Schottky Tunnel Contacts Research Articles

Electrical properties of a low-temperature fabricated Ge-based top-gate MOSFET structure with epitaxial ferromagnetic Heusler-alloy Schottky-tunnel source and drain

We show electrical properties of a Ge-based top-gate metal-oxide-semiconductor field-effect transistor (MOSFET) structure with epitaxial ferromagnetic Heusler alloy/Ge Schottky-tunnel contacts, fabricated with a low-temperature gate-stack process. For shaping the Ge-MOSFET with the ferromagnetic source and drain (S/D) contacts, we use a combination process of Ar+ ion milling and reactive ion etching (RIE). As a consequence of the formation of a gate stack (SiO2/GeO2) using a process temperature of 240 °C, we demonstrate inversion channel MOSFETs with a low gate-voltage operation (VG ∼5 V). Furthermore, we achieve a relatively high field-effect mobility (μFE) of ∼300 cm2/Vs compared to those in a Si-based back-gate spin-MOSFET reported previously [Phys. Rev. B 102, 035305 (2020).]. This study will open a way for developing a Ge-based top-gate spin-MOSFET on the Si platform.

Temperature Dependence of Two-Terminal Local Magnetoresistance in Co-Based Heusler Alloy/Ge Lateral Spin-Valve Devices

We investigate temperature dependence of two-terminal local magnetoresistance (MR) effect for germanium (Ge)-based lateral spin-valve (LSV) devices with three different ferromagnet (FM)/Ge Schottky tunnel contacts. When we insert a 0.7-nm-thick Fe layer between Co-based Heusler alloy and Ge interfaces in LSV devices, the magnitude of the local MR signals is significantly improved from low temperatures to room temperature. However, the temperature dependence of the MR ratio is still large even for LSV devices with Fe insertion. From the analysis based on the standard theory, the temperature-dependent MR ratio can be interpreted in terms of thermally excited spin waves ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$T^{3/{2}}$ </tex-math></inline-formula> law) in FM contacts. The physical origin of the decay of the two-terminal MR ratio in Ge-based LSV devices is the same as that of the four-terminal nonlocal spin signals because small temperature dependence of contact resistance is demonstrated in FM/Ge Schottky tunnel contacts which show almost linear <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$|J|$ </tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V$ </tex-math></inline-formula> characteristics and low interface resistance.

Room-temperature two-terminal magnetoresistance ratio reaching 0.1% in semiconductor-based lateral devices with L21-ordered Co2MnSi

We report on the highest two-terminal magnetoresistance (MR) ratio at room temperature in semiconductor-based lateral spin-valve devices. From first-principles calculations, we predict energetically stable ferromagnet–semiconductor heterointerfaces consisting of Co2MnSi (CMS) and Ge(111) upon insertion of Fe atomic layers. Using low-temperature molecular beam epitaxy, we demonstrate L21-ordered CMS epilayers at 80 °C on Ge(111), where the CMS layer can be utilized as a spin injector and detector. Two-terminal MR ratios as high as 0.1% are achieved in n-Ge-based lateral spin-valve devices with CMS/Fe/Ge Schottky tunnel contacts annealed at 200 °C. This study will open a path for semiconductor-based spintronic devices with a large MR ratio at room temperature.

Thermal Stability at the Interface between Ferromagnetic Alloys and Germanium for Semiconductor Spintronics Devices

