Co^{2+} centre in ZnTe has been proved to be an acceptor, contrary to the previous assignment, but in agreement with the general trend in the behaviour of the transition metal impurities in the II-VI compounds. The threshold of the Co^{2+} + hν → Co^{+1} + h transition has been found in the spectrum of photoconductivity at about 1.4 eV, close to the value already reported.
High quality, large (25 mm in diameter) crystals of ZnSe and Zn(Se,S) were grown by low-temperature physical vapour transport method and characterized by various techniques, i.e. energy dispersive X-ray fluorescence spectrometry, double-crystal X-ray diffraction rocking curve FWHM, reflectivity measurements in the region of free exciton, electrical measurements and photoluminescence measurements for both low and high excitation densities. The measurement of the photoluminescence related to the neutral-donor-bound exciton revealed superlinear log(photoluminescence intensity) versus log(excitation intensity) dependence, which indicates stimulated emission phenomena.
Two different types of optically pumped stimulated emission were observed at 1.8 K in the high quality Zn(Se,S) crystals grown by low temperature physical vapour transport. One type, occurring at moderate excitation levels, is ascribed to the recombination of excitons localized because of chemical disorder in the two-anion mixed crystal. The other type, occurring at high excitation levels, is related to the inelastic exciton-exciton scattering.
The process of growth of single crystals of Cd_{1-x}Zn_{x}Te (x ≤ 0.25) and ZnTe by physical vapour transport has been optimized and the twin-free single crystals with a very good crystal structure and low density of dislocations are grown as substrates for MBE and other techniques of epitaxy. Characterization of the crystals is described.
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