The composition profile along the [001] growth direction of low-growth-rate InAs–GaAs quantum dots (QDs) has been determined using medium-energy ion scattering (MEIS). A linear profile of In concentration from 100% In at the top of the QDs to 20% at their base provides the best fit to MEIS energy spectra

Al-Harthi, S.

Bailey, P.

Bell, Gavin R.

Grad, F.

Hatfield, S. A.

McConville, C. F. (Chris F.)

Noakes, T. C. Q.

Quinn, P. D.

Wilson, Neil R.

English

P. D. Quinn

N. R. Wilson

S. A. Hatfield

C. F. McConville

G. R. Bell

T. C. Q. Noakes

P. Bailey

S. Al-Harthi

F. Gard

Crossref

Composition profiles of InAs–GaAs quantum dots determined by medium-energy ion scattering

Warwick Research Archives Portal Repository

  University of Warwick institutional repository: http://go.warwick.ac.uk/wrapThis paper is made available online in accordance with publisher policies. Please scroll down to view the document itself. Please refer to the repository record for this item and our policy information available from the repository home page for further information.  To see the final version of this paper please visit the publisher’s website. Access to the published version may require a subscription.  Author(s): P. D. Quinn, N. R. Wilson, S. A. Hatfield, C. F. McConville, G. R. Bell, T. C. Q. Noakes, P. Bailey, S. Al-Harthi, and F. Gard Article Title: Composition profiles of InAs–GaAs quantum dots determined by medium-energy ion scattering   Year of publication: 2005 Link to published version: http://dx.doi.org/10.1063/1.2099533 Publisher statement: None    APPLIED PHYSICS LETTERS 87, 153110 2005Composition profiles of InAs–GaAs quantum dots determinedby medium-energy ion scatteringP. D. Quinn, N. R. Wilson, S. A. Hatfield, C. F. McConville, and G. R. BellaDepartment of Physics, University of Warwick, Coventry CV4 7AL, United KingdomT. C. Q. Noakes and P. BaileyCCLRC Daresbury Laboratory, Daresbury, Warrington Cheshire WA4 4AD, United KingdomS. Al-Harthi and F. GardDepartment of Physics, Sultan Qaboos University, PO Box 36, Code 123, Sultanate of OmanReceived 1 June 2005; accepted 8 September 2005; published online 6 October 2005The composition profile along the 001 growth direction of low-growth-rate InAs–GaAs quantumdots QDs has been determined using medium-energy ion scattering MEIS. A linear profile of Inconcentration from 100% In at the top of the QDs to 20% at their base provides the best fit to MEISenergy spectra. © 2005 American Institute of Physics. DOI: 10.1063/1.2099533InAs quantum dots QDs grown by self-assembly onGaAs001 substrates continue to attract great interest due totheir applicability in optoelectronic devices. The opticalproperties of a QD depend on the detailed shape of its po-tential well, which can be determined only if the distributionof In in the In/Ga sublattice is known. For the InAs–GaAssystem, it is well established that a an alloyed InGaAs wet-ting layer WL is first formed during InAs deposition;1 bthe final state of the QDs may be an alloy even for depositionof pure InAs;2 and c further changes to QD shape andcomposition take place during the capping of a QD arraywith GaAs or InGaAs.3–5 These processes are controlled by acomplicated combination of surface segregation6 and locallystrain-dependent surface migration and attachment.5 Severalexperimental techniques have been used to probe the In dis-tribution in QDs, including grazing incidence x-ray diffrac-tion XRD,7 cross-sectional scanning tunneling microscopyXSTM,5,8 transmission electron microscopy TEM andscanning TEM energy dispersive x-ray analysis.9 Both XRDand XSTM suggest that the In composition varies withheight through the QDs, with pure InAs at the top of the QDsand progressively more Ga in the alloy towards the base ofthe QDs.7,8 This alloying effect depends on growth condi-tions, particularly