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 Appl. Phys. Lett. 100, 093111 (2012) and may be found at https://doi.org/10.1063/1.3691251.We present a “bottom-up” approach for the lateral alignment of semiconductor quantum dots (QDs) based on strain-driven self-organization. A buried stressor formed by partial oxidation of (Al,Ga)As layers is employed in order to create a locally varying strain field at a GaAs(001) growth surface. During subsequent strained layer growth, local self-organization of (In,Ga)As QDs is controlled by the contour shape of the stressor. Large vertical separation of the QD growth plane from the buried stressor interface of 150 nm is achieved enabling high optical quality of QDs. Optical characterization confirms narrow QD emission lines without spectral diffusion.DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, Bauelement

Bimberg, Dieter

Dreismann, A.

Germann, Tim David

Haisler, V.

Hitzemann, Ole

Hoffmann, Axel

Ostapenko, Irina

Pohl, Udo W.

Rodt, Sven

Schliwa, Andrei

Schulze, J.-H.

Stock, Erik

Strittmatter, André

Unrau, Waldemar

Applied Physics Letters

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Appl. Phys. Lett. 100, 093111 (2012); https://doi.org/10.1063/1.3691251 100, 093111© 2012 American Institute of Physics.Lateral positioning of InGaAs quantum dotsusing a buried stressorCite as: Appl. Phys. Lett. 100, 093111 (2012); https://doi.org/10.1063/1.3691251Submitted: 30 September 2011 • Accepted: 14 February 2012 • Published Online: 01 March 2012A. Strittmatter, A. Schliwa, J.-H. Schulze, et al.ARTICLES YOU MAY BE INTERESTED INDevelopment of site-controlled quantum dot arrays acting as scalable sources ofindistinguishable photonsAPL Photonics 5, 096107 (2020); https://doi.org/10.1063/5.0013718Lithographic alignment to site-controlled quantum dots for device integrationApplied Physics Letters 92, 183101 (2008); https://doi.org/10.1063/1.2920189 Resonance fluorescence of a site-controlled quantum dot realized by the buried-stressorgrowth techniqueApplied Physics Letters 110, 111101 (2017); https://doi.org/10.1063/1.4978428Lateral positioning of InGaAs quantum dots using a buried stressorA. Strittmatter,1,a) A. Schliwa,1 J.-H. Schulze,1 T. D. Germann,1 A. Dreismann,1O. Hitzemann,1 E. Stock,1 I. A. Ostapenko,1 S. Rodt,1 W. Unrau,1 U. W. Pohl,1A. Hoffmann,1 D. Bimberg,1 and V. Haisler21Technische Universität Berlin, Institut für Festkörperphysik, Sekr. EW 5-2, Hardenbergstrasse 36, D-10623Berlin, Germany2Russian Academy of Sciences, Institute of Semiconductor Physics, pr. Lavrenteva 13, RU-630090 Novosibirsk,Federation of Russia(Received 30 September 2011; accepted 14 February 2012; published online 1 March 2012)We present a “bottom-up” approach for the lateral alignment of semiconductor quantum dots(QDs) based on strain-driven self-organization. A buried stressor formed by partial oxidation of(Al,Ga)As layers is employed in order to create a locally varying strain field at a GaAs(001)growth surface. During subsequent strained layer growth, local self-organization of (In,Ga)As QDsis controlled by the contour shape of the stressor. Large vertical separation of the QD growth planefrom the buried stressor interface of 150 nm is achieved enabling high optical quality of QDs.Optical characterization confirms narrow QD emission lines without spectral diffusion. VC 2012American Institute of Physics. [http://dx.doi.org/10.1063/1.3691251]The deterministic alignment of quantum dots (QDs) dur-ing an epitaxial growth process is mandatory for electronicand optoelectronic devices1 based on single QDs, for exam-ple, single photon detectors2 and non-classical light emit-ters.3 The self-organized formation of coherently strainedislands, e.g., QDs, by the growth of strained layers in the“Stranski-Krastanow” growth regime is a consequence of thetotal energy minimization of the strained layer system.4–6QDs are formed if the strain energy relieved by island forma-tion surpasses the energy cost associated with newly formedsurfaces and edges.7 Therefore, a selective formation of QDson a surface will occur if the surface exhibits sites ofincreased strain energy, higher strain energy relief, or lowerfacet formation energy during growth of a strained layer.Current techniques for QD positioning generally deploynanometer-scale lithography techniques like electron beamlithography,8 focused ion beam lithography,9 local oxida-tion,10 or nano-imprinting11 in order to define nanometer-sized areas as exclusive nucleation sites prior to the growthof quantum dots. All these “top-down” approaches share anumber of difficulties, which impact the structural and opti-cal properties of the quantum dots. First, deterministic quan-tum dot nucleation is possible only within very close verticalproximity to the structural patterning. An often reportedproblem is the missing of QDs at shallow holes patterned