First-principles calculations of elastic and optical properties of Aluminum Nitride (AlN) in cubic and hexagonal phase

Investigation of elastic and optical properties of AlN in rock salt, zinc blende, and wurtzite phase is done under the framework of Density Functional Theory (DFT) with modified Becke Johnson Generalised Gradient Approximation (mBJ-GGA) as exchange-correlation functional. Elastic properties conclude the bonding nature of the AlN in the rock salt phase is covalent and stiffest, while the bonding nature in the zinc blende and wurtzite phase is found to be ionic and less stiff. The ratio of bulk modulus to shear modulus indicates AlN is brittle in all three phases. The calculated Debye temperature in all three phases is in good agreement with the available theoretical and experimental works. The optical properties calculation shows the AlN is transparent in the low energy range and it has the metallic behavior in the energy range 7.5eV to 10 eV. At the same time, the compound loses its transparency at the high energy range. Our calculated value of the refractive index of AlN in the rock salt, zinc blende, and wurtzite phases is in good agreement with the available experimental and theoretical works.