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Ultraclean Single Photon Emission from a GaN Quantum Dot Munetaka Arita, Florian Le Roux, Mark J. Holmes, Satoshi Kako, and Yasuhiko Arakawa Nano Lett., Just Accepted Manuscript • Publication Date (Web): 24 Apr 2017 Downloaded from http://pubs.acs.org on April 25, 2017

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Ultraclean Single Photon Emission from a GaN Quantum Dot Munetaka Arita,*,†,‡ Florian Le Roux, ‡ Mark J. Holmes,†,‡ Satoshi Kako,†,‡ and Yasuhiko Arakawa*,†,‡ †

Institute for Nano Quantum Information Electronics, The University of Tokyo, 4-6-1 Komaba,

Meguro-ku, Tokyo 153-8505, Japan. ‡

Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo

153-8505, Japan

ABSTRACT: Wide bandgap III-nitride quantum dots (QDs) are promising materials for the realization of solid-state single-photon sources, especially operating at room temperature. However, so far a large degree of inhomogeneous broadening induced by spectral diffusion has compromised their use. Here, we demonstrate the ultraclean emission from single GaN QDs formed at macro-step edges in a GaN/AlGaN quantum well. As a likely consequence of the high growth temperature and hence a reduced defect density, spectral diffusion is heavily suppressed to levels at least one order of magnitude lower than conventional GaN QDs. A record narrow linewidth of as small as 87 µeV is obtained, while the low inhomogeneous broadening enables us to assess an upper limit of homogeneous broadening in the QDs (27 µeV). Furthermore, the

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uncontaminated emission facilitates the generation of ultraviolet single-photons with unprecedented purity (g(2)(0) = 0.02). The realization of high-quality GaN QDs will enable exploration of optoelectronic properties of III-nitrides, opening up the possibility of realizing single-photon quantum information systems operating at room temperature.

KEYWORDS: III-nitride, quantum dot, single photon source, inhomogeneous broadening, finestructure splitting

Single photon sources (SPSs) will be crucial system components in a number of quantum information processing applications including linear optical quantum computing,1 quantum key distribution2 and quantum memories.3 Thanks to their outstanding potential as a material for high-speed and integrated solid-state SPSs, semiconductor quantum dots (QDs) have attracted considerable attention in recent years.4,5 Wide bandgap III-nitride QDs, in particular, will offer significant advantages since they enable high-temperature operation of solid-state SPSs6–8 with wavelengths spanning from the ultraviolet (UV) to the infrared.9–11 It is obvious that both an indepth understanding and a precise level of control over the optoelectronic properties of III-nitride QDs are crucial for the development of sophisticated devices—especially for applications that will rely on single dots. However, spectral diffusion, which is exacerbated by the existence of an inherent strong electric field in wurtzite III-nitride QDs, results in a considerable level of inhomogeneous broadening of the emission linewidths,12 thus hindering both device development and fundamental studies. To date, various methods or structures have been proposed to suppress the spectral diffusion in GaN QDs. These include reducing the exciton permanent dipole moment by employing small


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and thin QDs,13,14 or reducing the strength of the internal electric field by using either nonpolar substrates15 or zinc-blende crystals.16 Although relatively small linewidths (440–500 µeV) have been recently reported,13–16 the remaining inhomogeneous broadening leads to the unavoidable conclusion that it is necessary to improve the quality of the crystal around the QDs. In principle this will require a reduction in the density of defects, which are known to induce spectral diffusion by the capture/release of charge. Nevertheless, this is a challenging task for GaN/Al(Ga)N QDs, which are usually grown at rather low temperatures (895–970 °C in the case of growth by metalorganic chemical vapor deposition (MOCVD),17,18 715–800 °C in the case of growth by molecular beam epitaxy (MBE)10,19) in order to facilitate the Stranski–Krastanov growth mode. Unfortunately, AlxGa1-xN exhibits a higher point defect density when it is grown at a temperature lower than optimum (between 1050–1100 °C for GaN (x = 0)20 and 1300–1600 °C for AlN (x = 1)21,22 in MOCVD).23,24 Therefore, during any standard III-nitride growth process, the Al(Ga)N capping layer inevitably consists of high density of point defects due to the serious mismatch of the growth temperature. In this letter, we report the formation of interface-fluctuation GaN QDs (that do not form via the Stranski–Krastanov growth mode and therefore do not require low growth temperatures) in a thin single GaN/AlxGa1-xN (x < 0.25) quantum well (QW) grown on sapphire (0001) substrates by MOCVD. The high temperature growth process leads to a striking suppression of spectral diffusion, enabling us to access an upper limit of homogeneous broadening in the QDs as 27 ± 13 µeV. Also, the clean exciton emission (without contamination from multi-excitonic or higherorder transitions or background emission) facilitates the generation of ultraviolet single-photons with very high purity. Moreover, asymmetry-induced fine-structure splitting not only in


