Conductivity in transparent oxide semiconductors

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Despite an extensive research effort for over 60 years, an understanding of the origins ofconductivity in wide band gap transparent conducting oxide (TCO) semiconductorsremains elusive. While TCOs have already found widespread use in device applicationsrequiring a transparent contact, there are currently enormous efforts to (i) increase theconductivity of existing materials, (ii) identify suitable alternatives, and (iii) attempt to gainsemiconductor-engineering levels of control over their carrier density, essential for theincorporation of TCOs into a new generation of multifunctional transparent electronicdevices. These efforts, however, are dependent on a microscopic identification of the defectsand impurities leading to the high unintentional carrier densities present in thesematerials. Here, we review recent developments towards such an understanding. Whileoxygen vacancies are commonly assumed to be the source of the conductivity,there is increasing evidence that this is not a sufficient mechanism to explainthe total measured carrier concentrations. In fact, many studies suggest thatoxygen vacancies are deep, rather than shallow, donors, and their abundance inas-grown material is also debated. We discuss other potential contributions to theconductivity in TCOs, including other native defects, their complexes, and inparticular hydrogen impurities. Convincing theoretical and experimental evidence ispresented for the donor nature of hydrogen across a range of TCO materials, andwhile its stability and the role of interstitial versus substitutional species arestill somewhat open questions, it is one of the leading contenders for yieldingunintentional conductivity in TCOs. We also review recent work indicating that thesurfaces of TCOs can support very high carrier densities, opposite to the case forconventional semiconductors. In thin-film materials/devices and, in particular,nanostructures, the surface can have a large impact on the total conductivity inTCOs. We discuss models that attempt to explain both the bulk and surfaceconductivity on the basis of bulk band structure features common across the TCOs, andcompare these materials to other semiconductors. Finally, we briefly considertransparency in these materials, and its interplay with conductivity. Understanding thisinterplay, as well as the microscopic contenders for providing the conductivityof these materials, will prove essential to the future design and control of TCOsemiconductors, and their implementation into novel multifunctional devices.