Solid Phase Epitaxy of Single Phase Two-Dimensional Layered InSe Grown by MBE.

Single-phase two-dimensional (2D) indium monoselenide (γ-InSe) film is successfully grown via solid phase epitaxy in the molecular beam epitaxy (MBE) system. Having high electron mobility and high photoresponsivity, ultrathin 2D γ-InSe semiconductors are attractive for future field-effect transistor and optoelectronic devices. However, growing single-phase γ-InSe film is a challenge due to the polymorphic nature of indium selenide (γ-InSe, α-In2Se3, ß-In2Se3, γ-In2Se3, etc.). In this work, the 2D α-In2Se3 film was first grown on a sapphire substrate by MBE. Then, the high In/Se ratio sources were deposited on the α-In2Se3 surface, and an γ-InSe crystal emerged via solid-phase epitaxy. After 50 min of deposition, the initially 2D α-In2Se3 phase was also transformed into a 2D γ-InSe crystal. The phase transition from 2D α-In2Se3 to γ-InSe was confirmed by Raman, XRD, and TEM analysis. The structural ordering of 2D γ-InSe film was characterized by synchrotron-based grazing-incidence wide-angle X-ray scattering (GIWAXS).

Enabling Long-Cycling Life of Si-on-Graphite Composite Anodes via Fabrication of a Multifunctional Polymeric Artificial Solid-Electrolyte Interphase Protective Layer.

The energy density of lithium-ion batteries (LIBs) can be meaningfully increased by utilizing Si-on-graphite composites (Si@Gr) as anode materials, because of several advantages, including higher specific capacity and low cost. However, long cycling stability is a key challenge for commercializing these composites. In this study, to solve this issue, we have developed a multifunctional polymeric artificial solid-electrolyte interphase (A-SEI) protective layer on carbon-coated Si@Gr anode particles (making Si@Gr/C-SCS) to prolong the cycling stability in LIBs. The coating is made of sulfonated chitosan (SCS) that is crosslinked with glutaraldehyde promoting good ionic conduction together with sufficient mechanical strength of the A-SEI. The focused ion beam-scanning electron microscopy and high-resolution transmission electron microscopy images show that the SCS is uniformly coated on the composite particles with thickness in nanometer. The anodes are investigated in Li metal cells Si@Gr/C-SCS||Li metal) and lithium-ion full-cells (LiNi0.6Co0.2Mn0.2O2 (NCM-622)||Si@Gr/C-SCS) to understand the material/electrode intrinsic degradation as well as the impact of the polymer coating on active lithium losses because of the continuous SEI (re)formation. The anode composites exhibit a high capacity reaching over 600 mAh g-1, and even without electrolyte optimization, the Si@Gr/C-SCS illustrates a superior long cycle life performance of up to 1000 cycles (over 67% capacity retention). The excellent long-term cycling stability of the anodes was attributed to the SCS polymer coating acting as the A-SEI. The simple polymer coating process is highly interesting in guiding the preparation of long-cycle-life electrode materials of high-energy LIB cells.

Visualizing Ferroelectric Uniformity of Hf1-xZrxO2 Films Using X-ray Mapping.

Hf1-xZrxO2 (HZO) is a complementary metal-oxide-semiconductor (CMOS)-compatible ferroelectric (FE) material with considerable potential for negative capacitance field-effect transistors, ferroelectric memory, and capacitors. At present, however, the deployment of HZO in CMOS integrated circuit (IC) technologies has stalled due to issues related to FE uniformity. Spatially mapping the FE distribution is one approach to facilitating the optimization of HZO thin films. This paper presents a novel technique based on synchrotron X-ray nanobeam absorption spectroscopy capable of mapping the three main phases of HZO (i.e., orthorhombic (O), tetragonal (T), and monoclinic (M)). The practical value of the proposed methodology when implemented in conjunction with kinetic-nucleation modeling is demonstrated by our development of a T → O annealing (TOA) process to optimize HZO films. This process produces an HZO film with the largest polarization values (Ps = 64.5 µC cm-2; Pr = 35.17 µC cm-2) so far, which can be attributed to M-phase suppression followed by low-temperature annealing for the induction of a T → O phase transition.

Pulse-Driven Nonvolatile Perovskite Memory with Photovoltaic Read-Out Characteristics.

