The authors studied the structural and point defect distributions of hydride vapor phase epitaxial GaN film grown in the [11−20] a direction on (1−102) r-plane sapphire with metal-organic vapor phase deposited a-GaN template using transmission electron microscopy, secondary ion mass spectrometry, and positron annihilation spectroscopy. Grown-in extended and point defects show constant behavior as a function of thickness, contrary to the strong nonuniform defect distribution observed in GaN grown along the [0001] direction. The observed differences are explained by orientation-dependent and kinetics related defect incorporation.Peer reviewe

B. Monemar

D. Hommel

F. Tuomisto

Kröger R.

R. Kersting

R. Kröger

S. Figge

T. Paskova

Tuomisto, Filip

Paskova, T.

Kröger, R.

Figge, S.

Hommel, D.

Monemar, B.

Kersting, R.

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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): Tuomisto, Filip & Paskova, T. & Kröger, R. & Figge, S. & Hommel, D.& Monemar, B. & Kersting, R.Title: Defect distribution in a-plane GaN on Al2O3Year: 2007Version: Final published versionPlease cite the original version:Tuomisto, Filip & Paskova, T. & Kröger, R. & Figge, S. & Hommel, D. & Monemar, B. &Kersting, R. 2007. Defect distribution in a-plane GaN on Al2O3. Applied Physics Letters.Volume 90, Issue 12. 121915/1-3. ISSN 0003-6951 (printed). DOI: 10.1063/1.2715128Rights: © 2007 American Institute of Physics. This article may be downloaded for personal use only. Any other userequires prior permission of the authors and the American Institute of Physics. The following article appearedin Applied Physics Letters, Volume 90, Issue 12 and may be found athttp://scitation.aip.org/content/aip/journal/apl/90/12/10.1063/1.2715128.All 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)Defect distribution in a -plane GaN on Al 2 O 3F. Tuomisto, T. Paskova, R. Kröger, S. Figge, D. Hommel, B. Monemar, and R. Kersting  Citation: Applied Physics Letters 90, 121915 (2007); doi: 10.1063/1.2715128 View online: http://dx.doi.org/10.1063/1.2715128 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/90/12?ver=pdfcov Published by the AIP Publishing  Articles you may be interested in Impacts of anisotropic tilt mosaics of state-of-the-art m -plane freestanding GaN substrates on the structural andluminescent properties of m -plane Al x Ga 1 − x N epilayers J. Vac. Sci. Technol. B 29, 021208 (2011); 10.1116/1.3566010  Structural defects and luminescence features in heteroepitaxial GaN grown on on-axis and misorientedsubstrates J. Appl. Phys. 97, 116101 (2005); 10.1063/1.1914956  Effect of growth polarity on vacancy defect and impurity incorporation in dislocation-free GaN Appl. Phys. Lett. 86, 031915 (2005); 10.1063/1.1854745  Transmission electron microscopy study of a defected zone in GaN on a SiC substrate grown by hydride vaporphase epitaxy J. Appl. Phys. 94, 1676 (2003); 10.1063/1.1589169  Structural, electrical, and optical properties of defects in Si-doped GaN grown by molecular-beam epitaxy onhydride vapor phase epitaxy GaN on sapphire J. Appl. Phys. 92, 786 (2002); 10.1063/1.1488241    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:130.233.216.22 On: Tue, 18 Aug 2015 09:04:59Defect distribution in a-plane GaN on Al2O3F. TuomistoaLaboratory of Physics, Helsinki University of Technology, FI-02015 TKK, FinlandT. Paskova, R. Kröger, S. Figge, and D. HommelInstitute of Solid State Physics, University of Bremen, D-28359 Bremen, GermanyB. MonemarDepartment of Physics and Measurement Technology, Linköping University, S-58183 Linköping, SwedenR. KerstingTascon GmbH, D-48148 Münster, GermanyReceived 21 December 2006; accepted 15 February 2007; published online 22 March 2007The authors studied the structural and