Letter pubs.acs.org/NanoLett
Single Photons on Demand from Novel Site-Controlled GaAsN/ GaAsN:H Quantum Dots Simone Birindelli,† Marco Felici,†,* Johannes S. Wildmann,‡ Antonio Polimeni,† Mario Capizzi,† Annamaria Gerardino,§ Silvia Rubini,∥ Faustino Martelli,∥,⊥ Armando Rastelli,‡ and Rinaldo Trotta‡ †
Dipartimento di Fisica and CNISM, Sapienza Università di Roma, Piazzale Aldo Moro 5, 00185 Roma, Italy Institute of Semiconductors and Solid State Physics, Johannes Kepler University, Altenbergerstr. 69, A-4040 Linz, Austria § Istituto di Fotonica e Nanotecnologie, IFN-CNR, Via Cineto Romano 42, 00156 Roma, Italy ∥ IOM-CNR Laboratorio TASC, S. S 14, Km 163.5, I-34149 Trieste, Italy ⊥ IMM-CNR, Via del Fosso del Cavaliere 100, 00133 Roma, Italy ‡
ABSTRACT: We demonstrate triggered single-photon emission from a novel system of site-controlled quantum dots (QDs), fabricated by exploiting the hydrogen-assisted, spatially selective passivation of N atoms in dilute nitride semiconductors. Evidence of this nonclassical behavior is provided by the observation of strong antibunching in the autocorrelation histogram of the QD exciton emission line. This class of site-controlled quantum emitters can be exploited for the fabrication of new hybrid QD-nanocavity systems of interest for future quantum technologies. KEYWORDS: Site-controlled quantum dots, single-photon emission, dilute nitrides, spatially selective hydrogenation
S
Ga(AsN),28−31 Ga(PN),32 and (InGa)(AsN)33,34). The latter is due to the formation of stable N-2H−H complexes,35,36 which gradually and reversibly transmute dilute nitrides in (effectively) N-free alloys by neutralizing the effects of nitrogen on the properties of the host crystal. By spatially controlling the hydrogen−nitrogen interaction at the nanometer scale (and consequently the band gap energy in all spatial directions), sitecontrolled nanostructures with optical spectra resembling those of self-assembled semiconductor QDs have been fabricated.37 However, single photon emission, that is, the ultimate proof of quantum confinement, is yet to be demonstrated. In this Letter, we report on the fabrication and characterization of ordered arrays of Ga(AsN)/Ga(AsN):H quantum dots obtained via spatially controlled hydrogenation of Ga(AsN) quantum wells. We demonstrate for the first time that these nanostructures, whose emission spectrum at low excitation power consists of several lines originating from single excitons, biexcitons, and charged excitons, emit single photons on demand. The capability of these QDs to generate nonclassical light states, combined with a perfect control over the QD position, opens up new avenues for the possible exploitation of this simple fabrication method in quantum technologies.
emiconductor quantum dots (QDs) have attracted much interest for the realization of single1 and entangled2 photon sources, which are key elements for quantum cryptography3 and optical quantum computing schemes.4 However, most of the envisioned applications will demand a full control over the size, shape, and position of the fabricated quantum emitters. In particular, the realization of integrated QD-optical microcavity systems, of great interest for the observation of cavity-QED effects5−7 and for the fabrication of high-performance nanolasers,8−11 requires a precise spatial (within 50 nm) and spectral (better than ∼5 meV) matching of the embedded QDs with the confined optical modes of the microcavity. This level of control is probably impossible to achieve via the simple self-assembly processes commonly exploited for QD growth.12 On the other hand, even the most successful attempts to control the QD position and template by lithographic means13−24 often result in a reduced control over the nanostructure nucleation site25 and/or in the formation of surface defects, which decrease the optical quality/efficiency of the QDs and are only partially eliminated via additional postgrowth steps.26 Recently, a novel route for the fabrication of site-controlled nanostructures based on hydrogen-assisted defect engineering in dilute nitride semiconductors has been demonstrated.27 This novel and original fabrication method combines the precision of modern electron-beam-lithography systems, which allow for the deposition of ordered arrays of nanometer-sized metallic structures on the surface of a sample, with the striking capability of hydrogen to modulate the electronic, optical, electric, and structural properties of dilute nitrides (such as © XXXX American Chemical Society
Received: November 12, 2013 Revised: January 23, 2014
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laser having an 80 MHz repetition rate and focused by a microscope objective with 0.55 numerical aperture. For power dependence studies, a frequency-doubled Nd:YVO4 laser having wavelength λexc = 532 nm was focused by a microscope objective with 0.8 numerical aperture. The photoluminescence signal was spectrally analyzed by single or double spectrometers with 0.75 m focal length and was detected with a nitrogencooled Si charge-coupled device. Photon-correlation measurements on single quantum dots were performed with a Hanbury Brown and Twiss setup (HBT). The signal was split in two detection arms using a nonpolarizing beam splitter, spectrally filtered with independent spectrometers, and finally detected with two silicon avalanche photodiodes (APDs). The number of counts as a function of the time interval between detection events on the two APDs was measured to determine the second-order correlation function. The time resolution of the experimental apparatus is about 500 ps, mainly limited by the time jitter of the APDs. Figure 2a shows a comparison between a conventional macro-PL spectrum (spatial resolution ∼300 μm) recorded on
The QD fabrication process is sketched in Figure 1. Titanium dots with a diameter of 80 nm were deposited by electron beam
Figure 1. (a−c) Sample processing steps required for the fabrication of GaAs1−xNx/GaAs1−xNx:H quantum dots, QDs. (a) An 80 nm metallic mask is deposited on the sample surface. (b) Hydrogenation (ion current density = 25 μA/cm2, sample temperature = 190 °C) is performed on the masked sample. (c) Following the formation of the QD, the mask is removed. The spatial distribution of the concentration (x) of unpassivated [i.e., not involved in the formation of the N-2H-H complexes responsible for N passivation35,36] N atoms, displayed as a grayscale in panels (b,c), was obtained from Finite-Elements calculations of the H diffusion process on the grounds of the model introduced in ref 38.
