Self-assembled quantum dots are typically fabricated from compressive-strained material systems, e.g., InAs on GaAs. In this letter, self-assembled quantum dots from tensile-strained GaAsN on InP are demonstrated. GaAsN on InP has type-I band alignment. Stranski-Krastanov growth mode is not observed, but in situannealing of the uncapped samples results in the formation of islands. Photoluminescence spectra from the buried GaAsN show separate peaks due to a wetting layer and islands around the energies of 1.3 and 1.1eV, respectively.Peer reviewe

Hakkarainen, T.

Koskenvaara, H.

Lipsanen, Harri

Pohjola, P.

Sainio, J.

Sopanen, Markku

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This is an electronic reprint of the original article.This reprint may differ from the original in pagination and typographic detail.Author(s): Pohjola, P. & Hakkarainen, T. & Koskenvaara, H. & Sopanen, M. &Lipsanen, Harri & Sainio, J.Title: Tensile-strained GaAsN quantum dots on InPYear: 2007Version: Final published versionPlease cite the original version:Pohjola, P. & Hakkarainen, T. & Koskenvaara, H. & Sopanen, M. & Lipsanen, Harri &Sainio, J. 2007. Tensile-strained GaAsN quantum dots on InP. Applied Physics Letters.Volume 90, Issue 17. P. 172110/1-3. ISSN 0003-6951 (printed). DOI:10.1063/1.2719662.Rights: © 2007 American Institute of Physics. This article may be downloaded for personal use only. Any other userequires prior permission of the author and the American Institute of Physics.http://scitation.aip.org/content/aip/journal/japAll material supplied via Aaltodoc is protected by copyright and other intellectual property rights, andduplication or sale of all or part of any of the repository collections is not permitted, except that material maybe duplicated by you for your research use or educational purposes in electronic or print form. You mustobtain permission for any other use. Electronic or print copies may not be offered, whether for sale orotherwise to anyone who is not an authorised user.Powered by TCPDF (www.tcpdf.org)Tensile-strained GaAsN quantum dots on InPP. Pohjola, T. Hakkarainen, H. Koskenvaara, M. Sopanen, H. Lipsanen, and J. Sainio  Citation: Applied Physics Letters 90, 172110 (2007); doi: 10.1063/1.2719662 View online: http://dx.doi.org/10.1063/1.2719662 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/90/17?ver=pdfcov Published by the AIP Publishing  Articles you may be interested in Anomalous temperature dependence of photoluminescence in self-assembled InGaN quantum dots Appl. Phys. Lett. 101, 131101 (2012); 10.1063/1.4754533  Electron delocalization of tensily strained GaAs quantum dots in GaSb matrix J. Appl. Phys. 108, 123503 (2010); 10.1063/1.3520669  Growth of InP self-assembled quantum dots on strained and strain-relaxed In x ( Al 0.6 Ga 0.4 ) 1 − x P matricesby metal-organic chemical vapor deposition J. Appl. Phys. 100, 043511 (2006); 10.1063/1.2244519  Stranski-Krastanov Ga N ∕ Al N quantum dots grown by metal organic vapor phase epitaxy J. Appl. Phys. 99, 083509 (2006); 10.1063/1.2189975  Temporal evolution of GaSb/GaAs quantum dot formation Appl. Phys. Lett. 79, 1027 (2001); 10.1063/1.1394715    This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:130.233.216.27 On: Mon, 04 May 2015 08:56:25Tensile-strained GaAsN quantum dots on InPP. Pohjola, T. Hakkarainen, H. Koskenvaara,a M. Sopanen, and H. LipsanenMicro- and Nanosciences Laboratory, Micronova, Helsinki University of Technology, Tietotie 3, EspooFIN-02015 HUT, FinlandJ. SainioLaboratory of Physics, Helsinki University of Technology, Otakaari 1, Espoo FIN-02015 HUT, FinlandReceived 18 December 2006; accepted 2 March 2007; published online 26 April 2007Self-assembled quantum dots are typically fabricated from compressive-strained material systems,e.g., InAs on GaAs. In this letter, self-assembled quantum dots from tensile-strained GaAsN on InPare demonstrated. GaAsN on InP has type-I band alignment. Stranski-Krastanov growth mode is notobserved, but in situ annealing of the uncapped samples results in the formation of islands.Photoluminescence spectra from the buried GaAsN show separate peaks due to a wetting layer andislands around the energies of 1.3 and 1.1 eV, respectively. © 2007 American Institute of Physics.DOI: 10.1063/1.2719662The Stranski-Krastanov SK growth of coherentlystrained islands has been under extensive study during thelast two decades. In this growth mode a wetting layer andcoherently strained three-dimensional islands are formedduring the growth process. The research has been concen-trated on compressively strained material systems such asGe/Si,1 