Porous SiC (PSiC) substrates were used for the growth of GaN by reactive molecular-beam epitaxy with ammonia as the nitrogen source. Improved quality of GaNfilms has been demonstrated for growth on PSiC substrates, as compared to that on standard 6H–SiC substrates. Cross-sectional transmission electron microscopy and electron diffraction showed a reduction in dislocation density and a higher degree of lattice and thermal relaxation in the GaNfilmsgrown on porous substrates. The submicron GaNfilms exhibit a rocking curve linewidth of 3.3 arcmin for (0002) diffraction and 13.7 arcmin for (101̄2) diffraction. Low-temperature photoluminescence showed an excitonic transition with a full width at half maximum of 9.5 meV at 15 K, as well as high quantum efficiency, on the GaN layer grown on PSiC when the thin skin layer on porous SiC was removed before growth

He, L.

Inoki, C. K.

Kuan, T. S.

Morkoç, Hadis

Reshchikov, Michael A.

Yun, F.

VCU Scholars Compass

Virginia Commonwealth UniversityVCU Scholars CompassElectrical and Computer Engineering Publications Dept. of Electrical and Computer Engineering2002Growth of GaN films on porous SiC substrate bymolecular-beam epitaxyF. YunVirginia Commonwealth UniversityMichael A. ReshchikovVirginia Commonwealth University, mreshchi@vcu.eduL. HeVirginia Commonwealth UniversitySee next page for additional authorsFollow this and additional works at: http://scholarscompass.vcu.edu/egre_pubsPart of the Electrical and Computer Engineering CommonsYun, F., Reshchikov, M.A., He, L., et al. Growth of GaN films on porous SiC substrate by molecular-beam epitaxy.Applied Physics Letters, 81, 4142 (2002). Copyright © 2002 AIP 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/56AuthorsF. Yun, Michael A. Reshchikov, L. He, Hadis Morkoç, C. K. Inoki, and T. S. KuanThis article is available at VCU Scholars Compass: http://scholarscompass.vcu.edu/egre_pubs/56Growth of GaN films on porous SiC substrate by molecular-beam epitaxyF. Yun,a) M. A. Reshchikov, L. He, and H. Morkoc¸Virginia Commonwealth University, Department of Electrical Engineering, Richmond, Virginia 23284C. K. Inoki and T. S. KuanUniversity at Albany, SUNY, Dept of Physics, Albany, New York 12222~Received 26 July 2002; accepted 7 October 2002!Porous SiC ~PSiC! substrates were used for the growth of GaN by reactive molecular-beam epitaxywith ammonia as the nitrogen source. Improved quality of GaN films has been demonstrated forgrowth on PSiC substrates, as compared to that on standard 6H–SiC substrates. Cross-sectionaltransmission electron microscopy and electron diffraction showed a reduction in dislocation densityand a higher degree of lattice and thermal relaxation in the GaN films grown on porous substrates.The submicron GaN films exhibit a rocking curve linewidth of 3.3 arcmin for ~0002! diffraction and13.7 arcmin for (101¯2) diffraction. Low-temperature photoluminescence showed an excitonictransition with a full width at half maximum of 9.5 meV at 15 K, as well as high quantum efficiency,on the GaN layer grown on PSiC when the thin skin layer on porous SiC was removed beforegrowth. © 2002 American Institute of Physics. @DOI: 10.1063/1.1524304#GaN has been receiving much attention in the applica-tions to high-temperature high power electronic devices aswell as visible to UV optoelectronic devices. Therefore, in-tense effort has been made toward the growth of high-qualityGaN films in terms of electrical, optical, and structural prop-erties. It is well known that the biggest obstacle is the rela-tively high density of defects including threading disloca-tions associated with the growth on non-native substrates.Recently, researchers have been intrigued by the epitaxialgrowth on porous substrates, both porous GaN and porousSiC.1–9 They have explored the growth of SiC on porous SiC~PSiC! substrates,1,6 and the growth of GaN on porous GaNlayer on top of SiC substrates by chemical vapordeposition.7–9 These preliminary experiments have shownsigns of improvement in the epitaxial layer.The purpose of using a porous template for growth ema-nates from the belief that the nanopatterned