a1 Center for Materials Science and Engineering, Departments of Electrical Engineering and Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
The electronic transition from localized to delocalized states of carriers in a disordered carbon material is investigated by photoconductivity measurements. Phenol-derived activated carbon fibers, where the carriers are strongly localized due to disorder, are heat treated in the range 300–2500 °C to give rise to the insulator-metal transition. Dark conductivity, Raman spectra, and x-ray diffraction patterns are also measured to characterize their structural changes. As a result, the transition temperature was determined to be rather low, around 1000 °C, considering the rapid decrease in the photoconductivity above this temperature. This decrease was ascribed to a fast recombination between the photoexcited carriers and the delocalized carriers generated by heat treatment.
(Received May 13 1991)
(Accepted December 11 1991)
c1 Permanent address: R & D Division, Sumitomo Metal Industries Ltd., Fuso 1-8, Amagasaki, Hyogo, 660 Japan.