We report on the reduction of threading dislocations in GaN overlayers grown by organometallic vapor phase epitaxy on micro-porous TiN networks. These networks were obtained by in situannealing of thin Ti layers deposited in a metalization chamber, on the (0001) face of GaN templates. Observations by transmission electron microscopy indicate dislocation reduction by factors of up to 10 in GaN layers grown on TiN networks compared with the control GaN.X-ray diffraction shows that GaNgrown on the TiN network has a smaller (102) plane peak width (4.6 arcmin) than the control GaN (7.8 arcmin). In low temperature photoluminescence spectra, a narrow excitonic full-width-at-half-maximum of 2.4 meV was obtained, as compared to 3.0 meV for the control GaN, confirming the improved crystalline quality of the overgrown GaN layers

Doğan, S.

Fu, Y.

Inoki, C. K.

Kuan, T. S.

Moon, Y.-T.

Morkoç, Hadis

Smith, David J.

Xie, J.

Yun, F.

Zhou, Lin

Özgür, Ü.

VCU Scholars Compass

Virginia Commonwealth UniversityVCU Scholars CompassElectrical and Computer Engineering Publications Dept. of Electrical and Computer Engineering2005Effectiveness of TiN porous templates on thereduction of threading dislocations in GaNovergrowth by organometallic vapor-phase epitaxyY. FuVirginia Commonwealth UniversityY.-T. MoonVirginia Commonwealth UniversityF. YunVirginia Commonwealth University, fyun@vcu.eduSee next page for additional authorsFollow this and additional works at: http://scholarscompass.vcu.edu/egre_pubsPart of the Electrical and Computer Engineering CommonsFu, Y., Moon, Y.-T., Yun, F., et al. Effectiveness of TiN porous templates on the reduction of threading dislocations inGaN overgrowth by organometallic vapor-phase epitaxy. Applied Physics Letters, 86, 043108 (2005). Copyright © 2005AIP Publishing LLC.This Article is brought to you for free and open access by the Dept. of Electrical and Computer Engineering at VCU Scholars Compass. It has beenaccepted for inclusion in Electrical and Computer Engineering Publications by an authorized administrator of VCU Scholars Compass. For moreinformation, please contact libcompass@vcu.edu.Downloaded fromhttp://scholarscompass.vcu.edu/egre_pubs/129AuthorsY. Fu, Y.-T. Moon, F. Yun, Ü. Özgür, J. Xie, S. Doğan, Hadis Morkoç, C. K. Inoki, T. S. Kuan, Lin Zhou, andDavid J. SmithThis article is available at VCU Scholars Compass: http://scholarscompass.vcu.edu/egre_pubs/129Effectiveness of TiN porous templates on the reduction of threadingdislocations in GaN overgrowth by organometallic vapor-phase epitaxyY. Fu, Y. T. Moon, F. Yun,a! Ü. Özgür, J. Q. Xie, S. Doğan,b! and H. MorkoçDepartment of Electrical and Computer Engineering, Virginia Commonwealth University, Richmond,Virginia 23284C. K. Inoki and T. S. KuanDepartment of Physics, SUNY at Albany, Albany, New York 12222Lin Zhou and David J. SmithCenter for Solid State Science, Arizona State University, Tempe, Arizona 85287sReceived 6 August 2004; accepted 10 November 2004; published online 21 January 2005dWe report on the reduction of threading dislocations in GaN overlayers grown by organometallicvapor phase epitaxy on micro-porous TiN networks. These networks were obtained by in situannealing of thin Ti layers deposited in a metalization chamber, on the s0001d face of GaNtemplates. Observations by transmission electron microscopy indicate dislocation reduction byfactors of up to 10 in GaN layers grown on TiN networks compared with the control GaN. X-raydiffraction shows that GaN grown on the TiN network has a smaller s102d plane peak width s4.6arcmind than the control GaN s7.8 arcmind. In low temperature photoluminescence spectra, a narrowexcitonic full-width-at-half-maximum of 2.4 meV was obtained, as compared to 3.0 meV for thecontrol GaN, confirming the improved crystalline quality of the overgrown GaN layers. © 