Dielectric Gate Applications of PMMA-TiO₂ Hybrid Films in ZnO-Based Thin Film Transistors

In this paper we report a low temperature sol-gel deposition process of organic-inorganic PMMA-TiO₂ hybrid films for applications to gate dielectric layers in field-effect (FE) thin film transistors (TFT), using sputtered n-type ZnO as semiconductor active layer . The PMMA-TiO₂ hybrid thin films were prepared by a modified sol-gel route using titanium butoxide (TBT) as the inorganic (titania) source, methyl methacrylate (MMA) as the organic source, and 3-trimetoxy-silyl-propyl- methacrylate (TMSPM) as the coupling agent between organic and inorganic phases. The hybrid precursor solution for the deposition of the films contained the three precursors with molar ratio 1:0.25:0.25 for TBT, TMSPM and MMA, respectively. For characterization purposes, the hybrid thin films were deposited by dip coating on glass slides substrates and subsequently heat-treated at 100 °C for 24 h. Previous to the device applications, the hybrid films were analyzed by scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier Transform Infra-Red (FTIR) spectroscopy, transmission and reflection spectroscopy and thermogravimetric analysis (TGA) measurements. The macroscopic characteristics of the hybrid films such as high homogeneity and high optical transparence evidenced the formation of a cross-linked, interpenetrated organic-inorganic network. The dielectric characteristics of the PMMA-TiO₂ hybrid films were studied by measuring capacitance-voltage (C-V) and current-voltage (I-V) curves in metal-insulator-metal (MIM) structures, using gold as metal contacts. Finally, the hybrid films were tested as gate dielectric layers in thin film transistors with structure ZnO/PMMA-TiO₂/ITO/Glass, with a common bottom gate and patterned Al source/drain contacts. We analyzed the output electrical response and transfer characteristics of the hybrid dielectric gate TFTs to determine their performance parameters.