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excitonic but also in biexcitonic transition is unambiguously observed in these high-quality GaN QDs, in contrast to typical III-nitride QDs. Samples were grown using MOCVD on sapphire (0001) substrates. Trimethylgallium and trimethylaluminum were used for the group-III precursors, while NH3 was used for the nitrogen source. Intentional doping was not performed throughout the growth. After a 4-minute thermal cleaning of the substrates at 930 °C, a thin (~40 nm) GaN nucleation layer was first deposited at 480 °C, followed by a 1.5 µm-thick GaN buffer layer grown at 1071 °C. The growth was carried out under atmospheric pressure from the initial stage until this buffer layer had formed, and the reactor was then rapidly pumped down to 2.67 × 104 Pa immediately after the growth of the buffer layer. Subsequently, a ~1.5 nm-thick GaN single quantum well sandwiched by two 110 nm-thick Al0.2Ga0.8N barrier layers was grown at 1100 °C. The optical properties of the samples were investigated by micro-photoluminescence (micro-PL). A continuous-wave diode pumped solid state laser (CryLas FQCW266-100, wavelength: 266 nm) was used as an excitation source. The samples were loaded in a liquid helium flow cryostat into which the laser was incident at a glancing angle of 20°. An excitation power of 1 mW corresponds to a power density of 40 W/cm2 at the center of the irradiated spot. Emitted photons were collected through a 50× ultraviolet infinity-corrected objective lens (NA 0.4). For micro-PL measurements, a confocal pinhole with diameter of 100 µm was inserted into the optical path as a spatial filter to restrict the inspection area down to a 2-µm diameter spot on the sample surface. The photoluminescence was dispersed by a 0.75m monochromator, and finally detected by a liquid nitrogen cooled charge coupled device (CCD) array. The spectral resolution of our setup when using a 2,400 lines/mm grating is approximately 140 µeV at around 340 nm, which is


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further enhanced to ~35 µeV by utilizing second order diffraction in high-resolution measurements. Note that because of the low diffraction efficiency, both a high excitation power and a long accumulation time are required for the high-resolution measurements. The cryostat was held on a motorized 2-axis stage which enabled us to map the PL spectra with positioning accuracy of 50 nm. Photon autocorrelation measurements were performed using a HanburyBrown Twiss Setup comprised of photomultiplier tubes designed for operation in the UV. The surface morphologies of the samples were studied by atomic force- and scanning electronmicroscopy, and also conventional optical microscopy. The Al composition, QW thickness, and the strain status of the epitaxial layers were analyzed using high resolution X-ray diffraction measurements. The cross-sectional microstructure of the samples was characterized in detail by an aberration-corrected scanning transmission electron microscope (Cs-STEM). We studied the optical properties of several single QDs. Thanks to the low density, it is possible to optically access individual QDs without any additional sample processing (cf. Fig. S1e, position C in the Supporting Information). First, we investigated the excitation power dependence of the PL from an isolated dot as shown in Fig. 1. It is clear that the emission from both excitonic (X) and biexcitonic (XX) state transitions are observed, evidenced via the linear and quadratic excitation power dependences of the peak intensities, the narrower linewidth of the biexciton peak,25 the saturation of peak X as peak XX approaches an appreciable intensity, and the measured biexciton binding energy (+12.4 meV) that is comparable to previously reported values for various GaN QDs (see Supporting Information).26 The other peak that appears in between the two peaks may be related to some charged states, although its origin is yet to be identified.


