Morphological study of InN films and nanorods grown by H-MOVPE
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Morphological study of InN films and nanorods grown by H-MOVPE H.J. Park, S.W. Kang, O. Kryliouk, and T. Anderson Department of Chemical Engineering, University of Florida, Gainesville, FL 32611 U.S.A. ABSTRACT Hydride-Metalorganic Vapor Phase Epitaxy (H-MOVPE) was used to grow a series of films on c-Al2O3 substrates. Depending on the growth temperature and HCl/TMIn molar ratio, InN deposited as a continuous film or a collection of micro or nanorods, or no InN growth was observed. A chemical equilibrium analysis of the In-N-H-Cl system predicts both InN growth and etching regimes with the nanorod growth observed near the growth-etching transition. All InN rod structures demonstrated well faceted hexagonal structure with a near random orientation of the rods, while the films were polycrystalline. INTRODUCTION Although InN is the least studied of the group III-nitrides, it exhibits some interesting optoelectronic and electronic characteristics that make it suitable for a variety of applications. Device structures based on InN that have already been fabricated and suggested include MISFETs [1], gas and liquid sensors [2, 3], heterojunction solar cells [4], and terahertz radiation microwave devices [5]. Interest in InN has intensified due to recent reports that the value of the bandgap energy (0.6 to 0.8 eV [6 – 11]) may be considerably lower than the previously accepted value (~1.9 eV [12 – 20]). The growth of device quality InN, however, is challenging. A low InN growth temperature (~600 to 700 °C) is required due to its low thermal stability relative to the other group III-nitrides. At low growth temperature, however, the amount of reactive nitrogen available for growth from NH3 is very low and In droplet formation can occur. Therefore a very high NH3 partial pressure is required to avoid nucleation of In liquid. Excessively high NH3 will in turn generate considerable H2 as a by product of NH3 decomposition. H2 is also a product of the apparently reversible InN deposition reaction, which will reduce the growth rate. Furthermore, the low deposition temperature makes it difficult to grow material of high structural quality due to the reduced surface mobilities of adatoms. Add in the lack of a suitable latticematched substrate, it is not surprising that the structural quality of InN has been relatively poor. There have been several morphological studies of InN growth, but all have used either MBE or MOCVD [21 – 29]. In the study reported here, Hydride-Metal Organic Vapor Phase Epitaxy (H-MOVPE) was used to grow InN. H-MOVPE is a growth technique that generates volatile InCl by reacting an organometallic precursor (TMIn in this study) with HCl in the source zone of a HVPE hot-walled reactor, and then combines the InCl with the group V hydride (NH3) as in conventional HVPE. The advantage of H-MOVPE or HVPE is that the added HCl can prevent formation of In droplets [30] and thus reduce the need for excess NH3 to allow growth of InN at a reasonable rate. It was also hoped that the surface morphology would im
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