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## Creator

Neul Ha, Xiangming Liu, [Takaaki Mano](https://orcid.org/0000-0002-6955-260X), [Takashi Kuroda](https://orcid.org/0000-0001-6445-7673), [Kazutaka Mitsuishi](https://orcid.org/0000-0002-9361-4057), Andrea Castellano, Stefano Sanguinetti, [Takeshi Noda](https://orcid.org/0000-0002-6705-8552), [Yoshiki Sakuma](https://orcid.org/0000-0001-6804-7217), [Kazuaki Sakoda](https://orcid.org/0000-0002-5530-3020)

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This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Neul Ha, Xiangming Liu, Takaaki Mano, Takashi Kuroda, Kazutaka Mitsuishi, Andrea Castellano, Stefano Sanguinetti, Takeshi Noda, Yoshiki Sakuma, Kazuaki Sakoda; Droplet epitaxial growth of highly symmetric quantum dots emitting at telecommunication wavelengths on InP(111)A. Appl. Phys. Lett. 7 April 2014; 104 (14): 143106 and may be found at https://doi.org/10.1063/1.4870839.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[Droplet epitaxial growth of highly symmetric quantum dots emitting at telecommunication wavelengths on InP(111)A](https://mdr.nims.go.jp/datasets/bbc63f55-176c-4480-b61e-3d61c3e7493d)

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Droplet epitaxial growth of highly symmetric quantum dots emitting at telecommunication wavelengths on InP(111)ADroplet epitaxial growth of highly symmetric quantum dots emitting attelecommunication wavelengths on InP(111)ANeul Ha, Xiangming Liu, Takaaki Mano, Takashi Kuroda, Kazutaka Mitsuishi, Andrea Castellano, StefanoSanguinetti, Takeshi Noda, Yoshiki Sakuma, and Kazuaki Sakoda  Citation: Applied Physics Letters 104, 143106 (2014); doi: 10.1063/1.4870839 View online: http://dx.doi.org/10.1063/1.4870839 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/104/14?ver=pdfcov Published by the AIP Publishing  Articles you may be interested in Elemental diffusion during the droplet epitaxy growth of In(Ga)As/GaAs(001) quantum dots by metal-organicchemical vapor deposition Appl. Phys. Lett. 104, 022108 (2014); 10.1063/1.4859915  Room temperature magnetoelectric properties of type-II InAsSbP quantum dots and nanorings Appl. Phys. Lett. 100, 033104 (2012); 10.1063/1.3676437  Quantitative investigations of optical absorption in InAs ∕ InP ( 311 ) B quantum dots emitting at 1.55 μ mwavelength Appl. Phys. Lett. 85, 5685 (2004); 10.1063/1.1832750  InAs ∕ InP quantum dots emitting in the 1.55 μ m wavelength region by inserting submonolayer GaP interlayers Appl. Phys. Lett. 85, 1404 (2004); 10.1063/1.1785859  Midinfrared photoluminescence of InAsSb quantum dots grown by liquid phase epitaxy Appl. Phys. Lett. 77, 3791 (2000); 10.1063/1.1329168    Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP:  144.213.253.16 On: Thu, 08 Sep 201606:53:23http://scitation.aip.org/content/aip/journal/apl?ver=pdfcovhttp://oasc12039.247realmedia.com/RealMedia/ads/click_lx.ads/www.aip.org/pt/adcenter/pdfcover_test/L-37/1590731600/x01/AIP-PT/Continuum_APlArticleDL_090716/Applied_Physics_Letters_7_13_Sept_High_Energy_banner_ad_1.jpg/434f71374e315a556e61414141774c75?xhttp://scitation.aip.org/search?value1=Neul+Ha&option1=authorhttp://scitation.aip.org/search?value1=Xiangming+Liu&option1=authorhttp://scitation.aip.org/search?value1=Takaaki+Mano&option1=authorhttp://scitation.aip.org/search?value1=Takashi+Kuroda&option1=authorhttp://scitation.aip.org/search?value1=Kazutaka+Mitsuishi&option1=authorhttp://scitation.aip.org/search?value1=Andrea+Castellano&option1=authorhttp://scitation.aip.org/search?value1=Stefano+Sanguinetti&option1=authorhttp://scitation.aip.org/search?value1=Stefano+Sanguinetti&option1=authorhttp://scitation.aip.org/search?value1=Takeshi+Noda&option1=authorhttp://scitation.aip.org/search?value1=Yoshiki+Sakuma&option1=authorhttp://scitation.aip.org/search?value1=Kazuaki+Sakoda&option1=authorhttp://scitation.aip.org/content/aip/journal/apl?ver=pdfcovhttp://dx.doi.org/10.1063/1.4870839http://scitation