Patent ID: 7160822

Claim:
A method of forming the active region of an optoelectronic device incorporating semiconductor quantum dots whose ground state emission occurs at wavelengths greater than 1350 nm at substantially 293 K, said method comprising the steps of: growing a first layer of quantum dots formed on one of a substrate layer or a buffer layer, said quantum dots of said first layer being subject to a strain due to a lattice mismatch between said substrate layer and said quantum dots of said first layer; growing a spacer layer over said first layer and said spacer layer being subject to a strain in strained areas overlying quantum dots of said first layer due to a lattice mismatch between said quantum dots of said first layer and said spacer layer; growing an active layer of quantum dots on said spacer layer, quantum dots of said active layer predominately forming upon strained areas of said spacer layer such that the surface density of quantum dots of said active layer is substantially determined by the surface density of quantum dots of said first layer, quantum dots of said active layer being in a strain-relaxed state in which said quantum dots of said active layer are subject to less strain than quantum dots grown on an unstrained surface, growth conditions for said active layer being different to those of the first layer and chosen appropriately, in particular substrate temperature being low enough, such as to substantially preserve said strain-relaxed state and to limit intermixing of said quantum dots of said active layer with said spacer layer; and growing a capping layer on said active layer, growth conditions for said capping layer chosen appropriately, in particular substrate temperature being low enough, such as to substantially preserve said strain-relaxed state and to limit intermixing of said quantum dots of said active layer with said spacer layer and with said capping layer.