3 páginas, 3 figuras.-- PACS: 778.66.Fd, 78.55.Cr, 81.15.Hi, 81.05.Ea, 73.21.-b, 85.35.BeWe demonstrate the dimensional tuning of InAs self-assembled quantum dots (QDs) by changing the growth kinetics during the capping of InAs islands with GaAs. Modifying the growth sequence during the capping of InAs islands, allows us to tune the thickness and lateral dimensions of the QDs while keeping the wetting layer thickness constant. Using the same method but embedding the tuned InAs islands into AlAs layers allows to further blueshift the photoluminescence emission to higher energies while keeping the wetting layer thickness constant. The main process responsible for the QDs size modification is consistent with a kinetically controlled materials redistribution of the InAs islands that minimizes the energy of the epitaxial layers at the start up of the GaAs capping deposition.The authors want to acknowledge the financial support
of QUEST an NSF-Science and Technology center (DMR
No. 91-20007), the Linköping University (POH), the Deutsche Forschungsgemeinschaft (PJW) and the Spanish Ministry
of Education and Science (JMG) for financial support.Peer reviewe

García Martínez, Jorge Manuel

Holtz, P. O.

Mankad, T.

Petroff, Pierre M.

Wellmann, P. J.

Applied Physics Letters

Digital.CSIC

APPLIED PHYSICS LETTERS VOLUME 72, NUMBER 24 15 JUNE 1998DownElectronic states tuning of InAs self-assembled quantum dotsJ. M. Garcı´a,a) T. Mankad, P. O. Holtz, P. J. Wellman, and P. M. Petroffb)Department of Materials and QUEST, University of California, Santa Barbara, California 93106~Received 16 December 1997; accepted for publication 14 April 1998!We demonstrate the dimensional tuning of InAs self-assembled quantum dots ~QDs! by changingthe growth kinetics during the capping of InAs islands with GaAs. Modifying the growth sequenceduring the capping of InAs islands, allows us to tune the thickness and lateral dimensions of the QDswhile keeping the wetting layer thickness constant. Using the same method but embedding the tunedInAs islands into AlAs layers allows to further blueshift the photoluminescence emission to higherenergies while keeping the wetting layer thickness constant. The main process responsible for theQDs size modification is consistent with a kinetically controlled materials redistribution of the InAsislands that minimizes the energy of the epitaxial layers at the start up of the GaAs cappingdeposition. © 1998 American Institute of Physics. @S0003-6951~98!02024-5#With the steady progress on the understanding of self-assembled quantum dots ~QDs!1–3 physical and electronicproperties, more device applications are currently under in-vestigation. QDs lasers,4 detectors,5 or charge storagedevices6 have been reported. More than often, to fullyachieve the device potential has remained elusive because ofthe lack of control over the dimensions of the self-assembledQDs. Here, we propose a method which allows to tune theQDs sizes in an easy and reproducible fashion for the InAs/GaAs QDs system while keeping the wetting layer ~WL!thickness constant to a minimum (;1.7 ML). This could bea desirable characteristic in some devices where the quantumwell ~QW! like characteristics of the WL could adverselyaffect the device.Previous studies have shown that by changing the QDscompositions,7 e.g., using AlInAs QDs imbedded in AlGaAs,it is also possible to tune their photoluminescence ~PL!wavelength emission. However, this was achieved at the ex-pense of a thicker WL ('22 Å) since the misfit strain of theAlInAs/AlGaAs structures is smaller than that of the InAs/GaAs QDs system. Post-growth annealing ~900 °C! of InAs/GaAs