point defect distributions of hydride vapor phase epitaxialGaN film grown in the 11−20 a direction on 1−102 r-plane sapphire with metal-organic vaporphase deposited a-GaN template using transmission electron microscopy, secondary ion massspectrometry, and positron annihilation spectroscopy. Grown-in extended and point defects showconstant behavior as a function of thickness, contrary to the strong nonuniform defect distributionobserved in GaN grown along the 0001 direction. The observed differences are explained byorientation-dependent and kinetics related defect incorporation. © 2007 American Institute ofPhysics. DOI: 10.1063/1.2715128Recently, there has been an emerging interest in thegrowth of nonpolar III-nitrides for optoelectronic devicestructures free of polarization-induced electric fields. Com-bining this with the possibility of growing thick GaN filmsby hydride vapor phase epitaxy HVPE is expected to leadto nonpolar GaN layers with low dislocation density. A largeeffort has been devoted to structural characterization of thismaterial and to the improvement of the crystal quality.1–5However, the point defect distribution in nonpolar GaN isrelatively poorly understood.The growth polarity has a dramatic effect on the in-grown point defects in HVPE-grown c-plane GaN. The in-corporation of impurities and formation of vacancy defectsare significantly enhanced in growth in the N polar as op-posed to Ga polar direction.6 These defects define the elec-trical, optical, and structural qualities of the material. In ad-dition, in heteroepitaxial c-plane GaN grown on c-planesapphire the distribution of impurities and native defects isnonuniform and their concentration decreases with the dis-tance from the GaN/sapphire interface.7–10In this work we use transmission electron microscopyTEM, secondary ion mass spectrometry SIMS, and posi-tron annihilation spectroscopy PAS to study the distributionof both extended and point defects in a-plane HVPE GaNgrown on an a-plane GaN template grown by metal-organicchemical vapor deposition MOCVD on r-plane sapphire.We compare the results to those obtained earlier in c-planeHVPE GaN.6 We propose an orientation-dependent defectincorporation model based on kinetics considerations to ac-count for the differences in the defect distributions.We studied a series of four a-plane GaN samples, oneMOCVD a-GaN layer with a thickness of 1.2 m and threeHVPE a-GaN films with thicknesses of about 5, 18, and25 m, grown on r-plane sapphire by employing the samethin a-plane MOCVD GaN template. The HVPE growth wascarried out at Linköping University in a conventional hori-zontal HVPE reactor at 1080 °C Ref. 5 and V/III ratio of 7.The GaN templates, including SiN micromask aiming atthreading dislocation reduction, were grown at Bremen Uni-versity in a Thomas Swan vertical reactor at 1050 °C andV/III ratio in the range of 2400–3000.A three-dimensional impurity analysis was performed bytime-of-flight secondary ion mass spectrometry using an IONTOF TOF.SIMS 5 instrument. For a quantitative analysis ofthe oxygen concentration in the GaN film Cs+ sputtering andBi+ analyzing beams were used. Figure 1 shows an oxygenSIMS image of the cross section of one representativesample with total GaN thickness of about 18 m. The Oconcentration remains almost constant in each of the sublay-ers with a very strong contrast between them, it is 11017 and 31018 cm−3 in the MOCVD GaN and HVPEGaN, respectively. The higher O concentration in the HVPEmaterial as compared to the MOCVD material is in agree-ment with previously observed trends in the residual impu-rity concentrations in c-plane GaN.11,12 The increase of the Oconcentration at the MOCVD-HVPE GaN interface could betentatively attributed to nonfully optimized surface pretreat-ment