Applied Physics Letters

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Temperature-dependent nonmonotonous evolution of excitonic blue luminescence and Stokes shift in chlorine-based organometallic halide perovskite film
Volume 116, Issue 7, February 2020. The chlorine-based organometallic halide perovskite (Cl-OHP) film with a (001)-preferred orientation and good crystallization has been synthesized by a hybrid sequential deposition process. The photoluminescence and absorption spectra of the Cl-OHP film in the blue light region have been investigated at operating temperatures ranging from 10 to 350 K. The Cl-OHP film shows a strong exciton-related emission of which the exciton binding energies at low temperature and high temperature are 136 meV and 41 meV, respectively. It is found that the blueshift from excitonic luminescence is initially observed at temperature below 175 K, and then, the redshift occurs from 175 to 350 K. Meanwhile, the bandgap of the Cl-OHP film widens with the increase in operating temperature. The nonmonotonous shifts on the emission peak energy are attributed to the competition between the Stokes effect and bandgap widening. This should contribute to the understanding of photophysical processes in Cl-OHP materials and devices.
Carbon dangling-bond center (carbon Pb center) at 4H-SiC(0001)/SiO2 interface
Volume 116, Issue 7, February 2020. We identify a carbon dangling-bond center intrinsically formed at thermally oxidized 4H-SiC(0001)/SiO2 interfaces. Our electrically detected-magnetic-resonance spectroscopy and first-principles calculations demonstrate that this center, which we name “the PbC center,” is formed at a carbon adatom on the 4H-SiC(0001) honeycomb-like structure. The PbC center (Si3≡C-, where “-” represents an unpaired electron) is determined to be a just carbon version of the famous Pb center (Si dangling-bond center, Si3≡Si-) at Si(111)/SiO2 interfaces because we found close similarities between their wave functions. The PbC center acts as one of the major interfacial traps in 4H-SiC(0001) metal-oxide-semiconductor field-effect transistors (MOSFETs), which decreases the free-carrier density and the field-effect mobility of 4H-SiC(0001) MOSFETs. The formation of the PbC centers has the role of reducing the oxidation-induced strain, similar to the case of the formation of the Pb centers.
Ultrasound beam steering with flattened acoustic metamaterial Luneburg lens
Volume 116, Issue 7, February 2020. We report ultrasound beam steering based on 2D and 3D flattened acoustic metamaterial Luneburg lenses at 40 kHz. The effective properties of the lenses are obtained by using the quasi-conformal transformation technique and solving the Laplace equation with Dirichlet and Neumann boundary conditions. A 2D lens and a 3D lens were designed and fabricated. The numerical and experimental results with these lenses demonstrate excellent beam steering performance of ultrasonic waves in both near field and far field.
Current-induced spin–orbit torque efficiencies in W/Pt/Co/Pt heterostructures
Volume 116, Issue 7, February 2020. We study the damping-like spin–orbit torque (DL-SOT) efficiencies in W/Pt/Co/Pt multilayer structures by the current-induced hysteresis loop shift measurement and current-induced magnetization switching measurement. It is known that transition metals W and Pt possess spin Hall ratios with opposite signs, and therefore, the DL-SOT efficiencies in these multilayer structures may become zero with a certain W/Pt thickness combination. In this work, we show that indeed the zero DL-SOT efficiency can be achieved in such a structure, and the efficiency can evolve from negative (W-dominated) to positive (Pt-dominated) depending on the relative thickness of W and Pt. More importantly, we did not observe field-free switching when the W/Pt combination gives zero DL-SOT efficiency, which is in contrast to a recent report [Ma et al., Phys. Rev. Lett. 120, 117703 (2018)]. By further considering a simple spin diffusion model, we find that DL-SOT efficiencies [math] and [math] for the Pt and W layer, respectively, in our multilayer system. We also show that the Pt(2)/Co(0.5)/Pt(2) symmetric structure is a robust perpendicular magnetization anisotropy multilayer that can be employed on W or other spin Hall materials to characterize their DL-SOT efficiencies.
