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  • Progress, challenges, and perspective on metasurfaces for ambient radio frequency energy harvesting
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-11
    Long Li, Xuanming Zhang, Chaoyun Song, Yi Huang

    In this paper, wireless power transfer (WPT) and energy harvesting (EH) technologies are reviewed in detail, and the application of metamaterials and metasurfaces for WPT and EH is discussed. Specifically, we focus on the metasurfaces for ambient radio frequency energy harvesting (AEH) in recent advances, comments, existing challenges, and future directions. The performance of metasurface- and antenna-based AEH systems is compared. The metasurfaces not only enable the efficient operation of the AEH system but also extend the potential function to various kinds of energy harvesting devices, which is influential progress of ambient electromagnetic energy harvesting.

    更新日期:2020-02-14
  • In-plane quartz-enhanced photoacoustic spectroscopy
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-10
    Yufei Ma, Shunda Qiao, Pietro Patimisco, Angelo Sampaolo, Yao Wang, Frank K. Tittel, Vincenzo Spagnolo

    An optical gas sensing technique based on in-plane quartz-enhanced photoacoustic spectroscopy (IP-QEPAS) is reported. In IP-QEPAS, the laser beam is aligned in the plane of the quartz tuning fork (QTF) to increase the interaction area between the acoustic wavefront and the QTF. A custom T-shaped QTF with a prong length of 9.4 mm and a resonance frequency of 9.38 kHz was designed and employed in the IP-QEPAS sensor. For comparison, the traditional QEPAS sensor in which the laser beam is perpendicular to the QTF plane (PP-QEPAS) is investigated with the same operating conditions. Theoretical calculations of strain and displacement of the QTF prong were performed to support the advantage of using the IP-QEPAS technique. By selecting water vapor as the gas target, the IP-QEPAS sensor results in a signal more than 40 times higher than that measured with the PP-QEPAS configuration, confirming the potential of this approach.

    更新日期:2020-02-14
  • A semiconductor topological photonic ring resonator
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-11
    M. Jalali Mehrabad, A. P. Foster, R. Dost, E. Clarke, P. K. Patil, I. Farrer, J. Heffernan, M. S. Skolnick, L. R. Wilson

    Unidirectional photonic edge states arise at the interface between two topologically distinct photonic crystals. Here, we demonstrate a micrometer-scale GaAs photonic ring resonator, created using a spin Hall-type topological photonic crystal waveguide. Embedded InGaAs quantum dots are used to probe the mode structure of the device. We map the spatial profile of the resonator modes and demonstrate the control of the mode confinement through tuning of the photonic crystal lattice parameters. The intrinsic chirality of the edge states makes them of interest for applications in integrated quantum photonics, and the resonator represents an important building block toward the development of such devices with embedded quantum emitters.

    更新日期:2020-02-14
  • Mid-infrared electroluminescence from type-II In(Ga)Sb quantum dots
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-11
    Andrew F. Briggs, Leland J. Nordin, Aaron J. Muhowski, Priyanka Petluru, David Silva, Daniel Wasserman, Seth R. Bank

    There exists significant interest in the demonstration and development of alternative mid-infrared emitters, with future applications for thermal scene projection, low-cost infrared sensing, and possible long-wavelength quantum communication applications. Type-II In(Ga)Sb quantum dots grown in InAs matrices have the potential to serve as a viable material system for wavelength-flexible, mid-infrared sources. Here, we dramatically expand the range of potential applications of these mid-infrared quantum emitters through the demonstration of surface-emitting electrically pumped mid-infrared light-emitting diodes with active regions utilizing type-II In(Ga)Sb quantum dots. Two device structures were studied, the first iteration being a single In(Ga)Sb insertion layer within a simple PIN structure and the second being a design engineered for improved room temperature emission with the addition of lattice matched AlAsSb cladding at the anode to block electrons and five layers of In(Ga)Sb dots to increase the effective volume of active material. Samples were grown by molecular beam epitaxy and the electrical and optical properties for each design were characterized as a function of temperature.

    更新日期:2020-02-14
  • Deep ultraviolet monolayer GaN/AlN disk-in-nanowire array photodiode on silicon
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-11
    Anthony Aiello, Yuanpeng Wu, Zetian Mi, Pallab Bhattacharya

    Extreme confinement of carriers in GaN layers of thickness of the order of a monolayer leads to a large quantum confinement energy and very large electronic and optical bandgaps. We have exploited this to realize a photodiode with AlN nanowire arrays, grown on silicon substrates by plasma-enhanced molecular beam epitaxy, wherein multiple ∼2 monolayer disks are inserted as the light absorbing region. Photoluminescence and photocurrent spectra confirm the optical gaps of the monolayer GaN. The photocurrent spectra show a peak at ∼240 nm in the deep-ultraviolet region of the optical spectrum. The dark current of the photodiodes is ∼10 nA at −6 V at room temperature. The peak quantum efficiency is 0.6%, and the noise-equivalent power is estimated to be 4.3 × 10−11W/Hz1/2. The bandwidth of the device is estimated to be limited to ∼3 MHz by the series resistance and diode capacitance.

    更新日期:2020-02-14
  • Tunable valleytronics with symmetry-retaining high polarization degree in SnSxSe1−xmodel system
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-12
    Shuren Lin, Zixuan Fang, Tingzheng Hou, Ting Wan Hsu, Chi H. So, Cher Yeoh, Roger Li, Yin Liu, Emory M. Chan, Yu-Lun Chueh, Bin Tang, Kristin Persson, Jie Yao

    SnS has recently been shown to possess unique valleytronic capability with a large polarization degree, where non-degenerate valleys can be accessed using linearly polarized light, bestowed upon by the unique anisotropy and wavefunction symmetry. It is thus of utmost importance to demonstrate the extension of such effects for the IV–VI system in general, thereby elucidating the generality and tunability of such valleytronics. We show the highly tunable valleytronics via gradual compositional control of the tin(II) sulfo-selenide (SnSxSe1−x) alloy system with excellent retainment of symmetry-determined selection rules. We show the presence of both ΓY and ΓX valleys in all alloy compositions via selectivity in absorption and emission of linearly polarized light by optical reflection (R)/transmission (T) and photoluminescence measurements and tuned the bandgaps of the valleys within a range of 1.28 eV–1.05 eV and 1.48 eV–1.24 eV, respectively. This simultaneous tuning of non-degenerate valleys agrees well with theoretical calculations. We then fitted the bandgap values in compositional space, obtaining bowing parameters as a useful database. We further demonstrated the feasibility of using IV–VI valleytronics systems in general by elucidating the retainment of strong polarization degrees of as high as 91% across all compositions. The generalization of such purely symmetry-dependent valleytronics also opens up opportunities for the discovery of more multi-functional materials.

    更新日期:2020-02-14
  • Multiple scattering in scanning helium microscopy
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-10
    S. M. Lambrick, L. Vozdecký, M. Bergin, J. E. Halpin, D. A. MacLaren, P. C. Dastoor, S. A. Przyborski, A. P. Jardine, D. J. Ward

    Using atom beams to image the surface of samples in real space is an emerging technique that delivers unique contrast from delicate samples. Here, we explore the contrast that arises from multiple scattering of helium atoms, a specific process that plays an important role in forming topographic contrast in scanning helium microscopy (SHeM) images. A test sample consisting of a series of trenches of varying depths was prepared by ion beam milling. SHeM images of shallow trenches (depth/width < 1) exhibited the established contrast associated with masking of the illuminating atom beam. The size of the masks was used to estimate the trench depths and showed good agreement with the known values. In contrast, deep trenches (depth/width > 1) exhibited an enhanced intensity. The scattered helium signal was modeled analytically and simulated numerically using Monte Carlo ray tracing. Both approaches gave excellent agreement with the experimental data and confirmed that the enhancement was due to localization of scattered helium atoms due to multiple scattering. The results were used to interpret SHeM images of a bio-technologically relevant sample with a deep porous structure, highlighting the relevance of multiple scattering in SHeM image interpretation.

    更新日期:2020-02-14
  • Epitaxial growth of single tellurium atomic wires on a Cu2Sb surface alloy
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-10
    Dechun Zhou, Heping Li, Nan Si, Yixuan Jiang, Han Huang, Hui Li, Tianchao Niu

    One-dimensional (1D) zigzag tellurium (Te) wires on an alloyed Cu2Sb (111) surface are produced and characterized by combining molecular beam epitaxy and high-resolution scanning tunneling microscopy, respectively. These 1D Te wires with zigzag shapes exhibit a uniform width and well-defined periodicity grown in registry on a Cu2Sb (111) substrate. Density functional theory (DFT) calculations revealed a remarkable bandgap of 0.3 eV induced by spin–orbit coupling. Interestingly, the resulting Te wires became metallic on the substrate, as confirmed by scanning tunneling spectroscopy and DFT. The present study provides a platform for constructing periodic atomic wires on alloyed surfaces, which may be extended further to integrate them into circuits.

