Triggered Indistinguishable Single Photons with Narrow Line Widths

Letter pubs.acs.org/NanoLett

Triggered Indistinguishable Single Photons with Narrow Line Widths from Site-Controlled Quantum Dots K. D. Jöns,*,† P. Atkinson,‡,§ M. Müller,† M. Heldmaier,† S. M. Ulrich,† O. G. Schmidt,‡ and P. Michler† †

Institut für Halbleiteroptik und Funktionelle Grenzflächen and Research Center SCoPE, University of Stuttgart, Allmandring 3, 70569 Stuttgart, Germany ‡ Institute for Integrative Nanosciences, IFW Dresden, Helmholtzstraße 20, 01069 Dresden, Germany § Institut des NanoSciences de Paris, UPMC, CNRS UMR 7588, 4 Place Jussieu, 75252 Paris Cedex 05, France S Supporting Information *

ABSTRACT: In this Letter, we present narrow line width (7 μeV), nearly background-free single-photon emission (g(2)(0) = 0.02) and highly indistinguishable photons (V = 0.73) from site-controlled In(Ga)As/GaAs quantum dots. These excellent properties have been achieved by combining overgrowth on ex situ pit-patterned substrates with vertical stacking of spectrally distinct quantum dot layers. Our study paves the way for largescale integration of quantum dots into quantum photonic circuits as indistinguishable single-photon sources. KEYWORDS: Quantum dots, site-controlled, single photons, indistinguishability

I

to a decrease in intensity and broadening of the line width of site-controlled QD luminescence, depending on the QD proximity to the interface. In this Letter we demonstrate that, by vertical stacking of two QD layers, we can increase the distance to the regrowth interface and obtain site-controlled QDs with optical properties (such as line width, intensity, coherence time, carrier lifetime, and indistinguishability) comparable to randomly grown QDs. To achieve site-controlled growth of the QDs the GaAs (100) substrate was patterned with a 12.5 μm periodic array of small pits by electron beam lithography and wet-etching. During overgrowth, InAs seed QDs nucleate over the patterned pit site at coverages lower than the critical thickness for dot formation on an unpatterned area.22 After capping with GaAs, the local strain field above these seed QDs drives preferential nucleation of a second layer of spectrally distinct In(Ga)As/GaAs QDs. The second-layer QDs measured here were located 22 nm from the regrowth interface. The detailed device growth and patterning was as follows. Twenty repeats of 65.7 nm/77.5 nm GaAs/AlAs, topped by 101.4 nm GaAs, were grown by molecular beam epitaxy on a GaAs (100) substrate. This corresponds to a DBR mirror and the bottom reflector of a λ/2 GaAs cavity at 930 nm wavelength, where the cavity growth is stopped 30 nm before completion. The buffer was then patterned by electron-beam lithography and wet-etched using 1:8:800 H2SO4/H2O2/H2O, resulting in an array of pits ∼100 nm wide and ∼18 nm deep,

ntegrated single-photon sources are regarded as essential building blocks in various applications in quantum information science. A high degree of indistinguishability is a precondition for most of these applications, for example, for linear optical quantum computing and quantum repeaters. As solid-state single-photon sources,1 self-assembled In(Ga)As/ GaAs quantum dots (QDs) have proven to be promising candidates for quantum information processing.2,3 Their ability to emit triggered and highly indistinguishable photons4 as well as entangled photon pairs5−7 is paving the way for their applications in different quantum information schemes. Stateof-the-art coupling of QDs with photonic cavities has been based on precisely locating randomly grown QDs,8,9 allowing a detailed study of fundamental physics. However, this technique is not suitable for mass production of, for example, indistinguishable single-photon sources on a single wafer or for their integration into a photonic quantum circuit,10,11 due to the time needed for the search for suitable QDs, and the poor probability of locating randomly positioned QDs with the correct spacing between them for integration into quantum circuit designs containing more than one QD. On the other hand, site-controlled QDs promise large scale integration into devices.12 Recently, they have been successfully embedded into microcavities,13,14 showing reasonable optical properties.15,16 Additionally, very high efficiencies have been achieved from QDs embedded in nanowires.17,18 However, in all previous work site-controlled QD line widths have not reached those of randomly grown QDs, and the generation of indistinguishable photons has not yet been reported. One method to control the QD nucleation site is to prepattern the substrate with small pits.12,15,19−22,24 However, defects at the regrowth interface, from the ex-situ patterning and subsequent oxide removal, lead © 2012 American Chemical Society

