Hydrogen complexes in ZnO grown by chemical vapor transport

The renewed interest in zinc oxide (ZnO) has provided a great number of details that could lead to a commercially viable alternative to gallium nitride (GaN) for use in high-efficiency, wide band-gap devices. The difficulty in obtaining such devices is that as-grown, large single crystals of ZnO are n type. Past research has shown that hydrogen plays an important role in the n-type conductivity of ZnO. However, other impurities cannot be eliminated as possible donors in as-grown ZnO. In an attempt to study the controlled doping of ZnO, we grew polycrystalline samples via the chemical vapor transport (CVT) method, using graphite as the transport agent. After diffusing hydrogen into these crystals, some clearly showed both the OH-related infrared (IR) absorption peak at 3327 cm−1, as reported in our earlier publications, and the 3611 cm−1 absorption peak reported by Lavrov [Physica B 340–342 (2003) 195]. Additionally, a third hydrogen-related absorption peak appears at 3150 cm−1. Suspecting a contaminant in our original source of graphite, we changed to a high-purity source of graphite. The resulting growth yielded no hydrogen-related absorption peaks after hydrogen diffusion into the sample. However, growing samples in ammonia recreated the 3150 cm−1 IR absorption peak.