Ge1−xMnx heteroepitaxial quantum dots: Growth, structure and magnetism

Group IV dilute magnetic semiconductors (DMS) are candidates for the development of spin based devices due to their compatibility with the traditional semiconductor technology. Ge:Mn alloy thin films grown homoepitaxially on Ge (001) exhibit dilute ferromagnetic behavior, but growth temperatures must be kept below about 200°C to prevent second phase formation. We have grown heteroepitaxial Ge_1-xMn_x quantum dots (QDs) on Si (001) by molecular beam epitaxial co-deposition, with x nominally ranging from 0.02-0.22. In order for strain-induced quantum dots to self-assemble, temperatures of 450°C are used, raising concerns over the unwanted formation of germanide phases. For Mn atomic fractions up to 5 at. %, QD morphologies look surprisingly similar to those of pure Ge QDs grown at identical conditions. M vs. H loops demonstrate ferromagnetic hysteretic behavior below 20K and superparamagnetism up to 70K. These transition temperatures are largely independent of Mn content. Key challenges include determination of the actual Mn content and the location of the Mn, given the very small total amount of Mn present. Ex situ x-ray photoelectron spectroscopy detects Mn, although typically at levels somewhat below those expected from the deposition flux ratio. Using atomic force microscopy, in situ scanning tunneling microscopy, transmission electron microscopy, and in situ scanning Auger mapping, our goal is to clearly ascertain how and where Mn incorporates in our films, especially where the magnetically-active Mn resides, and in so doing to contribute to our understanding of the basic origin of ferromagnetic (FM) ordering in this system. This work is supported by the National Science Foundation under grant number DMR-0907234.