Spin MOSFET has drawn much attention as an attractive candidate for future semiconductor devices because of its nonvolatility and reconfigurability [1]. For realization of the spin MOSFET, highly efficient spin injection from ferromagnets to semiconductors with low parasitic resistance is one of the most important key technologies. Recently, we demonstrated room-temperature spin transport in germanium (Ge) based lateral spin valve devices with Schottky-tunnel contact, where the interface resistance at the ferromagnet/Ge is much smaller than that with conventional insulating tunnel barriers [2, 3]. However, when the thermal annealing was conducted to fabricate gate-stack structures for the spin MOSFET, the spin injection/detection efficiency was degraded due to the formation of reaction layers at the ferromagnetic alloys/Ge interface [4]. In this study, we explore the enhancement in the thermal stability of the ferromagnet/Ge interface and demonstrate room-temperature spin signals even for the lateral spin device annealed at 250 oC by an insertion of five Fe atomic layers between the ferromagnetic alloys and Ge.We fabricated lateral spin valve devices with Co2FeAl0.5Si0.5(CFAS)/Fe (5 atomic-layers)/Ge Schottky tunnel junction [Fig. 1(a)]. A 70-nm-thick phosphorus (P) doped Ge layer was grown on Ge/Si(111) virtual substrate at 350 oC as a spin transport layer by molecular beam epitaxy (MBE). On the P doped Ge layer, five Fe atomic layers and an epitaxial CFAS layer were formed using low temperature MBE. Beneath the ferromagnetic contacts, the P δ-doping with a thin Si layer was inserted to promote the tunnel conduction. The CFAS/Fe and δ-doped Ge layers were patterned into the contacts with 0.4 × 5.0 μm2 and 1.0 × 5.0 μm2, where the edge-to-edge distances between the CFAS/n+-Ge contacts are designed to be 0.45 μm. As a reference sample, a device without the Fe atomic layers was also fabricated. The devices with and without the Fe atomic layers were annealed at 250 oC in N2 atmosphere.Figures 1(b) and (c) show the nonlocal spin signals at 77 K for devices annealed at 250 oC without and with an Fe insertion, respectively, together with schematics of the CFAS/Ge and CFAS/Fe/Ge interfaces after the annealing. For the device without an Fe insertion [Fig. 1(b)], the spin signal is significantly degraded (~97 %) compared with the as prepared device, where the Ge out-diffusion causes the degradation of the spin injection/detection efficiency [4]. On the other hand, even after the annealing, clear hysteretic curves related to the magnetization states of the ferromagnetic injector and detector contacts can be observed for the device with an insertion of Fe atomic layers [Fig. 1(c)]. Although there is ~50 % decrease of the spin signal by the annealing, the magnitude of nonlocal spin signals is ~ 140 mΩ due to the significant improvement of the spin injection efficiency [5] and of the thermal stability of the CFAS/Fe/Ge interface. As shown in schematics of Figs 1(b) and 1(c), the inserted Fe atomic layers can suppress the Ge out-diffusion into CFAS and maintain the highly efficient spin injection. For the device with an Fe atomic insertion annealed at 250 oC, a spin signal of ~7 mΩ can be obtained even at room temperature.This work was partly supported by Grants-in-Aid for Scientific Research (A) (No. 16H02333) and (S) (No. 17H06120 and 19H05616) from the Japan Society for the Promotion of Science (JSPS). M.Y. acknowledges JSPS Research Fellowships for Young Scientists (No.18J00502).[1] S. Sugahara and M. Tanaka, Appl. Phys. Lett. 84, 2307 (2004).[2] M. Yamada et al., Appl. Phys. Express 10, 093001 (2017).[3] M. Tsukahara et al., Appl. Phys. Express 12, 033002 (2019).[4] B. Kuerbanjiang et al., Phys. Rev. B 98, 115304 (2018).[5] M. Yamada et al., (submitted). Figure 1

Nonmonotonic bias dependence of local spin accumulation signals in ferromagnet/semiconductor lateral spin-valve devices

We find extraordinary behavior of the local two-terminal spin accumulation signals in ferromagnet (FM)/semiconductor (SC) lateral spin-valve devices. With respect to the bias voltage applied between two FM/SC Schottky tunnel contacts, the local spin-accumulation signal can show nonmonotonic variations, including a sign inversion. A part of the nonmonotonic features can be understood qualitatively by considering the rapid reduction in the spin polarization of the FM/SC interfaces with increasing bias voltage. In addition to the sign inversion of the FM/SC interface spin polarization, the influence of the spin-drift effect in the SC layer and the nonlinear electrical spin conversion at a biased FM/SC contact are discussed.

Room-temperature local magnetoresistance effect in n-Ge devices with low-resistive Schottky-tunnel contacts

Two-terminal local magnetoresistance (MR) effect in n-type germanium (Ge) based lateral spin-valve (LSV) devices can be observed at room temperature. By using phosphorus δ-doped Heusler-alloy/Ge Schottky-tunnel contacts, the resistance-area product of the contacts is able to be less than 0.20 kΩ μm2, which is the lowest value in semiconductor based LSV devices. From the one-dimensional spin drift-diffusion model, the interface spin polarization of the Heusler-alloy/Ge contacts in the present LSV devices can be estimated to be ∼0.018 at room temperature. We experimentally propose that it is important for enhancing the local MR ratio in n-Ge based LSV devices to improve the interface spin polarization of the Heusler-alloy/Ge contacts.

