We demonstrate that molecular beam epitaxy-grown InAs quantum dots (QDs) on (100) GaAs obtained by conversion of In nanocrystals enable long wavelength emission in the InAs/GaAs material system. At room temperature they exhibit a broad photoluminescence band that extends well beyond 1.55 mu m. We correlate this finding with cross-sectional scanning tunneling microscopy measurements. They reveal that the QDs are composed of pure InAs which is in agreement with their long-wavelength emission. Additionally, the measurements reveal that the QDs have an anomalously undulated top surface which is very different to that observed for Stranski-Krastanow grown QDs

Keizer, J.G.

Koenraad, P.M.

Nötzel, R.

Urbanczyk, A.J.

English

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 Long wavelength (> 1.55 mu m) room temperature emissionand anomalous structural properties of InAs/GaAs quantumdots obtained by conversion of In nanocrystalsCitation for published version (APA):Urbanczyk, A. J., Keizer, J. G., Koenraad, P. M., & Nötzel, R. (2013). Long wavelength (> 1.55 mu m) roomtemperature emission and anomalous structural properties of InAs/GaAs quantum dots obtained by conversionof In nanocrystals. Applied Physics Letters, 102(7), 073103-1/4. Article 073103.https://doi.org/10.1063/1.4792700DOI:10.1063/1.4792700Document status and date:Published: 01/01/2013Document Version:Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers)Please check the document version of this publication:• A submitted manuscript is the version of the article upon submission and before peer-review. There can beimportant differences between the submitted version and the official published version of record. Peopleinterested in the research are advised to contact the author for the final version of the publication, or visit theDOI to the publisher's website.• The final author version and the galley proof are versions of the publication after peer review.• The final published version features the final layout of the paper including the volume, issue and pagenumbers.Link to publicationGeneral rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.            • Users may download and print one copy of any publication from the public portal for the purpose of private study or research.            • You may not further distribute the material or use it for any profit-making activity or commercial gain            • You may freely distribute the URL identifying the publication in the public portal.If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license above, pleasefollow below link for the End User Agreement:www.tue.nl/taverneTake down policyIf you believe that this document breaches copyright please contact us at:openaccess@tue.nlproviding details and we will investigate your claim.Download date: 04. Oct. 2023Long wavelength (>1.55μm) room temperature emission and anomalousstructural properties of InAs/GaAs quantum dots obtained by conversion ofIn nanocrystalsA. Urbańczyk, J. G. Keizer, P. M. Koenraad, and R. Nötzel  Citation: Appl. Phys. Lett. 102, 073103 (2013); doi: 10.1063/1.4792700 View online: http://dx.doi.org/10.1063/1.4792700 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v102/i7 Published by the AIP Publishing LLC.  Additional information on Appl. Phys. Lett.Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors Downloaded 27 Jun 2013 to 131.155.151.167. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissionsLong wavelength (>1.55 lm) room temperature emissionand anomalous structural properties of InAs/GaAsquantum dots obtained by conversion of In nanocrystalsA. Urbanczyk,1,a) J. G. Keizer,1 P. M. Koenraad,1 and R. N€otzel21COBRA Research Institute on Communication Technology, Department of Applied Physics,Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands2Institute for Systems Based on Optoelectronics and Microtechnology (ISOM), ETSI Telecommunication,Technical University of Madrid, Ciudad Universitaria s/n, 28040 Madrid, Spain(Received 28 November 2012; accepted 5 February 2013; published online 19 February 2013)We demonstrate that molecular beam epitaxy-grown InAs quantum dots (QDs) on (100) GaAsobtained by conversion of In nanocrystals enable long wavelength emission in the InAs/GaAsmaterial system. At room temperature they exhibit a broad photoluminescence band that extendswell beyond 1.55 