Novel Semiconducting Phase of Amorphous Carbon Nickel Composite Films
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0910-A12-04
Novel Semiconducting Phase of Amorphous Carbon Nickel Composite Films Somnath Bhattacharyya, S. J. Henley, N. P. Blanchard, and S. R. P. Silva Nano-electronics Centre, Advanced Technology Institute, University of Surrey, Guildford, GU2 7XH, United Kingdom ABSTRACT A homogeneously mixed phase of carbon and 10% nickel yielding amorphous carbonnickel composite (a-C-Ni) films is prepared by an excimer UV pulsed laser ablation. Conductivity study of these films shows a nearly activated conduction. Also a saturation of conductivity below a temperature of 25 K explains the presence of an additional density of states at the Fermi level in these samples. Our experiments demonstrate a very different behaviour of electronic properties of these novel materials compared to undoped diamond-like carbon (DLC) films, which was directly confirmed using valence band spectroscopy. INTRODUCTION Numerous attempts have been made to increase electrical conduction in DLC or amorphous carbon films by incorporating various elements and to establish “substitution doping of carbon” [1,2]. However, most of the cases it resulted in the formation of graphitic structures with a high defect density of states (DOS) at the Fermi level (EF). Therefore, a true signature of activated conduction with low activation energy in these films has only been demonstrated rarely [2]. Carbon has a unique ability to mix with a large number of heavy elements however, synthesis of a homogeneous amorphous phase of metal-carbon remains largely unsuccessful [3,4]. In the microstructure of metal and carbon multi-layers fabricated by ion-bombardment or sputtering, the formation of metal and carbon clusters was attributed to non-homogeneous structures in the films [3]. Due to the formation of metal clusters and nanostructures, tunnelling of electrons was found to dominate the conduction processes even at high temperatures [4]. Here we show that to achieve a homogeneous amorphous phase or embedded nanostructures, laser ablation can be utilized to mix insulators and metals. Keeping in mind that the initial state of carbon is important to transform the film structure, we introduce Ni into highly insulating sp3 carbon films by laser ablation to produce films of intermediate nature between DLC and metal carbides and show a large reduction of the resistivity of the undoped DLC films. EXPERIMENTAL DETAILS Periodic ablation of a rapidly rotating, high purity, graphite target and nickel foils (both are 99.99% pure) has been performed using a 248 nm pulsed UV excimer laser (Lambda Physik LPX 210) delivering a laser fluence of ~ 10 J/cm2 at a repetition rate of 10 Hz. The targets were rotated at a constant speed of ~ 100 rpm and a pressure of ~2 x 10-7 Torr was maintained during the deposition at room temperature. The target to substrate distance was fixed at 6 cm. These conditions ensured that during the periodic ablation of each component, the thickness deposited was always less than a monolayer. These samples are approximately 40 nm thick. From transmission electron mi
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