Tomoyuki Nouda

Study of Crystal Growth Mechanism for Poly-Si Film Prepared by Excimer Laser Annealing

Recrystallization of polycrystalline silicon (poly-Si) film by excimer laser annealing (ELA) is discussed by considering the experimental results that the three stages of nucleation, textured grain growth and secondary grain growth were observed. Although the phenomenon of nucleation in the amorphous silicon (a-Si) is understood by considering crystallization from the super cooled liquid, the growth mechanisms of the textured grain and secondary grain are not understood by this, because the melting point of poly-Si which has already been formed on the entire surface during these growth stages is higher than that of a-Si. The recrystallization mechanism considering the dislocation movement is introduced to investigate the present phenomenon. It also clarifies the reason why secondary grain growth occurs under the critical conditions of laser irradiation energy and shot number. The feasibility of nucleation through the super cooled liquid is also discussed.

Mechanisms of Electrical Stress-Induced Degradation in H2/Plasma Hydrogenated n- and p-Channel Polysilicon Thin Film Transistors

The degradation characteristics under various bias stress conditions are systematically investigated in hydrogenated n- and p-channel polysilicon thin film transistors (poly-Si TFTs) with a sidewall spacer. The device characteristics after a relatively long stress in the p-channel TFTs exhibit more serious degradation than those in the n-channel TFTs. Moreover, the stress time dependence of the threshold voltage shift in the former devices shows more complicated behavior than that in the latter. These serious and complicated time-dependent degradation characteristics (induced only in p-channel devices) can be explained by the generation of two defect states in which hydrogen plays an important role: one is positively charged traps that correspond to Si+ atoms generated by the injection of H+ ions into the gate oxide near the source and appear after a certain stress time. The other is deep traps that correspond to dangling bonds generated by the dissociation of Si–H bonds at/near the interface near the source and appear after a long stress time.

Effects of Various Hydrogenation Processes on Bias-Stress-Induced Degradation in p-Channel Polysilicon Thin Film Transistors

The bias-stress-induced degradation characteristics of p-channel polysilicon (poly-Si) thin film transistors (TFTs) hydrogenated by plasma, H2 and H2/plasma processes are investigated. It is found that the stress time dependence of the transfer characteristics and the threshold voltage shift is affected by the hydrogenation processes and the quantity of hydrogen in the active channel layer of TFTs. These behaviors are mainly caused by the difference in the generation of defect states, that is, band tail states, deep traps corresponding to dangling bonds at/near the poly-Si/SiO2 interface, and positively charged traps corresponding to Si+ atoms created by H+ ions in the oxide. We propose a model to explain these behaviors.