Semiconductor spintronics with Co2-Heusler compounds

Abstract

Ferromagnetic Co2-Heusler compounds showing high spin polarization have been utilized as spin injectors and detectors for III–V and Group-IV semiconductors. In this article, we first describe the progress in the crystal growth of Co2-Heusler films on GaAs(001) and Ge(111) by low-temperature molecular beam epitaxy. Next, some examples of electrical spin injection from Co2-Heusler contacts into GaAs and Ge through Schottky-tunnel barriers are introduced. Thanks to those efforts, it was found that Co2-Heusler compounds are useful for the realization of spin injection, transport, and detection in GaAs and Ge at room temperature. However, to achieve highly efficient spin transport, it is very important to suppress the interfacial out-diffusion of GaAs or Ge into the Co2-Heusler layer near the heterojunction. Recent progress in high-quality Co2-Heusler/Ge heterojunctions has demonstrated two-terminal magnetoresistance ratios of more than 0.1% at room temperature even for semiconductor spintronic device structures. The approach using Co2-Heusler compounds provides a powerful solution to the need for simultaneous achievement of highly efficient spin injection and low electric power at the source and drain contacts in semiconductor devices such as a spin MOSFET. Graphical abstract

Half‐Metallic Heusler Alloy/GaN Heterostructure for Semiconductor Spintronics Devices

“…The epitaxial CFAS/GaN heterostructure is expected to be used as a highly efficient spin injector. [ 41,42,55 ] These structural and magnetic characterizations show that sufficiently high‐quality L 2 1 ‐ordered CFAS/GaN heterostructures were realized by inserting an ultrathin Co layer between the CFAS and the GaN(0001) surface during growth. Therefore, we can use the L 2 1 ‐ordered CFAS/GaN heterostructures as a Schottky‐tunnel‐type spin injector for highly efficient spin injection into GaN.…”

“…To solve the spin resistance mismatch problem between ferromagnetic metals and GaN, [30][31][32] it is very important to utilize a Schottky-tunnel barrier with half-metallic spin injector and detector, [39][40][41][42] as described in section I. Furthermore, if we need to use a Co-based Heusler alloy as a theoretically expected halfmetallic material, we should obtain the L2 1 -type structure even on GaN.…”

“…To achieve room-temperature electrical spin injection into III-V GaAs and group-IV Ge semiconductors without using an insulating tunnel barrier, half-metallic Heusler alloy/semiconductor Schottky tunnel contacts have been investigated. [39][40][41][42] Co-based Heusler alloys have the chemical formula Co 2 YZ, where Y is a transition metal and Z is a main-group element, are candidate materials for half-metals with a fully spin-polarized density of states at the Fermi level. [43][44][45][46][47] As a result, certain half-metallic Co 2 YZ alloys with an L2 1 -ordered structure (Figure 1a) can solve the spin resistance mismatch problem.…”

“…[43][44][45][46][47] As a result, certain half-metallic Co 2 YZ alloys with an L2 1 -ordered structure (Figure 1a) can solve the spin resistance mismatch problem. [40][41][42] Because GaAs and Ge also have cubic lattice structures known as zincblende and diamond structures, respectively, there was good matching between the atomic arrangements of the half-metallic Co 2 YZ alloys and GaAs or Ge. [42] Also, there was almost no lattice mismatch (less than 1%) between some of the half-metallic Heusler alloys (0.564-0.570 nm) [42] and GaAs or Ge (0.566 nm).…”

“…[40][41][42] Because GaAs and Ge also have cubic lattice structures known as zincblende and diamond structures, respectively, there was good matching between the atomic arrangements of the half-metallic Co 2 YZ alloys and GaAs or Ge. [42] Also, there was almost no lattice mismatch (less than 1%) between some of the half-metallic Heusler alloys (0.564-0.570 nm) [42] and GaAs or Ge (0.566 nm). Thus, highquality epitaxial half-metallic Co 2 YZ/GaAs [40] or half-metallic Co 2 YZ/Ge heterostructures [41,42,48] have been experimentally obtained, resulting in highly efficient spin injection even at room temperature.…”