    更新日期:2020-02-14
  • Emerging edge states on the surface of the epitaxial semimetal CuMnAs thin film
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-11
    Giang D. Nguyen, Krishna Chaitanya Pitike, Peter Wadley, Valentino R. Cooper, Mina Yoon, Tom Berlijn, An-Ping Li

    Epitaxial thin films of CuMnAs have recently attracted attention due to their potential to host relativistic antiferromagnetic spintronics and exotic topological physics. Here, we report on the structural and electronic properties of a tetragonal CuMnAs thin film studied using scanning tunneling microscopy (STM) and density functional theory (DFT). STM reveals a surface terminated by As atoms, with the expected semi-metallic behavior. An unexpected zigzag step edge surface reconstruction is observed with emerging electronic states below the Fermi energy. DFT calculations indicate that the step edge reconstruction can be attributed to an As deficiency that results in changes in the density of states of the remaining As atoms at the step edge. This understanding of the surface structure and step edges on the CuMnAs thin film will enable in-depth studies of its topological properties and magnetism.

    更新日期:2020-02-14
  • Frictional properties of polymer binders for Li-ion batteries
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-11
    Quang Dang Nguyen, Koo-Hyun Chung

    In this work, the frictional properties of various polymer binders were studied using atomic force microscopy to enhance the mechanical integrity of the silicon (Si) anode for lithium-ion batteries (LIBs). The binders used include polyacrylic acid (PAA), polyacrylonitrile (PAN), polyvinyl alcohol (PVA), carboxymethyl cellulose (CMC), and polyvinylidene fluoride. The interfacial shear strength of the polymer binder was determined against Si to understand the contact shearing and sliding behaviors. The results showed that due to electrolyte immersion, the intrinsic interfacial shear strengths and pressure coefficients decreased by factors of 1.3–7.5 and 1.6–23.7, respectively. Decreases in mechanical and adhesion properties in the electrolyte may be responsible for the decrease in the frictional properties. The results also showed that topography-induced friction fluctuation became significant in the electrolyte due to an increase in the slope of the surface asperities, which is associated with polymer swelling. The Si anodes of LIBs containing polymer binders with relatively large intrinsic interfacial shear strengths and pressure coefficients in the electrolyte (PAA, PAN, PVA, and CMC) had high capacity retention. Given that a polymer binder with high interfacial shear strength helps maintain the mechanical integrity of the Si anode, PAA with high intrinsic interfacial shear strength may be preferred when the stress level is low during lithiation, while PVA with a large pressure coefficient may be advantageous when the stress level is high during lithiation. The outcome of this work suggests that the interfacial shear strength is a better polymer binder performance indicator than just considering mechanical and adhesion properties.

    更新日期:2020-02-14
  • Saturable absorption properties and femtosecond mode-locking application of titanium trisulfide
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-12
    Wenjun Liu, Mengli Liu, Ximei Liu, Xiaoting Wang, Hao Teng, Ming Lei, Zhongming Wei, Zhiyi Wei

    Titanium trisulfide (TiS3) is regarded as a candidate material for optoelectronic devices and nano-transistors due to its photoresponse. However, its nonlinear optical response in a mode-locked laser is yet to be investigated. Here, the performance of TiS3 as a saturable absorber in a mode-locked laser is demonstrated. The generated mode-locked pulses achieve pulse duration as short as 147.72 fs at 1555 nm, which indicates that TiS3 as a potential functional material has applications in nanomaterial-related photonics.

    更新日期:2020-02-14
  • Suppression of dislocation-induced spiral hillocks in MOVPE-grown AlGaN on face-to-face annealed sputter-deposited AlN template
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-10
    Kenjiro Uesugi, Kanako Shojiki, Yuta Tezen, Yusuke Hayashi, Hideto Miyake

    AlGaN films were grown on face-to-face annealed sputter-deposited AlN/sapphire (FFA Sp-AlN) templates via metalorganic vapor phase epitaxy (MOVPE), and the growth behavior of the AlGaN films was investigated. The sapphire substrates with small off-cut exhibited poor surface flatness of AlGaN grown on the FFA Sp-AlN templates owing to the formation of large hillock structures. To understand the origin of these hillock structures, the crystallinity and surface morphology of conventional fully MOVPE-grown AlN/sapphire (MOVPE-AlN) templates and the FFA Sp-AlN template were comprehensively studied. The screw- and mixed-type threading dislocation density of the FFA Sp-AlN template was estimated to be approximately 1.8 × 106 cm−2, which was two orders of magnitude lower than that of the MOVPE-AlN template. Consequently, the uniquely observed growth of the hillock structures in the FFA Sp-AlN templates was attributed to their low density of screw- and mixed-type threading dislocations. The large surface off-cut sapphire substrates suppressed the hillock structures on the FFA Sp-AlN templates. The improvement in surface flatness resulted in better optical properties of multiple quantum wells grown on the AlGaN layer. These results demonstrate a promising method for achieving highly efficient and cost effective AlGaN based deep ultraviolet light-emitting diodes.

    更新日期:2020-02-14
  • Electrical characterization of GaN Schottky barrier diode at cryogenic temperatures
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-10
    Jiaxiang Chen, Min Zhu, Xing Lu, Xinbo Zou

    In this report, electrical characteristics of the Ni/GaN Schottky barrier diode grown on sapphire have been investigated in the range of 20 K–300 K, using current–voltage, capacitance–voltage, and deep level transient spectroscopy (DLTS). A unified forward current model, namely a modified thermionic emission diffusion model, has been developed to explain the forward characteristics, especially in the regime with a large ideality factor. Three leakage current mechanisms and their applicability boundaries have been identified for various bias conditions and temperature ranges: Frenkel–Poole emission for temperatures above 110 K; variable range hopping (VRH) for 20 K–110 K, but with a reverse bias less than 20 V; high-field VRH, in a similar form of Fowler–Nordheim tunneling, for cryogenic temperatures below 110 K, and relatively large bias (>25 V). Four trap levels with their energy separations from the conduction band edge of 0.100 ± 0.030 eV, 0.300 eV, 0.311 eV, and 0.362 eV have been tagged together with their capture cross sections and trap concentrations. The significantly reduced DLTS signal at 100 K suggested that traps practically became inactive at cryogenic temperatures, thus greatly suppressing the trap-assisted carrier hopping effects.

    更新日期:2020-02-14
  • Raman spectra of Si nanocrystals under high pressure: Metallization and solid state amorphization
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-11
    N. N. Ovsyuk, S. G. Lyapin

    We have observed and identified two crystalline peaks at ∼270 cm−1 and ∼400 cm−1 in nanocrystalline silicon during both compression and decompression. We attribute the first peak to the orthorhombic Imma phase (Si-XI) and the second one to the TO mode of the silicon metallic β-Sn phase (Si-II). Also, in the process of decompression, we observed a first-order-like transition from a highly coordinated metallic crystalline β-Sn phase to the normal tetrahedral LDA phase of the amorphous semiconductor. When the pressure is completely relieved, the powder regains its initial crystalline structure, which suggests that the amorphous phase has retained some structural features of the parent crystalline phase.

    更新日期:2020-02-14
  • Do all screw dislocations cause leakage in GaN-based devices?
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-12
    Jin Wang, Haifan You, Hui Guo, Junjun Xue, Guofeng Yang, Dunjun Chen, Bin Liu, Hai Lu, Rong Zhang, Youdou Zheng

    Screw dislocations are generally considered to be one of the main causes of GaN-based device leakage, but so far, nearly no reports have focused on the effects of open-core screw dislocations on device leakage currents experimentally. In this paper, we use a conductive atomic force microscope to characterize the electronic properties of threading dislocations (TDs) in the GaN layer. The full-core screw dislocations and mixed dislocations are found to provide conductive paths for device leakage currents. In terms of the contribution to device leakage currents, the edge and open-core screw dislocations have smaller effects than the full-core screw dislocations and mixed dislocations. We use isotropic linear elasticity theory and density functional theory calculations to model the core atomic structures of TDs and calculate the corresponding electronic structures. The results show that screw dislocations with full-core structures are found to introduce both deep and shallow energy states within the energy gap dispersedly, while the open-core screw dislocations and the most edge dislocations introduce only shallow energy states. The calculated electronic structures of each type of dislocation are systematically compared and correlated with experimental observations. Our findings demonstrate that full-core screw dislocations and mixed dislocations in the GaN layer have a far more detrimental impact on device leakage than edge and open-core screw dislocations.