Received: October 2, 2012 Revised: November 20, 2012 Published: November 30, 2012 126

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Letter

spaced 12.5 μm apart and surrounded by large alignment markers visible under an optical microscope. The pit arrays were then protected by a photoresist layer, and the alignment markers were etched a further ∼200 nm by SiCl4 reactive ion etching (RIE), to ensure that they remain clearly visible after overgrowth. Resist removal is one of the most critical stages of the substrate preparation, since any residual contaminant, either from the resist itself or the chemicals used to clean the surface can result in degraded overgrowth both in terms of the surface morphology (leading to unintentional dot formation between patterned sites) and in terms of the amount of defects at the interface and in the overgrown epilayer which degrade the dot optical qualities. Our process flow has the advantage that, by patterning the alignment markers by e-beam lithography at the same time as the pit arrays, we only need to strip the resist from the substrate once. This improves the process yield and reduces damage to the substrate at the same time. The resist was removed by ultrasonic agitation in a series of solvents (acetone, dimethyl sulfoxide, and isopropanol) followed by a final UV ozone ash and a dip in 1:3 HCl/H2O shortly before being loaded into the ultrahigh vacuum system. The remaining native oxide was then removed by exposure to atomic hydrogen in a two-stage process. Initially at 180 °C the arsenic oxides and subsequently at ∼400 °C the gallium oxides23 are reduced. The process was stopped when the surface showed a clear As-rich RHEED pattern. Overgrowth consisted of a brief anneal under As4 overpressure at 570 °C to remove any residual oxides or hydrocarbons on the surface. A 5 nm Al0.75Ga0.25As/5 nm GaAs buffer was then grown at 510 °C, followed by 1.5 ML InAs at 0.01 ML/s. The thin Al0.75Ga0.25As blocking layer in the buffer increases the brightness of the seed dot layer emission by reducing nonradiative recombination paths to the regrowth interface. After a 3 min growth interrupt, the dots were capped with 8 nm GaAs, followed by a 10 min smoothing interrupt at 550 °C and a growth of a 2 nm Al0.83Ga0.17As/2 nm GaAs barrier. 1.6 ML InAs at 510 °C, followed by a 3 min growth interrupt, formed the second dot layer. This layer was subsequently capped with 1.9 nm GaAs and annealed at 550 °C to limit the dot wavelength to less than 950 nm. By contrast, the lower dot layer emitted at wavelengths >1000 nm. The structure was then completed by 137.4 nm GaAs topped by 2 repeats of 77.5 nm/65.7 nm AlAs/GaAs. Figure 1a shows an atomic force microscope(AFM) image of a higher density pit array patterned at the same time as the dilute array, where the growth was stopped after the seed QD layer. As can be seen from the statistics in Figure 1b, there is a certain spread in occupancy from zero to four seed QDs per patterned site with only ∼40% of sites containing dots. The unoccupied sites are due to some of the initial pattern sites being too small to serve as preferential nucleation sites for the amount of InAs deposited in the seed layer. The spread in occupancy can be attributed to the observed spread in the sizes of the initially patterned pits due to both, instability in the electron beam current during patterning, and undercutting of the resist during the wet-etching which increased the pattern width. A higher, more uniform, occupancy has been reported elsewhere with wet-etched arrays,20 narrower and deeper (35 nm deep) RIE etched pits,21,22,24,25 or by the use of very low InAs growth rates.15,16 The microphotoluminescence (μ-PL) map in Figure 1c shows the positioning of the second QD layer. The sample was cooled to T = 4 K and excited nonresonantly with a Ti:Sapphire laser at λ = 800 nm focused down to a 1.5 μm