2005American Institute of Physics. fDOI: 10.1063/1.1849833gGaN and related nitride compounds have successfullypenetrated the marketplace in terms of light-emitting devicesand, to a lesser extent, light-detecting devices. However, op-tical and electrical properties of the nitride materials are ad-versely affected by the non-native substrates on which theyare normally grown.1 The resulting heteroepitaxial GaN hasa high density of threading dislocations sTDsd and associatedpoint defects which scatter charge carriers, hamper radiativerecombination efficiency, and cause device instabilities.Thus, TDs are detrimental to the operational lifetime andperformance of GaN-based devices.2 To mitigate the TDproblem, the epitaxial lateral overgrowth sELOd techniquehas been developed and widely used to reduce TD densityand also to obtain device-quality GaN epilayers.3 However,the ELO process requires ex situ photolithographic stepssd,where the frequency of the steps depends on how manytimes the process needs to be repeated in a given structure,which is both cumbersome and costly. To overcome thisdrawback, several groups have reported on the micro-ELOmethod using a discontinuous SiNx layer in situ grown asmask.4–6The self-separation from sapphire of a thick GaN tem-plate sa few hundred micronsd grown by hydride vapor phaseepitaxy sHVPEd, with the aid of a TiN interlayer, has re-cently been reported.7 A very low TD density of 53106 cm−2 was achieved for GaN grown on the TiN inter-layer. However, the thickness of the HVPE GaN reached 300µm in these experiments, and dislocation densities on theorder of 106 cm−2 are already possible in GaN layers grownby the same method without any TiN network.8 It is thusunclear whether the improved results are due solely to theTiN network. It is, therefore, worthwhile to investigate theusefulness of a TiN network for dislocation reduction inmuch thinner GaN films. In this letter, we report on thegrowth and charcterization of GaN grown by OMVPE onTiN micro-network.The process began with the growth of a 0.7 µm GaNtemplate on a sapphire substrate. A 20-nm-thick Ti film wasthen deposited on the GaN surface by e-beam evaporation.This step was followed by in situ annealing of the Ti filmabove 1000 °C in a mixture of NH3 and H2 gases, to form aTiN network. This annealing step and the subsequent GaNgrowth were carried out in a OMVPE reactor. A relativelythick layer of GaN was then grown on the TiN network at1030 °C, with a constant TMGa flow rate of 78 µmol/minand a NH3 flow rate of 7.6 l/min. For comparison, a controlGaN layer was grown on the same 0.7 µm GaN templateusing identical growth conditions but without the TiN net-work.During the initial stages of annealing, it was observedthat close chains of small openings formed on the Ti film.The typical size of the smooth Ti sTiNd surface surroundedby these chains is ,2 mm, which is similar to that reportedby Mikaye et al.9 As annealing progresses, both the densityand size of these windows increase. Smaller windows alsobegin to form inside the initially closed chains. Figures 1sadand 1sbd show the surface morphology of TiN networks onGaN templates labeled T63 and T68, respectively, after insitu annealing. Template T63 was annealed for 30 min inNH3:H2=1:1 at 1050 °C and template T68 was annealed for60 min in NH3:H2=1:3, at the same tempeature. Most win-dows on template T63 have smaller sizes than those on T68,and they are separated from each other by a TiN network.Sample T68 has less TiN coverage due to the longer anneal-ing time and higher H2 flow rate, which acts as an etchant forthe Ti layer.The GaN growth on these TiN networks originates asGaN islands at the microscopic windows of the discontinu-ous TiN network. Further GaN growth then continues fromadAuthor to whom correspondence should be addressed; electronic mail:fyun@vcu.edubdAlso with Atatürk University, Faculty of Art & Science, Department ofPhysics, 25240 Erzurum, Turkey.APPLIED PHYSICS LETTERS 86, 043108 s2005d0003-6951/2005/86~4!