MODIFICATIONS OF THE OPTICAL AND STRUCTURAL-PROPERTIES OF HYDROGENATED AMORPHOUS-SILICON BY ALLOYING WITH GERMANIUM AND CARBON

The effect of the Ge and C in a-Si1-xGex:H and a-Si1-xCx:H thin films on the optical gap and valence and conductivity band tails (VBT and CB T) from one side and on the short-range order (SRO) from the other, ha ve been studied by optical absorption, photothermal deflection spectro scopy and Raman scattering measurements. The results show that the VET is very sensitive to the geometric disorder as a consequence of the i ncorporation of Ge, whereas the CBT is not. CBT increases with Ge conc entration when x > 0.33 and this increase is due to the alloying rathe r than to the short-order deterioration. Both the CBT and VET, as well as the short-range order deterioration increases with the C content i n a-Si1-xCx:H films and the effect of C incorporation on these values is stronger than that of Ge in a-Si1-xGex:H films. Probably this is du e to the greater difference in electronegativity between silicon and c arbon than between silicon and germanium. A comparison of the correlat ion between Raman Si-Si TO-like peak width and the optical gap for fil ms with different Ge and C concentrations allows to separate the roles of Ge- and C-induced bond angle disordering and alloy energy band cha nges. (C) 1998 Elsevier Science B.V. All rights reserved.