ELECTRICAL PROPERTIES OF PHOSPHORUS INCORPORATED TETRAHEDRAL AMORPHOUS CARBON FILMS

Great attention has been given to diamond-like carbon (DLC) or tetrahedral amorphous carbon (ta-C) films due to their potential applications in electronic devices as semiconductor materials. The controlled variation of electrical conductivity through doping is of primary importance. Active phosphorus element could be introduced into ta-C films to achieve n-type doping and the resulted films show potential applications as photovoltaic solar cells, semiconductor field emitters or biomedical coatings. However, an inconsistency of doping effect and graphitization of the bonding still exists in the role of phosphorus atoms in carbon films. A detailed study on the conduction mechanism related to the structural changes should be attempted to further understand the conductive behaviour of the films. Based on this purpose, phosphorus incorporated tetrahedral amorphous carbon (ta-C:P) films were desposited using filtered cathodic vaccum are technology with PH(3) as a dopant source under negative substrate biases of 0-200 V. The structural characteristics of ta-C:P films were investigated by x-ray photoelectron spectroscopy (XPS) and Raman spectroscopy, and the electral behaviours of the films were examined by measuring electrical conductivity at the temperature range of 293-573 K and current-voltage curves. Results indicate that phosphorus implantation enhances the contents of sp(2) sites in ta-C films and the numbers of localized electronic pi and pi* states as hopping sites, and improves the conductive ability of the films ta-C:P film obtained at -80 V shows the best conductive propety. The carriers of ta-C:P films represent the hopping conduction in localized band tail states and the thermally activated conduction in extended states in the temperament range of 293-573 K. The results of current-voltage curves indicate that ta-C:P films are n type semiconductor materials.