Detecting current-induced quantum magnetization fluctuations with a spin-torque nano-oscillator
Volume 116, Issue 7, February 2020. Interactions between conduction electrons and quantum fluctuations of ferromagnetic order have seldom been observed in magnetoelectronic devices. We show that current-induced quantum magnetization fluctuations can be detected using a spin-torque nano-oscillator by measuring its linewidth at different temperatures. The relative linewidth in a special dynamic region of the device can distinguish quantum magnetization fluctuations from their thermal counterparts, which is important in understanding magnetization dynamics beyond the mean-field level in magnetoelectronic devices.
Growth of visible-light-responsive ferroelectric SbSI thin films by molecular beam epitaxy
Volume 116, Issue 7, February 2020. Photoresponsive ferroelectrics are recently under intense study due to their potential application to photovoltaic devices. Antimony sulfoiodide (SbSI) is a prototypical compound that possesses both ferroelectricity and a strong visible-light-response. However, most of the SbSI films reported so far have a polycrystalline structure with a randomly oriented polarization axis. In this study, we have fabricated c-axis textured SbSI thin films through annealing of amorphous films deposited in a molecular beam epitaxy system, employing Sb2S3 and SbI3 sources. The fabricated films are highly uniform and have the polarization axis ordered vertical to the film plane. We have confirmed that the films show a strong visible-light-response and ferroelectricity in accord with bulk samples. These results will stimulate the development of photovoltaics employing narrow bandgap ferroelectric compounds.
Heterogeneous catalysis at the surface of topological materials
Volume 116, Issue 7, February 2020. Intriguing properties are frequently reported in various topologically non-trivial materials. They include robust metallic surface states, high carrier mobility, chiral fermions, and ultralong Fermi arcs. An exciting recent finding is that these properties are strongly related to adsorption and electron transfer in various heterogeneous catalysis reactions, such as hydrogen evolution, oxygen evolution, oxygen reduction, enantiospecific adsorption, and hydrometallation. Thus, we expect that the introduction of non-trivial symmetry-protected topological order will offer important freedom for designing high-performance heterogeneous catalysts. To uncover the contribution of the topologically non-trivial electronic structure to the heterogeneous reactions, in situ techniques are urgently needed to detect the interaction between surface states, topological electrons, and reaction intermediates.
Comparison of size-dependent characteristics of blue and green InGaN microLEDs down to 1 μm in diameter
Volume 116, Issue 7, February 2020. There is growing interest in microLED devices with lateral dimensions between 1 and 10 μm. However, reductions in external quantum efficiency (EQE) due to increased nonradiative recombination at the surface become an issue at these sizes. Previous attempts to study size-dependent EQE trends have been limited to dimensions above 5 μm, partly due to fabrication challenges. Here, we present size-dependent EQE data for InGaN microLEDs down to 1 μm in diameter fabricated using a process that only utilizes standard semiconductor processing techniques (i.e., lithography and etching). Furthermore, differences in EQE trends for blue and green InGaN microLEDs are compared. Green wavelength devices prove to be less susceptible to reductions in efficiency with the decreasing size; consequently, green devices attain higher EQEs than blue devices below 10 μm despite lower internal quantum efficiencies in the bulk material. This is explained by smaller surface recombination velocities with the increasing indium content due to enhanced carrier localization.
Phonon-induced near-field resonances in multiferroic BiFeO3 thin films at infrared and THz wavelengths
Volume 116, Issue 7, February 2020. Multiferroic BiFeO3 (BFO) shows several phonon modes at infrared (IR) to THz energies, which are expected to carry information on any sample property coupled to crystal lattice vibrations. While macroscopic IR studies of BFO are often limited by single-crystal size, scattering-type scanning near-field optical microscopy (s-SNOM) allows for IR thin film spectroscopy of nanoscopic probing volumes with negligible direct substrate contribution to the optical signal. In fact, polaritons such as phonon polaritons of BFO introduce a resonant tip–sample coupling in s-SNOM, leading to both stronger signals and enhanced sensitivity to local material properties. Here, we explore the near-field response of BFO thin films at three consecutive resonances (centered around 5 THz, 13 THz, and 16 THz), by combining s-SNOM with a free-electron laser. We study the dependence of these near-field resonances on both the wavelength and tip–sample distance. Enabled by the broad spectral range of the measurement, we probe phonon modes connected to the predominant motion of either the bismuth or oxygen ions. Therefore, we propose s-SNOM at multiple near-field resonances as a versatile and very sensitive tool for the simultaneous investigation of various sample properties.
