Prediction of a Strain Induced Conduction Band Minimum in Embedded   Quantum Dots
  Free standing InP quantum dots have previously been theoretically and experimentally shown to have a direct band gap across a large range of experimentally accessible sizes. We demonstrate that when these dots are embedded coherently within a GaP barrier material, the effects of quantum confinement in conjunction with coherent strain suggest there will be a critical diameter of dot (60A), above which the dot is direct, type I, and below which it is indirect, type II. However, the strain in the system acts to produce another conduction state with an even lower energy, in which electrons are localized in small pockets at the interface between the InP dot and the GaP barrier. Since this conduction state is GaP X_1c-derived and the highest occupied valence state is InP, Gamma-derived, the fundamental transition is predicted to be indirect in both real and reciprocal space (``type II'') for all dot sizes. This effect is peculiar to the strained dot, and is absent in the free-standing dot. 
A.J.Williamson, Alex Zunger and A.Canning