Method of forming quantum dots for extended wavelength operation

A method of forming the active region of an optoelectronic device incorporating semiconductor quantum dots whose ground state emission occurs at wavelengths beyond 1350 nm at a temperature of substantially 293 K is provided by forming a first layer of quantum dots covered by a spacer layer with strained areas extending there through. The spacer layer then forms a template upon which quantum dots of an active layer may be formed with a surface with a surface density and formation that is influenced by the underlying first layer of quantum dots. This allows a choice of growth parameters more favourable to the formation of quantum dots in the active layer emitting at long wavelengths with a narrow inhomogeneous broadening. As an example, the active layer of quantum dots may be formed at a lower temperature than the first layer of quantum dots. The quantum dots of the active layer are then subject to less intermixing with the surrounding spacer and capping layers, and can also preserve a more strain-relaxed state, which results in a longer wavelength emission with a narrower inhomogeneous broadening. This method is particularly well suited to the growth of the active region of an optoelectronic device on a GaAs substrate.