Two negative-U centers in 6H-type material were characterized by using capacitance-transient techniques. The 2 defects gave rise to 1 acceptor level (-/0) and 1 donor level (+/0) each in the band-gap. The donor and acceptor levels had an inverted ordering, in that the thermal ionization energy of the acceptor level was higher than that of the donor level. The negative-U centers observed in 4H-type and 6H-type material had many properties in common. Both annealed out at approximately the same temperature of 1400C, and they had very similar electronic properties. It was suggested that the 2 negative-U defect systems in 4H-type and 6H-type material had a similar structure. Levels at Ec - (0.27 ... 0.29)eV and Ec - (0.19 ... 0.21)eV corresponded to the donor levels, E10 and E20, respectively. The levels at Ec - 0.38eV and Ec - 0.44eV corresponded to the acceptor levels, E1- and E2-, respectively.

Observation of Negative-U Centers in 6H Silicon Carbide. C.G.Hemmingsson, N.T.Son, E.Janzén: Applied Physics Letters, 1999, 74[6], 839-41