    更新日期:2020-02-14
  • Integration of polycrystalline Ga2O3on diamond for thermal management
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-12
    Zhe Cheng, Virginia D. Wheeler, Tingyu Bai, Jingjing Shi, Marko J. Tadjer, Tatyana Feygelson, Karl D. Hobart, Mark S. Goorsky, Samuel Graham

    Gallium oxide (Ga2O3) has attracted great attention for electronic device applications due to its ultra-wide bandgap, high breakdown electric field, and large-area affordable substrates grown from the melt. However, its thermal conductivity is significantly lower than that of other wide bandgap semiconductors such as SiC, AlN, and GaN, which will impact its ability to be used in high power density applications. Thermal management in Ga2O3 electronics will be the key for device reliability, especially for high power and high frequency devices. Similar to the method of cooling GaN-based high electron mobility transistors by integrating it with high thermal conductivity diamond substrates, this work studies the possibility of heterogeneous integration of Ga2O3 with diamond for the thermal management of Ga2O3 devices. In this work, Ga2O3 was deposited onto single crystal diamond substrates by atomic layer deposition (ALD), and the thermal properties of ALD-Ga2O3 thin films and Ga2O3–diamond interfaces with different interface pretreatments were measured by Time-domain Thermoreflectance. We observed a very low thermal conductivity of these Ga2O3 thin films (about 1.5 W/m K) due to the extensive phonon grain boundary scattering resulting from the nanocrystalline nature of the Ga2O3 film. However, the measured thermal boundary conductance (TBC) of the Ga2O3–diamond interfaces is about ten times larger than that of the van der Waals bonded Ga2O3–diamond interfaces, which indicates the significant impact of interface bonding on TBC. Furthermore, the TBC of the Ga-rich and O-rich Ga2O3–diamond interfaces is about 20% smaller than that of the clean interface, indicating that interface chemistry affects the interfacial thermal transport. Overall, this study shows that a high TBC can be obtained from strong interfacial bonds across Ga2O3–diamond interfaces, providing a promising route to improving the heat dissipation from Ga2O3 devices with lateral architectures.

    更新日期:2020-02-14
  • Polarization fields in semipolar(202¯1¯)and(202¯1)InGaN light emitting diodes
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-12
    Stefan Freytag, Michael Winkler, Rüdiger Goldhahn, Tim Wernicke, Monir Rychetsky, Ingrid L. Koslow, Michael Kneissl, Duc V. Dinh, Brian Corbett, Peter J. Parbrook, Martin Feneberg

    InxGa1−xN/GaN multiple quantum well structures (x = 0.13 and 0.18) embedded into p–i–n diodes on ( 20 2 ¯ 1 ¯) and ( 20 2 ¯ 1) oriented GaN substrates were investigated by electroreflectance, photocurrent, and electroluminescence. Transition energies in absorption and emission experiments were measured as a function of the polarization orientation of light and applied bias voltage. The results were analyzed by a perturbation theoretical model to determine polarization fields. For the ( 20 2 ¯ 1 ¯) sample (x = 0.18), the flatband voltage is found at + 1   V corresponding to a polarization field of − 458   kV / cm. For the ( 20 2 ¯ 1 ) sample (x = 0.13), the polarization field is estimated to be ≈ + 330   kV / cm at flatband voltage higher than turn-on voltage of this light emitting diode.

    更新日期:2020-02-14
  • Mechanisms of GaN quantum dot formation during nitridation of Ga droplets
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-13
    H. Lu, C. Reese, S. Jeon, A. Sundar, Y. Fan, E. Rizzi, Y. Zhuo, L. Qi, R. S. Goldman

    We have examined the formation mechanisms of GaN quantum dots (QDs) via annealing of Ga droplets in a nitrogen flux. We consider the temperature- and substrate-dependence of the size distributions of droplets and QDs, as well as the relative roles of Ga/N diffusivity and GaN nucleation rates on QD formation. We report on two competing mechanisms mediated by Ga surface diffusion, namely, QD formation at or away from pre-existing Ga droplets. We discuss the relative roles of nucleation- and coarsening-dominant growth, as well as zincblende vs wurtzite polytype selection, on various substrates. These insights provide an opportunity for tailoring QD size distributions and polytype selection for a wide range of III-N semiconductor QDs.

    更新日期:2020-02-14
  • Sub-micrometer near-field focusing of spin waves in ultrathin YIG films
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-10
    B. Divinskiy, N. Thiery, L. Vila, O. Klein, N. Beaulieu, J. Ben Youssef, S. O. Demokritov, V. E. Demidov

    We experimentally demonstrate tight focusing of a spin wave beam excited in extended nanometer-thick films of yttrium iron garnet by a simple microscopic antenna functioning as a single-slit near-field lens. We show that the focal distance and the minimum transverse width of the focal spot can be controlled in a broad range by varying the frequency/wavelength of spin waves and the antenna geometry. The experimental data are in good agreement with the results of numerical simulations. Our findings provide a simple solution for the implementation of magnonic nanodevices requiring a local concentration of the spin-wave energy.

    更新日期:2020-02-14
  • Epitaxially grown Cu2Sb-type MnGaGe films with large perpendicular magnetic anisotropy
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-10
    Mingling Sun, Takahide Kubota, Keita Ito, Shigeki Takahashi, Yoshiyuki Hirayama, Yoshiaki Sonobe, Koki Takanashi

    A perpendicularly magnetized film is essentially important for spintronics applications, such as spin-transfer-torque type magnetoresistive random access memories, STT-MRAMs, and large perpendicular anisotropy energy, K u, with small saturation magnetization, M s, is required for reducing the critical current for the STT induced magnetization switching phenomenon. In this study, an equiatomic intermetallic compound, MnGaGe, was investigated in epitaxially grown film samples. Perpendicularly magnetized films with an M s value of about 260 emu/cm3 at 300 K were fabricated onto MgO (100) single crystal substrates. The maximum value of K u was 8.1 × 106 erg/cm3 at room temperature. Microstructure analysis using a scanning transmission electron microscope atomically revealed the Cu2Sb type crystal structure in the film sample. First principles calculations were also carried out to discuss the electronic structure. Using the analysis of the projected density of states for 3d-orbitals, the hybridization of the 3d-orbitals was suggested as a possible factor for promoting magnetocrystalline anisotropy. The MnGaGe film with the large K u and relatively small M s would be potentially used for spintronics applications.

    更新日期:2020-02-14
  • Controlling vertical magnetization shift by spin–orbit torque in ferromagnetic/antiferromagnetic/ferromagnetic heterostructure
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-11
    Z. P. Zhou, X. H. Liu, K. Y. Wang

    We report the control of the vertical magnetization shift (VMS) and exchange bias through spin–orbit torque (SOT) in Pt/Co/Ir25Mn75/Co heterostructure devices. The exchange bias accompanying with a large relative VMS of about 30% is observed after applying a single pulse of 40 mA in a perpendicular field of 2 kOe. Furthermore, the field-free SOT-induced variations of VMS and exchange bias are also observed, which would be related to the effective built-in out-of-plane field due to unequal upward and downward interfacial spin populations. The SOT-induced switched fraction of out-of-plane interfacial spins shows a linear dependence on relative VMS, indicating that the number of uncompensated pinned spins is proportional to the switched interfacial spins. Our finding offers a comprehensive understanding for electrically manipulating interfacial spins of antiferromagnetic materials.

    更新日期:2020-02-14
  • Broadband optical measurement of AC magnetic susceptibility of magnetite nanoparticles
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-13
    R. Soucaille, M. E. Sharifabad, N. D. Telling, R. J. Hicken

    Characterization of magnetic nanoparticles in solution is challenging due to the interplay between magnetic relaxation and agglomeration. The AC magnetic susceptibility of magnetite nanoparticles in water has been studied using magneto-optical methods in the frequency range of 10 Hz–250 kHz. The Faraday effect is detected simultaneously with changes in the fluid configuration. It is shown that the relative sensitivity to the magnetic and structural response can be adjusted by varying the wavelength, paving the way toward spatially resolved studies at the micro-scale.