porous structurewould favor lateral growth, which would lead to a reducedextended defect density. Due to the electrical conductive na-ture of a 6H–SiC substrate, it is easy to convert it into porousmaterial through electrochemical anodization. Therefore, it isof practical interest to study the growth of GaN on top of thisPSiC substrate. A recent study5 revealed a factor of 2 reduc-tion of dislocation density in GaN film grown on PSiC bymolecular-beam epitaxy ~MBE!, using rf plasma-assisted ni-trogen source. In this work, we report the growth and defectreduction of GaN films on PSiC by reactive MBE utilizingammonia (NH3) as the nitrogen source.The nanometer-scale pores on 6H–SiC substrate wereformed by anodization in hydrofluoric acid under UVillumination.10 Pt was used as the cathode, while the sub-strate served as the anode. The as-prepared substrates usuallyshow very few pores on the surface, while most of the poresare buried under the so-called ‘‘skin layer.’’ Figure 1~a! is thecross-sectional TEM image of an as-prepared PSiC substrate,showing such a skin layer. The porous structure underneaththe skin layer is clearly observed, with a porous templatethickness of ;2.8 mm. Two types of discernable pores arepresent, sizing around 10 and 100 nm each. It is interestingto note that most of these pores do not run along the c di-rection, instead, they start from the substrate surface andpenetrate into the substrate like cones, the opening angle ofwhich depends on the anodization parameters. A skin layerof at least 60 nm is present at the substrate surface. This skinlayer prevents most of the pores from penetrating into thesurface. It can be removed by H annealing at 1700 °C. Fig-ure 1~b! is an atomic force microscopic ~AFM! image of aPSiC surface after the removal of the skin layer. Pores can beseen exposed at the surface with much higher density, whilethe dimension of pores has been enlarged and modified bythis method of annealing.GaN films were grown under Ga-rich conditions byMBE using NH3 as reactive nitrogen source. Four sampleswere used for this study, as listed in Table I. The first sample,A, is grown on a standard ~0001! 6H–SiC substrate; the sec-ond sample, B, is grown on a ~0001! PSiC substrate with aa!Electronic mail: fyun@vcu.eduFIG. 1. ~a! Cross-sectional TEM image showing the porous structure ofanodized 6H–SiC substrate. The skin layer is present which blocks mostpores from reaching the surface. ~b! AFM surface morphology of PSiCsubstrate after H annealing for 1 min at 1700 °C.APPLIED PHYSICS LETTERS VOLUME 81, NUMBER 22 25 NOVEMBER 200241420003-6951/2002/81(22)/4142/3/$19.00 © 2002 American Institute of Physics 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:128.172.48.58 On: Tue, 21 Apr 2015 19:37:23skin layer of ;60 nm. The thickness of the GaN in samplesA and B are thin ~0.11–0.15 mm!, and they were grownwithout any buffer layers. The third and fourth samples arethicker ~0.77–1.40 mm!, and were grown on 4H–PSiC with askin layer for sample C, and on 6H–PSiC with its skin layerremoved for sample D, respectively. Both samples C and Dare grown with a thin layer of AlN buffer (;40 nm) betweenGaN epilayer and the PSiC substrate. The substrate ofsample C has 8° miscut toward (112¯0) orientation. GaNgrowth temperature was about 650 °C for growth on PSiCsubstrate, and the NH3 flow rate was fixed at 10 sccm. Nomi-nally similar growth parameters were employed, which wereoptimized for GaN/6H–SiC growth. The removal of skinlayer is performed by H annealing at 1700 °C for 1 min.11Figure 2 shows the cross-sectional TEM images forsamples A and B. For the thin GaN layer (;0.15 mm) grownon a standard 6H–SiC substrate, as shown in Fig. 2~a!, adislocation density of ;331010 cm22 is observed, togetherwith twins forming on the c-plane of GaN layer. The thinnerGaN layer ~;0.11 mm! grown on a PSiC substrate with askin layer @Fig. 2~b!