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42119-G10
Synthetic Approach to Magnetic Semiconductor Nanostructures
Research Summary
The potential advantages of using electron spin to both store and transfer information has motivated research into magnetic semiconductor materials and hybrid structures of ferromagnetic metals with semiconductors. In addition, the potential advantages of bottom-up approaches to the synthesis and assembly of electronic materials has motivated research into one-dimensional materials such as carbon nanotubes and semiconductor nanowires. The high aspect ratios of nanowires and the tunability of their diameters offer additional means to tune magnetic properties through shape anisotropy and quantum confinement. The primary challenges in spintronic materials- doping to very high impurity levels and the formation of well-defined heterointerfaces- are also basic challenges in nanowire synthesis and device fabrication. Here we describe recent progress in the synthesis of nanowire heterostructures and new measurements of the paramagnetic impurity concentration.
We discovered that manganese, a typical paramagnetic dopant atom in magnetic semiconductors, can itself be used to generate one-dimensional germanium nanowires. Low pressure thermal chemical vapor deposition was used to realize the sequential synthesis of solid phase manganese germanide seed particles and crystalline germanium nanowires. Mn islands were formed on inert substrates using tricarbonyl-(methylcyclopentadienyl) manganese (TCMn). Upon exposure to germane, these islands formed manganese germanide nuclei which generated the formation of Ge nanowires. The phase of the solid germanide growth ‘seed' was determined to be Mn11Ge8 by electron diffraction. This approach to Ge nanowire growth has at least three compelling advantages: (1) the seed particle is not Au, avoiding the potentially negative influence on electrical properties; (2) the vapor-phase deposition and self-assembly of the seed greatly simplifies the nanowire synthesis process; and (3) the self-assembled seed naturally produces a narrow distribution of nanowire diameters. An additional compelling result of this growth process is that the germanide seed grows in a one-dimensional manner. This enables an epitaxial metallic contact to be formed directly on the nanowire during growth. A perfectly ordered and contamination-free interface should greatly improve spin-injection across the heterojunction interface.
The manganese content of the germanide-grown Ge nanowires was analyzed using atom probe tomography. We recently reported the first application of atom probe tomography to the analysis of individual nanowires (Nano Lett. 6, 181 (2006)). Concentrations of 0.05 to 0.3 % were detected in Ge nanowires grown at 350 degrees C. The equilibrium concentration of Mn in Ge at this growth temperature is not known, but was expected to be less than the observed value. This level of Mn doping may be insufficient to achieve ferromagnetism in Ge nanowires, but investigations are ongoing. The germanide seed itself offers an alternative approach to spin-dependent functionality in nanowire structures if the ferromagnetic Mn5Ge3 phase can be formed.
Impact on the field and personnel
The discovery of the syntaxial growth (simultaneous epitaxial growth of two phases) of semiconducting-metallic heterojunction nanowires provides a new approach to the realization of electronic and spintronic devices in low-dimensional systems. In addition, it suggests a rich area for exploration of solid-phase-mediated crystal growth with intriguing parallels in bulk materials, including metal-induced crystallization of amorphous silicon and directional silicidation in metal-implanted silicon. There is rapidly increasing interest in silicide nanowires, and studies on germanide nanowires will be highly complementary.
Three students have worked on the project. One graduate student is continuing towards her Ph.D. One current undergraduate has enrolled in a dual degree B.S./M.S. program and will continue the research towards a masters thesis. Another undergraduate previously involved in the project is studying renewable energy in China under a Fulbright fellowship.
