The electrical and optical properties of amorphous films of Si:Au and Si:Cu.
thesisposted on 19.11.2015, 09:17 by John. MacNeil
Amorphous films of a-Si:Au and a-Si:Cu have been prepared by radio-frequency sputtering in an argon/hydrogen plasma and their properties studied by various techniques. The object of the investigation was to study the behaviour of deep impurity states in a-Si:H and to explore the nature of the metal-insulator transition which occurs in these alloys for sufficiently high concentrations of incorporated metal atoms. A transmission electron microscope investigation of the a-Si1-xAux alloys confirmed that the films were amorphous in a composition range up to and beyond the metal-insulator transition, which occurs at x=0.14. For low concentration of Au, the electrical conductivity rises with increasing x and the temperature dependence shows activated behaviour. Beyond x ~ 0.05.T-1/4 behaviour, indicative of hopping conduction, is observed, The metal-insulator transition appears to be continuous and values of the conductivity less than the "minimum metallic conductivity" are observed at helium temperatures. Optical absorption measurements show that an optical gap remains when x > 0.14 and it is concluded that the transition to the metallic state occurs in Au-related impurity levels. Photoemission studies suggest that these levels lie at the top of the host Si valence band. The a-Si1-xCux alloys also display an increase in electrical conductivity with increasing x but evidence from electrical and optical measurements, suggests that in this alloy the impurity states lie near the centre of the gap. For x greater than a few percent the films exhibit phase separation and crystallization.