Ultrafast Room Temperature Single-Photon Source from Nanowire

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Ultrafast Room Temperature Single-Photon Source from Nanowire-Quantum Dots Samir Bounouar, Miryam Elouneg-Jamroz, Martien Ilse Den Hertog, Claudius Morchutt, Edith Bellet-Amalric, Régis André, Catherine Bougerol, Yann Genuist, Jean-Philippe Poizat, Serge Tatarenko, and Kuntheak KHENG Nano Lett., Just Accepted Manuscript • DOI: 10.1021/nl300733f • Publication Date (Web): 02 May 2012 Downloaded from http://pubs.acs.org on May 8, 2012

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Ultrafast Room Temperature Single-Photon Source from Nanowire-Quantum Dots S. Bounouar†,‡, M. Elouneg-Jamroz†, M. den Hertog†, C. Morchutt‡, E. Bellet-Amalric‡, R. André†, C. Bougerol†, Y. Genuist†, J.-Ph. Poizat†, S. Tatarenko† and K. Kheng‡,* CEA/CNRS/Université Joseph Fourier group “Nanophysique et Semiconducteurs”, †CNRSInstitut Néel, 25 rue des Martyrs, 38042 Grenoble cedex 9, France, ‡CEA-INAC 17 rue des Martyrs 38054 Grenoble cedex 9, France ABSTRACT Epitaxial semiconductor quantum dots are particularly promising as realistic single-photon sources for their compatibility with manufacturing techniques and possibility to be implemented in compact devices. Here, we demonstrate for the first time single-photon emission up to room temperature from an epitaxial quantum dot inserted in a nanowire, namely a CdSe slice in a ZnSe nanowire. The exciton and biexciton lines can still be resolved at room temperature and the biexciton turns out to be the most appropriate transition for single-photons emission due to a large non-radiative decay of the bright exciton to dark exciton states. With an intrinsically short radiative decay time (≈ 300 ps) this system is the fastest room temperature single-photon emitter, allowing potentially GHz repetition rates.

* [email protected] S.B. and M. E-J. have equally contributed to the present work. 1/15 ACS Paragon Plus Environment

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KEYWORDS: nanowire, quantum dot, single-photon emission, biexciton, dark exciton, II-VI semiconductor

MAIN TEXT

Single-photons are crucial for applications in quantum information processing1, quantum cryptography2 or metrology. A wide variety of emitters have demonstrated single-photon emission, but for a practical use, stable, fast and room temperature operation are key prerequisites. Single-photon emission has been demonstrated up to room temperature for dye molecules3, semiconductor nanocrystals4,5, nitrogen-vacancy centres in diamond6,7 but it was limited so far to a temperature of 200K for self-assembled QDs8,9. Epitaxially grown QDs such as self-assembled QDs are nevertheless particularly attractive as single-photon emitters10 for their stability, narrow spectral linewidths and short radiative lifetimes (≤ 1 ns, significantly shorter than diamond nitrogen-vacancy centres7 or nanocrystals5). Morever, they can potentially be integrated into monolithic structures (such as optical microcavities11,12) and even electrically pumped13,14 to realize practical devices. Recently, semiconductor nanowires (NWs) have appeared as promising building blocks for nanoscale devices owing to their crystalline quality and their integration possibilities. With NWs, a QD structure can be constructed by inserting a slice of a lower gap semiconductor within the NW15. A few years ago, some of us had shown the potentiality of such a NW-QD system with a CdSe QD inserted in ZnSe NWs grown on silicon substrates16. Although the diameter and the orientation of the NWs were not well controlled (in particular there was no epitaxial relationship between the NWs and the silicon substrate) single-photon emission was obtained up to 220K16. In the present work, by developing the homoepitaxial growth of ZnSe NWs on high quality ZnSe pseudo-substrates, we show that this NW-QD system can actually generate single-photons efficiently up to room temperature. Homoepitaxy is bound to provide higher 2/15 ACS Paragon Plus Environment

