Two negative-U centers in 6H-type material were observed and 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.

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