A Nanomembrane-Based Wavelength-Tunable High-Speed Single

Letter pubs.acs.org/NanoLett

A Nanomembrane-Based Wavelength-Tunable High-Speed SinglePhoton-Emitting Diode Jiaxiang Zhang,*,† Fei Ding,*,† Eugenio Zallo,† Rinaldo Trotta,‡ Bianca Höfer,† Luyang Han,† Santosh Kumar,† Yongheng Huo,† Armando Rastelli,‡ and Oliver G. Schmidt† †

Institute for Integrative Nanosciences, IFW Dresden, Helmholtzstraße 20, 01069 Germany Institute of Semiconductor and Solid State Physics, Johannes Kepler University Linz, Altenbergerstraße 69, 4040, Austria

‡

S Supporting Information *

ABSTRACT: We demonstrate an all-electrically operated wavelength-tunable on demand single-photon source for the first time. The device consists of a light-emitting diode in the form of a semiconductor nanomembrane containing selfassembled quantum dots integrated onto a piezoelectric crystal. Triggered single photons are generated via injection of ultrashort electrical pulses into the diode, while their energy can be precisely tuned over a broad range by varying the voltage applied to the piezoelectric crystal. High speed operation of this single-photon-emitting diode up to 0.8 GHz is demonstrated. These results represent an important step toward the realization of electrically driven sources of indistinguishable photons on demand. KEYWORDS: Single-photon source, quantum dots, light-emitting diode, nanomembrane, piezoelectric crystal, excitation repetition rate

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tuning16 have been demonstrated in order to tune the optical properties of the self-assembled QDs. However, thermal annealing is an irreversible coarse tuning technique, while the magnetic field tuning requires complex and bulky setup which renders a practical implementation inconvenient. Although applying an electric field is a promising “tuning knob” to tune the energy emission of QDs based on the quantum-confined Stark effect,17 the main drawback is the difficulty of combining it with electrical excitation at the same time. Recently, elastic strain fields induced by a piezoelectric crystal have been successfully employed as a reliable tuning knob to control the optical properties of QDs.18−24 But until now wavelengthtunable triggered single-photon emission from quantum LEDs, which is crucial for a nonpostselective TPI measurement with remote SPSs,19 has not been demonstrated yet. In this Letter, we report high speed, electrically triggered single-photon emission from a wavelength tunable quantum LED. The device is based on InGaAs QD-containing n-i-p nanomembranes integrated onto a piezoelectric crystal. Triggered single-photon emission is realized by applying subnanosecond electrical pulses to excite the InGaAs QDs. The wavelength tuning is achieved by applying a variable external strain field to the QDs using the piezoelectric crystal. We carry out the second-order autocorrelation [g(2)(τ)] measurements with a Hanbury-Brown Twiss (HBT) setup to

ingle-photon sources (SPS) have been extensively explored over the past decade due to abundant applications in scalable quantum information processing such as distributed quantum computing and quantum cryptography. Most of the envisioned schemes strongly rely on the “Hong-Ou-Mandel” type two-photon interference (TPI) between indistinguishable photons emitted from independent SPSs.1−4 Until now, the TPI has been realized successfully with a variety of remote quantum emitters such as atoms,5 trapped ions, 6 and molecules7 that benefit from their intrinsically identical photon emission properties. Self-assembled semiconductor quantum dots (QDs), which allow monolithic integration with electronic and photonic elements in mature semiconductor technologies, are among the most promising sources for single photons. In particular, for practical device applications, epitaxially grown QDs can be easily embedded in a light-emitting diode (LED) to achieve electrically driven SPS,8,9 which could eliminate the use of the bulky laser system. Therefore, one would expect a full-fledged optoelectronic quantum network that is running on macroscopically separated, QD-based single-photon LEDs.10 However, an inhomogeneous spectral broadening of about 10 meV is commonly induced by the random distributions in shape and composition of QDs during the self-assembled growth process. This gives rise to the distinguishability of photons from different QDs and to date becomes the main obstacle to be overcome for realizing the TPI from remote SPSs based on selfassembled QDs. Many techniques including in situ thermal annealing and temperature tuning,11−14 magnetic field,15 and electric field © 2013 American Chemical Society

