Strongly Coherent Single-Photon Emission from Site-Controlled

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Strongly coherent single photon emission from site-controlled InGaN quantum dots embedded in GaN nano-pyramids Jong-Hoi Cho, Youngmin M Kim, Seung-Hyuk Lim, HwanSeop Yeo, Sejeong Kim, Suhyun Gong, and Yong-Hoon Cho ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00922 • Publication Date (Web): 15 Nov 2017 Downloaded from http://pubs.acs.org on November 15, 2017

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Strongly coherent single photon emission from site-controlled InGaN quantum dots embedded in GaN nano-pyramids

Jong-Hoi Cho† , Youngmin M. Kim †, Seung-Hyuk Lim, Hwan-Seop Yeo, Sejeong Kim, Su-Hyun Gong, and Yong-Hoon Cho*

† : Equally contributed

Department of Physics and KI for the Nanocentury, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea

Group III-nitride materials have drawn a great deal of renewed interest due to their versatile characteristics as quantum emitters including room temperature operation, widely-tunable wavelengths from ultraviolet to infrared and a high degree of linear polarization. However, most reported results for III-nitride based quantum emitters show large inhomogeneous linewidth broadening in comparison to their lifetime-limited values, which is detrimental to achieving indistinguishability with high visibility. To overcome this, we propose an unprecedented InGaN quantum dots formation technique at the apex of GaN nano-pyramid structures, which significantly suppresses inhomogeneous linewidth broadening. Using high resolution transmission electron microscopy, a site-controlled InGaN quantum dot with small height (< 2 nm) was estimated. No measurable screening effect or frequency jitter of the single photon emission were observed, which leads to the narrow homogeneous emission linewidth (64 ± 8 eV) beyond the spectral resolution limit via Fourier-

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transform spectroscopy. The emitted photons exhibited superb antibunching characteristics with a near-unity degree of linear polarization, which is highly relevant for polarized non-classical light sources for applications in quantum information processing.

Keywords: III-nitrides, site-controlled quantum-dot, nanostructure, single-photon source, spectral diffusion, Fourier-transform spectroscopy


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The robust generation of single photons on demand is one of the key elements required for quantum optical information processing. Among many candidates,1-7 semiconductor quantum-dots (QDs) are promising for their near-unity quantum efficiency,8 indistinguishability,9 and their compatibility with quantum photonic networks.10 In particular, the III-nitride QDs possess a variety of attractive properties for engineering a solid-state quantum device. These include strong mechanical and chemical stability, room temperature operation, p-type doping and a high degree of linear polarization. Exploiting these advantages, optically and electrically driven single photon emissions at room temperature have been reported in different nanostructures,11-12 and the controllability of their polarization properties has also been demonstrated in recent years, using micro-pyramid structures.13-14 Although these are highly motivating results, challenges remain for the III-nitride QDs. Mainly, the presence of a large built-in electric field weakens the radiative recombination process through the quantum-confined Stark effect (QCSE),15 and causes considerable fast and slow spectral diffusion. This tends to broaden the emission linewidth inhomogeneously, and to randomize the coherence property of the emitted photons as reported in a single GaN QD in a nanowire.16 This inhomogeneous broadening is strongly related to the interaction of the permanent dipole moment with electronic field fluctuations in the vicinity of the emitter, which is known to be proportional to the fourth power of the QD height.17 These effects are further magnified by the high density of defects and dislocations in the medium, typically yielding meV order of emission linewidth.18 Overcoming these obstacles is of utter importance for III-nitrides16 to achieve a sufficient level of coherence required for many quantum information applications. Previous approaches to overcome these obstacles, used non-polar substrates19,