Figure 1. Spatial mapping of site-controlled QD growth and photon emission. (a) AFM image showing dot occupancy over a 500 nm spacing patterned site array. (b) Statistics from 100 patterned sites of the number of dots per patterned pit for a 500 nm spacing array. The spread in positioning accuracy of ±80 nm in the [110] direction is due to the hole width after GaAs buffer growth. (c) μ-PL map of the integrated intensity of the second QD layer (integration range 885− 934 nm). At each intersection of the grid lines (guide to the eye), a patterned pit is located which acts as a nucleation site for the first QD layer. The positioning yield of the high quality second QD layer is ∼40%.

diameter spot. While the sample was moved relative to the objective in steps of 1.25 μm, the PL signal was integrated from λ = 885 to 934 nm. This detection range enables us to select the light emitted by the second QD layer only. Similar to the seed QD layer occupancy in Figure 1b, we observe a positioning yield of ∼40% from our μ-PL map. The observed lateral width of the luminescence is due to both our experimental setup, since we collect luminescence even when our objective is slightly offset from the QD position, as well as the presence of clusters of dots over some patterned sites as seen in Figures 1a and 2a. Figure 2a−c shows three typical μ-PL spectra under nonresonant excitation. The upper graph (a) shows luminescence from a cluster of QDs above a nanohole position. Even in such an extreme example of a cluster, each QD emission line can be spectrally well resolved. Figure 2b and c shows very bright PL from one single QD, with nearly no background and no PL in the rest of the spectrum. In most cases we find PL from zero to three QDs at one pattern site. To investigate the QD transition line widths we carried out high-resolution microphotoluminescence (HRPL) with 0.5 μeV spectral resolution (details in Supporting Information). Figure 2d shows a histogram of the line widths for all of the QDs measured. Around 30% of the investigated transitions were neutral excitons (X) and the rest trion (T) lines. We observe narrow line widths as low as 7 μeV under nonresonant excitation (inset of Figure 2d). These line widths are comparable to those of self-assembled In(Ga)As/GaAs QDs.26,27 The emission line width for QDs strongly depends on spectral diffusion28,29 caused by charge carriers trapped at defects at the regrowth interface. In this work the median line width value for 40 site-controlled dots of 13 μeV reflects the relatively large distance to the regrowth interface. This is an improvement by a factor of 3 compared to published state-ofthe-art site-controlled QDs grown on pit-patterned (001) GaAs substrates30 and is comparable to site-controlled QDs grown by metal−organic−vapor phase epitaxy on (111)B substrates, 127

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= 212 ± 5 ps. The Fourier transform gives a Gaussian line shape with a line width of 13.0 ± 0.4 μeV, in good agreement with the 11.6 ± 0.5 μeV line width measured by HRPL. Timecorrelated single-photon counting measurements of this QD reveal a lifetime of T1 = 0.71 ± 0.04 ns (see Supporting Information). This lifetime is comparable to stochastically grown self-assembled QDs. We furthermore perform pulsed autocorrelation measurements to test the quality of singlephoton emission. We observe nearly background-free triggered single-photon emission with a value of g(2)(0) = 0.02 (Figure 3b) without any background subtraction. This excellent value can be attributed to the predetermined low density of the sitecontrolled dot array. An additional important figure of merit for quantum emitters and a benchmark for the applicability of these site-controlled QDs for quantum information processing is how identical consecutively emitted photons are, that is, their indistinguishability. This quality factor can be extracted from two-photon interference measurements (HOM).32 We create excitation laser pulse pairs with a width of 2 ps for each individual pulse and a variable delay time between these pulses of Δtlaser. The photons emitted from the QD are analyzed with a fiber-coupled Mach−Zehnder interferometer (MZI) (see Supporting Information) with an internal delay ΔtMZI. Figure 4a depicts three HOM measurements taken at different total delays Δt = Δtlaser − ΔtMZI. When the photons overlap in time (Δt = Δtlaser − ΔtMZI = 0) the middle peak, labeled 3, is suppressed as a signature of strong two-photon interference. We can extract the degree of two-photon interference for all measured total delay times Δt by the ratio M(Δt) between coincidences of the twophoton interference peak 3 and the sum of the side peaks denoted as 2 and 4 in Figure 4a: M(Δt) = A3/(A2 + A4), where A2, A3, and A4 describe the integrated intensities of peaks 2, 3, and 4, respectively. In Figure 4b−d M(Δt) is plotted for three different QDs. To describe the data we used M(Δt) = 0.5 − a·exp(−|Δt − t0|/τm), with a = 0.5 − M(0) corresponds to the depths of the dip and τm is the two-photon interference time. t0 is used to compensate the uncertainty in the total time delay Δt. We derive the visibility from the value of M(Δt = 0). For further details see the Supporting Information. The visibility of the two-photon interference are found to be V(0) = 0.54 ± 0.03, V(0) = 0.73 ± 0.04, and V(0) = 0.45 ± 0.04, for QD 1, QD 2, and QD 3, respectively. The V(0) value depends on the coherence time T2, the lifetime T1, and relaxation rate of the carriers into the s-shell.33 These parameters are influenced by the environment, for example, temperature, phonon scattering, and carrier−carrier scattering. Therefore, different V(0) values are expected for the three investigated QDs. The best visibility values reported so far from randomly grown QDs4,34 under quasi-resonant excitation are only 11% higher than our best values for site-controlled QDs. Thus, the investigated sitecontrolled QDs are suitable for triggered generation of indistinguishable photons. The two-photon coherence times τm determined from the Hong−Ou−Mandel dips in Figure 4 are 29 ± 6 ps, 35 ± 6 ps, and 36 ± 9 ps for QD 1, QD 2, and QD 3, respectively. For QD 3 this result is in good agreement with the coherence time T2 = 34 ± 1 ps observed in the onephoton interference measurements. QD 2 shows a slightly longer coherence time of T2 = 66 ± 2 ps. Surprisingly, T2 = 212 ± 5 ps is 7 times longer than τm for QD 1. The differences are not yet clear but might be related to the different type of laser excitation, that is, pulsed p-shell excitation for the two-photon interference and cw p-shell for the one-photon interference