    更新日期:2020-02-14
  • Modeling of demagnetization processes in permanent magnets measured in closed-circuit geometry
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-14
    J. Fliegans, O. Tosoni, N. M. Dempsey, G. Delette

    The hysteresis loops of nucleation-type magnets made of exchange-decoupled grains (i.e., sintered Nd–Fe–B magnets) reflect the discrete character of magnetization switching in such materials. Due to this discrete character, the experimental determination of coercivity depends on the measurement protocol. Finite element modeling allows us to investigate how the pattern of reversed grains develops during sample demagnetization performed under closed-circuit conditions, provided that the basic features of the hysteresigraph are known. Numerical modeling provides a quantitative understanding of the collective effects that are very pronounced in the closed-circuit configuration and shows how they affect both the slope of the demagnetizing curve and the sample coercivity. With a grain coercive field standard deviation adjusted to 0.1 T, it is numerically found that the difference in coercivity between closed- and open-circuit configurations is 40 kA/m, in good agreement with previous experimental data.

    更新日期:2020-02-14
  • Enhanced surface superconductivity in Ba(Fe0.95Co0.05)2As2
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-12
    Christopher T. Parzyck, Brendan D. Faeth, Gordon N. Tam, Gregory R. Stewart, Kyle M. Shen

    We present direct evidence for an enhanced superconducting Tc on the surface of cleaved single crystals of Ba( Fe 0.95 Co 0.05)2As2. Transport measurements performed on samples cleaved in ultra-high vacuum show a significantly enhanced superconducting transition when compared to equivalent measurements performed in air. Deviations from the bulk resistivity appear at 21 K, well above the 10 K bulk Tc of the underdoped compound. We demonstrate that the excess conductivity above the bulk Tc can be controllably suppressed by application of potassium ions on the cleaved surface, indicating that the enhanced superconductivity is strongly localized to the sample surface. Additionally, we find that the effects of the potassium surface dosing are strongly influenced by the presence of residual gas absorbates on the sample surface, which may prevent effective charge transfer from the potassium atoms to the FeAs plane. This further supports the conclusion that the effects of the dosing (and enhanced superconductivity) are localized within a few layers of the surface.

    更新日期:2020-02-14
  • Room-temperature deposition of ferroelectric HfO2-based films by the sputtering method
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-11
    Takanori Mimura, Takao Shimizu, Hiroshi Uchida, Hiroshi Funakubo

    Ferroelectricity has been demonstrated in epitaxial 7%Y-doped HfO2 (0.07YO1.5–0.93HfO2, YHO7) films grown by the RF magnetron sputtering method at room temperature without any subsequent annealing. The x-ray diffraction patterns of such films suggested that the decrease in RF power and in the partial oxygen pressure changes the crystal structures of the films from the monoclinic phase to the tetragonal/orthorhombic phase. Clear polarization-electric-field (P–E) hysteresis loops were observed for these epitaxial films with the tetragonal/orthorhombic phase. The obtained remanent polarization (Pr) and coercive field (Ec) values were 14.5 and 12.8 μC/cm2 and 2300 and 2200 kV/cm for the epitaxial films on (111) indium tin oxide (ITO)//(111) yttria-stabilized zirconia (YSZ) and (100)ITO//(100)YSZ substrates, respectively. Moreover, ferroelectricity was also observed in room-temperature-deposited polycrystalline YHO7 films prepared on Pt/TiOx/SiO2/(100)Si, crystallized ITO/soda glass, and amorphous ITO/polyethylene terephthalate substrates, namely, crystalline ferroelectric HfO2-based films were prepared at room temperature on various substrates, including organic flexible substrates, by using the RF magnetron sputtering method. The present results open a path to novel applications of ferroelectric HfO2-based films such as ferroelectric flexible memory.

    更新日期:2020-02-14
  • Reduced annealing temperature for ferroelectric HZO on InAs with enhanced polarization
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-14
    Anton E. O. Persson, Robin Athle, Pontus Littow, Karl-Magnus Persson, Johannes Svensson, Mattias Borg, Lars-Erik Wernersson

    Deposition, annealing, and integration of ferroelectric Hf x Zr 1 − x O 2 (HZO) thin films on the high-mobility semiconductor InAs using atomic layer deposition are investigated. Electrical characterization reveals that the HZO films on InAs exhibit an enhanced remanent polarization compared to films formed on a reference TiN substrate, exceeding 20   μ C / cm 2 even down to an annealing temperature of 370   °C. For device applications, the thermal processes required to form the ferroelectric HZO phase must not degrade the high-κ/InAs interface. We find by evaluation of the capacitance–voltage characteristics that the electrical properties of the high-κ/InAs are not significantly degraded by the annealing process, and high-resolution transmission electron microscopy verifies a maintained sharp high-κ/InAs interface.

    更新日期:2020-02-14
  • A polymer film with ultra-broadband optical gain characteristics
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-10
    Yuya Hara, Youtaro Higase, Marie Taguchi, Shun Takahashi, Fumio Sasaki, Kenichi Yamashita

    Miniaturization of the tunable laser equipment is an important factor for further development in various optoelectronic technologies. To realize the compact tunable laser devices, an optical gain medium having a broadband optical gain characteristic is required. In this study, we propose a promising strategy for preparing an optical gain film that exhibits a ∼250 nm gain bandwidth in the visible wavelength region. This film consists of a polymer matrix co-doped with organic luminescent molecules that form a complex of the excited state, i.e., exciplex. The exciplex state can co-exist with the monomolecular exciton state, leading to broadband (400–650 nm) optical gain with a large Stokes shift. Optically pumped lasing action is also possible when the film is combined with an optical resonator. This strategy would be useful for developing a compact tunable laser device without active medium replacement.

    更新日期:2020-02-14
  • Role of host excimer formation in the degradation of organic light-emitting devices
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-11
    Robert Newcomb, John S. Bangsund, Kyle W. Hershey, Dominea C. K. Rathwell, Hong-Yeop Na, Jeong-Hwan Jeon, Peter Trefonas, Russell J. Holmes

    Host-guest structures are used in most state-of-the-art organic light-emitting devices, with the host transporting charge and confining excitons on the guest. While the host often plays a critical role in achieving high efficiency and stability, predicting and understanding these effects is a persistent design challenge which slows the discovery of new active materials. Closely related host molecules, which differ only by several functional groups, often show drastically different degradation behavior. Here, we explore this observation for the archetypical carbazole hosts 4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) and 4,4′-bis(carbazole-9-yl)-2,2′-dimethylbiphenyl (CDBP). While devices based on these hosts show similar efficiencies, CDBP-based devices show a tenfold lower lifetime than CBP devices when paired with phosphorescent or fluorescent emitters. Using optically and electrically pumped degradation tests, mass spectrometry, compositional analysis, and low-temperature phosphorescence spectroscopy, the lifetimes of devices containing CDBP are shown to correlate with the formation of intermolecular triplet excimer states. These findings suggest that candidate host molecules should be screened for excimer formation as host excimers may aggravate device degradation and lower device stability.

    更新日期:2020-02-14
  • How short is the runaway electron flow in an air electrode gap?
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-10
    G. A. Mesyats, M. I. Yalandin, N. M. Zubarev, A. G. Sadykova, K. A. Sharypov, V. G. Shpak, S. A. Shunailov, M. R. Ulmaskulov, O. V. Zubareva, A. V. Kozyrev, N. S. Semeniuk

    We present and analyze characteristics of the runaway electron flow in a high-voltage (the voltage rise rate of up to 1.5 MV/ns) air-filled electrode gap with a strongly nonuniform electric field. It is demonstrated that such a flow contains a high-energy electron component of duration not more than 10 ps. According to numerical simulations, runaway electron generation/termination is governed by impact ionization of the gas near the cathode and switching on/off a critical (sufficient for electrons to run away) electric field at the boundary of the expanding cathode plasma. The corresponding characteristic time estimated to be 2–3 ps is defined by the ionization rate at a critical field.

    更新日期:2020-02-14
  • Large memcapacitance and memristance at Nb:SrTiO3/La0.5Sr0.5Mn0.5Co0.5O3-δtopotactic redox interface
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-10
    W. Román Acevedo, C. A. M. van den Bosch, M. H. Aguirre, C. Acha, A. Cavallaro, C. Ferreyra, M. J. Sánchez, L. Patrone, A. Aguadero, D. Rubi

    The possibility to develop neuromorphic computing devices able to mimic the extraordinary data processing capabilities of biological systems spurs the research on memristive systems. Memristors with additional functionalities such as robust memcapacitance can outperform standard devices in key aspects such as power consumption or miniaturization possibilities. In this work, we demonstrate a large memcapacitive response of a perovskite memristive interface, using the topotactic redox ability of La0.5Sr0.5Mn0.5Co0.5O3-δ (LSMCO, 0 ≤ δ ≤ 0.62). We demonstrate that the multi-mem behavior originates at the switchable n-p diode formed at the Nb:SrTiO3/LSMCO interface. We found for our Nb:SrTiO3/LSMCO/Pt devices a memcapacitive effect CHIGH/CLOW ∼ 100 at 150 kHz. The proof-of-concept interface reported here opens a promising venue to use topotactic redox materials for disruptive nanoelectronics, with straightforward applications in neuromorphic computing technology.

