Cubic GaN Heteroepitaxy on Thin-SiC-Covered Si(001)
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INTRODUCTION The combination of optoelectronic Ill-nitrides and highly advanced Si technology has the potential to be a key technology for fabricating optoelectronic integrated circuits. The luminescence in the blue and ultraviolet regions of GaN has attracted a lot of interest from many scientists and engineers. It has been reported that cubic GaN (0i-GaN) epitaxial films can be grown on GaAs(001) and 3C-SiC(001l) substrates by chemical vapor deposition (CVD) I lI or gas-source molecular beam epitaxy (MBE) [2, 31. The successful growth of P-GaN on Si, however, has been reported only by a group from Boston University 141. In a previous paper we reported that thin cubic SiC(fl-SiC) formation on Si(001) substrates is effective for the epitaxial growth of fi-GaN on Si(001) 151. We confirmed that thin SiC layers not only play the role as a buffer layer to reduce the large lattice mismatch between Si and P-GaN but also as a mask to protect the Si substrates against the irradiation of the active nitrogen, e.g., the nitrogen radicals and atomic nitrogen those were supplied during GaN growth. The SiC formation by using a carbonization technique on Si substrates has been studied as an initial process of heteroepitaxy of SiC on Si [6]. Carbonization using hydrocarbons is a very easy way to obtain SiC layers on Si substrates: in the conventional CVD the carbonization was performed by using C.AH1[71 as a source gas and at very high processing temperatures between 1000 and 1300 "C. On the other hand, it was reported that SiC was obtained at lower temperatures (750 - 1000 °C) using acetylene (C,H,) 181, although pit formation in Si substrates was also observed. However low temperature carbonization is desired not only for avoiding the high-density pit formation as much as possible but also for device fabrication. In this paper we report the optimum SiC formation and 13-GaN growth conditions, such as V/Ill ratios and substrate temperatures by RF-activated MBE. We also discuss the quality of f3-GaN layers.
EXPERIMENT Hydrogen-terminated Si(001) substrates were placed into a preparation chamber ( having a base pressure of about 5 x 10' Torr) which was connected to an MBE growth chamber (base pressure of about 5 x 10" Torr). The Si substrates were heated up to 850 to 950 °C for I to 10 min under a C2 H, pressure of 5 x 10' to 5 x 10.5 Torr in order to form SiC layers. The SiC-formed Si(001) substrates were then cooled down to room temperature and immediately transferred to the MBE growth chamber . They were then annealed at 1000 'C for 1 min and cooled G 3.9 Mat. Res. Soc. Symph. Proc. Vol. 537 C 1999 Materials Research Society
down to a growth temperature of 600 to 900 'C. GaN films were grown using RF-MBE under the following growth conditions: the nitrogen gas flow rate was set at 2 sccm and the RF-power was set at 300 W. Gallium molecular beam was supplied to the substrate by using a Knudsen-cell operating at 850 to 1000 °C. The surface morphology and the quality of GaN films were characterized by using in-situ reflection high
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