Formation of self-assembled quantum dots on AlInAs and GaInAs matrices using aGaSb sublayerRoland Enzmann, Susanne Dachs, Ralf Meyer, Jonathan Finley, and Markus-Christian Amann Citation: Applied Physics Letters 91, 083111 (2007); doi: 10.1063/1.2773754 View online: http://dx.doi.org/10.1063/1.2773754 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/91/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Growth by molecular beam epitaxy of self-assembled InAs quantum dots on InAlAs and InGaAs lattice-matchedto InP J. Vac. Sci. Technol. B 25, 1044 (2007); 10.1116/1.2731334 Formation and property of InSb self-assembled quantum dots on GaAsSb lattice matched to InP J. Vac. Sci. Technol. B 24, 1660 (2006); 10.1116/1.2190667 Gallium diffusion into self-assembled InAs quantum dots grown on indium phosphide substrates Appl. Phys. Lett. 85, 3578 (2004); 10.1063/1.1806277 Enhanced photoluminescence of InAs self-assembled quantum dots grown by molecular-beam epitaxy using a“nucleation-augmented” method Appl. Phys. Lett. 85, 567 (2004); 10.1063/1.1773914 Effect of growth temperature on luminescence and structure of self-assembled InAlAs/AlGaAs quantum dots J. Appl. Phys. 90, 2048 (2001); 10.1063/1.1388021

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Formation of self-assembled quantum dots on AlInAs and GaInAs matricesusing a GaSb sublayer

Roland Enzmann,a� Susanne Dachs, Ralf Meyer,Jonathan Finley, and Markus-Christian AmannWalter Schottky Institut, Technische Universität München, D-85748 Garching, Germany

�Received 18 December 2006; accepted 27 July 2007; published online 23 August 2007�

In this letter, the authors present the formation of InAs quantum dots on GaInAs and AlInAs latticematched on InP �001� substrates via molecular beam epitaxy by inserting a two monolayer GaSbsublayer below the InAs quantum dot material. They show that the formation of quantum dots isfavored on indium-free and antimony-rich surfaces while quantum dashes are preferentially formedon indium-rich and antimony-free surfaces. Using a thin layer of GaSb between the InAs quantumdot material and the AlInAs or GaInAs matrix, single quantum dots were formed even with lowquantum dot densities ��1/�m2�. These quantum dots give rise to photoluminescence between1100 and 1500 nm depending on the matrix material. © 2007 American Institute of Physics.�DOI: 10.1063/1.2773754�

Self-organized quantum dots are a promising approachfor the fabrication of light emitting devices that producesingle photons on demand. Such devices could be the keycomponents in future applications such as quantum cryptog-raphy and quantum information processing.1 For an effectivephoton transfer by fiber optics, the emission wavelength hasto be close to 1.55 �m �0.8 eV�. The technology for thefabrication of InAs quantum dots embedded in GaAs sub-strate is well known, but the emission wavelength of1.55 �m is difficult to reach for samples with low dot den-sities and at low temperature. In contrast to this, InP basedmaterials, in general, provide the emission wavelength of1.55 �m.2 Even the growth of InAs quantum dots on InPsubstrate is already possible by metal-organic vapor-phaseepitaxy,3 but effort of fabrication of InAs nanostructures onAlInAs or GaInAs by molecular beam epitaxy �MBE� leadsnot to quantum dots but to quantum dashes which are alignedalong the �1−10� direction.4–6

Two potential mechanisms may drive the formation ofelongated nanostructures in the InAs/GaInAs/ InP orInAs/AlInAs/ InP material systems. One is the lower strainwhen compared with the InAs/GaAs material system. In thiscase, a small asymmetry in the strain will cause a directiondependent relaxation,7,8 as shown for the GaInAs/GaAs ma-terial system.9 Another important influence for the inhomo-geneous relaxation of InAs may have been the indium in theuppermost layers. To clarify this point so as to establish anMBE growth process that allows the deposition of quantumdots with low density and an emission wavelength around1.55 �m was the main purpose of this study.