.aip.org/content/aip/journal/apl/104/14?ver=pdfcovhttp://scitation.aip.org/content/aip?ver=pdfcovhttp://scitation.aip.org/content/aip/journal/apl/104/2/10.1063/1.4859915?ver=pdfcovhttp://scitation.aip.org/content/aip/journal/apl/104/2/10.1063/1.4859915?ver=pdfcovhttp://scitation.aip.org/content/aip/journal/apl/100/3/10.1063/1.3676437?ver=pdfcovhttp://scitation.aip.org/content/aip/journal/apl/85/23/10.1063/1.1832750?ver=pdfcovhttp://scitation.aip.org/content/aip/journal/apl/85/23/10.1063/1.1832750?ver=pdfcovhttp://scitation.aip.org/content/aip/journal/apl/85/8/10.1063/1.1785859?ver=pdfcovhttp://scitation.aip.org/content/aip/journal/apl/77/23/10.1063/1.1329168?ver=pdfcovDroplet epitaxial growth of highly symmetric quantum dots emittingat telecommunication wavelengths on InP(111)ANeul Ha,1,2 Xiangming Liu,1 Takaaki Mano,1,a) Takashi Kuroda,1,2 Kazutaka Mitsuishi,1Andrea Castellano,1,3 Stefano Sanguinetti,3 Takeshi Noda,1 Yoshiki Sakuma,1and Kazuaki Sakoda11National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan2Graduate School of Engineering, Kyushu University, NIMS, Tsukuba 305-0044, Japan3Dip. di Scienza dei Materiali, Universit�a di Milano Bicocca, Via Cozzi 55, I-20125 Milano, Italy(Received 2 February 2014; accepted 28 March 2014; published online 8 April 2014)We demonstrate the formation of InAs quantum dots (QDs) on InAlAs/InP(111)A by means ofdroplet epitaxy. The C3v symmetry of the (111)A substrate enabled us to realize highly symmetricQDs that are free from lateral elongations. The QDs exhibit a disk-like truncated shape withan atomically flat top surface. Photoluminescence signals show broad-band spectra attelecommunication wavelengths of 1.3 and 1.5 lm. Strong luminescence signals are retained up toroom temperature. Thus, our QDs are potentially useful for realizing an entangled photon-pair sourcethat is compatible with current telecommunication fiber networks. VC 2014 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4870839]Quantum dots (QDs) are promising candidates for on-demand entangled photon emitters.1–6 A single QD emits apair of photons associated with the transition cascade fromthe biexciton-state to the exciton-state, and subsequently to theground state. This transition has two paths depending on theexciton angular momentum. When these two paths are degen-erate and are energetically indistinguishable, two photons emit-ted along the cascade become entangled on a polarizationbasis. In conventional QDs grown on a cubic semiconductoralong the [100] crystal axis, symmetry breaking occurs owingto the C2v symmetry of the semiconductor surfaces.7–10 To cir-cumvent symmetry breaking, QDs formed along the [111]crystal axis with C3v symmetry are promising. However,self-assembled QDs cannot be formed on the {111} surfacesby means of conventional Stranski–Krastanov (SK) mode (i.e.,two-dimensional growth followed by island formation).11 Onthese surfaces, dislocations form at the interfaces preferentiallyin order to relax strain.12,13 Therefore, other techniques, suchas droplet epitaxy and pyramidal site control, have been usedto create symmetric QDs in InGaAs/GaAs, GaAs/AlGaAs, andInGaAsN/GaAs systems.14–16 Entangled photons at wave-lengths near 850 nm have been observed in pyramidal QDs.17Moreover, we have recently demonstrated filtering-free viola-tion of Bell’s inequality using self-assembled QDs grown onGaAs(111)A by droplet epitaxy.18Extension of the emission wavelengths of these symmet-ric QDs into the optical fiber telecommunication wavelengthranges of 1.3 and 1.5 lm is highly desired. To achieve suchextension, the use of InAs QDs on InP substrates issuitable19–21 because highly strained InAs QDs formed onGaAs typically emit photons at the wavelengths of less than1 lm.3,8,9,11,15In this study, we investigated the droplet epitaxy of InAsQDs on InAlAs/InP(111)A. Owing to the three-fold rota-tional symmetry of the growth plane, highly symmetric InAsQDs were realized. Efficient photoluminescence (PL) emis-sions from the QDs at both 1.3 and 1.5 lm were realized attemperatures up to room temperature.Samples were grown on semi-insulating (Fe-doped)InP(111)A substrates using a solid-source molecular beamepitaxy. After the growth of an In0.52Al0.48As buffer layer of150-nm thickness at 470 �C, we supplied 0.8 monolayers(ML) or 1.6 ML of indium with a flux of 0.2 ML/s at 270 �C.The supply of indium without As4 enabled the formation ofliquid indium droplets, which were confirmed by observationof a halo pattern in reflection high-energy electron diffrac-tion (RHEED) measurements.14 Next, we supplied an As4flux of 3� 10�5 Torr at 270 �C, which led the RHEED pat-tern to change from halo to spotty due to the crystallizationof indium droplets into InAs QDs.14 After annealing at370 �C for 5 min under As4 supply,22 InAs QDs were cappedwith an In0.52Al0.48As layer of 75-nm thickness at 370 �C.The samples were then annealed at 470 �C for 5 min forimproving crystal quality.The surface morphology of uncapped QDs was studiedby atomic force microscope (AFM). The morphology ofcapped QDs was studied by cross-sectional transmissionelectron microscopy (X-TEM) in annular dark field-scanningtransmission electron microscopy (ADF-STEM) mode. PLspectra were measured at 9 K using the 532-nm line of acontinuous-wave diode-pumped laser. The PL signals weredispersed by a polychromator with a focal length of 50 cmand were detected by a cooled InGaAs photodiode array or aSi charge-coupled device camera.Figures 1(a) and 1(b) show AFM images of the samplesgrown with an indium supply of 0.8 and 1.6 ML, respec-tively, after crystallization at 270 �C. The images revealwell-defined QDs with a density of (a) 1.1� 1011 cm�2 and(b) 5.4� 1010 cm�2. Thus, with increasing indium, the QDdensity decreases roughly by a factor of two. In parallel, theaverage size of QDs increases from 18 nm to 30 nm in basediameter and from 0.83 to 1.6 nm in height. These changesin QD size and density imply that coalescence and/ora)Author to whom correspondence should be addressed. Electronic mail:mano.takaaki@nims.go.jp0003-6951/2014/104(14)/143106/4/$30.00 VC 2014 AIP Publishing LLC104, 143106-1APPLIED PHYSICS LETTERS 104, 143106 (2014) Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP:  144.213.253.16 On: Thu, 08 Sep 201606:53:23http://dx.doi.org/10.1063/1.4870839http://dx.doi.org/10.1063/1.4870839http://dx.doi.org/10.1063/1.4870839mailto:mano.takaaki@nims.go.jphttp://crossmark.crossref.org/dialog/?doi=10.1063/1.4870839&domain=pdf&date_stamp=2014-04-08ripening of the small indium droplets occurred with increas-ing indium supply.23 Notably, QDs were formed even whenthe indium supply was less than 1 ML, suggesting that the drop-let nucleation occurred immediately after supplying indium onthe surface (without forming a two-dimensional layer).16 Thisimmediate nucleation of droplets is likely facilitated by the ab-sence of any excess As atoms on the (111)A-(2� 2) surface, ashas been reported for GaAs surfaces.24Figures 1(c) and 1(d) show AFM images of a surface af-ter annealing at 370 �C for samples with an indium of 0.8and 1.6 ML, respectively. QDs are still visible even afterannealing. For the 0.8 ML indium sample, however, the QDdensity was found to decrease drastically after annealing to8.8� 109 cm�2 (Fig. 1(c)). In contrast, for the 1.6 ML in-dium sample, the density remained almost unchanged at5.4� 1010 cm�2 (Fig. 1(d)).The impact of annealing on the surface morphology isfurther depicted in the size distribution of QDs in Fig. 2.Here, we analyzed the QD volume with the assumption thateach QD had a hemispherical shape for simplicity. For the0.8 ML indium sample (Fig. 2(a)), more than 80% of QDsexhibit a volume lower than 150 nm3 (highlighted by thebroken line) after crystallization (before annealing).However, these small QDs mostly disappear in the sampleafter annealing, which leads to the observation of a decrease inthe QD density. For the 1.6 ML indium