Preparation and Properties of Gallium Phosphide Doped by Rare-Earth Elements
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PREPARATION AND PROPERTIES OF GALLIUM PHOSPHIDE DOPED BY RARE-EARTH ELEMENTS SERGEI L.PYSHKIN AND ALBERTO ANEDDA Institute of Applied Physics, Academy of Sciences, Academy Str.5, 277028 Kishinev, Moldova Department of Physics, University of Cagliari, Ospedale Str. 72, 1-09124 Cagliari, Italy
ABSTRACT GaP single crystals have been grown from 5at.% solution of P in Ga with (0.01-0.1)at.% of La, Sm or Gd as dopants. Electric properties, photoconductivity and luminescence of the crystals have been investigated by standard methods. Energy levels of impurities, mobility and concentration of current carriers have been determined in wide temperature region. It was shown that nitrogen (N) and other uncontrolled impurities present in the crystals due to some peculiarities of crystal growth. The intrinsic RE impurity luminescent spectra can be seen only together with spectra of excitons bounded on N traps. The evaluation of quantum eficiency for RE impurity made on the base of luminescence decay measurements gives the value close to 1. The interpretation of GaP:RE luminescence spectra is presented as intrashell transitions activated by bound exciton recombination as well as D-A and D-valency band transitions between some contaminating impurity (donor) and RE element (acceptor). INTRODUCTION (acceptors) The results on luminescence of GaP doped by elements of II and VI (donors), III and V (isoelectronic traps) groups demonstrate the unic properties of this semiconductor which are useful for development of basic models of irradiational recombination mechanisms through bound excitons and donor-acceptor pairs. It should be noted that the set of possible irradiational transitions as well as other prospective properties of GaP can not be realized completely on the base of the mentioned above dopants. Indeed, the quantum efficiency of the GaP light emitting diodes is very low up to now due to the non-radiational thermal processes as well as due to the low degradation parameters. It is obvious, that for GaP and other broad-band semiconductors the best impurities are those which create the irradiational transitions effective up to the room temperature. The most interesting are the intrashell transitions in Rare-Earth (RE) elements which are well scrinned from the influence of crysral field and temperature. The electronic states of some of these elements (Sm, for instance) are very well match with the band structure of GaP as well as with bound exciton states in this material. A laser system in which the centers of recombination and activators are uniformely intermixed in a crystal matrix was proposed by Bell in 1963 [E]. In such a system the recombination of non-equilibrium electron-hole pairs creates photons which are effectively absorbed by the activation centers. In semiconductors with indirect band gap, such as GaP, the irradiational transitions between valency and conductance bands are supressed, so the efficiency of activator excitation could be as high as 100. It was especially interesting to investigate the luminescence and th
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