INCORPORATION OF RARE-EARTHS INTO II-VI COMPOUNDS DURING MBE GROWTH -OPTICAL AND EXAFS STUDIES OF SM-DOPED ZNTE

Sm-doped ZnTe films with thicknesses several hundred nm were grown by molecular-beam epitaxy (MBE) on GaAs substrates at temperatures betwee n 300 and 400 degrees C. Sm concentrations between about 10(18) and ab out 10(22) cm(-3), as determined by X-ray microprobe analysis and SIMS , were incorporated by varying the temperature of the Sm source betwee n 400 and 650 degrees C. We report optical absorption spectra obtained from transmission measurements at photon energies below the interband edge. Two prominent absorption bands peak at about 1.45 and 2.1 eV ar e seen at concentrations above some 10(20) cm(-3). Based on a comparis on with SmTe data, we assign these bands to 4f(6) --> 4f(5) 5d transit ions in Sm2+ ions on sites with octahedral Te coordination. These cent ers are thought to be formed from zincblende interstitial sites during MBE growth. We have also performed extended X-ray absorption fine str ucture (EXAFS) studies at the Sm L-3 absorption edge using fluorescenc e yield detection. The results of these studies are consistent with ou r incorporation model, leading to a Sm-Te bond length of 3.25 Angstrom for the dominant Sm site. These Sm centers are obviously responsible for the transformation from zincblende to rocksalt structure which is seen in the RHEED patterns of such films as the total Sm concentration is increased above some 10(21) cm(-3). (C) 1998 Elsevier Science B.V. All rights reserved.