Figure 2. (a) PL spectrum of the unpatterned Ga(AsN) quantum well and micro-PL spectrum of a single Ga(AsN) quantum dot. (b) MicroPL image of a QD array, acquired by using a long wavelength pass filter to reject emission from GaAs.
lithography on top of a 6 nm thick GaAs1−xNx/GaAs quantum well (QW) with a nitrogen concentration x = 1.1%, capped by a 30 nm thick GaAs layer. The sample was then irradiated with hydrogen ions by means of a low-energy (100 eV) Kaufman source. The hydrogenations were performed with an ion current density of 25 μA/cm2, while keeping the sample temperature at 190 °C for the entire duration of the process. These conditions ensure a trap-limited diffusion with a hydrogen forefront of less than 5 nm/decade (see ref 38 for more details), finally resulting in very sharp interfaces between the hydrogenated and H-free regions of the sample. Because Ti is fully opaque to hydrogen, the deposited masks effectively screen the underlying regions from H diffusion. Finite-elements calculations of the spatial distribution of nonpassivated N atoms nearby (and underneath) a Ti mask are displayed in Figure 1b,c for two different hydrogenation times (tH = 250 and 500 s, respectively). At the end of the hydrogenation process [tH = 500 s, which is a typical hydrogenation time for the QD fabrication process; see Figure 1c] a well-defined region of Hfree GaAs1−xNx (approximately 50 nm in diameter and surrounded in all spatial directions by GaAs, or GaAs-like, barriers) is obtained underneath the mask. Single-QD microphotoluminescence (PL) and photoncorrelation measurements were then employed to address the optical properties of the site-controlled GaAs1−xNx QDs obtained with this method. The measurements were performed at low temperature (typically 4−10 K) in a helium flow cryostat. For time-resolved and photon-correlation measurements, the QDs were excited with a femtosecond Ti:Sapphire
the untreated GaAs1−xNx/GaAs quantum well and a micro-PL spectrum (spatial resolution ∼1 μm) acquired on a single GaAsN quantum dot (the QD barrier was irradiated with a hydrogen dose dH = 6 × 1016 ions/cm2). The micro-PL spectrum of the QD at low excitation power exhibits a single line corresponding to the recombination from the ground-state exciton (see below). Figure 2b shows a micro-PL image of an array of QDs obtained by rejecting the emission from the GaAs barriers by means of a high-pass 850 nm filter. This figure demonstrates our excellent control over the QD position, evidencing the high uniformity of the fabrication process. Typical exciton linewidths range between 150 and 800 μeV and are strongly dependent on the excitation power. The full width at half-maximum (fwhm) of the QD energy distribution (∼30 meV, as estimated from a sample of 10 dots) is consistent (and even compares favorably) with the initial fwhm of the untreated GaAs1−xNx QW peak (∼40 meV). On the one hand, this points out to the high quality of the fabrication process, which does not seem to introduce any additional spread in the QD energy distribution. On the other hand, this finding also suggests that the QD uniformity might be improved in a rather straightforward manner, by reducing the line width of the GaAs1−xNx/GaAs QW that represents the starting point of our nanofabrication method. Typical spectra acquired on a single dot at different excitation powers are shown in Figure 3a. At the lowest laser power, a B
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Nn =
⟨n⟩n exp( −⟨n⟩) n!
(1)
Here ⟨n⟩ is the average number of excitons, which, assuming a power-law dependence on the excitation power, can be expressed as ⟨n⟩ = αPβ, where α and β are phenomenological parameters depending on the carriers capture and relaxation processes. The results obtained by fitting eq 1 to the experimental data, using n = 1 for the X line and n = 2 for the XX line, are shown in Figure 3b. A value β = 1.02 is obtained from the fit, thus confirming the attribution of these emission lines to the recombination from a single exciton and from a biexciton, respectively. Even though eq 1 cannot be applied to the case of a charged exciton, a quasi-Poissonian distribution with a real parameter γ can be defined41 Nγ = ⟨n⟩γ exp( −⟨n⟩)
(2)
A good agreement with the data is observed also for the X* line. The value obtained from the fit, γ = 1.54 is consistent with the recombination from a trion. Time-resolved PL measurements were performed on bulk GaAs and on the excitonic line of two single QDs (the QD sample was irradiated with a hydrogen dose dH = 8 × 1016 ions/ cm2). Micro-PL spectra of the two dots are shown in Figure 4a, while time-resolved data are displayed in Figure 4b. The same data, as obtained after deconvolution from the instrumental response, are shown in Figure 4c with the corresponding fits. The transient behavior of bulk GaAs is well fitted by a double exponential function with an initial fast decay time τ1 = 0.23 ± 0.01 ns and a slower one τ2 = 1.01 ± 0.08 ns. The observation
Figure 3. (a) Micro-PL spectra of a single Ga(AsN) QD recorded at different excitation powers: A single line is visible at the lowest laser power, while emission lines originating from the excited states of the dot appear at high power. (b) Photoluminescence intensity as a function of the excitation power, with corresponding fits. Emission lines originating from single exciton (X), biexciton (XX), and charged exciton (X*) are identified.