Nonreciprocal normal-incidence lateral shift for transmitted wave beams through the magnetic photonic crystal slab
Volume 116, Issue 7, February 2020. Conventionally, there is no lateral beam shift (LBS) at normal incidence for a wave beam pass through a slab. However, by simultaneously breaking spatial inversion, time-reversal, and mirror symmetries of the photonic crystal slab, we realized nonreciprocal LBS for the transmitted wave beam with high transmission. We showed that the nonreciprocal LBS could be positive or negative, which could be tuned by the arrangement of a magnetic basis in the unit cell. We verified the nonreciprocal LBS at normal incidence by experiments. Our study provides a useful way to manipulate the wave propagation and wave-matter interaction by artificial materials and leads to a breakthrough in LBS, which has promising potential in optical devices, such as transducers, switches, and unidirectional couplers.
Speed dependence of friction on single-layer and bulk MoS2 measured by atomic force microscopy
Volume 116, Issue 7, February 2020. We perform atomic force microscopy (AFM) experiments on mechanically exfoliated, single-layer and bulk molybdenum disulfide (MoS2) in order to probe friction forces as a function of sliding speed. The results of the experiments demonstrate that (i) friction forces increase logarithmically with respect to sliding speed, (ii) there is no correlation between the speed dependence of friction and the number of layers of MoS2, and (iii) changes in the speed dependence of friction can be attributed to changes in the physical characteristics of the AFM probe, manifesting in the form of varying contact stiffness and tip-sample interaction potential parameters in the thermally activated Prandtl–Tomlinson model. Our study contributes to the formation of a mechanistic understanding of the speed dependence of nanoscale friction on two-dimensional materials.
Systematic investigation of the growth kinetics of β-Ga2O3 epilayer by plasma enhanced chemical vapor deposition
Volume 116, Issue 7, February 2020. β-Ga2O3 has attracted much attention due to its ultrawide-bandgap (∼4.9 eV) with a high breakdown field (8 MV/cm) and good thermal/chemical stability. In order for β-Ga2O3 to be used in electronic and optoelectronic devices, epitaxial growth technology of thin films should be given priority. However, challenges are associated with the trade-off growth rate with crystallization and surface roughness in conventional epitaxy. Herein, plasma enhanced chemical vapor deposition was used to grow the β-Ga2O3 epilayer, and the growth kinetics process has been systematically investigated. A high growth rate of ∼0.58 μm/h and a single [math] plane orientation with a full width at half maximum value of 0.86° were obtained when grown on the c-plane sapphire substrate at the growth temperature of 820 °C. Then, a proposed model for the mechanism of nucleation and growth of β-Ga2O3 epitaxial films is established to understand the precursor transport and gas phase reaction process. This work provides a cheap, green, and efficient epitaxial growth method, which is indispensable for device applications of β-Ga2O3.
Determination of background doping polarity of unintentionally doped semiconductor layers
Volume 116, Issue 7, February 2020. We present a method of determining the background doping type in semiconductors using capacitance–voltage measurements on overetched double mesa p–i–n or n–i–p structures. Unlike Hall measurements, this method is not limited by the conductivity of the substrate. By measuring the capacitance of devices with varying top and bottom mesa sizes, we were able to conclusively determine which mesa contained the p–n junction, revealing the polarity of the intrinsic layer. This method, when demonstrated on GaSb p–i–n and n–i–p structures, concluded that the material is residually doped p-type, which is well established by other sources. The method was then applied to a 10 monolayer InAs/10 monolayer AlSb superlattice, for which the doping polarity was unknown, and indicated that this material is also p-type.
