Hydrogen multicenter bond in oxides
Final Report Abstract
The main results of the project can be summarized as follows. • An innovative experimental approach has been proposed and applied for the detection of LVMs of defects in semiconductors, which is suitable for probing vibrational modes even in highly absorbing spectroscopic regions. It was shown that LVMs can be detected as Fano resonances in photoconductivity spectra. With this approach local vibrational modes of a number of hydrogen donors in ZnO and rutile Ti02 were investigated. In particular, it was concluded that ionization energy of the hydrogen shallow donor in rutile is less than 300 meV, which is at variance with the recent theoretical calculations predicting a deep donor behavior. • This technique applied on hydrogen substituting for oxygen in ZnO revealed that this defect acts as a shallow donor and has two local vibrational modes at 742 and 792 cm^-1, which were assigned to the A1 and E representations of the C3v point group, respectively. These results agree with the theoretically predicted properties of the fourfold coordinated hydrogen trapped in the oxygen vacancy, thus justifying the term “hydrogen multicenter bond”. • A Raman study on hydrogen donors in ZnO revealed the properties of bond-centered hydrogen HBc and hydrogen trapped in the oxygen vacancy H0. The donors were identified by their electronic Is —> 2s(2p) transitions and their characteristic local vibrational modes. The H0 donor was detected preferentially below the sample surface, where a high oxygen vacancy concentration is generated by thermal treatment of the samples. HBc and H0 exhibit different thermal stabilities and their concentrations depend strongly on the sample history. • The hydrogen molecule in ZnO was studied by Raman scattering spectroscopy. It was shown that H2 is practically a free rotator stable up to 700 °C, which is formed by two mobile interstitial hydrogen atoms. The concentration profile of the hydrogen molecule anticorrelates with H0 created at the sample surface during the high temperature hydrogenation. The H2 dissociation at elevated temperature in the bulk regenerates interstitial hydrogen HBC, whereas the formation of H0 and hydrogen out-diffusion are dominant decay channels of the molecule near the surface. The latter mechanisms are responsible for the lower stability of H2 in the subsurface region. An ortho-para conversion between the nuclear spin states 1 and 0 of the molecule at 79 K was found to occur within 7.5 h, whereas the back conversion at room temperature occurs faster than 0.5 h. A shift in frequency of the H2 LVM with annealing time and temperature is associated with the thermal anneal of lattice imperfections. The coupling of transitions between rotational states of the molecule and lattice phonons influences the ro-vibrational properties. Interstitial lattice sites and/or larger vacancy clusters are preferred trapping centers for H2 in ZnO. The results obtained during this study are too specialized to be advertised in the public media.
Publications
- “Photoconductive detection of tetrahedrally coordinated hydrogen in ZnO,” Phys. Rev. Lett. 108, p. 165501, 2012
S. G. Koch, E. V. Lavrov, and J. Weber
(See online at https://doi.org/10.1103/PhysRevLett.108.165501) - “Confirmation of freely rotating H2 in ZnO,” AIP Conference Proceedings 1583, p. 345, 2014
S. G. Koch, E. V. Lavrov, and J. Weber
(See online at https://doi.org/10.1063/1.4865667) - “Interplay between interstitial and substitutional hydrogen donors in ZnO,” Phys. Rev. B 89, p. 235203, 2014
S. G. Koch, E. V. Lavrov, and J. Weber
(See online at https://doi.org/10.1103/PhysRevB.89.235203) - “Towards understanding the hydrogen molecule in ZnO,” Phys. Rev. B 90, p. 205212, 2014
S. G. Koch, E. V. Lavrov, and J. Weber
(See online at https://doi.org/10.1103/PhysRevB.90.205212) - “Fano resonances in photoconductivity spectra of hydrogen donors in ZnO and rutile,” Semicond. Sci. Technol. XX, p. XXX, 2015
E. Lavrov, F. Herklotz, and J. Weber
(See online at https://doi.org/10.1088/0268-1242/30/2/024004)