Received: June 24, 2013 Revised: November 2, 2013 Published: November 7, 2013 5808

dx.doi.org/10.1021/nl402307q | Nano Lett. 2013, 13, 5808−5813

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the diode and for the piezoelectric crystal are independent, and they share the same ground contact on the gold layer. With this configuration, electric fields can be applied independently to the diode and to the piezoelectric crystal without any cross-talk, therefore achieving an all-electrically simultaneous excitation and tuning of the QDs. Figure 1c shows a cross-section scanning electron microscopy (SEM) image of the device prepared by focused ion beam cutting. The air free space, GaAs nanomembrane and gold layer are clearly seen. Here the gold layer acts not only as the common ground contact for the diode and the piezoelectric crystal, but also as a mirror to form a goldsemiconductor-air planar cavity with the aim to enhance the single-photon extraction efficiency. The reflectivity measurements on the nanomembrane reveal that this planar cavity has a cavity mode centered at 880 nm at cryogenic temperature. The thickness of the nanomembrane (1.67λm, λm is the cavity mode wavelength in GaAs medium) and the location of the InGaAs QDs layer (λm away from the semiconductor/air interface) are well-designed, from which the standing wave condition is satisfied.23,25 As a result, the extraction efficiency can be greatly increased and a ∼14-fold enhancement compared to that of a semiconductor/air planar structure has been achieved (see Supporting Information). The piezoelectric crystal used here is [Pb (Mg1/3Nb2/3) O3]0.72[PbTiO3]0.28 (PMN-PT). When a high voltage (Vp) is applied, an anisotropic in-plane biaxial stress (compressive or tensile) can be generated in a welldefined manner. Because the QDs are embedded in thin nanomembranes, the device facilitates the transfer of large strain fields from the PMN-PT crystal to the QDs and thus allows a broad tuning range of the QD emission energy.20−24 The device as shown in Figure 1b is then mounted in a liquid helium flow cryostat and all the experiments are carried out at 5 K. A simple circuit made of a 50 Ω resistor and a 1 nF capacitor is connected to the diode in parallel in order to ensure impedance-matching between the diode26 and the external fast electric pulse generator (Picosecond Lab, U.S.A, model 12010). With this pulse generator, the maximum achievable excitation repetition rate (ERR) is 800 MHz. In optical measurements, the electroluminescence (EL) is collected by a 50× microscope objective with numeric aperture of 0.42, which is placed on the top of the nanomembrane and collects the photon emission from the area close to the metal contact. It is noticeable that our sample has a low QD density of ∼4 × 106 cm−2. When considering the spatial resolution of the microscope objective (0.7 μm), we are able to collect the EL from a single QD (see Supporting Information). Subsequently, the EL is sent to a 750 mm focal length monochromator and detected by a liquidnitrogen-cooled Si-CCD camera, which allows a spectral resolution of ∼20 μeV. For the second-order time correlation measurements, a HBT setup, consisting of a 50:50 nonpolarizing beam splitter and two high efficiency single-photon counting avalanche photodiodes (SPADs), is used. The electrical signals corresponding to the photon detection event on each SPAD are sent to the start and stop inputs of a timecorrelated single-photon counting module (Pico-harp 300, PicoQuant GmbH) from which a histogram g(2)(τ) as a function of τ = tstop − t start is recorded. The time-resolved EL is analyzed by a time-correlated single-photon counting (TCSPC) technique. To determine the system response time, data were recorded from a train of 220 fs-long laser pulses at 880 nm, and the resolution times of about 495 ps for the g(2)(τ) measurements and about 280 ps for the time-resolved EL measurements were obtained respectively.

