Abstract
Non-equilibrium molecular dynamics (MD) simulation is performed to calculate the thermal conductivity of defect-free GaN along three high-symmetry directions. It is found that the thermal conductivity along [001] direction is about 25% higher than that along [100] or [120] direction. The calculated phonon dispersion relation and iso-energy surface from lattice dynamics show that the difference of the sound speeds among the three high-symmetry directions is quite small for the same mode. However, the variation of phonon irradiation with direction is qualitatively consistent with that of the calculated thermal conductivity. Our results indicate that the anisotropic thermal conductivity may partly result from the phonons in the low-symmetry region of the first Brillouin zone due to phonon focus effects, even though the elastic properties along the three high-symmetry directions are nearly isotropic. Thus, the phonon irradiation is able to better describe the property of thermal conductivity as compared to the commonly used phonon dispersion relation. The present investigations uncover the physical origin of the anisotropic thermal conductivity in defect-free GaN, which would provide an important guide for optimizing the thermal management of GaN-based device.
Highlights
GaN is a direct bandgap semiconductor material with wide application range in blue LED,1 laser diode2 and other high power optoelectronic devices.3 The performance and reliability of these devices strongly depend on the heat dissipation in the active region
Our results indicate that the anisotropic thermal conductivity may partly result from the phonons in the low-symmetry region of the first Brillouin zone due to phonon focus effects, even though the elastic properties along the three high-symmetry directions are nearly isotropic
The results show that the thermal conductivity along [001] is higher than that along both [100] and [120] directions at room temperature when the simulation thickness is from about 25 to 220 nm
Summary
GaN is a direct bandgap semiconductor material with wide application range in blue LED, laser diode and other high power optoelectronic devices. The performance and reliability of these devices strongly depend on the heat dissipation in the active region. With improving crystal growth technology and increasing demand for high-purity GaN, the density of the defects may become lower and lower. On this condition, the heat conduction may only depend on the intrinsic phonon transport properties. The present investigation indicates that the phonon irradiation is able to better describe the property of thermal conductivity as compared to the commonly used phonon dispersion relation. This is because the former considers all the phonon contribution in the first Brillouin zone (FBZ), while the latter only considers one specified direction
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