To avoid the direction-dependent growth, our approachwas to modify the surface properties underneath the InAsmaterial by inserting an indium-free sublayer. The materialfor the sublayer was chosen to be Ga�As,Sb�. In this materialsystem, we can examine whether strained sublayers or pri-marily indium-free surfaces have a higher impact on the for-mation of quantum dots on InP substrates.

The samples were grown in a solid-source MBE system�Varian Mod Gen II� on a rotating 2 in. InP �001� substrate at

515 °C �measured by thermal couple�. For the growth of theInAs quantum dots, we used a downward looking cell, sinceit allows growth with a very low indium flux �0.03 ML/s�.The rotation of the substrate was stopped during the growthof the quantum dots to obtain a gradient in the InAs coverageon a single wafer.

The sample structure is shown in Fig. 1. There are tworegions with quantum dots: an overgrown layer for photolu-minescence measurements and a surface layer for structuralinvestigations by atomic force microscopy �AFM�. As upperand lower matrix materials �thickness of 2 nm�Ga0.47In0.53As, Al0.48In0.52As, or any alloy of�AlxGa1−x�yIn1−yAs can be used for tuning the emissionwavelength of the quantum dots.10

Several samples were grown to separate the impact ofthe sublayer as well as the influence of the InAs coverage.The effect of the sublayer is shown in Fig. 2. Four sampleswith 3 ML InAs coverage were grown. Figure 2�a� showsInAs quantum dashes with an average width of 45 nm, alength of 250 nm, and a height of 4 nm, which appear with-out using any sublayer at all. By incorporating a tensilestrained GaAs sublayer �Fig. 2�b�� dashlike �average lengthof 93 nm� and also dotlike �average diameter of 53 nm�structures appear along the �1−10� direction. Intermixing ef-fects between the tensile strained sublayer, the indium con-taining matrix material �segregation�, and the compressivelystrained InAs of the quantum dot probably occur as proposedby Heyn et al.11

a�Electronic mail: Enzmann@wsi.tum.de FIG. 1. Sample structure.

APPLIED PHYSICS LETTERS 91, 083111 �2007�

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In contrast to this, on a compressively strained GaSbsublayer, the formation of circular shaped quantum dots canbe observed, as shown in Fig. 2�c�. These quantum dots havean average height of 7 nm and a diameter of 40 nm. We donot expect indium segregation from the matrix materialthrough this layer as well as no intermixing with the InAsmaterial of the quantum dots. Thus any alloy of GaInSb in-creases the strain in the sublayer even further. To separate theeffect of strain and an indium-free growth surface on theformation of InAs quantum dots we, also deposited 3 MLInAs on the lattice matched and indium-free matrix materialAl0.8Ga0.2As0.55Sb0.45. The fact that quantum dots are formedin preference to dashes �Fig. 2�d�� shows that the indium-freeand antimony-rich surface drives the quantum dot formationand not the strain.

Subsequently, we investigated the influence of the sur-face coverage with InAs on the formation of the nanostruc-tures using a GaSb sublayer �see AFM pictures in Fig. 3�. Byapplying an InAs amount of more than 3.5 ML �Fig. 3�a��,quantum dashes arise additionally to the quantum dots, whilebelow 3.5 ML only the formation of quantum dots occurs.Furthermore, the density of the quantum dots can be adjusted

by using an InAs amount below this value, as can be seenfrom Figs. 3�c� and 3�d�. The critical InAs coverage for theappearance of quantum dots was 2.2 ML on GaInAs matrixmaterial. This means that within a range of 3.5–2.2 ML thequantum dot density can be tuned from 150 dots/�m2 downto 1 dot/�m2 slightly above the critical thickness. At thislow density, the quantum dots have a height of 9 nm and anaverage diameter of 30 nm, as shown in Fig. 3�d�.

This finding holds in principle also for AlInAs matrixmaterial. The quantum dots grown on an AlInAs matrix aresmaller and appear in higher density and at a lower criticalthickness. This effect is also known for quantum dashesgrown on GaInAs and AlInAs matrix materials and is attrib-uted to a reduced mobility of indium atoms on an aluminumcontaining surface. AlInAs surfaces exhibit a higher surfaceroughness12 and, consequently, provide more sites of nucle-ation for quantum dots. A 2 ML GaSb sublayer is not suffi-cient to flatten the surface so that a higher surface roughnesson the samples with AlInAs �about 0.9 nm� matrix materialsis observed, whereas GaInAs matrix material shows onlyhalf of this roughness, and therefore larger dots in a lowerdensity.

