Yuki K. Wakabayashi

Ge/Si Heterojunction Tunnel Field-Effect Transistors and Their Post Metallization Annealing Effect

Ge/Si heterojunction tunnel field-effect transistors (TFETs) with an Al₂O₃ gate-stack are demonstrated. The high performances of steep subthreshold swing (SS) of 58 mV/decade and large I_ON/I_OFF ratio over 10⁷ are realized by a proper postmetallization annealing (PMA) process. We can obtain the high-quality Al₂O₃/Ge interface with Dit of as low as ~1011 cm^-2eV^-1 by electron cyclotron resonance oxygen plasma post oxidation. The electrical characteristics of the Ge/SOI TFETs are studied with changing PMA temperature from 200 °C to 400 °C. It is found that Dit between Al₂O₃ and the Si channel region is a critical factor for significantly reducing SS of Ge/Si heterojunction TFETs.

High I on /I off Ge-source ultrathin body strained-SOI tunnel FETs

High performance operation of Ge-source/strained-Si-channel hetero-junction tunnel FETs is demonstrated. It is found that tensile strain in Si-channels can enhance the tunneling current because of the reduced effective energy bandgap, Eg.eff. Nitrogen heat-treatment can improve the gate-to-channel MIS interface which causes SS improvement. The fabricated Ge/sSOI(1.1 %) tunnel FETs show high Ion/Ioff ratio over 107 and steep minimum SS of 28 mV/dec. Back biasing effects are also investigated and the Ion and average SS are improved by positive back biasing.

Annealing-induced enhancement of ferromagnetism and nano-particle formation in ferromagnetic-semiconductor GeFe

We report the annealing-induced enhancement of ferromagnetism and nano-particle formation in group-IV-based ferromagnetic-semiconductor GeFe. We successfully increase the Curie temperature of the Ge0.895Fe0.105 film up to ~220 K while keeping a single ferromagnetic phase when the annealing temperature is lower than 500{\deg}C. In contrast, when annealed at 600{\deg}C, single-crystal GeFe nano-particles with stacking faults and twins, which have a high Curie temperature nearly up to room temperature, are formed in the film. Our results show that annealing is quite effective to improve the magnetic properties of GeFe for high-temperature-operating spin-injection devices based on Si or Ge.

Carrier transport properties of the Group-IV ferromagnetic semiconductor Ge1-xFex with and without boron doping

We have investigated the transport and magnetic properties of group-IV ferromagnetic semiconductor Ge1-xFex films (x = 1.0 and 2.3%) with and without boron doping grown by molecular beam epitaxy (MBE). In order to accurately measure the transport properties of 100-nm-thick Ge1-xFex films, (001)-oriented silicon-on-insulator (SOI) wafers with an ultra-thin Si body layer (∼5 nm) were used as substrates. Owing to the low Fe content, the hole concentration and mobility in the Ge1-xFex films were exactly estimated by Hall measurements because the anomalous Hall effect in these films was found to be negligibly small. By boron doping, we increased the hole concentration in Ge1-xFex from ∼1018 cm−3 to ∼1020 cm−3 (x = 1.0%) and to ∼1019 cm−3 (x = 2.3%), but no correlation was observed between the hole concentration and magnetic properties. This result presents a contrast to the hole-induced ferromagnetism in III-V ferromagnetic semiconductors.

Important role of the non-uniform Fe distribution for the ferromagnetism in group-IV-based ferromagnetic semiconductor GeFe

We report the growth temperature dependence of the properties of the group-IV-based ferromagnetic semiconductor Ge1-xFex films (x = 6.5% and 10.5%), including the lattice constant, Curie temperature (TC), and Fe-atom locations. While TC strongly depends on the growth temperature, we find a universal relationship between TC and the lattice constant, which does not depend on the Fe content x. By using the channeling Rutherford backscattering and particle-induced X-ray emission measurements, it is clarified that about 15% of the Fe atoms exist in the tetrahedral interstitial sites in the Ge0.935Fe0.065 lattice and that the substitutional Fe concentration is not correlated with TC. Considering these results, we suggest that the non-uniformity of the Fe concentration plays an important role in determining the ferromagnetic properties of GeFe.We investigate the growth-temperature dependence of the properties of the group-IV-based ferromagnetic semiconductor Ge1−xFex films (x = 6.5% and 10.5%), and reveal the correlation of the magnetic properties with the lattice constant, Curie temperature (TC), non-uniformity of Fe atoms, stacking-fault defects, and Fe-atom locations. While TC strongly depends on the growth temperature, we find a universal relationship between TC and the lattice constant, which does not depend on the Fe content x. By using the spatially resolved transmission-electron diffractions combined with the energy-dispersive X-ray spectroscopy, we find that the density of the stacking-fault defects and the non-uniformity of the Fe concentration are correlated with TC. Meanwhile, by using the channeling Rutherford backscattering and particle-induced X-ray emission measurements, we clarify that about 15% of the Fe atoms exist on the tetrahedral interstitial sites in the Ge0.935Fe0.065 lattice and that the substitutional Fe concentration...

