Oxidation state of tungsten oxide thin films used as gate dielectric for zinc oxide based transistors

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Oxidation state of tungsten oxide thin films used as gate dielectric for zinc oxide based transistors Michael Lorenz1, Marius Grundmann1, Sandra Wickert2 and Reinhard Denecke2 1 Institut für experimentelle Physik II, Fakultät für Physik und Geowissenschaften, Universität Leipzig, Linnéstrasse 5, 04103 Leipzig, Germany 2 Wilhelm-Ostwald-Institut für Physikalische und Theoretische Chemie, Fakultät für Chemie und Mineralogie, Universität Leipzig, Linnéstrasse 2, 04103 Leipzig, Germany ABSTRACT We present an investigation of the degree of oxidization of tungsten oxide (WOx) thin films used as gate dielectric for metal-insulator-semiconductor field-effect transistors (MISFET). By means of X-ray photoelectron spectroscopy WOx thin films grown by pulsed-laser deposition at room temperature were investigated. The electrical and optical properties depend significantly on the oxygen pressure during deposition and are affected by the stoichiometric ratio of oxygen and tungsten. INTRODUCTION Tungsten trioxide (WO3) thin films have recently been demonstrated as high-ț gate dielectric for highly transparent and temperature stable thin-film transistors based on zinc oxide channel material [1,2]. WOx can also be used as an n-type semiconductor [3] or electrochromic material [4]. However the optical and electrical properties of the WOx thin films depend significantly on the oxygen pressure during deposition [1,5,6]. For pressures p(O2) > 0.1 mbar during PLD at room temperature highly insulating tungsten oxide is obtained, whereas p(O2) < 0.03 mbar gives rise to metallic like properties of the resulting thin film samples due to oxygen vacancies resulting in a high electron concentration. As gate dielectric, a highly insulating WOx layer with a stoichiometric ratio close to x = 3 is necessary. EXPERIMENT Tungsten oxide thin films have been prepared by pulsed-laser deposition (PLD) on aoriented sapphire substrates using a 248 nm KrF excimer laser. The substrate was kept at room temperature. The ablation target was pressed from 99.9% WO3 powder and sintered for several hours to obtain a ceramic target. Two samples, deposited at an oxygen pressure of p(O2) = 0.002 mbar (sample A) and p(O2) = 0.2 mbar (sample B) were grown (thickness about 250 nm). To investigate the effect of the PLD on the stoichiometric ratio O:W, a reference sample was prepared using WO3 powder on carbon tape. By means of an ESCALAB 220iXL XPS/AES device with a spectral resolution below 1.5 eV, using Al KĮ radiation (energy of 1486.6 eV) generated by an Al/Mg dual mode X-ray anode, X-ray photoelectron spectroscopy (XPS) was performed ex situ. Calibration of the spectrometer was performed according to ISO 15472. The quantification of the contribution of the different oxidized tungsten states of the samples was

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performed considering a product of the mean free path, the transmission factor and the ionization cross section.

DISCUSSION Figs. 1(a) and (b) depict the relative intensity of the W4f and O1s signal, respectively. The peak positions of the raw data show a

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