Materials Chemistry and Physics 100 (2006) 457–459Effect of nitridation on crystallinity of GaN grown on GaAs by MBEO. Mak...
458 O. Maksimov et al. / Materials Chemistry and Physics 100 (2006) 457–459Fig. 1. RHEED patterns for: (A) GaAs substrate ...
O. Maksimov et al. / Materials Chemistry and Physics 100 (2006) 457–459 459AcknowledgementThis material is based upon work...
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Effect of nitridation on crystallinity of GaN grown on GaAs by MBE


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Effect of nitridation on crystallinity of GaN grown on GaAs by MBE

  1. 1. Materials Chemistry and Physics 100 (2006) 457–459Effect of nitridation on crystallinity of GaN grown on GaAs by MBEO. Maksimova,∗, P. Fisherb, M. Skowronskib, V.D. Heydemannaa Electro-Optics Center, Pennsylvania State University, 559A Freeport Road, Freeport, PA 16229, United Statesb Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, United StatesReceived 6 October 2005; received in revised form 29 December 2005; accepted 23 January 2006AbstractGaN films are grown on [0 0 1] GaAs substrates by plasma-assisted molecular beam epitaxy using a three-step process that consists of a substratenitridation, deposition of a low-temperature buffer layer, and a high-temperature overgrowth. Films are evaluated by X-ray diffraction and thedependence of crystalline quality on the nitridation temperature is studied. It is demonstrated that nitridation has to be performed at low-temperatureto achieve c-oriented ␣-GaN. Higher nitridation temperature promotes formation of mis-oriented domains and ␤-GaN inclusions© 2006 Elsevier B.V. All rights reserved.Keywords: Molecular beam epitaxy; GaN; GaAsGaN materials are technologically important for a variety ofdevice application [1,2]. They are ideal candidates for fabri-cation of high power microwave devices, high frequency fieldeffect transistors, high electron mobility transistors, light emit-ters and detectors operating in the visible to UV spectral range.High quality hexagonal ␣-GaN films and heterostructures areusually grown either by metal organic chemical vapor deposition(MOCVD) or by molecular beam epitaxy (MBE) on sapphireand 6H-SiC substrates [3,4]. Growth on [0 0 1] GaAs is muchless studied, although these substrates provide several advan-tages, such as, low cost, easy cleavage along [0 1 1] direction,closer thermal expansion coefficient matching, and possibilityto stabilize cubic ␤-GaN.We have reported that direct deposition on a thermally des-orbed GaAs results in the growth of a polycrystalline poorly ori-ented ␣-GaN containing mis-oriented domains and large cubicinclusions. However, a significant improvement of the crys-tallinity is achieved by adopting the growth procedure that con-sists of a substrate nitridation, deposition of a low-temperaturebuffer layer, and epitaxial overgrowth at elevated temperature[5]. The nitridation conditions are extremely critical for this pro-cess and have to be carefully controlled to achieve high-qualityfilm.Here we investigate the influence of the substrate tempera-ture during nitridation on the structural properties of GaN film.∗ Corresponding author. Tel.: +1 724 295 6624; fax: +1 724 295 6617.E-mail address: (O. Maksimov).We observe that low-temperature (400 ◦C) nitridation promotesgrowth of c-oriented ␣-GaN. When nitridation is performedat higher temperature, crystalline quality degrades and filmbecomes polycrystalline.The samples are fabricated in a custom-built MBE systemequipped with a Ga effusion cell, a radio frequency (RF) excitedplasma source (SVT Associates, Inc.), a retractable ion gauge forflux calibration, and a reflection high-energy electron diffraction(RHEED) system. GaN is grown on semi-insulating epi-ready[0 0 1] GaAs substrates indium-mounted to molybdenum hold-ers. The substrate temperature is measured by a thermocouplein contact with the backside of the mounting block. To preventAs incorporation in the GaN alloy, the oxide layer is desorbed at500 ◦C in the absence of As flux. The GaAs wafer is exposed tosub-monolayer Ga pulses to facilitate oxide desorption throughthe conversion of Ga2O3 to a more volatile Ga2O [6]. This pro-cess results in a slightly distorted GaAs surface. Kikuchi linesare clearly visible in a RHEED pattern, indicating that GaAssurface is free of oxide layer, Fig. 