TEM Study of the Morphology Of GaN/SiC (0001) Grown at Various Temperatures by MBE
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W.L. Sarney1, L. Salamanca-Riba1, V. Ramachandran2, R.M Feenstra2, D.W. Greve3 1 Dept. of Materials & Nuclear Engineering, University of Maryland, College Park, MD 2 Dept. of Physics, 3Dept. of Computer & Electrical Engineering, Carnegie Mellon University, Pittsburgh, PA ABSTRACT GaN films grown on SiC (0001) by MBE at various substrate temperatures (600° 750° C) were characterized by RHEED, STM, x-ray diffraction, AFM and TEM. This work focuses on the TEM analysis of the films’ features, such as stacking faults and dislocations, which are related to the substrate temperature. There are several basal plane stacking faults in the form of cubic inclusions for samples grown at low temperatures compared to those grown at high temperatures. The dislocation density is greatest for the film grown at 600ºC, and it steadily decreases with increasing growth temperatures. Despite the presence of various defects, x-ray analysis shows that the GaN films are of high quality. The double crystal rocking curve full width at half maximum (FWHM) for the GaN (0002) peak is less than 2 arc-minutes for all of the films we measured and it decreases with increasing growth temperature. INTRODUCTION GaN films grown on SiC usually have high defect densities. Typical defects in GaN/SiC films include inversion domain boundaries, stacking faults, and unintended polytype transformations. The presence of these defects emphasizes that even though reducing the lattice mismatch improves film quality, other factors contribute to the nitride film defect morphology [1]. Further defect density reduction may be achieved by enhancing growth conditions. We examine the relationship between the SiC(0001) substrate temperature and the GaN film quality. EXPERIMENT The damage produced by polishing was removed from Si-face 6H-SiC(0001) substrates with ex-situ hydrogen etching [2]. The substrates were then placed into an ultra high vacuum environment (pressure < 10-10 Torr) and outgassed at about 800°C for 30 minutes. In order to replenish any surface Si that may have been lost during oxide removal, Si was deposited onto the substrate using an electron beam source. Oxide desorption was done by annealing the substrate at about 1000° C until a 3x1 reflection high energy electron diffraction (RHEED) pattern was obtained. GaN films were grown by MBE onto the substrates using a Ga effusion cell and a RF-plasma nitrogen source. The growth was a single step process with no nucleation layer growth. We grew four samples with substrate temperatures of 600° C, 650° C, 700°
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C, and 750° C. The temperature was monitored using a pyrometer and a thermocouple in contact with the back of the sample mounting stage. Growth was performed under highly Ga-rich conditions relative to the N concentration [3]. The films were characterized insitu with RHEED and STM and ex-situ with AFM, HRXRD, and TEM. The TEM results were obtained on a JEOL 4000FX microscope operated at 300 kV. Cross-sectional TEM samples were prepared using tripod polishing and ion milling at room temperature
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