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Structural properties of a-Si1-xCx:H alloys

The a-Si1-xCx:H thin films were deposited by means of DC magnetron sputtering source, using argon-hydrogen gas mixture and benzene vapour as a origin of carbon atoms. The specimens were analysed by visible, IR, SAXS (Small Angle x-ray Scattering), PL (Photoluminescence) and Raman spectroscopy, as a function of carbon concentration, up to x=0.3. Some specimens were annealed in vacuum at 400 deg C. The optical gap, obtained by Tauc plot, increases from 1.6 eV for x=0 to 1.86 eV for x=2. The further increase in carbon content does not change the optical gap indicating qualitative structural changes. The statical dielectric constant decreases gradualy with carbon incorporation but faster than expected by simple substitution of silicon by carbon atoms. In the same time, the frequency of stretching vibrations of Si-H bonds moves towards the higher values. The both effects was discussed as a consequence of individual voids volume contribution in the films and increasing in individual voids size. In the same way can be explained the changes in frequency of Si-C bonds vibrations during the thermal treatment. The SAXS spectra of a-Si:H exibit existence of structural defects which can be attributed to presence of voids with several A in size. The concentration and size of this type of defects increase with carbon content, in excellent agreement with above mentioned model. Besides microvoids, SAXS idicate the existence of additional, much larger structural units. By addition of carbon in a-Si:H matrix, the room-temperature luminescence appears, with maximum near 1.85 eV. The intensity of PL signal increases with increasing of carbon concentration, up to x=0.3. The frequency ofPL maximum does not change with carbon concentration, indicating the growth of new phase in a-Si1-xCx:H films. A specially large PL signal was obtained after thermal annealing of 400 deg C. The intensive PL signal appears together with a strong band in Raman spectra, attributed to carbon agglomerates in sp3 bond configuration.

amorphous silicon carbide; FTIR; visible spectroscopy; SAXS; PL; Raman;

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