Quantum dot strain-engineering of nanostructures for 1.3 µm light emission

Quantum dots (QDs) are zero-dimensional nanostructures where carriers are confined in regions with dimensions compa- rable to their De Broglie wavelengths, where therefore quan- tized energy levels form. Such systems are obtained by grow- ing self-assembled 3D nanoislands of highly lattice-mismatched (> 4 %) semiconductors on suitable substrates. QD structures proved to have significant advantages as active regions in laser cavities, both in terms of lower threshold currents and lower thermal sensitivities [1]. Therefore, it is of great interest to engineer such nanostructures to tune emission wavelengths to values suited for optoelectronic applications, such as 1.3 and 1.55 ?m. To date the most successful approach to red-shift the emission is to embed InAs QDs in InGaAs layers; however, though 1.3 ?m RT emission has been achieved, the interpreta- tion of physical mechanisms is still disputed [2-4]. The relevant parameters for the energy levels in QDs are: i) band disconti- nuities between QDs and confining layers (CL), ii) dimensions of QDs, iii) strain of QDs. This work is aimed at distinguishing the effects of these parameters on the PL emission energy. We prepared by Molecular Beam Epitaxy and studied structures with InAs QDs embedded in InxGa1-xAs CLs (x=0.15-0.35); the structures were designed so that the strain of QDs could be controlled independently of In composition of CLs. This was realized by changing the thickness-dependent residual strain of the lower CLs and, therefore, the mismatch between QDs and CLs, as this parameter determines univocally the strain in QDs, for given shape and composition. In such a way, we singled out the effect of strain in QDs. We obtained RT emis- sion at 1.3 ?m using CLs of In0.15 Ga0.85 As; furthermore, with CLs of In0.35 Ga0.65 As, we observed emission energies at 10 K (not yet reported for MBE QDs) that would correspond to RT emission wavelengths beyond 1.55 ?m. However, the max- imum temperature for PL emission in these structures was 120 K, when excitation power densities as low as 0.5 Wcm-2 are used; the more effective thermal quenching of PL as x increases is interpreted as due to: i) increased carrier 'evaporation', as band discontinuities are reduced and ii) higher defect concen- tration, due to the higher lattice mismatch between substrates and CLs. To analyse the PL results, we developed a sim- ple and yet effective model to calculate the QD fundamental energy levels. We use a single band effective mass approxi- mation, considering QD dimensions derived from AFM image analysis and we include the strain in QDs following the analyt- ical approach of Ref.[5]. From experimental results and model calculations we show that strain can be effectively used to tune the emission energy of QDs; in particular our results suggest that by quantum-dot strain engineering it will be possible to extend RT emission wavelengths beyond the 1.55 ?m window of lightwave communications.