lm. We correlate this finding with cross-sectional scanning tunneling microscopymeasurements. They reveal that the QDs are composed of pure InAs which is in agreement withtheir long-wavelength emission. Additionally, the measurements reveal that the QDs have ananomalously undulated top surface which is very different to that observed for Stranski-Krastanowgrown QDs. VC 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4792700]Molecular beam epitaxy (MBE) growth of InAs/GaAsbased materials is a very well established technology capableof producing high-quality low-dimensional structures suchas quantum wells and quantum dots (QDs) which constituteessential building blocks of modern optoelectronic devices.One of the long pursued research goals is the extension ofthe QD emission wavelength up to or even beyond 1.55 lm,where the absorption of silica optical fibers is lowest. Thiswould allow for a cheaper alternative to the presently usedInP-based devices for long-distance optical communication.The QDs are usually obtained by the Stranski-Krastanow(SK) growth mode. It is well known that at typical QDgrowth temperatures significant mass transport occurs result-ing in pronounced In/Ga intermixing.1 This increases theband gap energy and, hence, is clearly not desirable whenlong-wavelength emission is to be obtained. Intermixing canbe minimized by growth at very low temperature,2 however,at the cost of introducing non-radiative recombination cen-ters, degrading the optical quality of the QDs. Anotherapproach to achieve long emission wavelengths is embed-ding the QDs in InGaAs strain reducing layers.3–5 But thisreduces the depth of the confinement potential havingadverse effects on the temperature characteristics of laserscontaining such structures.6 Therefore, we consider an alter-native approach to grow high In-composition QDs which isderived from droplet epitaxy (DE).7–9 DE is an inherentlylow growth temperature process made possible by the decou-pling of the group III and group V fluxes. We recently foundthat in order to obtain small and uniform InAs QDs follow-ing DE, it is actually necessary to begin with solid In nano-crystals (NCs)9 and not with liquid droplets. In this case theinitial exposure to As4 flux and conversion process takeplace when the NCs are still solid, allowing in principle forextremely In-rich QDs due to the lack of intermixing withthe substrate. Obviously some intermixing does occur duringthe necessary subsequent annealing and capping steps; how-ever, it can be minimized when those steps are performed atas low as possible temperature.Here we report the growth of InAs/GaAs QDs by con-version of epitaxial, solid In NCs that emit over a widewavelength range extending beyond 1.55 lm. This result iscomplemented with atomic force microscopy (AFM) investi-gations of the morphology of uncapped QDs, as well asmeasurements of the shape and composition of buried QDsby means of cross-sectional scanning tunneling microscopy(XSTM) with atomic-scale resolution. The latter measure-ments reveal that the QDs are composed of pure InAs andhave a complex undulated top surface which is qualitativelydifferent to that of SK-grown QDs. The analysis of both theAFM and XSTM data reveals that there is a significantchange of the QD shape and large material redistributionduring the capping process.All samples were grown by solid-source MBE on(100)-oriented GaAs substrates. The growth commenced witha 100-nm-GaAs buffer layer at 580 C with As4 beam-equivalent pressure of 5 106 mbar. The samples weresubsequently cooled down to 450 C, and the As4 flux wasswitched off. This established a (2 4) surface reconstruction,monitored by reflection high-energy electron diffraction(RHEED), which was frozen-in during cooling down to 80 C(thermocouple reading). After 30 min temperature stabilization0.5 nm In was deposited at a growth rate of 0.045 nm/s inorder to obtain In NCs. The background pressure during thatprocess was lower than 2 109 mbar allowing the (2 4)surface reconstruction to be preserved during the long temper-ature stabilization step. Then the As flux was turned on, andthe substrate temperature was increased to 450 C at a rate of25 C per minute. After additional 5 min annealing the QDswere capped with 20-nm-GaAs at the same temperature.Next, the temperature was further increased to 580 C and180-nm-GaAs was deposited. For surface morphology studiesa)Author to whom correspondence should be addressed. Electronic mail:adam.jan.urbanczyk@gmail.com.0003-6951/2013/102(7)/073103/3/$30.00 VC 2013 American Institute of Physics102, 073103-1APPLIED PHYSICS LETTERS 102, 073103 (2013)Downloaded 27 Jun 2013 to 131.155.151.167. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissionsby AFM (tapping-mode in air) a sample was grown under thesame growth conditions without capping. The structure andcomposition of the capped QDs were accessed by XSTMat room temperature (RT) with atomic-scale resolution.10,11The STM was operated in constant current mode (I¼ 40 pA)on in situ cleaved (011)-surfaces at negative bias voltage(Ubias¼3 V). Under such conditions InAs (GaAs) richregions appear bright (dark) in the topographic XSTM images.The optical properties of the capped QDs were investigated byRT photoluminescence (PL) spectroscopy. A green 532-nmsolid-state laser was used for excitation. The PL was dispersedby a quarter-meter monochromator and detected by a liquidnitrogen cooled InGaAs photodiode array detector.Figure 1(a) shows the AFM image of the uncappedQDs, and Fig. 1(b) presents a histogram of the measured QDheights. The QDs are dome-shaped with density of 160 perlm2, which is a moderate value despite the low depositiontemperature of the In NCs. This is the consequence of highIn ad-atom mobility on GaAs in the absence of As flux.12 Asevident from Fig. 1(b) the average QD height is 6.5 nm, andthe half-width of the height distribution equals to 4 nm. Suchlarge heights are maintained due to the relatively short andlow-temperature annealing.Most relevant, however, is the final QD shape and com-position after capping, which is known to cause significantmaterial redistribution to flatten the QDs.13 Figure 2(a)shows the XSTM image of an individual QD after cappingwith GaAs. The QD is on average 3.5 nm high and 15 nmwide and has an undulated top surface with height maximain the center and at the periphery. This is in distinct contrastto SK-grown QDs which maintain a lens or truncated pyra-mid like shape.13,14 Such modulated QD shape is commonlyfound in the studied sample, as shown in Fig. 2(b). More-over, the QDs are composed of pure InAs. There is no signa-ture of individual Ga atoms inside the QDs which wouldappear as dark spots. This is in agreement with the initiallow-temperature growth of the In NCs and their immediateconversion to InAs QDs, preventing intermixing with theGaAs substrate as well as etching into the underlying layerwhich happens for GaAs/AlGaAs QDs grown by dropletepitaxy at higher temperatures.15We attribute the anomalously undulated shape of theburied QDs to significant morphological changes occurringduring the capping process. Such changes are evident fromthe large difference in height of the QDs observed withXSTM compared to the average height of the surface QDsinvestigated by AFM. A possible explanation of such behav-ior is as follows: due to the pure In content, the freshly con-verted QDs are highly strained, preventing completeovergrowth to leave the central part of the QDs exposed dur-ing the initial capping. This induces significant materialredistribution during heating up for the final thick GaAslayer growth at 580 C. At this temperature, the partiallyexposed top of the QDs readily decomposes and In desorbs,leaving the final buried QDs greatly flattened. Similar to thecase of SK-grown QDs, and intentionally enhanced by the socalled In-flush technique,16 this process certainly takes placeto reduce the overall height of the QDs, but does not explainthe modulated shape of our QDs. Therefore, we believe thatthe final shape of the QDs is additionally determined by theformation process of the QDs itself. For QDs formed by DEit has been found that the recrystallization process startsfrom the side of the droplets, proceeding to the top. ThisFIG. 1. (a) AFM image of uncapped InAs QDs and (b) histogram of the QDheights. The AFM scan area is 1 1 lm2 and the full height contrast is 12 nm.FIG. 2. (a) XSTM image of an individual InAs QD obtained by conversionof an In NC. (b) XSTM image of two InAs QDs confirming the characteris-tic wavy shape. The dark contrast on the right side visible in (a) is an artifactdue to the presence of a step edge.073103-2 Urbanczyk et al. Appl. Phys. Lett. 102, 073103 (2013)Downloaded 27 Jun 2013 to 131.155.151.167. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissionssuggests a better crystal quality at the side and, hence, higherstability during overgrowth. Together with the initially high-est part in the center this might lead to a buried QD with theobserved shape, i.e., highest in the center and the periphery.Of course, other mechanisms leading to undulated topogra-phy of strained layers should not be excluded.17,18The result of the RT PL measurement is shown inFig. 3. The QDs exhibit a very broad emission band withintensity maximum at 1285 nm and half-width of 320 nm.Such a large inhomogeneous broadening is most likely aconsequence of the broad distribution of initial QD heightsand the complex overgrowth process. What is most impor-tant, the emission extends significantly beyond 1.55 lm,even beyond the detector cut-off at 1.6 lm, notably withoutthe need for strain reducing layers. This long emission wave-length is mainly attributed to the QDs being composed ofpure InAs which has proven to be difficult to obtain forSK-grown QDs of high optical quality. Along these lines,further optimization of the QDs for specific applications isstraight-forward due to the excellent control of size anddensity of the epitaxial In NCs,19 much better than thecontrol achievable for directly grown SK QDs.To conclude, we have demonstrated room-temperaturelong-wavelength emission beyond 1.55 lm of InAs QDs onGaAs obtained by conversion of epitaxial In NCs. We alsopresented the results of AFM and XSTM measurements forstructural analysis. These measurements showed that theinvestigated QDs are composed of pure InAs and exhibit anundulated top surface, very different to that of SK-grownQDs, which has been related to the distinct QD formationprocess. We believe that the presented findings are a step-ping stone for the usage of the InAs/GaAs material systemfor optoelectronic devices operating in the second telecom-munication window and beyond. Furthermore, we believethat our findings can be directly applied to other materialsystems, such as InAs/InP, for extending the emission intothe mid-infrared.The authors would like to acknowledge the technicalsupport of F.W.M. van Otten in the growth of the samples.1P. B. Joyce, T. J. Krzyzewski, G. R. Bell, B. A. Joyce, and T. S. Jones,Phys. Rev. B 58, R15981 (1998).2M. V. Maximov, A. F. Tsatsul’nikov, B. V. Volovik, D. A. Bedarev, A. Y.Egorov, A. E. Zhukov, A. R. Kovsh, N. A. Bert, V. M. Ustinov, P. S.Kop’ev, Z. I. Alferov, N. N. Ledentsov, D. Bimberg, I. P. Soshnikov, andP. Werner, Appl. Phys. Lett. 75, 2347 (1999).3E. C. Le Ru, P. Howe, T. S. Jones, and R. Murray, Phys. Rev. B 67,165303 (2003).4J. Tatebayashi, M. Nishioka, and Y. Arakawa, Appl. Phys. Lett. 78, 3469(2001).5B. Alloing, C. Zinoni, V. Zwiller, L. H. Li, C. Monat, M. Gobet, G. Buchs,A. Fiore, E. Pelucchi, and E. Kapon, Appl. Phys. Lett. 86, 101908 (2005).6Y. Q. Wei, S. M. Wang, F. Ferdos, J. Vukusic, A. Larsson, Q. X. Zhao,and M. Sadeghi, Appl. Phys. Lett. 81, 1621 (2002).7T. Chikyow and N. Koguchi, Jpn. J. Appl. Phys. 29, L2093 (1990).8J. S. Kim and N. Koguchi, Appl. Phys. Lett. 85, 5893 (2004).9A. Urbanczyk, G. J. Hamhuis, and R. N€otzel, J. Appl. Phys. 107, 014312(2010).10E. T. Yu, MRS Bull. 22, 22 (1997).11B. Z. Nosho, W. Barvosa-Carter, M. J. Yang, B. R. Bennett, and L. J.Whitman, Surf. Sci. 465, 361 (2000).12Y. Horikoshi, Semicond. Sci. Technol. 8, 1032 (1993).13Q. Gong, P. Offermans, R. N€otzel, P. M. Koenraad, and J. H. Wolter,Appl. Phys. Lett. 85, 5697 (2004).14D. Litvinov, D. Gerthsen, A. Rosenauer, M. Schowalter, T. Passow,P. Fein€augle, and M. Hetterich, Phys. Rev. B 74, 165306 (2006).15J. G. Keizer, J. Bocquel, P. M. Koenraad, T. Mano, T. Noda, andK. Sakoda, Appl. Phys. Lett. 96, 062101 (2010).16Z. Wasilewski, S. Fafard, and J. McCaffrey, J. Cryst. Growth 201–202,1131 (1999).17D. J. Seol, S. Y. Hu, Z. K. Liu, L. Q. Chen, S. G. Kim, and K. H. Oh,J. Appl. Phys. 98, 044910 (2005).18W. Barvosa-Carter, M. J. Aziz, L. J. Gray, and T. Kaplan, Phys. Rev. Lett.81, 1445 (1998).19A. Urbanczyk and R. N€otzel, J. Cryst. Growth 341, 24 (2012).FIG. 3. RT PL spectrum of the InAs QDs. The detector cut-off is at 1.6 lm.073103-3 Urbanczyk et al. Appl. Phys. Lett. 102, 073103 (2013)Downloaded 27 Jun 2013 to 131.155.151.167. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions

Long wavelength (&gt; 1.55 mu m) room temperature emission and anomalous structural properties of InAs/GaAs quantum dots obtained by conversion of In nanocrystals

Urbanczyk, AJ Adam

Keizer, JG Joris

Koenraad, PM Paul

Nötzel, R Richard

Repository TU/e

NARCIS 

Long wavelength (> 1.55 mu m) room temperature emission and anomalous structural properties of InAs/GaAs quantum dots obtained by conversion of In nanocrystals

A. Urbańczyk

J. G. Keizer

P. M. Koenraad

R. Nötzel

Crossref

Applied Physics Letters

Long wavelength (&gt;1.55 μm) room temperature emission and anomalous structural properties of InAs/GaAs quantum dots obtained by conversion of In nanocrystals

 Long wavelength (> 1.55 mu m) room temperature emissionand anomalous structural properties of InAs/GaAs quantumdots obtained by conversion of In nanocrystalsCitation for published version (APA):Urbanczyk, A. J., Keizer, J. G., Koenraad, P. M., & Nötzel, R. (2013). Long wavelength (> 1.55 mu m) roomtemperature emission and anomalous structural properties of InAs/GaAs quantum dots obtained by conversionof In nanocrystals. Applied Physics Letters, 102(7), 073103-1/4. Article 073103.https://doi.org/10.1063/1.4792700DOI:10.1063/1.4792700Document status and date:Published: 01/01/2013Document Version:Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers)Please check the document version of this publication:• A submitted manuscript is the version of the article upon submission and before peer-review. There can beimportant differences between the submitted version and the official published version of record. Peopleinterested in the research are advised to contact the author for the final version of the publication, or visit theDOI to the publisher's website.• The final author version and the galley proof are versions of the publication after peer review.• The final published version features the final layout of the paper including the volume, issue and pagenumbers.Link to publicationGeneral rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.            • Users may download and print one copy of any publication from the public portal for the purpose of private study or research.            • You may not further distribute the material or use it for any profit-making activity or commercial gain            • You may freely distribute the URL identifying the publication in the public portal.If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license above, pleasefollow below link for the End User Agreement:www.tue.nl/taverneTake down policyIf you believe that this document breaches copyright please contact us at:openaccess@tue.nlproviding details and we will investigate your claim.Download date: 17. Nov. 2023Long wavelength (>1.55μm) room temperature emission and anomalousstructural properties of InAs/GaAs quantum dots obtained by conversion ofIn nanocrystalsA. Urbańczyk, J. G. Keizer, P. M. Koenraad, and R. Nötzel  Citation: Appl. Phys. Lett. 102, 073103 (2013); doi: 10.1063/1.4792700 View online: http://dx.doi.org/10.1063/1.4792700 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v102/i7 Published by the AIP Publishing LLC.  Additional information on Appl. Phys. Lett.Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors Downloaded 27 Jun 2013 to 131.155.151.167. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissionsLong wavelength (>1.55 lm) room temperature emissionand anomalous structural properties of InAs/GaAsquantum dots obtained by conversion of In nanocrystalsA. Urbanczyk,1,a) J. G. Keizer,1 P. M. Koenraad,1 and R. N€otzel21COBRA Research Institute on Communication Technology, Department of Applied Physics,Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands2Institute for Systems Based on Optoelectronics and Microtechnology (ISOM), ETSI Telecommunication,Technical University of Madrid, Ciudad Universitaria s/n, 28040 Madrid, Spain(Received 28 November 2012; accepted 5 February 2013; published online 19 February 2013)We demonstrate that molecular beam epitaxy-grown InAs quantum dots (QDs) on (100) GaAsobtained by conversion of In nanocrystals enable long wavelength emission in the InAs/GaAsmaterial system. At room temperature they exhibit a broad photoluminescence band that extendswell beyond 1.55 lm. We correlate this finding with cross-sectional scanning tunneling microscopymeasurements. They reveal that the QDs are composed of pure InAs which is in agreement withtheir long-wavelength emission. Additionally, the measurements reveal