Vacuum 80 (2006) 1042–1045 Short communication Structural and optical properties of GaN films grown on GaAs substrates by molecular beam epitaxy O. Maksimov a,Ã , Y. Gong b , H. Du c , P. Fisher c , M. Skowronski c , I.L. Kuskovsky b , V.D. Heydemann a a Electro-Optics Center, Pennsylvania State University, Freeport, PA 16229, USA b Department of Physics, Queens College of City University of New York, Flushing, NY 11367, USA c Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA Received 24 September 2005; accepted 3 January 2006 Abstract GaN films are grown on [0 0 1] GaAs substrates by plasma-assisted molecular beam epitaxy using a three-step process that consists of a substrate nitridation, deposition of a low-temperature buffer layer, and a high-temperature overgrowth. X-ray diffraction and transmission electron microscopy indicate that this method promotes prismatic growth of c-oriented a-GaN. Photoluminescence studies show that the emission from cubic b-GaN inclusions dominates the spectrum. r 2006 Elsevier Ltd. All rights reserved. Keywords: Molecular beam epitaxy; GaN; GaAs GaN materials are technologically important for a variety of device application [1,2]. They are ideal candi- dates for fabrication of high power microwave devices, high-frequency field effect transistors, high electron mobi- lity transistors, light emitters and detectors operating in the visible to UV spectral range. High-quality hexagonal GaN (a-GaN) films and heterostructures are usually grown either by metal organic chemical vapor deposition (MOCVD) or by molecular beam epitaxy (MBE) on sapphire (a-Al 2 O 3 ) and 6H-SiC substrates [3,4]. Growth on [0 0 1] GaAs is much less studied, although these substrates provide several advantages, such as, low cost, easy cleavage along [0 1 1] direction, closer thermal expan- sion coefficient matching, and possibility to stabilize cubic b-GaN. When GaN is grown on GaAs, an optimization of the GaAs surface, by growing a GaAs buffer layer, always precedes GaN deposition [5–7]. However, As gets incorpo- rated into the GaN alloy, degrading luminescent and transport properties of the film [8,9]. Therefore, it is of interest to explore GaN growth in the As-free environment. We have previously observed that direct deposition on a thermally desorbed GaAs results in the growth of a polycrystalline poorly oriented a-GaN containing misor- iented domains and large cubic inclusions. However, a significant improvement of the crystallinity is achieved by adopting the growth procedure that consists of a substrate nitridation, deposition of a low-temperature buffer layer, and epitaxial overgrowth at elevated temperature. These steps effectively suppress misorientation and improve morphology of the film [10]. Here we present a detailed study on the structural and optical properties of the GaN grown under these condi- tions. The crystalline quality and the morphology of the films are examined with X-ray diffraction (XRD), trans- mission electron and atomic force microscopes (TEM and AFM). Optical properties are evaluated with the contin- uous wave photoluminescence (PL). The samples are fabricated in a custom-built MBE system equipped with a Ga effusion cell, a radio frequency excited nitrogen plasma source, a retractable ion gauge for flux calibration, and a reflection high-energy electron diffraction (RHEED) system. For the growth we use ARTICLE IN PRESS www.elsevier.com/locate/vacuum 0042-207X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2006.01.001 Ã Corresponding author. Tel.: +1 724 295 6624; fax: +1 724 295 6617. E-mail address: Maksimov@netzero.net (O. Maksimov).