Optical Properties of Semimagnetic Quantum Dots
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Optical Properties of Semimagnetic Quantum Dots S. Mackowski*, T. A. Nguyen, H. E. Jackson, and L. M. Smith Dept. of Physics, Univ. of Cincinnati, Cincinnati, OH 45221-0011, United States * corresponding author, electronic mail: [email protected] J. Kossut, and G. Karczewski Institute of Physics, Polish Academy of Science, Warsaw, Poland W. Heiss Institut für Festkörperphysik, Johannes Kepler Universität Linz, Austria ABSTRACT Measurements of optical properties of epitaxially grown CdMnTe quantum dots are presented. Magnetic ions are incorporated into the structure by passivating the substrate surface with Mn prior the quantum dot deposition. As expected, the intensity of intra-Mn transition increases with the time of manganese passivation. The formation of magnetic quantum dots is evidenced by presence of single emission lines in the micro-photoluminescence spectrum. The width of single dot emission lines is much broader than observed for non-magnetic quantum dots. The broadening is caused by thermal fluctuations of magnetization in quantum dots. Resonant spectroscopy results indicate that the exciton-optical phonon scattering is the main energy relaxation mechanism in these structures. The measurements suggest that post-growth thermal processing offers the possibility of tuning the magnetic properties of these structures. INTRODUCTION The ability to dope self-assembled quantum dots (QDs) with magnetic ions, provides a unique opportunity to study strongly confined carriers in a magnetic environment [1]. Quantum dots composed of II-VI semiconductor compounds are well suited for this purpose since both the technology and physical properties of II-VI semimagnetic compounds are well known and understood [2]. Very recent studies of such structures indicate that the presence of magnetic ions in the vicinity of QDs strongly influences the optical properties of excitons in these structures [3,4,5,6]. In particular, magnetic polarons could be formed in single semimagnetic QDs [1]. Moreover, the incorporation of Mn influences the dynamics of excitons due to the presence of intra-Mn transitions. Analogous to the case of semimagnetic quantum structures with higher dimensionality, this transition acts as an efficient recombination channel. This leads in turn to a dramatic decrease of the photoluminescence (PL) intensity of QDs and a decrease of the exciton decay time [4,7]. Here we show that it is possible to sidestep this difficulty by exciting QDs resonantly, below this transition. Resonant spectroscopy is also used in the present work to study the mechanism of carrier relaxation in semimagnetic CdMnTe QDs. The interaction of zerodimensional carriers with magnetic ions is evidenced by appearance of broad emission lines associated with single QDs when probing a very limited number of dots. This broadening originates from magnetic moment fluctuations in a single dot [8]. Lastly we show that it is possible to change the magnetic properties of CdMnTe QDs by subjecting them to rapid thermal
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processing (RTP). Thi
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