Improved Long-Term Stability of Low-Temperature Polysilicon Thin-Film Transistors by Using a Tandem Gate Insulator with
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Improved Long-Term Stability of Low-Temperature Polysilicon Thin-Film Transistors by Using a Tandem Gate Insulator with an Atomic Layer of Deposited Silicon Dioxide Sungsoo Lee Department of Electrical and Computer Engineering, Seoul National University, Seoul 08826, Korea
Jin-Seong Park∗ Department of Material Science & Engineering, Han Yang University, Seoul 04763, Korea
Yongtaek Hong† Department of Electrical and Computer Engineering, Seoul National University, Seoul 08826, Korea (Received 1 May 2020; revised 20 May 2020; accepted 22 May 2020) In this study, we report a substantial improvement in the long-term stability of low-temperature polycrystalline silicon (LTPS) thin-film transistors (TFTs) with a tandem gate insulator composed of silicon dioxide (SiO2 ) deposited by using atomic layer deposition (ALD) and plasma-enhanced chemical vapor deposition (PECVD). Negative-bias temperature instability (NBTI) tests showed that threshold-voltage (ΔVth ) shifts were significantly smaller than when only a plasma-enhanced chemical vapor deposition (PECVD) structure was used. We believe that the unique stoichiometric characteristics and the reduction in the interfacial trap density (Dit ) produced by the SiO2 gate insulator that had been fabricated using ALD enhanced the long-term stability of the LTPS TFTs. These results suggest a tandem structure gate insulator with high-quality ALD-based SiO2 thin film can provide an important improvement in the characteristics of the p-channel LTPS TFTs required for advanced active matrix organic light-emitting diodes (AMOLEDs) applications. Keywords: LTPS TFT, ELA, Poly-Si, ALD, NBTI DOI: 10.3938/jkps.77.277
I. INTRODUCTION Low-temperature polycrystalline silicon thin-film transistors (LTPS TFTs) are specialized devices used in mobile flat panel displays, such as active matrix liquidcrystal displays (AMLCDs), and in active matrix organic light-emitting diodes (AMOLEDs) [1,2]. Although LTPS TFTs exhibit mobility several orders of magnitude higher than amorphous silicon TFTs (a-Si TFTs), in practical application, device degradation remains an important issue [3]. The grain boundaries in LTPS TFTs can lead to device degradation and have a detrimental effect on performance, a problem that has been investigated and is well recognized. Extensive work has been done to explore ways to improve the quality of the polycrystalline thin films by passivating dangling bonds at the grain boundaries or by replacing the gate insulator material, but less emphasis has been given to the morphology of the poly-Si protrusions [4]. ∗ E-mail: † E-mail:
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pISSN:0374-4884/eISSN:1976-8524
Excimer laser annealing (ELA) is a widely investigated alternative method of crystallizing a-Si thin films. The major advantage of this technique is the formation of polycrystalline grains with excellent structural quality without thermal damage to the glass substrate. However, the high surface roughness resulting from protrusions at the poly-Si grain boundaries is an inherent problem o
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