(Invited) Low-Resistance Ferromagnet/Germanium Schottky-Tunnel Contacts for Spintronic Applications

For CMOS-compatible spintronic devices, electrical spin injection and detection in silicon (Si) [1] and germanium (Ge) [2] have been explored. Due to the conductivity mismatch problem [3], the insulator tunnel barriers such as MgO [1,2] were generally used for electrical spin injection from ferromagnets (FM) into Si and Ge, where the resistance area products (RA) values of the tunnel contacts were approximately 104 Ωμm2 (10-4 Ωcm2) [1,2]. However, these contacts with large RA values cannot be utilized as a source and drain for spin-based MOS transistors [4]. If spintronic technologies are integrated with Ge-CMOS and/or Ge-based photonics, novel spin-injection techniques with low- RA contacts should be developed. In this paper, we have developed high-quality FM/n-Ge heterointerfaces with a delta-doped Ge interlayer for low-RA Schottky-tunnel contacts [5,6]. Because of the tunable doping concentration of the delta-doped layer, we can obtain the contacts with relatively low-RA values of ~102 Ωμm2 (~10-6 Ωcm2) for n-Ge. Using the Schottky-tunnel contacts, we can observe lateral transport of pure spin currents in n-Ge up to room temperature [7]. In addition, by using the low-RA contacts, the two-terminal magnetoresistance effect can be observed even at room temperature [8]. Although we should further reduce the RA value as low as possible, our results with the low-RA Schottky-tunnel contacts will open a road to integrate the spintronic technologies into next generation Ge-based devices on the Si platform. [1] T. Suzuki et al., Appl. Phys. Exp. 4, 023003 (2011). [2] Y. Zhou et al., Phys. Rev. B 84, 125323 (2011). [3] G. Schmidt et al., Phys. Rev. B 62, R4790 (2000). [4] M.Tanaka and S. Sugahara, IEEE Trans. Electron. Devices 54, 961 (2007). [5] M. Yamada, KH et al., Mat. Sci. Semicon. Proc. 70, 83 (2017). [6] Y. Fujita, KH et al., Phys. Rev. Appl. 8, 014007 (2017). [7] M. Yamada, KH et al., Appl. Phys. Exp. 10, 093001 (2017). [8] Y. Fujita, KH et al., (submitted).

Room Temperature Spin Accumulation Effect in Boron Doped Si Created by Epitaxial Fe3Si/p-Si Schottky Contact

To study spin-dependent transport phenomena in Fe3Si/p-Si structures we fabricated 3-terminal planar microdevices and metal/semiconductor diode using conventional photolithography and wet chemical etching. I‒V curve of prepared diode demonstrates rectifying behavior, which indicates the presence of Schottky barrier in Fe3Si/p-Si interface. Calculated Schottky barrier height is 0.57 eV, which can provide necessary conditions for spin accumulation in p-Si. Indeed, in 3-terminal planar device with Fe3Si/p-Si Schottky contact Hanle effect was observed. By the analysis of Hanle curves spin lifetime spin diffusion length in p-Si were calculated, which are 145 ps and 405 nm, respectively (at T = 300 K). Spin lifetime strongly depends on temperature which can be related to the fact that spin-dependent transport in our device is realized via the surface states. This gives a perspective of creation of spintronic devices based on metal/semiconductor structure without need for forming tunnel or Schottky tunnel contact.

Room-temperature spin transport in n-Ge probed by four-terminal nonlocal measurements

We demonstrate electrical spin injection and detection in n-type Ge (n-Ge) at room temperature using four-terminal nonlocal spin-valve and Hanle-effect measurements in lateral spin-valve (LSV) devices with Heusler-alloy Schottky tunnel contacts. The spin diffusion length (λGe) of the Ge layer used (n ∼ 1 × 1019 cm−3) at 296 K is estimated to be ∼0.44 ± 0.02 µm. Room-temperature spin signals can be observed reproducibly in the low bias voltage range (≤0.7 V) for LSVs with relatively low resistance–area product (RA) values (≤1 kΩ µm2). This means that the Schottky tunnel contacts used here are more suitable than ferromagnet/MgO tunnel contacts (RA ≥ 100 kΩ µm2) for developing Ge spintronic applications.

Control of electrical properties in Heusler-alloy/Ge Schottky tunnel contacts by using phosphorous δ-doping with Si-layer insertion

We experimentally demonstrate the control of electrical properties of Heusler-alloy/Ge contacts by phosphorous (P) δ-doping techniques with Si-layer insertion. Low-temperature molecular beam epitaxy methods enable us to obtain the high quality Heusler-alloy/Ge heterointerfaces with P δ-doped layers. Although the resistance area product (RA) values are scattered in the Heusler-alloy/Ge interfaces with conventional δ-doping techniques, we precisely adjust the RA values in the newly developed interface with Si-layer insertion. This method will open a way for developing source and drain structures in Ge-based spintronics devices.

