Applied Physics Letters

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ISSN / EISSN : 0003-6951 / 1077-3118
Published by: AIP Publishing (10.1063)
Total articles ≅ 128,374
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Qiu-Qiu Li, Si Li, Dan Wu, Zhong-Ke Ding, Xuan-Hao Cao, Lin Huang, Hui Pan, Bo Li, , Xi-Dong Duan
Published: 18 October 2021
Applied Physics Letters, Volume 119; https://doi.org/10.1063/5.0068018

Abstract:
Two-dimensional van der Waals magnetic crystals have been attracting significant research interest in recent years, and the manipulation of their magnetism is important for understanding their physical property and achieving their actual applications. Here, we systematically studied the manipulation of magnetic properties of a CrTe2 bilayer through in-plane strain and self-intercalation. We found that the magnetic ground state of the CrTe2 bilayer varies from intralayer antiferromagnetic coupling to ferromagnetic coupling and then to interlayer antiferromagnetic coupling when the strain changes from −6% to 4%, which should result from the coupling between intralayer Cr atoms tuned from direct Cr–Cr exchange to indirect Cr–Te–Cr superexchange. The magnetic easy axis of the CrTe2 bilayer varies from the in-plane to the out-of-plane owing to the change of pz orbital occupation from Te atoms near the Fermi level. Moreover, the magnetic ground states of different Cr-intercalated concentrations for the CrTe2 bilayer are all ferromagnetic, and the magnetic easy axis is in-plane, which are different from the intrinsic one. Our results indicate that the magnetic property of the CrTe2 bilayer is sensitive to the in-plane strain and self-intercalation, which provides important guidance for the further magnetic manipulation of the CrTe2 bilayer in theoretical research and application of magnetic strain sensors and spin transistors.
Cunyuan Jiang, Zhiyao Yang, Wen Xiong,
Published: 18 October 2021
Applied Physics Letters, Volume 119; https://doi.org/10.1063/5.0065762

Abstract:
Two-dimensional, honeycomb, and sandwich-structured transition metal dichalcogenides (TMDs) have two nonequivalent energy valleys at the six corners of the hexagonal first Brillouin zone, resulting in promising applications in valleytronics. Here, based on the WSe2/CrSe2 heterojunction, biaxial and uniaxial tensile strains with magnitudes of 0%–6% are demonstrated to have a similar effect on magnetism-induced valley splitting in the lowest conduction band of WSe2. However, at larger magnitudes of 6%–10%, uniaxial strain dramatically decreases the valley splitting. This decrease in valley splitting can be understood by the spin-orbit coupling induced different spin splitting between the two valleys. The findings provide valuable guidance for the valleytronic applications of TMDs.
Published: 18 October 2021
Applied Physics Letters, Volume 119; https://doi.org/10.1063/5.0065649

Abstract:
Frequency-domain electron spin resonance (FDESR) spectroscopy in the terahertz (THz) region using continuously tunable photomixers was demonstrated. Spectral resolution was greatly improved with the use of a pair of fiber stretchers. In this setup, the amplitude of the THz electric field was determined at each frequency by externally sweeping the optical path difference, resulting in a spectral resolution of about 1 MHz. With this technique, we observed narrow ESR spectra with a 20-MHz linewidth, enabling high-resolution FDESR spectroscopy in a broad frequency range.
, Victor V. Galushka, Ilya O. Kozhevnikov, ,
Published: 18 October 2021
Applied Physics Letters, Volume 119; https://doi.org/10.1063/5.0058572

Abstract:
Ongoing active development of modern radio frequency electronic devices operating in the millimeter (V) band, such as fifth-generation wireless communications, demands new materials to control electromagnetic interference, compatibility, and reliability of such systems. This work investigates feasibility absorptive non-reflective thin coatings deposition on dielectric substrates using simultaneous magnetron co-deposition. For this, electromagnetic waves propagation in the millimeter band through in micrometer-thick Al–Si films of varied composition was studied. The co-deposition process was controlled by the ratio of sputtered atoms fluxes. Graded segregation was observed under certain parameters of the co-deposition process, resulting in a depth gradient of an aluminum content, as confirmed by the secondary ion mass spectrometry study. A qualitative model was proposed involving aluminum-induced silicon recrystallization happening in the course of a known aluminum interlayer exchange process. The observed Al–Si segregation effect in micrometer-thick films allows for preparation of the non-reflective and absorptive material for operation in the V-band with reflection losses more than 10 dB and transmission losses around 5 dB in the bandwidth of up to 20 GHz.
Ana C. Feltrin, ,
Published: 18 October 2021
Applied Physics Letters, Volume 119; https://doi.org/10.1063/5.0066698

Abstract:
Transition metal borides have a unique combination of high melting point and high chemical stability and are suitable for high temperature applications (>2000 °C). A metastable dual-phase boride (Ti0.25V0.25Zr0.25Hf0.25)B2 with distinct two hexagonal phases and with an intermediate entropy formation ability of 87.9 (eV/atom)−1 as calculated via the density functional theory (DFT) was consolidated by pulsed current sintering. Thermal annealing of the sintered dual-phase boride at 1500 °C promoted the diffusion of metallic elements between the two boride phases leading to chemical homogenization and resulted in the stabilization of a single-phase high-entropy boride. Scanning electron microscopy, in situ high temperature x-ray diffraction, and simultaneous thermal analysis of the as-sintered and annealed high-entropy borides showed the homogenization of a dual-phase to a single-phase. The experimentally obtained single-phase structure was verified by DFT calculations using special quasirandom structures, which were further used for theoretical investigations of lattice distortions and mechanical properties. Experimentally measured mechanical properties of the single-phase boride showed improved mechanical properties with a hardness of 33.2 ± 2.1 GPa, an elastic modulus of 466.0 ± 5.9 GPa, and a fracture toughness of 4.1 ± 0.6 MPa m1/2.
, , , Irfan Saadat
Published: 18 October 2021
Applied Physics Letters, Volume 119; https://doi.org/10.1063/5.0063515