before the HVPE overgrowth. The almost constant be-havior of the O concentration along the thickness, which is instrong contrast with the strong nonuniform distribution of theO content, ranging over a few orders of magnitude, observedin GaN grown in the c direction.7,11For the microstructural analysis of extended defects suchas basal plane stacking faults BSFs, a cross-section TEMspecimen was prepared in the 1−100 orientation by me-chanical polishing followed by Ga ion milling in a NOVANanolab FEI dual beam focused ion beam system operatedat 30 kV. Three cross-sectional TEM images taken at theHVPE GaN spot 1, HVPE-MOCVD GaN interface spot2, and MOCVD GaN spot 3 areas are shown in Figs.1b–1d, respectively. Since the imaging was performed inthe 1−100 zone axis and the sample was slightly tiltedaElectronic mail: filip.tuomisto@tkk.fiAPPLIED PHYSICS LETTERS 90, 121915 20070003-6951/2007/9012/121915/3/$23.00 © 2007 American Institute of Physics90, 121915-1 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:130.233.216.22 On: Tue, 18 Aug 2015 09:04:59towards the 0001 direction, the BSFs are clearly visualized.Their density remains constant and comparable along thetwo MOCVD and HVPE layers with no noticeable change atthe interface Fig. 1c. The density of the BSFs was deter-mined to be about 5105 cm−2. As the BSFs are bonded byFrank-Shockley partial dislocations,13 the dislocation densitywas estimated to be in the order of 1010 cm−2, maintainingthis value throughout the whole thickness.We studied the distribution of the vacancy defects alongthe growth a axis with a variable energy E=0–38 keVpositron beam. The trapping of positrons at vacancy defectscan be observed as the narrowing of the Doppler-broadened511 keV annihilation peak. The measured spectra were char-acterized by the conventional line shape parameters S andW.14,15 Figure 2 shows the S parameter measured in the GaNsamples as a function of positron implantation energy depthscan. The S parameter measured in the a-plane GaN layersis above the value in the defect-free GaN lattice, indicatingthat positrons are trapped at vacancy defects. Interestingly,the S parameters measured in the 5–25 m thick HVPEsamples are similar and saturate at high positron implanta-tion energies to a value slightly lower than that in theMOCVD GaN template at the region at 5–15 keV. The Sparameter measured in the HVPE layers decreases with thedistance from the sample surface clearly slower than in theGaN reference, indicating that the near-surface region firstfew hundred nanometers contains vacancy defects with aconcentration significantly higher than that in the bulk of thelayer.The vacancy defects observed in the a-plane GaN layershave been already identified using both the S and W param-eters as VGa-ON pairs.15 This is in line with previousstudies6,9,10,16 that have shown that the VGa-ON pair is thedominant in-grown vacancy defect observed with positronsin HVPE GaN. The concentrations of the observed VGa-ONpairs in the a-plane GaN layers can be estimated from theS ,W parameters 14 using a typical positron trapping coeffi-cient of V=31015 s−1 for the Ga vacancy. The estimatedvalues are VGa=11018 cm−3 in the MOCVD GaN tem-plate and VGa=21017 cm−3 in the 5–25 m thick HVPElayers. The behavior of the VGa concentration as a function ofdistance from the GaN/sapphire interface in these a-planeHVPE GaN layers is strikingly different from that in thec-plane HVPE GaN see Fig. 3.In the c-plane HVPE GaN, the VGa concentration de-creases by more than two orders of magnitude when the dis-tance from the interface increases from 1 to 30 m,9 andthere is a strong correlation with the distributions of the Oimpurities and dislocations. Our results in a-plane HVPEGaN show that the distributions of the BSFs, O impurities,and VGa are all constant as a function of the distance from5 to 25 m from the interface. Therefore, point defect den-sities follow the density of extended defects for both a-planeand c-plane HVPE GaN. Interestingly, there are significantFIG. 