Infrared detection of hydrogen-generated free carriers in polycrystalline ZnO thin films

The changes in the free-carrier concentration in polycrystalline ZnO films during exposure to H ₂ and O ₂ plasmas were studied using in situ attenuated total reflection Fourier transform infrared spectroscopy. The carrier concentration and mobility were extracted from the free-carrier absorption in the infrared using a model for the dielectric function. The electron density in polycrystalline zinc oxide films may be significantly increased by > 10 ¹⁹ cm ^-3 by brief exposures to hydrogen plasma at room temperature and decreased by exposure to O ₂ plasmas. Room-temperature oxygen plasma removes a fraction of the H at donor sites but both elevated temperatures (∼225 °C) and O ₂ plasma were required to remove the rest. We demonstrate that combinations of O ₂ and H ₂ plasma treatments can be used to manipulate the carrier density in ZnO films. However, we also show the existence of significant drifts (∼15%) in the carrier concentrations over very long time scales (hours). Possible sites for H incorporation in polycrystalline films and reasons for the observed carrier-concentration changes are proposed.