Room-Temperature Defect Tolerance of Shape Engineered Quantum Dot Structures
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Room-Temperature Defect Tolerance of Shape Engineered Quantum Dot Structures Matthew Lamberti, Alex Katsnelson, Michael Yakimov, Gabriel Agnello, Vadim Tokranov, and Serge Oktyabrsky School of NanoSciences and NanoEngineering, University at Albany - SUNY, 251 Fuller Road, Albany, NY 12203, U.S.A. ABSTRACT A quantum dot (QD) medium is expected to demonstrate superior performance in various devices when compared with quantum wells (QWs). One area of interest has been the improved defect tolerance of QD media, though it was demonstrated at low temperatures so far. In this study, the defect tolerance of shape-engineered QD structures is compared with that of a QW structure at temperatures up to 300 K. To create high defect densities both QD and QW structures were irradiated with high energy (1.5 MeV) protons (with doses up to 3x1014 cm-2). Then, the relative luminescence efficiency was measured by variable temperature photoluminescence. The shape-engineered QD structure withstood two orders of magnitude higher defect density than the QWs at room temperature. This improvement is correlated with the activation energy for thermal evaporation of 390 meV, acquired through a kinetic model.
INTRODUCTION Optoelectronic devices using quantum dots (QDs) as the gain media are predicted to have advantages over quantum well (QW) devices. One early prediction was that QD-based lasers would be less sensitive to temperature than those utilizing QW active medium [1], which recent studies have proven experimentally [2]. Yet another advantage of QDs that has been verified by experiment is improved defect tolerance [3,4]. Carriers in QWs and QDs interact differently with defects due to quantum confinement. Carriers captured in a QW have two degrees of freedom for motion and, therefore, can diffuse within the plane of the well. In a QD system, the carriers are localized in all three dimensions by quantum confinement. The difference in kinetic properties after capture between a QW and QD system is significant. In a QW the motion of carriers gives them a higher chance of finding a defect, compared with QDs, in which the carriers are localized, and can only find a defect if there is one present in the QD or if the carriers are evaporated out of the QD. The carriers are able "to sample" much bigger volumes for defects in the QW structures making this medium more defect sensitive than the QDs [5]. Carriers can leave the QD or QW through thermal evaporation. Thermal evaporation and subsequent capture at non-raditative recombination centers (NRRCs) is the generally accepted mechanism for the decrease in radiative efficiency as temperature is increased in both QW and QD systems. The activation energies that control thermal evaporation are correlated with barrier heights, both for single carriers and electron-hole pairs [6-8]. Therefore, the barrier height will contribute to the onset of thermal evaporation, and the rate at which carriers are quenched will be proportional to the density of NRRCs. In QD systems, [9,10] activation energies h
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