This paper presents a unique GdFe0.8Ni0.2O3 perovskite thin film for use in pulse-controlled nonvolatile memory devices (combined with a SrTiO3 (STO) substrate) without the need for an electrical-stressing read-out process. The use of pulse voltage imposes permanent downward/upward polarization states on GFNO, which enables greater energy density and higher energy efficiency than the unpoled state for memory. The two polarization states produce carrier migrations in opposing directions across the GFNO/STO interface, which alter the depletion region of the device, as reflected in photovoltaic short-circuit current density (Jsc) values. Modulating the duration (varying the number of sequential pulses but fixing the pulse width and delay time) and direction of continuous pulse voltage is an effective method for controlling Jsc, thereby allowing the fabrication of nondestructive, light-tunable, nonvolatile memory devices. In experiments, Jsc in the downward polarized state was approximately 6 times greater than that in the upward polarized state. It is promising that more memory states can be enabled by the proposed heterostructure by selecting appropriate pulse trains. Real-time interfacial changes (relative to the nonvolatile characteristics of the device) were obtained by applying synchrotron X-ray techniques simultaneously with pulse characterization. This made it possible to separately probe the electronic and chemical states of the GFNO (a p-type-like semiconductor) and STO (an n-type-like semiconductor) while varying the pulse direction, thereby making it possible to identify the mechanisms underlying the observed phenomena.

GdFe0.8Ni0.2O3: A Multiferroic Material for Low-Power Spintronic Devices with High Storage Capacity.

Multiferroic materials are strong candidates for reducing the energy consumption of voltage-controlled spintronic devices because of the coexistence of ferroelectric (FE) and magnetic orders in a single phase. In this article, we present a new multiferroic perovskite, GdNixFe1-xO3 (GFNO), produced via sputtering on a SrTiO3 substrate. The proposed GFNO is FE and canted antiferromagnetic (AFM) within a monoclinic framework at room temperature. The FE polarization of the GFNO is up to 37 µC/cm2. When capped with a Co layer, the resulting heterostructure exhibits voltage-controlled magnetism (VCM). The heterostructured device exhibits two distinct features. First, its VCM depends on the magnitude as well as the polarity of the applied bias, thereby doubling the number of available magnetic readout states under a fixed voltage. Furthermore, the magnetic order of the device can be controlled very effectively within ±1 V. These two characteristics satisfy the requirements for low-power and high-storage technology. Theoretical analysis and experimental results indicate the importance of Ni dopant in regulating the polarity-dependent multiferroicity of this gadolinium ferrite system.

Heterostructured ferromagnet-topological insulator with dual-phase magnetic properties.

The introduction of ferromagnetism at the surface of a topological insulator (TI) produces fascinating spin-charge phenomena. It has been assumed that these fascinating effects are associated with a homogeneous ferromagnetic (FM) layer possessing a single type of magnetic phase. However, we obtained phase separation within the FM layer of a Ni80Fe20/Bi2Se3 heterostructure. This phase separation was caused by the diffusion of Ni into Bi2Se3, forming a ternary magnetic phase of Ni:Bi2Se3. The inward diffusion of Ni led to the formation of an FeSe phase outward, transforming the original Ni80Fe20/Bi2Se3 into a sandwich structure comprising FeSe/Ni:Bi2Se3/Bi2Se3 with dual-phase magnetic characteristics similar to that driven by the proximity effect. Such a phenomenon might have been overlooked in previous studies with a strong focus on the proximity effect. X-ray magnetic spectroscopy revealed that FeSe and Ni:Bi2Se3 possess horizontal and perpendicular magnetic anisotropy, respectively. The overall magnetic order of the heterostructure can be easily tuned by adjusting the thickness of the Bi2Se3 as it compromises the magnetic orders of the two magnetic phases. This discovery is essential to the quantification of spin-charge phenomena in similar material combinations where the FM layer is composed of multiple elements.

Voltage-induced Interface Reconstruction and Electrical Instability of the Ferromagnet-Semiconductor Device.

