Oxygen vacancies in PbTiO 3 thin fi lms probed by resonant Raman spectroscopy Ken NISHIDA, ³ Minoru OSADA, * Joe SAKAI, ** Nobuaki ITO, *** Takashi KATODA, **** Rikyu IKARIYAMA, ***** Hiroshi FUNAKUBO, ***** Hiroki MORIWAKE ****** and Takashi YAMAMOTO Department of Communications Engineering, National Defense Academy, Yokosuka, Kanagawa 239–8686, Japan * International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba, Ibaraki, 305–0044, Japan ** Laboratoire GREMAN, UMR 7347 CNRS, Université François Rabelais de Tours, Parc de Grandmont 37200 Tours, France *** Center for Nano Materials and Technology, Japan Advanced Institute of Science and Technology, Nomi, Ishikawa 923–1292, Japan **** Department of Electronic and Photonic Engineering, Kochi University of Technology, Kami, Kochi 780–8502, Japan ***** Department of Innovative and Engineered Materials, Tokyo Institute of Technology, Yokohama 226–8502, Japan ****** Nanostructures Research Laboratory, Japan Fine Ceramics Center, Nagoya 456–8587, Japan Resonant Raman spectroscopy was applied to evaluate oxygen vacancies in PbTiO 3-x thin films that were heat treated in a hydrogen atmosphere at various temperatures. Additional mode related to oxygen vacancies occurred in the resonant Raman measurement condition, and its intensity was in proportion to the oxygen vacancy concentration. This correlation offers a simple and useful probe for oxygen vacancies in oxide-based devices. ©2013 The Ceramic Society of Japan. All rights reserved. Key-words : PbTiO 3 thin films, Oxygen vacancy, Heat treatment, Resonant Raman spectroscopy, Photoluminescence, Non-Rutherford elastic resonance scattering [Received April 15, 2013; Accepted April 29, 2013] 1. Introduction Due to the excellent dielectric, pyroelectric, piezoelectric, and ferroelectric properties, 1)3) Pb-based ferroelectric materials such as PbTiO 3 and Pb(Zr,Ti)O 3 with perovskite-type structures are very successful in electric device applications, including infra- red radiation sensors, microactuators, and nonvolatile memories. However, Pb or oxygen vacancies frequently occur during crystal growth or device fabrication because Pb and their oxides have high vapor pressures. These vacancies affect the ferroelectric properties such as degradation of the insulation property or pinning of polarization switching. 4)8) Pb or oxygen vacancies have been evaluated in bulk materials using X-ray diffraction or neutron radiation, and thermogravimetry. 9),10) Additionally, the relationship between vacancy concentration and properties has been discussed. 9) As electronic devices become highly integrated and miniatur- ized, the importance of thin film applications using ferroelectric materials has increased. Unfortunately, methods used to evaluate vacancies in the bulk are not applicable to films because films have a much smaller absolute amount of vacancies compared to the bulk. Although X-ray fluorescence (XRF) can evaluate Pb vacancies in PbTiO 3 with a relatively high sensitivity and reliability even in films, detecting oxygen vacancies in oxides is quite difficult. Only a few reports have evaluated oxygen vacancies in perovskite-type oxide films. The oxygen vacancies of a SrRuO 3 thin film and other materials have been reported using a non- Rutherford elastic resonance scattering (NRERS) method. 11),12) However, NRERS has some disadvantages; the equipment is expensive, the spatial resolution is limited to ³mm scale, and samples are occasionally damaged during measurements. Raman spectroscopy is a versatile, highly sensitive technique used to evaluate the crystal structure. Additional advantages include a relatively high spatial resolution and a nondestructive measurement without direct specimen contact. Moreover, the phase transition and identification of the substitution site in ferro- electric materials have been reported using Raman spectroscopy analysis. 13),14) Although Raman spectroscopy is a powerful tool for crystal structure evaluations, reports on the film defects such as oxygen vacancies are limited. We have employed polarized Raman spectroscopy to evaluate oxygen vacancies in epitaxial PbTiO 3 thin films. 15)17) Herein a resonant Raman spectroscopy technique is applied as a highly sensitive and accurate method to evaluate oxygen vacancies. An additional mode related to oxygen vacancies is dominant in the resonant Raman measurement condition. The intensity of this additional mode increases as the oxygen vacancy concentration increases. The relationship between Raman intensity of the addi- tional phonon mode and oxygen vacancies is used to estimate the oxygen content in PbTiO 3 thin films. 2. Experimental procedure Metal organic chemical vapor deposition (MOCVD) was used to grow 300400 nm thick PbTiO 3 thin films on (100) MgO substrates. The details of the growth conditions are described elsewhere. 18) High-resolution X-ray diffraction (XRD) measure- ments (PANalytical, X’Pert MRD) confirmed epitaxial growth of PbTiO 3 thin films with a (001)/(100) orientation. Various oxygen vacancy concentrations were generated by heat treatment of ³ Corresponding author: K. Nishida; E-mail: nishida@nda.ac.jp Journal of the Ceramic Society of Japan 121 [8] 598-601 2013 Paper 598 ©2013 The Ceramic Society of Japan DOI http://dx.doi.org/10.2109/jcersj2.121.598