The electronic levels of vanadium in semi-insulating 4H- and 6H-SiC have been reinvestigated using temperature dependent Hall effect and resistivity measurements at temperatures up to 1000K in conjunction with electron paramagnetic resonance (EPR) and optical absorption measurements which were used to identify the charge state of vanadium in the material. Two distinct thermal activation energies were found for each polytype. The shallower of the two levels correlated with the presence of both V3+ and V4+ in the EPR and absorption experiments, demonstrating that this level is the vanadium acceptor level while the deeper level is the donor level for which the V4+ charge state was observed. The results for the V4+∕5+ donor level, EC−1.57±0.09eV for 4H-SiC and EC−1.54±0.06eV for 6H-SiC, are in agreement with the generally accepted values. However, the results for the V3+∕4+ acceptor level, EC−0.85±0.03eV in 6H-SiC and EC−1.11±0.08eV in 4H-SiC, are significantly higher than previously assumed. Variations in crystal quality and purity may explain the differences in the previously reported values for the donor and acceptor levels.

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Electron paramagnetic resonance (EPR) spectroscopy and photo-induced EPR are used to examine the point defects in vanadium-doped 4H-SiC and high-purity semi-insulating (HPSI) 4H-SiC grown by physical vapor transport. Both types of samples often exhibit a 1.1-eV activation energy, E a , as extracted from the temperature-dependent Hall/resistivity measurements; however, different defects are related to the same E a in each case. In the vanadium-doped wafers, the EPR data reveal both V 4+ and V 3+ in the same sample; thus, the 1.1-eV Hall activation energy is tentatively interpreted as the V 3+/4+ acceptor level. However, this conclusion cannot be confirmed because additional defects complicate the photoresponse of vanadium. The carbon vacancy, which is detected in all the HPSI wafers, exhibits a range of photothresholds similar to the various values measured for E a . The photo-EPR and temperature-dependent Hall/resistivity measurements made before and after a 1600°C anneal suggest that several different types of nonuniformly distributed defects participate in compensation of the HPSI material and that annealing selectively removes those with different defect levels.

We report demonstration of efficient continuous-wave lasing from chromium-doped zinc selenide using anti-reflection microstructures (ARMs) in place of thin-film AR coatings or Brewster angle cavity geometries. ARM textures are more resistant to laser-induced damage than coatings, exhibit low-loss, wide angular acceptance, broad wavelength effectiveness, and are not susceptible to water absorption. Slopeefficiencies of 68% were achieved, which compares favorably to the thinfilm control samples at 58% for the same cavity. ARMs hold promise for near-term power scaling and wavelength agility of transition-metal-ion doped II-VI lasers.

Defect levels and types of point defects in high-purity and vanadium-doped semi-insulating 4H-SiC J. Appl. Phys. 96, 5484 (2004); 10.1063/1.1797547 Fermi level control and deep levels in semi-insulating 4H-SiCA study of deep levels in high purity semi-insulating 4H-SiC has been made using temperature dependent Hall effect ͑TDH͒, thermal and optical admittance spectroscopies, and secondary ion mass spectrometry ͑SIMS͒. Thermal activation energies from TDH varied from a low of 0.55 eV to a high of 1.65 eV. All samples studied showed n-type conduction with the Fermi level in the upper half of the band gap. Fits of the TDH data to different charge balance equations and comparison of the fitting results with SIMS measurements indicated that the deep levels are acceptorlike even though they are in the upper half of the band gap. Carrier concentration measurements indicated that the deep levels are present in concentrations in the low 10 15 cm −3 range, while SIMS results demonstrate nitrogen and boron concentrations in the low to mid-10 15 -cm −3 range. The results suggest that compensation in this material is a complex process involving multiple deep levels.