    更新日期:2020-02-14
  • Resistive switching behaviors and mechanisms of HfS2film memory devices studied by experiments and density functional theory calculations
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-11
    Naifeng Li, Yue Wang, Haifeng Sun, Junjie Hu, Maoyuan Zheng, Sihao Ye, Qi Wang, Yingtao Li, Deyan He, Jiatai Wang, Guangan Zhang, Jing Qi

    Energy band diagrams are widely utilized to explain the switching mechanism of resistance random access memory (RRAM). However, a precise and quantitative band theory is still lacking in this field. Although HfS2 has good applications in many fields because of its good electrical and optical properties, its applications in RRAM have seldom been reported. In this work, the exfoliated nanosheets of HfS2 were utilized to fabricate memory devices with a structure of Pt/Al/HfS2/p+-Si, which show typical bipolar resistive switching behavior with high switching voltage and a small ratio of high and low resistive states (R-ratio). According to the density functional theory (DFT) calculation results of energy band diagrams, instead of conductive filament formation in other resistive switching materials, the doping of sulfur vacancy (VS) of 3.8% is already enough to change the whole HfS2 layer from the semiconductor to the metal. The transition is caused by the change in the VS doping concentration from low to high, which is the result of the generation and movement of VS under an electric field. The DFT also calculated that HfS2 devices utilizing Indium Tin Oxide as the bottom electrode can show bipolar resistive switching behavior with lower switching voltage and a higher R-ratio than those utilizing p+-Si, which is confirmed by the experimental results. The DFT calculation can be utilized for both explaining the switching mechanism and designing the device structure to optimize the switching characteristics.

    更新日期:2020-02-14
  • Impact of the thermal environment on the analog temporal response of HfOx-based neuromorphic devices
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-11
    Matthew P. West, Pradip Basnet, Darshan G. Pahinkar, Robert H. Montgomery, Samuel Graham, Eric M. Vogel

    Filamentary adaptive oxide devices based on HfOx are a promising technology for neuromorphic computing applications. The resistance of these devices depends on the concentration of oxygen vacancies in the filament region. A local temperature rise from joule heating plays a significant role in the movement of oxygen ions, making thermal management crucial to reliable performance. In this work, the role of the substrate thermal conductivity on the analog performance was investigated at biologically realistic pulse widths. Au/Ti/HfOx/Au adaptive oxide devices were fabricated on substrates with two orders of magnitude difference in thermal conductivity. A lower thermal conductivity substrate dissipates heat more slowly, resulting in a large initial change in resistance from a single operation pulse, which is detrimental to the desired analog behavior. The results were validated by a COMSOL Multiphysics® model that models the flow of heat in both samples.

    更新日期:2020-02-14
  • Defect proliferation in CsPbBr3crystal induced by ion migration
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-12
    Bin-Bin Zhang, Fangbao Wang, Hongjian Zhang, Bao Xiao, Qihao Sun, Jun Guo, Ahmed Ben Hafsia, Aihui Shao, Yadong Xu, Jian Zhou

    Ion migration in halide perovskite materials usually brings an intractable problem in the working stability of solar cells and photoelectrical detectors. The mechanism of ion migration and its impact on physical properties are still open questions. In this work, the ion migration behavior in solution-grown CsPbBr3 crystals was observed by the hysteresis in current–voltage curves and the temperature dependent reversed current–time measurements. Defect proliferation phenomena (new defects of [VCs]− and [PbBr]2+) originating from ion migration were verified by thermally stimulated current spectroscopy. Our results also give evidence that Cs+ ions also participate in the process of ion migration except the widely considered Br− ions. Furthermore, the photoelectric properties of the CsPbBr3 device were found to be seriously deteriorated after the ion migration. Our work demonstrates the strong correlation between the ion migration and physical properties in halide perovskites.

    更新日期:2020-02-14
  • Anomalous magnetorheological effect in unstructured magnetoisotropic magnetoactive elastomers
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-10
    V. M. Kalita, Yu. I. Dzhezherya, G. G. Levchenko

    It was found that the anomalous magnetorheological effect observed in initially unstructured magnetoisotropic magnetoactive elastomers, i.e., the growth of their effective shear modulus in an external magnetic field by almost two orders of magnitude, is a result of the uniaxial magnetic anisotropy induced by the magnetic field. An additional magnetoelastic contribution to the mechanical stress created by the induced magnetic anisotropy counteracts the shear and increases the effective shear modulus of the magnetoactive elastomer when the latter is magnetized. Numerical estimates made for the magnitude of magnetorheological effect are in good agreement with known experimental data.

    更新日期:2020-02-14
  • Resemblance between motile and magnetically actuated sperm cells
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-10
    Islam S. M. Khalil, Veronika Magdanz, Juliane Simmchen, Anke Klingner, Sarthak Misra

    The active flagellum propels a motile sperm cell by traveling bending waves. Here, we demonstrate that non-motile cells have the capacity to be wirelessly actuated by external magnetic fields and reveal insights into their propulsion characteristics. Partial coating of the sperm head with nanoparticle aggregates is achieved by electrostatic-based self-assembly. The coating enables propagation of helical traveling waves along the passive flagellum under the action of a periodic magnetic field. We compare the waveforms of active and passive flagellated motion and show noticeable asymmetry in the case of magnetically actuated cells, leading to lower linearity ( LIN = VSL / VCL) of the taken pathway. The average curvature of the flagellar beat cycle is 10.4 ± 8.1 rad mm−1 ( mean   ±   s . d .) for an active flagellum, whereas the curvature of a passive flagellum exhibits a linear increase (37.4 ± 18.1 rad mm−1) and decreases toward the distal end. We also show that the maximum amplitude of the bending wave occurs at the distal end of the active flagellum and at the middle of the passive flagellum. Our experiments also show the ability of the actuating field to control the rate of progression of the bending waves along the passive flagellum to match that of motile cells.

    更新日期:2020-02-14
  • A dual-effect solution for broadband piezoelectric energy harvesting
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-10
    Bao Zhao, Jiahua Wang, Junrui Liang, Wei-Hsin Liao

    Literature has shown that the bandwidth of a piezoelectric energy harvesting (PEH) system can be effectively broadened by introducing nonlinear mechanical dynamics. However, when connected to a practical PEH interface circuit for ac-to-dc conversion, the bandwidth improvement will be impaired because of the electrically induced damping effect. On the other hand, there is another bandwidth broadening solution, in which the resonant frequency is tunable to some extent by carrying out the phase-variable synchronized switch (PVSS) control in practical interface circuits. This Letter reports a synergistic design by combining these two mechanisms toward a dual-effect broadband PEH solution. The theoretical model of this nonlinear and electromechanically coupled system is derived. The experimental result shows that, by introducing the PVSS control to a PEH system, which is composed of a monostable piezoelectric cantilever beam and a practical synchronized switch interface circuit, its bandwidth can be broadened by 18.7%. This dual-effect solution incorporates two bandwidth broadening mechanisms toward practical broadband PEH systems.

    更新日期:2020-02-14
  • Energy-tunable photon-enhanced thermal tunneling electrons for intrinsic adaptive full spectrum solar energy conversion
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-10
    Yicong Chen, Shaozhi Deng, Ningsheng Xu, Jun Chen

    Considering that the actual terrestrial solar irradiance is dynamically changing with the atmosphere, the overall efficiencies of most current solar cells are much lower than the reported static values that are based on a standard solar spectrum. The realizations of solar cells, which can maintain high efficiency under variable solar irradiance, are necessary for further improvement of solar energy conversion. In this work, a metal-insulator-semiconductor (MIS) structure based photon-enhanced thermionic energy converter (PETEC) has been proposed for intrinsic adaptive full spectrum solar energy conversion. The basic idea is to form the thermionic electron with desired energy by the photon-enhanced thermal tunneling process. Investigations on its fundamental performance indicate that the MIS-PETEC can have an overall output energy improvement of at least 0.8%, 2.5%, and 3% in typical sunny, rainy, and cloudy days compared to the normal PETEC. Additionally, the total output energy over a year can be improved by at least 3%. These results offer an alternate technique for intrinsic adaptive full spectrum solar energy conversion, which is helpful for the development of next generation high performance solar cells.

