In(Ga)As/GaAs site-controlled quantum dots with tailored morphology and high optical quality

Phys. Status Solidi A, 1–8 (2012) / DOI 10.1002/pssa.201228373 p s sa

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ical Section onQuantum Dots

Part of TopSite-selective Growth of Single

pplications and materials science

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eature Article
In(Ga)As/GaAs site-controlledquantum dots with tailoredmorphology and high optical quality
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Christian Schneider*,1, Alexander Huggenberger1, Manuel Gschrey2, Peter Gold1, Sven Rodt2,Alfred Forchel1, Stephan Reitzenstein2, Sven Hofling1, and Martin Kamp1

1 Technische Physik, Physikalisches Institut and Wilhelm Conrad Rontgen-Research Center for Complex Material Systems,

Universitat Wurzburg, Am Hubland, 97074 Wurzburg, Germany2 Institut fur Festkorperphysik, Technische Universitat Berlin, Hardenbergstraße 36, 10623 Berlin, Germany

Received 7 June 2012, revised 23 July 2012, accepted 17 August 2012

Published online 19 September 2012

Keywords site-controlled quantum dots, cathodoluminescence, semicondoctor

* Corresponding author: e-mail [email protected], Phone: þ49 931 31 88021, Fax: þ49 931 31 85143

In this article, we describe epitaxial growth and investigations

of optical properties of In(Ga)As/GaAs site-controlled quantum

dots (QDs) fabricated on (001)-oriented GaAs substrates. The

QD nucleation is directed by pre-patterning planar GaAs

surfaces with shallow nanoholes. The focus of this work lies on

the realization of arrays of site-controlled QDs (SCQDs) with a

tailored morphology and optical properties comparable to QDs

fabricated on planar substrates. By maximizing the migration

length during QD deposition, we were able to increase the QD

pitches to values exceeding device dimensions of typical

semiconductor microresonators. The introduction of a seeding

layer in our growth scheme allows us to extend the vertical

distance between the QDs and the etched nucleation centres to

about 20 nm without suffering from nanoholes being occupied

by multiple QDs. Furthermore, the extended distance between

the QD layer and the re-growth interface allows us to preserve

excellent optical properties of the single QDs as probed in

photoluminescence with an average single QD related line-

width of 133 meV and minimum values as low as 25 meV for

non-resonant excitation. The high yield of optically active QDs

on the pre-defined nucleation positions is studied by cathodo-

luminescence (CL) with high spatial resolution. We find

emission from single SCQDs on more than 90% of the

nucleation centres, which is a pre-requisite for any scalable

QD-device fabrication scheme.

� 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction The rapid progress in the fabricationtechnology of high quality semiconductor microresonators[1–3] as well as quantum emitters (so called quantum dots,QDs) in the solid state environment [4] has triggered avariety of breakthrough experiments in solid state quantumoptics. To date, the effects of semiconductor cavity quantumelectrodynamics (cQED) and light–matter coupling innanostructures have been widely investigated, resulting inthe observation of phenomena which are at the heart ofdevices for future quantum communication and dataprocessing. On the one hand, the Purcell effect experiencedby QDs in a spectrally resonant microcavity (i.e., the weaklight–matter coupling), observed by Gerard et al. [5]significantly enhances the spontaneous emission rate of thequantum emitter and can therefore be used to boost theefficiency of optical [6] and electrically driven single photon

sources [7, 8]. First proof of principle experiments ofquantum communication with such devices relying onweakly coupled QD-micropillars were recently demon-strated, underlining the maturity of the technology [9, 10].Moreover, Purcell enhanced efficient sources of entangledphotons based on single QDs in micropillar molecules [11]are promising candidates as building blocks for semicon-ductor quantum repeaters, which are essential devices forlong distance quantum communication. On the other hand,the strong coupling regime between a zero-dimensionalconfined QD exciton and a cavity resonance [12–14] canensure a coherent exchange of information between light andmatter and can be the basis for the interconversion between astationary QuBit (such as an exciton, or an electron or holespin) and a flying QuBit (photon) [15]. While microcavitiesare typically fabricated by means of high-resolution


