Stark Splitting in Photoluminescence Spectra of Er in a-Si:H
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A18.1.1
Stark Splitting in Photoluminescence Spectra of Er in a-Si:H Minoru Kumeda1, Mitsuo Takahashi1, Akiharu Morimoto1 and Tatsuo Shimizu2, 1 Division of Electrical Engineering and Computer Science, Graduate School of Natural Science & Technology, Kanazawa University, Kanazawa 920-8667, Japan. 2 NTT Microsystem Integration Labs., Atsugi 243-0198, Japan. ABSTRACT The photoluminescence spectra due to Er ions doped in a-Si:H were decomposed into several lines. The 19-K spectrum was reproduced by adding four Gaussian lines whose linewidths were increased with decreasing the photon energy. Since only the lowest energy level of 4I13/2 contributes to the radiative transition at this low temperature, the energy levels in 4I15/2 split by the Stark effect can be determined. These splittings are not largely different from those for Er in aluminosilicate glass. This implies that the all nearest neighbors of Er are oxygens which have been introduced unintentionally in the sample during preparation. The change in the photoluminescence intensity by annealing is discussed in relation with the spectral change and the results of ESR measurements. INTRODUCTION The wavelength 1540 nm of the optical transition of Er ions coincides with a minimum loss of silica optical fibers and many works on Er-doped materials have been done aiming applications to optical devices [1]. Since the optical transition between states with f orbitals is inhibited by symmetry, we need a lack of inversion symmetry around the Er ion to make mixing of orbitals with different symmetries (say d orbitals) possible. a-Si:H films are useful as a host material because the disorder of the amorphous network promotes mixing of the orbitals with different symmetries and the flexibility of amorphous network makes possible a higher amount of Er ions incorporated in the host. Understanding of the excitation mechanism for Er ions [2] is important to get a strong emission. Fuhs et al. reported that the excitation spectrum of the photoluminescence (PL) has a correlation with the absorption coefficient, indicating that Er centers are not excited by direct resonant absorption process but by the absorption in the matrix, and they compared their results with the theoretical work [3]. The incorporation scheme of Er ions is useful for clarifying the excitation mechanism, and should be reflected in the PL spectra. We tried to decompose the PL spectra and pursue the change by annealing. EXPERIMENTAL Er-doped a-Si:H films were prepared by magnetron cosputtering. In order to get a small ratio of Er to Si, a very small chip of Er metal should be placed on the c-Si target.
A18.1.2
However, it is difficult to deal with such a small chip of Er metal. Therefore, we prepared thick Si-Er films in advance by cosputtering in an atmosphere of Ar using Er metals on the c-Si target. The content of Er in the Si-Er films was determined by an electron probe micro-analysis. Then small chips of the Si-Er films were placed on the c-Si target and the cosputtering was performed in an atmosphere of Ar
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