Optical and structural properties of siliconlike films prepared by plasma-enhanced chemical-vapor deposition

摘要

Amorphous siliconlike thin films (Si:O_x:C_y:H_z), deposited by plasma-enhanced chemical-vapor deposition using hexamethyldisiloxane as monomer and Ar as feed gas, have been investigated for their optical and structural properties as a function of the deposition power, in the range of 100-400 W. The films have been analyzed by Fourier transform infrared spectroscopy (FTIRS), UV-VIS-NIR spectrophotometry, and atomic force Microscopy (AFM). From the analysis of FTIR spectra it results that films assume a marked inorganic character as the power increases. Indeed, at high power, Si-O-Si groups prevail over Si(CH_3)_x groups, so that the film chemistry approaches the SiO_2 one. Reflectance/transmittance spectra, acquired in the range of 200-2500 nm, allow us to describe the film absorption edge for interband transitions. The relationship between the optical energy band gap, deduced from the absorption coefficient curve, and the deposition power has been investigated. The reduction of the optical energy gap from 3.86 to 3.61 eV and the broadening of the optical-absorption tail with the increase of power from 100 to 400 W are ascribed to the growth of thermal and structural disorders. Moreover, the refractive index has been evaluated and related to the film morphology. The AFM analysis confirms the amorphous character of the films and shows how the deposited layers become flatter and more compact when power increases. We consider the densification of the film responsible for the growth of the refractive index from 1.90 to 1.97 in the power range 100-400 W.