Excimer-laser-annealed poly-Si thin-film transistors

The crystallization of alpha -Si:H into poly-Si using an excimer laser has been examined. The resulting microstructure was found to be stratified into a large-grain surface region, formed from the liquid phase, and a fine-grain underlying layer, thought to be formed by solid phase crystallization. The threshold beam energies for these sequential phase changes were identified from surface reflectance measurements after crystallization and the energies increased with diminishing hydrogen content of the material. The electrical characteristics of thin-film transistors made with material crystallized at energies close to the melt threshold could be correlated with the limited depth of large-grain material. For significantly higher beam energies, coplanar structures showed a severe degradation in leakage current due to lateral diffusion of phosphorus, across the channel from the source and drain regions. When this effect was avoided, thin-film transistors with field-effect mobilities up to 160 cm/sup 2//V-s and on/off current ratios up to 10/sup 8/ were obtained.>