Influence of thermal annealing on the spin injection and spin detection through Fe/GaAs interfaces

A strong bias asymmetry of the spin-injection efficiency through an epitaxial Fe/GaAs Schottky tunnel contact is observed. Low-temperature post-growth thermal annealing is shown to strongly affect the spin-injection efficiency. The annealing leads either to a reduction or an enhancement. The spin accumulation is addressed electrically in a lateral spin-valve geometry using a non-local spin-valve setup at liquid helium temperatures. A spin-injection efficiency of up to 5.5% is estimated from experimental results. The electrical properties of the Schottky tunnel diode do not reflect the bias asymmetry and the changes in the spin-injection efficiency during annealing. Formation of spin-polarized interface states (IS) close to the Fermi-level is a possible explanation. The IS will not only radically affect the spin-injection but also the spin-detection process.

A low-temperature fabricated gate-stack structure for Ge-based MOSFET with ferromagnetic epitaxial Heusler-alloy/Ge electrodes

A possible low-temperature fabrication process of a gate-stack for Ge-based spin metal–oxide–semiconductor field-effect transistor (MOSFET) is investigated. First, since we use epitaxial ferromagnetic Heusler alloys on top of the phosphorous doped Ge epilayer as spin injector and detector, we need a dry etching process to form Heusler-alloy/n+-Ge Schottky-tunnel contacts. Next, to remove the Ge epilayers damaged by the dry etching process, the fabricated structures are dipped in a 0.03% diluted H2O2 solution. Finally, Al/SiO2/GeO2/Ge gate-stack structures are fabricated at 300 °C as a top gate-stack structure. As a result, the currents in the Ge-MOSFET fabricated here can be modulated by applying gate voltages even by using the low-temperature formed gate-stack structures. This low-temperature fabrication process can be utilized for operating Ge spin MOSFETs with a top gate electrode.

Greatly enhanced generation efficiency of pure spin currents in Ge using Heusler compound Co2FeSi electrodes

We show nonlocal spin transport in n-Ge-based lateral spin-valve devices with highly ordered Co2FeSi/n+-Ge Schottky tunnel contacts. Clear spin-valve signals and Hanle effect curves are demonstrated at low temperatures, indicating the generation, manipulation, and detection of pure spin currents in n-Ge. The obtained spin generation efficiency of ∼0.12 is about two orders of magnitude larger than that for a previously reported device with Fe/MgO tunnel barrier contacts. Considering the spin-related behavior with temperature evolution, we infer that it is necessary to simultaneously demonstrate a high spin generation efficiency and improve the quality of the transport channel to realize Ge-based spintronic devices.

Room-temperature electrical creation of spin accumulation in n-Ge using highly resistive Fe3Si/n+-Ge Schottky-tunnel contacts

We have explored an effect of the interface resistance of Fe3Si/n+-Ge Schottky-tunnel contacts on spin accumulation created electrically in n-Ge. Using the Schottky-tunnel contacts with a relatively high interface resistance, we can clearly detect spin-accumulation signals in n-Ge even at room temperature, meaning that the use of highly resistive Schottky tunnel contacts is effective to electrically create large spin accumulation. Since such highly resistive source and drain electrodes are not available for next-generation Ge-based spintronic applications with ultra-low power consumption, the development of highly spin-polarized spin injectors such as half-metallic materials is essential from now on.

Qualitative study of temperature-dependent spin signals in n-Ge-based lateral devices with Fe3Si/n+-Ge Schottky-tunnel contacts

We study electrical spin injection and detection in n-Ge across Fe3Si/n+-Ge Schottky tunnel barriers. Spin-accumulation signals detected electrically by the three-terminal Hanle-effect measurements have large temperature dependence, and the spin signals disappear at around 200 K. We find that the temperature variation in the spin signals is strongly related to that in the interface resistance of the Fe3Si/n+-Ge contacts. We also observe marked reduction in nonlocal spin-valve signals with increasing temperature in the four-terminal device with almost the same Fe3Si/n+-Ge/n-Ge heterostructure. Note that the nonlocal spin signals depend on not only the measurement temperature but also on the electrical characteristics of the spin injector. Considering the results of three-terminal and four-terminal measurements, we conclude that temperature dependence of the spin signals in our Fe3Si/n+-Ge/n-Ge devices is governed by the electrical characteristics of the spin injector. This study means that it is necessary to consider the effective Schottky-tunnel contacts with the stability on temperature variations for next-generation Ge-based spintronic applications.