# exhibits a slightly lower density ofdislocations in the order of ;231010 cm22, which may berelated to the smaller film thickness. Strain condition of GaNwith respect to SiC can be obtained by measuring the relativelattice constant from electron diffraction pattern which in-cludes both GaN and SiC reflections. The in-plane latticemismatch for samples A and B are, therefore, estimated to beDa/uau53.1% and Da/uau53.3%, respectively. Consideringthe in-plane lattice mismatch between GaN and SiC crystals~bulk material! is Da/uau53.48%, we found the degree ofstrain relaxation to be 90% and 95% for samples A and B,respectively. It is clear that for the same thickness of GaNlayer, it is easier to achieve lattice and/or thermal relaxationwhen grown on PSiC substrate, even with the presence of askin layer.The effect of a skin layer is significant on the dislocationdistribution. First, we examine sample C, which is GaNgrown on PSiC with a skin layer. The dislocation density isabout 53109 cm22 for a GaN thickness up to 1.40 mm, ascan be seen in Fig. 3~a!. However, when the nonporous skinlayer is removed from the PSiC surface, the growth qualityof GaN epilayer has significantly improved. As shown forsample D in Fig. 3~b!, the dislocation density near the top ofTABLE I. GaN layers grown on PSiC and standard 6H–SiC substrate by MBE using NH3 source.SampleNo. Substrate Growth parameter Da/uauDislocation densitynear top GaNA Standard ~0001! 6H–SiCsubstrateNH3510 sccm, Ts5585 °C,thickness ;0.15 mm3.1%~90%!;331010 cm22B ~0001! PSiC on axis,with skin layer ;60 nmNH3510 sccm, Ts5655 °C,thickness ;0.11 mm3.3%~95%!;231010 cm22C ~0001! PSiC off 8°,with skin layer ;60 nmNH3510 sccm, Ts5655 °C,thickness ;1.40 mm3.6%~100%!;53109 cm22D ~0001! PSiC, H anneal,1700 °C/1 min. No skinlayerNH3510 sccm, Ts5650 °C,thickness ;0.77 mm3.6%~100%!;13109 cm22FIG. 2. Cross-sectional TEM images for ~a! GaN layer ~0.15 mm—sampleA! grown on standard 6H–SiC substrate, and ~b! GaN layer ~0.11 mm—sample B! grown on PSiC substrate with skin layer. The insets show theelectron diffraction patterns for GaN layer and the SiC substrates.FIG. 3. Cross-sectional TEM images for ~a! GaN layer ~1.40 mm—sampleC! grown on PSiC substrate with skin layer, and ~b! GaN layer ~0.77 mm—sample D! grown on PSiC substrate after the skin layer removal by high-temperature (1700 °C) anneal for 1 min. The insets show the electron dif-fraction patterns for GaN layer and the PSiC substrates.4143Appl. Phys. Lett., Vol. 81, No. 22, 25 November 2002 Yun et al. 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:128.172.48.58 On: Tue, 21 Apr 2015 19:37:23a 0.77 mm GaN grown on PSiC without a skin layer has beenreduced to ;13109 cm22. If the same layer were grown toa thickness of 1.40 mm, it can be predicted that the disloca-tion density will be further reduced. The effect of the Hannealing at a high temperature can be clearly observed inFig. 3~b!. Many pores extend to the surface where the AlNbuffer layer and GaN epilayer were grown, though the inter-face is greatly roughened due to the presence of a large quan-tity of pores exposed on the surface after etching. It can bealso observed that some of the exposed pores are backfilledwith Al and/or Ga droplets during the MBE growth ~darkarea inside pores!. For samples C and D, the electron diffrac-tion patterns indicate 100% relaxation based on the measuredvalue of Da/uau53.6% for both. As discussed in Ref. 5,when Da/uau exceeds the strain-free value ~3.48%!, the GaNfilm is likely to be slightly under tensile strain. It is alsopossible that the lattice constant of PSiC is slightly differentfrom SiC, as is the case with porous Si material.A tentative growth mechanism can be suggested, thoughspeculative at this juncture, for GaN growth on PSiC. At thebeginning of growth, AlN buffer grows on the nonporousregions of SiC, followed by GaN growth on top of the AlNcolumns. This is supported by the observation of open tubesin the initial stages of GaN growth, especially on the tiltedPSiC substrates.5,12 As the growth progresses, lateral epitaxyis enhanced, and the open tubes are closed at the upper partof the GaN layer. These tubes may serve as a relief mecha-nism of strain caused by the lattice and thermal mismatchduring growth. In addition, some threading dislocations startto bend, merge, and bury in the midlayer.The crystalline quality of