single narrow line is observed, while other features appear with increasing power, both at lower and higher energies. At even higher excitation power additional lines, due to the recombination of carriers from the excited states of the dot, appear at high energy (∼20 meV from the ground-state exciton peak). Three main emission lines, labeled X, XX, and X*, can be observed in the spectra. They were attributed, respectively, to radiative recombinations from the neutral exciton, biexciton, and charged exciton states of the QD, as determined from the study of the PL intensities as a function of laser power presented in Figure 3b. When performing such studies, simple power-law dependences are generally fitted to the data,39 thus disregarding the saturation of the PL intensity usually observed at high excitation powers and due to state-filling effects. A more complete approach is based on random-population theory,40 which in contrast to a simple model based on rate equations describes the probability of finding the dot in a particular charge configuration (microstate). When the model is limited to taking into account only excitonic states and stationary conditions are assumed, the number of excitons confined in a single dot follows a Poissonian statistics41
Figure 4. (a) Micro-PL spectra of two single Ga(AsN) quantum dots. (b) Time-resolved measurements performed on bulk GaAs and on the two quantum dots. (c) The same data shown in (b) after deconvolution from the instrumental response, along with the corresponding fits. C
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of two different decay times for GaAs can be explained by noting that our samples contain both untreated GaAs and hydrogenated Ga(AsN). Although they have the same electronic and optical properties, carrier dynamics on this time scale could be different. However, the slower decay time agrees reasonably well with previously reported exciton lifetimes in high-purity GaAs.42 A single exponential function could instead be fitted to the time-resolved data of QD1 and a value of the excitonic lifetime τ = 1.61 ± 0.04 ns was extracted from the fit. On the other hand, QD2 is again characterized by a biexponential decay with lifetimes τ1 = 0.52 ± 0.02 ns and τ2 = 2.2 ± 0.2 ns. This behavior is probably related to carrier relaxation phenomena between the excited states and the exciton ground state of the dot, as reported elsewhere.43−45 Additionally, a biexponential decay could also suggest the presence of charged traps in the dot surroundings. Indeed, carriers excited above the barrier band gap could be trapped in charged states around the dot and subsequently relax into the exciton ground state, giving rise to the observation of a slower decay time.46 Finally, biexponential decays have also been ascribed to the presence of dark excitons, which might serve as an additional feeding channel for the observed bright exciton state via spin-flip processes.47,48 In order to investigate the photon statistics of the light emitted by our QD system, thus unambiguously confirming the occurrence of single-photon emission and of 3D quantum confinement, we performed photon-correlation measurements with the HBT setup positioned at the wavelength of the X line. For the neutral-exciton transition of a single QD, the statistics of the emitted photons is described by the second-order autocorrelation function49 g(2)(τ ) =
⟨I(t )I(t + τ )⟩ ⟨I(t )⟩2
Figure 5. Normalized second-order correlation function for the excitonic emission of two single quantum dots with the corresponding fits (thick lines) based on eq 5. The same plot for bulk GaAs is shown as a reference. For the QDs the amplitude of the central peak (respectively 0.24 and 0.26, according to the fit) is well below the threshold value of 0.5, which points out to the emission of single photons.
emission threshold, demonstrate that the investigated QD emission lines correspond to the radiative recombination of a single exciton, and that the nanostructures fabricated with this method generate nonclassical light states. It is worth noting here that the residual nonzero value of the g(2)(0) measured for our QDs could be ascribed to several physical mechanisms. Along with the finite instrumental response, the presence of carrier recapture phenomena on a time scale comparable to the exciton lifetime (as also suggested by the observation of a biexponential decay in the PL transient of QD2, see above) can lead to an increase in the value of the g(2)(0). Indeed, carrier relaxation from multiexcitons and/or excited states to the exciton ground state of the dot has been reported52 as a possible cause for the observation of multiphoton events. Repopulation of the exciton ground state from charged traps in the dot surroundings can also result in a nonzero value of g(2)(0).53 A significant reduction of these effects is expected by employing resonant excitation schemes54,55 and/or for decreasing excitation power, due to the decreasing average number of electron−hole pairs confined in the dot at any given time.52 The excitation conditions employed in this work (nonresonant excitation, laser power ∼0.5 μW), however, were carefully chosen to obtain reasonably fast (collection times of ∼3 h), yet reliable, g(2)(τ) measurements, while maintaining the g(2)(0) value well below the critical value of 0.5. In summary and conclusion, we have demonstrated a novel route for the fabrication of single-photon emitters by means of spatially selective hydrogen irradiation of dilute nitrides. Our approach leads to the realization of highly uniform and reproducible arrays of quantum dots, whose size, shape, and position can be controlled at the nanometer scale. Single dots were characterized by micro-PL spectroscopy and emission lines originating from single excitons, as well as multiple and charged excitons, were identified. Photon-correlation measure-
(3)
where I(t) and I(t + τ) are the intensities of light emitted at the time t and after a time delay τ, respectively. The value of the function at zero delay is related to the number of photons N of the electromagnetic field49 g(2)(0) = 1 −
1 N
(4)
Ideally, we would expect the highly nonclassical value of g(2)(0) = 0 for a true single photon source (N = 1); in practice, the presence of a single quantum emitter is confirmed by g(2)(0) < 1/2 (N < 2). Pulsed-excitation autocorrelation plots of the excitonic emission from two single Ga(AsN) QDs, see Figure 4b for the corresponding PL spectra, are presented in Figure 5 (the autocorrelation plot of the PL emission from bulk GaAs is also shown for comparison). For both QDs we observe a strong suppression of the peak at zero delay, which can be quantified by fitting the following equation50,51 to the data ⎡ ⎢ τ− g (τ ) = B + ∑ an exp⎢ − τX ⎢ n ⎣ (2)
( ) ⎤⎥⎥ n f
⎥ ⎦
(5)
Here B is the background, f is the repetition rate of the excitation laser and τX is the exciton lifetime. From these fits, values of g(2)(0) = 0.24 ± 0.06 and g(2)(0) = 0.26 ± 0.07 are obtained for QD1 and QD2, respectively. These values, more than three standard deviations below the single-photon D
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ments clearly indicate that this novel class of site-controlled nanostructures emits single photons on demand, thus holding strong promise for the realization of site-controlled single photon sources integrated in nanophotonic devices. In particular, the high spatial resolution made available by electron-beam lithography (positioning accuracy in the 20−50 nm range22,56,57) could be exploited to precisely position one (or several) of the QDs fabricated with this technique within a photonic crystal (PhC) microcavity system, thereby achieving a near-perfect match of the QD position(s) with the cavity electric field and ensuring the ideal conditions for the observation of cavity-QED effects. It is also worth mentioning that the nanofabrication method presented here could be extended to obtain site-controlled nanostructures emitting in the region of interest for telecommunications simply by using samples with larger nitrogen content or, alternatively, by using an (InGa)(AsN) quantum well58,59 instead of a Ga(AsN) one.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Author Contributions
S.R. and F.M. grew the GaAs1−xNx/GaAs quantum wells used in the present study, while A.G. performed electron beam lithography and S.B. and M.F. hydrogenated the samples. S.B., M.F., J.W., and R.T. performed the measurements with help from A.P, A.R., and M.C. Data analysis was carried out primarily by S.B., with contributions from M.F, A.P., M.C., and R.T. The manuscript was written by S.B., M.F., A.P., A.R., M.C., and R.T. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS M.F. would like to acknowledge funding by the EU under Grant Agreement No. PIEF-GA-2011-301363. M.F., A.P., and M.C. also acknowledge support from the Italian Ministry for Education, University and Research within the Futuro in Ricerca (FIRB) program (project DeLIGHTeD, Prot. RBFR12RS1W). R.T. and A.R. would like to acknowledge F. Binder, A. Halilovic, U. Kainz, E. Vorhauer, and S. Brauer for technical assistance.
