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

摘要

Amorphous hydrogenated germanium carbon (a-Ge_(1-x)C_x:H) films were prepared by radio frequency (RF) reactive magnetron sputtering of a pure Ge (111) target in a CH_4 + H_2+Ar mixture and their composition, optical properties, chemical bonding were investigated as a function of gas flow rate ratio of CH_4/(Ar + H_2). The results showed that the deposition rate first increased and then decreased as gas flow rate ratio of CH_4/(Ar + H_2) was increased from 0.125 to 0.625. And the optical gap of the a-Ge_(1-x)C_x: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 CH_4 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 CH_4 flow rate which is connected with relatively high H_2 concentration and Ge content. Especially in low CH_4 concentration, the formation of sp ~2-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 sp~3 hybridization.