Mechanical Interaction in Near-Field Spectroscopy of single Semiconductor Quantum Dots.
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Mechanical Interaction in Near-Field Spectroscopy of single Semiconductor Quantum Dots. A. M. Mintairov1, P. A. Blagnov2, O. V. Kovalenkov2, C. Li3, J. L. Merz1, S. Oktyabrsky4, V. Tokranov4, A. S. Vlasov2, D. A. Vinokurov2. 1
Electrical Engineering Department, University of Notre Dame, Notre Dame, IN 46556, USA Ioffe Physico-Technical Institute, RAS, 194021 St. Petersburg, Russia 3 Aerospace and Mechanical Engineering Department, University of Notre Dame, IN 46556, USA 4 UAlbany Inst. for Materials University at Albany-SUNY, 251 Fuller Rd. Albany, NY 12203 2
ABSTRACT We have studied high-energy shifts of single quantum dot (QD) emission lines induced by contact pressure exerted by a near-field optical fiber tip. “Pressure” coefficients of 0.65-3 meV/nm have been measured for self-organized InAs/GaAs, InAs/AlAs and InP/GaInP QDs in agreement with numerical calculations of the local strain field. We found an increase of the tipinduced pressure with increasing aperture diameter from 50-300 nm. A correlation between the shift rate and QD stiffness was obtained. We also observed an order of magnitude increase of single QD emission intensity with increased pressure. INTRODUCTION Near-field scanning optical microscopy allows one to extend spatial resolution of optical experiments far beyond the light diffraction limit, which opens new possibilities for nanoscale characterization. In the present paper we have studied the shift of the emission lines of single InAs and InP quantum dots (QDs) induced by the pressure produced by a near-field tip [1, 2]. Using 50-300 nm tips we observed an increase of the energy shift with increasing aperture diameter. Unexpectedly, a strong increase of the emission intensity of single QDs has also been observed. EXPERIMENTAL DETAILS The InAs QDs were formed within a short-period superlattice consisting of two/eight monolayers (ML) of AlAs/GaAs by MBE at 500 o C [3]. The thickness of the cap layer was 40 nm. In the first sample, called InAs/AlAs, InAs QDs were grown between 2ML of AlAs. In the second, called InAs/GaAs, the dots were grown between 8 ML of GaAs. The InAs/AlAs and InAs/GaAs dots have base 22 and 14 nm and density 300 and 100 µm-2, respectively. The height of the dots is ~5 nm. As can be seen from the plan-view transmission electron micrograph of the InAs/AlAs sample presented in Fig.1a, there is a bimodal distribution of dot sizes - in addition to the 20 nm base dots, the QD layer also contains smaller dots having base ~10nm. The density of these small dots is half that of the larger ones. InAs/GaAs sample has a similar bimodal distribution. Some measurements were done using InP QDs embedded in GaInP, which were studied by us [4]. The dots have a base ~100 nm, height 10 nm and density 20 µm-2 (see Fig.1b). The near-field photoluminescence (NPL) spectra were taken in collection-illumination mode at 10 K under excitation of the 514.5 nm Ar-laser line. The spectra were measured using a CCD detector together with a 280 mm focal length monochromator. The excitation power was 150 µW,
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