2 C. Schneider et al.: In(Ga)As/GaAs site-controlled quantum dots with tailored morphologyp

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lithography and etching (i.e., in a top-down approach), thenano-emitters are usually fabricated via bottom-up tech-niques. Especially in GaAs, the material system of choice formost of the experiments related to semiconductor quantumoptics, the QD-quantum light sources are usually fabricatedin the Stranski–Krastanov growth mode. This growth modeallows for the realization of nanostructures with excellentcrystalline and optical properties [4, 16, 17], however itresults in arrays of almost completely random in-plane QDpositions. This is a serious drawback since the couplingstrength between a photonic cavity resonance and the QDtransition strongly depends on the QD position in the device[18]. Consequently, in most reports dealing with coupledQD–microcavity systems, the devices of interest had to beselected by scanning over a large number of resonators (i.e.,[19]). In order to circumvent such time-consuming scanningefforts, techniques to fabricate micro- or nanocavities aroundselected QDs have been developed, allowing for an accuratecontrol of the relative QD-resonator position and hence acontrolled QD-resonator coupling [20–22]. However, thesetechniques do not allow for a scalable device processing andcannot completely ensure the integration of a single QD inthe microresonator of choice, which is of high interest, forexample in the fabrication of cavity resonant single photonsources. In fact, spectator QDs can have a detrimentalinfluence on the device performance, since the purity interms of the g(2)(0)-value of a single photon source criticallydepends on the number of quantum emitters contributing tothe emission event [4]. One technique which can ensure theintegration of single dots in microcavities, and furthermorecontrolling their position in the device exploits site-controlled QD (SCQD) growth and a subsequent devicealignment process [23]. Moreover, such an approach is fullyscalable, allowing for the simultaneous fabrication of a largenumber of spatially resonant single-QD resonator systems[24–26].

Even though encouragingly bright and spectrally pureemission from lithographically defined QDs in an etched in aGaAs quantum well has been reported on recently [27], themost promising approaches to realize structures with highoptical quality rely on growth on pre-patterned substrates[28–32]. These substrates are typically patterned either by amask with holes exposing the surface, and position

Christian Schneider received hisDipl. Ing. Degree (Nanotechnology)in 2007 and his PhD degree (Physics)in 2012 at the Department ofTechnische Physik, University ofWuerzburg. His current researchinterest includes the investigationof light–matter coupling in semi-conductor nanostructures. He is a

member of the Deutsche Physikalische Gesellschaft(DPG).


controlled QDs are realized by means of selective areagrowth [26, 33, 34], or the surfaces are structured directly vialithography and etching techniques [32, 35–37]. Unfortu-nately, the optical properties of the QDs can suffer from theirclose proximity to the re-growth surface [28, 38], which isusually manifested in spectrally broadened single QD-related emission features. In this article, we describe afabrication and growth procedure which results in well-ordered QD-arrays with close-to-perfect site-control, whilethe optical properties of the QD-samples remain excellent.The QDs are grown on nanoholes fabricated by electronbeam lithography and etching techniques, acting as nuclea-tion centres for the QD growth.

The manuscript is structured as follows: We will firstdescribe the fabrication and growth technology which weapply to obtain SCQD arrays with highest positioningaccuracies. The influence of the hole size and shape, as wellas the growth conditions on the QD morphology andoccupancy per nanohole are discussed in detail.

In the second part of the paper we will focus on the QDs’emission properties. We will show that the site-selective QDgrowth results in ensemble broadenings superior torandomly grown QDs. Cathodoluminescence (CL) investi-gations of buried QDs simultaneously confirm the excellentlateral QD ordering and demonstrate the high yield ofoptically active SCQDs in our samples. The emissionproperties of single SCQDs are investigated in micro-photoluminescence studies, yielding average values for thesingle QD related linewidth of �130 meV and best valuesdown to 25 meV for our QDs.

