Effect of nonuniform doping on electrical transport intrans-(CH)x: Studies of the semiconductor-metal transition

Owing to the strong dependence of the resistivity on the dopant concentration, nonuniformity across the thickness of a sample leads to an apparent anisotropy in the resistivity (measured parallel to and perpendicular to the plane of the film). Thus the magnitude of the apparent anisotropy (for nonoriented samples) offers a convenient means for quantifying the degree of dopant nonuniformity in trans-${\left(\mathrm{CH}\right)}_x$. This technique is used to evaluate the uniformity achieved in ${\left[\mathrm{CH}{\left(\mathrm{As}{\mathrm{F}}_5\right)}_y\right]}_x$ by conventional vapor-phase doping as compared with the slow-doping technique developed earlier for susceptibility studies. Using samples which have been demonstrated in this manner to be uniform, we reexamine the semiconductor-metal transition. From the transport data, we find that the better the uniformity in the distribution of dopant, the more abrupt is the transition at $y_c\simeq 0.002$. A generalized soliton picture, involving delocalized carriers, appears to be implied by the combination of optical, magnetic, and transport data.