| As-received commercial 6H-SiC wafers contain surface defects such as scratches from the polishing process, which would degrade the crystal quality and change the polytype of epitaxial films. A comparison was made of on- and off-axis 6H-SiC (0001) substrate surfaces etched in H2, atomic hydrogen, C2H4/H2, and HCl/H2 at relatively low temperature range of 1400--1500°C. All etches effectively removed surface defects such as remnant polishing scratches and produced surface step structures with varying regularity and step heights as well as shallow and deep etch pits, depending on the etching conditions such as time, temperature, etchant concentrations, and the residual SiC coatings on the susceptor.; Subsequently, GaN, AlN, and SiC were deposited on both etched substrates with stepped structures and etch pits via metalorganic chemical vapor deposition (MOCVD) and atmospheric pressure CVD. The effects of etched surfaces on the crystal quality, surface morphology, type and density of defects, and polytype of resulting films were investigated using atomic force microscopy (AFM), NDIC microscopy and x-ray rocking curves. The crystal quality of epitaxial GaN films was improved on smooth substrate surfaces, and the density and size of surface morphological defects such as pinholes decrease with the improved substrate surface quality. The AlN films grown on the as-received on-axis substrates had quasi two-dimensional growth surfaces, while the AlN films grown on stepped substrates had island-like surface features. The crystal quality of the AlN films as determined by screw and edge dislocation densities depends on the substrate surface step regularity, height, and density, with the best film grown on the 3.5° off-axis substrates due to the highest step regularity and density and the lowest step height. A mechanism based on stress-relieving defects at the interface was proposed to explain the above relationship. The 3C-SiC films grown on etched substrates had a higher crystal quality and a lower density of DPBs compared to those films grown on as-received substrates. In addition, the polytype controlled growth and DPBs-free growth of 3C-SiC were also achieved via step flow growth on etched substrates with well-defined three-bilayer steps under the relatively high temperature (1475°C) and low silane concentrations (<50 ppm. SiH4/H2).; Finally, polytype controlled SiC epitaxy was achieved on on-axis 6H-SiC (0001) substrates by adding HCl during growth to modify the growth mode, leading to the step formation. All homoepitaxial growth methods of SiC at relatively low temperatures (<1500°C) are unified by the step flow growth mechanism, differing only in the manipulation of surface steps: that is the pre-growth and in-situ growth step manipulation. |
