Plasma-Assisted MBE of GaN and AlGaN on 6H SiC(0001)
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EXPERIMENTAL GaN films were grown on 6H SiC(0001) substrates with 3' tilt towards (hereafter denoted "off axis"). High resistivity (>1000 Q-cm) substrates were used except for the n-p 157 Mat. Res. Soc. Symp. Proc. Vol. 395 01996 Materials Research Society
structure described below. The experimental set up and the SiC substrate cleaning procedure have been described in a separate publication'a and will not be repeated here. GaN films were grown at temperatures ranging from 700'C to 850'C after thermal cleaning of the SiC substrate. Film growth experiments were performed with 100A to 500A AIN or GaN buffer layers grown at temperatures ranging from 450'C to 800°C. A thin (-400A) layer of AIN grown at 800'C was found to provide the optimum buffer layer for the subsequent GaN film
growth. GaN films grown at 800'C were found to be of the best structural and morphological quality. Other optimum growth parameters for these films were as follows: plasma power at 25 W, magnet current at 17 A, nitrogen flow rate of 2.5 sccm with chamber pressure at 2.5x10 4 mbar, Ga effusion cell at 900'C, and Al effusion cell at 950'C. The film growth rate ranged from 600 to 1000A per hour. Typical film thickness was about 1 jim. The structural and morphological quality of the films were found to improve with increasing film thickness. Structural quality was determined by x-ray rocking curve measurement of the GaN (0002) reflection using CuKox radiation. Morphological quality was determined using optical microscopy and scanning electron microscopy (SEM). Secondary electron mass spectrometry (SIMS) was used to determine the compositional profile, including dopant and impurity concentrations. Hall effect measurements were performed in the van der Pauw configuration with indium contacts at the comers of each sample. In view of the success of Pankove et al. 9 in fabricating an HBT on a 6H SiC substrate using GaN as the emitter, we have investigated the n-p junction characteristics of a n-Al0 ,,Ga, DN/p-SiC diode. The test structure and the results are shown in the next section.
RESULTS AND DISCUSSION SEM micrographs of 1 p.m thick GaN films grown on an off-axis 6H-SiC substrate under the optimized conditions described in the previous section showed a smooth, crack-free surface. X-ray rocking curve full width at half maximum (FWHM) of the GaN(0002) reflection ranged between 9 and 20 arc-min. This is indicative of reasonably good crystalline quality, and is comparable to FWHM values reported for MBE-grown GaN films on 6H-SiC by other researchers. '"6 Hughes et al." have recently reported a FWHM value of 156 arcsec for the GaN(0002) reflection of a homoepitaxial GaN film on 6H-SiC by MBE. They reported that the GaN film quality depended on the type of nitrogen plasma source; a rf source providing better juality films than an ECR source. The FWHM value of 156 arcsec reported by Hughes et al.' is comparable to the value of 66 arc-sec reported recently by Weeks et al."7 for the GaN(0004) reflection of their MOVPE-grown GaN film on vicinal 6H-SiC. R
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