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purity materials than heteroepitaxy and this has allowed us to show that, in this system, the exciton and biexciton lines can be still resolved at room temperature. Furthermore we show that in this system the most appropriate transition for single-photon emission is the biexciton transition, which is quite uncommon, and we clearly identify the underlying mechanism. As compared to self-assembled QDs, there are several important advantages to using a NW heterostructure to construct a QD. First of all, the strain of the heterostructure can relax efficiently on the sidewalls due to the narrow dimensions of the NW, offering the prospect of improved optical properties. Secondly, NW-QDs are not subject to a wetting layer which is a pathway for carriers to escape easily from the QD when the temperature increases, drastically degrading the QD emission intensity17. And if necessary, it is very easy to detach nanowires from the as-grown sample to get well isolated emitters whose emission spectrum is not polluted by nearby QDs. The ZnSe/CdSe/ZnSe NW-QDs in this study were grown by molecular beam epitaxy (see Supporting Information for details) using Au-catalysts (gold nano-droplets) to nucleate the NWs growth18. A 40 nm thick ZnSe (100) buffer layer was first epitaxially grown on a GaAs (100) substrate to act as a pseudo-substrate and a shield to potential Ga and As dopants. We use a low density of Au-catalyst seeds (about 15/µm2) to reduce competition between NWs for impinging material during growth, leading to more uniform NWs. Figure 1 shows that the homoepitaxy on ZnSe (100) promotes the growth of oriented NWs as expected. The NWs grow mostly along the [111]B direction of the substrate but a significant fraction (30%) grows vertically along the [100] direction. The NWs are fairly uniform, with a diameter of 10 nm and a length around 250 nm. NWs from the sample in Fig. 1 were studied by high resolution transmission electron microscopy (HRTEM). Figure 2a presents the HRTEM image of a full NW from the ZnSe substrate to the gold particle. Its diameter, constant all along the NW, is of 10 nm. In order to determine the QD size, Geometrical Phase Analysis (GPA)19,20 was used on HRTEM images of NWs to map the interplanar spacing along the wire axis (see Supporting Information for 3/15 ACS Paragon Plus Environment

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details). The GPA analysis of the whole NW reveals no change of lattice spacing all along the wire except in a small region (Fig. 2c) which is attributed to the CdSe insertion. The size of this region (Fig. 2d) gives the QD size. On the same sample we have found QDs with lengths ranging from 2.4 to 4 nm. As the lateral and longitudinal sizes are smaller than the 11 nm mean electron-hole distance in bulk CdSe (i.e. the Bohr diameter), we have full threedimensional quantum confinement and a strong Coulomb interaction between carriers. These QDs emit in the visible spectrum with a peak centered at an energy of 2.3 eV (wavelength λ≈540 nm) and an inhomogeneous linewidth of 0.14 eV (∆λ≈32 nm) attributable to size and composition dispersion. Depending on the QD charge state, neutral or charged excitons can be observed in single QDs photoluminescence spectra21. Figure 3a presents four selected neutral QDs, each showing two sharp emission lines (full width at half maximum (FWHM) smaller than 1 meV (0.25 nm) at 4K) corresponding to the biexciton (XX) and the single-exciton (X) emission. The biexciton state corresponds to a doubly-excited QD with completely filled lowest electron and hole energy levels. When the biexciton decays, the final state is a single-exciton and the emitted photon (XX) is red-shifted with respect to the singleexciton emission (X) due to Coulomb interaction between the carriers. These transitions can be unambiguously identified by recording the cross-correlation histogram (Fig. 3b) whose asymmetric shape is characteristic of the radiative cascade sequence22 sketched in the inset of Fig. 3a. At a temperature of 4K, the biexciton decay time amounts to its radiative lifetime and we have measured a value of 300±50 ps. The photoluminescence spectra (Fig.3a) show a striking intensity difference between lines X and XX. The QDs were excited with 1 ps laser pulses at intensities high enough to fill the QD with at least two excitons, i.e. in the biexciton saturation regime. After each laser pulse, one would expect two photons to be emitted: one for the biexciton decay followed by one for the exciton decay. If that were the case then the X and XX lines on the photoluminescence spectra would have equal intensities. Figure 3a shows clearly that the biexciton line XX is 4/15 ACS Paragon Plus Environment