    更新日期:2020-02-14
  • Exploiting the advantages of the centrifugal softening effect in rotational impact energy harvesting
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-10
    Shitong Fang, Suo Wang, Shengxi Zhou, Zhichun Yang, Wei-Hsin Liao

    This Letter presents a rotational impact energy harvester by utilizing the centrifugal softening effect of an inverted driving beam in improving the energy harvesting performance of two piezoelectric beams at low rotational frequencies. By our proposed structure, the static divergence of the inverted driving beam in the deflected mode can not only be avoided but also be utilized. Numerical and experimental results show that the centrifugal softening effect can amplify the relative motion between the driving and generating beams and increase the impact force, which in turn improves the output power significantly. The maximum output power of the harvester is increased by 212.5%, 258.7%, and 682.8% for the impact gaps of 1.07 mm, 1.43 mm, and 2.14 mm, respectively. Moreover, the inverted driving beam can be prevented from continuously deflecting by introducing large impact stiffness at the contact instant.

    更新日期:2020-02-14
  • Magnetocaloric effect in Ni–Fe–Mn–Sn microwires with nano-sized γ precipitates
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-11
    Hehe Zhang, Xuexi Zhang, Mingfang Qian, Limeng Yin, Longsha Wei, Dawei Xing, Jianfei Sun, Lin Geng

    Ni45.6Fe3.6Mn38.4Sn12.4 microwires, with nanoscale γ-phase precipitates that enhance the magnetocaloric effects (MCEs) and mechanical properties, were prepared by a melt-extraction technique and subsequent high temperature annealing. The atomic ordering degree significantly increased after annealing, leading to a considerable increment in the structural entropy change (ΔStr) from 4.5 J/kg·K in the as-extracted microwire to 26.6 J/kg·K in the annealed one, and the magnetization difference (ΔM) from 35 A·m2/kg to 51 A·m2/kg under a magnetic field of 5.0 T. Consequently, a positive magnetic entropy change (ΔSM) peak of 15.2 J/kg·K with working temperature span (ΔTFWHM) of 12 K for the first-order martensite transformation followed by a negative ΔSM peak of −4.3 J/kg·K with ΔTFWHM = 50 K for the second-order magnetic transition under μ0ΔH = 5.0 T was achieved. By employing both magnetizing and demagnetizing processes for magnetic cooling, the two successive inverse and conventional MCEs in Ni–Fe–Mn–Sn microwires may show potential applications for micro-devices and systems.

    更新日期:2020-02-14
  • Velocity-amplified monostable dual-charged electret dome energy harvester using low-speed finger tapping
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-12
    Yue Feng, Zilong Zhou, Dongxiao Fu, Wei Ren

    Power generation from linear finger-tapping-based electrostatic energy harvesting (FTEEH) devices is hindered by the slow capacitance variation under low-speed finger-tapping (FT) motion. Herein, a velocity amplification mechanism is proposed, which exploits the snap-through behavior of a dual-charged electret monostable dome structure and thus greatly enhances the power generation of FTEEH devices from slow FT motion. The kinetic energy and velocity amplification during the buckling event were effectively predicted for various specimens using the modified Föppl–von Kármán equations and Hamilton's principle. A high degree of dynamic velocity amplification was demonstrated both theoretically and experimentally and quantified with respect to the velocity gain and power gain. Specifically, the velocity of the capacitance variation of the designed FTEEH device, driven by a slow FT motion at 2.7 cm/s, was substantially increased to 18.5 cm/s, affording a high velocity gain of 6.9 and a correspondingly large power gain of 6.8. The proposed velocity-amplified nonlinear FTEEH device was compared with recently developed linear FTEEH devices that do not utilize this velocity amplification mechanism and found to yield a large pulse width of 90.0 ms (full width) and a high volumetric power density of 1015.7 μW/cm3.

    更新日期:2020-02-14
  • Long optical coherence times of shallow-implanted, negatively charged silicon vacancy centers in diamond
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-10
    Johannes Lang, Stefan Häußler, Jens Fuhrmann, Richard Waltrich, Sunny Laddha, Jochen Scharpf, Alexander Kubanek, Boris Naydenov, Fedor Jelezko

    The creation of single, negatively charged silicon vacancy ( SiV −) centers in well-defined diamond layers close to the host surface is a crucial step for the development of diamond-based quantum optic devices with many applications in nanophotonics, quantum sensing, or quantum information science. Here, we report on the creation of shallow (10 nm below the surface), single SiV − centers in diamond using low energy Si + ion implantation with subsequent high temperature annealing at 1500 °C. We show transition linewidths down to 99 MHz and narrow inhomogeneous distributions. Furthermore, we achieved a reduction of homogeneous linewidths by a factor of 2 after removing subsurface damage using oxygen plasma processing. These results not only give insights into the formation process of SiV − centers but also indicate a favorable processing method to fabricate shallow single quantum emitters in diamond perfectly suited for coupling to nanostructures on the diamond surface.

    更新日期:2020-02-14
  • Time-resolved Langmuir probe diagnostics of a bipolar high power impulse magnetron sputtering discharge
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-10
    Rainer Hippler, Martin Cada, Zdenek Hubicka

    High power impulse magnetron sputtering (HiPIMS) of a cobalt cathode in argon gas was investigated by time-resolved electrical (Langmuir) probe diagnostics and by time-integrated energy-resolved mass spectrometry. The HiPIMS discharge was operated with a bipolar pulsed power supply, providing a large negative voltage with a typical pulse width of 100 μs followed by a long positive pulse with a pulse width of about 310 μs. The time-resolved Langmuir probe results yield a small negative plasma potential in the negative pulse regime and a large positive floating potential and plasma potential in the positive pulse regime. The electron density is significantly reduced during the positive pulse regime. The probe results are supported by ion energy measurements.

    更新日期:2020-02-14
  • Direct observation of ultrafast electrons generated by high-intensity laser-matter interaction
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-11
    M. Galletti, F. G. Bisesto, M. P. Anania, M. Ferrario, R. Pompili, A. Poyé, V. Tikhonchuk, A. Zigler

    High intensity ultrashort laser pulses interacting with thin solid targets are able to produce energetic protons and ions by means of extremely large accelerating fields, generated by escaping electrons. The characterization of such electrons is thus a key factor for the understanding of the accelerating potential temporal evolution. Here, we present temporally resolved measurements of the ultrafast escaping electron component. The charge, electric field, and temporal duration of the emitted ultrafast electron beams are determined using temporal diagnostics with a 100 fs temporal resolution. Experimental evidence of scaling laws for the ultrafast electron beam parameters with respect to the incident laser pulse energy is retrieved and compared with theoretical models, showing an excellent agreement.

    更新日期:2020-02-14
  • Solid-state cooling by stress: A perspective
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-03
    Lluís Mañosa, Antoni Planes

    Materials with both giant and colossal mechanocaloric effects together with giant electrocaloric and magnetocaloric materials are expected to replace harmful fluids in more efficient and environmentally friendly refrigeration and heat pumping technologies. While mechanocaloric materials have only received attention in the last decade, they have already shown better caloric performances than their magnetic and polar counterparts. In particular, the recent discovery of colossal barocaloric and elastocaloric materials opens up bright perspectives for this class of materials. We envisage great promise in the use of mechanocaloric materials for future energy applications.

    更新日期:2020-02-07
  • Terahertz electron paramagnetic resonance spectroscopy using continuous-wave frequency-tunable photomixers based on photoconductive antennae
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-03
    Eiji Ohmichi, Tatsuya Fujimoto, Keisuke Minato, Hitoshi Ohta

    In this article, we demonstrate terahertz (THz) electron paramagnetic resonance (EPR) spectroscopy using a frequency-tunable photomixing source/detection system to investigate the electronic structure of solid-state samples in a microscopic manner. Fiber-coupled photoconductive antennae were used to generate and detect continuous THz waves obtained as the beat signals from two laser beams with different wavelengths. We will show some examples of EPR spectroscopy obtained by multi-frequency EPR measurements at room temperature. The technique reported here has several advantages, including high spectral resolution, wide frequency-range operation, high dynamic range, owing to the continuous-wave (cw) and tunable generation/detection of THz waves.

