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

Sm-doped ZnTe and CdTe films with thicknesses of several hundred nanom etres were grown by molecular beam epitaxy on GaAs substrates at tempe ratures between 300 and 400 degrees C. Sm concentrations between about 10(18) and 10(22) cm(-3) (determined by x-ray microprobe analysis and SIMS) were incorporated by varying the temperature of the Sm source b etween 400 and 650 degrees C. The RHEED patterns show that the lattice structure changes from zinc blende to rocksalt type at about some 10( 21) cm(-3). This phase transition is also found to manifest itself in the interband reflection spectra studied for ZnTe:Sm. Optical transmis sion measurements reveal two prominent absorption bands peaking at abo ut 1.45 eV (ZnTe and CdTe) and 2.1 eV (ZnTe) respectively and appearin g at Sm concentrations above some 10(20) cm(-3). On the basis of SmTe data, we assign these features to Sm2+ 4f(6) --> 4f(5) 5d transitions. From their high-energy shift, we conclude that, in the zinc blende ra nge, Sm is incorporated on sites surrounded by an octahedron of Te ato ms, which are formed from zinc blende interstitial sites during MBE gr owth. These centres are thought to initiate the transition to the rock salt phase when the critical concentration is reached.