GaN grown on PSiC substratewas accessed by high-resolution x-ray diffraction ~XRD!rocking curve ~v scan!. The best full width at half maximum~FWHM! of the ~0002! rocking curve diffraction obtained~sample B! was 3.3 arcmin, and that of (101¯2) was 13.7arcmin. This implies that device quality GaN film can beachieved in thin layers grown on PSC substrate.Moreover, low-temperature photoluminescence ~PL!measurements indicate a good quality of GaN films grownon PSiC substrates. Figure 4 shows the PL spectra measuredat 15 K for samples B, C, and D. The spectra show the bestFWHM of GaN excitonic peak to be ;9.5 meV for GaNgrown on PSiC with the skin layer removed ~sample D!,while GaN films with the skin layer exhibit FWHMs of14–16 meV ~samples B and C!. The quantum efficiency ofPL from thicker layers ~samples C and D, both with andwithout skin layer! is quite high ~about 1.6%!, whereas thatfrom the thin layer is low ~about 0.1%!, indicating a reduc-tion of nonradiative defects ~presumably dislocations! inthicker layers. The highest optical quality of sample D fromPL spectra agrees with the lowest dislocation density (;13109 cm22) of this GaN layer grown on top of the porousSiC with the skin layer removed.In conclusion, we have grown GaN epitaxial layers us-ing PSiC substrates, both with and without the nonporousskin layer, by reactive MBE using an ammonia source. Dis-cernable reduction in dislocation density was demonstratedin GaN grown on a PSiC template especially with the skinlayer removed prior to the growth of GaN. Specifically, thedislocation density is reduced by an order of magnitude ~to;13109 cm22) when compared with GaN grown on stan-dard 6H–SiC substrates. The narrow linewidth from x-rayrocking curves, and high-quality PL spectra indicate an im-proved quality of GaN films grown on porous SiC substrates.The authors would like to thank Dr. C. D. Lee and Pro-fessor R. M. Feenstra for cutting and H etching of SiC sub-strates. The work at both institutions ~VCU and SUNY! wasfunded under a DoD DURINT program and monitored by C.E. C. Wood of ONR. The work at VCU also benefited fromgrants by AFOSR, NSF and ONR.1 M. Mynbaeva, S. E. Saddow, G. Melnychuk, I. Nikitina, M. Scheglov, A.Sitnikova, N. Kuznetsov, K. Mynbaev, and V. Dmitriev, Appl. Phys. Lett.78, 117 ~2001!.2 S. E. Saddow, M. Mynbaeva, W. J. Choyke, S. Bai, G. Melnychuk, Y.Koshka, V. Dimitriev, and C. E. C. Wood, Mater. Sci. Forum 353, 115~2001!.3 X. Li, Y.-W. Kim, P. W. Bohn, and I. Adesida, Appl. Phys. Lett. 80, 980~2002!.4 K. Kusakabe, A. Kikuchi, and K. Kishino, J. Cryst. Growth 237, 988~2002!.5 C. K. Inoki, T. S. Kuan, C. D. Lee, A. Sagar, and R. M. Feenstra, Mater.Res. Soc. Symp. Proc. 722, K1.3 ~2002!.6 J. E. Spanier, G. T. Dunne, L. B. Rowland, and I. P. Herman, Appl. Phys.Lett. 76, 3879 ~2000!.7 K. Kusakabe, A. Kikuchi, and K. Kishino, Jpn. J. Appl. Phys., Part 2 40,L192 ~2001!.8 M. Mynbaeva, A. Titkov, A. Kryganovskii, V. Ratnikov, K. Mynbaev, H.Huhtinen, R. Laiho, and V. Dmitriev, Appl. Phys. Lett. 76, 1113 ~2000!.9 M. Mynbaeva, A. Titkov, A. Kryzhanovski, I. Kotousova, A. S. Zubrilov,V. V. Ratnikov, V. Y. Davydov, N. I. Kuznetsov, K. Mynbaev, D. V. Ts-vetkov, S. Stepanov, A. Cherenkov, and V. A. Dmitriev, MRS Internet J.Nitride Semicond. Res. 4, 14 ~1999!.10 PSiC substrates from TDI, Inc., Silver Spring, MD 20904.11 A. Sagar, C. D. Lee, R. M. Feenstra, C. K. Inoki, and T. S. Kuan, J. Appl.Phys. 92, 4070 ~2002!.12 F. Yun, M. A. Reshchikov, L. He, T. King, D. Huang, H. Morkoc¸, C. K.Inoki, and T. S. Kuan, Mater. Res. Soc. Symp. Proc. 719, F1.3 ~2002!.FIG. 4. Low-temperature PL spectra of GaN samples B, C, and D. SampleD shows the best PL results with the narrowest FWHM of excitonic peak~9.5 meV!.4144 Appl. Phys. Lett., Vol. 81, No. 22, 25 November 2002 Yun et al. 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:128.172.48.58 On: Tue, 21 Apr 2015 19:37:23

Growth of GaN films on porous SiC substrate by molecular-beam epitaxy

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Growth of GaN films on porous SiC substrate by molecular-beam epitaxy

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