    更新日期:2020-02-07
  • Electric-field-induced phase transition in 2D layered perovskite (BA)2PbI4microplate crystals
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-03
    Jiaqi Wang, Hongzhi Shen, Junze Li, Wancai Li, Chen Fang, Jiaqi Ma, Xue Cheng, Dehui Li

    Two-dimensional (2D) lead halide perovskite materials are solution-processable semiconductor materials, which would find promising applications in optoelectronic devices. The fundamental understanding of the structural phase transition in two-dimensional perovskites is of great importance for fully exploiting their potential applications in electronic and optoelectronic devices. Here, we report on how the external electric field affects the structural phase transition in 2D perovskite (BA)2PbI4 microplates via temperature-dependent photoluminescence spectroscopy. A high-temperature phase and a low-temperature phase can coexist in a wider range of temperatures. The external electric field would enhance the phase transition to the dominant phase depending on the surrounding temperature. This field-induced phase transition might be ascribed to the release of strain in the perovskite microplate induced by the applied electric field, leading to the change in the structural phase transition temperature. Our findings are of great significance not only to the fundamental understanding of phase transition but also to the design and optimization of two-dimensional perovskite based electronic and optoelectronic devices.

    更新日期:2020-02-07
  • Ultra-compact visible light depolarizer based on dielectric metasurface
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-04
    Yilin Wang, Wenqi Zhu, Cheng Zhang, Qingbin Fan, Lu Chen, Henri Lezec, Amit Agrawal, Ting Xu

    With rapid development toward shrinking the size of traditional photonic systems such as cameras, spectrometers, displays, and illumination systems, there is an urgent need for high-performance and ultra-compact functional optical elements. Metasurfaces, consisting of an array of subwavelength nanoscatterers with spatially varying geometries, have shown remarkable performance as ultrathin multifunctional optical elements. Here, based on an all-dielectric metasurface, we propose and experimentally demonstrate a spatial domain optical depolarizer capable of efficiently depolarizing linearly polarized light in the visible spectral band from 450 nm to 670 nm, with a degree of polarization of less than 10%. Remarkably, it is capable of depolarizing the light beam with a diameter down to several micrometers, about two orders of magnitude smaller than commercial liquid-crystal-based depolarizers. We envision the metasurface depolarizer to find its applications in the next generation ultra-compact grating spectrometers and illumination systems.

    更新日期:2020-02-07
  • Smart optically induced nonlinear photonic crystals for frequency conversion and control
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-04
    Dawei Liu, Shan Liu, Leszek Mateusz Mazur, Bingxia Wang, Peixiang Lu, Wieslaw Krolikowski, Yan Sheng

    We extend the functionality of nonlinear photonic crystals by fabricating a structure combining a few individual optical transformations in second harmonic generation. In particular, we employed all-optical spontaneous polarization reversal in ferroelectrics to create nonlinear holograms with curved fork-like spatial modulation of nonlinearity for the generation of second harmonic optical vortices. The curved fork-like structure represents a combination of a standard vortex harmonic generator (fork structure) and an axicon, thereby allowing us to realize the second harmonic “perfect vortices” whose diameter remains constant independently of their topological charge.

    更新日期:2020-02-07
  • Polarization controlled fine structure of diffraction spots from an optically induced grating
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-04
    Joachim Jelken, Carsten Henkel, Svetlana Santer

    We report on the remote control of the fine structure of a diffraction spot from optically induced dual gratings within a photosensitive polymer film. The material contains azobenzene in the polymer side chains and develops a surface relief under two-beam holographic irradiation. The diffraction of a polarized probe beam is sensitive to the orientation of the azobenzene groups forming a permanently stored birefringence grating within the film. We demonstrate that the fine structure of the probe diffraction spot switches from a Gaussian to a hollow or a hollow to a “Saturn”-like structure by a change in polarization. This makes it potentially useful in photonic devices because the beam shape can be easily inverted by an external stimulus.

    更新日期:2020-02-07
  • Quantifying the pressure-dependence of work of adhesion in silicon–diamond contacts
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-04
    Rimei Chen, Sai Bharadwaj Vishnubhotla, Subarna R. Khanal, Tevis D. B. Jacobs, Ashlie Martini

    Continuum mechanics models for contacting surfaces assume a constant interfacial energy, or work of adhesion, between materials. Recent studies have challenged this assumption, instead demonstrating that stress-dependent chemical reactions across the interface modify the work of adhesion. Here, we perform 77 adhesion tests on diamond–silicon contacts using in situ transmission electron microscopy and atomistic simulations to quantify how the adhesion changes as a function of applied pressure. The results show a sevenfold increase in the work of adhesion (from approximately 1 to 7 J/m2) with an increase in the mean applied pressure from 0 to 11 GPa, where the most significant increase occurs above 5 GPa. We rule out alternative explanations for the changing work of adhesion, such as electron-beam artifacts, bulk shape change by inelastic deformation, and time-dependent processes such as creep. Therefore, these results confirm the presence of stress-driven chemical reactions in the contact and quantify the resulting change in the adhesion of these materials with applied pressure.

    更新日期:2020-02-07
  • LaScO3/SrTiO3: A conducting polar heterointerface of two3dband insulating perovskites
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-04
    Sumit Kumar, Jyoti Kaswan, Biswarup Satpati, A. K. Shukla, Bhasker Gahtori, J. J. Pulikkotil, Anjana Dogra

    This work reports a quasi-two-dimensional electron gas (q-2DEG) system at the interface of two wideband-gap insulators, (TiO2-terminated) SrTiO3 and LaScO3, with a minimum thickness of 4-unit cell (uc). The highly crystalline and abrupt heterointerface is confirmed with high-resolution electron microscopy. The mixed Ti4+ and Ti3+ valence states (for 4 uc of LaScO3) obtained from the x-ray photoelectron spectroscopy study suggest an intrinsic electronic reconstruction at the interface, leading to a metallic nature. This origin is well supported by density functional theory calculations that reveal an emergence of 3.3 states/eV/spin at the Fermi level for 4 uc in accordance with the polar catastrophe model. The study offers one more perovskite heterostructure, like LaAlO3/SrTiO3, for unraveling the q-2DEG phenomena toward a clear mechanism and futuristic applications.

    更新日期:2020-02-07
  • Sensing surface lattice strain with Kondo resonance of single Co adatom
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-05
    Kota Iwata, Toshio Miyamachi, Emi Minamitani, Fumio Komori

    Detection of lattice strain is crucial for various studies in a nanometer scale because it largely modifies the local electronic states and thus various physical and chemical properties. Here, we demonstrate that the Kondo effect in a single magnetic atom on a metal surface can be a quantum sensor for the local lattice strain. Using low-temperature scanning tunneling spectroscopy, we measured the Kondo resonance in a Co adatom on partially N-adsorbed Cu(001) surfaces, which consist of nanoislands of the Cu 2N monolayer and the clean Cu(001) surface compressed by the surrounding Cu 2N nanoislands. The observed Kondo temperature at the compressed clean surface depends on the area size of the surface, i . e ., the strength of the local lattice strain. This behavior is attributed to the change in the distance between the Co adatom and Cu surface due to the surface lattice strain, which is supported by our density functional calculations. These results provide a way to detect the local strain on the sub-angstrom scale by using the sensitivity of quantum many-body effects.

    更新日期:2020-02-07
  • The effect of rare-earth substitution on the Debye temperature of inorganic phosphors
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-03
    Shruti Hariyani, Anna C. Duke, Thorben Krauskopf, Wolfgang G. Zeier, Jakoah Brgoch

    In the quest to predict photoluminescent efficiency in rare-earth substituted inorganic phosphors, research has shown that materials consisting of ordered, rigid crystal structures tend to possess the highest photoluminescent quantum yields. A compound's Debye temperature (ΘD), which can be calculated using ab initio calculations, is an ideal proxy for quantitatively comparing structural rigidity among different inorganic compounds, allowing potentially efficient phosphors to be selected from large crystal structure databases. However, the high computational cost of these calculations limits estimating ΘD for unsubstituted host crystal structures only. It is assumed that the low substitution concentration of the rare-earth luminescent center does not significantly influence a material's Debye temperature. This work evaluates the validity of this approximation by examining the effect of luminescent center substitution on a host structure's ΘD. Two well-known phosphors, (Y1–xCex)3Al5O12 (x = 0 − 0.05) and Ba1-δEuδMgAl10O17 (δ = 0 − 0.15), were synthesized with varying rare-earth concentrations, while ΘD was computationally estimated and then determined by ultrasonic pulse-echo speed-of-sound and low-temperature heat capacity measurements. The ensuing results provide key implications for using ΘD as a proxy for structural rigidity in substituted inorganic compounds.

