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

掺杂在半导体中的稀土离子的发光是由4f^n壳层中的电子跃迁引起的。由于4f^n电子在(5s)^2(5p)^6闭合壳层中被外层电子屏蔽，稀土离子在各种基质材料中的4f^n能态几乎不受晶场的影响。因此，稀土离子具有尖锐的谱线，发射波长几乎不受晶格温度的影响。稀土离子的激发可能由下列过程引起。由于带对带激发而产生的电子空穴对首先发生。随后在稀土离子中心的电子-空穴对的俘获和复合导致了4f^n电子的激发。最后，稀土离子由于4f^n电子组态的跃迁而发光。为了阐明稀土离子在半导体中的激发和发光过程，了解能量传递…是必不可少的。从电子空穴对到4f^n电子的更多r过程。对于Ce^&lt；3+&gt；离子，已知Ce^&lt；3+&gt；中心通过5d激发态电离，形成Ce^&lt；4+&gt；离子。本文研究了Ce~(2+)和Lt；~(3+)离子的电离和电子俘获在激发过程中的作用。研究结果描述如下1.SRS：Ce作为一种电致发光材料一直受到人们的广泛关注。已知Ce^&lt；3+&gt；离子5d激发态电子与导带电子之间存在较强的相互作用，在激发过程中Ce^&lt；3+&gt；离子的电离是必不可少的。为了阐明这一过程，研究了发光强度与温度的关系，发现电子对Ce~(2+)和Lt；3+&Gt~(2+)离子的俘获受到晶格中载流子迁移的强烈影响。为了进一步研究激发过程，用准分子激光激发的染料激光脉冲测量了光生电流和发光。结果表明，当Ce^&lt；3+&gt；中心受光激发到5d态时，Ce^&lt；3+&gt；中心被电离。这一结果表明，在SrS：Ce2中存在5d态激发过程。对SrGa2S4：Ce薄膜也进行了类似的实验。Ce^lt；3+&gt；取代了sr^&lt；2+&gt；站点。虽然SrGa_2S_4中锶位的离子性质比锶的强，但发现Ce~(2+)-Lt；3+-Gt~(3+)中电子-空穴对与5d电子之间存在相互作用。研究还发现，通过主体晶格的激发是有效的，Ce^&lt；3+&gt；产生强烈的蓝色发光。较少

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