In our experiments, no quantum dots occur on GaInAs orAlInAs for any InAs coverage without GaSb sublayer. Thegrowth of InAs quantum dots on the indium-free and latticematched matrix material AlGaAsSb shows that not the strainat the surface but the indium and antimony content play amajor role in the formation process of the InAs nanostruc-tures. The separation between indium-free and antimony-richsurfaces was not unambiguously possible. Indeed, we alsohave quantum dot formation onto InP substrate latticematched AlGaAsSb, which has only an antimony content of45%. Therefore, we attribute to the indium of the uppermostlayer of the matrix material the main influence on the appear-ance of quantum dots or quantum dashes.

Several publications report on a suppression of the coa-lescence of quantum dots using antimony surfactant-mediated metal organic chemical vapor deposition13 as wellas the growth of InAs�Sb� quantum dots by solid sourcemolecular beam epitaxy14 in the InAs/GaAs material system.However, these approaches lead to a high quantum dot den-sity, explained by a reduced indium migration length due toantimony. However, in our experiments, we also obtained alow quantum dot density, which argues not for a reduceddiffusion length of indium in the InAs/AlInAs/ InP andInAs/GaInAs/ InP material system.

In the preceding section, we analyzed the structuralproperties of the InAs quantum dots grown on a GaSb sub-layer. We now proceed with their optical properties. Photo-luminescence measurements of quantum dot ensembles �Fig.4� show light emission between 1100 and 1500 nm depend-ing on the upper and lower matrix materials. For AlInAs asupper and lower matrix materials, the emission of the quan-tum dots is around 1100 nm �Fig. 4�a��. Changing the lowermatrix material to GaInAs results in larger dots and a lowerbarrier. Thereby a redshift of about 200 nm occurs �Fig.4�b��. For GaInAs as upper and lower matrix materials theemission wavelength is around 1500 nm �Fig. 4�c��. So it ispossible to tune the emission over a wide wavelength rangeby adjusting the matrix material. However, compared toquantum dots grown on GaAs substrates the photolumines-cence intensity of the InP based quantum dots is weak. In-vestigation of the band structure and photoluminescence

FIG. 2. AFM picture �1 �m2� of 3 ML InAs grown on �a� GaInAs matrixmaterial without sublayer, �b� GaInAs matrix material with 2 ML GaAssublayer, �c� GaInAs matrix material with 2 ML GaSb sublayer, and �d�AlGaAsSb matrix material.

FIG. 3. AFM picture �1 �m2� of GaInAs matrix material with GaSb sub-layer: �a� 4, �b� 3.5, �c� 3, and �d� 2.3 ML InAs.

083111-2 Enzmann et al. Appl. Phys. Lett. 91, 083111 �2007�

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measurements with an electric field indicates that the nano-structures may have a type-II band alignment, but furtherinvestigations are necessary to clarify this.

In summary, we showed that the growth of InAs islandson GaInAs and AlInAs on InP substrates is possible by in-serting a 2 ML GaSb sublayer. By growing InAs quantumdots on the Indium-free and lattice matched material systemAlGaAsSb, we separated the influence of strain and of in-dium in the uppermost layers. This examination showed thatan indium-free growth surface is required for the formationof InAs quantum dots. Low dot densities down to 1 �m−2

could be reached. Shifting the emission wavelength of the

quantum dot by changing the matrix material is a promisingapproach to adjust the emission wavelength of quantum dots.Thus, low dot densities emitting at 1.55 �m on AlInAs orGaInAs are possible.

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FIG. 4. Photoluminescence of three samples. �a� Upper and lower matrix:AlInAs; �b� upper matrix: AlInAs and lower matrix: GaInAs; and �c� upperand lower matrix: GaInAs.

083111-3 Enzmann et al. Appl. Phys. Lett. 91, 083111 �2007�

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