Electrical properties of K-doped superfulleride thin films

K-doped superfulleride films (KxC60, x>6) of an average homogeneous composition of K11+/-0.3C60 and a strong [111] texture were prepared by depositing C-60 on K multilayers at 200 K, in a proportion such that K:C-60>12:1, and then annealing these layers to 300 K. This K11C60 superfulleride film was doped with C-60 to form another superfulleride phase of average composition K8C60 and then doped further with C-60 to form homogeneous K6C60 and K3C60 fulleride phases. The electrical properties of these superfulleride films are compared to those of alkali metal fulleride and alkaline earth metal fulleride films. The doping-resistivity profile showed a resistivity minimum, rho(min1)=4.0x10(-3) Omega cm due to the half filled t(1u) band in K3C60 and another dip in resistivity to 9.5x10(-3) Omega cm at higher K-doping levels of K11C60, presumably due to the unfilled t(1g) band, as in the alkaline earth metal fullerides. The resistivity of the K3C60 and K11C60 films showed only minimal variations with film thickness, probably because the films prepared in this study had large grain sizes, and hence, resistivity was invariant with film thickness. The temperature dependence of the resistivity for 300 Angstrom films of K3C60, K8C60, and K11C60 was also studied in the 200-350 K temperature range. K11C60 films showed a metallic behavior like K3C60 films, while conduction in K8C60 films exhibited an activated behavior that could be described by a granular metal model for resistivity in the dielectric region. (C) 1999 American Institute of Physics. [S0021-8979(99)05403-1].