Non‐Classical Excitonic Reflectivity Spectra of Mixed Single Crystals of the II–VI Group

Abstract

In studies 011 excitonic reflectivity spectra up to now tarious modifications of the symmetric curve R(hw ) corresponding to the c l a s s i a l harmonic oscillator have been reported for a number of direct-gap pure 11-VI cryslals (see, for example, /1/ for references). In this paper we report the first experimental observation of the Lion-classical excitonic reflectivity f o r mixeJ crystals, namely, ZnxCdl -xS and ZnxCdl -xSe. ductivity measurements indicated 1 0' to 1 J g X d mors/cm3. These and other impurities manifest themselves also in luminescence /2, . Due to the rather poor qkality of our crystals it was possible to observe excitonic structures only in spectra taken from cleaved faces and not from as-grown ones. The presence of impurities and imperfections caused the excitmic b m d s to be 3 to 5 times wider than those of "pure" melt-grown CdS and (cubic) ZnS, though the lineshapes generally remained classical. For a few samples, however, some non-classical features occurred: the "inverse" excitonic reflectivity band (Fig. 1 , 2), the band without minimurn (Fig. 1 to 3 ) . It should be noted that at higher temperaturesup to T 25 80 Kthese features remained pretty the same whereas at T i % 90 K no structure was seen at all.We studied melt-grown hexagonal ZnxCd S samples with 0 6 x 20.8. Con-Our ZnxCdl -xSe bulk crystals grown from vapour phase appeared of better quality than ZnxCdl-xS samples for we d e r e able to observe excitonic structures in spectra taken from as-grown faces, a o . A s well as in /3/ we lound that in general the excitonic reflectivity spectra of ZnxCdl -xSe followed the wurtzitezincblende transition that took place with growing x. As for non-classical features we succeeded in observing them only for some hexagonal samples comparatively 1) Prospekt Nauki 144, 252650 Uev-28, USSR.