2 Sample fabrication technology2.1 Sample pre-patterning We will now briefly

describe the technology applied for sample pre-patterningprior to the growth of SCQDs by solid source molecularbeam epitaxy (MBE). First, several hundred nanometers ofGaAs are grown on epi-ready (100)-oriented GaAs surfaces.Then, the samples are spin coated by poly (methyl-methacrylate) and small circles with a diameter of �30–40 nm are defined in the resist by electron beam (e-beam)lithography. The circles are subsequently transferred into thesemiconductor surface by either wet-chemical etching(WCE, the etch solution consists of a mixture of H2O,H2O2 and H2SO4) or electron cyclotron resonance reactiveion etching (ECR-RIE) using a Cl2/Ar plasma. The durationof the etching step was adapted to generate nanoholes with adepth of �15–20 nm. As can be seen from the scanningelectron microscopy (SEM) images in Fig. 1a and b, theetching technique has a significant influence on the nanoholesize. All SEM pictures in this article were collected with asample tilted by 708 to increase the contrast. While theisotropic wet chemical etch solution generates significantlylarger, laterally widened nanoholes, the reactive ion etchingallows for the fabrication of nanoholes with steep sidewallsand smaller diameter. The dependence of the nanohole widthon the chosen etching technique and the e-beam exposuredose for electron energies of 80 keV is shown in Fig. 1c.

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Figure 1 SEM image of a surface with nanoholes defined bya) WCE and b) RIE. The samples were tilted by 708 in the SEMto increase the image contrast. c) Diameter of the nanoholes depend-ing on the e-beam dose and etching technique.

Figure 2 (online colour at: www.pss-a.com) Schematic structure ofuncapped site-controlled QD samples and tilted SEM image of anarray of SCQDs (on top of the structure). GaAs is shown in grey,AlGaAs in blue, and In(Ga)As in red colour.

Especially for the smaller nanoholes defined by ECR-RIE,there is a consistent trend towards larger nanoholes forincreasing exposure doses. As we will show later in thiswork, this can have a direct influence on the QD morphology.

2.2 Site-controlled QD growth The SCQD-growthtechnique, which is carried out on the pre-patternedsubstrates is briefly described in the following passage:Prior to the re-growth on the patterned surface, the samplesare cleaned by the chemicals HCl, H2SO4 and pyrrolidonein order to remove contaminations and oxides from thesample surface. In the load lock chamber of the MBE system,the remaining oxides are removed by a 30 min exposureof the sample surface to an activated hydrogen beam atmoderate sample temperatures of 360 8C. The standardthermal oxide removal carried out at temperatures above580 8C would unavoidably destroy the pre-patterned struc-tures, and additionally roughen the GaAs surface betweenthe intentionally defined nucleation centres [39]. On top ofthe de-oxidized surface, at first a thin layer of 8 nm GaAs isdeposited. Note that it is crucial not to exceed a thicknessmuch above 10 nm, since the nanohole size and shapeunavoidably changes with the thickness of the depositedlayer. The initially circular holes strongly elongate along the[0,1,1] direction due to the anisotropic surface migrationdynamics of GaAs, as was also reported in, for example [40].On top of the 8 nm GaAs, a nominally �0.75–1.5 nm thinInAs layer is deposited at a substrate temperature of 530–545 8C. Due to the high substrate temperature, intermixingwith the barrier material as well as desorption during theInAs growth becomes important. This leads to an effective

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reduction of the deposited material, together with amodification of the composition dependent critical thicknessfor QD nucleation [41]. Hence, for each given substratetemperature, the amount of deposited InAs needs to be re-calibrated carefully. In this first InAs layer, the so-calledseeding layer, the amount of InAs is reduced to a value belowthe critical thickness for QD nucleation. However, somepreferential InAs material accumulation in the nanoholes istaking place, which creates a strain field upon overgrowth ofthe layer with GaAs. After capping the seeding layer by 2 nmof GaAs and annealing it for 90 s at a substrate temperature of560 8C, a 10 nm thick sequence, typically consisting of 5 nmGaAs, 3 nm AlGaAs and 2 nm GaAs is deposited. On top ofthis layer sequence, the SCQDs are deposited by growing�1–2 nm of InAs at a growth rate of �0.006 nm/s. Formorphological investigations, the QDs are left uncapped andthe sample temperature is immediately ramped down toroom temperature. For optical characterization, the QDs arecapped by 70–100 nm of GaAs after a partial capping andannealing step [42, 43] with a cap thickness of 1.5–2 nm andan annealing duration of 90 s at 560 8C. The sample structureis sketched in Fig. 2, together with an SEM image of anuncapped QD sample grown under the described conditions.

