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ABSTRACT In this paper GaN films are examined, which are grown on basal plane (0001) sapphire substrates. Growth is performed in a novel type of vertical rotating disk reactor. Results on the effect of a GaN nucleation layer on the properties of the overgrown GaN epilayer are presented. Characterisation includes surface morphology studies, DC X-ray diffraction and optical characterisation. Best film quality so far has a double crystal X-ray half width of 85 arcsec at approximately 1 ýtm thickness. INTRODUCTION With a wide direct bandgap of 3.39 eV and excellent physical properties gallium nitride (GaN) is a very promising material for fabricating blue light emitting devices. A few possible applications are high density optical data storage, full color video-screens and even trafficlights.[l,2] Recently significant progress in epitaxial growth of GaN has already led to commercially available nitride-based high brightness light emitting diodes [3,4]. In spite of this recent success many fundamental problems still remain in the epitaxial growth of GaN and it's alloys. GaN is usually grown heteroepitaxially on sapphire (0001) substrates, which have a lattice mismatch of 13.8 % [5]. When GaN is grown directly on the sapphire however, this results in three-dimensional growth and a film quality which is not suitable for realising devices. Recently experiments have shown tha a two-step growth procedure leads to two-dimensional growth and improved film quality [6]. First a thin polymorphic GaN or AIN nucleation layer is grown on the sapphire surface at low temperature (450 to 600 °C). After raising the temperature to between 900 to 1100 'C a GaN layer is deposited. The growth temperature and thickness of this nucleation layer have a great influence on the properties of the overgrown GaN film. The purpose of this paper is to determine optimum growth parameters for GaN films in a novel type of vertical rotating disk reactor. EXPERIMENT The GaN films were grown by metalorganic chemical vapour deposition on sapphire (0001) substrates with H2 and mixed H2/N2 carrier gas. Ammonia (NH3) and trimethylgallium (TMG) were used as source materials. The growth was performed in a vertical rotating disk reactor, manufactured by Thomas Swan & Co. The group III and group V elements are injected separately into the reactor through a water cooled "showerhead" to avoid undesired upstream pre-reactions. A graphite resistance heater, is used to raise the temperature of a graphite susceptor up to 1200 'C. The temperature is measured with an optical pyrometer looking directly at the substrate (fig. 1). The rotation speed of the susceptor can be varied between 0 and 1500 rpm. All the deposition was performed under atmospheric pressure (700 Torr).

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Mat. Res. Soc. Symp. Proc. Vol. 395 * 1996 Materials Research Society

group III water cooling group V

graphite susceptor heater quartz liner water cooled "stainlesssteel wall

fig. I :Novel type of vertical rotatingdisk reactor The substrates were degreased in organic solvents (Trichlorethane,