Selectively Populating a Quantum Dot Ensemble Using a Tunnel Injection Structure

Abstract

We have analysed the carrier distribution of a tunnel injection quantum dot laser to reveal features which suggest dots of a particular size are preferentially populated during the tunnel injection process.Self-assembled quantum dot (QD) lasers have been developed with low threshold current densities, high differential gain and low temperature sensitivity as compared to lasers containing quantum wells (QW) [1]. Further improvements to their performance can be made by reducing the size non uniformity of the dot ensemble and the influence of the closely spaced wetting layer on the carrier population of the lower dot states.A tunnel injection structure similar to that which has previously been used to reduce the temperature sensitivity of QW structures[2] has recently been applied to QD structures [3]. The tunnelling process is designed to inject carriers from an adjacent QW into the lower energy dot states so that relaxation from the wetting layer is no longer required. Previous analysis of the carrier dynamics in tunnel injection quantum dot structures [4] has shown that these devices have low temperature sensitivity and, compared to other self assembled QD structures, they have a large small-signal bandwidth. In this paper we analyse the carrier distribution within a 1.24µm tunnel injection QD laser and reveal a selection process in which dots of a particular size are preferentially occupied.The devices were grown by molecular beam epitaxy on a (001) GaAs substrate and contain five repeat layers within the active region. Each layer consists of 2.6ML of InAs QDs capped with 45Å In 0.15 Ga 0.85 As. These are grown on top of a 95Å In 0.27 Ga 0.73 As injector well which is separated form the dot by a 15Å Al 0.25 Ga 0.75 As barrier. Further details of the structure and growth can be found in [5]. The tunnelling process is illustrated in Figure 1.The samples were tested using the segmented contact method[6] to produce carrier distribution (P f ) spectra as a function of photon energy. The P f spectra are produced from the ratio of the gain and spontaneous emission spectra as a function of photon energy (E=hv). The value of P f over a particular energy range reflects the degree of inversion of a state over that range. If all the carriers are distributed according to Fermi-Dirac statistics (thermally distributed) and described by the same quasi Fermi level separation ? E f, then P f at a particular temperature T can be described by:The spontaneous emission spectra at 300 K (figure 2) shows that the peak energies of the ground and excited state do not vary with carrier density. This suggests there is a non-thermal carrier distribution present. In addition, there is little recombination from the dot second excited state, which is consistent with the injection of carriers into the first excited state with little subsequent redistribution.In Figure 3 we plot the carrier distribution function at 300K , we also include the peak energies of the spontaneous emission ground and 1 st excited states on these spectra for reference. ...