Effects of methane flow rate on the optical properties and chemical bonding of germanium carbon films deposited by reactive sputtering

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Amorphous hydrogenated germanium carbon (a-Ge1−xCx:H) films were prepared by radio frequency (RF) reactive magnetron sputtering of a pure Ge (111) target in a CH4 + H2+Ar mixture and their composition, optical properties, chemical bonding were investigated as a function of gas flow rate ratio of CH4/(Ar + H2). The results showed that the deposition rate first increased and then decreased as gas flow rate ratio of CH4/(Ar + H2) was increased from 0.125 to 0.625. And the optical gap of the a-Ge1−xCx:H films increased from 1.1 to 1.58 eV accompanied with the increase in the carbon content and the decrease in the relative content of Ge–C bonds of the films as the CH4 flow rate ratio was increased, while refractive index of the films decreased and the absorption edge shifted to high energy. Through the analysis of X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy, it was found that the formation of Ge–C bonds in the films was promoted by low CH4 flow rate which is connected with relatively high H2 concentration and Ge content. Especially in low CH4 concentration, the formation of sp2-hybridised C–C bonds was suppressed considerably both due to the etching effect on weak bonds of H and the fact that chemical bonding for germanium can be only sp3 hybridization.