Magnetostriction enhancement in ferromagnetic strain glass by approaching the crossover of martensite
Volume 116, Issue 7, February 2020. Owing to its unique nanostructure with nanosized strain domains embedded in the austenite matrix, ferromagnetic strain glass has recently been found to yield low-field large magnetostriction, providing an important principle for designing magnetostrictive materials. Considering that magnetostriction maximizes in the vicinity of the strain glass transition temperature, Tg that is usually below room temperature; it has inspired the search for a feasible approach to further enhance room temperature magnetostriction from an application point of view. Here, we report that approaching the martensite crossover through applying proper stress during annealing can effectively enhance room temperature magnetostriction of a random polycrystalline Fe67.7Pd32.3 strain glass alloy with Tg of 133 K from 73 to 95 ppm by ∼30%. The comparative results reveal that annealing with higher stress, e.g., 15 MPa, will deteriorate magnetostriction performance due to stress-induced martensites. Further transmission electron microscopy study reveals that enhanced magnetostriction is due to slightly enlarged strain nanodomains because the proper bias stress provides an extra driving force toward the martensite and helps to overcome the kinetic limitation, which may be a universal approach to achieve large magnetostriction in ferromagnetic strain glass.
Reduced spin torque nano-oscillator linewidth using [math] irradiation
Volume 116, Issue 7, February 2020. We demonstrate an approach for improving the spectral linewidth of a spin torque nano-oscillator (STNO). Using [math] ion irradiation, we tune the perpendicular magnetic anisotropy (PMA) of the STNO free layer such that its easy axis is gradually varied from strongly out-of-plane to moderate in-plane. As the PMA impacts the non-linearity [math] of the STNO, we can, in this way, control the threshold current, the current tunability of the frequency, and, in particular, the STNO linewidth, which dramatically improves by two orders of magnitude. Our results are in good agreement with the theory for nonlinear auto-oscillators, confirm theoretical predictions of the role of the nonlinearity, and demonstrate a straightforward path toward improving the microwave properties of STNOs.
Effect of sample size on anomalous Nernst effect in chiral antiferromagnetic Mn3Sn devices
Volume 116, Issue 7, February 2020. We investigate the effect of sample size on the anomalous Nernst effect (ANE) in a device formed from chiral antiferromagnetic Mn3Sn. We also investigate its magnetic domains by employing focused ion beam lithography. Mn3Sn is a suitable material for studying the thermoelectric effect in the presence of antiferromagnetic domains because it exhibits a large ANE. In the Mn3Sn device used in this study, a Ta layer acts as a heater; the heat produced via Joule heating diffuses through a sapphire substrate into the thin flake of Mn3Sn. The Nernst signal exhibits a stepwise hysteresis when the sample is subjected to a temperature gradient and magnetic field at 290 K. The stepwise hysteresis depends on the sample shape and size—which affect nucleation, pinning, and depinning processes—but the temperature difference also has a significant effect on the switching process. The domain ratios calculated using the ANE results indicate that the domain size is smaller than 20 μm2. This obtained domain size is in good agreement with the reported experimental values of 10–100 μm2 for the magneto-optical Kerr effect in bulk single-crystal Mn3Sn. Thus, the ANE is a powerful means of obtaining information about the magnetic domains in samples under a temperature gradient, thereby promising a reliable approach to study magnetic domains and spintronics using antiferromagnets.
Nonlinear magnetoelectric effect in a layered ferromagnetic-piezoelectric heterostructure excited by transverse magnetic field
Volume 116, Issue 7, February 2020. The nonlinear magnetoelectric effect in a heterostructure containing layers of amorphous ferromagnet FeBSiC and piezoelectric lead zirconate titanate ceramics has been investigated. The heterostructure was subjected to permanent, H0, and alternating, h, magnetic fields applied in the structure plane. In contrast to previous studies, the excitation field was directed perpendicular to the permanent field ([math]). The generation of even voltage harmonics across the piezoelectric layer was observed for excitation fields in the range of 0–3 Oe. The dependence of the second harmonic amplitude on the permanent field strength was found to differ significantly from a similar dependence upon longitudinal excitation, h//H0. A theory was developed, which describes the field dependence of voltage harmonic amplitudes for the magnetoelectric effect excited by a transverse magnetic field.