temperature2 and growth rate.10In this letter we describe the use of medium-energy ionscattering MEIS to produce composition profiles of InAsQDs. The MEIS technique is able to probe layered materialswith near atomic layer resolution. Here we demonstrate itsability to probe an array of three-dimensional 3D islandstructures and, when combined with data from atomic forcemicroscopy AFM, produce compositional profiles along thegrowth direction.InAs QDs were grown on GaAs001 by molecular beamepitaxy at a substrate temperature of 480 °C. The totalamount of InAs deposited was 2.2 monolayers 1 ML=0.3 nm and the growth rate was 0.012 ML s−1.10 Here, wediscuss free-standing QDs uncapped by GaAs. Samples weretransferred through air between the MBE chamber and theMEIS system, with an arsenic capping-decapping procedureaElectronic mail: gavin.bell@physics.org; URL: http://www.warwick.ac.uk/go/grbell0003-6951/2005/8715/153110/3/$22.50 87, 15311Downloaded 06 Jul 2009 to 137.205.202.8. Redistribution subject to employed to minimize oxidation.11 The arsenic decappinganneal was approximately 400 °C for 15 min, a temperaturewell below that used to modify the optical properties ofInAs–GaAs QDs.12 Residual submonolayer contaminationby oxygen and carbon was normally detected by Auger elec-tron spectroscopy, but neither this nor the anneal itself mate-rially affect the In profile results presented here. Ion scatter-ing experiments were performed using the UK MEIS Facilityat Daresbury Laboratories.13 Helium ions at 100 keV energywere incident a pseudorandom direction near 110. The to-roidal electrostatic detector monitors an angular range of 27°and has an energy resolution 400 eV. In the present experi-ments the scattering angle was 90°. Integration of the sig-nal over an appropriate angular range allowed energy spectrato be obtained. For uncapped QDs, the entire In signal at90–94 keV was separate from the GaAs surface peaks andbackground, allowing the peak shape to be analyzed inde-pendently. Ion beam damage was minimized by regularmovement of the sample under the beam. The ion dose ateach location 5 C was below the damage threshold forGaAs measured in separate dechanneling experiments.An AFM topograph of a typical QD array is shown inFig. 1. The number density of QDs is 51010 cm−2 whilethe mean height and diameter are 2.8 and 27 nm, respec-tively. Large 3D islands L3DI of mean height 8 nm arepresent at much lower number density, typically 5108 cm−2.10The pseudorandom MEIS energy spectrum in the In re-gion is shown in Fig. 2 with several simulated spectra super-imposed on the data. Slightly different models were em-ployed in Figs. 2a and 2b, as explained subsequently. Theasymmetric shapes of the peaks in both the experimental andsimulated data are caused by the distribution of In throughthe QDs. Because of the discrete 3D nature of the QDs, arange of different path lengths, and hence stopping and strag-gling energy losses, is possible for backscattering from Innuclei at a particular depth. This energy loss process wassimulated by placing a single model QD, whose size is de-termined by AFM, in the center of a section of WL whosearea is determined by AFM via the QD number density. Oursimulations employ an idealized QD shape but are not sen-sitive to the precise geometrical figure used e.g., sphericalcap versus truncated pyramid. The whole structure is then© 2005 American Institute of Physics0-1AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp153110-2 Quinn et al. Appl. Phys. Lett. 87, 153110 2005divided into a grid of 400 possible ion entry points and theenergy spectrum calculated for each of the resulting ion tra-jectories. The simulated spectrum is a sum over all thesepossible paths. A similar method has been used for calculat-ing MEIS energy spectra of metal alloy nanoparticles.14We employ the SRIM2000 code and analytic formulae15 toobtain stopping powers. The total ion yield Y from element ias a function of energy E is given by16,17Y = xii fE − Eout;NE . 