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Zinc-Blende GaN QDs20-21 or interface-fluctuation GaN QDs22. While these approaches enable to reduce inhomogeneous linewidth broadening, the limitations of QDs embedded in film structures such as lack of site-controllability and low light extraction remain. In this study, we propose a structural approach using a single InGaN QD formed at the apex of GaN nano-pyramid structure, which leads to strongly coherent single photon emission with homogeneous linewidth of 64 ± 8 μeV(corresponding timescale: 20.5 ± 3.2 ps). This structure was grown by the metal-organic chemical vapor deposition (MOCVD) method on a conventional (0001) sapphire substrate. The tapered side-walls of the nano-pyramid structure not only act to deflect threading 2} facets23 significantly reducing the dislocation density dislocations towards the {112 near the apex, but also to reduce the overall built-in electric field at the apex via strain-release.24 Moreover, the superb apex quality ensures high light extraction efficiency in comparison to the QDs embedded in film structures. (See Figure S1 in the Supporting Information) Here, systematic analysis methods of optical as well as structural characteristics of InGaN QD in GaN nano-pyramid structures are introduced. Using the high-resolution transmission electron microscopy (HRTEM), the superb crystal quality and small height of the InGaN QD were observed, which can reduce the dipole moment of exciton interactions, and electronic fluctuations near the QD. As a result, a remarkably sharp emission linewidth was observed with virtually no measurable screening effect or frequency jitter. Furthermore, we performed Fouriertransform spectroscopy on a single quantum emitter to estimate the homogeneous linewidth beyond the spectral resolution limit. These emitted photons exhibited subPoissonian statistics at an elevated temperature with the degree of linear polarization


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reaching unity. Our results present the advantages of utilizing high-quality nano-pyramid structures for bypassing a well-known issue in III-nitrides single photon sources, and subsequently demonstrate the potential of these quantum emitters as viable building blocks for solid-state quantum information technology.

Results and discussion The proposed 3D nano-pyramid structure offers several advantages, including the following: (i) the apex geometry of the pyramid ensures high light extraction efficiency of the QD, as compared to a QD embedded in a planar structure (See Figure S1 in the Supporting Information). (ii) In our apex-QD, unlike Stranski– Krastanov QDs, the QD-like potentials are formed at the apex of the pyramid defined by geometry and strain,25 which naturally leads to site-controllability. (iii) By manipulating the elongated direction of the apex of the pyramid, the orientation of the linear polarization can be controlled.13 (iv) Due to the bending nature of the threading dislocation lines towards the side-walls of the pyramids, the defect density at the apex, and hence the non-radiative recombination center, can be significantly suppressed.23 (v) The piezoelectric field at the apex is calculated to decrease due to the pyramid geometry in nano-scale.24 Although previous studies on similar pyramid strutures in micro-scale have shown some promising results, the reported linewidth of single emissions are still in the order of 1 meV.26-27 This broadended linewidth is related to the presence of a large built-in electric field interacting with the large dipole moment of the exciton due to the extended QD height17-18 as a result of Indium accumulation at the apex. To overcome this, we fabricated a nano-pyramid structure grown on a (0001)


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sapphire substrate, with a single InGaN layer at the apex of the nano-pyramid structure, without accumulating Indium above a layer thickness of ca. 2 nm as shown Figure 1. Figure 1a represents the schematic image of whole structures. Using scanning electron microscopy (SEM), the tiltied view of InGaN/GaN nano-pyramid structure was observed in Figure 1b. Subsequent morphological characterizations were performed via cross-sectional HRTEM images. We obatined the HRTEM image of the apex of nano-pyramid structure in Figure 1c, which was taken along the direction [21 1 0]. Our Interpretation of HRTEM Images carefully considered the cross-sectional orientation and the thickness of the sample slab (See Figure S2 in the Supporting Information). The inset of Figure 1c shows a high-magnification HRTEM image of the InGaN/GaN interface layer, which indicates a superbly uniform crystallinity. The estimated lateral length and height of the apexQD were 18 ± 2 nm and 1.8 ± 0.2 nm, respectively. Although it is challenging to measure definite Indium contents at the apex due to tapered and nano-scale geometry, we estimated the Indium content of the side-wall InGaN layer near the apex of these structures to be 15.6 ± 3.2%. (See Figure S3 in the Supporting Information) Moreover, no Indium accumulation within the apex-QD was observed. In Figure 2, we show spatially resolved photoluminescence (μ-PL) spectroscopy results of the apex-QD. Figure 2a shows a μ-PL spectrum of the apex-QD, while the inset shows the spectrum taken by the 2nd-order of the same grating (1800 grooves/mm grating) with 146 μeV in linewidth, which is resolution limited. In Figure 2b, we probed the degree of linear polarization as well as the orientation by rotating the half-wave plate with respect to the axis of the analyzer. We observed a nearly 100% inherently polarized emission towards one of the hexagonal crystal axis 0]. This naturally high degree of polarization could be of benefit, as polarized [21 1