    更新日期:2020-02-07
  • Correlation transports atp-/n-types in electron metastable perovskite family of rare-earth nickelates
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-03
    Jinhao Chen, Fengbo Yan, Haiyang Hu, Jiaou Wang, Yong Jiang, Nuofu Chen, Jikun Chen

    It is important to achieve both donor and acceptor doping for correlated oxide semiconductors to cater for elementary device constructions, e.g., establishing a p-n junction and a thermal couple or a thermoelectric π-joint in correlated electronics. The perovskite family of rare-earth nickelates (ReNiO3) exhibits correlated transportation characters (e.g., metal to insulator transitions and thermistor transportations) dominated by electron conductions, as indicated by their negative thermopower for n-type materials. Herein, we demonstrate the presence of positive magnitude of thermopower as achieved in GdNiO3 single crystalline thin films, indicating a hole dominated transportation for p-type correlated semiconductors. Probing the Ni-L edge of GdNiO3 via near edge x-ray absorption fine structure indicates its distinguished intermediate acceptor energy states that are more easily occupied by the thermal excited valence band electrons. The hole-dominated transportation for GdNiO3 was further confirmed by its increased localization length and opposite sign in magnetoresistance, which are derived from the general tendency of ReNiO3, owing to the larger effective mass of holes compared to electrons. The discovery of p-types in GdNiO3 will further promote establishing ReNiO3-based elementary semiconductive devices in the field of correlated electronics.

    更新日期:2020-02-07
  • Buckling-induced reconfigurability in underwater acoustic scatterers
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-04
    Stephanie G. Konarski, Christina J. Naify, Charles A. Rohde

    In this work, we explore switchable acoustic scattering from underwater particles via instability-induced internal pattern transformation in the 50 kHz–80 kHz frequency range. Our wavelength scale aqueous scatterer is designed based on modeling using the finite element method for a square lattice of air-filled voids within a shape memory polymer and is directly 3D printed. The structure undergoes a buckling transformation when subjected to an external deformation while simultaneously being heated. Through computational and experimental results, we demonstrate that the deformation state change leads to programmable acoustic transparency, or opacity, for the scattering particle. Underwater propagation experiments resolved in the near field illustrate that the switchable acoustic characteristics are frozen in the structure with rapid cooling after compression, and the initial acoustic state can be automatically recovered through reheating.

    更新日期:2020-02-07
  • Intense green elastico-mechanoluminescence from KZn(PO3)3:Tb3+
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-04
    Huimin Chen, Li Wu, Tongqing Sun, Rui Dong, Zhongzhong Zheng, Yongfa Kong, Yi Zhang, Jingjun Xu

    Mechanoluminescent (ML) materials have important applications in the field of mechanical detection. To accelerate the application of ML materials, the expansion of ML materials is essential. Herein, a green ML phosphor KZn(PO3)3:Tb3+ was prepared via a high temperature solid state reaction method. Tb3+ ions occupy the Zn2+ sites, and the defects are generated due to charge compensation, which can serve as the trap center for capturing carriers and are verified by thermoluminescence (TL). The flexible structure framework of KZn(PO3)3 not only provides enough space for defects in the lattice, but also generates great strain energy when applied with forces. KZn(PO3)3:Tb3+ shows a sensitive reaction to different mechanical forces, which indicates it a potential ML material for use in intelligent sensors.

    更新日期:2020-02-07
  • Demonstration ofn-type behavior in catalyst-free Si-doped GaAs nanowires grown by molecular beam epitaxy
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-04
    Daniel Ruhstorfer, Simon Mejia, Manfred Ramsteiner, Markus Döblinger, Hubert Riedl, Jonathan J. Finley, Gregor Koblmüller

    The realization of n-type conduction in directly bottom-up grown Si-doped GaAs nanowires (NWs) by molecular beam epitaxy has remained a long-standing challenge. Unlike the commonly employed vapor–liquid−solid growth, where the amphoteric nature of Si dopants induces p-type conduction, we report a completely catalyst-free, selective area molecular beam epitaxial growth that establishes n-type behavior under Si doping. The vapor–solid selective area growth on prepatterned Si (111) substrates is enabled by an important in situ substrate pretreatment to create an As-terminated 1 × 1-Si(111) substrate necessary for the growth of [111]-oriented GaAs:Si NWs with a large aspect ratio and high yield. Correlated resonant Raman scattering and single-NW micro-photoluminescence (μPL) experiments confirm the n-type nature of the Si-doped GaAs NWs evidenced by a dominant SiGa local vibrational Raman mode, a distinct band filling effect (up to > 10 meV) along with increased PL peak broadening upon increased Si concentration. Excessive Si doping is further found to induce some auto-compensation evidenced by red-shifted PL and the appearance of minor SiAs and SiGa–SiAs pair-like local vibrational Raman modes. Employing excitation power dependent μPL, we further discern signatures in below-gap defect luminescence (∼1.3–1.45 eV) arising from structural defects and Si dopant-point defect complexes.

    更新日期:2020-02-07
  • Investigation of the electronic structure of amorphous SnO film using x-ray absorption spectroscopy
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-04
    Ahmed Yousef Mohamed, Seung Yeon Lee, Seung Jun Lee, Cheol Seong Hwang, Deok-Yong Cho

    The electronic structure of an amorphous SnO (a-SnO) thin film was examined by using spectroscopic methods including tender and soft x-ray absorption spectroscopies (XAS) and spectroscopic ellipsometry (SE). XAS at the Sn L1−, L3−, and O K-edges revealed that in a-SnO, the Sn 5px/y orbital states, which comprise the conduction band minimum (CBM), are broadened significantly compared to the case of crystalline SnO, whereas the hybridized Sn 5spz-O 2p states above the CBM are persistent. A lowering of the 5px/y states at the CBM by −0.4 eV and a reduction of the indirect bandgap were also observed. These orbital-dependent evolutions upon amorphization were caused by weakened interlayer couplings in the disordered quasi-2-dimensional semiconductor. However, the functionality of a-SnO as a p-type semiconductor would not be degraded significantly because the isotropic Sn 5s orbital states dominate in the valence band states.

    更新日期:2020-02-07
  • A unified mid-gap defect model for amorphous GeTe phase change material
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-05
    Huanglong Li, John Robertson

    Understanding the nature of the mid-gap defects in amorphous chalcogenide alloy-based phase change materials is crucial to guide the development of multi-level memories, selectors, and neuromorphic devices. Currently, however, the knowledge of the mid-gap defects is fragmented and the known structural and electrical characteristics of the defects cannot be unified in a single atomic model. In this work, a Ge–Ge chain defect model is built by artificial bond rearrangement in an amorphous GeTe network generated by atomic distortion, revealing a possible valence alternation mechanism in its formation. Atoms in the Ge–Ge chain are found to have a crystalline-like environment. The chain is found to have a negative-U property and to introduce mid-gap states. This model unifies the main known properties of the mid-gap defects in amorphous chalcogenide alloy-based phase change materials.

    更新日期:2020-02-07
  • Enhanced thermally aided memory performance using few-layerReS2transistors
    Appl. Phys. Lett. (IF 3.521) Pub Date : 2020-02-06
    Natasha Goyal, David M. A. Mackenzie, Vishal Panchal, Himani Jawa, Olga Kazakova, Dirch Hjorth Petersen, Saurabh Lodha

    Thermally varying hysteretic gate operation in few-layer ReS 2 and MoS 2 back gate field effect transistors (FETs) is studied and compared for memory applications. Clockwise hysteresis at room temperature and anti-clockwise hysteresis at higher temperature (373 K for ReS 2 and 400 K for MoS 2) are accompanied by step-like jumps in transfer curves for both forward and reverse voltage sweeps. Hence, a step-like conductance (STC) crossover hysteresis between the transfer curves for the two sweeps is observed at high temperature. Furthermore, memory parameters such as the RESET-to-WRITE window and READ window are defined and compared for clockwise hysteresis at low temperature and STC-type hysteresis at high temperature, showing better memory performance for ReS 2 FETs as compared to MoS 2 FETs. Smaller operating temperature and voltage along with larger READ and RESET-to-WRITE windows make ReS 2 FETs a better choice for thermally aided memory applications. Finally, temperature dependent Kelvin probe force microscopy measurements show decreasing (constant) surface potential with increasing temperature for ReS 2 ( MoS 2). This indicates less effective intrinsic trapping at high temperature in ReS 2, leading to earlier occurrence of STC-type hysteresis in ReS 2 FETs as compared to MoS 2 FETs with increasing temperature.

    更新日期:2020-02-07
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