sample (Fig. 2(b)), theQD volume is distributed around an average value of 740 nm3before annealing, and around 350 nm3 after annealing.Thus, for both samples, the average volume was signifi-cantly reduced by annealing. For the 0.8 ML sample, thisvolume reduction resulted in a significant density reductionsince most of the QDs were already of low volume beforeannealing.We attribute this annealing-induced volume reduction tothe growth of a two-dimensional layer caused by diffusion ofatoms from InAs QDs. In the case of droplet epitaxy, QDsare grown by a kinetically limited processes without forminga two-dimensional layer. At the annealing step, however,some amount of InAs flows out from the QDs and forms atwo-dimensional layer, which might reduce the local strainenergy in the vicinity of the QDs. This emergence of a two-dimensional layer following the QD formation is in thereverse order of SK growth, for which a two-dimensionallayer (i.e., a wetting layer) appears before QDs.The presence of a two-dimensional layer was confirmedby the X-TEM image of the 1.6 ML QDs embedded in theInAlAs barrier in Fig. 3(a). A two-dimensional layer with athickness of around 1 ML is clearly visible, as indicated withan arrow. The buried QD shows a truncated disk-like shapewith an atomically smoothed surface on the top. No disloca-tion was found at the interface between the QD and theInAlAs barrier, indicating the high crystalline quality of thepresent samples. The clear contrast between the QD and theInAlAs barrier evidences the formation of abrupt interface.The formation of disk-like QDs with flat surfaces has beenalso observed in droplet epitaxial GaAs QDs formed on anAlGaAs (111)A surface.16Figure 3(b) shows the three-dimensional view of anAFM image of uncapped InAs QDs grown with 1.6 ML in-dium. Despite a finite spatial resolution, the flat-top shape isFIG. 1. AFM images of (a) 0.8 and (b) 1.6 ML InAs QDs on InAlAs aftercrystallization at 270 �C (left). (c) and (d) AFM images after annealing at370 �C (right).FIG. 2. Histograms of QD volume for (a) 0.8 and (b) 1.6 ML InAs after crys-tallization at 270 �C and after annealing at 370 �C. They are plotted with avolume bin of 50 nm3.FIG. 3. (a) X-TEM image of 1.6 ML QDs after capping. The white arrowindicates the two-dimensional layer. (b) Three-dimensional view of AFMimage of InAs QDs after annealing. (c) Cross-sectional profiles of a QDsalong the [�211] and [01�1] directions after annealing at 370 �C.143106-2 Ha et al. Appl. Phys. Lett. 104, 143106 (2014) Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP:  144.213.253.16 On: Thu, 08 Sep 201606:53:23clearly visible for relatively large QDs. Figure 3(c) showstypical cross-sectional profiles obtained along two orthogo-nal directions ([�211] and [01�1]) of a QD shown inFig. 3(b). The profiles are nearly identical. This isotropicfeature is a direct consequence of the three-fold rotationalsymmetry of the (111)A surface,24 on which equivalentdirections appear with respect to every 120 � rotation. As aconsequence of this symmetry, in-plane anisotropy is elimi-nated in QDs on (111)A surfaces.Figures 4(a) and 4(b) show the PL spectra of ensemblesof QDs prepared with 0.8 and 1.6 ML indium, respectively.At short wavelengths, emission from the two-dimensionallayer was observed around 840 nm, together with emissionsfrom the InP substrate at 875 and 900 nm and the InAlAsbarrier at �800 nm. The two-dimensional layer signal is rela-tively higher for the 0.8 ML sample than for the 1.6 MLsample due to differences in the QD density, as shown inFigs. 