Selective optical pumping of charged excitons in unintentionally doped InAs quantum dots

Abstract

As an alternative to commonly used electrical methods, we have investigated the optical pumping of charged exciton complexes addressing impurity related transitions with photons of the appropriate energy. Under these conditions, we demonstrate that the pumping fidelity can be very high while still maintaining a switching behavior between the different excitonic species. This mechanism has been investigated for single quantum dots of different size present in the same sample and compared with the direct injection of spectator electrons from nearby donors.PACS numbers: 73.63. Kv, 81.07.Ta, 78.67.Hc Nowadays, InAs/GaAs self-assembled quantum dots (QDs) are well known nanostructures with important applications envisaged within the quantum computation and cryptography fields.1,2 The singly charged exciton state (trion), either positive or negative, is of particular importance because it lacks fine structure splitting, enabling the efficient generation of single photons, and also because, after radiative recombination, it leaves behind a single charge with well defined spin. Therefore, there is an increasing interest in the electrical or optical control of the exciton charge state as a necessary step for the spin manipulation.3 The charge in QD states can be electrically controlled by tuning the gate voltage in field effect structures embedding intrinsic QD layers. 4,5,6 However, this method can produce undesired effects like the reduction of the oscillator strength induced by the external field.7 The charge state can also be controlled by optical injection, and different charging schemes have been proposed using above or below barrier excitation. 8,9,10,11,12 In this work, we demonstrate the selective formation of charged exciton complexes in initially empty QDs under the presence of unintentional acceptor and donor impurities. Furthermore, the optical pumping mechanism is investigated for two ensembles of InAs QDs with very different size present in the same sample: small QDs emitting below 970 nm and large QDs emitting at 1165 nm at 4 K.The MBE (molecular beam epitaxy) growth starts with a 100 nm-thick GaAs buffer grown at 600 C, followed by InAs deposition at 505 C and at very low growth rate (0.009 ML/s) and ends with a 100 nm-thick GaAs cap grown by atomic layer MBE at 360 C.13 During the InAs deposition, the substrate was not azimuthally rotated, thus producing a continuous variation of InAs coverages on the sample surface.14 The combination of low growth rate (LGR) and graded coverage allowed us to obtain particularly low surface density values, down to 2 µm −2 , suitable for optical investigation of isolated QDs. In particular, the coverage of the sample under consideration here is 2.5 MLs, with a density of about 16 µm −2 , as estimated by atomic force microscopy (AFM) measure- ments shown in Fig. 1(a) carried on uncapped samples. The AFM images also evidence a bimodal distribution of QD sizes, with most frequent values of 9 and 14 nm for the heights and 36 and 54 nm for the diameters of small (SQDs) and ...

Single quantum dot emission at telecom wavelengths from metamorphic InAs/InGaAs nanostructures grown on GaAs substrates

“…In InAs/GaAs QDs emitting in the same spectral region positive and negative trions are usually blue and redshifted with respect to X 0 by 1-2 meV and 5-7 meV, respectively. 23,27 Thus, results obtained here are in agreement with the ones measured in previous works. Finally, weak lines observed at 1.0560 and 1.0540 eV for medium-high excitation powers can have their origin on charged biexcitons and hence are labeled as XX ‫ء‬ but there is not enough information for a clearer identification.…”

“…2͑c͔͒ associated to local unintentional impurities. [23][24][25] Neutral exciton ͑X 0 ͒ is identified at 1.0587 eV ͑around 1170 nm͒, where the smaller than unity ͑0.705͒ slope of the transition comes from the statistical measurement process and the existence of the residual impurities. 26 The line centered at 1.0575 ͑1.2 meV redshifted from X 0 ͒ was assigned to neutral biexciton ͑XX͒, as its slope ͑1.455͒ is double of neutral exciton.…”

“…Positive and negative trions ͑X +1 at 1.0600 eV and X −1 at 1.0553 eV͒ are identified by a slope close to one and the greater importance of the negative trion lines, when the excitation energy is resonant with residual acceptor impurity levels ͑see Ref. 23 for more information on charge initialization by selective optical pumping͒. In InAs/GaAs QDs emitting in the same spectral region positive and negative trions are usually blue and redshifted with respect to X 0 by 1-2 meV and 5-7 meV, respectively.…”

Size dependent carrier thermal escape and transfer in bimodally distributed self assembled InAs/GaAs quantum dots

“…49 More details concerning excited state assignation and evaluation could be found in Ref. 27. In summary, by this micro-PL study we demonstrate the presence of thermal carrier promotion to SQD and LQD bands in both samples.…”

“…8 Single QD emission spectra are characterized by the coexistence of exciton complexes of different charge state and particle number. 27 They correspond to dynamical configurations, which cannot be explained with conventional rate equations for the ensemble averaged level occupations. 28 The stochastic capture/escape processes can be better described using a random population model.…”