Hydrogen-related defects in ZnO and TiO2

• Main Author: Herklotz, Frank
• Other Authors: Technische Universität Dresden, Fakultät Mathematik und Naturwissenschaften
• Format: Doctoral Thesis
• Language: deu
• Published: Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden 2012
• Subjects: Zinkoxid; Rutil; Wasserstoff; Absorption; Photoluminszenz; Raman; zinc oxide; rutile; hydrogen; absorption; photoluminescence; ddc:530; rvk:UP 2800
• Online Access: http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-83311; http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-83311; http://www.qucosa.de/fileadmin/data/qucosa/documents/8331/DissertationOfficial.pdf

Hydrogen-related defects in single-crystal ZnO and rutile TiO2 are investigated by means of infrared absorption, Raman scattering, photoluminescence and photoconductivity. Four different defect centers in ZnO are considered: bond-centered hydrogen (HBC ), hydrogen bound within the oxygen vacancy (HO), hydrogen molecules, and a defect, which gives rise to a local vibrational mode at 3326 cm−1 . The measurements identify HBC as a shallow donor with an ionization energy of 53 meV. The internal 1s → 2p transition of HBC is detected at 330 cm−1 in the Raman scattering and photoconductivity spectra. The decay of an exciton bound to HBC results in the photoluminescence line at 3360.1 ± 0.2 meV. The local vibrational mode of the O–H bond for bond-centered hydrogen has a frequency of 3611 cm−1 (H-I) and an effective charge of 0.28±0.03e. It is found that bond-centered hydrogen is unstable against annealing at 190 °C due to diffusion and trapping by other defects. The dominant sink is the hydrogen molecule. It is demonstrated that the well-known I4 photoluminescence line at 3362.8 meV is due to the recombination of excitons bound to the HO donor. The ionization energy of the HO donor is determined to be 47 meV. The 1s → 2pz (2pxy) electronic transition of HO is detected at 265 cm−1 in photoconductivity spectra. The formation of HO occurs via trapping of HBC at vacancies left by out-diffusing oxygen. It is shown that sub-band gap illumination leads to an intensity reduction of the O–H local vibrational mode at 3326 cm−1 and the appearance of a previously unreported infrared absorption line at 3358 cm−1. The signals are identified as stretch modes of an O–H bond associated with the same defect in different charge states. The measurements indicate that this defect has a deep level in the band gap of ZnO at roughly Ec − 1.7 eV. Additionally, results on the thermal stability, uniaxial stress response, and temperature dependence of the transition rates between the two charge states of this defect are presented. Interstitial hydrogen in rutile TiO2 is studied by infrared absorption. It is shown that the defect is a shallow donor with an ionization energy of 10 meV. The absorption lines at about 3290 cm−1 consists of local vibrational modes due to the neutral and the positive charge states of the donor with relative intensities depending on the measurement conditions. In the neutral charge state, the defect reveals two modes at 3288.3 and 3292 cm−1 (10 K), whereas the positive charge state has a vibrational mode at 3287.4 cm−1. An unknown hydrogen complex was found to contribute to the 3288 cm−1 feature.