Resonant photoluminescence and excitation spectroscopy of CdSe/ZnSe and CdTe/ZnTe self-assembled quantum dots
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Resonant photoluminescence and excitation spectroscopy of CdSe/ZnSe and CdTe/ZnTe self-assembled quantum dots T. A. Nguyen*, S. Mackowski, H. Rho, H. E. Jackson, L. M. Smith Department of Physics, University of Cincinnati, OH 45221-0011, United States * corresponding author, electronic mail: [email protected] J. Wrobel, K. Fronc, J. Kossut, and G. Karczewski Institute of Physics, Polish Academy of Science, Warsaw, Poland. M. Dobrowolska and J. Furdyna Department of Physics, University of Notre Dame, United States ABSTRACT We show that two major carrier excitation mechanisms are present in II-VI self-assembled quantum dots. The first one is related to direct excited state – ground state transition. It manifests itself by the presence of sharp and intense lines in the excitation spectrum measured from single quantum dots. Apart from these lines, we also observe up to four much broader excitation lines. The energy spacing between these lines indicates that they are associated with absorption related to longitudinal optical phonons. By analyzing resonantly excited photoluminescence spectra, we are able to separate the contributions from these two mechanisms. In the case of CdTe dots, the excited state – ground state relaxation is important for all dots in ensemble, while phononassisted processes are dominant for the dots with smaller lateral size. INTRODUCTION The results of optical spectroscopy reported for III-V semiconductor self-assembled quantum dots (QDs) show that both excited states as well as electron-phonon interaction play an important role in carrier excitation and relaxation in these structures [1-4]. However, due to relatively weak confinement, it is sometimes hard to distinguish between these two processes, especially if the ground state – excited state energy interval equals to a multiple of the optical phonon energy [2]. In this work we investigate excitation and relaxation mechanisms in CdTe and CdSe QDs. As reported by several groups, QDs made of II-VI compounds are usually much smaller than InAs/GaAs structures [5-7]. This leads to stronger electronic confinement which, in turn, should give larger value of excited state – ground state splitting. Moreover, due to the different growth mechanism, no wetting layer formation is observed in the case of fully developed II-VI QDs [5.6]. The results of resonantly excited photoluminescence (PL) together with photoluminescence excitation (PLE) data obtained for single QDs feature unambiguously both types of mechanisms mentioned above. In particular, we show that the excited state (ES) – ground state (GS) processes are important for all QDs. In contrast, optical phonon assisted absorption is dominant only for QDs with high emission energy, i.e. presumably with smaller size. This finding agrees with recently predicted size dependence of the strength of electron – phonon coupling in QDs [8].
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SAMPLES AND EXPERIMENTAL DETAILS The samples studied were grown by molecular beam epitaxy on GaAs substrate. After a 1µm thick ZnTe or ZnSe buffer layer
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