The authors report on the fabrication and detailed structural characterization of ordered arrays of vertically stacked SiGe/Si(001) island pairs. By a proper choice of growth parameters, islands which have both large sizes and high Ge fraction are obtained in the upper layer. Finite element method calculations of the strain distribution reveal that (i) the Si spacer between a pair of islands can act as a lateral quantum dot molecule made of four nearby dots for electrons and (ii) the tensile strain in a Si cap deposited on top of the stack is significantly enhanced with respect to a single layer.

Abstract-The phonon dispersion relations and density of states of the nitrides BN, AlN, GaN and InN in the wurtzite structure are investigated by using separately the local density approximation (LDA) and the generalized gradient approximation (GGA) within the framework of density functional theory. The results show that the highest optical phonon frequency at the Brillouin zone center decreases, but the phonon band gap increases, with increasing the cation/anion mass ratio. The frequencies calculated with GGA are lower than those with LDA. The GGA is more suitable for AlN and GaN, but the LDA for BN and InN.Index Terms-Phonon dispersion, phonon density of states, III-N compound semiconductor, wurtzite structure.