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REFERENCES
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(39) Thompson, R. M.; Stevenson, R. M.; Shields, A. J.; Farrer, I.; Lobo, C. J.; Ritchie, D. A.; Leadbeater, M. L.; Pepper, M. Phys. Rev. B 2001, 64, 201302. (40) Grundmann, M.; Bimberg, D. Phys. Rev. B 1997, 55, 9740− 9745. (41) Abbarchi, M.; Mastrandrea, C.; Kuroda, T.; Mano, T.; Vinattieri, A.; Sakoda, K.; Gurioli, M. J. Appl. Phys. 2009, 106, 053504. (42) Hwang, C. J. Phys. Rev. B 1973, 8, 646−652. (43) Birkedal, D.; Leosson, K.; Hvam, J. M. Superlattices Microstruct. 2002, 31, 97−105. (44) Adler, F.; Geiger, M.; Bauknecht, A.; Scholz, F.; Schweizer, H.; Pilkuhn, M. H.; Ohnesorge, B.; Forchel, A. J. Appl. Phys. 1996, 80, 4019. (45) Grundmann, M.; Heitz, R.; Bimberg, D.; Sandmann, J. H. H.; Feldmann, J. Phys. Status Solidi B 1997, 203, 121−132. (46) Verma, V. B.; Stevens, M. J.; Silverman, K. L.; Dias, N. L.; Garg, A.; Coleman, J. J.; Mirin, R. P. Opt. Express 2011, 19, 4182−4187. (47) Zinoni, C.; Alloing, B.; Monat, C.; Zwiller, V.; Li, L. H.; Fiore, A.; Lunghi, L.; Gerardino, A.; de Riedmatten, H.; Zbinden, H.; Gisin, N. Appl. Phys. Lett. 2006, 88, 131102. (48) Patton, B.; Langbein, W.; Woggon, U. Phys. Rev. B 2003, 68, 125316. (49) Walls, D. F.; Milburn, G. G. J. Quantum Optics; Springer: Berlin, 2008. (50) Nakajima, H.; Ekuni, S.; Kumano, H.; Idutsu, Y.; Miyamura, S.; Kato, D.; Ida, S.; Sasakura, H.; Suemune, I. Phys. Status Solidi C 2011, 8, 337−339. (51) Rivoire, K.; Buckley, S.; Majumdar, A.; Kim, H.; Petroff, P.; Vuckovic, J. Appl. Phys. Lett. 2011, 98, 083105. (52) Regelman, D. V.; Mizrahi, U.; Gershoni, D.; Ehrenfreund, E.; Schoenfeld, W. V.; Petroff, P. M. Phys. Rev. Lett. 2001, 87, 257401. (53) Aichele, T.; Zwiller, V.; Benson, O. New. J. Phys. 2004, 6, 90. (54) Santori, C.; Fattal, D.; Vuckovic, J.; Solomon, G. S.; Yamamoto, Y. New. J. Phys. 2004, 6, 89. (55) Bennett, A. J.; Unitt, D. C.; Atkinson, P.; Ritchie, D. A.; Shields, A. J. Opt. Express 2005, 13, 50−55. (56) Gallo, P.; Felici, M.; Dwir, B.; Atlasov, K. A.; Karlsson, K. F.; Rudra, A.; Mohan, A.; Biasiol, G.; Sorba, L.; Kapon, E. Appl. Phys. Lett. 2008, 92, 263101. (57) Calic, M.; Gallo, P.; Felici, M.; Atlasov, K. A.; Dwir, B.; Rudra, A.; Biasiol, G.; Sorba, L.; Tarel, G.; Savona, V.; Kapon, E. Phys. Rev. Lett. 2011, 106, 227402. (58) Liu, H. Y.; Hopkinson, M.; Navaretti, P.; Gutierrez, M.; Ng, J. S.; David, J. P. R. Appl. Phys. Lett. 2003, 83, 4951−4953. (59) Fekete, D.; Carron, R.; Gallo, P.; Dwir, B.; Rudra, A.; Kapon, E. Appl. Phys. Lett. 2011, 99, 072116.