Room-temperature sign reversed spin accumulation signals in silicon-based devices using an atomically smooth Fe3Si/Si(111) contact

We demonstrate a reliable sign-reversed spin signal detected by three-terminal Hanle effect measurements at room temperature in the Si-based lateral devices with one Fe3Si/Si(111) Schottky-tunnel contact. Theoretical calculations of the spin polarized density of states suggest the sign difference in the spin polarization (P) between the two types of Fe3Si/Si(111) interfaces. Actually, we directly observe the atomic steps at the Fe3Si/Si(111) interface, implying that there is a possible origin of the sign difference in the spin polarization (P) between spin-injection region and spin-detection one in one contact. The reliable sign-reversed spin signals support that the injected spins are transported laterally in the Si channel even for the three-terminal Hanle-effect measurements.

Spin accumulation created electrically in an n-type germanium channel using Schottky tunnel contacts

Using high-quality Fe3Si/n+-Ge Schottky-tunnel-barrier contacts, we study spin accumulation in an n-type germanium (n-Ge) channel. In the three- or two-terminal voltage measurements with low bias current conditions at 50 K, Hanle-effect signals are clearly detected only at a forward-biased contact. These are reliable evidence for electrical detection of the spin accumulation created in the n-Ge channel. The estimated spin lifetime in n-Ge at 50 K is one order of magnitude shorter than those in n-Si reported recently. The magnitude of the spin signals cannot be explained by the commonly used spin diffusion model. We discuss a possible origin of the difference between experimental data and theoretical values.

Bias current dependence of spin accumulation signals in a silicon channel detected by a Schottky tunnel contact

We study the electrical detection of spin accumulation at a ferromagnet-silicon interface, which can be verified by measuring a Hanle effect in three-terminal lateral devices. The device structures used consist of a semiconducting Si channel and a Schottky tunnel contact. In a low current-bias region, the Hanle-effect curves are observed only under forward bias conditions. This can be considered that the electrical detectability at the forward-biased contact is higher than that at the reverse-biased contact. This is possible evidence for the detection of spin-polarized electrons created in a Si channel.

Source–Drain Engineering Using Atomically Controlled Heterojunctions for Next-Generation SiGe Transistor Applications

Using low-temperature molecular-beam epitaxy techniques on the (111) plane of Si or Ge, we can realize an atomically controlled Fe3Si/Si or Fe3Si/Ge heterojunction and simultaneously obtain DO3-ordered crystal structures of Fe3Si films. First, high-quality Fe3Si/Si(111) Schottky tunnel contacts enable us to inject and detect spin-polarized electrons in Si conduction channels at ∼180 K, where Fe3Si is a ferromagnetic spin injector and detector. This may lead to the accelerated development of next-generation Si-based spin metal–oxide–semiconductor field-effect transistors (MOSFETs). Next, for the atomically controlled Fe3Si/Ge(111) Schottky contacts, we find the unexpected suppression of the Fermi level pinning (FLP) effect. This indicates that there is an influence of extrinsic contributions such as dangling bonds and disorder on the strong FLP effect at metal/Ge interfaces. We expect that the Fe3Si/Ge(111) contacts can be used to control the Schottky barrier height of future ultra scaled Ge-channel MOSFETs. These two novel and interesting results are expected to form the basis of a key technology toward developing next-generation SiGe transistors.

Electrical Detection of Spin Transport in Si Using High-quality Fe3Si/Si Schottky Tunnel Contacts

We investigated the epitaxial growth of high-quality Fe3Si thin films on Si(111) and demonstrated electrical injection and detection of spin-polarized electrons in Si through the Fe3Si/Si Schottky-tunnel contacts. TEM observations and analyses of electron diffraction patterns revealed that precisely controlling the Fe/Si ratio during growth is important to obtain well ordered Fe3Si structures and atomically abrupt heterointerfaces at the same time. The electrical properties of Fe3Si/Si Schottky diodes with various carrier concentrations were also examined, and highly transparent tunnel contacts were demonstrated. We were able to clearly detect nonlocal spin signals in Si at 150K using Fe3Si/Si lateral four-probe devices.