Nano-Floating Gate Memory Devices Composed of ZnO Thin-Film Transistors on Flexible Plastics
© Park et al. 2010
Received: 8 July 2010
Accepted: 9 September 2010
Published: 28 September 2010
Nano-floating gate memory devices were fabricated on a flexible plastic substrate by a low-temperature fabrication process. The memory characteristics of ZnO-based thin-film transistors with Al nanoparticles embedded in the gate oxides were investigated in this study. Their electron mobility was found to be 0.18 cm2/V·s and their on/off ratio was in the range of 104–105. The threshold voltages of the programmed and erased states were negligibly changed up to 103 cycles. The flexibility, memory properties, and low-temperature fabrication of the nano-floating gate memory devices described herein suggest that they have potential applications for future flexible integrated electronics.
Flexible electronic devices have attracted considerable attention because they enable remarkable applications such as flexible display backplanes, electronics papers, and radio frequency identification tags, due to their light weight, low cost, and low profile [1, 2]. Currently, amorphous silicon (a-Si), polycrystalline silicon and organic semiconductors are the dominant materials used for transistors in these systems [3, 4]. However, an emerging trend in flexible electronics is the development of alternative materials with the goal of increasing their mobilities, decreasing their cost, ensuring their chemical stability, and enabling them to be fabricated on low-cost, flexible substrates with a low-temperature process.
To realize high carrier mobilities, excellent chemical stability, and a low-temperature process, many inorganic semiconductors have been researched [5, 6]. Especially, oxides of transition metals including ZnO and IGZO provide the possibility for transparent and flexible electronics to be developed [7, 8]. These oxides are usually transparent in the visible region, owing to their large band-gaps (over 3 eV), and their optical properties provide for outstanding applications, such as invisible electronics for transparent displays on automobile windshields, when the oxide films are combined with transparent conducting oxides and insulators. Such oxide materials with properties suitable for thin-film transistors (TFTs) can be formed on flexible plastic substrates by the rf-sputtering method, pulsed-laser deposition, and solution processes at low temperature [9–11]. The mobilities that can be achieved using the above-mentioned processes are in the range of 11–80 cm2/V·s [12, 13]. However, the focus of recently reported studies has been on enhancing the TFT performance, such as the carrier mobility and on/off ratio. It is useful to investigate other circuit components, such as transparent diodes and memory elements, to extend the concept of transparent or flexible transistors to circuits and electronics.
In this paper, we demonstrate the fabrication processes and electrical characteristics of flexible nonvolatile memory devices based on bottom-gate ZnO TFTs consisting of transparent floating gate transistors that incorporate Al NPs which act as the floating gate layers in the gate dielectric. The bottom-gate nano floating gate memory (NFGM) devices could be fabricated on any plastic substrates so that the application area of NFGM could be expanded to wearable electronic devices.
Bottom-gate TFTs with 60-nm-thick ZnO channels and 50-nm-thick SiO2 gate insulators embedded with Al NPs were fabricated on PES (polyether sulfone) plastic substrates at room temperature. The SiO2 layers were deposited on the unheated plastic substrates at an rf-power of 100 W, gas mixing ratio of Ar/O2 of 4/5, and total pressure of 3.5 mTorr. The sputtering durations were 6 min for the ZnO and 60 min for the SiO2 layers. The TFT structure was defined using photolithography and lift-off processes. The electrodes were formed with sputtered Al layers. High-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray (EDX) spectroscopy were used to analyze the cross-section of the film. The output and transfer characteristics of the fabricated ZnO/Al-NPs memory TFTs were measured with a semiconductor parameter analyzer (Agilent 4155C) and a pulse generator (Agilent 41501B) and the bending properties of the flexible memory device were also measured. The bending tests were performed using a home-made bending machine, and the memory characteristics were measured after the devices suffered from the bending cycles consisting of tensile strained and unstrained states.
Results and Discussion
NFGM devices were fabricated on a flexible plastic substrate for the first time. The process used for making the Al NPs-embedded TFTs on the gate oxides was performed at room temperature. Their electron mobility was found to be 0.18 cm2/V·s and their on/off ratio was of the order of 104. The threshold voltages of the programmed and erased states were negligibly changed up to 103 cycles. The techniques developed in this work demonstrate that flexible electronic devices, including TFTs, nonvolatile memory chips and others, can be fabricated on temperature sensitive substrates.
This work was partly supported by the IT R&D program of MKE/KEIT[10030559, Development of next generation high performance organic/nano materials and printing process technology], Industry and Energy, the Nano R&D Program (M10703000980-08M0300-98010), and World Class University (WCU, R32-2008-000-10082-0) Project of the Ministry of Education, Science and Technology (Korea Science and Engineering Foundation).
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