Environment of Er Doped in a-Si:H and Its Relation with Photoluminescence Spectra
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0910-A06-02
Environment of Er Doped in a-Si:H and Its Relation with Photoluminescence Spectra Minoru Kumeda1, Yoshitaka Sekizawa1, Akiharu Morimoto1, and Tatsuo Shimizu2 1 Div. Electrical Engineering and Computer Science, Grad. School of Natural Sci. & Tech., Kanazawa Univ., Kanazawa, 920-1192, Japan 2 Professor Emeritus, Kanazawa Univ., Kanazawa, 920-1192, Japan ABSTRACT The crystal-field potential at the Er3+ ion surrounded by six oxygen ions is expanded in terms of polynomials. After converting it into equivalent angular momentum operators, the Stark-splitting of the 4I15/2 ground state of the Er3+ ion is calculated. Influence of the change in the environment of the Er3+ ion on the shift of the energy levels is investigated and compared with the observed Er photoluminescence spectrum in a-Si:H. The scattering of the calculated energy levels by the structural fluctuation around the Er3+ ion is also compared with the linewidth of the component photoluminescence lines. INTRODUCTION Emission of the light at 1540 nm of an Er ion is matched with the wavelength region of the lowest loss of the optical fiber, and many studies have been done on Er ions doped in various host materials. Hydrogenated amorphous silicon (a-Si:H) films are a suitable host material because many Er atoms can be added without segregation and the inversion symmetry inhibiting the optical transition among 4f electronic states is destroyed in the amorphous network. Er atoms tends to get O atoms as nearest neighbors and three- to six-fold coordinations with O are reported by EXAFS measurements[1]. A DV-Xα calculation also suggests that the Er atom is slightly lifted from the plane where the four O atoms are located [2]. However, it is not clear whether such an Er environment is consistent with the observed Er photoluminescence (PL) spectrum. The environment of the Er atom has been analyzed in crystalline materials by fitting the observed PL spectra or ESR spectra with calculated results [3-6]. The calculations are based on the Stevens equivalent operator method [7] and parameters associated with the crystal-field potential are adjusted to fit the observed spectra. Although a fairly well fitted result is obtained, it is not directly evident whether the obtained values of the parameters are reasonable in connection with the wavefunction of the Er atom or the bond-lengths between the neighboring atoms. Especially for Er atoms which are incorporated in amorphous network, the environmental fluctuation gives rise to changes in the parameters of the crystal-field potential, but the extent of the effect on the PL spectra has not been clear. We try to fit the PL spectra of Er which is doped in a-Si:H, starting with the simple
crystal-field potential and using the wavefunction of 4f electrons of the Er atom. This is the first study to fit the Er PL spectrum based on the wavefunction to the authors knowledge. We investigate the effect of the change in the location of the neighboring atoms on the PL spectra. EXPERIMENTS AND CALCULATIONS Er-doped a-Si:H films w
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