CdZnSe/Zn(Be)Se Quantum Dot Structures: Size, Chemical Composition and Phonons
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CdZnSe/Zn(Be)Se Quantum Dot Structures: Size, Chemical Composition and Phonons Y. Gu1, Igor L. Kuskovsky1, J. Fung1, R. Robinson1, I. P. Herman1, G. F. Neumark1, X. Zhou2, S. P. Guo2, and M. C. Tamargo2 1 Department of Applied Physics & Applied Mathematics, Columbia University, New York, NY 10027 2 Department of Chemistry, City College of CUNY, New York, NY 10031 ABSTRACT The size and chemical composition of optically active CdZnSe/ZnSe and CdZnSe/Zn0.97Be0.03Se quantum dots (QDs) are determined using photoluminescence, photoluminescence excitation and polarized Raman scattering spectroscopies. We show that the addition of Be into the barrier enhances the Cd composition and the quantum size effect of optically active QDs. Additionally, surface phonons from QDs are observed in CdZnSe/ZnBeSe nanostructures. INTRODUCTION Self-assembled quantum dot (QD) systems have been of great interest due to both interesting physics and such promising applications as QD laser diodes and quantum computing. For optimum device performance, it is often necessary to have QDs with uniform size and chemical composition. However, due to inter-diffusion that usually occurs during the growth, the QDs obtained so far often have a relatively broad distribution in both size and chemical composition. It is thus desirable to determine the specific size and chemical nature of the QDs that dominate the optical properties of the system. This has been attempted (see, e.g. [1]) by studying the photoluminescence (PL) of samples that are also characterized by transmission electron microscopy (TEM). However, such an approach is inconclusive since the QDs observed by TEM do not necessarily participate in optical processes (i.e. optically inactive). In this paper, we show how to determine the size and chemical composition of optically active QDs directly from their optical properties. Specifically, we investigate PL, PL excitation (PLE) and Raman scattering properties, complemented by model calculations, and use CdSe/Zn(Be)Se QDs as an example. In addition, by comparing the results for the CdSe/ZnSe and CdSe/ZnBeSe systems, we show that the addition of Be into the barrier enhances the Cd composition of QDs as well as quantum size effect. RESULTS AND DISCUSSION In Figs. 1-(a) and (b) we plot the PLE as well as PL spectra of CdxZn1-xSe/ZnSe (sample A) and CdxZn1-xSe/Zn0.97Be0.03Se (sample B) QD structures, respectively. The PL spectra of both samples show a single relatively broad peak, whose energy depends on the size and Cd composition (x) of the optically active QDs. The PLE spectra for both samples are similar, showing free excitons from the barriers and broad features associated with the excitation via the wetting layers. In addition, the PLE spectrum from sample B (Fig. 1-(b)) also shows a small peak (not observed for sample A) separated from the detection energy (indicated by the arrow) by ~28meV. This type of feature is consistently observed across this sample’s PL band; moreover, above 2.43eV up to three equally spaced peaks can be observed (
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