Growth of Ultra-thin Titanium Dioxide Films by Complete Anodic Oxidation of Titanium Layers on Conductive Substrates
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Growth of Ultra-thin Titanium Dioxide Films by Complete Anodic Oxidation of Titanium Layers on Conductive Substrates Karsten Wolff1, Petri Heljo1 and Donald Lupo1 1
Tampere University of Technology, Department of Electronics, Printed and Organic Electronics Group, P.O. Box 692, 33101 Tampere, Finland
ABSTRACT The growth of thin and ultra-thin titanium dioxide layers was investigated. Oxide films were grown by galvanostatic and potentiodynamic anodisation of evaporated titanium layers on conductive substrates. It is shown that thin-film oxidation differs significantly from anodic oxidation of solid foils or plates, due to the sudden stop of anodisation process before complete oxidation of the thick films. Depending on the pH value and the potential sweep rate, the effective defect density and the dielectric constant of the anodized layers vary from 3ยท10 19 cm-3 to 1020 cm-3 and from 16 to 27, respectively, whereas the electrolyte temperature plays only a minor role. INTRODUCTION Titanium dioxide (TiO2) is a widely studied material and it is used in many applications including gate insulators, dye-sensitized solar cells, photo-catalysis, anti-corrosion coatings as well as sensors. In contrast to oxides of other valve metals (e.g. Al or Hf), TiO2 exhibits rather semiconducting than insulating properties. It has a relatively small bandgap (Eg 3.2 eV) and oxygen deficiencies cause n-type conductivity. Very thin layers of TiO2 are needed for some applications like gate dielectrics or organic tunnelling diodes. Anodisation is known to be a costeffective technique to grow the thin oxide layers [1, 2]. In some applications the full oxidation of the Ti layer is required. Since we do not know any report on the complete anodisation of Ti on conductive substrates, we have focused on the growth behavior and the influence of the process parameters on the effective defect density and the dielectric constant. EXPERIMENT Titanium electrodes were fabricated on glass substrates (2.5cm x 2.5cm) coated with a conductive layer of either gold, platinum or indium tin oxide. For Au and Pt as conductive film, microscope slides were degreased in isopropanol, rinsed with water and dried using an ionized air gun. Prior to deposition of the conductive films, UV/ozone treatment was performed. Then, 5 nm adhesion layer of Ti and 50 nm of Au or 100 nm of Pt were e-beam evaporated. The platinum layer was thicker in order to compensate the lower conductivity compared to gold. Polished and ITO-coated glass substrates were purchased from Delta Technologies, Ltd. (USA) and cleaned following the previously mentioned procedure. On top of the respective conductive layer, Ti was e-beam evaporated at different layer thicknesses. Measurement of the sheet resistances revealed values of R sh 0.3 /sq and Rsh 21 /sq for the Au/Pt and ITO samples, respectively.
Anodisation was performed in citric acid solutions (1 mM and 0.1 M) using a Zennium electrochemical workstation (Zahner Elektrik GmbH, Germany). The samples were mounted in a sample holder, which provide
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