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The effect of hydrogenation on the optical and electrical properties of amorphous silicon - tantalum alloys near the metal - insulator transition

In order to study the effect of hydrogenation on the optical and electrical properties of amorphous silicon - tantalum alloys two sets of samples (one unhydrogenated and the other hydrogenated) were prepared by sputtering. Films (about thick) were deposited on substrates kept at room temperature in an atmosphere of pure Ar for the unhydrogenated samples and a 90% Ar - 10% mixture for the hydrogenated ones. Both optical and electrical measurements were carried out on `as-deposited' samples. Optical measurements were made with photon energies ranging from 0.5 to 3.0 eV while the electrical data were recorded over a range of temperatures from room temperature down to about 6 K. The experimental conductivity versus temperature curves were fitted to theoretical expressions corresponding to both insulating and metallic regimes, from which useful information about carrier transport in such samples is extracted. Based on the comparison between electrical and optical results and on the computer analysis of experimental data, five different methods are used to estimate the position of the metal - insulator transition in both kinds of sample. The results show a shift of about 2% to higher values of the critical tantalum concentration for the hydrogenated samples in comparison with the unhydrogenated ones. It is concluded that the metal - insulator transition lies at -values of about 0.15 and 0.17 in the unhydrogenated and hydrogenated samples respectively.