QDs has been shown to also blueshift the PLemission.8 This effect was attributed to compositional inter-diffusion between the islands and the cladding film. Sincethe electronic properties of the QDs,1–3 are sensitive to theirshape, size and composition,9–12 it is not surprising that dif-ferent growth conditions ~and therefore different growth ki-netics! yield different electronic structures. We have alreadyshown that during the overgrowth of InAs islands withGaAs, diffusion and intermixing13 with the top claddingGaAs epitaxial layer takes place and produce large changesin the original island sizes and shapes. In this letter we pro-pose to use this effect for tuning the energy levels of theQDs.To demonstrate the possibility of tuning the wavelengthof the dots we have grown by molecular bean epitaxy fivedifferent samples on semi-insulating GaAs~100! under 1a!Electronic mail: jorge.garcia@uam.esb!Electronic mail: petroff@engineering.ucsb.edu3170003-6951/98/72(24)/3172/3/$15.00loaded 05 Nov 2010 to 161.111.180.191. Redistribution subject to AIP l31025 Torr Arsenic (As2) pressure. In all cases a GaAsbuffer layer is prepared with a short period superlattice (40320 Å/20 Å AlAs/GaAs! at a growth temperature of Tg5630 °C. The dots are formed by depositing 1.7 InAsmonolayers at 530 °C. A first growth interruption of 30 s isused to narrow down the size distribution. Then the dots arecovered at 530 °C with a thin GaAs cap of thickness tcapadjusted between 10 and 50 Å followed by a second 30 sgrowth interruption and a final capping with GaAs or AlAs.Samples A, B, and C are metal insulator semiconductorfield effect transistor ~MISFET! structures in which the InAsdots imbedded in GaAs have been grown using this newgrowth technique with tcap550, 30, and 10 Å, where a 200 Åheavily (50310218 cm23) Si-doped GaAs layer 250 Å be-low the dots acts as a back contact. The QDs layer in sampleD is grown on the undoped GaAs buffer layer without theSi1 back contact and with tcap510 Å ~similar to sample C!and covered with thick GaAs (.200 Å). Sample E includesfive layers of QDs grown with continuous rotation in whichthe dots are embedded in AlAs ~see inset in Fig. 3!. The firstfour layers (tcap510, 20, 30, 40 Å! are grown repeating fourtimes the following sequence ~see insert in Fig. 3!: 2000 Åspace GaAs (Tg5600 °C), 50 Å AlAs (Tg5630 °C), 2monolayers ~ML! GaAs (Tg5530 °C), 1.7 ML InAs (Tg5530 °C), 30 s pause, tcap GaAs (Tg5530 °C), 30 s pause,60 Å AlAs rising Tg from 530 to 630 °C, 2000 Å spaceGaAs layer (Tg5600 °C) and 300 Å Al0.4Ga0.7As (Tg5630 °C). The last layer of QDs (tcap550 Å) is embeddedin AlAs and covered with 300 Å of GaAs.The PL measurements were done at low temperature ~10K! using an Ar1 laser tuned at 514.4 nm and focused on aspot of '100 mm. The emitted light is dispersed by a 0.85 mdouble spectrometer and detected with a liquid nitrogencooled Ge detector with a spectral resolution of 0.5 nm. Thetop layers of sample E are etched away14 at a slow etch rate~60 Å/s! solution (H3PO4:H2O2:H2O) to expose successivelyeach layer of QD for carrying out the PL spectroscopy.Figure 1 shows the PL spectra of samples A, B, and C.As tcap decreases a blueshift of the ground state emission isobserved. For sample A (tcap550 Å) and low pump intensity2 © 1998 American Institute of Physicsicense or copyright; see http://apl.aip.org/about/rights_and_permissions3173Appl. Phys. Lett., Vol. 72, No. 24, 15 June 1998 Garcı´a et al.Down(;1 mW), the spectrum shows a single peak at 1.057 eV~not shown!, while at 50 mW, four luminescence peaksequally spaced approximately 75 meV are observed ~Fig. 1!.When tcap is reduced to 30 Å, the PL