1A.After oxide desorption wafer temperature is adjusted to thedesired setting and nitridation is performed by exposing sub-strate to nitrogen plasma. The nitridation rate is controlled witha mass flow controller through which a high purity (6N) N2gas (gas flow is ∼2.5 sccm) is introduced into the RF-plasmasource (input power is ∼400 W). The nitridation is performedfor 15 min with the substrate temperature kept constant.When the wafer is exposed to nitrogen plasma, surfacereconstruction disappears during the first few minutes sug-gesting formation of an amorphous GaAsN layer. We observe0254-0584/$ – see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.matchemphys.2006.01.024
  2. 2. 458 O. Maksimov et al. / Materials Chemistry and Physics 100 (2006) 457–459Fig. 1. RHEED patterns for: (A) GaAs substrate after oxide desorption at 500 ◦C, (B) GaAs substrate after 5 min of nitridation at 400 ◦C, (C) GaN buffer layerdeposited at 600 ◦C, (D) GaN film grown at 750 ◦ arc pattern after approximately 5 min indicating develop-ment of a preferred out-of-plane orientation in a disorderedlayer, Fig. 1B. Since it does not change when the wafer isrotated around the surface normal, layer is not oriented in-plane. Spot-like features with hexagonal symmetry developafter, approximately, 10 min. This reconstruction does not sig-nificantly change when the wafer is exposed to nitrogen plasmafor a longer period of time. Therefore, we limit nitridationto 15 min.In the next step we close nitrogen plasma source shutterand increase wafer temperature to 600 ◦C. Annealing, ∼30 min,sharpens diffraction spots demonstrating recrystallization of ␣-GaN phase, Fig. 1C. The diffraction spots are broad signifyingthat very defective GaN layer forms at the beginning. How-ever, they become significantly sharper and elongated duringthe growth of a relatively thin (50-nm) buffer layer, indicatingthat GaN buffer has a better crystalline quality and a smoothersurface.Finally, wafer temperature is raised to 750 ◦C for GaNgrowth. A slightly diffused (1 × 1) reconstruction is observedduring the film growth, Fig. 1D.Crystalline quality of the GaN films is studied by X-raydiffraction (XRD). All the films are deposited in one growthrun under identical conditions and differ only in the nitridationtemperature (A 400 ◦C, B 500 ◦C, C 550 ◦C, D 600 ◦C). A XRDθ–2θ scan demonstrates that low temperature nitridation pro-motes growth of c-oriented ␣-GaN, Fig. 2A. Mis-oriented grains( 1 0 1 1 , 1 0 1 2 , 1 1 2 0 , 1 0 1 3 ) and cubic ␤-GaN inclu-sions ( 0 0 2 ) develop when nitridation is performed at 500 ◦C,Fig. 2B. The intensity of 0 0 0 2 diffraction decreases whileother peaks become more pronounced with the further increaseof nitridation temperature indicating degradation of crystallinequality of the film, Fig. 2C and D. This trend is, most prob-ably, due to the surface etching that is activated by substratetemperature during nitridation [7]. It results in a rough defec-tive epilayer/substrate interface and can promote polycrystallinegrowth.In conclusion, we demonstrate that crystalline quality of GaNfilms grown on [0 0 1] GaAs substrates is extremely sensitive tonitridation conditions. Nitridation has to be performed at low-temperature (400 ◦C) to achieve c-oriented ␣-GaN. Higher sub-strate temperature promotes formation of mis-oriented domainsand ␤-GaN inclusions.Fig. 2. XRD θ–2θ scans of ∼2 ␮m thick GaN films grown on a GaAs substrate.Substrate nitridation is performed at (A) 400 ◦C, (B) 500 ◦C, (C) 550 ◦C, (D)600 ◦C.
  3. 3. O. Maksimov et al. / Materials Chemistry and Physics 100 (2006) 457–459 459AcknowledgementThis material is based upon work supported by Dr. ColinWood, ONR under Contract No. N00014-05-1-0238.References[1] S.N. Mohammad, A.A. Salvador, H. Morkoc, Proc. IEEE 83 (1995) 1306.[2] R. Fen, J.C. Zolper, Wide Bandgap Electronic Devices, World Scientific,Singapore, 2003.[3] H.X. Jiang, J.Y. Lin, Opto-Electron. Rev. 10 (2002) 271.[4] O. Brandt, R. Muralihadharan, A. Thamm, P. Waltereit, K.H. Ploog, Appl.Surf. Sci. 175 (2001) 419.[5] O. Maksimov, P. Fisher, H. Du, M. Skowronski, V. D. Heydemann, NorthAmerican MBE Conference, Santa Barbara, CA, 2005.[6] Z.R. Wasilewski, J.M. Baribeau, M. Beaulieu, X. Wu, G.I. Sproule, J. Vac.Sci. Technol. B 23 (2004) 1534.[7] I. Aksenov, H. Iwai, Y. Nakada, H. Okumura, J. Vac. Sci. Technol. B 17(1999) 1525.