Stability of unhydrogenated polysilicon thin film transistors and structural quality of the channel material

Abstract

The performance of polysilicon thin film transistors used in large-area electronics applications, directly depends on the structural quality of the channel material. Moreover, their stability under electrical stress is shown, in this work, to also depend on the quality of the channel material. TFTs were fabricated using several channel materials, deposited as an amorphous film by low-pressure chemical vapor deposition (LPCVD), and then crystallized using solid-phase annealing, a large-area pulsed excimer laser, or scanning with a 532-nm beam of a pulsed diode pumped Nd:YVO4 laser. The stability of the TFTs, determined from the increase in the subthreshold slope S, is shown to be related to the importance of the surface roughness and to the structural quality of the crystallized active layer. With similar surface roughness, the stability is better when the structural quality of the active layer is improved. The increase in S is then explained by the creation of a state in the channel material that is more effective when the structure of the polysilicon is more disordered.