Study of Excitation and Luminescence Process of Rare-Earth Ions Doped in Semiconductors

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Luminescence of rare-earth ions doped in semiconductors arises from electronic transition in an open 4f^n shell. Because 4f^n electrons is screened by outer electrons in (5s) ^2 (5p) ^6 closed shells, the energy states of the 4f^n of rare-earth ions in the various host materials are hardly influenced by the crystal field. Therefore, rare-earth ions give specific luminescence with sharp line spectra and emission wavelength is hardly dependent on the lattice temperatures. Exciation of rare-earth ions may result from the following processes. Creation of electon hole pairs due to band to band excitation occurs first. Subsequent capture and recombination of electron-hole pairs at rare-earth ion site resulting in the exciation of 4f^n electrons follows. Finally rare-earth ions give rise to luminescence due to the transition in the 4f^n electron configutations. To clarify excitation and luminescence process of rare-earth ions in semiconductors, it is essential to understand the energy transfe … More r process from electron hole pairs to 4f^n electrons. For Ce^<3+> ions, it is known that Ce^<3+> centers are ionized through 5d excited state resulting in Ce^<4+> ions. In this study, role of ionization of Ce^<3+> ions and capture of electrons in the excitation process is investigated. The results are described in the following.1. SrS : Ce, which has been receiving a great deal of attention as electroluminescent materials, has been studied. It is known that strong interaction between electron in 5d excited state of Ce^<3+> ions and electrons in conduction band exists, and ionization of Ce^<3+> is essential in the excitation process. To clarify this process, temperature dependence of luminescence intensity was investigated, and it is found that capture of electron of electron to Ce^<3+> ions is strongly affected by carriar migration in the lattice. To further investigate the excitation process, the photo-induced current and luminescence were measured using excimer laser excited dye laser pulses. It is found that Ce^<3+> centers are ionized when Ce^<3+> centers are excited to 5d state due to light irradiation. This result implies that excitation process through 5d state exists in SrS : Ce.2. Similar experiments on SrGa_2S_4 : Ce thin films were carried out. Ce^<3+> substitutes the Sr^<2+> site. Although ionic nature of Sr site in SrGa_2S_4 is stronger than that for SrS,it is found that the interactionin between electron-hole pairs and 5d electron of Ce^<3+> exists. It is also found that the excitation through host lattice takes place efficiently, and Ce^<3+> gives rise strong blue luminencence. Less