Understanding the Effect of MgO Interfacial Layer on ZnO/High-K/FTO Transparent Thin Film Transistors for Large-Area Transparent Electronics Applications

ZnO and doped-ZnO based thin film transistors (TFT) have been widely studied for transparent electronics applications [1-2]. However, majority of the previous work have evaluated the performance of TFT using Indium Tin Oxide (ITO) as transparent gate electrodes which is expensive and may not be sustainable for large-area transparent electronics applications, such as transparent electronics integrated glass windows and glass walls in buildings [3-4]. In addition, replacing ITO with low-cost and scalable material such as Fluorine-Doped Tin Oxide (FTO) coated glass substrates also has potential to reduce the cost of displays in consumer electronic devices. Towards addressing this issue and extending the application of ZnO-TFT in large-area transparent electronics that is cost-effective and sustainable, we have evaluated the performance of ZnO TFT with commercially available FTO as gate electrodes. ZnO was deposited at room temperature using RF magnetron sputtering while reactively sputtered HfO₂, deposited at 300◦C, was used as high-k gate dielectric.  The devices indicated well-behaved transistor characteristics with field effect mobility of 0.84 cm²V^-1s^-1, the ON/OFF current ratio of 2.5×10³ and sub-threshold swing 140 mV/decade. To understand the cause of low field effect mobility and large sub-threshold slope, the defect characteristics at ZnO/HfO₂ interface was studied using admittance spectroscopy (Gp/ω vs. ω) measurements. The interface state density was found to be on the order of 10¹³ eV^-1 cm^-2 which is 1-2 orders of magnitude higher than typically reported for ZnO-TFT [5-6]. This work will present our systematic understanding on the role played by FTO electrode in governing the FTO/HfO₂ and HfO₂/ZnO interface characteristics which finally plays an important role in governing the performance of the transistor. In particular, the role of fluorine diffusion in the stack and possibility of engineering the FTO/HfO₂ and HfO₂/ZnO interfaces via the insertion of interfacial layers such as MgO will be presented and supported with experimental results.

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