Optical constants of a series of amorphous hydrogenated silicon-carbon alloy films: Dependence of optical response on film microstructure and evidence for homogeneous chemical ordering

Abstract

The optical constants (n =n +ik, el -nk, e2 --2nk) of a series of amorphous hydrogenated silicon-carbon alloy films (a-Sil C:H) have been determined for photon energies between 1.5 and 4.75 eV. These films have been prepared via the rf glow-discharge decomposition of SiH4 and CqH2.The index of refraction n at 1.5 eV increases smoothly from n =1.67 for a-C:H (~=1) to n =3.18 for a-Si:H (x =0), while the optical energy gap Eopt reaches a maximum value of 2.68 eV for a carbon fraction of x =0.68. The films in this alloy series are proposed to be macroscopically homogeneous, while having a heterogeneous microstructure. Their optical response has been modeled, via the Bruggemann effective-medium approximation (EMA), as arising from three amorphous components: polymeric (a-CH, m -2), graphitic (a-C), and tetrahedral (a-Si:C:H), and a void component. A Siand C-centered tetrahedron model developed in the preceding paper has been used to predict the optical response of the amorphous tetrahedral component as a function of its composition. This EMA approach based on four components in the films gives a good description of the observed dependences of el and e2 on composition and provides a convincing demonstration that the appearance of the amorphous graphitic component in the films limits the attainable value of E"p, in this alloy series as the carbon content increases. In addition, the model provides strong evidence that complete chemical ordering with homogeneous dispersion exists within the amorphous tetrahedral (a-Si:C:H) component across the entire alloy series.

Distribution of Pores in a-Si1−x C x :H Thin Films

“…The growth of a-Sil-xCx:H thin films with very low conductivity (<10-14 ~-1 cm-l) and high optical gap (higher than ,~3 eV) is particularly important for thin-film transistor (TFT) technology based on amorphous materials (Madan & Shaw, 1988). It is well known that the increase in optical gap is limited by the formation of graphite-like carbon clusters, which provoke a decrease in the optical gap (Sussmann & Ogden, 1981;Bullot & Schmidt, 1987;Mui, Basa & Smith, 1987). Furthermore, hydrogenated amorphous silicon and its alloys, grown as thin films by different methods, present pores in the nanometer size range (Bullot & Schmidt, 1987;Mahan, Williamson, Nelson & Crandall, 1989).…”

Aging of Oxygen-Treated Trimethylsilane Plasma-Polymerized Films Using Spectroscopic Ellipsometry

“…First, the films are presumed to be macroscopically homogenous, while having a heterogeneous microscopic structure [14]. Second, the effective index of these films is modeled according to the Bruggemann effective medium approximation (EMA) [15].…”

Study on the excimer laser annealed amorphous hydrogenated silicon carbon films deposited by PECVD

“…The excimer laser annealing represents a promising technique [7,8] for the crystallization because the excimer laser has larger beam size and better density homogeneity than other lasers. Further, because the absorption coefficient of silicon rich amorphous silicon carbon (a-Si 1-x C x :H) is relatively high in the UV range (≈10 6 cm -1 ) [1], the crystallization takes place without damage of the substrate in these samples. Since excimer laser annealing are reported to produce crystalline films of interest [5,[9][10][11][12][13], in this paper we have undertaken excimer laser annealing of hydrogenated amorphous silicon carbon films of different carbon content deposited by plasma enhanced chemical vapour deposition (PECVD).…”

“…Hydrogenated amorphous and crystalline silicon carbon films have emerged as semiconductor materials of considerable technological interest because of their tunable optical and electronic properties [1][2][3][4]. Accordingly, these materials are finding applications in optoelectronic and microelectronic devices such as thin film solar cells, light emitting diodes and thin film transistors [2,5].…”