Surface Driven Mn-Doping of Ge Quantum Dots - Mn-Interaction with the Ge QD{105} Facet and the Wetting Layer

Abstract

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). Mn is deposited on the Ge QD and WL surface in sub-monolayer concentrations, and subsequently annealed up to temperature of 400 C. Bonding and surface topography were measured with STM during the annealing process. Mn forms flat islands on the Ge {105} facet, whose shape and position is guided by the rebonded step reconstruction. The images show a hybridization of Mn-d band and empty states of the Ge{105} facet. A statistical analysis of Mn-islands on the QD yields a slight preference for edge positions, whereas the QD strain field does not impact Mn-islands. The formation of ultra-small Mn-clusters dominates on the Ge(001) WL, which contrasts the Mn-interaction with unstrained Ge(001) surfaces. Annealing (<160 C) leaves the Mn-clusters on the WL unchanged, while the Mn-islands on the Ge{105} facet undergo a ripening process, followed by a volume gain attributable to the onset of intermixing with Ge. This is supported by a statistical analysis of island volume, and size distribution. Increasing the annealing temperature (220- 375 C) leads to a rapid increase in Mn-surface diffusion evidenced by the formation of nanometer size clusters, which are identified as germanide Mn5Ge3 by a mass balance analysis. This reaction is accompanied by the disappearance of the original Mn-surface structures completing Mn de-wetting. 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.