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 (similar to225 degreesC) 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 (similar to15%) 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. (C) 2005 American Institute of Physics.