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dx.doi.org/10.1021/nl404196y | Nano Lett. XXXX, XXX, XXX−XXX
Single Photons on Demand from Novel Site-Controlled GaAsN/ GaAsN:H Quantum Dots Simone Birindelli,† Marco Felici,†,* Johannes S. Wildmann,‡ Antonio Polimeni,† Mario Capizzi,† Annamaria Gerardino,§ Silvia Rubini,∥ Faustino Martelli,∥,⊥ Armando Rastelli,‡ and Rinaldo Trotta‡ †
Dipartimento di Fisica and CNISM, Sapienza Università di Roma, Piazzale Aldo Moro 5, 00185 Roma, Italy Institute of Semiconductors and Solid State Physics, Johannes Kepler University, Altenbergerstr. 69, A-4040 Linz, Austria § Istituto di Fotonica e Nanotecnologie, IFN-CNR, Via Cineto Romano 42, 00156 Roma, Italy ∥ IOM-CNR Laboratorio TASC, S. S 14, Km 163.5, I-34149 Trieste, Italy ⊥ IMM-CNR, Via del Fosso del Cavaliere 100, 00133 Roma, Italy ‡
ABSTRACT: We demonstrate triggered single-photon emission from a novel system of site-controlled quantum dots (QDs), fabricated by exploiting the hydrogen-assisted, spatially selective passivation of N atoms in dilute nitride semiconductors. Evidence of this nonclassical behavior is provided by the observation of strong antibunching in the autocorrelation histogram of the QD exciton emission line. This class of site-controlled quantum emitters can be exploited for the fabrication of new hybrid QD-nanocavity systems of interest for future quantum technologies. KEYWORDS: Site-controlled quantum dots, single-photon emission, dilute nitrides, spatially selective hydrogenation
S
Ga(AsN),28−31 Ga(PN),32 and (InGa)(AsN)33,34). The latter is due to the formation of stable N-2H−H complexes,35,36 which gradually and reversibly transmute dilute nitrides in (effectively) N-free alloys by neutralizing the effects of nitrogen on the properties of the host crystal. By spatially controlling the hydrogen−nitrogen interaction at the nanometer scale (and consequently the band gap energy in all spatial directions), sitecontrolled nanostructures with optical spectra resembling those of self-assembled semiconductor QDs have been fabricated.37 However, single photon emission, that is, the ultimate proof of quantum confinement, is yet to be demonstrated. In this Letter, we report on the fabrication and characterization of ordered arrays of Ga(AsN)/Ga(AsN):H quantum dots obtained via spatially controlled hydrogenation of Ga(AsN) quantum wells. We demonstrate for the first time that these nanostructures, whose emission spectrum at low excitation power consists of several lines originating from single excitons, biexcitons, and charged excitons, emit single photons on demand. The capability of these QDs to generate nonclassical light states, combined with a perfect control over the QD position, opens up new avenues for the possible exploitation of this simple fabrication method in quantum technologies.
emiconductor quantum dots (QDs) have attracted much interest for the realization of single1 and entangled2 photon sources, which are key elements for quantum cryptography3 and optical quantum computing schemes.4 However, most of the envisioned applications will demand a full control over the size, shape, and position of the fabricated quantum emitters. In particular, the realization of integrated QD-optical microcavity systems, of great interest for the observation of cavity-QED effects5−7 and for the fabrication of high-performance nanolasers,8−11 requires a precise spatial (within 50 nm) and spectral (better than ∼5 meV) matching of the embedded QDs with the confined optical modes of the microcavity. This level of control is probably impossible to achieve via the simple self-assembly processes commonly exploited for QD growth.12 On the other hand, even the most successful attempts to control the QD position and template by lithographic means13−24 often result in a reduced control over the nanostructure nucleation site25 and/or in the formation of surface defects, which decrease the optical quality/efficiency of the QDs and are only partially eliminated via additional postgrowth steps.26 Recently, a novel route for the fabrication of site-controlled nanostructures based on hydrogen-assisted defect engineering in dilute nitride semiconductors has been demonstrated.27 This novel and original fabrication method combines the precision of modern electron-beam-lithography systems, which allow for the deposition of ordered arrays of nanometer-sized metallic structures on the surface of a sample, with the striking capability of hydrogen to modulate the electronic, optical, electric, and structural properties of dilute nitrides (such as © XXXX American Chemical Society
Received: November 12, 2013 Revised: January 23, 2014
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laser having an 80 MHz repetition rate and focused by a microscope objective with 0.55 numerical aperture. For power dependence studies, a frequency-doubled Nd:YVO4 laser having wavelength λexc = 532 nm was focused by a microscope objective with 0.8 numerical aperture. The photoluminescence signal was spectrally analyzed by single or double spectrometers with 0.75 m focal length and was detected with a nitrogencooled Si charge-coupled device. Photon-correlation measurements on single quantum dots were performed with a Hanbury Brown and Twiss setup (HBT). The signal was split in two detection arms using a nonpolarizing beam splitter, spectrally filtered with independent spectrometers, and finally detected with two silicon avalanche photodiodes (APDs). The number of counts as a function of the time interval between detection events on the two APDs was measured to determine the second-order correlation function. The time resolution of the experimental apparatus is about 500 ps, mainly limited by the time jitter of the APDs. Figure 2a shows a comparison between a conventional macro-PL spectrum (spatial resolution ∼300 μm) recorded on
The QD fabrication process is sketched in Figure 1. Titanium dots with a diameter of 80 nm were deposited by electron beam
Figure 1. (a−c) Sample processing steps required for the fabrication of GaAs1−xNx/GaAs1−xNx:H quantum dots, QDs. (a) An 80 nm metallic mask is deposited on the sample surface. (b) Hydrogenation (ion current density = 25 μA/cm2, sample temperature = 190 °C) is performed on the masked sample. (c) Following the formation of the QD, the mask is removed. The spatial distribution of the concentration (x) of unpassivated [i.e., not involved in the formation of the N-2H-H complexes responsible for N passivation35,36] N atoms, displayed as a grayscale in panels (b,c), was obtained from Finite-Elements calculations of the H diffusion process on the grounds of the model introduced in ref 38.
Figure 2. (a) PL spectrum of the unpatterned Ga(AsN) quantum well and micro-PL spectrum of a single Ga(AsN) quantum dot. (b) MicroPL image of a QD array, acquired by using a long wavelength pass filter to reject emission from GaAs.