ona growth surface.8,9 Since the patterning involves etching ofthe surface or other invasive means, the quantum dots willbe surrounded by defect sites which degrade their structuraland optical quality.12 Even though sophisticated cleaningprocedures are applied prior to growth, the QDs show infe-rior optical qualities in terms of spectral linewidth and radia-tive efficiency as compared to QDs grown on planarsubstrate surfaces.8,9 Second, these technologies are not eas-ily scalable to large wafer sizes because of the involved deli-cate nanometer-scale processing steps. Third, there is noself-alignment of an electrical current path with respect tothe QDs in order to efficiently inject charge carriers into sin-gle quantum dots.We report on a “bottom-up” approach employing a localmodification of the free energy of a GaAs(001) surface usingspatially modulated strain fields. These strain fields emanatefrom the interfaces of a buried AlxGa1xAs/AlOx stressorobtained by partial oxidation of AlGaAs.13,14 The strain fieldis created by altering the volume of the initial AlGaAs uponoxidation. During subsequent InGaAs strained layer growththe InAs growth is preferred at the local tensile strain max-ima of the GaAs surface. Consequently, local QD formationis observed. For properly reduced size of the stressor, con-trolled growth of single QDs at the center of the mesa isobtained. We first analyze the stressor-induced strain distri-bution at a GaAs(001) surface and report afterwards on thepositioning of InGaAs QD using buried stressors.The strain distribution as obtained by a continuum-mechanical model at the surface of a circular mesa structurefor a rectangular AlxGa1xAs/AlOx stressor buried 100 nmbeneath a GaAs(001) surface is shown in Fig. 1. The upperpanel shows a schematic of the structure used for modeling.An isotropic contraction of the unit cell volume by 7% uponoxidation is assumed for all calculations. As a result of thevolume contraction, the region above the oxidized buriedlayer is compressively strained. However, the volume con-traction exerts a tensile force onto the region at the bounda-ries between the oxide and the AlAs. The result as shown inthe lower panel of Fig. 1 is an inhomogeneous strain distri-bution e(x,y) at the GaAs(001) surface. On the vertical axisthe strain distribution e¼ exxþ eyy at the surface is displayed.Two cross-sectional line scans of the strain distribution ofrectangular apertures with 2 and 1 lm side length reveal theimpact of the aperture size on the strain distribution.Whereas two separate maxima of the tensile in-plane strainare located above the aperture region with 2 lm side lengthonly a single tensile strain maximum is found for the aper-ture 1 lm side length. For the given structure with a 100 nmGaAs layer thickness on top of the oxide, the magnitude ofa)Electronic mail: strittma@sol.physik.tu-berlin.de.0003-6951/2012/100(9)/093111/3/$30.00 VC 2012 American Institute of Physics100, 093111-1APPLIED PHYSICS LETTERS 100, 093111 (2012)the interfacial strain (exx or eyy) is of the order of 1%–1.5%.As the QD formation in the InxGa1xAs/GaAs system isdriven by interfacial lattice mismatch (e 3%–7% forx> 0.30) this magnitude of the surface strain is significantfor the QD growth. Since InAs has a larger lattice constantthan GaAs the points of maximum tensile strain at the GaAssurface correspond to points of lowest strain for the InAslayer. The inhomogeneous strain distribution will thereforeaffect growth of both GaAs and InAs layers since atoms willmove to the minima of their respective surface strain energyduring growth. As the total amount of InAs triggers the onsetof quantum dot formation,15 selective growth of QDs at theaperture boundaries is expected. From the simulation data,the width of the tensile strain maximum is about 300 nmwhich sets the spatial resolution for QD alignment to a circleof about 300 nm diameter. Since the oxide layer defines acurrent path through the aperture in vertical pn-junctiondevices, the QDs are inherently self-aligned to this currentpath.The process for the stressor fabrication starts with thegrowth of the epitaxial stressor structure consisting of 40 nmAl0.90Ga0.10As/40 nm AlAs/40 nm Al0.90Ga0.10As which iscovered by 50 nm GaAs. After growth, circular mesas areformed by dry etching and finally, the AlAs layer is oxidizedin a water vapor/nitrogen ambient from the mesa edgestowards the center (see Fig. 1). The fabricated circular mesasare 10–25 lm in diameter. Upon an oxidation time ofapproximately 10 min, the oxidation depth is about 7 lmleading to apertures in the oxide layer for dmesa> 14 lm,which are centered with respect to the mesa boundaries. Theaperture sizes scale with the mesa diameter from< 1 lm fordmesa< 16 lm to 2 lm for dmesa¼ 20 lm. Prior to QD for-mation sample surfaces are annealed at 720 C and a 50 nmGaAs buffer layer is grown at 690 C which is standard