Metamorphic self-assembled quantum dot nanostructures

The light-emission energy E of self-assembled semiconductor quantum dots (QDs) is determined by the complex interplay of parameters such as compositions of QDs and confining layers (CLs), strain of QDs (imposed by the QD mismatch to CLs) and sizes and shapes of QDs. In order to have RT emission in the 1.55 µm photonic window from InAs QDs, the QD-CL lattice mismatch should be in the 4-5% range, values much lower than that of pseudomorphic InAs on GaAs (7%). We show that by: i) growing InAs QDs on virtual substrates consisting of metamorphic InGaAs buffers on GaAs and ii) using the thickness-dependent partial relaxation of buffers (acting also as lower CLs, LCLs) and suitable InGaAs compositions, the QD-CL mismatch can be tuned in the 5-7% range. Our experimental results on MBE-grown metamorphic InAs/InxGa1-xAs QD structures show that for x and LCL thicknesses d in the 0.09-0.35 and 20 nm-1000 nm ranges, respectively, the band-gap of the QD material and the band-discontinuities that confine carriers are such that the RT emission wavelengths range from 1.3 µm up to values that may exceed 1.55 µm. By using x and d as two degrees-of-freedom, not only that E can be selected but also the barrier energy for confined carriers' thermal escape can be maximised, in order to achieve efficient emission at RT.