Homoepitaxial Growth of GaN Using Seeded Supersonic Molecular Beams
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E. Chen*, S. Zhang**, A. Michel*, R.F. Davis** and H.H. Lamb*
*Department of Chemical Engineering, North Carolina State University, Raleigh, NC 27695-7905 "**Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-7907
ABSTRACT Homoepitaxial growth of GaN on MOCVD-grown GaN/AlN/6H-SiC substrates was investigated using NH3 -seeded supersonic molecular beams and an effusive Ga source. 1.2x10
Ga-limited growth is observed at 730 and 770'C for incident Ga fluxes < 15
cm- 2 s-1 using a 0.25 eV NH 3 beam. A Ga incorporation efficiency of 20-25%
is observed under these conditions. Increasing NH 3 kinetic energy in the 0.25 to 0.61 eV range results in a modest increase in the GaN growth rate which we ascribe to an enhancement in NH 3 reactivity. A concomitant increase in surface roughness is observed with increasing GaN growth rate.
INTRODUCTION Gallium nitride is a wide bandgap semiconductor (Eg=3.4 eV) with many potential optoelectronics and high-temperature, high-frequency, microelectronics applications' 2 . GaN films have been used to fabricate blue light emitting diodes (LEDs) and laser diodes. Heteroepitaxial growth of high-quality monocrystalline GaN films has been problematic due to the lack of a suitable lattice-matched substrate and the thermodynamic instability of GaN under high-temperature growth conditions. Substrate temperatures in excess of 1000°C are employed for growth of monocrystalline GaN films by organometallic vapor phase epitaxy (OMVPE) using trimethylgallium and NH3. In MOCVD, substrate thermal energy is used to overcome activation barriers for precursor decomposition and adatom surface migration 3; however, GaN decomposition above 6200C in vacuo necessitates the 4 use of large V/II flux ratios4.
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Mat. Res. Soc. Symp. Proc. Vol. 482 © 1998 Materials Research Society
The use of energetic neutral beams of precursor molecules is a new approach to the epitaxial growth of GaN films at lower substrate temperatures 5-7 . In selected energy epitaxy (SEE), heavy reactant molecules are seeded in a supersonic expansion of light molecules and thereby accelerated to hyperthermal energies. The precursor molecules attain kinetic energies of 0.5 - 3 eV which can provide the necessary energy for activated surface processes, such as dissociative chemisorption and adatom migration. Hence, in prospect, monocrystalline GaN films may be grown at lower substrate temperatures by SEE than by conventional molecular beam epitaxy (MBE). To demonstrate the potential advantages of SEE, homoepitaxial growth of GaN was
investigated, obviating the substrate lattice-mismatch issue associated with heteroepitaxy and allowing us to better isolate the effects of precursor kinetic energy on growth kinetics and film morphology. In this paper, results of GaN homoepitaxial growth using an NH 3 seeded supersonic beam and effusive Ga source are presented, and the effects of substrate temperature and Ga flux on growth rate and morphology are elucidated.
EXPERIMENTAL METHODS The SEED/XP
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