Exciton footprint of self-assembled AlGaAs quantum dots in core-shell nanowires

Yannik Fontana, Pierre Corfdir, Barbara Van Hattem, Eleonora Russo-Averchi, Martin Heiss, Samuel Sonderegger, Cesar Magen, Jordi Arbiol, Richard T. Phillips, and Anna Fontcuberta i Morral

Phys. Rev. B 90, 075307 – Published 15 August 2014

Abstract

Quantum-dot-in-nanowire systems constitute building blocks for advanced photonics and sensing applications. The electronic symmetry of the emitters impacts their function capabilities. Here we study the fine structure of gallium-rich quantum dots nested in the shell of $GaAs − {Al}_{0.51} {Ga}_{0.49}$ As core-shell nanowires. We used optical spectroscopy to resolve the splitting resulting from the exchange terms and extract the main parameters of the emitters. Our results indicate that the quantum dots can host neutral as well as charged excitonic complexes and that the excitons exhibit a slightly elongated footprint, with the main axis tilted with respect to the long axis of the host nanowire. $GaAs − {Al}_{x} {Ga}_{1 - x} As$ emitters in a nanowire are particularly promising for overcoming the limitations set by strain in other systems, with the benefit of being integrated in a versatile photonic structure.

Received 25 May 2014
Revised 21 July 2014

DOI:https://doi.org/10.1103/PhysRevB.90.075307

Authors & Affiliations

Yannik Fontana^1,*, Pierre Corfdir^2,*, Barbara Van Hattem², Eleonora Russo-Averchi¹, Martin Heiss¹, Samuel Sonderegger³, Cesar Magen⁴, Jordi Arbiol⁵, Richard T. Phillips², and Anna Fontcuberta i Morral^1,†

¹Laboratoire des Matériaux Semiconducteurs, Institut des Matériaux, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland
²Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE, United Kingdom
³Attolight AG, EPFL innovation Square/PSE D, 1015 Lausanne, Switzerland
⁴Laboratorio de Microscopias Avanzadas (LMA) - Instituto de Nanociencia de Aragon (INA) and Departamento de Fisica de la Materia Condensada, Universidad de Zaragoza, 50018 Zaragoza, Spain
⁵Institució Catalana de Recerca i Estudis Avançats (ICREA) and Institut de Ciència de Materials de Barcelona, ICMAB-CSIC, E-08193 Bellaterra, Catalonia, Spain

^*These authors contributed equally to this work.
^†anna.fontcuberta-morral@epfl.ch

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Vol. 90, Iss. 7 — 15 August 2014

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Images

Figure 1
(a) ADF STEM image of a segregated island nested in the AlGaAs shell of a NW. Darker contrast corresponds to higher aluminum fraction. The triangular feature corresponds to the Al-rich shell enclosing an Al-depleted island. A wide-scale drawing of such a cross section is provided on the right; the space frame up-right indicates the important crystallographic directions. (b) Possible processes leading to the co-observation of exciton and charged-exciton (trion), here described for positive charging due to unintentional background doping (left) or an imbalance in free carrier capture rate (right). (c) 10 K CL-SEM of two NWs. GaAs emission at 1.49–1.51 eV is color coded in blue. Red is used to encode the signal recorded at 1.85 eV. The QDs luminescence at 1.85 eV is efficiently generated by the electron beam only very locally. (d) Tridimensional sketch of a segregated island embedded in the shell of a NW. The direction of the magnetic field used in this study is drawn, as well as the observation direction in the MPL experiment ( $k_{∥}$ label). The observation direction for the polarization measurements done on NWs transferred on a substrate is also indicated ( $k_{⊥}$ label).
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Figure 2
Photoluminescence of an isolated shell QD. (a) The emission intensities from both GaAs core and QD can be compared. The antenna effect expresses itself in a very bright emission from the nanoscale QD. (b) Spectral closeup of the QD outlined in (a). Several emission peaks can clearly be seen with an excitation power of $5 μ W$ . (c) Evolution of the intensity of the four peaks marked in (b). The exciton ( $X$ , black circles) and biexciton ( $X X$ , orange squares) can be identified thanks to their linear and quadratic dependence on laser input power (fitted with a linear function of slope $s$ ). The two intermediate peaks show intermediate (superlinear) dependencies, typical of charged excitons ( $C X$ and $C X$ , red and blue triangles).
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Figure 3
Polarization analysis of $X$ and $C X$ emission lines. (a) Normalized emission from $C X$ . The peak does not shift with polarization thanks to spin pairing. The polarizer orientation in the laboratory reference frame is represented by the left-hand arrow quadrant. (b) Emission from $X$ . In contrast to its charged counterpart, the $X$ emission peak shape varies as a function of the polarization as a result of the anisotropic exchange interaction between the hole and electron forming the exciton. (c) Example of an $X$ spectrum fitted in order to retrieve the energy difference between the exchange interaction admixed states. Both linewidth and splitting result from a global fit on the full data set. (d) Polar representation of the intensity of the two AEI-split states $H$ and $V$ . As expected, the states are orthogonal one to the other. (e) Intensity of the $C X$ as a function of polarization. Due to spin pairing, $C X$ is protected against the exchange interaction. In this case, the intensity modulation comes from the photonic effect of the nanowire through the antenna effect. (f) Cartoon of the orientation of the different states. $H$ and $V$ states are orthogonal to each other, but neither of them coincide with the orientation given by the $C X$ emission.
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Figure 4
MPL of two single QDs in a nanowire. (a) and (c) Color scans showing the evolution of the spectra of QD $A$ (a) and QD $B$ (c) as the magnetic field is progressively increased. In both cases, $X$ and $C X$ are visible. The zero field peaks are split in groups of two doublets by the Zeeman effect and shifted because of diamagnetism. The bright and dark doublets are particularly obvious for the $C X$ , as the splitting between dark and bright states at zero field is nonexistent. (b) and (d) Spectra at different magnetic fields for the QD $A$ and $B$ . the spectra are shifted in intensity for clarity. Black (gray) arrows denote the position of the dark peaks of the $X (C X)$ . The behavior of the dots is very similar, differing marginally because of the different magnitude of the Landé factors, diamagnetic coefficients, and $X − C X$ binding energies.
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Figure 5
(a) Fitting of the energetic splitting and shift for QD $A$ with 1. (b) $X$ and $C X$ Landé factors from different QDs, showing consistent values between different QDs. The dark states $| g_{D}^{X / C X} |$ are systematical larger than the bright ones and the Landé factors of the bright and the dark states slightly increase with the emission energy. (c) Schematic depicting the splittings for $X$ (right) and $C X$ (left). For zero magnetic field, $X$ states are split by the isotropic and anisotropic exchange interactions, while the spin-paired $C X$ levels are still degenerated. The application of an external magnetic field lifts the degeneracy and mixes the pure states, leading to the observation of bright and dark optical transitions.
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