Electronic structure and magnetic properties of magnetically dead layers in epitaxial CoFe2O4/Al2O3/Si(111) films studied by x-ray magnetic circular dichroism

Epitaxial CoFe2O4/Al2O3 bilayers are expected to be highly efficient spin injectors into Si owing to the spin filter effect of CoFe2O4. To exploit the full potential of this system, understanding the microscopic origin of magnetically dead layers at the CoFe2O4/Al2O3 interface is necessary. In this paper, we study the crystallographic and electronic structures and the magnetic properties of CoFe2O4(111) layers with various thicknesses (thickness d = 1.4, 2.3, 4, and 11 nm) in the epitaxial CoFe2O4(111)/Al2O3(111)/Si(111) structures using soft X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) combined with cluster-model calculation. The magnetization of CoFe2O4 measured by XMCD gradually decreases with decreasing thickness d and finally a magnetically dead layer is clearly detected at d = 1.4 nm. The magnetically dead layer has frustration of magnetic interactions which is revealed from comparison between the magnetizations at 300 and 6 K. From analysis using configuration-interaction cluster-model calculation, the decrease of d leads to a decrease in the inverse-to-normal spinel structure ratio and also a decrease in the average valence of Fe at the octahedral sites. These results strongly indicate that the magnetically dead layer at the CoFe2O4/Al2O3 interface originates from various complex networks of superexchange interactions through the change in the crystallographic and electronic structures. Furthermore, from comparison of the magnetic properties between d = 1.4 and 2.3 nm, it is found that ferrimagnetic order of the magnetically dead layer at d = 1.4 nm is restored by the additional growth of the 0.9-nm-thick CoFe2O4 layer on it.

Room-temperature local ferromagnetism and its nanoscale expansion in the ferromagnetic semiconductor Ge 1– x Fe x

We investigate the local electronic structure and magnetic properties of the group-IV-based ferromagnetic semiconductor, Ge1−xFex (GeFe), using soft X-ray magnetic circular dichroism. Our results show that the doped Fe 3d electrons are strongly hybridized with the Ge 4p states, and have a large orbital magnetic moment relative to the spin magnetic moment; i.e., morb/mspin ≈ 0.1. We find that nanoscale local ferromagnetic regions, which are formed through ferromagnetic exchange interactions in the high-Fe-content regions of the GeFe films, exist even at room temperature, well above the Curie temperature of 20–100 K. We observe the intriguing nanoscale expansion of the local ferromagnetic regions with decreasing temperature, followed by a transition of the entire film into a ferromagnetic state at the Curie temperature.

Origin of robust nanoscale ferromagnetism in Fe-doped Ge revealed by angle-resolved photoemission spectroscopy and first-principles calculation

Ge1−xFex (Ge:Fe) shows ferromagnetic behavior up to a relatively high temperature of 210 K, and hence is a promising material for spintronic applications compatible with Si technology. We have studied its electronic structure by soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) measurements in order to elucidate the mechanism of the ferromagnetism. We observed finite Fe 3d components in the states at the Fermi level (EF) in a wide region in momentum space and EF was located above the valenceband maximum (VBM). First-principles supercell calculation also suggested that the EF is located above the VBM, within the narrow spin-down d(e) band and within the spin-up impurity band of the deep acceptor-level origin derived from the strong p-d(t2) hybridization. We conclude that the narrow d(e) band is responsible for the ferromagnetic coupling between Fe atoms while the acceptor-level-originated band is responsible for the transport properties of Ge:Fe.

Tunneling magnetoresistance in trilayer structures composed of group-IV-based ferromagnetic semiconductor Ge1-xFex, MgO, and Fe

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