Superlattices and Microstructures 43 (2008) 494–499www.elsevier.com/locate/superlattices

Optical switching of quantum states insideself-assembled quantum dots

G. Munoz-Matutanoa,∗, B. Alena,1, J. Martınez-Pastora, L. Seravallib,P. Frigerib, S. Franchib

aInstituto de Ciencias de los Materiales, Universitat de Valencia, PO Box 2085, 46071 Valencia, Spain

bCNR e IMEM Institute, Parco delle Scienze 37/a, I-43100 Parma, Italy

Available online 4 September 2007

Abstract

Photoluminescence and excitation of photoluminescence spectroscopy have been performed for twokinds of single InAs self-assembled quantum dots grown on GaAs. The presence of unintentional impurities(donors and acceptors) offers the possibility to switch from negative to positively charged excitons byselectively exciting impurity related optical transitions.c� 2007 Elsevier Ltd. All rights reserved.

Keywords: Single QD spectroscopy; Trion recombination; Impurity related absorption; Charge transfer

Nowadays, research on semiconductor self-assembled quantum dots is not just a promisingtopic, but a consolidated technological option for many optoelectronic devices. In particular,optical spectroscopy has been recently the subject of intensive investigation. Applications such assingle-photon sources, entangled photon pair emitters, quantum logic gates, and spin-flip controlin nanostructures [1,2] show strong evidence of the intensive research in this topic.

Optical microspectroscopy is the ideal tool for managing quantum states inside QuantumDots (QD), such as neutral and charged few-particle excitonic states. The charge in QDs can beelectrically controlled by tuning the gate voltage in Schottky diodes embedding intrinsic QD

∗ Corresponding author.E-mail address: guillermo.munoz@uv.es (G. Munoz-Matutano).

1 Permanent address: Instituto de Microelectronica de Madrid (CNM-CSIC), Isaac Newton 8, 28760 Tres Cantos,Madrid, Spain.

0749-6036/$ - see front matter c� 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.spmi.2007.07.028

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Fig. 1. (a) PL spectra of the QD ensemble in the sample studied. The different sizes of the two QD families coexistingin the sample can be observed. (b) PL spectra analysis on the energy range between WL and GaAs for the same sample.

layers [3]. The Pauli exclusion principle and the effect of the Coulomb blockade allow oneto fill the states of the QD with single electrons. There are other filling options for runningoptical pumping but without any electronic device implementation on the sample [4–9]. Thesemechanisms are based on the QD surrounding impurities (basically acceptors) and on thedifferent diffusion lengths for the electrons and holes. In this work we prove the selectiveformation of either positively or negatively charged excitons in initially empty QDs in thepresence of both unintentional acceptor and donor impurities. This mechanism is compared fortwo families or ensembles of QDs present in the same sample: larger QDs emitting in the region1160–1170 nm at 4 K (close to 1300 nm at room temperature) and smaller QDs emitting atshorter wavelengths (below 970 nm at low temperatures).

The InAs QDs investigated in this paper were grown by MBE (Molecular Beam Epitaxy)on GaAs semi-insulating substrate. A GaAs buffer layer was grown at 600 ◦C by MBE. Afterthis, the InAs QDs were deposited at 505 ◦C at very low growth rate (0.009 ML/s) and cappedby a 100 nm thick GaAs layer grown by ALMBE [13] at 360 ◦C. During the deposition ofInAs QDs, the substrate was not azimuthally rotated in order to have a continuous variation ofInAs coverages for the 2D–3D transition, as discussed in [12] and references there quoted. Thecombination of a low growth rate (LGR) and graded coverage give rise to low density samples.The coverage of the sample studied in this work is 2.5 MLs, with a density of 16.5 QD/µm−2,as estimated by Atomic Force Microscopy (AFM) measurements.

Conventional photoluminescence (PL) and photoluminescence excitation (PLE) characteriza-tion was carried out with the sample held in the cold finger of a closed-cycle He cryostat. Singlespectroscopy (microPL and microPLE) was performed using a confocal arrangement based onan optical fiber inserted in the He exchange gas chamber of an immersion cryostat. The PL sig-nal, excited by a tunable Ti:sapphire laser, was dispersed by a 0.5 (0.3) m focal length gratingspectrograph and detected with a cooled InGaAs focal plane array (Si CCD) for wavelengthsabove (below) 1000 nm.

Fig. 1(a) shows the PL spectra recorded with an excitation density of 0.5 W/cm2 at 790 nm.Two PL emission bands are observed at around 1.08 eV and 1.38 eV, which we attribute to

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different families of Large (L-QD) and Small (S-QD) dot ensembles, respectively. A bimodal sizedistribution has been previously reported for LGR grown QDs [11]. Atomic force microscopyperformed on uncapped samples revealed such a bimodal QD size distribution, with mostfrequent values of 9 and 14 nm for heights and 36 and 54 nm for diameters in S-QDs andL-QDs, respectively. Fig. 1(b) displays the PL spectra for the energy range from 1.25 eV to1.55 eV showing the WL (Wetting Layer; 1.42 eV), GaAs (1.51 eV) and some other emissionbands (excitation density of 1.6 W/Cm2). Among them we can recognize a strong contributionat around 1.496 eV, very close to the GaAs excitonic feature. It can be attributed to electronrecombination with neutral acceptor impurities (e-A0; most probably carbon) and neutraldonor–acceptor (D0–A0) recombination. Hall measurements on similar GaAs layers revealthat although they are not intentionally doped, they have a net electron carrier concentration(i.e., donors minus acceptors) in the order of 1015 cm−3. A relevant point is that, even if theGaAs layers have been grown at standard velocity, there are growth interruptions that may causean increase of the impurity concentration in the GaAs close to the QDs. Moreover, the presenceof these impurities will not be important for normal high dot density samples, but will play animportant role for our low dot density sample, as discussed below.

