Low-Temperature Homoepitaxial Growth of Gan Using Hyperthermal Molecular Beams
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INTRODUCTION Gallium nitride is a wide bandgap semiconductor (Eg = 3.4 eV) with many potential optoelectronics and high-temperature, high-frequency, microelectronics applications[l], [2]. MOCVD-grown GaN films have been employed in the fabrication of blue light emitting diodes (LEDs) and laser diodes. Molecular beam epitaxy (MBE) techniques can reduce the growth temperature and provide atomic layer control of film composition. Selected energy epitaxy (SEE) is a new approach to GaN MBE growth that employs energetic neutral beams of precursor molecules[3], [4], [5]. Heavy reactant molecules, seeded in a supersonic expansion of light molecules are accelerated to hyperthermal kinetic energies. Typical precursor molecules (e.g. NH 3 and Et3Ga) can attain kinetic energies of 0.5-5 eV providing energy for activated surface processes, such as dissociative chemisorption and adatom migration. Hence, monocrystalline GaN films may be grown at lower substrate temperatures by SEE than by MBE using conventional effusive sources. To demonstrate the potential advantages of SEE, we are investigating homoepitaxial growth of GaN on high-temperature MOCVD-grown GaN substrates. This approach obviates the substrate lattice-mismatch issue, allowing the effects of precursor kinetic energy and film morphology to be studied in isolation. It has become increasingly apparent, however, that in order to achieve 2-D, step-flow growth, GaN in situ surface cleaning methods must be perfected. In this paper, detailed results of in situ GaN cleaning using NH 3-seeded supersonic beams are presented. Preliminary results of GaN homoepitaxial growth using a hyperthermal NH 3 beam and a Ga effusive source are also reported.
EXPERIMENT The SEE/XPS multi-chamber system has been described previously [6]. The substrates were 0.5-jtm thick GaN films grown at 1050'C by MOCVD on on-axis 6H-SiC employing a 0. 1gtm thick AIN buffer layer. The substrates were used as received. Substrates were mounted on solid Mo sample holders, and Ag paste was used to ensure good thermal contact. The sample holder was introduced via the load-lock chamber and transferred in vacuo into the growth chamber. Each sample was heated slowly to 400'C under an NH 3 flux for outgassing prior to cleaning. Films were cleaned in situ by heating at 730'C-850°C under a NH 3-seeded supersonic beam for times ranging from 15-60 min. The NH3 nozzle was heated to 200'C, and the stagnation pressure was 745-755 Torr for typical cleaning experiments. These conditions correspond to a 437 Mat. Res. Soc. Symp. Proc. Vol. 512 ©1998 Materials Research Society
NH 3 flow rate of 60 sccm and a He flow rate of 200 sccm through a 150-jim orifice. A He flow of 200 sccm with 730-750 Torr stagnation pressure through the metal-organic source nozzle was used to prevent contaminant diffusion from the source to deposition chamber through skimmer and collimation aperture. Films were grown using a hot-lip Ga Knudsen cell (K-cell) and a NH 3-seeded supersonic molecular beam. A substrate temperature of 700'C and Ga K-cell
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