Crystallization of a-Si:H on glass for active layers in thin film transistors effects of glass coating

One hundred nanometer thick a-Si:H films deposited by plasma-enhanced chemical vapor deposition from hydrogen diluted silane at temperatures in the range 150-320°C were used as precursor materials for solid-phase crystallization (SPC) of polycrystalline silicon (poly-Si) on which n-channel thin film transistors (TFTs) were fabricated. Furnace annealing at 600°C was used in order to study the SPC kinetics as a function of deposition temperature and substrate coating. The substrates were either bare or silicon nitride coated Corning 7059 glass. The poly-Si films were characterized by using ultraviolet reflectance spectrophotometry to determine the annealing time for crystallization and by using transmission electron microscopy for grain size measurement and nuclei density analysis. It was found that there is a strong relation between grain size and deposition temperature for films deposited on bare glass substrates. For films deposited on silicon nitride coated glass substrates this correlation is not as strong; however, upon full crystallization, a-Si:H deposited on nitride coated glass yielded grains that were larger than those in a-Si:H deposited on bare glass. The presence or absence of a SiN_x barrier layer between an a-Si precursor film and substrate glass is observed to greatly affect the SPC process. TFT parameter measurements, on the other hand, have shown that the SiN_x-coating of glass has a significant impact on TFT performance: transistor leakage current is observed to be markedly reduced by coating. This is attributed to the increase in grain size and the decrease in the impurity content of active poly-Si caused by SiN_x-coating of glass.