lithography on top of a 6 nm thick GaAs1−xNx/GaAs quantum well (QW) with a nitrogen concentration x = 1.1%, capped by a 30 nm thick GaAs layer. The sample was then irradiated with hydrogen ions by means of a low-energy (100 eV) Kaufman source. The hydrogenations were performed with an ion current density of 25 μA/cm2, while keeping the sample temperature at 190 °C for the entire duration of the process. These conditions ensure a trap-limited diffusion with a hydrogen forefront of less than 5 nm/decade (see ref 38 for more details), finally resulting in very sharp interfaces between the hydrogenated and H-free regions of the sample. Because Ti is fully opaque to hydrogen, the deposited masks effectively screen the underlying regions from H diffusion. Finite-elements calculations of the spatial distribution of nonpassivated N atoms nearby (and underneath) a Ti mask are displayed in Figure 1b,c for two different hydrogenation times (tH = 250 and 500 s, respectively). At the end of the hydrogenation process [tH = 500 s, which is a typical hydrogenation time for the QD fabrication process; see Figure 1c] a well-defined region of Hfree GaAs1−xNx (approximately 50 nm in diameter and surrounded in all spatial directions by GaAs, or GaAs-like, barriers) is obtained underneath the mask. Single-QD microphotoluminescence (PL) and photoncorrelation measurements were then employed to address the optical properties of the site-controlled GaAs1−xNx QDs obtained with this method. The measurements were performed at low temperature (typically 4−10 K) in a helium flow cryostat. For time-resolved and photon-correlation measurements, the QDs were excited with a femtosecond Ti:Sapphire
the untreated GaAs1−xNx/GaAs quantum well and a micro-PL spectrum (spatial resolution ∼1 μm) acquired on a single GaAsN quantum dot (the QD barrier was irradiated with a hydrogen dose dH = 6 × 1016 ions/cm2). The micro-PL spectrum of the QD at low excitation power exhibits a single line corresponding to the recombination from the ground-state exciton (see below). Figure 2b shows a micro-PL image of an array of QDs obtained by rejecting the emission from the GaAs barriers by means of a high-pass 850 nm filter. This figure demonstrates our excellent control over the QD position, evidencing the high uniformity of the fabrication process. Typical exciton linewidths range between 150 and 800 μeV and are strongly dependent on the excitation power. The full width at half-maximum (fwhm) of the QD energy distribution (∼30 meV, as estimated from a sample of 10 dots) is consistent (and even compares favorably) with the initial fwhm of the untreated GaAs1−xNx QW peak (∼40 meV). On the one hand, this points out to the high quality of the fabrication process, which does not seem to introduce any additional spread in the QD energy distribution. On the other hand, this finding also suggests that the QD uniformity might be improved in a rather straightforward manner, by reducing the line width of the GaAs1−xNx/GaAs QW that represents the starting point of our nanofabrication method. Typical spectra acquired on a single dot at different excitation powers are shown in Figure 3a. At the lowest laser power, a B
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Nn =
⟨n⟩n exp( −⟨n⟩) n!
(1)
Here ⟨n⟩ is the average number of excitons, which, assuming a power-law dependence on the excitation power, can be expressed as ⟨n⟩ = αPβ, where α and β are phenomenological parameters depending on the carriers capture and relaxation processes. The results obtained by fitting eq 1 to the experimental data, using n = 1 for the X line and n = 2 for the XX line, are shown in Figure 3b. A value β = 1.02 is obtained from the fit, thus confirming the attribution of these emission lines to the recombination from a single exciton and from a biexciton, respectively. Even though eq 1 cannot be applied to the case of a charged exciton, a quasi-Poissonian distribution with a real parameter γ can be defined41 Nγ = ⟨n⟩γ exp( −⟨n⟩)
(2)
A good agreement with the data is observed also for the X* line. The value obtained from the fit, γ = 1.54 is consistent with the recombination from a trion. Time-resolved PL measurements were performed on bulk GaAs and on the excitonic line of two single QDs (the QD sample was irradiated with a hydrogen dose dH = 8 × 1016 ions/ cm2). Micro-PL spectra of the two dots are shown in Figure 4a, while time-resolved data are displayed in Figure 4b. The same data, as obtained after deconvolution from the instrumental response, are shown in Figure 4c with the corresponding fits. The transient behavior of bulk GaAs is well fitted by a double exponential function with an initial fast decay time τ1 = 0.23 ± 0.01 ns and a slower one τ2 = 1.01 ± 0.08 ns. The observation
Figure 3. (a) Micro-PL spectra of a single Ga(AsN) QD recorded at different excitation powers: A single line is visible at the lowest laser power, while emission lines originating from the excited states of the dot appear at high power. (b) Photoluminescence intensity as a function of the excitation power, with corresponding fits. Emission lines originating from single exciton (X), biexciton (XX), and charged exciton (X*) are identified.
single narrow line is observed, while other features appear with increasing power, both at lower and higher energies. At even higher excitation power additional lines, due to the recombination of carriers from the excited states of the dot, appear at high energy (∼20 meV from the ground-state exciton peak). Three main emission lines, labeled X, XX, and X*, can be observed in the spectra. They were attributed, respectively, to radiative recombinations from the neutral exciton, biexciton, and charged exciton states of the QD, as determined from the study of the PL intensities as a function of laser power presented in Figure 3b. When performing such studies, simple power-law dependences are generally fitted to the data,39 thus disregarding the saturation of the PL intensity usually observed at high excitation powers and due to state-filling effects. A more complete approach is based on random-population theory,40 which in contrast to a simple model based on rate equations describes the probability of finding the dot in a particular charge configuration (microstate). When the model is limited to taking into account only excitonic states and stationary conditions are assumed, the number of excitons confined in a single dot follows a Poissonian statistics41
Figure 4. (a) Micro-PL spectra of two single Ga(AsN) quantum dots. (b) Time-resolved measurements performed on bulk GaAs and on the two quantum dots. (c) The same data shown in (b) after deconvolution from the instrumental response, along with the corresponding fits. C
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of two different decay times for GaAs can be explained by noting that our samples contain both untreated GaAs and hydrogenated Ga(AsN). Although they have the same electronic and optical properties, carrier dynamics on this time scale could be different. However, the slower decay time agrees reasonably well with previously reported exciton lifetimes in high-purity GaAs.42 A single exponential function could instead be fitted to the time-resolved data of QD1 and a value of the excitonic lifetime τ = 1.61 ± 0.04 ns was extracted from the fit. On the other hand, QD2 is again characterized by a biexponential decay with lifetimes τ1 = 0.52 ± 0.02 ns and τ2 = 2.2 ± 0.2 ns. This behavior is probably related to carrier relaxation phenomena between the excited states and the exciton ground state of the dot, as reported elsewhere.43−45 Additionally, a biexponential decay could also suggest the presence of charged traps in the dot surroundings. Indeed, carriers excited above the barrier band gap could be trapped in charged states around the dot and subsequently relax into the exciton ground state, giving rise to the observation of a slower decay time.46 Finally, biexponential decays have also been ascribed to the presence of dark excitons, which might serve as an additional feeding channel for the observed bright exciton state via spin-flip processes.47,48 In order to investigate the photon statistics of the light emitted by our QD system, thus unambiguously confirming the occurrence of single-photon emission and of 3D quantum confinement, we performed photon-correlation measurements with the HBT setup positioned at the wavelength of the X line. For the neutral-exciton transition of a single QD, the statistics of the emitted photons is described by the second-order autocorrelation function49 g(2)(τ ) =
⟨I(t )I(t + τ )⟩ ⟨I(t )⟩2
Figure 5. Normalized second-order correlation function for the excitonic emission of two single quantum dots with the corresponding fits (thick lines) based on eq 5. The same plot for bulk GaAs is shown as a reference. For the QDs the amplitude of the central peak (respectively 0.24 and 0.26, according to the fit) is well below the threshold value of 0.5, which points out to the emission of single photons.
