Ultrahigh-Temperature Semiconductors Made from Polymer-Derived Ceramics

We report the semiconductor behavior of polymer-derived ceramics at high temperatures extending up to 1300 degrees C, far above that of any known material. The conductivity depends strongly on the N/O molar ratio, reaching its highest value when the ratio is approximately unity. The temperature dependence of the conductivity for these specimens, sigma, shows good agreement with the Mott's variable range hopping (VRH) mechanism for three-dimensional conduction in amorphous materials as described by sigma proportional to exp - (T-0/T)(1/4). The comparison yields the following range of values for the density of states, N(E) = 4.9 x 10(17)-5.9 x 10(18) (eV.cm(3))(-1), hopping energy, W = 0.017-0.047 eV, and hopping distance, R = 13.4-21.8 nm. The charge carrier mobilities predicted by the VRH model are in excellent agreement with the values measured in the Hall experiment. The long hopping distances are an unusual feature of this ceramic, suggesting longrange wave functions that may arise from clusters of SiCNO atoms that can exist in the form of a nanodomain network. Specimens that are either rich in oxygen (at the expense of nitrogen) or rich in nitrogen, have conductivities that are four to eight orders of magnitude lower than the similar to equimolar compositions. One oxygen-rich specimen shows band-gap controlled semiconductivity with an activation energy of 1.1 eV. Taken together, these results suggest that the electronic properties of the SiCNO ceramics are controlled by complex interactions between C and other atoms (Si, N, and O). These results are at variance with the simple picture where free carbon'' is assumed to determine the electronic behavior.