First-principles Study On The Physical Properties Of Cubic Ca₂Ge

Ca₂Ge has been researched as next-generation semiconductor,which has better structural stability and higher carriers'mobility;besides,it can be epitaxial growth on Ge substrate with small lattice mismatch,therefore,Ca₂Ge is one of the key materials to meet the new generation of high performance and low power devices.Temperature and stress are the two main factors affecting semiconductor performance in the process of preparation and application.In order to explore the performance changes of cubic Ca₂Ge under the different temperatures and stresses,we used the pseudo-potential plane wave method to investigate the structural stability,electric structure,optical properties and thermodynamic properties of cubic Ca₂Ge,the main research contents and results are described in the following.Firstly,the elastic properties,population analysis,electronic structures,density of states and charge densities of cubic Ca₂Ge have been calculated by the pseudo-potential plane wave method.Analyzing the elastic properties revealed that the structure of cubic Ca₂Ge is stable at the stress range from-6 to 8 GPa,meanwhile,when the stress is less than 4 GPa,the cubic Ca₂Ge semiconductor exhibits brittleness by the Pugh criterion,and it's ductile at the other stress range.According to the calculation of population analysis,there are covalent bonds in the Ge compound at high stress,indicated that the compound ionicity is decrease with the stress increasing.The calculations of energy bands,density of states and charge densities shown that the Ca s state electrons do not participate in bonding,the bonding strength between electrons is strengthened with stress increasing,at-6⁸ GPa.Secondly,the calculated results of energy bands revealed that the cubic Ca₂Ge is direct band-gap semiconductor at G point with the pressure range from-6 to 8 GPa,and the band-gap value is quasi linear decreasing with the pressure increasing when the pressure is greater than-4 GPa.At 0 GPa pressure,the band gaps are 0.55 eV and0.426 eV when using the GGA and LDA,respectively.The dielectic function results shown the dielectric constant of 0 GPa obtain maximum value at the point of E=0 eV,and in other energy ranges,the dielectric constants?₁ achieve maximum peak value when the energy is near 0.7 eV.Besides,according to the informations of refractive index,reflectivity and absorption,the phenomenon of strong metallic reflection be found in the energy range from 5.5 to 6.5 eV except for 0 GPa.Finally,the study results of phonon dispersion and phonon density states of cubic Ca₂Ge shown that the acoustic waves of Ca₂Ge are main caused by the vibration of Ca and Ge atoms,the optical branch are contributed from Ge atoms in the vibration frequency range of 4.6⁵.2 THz,and are contributed from Ca atoms in the vibration frequency range of 5.5⁷.3 THz.The results of thermodynamic properties of cubic Ca₂Ge were obtained via the Quasi-harmonic Debye model,the calculations shown that the volume change rate,heat capacity and expansion coefficient are increased and the elastic modulus is decreased with the stress increasing under the constant temperature.The volume change rate and the elastic modulus are decreased with the temperature increasing,the expansion coefficient and heat capacity are increased with the stress increasing,under the constant stress.The effect of heat capacity is little which from the stress,and the heat capacity closed to the Dulong-Petit limited,when the temperature rise to 800 K.