Excimer Laser Crystallized Polycrystalline Silicon Thin Film Transistors

Polycrystalline silicon (polysilicon) thin film transistors (TFTs) have been widely applied to active matrix liquid crystal displays (AMLCDs) and active matrix organic light emitting displays (AMOLEDs), allowing the integration of both active matrix and driving circuitry on the same substrate. Recently, the decrease of maximum process temperature allowed the application of polysilicon TFTs as driving transistors in OLED displays on flexible metal and polymeric substrates. The high device performance required by these applications have been obtained by using laser crystallization (ELC) of amorphous silicon leading to polysilicon with good crystallinity with very few in-grain defects, due to the melt-regrowth process. In particular, when the active layers are crystallized under the Super Lateral Growth (SLG) regime, large (>1 ?m) grain materials are formed and high performance TFTs (electron field-effect mobility >300 cm2/Vs) can be obtained. However, the SLG mechanism has a very narrow energy density window and, consequently, highly uniform beam profiles and pulse-to-pulse stability better than 2% are required. This implies that standard ELC-process is technologically quite critical and several approaches have been proposed to improve the process uniformity as well as device performance. Several crystallization methods have been proposed, mainly based on lateral growth technique, such as two pass annealing, selectively enlarging laser X'tallization (SELAX), ?-Czochralski (grain-filter) process, CW laser lateral crystallization (CLC) and sequentiallateral solidification (SLS). In particular, SLS process exploits controlled super lateral growth (C-SLG), inducing, through a patterned mask, complete melting of a selected region of the Si film. This process can produce grains of almost arbitrary length from micrometers to centimetres, but these long grains will always be terminated by a grain boundary. In the direction perpendicular to the grain growth, the grains results very narrow (100-500 nm), depending upon film thickness, substrate temperature and laser energy, with subgrain boundaries at this pitch. Hence the properties of this material, such as the field effect mobility, result highly anisotropic. The location-controlled distribution of grain boundaries (GBs) of SLS-polysilicon makes this material particularly useful for the investigation of influence of GBs on the performance of poly-Si TFTs. Indeed, by studying TFTs with the channel orthogonal and parallel to grain growth direction, where carriers flow parallel and perpendicular to sub-grain-boundaries, respectively, it is possible to distinguish grain boundary carrier trapping from intragrain trapping. We have characterized SLS-polysilicon TFTs by measuring the thermal activation energy of the channel current in the two orthogonal directions, and the difference in activation energy has been related to carrier flow over perpendicular sub-GBs. Experimental results have been analysed through two-dimensional numerical simulations. This analysis showed that a planar barrier GB, which simply resulted in a potential barrier within the channel, is fundamentally incompatible with the experimental drain current activation energy data. A more satisfactory representation of all the experimental data can be obtained by using a finite width GB, in which carrier flow was controlled by transport across the resistive GB region, rather than by emission over a barrier.