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 chemi cal vapor deposition from hydrogen diluted silane at temperatures in the ra nge 150-320 degrees C were used as precursor materials for solid-phase crys tallization (SPC) of polycrystalline silicon (poly-Si) on which n-channel t hin film transistors (TFTs) were fabricated. Furnace annealing at 600 degre es C was used in order to study the SPC kinetics as a function of depositio n temperature and substrate coating. The substrates were either bare or sil icon nitride coated Coming 7059 glass. The poly-Si films were characterized by using ultraviolet reflectance spectrophotometry to determine the anneal ing time fdr crystallization and by using transmission electron microscopy for grain size measurement and nuclei density analysis. It was found that t here is a strong relation between grain size and deposition temperature for films deposited on bare glass substrates. For films deposited on silicon n itride 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 SiNx barrier layer between an a-Si precursor film and substrate glass is observed to greatly affect the SPC process. TFT para meter measurements, on the other hand, have shown that the SiNx-coating of glass has a significant impact on TFT performance: transistor leakage curre nt 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 SiNx-coating of glass. (C) 1999 The Electrochemical Socie ty. S0013-4651(98)03-094-8. All rights reserved.