1(c) and 1(d).At long wavelengths, high-yield QD emissions wereobserved in both samples at a wide spectral range of950–1600 nm at 9 K. This broad-band emission is consistentwith a large size distribution of QDs observed by AFM. ThePL spectrum consists of multiple peaks, rather than a broadsingle peak. The relative intensity between the multiplepeaks was independent of excitation intensity (data notshown). Thus, the observed spectral multiplet is ascribed todifferent families of QDs whose heights vary by a monolayerstep.16,19 The split multiple peaks indicate that the QDs havea truncated shape with a flat top, as shown in Fig. 3. AFMmeasurements indicate that the average height of QDs pre-pared with an indium supply of 0.8 ML is 1.6 nm, which cor-responds to 5 ML InAs.With increasing temperature, the multiple peaks shift tolonger wavelengths in unison. Even at 300 K, the PL signalremains, although its intensity is 1/10 of that observed at9 K. The integrated PL intensity change as a function of thetemperature was well reproduced by the Arrhenius-typeequation25,26 using two activation energies of 160 meV and36 meV, as shown in inset of Fig. 4(b). These higher(160 meV) and lower (36 meV) activation energies might berelated to the carrier escape from the QDs and nonradiativescattering in the barrier, respectively.26,27 The observedroom temperature PL emissions reflect the high crystal qual-ity and strong confinement in this QD system.The inset of Fig. 4(a) shows a typical micro-PL (lPL)spectrum of an isolated single QD, which emits at a wave-length of around 1.5 lm.28 Bright emission from the singleQD is visible and it consists of four sharp lines, which areidentified from the shortest to longest wavelengths as neutralexcitons (X), positively charged excitons (Xþ), neutral biex-citons (XX), and negatively charged excitons (X�), respec-tively. These lines were assigned by careful analysis of theexcitation power dependence and linear and circular polar-ization characteristics.29 For some QDs, we observed signifi-cant line broadening with an FWHM as large as �300 leV,which is due to spectral diffusions caused by random chargetrapping in the vicinity of QDs.30 The observed excitonicfeatures are of importance for the application of our QD sys-tems to single-photon and entangled photon-pair sources.In conclusion, we demonstrated the formation of InAsQDs on InAlAs/InP(111)A substrates by means of dropletepitaxy. AFM and X-TEM images reveal that symmetricQDs with flat tops were formed. During the annealing ofQDs, a two-dimensional layer was formed by flowing out ofInAs from the QDs. High-yield PL emission was observedfrom the QDs up to room temperature, indicating high QDquality. In lPL, bright excitonic emission from the singleQDs was observed. Using droplet epitaxy, we can easily tunethe size and density of the QDs during the droplet formationprocess. Therefore, we believe that the present system ishighly promising to realize on-demand entangled photonemission at telecommunication wavelengths.We acknowledge B. Urbaszek, H. Nakajima, H.Kumano, and I. Suemune for fruitful discussions. This workwas partially supported by a Grant-in-Aid for ScientificResearch (C), No. 2539011.1Y. Arakawa and H. Sakaki, Appl. Phys. Lett. 40, 939 (1982).2O. Benson, C. Santori, M. Pelton, and Y. Yamamoto, Phys. Rev. Lett. 84,2513 (2000).3R. M. Stevenson, R. J. Young, P. Atkinson, K. Cooper, D. A. Ritchie, andA. J. Shields, Nature (London) 439, 179 (2006).4M. Akopian, N. H. Lindner, E. Poem, Y. Berlatzky, J. Avron, D. Gershoni,B. D. Gerardot, and P. M. Petroff, Phys. Lev. Lett. 96, 130501 (2006).5A. Muller, W. Fang, J. Lawall, and G. S. Solomon, Phys. Rev. Lett. 103,217402 (2009).6A. Dousse, J. Suffczynski, A. Beveratos, O. Kreb, A. Lemaitre, I. 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In order to reduce the density, indium droplets wereformed by a supply of 0.4 ML In at 320 �C. The other conditions wereunchanged. The density of QDs is 2.6� 109 cm�2.29M. Abbarchi, T. Kuroda, T. Mano, K. Sakoda, C. A. Mastrandrea, A.Vinattieri, M. Gurioli, and T. Tsuchiya, Phys. Rev. B 82, 201301 (2010).30M. Abbarchi, T. Kuroda, T. Mano, M. Gurioli, and K. Sakoda, Phys. Rev. B86, 115330 (2012).143106-4 Ha et al. Appl. Phys. Lett. 104, 143106 (2014) Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. 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