Stability, Electrical Characteristics and Flexibility of ZnO Thin Film Transistors Fabricated by Atomic Layer Deposition
Date Issued
2014
Date
2014
Author(s)
Lin, Yuan-Yu
Abstract
Metal-oxide thin film transistors (TFTs) are promising for display applications. However, metal-oxide TFTs usually require high-temperature processing to reduce and remedy defects produced during their manufacturing process, which increased complexity and cost of production and limit their suitability for flexible electronics applications. We developed a low-temperature atomic layer deposition (ALD) process to fabricate flexible ZnO TFTs, where defects in the TFTs were minimized by depositing the dielectric, ZnO channel and passivation layers of the devices in the integrated ALD process. The ZnO TFTs achieved high mobility (20.2 cm2/Vs), good bias-stress stability (0.2 V threshold voltage shift after 8 V bias 10800 s) and low sub-threshold swing (0.34 V/dec), and could be bended to 1.3 cm of radius for 1400 times without any degradation, which was presented for the first time. Our approaches were two-fold: (1) prevent intrinsic defects from forming in the deposition processes of the dielectric, channel, and passivation layers; (2) protect the devices from developing extrinsic defects under the influence of the surroundings by optimizing the passivation layer. Our key findings were as follows: (1) the dielectric surface should have a high concentration of hydroxyl groups to allow it to thoroughly bond with the ZnO channel; for this we found HfO2 to be superior to ZrO2 and Al2O3, and lower deposition temperatures to be superior to higher ones for the dielectric layer; (2) in the first few ALD cycles of the ZnO channel layer, soaking steps should be employed to allow the precursors to fully react with the hydroxyl groups on the dielectric surface, again to ensure complete bonding the interface; (3) the deposition temperature of the ZnO channel should be high, and the purge times of the ALD precursors should be long to obtain high crystallinity and low residual water of the ZnO channel; (4) H2O2 can be used instead of H2O for higher-temperature ZnO processes to reduce oxygen vacancies, which become abundant at higher deposition temperatures with the H2O process; (5) the typically severe passivation-process-induced degradation to ZnO TFTs can be avoided by using a ALD TiO2 passivation process with titanium isopropoxide (TTIP) and H2O as the precursors, where the low reactivity of TTIP prevented it from inducing oxygen vacancy in the ZnO channel as do other ALD precursors; (6) exceptional gas-barrier performance—WVTR ~1×10-6 g/m2-day—can be obtained by combining the TiO2 process with the Al2O3 process to form a TiO2/Al2O3 nano-laminated passivation, which effectively eliminated environment-induced degradations to the ZnO TFTs; (7) the high quality of the ALD dielectric, channel, and passivation layers allowed excellent device functions to be retained even at substantially reduced layer thicknesses (from 120-nm to54-nm), which in turn enabled the devices to obtain high mechanical flexibility.
Subjects
Thin film transistor
Zinc oxide
Stability
Atomic layer deposition
Passivation
Type
thesis
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