that the QDs have ananomalously undulated top surface which is very different to that observed for Stranski-Krastanowgrown QDs. VC 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4792700]Molecular beam epitaxy (MBE) growth of InAs/GaAsbased materials is a very well established technology capableof producing high-quality low-dimensional structures suchas quantum wells and quantum dots (QDs) which constituteessential building blocks of modern optoelectronic devices.One of the long pursued research goals is the extension ofthe QD emission wavelength up to or even beyond 1.55 lm,where the absorption of silica optical fibers is lowest. Thiswould allow for a cheaper alternative to the presently usedInP-based devices for long-distance optical communication.The QDs are usually obtained by the Stranski-Krastanow(SK) growth mode. It is well known that at typical QDgrowth temperatures significant mass transport occurs result-ing in pronounced In/Ga intermixing.1 This increases theband gap energy and, hence, is clearly not desirable whenlong-wavelength emission is to be obtained. Intermixing canbe minimized by growth at very low temperature,2 however,at the cost of introducing non-radiative recombination cen-ters, degrading the optical quality of the QDs. Anotherapproach to achieve long emission wavelengths is embed-ding the QDs in InGaAs strain reducing layers.3–5 But thisreduces the depth of the confinement potential havingadverse effects on the temperature characteristics of laserscontaining such structures.6 Therefore, we consider an alter-native approach to grow high In-composition QDs which isderived from droplet epitaxy (DE).7–9 DE is an inherentlylow growth temperature process made possible by the decou-pling of the group III and group V fluxes. We recently foundthat in order to obtain small and uniform InAs QDs follow-ing DE, it is actually necessary to begin with solid In nano-crystals (NCs)9 and not with liquid droplets. In this case theinitial exposure to As4 flux and conversion process takeplace when the NCs are still solid, allowing in principle forextremely In-rich QDs due to the lack of intermixing withthe substrate. Obviously some intermixing does occur duringthe necessary subsequent annealing and capping steps; how-ever, it can be minimized when those steps are performed atas low as possible temperature.Here we report the growth of InAs/GaAs QDs by con-version of epitaxial, solid In NCs that emit over a widewavelength range extending beyond 1.55 lm. This result iscomplemented with atomic force microscopy (AFM) investi-gations of the morphology of uncapped QDs, as well asmeasurements of the shape and composition of buried QDsby means of cross-sectional scanning tunneling microscopy(XSTM) with atomic-scale resolution. The latter measure-ments reveal that the QDs are composed of pure InAs andhave a complex undulated top surface which is qualitativelydifferent to that of SK-grown QDs. The analysis of both theAFM and XSTM data reveals that there is a significantchange of the QD shape and large material redistributionduring the capping process.All samples were grown by solid-source MBE on(100)-oriented GaAs substrates. The growth commenced witha 100-nm-GaAs buffer layer at 580 C with As4 beam-equivalent pressure of 5 106 mbar. The samples weresubsequently cooled down to 450 C, and the As4 flux wasswitched off. This established a (2 4) surface reconstruction,monitored by reflection high-energy electron diffraction(RHEED), which was frozen-in during cooling down to 80 C(thermocouple reading). After 30 min temperature stabilization0.5 nm In was deposited at a growth rate of 0.045 nm/s inorder to obtain In NCs. The background pressure during thatprocess was lower than 2 109 mbar allowing the (2 4)surface reconstruction to be preserved during the long temper-ature stabilization step. Then the As flux was turned on, andthe substrate temperature was increased to 450 C at a rate of25 C per minute. After additional 5 min annealing the QDswere capped with 20-nm-GaAs at the same temperature.Next, the temperature was further increased to 580 C and180-nm-GaAs was deposited. For surface morphology studiesa)Author to whom correspondence should be addressed. Electronic mail:adam.jan.urbanczyk@gmail.com.0003-6951/2013/102(7)/073103/3/$30.00 VC 2013 American Institute of Physics102, 073103-1APPLIED PHYSICS LETTERS 102, 073103 (2013)Downloaded 27 Jun 2013 to 131.155.151.167. