Microstructure and its effect on the conductivity of magnetron sputtered carbon thin films

Carbon thin films were grown by magnetron sputtering at room temperature on silicon substrates, with the substrate bias voltage varying from +10 to -200 V. Transmission electron microscopy analysis has shown that films deposited at V-b = +10 and -40 V are amorphous (alpha -C), while films deposited at V-b = -200 V are nanocrystalline (nc-C). Temperature dependent conductivity measurements were carried out in the temperature range 300-77 K. With respect to conductivity, the results indicate that the investigated carbon films are classified in three groups: (i) In alpha -C films deposited at V-b = +10 V (sp(2) rich bonds), the variable range hopping (VRH) conduction dominates below 300 K. (ii) In alpha -C films deposited at negative V-b up to -100 V (sp(3) rich bonds), VRH conduction dominates at low temperatures (T < 150 K) and a thermally activated process satisfying the Meyer-Neldel rule at higher temperatures (T > 150 K). (iii) In nc-C film deposited at V-b = -200 V, the conductivity is explained by a heteroquantum-dots model based on a thermal-assisted tunneling process. The earlier differentiation in the conductivity mechanisms may play a significant role in the field electron emission properties of the films. (C) 2001 American Institute of Physics.