Istituto dei materiali per l'elettronica ed il magnetismo     
Seravalli L., Trevisi G., Minelli M., Frigeri P., Franchi S. Engineering of Quantum Dot Nanostructures for Photonic Devices. In: HANDBOOK OF SELF ASSEMBLED SEMICONDUCTOR NANOSTRUCTURES FOR NOVEL DEVICES IN PHOTONICS AND ELECTRONICS. pp. 504 - 528. Mohamed Henini. New York: Elsevier Science, 2008.
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 chapter we review some approaches used to design and prepare structures for both the 1.3-1.6 μ m spectral window of low hydroxyl-content optical fi bres, and the 0.98-1.04 μ m one for telecom and medical applications. As for long wavelength applications, we discuss fi rst the approach termed as QD strain engineering : in InAs/InGaAs structures on GaAs substrates the composition of InGaAs confi ning layers and the thickness of the metamorphic lower one determine both the band discontinuities between QDs and confi ning 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 effi ciency. By using QD strain engineering we obtained from structures grown by molecular beam epitaxy photoluminescence emission at RT up to 1.44 μ m under excitation power densities as low as 5 W/cm 2 . A simple effective-mass model, validated by our experimental results, offers a rationale for the achievement of effi cient 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 operation. We review our results on the insertion of InAlAs additional barriers embedding QDs and set amid the InGaAs confi ning 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 signifi cantly increased so that RT emission wavelengths in excess of 1.5 μ m are experimentally obtained. AlGaAs confi ning 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 μ m emitting structures. Experimental results and model calculations allow us to discuss the effect of QD and confi ning 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 μ m RT emission was achieved from structures consisting of InGaAs QDs embedded in Al 0.30 Ga 0.70 As CLs.
Subject molecular beam epitaxial growth
semiconductor quantum dots

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