1In Eq. 1, the Gaussian f is centered on the energy ofthe ion Eout as it leaves the surface; Eout is given by theappropriate stopping powers and elastic energy loss. Thegaussian width  is the sum of inward and outward strag-gling terms18 and the instrumental resolution. The resolutionand detector normalization  are determined by experimentsFIG. 1. A 1 m1 m AFM topograph of uncapped QDs produced bydepositing 2.2 ML InAs on GaAs001. A height cross section shows twoL3DIs and several QDs.FIG. 2. The MEIS energy spectrum in the In region for uncapped QDstogether with illustrative simulations based on fixed QD number density andheight, with no L3DI. a In compositions constant throughout the QDs arecompared; namely, 20%, 40%, 60%, 80%, and 100% from bottom to top. bSeveral linear composition profiles are shown, with 100% In at the top ofthe QD falling to a value at the base of 0% bottom curve, 20%, 40%, 60%,or 80% top curve.Downloaded 06 Jul 2009 to 137.205.202.8. Redistribution subject to on clean metal surfaces.19 The atomic density and cross sec-tion are given by xi and i, respectively, while the neutral-ization probability NE is determined from tabulated data.16The simulated curves in Fig. 2 do not include any L3DIsand employ fixed parameters to illustrate the sensitivity ofthe method. In Fig. 2a, the model QD has a uniform Incomposition that does not vary with height, while in Fig.2b, the In fraction varies linearly from 100% at the top ofthe QD to different values at the base. The plots clearly showthe sensitivity of the technique to the composition profile.Similar sensitivity to the number density and QD sizes areobserved, but these parameters are easily determined byAFM imaging. The simulations allow several parameters tobe optimized simultaneously, typically converging in around10 h on a standard desktop computer.Figure 3 shows the best fit to the MEIS energy spectrumincluding the QD, L3DI, and WL components. The WL com-position and the compositions at the top and bottom of theQDs are all free parameters. The WL thickness and L3DIcomposition are fixed at 2 ML and pure InAs, respectively. Alinear variation with height in the QDs is assumed as thesimplest nonconstant composition.7,8 The contribution of theWL is important, particularly at the leading edge of the en-ergy spectrum. The WL is expected to be an InGaAs alloy ofthickness 1–2 ML,1 and indeed the optimized WL comprisesa 2 ML thick In0.05Ga0.95As alloy. A higher In composition inthe WL would result in too steep a leading edge in the energyspectrum. By contrast, the L3DIs contribute a very broadbackground extending to low ion energies due to the longpath lengths available through them. This background is notstrongly affected by the size distribution of the L3DIs; thedensity is more important, and is fixed by AFM. Havingaccounted for the L3DI and WL contributions, the optimizedQD model corresponds to a linear InGaAs profile, from100% In at the top to 20% In at the base. As shown in Fig. 3,an excellent fit can be obtained using the linear profile. Sat-isfactory fits could not be obtained using any uniform com-position: it is clear from Fig. 2 that the shape of the spectrumin the 92–93 keV region is incorrect for uniform In profiles.It was inferred from total volume measurements10 thatpure InAs QDs could be obtained for substrate temperaturesaround 500 °C by using growth rates 0.02 ML s−1. How-ever, the present study demonstrates clearly that Ga is stillfound in significant quantities in the QDs for these growthconditions. Compositional variation is to be expectedthrough nominally pure InAs QDs even when they are grownat low deposition rates. The precise composition profile ofFIG. 