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissionsby AFM (tapping-mode in air) a sample was grown under thesame growth conditions without capping. The structure andcomposition of the capped QDs were accessed by XSTMat room temperature (RT) with atomic-scale resolution.10,11The STM was operated in constant current mode (I¼ 40 pA)on in situ cleaved (011)-surfaces at negative bias voltage(Ubias¼3 V). Under such conditions InAs (GaAs) richregions appear bright (dark) in the topographic XSTM images.The optical properties of the capped QDs were investigated byRT photoluminescence (PL) spectroscopy. A green 532-nmsolid-state laser was used for excitation. The PL was dispersedby a quarter-meter monochromator and detected by a liquidnitrogen cooled InGaAs photodiode array detector.Figure 1(a) shows the AFM image of the uncappedQDs, and Fig. 1(b) presents a histogram of the measured QDheights. The QDs are dome-shaped with density of 160 perlm2, which is a moderate value despite the low depositiontemperature of the In NCs. This is the consequence of highIn ad-atom mobility on GaAs in the absence of As flux.12 Asevident from Fig. 1(b) the average QD height is 6.5 nm, andthe half-width of the height distribution equals to 4 nm. Suchlarge heights are maintained due to the relatively short andlow-temperature annealing.Most relevant, however, is the final QD shape and com-position after capping, which is known to cause significantmaterial redistribution to flatten the QDs.13 Figure 2(a)shows the XSTM image of an individual QD after cappingwith GaAs. The QD is on average 3.5 nm high and 15 nmwide and has an undulated top surface with height maximain the center and at the periphery. This is in distinct contrastto SK-grown QDs which maintain a lens or truncated pyra-mid like shape.13,14 Such modulated QD shape is commonlyfound in the studied sample, as shown in Fig. 2(b). More-over, the QDs are composed of pure InAs. There is no signa-ture of individual Ga atoms inside the QDs which wouldappear as dark spots. This is in agreement with the initiallow-temperature growth of the In NCs and their immediateconversion to InAs QDs, preventing intermixing with theGaAs substrate as well as etching into the underlying layerwhich happens for GaAs/AlGaAs QDs grown by dropletepitaxy at higher temperatures.15We attribute the anomalously undulated shape of theburied QDs to significant morphological changes occurringduring the capping process. Such changes are evident fromthe large difference in height of the QDs observed withXSTM compared to the average height of the surface QDsinvestigated by AFM. A possible explanation of such behav-ior is as follows: due to the pure In content, the freshly con-verted QDs are highly strained, preventing completeovergrowth to leave the central part of the QDs exposed dur-ing the initial capping. This induces significant materialredistribution during heating up for the final thick GaAslayer growth at 580 C. At this temperature, the partiallyexposed top of the QDs readily decomposes and In desorbs,leaving the final buried QDs greatly flattened. Similar to thecase of SK-grown QDs, and intentionally enhanced by the socalled In-flush technique,16 this process certainly takes placeto reduce the overall height of the QDs, but does not explainthe modulated shape of our QDs. Therefore, we believe thatthe final shape of the QDs is additionally determined by theformation process of the QDs itself. For QDs formed by DEit has been found that the recrystallization process startsfrom the side of the droplets, proceeding to the top. ThisFIG. 1. (a) AFM image of uncapped InAs QDs and (b) histogram of the QDheights. The AFM scan area is 1 1 lm2 and the full height contrast is 12 nm.FIG. 2. (a) XSTM image of an individual InAs QD obtained by conversionof an In NC. (b) XSTM image of two InAs QDs confirming the characteris-tic wavy shape. The dark contrast on the right side visible in (a) is an artifactdue to the presence of a step edge.073103-2 Urbanczyk et al. Appl. Phys. Lett. 102, 073103 (2013)Downloaded 27 Jun 2013 to 131.155.151.167. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissionssuggests a better crystal quality at the side and, hence, higherstability during overgrowth. Together with the initially high-est part in the center this might lead to a buried QD with theobserved shape, i.e., highest in the center and the periphery.Of course, other mechanisms leading to undulated topogra-phy of strained layers should not be excluded.17,18The result of the RT PL measurement is shown inFig. 