Physica E 40 (2008) 2100–2103 Exciton, biexciton and trion recombination dynamics in a single quantum dot under selective optical pumping Guillermo Mun˜oz-Matutano a , Jordi Gomis a,Ã , Benito Ale´n b , Juan Martı´nez-Pastor a , Luca Seravalli c , Paola Frigeri c , Secondo Franchi c a Instituto de Ciencias de los Materiales, Universitat de Valencia, P.O. Box 22085, Valencia 46071, Spain b Instituto de Microelectro´nica de Madrid (CNM-CSIC), Isaac Newton 8, Tres Cantos 28760, Madrid, Spain c Istituto dei Materiali per l’Elettronica e il Magnetismo (CNR), Parco delle Scienze 37/a, Parma I-43100, Italy Available online 11 October 2007 Abstract Continuous wave- and time-resolved micro-photoluminescence spectroscopy has been performed on single InAs self-assembled quantum dots grown on GaAs. The presence of residual impurities (donors and acceptors) in samples with low dot density opens the possibility to switch from trion to neutral exciton states inside quantum dots by selective optical pumping. We propose a microstate model to describe the recombination dynamics of all the excitonic especies (neutral exciton, positive/negative trion and biexciton) under the considered optical pumping conditions when increasing the excitation power. r 2007 Elsevier B.V. All rights reserved. PACS: 73.63.Kv; 81.07.Ta; 78.67.Hc Keywords: Optical switching; Trion and exciton recombination; Micro-photoluminescence; Microstate theory; Time-resolved photoluminescence Nowadays, the control of positive and negative charge inside single quantum dots (QDs) is a subject of intense research activity. In the recent past, the use of field effect samples opened the possibility to control the number of electrons inside a QD by Coulomb Blockade effect [1]. More recently, optical pumping has been also used to have different charge conditions in the QD. This is the case of the photodepletion mechanism [2], in which the initial situation is a charged dot and optical excitation depletes this charge, and a more simple charge mechanism based on the different mobility of electrons and holes [3]. In this paper we analyze charging phenomena based on the existence of residual impurities. Micro-pholuminescence (mPL) experiments are able to evidence the optical switch- ing of charged excitonic species. We propose a master equation microstate (MEM) model to reproduce the capture and recombination dynamics of the different exciton species in single QD. Our model was parameterized by using like inputs the time constants directly deduced from time-resolved photoluminescence (TRPL) experi- ments in the same single QD studied under continuous wave (CW) conditions. The InAs QDs investigated in this paper were grown by MBE on a GaAs semi-insulating substrate. The combina- tion of low growth rate (LGR) and graded coverage allows obtaining very low-density samples. In particular, the sample under study has a density of 16.5 mm 2 , as estimated by atomic force microscopy images. Hall measurements of similarly grown GaAs buffer layers reveal a residual doping concentration n ¼ N D N A 10 15 cm 3 . This residual impurity concentration is not important in usual samples with higher dot densities, but it will play an important role in low-density samples. The PL at the single QD level (mPL) was performed by using a confocal microscope inserted in the He exchange gas chamber of an immersion cryostat. Two optical fiber connections to the microscope were used for optical excitation and detection light. Collected light for lo1000 nm was dispersed by a 0.3 m focal length double spectrograph ARTICLE IN PRESS www.elsevier.com/locate/physe 1386-9477/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2007.09.182 Ã Corresponding author. E-mail address: jordi.gomis@uv.es (J. Gomis).