DONOR IMPURITIES AND DX CENTERS IN THE IONIC SEMICONDUCTOR CDF2

Group-III impurities in the wide-gap ionic crystal CdF2 are examined. After being heated in a reducing atmosphere, crystals with these impur ities acquire semiconductor properties, which are determined by electr ons bound in hydrogen-like orbitals near an impurity. Besides these do nor states, nontransition impurities form ''deep'' states accompanied by strong lattice relaxation, i.e. they are strongly shifted along the configuration coordinate. These states are a complete analog of DX ce nters in covalent and ionic-covalent semiconductors. The difference of the behavior of nontransition impurities from that of transition and rare-earth impurities is analyzed. This difference is attributed to th e character of the filling of their valence shells by electrons. A dee p, multilevel analogy is drawn between the properties of deep centers in typical semiconductors with an appreciable fraction of a covalent b ond component and in predominantly ionic crystal CdF2 with semiconduct or properties. (C) 1997 American Institute of Physics.