3. The MEIS energy spectra from experiments and optimized simula-tions. The QD, L3DI, and WL contributions to the simulated spectrum arealso shown.QDs should depend on growth conditions because of theAIP license or copyright; see http://apl.aip.org/apl/copyright.jsp153110-3 Quinn et al. Appl. Phys. Lett. 87, 153110 2005complex interplay of kinetic factors involved segregation,migration, attachment. Indeed, in previous studies based onXSTM,8 where the growth rate was 0.01 ML s−1 and the sub-strate temperature was 512 °C, a linear In profile from 60%to 100% was inferred. This result depends on finite elementcalculations to fit the measured outward relaxation of theQDs and requires that the QDs be fully capped with GaAs.Similarly, the work of Kegel et al.7 produced a roughly linearprofile from 5% to 100% for QDs grown by periodic Insupply at an average rate of 0.035 ML s−1 and temperature530 °C. These XRD studies depend on the QDs being un-capped. By contrast, MEIS provides a direct method for de-riving In profiles in InGaAs structures including both cappedand uncapped QDs.One of the authors G.R.B. is grateful to the Royal So-ciety for financial support. The authors thank T. S. Jones foraccess to the MBE growth facility at Imperial College Lon-don. The EPSRC is gratefully acknowledged for access tothe MEIS facility.1J. Belk, C. McConville, J. Sudijono, T. Jones, and B. Joyce, Surf. Sci.387, 213 1997.2P. Joyce, T. Krzyzewski, G. Bell, B. Joyce, and T. Jones, Phys. Rev. B 58,R15981 1998.3P. Joyce, T. Krzyzewski, G. Bell, and T. Jones, Appl. Phys. Lett. 79, 3615Downloaded 06 Jul 2009 to 137.205.202.8. Redistribution subject to 2001.4P. Joyce, T. Krzyzewski, P. Steans, G. Bell, J. Neave, and T. Jones, Surf.Sci. 492, 345 2001.5D. Bruls, P. Koenraad, H. Salemink, J. Wolter, M. Hopkinson, andM. Skolnick, Appl. Phys. Lett. 82, 3758 2003.6P. Howe, E. L. Ru, R. Murray, and T. Jones, J. Cryst. Growth 278, 572005.7I. Kegel, T. Metzger, A. Lorke, J. Peisl, J. Stangl, G. Bauer, J. Garcia, andP. Petrof, Phys. Rev. Lett. 85, 1694 2000.8D. Bruls, J. Vugs, P. Koenraad, H. Salemik, J. Wolter, M. Hopkinson,M. Skolnick, F. Long, and S. Gill, Appl. Phys. Lett. 81, 1708 2002.9P. Siverns, S. Malik, G. McPherson, D. Childs, C. Roberts, R. Murray,B. Joyce, and H. Davock, Phys. Rev. B 58, R10127 1998.10P. Joyce, T. Krzyzewski, G. Bell, T. Jones, E. L. Ru, and R. Murray, Phys.Rev. B 64, 235317 2001.11G. Bell, C. McConville, and T. Jones, Phys. Rev. B 54, 2654 1996.12S. Malik, E. L. Ru, D. Childs, and R. Murray, Phys. Rev. B 63, 1553132001.13P. Bailey, T. Noakes, and D. Woodruff, Surf. Sci. 426, 358 1999, http://www.dl.ac.uk/MEIS14I. Konimi, S. Hyodo, and T. Motohiro, J. Catal. 192, 11 2000.15J. Ziegler and J. Biersack, http://www.srim.org16Y. Kido and T. Koshikawa, J. Appl. Phys. 67, 187 1990.17E. Gusev, H. Lu, T. Gustafsson, and E. Garfunkel, Phys. Rev. B 52, 17591995.18Q. Yang and D. O’Connor, Nucl. Instrum. Methods Phys. Res. B 61, 1491991.19D. Brown, P. Quinn, D. Woodruff, P. Bailey, and T. Noakes, Phys. Rev. B61, 7706 2000.AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

Quinn P D

Wilson NR

Hatfield SA

McConville CF

Bell GR

Noakes T C Q

Bailey P

Al-Harthi SH

Gard FS

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Composition profiles of InAs-GaAs quantum dots determined by medium energy ion scattering

The composition profile along the [001] growth direction of low-growth-rate InAs–GaAs quantum dots (QDs) has been determined using medium-energy ion scattering (MEIS). A linear profile of In concentration from 100% In at the top of the QDs to 20% at their base provides the best fit to MEIS energy spectr

Composition profiles of InAs–GaAs quantum dots determined by medium-energy ion scattering

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