Growth and Doping of GaN Directly on 6H-SiC by MBE
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"**PhysicsDepartment, Boston University, 590 Commonwealth Avenue, Boston MA 02215 ABSTRACT We report on methods for the growth of GaN by MBE directly on 6H-SiC substrates. The films were doped p-type by the incorporation of Mg and the samples were characterized by studying their structure and morphology by RHEED, XRD and SEM and their recombination properties by photoluminescence measurements. The undoped films were found to be atomically smooth with 2x2 surface reconstruction and have an x-ray rocking curve with a FWHM of 3.5 arcmin. The photoluminescence spectra indicate that recombination is dominated by transition across the gap. The p-type doped films have a rocking curve with FWHM of 6.5 arcmin. and the majority of recombination occurs through D-A transitions at 3.26 eV. INTRODUCTION Employment of 6H-SiC substrates for the growth of GaN provides a number of advantages such as small lattice mismatch (-3%), compared to sapphire's (-13.8 %), as well as potential for the formation of Fabry-Perot cavities by cleaving. The epitaxial growth of GaN on 6H-SiC substrates generally follows the predeposition of an AIN buffer whose growth temperature was reported to vary from 500' - 1100 °C' 2 . The epitaxial growth of the II-V nitrides directly on doped SiC is important for the fabrication of vertical devices. The direct growth of GaN on SiC for the fabrication of heterojunction bipolar transistors has been demonstrated3 . However, for the fabrication of blue GaN LEDs an AlGaN buffer was employed 4 . Still, for the realization of efficient devices it is desirable to eliminate any buffer layer since as such it hinders the operation of the device by introducing an additional series resistance. It is therefore of interest to identify conditions that allow AlGaN epitaxial growth directly on 6H-SiC. In this paper we report on the optimization of the growth of GaN directly on n-6H-SiC without the employment of an intermediate buffer. Furthermore, the films have been doped ptype and preliminary studies of the junction properties of the p-GaN/n-SiC heterojunctions were conducted. EXPERIMENTAL METHODS A Varian Gen II MBE unit equipped with an ASTEX compact ECR plasma source was used for the work reported in this paper. The apparatus has been described in detail elsewhere . The substrates employed in this study were Si-terminated (0001) 6H-SiC 6. Prior to growth the substrates were chemically cleaned using the RCA procedure, passivated by immersing them
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Mat. Res. Soc. Symp. Proc. Vol. 395 01996 Materials Research Society
in buffered HF for 5 minutes, and mounted to a Mo blocks with Mo wires. The SiC substrates were degassed in the MBE buffer chamber at 400 °C and subsequently heated in the growth chamber to 900 °C just prior to growth. Although other workers found that H-plasma cleaning is necessary to form a good SiC surface, we found that the procedure described previously lead to surfaces with good RHEED patterns, including Kikuchi lines (see Fig. 1). All investigated films were grown at 800 °C measured by a thermocoupl
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