Magnetic anisotropy controlled FeCoSiB thin films for surface acoustic wave magnetic field sensors
Volume 116, Issue 7, February 2020. Surface acoustic wave magnetic field sensors based on guided Love waves using the ΔE effect of a magnetostrictive thin film have been shown to be promising candidates for the measurement of weak fields at low frequencies as required for biomagnetic applications or as current sensors benefitting from the large dynamic range and bandwidth. The deposition of soft magnetic films with high magnetostriction is, however, more challenging on piezoelectric substrates such as quartz than on silicon. Thermally induced anisotropic expansion during the deposition process or during post-deposition magnetic field annealing leads to uniaxial stresses acting on the films, which makes the precise control of magnetic anisotropy difficult. Accordingly, this work analyzes the influence of the deposition process and heat treatment on the performance of Love wave devices. ST-cut quartz based delay line surface acoustic wave sensors with a SiO2 guiding layer are employed, and a 200 nm layer of amorphous magnetostrictive (Fe90Co10)78Si12B10 is used as the sensitive element. Magneto-optical imaging is performed for magnetic domain characterization, and the sensor performance is characterized in terms of bias field dependent phase sensitivity and frequency dependent phase noise. By performing a low temperature deposition in an external magnetic field, considerable improvement in limits of detection at biomagnetic relevant frequencies down to 70 [math] at 10 Hz and 25 [math] at 100 Hz is achieved.
Increased droplet coalescence using electrowetting on dielectric (EWOD)
Volume 116, Issue 7, February 2020. Small-scale electrodes and gaps subjected to repeated short bursts of AC voltage were used to improve droplet coalescence and growth for water harvesting by actively bashing smaller droplets together to form larger ones. Several different electrode patterns were tested under the same conditions. The results indicate that condensation on a cooled flat surface was increased using electrowetting (EW) by accelerating the slow coalescence process where smaller droplets join to form larger droplets and leave behind a dry surface for new droplets to form. A pattern consisting of 100-μm wide interdigitated electrodes separated by 100-μm gaps showed the fastest growth in droplet size. The largest droplets formed with such a pattern had approximately 30 times larger volume than the largest droplets formed on the surface when electrowetting was not applied. Finer patterns exhibited a larger overall condensation rate, where the electrowetting method showed up to a 56% increase in overall water condensation.
Switchable optical and acoustic resolution photoacoustic dermoscope dedicated into in vivo biopsy-like of human skin
Volume 116, Issue 7, February 2020. As a promising branch of optical absorption-based photoacoustic microscopy, photoacoustic dermoscopy (PAD) can provide manifold morphologic and functional information in clinical diagnosis and the assessment of dermatological conditions. However, most PAD setups are insufficient for clinical dermatology, given their single optical resolution (OR) or acoustic resolution (AR) mode, which results in poor spatiotemporal resolution or imaging depth for visualizing the internal texture of skin. Here, a switchable optical and acoustic resolution photoacoustic dermoscope (S-OR-ARPAD) system is developed, which provides a smooth transition from OR mode in microscopic imaging of superficial skin layers to AR mode when imaging at greater depths within intensely scattering deep skin layers. The lateral resolution can be seamlessly switched between 4.4 and 47 μm as the maximum imaging depth is switched between 1.2 and 1.8 mm for skin imaging. Using the S-OR-ARPAD, we identified the two distinct resolution modes responsible for resolving features of different skin layers and demonstrated the fine structures with strong contrast in the stratum corneum, dermal papillae, and microvascular structures in the horizontal plexus by imaging the healthy human skin at different locations.
Erratum: “Non-magnetic origin of spin Hall magnetoresistance-like signals in Pt films and epitaxial NiO/Pt bilayers” [Appl. Phys. Lett. 116, 022410 (2020)]
Volume 116, Issue 7, February 2020.
High-transition-temperature nanoscale superconducting quantum interference devices directly written with a focused helium ion beam
Volume 116, Issue 7, February 2020. In this work, we present nanoscale superconducting quantum interference devices (SQUIDs) with dimensions as small as 10 nm from the high-transition-temperature superconductor YBa2Cu3[math] (YBCO). The SQUID features and Josephson junctions are directly written into a 35-nm thick YBCO film with a focused helium ion beam. We integrate these nano-SQUIDs with directly written nano-isolated inductively coupled control lines to demonstrate a low power superconducting output driver capable of transimpedance conversion over a very wide temperature range of 4–50 K.
