A-Si_xC_1-x:H thin films with subnanometer surface roughness for biological applications

The characterization of a-Si_xC_1-x:H thin films by plasma-enhanced chemical vapor deposition with high hydrogen dilution for biological applications is addressed. A root mean square roughness less than 1-nm was measured via atomic force microscopy for an area of 25-μm². Structural analysis was done using Fourier transform infrared spectroscopy in the middle infrared region. It was found that under the deposition conditions, the formation of Si-C bonds is promoted. Electrical dark conductivity measurements were performed to evaluate the effect of high hydrogen dilution and to find the relation between carrier transport properties and the structural arrangement. Conductivities of the order of 10^-7 to 10^-9 S/cm at room temperature for methane-silane gas flow ratio from 0.35 to 0.85 were achieved, respectively. UV-visible spectra were used to obtain the optical band gap and the Tauc parameter. Optical band gap as wide as 3.55-eV was achieved in the regime of high carbon incorporation. Accordingly, deposition under low power density and high hydrogen dilution reduces the roughness, improves the structure of the network, and stabilizes the film properties as a greater percentage of carbon is incorporated. The biofunctionalization of a-Si_xC_1-x:H surfaces with NH₂-terminated self-assembled monolayers was obtained through silanization with 3-aminopropyltrimethoxysilane. This knowledge opens a window for the inclusion of these a-Si_xC_1-x:H thin films in devices such as biosensors.