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single photon emitters for certain quantum information schemes.28-29 Next, we investigated the presence of QCSE and frequency jitter in time, to monitor the influence of spectral diffusion. In Figure 2c, we probed the variation in emission energy in time where 1000 consecutive measurements with 100 ms were compiled to construct the results shown in Figure 2c. The width of the stripe represents the system resolution limit, beyond which no measurable slow jittering can be seen, up to 100 s. The presence of a strong built-in electric field induces a red-shift in the emission energy via QCSE, which may be reverted by pumped carriers, known as the screening effect.30 In this process, the changing of the band edges is manifested as a continuous blue-shift of the emission energy, typically on the order of a few meV to tens of meV.30-31 Figure 2d shows the relative peak energy shift of excitons as a function of the excitation power density ratio: ≡ / , where is the excitation power density and is the saturation power density. We observed no measurable blue-shift beyond the saturation excitation power. These results confirm the negligible influence of QCSE and spectral diffusion in our emission linewidth, leading to the suppression of inhomogeneous broadening. To determine the emission linewidth beyond the spectral resolution limit, we probed Fourier-transform spectroscopy via Michelson-interferometer. The interference fringes were obtained from constructive and destructive extrema by delaying one arm of the interferometer using a piezo-stage.

Figure 3a shows the fringe visibility as a function of delay time: = ,

where and are the interference maximum and minimum intensities, respectively. To fit the visibility curve, we employed the Fourier-transformed Voigt


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function, which is a convolution of a Gaussian and Lorentzian function: = $

'

%

|'|

! "− & ) − ,32. From the converged fit, we inferred a homogeneous linewidth % '(

'+

(-./ of 64 ± 8 μeV using the relation -./ =

0

$'+

, indicating a linewidth ratio of

1:126 between the lifetime-limited linewidth (~0.50 μeV corresponding to radiative decay-time of 1.3 ns at 10 K) to the broadened linewidth. This is a significant improvement over previously reported InGaN QD single photon sources, typically yielding more than 1:1,000.27, 33-35 In Figure 3b, we performed excitation power-dependent measurements of the fringe visibility. The obtained Gaussian linewidth was plotted as a function of the excitation power density ratio: ≡ / . Two power regimes can be deduced, delineated by = 1. Above = 1 (blue-curve), the linewidth broadening is in the powerbroadening regime as reported for single III-V or III-nitrides QDs16, 36 with -.3 ≅ + 6 7 − 1 for ≥ 1 where 6 is a coefficient. Below = 1 (red-line), the linewidth broadening occurred more gradually, decoupled from the powerbroadening regime, indicating a power density regime, where the inhomogeneous linewidth broadening via spectral diffusion could be suppressed. By enhancing the radiative recombination rate via plasmonic coupling, as shown in our previous work37 and through resonant pumping, we expect to achieve a sufficient level of overlap between emitted single photons towards indistinguishability in future experiments. The Hanbury Brown-Twiss experiment was performed under continuous-wave excitation at a wavelength of 375 nm. The left-hand column in Figure 4a shows the µ-PL of the main exciton peak at three temperature levels. As can be seen, the overall QW background level coming from the side-wall of the pyramid tends to rise with increasing temperature, which consequently reduces the signal to noise ratio,


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9≡

:

:;