3 Quantum dot properties3.1 Morphologic properties Since the size and the

shape of the nanoholes is not only a sensitive function of theetching technique but also the e-beam exposure dose, it canbe expected that these parameters also have a significantimpact on the QD properties. At first, we want to clarify theinfluence of the exposure dose on the QD morphology whenthe growth scheme of Section 2.2 is applied. The sample pre-patterning via ECR-RIE is chosen in this experiment,since the hole size can be varied more precisely via theexposure does if a directional etching process is applied(compare Fig. 1c). The resulting SEM images of the SCQDs



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Figure 3 Investigation of the SCQD morphology dependent on thenanohole size. The (tilted) SEM images show that the occupancywith multiple QDs per nanoholes is increased for larger nucleationcentres (from a to d).

on the sample surface are depicted in Fig. 3a–d. The e-beamdose is gradually increased from Fig. 3a–d, and a systematicchange in the nucleation properties can be observed. Whilefor the lowest e-beam dose, the occupancy of the nanoholeswith single QDs dominates, for the largest hole diametersmost of the nanoholes are occupied by two QDs arrangedalong the [1,1,0] direction. This implies that the strain fieldmediated from the seeding layer is anisotropic for largerholes and follows the elongation direction of the nanohole.

A quantitative evaluation of the occupancy per nanohole isshown in Table 1. This investigation shows that, similar toSCQDs grown directly on nanoholes without the seeding layer[44] the diameter of the nanoholes is a crucial parameter whichhas to be taken into account when designing these kinds ofSCQDs. However, as discussed, for example in [28], thefabrication process of the nanoholes has important con-sequences on the optical QD properties. The optical quality isvery sensitive to the semiconductor environment, and agentle etching technique allowing for the fabrication ofnucleation centres with very few crystal defects is crucial inthis respect. Therefore, it is important to achieve a highdegree of control over the morphologic QD properties (i.e., ahigh single-QD occupancy per nanohole) also for growth on

Table 1 Probablitiy for a nanohole occupied by a single QD forvarious e-beam dose values.

e-beam dose (mC/cm2) 17,500 20,000 22,500 25,000single QD occupancy 77.5% 64.3% 57.1% 48.8%


the much larger wet chemically defined nanoholes. In orderto accomplish accurate QD ordering on such nanoholes, thesubstrate temperature was increased during QD deposition.A higher substrate temperature increases both the migrationlength of the adatoms during QD deposition, but alsoincreases the size of the QDs, typically resulting in stronglylaterally enlarged and elongated QD structures [45]. In orderto monitor the influence of the substrate temperature on thenucleation behaviour of the SCQDs grown on wet chemi-cally defined nanoholes, two samples were compared. Forthe sample in Fig. 4a, the substrate temperature was set to530 8C during QD deposition. As expected, a significantfraction of nanoholes with a pitch of 1 mm is occupied by aQD molecule, whereas the remaining holes are occupiedwith single QDs. The QD shape is investigated by recordingSEM images under different directions. The sampleorientation is indicated in the images. A pronouncedelongation of the QDs can be clearly observed along the[0,�1,1] direction, that is in agreement with investigationson low strain self-assembled InGaAs QDs [46] and InGaAsQDs grown at elevated substrate temperatures [45].

In order to suppress the multiple occupations of thenanoholes, we increased the substrate temperature duringQD deposition up to 545 8C. In this regime, the amount ofdeposited material had to be increased up to 1.92 nm tocompensate for the significantly stronger material desorptionat such high temperatures. The resulting SEM images areshown in Fig. 4b. Due to the further enlarged QD dimensions,the multiple occupations of the nanoholes are now almostentirely suppressed, and large single dots with a length of135 nm and a width of �60 nm occupy the nanoholepositions. Furthermore, in this regime, SCQDs can befabricated on nanohole pitches in a wide range, that is for a

Figure 4 (online colour at: www.pss-a.com) Site-controlled QDsgrown on wet chemically etched nanoholes. (a) QD deposition at530 8C substrate temperature. (b) QD deposition at 545 8C. Thesample was tilted by 708 in the SEM to increase the contrast.

