Single quantum dot nano‐LEDs – spectroscopy of an electrically controlled few‐particle system

Abstract

We present a new device concept for nano-structured LEDs with active areas smaller than 1µm 2 , which are fabricated by focused ion beam (FIB) implantation and successive MBE overgrowth. These LEDs contain a layer of InAs self-assembled quantum dots embedded in the intrinsic layer and are therefore an ideal tool for both electrical and optical investigations on single quantum dots. We report results from electroluminescence (EL) measurements which prove the single-dot character of our devices. The EL spectra consist of sharp emission lines, which can be assigned to charged (multi-) exciton recombination and their dependence on the injection current shows linear behaviour for exciton and quadratic behaviour for biexciton recombination. Highly resolved current vs. voltage spectroscopy provides additional information on the population of electron and hole levels of the quantum dot. 1 Introduction During the last years the numerous investigations of self-assembled quantum dots focused on experiments with a single dot, mainly to avoid the disturbing effect of inhomogeneous broadening caused by the size fluctuations within an ensemble of dots. Photoluminescence (PL) experiments provided insight into the excitonic structure and led to a deeper understanding of many-particle effects in quantum dots [1,2]. Nevertheless, more difficult to realize are electrical measurements on single quantum dots. An electrically contacted dot is not only of interest with regard to controlled single photon emission (e.g. turnstile devices), but also for a deeper understanding of electron and hole kinetics in quantum dots. This is particularly true, if the distances between the quantum dot and the carrier injecting layers are short enough to allow for efficient and controlled tunnelling directly into the dot. Under these conditions the number of electrons and holes in the quantum dot only depends on the applied voltage and the ratio of these distances. In this way the formation of neutral, positively or negatively charged single, bi-and multiple-excitons can be controlled. So far, only a few groups have reported on electroluminescence (EL) from a single self-assembled quantum dot. This is mainly due to the challenge of placing a single dot within the active region of a diode, as common dot densities are in the range of 10 9 cm -2 to 10 11 cm -2 . Several approaches to single-dot EL are published. Itskevich et al. fabricated small diodes by using a sophisticated underetching technique [3], whereas Fiore and coworkers built diodes with a submicron oxidized current aperture [4,5]. Recently

Optical Properties of Semiconductor Nanomaterials

“…The basic mechanism is electron-hole recombination following electrical injection of electrons and holes into a semiconductor [176][177][178][179][180][181]. Figure 5.12 illustrates the basic idea behind EL in a semiconductor QD.…”