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We find that the emission peak energies are extremely stable, exhibiting negligible spectral wandering (long-term spectral diffusion) over a 10 minute period of continuous data acquisition as shown in Fig. 1c (see also Fig. S4). Although the typical emission full width at half-maximum (FWHM) measured for these single QDs is 200–400 µeV and the linewidth generally increases with the excitation power (Fig. 1d), we note that it is actually possible to measure linewidths lower than 100 µeV. Figure 2 shows a high-resolution PL spectrum of the exciton emission recorded from a QD in which the FWHM extracted via a Voigt lineshape fitting is 87 ± 4 µeV: the smallest value ever reported for a III-nitride QD. This is one to two orders of magnitude smaller than those of conventional self-assembled polar hexagonal GaN/AlN QDs,12,27 and is even narrower than those of state-of-the-art InGaN/GaN QDs (170 µeV).28 The Lorentzian and Gaussian component linewidths (FWHM) for the Voigt profile are 27 ± 13 µeV and 71 ± 11 µeV, respectively. It is likely that the homogeneous linewidth is determined by some rapid dephasing process, such as the interaction with low energy phonons, and we believe that it could be further suppressed if the excitation power is further reduced.13 In general, the degree of longterm spectral wandering, and indeed linewidth broadening due to spectral diffusion, depends on the exciton permanent dipole moment (which itself is determined by the internal electric field and the size and shape of the QD), the defect density, and the occupation probability of those defects.13,29 Judging from the PL emission energy, it can be assumed that the typical height of the dots (and hence the exciton permanent dipole moment) is larger than that of recently reported small GaN nanowire QDs,13 and similar to that of previously reported Stranski–Krastanov dots.6 Therefore, the suppression of the long-term spectral diffusion and linewidth broadening observed in the current experiment may be ascribed to a low density of defects that interact with the dots; a likely consequence of the relatively high growth temperature (1100 °C). We would like to


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mention that the possibility of a low trap occupancy is not fully excluded at present: further investigations are necessary to conclude whether this is the case. If so, although the absolute density of the defects may still be high, we can still say the environment (the density of the active traps) in the immediate vicinity of the QDs is cleaner (lower) than those of conventional GaN QDs. As the inhomogeneous broadening may deteriorate the purity of single photons generated from a single QD through possible contaminations from multi-excitonic or higher-order transitions,6 a suppressed spectral diffusion is also highly beneficial in terms of single photon generation. Note that a limited impact on the single photon purity is expected due to a homogeneous broadening which, as we have shown, is much smaller compared to the inhomogeneous one. Figure 3a shows an intensity autocorrelation histogram measured from an isolated QD under continuous wave excitation using a low excitation power density of 1.0 W/cm2. The corresponding PL spectrum (Fig. 3b) indicates that any spectral background can be removed with the low power for this QD. The measured g(2)(0) value of 0.085 (raw value with no correction) reveals the single photon emission from a single quantum emitter, providing unequivocal evidence of the zerodimensional density of states in the observed structures. This value is the lowest ever reported for a III-nitride QD,7 and is actually limited by the detector response function. A fitting which takes this into account (green and red curves in Fig. 3a) reveals a deconvolved g(2)(0) value as low as 0.02 ± 0.05, showing the remarkable nature of these dots. Again, this corrected value is the lowest reported to date for a III-nitride QD.14 It should be stressed here that we have not performed any correction for background emission for the values quoted above; the value we report is a true representation of the single photon purity.


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Finally, we present an observation of asymmetry-induced excitonic fine-structure splitting (FSS) in the emission spectrum of a QD (see Fig. 4). For this particular dot, both the X emission and the XX emission (which are assigned from their power dependences, Fig. 4a) exhibit orthogonally polarized doublets in their emission spectra. As expected, the energy separation in the doublets is the same for both emission states: 1100 µeV, and furthermore, the polarization behaviors are complementary for X and XX, as can be seen in Fig. 4b. These features are typical fingerprints of FSS. Although there have been a handful of reports on FSS observed in III-nitride QDs to date,30-34 and in particular Amloy et al. have reported that FSS (in both X and XX transitions) of zero-dimensional InxGa1-xN excitons is exclusively observed from those with vertical polarization vectors (in the plane parallel to the c axis),31 no experimental evidence of FSS in the XX transition of usual excitons –consisting of two optically active states with polarization vectors in the xy plane perpendicular to the c axis– has been reported so far. Indeed, Hönig et al. discussed this issue in great detail, proposing an unconventional biexciton state which does not exhibit splitting due to its anomalous spin configurations and the existence of a phonon-assisted spin-flip cascade process to the bright exciton state.32 In this study, however, the splitting is clearly resolved in both the X and XX transitions in the plane perpendicular to the c axis, leading us to the conclusion that the biexcitons formed in these QDs are of a more conventional nature. Although the fairly large value of the splitting shown in this example would hinder the emission of polarization entangled photon-pairs, we note that such a large splitting may provide a valuable platform for the isolation and study of individual spin states in these QDs. In summary, we have reported the formation of ultraviolet-emitting interface-fluctuation GaN QDs with extremely small degrees of spectral diffusion. The narrow emission linewidths, single


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photon emission with very high purity, and clearly resolved fine structure splitting suggests the high quality of the QDs, showing that III-nitride QDs with high quality optical properties can indeed be created. The superior characteristics of these novel QDs will boost further development of quantum optoelectronic devices based on the III-nitride material system, and will allow us to obtain further insight into the physics of III-nitride nanostructures. It is believed that further development of nanofabrication technologies will allow us to precisely control the position of the QDs, and that incorporating these GaN QDs into air-bridged photonic crystal nanocavities will also be possible in order to control the emission properties.35 Further control over the homogeneous broadening would be a key technology to realize indistinguishable singlephoton sources using III-nitride QDs.