emission threshold, demonstrate that the investigated QD emission lines correspond to the radiative recombination of a single exciton, and that the nanostructures fabricated with this method generate nonclassical light states. It is worth noting here that the residual nonzero value of the g(2)(0) measured for our QDs could be ascribed to several physical mechanisms. Along with the finite instrumental response, the presence of carrier recapture phenomena on a time scale comparable to the exciton lifetime (as also suggested by the observation of a biexponential decay in the PL transient of QD2, see above) can lead to an increase in the value of the g(2)(0). Indeed, carrier relaxation from multiexcitons and/or excited states to the exciton ground state of the dot has been reported52 as a possible cause for the observation of multiphoton events. Repopulation of the exciton ground state from charged traps in the dot surroundings can also result in a nonzero value of g(2)(0).53 A significant reduction of these effects is expected by employing resonant excitation schemes54,55 and/or for decreasing excitation power, due to the decreasing average number of electron−hole pairs confined in the dot at any given time.52 The excitation conditions employed in this work (nonresonant excitation, laser power ∼0.5 μW), however, were carefully chosen to obtain reasonably fast (collection times of ∼3 h), yet reliable, g(2)(τ) measurements, while maintaining the g(2)(0) value well below the critical value of 0.5. In summary and conclusion, we have demonstrated a novel route for the fabrication of single-photon emitters by means of spatially selective hydrogen irradiation of dilute nitrides. Our approach leads to the realization of highly uniform and reproducible arrays of quantum dots, whose size, shape, and position can be controlled at the nanometer scale. Single dots were characterized by micro-PL spectroscopy and emission lines originating from single excitons, as well as multiple and charged excitons, were identified. Photon-correlation measure-
(3)
where I(t) and I(t + τ) are the intensities of light emitted at the time t and after a time delay τ, respectively. The value of the function at zero delay is related to the number of photons N of the electromagnetic field49 g(2)(0) = 1 −
1 N
(4)
Ideally, we would expect the highly nonclassical value of g(2)(0) = 0 for a true single photon source (N = 1); in practice, the presence of a single quantum emitter is confirmed by g(2)(0) < 1/2 (N < 2). Pulsed-excitation autocorrelation plots of the excitonic emission from two single Ga(AsN) QDs, see Figure 4b for the corresponding PL spectra, are presented in Figure 5 (the autocorrelation plot of the PL emission from bulk GaAs is also shown for comparison). For both QDs we observe a strong suppression of the peak at zero delay, which can be quantified by fitting the following equation50,51 to the data ⎡ ⎢ τ− g (τ ) = B + ∑ an exp⎢ − τX ⎢ n ⎣ (2)
( ) ⎤⎥⎥ n f
⎥ ⎦
(5)
Here B is the background, f is the repetition rate of the excitation laser and τX is the exciton lifetime. From these fits, values of g(2)(0) = 0.24 ± 0.06 and g(2)(0) = 0.26 ± 0.07 are obtained for QD1 and QD2, respectively. These values, more than three standard deviations below the single-photon D
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(10) Atlasov, K. A.; Calic, M.; Karlsson, K. F.; Gallo, P.; Rudra, A.; Dwir, B.; Kapon, E. Opt. Express 2009, 17, 18178−18183. (11) Nomura, M.; Kumagai, N.; Iwamoto, S.; Ota, Y.; Arakawa, Y. Nat. Phys. 2010, 6, 279−283. (12) Stangl, J.; Holy, V.; Bauer, G. Rev. Mod. Phys. 2004, 76, 725− 783. (13) Baier, M. H.; Pelucchi, E.; Kapon, E.; Varoutsis, S.; Gallart, M.; Robert-Philip, I.; Abram, I. Appl. Phys. Lett. 2004, 84, 648−650. (14) Hartmann, A.; Ducommun, Y.; Kapon, E.; Hohenester, U.; Molinari, E. Phys. Rev. Lett. 2000, 84, 5648−5651. (15) Baier, M.; Malko, A.; Pelucchi, E.; Oberli, D.; Kapon, E. Phys. Rev. B 2006, 73, 205321. (16) Leifer, K.; Pelucchi, E.; Watanabe, S.; Michelini, F.; Dwir, B.; Kapon, E. Appl. Phys. Lett. 2007, 91, 081106. (17) Felici, M.; Gallo, P.; Mohan, A.; Dwir, B.; Rudra, A.; Kapon, E. Small 2009, 5, 938−943. (18) Mohan, A.; Gallo, P.; Felici, M.; Dwir, B.; Rudra, A.; Faist, J.; Kapon, E. Small 2010, 6, 1268−1272. (19) Surrente, A.; Gallo, P.; Felici, M.; Dwir, B.; Rudra, A.; Kapon, E. Nanotechnology 2009, 20, 415205. (20) Kiravittaya, S.; Rastelli, A.; Schmidt, O. G. Appl. Phys. Lett. 2006, 88, 043112. (21) Huggenberger, A.; Heckelmann, S.; Schneider, C.; Hofling, S.; Reitzenstein, S.; Worschech, L.; Kamp, M.; Forchel, A. Appl. Phys. Lett. 2011, 98, 