Conductivity of weakly insulating amorphous nickel - silicon films below the metal - insulator transition

Abstract

The electronic conductivity has been measured in homogeneous, weakly insulating, amorphous nickel-silicon films located just below the metal-insulator transition (MIT). The conductivity follows a simple CT z power-law dependence with z ≈ 1/2 over a large temperature interval. In contrast, a Mott variable-range hopping expression could not be fitted successfully through these zero-field conductivity data. The CT z behaviour can be explained using the three-dimensional (3D) electron-electron interaction (EEI) theory. The negative magnetoconductance data observed in these weakly insulating films can be fitted nicely using only the 3D EEI theory. A crossover of the conductivity from the simply power-law CT z dependence at high temperatures to an activated hopping-law dependence in the liquid helium temperature region is observed; this transition is attributed to changes in the energy dependence of the density of states near the Fermi level. The conductivity of these weakly insulating films can be fitted well over three decades of temperature using an empirical scaling expression suggested by Möbius et al.

Metal-insulator transition in amorphousSi1−xNix:Evidence for Mott’s minimum metallic conductivity

“…The sample names in parentheses are the notations used in Ref. [55,56]. Sample j, positioned between samples f and g during film deposition, was prepared specifically for EDX and RBS analyses within that work.…”

“…Energy dispersive X-ray analysis (EDX) by means of a LINK AN10000 EDS system attached to a JSM-840 SEM was used in determining the composition for sample set B in Witwatersrand, South Africa, within Refs. [55,56]. The results of these analyses are given in Tables I and II. In order to compare both x scales, we performed additional EDX analyses for samples 3, 6, f, h, and i by means of a Tracor/Noran spectrometer Voyager IIa, attached to a transmission electron microscope (TEM) Philips CM20FEG, in Dresden.…”

“…Measurements (a) of the B samples are repetitions with increased accuracy of the investigations in Refs. [55,56]. Some of the measurements (b) were already published in Refs.…”

“…For a-Si 1−x Ni x :H, a structure study [51], optical conductivity measurements [52], also under pressure [53], and electronic structure investigations by XPS and XES [54] were reported in the literature. Last but not least, some of the authors have gained detailed experience in trying fits of σ(T ) by conventional theories for a-Si 1−x Ni x samples prepared by electron-beam evaporation in Tel Aviv [55,56]. The present paper is organized as follows: Section II describes the sample preparation.…”

Electric and Magnetic Evolution in Sputter-Deposited Fe_<i>x</i>Si_1−<i>x</i> Alloy Films

“…For x > 0.42, the mobility gap diminishes, but σ is heavily suppressed by the structure-randomness and impurity scattering 7,12) . The detailed experiment and systematic analyses of electrical resistivity data indicate the semiconductor to metal transition threshold x C = 0.24 for Ni x Si 1−x alloy lms 13,14) and x C = 0.14 for Cr x Si 1−x alloy lms 15,16) where an electron-localization effect and an electron-electron interaction are predominant at low temperature. These value are roughly consistent with x C ≅ 0.22 at 300 K (high temperature) in the present Fe x Si 1−x alloy lms.…”