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always more intense. More precisely, line XX always reaches higher saturation intensity than line X (Fig. 3c). This intensity difference can be explained by the fine structure of the exciton state. The biexciton is a singlet state with total momentum M=0 and the transition XX leading to the bright-exciton state with M=±1 is always optically allowed. The bright-exciton, on the other hand, can either recombine radiatively or transfer very efficiently (and non-radiatively) to the lower lying dark-exciton state M=±2 through a spin-flip process. The dark-exciton, which typically lies an amount ∆E=4 to 9 meV below the bright-exciton state21, 23 can only decay through non-radiative mechanisms because its optical recombination is forbidden. The dark exciton thus represents a non-radiative decay channel that leads to a decrease of the X line intensity. In the spectra of Fig. 3a, higher emission energies correspond to smaller QDs (higher confinement energies). The phonon-induced spin-flip between bright-exciton and dark-exciton levels is faster for smaller QDs23 leading to a higher non-radiative decay rate. This explains why the exciton intensity, relatively to that of the biexciton, decreases with increasing emission energy. Increasing the temperature can repopulate the bright-exciton state but we observe that line X is significantly weaker than the biexciton line XX at saturation whatever the temperature23. Thus, the biexciton transition is clearly more appropriate as single-photon source. Fig. 4b shows indeed that the photon antibunching can be observed on this transition up to 300K. The raw correlation function at zero delay is g2(0)=0.48 at T=300K. The normalized value corrected for the background random coincidences24 is 0.22. This non-zero value is partly due to contamination from the thermally broadened exciton line. Let us emphasize that the X and XX lines correspond to single-photons both emitted after each pulse excitation but at slightly different wavelengths. For operation as single-photon source one has to be able to filter out one line from the other because otherwise two photons are detected. The width of the lines must then be smaller than their spectral separation. 5/15 ACS Paragon Plus Environment

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Increasing the temperature usually leads to an overlap of the two lines due to the unavoidable linewidth broadening25. Usually, single-photon emission in epitaxial QDs is demonstrated with the single-exciton recombination, and the deleterious effect of the biexciton at high temperature can only be diminished by pumping below saturation8 (which reduces the count rate and affects the deterministic generation of single-photons). In CdSe nanocrystals, the biexciton transition is usually quenched by non-radiative Auger effects so that single-photon detection is still possible even when the linewidth is too large (FWHM ∼25 nm at 300K) to resolve the X and XX lines; an enhancement of the biexciton emission therefore will result in a degradation of the anti-bunching26. Interestingly in our system, the exciton and biexciton lines can still be resolved at room temperature (3.4 nm FWHM as compared to 3.9 nm X-XX line separation in Fig. 4a). Additionally, the decay through dark-exciton state helps to considerably weaken the exciton line at saturation, further reducing its effect on the degradation of the biexciton anti-bunching. The zero delay autocorrelation value is an important figure of merit but the photon count rate is also a key parameter. The thermal activation of non-radiative channels decreases this count rate (from 105 s-1 at 4K to 8. 103 s-1 at 300 K in our case, with a collection numerical aperture NA=0.4). This effect can be quantitatively evaluated by investigating the temperature dependence of the biexciton lifetime. At low temperature, the biexciton decays radiatively with a decay time τ=τrad=1/ Γrad ≈ 300 ps. The decay rate Γ=1/τ is constant up to ∼ 100K and then increases due to the appearance of non-radiative channels represented by rate γnr so that Γ(Τ) = Γrad + γnr(T). The measured values of γnr(Τ) follow an Arrhenius law (Fig. 4c) with activation energy Ea ranging from 40 to 170 meV for different QDs (Fig. 4d). Roughly speaking, the activation energy quantifies the energy barrier for the biexciton to disappear non-radiatively. The actual mechanism is likely to be an escape of a hole to a NW surface state or to the ZnSe barrier (holes are less confined than electrons in CdSe/ZnSe heterostructures). With the latter assumption, the largest observed values of Ea are not 6/15 ACS Paragon Plus Environment

