The Study On Excimer Laser Crystallization Of Nanocrystalline Silicon Thin Films

It is well known that silicon is the main semiconductor materials of today’s microelectronic industry in the world. The technology of low-temperature polycrystalline silicon(poly-Si) thin film transistors(TFTs) which can be fabricated by excimer laser crystallization(ELC) has been widely investigated for application in active matrix flat panel display.However, the hydrogenated amorphous silicon(a-Si:H) used as starting material for ELC is with disordered structure, no crystallinity, and no silicon grains in its tissue. Meanwhile, hydrogenated nanocrystalline silicon thin films(nc-Si:H),as mixture of various amounts of different structural components where Si nanocrystals are embedded in the amorphous silicon network,have attracted considerable interests due to their potential applications to optoelectronic devices including thin film solar cells,thin film transistors,and tunneling diodes.The research on ELC of nc-Si:H films has seldom been reported in the literatures. The effect of varying excimer laser energy density onnanocrystalline silicon grains in nc-Si:H thin films is not known yet.In our work the nc-Si:H thin film on glass substrate prepared from plasma enhanced chemical vapor deposition(PECVD) system is crystallized by excimer laser with a laser energy density of 50-600 m J/cm2.The crystallized thin films have been characterized by Raman spectra,XRD, AFM and SEM. Poly-Si films with higher crystallinity can be obtained by ELC method. A threshold value of 200 m J/cm2 is estimated from plots of crystalline fraction and surface roughness versus laser energy density. The amorphous part in the nc-Si:H film is partially melted before the threshold. Then the nc-Si grains begin to melt after energy density increases threshold. The nc-Si grains in the films serve as seeds for lateral growth to larger grains during the process of ELC. The existence of nc-Si grains in the films widens the process window of the super lateral growth(SLG).