Study of oxygen vacancies in SrTiO3 by positron annihilation

Abstract

Suppression of oxygen diffusion by thin Al 2 O 3 films grown on Sr Ti O 3 studied using a monoenergetic positron beam J. Appl. Phys. 97, 033508 (2005); 10.1063/1.1836010 Defects in CeO 2 / SrTiO 3 fabricated by automatic feeding epitaxy probed using positron annihilation J. Appl. Phys. 94, 5193 (2003); 10.1063/1.1606112Vacancy-type defects in BaTiO 3 / SrTiO 3 structures probed by monoenergetic positron beams Oxygen vacancies introduced by homoepitaxial growth of thin films on strontium titanate (SrTiO 3 ) substrates were studied by means of positron annihilation. The SrTiO 3 films were grown by molecular-beam epitaxy without using an oxidant. The Doppler broadening spectra of the annihilation radiation were measured as a function of incident positron energy for the SrTiO 3 /SrTiO 3 samples fabricated by various growth conditions. The line shape parameter S, corresponding to the annihilation of positrons in the substrate, was found to be increased by the growth of the film. This increase was attributed to the diffusion of oxygen from the substrate into the film, and the resultant introduction of oxygen vacancies in the substrate. Two different types of defects in the substrate were identified: one is oxygen multivacancies, such as oxygen divacancies, and the other is their complexes with Sr vacancies. The concentration of oxygen vacancies in the subsurface region increased as the substrate temperature during the growth decreased (р320°C). This fact was attributed to the decrease in the diffusion length of oxygen at low temperatures, and the resultant accumulation of oxygen vacancies in the subsurface region.

The dependence of oxygen vacancy distributions in BiFeO3 films on oxygen pressure and substrate

“…18 For the 760-BFO/SRO/STO heterostructure ͓Fig. 2͑d͔͒, the S BFO at E ϳ 12 keV is lower than the S BFO at E ϳ 2.7 keV in region B, and the S STO is lower than the other two S STO For BFO/SRO/STO, their rougher surface should be due to their differences from BFO/SRO/DSO, i.e., the stronger compressive strain and/or the STO absorbing V O s. As mentioned before, with the P O 2 decreasing from 760 to 0.001 Torr, BFO surface creates a large amount of V O to satisfy the increased S STO in region D. 9,10 Simultaneously, a large amount of oxygen atoms in STO substrate diffused to BFO surface and contribute more O 2 . These V O s diffusion, together with the V O s-induced crystal expansion, should be the main reason of the BFO surface worsening.…”

“…With a monoenergetic positron beam, the parameter ͑S͒ of positron annihilation can be measured as a function of the incident positron energy ͑E, keV͒, where S is defined as S = N s / N T , and N T and N s are the numbers of annihilation events occurring in the range of 503.8 keVՅ E ␥ Յ 518.2 keV or 510.24 keVՅ E ␥ Յ 511.76 keV, respectively. [8][9][10][11][12] Compared with the S of vacancy-free material, the S of a material may increase when open volume and/or density of vacancy-type defects increase. [8][9][10][11][12] In this article, we have found that the partial pressure of oxygen ͑P O 2 ͒ during cooling after growth, strain-state, and substrate material are very important in manipulating the density, diffusion, and distribution of V O s.…”

“…The S was analyzed by the VEPFIT method S͑E͒ = S s F s ͑E͒ + ͚ S i F i ͑E͒ and F s ͑E͒ + ͚ F i ͑E͒ = 1, where F s ͑E͒ is the fraction of positrons annihilated at the surface and F i ͑E͒ is that in the ith layer, S s and S i are the S parameters corresponding, respectively, to the annihilation of positrons on the surface and that in the ith layer. [9][10][11][12] Three layers were chosen for all samples except two layers for 0.1-BFO/SRO/STO, and one fixed boundary at 300 nm was used during simulation. BFO surfaces were studied using atomic force microscopy ͑AFM͒ and ferroelectric domains of the BFO films were studied using piezoelectric force microscopy ͑PFM͒.…”

Lattice distortion mechanism study of TiO2 nanoparticles during photocatalysis degradation and reactivation

“…Each spectrum was collected with a total count of 2×10 6 . The positron source 22 Na on a kapton foil backing was sandwiched between two pellet samples and was placed between the two detectors of the spectrometer. The components of the positron annihilation in the source and the Kapton films were determined to be 385 ps with an intensity of 17.4% and 1100 ps with an intensity of 1.0%.…”

“…11,19,20 Recently, PALS has been widely applied in the fields of solid materials, especially in condensed substances, materials physics, and also in physics, chemistry, materials science and biology, etc. 19,21,22 In this research, in addition to PALS and HRTEM, photoluminescence spectroscopy (PL) and UV Raman spectroscopy are also used to systematically study the P25 photocatalytic process under the Uv-visible light irradiation. It expects to reveal the defects changes within TiO 2 nanoparticles from PALS by comparing the different samples involving pristine, adsorption, degradation and recovery samples, and deeply understand the photocatalytic process.…”