| The unique properties of silicon carbide (SiC) allow it to operate under severe conditions that are not suitable for conventional semiconductors. As such, electronic devices are being electronically pursued for high-temperature, high-power, and high-frequency applications. For example, SiC Schottky diodes are commercially for high-power switches. However, further development of these and other devices are limited by problems associated with metal contacts. This research investigates the source(s) of non-ideal rectifying, or Schottky, contacts to SiC epitaxial layers. Hundreds of current-voltage (I-V) measurements were performed at low applied voltages. While near-ideal contacts were fabricated on all samples, a significant percentage of diodes (∼7-50% depending on epitaxial growth method and diode size) displayed a "non-ideal," or inhomogeneous barrier height. These "non-ideal" diodes occurred regardless of growth technique, pre-deposition cleaning, and contact metal. A theoretical model, based on two parallel diodes with different barrier heights, matched the experimental data extremely well, indicating that the "double-barrier" contacts consisted of an inhomogeneous energy barrier caused by defects in the SiC material.;The sources of these non-idealities were investigated with a variety of spectroscopic and imaging techniques. Comparisons of X-ray topographic (XRT) and polarized light microscopy (PLM) images with the I-V data revealed no correlations with specific one-or two-dimensional defects, such as screw dislocations or micropipes. Using electron-beam induced current (EBIC), recombination centers associated with screw dislocations and stacking faults were observed. Although there was no direct relationship between the number of defects and the electrical characteristics, clusters of defects were observed in all "non-ideal" diodes. Cathodoluminescence spectra revealed additional peaks in the non-ideal diodes at 2.65 and 2.20 eV that were not observed in the "near-ideal" diodes. Additional site-specific cathodoluminescence of the stacking fault clusters revealed a peak at 2.40 eV. With reference to the 4H-SiC bandgap of 3.24 eV, these peaks complement the low-barrier Schottky barrier heights determined from the theoretical model of 0.60, 0.85 and 1.05 eV. It is proposed that defect clusters act to locally pin the Fermi level, creating localized low-barrier patches, which account for the "non-ideal" electrical characteristics. |
