Showing papers in "Applied Physics Letters in 2011"
TL;DR: In this paper, it was shown that crystalline phases with ferroelectric behavior can be formed in thin thin films of SiO2 doped hafnium oxide, which is suitable for field effect transistors and capacitors due to its excellent compatibility to silicon technology.
Abstract: We report that crystalline phases with ferroelectric behavior can be formed in thin films of SiO2 doped hafnium oxide. Films with a thickness of 10 nm and with less than 4 mol. % of SiO2 crystallize in a monoclinic/tetragonal phase mixture. We observed that the formation of the monoclinic phase is inhibited if crystallization occurs under mechanical encapsulation and an orthorhombic phase is obtained. This phase shows a distinct piezoelectric response, while polarization measurements exhibit a remanent polarization above 10 μC/cm2 at a coercive field of 1 MV/cm, suggesting that this phase is ferroelectric. Ferroelectric hafnium oxide is ideally suited for ferroelectric field effect transistors and capacitors due to its excellent compatibility to silicon technology.
1,631 citations
TL;DR: In this article, the effects of residues introduced during the transfer of chemical vapor deposited graphene from a Cu substrate to an insulating (SiO2) substrate on the physical and electrical properties of the transferred graphene are studied X-ray photoelectron spectroscopy and atomic force microscopy.
Abstract: The effects of residues introduced during the transfer of chemical vapor deposited graphene from a Cu substrate to an insulating (SiO2) substrate on the physical and electrical of the transferred graphene are studied X-ray photoelectron spectroscopy and atomic force microscopy show that this residue can be substantially reduced by annealing in vacuum The impact of the removal of poly(methyl methacrylate) residue on the electrical properties of graphene field effect devices is demonstrated, including a nearly 2 × increase in average mobility from 1400 to 2700 cm2/Vs The electrical results are compared with graphene doping measurements by Raman spectroscopy
936 citations
TL;DR: In this article, a material possessing a very small energy gap between its singlet and triplet excited states, ΔE1−3, which allows efficient up-conversion of triplet excitons into a singlet state and leads to efficient thermally activated delayed fluorescence (TADF), is reported.
Abstract: A material possessing a very small energy gap between its singlet and triplet excited states, ΔE1−3, which allows efficient up-conversion of triplet excitons into a singlet state and leads to efficient thermally activated delayed fluorescence (TADF), is reported. The compound, 2-biphenyl-4,6-bis(12-phenylindolo[2,3-a] carbazole-11-yl)-1,3,5-triazine, breaks the restriction of a large energy gap, with a ΔE1−3 of just 0.11 eV, while maintaining a high fluorescent radiative decay rate (kr∼107). The intense TADF provides a pathway for highly efficient electroluminescence.
906 citations
TL;DR: The residual defects and groups in chemically reduced graphene oxide cannot not only improve the impedance match characteristic and prompt energy transition from contiguous states to Fermi level, but also introduce defect polarization relaxation and groups' electronic dipole relaxation, which are all in favor of electromagnetic wave penetration and absorption as mentioned in this paper.
Abstract: The residual defects and groups in chemically reduced graphene oxide cannot only improve the impedance match characteristic and prompt energy transition from contiguous states to Fermi level, but also introduce defect polarization relaxation and groups’ electronic dipole relaxation, which are all in favor of electromagnetic wave penetration and absorption The chemically reduced graphene oxide shows enhanced microwave absorption compared with graphite and carbon nanotubes, and can be expected to display better absorption than high quality graphene, exhibiting a promising prospect as microwave absorbing material
728 citations
TL;DR: In this paper, the authors investigated spin torque switching in perpendicular magnetic tunnel junctions using Ta∣CoFeB∣MgO free layers and a synthetic antiferromagnet reference layer.