spectrum is blueshiftedwith a significant broadening of the lines that is probablyassociated with a change in the shape and size of the QDs.This effect has been reproduced in other samples and canalso be seen in Fig. 2 for sample E. For sample C (tcap510 Å), the emission is further blueshifted. The groundstate luminescence is observed at 1.311 eV and the energydifference between higher order recombination lines is re-duced to ;42 meV.For all these samples ~Fig. 1!, the wetting layer PL emis-sion appears at the same energy ~1.43 eV! in agreement withan expected constant WL layer thickness. The observedGaAs donor-acceptor pair ~DAP! exciton recombination insamples A, B, and C appears at 1.55 eV because of theconduction band degeneracy of the heavily doped back con-tact GaAs layer.FIG. 1. Photoluminescence spectra taken at 10 K and 50 mW/cm2 forsamples A, B, and C. The emission from the QDs is blueshifted closer to theWL emission line (;1.43 eV) as tcap decreases from 50 to 10 Å.FIG. 2. Quantum dots photoluminescence for sample E (50 mW/cm2, T510 K). The ground state PL line emission is blueshifted beyond the GaAsexciton emission (;1.52 eV) as tcap decreases from 50 to 10 Å. The spec-trum for tcap510 Å is plotted 310.loaded 05 Nov 2010 to 161.111.180.191. Redistribution subject to AIP lSince covering partially the InAs islands with tcap re-duces their vertical size, it is reasonable to question whetherthe QDs obtained with tcap'10 Å ~;3 monolayers! of GaAsstill exhibit QDs with 0D confinement characteristics. Withincreased optical pump power ~from 0.1 to 50 mW/cm2), weverified that the electronic state filling effects observed in thenormally grown QDs are also seen with these QDs. Thehigher energy luminescence lines corresponding to thehigher excited states intensities are becoming dominant asthe pump power increase and the lower subband lumines-cence is saturated. Confinement in locally 0D areas due toQW monolayer fluctuation can be excluded since the calcu-lated electronic energy splitting due to InAs/GaAs QWmonolayer fluctuations are much bigger (;110 meV) thanthe measured values. We also note that for the sample withthe QDs corresponding to tcap510 Å, the wetting layer ismuch more intense. We attribute this to the reduced thick-ness of the QDs which induces higher energy quantum con-fined states and a smaller confinement barrier with the GaAsbarrier continuum.When the QDs are embedded in AlAs ~sample E! a blue-shift is also observed as tcap decreases ~Fig. 2!. Compared tosamples A, B, C, and D, the ground state PL emission linesare shifted to higher energy values. As in sample B, for tcap'30 Å, an abrupt change in the energy position and PL lineshape is observed. For tcap'10 Å the emission is shiftedbeyond the GaAs free exciton around 1.56 eV. As expectedfrom the larger quantum confinement provided by embed-ding the QDs in AlAs, all the spectra in Fig. 2 are blueshifted from those in Fig. 1.As we have previously shown,13 the vertical and lateralsize and shape of the InAs islands and partially covered is-lands ~PCI! are primarily controlled by the thickness (tcap) ofthe GaAs capping layer which is deposited before the islandsare completely covered. As seen in Fig. 3, for the QDs, theobserved shift in the ground state luminescence and changesin the energy level spacing of the higher excited states lumi-nescence lines are consistent with the previously reportedshape changes of the PCI. For tcap'20 Å, atomic force mi-croscope measurements show InAs PCI which are ;15 Åhigh and have an approximately ellipsoidal base with dimen-sions ;11003700 Å2. The total InAs QD vertical size isFIG. 