Charged Excitons in Self-assembled Quantum Dots
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Charged Excitons in Self-assembled Quantum Dots R. J. Warburton1 , B. Urbaszek1 , E. J. McGhee1 , C. Schulhauser2 , A. H¨ogele2 , K. Karrai2 , A. O. Govorov3,4 , J. M. Garcia5 , B. D. Gerardot6 and P. M. Petroff6 1 Department of Physics, Heriot-Watt University, Edinburgh, UK 2 Sektion Physik, Ludwig-Maximilians-Universit¨at, Munich, Germany 3 Department of Physics and Astronomy, Ohio University, Athens, USA 4 Institute of Semiconductor Physics, 630090 Novosibirsk, Russia 5 Instituto de Microelectr´onica, Madrid, Spain 6 Materials Department, University of California, Santa Barbara, USA ABSTRACT We have succeeded in generating highly charged excitons in InAs self-assembled quantum dots by embedding the dots in a field-effect heterostructure. We discover an excitonic Coulomb blockage: over large regions of gate voltage, the exciton charge remains constant. We present here a summary of the emission properties of the charged excitons. INTRODUCTION An exciton is the elementary excitation in a semiconductor consisting of an electron and a valence level hole, bound together by the Coulomb potential. The exciton decays by photon emission when the electron and hole recombine, which is a strong optical process because the exciton and vacuum states are connected by the electric dipole operator. The exciton ionization energy can be increased in a quantum well relative to that in bulk material by the quantum confinement. An increase up to about 10 meV from a bulk value of about 4 meV is possible in a GaAs quantum well [1]. A charged exciton, consisting of two electrons and a hole, is just bound in a quantum well. More highly-charged excitons are however unbound and do not exist. This state-of-affairs changes in a self-assembled quantum dot where in addition to a vertical confinement, there is also a lateral confinement arising from the nanometer-sized dot. In this case, the exciton binding energy (defined as the energy required to separate an electron and a hole and place them in identical but well separated quantum dots) increases to, say, 30 meV for an InAs/GaAs quantum dot [2]. In fact, the quantum confinement is so strong in these systems that it is more accurate to refer to a highlyconfined electron-hole pair than to an exciton. The strong confinement in a self-assembled quantum dot opens up the possibility for the first time of investigating highly-charged excitons because even excitons with several excess electrons are stable. We have shown how it is possible to generate these highly-charged excitons in self-assembled quantum dots by embedding the dots in an appropriate field-effect heterostructure [4]. We label the excitons with Xn− where n is the excess charge. We have detected photoluminescence (PL) from the neutral exciton X0 up to the quadruply-charged exciton X4− . Each time an exciton gains an additional electron, its PL red-shifts through the Coulomb interaction, with the jumps in wavelength revealing a shell structure. Furthermore, for the X2− and X3− , splittings arise, related to electron-hole exchange
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