K. Tanikawa

Atomically Controlled Epitaxial Growth of Single-Crystalline Germanium Films on a Metallic Silicide

We demonstrate high-quality epitaxial germanium (Ge) films on a metallic silicide, Fe3Si, grown directly on a Ge(111) substrate. Using molecular beam epitaxy techniques, we can obtain an artificially controlled arrangement of silicon (Si) or iron (Fe) atoms at the surface on Fe3Si(111). The Si-terminated Fe3Si(111) surface enables us to grow two-dimensional epitaxial Ge films, whereas the Fe-terminated one causes the three-dimensional epitaxial growth of Ge films. The high-quality Ge grown on the Si-terminated surface has almost no strain, meaning that the Ge films are not grown on the low-temperature-grown Si buffer layer but on the lattice matched metallic Fe3Si. This study will open a new way for vertical-type Ge-channel transistors with metallic source/drain contacts.

Improvement of magnetic and structural stabilities in high-quality Co2FeSi1−xAlx/Si heterointerfaces

We study high-quality Co2FeSi1−xAlx Heusler compound/Si (0 ≤ x ≤ 1) heterointerfaces for silicon (Si)-based spintronic applications. In thermal treatment conditions, the magnetic and structural stabilities of the Co2FeSi1−xAlx/Si heterointerfaces are improved with increasing x in Co2FeSi1−xAlx. Compared with L21-ordered Co2FeSi/Si, B2-ordered Co2FeAl/Si can suppress the diffusion of Si atoms into the Heusler-compound structure. This experimental study will provide an important knowledge for applications in Si-based spin transistors with metallic source/drain contacts.

An ultra-thin buffer layer for Ge epitaxial layers on Si

Using an Fe3Si insertion layer, we study epitaxial growth of Ge layers on a Si substrate by a low-temperature molecular beam epitaxy technique. When we insert only a 10-nm-thick Fe3Si layer in between Si and Ge, epitaxial Ge layers can be obtained on Si. The detailed structural characterizations reveal that a large lattice mismatch of ∼4% is completely relaxed in the Fe3Si layer. This means that the Fe3Si layers can become ultra-thin buffer layers for Ge on Si. This method will give a way to realize a universal buffer layer for Ge, GaAs, and related devices on a Si platform.

Molecular Beam Epitaxy of Co2MnSi Films on Group-IV Semiconductors

We explore epitaxial growth of Co2MnSi films on Si(111) or Ge(111) by means of low-temperature molecular beam epitaxy. We find that as-grown Co2MnSi films consist of mixed phases with L21-ordered structures and microcrystalline ones. As a result, the magnetic moment, which is nearly half of the ideal value, can be obtained even at very low growth temperature. Post-growth annealing was effective to crystallize the microcrystalline phases observed in the as-grown layer, leading to a further enhancement in the magnetic moment. We discuss a difference in growth mechanism between Co2MnSi and other Heusler alloys examined in our previous works.

Lateral spin valves with two-different Heusler-alloy electrodes on the same platform

Using room-temperature molecular beam epitaxy on Si(111), we demonstrate Heusler-alloy bilayers consisting of L21-Co2FeSi (CFS) and D03-Fe3Si (FS). By fabricating lateral spin valves with L21-CFS and D03-FS electrodes, we can see ideal spin signals even though we use one L21-CFS as a spin injector and another D03-FS as a spin detector. The difference in the spin absorption between L21-CFS and D03-FS can also be examined, and we find that the spin resistance of D03-FS is larger than that of L21-CFS. This work will be useful for understanding spin transport in lateral spin-valve devices with different Heusler-alloy electrodes.

An All-Epitaxial Fe 3 Si/FeSi/Co 2 FeSi Trilayer Grown by Room-Temperature Molecular Beam Epitaxy

We present a heat-treatment-free fabrication technique for pseudo current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) structures with a highly spin-polarized Co-based Heusler-compound electrode. Room-temperature molecular beam epitaxy enables an all-epitaxial Heusler compound/metallic spacer/Heusler compound, i.e., Fe3Si/FeSi/Co2FeSi, trilayer structure. Since each layer includes ideally ordered structures, the magnetization reversal process of the trilayer is changed from two-step switching to one-step switching upon cooling through the paramagnetic-ferromagnetic transition of the ordered FeSi spacer layer. This paper will open a way for high-performance CPP-GMR devices with Co-based Heusler compounds.

Exchange coupling in metallic multilayers with a top FeRh layer

We study magnetic properties of metallic multilayers with FeRh/ferromagnet interfaces grown by low-temperature molecular beam epitaxy. Room-temperature coercivity of the ferromagnetic layers is significantly enhanced after the growth of FeRh, proving the existence of the exchange coupling between the antiferromagnetic FeRh layer and the ferromagnetic layer. However, exchange bias is not clearly observed probably due to the presence of disordered structures, which result from the lattice strain at the FeRh/ferromagnet interfaces due to the lattice mismatch. We infer that the lattice matched interface between FeRh and ferromagnetic layers is a key parameter for controlling magnetic switching fields in such multilayer systems.

Marked difference in structural stability between Co 2 FeSi/Si(111) and Co 2 FeAl/Si(111) heterointerfaces in post-growth annealing conditions

Si-based spintronics have been widely studied for realizing low-power-consumption devices compatible with the Si large-scale integrated circuits technologies. For highly efficient spin injection from ferromagnets into Si across Schottky-tunnel barriers, high-quality formation of ferromagnet/Si heterointerfaces is essential. Here we find a marked difference in structural stability between Co2FeSi (CFS)/Si and Co2FeAl (CFA)/Si heterointerfaces in post-annealing conditions. For CFA/Si interfaces, the diffusion of Si atoms from the substrate is markedly suppressed compared to CFS/Si ones. This study will provide an important knowledge for source-drain technologies in Si-based spin transistors.

Lateral spin-valve devices with two different epitaxial Heusler-alloy electrodes

Applications of spintronic technologies to the existing Si ones are required to overcome the limitless increment of power consumption in electronic devices. The use of a pure spin current, the flow of spin angular momentum without a charge current, is prospective approaches to realize ultra-low power consumption for spintronic devices. Here we demonstrate generation and detection of pure spin currents in laterally fabricated spin-valve devices (LSVs) with L21-Co2FeSi (CFS) and D03-Fe3Si (FS) electrodes. Even in a microscopic area in the same Si platform, we can obtain two different spin injectors consisting of single-crystalline Heusler compounds with the different spin functionalities. This work will open a way for intentional arrangement of various spin injectors for generation of pure spin currents in the