Tungsten in silicon carbide: Band-gap states and their polytype dependence

Band-gap states of tungsten in silicon carbide (polytypes $4H,$ $6H,$ and $15R)$ are investigated by deep-level transient spectroscopy (DLTS) and admittance spectroscopy on n-type SiC. Doping with W is done by ion implantation and annealing. To establish a definite chemical identification of band-gap states, the radioactive isotope ${}^{178}\mathrm{W}$ is used as a tracer: band-gap states involving a ${}^{178}\mathrm{W}$ isotope are uniquely identified by their decreasing concentration during the nuclear transmutation of ${}^{178}\mathrm{W}$ to Hf. In addition, conventional doping studies with stable W isotopes are performed. Within the part of the band gap accessible by DLTS on n-type SiC, there is one tungsten-related deep level with a large capture cross section ${(10\mathrm{}}^{-12}{\mathrm{cm}}^2)$ for electrons. In the polytypes $4H,$ $6H,$ and $15R,$ its energy is 1.43, 1.16, and 1.14 eV below the conduction-band edge ${(E}_C),$ respectively. The polytype dependence of this level position directly reflects the conduction-band offset. In the $4H$ polytype, an additional level close to the conduction band ${(E}_C-0.17\mathrm{eV})$ exists that is absent in the other polytypes because of their smaller band gap. Due to the acceptorlike deep band-gap state, tungsten is a good candidate for a compensating center to produce semi-insulating SiC.