Transverse localization of light in laser written designed disorder
Volume 116, Issue 7, February 2020. Transverse Anderson localization provides the lateral confinement of electromagnetic waves in disordered systems that are invariant along the propagation direction. Here, we demonstrate a disorder induced confinement in glass microstructures where disorder is fabricated ad hoc by the femtosecond direct laser writing technique. By employing a high numerical aperture objective, we are able to write parallel arrays of tiny tubes with a refractive index higher than the surrounding glass and to arrange them in a disordered fashion in the transversal plane. We demonstrate that these paraxial scatterers are supporting transverse localization and that the confinement strength depends on the disorder properties. The proposed approach, which relies on a user-controlled positioning of individual scatterers, allows us to finely tune the structural design, maximizing the transversal confinement.
Epitaxial stabilization of ultra thin films of high entropy perovskite
Volume 116, Issue 7, February 2020. High entropy oxides (HEOs) are a class of materials, containing equimolar portions of five or more transition metal and/or rare-earth elements. We report here about the layer-by-layer growth of HEO [([math])NiO3] thin films on NdGaO3 substrates by pulsed laser deposition. The combined characterizations with in situ reflection high energy electron diffraction, atomic force microscopy, and x-ray diffraction affirm the single crystalline nature of the film with smooth surface morphology. The desired +3 oxidation of Ni has been confirmed by an element sensitive x-ray absorption spectroscopy measurement. Temperature dependent electrical transport measurements revealed a first order metal-insulator transition with the transition temperature very similar to the undoped NdNiO3. Since both these systems have a comparable tolerance factor, this work demonstrates that the electronic behaviors of A-site disordered perovskite-HEOs are primarily controlled by the average tolerance factor.
Development of a silicon–diamond interface on (111) diamond
Volume 116, Issue 7, February 2020. We report the preparation of a silicon terminated (111) diamond surface. Low energy electron diffraction and core level photoemission demonstrate that this surface is highly ordered and homogeneous and possesses a negative electron affinity. Our analysis suggests that the surface reconstruction begins with the formation of silicon trimers that coalesce into a rhombohedral 2D silicon layer reminiscent of rhombohedral silicene.
A composite phase change material thermal buffer based on porous metal foam and low-melting-temperature metal alloy
Volume 116, Issue 7, February 2020. Composite phase change materials consisting of a high-latent-heat phase change material (PCM) embedded in a high-thermal-conductivity matrix are desirable for thermally buffering pulsed heat loads via rapid absorption and release of thermal energy at a constant temperature. This paper reports a composite PCM thermal buffer consisting of a Field's metal PCM having high volumetric latent heat (315 MJ/m3) embedded in a copper (Cu) matrix having high intrinsic thermal conductivity [384 W/(m·K)]. We demonstrate thermal buffer samples fabricated with Cu volume fractions from 0.05 to 0.2 and sample thicknesses ranging between 1 mm and 4 mm. Experiments coupled with finite element method simulations were used to determine the figures of merit (FOMs), cooling capacity ηeff, energy density Eeff, effective thermal conductivity keff, and the buffering time constant τ. The cooling capacity was measured to be as high as ηeff = 72 ± 4 kJ/(m2·K1/2·s1/2) for the 1.45 mm thick thermal buffer sample having a Cu volume fraction of 0.13, significantly higher than theoretical values for aluminum–paraffin composites [45 kJ/(m2·K1/2·s1/2)] or pure paraffin wax [8 kJ/(m2·K1/2·s1/2)]. Our work develops design guidelines for high-FOM thermal buffer devices for pulsed heat load thermal management.
Ti- and Fe-related charge transition levels in [math]
Volume 116, Issue 7, February 2020. Deep-level transient spectroscopy measurements on β-[math] crystals reveal the presence of three defect signatures labeled [math], and [math] with activation energies at around 0.66 eV, 0.73 eV, and 0.95 eV below the conduction band edge. Using secondary ion mass spectrometry, a correlation between the defect concentration associated with [math] and the Ti concentration present in the samples was found. Particularly, it is found that [math] is the dominant Ti-related defect in β-[math] and is associated with a single Ti atom. This finding is further corroborated by hybrid functional calculations that predict Ti substituting on an octahedral Ga site, denoted as [math], to be a good candidate for [math]. Moreover, the deep level transient spectroscopy results show that the level previously labeled [math] and attributed to Fe substituting on a gallium site ([math]) consists of two overlapping signatures labeled [math] and [math]. We tentatively assign [math] and [math] to Fe substituting for Ga on a tetrahedral or an octahedral site, respectively.
