Composition dependence of phonon and thermodynamic properties of the ternary AlGaN mixed crystal

Although ternary AlGaN has recently been studied, an accurate phonon spectrum of AlGaN mixed crystal is still scarcity. Based on the density functional theory ( DFT), the atomic geometry, phonons and thermodynamic properties of AlxGa1-xN mixed crystal over the entire Al concentration range ( 0 <= x <= 1) are investigated. The properties are analyzed by using the generalized gradient approximation ( GGA) and the local density approximation ( LDA). The optimal doping position of each component is given explicitly. The accurate phonon spectrums of AlGaN mixed crystal under the entire concentration range are shown. The present phonon calculations of the stabled structures for AlxGa1-xN show that the vibration frequency increases to a higher frequency with the Al content increasing. The Debye temperature depends approximately linearly on the Al content at zero temperature and room temperature. Additionally, composition dependence of the high-frequency dielectric constants, Helmholtz free energy, zero point energy and specific heat are determined.