Title:	Phonon dispersions in ScAlN and YAlN
DNr:	SNIC 2021/22-551
Project Type:	SNIC Small Compute
Principal Investigator:	Vanya Darakchieva <vanya.darakchieva@liu.se>
Affiliation:	Linköpings universitet
Duration:	2021-07-26 – 2021-10-01
Classification:	10304
Keywords:

Abstract

The goal of the project is to calculate the phonon properties in YxAl1-xN and ScxAl1-xN alloys needed for the interpretation of experimentally determined thermal conductivity of such alloy thin films. More specifically, we aim at calculating the phonon dispersions, Grüneisen parameters and group velocity as a function of Y and Sc content in Y(Sc)xAl1-xN up to x=0.5. The obtained results will be implemented in a semi-empirical Debye-Callaway formalism to model the experimentally obtained thermal conductivity parameters. Alloying ScN or YN with AlN allows to tune the bandgap and polarization [1,2], enabling various optoelectronic and electronic applications of ScxAl1-xN and YxAl1-xN alloys, such as in high-power and high-frequency high mobility field effect transistors. The thermal conductivity of individual layers in device structures is critical for their thermal management design and optimization. Very recently, we have determined the thermal conductivity of Y(Sc)xAl1-xN thin films with 0 < x < 0.22 using the transient thermoreflectance (TTR) technique. We observed a remarkable reduction of thermal conductivity of the Y(Sc)xAl1-xN layers with increasing the Sc and Y contents. For interpretation of the measurements, a modeling for the thermal conductivity is needed. The modeling is based on a semi-empirical Debye-Callaway formalism [3], where the possibility to implement various scattering rates in the model such as phonon-boundary, phonon-point defects, phonon-dislocation, phonon-alloy, and so forth allows a predictive calculation of thermal conductivity of real device structures. The accuracy of this modeling strongly depends on the provided phonon properties of the materials. Therefore, the accurate phonon properties including phonon dispersion, group velocity, and mode Grüneisen parameters as functions of Y and Sc content in Y(Sc)xAl1-xN up to x=0.5 need to be determined and implemented in the model. To achieve accurate phonon properties of the Y(Sc)xAl1-xN alloys, the phonon dispersions of these materials need to be calculated ab initio using the supercell approach and displacement method. The atomic forces of different atomic displacements of supercells will be calculated using density-functional-perturbation theories (DFPT) implemented in Quantum Espresso (QE) [4]. The second-harmonic interatomic force constants (IFCs) are then produced from the obtained forces and the phonon dispersions are calculated using PHONOPY [5]. From the results on the phonon properties calculated in this project we will interpret the experimentally observed effect of alloying on the thermal conductivity in Y(Sc)xAl1-xN and will establish a predictive model to be used in future works on both material development and devices. [1] C. Tholander at al., Phys. Rev. B 87 (2013) 094107 [2] A. Zukauskaite et al., J. Phys. D: Appl. Phys. 45 (2012) 422001 [3] J. Callaway, Phys. Rev. 113, 1046 (1959) [4] P. Giannozzi, et al., Journal of Physics: Condensed Matter 21, 395502 (2009). [5] A. Togo, L. Chaput, and I. Tanaka, Phys. Rev. B 91, 094306 (2015)