ELECTRONIC TRANSPORT BY ULTRATHIN MOLYBDENUM LAYERS BURIED IN AMORPHOUS-SILICON

Multilayers of the type Mo/a-Si:H and a-Si:H/Mo/a-Si:H were grown by m agnetron sputtering (silicon and molybdenum) and plasma chemical vapor deposition (silicon), respectively. For a nominal Mo layer thickness between 0.1 and 34 nm, the temperature-dependent conductivity is inves tigated in the range between 10 and 300 K. The conductivity at 300 K o f the films decreases from 5 x 10(4) to 1 x 10(-2) (Omega cm)(-1) with decreasing thickness. The thickness dependence of the conductivity is explained in terms of different mechanisms: grain boundary scattering for the thickest (nanocrystalline) films, surface scattering for the intermediate thickness (amorphous) films, and hopping between isolated Mo particles for the thinnest (discontinuous) films. For the interpre tation of experimental data by scattering, an analytical solution of t he Fuchs equation assuming pure diffusive scattering is presented. For Mo layers with 62% of bulk density, the conductivity of the nanocryst alline Mo layers is smaller than for amorphous Mo layers, which is rel ated to the existence of free standing columns of the nanocrystalline Mo. The temperature-dependent conductivity of nanocrystalline layers w ith 93% of bulk density has a negative slope. For all amorphous films with a thickness of approximately 3 nm and for the nanocrystalline mat erial with 62% of bulk density, the temperature-dependent conductivity is described by sigma(T)proportional to T-0.5. With decreasing thickn ess, a gradual change from this law to a dependence according to sigma (T)proportional to exp[-(T-0/T)](n) for the layers of minimum thicknes s is observed. [S0163-1829(98)00840-6].