/043108/3/$22.50 © 2005 American Institute of Physics86, 043108-1 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:128.172.48.59 On: Fri, 17 Apr 2015 17:17:47the islands by lateral and vertical expansion, leading eventu-ally to coalescence and overgrowth. Figures 2sad and 2sbdshow cross-sectional, bright-field TEM images for samplesT63 and T68 grown on TiN network, respectively. Thin andextended surface voids are formed above the discontinuousTiN layer, due to the lateral overgrowth of GaN and a pos-sible “anti-surfactant effect” of TiN. These voids result in theelimination of an interface between TiN and the laterallyovergrowth GaN layer, which may help to release interfacialstress and lead to enhanced quality of the coalesced fronts, asreported elsewhere.10 For both samples, the density ofthreading dislocations significantly decreases at/above theTiN/GaN interface.For template T63, both the windows and TiN betweenthose windows have an average size of ,1 mm, so that theTiN layer has a surface coverage of ,50%. Most TDs fromthe GaN template are blocked by the TiN layer, while thosepenetrating through the windows tend to bend and/or clusterinto dislocation arrays in the upper GaN layer fFig. 2sadg.The size of the windows in sample T68 increases to 3–4 µmwith reduced TiN coverage between the windows, becauselonger annealing times lead to increased TiN desorption. Inthis sample, we observe more straight screw dislocationspenetrating through TiN fFig. 2sbdg than in sample T63. Forcomparison, Fig. 2scd shows a cross-sectional TEM image ofa control GaN, which has no discernible dislocation reduc-tion above the initial GaN template.An assessment of the amount of dislocation reductioncan be made by directly counting the number of dislocationsin plan-view TEM micrographs. The plan-view image in Fig.3sad of a GaN control layer grown without TiN shows a highdensity of edge/mixed dislocation arrayss,1.53109/cm2d as marked by “e,” and a much lower den-sity of isolated end-on screw dislocations s,1.33108/cm2das marked by “s.” The end-on screw dislocations showstrong, characteristic contrast aligned with the imaging re-flection vector, and the contrast from edge/mixed disloca-tions is much weaker due to smaller Burgers vectors andstrain-relieving array configurations. The plan-view image ofthe sample T63 in Fig. 3sbd, on the other hand, shows asignificantly s,103 d lower density of edge/mixed disloca-tions s,1.63108/cm2d and ,23 reduction in screw dislo-cations s,0.73108/cm2d. In sample T68 fFig. 3scdg, we findeven fewer edge dislocations s,0.93108/cm2d and morescrew dislocations s,1.43108/cm2d than in sample T63,consistent with the observations in Figs. 2sad and 2sbd. Theseplan-view observations suggest that the thin TiN network canbe very effective in reducing the density of edge/mixed dis-locations sby an order of magnituded. As for screw disloca-tions, their number is already small sabout 10% of the totalFIG. 1. Scanning electron micrographs showing surface morphology of TiNon sad template T63, and sbd template T68.FIG. 2. Cross-sectional electron micrographs of GaN layers grown on TiNnetworks in sad T63, sbd T68, and grown without a TiN network in scd.043108-2 Fu et al. Appl. Phys. Lett. 86, 043108 ~2005! 