Dynamic Jahn-Teller interaction with Γ5 phonons in the ground state of Cu2+ in cubic II-VI semiconductors

Optical absorption and emission measurements of Cu2+ as a substitutional impurity in cubic ZnS and ZnTe rue analyzed by means of an electron-phonon coupling model. The D-2 term of Cu2+ is split by a crystal field of tetrahedral symmetry into a (2) Gamma(5) orbital triplet and a (2) Gamma(3) orbital doublet. Optical transitions have been observed between these two multiplets in ZnS:Cu2+ and within the (2) Gamma(5) ground state in ZnTe:Cu2+. The theoretical model is based on crystal-field theory and includes the spin-orbit interaction and a dynamic Jahn-Teller interaction between the electronic (2) Gamma(5) states and a transverse acoustic phonon of TI symmetry. Starting from the ten spin-orbit wave functions appropriate to the orbital tripler and doubler manifolds, the symmetry-adapted vibronic basis is constructed and used to diagonalize the Hamiltonian matrix. Phonon overtones up to n = 14 are included to ensure convergence of the energy eigenvalues. The measured positions and relative intensities of the spectral lines are described with good accuracy by the theoretical model, including covalency effects.. In ZnS, comparison between theory and experiment yields the following values of the physical parameters: the crystal-field splitting Delta = 5990.6 cm(-1), the spin-orbit coupling constants lambda(1) = -667 cm(-1) and lambda(2) = -830 cm(-1), the phonon energy h omega = 73.5 cm(-1), and the Jahn-Teller stabilization energy E-JT = 474.5 cm(-1) The corresponding parameters in ZnTe are Delta = 6000 cm(-1), lambda(1) = -888 cm(-1), lambda(2)= -830 cm(-1), h omega= 38.8 cm(-1), and E-JT= 468.5 cm(-1).