ELECTRONIC STRUCTURE AND CONDUCTIVITY OF HYDROCARBON FILMS OBTAINED IN PLASMA DISCHARGES OF TOKAMAK T-10

Abstract

Amorphous hydrocarbon films on silicon substrates obtained in the chamber of tokamak T-10 with space-bounded deuterium plasma by carbon diaphragms were studied. Using the methods of spectrophotometry, ellipsometry, X-ray photoemission spectroscopy and X-ray excited Auger electron spectroscopy, it was established that the refraction and absorption coefficients of films, as well as the parameters of the electronic structure such as the magnitude of the band gap, the fraction of sp2-hybridized carbon and the chemical composition of impurities depend on the characteristics of the discharge in the tokamak. It is shown that the deposited films refer to high-resistance dielectrics, and they can be classified by optical properties as hard or soft amorphous hydrocarbon films, depending on the type of the plasma discharge (pulse working discharge or long-term low-energy cleaning discharge). Wherein, the conductivity of hard films is less than that of soft films, which corresponds to a smaller fraction of sp2-states of carbon in these films and to a higher value of the band gap. The current-voltage characteristics and the temperature dependence of the direct current conductivity of hard and soft films were measured. It was shown that in the temperature range of 293–550 K, the conductivity is determined by the hopping conductivity mechanism over localized states near the Fermi level and the boundaries of the allowed bands. The hopping conductivity mechanism is also indicated by the power law obtained at room temperature at alternating current with a value of a power exponent close to 0.8. The measurement of the current-voltage characteristics and the temperature dependence of the conductivity of hard and soft films showed a significant difference in the activation energy of conductivity and the conductivity at an elevated temperature. The established dependences of the direct current conductivity and the activation energy value of the films on the discharge parameters can be used as diagnostic benchmarks of different types of plasma discharges in a tokamak. Data on the electrical conductivity of the films are analyzed within the framework of the concept of the electronic structure of amorphous non-crystalline materials.