Abstract: Spin torque switching is investigated in perpendicular magnetic tunnel junctions using Ta∣CoFeB∣MgO free layers and a synthetic antiferromagnet reference layer. We show that the Ta∣CoFeB interface makes a key contribution to the perpendicular anisotropy. The quasistatic phase diagram for switching under applied field and voltage is reported. Low switching voltages, Vc 50 ns=290 mV are obtained, in the range required for spin torque magnetic random access memory. Switching down to 1 ns is reported, with a rise in switching speed from increased overdrive that is eight times greater than for comparable in-plane devices, consistent with expectations from a single-domain model.
715 citations
TL;DR: In this paper, the authors report on scanning Raman and on temperature-dependent photoluminescence measurements on single-layer MoS2 flakes prepared by exfoliation.
Abstract: The band structure of MoS2 strongly depends on the number of layers, and a transition from indirect to direct-gap semiconductor has been observed recently for a single layer of MoS2. Single-layer MoS2 therefore becomes an efficient emitter of photoluminescence even at room temperature. Here, we report on scanning Raman and on temperature-dependent, as well as time-resolved photoluminescence measurements on single-layer MoS2 flakes prepared by exfoliation. We observe the emergence of two distinct photoluminescence peaks at low temperatures. The photocarrier recombination at low temperatures occurs on the few-picosecond timescale, but with increasing temperatures, a biexponential photoluminescence decay with a longer-lived component is observed.
714 citations
TL;DR: In this article, a tilted-pump-pulse front scheme was used to generate single-cycle terahertz (THz) pulses by optical rectification of femtosecond laser pulses in LiNbO3.
Abstract: Using the tilted-pump-pulse-front scheme, we generate single-cycle terahertz (THz) pulses by optical rectification of femtosecond laser pulses in LiNbO3. In our THz generation setup, the condition that the image of the grating coincides with the tilted-optical-pulse front is fulfilled to obtain optimal THz beam characteristics and pump-to-THz conversion efficiency. By using an uncooled microbolometer-array THz camera, it is found that the THz beam leaving the output face of the LN crystal can be regarded as a collimated rather than point source. The designed focusing geometry enables tight focus of the collimated THz beam with a spot size close to the diffraction limit, and the maximum THz electric field of 1.2 MV/cm is obtained.
712 citations
TL;DR: In this paper, Fowler-Nordheim tunneling was used for atomically flat and ultrathin hexagonal boron nitride (h-BN) on gold-coated mica using conductive atomic force microscopy.
Abstract: Electron tunneling through atomically flat and ultrathin hexagonal boron nitride (h-BN) on gold-coated mica was investigated using conductive atomic force microscopy. Low-bias direct tunneling was observed in mono-, bi-, and tri-layer h-BN. For all thicknesses, Fowler-Nordheim tunneling (FNT) occurred at high bias, showing an increase of breakdown voltage with thickness. Based on the FNT model, the barrier height for tunneling (3.07 eV) and dielectric strength (7.94 MV/cm) of h-BN are obtained; these values are comparable to those of SiO2.
476 citations
TL;DR: In this article, the effect of interlayer interactions on the band structure and density of states using the screened hybrid functional of Heyd, Scuseria, and Ernzerhof was investigated.
Abstract: Molybdenite (MoS2) undergoes a transition from an indirect to direct gap semiconductor exhibiting strong photoluminescence when confined in a 2D monolayer. We investigate the effect of interlayer interactions on the band structure and density of states using the screened hybrid functional of Heyd, Scuseria, and Ernzerhof. We show that for the bulk and monolayer systems, our short-range screened hybrid functional produces band gaps in good agreement with experiment. Our functional includes only interlayer interactions of non-van der Waals origin, predicts properties consistent with recent experiments, and provides predictions for few-layered systems.
473 citations
TL;DR: In this paper, the authors identify the origin of the droop in InGaN-based light-emitting diodes and provide a guide to addressing the efficiency issues in nitride LEDs and the development of efficient solid-state lighting.
Abstract: InGaN-based light-emitting diodes(LEDs) exhibit a significant efficiency loss (droop) when operating at high injected carrier densities, the origin of which remains an open issue. Using atomistic first-principles calculations, we show that this efficiency droop is caused by indirect Auger recombination, mediated by electron-phonon coupling and alloy scattering. By identifying the origin of the droop, our results provide a guide to addressing the efficiency issues in nitride LEDs and the development of efficient solid-state lighting.
