Deposition of ultraviolet photoconductive films of amorphous hydrogenated carbon

The structural, optical, and transport properties of hydrogenated amorphous carbon films deposited on the grounded electrode under different dilutions (pure CH4, CH4+H-2, and CH4+Ar) in radio frequency powered plasma enhanced chemical vapor deposition process have been studied. Though all the samples deposited were observed to be polymer-like carbon, they have different optical and transport properties. While some of the samples were highly conductive under dark (similar to 10(-1) S cm(-1) at room temperature), others exhibited very low (< 10(-12) S cm(-1)) dark conductivity and were photoconductive (similar to 10(-9)-10(-10) S cm(-1)) only under ultraviolet (UV) light (wavelength lambda < 250 nm). There appears to be a close link between the transport properties and optical absorption. Based on optical absorption, we propose two possible models for the distribution of the density of states involving the pi and sigma states. The evolution of these states and of the deep defect absorption has a profound impact on the transport properties and particularly on the UV photoconductivity. Comparing the two models, it seems more probable that the UV photoconductivity (for photon energy > 4.96 eV) is rather linked to transitions involving the sigma-sigma(*) states. (c) 2007 American Institute of Physics.