Structural Characterization of Thick GaN Films Grown by Hydride Vapor Phase Epitaxy
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OF THICK GAN
FILMS
GROWN BY
L.T. Romano, R.J. Molnar*, B.S. Krusor, G.A. Anderson, D.P. Bour, and P.Maki* Xerox Palo Alto Research Center, Palo Alto, CA 94304, [email protected] *Massachusetts Institute of Technology, Lincoln Laboratory, Lexington, MA 02173-9108 ABSTRACT The structural quality of GaN films grown by hydride vapor phase epitaxy (HVPE) was characterized by transmission electron microscopy (TEM), x-ray diffraction (XRD), and atomic force microscopy (AFM). F'tlms were grown up to 40[tm on sapphire with either a GaC1 pretreatment prior to growth or on a ZnO buffer layer. Dislocation densities were found to decrease with increasing film thickness. This is attributed to the mixed nature of the defects present in the film which enabled dislocation annihilation. The thickest film had a defect density of 5x 10 dislocations/cm 2. INTRODUCTION Defect densities in GaN blue LED devices are as high as 101° dislocations/cm 2 and appear not to introduce nonradiative recombination centers as with other III-V semiconductor systems. However the lifetimes of these devices may depend on the defects as shown with other III- V devices [1]. Achieving lower defect densities in the GaN material system has been difficult since substrates that are both lattice and chemically matched, and have the same symmetry as GaN are not available. It would also be desirable for the substrate to cleave in order to obtain facets for edge emitting lasers. Bulk GaN single crystals have been difficult to grow wider than a few millimeters [2]. Currently, sapphire is the most common substrate with a 16% lattice mismatch and thermal expansion difference 1.4 times greater than GaN. Silicon carbide substrates are also used which have a 3.5% lattice mismatch and a thermal expansion difference that is 2.0 times smaller than GaN [3]. In this paper we report the structural properties of thick GaN films grown on sapphire by hydride vapor phase epitaxy (HVPE) which is capable of growing films at a rate of several tens of microns/hour. Room temperature mobilities up to 768cm 2 /Vs have been obtained for films with a ZnO buffer layer [4]. These films could then be used as substrates for nitride devices. Homoepitaxial growth of GaN by metal organic chemical vapor deposition (MOCVD) and a A1GaN/GaN/AIGaN double heterostructure (DH) grown by electron-cyclotron-resonance molecular beam epitaxy (ECR-MBE) were made on 15•tm thick GaN f'ilms. EXPERIMENT The details of the HVPE process have been published previously [5]. GaN films were grown to thicknesses ranging from 10 - 40 jtm on sapphire at temperatures between 1050 1100°C2. The sapphire surface either had a G-aCI pretreatment at 1050°C or was grown on a sputter deposited ZnO buffer layer. Growth of the GaN/AIGaN DH by ECR-MBE was initiated at 700 - 7500°C without a low temperature buffer layer as discussed elsewhere [6]. Homoepitaxial layers of GaN were grown by MOCVD directly at high temperature without the need of a low temperature buffer layer. 245 Mat. Res. Soc. Symp. Proc. Vol. 423 ©1996 Materials
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