472 citations
TL;DR: In this paper, the authors demonstrate the generation of optical vortices with radial or azimuthal polarization using a space variant polarization converter, fabricated by femtosecond laser writing of self-assembled nanostructures in silica glass.
Abstract: We demonstrate the generation of optical vortices with radial or azimuthal polarization using a space variant polarization converter, fabricated by femtosecond laser writing of self-assembled nanostructures in silica glass. Manipulation of the induced form birefringence is achieved by controlling writing parameters, in particular, the polarization azimuth of the writing beam. The fabricated converter allows switching from radial to azimuthal polarization by controlling the handedness of incident circular polarization.
TL;DR: In this article, a wideband-tunable Q-switched fiber laser exploiting a graphene saturable absorber was demonstrated, with 2μs pulses, tunable between 1522 and 1555 nm.
Abstract: We demonstrate a wideband-tunable Q-switched fiber laser exploiting a graphene saturable absorber. We get ∼2 μs pulses, tunable between 1522 and 1555 nm with up to ∼40 nJ energy. This is a simple and low-cost light source for metrology, environmental sensing, and biomedical diagnostics.
TL;DR: In this article, the observation of ferroelectricity in capacitors based on hafnium-zirconium-oxide thin films of 7.5 to 9.5 nm thickness was reported.
Abstract: We report the observation of ferroelectricity in capacitors based on hafnium-zirconium-oxide. Hf0.5Zr0.5O2 thin films of 7.5 to 9.5 nm thickness were found to exhibit ferroelectric polarization-voltage hysteresis loops when integrated into TiN-based metal-insulator-metal capacitors. A remnant polarization of 16 μC/cm2 and a high coercive field of 1 MV/cm were observed. Further proof for the ferroelectric nature was collected by quasi-static polarization-voltage hysteresis, small signal capacitance-voltage, and piezoelectric measurements. Data retention characteristics were evaluated by a Positive Up Negative Down pulse technique. No significant decay of the initial polarization state was observed within a measurement range of up to two days.
TL;DR: In this paper, the electronic properties of hydrogenated silicene and germanene, so called silicane and Germanane, respectively, are investigated using first-principles calculations based on density functional theory.
Abstract: The electronic properties of hydrogenated silicene and germanene, so called silicane and germanane, respectively, are investigated using first-principles calculations based on density functional theory. Two different atomic configurations are found to be stable and energetically degenerate. Upon the adsorption of hydrogen, an energy gap opens in silicene and germanene. Their energy gaps are next computed using the HSE hybrid functional as well as the G0W0 many-body perturbation method. These materials are found to be wide band-gap semiconductors, the type of gap in silicane (direct or indirect) depending on its atomic configuration. Germanane is predicted to be a direct-gap material, independent of its atomic configuration, with an average energy gap of about 3.2 eV, this material thus being potentially interesting for optoelectronic applications in the blue/violet spectral range.
TL;DR: Using a boron nitride underlayer, this article achieved mobilities as high as 37'000'cm2/V's, an order of magnitude higher than commonly reported for CVD graphene and better than most exfoliated graphene.
Abstract: Chemical vapor deposited (CVD) graphene is often presented as a scalable solution to graphene device fabrication, but to date such graphene has exhibited lower mobility than that produced by exfoliation. Using a boron nitride underlayer, we achieve mobilities as high as 37 000 cm2/V s, an order of magnitude higher than commonly reported for CVD graphene and better than most exfoliated graphene. This result demonstrates that the barrier to scalable, high mobility CVD graphene is not the growth technique but rather the choice of a substrate that minimizes carrier scattering.
TL;DR: In this article, the authors present electronic transport measurements of single and bilayer graphene on commercially available hexagonal boron nitride and extract mobilities as high as 125'000 cm2 V−1 s−1 at room temperature and 275'000cm2 V −1 s −1 at 4.2'K.
