Top-gate ZnO-based TFTs by RF Sputtering
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1035-L11-32
Top-gate ZnO-based TFTs by RF Sputtering Shahrukh Khan, Abbas jamshidi-Roudbari, and Miltiadis Hatalis Department of Electrical Engineering, Lehigh University, Bethlehem, PA, 18015
Abstract: This work emphasizes room temperature deposition and fabrication of top-gated staggered structure ZnO-TFTs and integration of ZnO-TFT based simple logic circuits. We synthesized ZnO thin films by RF sputtering in an Ar/Oxygen ambience with no intentional heating of the substrates. The electrical, optical and structural properties of the ZnO thin films can be well-controlled by altering process parameters such as RF power density and relative Oxygen partial pressure. Typical deposition was carried out at a chamber pressure of 15 mTorr, Ar/Oxygen flow rates of 15 sccm/1 sccm and RF power density of 3W/cm2. The resistivity of the as-deposited films was between 104-106 Ω-cm with high optical transparency (>80%) in the visible spectrum and minimal surface roughness as detected by high-resolution AFM imaging. Gated van der Pauw and Kelvinbridge structures were lithographically patterned to asses ZnO channel resistance. In the completed devices, a dual-stack (Ta2O5/SiO2) dielectric layer was effective in suppressing gate-leakage current below 10 pA and enabled depletion-mode ZnO-TFT operation exhibiting hard saturation. A Ti/Au metallization scheme was adopted to provide good ohmic contact to ZnO. TFTs retained well-behaved transfer characteristics down to a channel length of 4 µm with on/off drain current ratio exceeding 105, threshold voltage between -15 V to -5 V and inverse sub-threshold slope of around 1.75 V/decade.
Introduction: The potential for lightweight and mechanically robust electronics for display and other devices on low-cost, flexible plastic substrates has motivated immense research on new materials and improved processes for fabricating thin film transistors (TFTs). Current industrial approaches to produce thin film transistors for display devices include hydrogenated amorphous silicon (a-Si:H) and low-temperature poly silicon (LTPS). The mainstay of today’s display devices based on a-Si:H although mature, is typically incompatible with plastic substrates and poses other limitations such as light-induced degradations and inherent low-mobility. On the other hand, LTPS offers high mobility but suffers from poor threshold voltage variation across display backplanes [1]. Oxide based semiconductor thin film transistors have advanced tremendously since they combine simultaneously high/low conductivity and optical transparency. ZnObased thin film transistors (TFTs) have gained considerable attention due to their excellent material properties including wide-bandgap, intrinsic n-type conductivity, optical transparency and relatively high field effect mobility. Furthermore, lowtemperature processing capability and compatibility with established IC processing techniques of such oxide TFTs provide very attractive alternative to conventional Sibased TFTs and may render possible fully integrated active-matrix displa
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