illustrate the triggered single-photon emission from this electrically operated, wavelength tunable single-photon-emitting diode. Together with a demonstration of the high speed operation up to 800 MHz, these results hold strong promise toward the nonpostselective TPI measurements with two remote electrically driven SPSs. The studied sample was grown by solid source molecular beam epitaxy. Thin membrane structure, which consists of a 178 nm-thick n-doped GaAs layer, a 160 nm-thick intrinsic GaAs layer containing a layer of self-assembled InGaAs QDs, and a 96 nm thick p-doped GaAs layer, was grown on a 100 nm thick Al0.75Ga0.25As sacrificial layer. The nanomembrane was then released from the substrate and became free-standing by selective etching of the sacrificial layer in diluted hydrofluoric acid. Subsequently, the nanomenbrane was transferred onto a gold-coated piezoelectric single crystal via a gold-to-gold thermo-compression bonding technique. The bonded gold layer on the bottom formed the p-contact, while the n-contact was made by bonding a 25 μm thick aluminum wire via a wedge bonder on the top of the nanomembrane. Further details of the sample growth are described in the Supporting Information and the device preparation has been discussed in our previous work.23 A strain-tunable single-photon-emitting diode is shown schematically in Figure 1a, together with the microscopic images of the device (see Figure 1b). The electrical contacts for

Figure 1. Sketch of a nanomembrane-based strain-tunable singlephoton-emitting diode. (a) The n-i-p GaAs nanomembrane-based LED including self-assembled InGaAs QDs is integrated onto a piezoelectric crystal, and therefore the emission energy of photons can be controlled by the strain fields. (b) Microscopic images of the devices. The right side shows nanomembranes bonded onto the PMNPT crystal as well as the n-GaAs contacts formed by the electrical bonding of the aluminum wires on the top of the nanomembrane. (c) Cross-sectional SEM image of the device. A planar cavity, consisting of air free space, a GaAs semiconductor nanomembrane, and a gold layer, is clearly identified. 5809


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Figure 2. (a) Scheme for electrically pulsed excitation. The electrical pulses are composed of a dc bias Vd (less than the EL threshold of about −1.9 V) and ultrashort consecutive electrical pulses with amplitude Vpp. (b) EL spectra from an InGaAs QD injected by electrical pulses with 80 MHz repetition rate, nominal pulse width of 300 ps, and a fixed Vpp of −0.7 V. (c) Time-resolved EL intensity of the neutral exciton X0 measured at different dc bias Vd. (d) Measured exciton decay time (τmeas) and estimated tunneling time (τtunneling).

Figure 3. (a) Tunable X0 emission under in-plane biaxial strain ε∥. Large compressive (ε∥ < 0) or tensile (ε∥ > 0) strain can be obtained by applying different electric fields to the PMN-PT. The QD is under electrically pulsed excitation with 80 MHz ERR, 300 ps pulse width and Vpp = −0.7 V, Vd = −1.7 V. (b−d) Autocorrelation measurements demonstrating electrically triggered single-photon emission from X0. The QD is driven with 300 ps long pulses at a ERR of 200 MHz. The dc bias and the pulse amplitude are Vd = −1.7 V and Vpp = −0.7 V, respectively. The single-photon emission characteristic of the QD, which is indicated by the suppression of the zero-delay peak, is well maintained even when the emission energy is tuned in a broad range of about 4.8 meV by the strain field.

In order to realize electrically triggered single-photon emission, electrical pulses with a pulse width of less than the decay time of the QDs are applied to the diode. Subsequently, the decay time of single-photon emission can be controlled by varying the direct current (dc) bias.27−29 Figure 2a shows the electrical excitation scheme in which the ultrafast voltage pulses (Vpp) from the pulse generator are added with the dc bias (Vd) by a high frequency bias tee. This pulsed electrical signal is then

sent to the diode via a homemade high-frequency feed-through. In the experiments, the dc bias is chosen below the EL threshold of about −1.9 V to enhance the band bending.29 The EL spectra from InGaAs QDs under pulsed excitation are shown in Figure 2b, and the emission lines are assigned to the neutral (X0) and charged exciton (X*) according to the polarization-resolved EL measurements. The extra line at higher energy probably arises from a neighboring QD. Because 5810


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Figure 4. Autocorrelation measurements of X0 at different ERR of (a) 400, (b) 600, and (c) 800 MHz. The diode is excited at fixed dc bias of −1.7 V. The pronounced antibunching dip at zero time-delay provides the evidence of the single-photon emission up to 800 MHz.