Abstract: We present electronic transport measurements of single and bilayer graphene on commercially available hexagonal boron nitride. We extract mobilities as high as 125 000 cm2 V−1 s−1 at room temperature and 275 000 cm2 V−1 s−1 at 4.2 K. The excellent quality is supported by the early development of the ν = 1 quantum Hall plateau at a magnetic field of 5 T and temperature of 4.2 K. We also present a fast, simple, and accurate transfer technique of graphene to hexagonal boron nitride crystals. This technique yields atomically flat graphene on boron nitride which is almost completely free of bubbles or wrinkles. The potential of commercially available boron nitride combined with our transfer technique makes high mobility graphene devices more accessible.
TL;DR: In this paper, an ab-initio study of electron mobility and electron-phonon coupling in chemically modified graphene, considering fluorinated and hydrogenated graphene at different percentage coverage, is presented.
Abstract: We present an ab-initio study of electron mobility and electron-phonon coupling in chemically modified graphene, considering fluorinated and hydrogenated graphene at different percentage coverage. Hexagonal boron carbon nitrogen is also investigated due the increased interest shown by the research community towards this material. In particular, the deformation potentials are computed by means of density functional theory, while the carrier mobility is obtained according to the Takagi model (S. Takagi, A. Toriumi, and H. Tango, IEEE Trans. Electron Devices 41, 2363 (1994)). We will show that graphene with a reduced degree of hydrogenation can compete, in terms of mobility, with silicon technology.
TL;DR: In this paper, an elastic metamaterial which exhibits simultaneously negative effective mass density and bulk modulus is presented with a single unit structure made of solid materials, which is achieved through a chiral microstructure that is capable of producing simultaneous translational and rotational resonances.
Abstract: In this letter, an elasticmetamaterial which exhibits simultaneously negative effective mass density and bulk modulus is presented with a single unit structure made of solid materials. The double-negative properties are achieved through a chiralmicrostructure that is capable of producing simultaneous translational and rotational resonances. The negative effective mass density and effective bulk modulus are numerically determined and confirmed by the analysis of wave propagation. The left-handed wave propagation property of this metamaterial is demonstrated by the negative refraction of acoustic waves.
TL;DR: In this paper, the roles of defects including monatomic vacancies and Stone-Wales dislocations in the mechanical and thermal properties of graphene were investigated through molecular dynamics simulations, and it was shown that Young's modulus of a defected graphene sheet has a gentle dependence with the concentration of defects.
Abstract: The roles of defects including monatomic vacancies and Stone-Wales dislocations in the mechanical and thermal properties of graphene are investigated here through molecular dynamics (MD) simulations. The results show that Young’s modulus of a defected graphene sheet has a gentle dependence with the concentration of defects, while the thermal conductivity is much more sensitive. Analysis based on the effective medium theory (EMT) indicates that this sensitivity originates from the scattering of phonons by defects and delocalized interaction between them, which leads to a transition from propagating to diffusive mode as the concentration increases.
TL;DR: In this article, the conduction mechanism of metal oxide resistive switching memory is investigated and an alternative viewpoint based on a trap-assisted-tunneling model is provided. But the model is not suitable for tunneling due to the fitted dielectric constant and trap energy.
Abstract: The conduction mechanism of metal oxide resistive switching memory is debated in the literature. We measured the I-V characteristics below the switching voltages through TiN/HfOx/Pt memory stack and found the conduction cannot be described by the commonly used Poole-Frenkel model, because the fitted dielectric constant and the trap energy are unreasonable as compared to their known values. Therefore, we provide an alternate viewpoint based on a trap-assisted-tunneling model. Agreement of the bias polarity/temperature/resistance state-dependent conduction behavior was achieved between this model and experimental data. And insights for the multilevel capability due to the control of tunneling distance were obtained.
TL;DR: In this paper, the authors demonstrate InP-based quantum cascade lasers (QCLs) emitting around 4.9μm with 27% and 21% wall plug efficiencies in room temperature (298 K) pulsed and continuous wave (cw) operations, respectively.
