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

Zinc oxide, a wide-band-gap semiconductor with many technological applications, typically exhibits n-type conductivity. The cause of this conductivity has been widely debated. A first-principles investigation, based on density functional theory, produces strong evidence that hydrogen acts as a source of conductivity: it can incorporate in high concentrations and behaves as a shallow donor. This behavior is unexpected and very different from hydrogen's role in other semiconductors, in which it acts only as a compensating center and always counteracts the prevailing conductivity. These insights have important consequences for control and utilization of hydrogen in oxides in general.

Most research to develop highly transparent and conductive thin films has focused onn-type semiconductors consisting of metal oxides. Historically, transparent conducting oxide (TCO) thin films composed of binary compounds such as SnO2and In2O3were developed by means of chemical- and physical-deposition methods. Impurity-doped SnO2(Sb- or F-doped SnO2, e.g., SnO2:Sb or SnO2: F) and In2O3: Sn (indium tin oxide, ITO) films are in practical use. In addition to binary compounds, ternary compounds such as Cd2SnO4, CdSnO3, and CdIn2O4were developed prior to 1980, but their TCO films have not yet been used widely.