Nonlinear Carrier Relaxation in a Single GaAs Self-organized Quantum Dot
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Nonlinear Carrier Relaxation in a Single GaAs Self-organized Quantum Dot T. Kuroda, S. Sanguinetti1,2, F. Minami, M. Gurioli1, K. Watanabe2, and N. Koguchi2 Department of Physics, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8551, JAPAN 1 I.N.F.M. and Dipartimento di Scienza dei Materiali, Università di Milano Bicocca, Via Cozzi 53, 20125 Milano, ITALY 2 Nanomaterials Laboratory, National Institute for Material Science, Tsukuba 305-0047, JAPAN
ABSTRACT We resolved a photoemission signal from a single self-organized GaAs/Al0.3Ga0.7As quantum dot (QD) with picosecond time resolution. The emission spectra consisting of the multiexciton structures are observed to depend on the delay time and the excitation intensity. Through the analysis we can determine the carrier relaxation time as a function of population of photoinjected carriers. Enhancement of the intra-dot carrier relaxation is demonstrated to be due to the carrier-carrier scattering inside a single QD.
INTRODUCTION Semiconductor quantum dots (QDs) are designable mesoscopic atoms easily integrable in bulk electronics. These zero-dimensional structures have attracted large interest as possible efficient replacements of other quantum heterostructures for standard optoelectronic devices. In this paper we study the ultrafast nonlinear properties on the carrier relaxation inside a single QD (SQD). The SQD emission transients are resolved with ps time resolution and interpreted in terms of the successive transition between the correlated few-exciton states. Quantitative analysis on the SQD emission allows us to determine the temporal dependence of the carrier population confined in a single dot. This permits us to show a strict correlation between the intrinsic carrier energy relaxation and the number of photoinjected carriers. We demonstrate that the intra-dot carrier-carrier scattering process is responsible for the efficient carrier relaxation in semiconductor QDs [1].
EXPERIMENTAL The samples are GaAs/Al0.3Ga0.7As self-assembled QDs grown by modified droplet epitaxy (MDE) [2,3]. MDE is a non-conventional growth method for self-assembling QDs even in lattice matched systems [3]. By modifying the surface reconstruction and the adatom mobility it is possible to obtain QD samples with a density as low as ~ 6 x 108 cm-2. Surface and cross sectional high-resolution scanning electron microscope images demonstrate the formation of pyramidal shape nanocrystals of 16 - nm height and 20 - nm base [4]. The optical measurement was performed with a fs mode-locked Ti-sapphire laser of 76-MHz
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repetition rate. A second-harmonic beam of the laser output was used for excitation. This beam was loosely focused on the sample by a conventional lens. The emitted signal was collected by a microscope objective of N.A. = 0.5. The present configuration allows one to irradiate the sample homogeneously inside the detection spot, and to determine the power density with precise accuracy. The signal was dispersed by a polychromator, and detected by a synch
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