MAGNETOOPTICAL EFFECTS ON SHALLOW DONOR STATES IN 6H-SIC IN HIGH MAGNETIC-FIELDS

Impurity excitation spectra in 6H-SiC doped with nitrogen donors are studied by infrared absorption measurements in the frequency range 200-1400 cm(-1) under the influence of high magnetic fields up to 20 T. No line splitting is observed for any orientation of the magnetic field. However, four transitions show a diamagnetic shift if the magnetic field is oriented along the crystal c axis. This diamagnetic shift is found to be anisotropic, depending on the orientation of the viewing direction towards c. From these results we conclude that the effective mass ellipsoids describing the conduction-band minima must be oriented along lines of the Brillouin zone, which are parallel to the crystal c axis, and that the effective mass tenser has three independent components. Using the symmetry properties of the Brillouin zone it is shown that the conduction-band minimum must be located somewhere along the line M-L in the Brillouin zone. From the experimental results at B = 20 T it is estimated that the average effective mass m* = (m(x)m(y)m(z))(1/3) must be m* > 0.3m(0).