Optical study of disorder and defects in hydrogenated amorphous silicon carbon alloys

We have studied the defect density and disorder in hydrogenated amorphous silicon carbon alloys produced by rf glow discharge of silane-methane-hydrogen mixtures, by combining spectroscopic ellipsometry and photothermal deflection spectroscopy measurements. Increasing the methane flow rate leads to a widening of the optical gap and to an increase of the apparent disorder, deduced from the standard analysis of the exponential absorption edge; the so-called Urbach energy. Interestingly, the subgap absorption decreases with increasing methane flow rate. This points towards a lower density of defects with increasing carbon content and is in contrast with the increased disorder. This apparent contradiction results from the presence of three absorption bands within the gap of this material, as reported by [Ivashchenko, J. Phys.: Condens. Matter 14, 1799 (2002)], and which make unreliable the standard analysis of the disorder in silicon carbon alloys.