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Wavelength (nm) 331

330.5

330

b

329.5

Integrated PL Intensity (arb. units)

a

5K

PL Intensity (arb. units)

XX

X [W/cm2] 5.1 1.3 0.32 3.745

3.750

3.755

3.760

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10-2

3.765

10-1

100

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Photon Energy (eV)

Excitation Power Density (W/cm )

c

d 450

600 6K 14 W/cm2

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Photon Energy (eV)

3.765

400

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400


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Time (sec.)

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350 300 250 X 200 150 XX

100 50 0 10-2

10-1

100

101

102 2

Excitation Power Density (W/cm )

Figure 1. Optical properties of a single GaN interface-fluctuation QD. (a) Excitation power dependence of micro-PL spectra (vertically offset for clarity). (b) Integrated PL intensity of the peaks labelled X and XX as a function of excitation power density, where the solid lines are linear (X) and quadratic (XX) dependences fitted to the data. (c) Temporal variation of spectra in (a), created from 200 consecutively measured spectra (accumulated for 3 seconds for each). (d) Excitation power dependence of measured (Voigt fitted) linewidths of peaks X and XX.


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Wavelength (nm) 338.84


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338.83

Voigt fit: FWHM 87 ± 4 µeV (Gaussian width: 71 ± 11 µeV Lorentzian width: 27 ± 13 µeV)

338.82

338.81

338.8

4.6 K 10 W/cm2

3.6590 3.6591 3.6592 3.6593 3.6594 3.6595

Photon Energy (eV)

Figure 2. A high resolution micro-PL spectrum of a QD. Ten spectra (accumulated for 120 seconds each) were averaged to produce the final spectrum. The statistics of each data point provide us the average and the standard deviation, and the red line is a fitting result assuming a Voigt lineshape.


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b

Wavelength (nm) 349

348

347

346

345

344

1.2 5000

1.0


Normalized Coincidence Counts (arb. units)

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0.8

0.6 10 K 1.0 W/cm2

0.4

0.2

-6

-4

-2

4000

3000

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1000

measured g(2)(0) = 0.085

deconvolved g(2)(0) = 0.02 ± 0.05

0.0 -10 -8

10 K 1.0 W/cm2

0

0

2

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8

10

3.55

3.56

Time Delay (ns)

3.57

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Photon Energy (eV)

Figure 3. Intensity autocorrelation measurement performed on a QD under a continuous wave excitation. (a) The statistics showing clear antibunching with a measured raw g(2)(0) value of 0.085. (b) The corresponding emission spectrum.


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Wavelength (nm)

a

337

336.5

336

335.5

XX


5K w/o pol.

X

[W/cm2]

3.2 1.6 3.680

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X


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0 3.680

3.685

3.690

3.695

Photon Energy (eV)

Figure 4. Polarization-resolved spectra of a QD exhibiting an asymmetry-induced fine structure splitting. (a) Typical spectra taken at two different excitation powers indicated in the figure (offset for clarity). (b) The polarization angle dependence of the excitonic (X) and biexcitonic (XX) transitions from the QD, where a fine structure splitting of 1100 µeV is clearly observed in both the exciton and biexciton emission peaks.


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ASSOCIATED CONTENT Supporting Information. Additional results for structural and optical characterizations; detailed discussion on possible structural origin of the localized emission; discussion on spectral diffusion; and discussion on the biexciton binding energy and the fine structure splitting. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. M.A. and Y.A. conceived and designed the experiments. M.A. grew the samples and performed structural characterization of them. M.A., F.LR. and M.H. performed optical characterization of the samples with assistance from S.K. All authors contributed to interpretation of the data and preparation of the manuscript. M.A. F.LR., M.H. and Y.A. wrote the paper. Y.A. supervised the entire project. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS


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This work was supported by the Project for Developing Innovation Systems of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) and the New Energy and Industrial Technology Development Organization (NEDO). Japan. The authors thank T. Iki, K. Gao and M. Nishioka for technical support. REFERENCES (1)

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