The metal-insulator transition in the amorphous silicon-nickel system

• Main Author: Asal, Rasool Abid
• Published: University of Leicester 1993
• Subjects: 900
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.674306

Amorphous thin films of Si1-yNiy:H have been prepared over a wide range of compositions by radio-frequency sputtering in an argon/hydrogen plasma and their properties studied by various techniques. Transmission electron microscope investigations confirmed that the films were amorphous and the composition of the films was determined by EDAX. The principal object of the study is to investigate the nature of the semiconductor-metal transition in the a-Si1-yNiy:H system. The system has been shown to exhibit a semiconductor-to-metal transition as a function of concentration at approximately y = 0.26 at which value the optical gap shrinks to zero and beyond which the reflectivity falls with increasing photon energy in the region 0.5 - 2 eV, i.e becomes Drude-like. D.C. electrical conductivity measurements as a function of temperature show an increase in conductivity and a decrease in activation energy with increasing nickel content which is close to zero for y 0.26. The optical joint density of states (OJDOS) is finite at all energies for y ~ 0.26, confirming the existence of overlap between the conduction and valence bands. Pressure-induced transitions from semiconductor-to-metallic behaviour of the a-Si1-yNiy:H films have been investigated by measurements of the optical absorption edge as a function of pressure in a diamond anvil cell and by measurements of the electrical conductivity in a Bridgman opposed-anvil apparatus, both at room temperature. The optical gap decreases with increasing pressure, becoming zero at pressures that are lower the higher the nickel content. The electrical conductivity increases with applied pressure for all samples studied, reaching a saturation value close to the Mott's minimum metallic conductivity; this also occurs at lower values of the pressure for films with higher nickel content. Information on the structure and the local bonding configurations for the a-Si1-yNiy:H films was obtained from EXAFS and IR measurements. The results indicate that there is a significant change in the local environment of the Ni atoms as their concentration is changed but the system appears to favour chemical ordering.