Christopher Nolph

Mn solid solutions in self-assembled Ge/Si (001) quantum dot heterostructures

Heteroepitaxial Ge0.98Mn0.02 quantum dots (QDs) on Si (001) were grown by molecular beam epitaxy. The standard Ge wetting layer-hut-dome-superdome sequence was observed, with no indicators of second phase formation in the surface morphology. We show that Mn forms a dilute solid solution in the Ge quantum dot layer, and a significant fraction of the Mn partitions into a sparse array of buried, Mn-enriched silicide precipitates directly underneath a fraction of the Ge superdomes. The magnetic response from the ultra-thin film indicates the absence of robust room temperature ferromagnetism, perhaps due to anomalous intermixing of Si into the Ge quantum dots.

Addition of Mn to Ge quantum dot surfaces--interaction with the Ge QD {105} facet and the Ge(001) wetting layer.

The interaction of Mn with Ge quantum dots (QD), which are bounded by {105} facets, and the strained Ge wetting layer (WL), terminated by a (001) surface, is investigated with scanning tunneling microscopy (STM). These surfaces constitute the growth surfaces in the growth of Mn-doped QDs. Mn is deposited on the Ge QD and WL surface in sub-monolayer concentrations, and subsequently annealed up to a temperature of 400 ° C. The changes in bonding and surface topography are measured with STM during the annealing process. Mn forms flat islands on the Ge{105} facet, whose shape and position are guided by the rebonded step reconstruction of the facet. Voltage-dependent STM images reflect the Mn-island interaction with the empty and filled states of the Ge{105} reconstruction. Scanning tunneling spectra (STS) of the Ge{105} facet and as-deposited Mn-islands show a bandgap of 0.8 eV, and the Mn-island spectra are characterized by an additional empty state at about 1.4 eV. A statistical analysis of Mn-island shape and position on the QD yields a slight preference for edge positions, whereas the QD strain field does not impact Mn-island position. However, the formation of ultra-small Mn-clusters dominates on the Ge(001) WL, which is in contrast to Mn interaction with unstrained Ge(001) surfaces. Annealing to T < 160 °C leaves the Mn-clusters on the WL unchanged, while the Mn-islands on the Ge{105} facet undergo first a ripening process, followed by a volume gain which can be attributed to the onset of intermixing with Ge. This development is supported by the statistical analysis of island volume, size and size distribution. Increasing the annealing temperature to 220° and finally 375 ° C leads to a rapid increase in the Mn-surface diffusion, as evidenced by the formation of larger, nanometer size clusters, which are identified as germanide Mn5Ge3 from a mass balance analysis. This reaction is accompanied by the disappearance of the original Mn-surface structures and de-wetting of Mn is complete. This study unravels the details of Mn-Ge interactions, and demonstrates the role of surface diffusion as a determinant in the growth of Mn-doped Ge materials. Surface doping of Ge-nanostructures at lower temperatures could provide a pathway to control magnetism in the Mn-Ge system.

Magnetic doping of Ge-quantum dots: growth studies exploring the feasibility of modulating QD properties

The magnetic doping of Ge-QDs is highly coveted and has been pursued for several years. The Ge- Mn-Si substrate system presents a complex challenge, and the competition between dopant integration and formation of silicides, germanides and metastable phases makes the magnetic doping a considerable, and maybe insurmountable challenge. We will discuss the interaction of Mn with all growth surface, Si(100), Ge(100) wetting layer, and Ge{105} QD facet, as well as the co-deposition of Mn and Ge. The monoatomic Mn-wires, which form on Si(100), and their magnetic signatures allow unique insight into the relation between bonding and magnetism. We will close with an outlook on the feasibility of QD manipulation by controlling dopant-surface interactions.

Ge 1−x Mn x 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 Ge1−xMnx 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.