Transpolyacetylene chains in DECR plasma deposited a-C:H films

摘要

A distributed electron cyclotron resonance plasma reactor powered by a microwave generator operating at 2.45 GHz was used to deposit a-C:H films at RT on RF biased <100> Si substrates. The sp~2 hybridized carbon structure was investigated using electron diffraction, visible Raman spectroscopy and Fourier transform infrared spectroscopy. Hydrogen content N_H (21 <=N_H <= 36 at. percent) and density rho (1.2 <= rho <= 2.3 g cm~(-3)) were assessed using nuclear reaction analysis and spectroscopic ellipsometry. Electron diffraction analysis shows that the sp~2 hybridized carbon content is >47 at. percent. The corresponding Raman spectra were successfully fitted using the G and D bands and the bands centred at 1140, 1233 and 1475 cm~(-1). The latter have been previously assigned to the transpolyacetylene (TPA) chains modes. This interpretation is supported here by three observations. The values of the bond angles and lengths (approx 116 deg and approx 1.52 A, respectively) and first neighbouring numbers (ranging between 2.3 and 2.8) derived from the electron diffraction analysis are consistent with the corresponding TPA data. The relative intensity of the bands is seen to decrease with the increasing substrate bias while N_H decreases. A significant sp~2-CH olefinic mode contribution to the infrared stretching band is observed for the low-density films (approx 1.2 g cm~(-3)). The results show that growth of the TPA chains is enhanced when ion current density and energy decrease. The evolution of the ratio of the D and G band intensities as a function of the substrate bias indicates that the average size of the sp~2 clusters is close to 2.5 nm whatever the plasma pressure.