In-Situ Observation of AlN Formation During Nitridation of Sapphire by Ultrahigh Vacuum Transmission Electron Microscopy
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M. YEADON, M.T. MARSHALL, F. HAMDANI, S. PEKIN, H. MORKOC, J.M. GIBSON Materials Research Laboratory, University of Illinois, Urbana, IL 61801 ABSTRACT Using a novel ultrahigh vacuum transmission electron microscope (UHV TEM) with insitu molecular beam epitaxy capability we have studied the nitridation of (0001) sapphire upon exposure to ammonia. Atomically flat sapphire surfaces for the experiments were obtained by high temperature annealing. Subsequent exposure to ammonia flow at 950'C led to the successful synthesis of epitaxial AIN; the films were characterized in-situ using TEM. Complimentary ex-situ atomic force microscopy (AFM) was also performed in order to characterize the surface morphology before and after nitridation. The experiments indicate that AIN grows by a 3D island growth mechanism. Electron diffraction patterns suggest an abrupt AIN/sapphire interface with no evidence of the formation of Al-O-N compounds. The rate limiting step in the nitridation reaction appears to be the diffusion of nitrogen and oxygen species between the free surface of the growing AIN film and the reaction interface. It is inferred from kinetic measurements that diffusion of these species occurs along the boundaries between coalescing AIN islands. INTRODUCTION Advances in the growth of epitaxial p- and n-type doped GaN thin films have enabled the recent realization of optoelectronic devices operating in the blue and ultraviolet regions of the electromagnetic spectrum [1-3]. Despite a large lattice mismatch with the I11-V nitride compounds, sapphire remains the most commonly used substrate for device fabrication. The growth of high quality epitaxial GaN on sapphire currently involves a number of key processing stages which have been determined largely empirically in recent years. The growth of a buffer layer of AIN or GaN at a temperature well below the regular GaN growth temperature leads to a substantial improvement in the quality of the GaN active layer [4-8] by promoting lateral film growth [4, 6]. Furthermore, the quality of the buffer layer, and in turn the active layer, can be improved by exposure of the sapphire surface to nitrogen species at temperature prior to growth [9-11]. This latter 'nitridation' stage has become a subject of considerable interest. A number of workers have speculated concerning the nitridation mechanism and reaction products with various conflicting reports for a range of deposition techniques. For example, reports of the formation of an amorphous A1-O-N compound and single crystal AIN have been made for MOCVD-grown material [12, 13]. Recent RHEED investigations have also provided strong evidence for the formation of AIN during nitridation by MBE [5-6, 11]. In this paper we present a study of the nitridation of the sapphire surface during ammonia exposure using a novel ultrahigh vacuum transmission electron microscope (UHV TEM). The system permits imaging of the nitridation process in addition to electron diffraction experiments thus enabling identification of the reaction products as they for
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