Ageing of laser crystallized and unhydrogenated polysilicon thin film transistors

Ageing of low temperature polysilicon Thin Film Transistors (TFTs) is reported in this study. The active layer of these high performances transistors is amorphous deposited using Low Pressure Chemical Vapor Deposition (LPCVD) technique and then laser crystallized using a single shot ECL (SSECL of SOPRA) with very large excimer laser. The drain and source regions are in-situ doped during the LPCVD deposition by using phosphine or diborane to fabricate n-type or p-type transistors respectively.These laser crystallized TFT's show poorer reliability properties than solid-phase crystallized TFT's. This poor stability is explained to originate from the high surface roughness produced by the laser crystallization, which is highlighted from Atomic Force Microscopy observations.Moreover to this conclusion, the behaviour of the threshold voltage shift ΔV_T during positive and negative stresses is checked to the light of a stretched exponential law that is, as supposed, a federative law. This law is explained in hydrogenated amorphous silicon TFT's by a dispersive diffusion coefficient of hydrogen in the disordered material. Taking into account that such relation appears as sufficiently general and, particularly, can describe the behaviour of monocrystalline silicon MOSFET and un-hydrogenated polysilicon TFT's where the hydrogen cannot involved, it can be supposed that it deals with disordered materials and disordered regions in crystalline materials (interface, grain boundary, …..).