Incorporation of Mn into Ge Quantum Dots: Growth Strategies to Control Structure and Magnetism

Dilute magnetic semiconductors are important building blocks towards the realization of low power, low thermal loss spintronics based devices. In this work, group IV based dilute magnetic semiconductors are investigated for their potential to function at temperature above room temperature and compatibility with the current microelectronics industry. The goal of this work is to understand and control the Mn environment within the Si(100), Ge wetting layer and Ge quantum dot (QD) systems and how it influences the magnetic properties. The combination of quantum confinement from the quantum dots and carrier mediated ferromagnetism make these structures of particular interest, but the materials related challenges are considerable and are the focal point of this work.

We investigated three methods for Mn doping of Ge QDs with the goal to overcome low solid solubility of Mn in Ge and suppress unwanted secondary phases, i.e. Mn germanides. The first method investigates the stability and evolution of Mn nanostructures on a Si(100)2x1 reconstructed surface as a function of annealing temperature up to temperatures typical for Ge QD growth. Annealing the Mn-Si(100)2x1 sample to a temperature of 316°C ± 38°C yields a structure characteristic of Mn doping in Si sub-surface substitutional sites. The second method uses a surface driven approach: Mn is deposited on the Ge QD surface, forms well-defined islands on the QD and wetting layer surface and their behavior during in-situ annealing is studied in detail. The as deposited Mn structures on Ge QDs were investigated using tunneling spectroscopy techniques and found to strongly interact with the underlying Ge. The third method utilized the co-deposition of Ge and Mn (i) only during the formation of the wetting layer, and (ii) throughout the entire QD growth process. The highest concentration of Mn studied was approximately 23%, which results in only minor perturbations in the Ge QD growth albeit Mn germanides or Mn-Si-Ge ternary compounds form. All processes are observed with scanning tunneling microscopy, which yields topographic and electronic structure information of the reaction sequence. Room tempereature magnetism results from one particular sample (Mn0.05Ge0.95 QD), obtained with a vibrating sample magnetometer and x-ray circular dichroism, indicates a ferromagnetic material with a Curie temperature above room temperature. Other co-deposited samples with Ge capping layers and increasing Mn concentrations, 8 % and 10 %, had Curie temperatures around 50 K.