3. Quantum dots ground state emission energy (T510 K) vs cappinglayer thickness, tcap , for InAs QDs embedded in AlAs or GaAs claddinglayers.icense or copyright; see http://apl.aip.org/about/rights_and_permissions3174 Appl. Phys. Lett., Vol. 72, No. 24, 15 June 1998 Garcı´a et al.Downthen tcap115 Å'35 Å. For tcap'10 Å, we estimate a verti-cal size of ;25 Å and similar lateral dimensions (;11003700 Å2), while those corresponding to tcap'50 Å have athickness of ;70 Å and a diameter of '200 Å.As we have previously shown intermixing reactions be-tween InAs and AlAs are important and we expect thesereactions to induce changes in the QDs composition whencapping the InAs islands with AlAs. Thus we cannot esti-mate quantitatively if the observed blueshifts observed be-tween Figs. 1 and 2 are entirely due to quantum confinementand/or compositional changes or a mixture of the two.These results clearly show the dramatic effects of thegrowth kinetics during the transformation of the InAs islandsinto InAs QDs during the GaAs capping deposition. Bothstrain relaxation and intermixing of the Ga or Al and In willplay a role in these shape and size changes of the QDs. Thevertical size reduction of the original InAs islands furtherindicates that In and Ga or Al diffusion is taking place duringthe partial capping of the islands. Much remains to be doneto understand these kinetic effects, however, as we haveshown they can be used to tune the QDs electronic structurewithout having recourse to changing the misfit strain in thesystem or the WL thickness.The authors want to acknowledge the financial supportof QUEST an NSF-Science and Technology center ~DMRNo. 91-20007!, the Linko¨ping University ~POH!, the Deut-loaded 05 Nov 2010 to 161.111.180.191. Redistribution subject to AIP lsche Forschungsgemeinschaft ~PJW! and the Spanish Minis-try of Education and Science ~JMG! for financial support.1 G. Madeiros-Ribeiro, F. G. Pikus, P. M. Petroff, and A. L. Efros, Phys.Rev. B 55, 1568 ~1997!.2 S. Raymond, S. Fafard, P. J. Poole, A. Wojs, P. Hawrylak, S. Charbooeau,D. Leonard, R. Leon, P. M. Petroff, and J. L. Merz, Phys. Rev. B 54,11548 ~1996!.3 H. Drexler, D. Leonard, W. Hansen, J. P. Kotthaus, and P. M. Petroff,Phys. Rev. Lett. 73, 2252 ~1994!.4 M. Grundmann, N. N. Ledentsov, R. Heitz, L. Eckey, J. Christen, J. Bo-hrer, D. Bimberg, S. S. Ruvimov, P. Werner, U. Richter, J. Heidenreich,V. M. Ustinov, A. Yu. Egorov, A. E. Zhukov, P. S. Kop’ev, and Zh. I.Alverov, Phys. Rev. Lett. 74, 4043 ~1995!.5 S. Sauvage, P. Boucaud, F. H. Julien, J. M. Gerard, and V. Thierry-Mieg,Appl. Phys. Lett. 71, 2785 ~1997!.6 G. Yusa and H. Sakaki, Appl. Phys. Lett. 70, 345 ~1997!.7 R. Leon, S. Fafard, D. Leonard, J. L. Merz, and P. M. Petroff, Appl. Phys.Lett. 67, 521 ~1995!.8 R. Leon, Y. Kim, C. Jagadish, M. Gal, J. Zou, and D. J. H. Cockayne,Appl. Phys. Lett. 69, 1888 ~1996!; S. Malik, C. Roberts, R. Murray, andM. Pate, ibid. 71, 1987 ~1997!.9 M. Grundmann, O. Stier, and D. Bimberg, Phys. Rev. B 52, 11969 ~1995!.10 K. H. Schmidt, G. Medeiros-Ribeiro, M. Osestreich, P. M. Petroff, and G.H. Do¨hler, Phys. Rev. B 54, 11346 ~1996!.11 M. A. Cusack, P. R. Briddon, and M. Jaros, Phys. Rev. Lett. 56, 4047~1997!.12 G. Medeiros-Ribeiro, D. Leonard, and P. M. Petroff, Appl. Phys. Lett. 66,1767 ~1995!.13 J. M. Garcia, G. Medeiros-Ribeiro, K. Schmidt, T. Ngo, and P. M. Petroff,Appl. Phys. Lett. 71, 2014 ~1997!.14 K. W. Berryman, S. A. Lyon, and M. Sergev, Appl. Phys. Lett. 70, 1861~1997!.icense or copyright; see http://apl.aip.org/about/rights_and_permissions

Electronic states tuning of InAs self-assembled quantum dots

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