First principles calculation of the dipole moments of small mixed Ge/Te semiconductor clusters

Abstract

We investigate the structural and electronic properties of the small mixed semiconductor clusters GeTe, Ge2Te, GeTe2 and Ge2Te2 by means of first principles supercell calculations using the local density approximation and the pseudopotential plane wave method. The cluster geometry is optimized using a conjugate gradient method. For GeTe2 we confirm the linear structure (D∞h) suggested by recent experiments. For Ge2Te we obtain an equilateral triangle as the equilibrium structure. For Ge2Te2 we obtain a deformed tetrahedrom (butterfly C2v symmetry). For each of these molecules we compute the dipole moment in the equilibrium geometry and in selected geometries near to equilibrium corresponding to states higher in energy by a few tens of meV. For the equilibrium state of Ge2Te and Ge2Te2 we obtain permanent dipole moments smaller than those estimated from recent experiments at room temperature. For selected low lying excited states of these clusters, which have elongated Ge-Te bond length, we obtain enhanced dipole moments, improving the agreement with the ones estimated from experiments.