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compatible with a pure CdSe composition in the QD (valence band offset ~ 300 meV), but rather with a ZnxCd1-xSe ternary alloy with x≈0.5 Zn alloying. This QD composition is consistent with the TEM composition analysis18 of Fig. 2 (see Supporting Information) and explains the observed ~ 400 meV blueshift of the emission energies (as compared to energies expected for pure CdSe QDs). Figure 4d shows that low emission energy QDs exhibit large activation energies, in agreement with the fact that more energy is required to escape from a deeper potential well. According to this criteria, low emission energy QDs should be more robust with respect to temperature. But these QDs also have a lower phonon-spin flip coupling, which increases the hindering exciton emission rate, as discussed earlier. A tradeoff must thus be found between these two trends by choosing average emission energy QDs. To conclude, this NW-QD architecture is a very promising single-photon emitter since it can emit single-photons up to room temperature with a high repetition rate (potentially larger than 1 GHz) thanks to a short decay time (300 ps). The emission efficiency at high temperature may be improved by the development of the core-shell growth27 (NWs surrounded by a radial shell). This shell would enable passivation of the NW sidewalls and enhancement of the emission efficiency as demonstrated for colloïdal QDs28. This possibility of radial growth also offers the appealing prospect to form photonic wires for very efficient extraction of single-photons29. It would be an elegant bottom-up approach to design an efficient single-photon source with a single emitter deterministically positionned at the centre of a photonic wire.

Acknowledgements We thank Le Si Dang and Henri Mariette for fruitful discussions. We acknowledge support from the French National Research Agency (ANR) through the Nanoscience and Nanotechnology Program (Project BONAFO n°ANR-08-NANO-031-01) that provided a 7/15 ACS Paragon Plus Environment

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research fellowship for MdH. MEJ acknowledges financial support from the Nanosciences Foundation «Nanosciences, aux limites de la nanoélectronique».

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FIGURES

Figure 1. ZnSe/CdSe NW-QDs grown on a ZnSe buffer. a, Top view Scanning Electron Microscope (SEM) image. b, Perpendicular SEM view of the same sample. Vertical and tilted NWs are observed corresponding to growth direction [100] and [n11] respectively.

Figure 2. Structural analysis by transmission electron microscopy (TEM) of a single ZnSe/CdS NW-QD. a, Series of HRTEM images along an entire tilted NW attached to the substrate. The CdSe QD is located about 70 nm from the gold particle, in the boxed region. b, HRTEM of the boxed region in a; the dashed rectangle in b, c and d corresponds to the CdSe slice. c, Geometrical phase analysis of the growth planes along the growth direction. d, Line profiles made in c along the solid box region. The (0002) interplanar spacing for ZnSe is indicated by the horizontal dashed line.


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Figure 3. Exciton and biexciton in single NW-QDs. a, Photoluminescence (PL) spectra of four selected single ZnSe/CdSe NW-QDs at saturation excitation (see Fig. 3c) at 5K. The vertical axes are shifted for clarity. As commented in the text, the X/XX intensity ratio decreases with increasing emission energy (from QD1 to QD4). The inset shows the QD energy levels. b, Exciton-biexciton cross-correlation under continuous-wave excitation for QD1. c, Exciton and biexciton photoluminescence intensity as a function of the pulsed laser power. The saturation intensity for XX is higher than for X (see explanations in the text).


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Figure 4. Temperature dependent emission properties. a, Photoluminescence spectra of QD4 at saturation excitation for temperatures T= 4 K, 220 K and 300 K. The exciton line X is faintly present on the side of the biexciton line XX (the XX-X line-separation is 16.4 meV). The biexciton linewidth increases from 0.9 meV (0.20 nm) at 5K to 14.3 meV (3.43 nm) at 300K. b, Corresponding intensity autocorrelation functions of the biexciton line XX under pulsed excitation. The numbers in the graph are the second-order correlation values g(2)(0) (corresponding to the normalized areas) and the numbers in brackets are the value corrected for signal to background ratio (see Methods). The total count rate is 105 s-1 at 4K and 8. 103 s-1 at 300K with a collection numerical aperture NA=0.4. The time bin is 0.39 ns for 4K and 1.6 ns for 220K and 300K measurements. c, Biexciton non-radiative decay rate γnr as a function of 1/T for three different QDs. The slope gives the activation energy. d, Activation energy as a function of the QD emission energy for seven QDs.


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Ultrafast Room Temperature Single-Photon Source from Nanowire

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