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Figure 5 (online colour at: www.pss-a.com) AFM images ofSCQDs on large periods of (a) 500 nm, (b) 2 mm and (c) 4 mm onthe same sample, exhibiting excellent long range ordering. Thevertical stripes in (a) stem from AFM scanning artefacts.

Figure 6 (online colour at: www.pss-a.com) (a) Tilted SEM imageof uncapped SCQDs ordered on a 300 nm period. (b) Surface AFMscan of SCQDs buried by 100 nm of GaAs.

large variety of QD densities. This is shown in the atomicforce microscopy (AFM) images in Fig. 5. Areas withdiffering QD pitches between 0.5 and 4 mm were fabricatedon this sample. Due to the elaborate growth conditions, theQD ordering is very accurate irrespective of the QD period.Even for the largest nanohole periods, the nucleation ofinterstitial QDs is absent, except on very few positions withnatural crystal defects (not shown in the figure). The growthconditions do not only allow for the realization of QD arrayswith very large pitches, but can also be adapted to fabricateSCQD arrays of high density. Figure 6a shows a QD arraygrown at such elevated temperature with a QD pitch as smallas 300 nm. It is interesting to note, that the long rangeordering in such QD samples can still be confirmed in AFMinvestigations after overgrowth of the QDs. Figure 6b depictsan AFM image recorded at the edge of an array of SCQDs.The QDs in this sample were capped by 100 nm of GaAs.It is still easily possible to recognize the QD positions by

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the small hills on the surface of the AFM scan, confirming thesuccessful growth of well-positioned QDs in this cappedsample.

3.2 Optical properties In this section of the manu-script, we will discuss the optical properties of the SCQDs ingreater detail. All measurements were taken at cryogenictemperatures (T� 10 K). For photoexcitation of the QDs,we used a CW-laser with a tunable wavelength. Thephotoluminescence signal was analysed either by a 0.75 mspectrometer for ensemble investigations, or a doublemonochromator equipped with a silicon detector allowingfor spectral resolutions down to 16 meV. Alternatively, inorder to investigate the QD-ordering with a high spatialresolution via luminescence scanning, we employed amodified scanning electron microscopy. The sample wasmounted in a helium flow cryostat built inside of themicroscope, and the excitation electron beam of the SEMunit was scanned over the sample surface. The incomingbeam of high energy electrons generates electron hole pairsvia electron–electron scattering processes, resulting inluminescence from the sample. The luminescence signal iscollected with an elliptical mirror, dispersed by a 0.45 mmonochromator and detected by a standard silicon CCDdetector allowing for a spectral resolution of 140 meV.

At first, we will examine the optical properties of the QDensemble, that is a large number of QDs simultaneouslyinvestigated by photoluminescence spectroscopy. Due tounavoidable fluctuations in the material distribution duringgrowth, the size, shape and composition of the QDs varystatistically. The height of the QDs, being the smallest QDdimension, can be expected to have the largest influence onthe emission energy. Using the partial capping and annealingstep mentioned in Section 2.2, the QD height can becontrolled by the thickness of the partial GaAs cap. Theremaining distribution in the lateral size of the QDs andcompositional inhomogeneities cause an approximatelyGaussian distribution in the emission from the QD groundstate.

Figure 7a shows the emission of an ensemble of morethan 100 QDs with a pitch of 300 nm. The emission was fittedby a Gaussian curve with a full width at half maximum of15.9 meV. The measured data show small peaks on top of thecurve which we attribute to intense light emitting QDs in the

Figure 7 (online colour at: www.pss-a.com)(a) EnsemblePL spectrum of SCQDsgrown ona nanohole pitch of 300 nm. (b) Emissionenergy of the SCQDs as a function of the QDperiod.