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:128.172.48.59 On: Fri, 17 Apr 2015 17:17:47dislocationsd in our OMVPE GaN, and they are not bent bythe TiN network fFig. 2sbdg. The impact of TiN in reducingthe density of screw dislocations is therefore smaller.High-resolution x-ray rocking curves sv scand from thesamples T63, T68, and the control sample, have a s102d peakwidth of 4.6, 5.4, and 7.8 arcmin, respectively sTable Id. Thistrend is consistent with the TEM analyses since the s102dpeak width reflects the total density of edge, screw, andmixed dislocations. The s002d peak width, which is sensitivemostly to the screw dislocations, remains roughly the same,also in agreement with the TEM results.Low temperature PL measurement s11 K, not shownd,provides evidence that the dominant emission line in all threesamples is related to donor-bound exciton recombinationsD0Xd. The FWHMs of D0X for samples T63, T68, and con-trol GaN are 2.4, 3.8, and 3.0 meV respectively, again con-sistent with the trend observed from x-ray and TEM analy-ses. Furthermore, the intensity of D0X in T63 is about 30times higher than that of the control sample. The narrowerexcitonic peak width, as well as the much stronger emissionintensity, correlates with the reduced density of defects inGaN grown on the TiN network.In summary, we have demonstrated that the density ofTDs, particularly the edge/mixed type, can be substantiallyreduced using a micron-scale TiN network. Electron micro-graphs show that the TiN network prevents most edge/mixedtype TDs in GaN templates from penetrating into the upperlayer, resulting in an order of magnitude reduction in thedislocation density. XRD and PL results are consistent withthe TEM investigations. Different annealing times produceTiN networks with different surface coverage, which in turnimpact the extent of the efficacy of TiN for reducing thedislocation density.The work at VCU and SUNY Albany has been fundedby ONR as part of a DURINT program, monitored by Dr. C.E. C. Wood. The authors thank Professor R. M. FeenstrasCarnegie Mellon Universityd, A. Baski sVCUd, and D.Johnstone sVCUd for helpful discussions.1H. Morkoç, Nitride Semiconductors and Devices sSpringer, 1999d.2M. A. Reshchikov and H. Morkoç, J. Appl. Phys. ssubmittedd.3See, for example, P. Gibart, B. Beaumont, and P. Vennéguès, in Handbookon Materials and Devices, edited by P. Ruterana, M. Albrecht, and J.Neugebauer sWiley-VCH, Weinheim, 2003d, Chap. 2, ISBN: 3-527-40387-6.4S. Haffouz, B. Beaumont, P. Vennéguès, and P. Gibart, Phys. Status SolidiA 176, 677 s1999d.5S. Sakai, T. Wang, Y. Morishima, and Y. Naoi, J. Cryst. Growth 221, 334s2000d.6A. Sagar, R. M. Feenstra, C. K. Inoki, T. S. Kuan, Y. Fu, Y. T. Moon, F.Yun, and H. Morkoç, Proceedings of the International Workshop on Ni-tride Semiconductors, Phys. Status Solidi sin pressd.7Y. Oshima, T. Eri, M. Shibata, H. Sunakawa, and A. Usui, Phys. StatusSolidi A 194, 554 s2002d.8P. Visconti, K. M. Jones, M. A. Reshchikov, F. Yun, R. Cingolani, H.Morkoç, S. S. Park, and K. Y. Lee, Appl. Phys. Lett. 77, 3743 s2000d.9H. Miyake, M. Yamaguchi, T. Maeda, and I. Akasaki, MRS Internet J.Nitride Semicond. Res. 551, W2.3 s2000d.10M. H. Kim, Y. Choi, J. Yi, M. Yang, J. Jeon, S. Khym, and S.-J. Leem,Appl. Phys. Lett. 79, 1619 s2001d.FIG. 3. Plan-view electron micrographs showing the top of sad a 13-µm-thick GaN control sample grown without TiN, sbd T63, and scd T68.TABLE I. List of TiN preparation conditions and XRD and PL data for GaNfilms grown on TiN templates.Sample Control T63 T68NH3 /H2 ratio 0 1:1 1:3Anneal time 0 30 min 60 minXRD s002d FWHM sarcmind 3.9 4.4 3.8XRD s102d FWHM sarcmind 7.6 4.6 5.4LT-PL exciton FWHM smeVd 3.0 2.4 3.8043108-3 Fu et al. Appl. Phys. Lett. 86, 043108 ~2005! 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:128.172.48.59 On: Fri, 17 Apr 2015 17:17:47

Effectiveness of TiN porous templates on the reduction of threading dislocations in GaN overgrowth by organometallic vapor-phase epitaxy

http://scholarscompass.vcu.edu/cgi/viewcontent.cgi?article=1078&context=egre_pubs

Effectiveness of TiN porous templates on the reduction of threading dislocations in GaN overgrowth by organometallic vapor-phase epitaxy

Abstract

Similar works

Full text

Available Versions

VCU Scholars Compass