Abstract
Incorporating spin functionality into a semiconductor core–shell nanowire that offers immunity from the substrate effect is a highly desirable step for its application in next generation spintronics. Here, using first-principles density functional theory that does not make any assumptions of the electronic structure, we predict that a very small amount of Mn dopants in the core region of the wire can transform the Ge–Si core–shell semiconductor nanowire into a half-metallic ferromagnet that is stable at room temperature. The energy band structures reveal a semiconducting behavior for one spin direction while the metallic behavior for the other, indicating 100% spin polarization at the Fermi energy. No measurable shifts in energy levels in the vicinity of Fermi energy are found due to spin–orbit coupling, which suggests that the spin coherence length can be much higher in this material. To further assess the use of this material in a practical device setting, we have used a quantum transport approach to calculate the spin-filtering efficiency for a channel made out of a finite nanowire segment. Our calculations yield an efficiency more than 90%, which further confirms the excellent spin-selective properties of our newly tailored Mn-doped Ge-core/Si-shell nanowires.
Highlights
Due to the valence band offset between the Ge and Si in core– shell nanowires, spin carriers in the Mn-doped core–shell structure can be guided through the spin active Ge core of the wires resulting in complete suppression of spin lifetime degradation due to scattering and recombination with the surface states
We begin by examining the energy of the Mn-doped Ge-core/Sishell nanowire when a Ge atom at various sites in the nanowire is replaced by a Mn atom
We predict that a small amount of Mn dopants in the core region of a Ge-core/Si-shell nanowire can transform the semiconducting Ge–Si core–shell nanowire into a half-metallic ferromagnet with 100% spin polarization at the Fermi energy
Summary
Since their inception,[1] core–shell semiconductor nanowires, built of group IV elements such as Ge and Si, are the subject of immense interest.[2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18] This level of interest in these nanostructures can be attributed to their multi-functional applications ranging from next-generation electronics,[3,12,19] to biosensors[20] to photovoltaics[21,22] to quantum computing devices.[4,11,17] For example, Ge-core/Si-shell nanowires, which are the materials of choice due to their compatibility with the current Si-based technology, have been successfully synthesized in high yield[1,2,3] and reported to exhibit ballistic transport at unlike these homogeneous nanowires, where the stabilization of the ferromagnetic phase at room temperature is a major challenge due to the substrate effect and o en requires alloying, doping Mn into the core region of a Ge–Si core–shell heterostructure nanowire would offer signi cant advantages. The un-doped nanowire of a similar diameter is reported to have a direct bandgap of 0.89 eV.[19,53] The majority spin carriers (Fig. 2b), show a metallic behavior.
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