desirable to have a reliable method to tune the emission energy of photons emitted from the QDs. In our devices, this has been achieved by applying a voltage Vp to the PMN-PT crystal that exerts variable strain fields on the QDs. This results in an outof-plane electric field Fp that leads to an in-plane strain ε∥ in the nanomembrane. As shown in Figure 3a, an in-plane tensile strain (ε∥ > 0) can be obtained when a negative electric field is applied, and vice versa. Figure 3a shows the tunable EL spectra from an InGaAs QD in the diode under pulsed excitation. Here the applied dc voltage Vd is −1.7 V, and the electrical pulse has the ERR of 80 MHz, a nominal pulse width of 300 ps and the amplitude (Vpp) of −0.7 V. We observe blue (red) shift of the QD emission when compressive (tensile) stresses are applied, which is consistent with previous observations.24,32 For the neutral exciton X0 emission, a total energy shift of approximately 4.8 meV has been achieved when Fp is changed from −10 to 20 kV/cm. To demonstrate the wavelength-tunable single-photon emission, the second-order autocorrelation function g(2)(τ) is measured and the results are shown in Figure 3b−d. The emission energy of X0 at Fp = 0 kV/cm, E0 = 1.3982 meV is defined as the zero-point, while the tuned energy Et at different Fp is defined relative to the zero-point by ΔE = Et − E0. At ΔE = 0 meV, the photon autocorrelation result is shown in Figure 3c. The periodic autocorrelation peaks together with the absent peak at zero time-delay provide the evidence of the triggered single-photon emission. The time separation between the neighboring peaks is 5 ns, which coincides with the repetition rate of 200 MHz. The normalized value of g(2)(0) provides a multiphoton emission probability of 0.14 ± 0.02. This finite probability most likely originates from the contributions of the background and the dark counts of the SPADs.33 We also measured the photon autocorrelation when the emission energy of the X0 line is tuned to different values by the strain field, and the results are shown in Figure 3b,d for ΔE = −1.6 and 3.2 meV, respectively. By comparing the g(2)(0) at different X0 emission energies, we do not observe significant changes in the multiphoton emission probability. This finding, together with the demonstrated capability of achieving energy tuning with microelectronvolts precision, proves that our technology provides a stable and precise tuning method to the photon emission from a triggered electrically driven QD-based SPS. In addition to the wavelength tuning, a high ERR is also of practical significance for the TPI measurements using singlephoton-emitting diodes. For an ERR of 400 MHz as shown in Figure 4a, we observe clear antibunching dip at the zero timedelay and a well-defined time separation of 2.5 ns between neighboring peaks. However, it is noticeable that the neighboring peaks in this photon autocorrelation experiment

of the cavity-enhancement, a photon collection efficiency of ∼4.8% for the X0 line at 1.3982 eV is achieved. This efficiency is 1 order of magnitude larger than that of a similar device without a gold mirror. Moreover, we observe a broadening of the line width for the X0 from 83 ± 5.6 to 118 ± 4 μeV as dc voltage varies from −1.80 to −1.65 V. Polarization-dependent measurements reveal that this line width broadening results from the fine structure splitting (FSS = 64 ± 1.5 μeV) and the time-varying Stark-shift as reported by Ward et al.30 More details about the photon collection efficiency and the line width broadening analysis can be found in the Supporting Information. To investigate the dc voltage-dependent decay time of the QD emission, we have performed TCSPC measurements on the X0 line and the results are shown in Figure 2c.The pulse amplitude Vpp is fixed at −0.7 V while the dc bias voltage Vd is systematically varied. We observe that the decay time of the X0 is reduced from 950 to 540 ps as Vd is varied from −1.80 to −1.65 V. This reduced decay time is mainly ascribed to the tunneling of charges out of the QD due to the band bending.27−29 More specifically, the quantum tunneling effect introduces an additional fast, nonradiative decay channel to the bright excitonic recombination in the QD. We use the relation 1/τmeas = 1/τradiative + 1/τtunneling to estimate the tunneling time of carriers in the considered QD, where τmeas is the measured fast decay time and it can be extracted with an exponential fit function. The radiative decay time τradiative was obtained from optical investigations of similar QDs in an undoped structure27 and we recorded a mean τradiative of about 1.3 ns. Therefore, the tunneling time, τtunneling, can be extracted from the above relation and is plotted in Figure 2d. We find that the tunneling time decreases from 3.52 to 0.92 ns when the dc bias is varied from −1.8 to −1.65 V. This means that a reduced time jitter of the single-photon emission can be achieved at a low dc bias (here, −1.65 V) due to the band bending. In addition, it is worth noticing that the rise time in Figure 2d increases when the dc bias increases in magnitude. This slow pump at higher dc bias (i.e., −1.8 V) is likely to arise from the population of higher occupied excitonic states, which occurs at high excitation powers (near QD saturation) during the pump pulse.31 In this case, emission of the neutral exciton X0 is inhibited within the pump pulse due to the presence of higher excited states. The delayed pumping process, together with the system response time and the EL decay time, limits the maximum ERR of the single-photon-emitting diode. Unlike single atoms or ions, no two self-assembled QDs have the same spectroscopic properties. This situation hampers the use of self-assembled QDs as independent SPSs to realize the TPI with indistinguishable photons. Therefore, it is highly 5811