131104. (22) Hughes, S.; Yao, P.; Milde, F.; Knorr, A.; Dalacu, D.; Mnaymneh, K.; Sazonova, V.; Poole, P. J.; Aers, G. C.; Lapointe, J.; Cheriton, R.; Williams, R. L. Phys. Rev. B 2011, 83, 165313. (23) Skiba-Szymanska, J.; Jamil, A.; Farrer, I.; Ward, M. B.; Nicoll, C. A.; Ellis, D. J. P.; Griffiths, J. P.; Anderson, D.; Jones, G. A. C.; Ritchie, D. A.; Shields, A. J. Nanotechnology 2011, 22, 165313. (24) Atkinson, P.; Kiravittaya, S.; Benyoucef, M.; Rastelli, A.; Schmidt, O. G. Appl. Phys. Lett. 2008, 93, 101908. (25) Jons, K. D.; Atkinson, P.; Muller, M.; Heldmaier, M.; Ulrich, S. M.; Schmidt, O. G.; Michler, P. Nano Lett. 2013, 13, 126−130. (26) Mereni, L. O.; Dimastrodonato, V.; Young, R. J.; Pelucchi, E. Appl. Phys. Lett. 2009, 94, 223121. (27) Trotta, R.; Polimeni, A.; Martelli, F.; Pettinari, G.; Capizzi, M.; Felisari, L.; Rubini, S.; Francardi, M.; Gerardino, A.; Christianen, P. C.; Maan, J. C. Adv. Mater. 2011, 23, 2706−2710. (28) Bissiri, M.; Baldassarri Höger von Högersthal, G.; Polimeni, A.; Gaspari, V.; Ranalli, F.; Capizzi, M.; Bonapasta, A. A.; Jiang, F.; Stavola, M.; Gollub, D.; Fischer, M.; Reinhardt, M.; Forchel, A. Phys. Rev. B 2002, 65, 235210. (29) Geddo, M.; Ciabattoni, T.; Guizzetti, G.; Galli, M.; Patrini, M.; Polimeni, A.; Trotta, R.; Capizzi, M.; Bais, G.; Piccin, M.; Rubini, S.; Martelli, F.; Franciosi, A. Appl. Phys. Lett. 2007, 90, 091907. (30) Alvarez, J.; Kleider, J. P.; Trotta, R.; Polimeni, A.; Capizzi, M.; Martelli, F.; Mariucci, L.; Rubini, S. Phys. Rev. B 2011, 84, 085331. (31) Berti, M.; Bisognin, G.; De Salvador, D.; Napolitani, E.; Vangelista, S.; Polimeni, A.; Capizzi, M.; Boscherini, F.; Ciatto, G.; Rubini, S.; Martelli, F.; Franciosi, A. Phys. Rev. B 2007, 76, 205323. (32) Polimeni, A.; Bissiri, M.; Felici, M.; Capizzi, M.; Buyanova, I.; Chen, W.; Xin, H.; Tu, C. Phys. Rev. B 2003, 67, 201303. (33) Polimeni, A.; Baldassarri, G.; Bissiri, H. V. M; Capizzi, M.; Fischer, M.; Reinhardt, M.; Forchel, A. Phys. Rev. B 2001, 63, 201304. (34) Baldassarri, H. v. H. G.; Bissiri, M.; Polimeni, A.; Capizzi, M.; Fischer, M.; Reinhardt, M.; Forchel, A. Appl. Phys. Lett. 2001, 78, 3472. (35) Wen, L.; Bekisli, F.; Stavola, M.; Fowler, W. B.; Trotta, R.; Polimeni, A.; Capizzi, M.; Rubini, S.; Martelli, F. Phys. Rev. B 2010, 81, 233201. (36) Wen, L.; Stavola, M.; Fowler, W. B.; Trotta, R.; Polimeni, A.; Capizzi, M.; Bisognin, G.; Berti, M.; Rubini, S.; Martelli, F. Phys. Rev. B 2012, 86, 085206. (37) Trotta, R.; Polimeni, A.; Capizzi, M. Phys. Status Solidi C 2009, 6, 2644−2648. (38) Trotta, R.; Giubertoni, D.; Polimeni, A.; Bersani, M.; Capizzi, M.; Martelli, F.; Rubini, S.; Bisognin, G.; Berti, M. Phys. Rev. B 2009, 80, 195206.
ments clearly indicate that this novel class of site-controlled nanostructures emits single photons on demand, thus holding strong promise for the realization of site-controlled single photon sources integrated in nanophotonic devices. In particular, the high spatial resolution made available by electron-beam lithography (positioning accuracy in the 20−50 nm range22,56,57) could be exploited to precisely position one (or several) of the QDs fabricated with this technique within a photonic crystal (PhC) microcavity system, thereby achieving a near-perfect match of the QD position(s) with the cavity electric field and ensuring the ideal conditions for the observation of cavity-QED effects. It is also worth mentioning that the nanofabrication method presented here could be extended to obtain site-controlled nanostructures emitting in the region of interest for telecommunications simply by using samples with larger nitrogen content or, alternatively, by using an (InGa)(AsN) quantum well58,59 instead of a Ga(AsN) one.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Author Contributions
S.R. and F.M. grew the GaAs1−xNx/GaAs quantum wells used in the present study, while A.G. performed electron beam lithography and S.B. and M.F. hydrogenated the samples. S.B., M.F., J.W., and R.T. performed the measurements with help from A.P, A.R., and M.C. Data analysis was carried out primarily by S.B., with contributions from M.F, A.P., M.C., and R.T. The manuscript was written by S.B., M.F., A.P., A.R., M.C., and R.T. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS M.F. would like to acknowledge funding by the EU under Grant Agreement No. PIEF-GA-2011-301363. M.F., A.P., and M.C. also acknowledge support from the Italian Ministry for Education, University and Research within the Futuro in Ricerca (FIRB) program (project DeLIGHTeD, Prot. RBFR12RS1W). R.T. and A.R. would like to acknowledge F. Binder, A. Halilovic, U. Kainz, E. Vorhauer, and S. Brauer for technical assistance.
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