this work, the first-principles calculation combined with the phonon BTE is used to investigate the thermal conductivities and size effect of wurtzite AIN, GaN, InN and their alloys.At room temperature, the in-plane and out-of-plane thermal conductivities of naturally isotopical wurtzite AIN are 301 Wm-1K-1 and 287 Wm-1K-1 respectively, while they are 244 Wm-1K-1 and 277 Wm-1K-1 for wurtzite GaN,133 Wm-1K-1 and 152 Wm-1K-1 for wurtzite InN. The thermal conductivities of wurtzite AIN at different temperatures are almost the same in the in-plane and out-of-plane directions, while the anisotropy of wurtzite GaN and InN cannot be neglected especially at low temperatures. The distribution of average squared group velocity reveals the anisotropy of thermal conductivity is mainly contributed by low-frequency phonons, and mainly from the high-frequency transverse acoustic modes. The cumulative thermal conductivity versus phonon mean free paths and the thickness dependent thermal conductivity of film show the size effect of wurtzite AIN, GaN and InN can persist up to a few tens of micrometers.Based on the first-principles calculated parameters of wurtzite AIN, GaN, InN and the virtual crystal model, the dependences of the thermal conductivities of wurtzite AlxGa1-xN?InxGa1-xN and InxAl1-xN on alloying concentration are investigated. It is found that a small amount of alloying can reduce the thermal conductivity significantly. For instance, with only 1% alloying of Al or In atoms, the thermal conductivity of wurtzite GaN decreases about 60%. With the alloying concentration between 0.2 and 0.8, the thermal conductivity remains almost unaffected. At room temperature, the minimal in-plane thermal conductivities are 18 Wm-1K-1,22 Wm-1K-1 and 8 Wm-1K-1, while the minimal out-of-plane thermal conductivities are 22 Wm-1K-1,27 Wm-1K-1 and 10 Wm-1K-1 for AlxGa1-xN, InxGa1-xN and InxAl1-xN, respectively. The anisotropy of alloys is larger than its components. This is because the alloy scattering has stronger effect on high-frequency phonons than low-frequency phonons, the relative contribution from low-frequency phonons which have larger anisotropy is increased. The size effect of alloys can still persist up to a few tens of micrometers, and the thermal conductivity can be reduced by half in about 100nm thick film.As the relaxation time approximation (RTA) of phonon BTE underestimates the thermal conductivity, the Callaway model regarded as an improvement is widely used. However, its accuracy in studying the lattice thermal conductivity is never known. Based on full first-principles calculations of silicon, diamond and wurtzite AlN, the accuracy of Callaway model is examined. The results clarify that the Callaway model cannot guarantee the accuracy of thermal conductivity calculation. In these three systems, it is also found that the relaxation times for U processes are scaled as l/?3 at low frequencies for both transverse acoustic (TA) and longitudinal acoustic (LA) modes, and those for N processes are scaled as 1/? and 1/?2 for TA and LA modes, respectively.