The transformation of silicon from an amorphous phase to a polycrystalline phase by excimer laser annealing was investigated by means of Raman spectroscopy and electron mobility measurements. It was shown that the phase transformation was mainly governed by the energy density of the laser, and that when the density was high enough to evaporate the surface layer of the amorphous silicon, a latent state of the multiple irradiated sample was formed, where the Raman intensity exceeded that of the bulk crystal silicon and the electron mobility of the film became very high. This observation was explained by the pyramidal grain growth structure of the crystalline silicon. The amorphous component at the grain boundary was also observed in measured Raman spectra and it was shown that the presence of this phase had a significant influence on reducing the field effect mobility.
A thin-film transistor (TFT) with a maximum field-effect mobility of 329 cmZ/V * s, an on/off current ratio of 7.6 x lo7, a threshold voltage of 6.7 V, and a subthreshold slope of 0.37 V/decade was fabricated by using pulse laser annealing processes. Amorphous silicon film (a-Si:H) with a very IOW impurity concentration of 4 x 10" for oxygen, 1.5 x lo'* for carbon, and 2 x l O I 7 cm-' for nitrogen was deposited by plasma CVD method and annealed by KrF Excimer laser (wavelength of 248 nm). The Raman spectroscopy technique was a useful tool for optimizing laser annealing conditions. Experimental results show that two factors are very important for fabricating very-high mobility TFT's: 1) utilizing high-purity as-deposited a-Si:H film, and 2) performing whole laser annealing processes sequentially in a vacuum container and optimizing illumination conditions.
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