Hydrogen local modes and shallow donors in ZnO

Shi, Gang; Stavola, Michael; Pearton, S. J.; Thieme, Mike; Lavrov, E. V.; Weber, J.

doi:10.1103/physrevb.72.195211

Cited by 162 publications

(133 citation statements)

References 25 publications

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“…By analogy, a shallow donor state of hydrogen would also be expected, which was soon-after confirmed by EPR measurements [88]. These works stimulated a flurry of studies on the presence and local bonding environment of interstitial hydrogen in ZnO [46,[89][90][91][92][93][94][95][96][97][98]. In addition, hydrogen has been shown as a viable intentional n-type dopant [99] and co-dopant with Al [100] and Ga [101] in ZnO grown by a variety of techniques ranging from sputtering methods [99] to chemical solution deposition [100].…”

Section: Donor Nature Of Hydrogenmentioning

confidence: 82%

Conductivity in transparent oxide semiconductors

King

Veal

2011

J. Phys.: Condens. Matter

224

180

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Abstract. Despite an extensive research effort for over 60 years, an understanding of the origins of conductivity in wide band-gap transparent conducting oxide (TCO) semiconductors remains elusive. While TCOs have already found widespread use in device applications requiring a transparent contact, there are currently enormous efforts to (i) increase the conductivity of existing materials, (ii) identify suitable alternatives, and (iii) attempt to gain semiconductor-engineering levels of control over their carrier density, essential for the incorporation of TCOs into a new generation of multifunctional transparent electronic devices. These efforts, however, are dependent on a microscopic identification of the defects and impurities leading to the high unintentional carrier densities present in these materials. Here, we review recent developments towards such an understanding. While oxygen vacancies are commonly assumed to be the source of the conductivity, there is increasing evidence that this is not a sufficient mechanism to explain the total measured carrier concentrations. In fact, many studies suggest that oxygen vacancies are deep, rather than shallow, donors, and their abundance in as-grown material is also debated. We discuss other potential contributions to the conductivity in TCOs, including other native defects, their complexes, and in particular hydrogen impurities. Convincing theoretical and experimental evidence is presented for the donor nature of hydrogen across a range of TCO materials, and while its stability and the presence of interstitial and substitutional species are still somewhat open questions, it is one of the leading contenders for unintentional conductivity in TCOs. We also review recent work indicating that the surfaces of TCOs can support very high carrier densities, opposite to the case of conventional semiconductors. In thin-film materials/devices and, in particular, nanostructures, the surface can have a large impact on the total conductivity in TCOs. We discuss models that attempt to explain both the bulk and surface conductivity based on features of the bulk band structure common across the TCOs, and compare these materials to other semiconductors. Finally, we briefly consider transparency in these materials, and its interplay with conductivity. Understanding this interplay, as well as the microscopic contenders for the conductivity of these materials, will prove essential to the future design and control of TCO semiconductors, and their implementation into novel multifunctional devices.

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Section: Donor Nature Of Hydrogenmentioning

confidence: 82%

Conductivity in transparent oxide semiconductors

King

Veal

2011

J. Phys.: Condens. Matter

224

180

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“…29 In ZnO that had been annealed in an H 2 ambient at 700 °C, approximately 85% of the donors that were formed were not thermally stable near room temperature and only 15% were stable up to near 500 °C. 31 In ZnO, H O was found to be formed in the near surface region of a sample treated in H 2 at temperatures near 700 °C. 29 For SnO 2 , we have introduced hydrogen with annealing treatments in H 2 that severely damage the sample surface.…”

Section: B Relationship Of Shallow Donors To Hydrogen Centersmentioning

confidence: 97%

“…30) and is not thermally stable near room temperature. 31 When H i decays upon annealing, H 2 molecules are formed that provide a reservoir of hydrogen in the ZnO sample that can be partially converted back to H i by thermal treatments. [31][32][33] H O is a more thermally stable defect than H i and decays upon annealing at ≈500 °C.…”

Section: Introductionmentioning

confidence: 99%

Hydrogen impurities and shallow donors in SnO2studied by infrared spectroscopy

et al. 2011

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Hydrogen has been found to be an important source of n-type conductivity in the transparent conducting oxide SnO 2 . We have studied the properties of H in SnO 2 single crystals with infrared spectroscopy.When H or D is introduced into SnO 2 by annealing in an H 2 or D 2 ambient at elevated temperature, several O-H and O-D vibrational lines are produced along with the low-frequency absorption that is characteristic of free carriers. To probe the relationship between H and the free carriers it introduces, the thermal stability of the free carrier absorption and its relationship to the thermal stabilities of the O-H lines have been examined. Two H-related donors are found, one that is stable at room temperature on a time scale of weeks and a second that is stable up to 600ºC. These electrically active defects are found to interact with other O-H centers and can be converted from one to another by thermal treatments. The vibrational modes have been found to have distinctive polarization properties that provide an important test of microscopic defect models for the several O-H and (O-H) 2 centers that we have observed.2

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“…Forming gas contains H, and isolated H atoms are known to move rather easily in ZnO. [8][9][10][11][12][13][14] In fact, even the most stable member of this class, substitutional H O , is believed to completely diffuse out of the sample for temperatures higher than about 475°C. 12 H can also attain stability by forming complexes with certain impurities and defects, 13 such as the Zn vacancy V Zn , 8,13 Cu, 8 and N. 14 For example, the neutral complex V Zn H 2 is stable to about 400°C.…”

Section: ͑4͒mentioning

confidence: 99%

Mobility analysis of highly conducting thin films: Application to ZnO

Look

Leedy

Tomich

et al. 2010

Applied Physics Letters

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Hall-effect measurements have been performed on a series of highly conductive thin films of Ga-doped ZnO grown by pulsed laser deposition and annealed in a forming-gas atmosphere (5% H2 in Ar). The mobility as a function of thickness d is analyzed by a simple formula involving only ionized-impurity and boundary scattering and having a single fitting parameter, the acceptor/donor concentration ratio K=NA/ND. For samples with d=3–100 nm, Kavg=0.41, giving ND=4.7×1020 and NA=1.9×1020 cm−3. Thicker samples require a two-layer formulation due to inhomogeneous annealing.

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Hydrogen local modes and shallow donors in ZnO

Cited by 162 publications

References 25 publications

Conductivity in transparent oxide semiconductors

Conductivity in transparent oxide semiconductors

Hydrogen impurities and shallow donors in SnO2studied by infrared spectroscopy

Mobility analysis of highly conducting thin films: Application to ZnO

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