Abstract:
Nonvolatile memory technology is a necessary component in many electronic devices. With the scaling down of memory devices to achieve high density and low power consumption, floating gate devices encounter various challenges like high leakage current, which leads to reliability issues and a decrease in charge density. Therefore, the use of metal nanoparticles (NPs) as charge storage centers is becoming a promising candidate due to their excellent scalability and favorable reliability. In this work, we demonstrate the charge storage dependency on the size of a gold-nanoparticle (Au-NP) by using a contact mode atomic force microscope. The individually dispersed Au-NPs are sandwiched between a thin layer (3 nm) of TiO2 blocking layer and SiO2 (2 nm) tunneling layer thin films. The consecutive I–V sweeps on a pristine device of stacking TiO2/Au-NP/SiO2/n-Si show that the threshold voltage (ΔV) increases with the increase in the Au-NP size, whereas the retention shows much more stability time with smaller size NPs, in the range of 10 nm.
Ke Jiang, , Yuxuan Chen, Shanli Zhang, Jianwei Ben, Yang Chen, , Yuping Jia, ,
Published: 18 October 2021
Applied Physics Letters, Volume 119; https://doi.org/10.1063/5.0064779

Abstract:
GaN-based ultraviolet (UV) detectors have a considerable application potential in many fields. In this Letter, we report an alternative strategy to realize a high-optical-gain bipolar UV phototransistor based solely on a GaN p-i-n epilayer. The device consists of two tightly adjacent vertical p-i-n structures with a common n-type layer as a floating base. The collector and emitter electrodes are deposited on the two p-type mesas, forming a three-dimensional metal–semiconductor–metal (MSM) like photodetector. As a result, a peak responsivity of 11.7 A/W at a wavelength of 358 nm at 5 V is realized, corresponding to an optical gain of 40 with the assumption of 100% internal quantum efficiency. Different from traditional GaN-based n-p-i-n phototransistors, the optical gain of this detector originates from the accumulated electrons in the n-type floating base upon illumination, which can lower the barrier height between the base and emitter, leading to hole emission from the emitter. Although the structure of this phototransistor is similar to a planar back-to-back Schottky-type MSM photodetector, the response speed is much faster because the gain mainly results from carrier emission rather than MS interface defects.
, Michael S. Shur
Published: 18 October 2021
Applied Physics Letters, Volume 119; https://doi.org/10.1063/5.0069072

Abstract:
A non-uniform capacitance profile in the channel of a terahertz (THz) field-effect transistor (TeraFET) could improve the THz detection performance. The analytical solutions and simulations of the hydrodynamic equations for the exponentially varying capacitance vs distance showed ∼10% increase in the responsivity for the 130 nm Si TeraFETs in good agreement with numerical simulations. Using the numerical solutions of the hydrodynamic equations, we compared three different Cg configurations (exponential, linear, and sawtooth). The simulations showed that the sawtooth configuration provides the largest response tunability. We also compared the effects of the non-uniform capacitance profiles for Si, III–V, and p-diamond TeraFETs. The results confirmed a great potential of p-diamond for THz applications. Varying the threshold voltage across the channel could have an effect similar to that of varying the gate-to-channel capacitance. The physics behind the demonstrated improvement in THz detection performance is related to the amplification of boundary asymmetry by the non-uniform device geometry.
Aleksandr Shabanov, Maxim Moskotin, Vsevolod Belosevich, Yakov Matyushkin, , , Dmitry Svintsov
Published: 18 October 2021
Applied Physics Letters, Volume 119; https://doi.org/10.1063/5.0063870

Abstract:
Detectors of terahertz radiation based on field-effect transistors (FETs) are among the most promising candidates for low-noise passive signal rectification both in imaging systems and wireless communications. However, it was not realized so far that geometric asymmetry of common FETs with respect to source-drain interchange is a strong objective to photovoltage harvesting. Here, we break the traditional scheme and reveal the optimally asymmetric FET structure, providing the maximization of THz responsivity. We fabricate a series of graphene transistors with variable top gate positions with respect to a mid-channel and compare their subterahertz responsivities in a wide range of carrier densities. We show that responsivity is maximized for input gate electrode shifted toward the source contact. Theoretical simulations show that for large channel resistance, exceeding the gate impedance, such a recipe for responsivity maximization is universal and holds for both resistive self-mixing and photo-thermoelectric detection pathways. In the limiting case of the small channel resistance, the thermoelectric and self-mixing voltages react differently upon changing the asymmetry, which may serve to disentangle the origin of nonlinearities in novel materials.
Published: 18 October 2021
Applied Physics Letters, Volume 119; https://doi.org/10.1063/5.0070483

Abstract:
Downscaling single magnetic bits to the ultimate size of individual atoms would open the possibility to maximize the magnetic storage density on a solid surface. However, realizing stable magnets of the size of one atom remained an elusive challenge for more than a decade. Recent advances show that single lanthanide atoms on suitable surfaces can preserve their magnetization on a timescale of days at a temperature of 1 K or below. Such properties enable the use of these atoms as stable magnets for low temperature experiments, opening a platform for testing magnetic recording techniques at the atomic scale. In this article, we describe the single atom magnets that have been investigated and the methods employed to address their magnetic properties. We will discuss the limitations of the present systems and techniques and identify the challenges to close the gap toward potential future technological applications.
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