engleski

The influence of thermal annealig on the structural and electrical properties of amorphous-nano-crystaline thin Si films

The a-nc-Si:H films were deposited by the PECVD method, using radio frequency discharge in silane highly diluted by hydrogen. In order to test the stability of this kind of material and examine the possibilities of post-deposition treatment, the as deposited samples were exposed to isochronal thermal annealing in vacuum and hydrogen atmosphere with a duration of 1 hour at 200, 300 and400 °C. The structure of as deposited and treated films was measured by HRTEM, GISAXS, GIWAXS and Raman spectroscopy. HRTEM micrographs showed that the initial films contained isolated nano-sized ordered domains, nano-crystals, with log-normal size distribution and average size between 8 and 9 nm. The Raman crystal fraction was about 30%. The Raman spectra of as deposited a-nc-Si:H samples showed a crystalline TO peak at frequencies lower than the mono- crystalline peak. After thermal treatment, this peak shifts further towards lower frequencies. GISAXS pattern showed that films contained particles with similar sizes as seen by HRTEM. The analysis of the GIWAXS diffraction lines confirmed that crystals are as small as expected from HRTEM and indicated that strain was present in the samples. After thermal treatment, the critical angle for total external reflection increased gradually indicating an increase of the mass density, and also the width of the diffraction lines increased indicating an increase of strain. Electrical properties were measured by impedance spectroscopy in the frequency range 0.01 Hz – 1 MHz and in the temperature range -100–120 °C. It showed two relaxation times and two activation energies for DC conductivity just confirming two phases. After annealing, the photoconductivity increased and showed inverse Staebler-Wronsky effect – the light versus dark conductivity increased during light exposure. The observed changes in the electrical and structural properties by annealing can be explained as a consequence of hydrogen effusion and redistribution in the film.

a-nc-Si:H; annealing; relaxation; activation energy

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