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Characteristics and stress-induced degradation of laser-activated low temperature polycrystalline silicon thin-film transistors

Abstract— A photodetector using a silicon‐nanocrystal layer sandwiched between two electrodes is proposed and demonstrated on a glass substrate fabricated by low‐temperature poly‐silicon (LTPS) technology. Through post excimer‐laser annealing (ELA) of silicon‐rich oxide films, silicon nanocrystals formed between the bottom metal and top indium thin oxide (ITO) layers exhibit good uniformity, reliable optical response, and tunable absorption spectrum. Due to the quantum confinement effect leading to enhanced phonon‐assisted excitation, these silicon nanocrystals, less than 10 nm in diameter, promote electron‐hole‐pair generation in the photo‐sensing region as a result resembling a direct‐gap transition. The desired optical absorption spectrum can be obtained by determining the thickness and silicon concentration of the deposited silicon‐rich oxide films as well as the power of post laser annealing. In addition to obtaining a photosensitivity comparable to that of the p‐i‐n photodiode currently used in LTPS technology, the silicon‐nanocrystal‐based photosensor provides an effective backlight shielding by the bottom electrode made of molybdenum (Mo). Having a higher temperature tolerance for both the dark current and optical responsibility and maximizing the photosensing area in a pixel circuit by adopting a stack structure, this novel photosensor can be a promising candidate for realizing an optical touch function on a LTPS panel.

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Thin-beam directional X'tallization (TDX) is a low-temperature-substrate compatible crystallization method that can form directionally solidified poly-si films directly on glass substrates. Without using a mask, extremely long uniformly-spaced poly-Si grains can be formed by TDX. In this paper, we have investigated the influence of laser energy and scan pitch on the properties of the resulting poly-Si films. Grain size and surface morphology of TDX-processed poly-Si films were observed by SEM and AFM. The AFM results demonstrate that, due to the lateral growth phenomenon, TDX poly-Si films are much smoother than films produced by the current industry standard method, Excimer Laser Annealing (ELA). The breakdown of the TDX method is also discussed in this paper. When the laser scan pitch is larger than the lateral growth length that is characteristic of a given poly-Si film and beam property combination, a discontinuous poly-Si film is formed with vertical protrusions and grain boundaries occurring perpendicular to the scan direction. Poly-Si films with vertical protrusions and grain boundaries perpendicular to the direction of current flow will degrade the resulting thin-film transistor (TFT) performance.