Paper MI-ThM11
A Surface-Driven Route to the Synthesis of Mn-Si and Mn-Ge-Quantum Dot Nanostructures

Thursday, October 23, 2008, 11:20 am, Room 206

The combination of the group IV semiconductors silicon and germanium with an element with a large magnetic moment, such as Manganese, is a critical step in the development of novel and versatile spintronics devices. The goal of our studies are to firstly, incorporate Mn as delta-doped layers in a crystalline Si matrix, which is predicted to present a ferromagnetic structure with a half-metallic character, and secondly, to magnetically dope Ge-quantum dots, which are fabricated by a strain-driven Stranski-Krastanov growth on a Si(100) surface. The synthesis of both types of nanostructures begins with the deposition of Mn on a Si(100)-2x1 surface, which serves as the template for the subsequent Si or Ge overlayer growth. The evolution of nanostructures is observed with scanning tunneling microscopy (STM), and photoelectron spectroscopy (PES) to study bonding and electronic structure at the surface. The prerequisite for a successful synthesis of the Mn-doped Si and Ge nanostructures is to control the Mn-surface structure on Si(100)-(2x1), which is achieved by establishing the surface phase diagram as a function of temperature and Mn-coverage. At room temperature the formation of short monoatomic Mn wires, oriented perpendicular to the Si-dimer rows, dominates. Upon heating the Mn-adatom wires are first transformed to subsurface Mn-Si then the formation of Mn-silicide crystallites occurs. At the same time, the defect density on the Si surface rises dramatically, including a loss of structural integrity at the terrace edges. The surface phase diagram establishes guidelines for the subsequent formation of Si-overlayers and Ge QD growth, and shows the variability in Mn-surface structures and bonding within the Mn-Si(100)-(2x1) system; the consideration of these factors will decisevely influence the resultant magnetism of Mn-delta doped Si-structures. A first assessment of the magnetism in the layered structure is obtained from a measurement of the anomalous Hall-effect contribution to transport and will be discussed. The deposition of Ge was explored in the low temperature and mobility regime and the Mn-nanostructure remains indeed unperturbed by the growth of the Ge-overlayer. After the room-temperature deposition of a thin Ge buffer layer in order to contain and protect the Mn-nanostructure, the transition is made to conditions which allow the formation of Ge quantum dots, and presumably will allow the Mn to move into the QD from the Si-Ge interface.