This entry is a comment on "Effect of growth interruptions on the light emission and indium clustering

Edwards, P R

Martin, R W

O'Donnell, K P

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R. W. Martin

K. P. O’Donnell

P. R. Edwards

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Comment on “Effect of growth interruptions on the light emission and indium clustering of InGaN/GaN multiple quantum wells” [Appl. Phys. Lett. 79, 2594 (2001)]

University of Strathclyde Institutional Repository

Strathprints Institutional RepositoryMartin, R W and O’Donnell, K P and Edwards, P R (2002) Comment on "Effect of growthinterruptions on the light emission and indium clustering of InGaN/GaN multiple quantum wells"[Appl. Phys. Lett. 79, 2594 (2001)]. Applied Physics Letters, 81 (16). pp. 3100-3101. ISSN0003-6951Strathprints is designed to allow users to access the research output of the University of Strathclyde.Copyright c© and Moral Rights for the papers on this site are retained by the individual authorsand/or other copyright owners. You may not engage in further distribution of the material for anyprofitmaking activities or any commercial gain. You may freely distribute both the url (http://strathprints.strath.ac.uk/) and the content of this paper for research or study, educational, ornot-for-profit purposes without prior permission or charge.Any correspondence concerning this service should be sent to Strathprints administrator:mailto:strathprints@strath.ac.ukhttp://strathprints.strath.ac.uk/COMMENTSComment on ‘‘Effect of growth interruptions on the light emissionand indium clustering of InGaNÕGaN multiple quantum wells’’Appl. Phys. Lett. 79, 2594 2001R. W. Martin,a) K. P. O’Donnell, and P. R. EdwardsDepartment of Physics, Strathclyde University, Glasgow, G4 0NG United Kingdom~Received 2 January 2002; accepted 29 August 2002!© 2002 American Institute of Physics. @DOI: 10.1063/1.1515886#The origin of the bright luminescence of nitride devicesand its relation to the nanostructure of InGaN films is thesubject of an intense and continuing debate. Using transmis-sion electron microscopy ~TEM! to investigate InGaN quan-tum wells ~QWs!, Chichibu et al.1 and Narukawa et al.2 werethe first to argue for the existence of naturally occurring In-rich quantum dots. Energy dispersive x-ray ~EDX! mi-croanalysis on and between ‘‘dot-like’’ regions of high con-trast gave InN mole fractions of approximately 17% and 8%,respectively.2 Subsequently some of the present authors ex-tended these ideas to argue for the formation of nearly pureInN quantum dots in InGaN QWs, with the size of the dots,rather than their composition, being primarily responsible fordetermining the spectral energies.3–5In a recent letter Cho et al.6 present energy filtered trans-mission electron microscopy ~EFTEM!, which offers infor-mation on elemental composition with extremely high spatialresolution, better even than that of TEM–EDX. Their dataindicate a spontaneous InN–GaN segregation in InGaNquantum wells which have not experienced an interruption ingrowth before the top barrier layer is deposited. We wish todraw attention to the almost complete absence of an indiumsignal within certain interrogated regions. In particular, Fig.4~c! of Ref. 6 shows a fluctuation between approximately40% and 5% InN, with a typical length scale of 2 nm. Wewill argue that this figure represents the lowest estimates forthe segregation.With any measurement of microcomposition, it is essen-tial to consider the volume addressed in the measurementprocess. For an optical technique this volume is defined bythe excitonic radius while in TEM-based microanalysis theinvestigated volume results from the spread of the electrontrajectories as the beam traverses the sample, as well as thesample thickness. For the EFTEM measurements, with abeam diameter of ;1 nm and a sample thickness in the re-gion of 20 nm, the measured InN mole fractions thereforecorrespond to values averaged within a volume of tens ofcubic nanometers. For their EDX experiments, Narukawaet al. used a 100 kV beam with a 1 nm diameter spot. Monte-Carlo simulations indicate that such a beam will spread to adiameter of approximately 3 nm while traversing a specimenof thickness about 30 nm.In the EFTEM traverse across the QWs grown withoutinterruption, the peaks of 40% InN content represent an av-erage over the excitation cone and similar fluctuations wouldbe expected to occur along the direction of the incident beam~perpendicular to the traverse and also in the plane of thequantum well!. The most likely nanostructure thus appears tobe a set of In-rich quantum dots, about 2 nm in diameter, thatoccupy only a small fraction of the total volume. To reach anaverage In content of 40% in any of the excitation conesrequires a