InAs/ InP,2 InP/GaInP,3 InGaAs/GaAs,4InP/GaAs,5 and GaInNAs/GaAs.6 Also tensile three-dimensional islands have been fabricated from, e.g., PbSe onPbTe.7 The formation of islands is not, however, always hap-pening in tensile-strained growth: when growingGa0.75In0.25As on InP only holes were observed.8 Anotherproblem with tensile-strained systems is that tensile islandshave typically larger band gap than the host material. There-fore, these islands cannot confine carriers and are not quan-tum dots. However, at least one tensile-strained system, i.e.,GaAs1−xNx on InP, theoretically has type-I band alignment atcertain nitrogen compositions and can confine both electronsand holes. GaAs/ InP heterojunction on InP substrate has atype-II band alignment in which only holes are confined inGaAs. The conduction and valence band offset energies areapproximately 172 and 550 meV, respectively.9 Because theincorporation of nitrogen reduces the band gap of GaAsNRefs. 10 and 11 by lowering the conduction band edge,12 itis expected that the transition to type-I interface would occurwith a nitrogen composition of about x0.013. In this letter,we present growth studies of the GaAsN-on-InP system witha goal to achieve type-I quantum dot structures.The sample growth was carried out in a horizontalmetal-organic vapor-phase epitaxy MOVPE reactor at at-mospheric pressure using hydrogen as carrier gas. The pre-cursors were trimethylindium, trimethylgallium, tertiarybu-tylphosphine TBP, tertiarybutylarsine TBAs, anddimethylhydrazine DMHy. First, a 100-nm-thick InP bufferlayer was grown at 610 °C on a vicinal semi-insulating InP100 substrate. After the growth of the buffer layer, the tem-perature was lowered under TBP flow to the growth tempera-ture of the GaAsN layer. The GaAsN layers were depositedat different temperatures varied in the range of 530–610 °C.The TBAs/III and DMHy/V ratios were 2 and 0.92, respec-tively. With these flows the DMHy/TBAs ratio correspondsto a nitrogen composition of 5.5% in the layer in thisreactor.10 All the growth temperatures mentioned are thermo-couple readings and all the layer thicknesses are nominaldeposition thicknesses.To study surface morphology by atomic force micros-copy AFM, part of the samples were left uncapped. Someof the uncapped samples were cooled down to room tem-perature directly after the growth of the GaAsN layer. For theother uncapped samples, a thermal annealing was performedby raising the temperature to 610 °C for 20 s under TBAsand DMHy partial pressure, after which the samples werecooled down. X-ray photoelectron spectroscopy XPS mea-surements were performed on the uncovered GaAsN layersto investigate if nitrogen had been incorporated.The rest of the samples were fabricated for optical mea-surements. The GaAsN layer was capped with an InP barrierlayer grown at 610 °C. The optical characterization was per-formed with low temperature photoluminescence LT-PL at9 K. The 488 nm line of an argon ion laser was used forexcitation. The luminescence light was dispersed with a0.5 m monochromator and a liquid-nitrogen-cooled germa-nium detector was used for detection.First, for reference, thin layers 1-6 ML of GaAs weregrown on the InP buffer layer. The TBAs/III ratio was variedin the range of 2-20 to study the effect of As/P exchange onthe surface morphology. The V/III ratio of 2 was found togive the smoothest initial surface indicating the smallestAs/P exchange. This TBAs/III ratio also gives a good incor-poration of nitrogen into GaAsN on GaAs and it is also theminimum for high quality GaAsN.10 The growth rate wasvaried between 2.8 and 8.5 Å/s, which corresponds to1–3 ML/s for GaAs. The growth rate of 5.0 Å/s was cho-sen because no significant differences were observed.Next, the growth of GaAsN was investigated by depos-iting 6 ML of GaAsN on InP at different temperatures be-tween 530 and 610 °C. At the temperatures of 530–580 °Cno islands were formed. At 610 °C few islands were formedbut the material had otherwise accumulated to wirelike struc-tures. Therefore, clear SK growth mode could not be ob-served. Furthermore, the growth temperature of 610 °C isaAuthor to whom correspondence should be addressed; FAX: 358-9-4513128; electronic mail: hannu.koskenvaara@tkk.fiAPPLIED PHYSICS LETTERS 90, 172110 20070003-6951/2007/9017/172110/3/$23.00 © 2007 American Institute of Physics90, 172110-1 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:130.233.216.27 On: Mon, 04 May 2015 08:56:25not very attractive to obtain type-I GaAsN/InP quantum dotsbecause the nitrogen composition has been found to decreaseradically with increasing growth