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Farida Selim

Bio: Farida Selim is an academic researcher from Bowling Green State University. The author has contributed to research in topics: Positron annihilation spectroscopy & Positron. The author has an hindex of 23, co-authored 109 publications receiving 1736 citations. Previous affiliations of Farida Selim include Lawrence Livermore National Laboratory & Washington State University.


Papers
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TL;DR: This study revealed the nature of native defects and their roles in ZnO through positron annihilation and optical transmission measurements and revealed oxygen vacancies are the origin for the shift in the optical absorption band that causes the red or orange coloration.
Abstract: This study revealed the nature of native defects and their roles in ZnO through positron annihilation and optical transmission measurements. It showed oxygen vacancies are the origin for the shift in the optical absorption band that causes the red or orange coloration. It also revealed experimental evidence that the donor nature of oxygen vacancy is approximately 0.7 eV. In addition, this work showed the Zn interstitial was not the donor in the as-grown ZnO and supported recent calculations that predicted hydrogen in an oxygen vacancy forms multicenter bonds and acts as a shallow donor.

326 citations

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TL;DR: From positron lifetime measurements, the persistent photoconductivity was observed in strontium titanate (SrTiO(3)) single crystals is attributed to the excitation of an electron from a titanium vacancy defect into the conduction band, with a very low recapture rate.
Abstract: Persistent photoconductivity was observed in strontium titanate (${\mathrm{SrTiO}}_{3}$) single crystals. When exposed to sub-bandgap light (2.9 eV or higher) at room temperature, the free-electron concentration increases by over 2 orders of magnitude. After the light is turned off, the enhanced conductivity persists for several days, with negligible decay. From positron lifetime measurements, the persistent photoconductivity is attributed to the excitation of an electron from a titanium vacancy defect into the conduction band, with a very low recapture rate.

133 citations

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TL;DR: In this article, vacuum as-sintered high quality Ce:YAG ceramics were subjected to various annealing conditions, and their microstructural evolution, and optical and luminescence properties were monitored and studied systematically.
Abstract: Ce:YAG transparent ceramic, a promising photo-converter for white LED lighting, faces a significant obstacle of post-annealing treatments, which allow the simultaneous elimination of defections and the regulation of the redox state of Ce. In this paper, vacuum as-sintered high quality Ce:YAG ceramics were subjected to various annealing conditions, and their microstructural evolution, and optical and luminescence properties were monitored and studied systematically. The results showed that air annealing efficiently benefited the microstructure and optical transmittance. In addition, the absorption band below 300 nm was strongly dependent on the annealing strategies due to the elimination of defects (e.g. F/F+-type color centers) and charge transfer (CT) between Ce(III) and Ce(IV). By constructing with 1350 °C air annealed Ce:YAG ceramics, a high-performance white LED with a luminous efficiency radiation (LER) exceeding 220 lm W−1 was achieved, thanks to the “protected”-annealing condition, which guaranteed moderate CT from Ce(IV) to Ce(III) due to weakened pump light absorption by defects. Differing from traditional Ce:YAG phosphors, excrescent oxygen vacancies were introduced by annealing Ce:YAG ceramics in hydrogen, leading to deteriorated luminescence properties. Therefore, it is expected that this refined preparation of Ce:YAG photo-convertors could drive the development of white LED lighting.

68 citations

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TL;DR: Positron annihilation, thermoluminescence, and optical absorption measurements were applied with the aid of several annealing and diffusion procedures to investigate the nature of point defects in Y3Al5O12 (YAG) single crystals as mentioned in this paper.
Abstract: Positron annihilation, thermoluminescence, and optical absorption measurements were applied with the aid of several annealing and diffusion procedures to investigate the nature of point defects in Y3Al5O12 (YAG) single crystals By annealing at 1500°C in air or O, and diffusing Al into a Ce doped YAG single crystal, a reduction of nearly two orders of magnitude in vacancy concentration was observed Scintillation measurements showed a significant improvement in energy resolution after Al diffusion This study revealed the presence of vacancy-defect complexes, most likely associated with cation antisites in YAG crystals

67 citations

Journal ArticleDOI
TL;DR: Positron annihilation spectroscopy (PAS) is the only probe that can detect individual atomic vacancies and small and large vacancy clusters induced by irradiation with remarkable sensitivity, providing information about their size, concentration, and chemical environment as discussed by the authors.