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ensemble. The inhomogeneous broadening of these QDscompares favourably to similarly grown InAs QDs onreference planar substrates. Indeed, such low density QDstreated by the partial capping and annealing method typicallyexhibit ensemble broadenings larger by a factor of 2–3 [43].The improved homogeneity probably results from a well-defined material distribution on the surface due to the QDordering. Further improvement of this figure of merit is keyfor a successful implementation in spectrally resonant singleQD–cavity systems, since spectral tuning techniques areusually limited to values much smaller than 10 meV, that issmaller than the present energetic variation from one QD toanother [47]. Only sophisticated growth sequences can allowfor the extension of QD tuning ranges via the giant Starkeffect [48].

We also investigated the influence of the SCQD-pitch onthe QD-emission wavelength (see Fig. 7b). A redshift ofabout 8 meV is observed when increasing the QD pitch from200 to 350 nm. This can be explained by taking into accountthe surface area per nucleation centre where InAs cancontribute to the QD growth. A greater pitch entails QDs oflarger size emitting at longer wavelengths. This demon-strates that a variation of the QD pitch has a directconsequence on the very fundamental emission behaviourof the SCQD, and can therefore be employed as a tool totailor their optical properties. We will now discuss theproperties of single SCQDs, with a focus on spatial orderingand their optical quality. Even though it is in some casespossible to reconfirm QD ordering after overgrowth by arather thick GaAs cap via surface scanning techniques(compare Fig. 6b), this cannot generally be employed toprove the successful site-control of the studied QDs: Forinstance, shallow SCQDs and interstitial QDs can possiblybe entirely planarized and therefore not be detected.Furthermore, the AFM scanning technique can only be usedfor capping thicknesses of several tens to 100 nm, and, mostimportantly, does not provide any information about whetherthe SCQDs are optically active or not. However, if one wantsto study the optical properties of SCQDs, it is absolutelyessential to ensure that the buried QDs under investigationare well ordered, that is to both confirm the successful QD-ordering and exclude the possibility that the emitters underinvestigation are interstitial QDs. Several complementarytechniques have been employed to facilitate the spatiallyresolved investigation of SCQDs, such as spatially resolvedphotoluminescence mapping [32, 49–51], one dimensionalPL-scans [34, 44, 52], or the post-growth fabrication ofmesa-structure around the SCQDs [23]. The PL-mapping


technique can be challenging for smaller QD periods, since itcan only be applied for QD samples with pitches on the orderof the excitation spot’s diameter (1–3 mm) and it furthermorerequires measurement equipment with highest long-termstability. One-dimensional PL-scanning is more convenientand less time-consuming and can also show the ordering ofQDs on nanohole pitches smaller than the diameter of theexcitation laser. However only a very small area of thesample can be probed, which is mostly insufficient tosatisfactory reflect the generally successful long-rangeordering of the buried QDs. The fabrication of mesastructures with small diameters around SCQDs is wellcompatible with smaller pitches and inherently removesinterstitial QDs during the mesa processing, however thefabrication process can have a direct (mostly negative)influence on the QD properties via enhanced spectraldiffusion from the mesa sidewalls [16]. Therefore weemploy CL scanning to investigate the spatial ordering ofemitters in our samples. This technique has previously beenemployed to investigate optical properties of SCQDsfabricated in inverted pyramides on (111)B oriented GaAssubstrates in [53]. Here, we will show that by CL scanning wecan investigate the emission properties of our SCQDs withboth moderate and large periods. The sample underinvestigation was as-grown to the uncapped QDs shown inFig. 5. The QDs were capped by 100 nm of GaAs, with anemission band centered around 900 nm. Figure 8a shows aCL map of a sample region with a QD pitch of 2 mm. Theemission spot of every single SCQD can be easily recognizeddue to the high spatial resolution of the scanning technique.The excellent spatial ordering of the buried SCQDs caneasily be extracted from the square pattern of the emissionspot, while the emission of interstitial QDs is entirely absenton this sample area. Note that the effective size of theemission spots are determined by the electron and holediffusion length in the semiconductor, which is in contrast toPL scanning techniques, where the excitation and collectionlaser focus determines the size of the features. Figure 8bshows a similar CL scan from a larger QD period of 4 mm ofthe same sample, reflecting the high positioning accuracyalso on such ultra-low density arrays.

