Er environment in a-Si:HEr> prepared by PECVD
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Er environment in a-Si:H〈〈Er〉〉 prepared by PECVD Cínthia Piamonteze1, Leandro R. Tessler1, Hélio Tolentino2, Maria do Carmo Martins Alves2, Gerhard Weiser3, Eugeny Terukov4 1 Instituto de Física “Gleb Wataghin”, UNICAMP, C. P. 6165, 13083-970 Campinas, SP, Brazil 2 Laboratório Nacional de Luz Síncrotron, C. P. 6093, 13083-970, Campinas, SP, Brazil 3 Marburg University, Marburg, Germany 4 A. F. Ioffe Institute, St. Petersburg, Russia ABSTRACT The Er local environment of a-Si:H prepared by PECVD using a metalorganic precursor was determined by EXAFS. We found that in as-deposited samples Er is coordinated to 6 oxygen atoms at 2.28±0.01Å, very similar to Er2O3. Annealing at 420ºC hardly affects the Er neighborhood, but higher annealing temperatures (starting at 600ºC up to 1033ºC) decrease the Er-O separation as much as 0.05Å, maintaining the Er average coordination around 6. This is interpreted as due to the formation of a carbon second neighbor shell. Our results show that the Er local environment is not related with the luminescence enhancement for annealing at moderate temperatures. INTRODUCTION Because of their luminescence at 1.54µm, Er3+ ions have been incorporated in different hosts. This is the wavelength of minimum absorption loss in silica based optical fibers and there is a great technological potential for Er3+-doped materials. The luminescence at 1.54µm is due to the transition from the first excited to the ground state of the ion 4f shell. Since it is a transition between states of the same parity, it is forbidden by dipole selection rules. However, when the Er3+ site in the solid host is non-centrosymmetric, the transition is partially allowed due to the mixture of wavefunctions of opposite parity. It is therefore generally believed that the Er local environment plays an important role in the luminescence intensity. Among the several hosts used to incorporate Er, silicon is one of the most studied. Since most microelectronic technology employs silicon, the developing of optical devices based on this element may lead to photonic-electronic integration. Er3+ luminescence has been studied in crystalline [1] (c-Si), polycrystalline [2] and hydrogenated amorphous silicon [3] (a-Si:H). It has been found that in a-Si:H Er3+ luminescence is more intense and has a smaller temperature quenching than in c-Si [4]. Until recently, a-Si:H had been prepared only by ion implantation [5] and by co-sputtering or similar techniques. The use of a suitable metalorganic carrier gas for Er has allowed the preparation of a-Si:H by PECVD [6], a technique which yields higher quality a-Si:H. The Er local environment in a-Si:H prepared by co-sputtering has been investigated by EXAFS [7,8] and by Mössbauer spectroscopy [9,10]. In the present work we report on an EXAFS study of the Er local environment in a-Si:H prepared by PECVD. Since thermal treatments can enhance the luminescence intensity [11] we investigated the evolution of Er local environment under annealing.
A11.2.1
EXPERIMENT Sample Preparation An a-Si:H thin film was
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