Engineering of Quantum Dot Nanostructures for Photonic Devices

In order to take full advantage of the peculiar optical properties of quantum dot (QD) nanostructures, their band structure must be engineered to optimize a few relevant parameters. In this paper we review some approaches used to design and prepare structures for both the 1.3-1.6 µm spectral window of low hydroxyl-content optical fibers, and the 0.98-1.04 µm one for telecom and medical applications. As for long wavelength applications, we discuss first the approach termed as QD strain engineering: In InAs/InGaAs structures on GaAs substrates the composition of InGaAs confining layers and the thickness of the metamorphic lower one determine both the band discontinuities between QDs and confining layers and the energy gap of the QD material, through its strain; the availability of the two degrees-of-freedom make it possible to tune both the emission energy and the activation energy for thermal quenching of emission, a parameter that determines the room temperature (RT) emission efficiency. By using QD strain engineering we obtained from structures grown by Molecular Beam Epitaxy photoluminescence emission at RT up to 1.44 µm under excitation power densities as low as 5 W/cm2. A simple effective-mass model, validated by our experimental results, offers a rationale for the achievement of efficient RT emission at long wavelength; the results also show that in long wavelength structures the inevitably low band discontinuities hamper the achievement of room temperature. We review our results on the insertion of InAlAs additional barriers embedding QDs and set amid the InGaAs confining layers; it is shown that the blue-shift of emission wavelength due to the additional barriers can be effectively counterbalanced by the red-shift induced by QD strain engineering; as a consequence, in QD strain engineered structures with enhanced barriers the activation energies can be significantly increased so that RT emission wavelengths in excess of 1.5 µm are experimentally obtained. AlGaAs confining layers and InGaAs QDs have been successfully used in order to respectively increase the band discontinuities and the QD energy gap for 0.98-1.04 µm emitting structures. Experimental results and model calculations allow us to discuss the effect of QD and confining layer composition on the QD morphology. In particular, we show how the CL composition, besides band discontinuities, affects also the QD dimensions and other details of the QD band structure. Furthermore, by studying the effect of the change in composition of InGaAs QDs, we identify different mechanisms contributing to the blue-shift of the emission; 0.98 µm RT emission was achieved from structures consisting of InGaAs QDs embedded in Al0.30Ga0.70As CLs.