59 citations


Cited by
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01 Apr 1988-Nature
TL;DR: In this paper, a sedimentological core and petrographic characterisation of samples from eleven boreholes from the Lower Carboniferous of Bowland Basin (Northwest England) is presented.
Abstract: Deposits of clastic carbonate-dominated (calciclastic) sedimentary slope systems in the rock record have been identified mostly as linearly-consistent carbonate apron deposits, even though most ancient clastic carbonate slope deposits fit the submarine fan systems better. Calciclastic submarine fans are consequently rarely described and are poorly understood. Subsequently, very little is known especially in mud-dominated calciclastic submarine fan systems. Presented in this study are a sedimentological core and petrographic characterisation of samples from eleven boreholes from the Lower Carboniferous of Bowland Basin (Northwest England) that reveals a >250 m thick calciturbidite complex deposited in a calciclastic submarine fan setting. Seven facies are recognised from core and thin section characterisation and are grouped into three carbonate turbidite sequences. They include: 1) Calciturbidites, comprising mostly of highto low-density, wavy-laminated bioclast-rich facies; 2) low-density densite mudstones which are characterised by planar laminated and unlaminated muddominated facies; and 3) Calcidebrites which are muddy or hyper-concentrated debrisflow deposits occurring as poorly-sorted, chaotic, mud-supported floatstones. These

9,929 citations

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TL;DR: A review of defects in ZnO is presented in this paper, with an emphasis on the physical properties of point defects in bulk crystals, and the problem of acceptor dopants remains a key challenge.
Abstract: Zinc oxide (ZnO) is a wide band gap semiconductor with potential applications in optoelectronics, transparent electronics, and spintronics. The high efficiency of UV emission in this material could be harnessed in solid-state white lighting devices. The problem of defects, in particular, acceptor dopants, remains a key challenge. In this review, defects in ZnO are discussed, with an emphasis on the physical properties of point defects in bulk crystals. As grown, ZnO is usually n-type, a property that was historically ascribed to native defects. However, experiments and theory have shown that O vacancies are deep donors, while Zn interstitials are too mobile to be stable at room temperature. Group-III (B, Al, Ga, and In) and H impurities account for most of the n-type conductivity in ZnO samples. Interstitial H donors have been observed with IR spectroscopy, while substitutional H donors have been predicted from first-principles calculations but not observed directly. Despite numerous reports, reliable p-t...

995 citations

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TL;DR: In this paper, the authors compared the results of different gap-correction methods and concluded that to date there is no universal scheme for band gap correction in general defect systems, and they turn instead to classification of different types of defect behavior to provide guidelines on how the physically correct situation in an LDA defect calculation can be recovered.
Abstract: Contemporary theories of defects and impurities in semiconductors rely to a large extent on supercell calculations within density-functional theory using the approximate local-density approximation (LDA) or generalized gradient approximation (GGA) functionals. Such calculations are, however, affected by considerable uncertainties associated with: (i) the ``band-gap problem,'' which occurs not only in the Kohn-Sham single-particle energies but also in the quasiparticle gap (LDA or GGA) calculated from total-energy differences, and (ii) supercell finite-size effects. In the case of the oxygen vacancy in ZnO, uncertainties (i) and (ii) have led to a large spread in the theoretical predictions, with some calculations suggesting negligible vacancy concentrations, even under Zn-rich conditions, and others predicting high concentrations. Here, we critically assess (i) the different methodologies to correct the band-gap problem. We discuss approaches based on the extrapolation of perturbations which open the band gap, and the self-consistent band-gap correction employing the $\text{LDA}+U$ method for $d$ and $s$ states simultaneously. From the comparison of the results of different gap-correction, including also recent results from other literature, we conclude that to date there is no universal scheme for band gap correction in general defect systems. Therefore, we turn instead to classification of different types of defect behavior to provide guidelines on how the physically correct situation in an LDA defect calculation can be recovered. (ii) Supercell finite-size effects: We performed test calculations in large supercells of up to 1728 atoms, resolving a long-standing debate pertaining to image charge corrections for charged defects. We show that once finite-size effects not related to electrostatic interactions are eliminated, the analytic form of the image charge correction as proposed by Makov and Payne leads to size-independent defect formation energies, thus allowing the calculation of well-converged energies in fairly small supercells. We find that the delocalized contribution to the defect charge (i.e., the defect-induced change of the charge distribution) is dominated by the dielectric screening response of the host, which leads to an unexpected effective $1/L$ scaling of the image charge energy, despite the nominal $1/{L}^{3}$ scaling of the third-order term. Based on this analysis, we suggest that a simple scaling of the first order term by a constant factor (approximately 2/3) yields a simple but accurate image-charge correction for common supercell geometries. Finally, we discuss the theoretical controversy pertaining to the formation energy of the O vacancy in ZnO in light of the assessment of different methodologies in the present work, and we review the present experimental situation on the topic.

968 citations