3. The QDs exhibit a very broad emission band withintensity maximum at 1285 nm and half-width of 320 nm.Such a large inhomogeneous broadening is most likely aconsequence of the broad distribution of initial QD heightsand the complex overgrowth process. What is most impor-tant, the emission extends significantly beyond 1.55 lm,even beyond the detector cut-off at 1.6 lm, notably withoutthe need for strain reducing layers. This long emission wave-length is mainly attributed to the QDs being composed ofpure InAs which has proven to be difficult to obtain forSK-grown QDs of high optical quality. Along these lines,further optimization of the QDs for specific applications isstraight-forward due to the excellent control of size anddensity of the epitaxial In NCs,19 much better than thecontrol achievable for directly grown SK QDs.To conclude, we have demonstrated room-temperaturelong-wavelength emission beyond 1.55 lm of InAs QDs onGaAs obtained by conversion of epitaxial In NCs. We alsopresented the results of AFM and XSTM measurements forstructural analysis. These measurements showed that theinvestigated QDs are composed of pure InAs and exhibit anundulated top surface, very different to that of SK-grownQDs, which has been related to the distinct QD formationprocess. We believe that the presented findings are a step-ping stone for the usage of the InAs/GaAs material systemfor optoelectronic devices operating in the second telecom-munication window and beyond. Furthermore, we believethat our findings can be directly applied to other materialsystems, such as InAs/InP, for extending the emission intothe mid-infrared.The authors would like to acknowledge the technicalsupport of F.W.M. van Otten in the growth of the samples.1P. B. Joyce, T. J. Krzyzewski, G. R. Bell, B. A. Joyce, and T. S. Jones,Phys. Rev. B 58, R15981 (1998).2M. V. Maximov, A. F. Tsatsul’nikov, B. V. Volovik, D. A. Bedarev, A. Y.Egorov, A. E. Zhukov, A. R. Kovsh, N. A. Bert, V. M. Ustinov, P. S.Kop’ev, Z. I. Alferov, N. N. Ledentsov, D. Bimberg, I. P. Soshnikov, andP. Werner, Appl. Phys. Lett. 75, 2347 (1999).3E. C. Le Ru, P. Howe, T. S. Jones, and R. Murray, Phys. Rev. B 67,165303 (2003).4J. Tatebayashi, M. Nishioka, and Y. Arakawa, Appl. Phys. Lett. 78, 3469(2001).5B. Alloing, C. Zinoni, V. Zwiller, L. H. Li, C. Monat, M. Gobet, G. Buchs,A. Fiore, E. Pelucchi, and E. Kapon, Appl. Phys. Lett. 86, 101908 (2005).6Y. Q. Wei, S. M. Wang, F. Ferdos, J. Vukusic, A. Larsson, Q. X. Zhao,and M. Sadeghi, Appl. Phys. Lett. 81, 1621 (2002).7T. Chikyow and N. Koguchi, Jpn. J. Appl. Phys. 29, L2093 (1990).8J. S. Kim and N. Koguchi, Appl. Phys. Lett. 85, 5893 (2004).9A. Urbanczyk, G. J. Hamhuis, and R. N€otzel, J. Appl. Phys. 107, 014312(2010).10E. T. Yu, MRS Bull. 22, 22 (1997).11B. Z. Nosho, W. Barvosa-Carter, M. J. Yang, B. R. Bennett, and L. J.Whitman, Surf. Sci. 465, 361 (2000).12Y. Horikoshi, Semicond. Sci. Technol. 8, 1032 (1993).13Q. Gong, P. Offermans, R. N€otzel, P. M. Koenraad, and J. H. Wolter,Appl. Phys. Lett. 85, 5697 (2004).14D. Litvinov, D. Gerthsen, A. Rosenauer, M. Schowalter, T. Passow,P. Fein€augle, and M. Hetterich, Phys. Rev. B 74, 165306 (2006).15J. G. Keizer, J. Bocquel, P. M. Koenraad, T. Mano, T. Noda, andK. Sakoda, Appl. Phys. Lett. 96, 062101 (2010).16Z. Wasilewski, S. Fafard, and J. McCaffrey, J. Cryst. Growth 201–202,1131 (1999).17D. J. Seol, S. Y. Hu, Z. K. Liu, L. Q. Chen, S. G. Kim, and K. H. Oh,J. Appl. Phys. 98, 044910 (2005).18W. Barvosa-Carter, M. J. Aziz, L. J. Gray, and T. Kaplan, Phys. Rev. Lett.81, 1445 (1998).19A. Urbanczyk and R. N€otzel, J. Cryst. Growth 341, 24 (2012).FIG. 3. RT PL spectrum of the InAs QDs. The detector cut-off is at 1.6 lm.073103-3 Urbanczyk et al. Appl. Phys. Lett. 102, 073103 (2013)Downloaded 27 Jun 2013 to 131.155.151.167. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions

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Long wavelength (> 1.55 mu m) room temperature emission and anomalous structural properties of InAs/GaAs quantum dots obtained by conversion of In nanocrystals

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Long wavelength (&gt; 1.55 mu m) room temperature emission and anomalous structural properties of InAs/GaAs quantum dots obtained by conversion of In nanocrystals

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Long wavelength (> 1.55 mu m) room temperature emission and anomalous structural properties of InAs/GaAs quantum dots obtained by conversion of In nanocrystals