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

TL;DR

This study investigates how manganese interacts with germanium quantum dots and wetting layers using STM, revealing surface diffusion, cluster formation, and intermixing processes that influence Mn doping in Ge nanostructures.

Contribution

It provides detailed insights into Mn-Ge interactions on different Ge surfaces, highlighting the role of surface diffusion and annealing in Mn doping and cluster formation.

Findings

Mn forms flat islands on Ge {105} facets guided by step reconstruction.

Mn clusters on the wetting layer are ultra-small and stable up to 160°C.

Higher annealing temperatures cause Mn de-wetting and formation of Mn5Ge3 germanide clusters.

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.