Fig. 2 shows several curves for µPL [Fig. 2(a) and (b); detected at two different excitationenergies], and µPLE [Fig. 2(c) and (d)] for these PL peaks, for both QD families. The transitionassignment to positive (X

+) and negative (X−) trions is justified for several reasons. There are

previous studies in the literature reporting the simultaneous observation of X+ and X

− opticaltransitions at both sides of the neutral exciton line under similar excitation conditions [6–9].The power dependence of their integrated intensity grows in a linear way, as also observed forthe neutral exciton. In the L-QD family this characteristic “three-line signature” (neutral pluscharged excitons) is found all along the sample surface and relative distances did not vary beyond∼1 meV. Polarization dependence experiments on these QDs help to assign the excitonic andbiexcitonic transitions by the identification of the fine structure splitting (not shown here). ThePL spectra in Fig. 2(a) and (b) also show us the switching of excitonic species by impurity carriertransfer to the dots. When the pumping excitation is resonant with the acceptor band, the negativeexcitonic species increases its relative intensity with respect to the neutral exciton transition, ascan be seen in the lower panels of Fig. 2(a) and (b). In contrast, excitation resonant with the WLproduces the opposite behavior, even cancelation of the negative excitonic species and givingrise to a small amount of intensity of the positive ones (upper panels in Fig. 2(a) and (b)). Clearconfirmation of this optical switching is definitely obtained from PLE results.

Performing the µPLE experience we have observed some broad resonances in the energyrange between the WL and GaAs absorption edges, shown in Fig. 2(c) and (d). Those resonancesappear around two energy regions: 1.42–1.46 and 1.47–1.50 eV. We can easily correlate thefirst region with the appearance of positively charged excitons and the second region with thedominance of the negatively charged excitons. At the same time, it is possible to observe aspecial and narrower resonance for the neutral exciton at 1.478 eV [arrow on the X0 PLE linein Fig. 2(d)]. Other studies in the literature reported resonances at the WL energy range whichare assigned to the HH and LH recombinations [8,10]. For these references, narrow resonancesare shown at around 1.44 and 1.47 eV, but added to a slowly increasing continuum from WL(1.42 eV) to GaAs (1.52 eV) barriers. Our experiments show different results: broad resonancescentered at ∼1.496 eV on negative species [X

− PLE line in Fig. 2(d)] and broad depressions inthe same energy region on positive species recombination [X

2+ PLE in Fig. 2(d)]. In our opinionthese broad resonances can be associated with the existence of the impurities. In Fig. 3 is plottedthe normalized population difference ηX± = (Int X

±−Int X0)/(Int X

±+Int X0) extracted from

G. Munoz-Matutano et al. / Superlattices and Microstructures 43 (2008) 494–499 497

Fig. 2. µPL spectra for a single QD of the L-QD (2(a)) and S-QD (2(b)) families. µPLE spectra for the same QDs (2(c),2(d)). The detection energies in PLE are selected to coincide with charged and neutral exciton optical transitions. Theexcitation energies in PL are selected to coincide with the observed resonances in the PLE spectra.

PLE (in the S-QD family), and the macroPL measured in this energy range for comparison [thisPL spectrum was shown in Fig. 1(b)]. It can be observed that the X

− recombination increasesits relative intensity over the neutral exciton when the laser is tuned to the maximum of theimpurity band. In contrast, the doubly positively charged exciton has a strong η maximum whenthe photon energy is resonant with the WL absorption edge, gaining even more intensity than theneutral exciton.

We suggest two kinds of charge transfers to the QD: net neutral charge transfer upon excitationon GaAs and WL barriers, which generates electron–hole pairs or excitons without kinetic energywhich are directly transferred to the QD, and net negative charge transfer when the excitation istuned to the impurity resonance. By adding tunneling phenomena from nearest neutral acceptorsto the valence band WL levels, it is also possible to understand a positive charge transfer into theQD. So, tuning the excitation source to one of the latest energy regions provides the possibilityof obtaining optical switching between charged quantum dot states.

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Fig. 3. Renormalized population difference (see the text) for negatively charged and doubly positively charged excitonsin the S-QD family. The ensemble PL in the WL–GaAs range is shown for comparison (shadow mask).

In summary, we have shown that in the presence of acceptors and donors in the surroundingsof a quantum dot, optical pumping of charged exciton complexes in self-assembled QDs offers analternative to electrical injection. At low power, their net charge can be nicely controlled by usingphotons of the appropriate energy, independently of other lines originated by multiple-carrier(namely biexciton) effects at high excitation power. The charge mechanism has been comparedfor two kinds of QDs coexisting in the same studied sample. In the two cases we demonstratethat the charge injection behaviors are consistently similar, but the phenomenology is clearer forsmall dots, where carrier energy levels are closer to those of the impurities.

Acknowledgements

The authors gratefully acknowledge financial support by the Spanish MEC through TEC-2005-05781-C03-03 and NAN 2004-09109-C04-04 projects, and by the European Commissionthrough SANDiE Network of Excellence (NMP4-CT-2004-500101).

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