Wide bandgap semiconductors are of interest for applications requiring devices capable of operating at elevated temperature and at high power, for example, in circuits used in hybrid cars and for electric power distribution. In addition to a bandgap of 3.26 eV, 4H-SiC is especially promising for high-frequency MOSFET applications because of the high electron mobility, ~800 cm²/Vs, and because an SiO₂ layer can be formed on the wafer surface by thermal oxidation. However, the high density of interface states near the conduction band edge reduces electron mobility to only ~5 cm²/Vs in n-channel MOSFETs. Recently it was shown that annealing in NO after thermal oxidation of 4H-SiC reduces the interface state density by a factor of ~10 with a corresponding increase in the electron mobility in MOSFETs. But the mobility remains about an order of magnitude lower than in bulk 4H-SiC. We use high frequency capacitance techniques, both temperature dependent capacitance-voltage and transient capacitance measurements, to investigate near-interface defects. These techniques allow us to detect electrons captured and emitted at defect energy states. From their trapping behavior, we can distinguish different defect species and investigate their properties. Our measurements revealed several diferent near-interface defect species in NO annealed samples. Two are located in the oxide, likely C dimers on oxygen sites and Si interstitials, and a third, possibly C di-interstitials, is located in the SiC [1,2]. Another method of delivering N to the SiO₂/SiC interface is by ion-implantation of N into the SiC prior to oxidation. In these samples we found the same oxide traps, but at a higher density than in NO annealed samples. The C-related defect in SiC was not observed, and this may account in part for the higher electron mobility in MOSFETS fabricated by this process [3]. Recently the effects of Na contamination on interface defects in the oxide were investigated [4]. The C-related defect in SiC was not observed in samples prepared by sodium enhanced oxidation.

The alloying of GaAs with small amounts of N or Bi produces large changes in the bandgap, making these materials of interest for optoelectronic applications including quantum well lasers, heterojunction bipolar transisters, and multi quantum well solar cells. Although N and Bi are isolelectronic with As, these atoms behave more like impurities than conventional alloying elements. The bandgap reduction in the dilute nitride alloy is due to a resonant interaction of a nitrogen state with the bottom of the conduction band. Similarly the bandgap reduction in the Bi alloy is believed to be due to a resonant interaction with the top of the valence band. Deep level transient spectroscopy (DLTS) measurements are employed to investigate bandgap electronic states in GaAsBi alloys. Such states, which originate from semiconductor defects, are of concern because GaAsBi alloys are grown at temperatures below 400˚C. DLTS measuremernts of p-i-n diodes having a thin (<50nm) GaAsBi layer in the center of the 100nm-thick i-layer, which was grown at temperatures close to 300˚C, reveal defects in signficant concentrations located in these i-layers. These defect studies complement measurements of the optical and transport properties of these alloys, which are grown by molecular beam epitaxy in Prof. Tom Tiedje's lab at UVic. More recently p⁺/n junction diodes were investigated and electron traps previously identified as As anti-site defect complexes were found in GaAs and GaAsBi layers at 300˚C.