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start to merge with each other. Bennett et al.27 pointed out that the width of the peaks in g(2)(τ) measurements was mainly determined by the total jitter on the time interval between the photon detection events registered on “start” and “stop” inputs of the time correlation module. In this context, the total time jitter in the case of 400 MHz ERR has to be considered. For 400 MHz ERR, with the constant dc voltage of −1.7 V and electrical pulse amplitude of −0.7 V the rise and decay time are 702 and 667 ps, respectively, see Figure 2c. This leads to a total time jitter of 1.369 ns and thus the expected width of the peaks in the autocorrelation measurement is ∼2.74 ns, which is longer than the temporal distance of 2.5 ns for the 400 MHz electrical pulses. As a consequence, a slight overlap between the neighboring peaks is observed. In order to obtain distinct separated peaks in autocorrelation measurements under this dc bias, the device should be driven using an ERR of below 370 MHz. Higher frequency operations at 600 and 800 MHz are also demonstrated in Figure 4b,c. In both cases, the suppressed peaks at the zero time-delay confirm the single-photon emission characteristic of the QD unambiguously. However the overlap between neighboring peaks becomes significant due to the above-mentioned reasons. In summary, an all-electrically driven single-photon-emitting diode has been demonstrated, which allows the generation of triggered, wavelength-tunable single photons. This device is based on a QD-containing LED nanomembrane integrated onto a 300 μm-thick PMN-PT piezoelectric crystal. The energy of the QD emission lines can be precisely and stably tuned over a broad range by varying the voltage applied to the PMN-PT. We show that the triggered single-photon-emitting characteristic of this diode is well maintained during the strain-controlled wavelength tuning. Together with the ability of shortening the exciton decay time by band-bending at low dc voltages, which can increase the ERR up to 800 MHz, our device represents a promising way to achieve indistinguishable single-photon emission from two remote single-photon-emitting diodes at high repetition rates.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Sample growth and electrically addressing a single quantum dot, photon collection efficiency from a gold-semiconductor-air planar cavity, line width broadening analysis, and Figures S1− S3. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: (J.Z.) [email protected]. *Email: (F.D.) [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported financially by BMBF QuaHL-Rep (No. 01BQ1032). J.X.Z. was supported by China Scholarship Council (CSC, No.2010601008). The authors thank S. Ulrich, P. Atkinson, S. Böttner, and J. W. Deng for fruitful discussions and B. Eichler, R. Engelhard, and S. Baunack for the technical support. 5812


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(31) Kessler, C. A.; Reischle, M.; Hargart, F.; Schulz, W. M.; Eichfelder, M.; Roßbach, R.; Jetter, M.; Michler, P.; Gartner, P.; Florian, M.; Gies, C.; Jahnke, F. Phys. Rev. B 2012, 86, 115326. (32) Plumhof, J. D.; Křaṕ ek, V.; Ding, F.; Jöns, K. D.; Hafenbrak, R.; Klenovský, P.; Herklotz, A.; Dörr, K.; Michler, P.; Rastelli, A.; Schmidt, O. G. Phys. Rev. B 2011, 83, 121302. (33) Bennett, A. J.; Atkinson, P.; See, P.; Ward, M. B.; Stevenson, R. M.; Yuan, Z. L.; Unitt, D. C.; Ellis, D. J. P.; Cooper, K.; Ritchie, D. A.; Shields, A. J. Phys. Status Solidi B 2006, 243, 3730.

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