FIRST-PRINCIPLES STUDY OF THE QUATERNARY SEMICONDUCTOR SUPERLATTICES (GAX)(1) (YAS)(1) (X=N, P Y=AL, IN)/

We have performed first-principles relativistic pseudopotential calcul ations to study the structural and electronic properties of quaternary semiconductor superlattices composed of GaAlAsN, InGaAsP, and GaAlAsP . Due to the complexity and low symmetry of the quaternary superstruct ure, the total energy as well as the force and stress in the system ar e calculated to determine the equilibrium structural parameters. The b and structures of these quaternary superlattices have been calculated and the energy band gaps have been obtained. A relativistic Hartree-Fo ck local density approximation pseudopotential calculation also has be en performed to reinforce the result of the energy band gap for the Ga AlAsN system that is predicted to be gapless. A systematic comparison of the calculated electronic structure of the quaternary superlattices with the related binary compounds is presented. It is found that the virtual-crystal approximation fails to provide a proper description fo r these systems and the effects of chemical bonding and ionicity of th e anion atoms are very important in determining the structural and ele ctronic properties of the quaternary compounds.