temperature.10,13It has been found out that thermal annealing can be usedto enhance island formation.14 To both optimize the nitrogenconcentration and to form islands, the samples were grown at530 °C with varying GaAsN deposition thicknesses 3-6ML and then were in situ annealed at 610 °C for 20 s. Fig-ure 1 shows AFM images from the annealed samples. Thesample with the 3-ML-thick GaAsN layer a has large two-dimensional islands with a height of 1-2 ML. When 4 ML ofGaAsN is deposited and annealed b, three-dimensional3D islands start to form. The areal density and the averageheight of the islands are 4109 cm−2 and 3.5 nm, respec-tively. The deposition of 5 ML c leads to 3D islands with alarger density of 3.81010 cm−2 and an average height of4.0 nm. When 6 ML is deposited and annealed d, the shapeof the larger islands becomes unsymmetrical. This is charac-teristic for islands that are relaxed.3,15,16 However, the largerislands are quite small, mainly 8–12 nm in height, for relax-ation. For comparison, compressively strained InP islands onInGaP lattice matched to GaAs 001 have been found to bedefect-free up to heights of 22.5 nm.3 The absolute value ofthe relative lattice mismatch a /a is almost the same in bothof these cases. To determine whether the GaAsN islands arerelaxed or not, further studies are needed.Figure 2 shows the effect of the deposition thickness onthe areal density a and average height b of the 3D islands.The 3D islands have been divided into smaller type-A height7 nm and larger type-B height 7 nm islands. Accord-ing to Fig. 2a the critical thickness for 3D island formationis somewhere between 3 and 4 ML. The areal density of thesmaller islands increases from 4.0109 to 3.81010 cm−2when the deposition thickness is increased from 4 to 5 ML.The average height of the small islands is about 4.0 nm. Inthe case of 6 ML, the areal density decreases to 1.51010 cm−2 because of island coalescence. At the same timethe areal density of the larger islands increases to 1.51010 cm−2. The average height of the large islands is9.9 nm. This kind of behavior has been reported, e.g., for SKInAs islands on InP.2Figure 3 shows LT-PL spectra of buried GaAsN layerswith different nominal thicknesses. The peaks in the energyrange of 1.25–1.37 eV seem to originate from the GaAsNwetting layer WL. The multiple peaks in the 3 ML samplecorrespond to different areas of the WL having 1 ML differ-ence in thickness. These 1 ML fluctuations are also observedin the AFM image of the uncovered layer in Fig. 1a. Withthe coverages of 4 and 5 ML, the critical thickness is reachedand islands are formed. The luminescence peak centered at1.05 eV originates from the islands. When the depositionthickness is increased to 6 ML, the PL intensity clearly de-creases. This can be explained by enhancement of the non-radiative recombination due to dislocation formation associ-ated with the relaxation of the type-B islands. In addition, thewetting layer luminescence begins to dominate.16 To studyeven thicker layers of GaAsN, two samples with coveragesof 10 and 20 ML were grown. Luminescence peaks originat-ing from the islands were not observed at all from thesesamples.To examine the nature of the radiative transitions ob-served in the GaAsN samples, a GaAs reference sample wasgrown. The reference sample had a 6-ML-thick layer ofGaAs buried between InP layers. Figure 4 shows the LT-PLspectra from this sample and a GaAsN sample grown withthe DMHy/V ratio of 0.92. WL peaks at about 1.34 eV canbe seen in both samples. In the GaAs reference sample, thepeak results from a type-II transition between the InP con-FIG. 1. AFM images of GaAsN layers with thicknesses of a 3 ML, b 4ML, c 5 ML, and d 6 ML deposited on InP at the temperature of 530 °C.The samples were annealed in situ at 610 °C for 20 s. The scan size is 11 m2.FIG. 2. a Areal density and b average height of the GaAsN islands as afunction of coverage. A and B stand for islands that are 7 and 7 nmhigh, respectively.FIG. 3. LT-PL spectra from the samples with various thicknesses of buriedGaAsN layers on InP.172110-2 Pohjola et al. Appl. Phys. Lett. 90, 172110 2007 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:130.233.216.27 On: Mon, 04 May 2015 08:56:25duction band and the GaAs WL quantum well QW. In theGaAsN sample the wetting layer peak is approximately at thesame wavelength as in the GaAs sample. This can be ex-plained by the fact that nitrogen modifies mainly the conduc-tion band and does not have a noticeable effect on the va-lence band.12 The band structure and the energy states of theGaAs/ InP and GaAsN/InP quantum wells are shown in theinset