Abstract
We report the effects of surface passivation by depositing a hydrogenated amorphous silicon (a-Si:H) layer on the electrical characteristics of low temperature polycrystalline silicon thin film transistors (LTPS TFTs). The intrinsic a-Si:H layer was optimized by hydrogen dilution and its structural and electrical characteristics were investigated. The a-Si:H layer in the transition region between a-Si:H and µc-Si:H resulted in superior device characteristics. Using a-Si:H passivation layer, the field-effect mobility of the LTPS TFT was increased by 78.4% compared with conventional LTPS TFT. Moreover, the leakage current measured at VGS of 5 V was suppressed because the defect sites at the poly-Si grain boundaries were well passivated. Our passivation layer, which allows thorough control of the crystallinity and passivation-quality, should be considered as a candidate for high performance LTPS TFTs.
Highlights
The display industry has been widely investigating to meet new market demands for low-cost, large-size, high-resolution, high-frame-rate, and 3D displays
Low temperature polycrystalline silicon (LTPS) Thin film transistors (TFTs) have been widely used because their field effect mobility is higher than that of hydrogenated amorphous silicon (a-Si:H) TFTs, so that high resolution displays with ultra-high densities (3840 × 2160, UHD) can be fabricated
Poly-Si TFTs have excellent and reliable electrical characteristics, but they still suffer from leakage currents under off state bias
Summary
The display industry has been widely investigating to meet new market demands for low-cost, large-size, high-resolution, high-frame-rate, and 3D displays. To reduce leakage currents in poly-Si TFTs, some additional structures have been proposed including lightly doped drain (LDD) and offset gate structures [3] These structures require additional processing and costs, so they have not been fully applied in the display industry. To reduce the off-state leakage current, Kim et al proposed a simple method based on the insertion of a thin (10 nm) a-Si:H layer [4] They explained the reason for the reduced leakage current using an energy band diagram and the current paths in the LTPS TFT with an a-Si:H passivation layer and a larger band gap. Materials 2019, 12, 161 without an a-Si:H passivation layer (58.3 cm2 /V·s) They solved the leakage current problem in LTPS TFTs through the simple insertion of a passivation layer with a larger band gap, the field-effect mobility was severely decreased, so that the high performance of poly-Si TFTs disappeared. The leakage current was successfully reduced without sacrificing the field-effect mobility by using an optimized a-Si:H layer
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