, where < and = are the signal and background level, respectively. This was

changed from near unity (9=0.97) at 10 K to 9= 0.57 at 80 K. The measured >% 0 value of 0.11 ± 0.05 at 10 K demonstrates the superb antibunching nature of this material system. This antibunching nature was preserved at 50 and 80 K, with measured >% 0 values of 0.42 ± 0.06 and 0.68 ± 0.08, respectively. The decreasing trend in 9 has a direct impact on the measured >% 0. In Figure 4b, we plotted the computed >% 0 by performing deconvolution38 as a function of 9 with the fixed timing resolution of a single photon detector (40 ps), and the histogram time-bin width of 64 ps. The measured >% 0 values were in good agreement with the computed >% 0 shown for exciton lifetimes of 1.3 ns (solid-line) and 600 ps (dotted-line) where the 600 ps was considered a shortened radiative lifetime at 80 K. This analysis indicates that the main limiting factor to achieving a low >% 0 value at elevated temperature was the contribution of the QW background level. However, our QDs could still act as single photon sources at high temperature, since antibunching at zero delay can only occur with non-classical light sources. We observed that the rising QW background level made the exciton emission difficult to observe at T ≥ 200 K in the present state. However, the proper optical technique, such as two-photon absorption39 or post-fabrication processes to suppress the QW background level27 may enable our single photon emission to be distinct above 200 K. The manifested high purity single photon characteristics, together with the near unity linear polarization, which is suitable for minimizing loss due to polarization selection, make this system an attractive polarized non-classical light source.


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Conclusion We successfully formed a single InGaN layer at the apex of a nano-pyramid structure. Using HRTEM, we observed the apex-QD (lateral length: 18 ± 2 nm, height: 1.8 ± 0.2 nm) of nano-pyramid structures with no further indium accumulation. By successfully forming the apex-QD of a nano-pyramid structure, we have experimentally demonstrated a significant reduction in the inhomogeneous broadening of emission linewidth. Using Fourier-transform spectroscopy, we estimated the homogeneous linewidth to be 64 ± 8 μeV (corresponding time-scale: 20.5 ± 3.2 ps), which is the-state-of-the-art among InGaN single quantum emitters. Furthermore, we confirmed the sub-Poissonian nature of our emitted photons at elevated temperature, which possessed an inherent degree of high linear polarization. The results presented in this work highlight the advantages of highquality nano-pyramid structures for achieving Fourier-transform limited linewidth in III-nitrides, and subsequently demonstrate their potential as polarized single photon sources and as viable building blocks for solid-state quantum information technology.

Methods InGaN/GaN nano-pyramid growth procedures. The n-GaN template was grown on a (0001) sapphire substrate by MOCVD. A Si3N4 layer with 100 nm thickness was deposited on a n-GaN template. Then, GaN nano-pyramid structures were grown on the Si3N4 hole patterns. The InGaN single QW layer was grown on these structures, where the Indium content near the apex of these structures was 15.6 ± 3.2%. (See


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Figure S3 in the Supporting Information) Finally, a 8 nm thickness of GaN layer was deposited as a capping layer. To investigate the morphology of the apex-QD FIB (Helios nanolab 450 F1) and HRTEM (Titan cubed G2 60-300) were employed. The average height and base diameter of the nano-pyramid structures were 272 ± 11 nm and 197 ± 10 nm, respectively.

Optical characterization. A μ -PL setup with a low-vibration cryostat having a temperature range between 10 to 300 K was employed to investigate the optical properties of the QD. For continuous-wave excitation, a semiconductor laser diode with a 375 nm wavelength was used, and for time-resolved photoluminescence spectroscopy a pulsed-mode Ti:sapphire laser with a full width at half-maximum of < 1 ps and a repetition rate of 80 MHz was utilized. An objective lens with a long working distance of 11 mm (Mitutoyo; 100×, N.A., 0.5) was employed to locally excite the nano-pyramid structures and to collect the photoluminescence. For the optical spectroscopy, a monochromator with a 750 mm focal length (Acton; SP2750) installed with 1,800 lines per mm gratings was used for the high resolution spectrum in the near ultra-violet regime with a CCD detector (PIXIS-400). To determine the degree of polarization, a half-wave plate was employed in front of an analyzer (GlanTaylor calcite ploarizer). For the 1st and 2nd-order coherence measurement, rapid avalanche photodiodes with 40 ps time-resolution (ID Quantique) coupled to a timecorrelated single-photon counting module (Picoharp300; Picoquant) were adopted where a flip-mirror switches the path either to the Hanbury Brown-Twiss setup or to Michelson interferometer setup with a 300 mm long travel linear stage and a piezoelectric translation stage defining the path-length difference.


