The present invention uses a p-type Gallium Arsenide substrate or an underlying p-type grown layer with the consequent inversion of the doping sequence in order to realise a n-i-p structure. This results in the heterostructure built-in electric field being opposite to the QD internal dipole electric field. As a consequence the separation of the ground state electron and hole wavefunctions is strongly reduced, resulting in a high efficiency ground state optical emission in the region of 1.4 microns.