One key advantage of the CL scanning technique is therapidness of the scanning technique, allowing for theconvenient investigation of much larger sample areas ascompared to experiments with optical excitation. A scan over alarge sample area of 6090 mm2 with a SCQD period of 4 mm isshown in Fig. 9. This representative investigation demonstratesthat more than 90% of all emission sites are occupied by an

Figure 8 (online colour at: www.pss-a.com) Cathodolumi-nescence map of the sample area with (a) the 2 mm SCQDperiod and (b) the 4 mm SCQD period.

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Figure 9 (online colour at: www.pss-a.com) Overview CL map ofan array of SCQDs on a 4 mm period.

optically active SCQD, which is a significant step towards thescalable exploitation of single SCQDs as quantum emitters, forexample in microresonator based devices.

Since the single QD-related emission linewidth is animportant figure of merit for the optical property ofsemiconductor QDs, we statistically investigated the emissionlinewidth of 57 QDs in a micro-photoluminescence exper-iment. Especially in experiments and applications exploitinglight–matter coupling effects between single QDs andmicrocavities in the regime of cQED, the single-QD relatedlinewidth should be as narrow as possible, ideally reachingdephasing limited values. This requirement is set by the factthat any spectral broadening, be it dephasing related orspectrally diffusive, effectively reduces the visibility of theRabi-doublet in the strong coupling regime of cQED, orbleaches the maximum Purcell factor in the weak couplingregime [54]. As shown in the statistical chart in Fig. 10a, inour sample the majority of QDs comprise values below113 meV with an average value of 133 meV. Both valuesare comparable with reported Rabi-splittings in coupledQD-micropillar resonators [12], and significantly lower thanRabi-splitting values reported for photonic crystal resonator[55] and microdisk resonators [14]. Therefore, such SCQDsare well suitable for cQED experiments even in the strongcoupling regime when integrated in high quality-low modevolume resonators. In Fig. 10b a selected mPL spectrum of a

Figure 10 (online colour at: www.pss-a.com) (a) Statistics ofemission linewidth in a high purity SCQD sample on the 2 mmperiod. (b) High-resolution spectrum of single SCQD emission linewith a linewidth of 25 meV.

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SCQD is plotted, acquired under non-resonant excitation withan excitation power of 1 mW at T¼ 5 K. The linewidth of theSCQD related emission feature has a value of only 25 meV,without deconvoluting the detector characteristics. Note, thatthis value marks the current state of the art for this importantspecies of SCQDs, namely single SCQDs grown on shallownanoholes, and is comparable to values of high qualityStranski–Krastanov InGaAs QDs [16] or state of the artpyramidal InGaAs QDs [31] studied under similar conditions.

4 Conclusions In conclusion, we have presented atechnology allowing us to fabricate SCQDs with tailoredmorphology, over a wide range of QD-pitches. In our growthand fabrication scheme, the occupancy of QDs per nanoholecan be flexibly manipulated by either the nanohole size (i.e.,the lithography technique) or the general growth conditions.This allows for the fabrication of site-selected QD molecules,which can potentially be employed to study lateral inter-QDcoupling effects on a scalable platform, and more importantlyfor the realization of single SCQD on QD pitches suitable forthe integration into photonic microresonators. Employing CLscanning, we demonstrated that the vast majority of our SCQDare optically active, and that the spatial QD-ordering and thesuppression of interstitial QD formation is not negativelyinfluenced by the capping procedure. Statistical evaluations ofthe single SCQD related linewidth reveals average values of133 meV, with minimum values of only 25 meV marking thestate of the art for SCQDs grown on (001) GaAs.

Acknowledgements Expert sample preparation by TheresaSteinl is acknowledged. We appreciate the support by MargitWagenbrenner in SEM-imaging and MBE-machine maintenance,and S. Heckelmann as well as C.R. Drescher in sample growth.Funding by the German ministry of education and research withinthe project QPENS is gratefully acknowledged.

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In(Ga)As/GaAs site-controlled quantum dots with tailored morphology and high optical quality

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