1. Color online a Cross-sectional SIMS image showing the oxygendistribution in the sapphire/MOCVD-grown a-GaN/HVPE-grown a-GaN.The quantification is only valid for the GaN film and the dimensions of theimage are 22 and 90 m in the x and y directions, respectively. b–dCross-sectional TEM images taken in spots 1–3, respectively, as indicated ina. The zone axis for the TEM imaging is 0001, showing nearly constantBSF distribution along the thickness of the structure.FIG. 2. S parameter in the a-plane GaN layers as a function of positronimplantation energy. The solid and dashed lines show the S parameter in theGaN lattice and at a Ga vacancy, respectively. Also the S parameters mea-sured in the GaN reference and in the sapphire substrate are shown.121915-2 Tuomisto et al. Appl. Phys. Lett. 90, 121915 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:130.233.216.22 On: Tue, 18 Aug 2015 09:04:59differences between the a-plane HVPE and MOCVD layers:the O concentration is almost one order of magnitude lowerin the a-plane MOCVD GaN layer than in HVPE GaN, whilethe concentrations of the stacking faults are similar and theconcentrations of the Ga vacancies are higher. Especially thebehavior of the O impurity and Ga vacancy concentrations isunexpected, as they have been shown to correlate individu-ally both in c-plane MOCVD and HVPE GaN.16,17The spatial correlation of the densities of O impuritiesand dislocations in c-plane HVPE GaN grown on sapphirehas been attributed to the dislocation-mediated diffusion ofoxygen from the substrate.7 On the other hand, a high Oimpurity about 1019 cm−3 concentration has been found toalso exist in homoepitaxial N-polar HVPE GaN, where thedislocation density is already several orders of magnitudelower at the interface than in heteroepitaxial GaN. This hasbeen attributed to the enhanced incorporation of impuritieson some growth surfaces relative to the others.6,18 As thegrowth of N-polar GaN proceeds with a strong contributionof nonpolar lateral growth modes, the efficient incorporationof impurities from the growth environment is a likely causefor the high O impurity concentration in a-plane HVPE GaNas well. Hence our results suggest that the nonpolar growthmode determines the O impurity incorporation in heteroepi-taxial a-plane GaN, while the impact of the BSF and partialdislocations connected with them is negligible.The reverse ratio of the O impurity and Ga vacancy con-centrations in the a-plane HVPE and MOCVD GaN layers ismore complicated. We propose that the differences in thepoint defect distributions in these materials arise from thedifferent growth environments and are additionally affectedby the interplay of the O impurities and dislocations. TheMOCVD growth is clearly more N rich than the HVPEgrowth, and hence from kinetics point of view the incorpo-ration of the ON should be more efficient in the latter in anotherwise defect-free crystal. With the same argument, theVGa formation should be more efficient in MOCVD-grownGaN. i In c-plane material intentionally doped with O tosimilar concentrations, the Ga vacancy concentration inMOCVD GaN is indeed at least one order of magnitudehigher than in HVPE GaN.15,16 However, the differences inthe incorporation of unintentional O from the growth envi-ronment are much less evident. This on the other hand can beexplained by high density of dislocations whose presencedominates the O incorporation as the O concentration hasbeen shown to be clearly lower in dislocation-free homoepi-taxial HVPE GaN.6 ii In a-plane GaN, where