Structure and Properties of III-N Semiconductor Thin Films Grown at Low Temperatures by N-Radical-Assisted Pulsed Laser
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ABSTRACT Thin films of nitride semiconductors are usually grown by means requiring high substrate temperatures. Deposition techniques providing higher kinetic energies of incident species offer an alternative route which might allow growth of good quality films at lower temperatures. Pulsed Laser Deposition can provide higher kinetic energies than most thin film growth methods. However, III-nitride thin films grown by PLD are often nitrogen deficient. We have been able to obtain good stoichiometry for aluminum nitride films even at room temperature by providing atomic nitrogen at low (thermal) energies during growth. Very good orientation can be obtained on (001) sapphire substrates at moderate temperatures (- 500 C). AIN films were grown from either AIN or Al targets. We also report on preliminary work by the same method with GaN film growth from a liquid Ga target. INTRODUCTION Of the many techniques which have been employed to date for thin film growth of wide bandgap nitrides, metal-organic vapor-phase epitaxy (MOVPE) and molecular beam epitaxy assisted by reactive nitrogen (RMBE) have produced the best quality materials. Both require the use of fairly high substrate temperatures (- 1,000' C for MOVPE, and over 700' C for RMBE) in order to surmount the kinetic barrier for formation.! The lower temperatures in RMBE are possible through provision of well controlled reactive-N species. A very recent review of RMBE results for GaN growth has been given by Mohammad et al. 2 The thermal energies available for the evaporated metal species in MBE imply that high substrate temperatures must still be used. An alternative which might allow further lowering of the growth temperature is the provision of higher energy of the metal species, such as can be achieved by pulsed laser deposition (PLD). While PLD growth of AIN in particular has been attempted in the past,3 4A these efforts, in which AIN targets were ablated in vacuum or in a N2 atmosphere, resulted in nitrogen-deficient films. More recent work has led to high quality AIN films grown by PLD, 5 but relatively high temperatures were required. Clearly, a more reactive source for the nitrogen is desirable. For this purpose, similar approaches to those recently employed in RMBE could be suitable. One of the better alternatives in RMBE appears to be the provision of a low-energy atomic nitrogen beam during growth.6' 7 One advantage over ion beam sources is that less damage of the growing film should occur. Accordingly, we have initiated an exploration of wide bandgap nitride growth by PLD assisted by atomic-N beam. The current report offers a summary of our initial results for AIN and GaN films.
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Mat. Res. Soc. Symp. Proc. Vol. 482 © 1998 Materials Research Society
EXPERIMENT We use an atomic nitrogen source (Oxford Applied Research MPD21) which breaks up the nitrogen molecules by coupling RF energy to the N2 gas flowing through a pyrolythic BN crucible. The crucible has an exit plate with 36 orifices, each 0.5 mm in diameter. In order to maintain stable operation
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