much higher local InN content within the dots,probably approaching 100%. The data cannot distinguish aGaN matrix containing nearly pure InN dots occupying 40%of this volume from a column with average compositionIn0.4Ga0.6N, but the equivalence of the two directions in theplane of the quantum well point to the former. The minimaare even more thought provoking, given that they indicate analmost complete absence of indium nitride within a volumeof tens of cubic nanometers in material where the overallmean InN content can be estimated at ;17% @by averagingthe entire trace in Fig. 4~c! of Ref. 6#. It is worth pointing outthat the globally averaged In content in the wells ~;17%!agrees well with the observed photoluminescence emissionenergy (Ep) of 2.8 eV, according to our published expressionfor Ep(x).7 Such a situation cannot be explained by anythingother than extreme InN–GaN segregation. $We are, however,unable to suggest an explanation at present for the apparentincrease in mean indium fraction in the QWs subject to agrowth delay or for the absence of a shift in Ep betweenQWs with differing average indium contents of 24% and31% @Fig. 4~d! of Ref. 6#.%Such considerations lead to an important re-evaluationof the data presented in Ref. 2. Here the areal density of theIn-rich dot-like regions is estimated to be 531011 cm22,giving a volume filling factor of 2% for the dots. An averagedot diameter of 3 nm corresponds to about ten dots per wellin the 100 nm by 30 nm area illustrated in Fig. 2~b! of Ref.2. Following the author’s assumption of no overlapping ofdots, the EDX probe beam will encounter at most one dot inits 30 nm path, for which the measured In-content is only anaverage. The local In content in the isolated dots thereforea!Electronic mail: r.w.martin@strath.ac.ukAPPLIED PHYSICS LETTERS VOLUME 81, NUMBER 16 14 OCTOBER 200231000003-6951/2002/81(16)/3100/2/$19.00 © 2002 American Institute of PhysicsDownloaded 11 Jan 2011 to 130.159.17.136. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsapproaches 100%. Figure 1 illustrates the situation, usingMonte–Carlo simulations of the trajectory of 100 000 elec-trons in the incident beam. As mentioned by the authors ofRef. 2, measuring the residual In content in the barriers willbe complicated by the spread of the electron beam intoneighboring wells.In conclusion, we suggest that the TEM microanalyses,presented recently by Cho et al. and almost 5 years ago byNarukawa et al., appear to support the idea of an extremeInN–GaN segregation in InGaN heterostructures, which re-sults in the spontaneous formation of nearly pure InN quan-tum dots. In both cases the reported In contents have beendrastically reduced by the volume averaging that is inherentin such measurements. In all techniques that probe the nano-structure of thin films we stress the importance of consider-ing the measured volume. To directly probe the local struc-ture of an InN quantum dot requires a technique that iscapable of addressing only a few cubic nanometers of mate-rial.1 S. Chichibu, T. Azuhata, T. Sota, and S. Nakamura, Appl. Phys. Lett. 69,4188 ~1996!.2 Y. Narukawa, Y. Kawakami, M. Funato, S. Fujita, and S. Nakamura, Appl.Phys. Lett. 70, 981 ~1997!.3 K. P. O’Donnell, R. W. Martin, and P. G. Middleton, Phys. Rev. Lett. 82,237 ~1999!.4 R. W. Martin and K. P. O’Donnell, Phys. Status Solidi B 216, 441 ~1999!.5 L. Nistor, H. Bender, A. Vantomme, M. F. Wu, J. Van Landuyt, K. P.O’Donnell, R. Martin, K. Jacobs, and I. Moerman, Appl. Phys. Lett. 79,2594 ~2001!.6 H. K. Cho, J. Y. Lee, N. Sharma, C. J. Humphreys, G. M. Yang, C. S. Kim,J. H. Song, and P. W. Yu, Appl. Phys. Lett. 79, 2594 ~2001!.7 K. P. O’Donnell, R. W. Martin, C. Trager-Cowan, M. E. White, K. Esona,C. Deatcher, P. G. Middleton, K. Jacobs, W. Van der Stricht, C. Merlet, B.Gil, A. Vantomme, and J. F. W. Mosselmans, Mater. Sci. Eng., B 82, 194~2001!.FIG. 1. Schematic representation of the measurement in Ref. 2 using aMonte–Carlo simulation of the trajectories of one hundred thousand 100keV electrons. Nearly pure InN dots, 3 nm in diameter and with areal den-sity 531011 cm22, are represented by the white dots. The shaded regionrepresents an area 30360 nm.3101Appl. Phys. Lett., Vol. 81, No. 16, 14 October 2002 Martin, O’Donnell, and EdwardsDownloaded 11 Jan 2011 to 130.159.17.136. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

Comment on "Effect of growth interruptions on the light emission and indium clustering of InGaN/GaN multiple quantum wells" [Appl. Phys. Lett. 79, 2594 (2001)]

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Comment on "Effect of growth interruptions on the light emission and indium clustering of InGaN/GaN multiple quantum wells" [Appl. Phys. Lett. 79, 2594 (2001)]

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