The stability of the ferromagnetic state in Cr-based diluted magnetic semiconductors (DMS) has been recently predicted. A few years ago, the low-temperature MBE technique resulted in the successful growth of II-VI-based DMS incorporating a relatively large Cr concentration, i.e., (Zn,Cr)Se epilayers with Cr concentration up to 2.1% were obtained . Evidence of a strong ferromagnetic coupling between Cr ions was given in Ref.. However, the possible domination of some magnetic properties of investigated layers by the presence of small grains of ZnCr2Se4 spinel was also suggested (direct proof of even partial substitution of Zn by Cr in ZnSe has not been found).
The goal of the present work is to look for efficient methods of characterisation of the structure of such layers and to check the possible presence of mixed (Zn,Cr)Se crystals in the zinc-blende phase. MBE-grown, Cr-doped ZnSe layers were deposited on (001) GaAs substrates (which, in some cases, were also covered with a pure ZnSe buffer layers). The structure of layers was investigated by means of the high-resolution X-ray diffraction, atomic force microscopy and Raman scattering. Presence of Cr at substitutional positions is demonstrated, and details of (Zn,Cr)Se/GaAs interface as well as that of the layer structure are discussed.
This work was supported in part within European Community program G1MA-CT-2002-4017 (Centre of Excellence CEPHEUS) and by SPINOSA project (IST-2001-33334).
 G. Karczewski, M. Sawicki, V. Ivanov et al., J. Superconductivity: Incorporating Novel Magnetism 16, 55 (2003).