of Fig. 4. Energy of the type-II WL peak is close to theband gap of InP because the carrier states in a very narrowQW are very near to the barrier potential. Several WL QWpeaks result from the monolayer thickness variations in theWL.In the LT-PL of the GaAs sample, a broad peak centeredat 1.28 eV with a full width at half maximum FWHM of162 meV is observed. In the GaAsN sample a peak related toislands is centered at 1.1 eV and has a FWHM of 82 meV.Narrower and more intense peak in the GaAsN sample sug-gests that there may be a type-I transition in GaAsN quantumdot QD, whereas type-II transition produces a weaker andbroader peak in the GaAs sample. Also the wetting layerpeak is strong in GaAsN sample. This can be explained bythe low confinement potential in the GaAsN conduction bandof the GaAsN QDs. Thermal excitation throws electrons outfrom the shallow QD potential well and they recombine inthe WL QW.The energetic positions of the peaks obtained from theGaAsN samples do not correspond to the values calculatedfrom a simple potential well model. The calculated values forthe assumed nitrogen composition of x=0.055 are somewhatlower. To investigate if any mixing of materials has occurredand if any nitrogen has really been incorporated, an un-capped 6-ML-thick GaAsN sample was investigated by XPS.XPS results show that there are nitrogen, phosphorus, andindium on the top atomic layers of the sample. This indicatesthat nitrogen has really been incorporated into the GaAsNlayer. It is also possible that mixing of materials in the inter-face of GaAsN and InP layers is occurring. This cannot beasserted for sure because the thickness of the wetting layer isnot known. Therefore, XPS signal could be partly originatedfrom the buffer layer between the islands. However, mixingof materials does explain quite naturally the reason for theobserved difference in the measured and calculated positionsof the PL peaks.In summary, tensile-strained thin GaAsN layers weregrown on InP by MOVPE. Stranski-Krastanov growth modewas not observed, but in situ annealing of the uncappedsamples at 610 °C resulted in the formation of islands. Thecritical thickness for island formation was found to be be-tween 3 and 4 ML. The LT-PL spectra of GaAsN samplesshowed separate peaks due to WL and islands around theenergies of 1.3 and 1.1 eV, respectively. The PL and XPSresults also indicate that nitrogen had been incorporated intothe layers and that some mixing between GaAsN and InPmay have occurred.1D. J. Eaglesham and M. Cerullo, Phys. Rev. Lett. 64, 1943 1990.2M. Taskinen, M. Sopanen, H. Lipsanen, J. Tulkki, and T. Tuomi, and J.Ahopelto, Surf. Sci. 376, 60 1997.3S. P. DenBaars, C. M. Reaves, V. Bressler-Hill, S. Varma, W. H. Weinberg,and P. M. Petroff, J. Cryst. Growth 145, 721 1994.4D. Leonard, M. Krishnamurthy, C. M. Reaves, S. P. DenBaars, and P. M.Petroff, Appl. Phys. Lett. 63, 3203 1993.5M. Sopanen, H. Lipsanen, and J. Ahopelto, Appl. Phys. Lett. 67, 37681995.6M. Sopanen, H. P. Xin, and C. W. Tu, Appl. Phys. Lett. 76, 994 2000.7M. Pinczolits, G. Springholz, and G. Bauer, Appl. Phys. Lett. 73, 2501998.8P. Krapf, Y. Robach, M. Gendry, and L. Porte, Phys. Rev. B 55, R102291997.9D. Gershoni, H. Temkin, J. M. Vandenberg, S. N. G. Chu, R. A. Hamm,and M. B. Panish, Phys. Rev. Lett. 60, 448 1988.10J. Toivonen, T. Hakkarainen, M. Sopanen, and H. Lipsanen, J. Cryst.Growth 221, 456 2000.11M. Weyers, M. Sato, and H. Ando, Jpn. J. Appl. Phys., Part 2 31, L8531992.12W. Shan, W. Walukiewicz, J. W. Ager III, E. E. Haller, J. F. Geisz, D. J.Friedman, J. M. Olson, and S. R. Kurtz, J. Appl. Phys. 86, 2349 1999.13A. Ougazzaden, Y. Le Bellego, E. V. K. Rao, M. Juhel, L. Leprince, andG. Patriarche, Appl. Phys. Lett. 70, 2861 1997.14J. F. Carlin, R. Houdré, A. Rudra, and M. Ilegems, Appl. Phys. Lett. 59,3018 1991.15S. Guha, A. Madhukar, and K. C. Rajkumar, Appl. Phys. Lett. 57, 21101990.16H. Li, T. Daniels-Race, and Z. Wang, Appl. Phys. Lett. 74, 1388 1999.FIG. 4. LT-PL spectra of samples with a 6-ML-thick buried layer of aGaAsN grown with the DMHy/V ratio of 0.92 and b GaAs. Energy bandstructures of the GaAs/ InP and the GaNAs/InP QWs and respective tran-sitions are shown in an inset.172110-3 Pohjola et al. Appl. Phys. Lett. 90, 172110 2007 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:130.233.216.27 On: Mon, 04 May 2015 08:56:25

Tensile-strained GaAsN quantum dots on InP

P. Pohjola

T. Hakkarainen

H. Koskenvaara

M. Sopanen

H. Lipsanen

J. Sainio

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Tensile-strained GaAsN quantum dots on InP

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