An Investigation of Electron Transport in Zinc-Tin oxide Thin Film Transistor and its Application to Charge-Trapping Memory

• Main Authors: Jeng-TingLi, 李政廷
• Other Authors: Jen-Sue Chen
• Format: Others
• Language: zh-TW
• Published: 2018
• Online Access: http://ndltd.ncl.edu.tw/handle/49n3af

博士 === 國立成功大學 === 材料科學及工程學系 === 106 === In this study, zinc-tin oxide (ZTO) was prepared by solution method as the active layer for thin film transistor (TFT). The microscopic electron conduction mechanism in the ultra-thin active layer and the causes of the gate-leakage current were discussed. Finally, the ZTO-TFT is fabricated into a charge trapping memory, and its write and erase characteristics are studied. The full text is divided into three parts: In the first part, the variation in gate-leakage current due to the Fowler–Nordheim (FN) tunneling of electrons through a SiO2 dielectric layer in zinc-tin oxide thin film transistors (ZTO TFTs). It is shown that the gate-leakage current is not related to the absolute area of the ZTO active layer, but it is reduced by reducing the ZTO/SiO2 area ratio. The ZTO/SiO2 area ratio modulates the ZTO-SiO2 interface dipole strength as well as the ZTO-SiO2 conduction band offset, and subsequently affects the FN tunneling current through the SiO2 layer, which provides a route that modifies the gate-leakage current. In the second part, carrier transport properties of solution processed ultra thin (4 nm) zin-tin oxide (ZTO) thin film transistor are investigated based on its transfer characteristics measured at the temperature ranging from 310K to 77K. As temperature decreases, the transfer curves show a parellel shift toward more postive voltages. The conduction mechanism of ultra-thin ZTO film and its connection to the density of band tail states have been substantiated by two approaches, including fitting logarithm drain current (log ID) to T-1/3 at 310K to 77K according to two-dimensional Mott variable range hopping theory and the extraction of density of localized tail states through the energy distribution of trapped carrier density. The linear dependency of log ID vs. T-1/3 indicates that the dominant carrier transport mechanism in ZTO is variable range hopping. The IV extracted value of density of tail states at the conduction band minimum is 4.75×1020 cm-3eV-1 through the energy distribution of trapped carrier density. The high density of localized tail states in the ultra thin ZTO film is the key factor leading to the room-temperature hopping transport of carriers among localized tail states. The third study addresses that the nonvolatile charge trapping memory is demonstrated on a thin film transistor (TFT) using an solution processed ultra-thin (~7 nm) zinc tin oxide (ZTO) semiconductor layer with an Al2O3/Ni-nanocrystals (NCs) /SiO2 dielectric stack. A positive threshold voltage (VTH) shift of 7 V is achieved at gate programming voltage of 40 V for 1 s but the state will not be erased by applying negative gate voltage. However, the programmed VTH shift can be expediently erased by applying a gate voltage of -10 V in conjunction with visible light illumination for 1 s. It is found that the sub-threshold swing (S.S.) deteriorates slightly under light illumination, indicating that photo-ionized oxygen vacancies (Vo+ and/or Vo ++) are trapped at the interface between Al2O3 and ZTO, which assists the capture of electrons discharged from Ni NCs charge trapping layer. The light-bias coupling action and the role of ultra-thin ZTO thickness are discussed to elucidate the efficient erasing mechanism.