Non-volatile Thin Film Transistors Using Ferroelectric/ITO Structures.
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Non-volatile Thin Film Transistors Using Ferroelectric/ITO Structures. Eisuke Tokumitsu, Takaaki Miyasako and Masaru Senoo Precision and Intelligence Laboratory, Tokyo Institute of Technology, R2-19, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan ABSTRACT We report ferroelectric-gate thin film transistors (TFTs) using indium tin oxide (ITO) as a channel material. Bottom-gate structure TFTs have been fabricated using ferroelectric Pb(Zr,Ti)O3 (PZT) film as a gate insulator and ITO channel. Ferroelectric and ITO layers were formed by the sol-gel technique and RF sputtering, respectively. Drain current-drain voltage (ID-VD) characteristics of PZT/ITO ferroelectric-gate TFTs exhibit typical n-channel transistor operations with clear current saturation and electrical properties were improved by the post annealing. Drain current-gate voltage (ID-VG) characteristics demonstrate clear counterclockwise hysteresis loop due to the ferroelectric gate insulator. The obtained memory window is 2V. The on/off current ratio of more than 102 has been obtained, which indicates that the ITO channel is sufficiently depleted by the ferroelectric polarization. INTRODUCTION Ferroelectric random access memory (FeRAM) has attracted much attention as a low-powerconsumption nonvolatile memory in mobile communication tools and wireless IC cards and so on [1,2]. There are two types of memory cell configuration in FeRAMs. The first one is the capacitor-type, where ferroelectric capacitors are used to store the data and FeRAM with 1T1C configuration is commercially available at present. The second type is the transistor-type, in witch ferroelectric-gate transistors are used to store the data [3,4]. In this type of FeRAM, nondestructive readout can be realized and it has an advantage for high-density implementation, because the ferroelectric-gate transistor obeys the scaling rule. In other words, an important parameter to realize non-volatility in transistor-type FeRAM is charge density (charge per unit area) of the ferroelectric polarization, whereas absolute value of the ferroelectric polarization is important for capacitor-type FeRAM. However, ferroelectric-gate transistors are not in practical use, because it is very difficult to obtain good interface between ferroelectric layer and silicon substrate. It is well known that the ferroelectric films need high temperature annealing in oxygen ambient to be crystallized and such a process produces thick interfacial layer between ferroelectric film and Si with poor electrical properties. Moreover, there is a problem in Si-based ferroelectric-gate transistors, which is “charge-mismatch problem”. The remanent polarization of most ferroelectric materials are more than 10 µC/cm2, while the charge amount required to control the channel conductivity of MOSFET is usually around or less than 1 µC/cm2. Even if the breakdown field of 10 MV/cm is applied to the SiO2 gate insulator, the induced charge is only 3.5 µC/cm2. Hence, many efforts have been made to equivalently reduce the remanent polarizat
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