forGaAs growth on un-patterned substrates. For QD formation,approximately 2 monolayers of In0.5Ga0.5As are depositedonto the GaAs surface by metalorganic vapor phase epitaxy.Thereby, the vertical separation of the QD growth planefrom the stressor is about 150 nm which is much larger thanfor QDs grown on patterned nano-holes, for example.Figures 2(a)–2(c) are atomic force microscopy (AFM)surface images taken at the center of mesas with diametersof 20, 14, and 13 lm exhibiting apertures of 2 lm, <1 lm,and <500 nm side length, respectively. For this experiment,the InGaAs QDs are buried beneath the surface by only 4 nmGaAs to distinguish the QDs by hillocks on the surface. Forthe 2 2 lm2 aperture (Fig. 2(a)), a square-like alignment ofthe QDs is observed while a QD cluster appears for the14 lm mesa (Fig. 2(b)). With an aperture size of less than500 nm (13 lm mesa diameter) only two QDs are left at theaperture center (Fig. 2(c)) in agreement with our theoreticalmodeling. Optical properties of the QDs are investigated bycathodoluminescence and micro-photoluminescence spec-troscopy. The QD luminescence distribution is very well cor-related to the alignment found in the AFM images. Forexample, 20 lm large mesas show QD luminescence distri-bution along the boundaries of a 2 2 lm2 square (notshown). Figure 3 shows the spatial distribution of QD emis-sion (1.28–1.32 eV) on a 15 lm mesa and a local spectrumtaken at the center to demonstrate the high optical quality ofthe QDs. From the AFM analysis (Fig. 2(b)), a cluster ofQDs is expected to be formed at the center of the mesa.Amongst other QD emission lines, a spectrally separated sin-gle emission line is selected for demonstration purposes.This line exhibits a full-width at half-maximum of 160 leVFIG. 1. (Color online) (Upper panel) Schematic of the buried stressor struc-ture as used for strain calculation. (Lower panel) Calculated sum of thein-plane strain components (e¼ exxþ eyy) at the surface of a GaAs(001) sub-strate for a buried AlAs/AlOx stressor structure covered by 100 nm GaAs.For square-like aperture of 2 lm side length (black line) the maximum in-plane strain is surrounding the inner part of the aperture. For an apertureside length of 1 lm (red line) maximum tensile strain is found at the centerof the aperture.FIG. 2. (Color online) Atomic-force microscopy images of the centerregions of mesa structures with (a) 20 lm, (b) 14 lm, and (c) 13 lm diameterovergrown with quantum dots. QDs appear as hillocks and show clear lateralordering in a square-like pattern (a) according to the shape of the 2 2 lm2rectangular stressor. For aperture sizes less than 1 lm, a cluster of QDs isfound (b). The number of aligned QDs at the mesa center is reduced to 2quantum dots for aperture sizes of less than 500 nm (c). Scale bars in the fig-ures indicate the height scale.FIG. 3. (Color online) Individual QD emission line in the center of a 15 lmmesa with 160 leV FWHM as indicated in the inset. Inset: spatial distribu-tion of QD luminescence between 1.28 and 1.32 eV. The local value of max-imum intensity is encoded in false colors. Additional QDs have formed inthe region of reduced strain at the mesa’s edge.093111-2 Strittmatter et al. Appl. Phys. Lett. 100, 093111 (2012)which is limited by the spectral resolution of the setup. Thevalue compares well with some of the best reported values todate.8,16 In Ref. 8, an average QD emission linewidth of132 leV is reported, and in Ref. 14, the linewidth is about100 leV. The intensity of the spectrum is comparable to thatof QD samples grown on planar substrates, demonstratinghigh optical quality of the aligned QDs. Very little spectraldiffusion of QD emission lines is observed on individualemission lines which indicates high quality of the surround-ing GaAs material.A “bottom-up” technique for QD positioning based onlateral self-organization above a buried stressor is developed.Deterministic alignment of InGaAs QDs at large vertical dis-tance from a buried AlxGa1xAs/AlOx stressor is achieved.QDs are located above the inner regions of the buried oxideaperture which inherently defines an electrical current paththrough the QDs in pn-junction devices. Optical characteri-zation yields narrow QD emission lines with high lumines-cence intensity and little spectral diffusion.We gratefully acknowledge helpful discussions with D.Gershoni, Technion, Haifa (Israel). Financial support for thiswork is received by the Deutsche Forschungsgemeinschaftwithin the Collaborative Research Center 787 “Nanophotonicdevices” (projects A2, A5, and C1).1J. V. Barth, G. Costantini, and K. 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Lateral positioning of InGaAs quantum dots using a buried stressor

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Lateral positioning of InGaAs quantum dots using a buried stressor

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