Excitation and De-Excitation Mechanisms of Rare-Earth Ions in III-V Compounds: Optically Detected Microwave-Induced Impa
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EXCITATION AND DE-EXCITATION MECHANISMS OF RARE-EARTH IONS IN III-V COMPOUNDS: OPTICALLY DETECTED MICROWAVE-INDUCED IMPACT IONIZATION OF Yb DOPANT IN InP T. GREGORKIEWICZ*, B.J. HEIJMINK LIESERTr, I. TSIMPERIDIS*, I. de MAAT-GERSDORF*, C.A.J. AMMERLAAN*, M. GODLEWSKI**, AND F. SCHOLZ*** *Van der Waals - Zeeman Laboratorium, Universiteit van Amsterdam, Valckenierstraat 65-67, NL-1018 XE Amsterdam, The Netherlands, ** Institute of Physics, Polish Academy of Sciences, Al. Lotnik6w 32/46, PL-06-668 Warszawa, Poland. ***4 Physikalisches Institut, Universitft Stuttgart, Pfaffenwaldring 57, D-7000 Stuttgart 80, Germany. ABSTRACT The excitation mechanisms of rare-earth dopants in III-V semiconductors are being reviewed. The discussion is focused on ytterbium-doped InP crystals for which a particularly large amount of experimental data has been gathered. Here, the results obtained recently by optically detected microwave-induced impact ionization are being examined in detail. On the basis of the experimental findings it is argued that the intrashell luminescence is excited by an intermediate state involving binding of an exciton. Direct evidence for the existence of such a state, of pseudoacceptor type, will be given. The nonradiative recombination channel responsible for the fast decay of Yb luminescence will also be discussed and, for the first time, evidence for an Auger process will be presented. It will also be shown that the nonradiative channel may be effectively blocked by impact ionization of a participating carrier. INTRODUCTION Over the past decades, many investigations have been devoted to the optical properties of rare-earth (RE) ions (lanthanides) in ionic solids such as wide-gap sulfides (ZnS, CaS) and garnet oxides (Y 20 2S, Y 3Al 5012). In these hosts the 4f ions exhibit strong visible and near-infrared luminescence. They find applications in laser materials such as YAG : Nd3+. Rare-earth-activated sulfides have the advantage of obtaining multicolor devices due to internal 4f-shell transitions of RE'+ ions. In recent years, also RE-doped III-V semiconductors have been intensively studied because of their possible applications in opto-electronic devices. The characteristic feature of the electronic structure of RE elements in solids is the presence of incompletely filled 4f-core orbitals, surrounded by closed 5s2 and 5p9 shells. The 4f electrons are thus effectively screened and are only weakly affected by their crystalline environment. As a consequence, rare-earth ions in solids may show sharp luminescence spectra arising from atomic-like, intra-4f-shell transitions. Semiconductor lasers based on these materials have a well defined lasing wavelength which, due to rare-earth-related internal transitions, is relatively insensitive to ambient temperature and almost independent of the III-V host material. ELECTRICAL AND OPTICAL PROPERTIES OF Yb IN InP The most extensively studied rare-earth-doped III-V semiconductor is ytterbiumdoped InP (InP:Yb). The electron configuration of the free Yb atom is [Xe]4f 1 46s 2
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