the O incor-poration is independent of the distance from the substrateand hence seems to be dominated by the higher reactivity ofthe nonpolar growth surface compared to the c-plane mate-rial, the behavior of both the O impurities and Ga vacanciesfollows the above-presented kinetic trends rather well.In summary, we have investigated the distributions ofextended and point defects in a-plane heteroepitaxial GaNusing SIMS, TEM, and PAS. We show that unlike inc-oriented GaN, the defect O impurity, Ga vacancy, andBSF distributions do not vary significantly in a-plane HVPEGaN as a function of distance from the GaN/sapphire inter-face. We propose an orientation-dependent defect incorpora-tion model based on kinetics considerations to account forthe anticorrelations of the point defect concentrations in thea-plane MOCVD GaN and a-plane HVPE GaN.1M. D. Craven, F. Wu, S. H. Lim, J. S. Speck, and S. P. Denbaars, Appl.Phys. Lett. 81, 1201 2002.2B. A. Haskell, F. Wu, M. D. Craven, S. Matsuda, P. T. Fini, T. Fujii, K.Fujito, S. P. Denbaars, J. S. Speck, and S. Nakamura, Appl. Phys. Lett. 83,644 2003.3B. A. Haskell, T. J. Baker, M. B. McLaurin, F. Wu, P. T. Fini, S. P.Denbaars, J. S. Speck, and S. Nakamura, Appl. Phys. Lett. 86, 1119172005.4B. A. Haskell, A. Chakraborty, F. Wu, H. Sasano, P. T. Fini, S. P. Den-baars, J. S. Speck, and S. Nakamura, J. Electron. Mater. 34, 357 2005.5T. Paskova, V. Darakchieva, P. P. Paskov, J. Birch, E. Valcheva, P. O. A.Persson, B. Arnaudov, S. Tungasmita, and B. Monemar, J. Cryst. Growth281, 55 2005.6F. Tuomisto, K. Saarinen, B. Lucznik, I. Grzegory, H. Teisseyre, T. Suski,S. Porowski, P. Hageman, and J. Likonen, Appl. Phys. Lett. 86, 0319152005.7D. C. Look, C. E. Stutz, R. J. Molnar, K. Saarinen, and Z. Liliental-Weber,Solid State Commun. 117, 571 2001.8X. L. Sun, S. H. Goss, L. J. Brillson, D. C. Look, and R. J. Molnar, J.Appl. Phys. 91, 6729 2002.9J. Oila, J. Kivioja, V. Ranki, K. Saarinen, D. C. Look, R. J. Molnar, S. S.Park, S. K. Lee, and J. Y. Han, Appl. Phys. Lett. 82, 3433 2003.10F. Tuomisto, K. Saarinen, T. Paskova, B. Monemar, M. Bockowski, and T.Suski, J. Appl. Phys. 99, 066105 2006.11T. Paskova, P. P. Paskov, V. Darakchieva, E. M. Goldys, U. Södervall, E.Valcheva, B. Arnaudov, and B. Monemar, Phys. Status Solidi C 0, 2092002.12A. E. Wickenden, D. D. Koleske, R. L. Henry, M. E. Twigg, and M.Fatemi, J. Cryst. Growth 260, 54 2004.13R. Kröger, T. Paskova, B. Monemar, S. Figge, D. Hommel, and A.Rosenauer, Proceedings of the International Workshop on Nitride Semi-conductors IWN2006, 22–27 October 2006, Kyoto, Japan unpublished.14K. Saarinen, P. Hautojärvi, and C. Corbel, in Identification of Defects inSemiconductors, edited by M. Stavola Academic, New York, 1998, Vol.51A, p. 209.15F. Tuomisto, T. Paskova, S. Figge, D. Hommel, and B. Monemar, J. Cryst.Growth 300, 251 2007.16S. Hautakangas, V. Ranki, I. Makkonen, M. J. Puska, K. Saarinen, X. Xu,and D. C. Look, Phys. Rev. B 73, 193301 2006.17J. Oila, V. Ranki, J. Kivioja, K. Saarinen, P. Hautojärvi, J. Likonen, J. M.Baranowski, K. Pakula, T. Suski, M. Leszczynski, and I. Grzegory, Phys.Rev. B 63, 045205 2001.18E. Frayssinet, W. Knap, S. Krukowski, P. Perlin, P. Wisniewski, T. Suski,I. Grzegory, and S. Porowski, J. Cryst. Growth 230, 442 2001.FIG. 3. Estimated Ga vacancy concentrations as a function of the distancefrom the sapphire interface. The values for c-plane HVPE GaN are takenfrom Ref. 9.121915-3 Tuomisto et al. Appl. Phys. Lett. 90, 121915 2007 This article is copyrighted as indicated in the article. 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Defect distribution in a-plane GaN on Al2O3

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Defect distribution in a-plane GaN on Al2O3

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