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.
Bibliographical noteFunding Information:
This work was supported by the National Science Foundation through Grant Nos. CTS-0093611 and ECS-0317459. The authors thank Dr. Tom Mates for assistance with the SIMS profile, which made use of the MRL Central Facilities supported by the MRSEC Program of the National Science Foundation under Award No. DMR00-80034. One of the authors (J.B.) was supported through a National Science Foundation Fellowship.