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In situ formation of tantalum oxide - PMMA hybrid dielectric thin films for transparent electronic application

: Valcu, E.E.; Musat, V.; Oertel, S.; Jank, M.P.M.; Leedham, T.

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Advanced materials letters 6 (2015), Nr.6, S.485-491
ISSN: 0976-3961
ISSN: 0976-397X
Zeitschriftenaufsatz, Elektronische Publikation
Fraunhofer IISB ()

Solution-processed high-k dielectric hybrid thin films prepared at temperatures below 200oC represent a subject of increasing scientific interest satisfying current requirements for printable thin film transistors used in transparent flexible electronics. In this work, we propose a new approach for the synthesis of new tantalum oxide-PMMA hybrid dielectric thin films at 160oC by modified sol-gel method, using as precursors tantalum ethoxide cluster (Ta(OC2H5)5) and methyl methacrylate monomer (MMA). So far it has not been reported in situ formation of tantalum oxide nanocrystals in hybrid dielectric materials at this low temperature. Hybrid sols with 1:1 and 4:1 (Ta(OC2H5)5):MMA molar ratios were used for spin-coating of thin films. The thermal behavior of these sols was observed in order to optimize the post-deposition treatment of the films. The hybrid films were investigated by scanning electron microscopy (SEM) for thickness and morphology, by grazing incidence x-ray diffraction (GIXRD) and high resolution transmission electron microscopy (HRTEM) for tantalum oxide phase formation. TaO2 single crystals with a diameter of about 2 nm embedded in an amorphous phase were identified. The dielectric properties of the hybrid thin films were derived from the characterization of Metal-Insulator-Metal (MIM) structures by current-voltage and capacitance-voltage measurements. I-V curves show a leakage current between 10-12 and 10-7A and a constant capacitance in bias range ± 50 V. For films with 1:1 and 4:1 molar ratio, the leakage current density ranges between 10-9 - 10-3A/cm2 and 10-9 - 10-4 A/cm2, and the limit of the current density goes to an electric field of ±1.2 MV/cm and ±2.5 MV/cm, respectively. In the case of films with 1:1 molar ratio, the applied voltage was up to 70 V in positive domain and no breakdown was observed for the dielectric layer. These results show higher current density for a larger voltage range, than the characteristics leakage current values reported for PMMA (10-8 A/cm2) at 0.3 MV/cm. The value of the permittivity ranges between 3.5 and 7.5 at 1 MHz, depending on the tantalum alkoxyde: MMA molar ratio, suggesting very promising future of these hybrid dielectric thin films for the fabrication of transparent TFTs, which can serve for next generation of transparent and flexible electronic devices.