AVS 51st International Symposium
    Thin Films Wednesday Sessions
       Session TF-WeA

Paper TF-WeA5
Direct Observation of Hydrogen Generated Free Carriers in ZnO Thin Films

Wednesday, November 17, 2004, 3:20 pm, Room 303C

Session: Transparent Conducting Oxides
Presenter: C.A. Wolden, Colorado School of Mines
Authors: C.A. Wolden, Colorado School of Mines
J.B. Baxter, University of California Santa Barbara
T.M. Barnes, Colorado School of Mines
E.S. Aydil, University of California Santa Barbara
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Zinc oxide is a versatile wide band-gap semiconductor that has generated tremendous interest due to its unique combination of optical, electronic and mechanical properties. Hydrogen is common to many ZnO processing environments, and recent first principle calculations have suggested that it behaves exclusively as an electron donor in ZnO. Although hydrogen in ZnO has been observed in bulk single crystals, its behavior has not been examined in commonly used polycrystalline thin films. We report on the use of in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy to probe the changes in free carrier absorption when thin ZnO films are exposed to H or O atoms generated by H@sub 2@ and O@sub 2@ plasmas, respectively. Polycrystalline ZnO films were deposited on ATR crystals using metalorganic chemical vapor deposition or a plasma-assisted CVD technique. For both types of films, room temperature exposure to hydrogen plasma resulted in a sharp increase in free carrier infrared absorption and free carrier density. Carrier concentrations and mobilities were extracted from the infrared spectra using a model for the complex dielectric function. Hydrogen uptake was rapid and the carrier concentration reached its saturation level (~10@super 18@ /cm@super 3@) in less than a minute of H@sub 2@ plasma exposure. Room temperature oxygen plasma exposure of the hydrogenated films decreased free carrier absorption, but this decrease was only a fraction of the free carrier density increase after H exposure. Extended exposure to oxygen plasma at elevated temperatures (~200°C) was required to return the carrier concentration to levels that are comparable to those in the as-deposited films. The results are contrasted with observations from single crystal experiments, and the ramifications for device processing are discussed.