Non-standing spin-waves in confined micrometer-sized ferromagnetic structures under uniform excitation
Volume 116, Issue 7, February 2020. A non-standing characteristic of directly imaged spin-waves in confined micrometer-sized ultrathin Permalloy ([math]) structures is reported along with evidence of the possibility to alter the observed state by modifications to the sample geometry. Using micromagnetic simulations, the presence of the spin-wave modes excited in the Permalloy stripes along with the quasi-uniform modes was observed. The predicted spin-waves were imaged in direct space using time resolved scanning transmission x-ray microscopy, combined with a ferromagnetic resonance excitation scheme (STXM-FMR). STXM-FMR measurements revealed a non-standing characteristic of the spin-waves. Also, it was shown by micromagnetic simulations and confirmed using STXM-FMR results that the observed characteristic of the spin-waves can be influenced by the local magnetic fields in different sample geometries.
Two-dimensional series connected photovoltaic cells defined by ferroelectric domains
Volume 116, Issue 7, February 2020. Recently, a large amount of effort has been devoted to bringing p- and n-type two-dimensional (2D) materials in close contact to promise a p–n junction for photodetectors and photovoltaic devices. However, all solar cells based on 2D materials are single p–n junctions so far, where the open circuit voltage is usually limited by the bandgap of semiconductor materials. Here, by using a scanning-probe domain patterning method to polarize the ferroelectric film, we demonstrate a series connected MoTe2 photovoltaic cell with an additive open circuit voltage and output electrical power. The nonvolatile MoTe2 p–n diodes exhibit a rectification ratio of 100. As a photodetector, the device presents a responsivity of 220 mA/W and an external quantum efficiency of 41% without any gate or bias voltages. The open circuit voltage increases linearly with the number of series connected p–n junctions and can be beyond the bandgap of the multilayer MoTe2.
Ferromagnetic behaviors in monolayer MoS2 introduced by nitrogen-doping
Volume 116, Issue 7, February 2020. Effective functionalization of magnetic properties through substitutional doping may extend the spintronic applications of two-dimensional (2D) semiconductor MoS2. Here, the magnetoelectric properties of nitrogen-doped monolayer MoS2 are investigated by first-principles calculations, revealing that the N-p and S-p states are strongly hybridized with the Mo-d states, thus leading to the appearance of magnetism as verified experimentally. We demonstrate in situ doping of monolayer MoS2 with nitrogen via a convenient chemical vapor deposition method. Incorporation of nitrogen into MoS2, leading to the evolution of magnetism, is evidenced by combining x-ray photoelectron spectroscopy and vibrating sample magnetometer measurements. By comparison with pristine monolayer MoS2, the distinct ferromagnetism behaviors of nitrogen-doped monolayer MoS2 are observed up to room temperature, while the semiconducting nature persists. Our work introduces an efficient and feasible approach to realize magnetism in the 2D limit and explores potential applications in semiconductor spintronics.
On-site tuning of the carrier lifetime in silicon for on-chip THz circuits using a focused beam of helium ions
Volume 116, Issue 7, February 2020. In this study, we demonstrate that a focused helium ion beam allows the local adjustment and optimization of the carrier lifetime in silicon-based photoswitches integrated in ultrafast on-chip terahertz-circuits. Starting with a carrier lifetime of 5.3 ps for as-grown silicon on sapphire, we monotonously reduce the carrier lifetime in integrated switches to a minimum of ∼0.55 ps for a helium ion fluence of 20 × 1015 ions/cm2. By introducing an analytical model for the carrier lifetimes in the photoswitches, we particularly demonstrate that the carrier lifetime can be adjusted locally even within single photoswitches. In turn, the demonstrated on-site tuning allows optimizing ultrafast high-frequency circuits, into which radiation-sensitive nanoscale materials, such as two-dimensional materials, are embedded.