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Edwards, P. R.; Martin, R. W.; Watson, I. M.; Liu, C.; Taylor, R. A.; Rice, J. H.; Na, J. H.;

Robinson, J. W.; Smith, J. D., Quantum dot emission from site-controlled InGaN∕GaN micropyramid arrays. Applied Physics Letters 2004, 85, 4281. 36.

Stufler, S.; Ester, P.; Zrenner, A.; Bichler, M., Power broadening of the exciton linewidth in a

single InGaAs∕GaAs quantum dot. Applied Physics Letters 2004, 85, 4202. 37.

Gong, S.-H.; Kim, J.-H.; Ko, Y.-H.; Rodriguez, C.; Shin, J.; Lee, Y.-H.; Dangd, L. S.; Zhange, X.;

Cho, Y.-H., Self-aligned deterministic coupling of single quantum emitter to nanofocused plasmonic modes. Proceedings of the National Academy of Sciences 2015, 112, 5280-5285. 38.

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single photon antibunching from a site-controlled InGaN quantum dot. Applied Physics Letters 2014, 105, 081901. 39.

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Author information Corresponding Author *E-mail: [email protected]

Acknowledgements


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ACS Photonics

This

work

was

supported

by

the

National

Research

Foundation

(NRF-

2016R1A2A1A05005320, NRF-2016K2A9A2A12003785) of the Korea government (MSIP), and the Climate Change Research Hub of KAIST (Grant No. N11170054)


ACS Photonics

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Table of Content graphic 80x39mm (300 x 300 DPI)


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Figure 1. (a) Schematic image of InGaN/GaN nano-pyramid structure. (b) Tilted-view SEM image of InGaN/GaN nano-pyramid structure. (c) HRTEM image at apex of nano-pyramid structure. (inset) magnified HRTEM image of side-wall of nano-pyramid structure 170x190mm (300 x 300 DPI)


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Figure 2. Optical characterizations using µ-PL setup. (a) µ-PL spectra of the apex-QD. The inset shows the spectrum taken by the 2nd-order of the same grating with a resolution limit. (b) The orientation and the degree of linear polarization of the apex-QD emission. The orientation points to one of the hexagonal axes [21 ̅1 ̅0] with nearly 100% degree of linear polarization. (c) Measurement of the temporal variation in emission energy showing no measurable frequency jitter. (d) Relative energy shift as a function of P ̃≡P/Ps, where Ps is the saturation excitation power. The inset shows PL intensity as a function of P ̃. 170x190mm (300 x 300 DPI)


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Figure 3. Fourier-transform spectroscopy. (a) Fringe visibility as a function of delay time. The inset shows the intensity oscillation of constructive and destructive interferences as the delay length of one arm is altered. (b) Gaussian full width at half-maximum extracted from power-dependent measurement of the fringe visibility as a function of P ̃≡P/Ps. 170x190mm (300 x 300 DPI)


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Figure 4. µ-PL spectra and HBT experiment as a function of temperature. (a) µ-PL spectra of exciton at 10, 50, 80 K (Left), and the second-order correlation measurement (Right) exhibiting sub-Poissonian nature at the three temperature levels. (b) Comparison between the computed (solid and dashed line) and the measured g2(0) (blue, green and red dot) as a function of ρ≡S/(S+B), where S and B are signal and background levels, respectively. 170x190mm (300 x 300 DPI)


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Strongly Coherent Single-Photon Emission from Site-Controlled

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