Figure 2. Photoluminescence of site-controlled QDs. (a) μ-PL spectrum of a cluster of In(Ga)As/GaAs QDs under nonresonant excitation. (b, c) Single QD emission at a patterned site. (d) Histogram on the observed line widths extracted from HRPL measurements under above-band excitation. A Poisson statistic (red curve) extracts a median value of 13 μeV for the QD line widths. The inset shows one HRPL spectra of a T transition with a narrow line width of 7 μeV. The measurement was performed with a scanning Fabry−Pérot interferometer of a free spectral range (FSR) of 62.035 μeV.

where the QDs have a distance greater than 100 nm from the patterned interface.31 Figure 3a shows a coherence time T2 measurement performed with a Michelson interferometer of a quasiresonantly excited QD. We observed a coherence time of T2

Figure 3. First- and second-order correlation measurements. (a) Coherence time measurement with a Michelson interferometer of a single In(Ga)As/GaAs QD under quasi-resonant excitation reveals a T2 = 212 ps. This corresponds to a line widths of 13 μeV. (b) Pulsed autocorrelation measurement of the same transition shows a nearly background-free antibunching signal with g(2)(0) = 0.02 without background correction. 128

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the fabrication of site-controlled QDs for solid-state singlephoton sources in quantum information technologies.

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ASSOCIATED CONTENT

S Supporting Information *

Schematic layer structure of the sample, seed QD layer−second QD layer correlation, experimental details concerning twophoton interference measurements, high-resolution photoluminescence, and time-correlated single-photon counting. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank R. Hafenbrak and A. Ulhaq for advice and fruitful discussions. Financial support by the DFG research group 730 “Positioning of single nanostructuressingle quantum devices” and the BMBF QuaHL-Rep is gratefully acknowledged.

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Figure 4. Two-photon interference measurement series on individual site-controlled QDs. (a, b) Results from the HOM measurements of the QD investigated in Figure 3. (a) Three HOM measurements taken at different total delay Δt between photon pairs at the second beam splitter of the MZI. When the photons overlap in time (Δt = Δtlaser − ΔtMZI = 0) the middle peak, labeled 3, is suppressed as a signature of strong two-photon interference. (b−d) The relative suppression of the middle peak M(Δt) is plotted for three different site-controlled QDs. The emitted photons show a visibility as high as V = 0.73. The twophoton coherence time of the photons for all three QDs is short, as expressed by the narrow HOM dips.

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