Gallium nitride tunneling field-effect transistors exploiting polarization fields
Volume 116, Issue 7, February 2020. This report showcases a vertical tunnel field effect transistor (TFET) fabricated from a GaN/InGaN heterostructure and compares it to a gated vertical GaN p-n diode. By including a thin InGaN layer, the interband tunneling in the TFET is increased compared to the gated homojunction diode. This leads to an increased drain current of 57 μA/μm and a reduced subthreshold swing of 102 mV/dec, from 240 mV/dec. However, trap assisted tunneling prevents devices from realizing subthreshold slopes below the Boltzmann limit of 60 mV/dec. Nevertheless, this work shows the capability of tunnel field effect transistors to be realized in GaN by taking advantage of the spontaneous and piezoelectric polarization in the III-N material system.
Microbubble enhanced acoustic tweezers for size-independent cell sorting
Volume 116, Issue 7, February 2020. Acoustic tweezers hold great promise for potential applications in cell sorting due to their noncontact, noninvasive, and simple characteristics. Acoustic tweezers, however, have difficulty in separating the cells of the same size distribution, which hampers their applications. In this paper, we demonstrate that assisted by the targeted microbubble, two kinds of cells with an overlap in size distribution can be efficiently separated by surface acoustic waves. By specifically adhering the targeted microbubbles to MDA-MB-231 cells, the acoustic sensitivity of cells can be improved significantly, leading to the isolation of MDA-MB-231 from MCF-7 cells with an efficiency of 91.2 ± 3.4%. This method extends the diversity of acoustic separation and is capable of separation of particles with the same density and diameter, proving a strategy for specific cell sorting.
Modeling of demagnetization processes in permanent magnets measured in closed-circuit geometry
Volume 116, Issue 6, February 2020. 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.
Reduced annealing temperature for ferroelectric HZO on InAs with enhanced polarization
Volume 116, Issue 6, February 2020. Deposition, annealing, and integration of ferroelectric [math] (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 [math] even down to an annealing temperature of [math]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.
Mechanisms of GaN quantum dot formation during nitridation of Ga droplets
Volume 116, Issue 6, February 2020. 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.
Broadband optical measurement of AC magnetic susceptibility of magnetite nanoparticles
Volume 116, Issue 6, February 2020. 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.
Tunable valleytronics with symmetry-retaining high polarization degree in SnSxSe1−x model system
Volume 116, Issue 6, February 2020. 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.
Saturable absorption properties and femtosecond mode-locking application of titanium trisulfide
Volume 116, Issue 6, February 2020. 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.
Do all screw dislocations cause leakage in GaN-based devices'
Volume 116, Issue 6, February 2020. 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.
Integration of polycrystalline Ga2O3 on diamond for thermal management
Volume 116, Issue 6, February 2020. 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.
Polarization fields in semipolar [math] and [math] InGaN light emitting diodes
Volume 116, Issue 6, February 2020. InxGa1−xN/GaN multiple quantum well structures (x = 0.13 and 0.18) embedded into p–i–n diodes on ([math]) and ([math]) 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 ([math]) sample (x = 0.18), the flatband voltage is found at [math] corresponding to a polarization field of [math]. For the [math] sample (x = 0.13), the polarization field is estimated to be [math] at flatband voltage higher than turn-on voltage of this light emitting diode.
Enhanced surface superconductivity in Ba(Fe0.95Co0.05)2As2
Volume 116, Issue 6, February 2020. We present direct evidence for an enhanced superconducting Tc on the surface of cleaved single crystals of Ba([math])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.
Defect proliferation in CsPbBr3 crystal induced by ion migration
Volume 116, Issue 6, February 2020. 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.
Velocity-amplified monostable dual-charged electret dome energy harvester using low-speed finger tapping
Volume 116, Issue 6, February 2020. 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.
Progress, challenges, and perspective on metasurfaces for ambient radio frequency energy harvesting
Volume 116, Issue 6, February 2020. 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.
A semiconductor topological photonic ring resonator
Volume 116, Issue 6, February 2020. 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.
Mid-infrared electroluminescence from type-II In(Ga)Sb quantum dots
Volume 116, Issue 6, February 2020. 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.
Deep ultraviolet monolayer GaN/AlN disk-in-nanowire array photodiode on silicon
Volume 116, Issue 6, February 2020. 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.
Emerging edge states on the surface of the epitaxial semimetal CuMnAs thin film
Volume 116, Issue 6, February 2020. 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.
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