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EXCITATION OF THE 4F-4F EMISSION OF DONOR-TYPE RARE-EARTH CENTERS THROUGH DONOR-ACCEPTOR PAIR STATES (ZNS:TM)

K. LOBE*, R. BOYN** AND H. ZIMMERMANN** *Fritz-Haber-Institut der Max-Planck-Gesellschafl, Berlin, FRG "**Humboldt-Universitt, Fachbereich Physik, Berlin, FRG

Abstract Energy transfer from donor-acceptor pair states is known to be an efficient excitation channel for 4f-4f emission, especially if the rare-earth centers themselves are the relevant donors. Information on this type of mechanism can be obtained in the case when it is possible to observe, parallel to the 4f-4f emission, luminescence arising from radiative deexcitation of the pairs. We have found these two kinds of emission, under electron-beam excitation, in ZnS:Tm crystals which, according to earlier site-selective studies performed by us, contain predominantly donor-type rare-earth centers. Characteristic differences between the two kinds of emission are found in the dependence of intensities on pumping rate; these arise from partial blocking of the energy transfer due to depopulation of the 4f ground levels. By quantitative studies of the correlation between those intensities we demonstrate that the same types of initial states are involved in the energy transfer and the pair transitions.

1. Introduction It is well known that the 4f-4f luminescence of rare-earth (RE) centers in semiconductors can be rather efficiently excited through energy transfer from donor-acceptor pair (DAP) states [1,21, especially if these centers themselves are the relevant donors [3,41. Such a situation is expected for RE's incorporated on metal sites in the II-VI materials. Information on that excitation mechanism can be obtained if it is possible to observe, parallel to the 4f-4f emission, luminescence arising from the radiative deexcitation of the pairs. The latter type of luminescence should become enhanced at high pumping rates at which the DAP to 4f energy transfer becomes partly blocked due to depopulation of the 4f ground state. We have observed those two kinds of emission for ZnS:Tm bulk crystals containing "donor-type" [3] Tm centers. A more detailed discussion of these phenomena including similar data for ZnS:Sm will be published elsewhere [5].

2. Spectra Fig. I shows the cathodoluminescence spectrum of a ZnS:Tm crystal (grown by the high-pressure Bridgman technique) extending from the range of 1G4 -+ 3H 6 to that of excitonic transitions. The band about 29000 cm-l, which is found to exhibit LO phonon structure and to shift toward higher energy with increasing electron-beam current (J), is obviously due to DAP transitions ("edge emission" [6]). We emphasize that this type of band was found only for ZnS :Tm samples containing two kinds of centers which we believe to have donor character [3]. For the sample of Fig. 1 these centers are the dominant ones. This is demonstrated in Fig. 2: The upper part shows the 1G4 -+ 3H6 spectrum at higher resolution (photoluminescence under 514.5nm AP laser excitation); in the two Mat. Res. Soc. Symp. Proc. Vol. 301. c 19