Using x-ray magnetic spectroscopy with in-situ electrical characterizations, we investigated the effects of external voltage on the spin-electronic and transport properties at the interface of a Fe/ZnO device. Layer-, element-, and spin-resolved information of the device was obtained by cross-tuning of the x-ray mode and photon energy, when voltage was applied. At the early stage of the operation, the device exhibited a low-resistance state featuring robust Fe-O bonds. However, the Fe-O bonds were broken with increasing voltage. Breaking of the Fe-O bonds caused the formation of oxygen vacancies and resulted in a high-resistance state. Such interface reconstruction was coupled to a charge-transfer effect via Fe-O hybridization, which suppressed/enhanced the magnetization/coercivity of Fe electronically. Nevertheless, the interface became stabilized with the metallic phase if the device was continuously polarized. During this stage, the spin-polarization of Fe was enhanced whereas the coercivity was lowered by voltage, but changes of both characteristics were reversible. This stage is desirable for spintronic device applications, owing to a different voltage-induced electronic transition compared to the first stage. The study enabled a straightforward detection of the spin-electronic state at the ferromagnet-semiconductor interface in relation to the transport and reversal properties during operation process of the device.

Phase-driven magneto-electrical characteristics of single-layer MoS2.

Magnetism of the MoS2 semiconducting atomic layer was highlighted for its great potential in the applications of spintronics and valleytronics. In this study, we demonstrate an evolution of magneto-electrical properties of single layer MoS2 with the modulation of defect configurations and formation of a partial 1T phase. With Ar treatment, sulfur was depleted within the MoS2 flake leading to a 2H (low-spin) → partial 1T (high-spin) phase transition. The phase transition was accompanied by the development of a ferromagnetic phase. Alternatively, the phase transition could be driven by the desorption of S atoms at the edge of MoS2via O2 treatment while with a different ordering magnitude in magnetism. The edge-sensitive magnetism of the single-layer MoS2 was monitored by magnetic force microscopy and validated by a first-principle calculation with graded-Vs (sulfur vacancy) terminals set at the edge, where band-splitting appeared more prominent with increasing Vs. Treatment with Ar and O2 enabled a dual electrical characteristic of the field effect transistor (FET) that featured linear and saturated responses of different magnitudes in the Ids-Vds curves, whereas the pristine MoS2 FET displayed only a linear electrical dependency. The correlation and tuning of the Vs-1T phase transition would provide a playground for tailoring the phase-driven properties of MoS2 semiconducting atomic layers in spintronic applications.

Competing Anisotropy-Tunneling Correlation of the CoFeB/MgO Perpendicular Magnetic Tunnel Junction: An Electronic Approach.

We intensively investigate the physical principles regulating the tunneling magneto-resistance (TMR) and perpendicular magnetic anisotropy (PMA) of the CoFeB/MgO magnetic tunnel junction (MTJ) by means of angle-resolved x-ray magnetic spectroscopy. The angle-resolved capability was easily achieved, and it provided greater sensitivity to symmetry-related d-band occupation compared to traditional x-ray spectroscopy. This added degree of freedom successfully solved the unclear mechanism of this MTJ system renowned for controllable PMA and excellent TMR. As a surprising discovery, these two physical characteristics interact in a competing manner because of opposite band-filling preference in space-correlated symmetry of the 3d-orbital. An overlooked but harmful superparamagnetic phase resulting from magnetic inhomogeneity was also observed. This important finding reveals that simultaneously achieving fast switching and a high tunneling efficiency at an ultimate level is improbable for this MTJ system owing to its fundamental limit in physics. We suggest that the development of independent TMR and PMA mechanisms is critical towards a complementary relationship between the two physical characteristics, as well as the realization of superior performance, of this perpendicular MTJ. Furthermore, this study provides an easy approach to evaluate the futurity of any emerging spintronic candidates by electronically examining the relationship between their magnetic anisotropy and transport.

Instrument for x-ray absorption spectroscopy with in situ electrical control characterizations.

We report a synchrotron-based setup capable of performing x-ray absorption spectroscopy and x-ray magnetic circular dichroism with simultaneous electrical control characterizations. The setup can enable research concerning electrical transport, element- and orbital-selective magnetization with an in situ fashion. It is a unique approach to the real-time change of spin-polarized electronic state of a material/device exhibiting magneto-electric responses. The performance of the setup was tested by probing the spin-polarized states of cobalt and oxygen of Zn(1-x)Co(x)O dilute magnetic semiconductor under applied voltages, both at low (~20 K) and room temperatures, and signal variations upon the change of applied voltage were clearly detected.