Abstract: Using the recently proposed shallow-well design, we demonstrate InP based quantum cascade lasers (QCLs) emitting around 4.9 μm with 27% and 21% wall plug efficiencies in room temperature (298 K) pulsed and continuous wave (cw) operations, respectively. The laser core consists of 40 QCL-stages. The highest cw efficiency is obtained from a buried-ridge device with a ridge width of 8 μm and a cavity length of 5 mm. The front and back facets are antireflection and high-reflection coated, respectively. The maximum single facet cw power at room temperature amounts to 5.1 W.
TL;DR: In this paper, the forward direction of the rectifying current can be reversed repeatedly with polarization switching, indicating a switchable diode effect and large ferroelectric resistive switching.
Abstract: Current-voltage hysteresis and switchable rectifying characteristics have been observed in epitaxial multiferroic BiFeO3 (BFO) thin films. The forward direction of the rectifying current can be reversed repeatedly with polarization switching, indicating a switchable diode effect and large ferroelectric resistive switching. With analyzing the potential barriers and their variation with ferroelectric switching at the interfaces between the metallic electrodes and the semiconducting BFO, the switchable diode effect can be explained qualitatively by the polarization-modulated Schottky-like barriers.
TL;DR: In this article, a waveguide single-photon detector based on superconducting nanowires on GaAs ridge waveguides is proposed for linear-optics quantum computing and quantum communications.
Abstract: The monolithic integration of single-photon sources, passive optical circuits, and single-photon detectors enables complex and scalable quantum photonic integrated circuits, for application in linear-optics quantum computing and quantum communications. Here, we demonstrate a key component of such a circuit, a waveguide single-photon detector. Our detectors, based on superconducting nanowires on GaAs ridge waveguides, provide high efficiency (∼20%) at telecom wavelengths, high timing accuracy (∼60 ps), and response time in the ns range and are fully compatible with the integration of single-photon sources, passive networks, and modulators.
TL;DR: In this article, a near-infrared absorbing organic photovoltaics that are highly transparent to visible light is constructed and shown to achieve power efficiencies of 1.3±0.1% with simultaneous average visible transmission of >65%.
Abstract: We fabricate near-infrared absorbing organic photovoltaics that are highly transparent to visible light. By optimizing near-infrared optical-interference, we demonstrate power efficiencies of 1.3±0.1% with simultaneous average visible transmission of >65%. Subsequent incorporation of near-infrared distributed-Bragg-reflector mirrors leads to an increase in the efficiency to 1.7±0.1%, approaching the 2.4±0.2% efficiency of the opaque cell, while maintaining high visible-transparency of >55%. Finally, we demonstrate that a series-integrated array of these transparent cells is capable of powering electronic devices under near-ambient lighting. This architecture suggests strategies for high-efficiency power-generating windows and highlights an application uniquely benefiting from excitonic electronics.
TL;DR: In this article, the in-depth resolved Raman scattering analysis with different excitation wavelengths of Cu2ZnSnS4 layers is presented, showing peaks that are not detectable by excitation in the visible.
Abstract: This work reports the in-depth resolved Raman scattering analysis with different excitation wavelengths of Cu2ZnSnS4 layers. Secondary phases constitute a central problem in this material, particularly since they cannot be distinguished by x-ray diffraction. Raman spectra measured with 325 nm excitation light after sputtering the layers to different depths show peaks that are not detectable by excitation in the visible. These are identified with Cu3SnS4 modes at the surface region while spectra measured close to the back region show peaks from ZnS and MoS2. Observation of ZnS is enhanced by resonant excitation conditions achieved when working with UV excitation.
TL;DR: In this article, the authors investigated the mechanical properties of graphene under shear deformation using molecular dynamics simulations and showed that the wrinkling behavior of graphene can be significant and proposed an analytical theory based on the kinetic analysis to predict shear strength and fracture shear strain.
Abstract: In this letter, we investigate the mechanical properties of graphene under shear deformation. Specifically, using molecular dynamics simulations, we compute the shear modulus, shear fracture strength, and shear fracture strain of zigzag and armchair graphene structures at various temperatures. To predict shear strength and fracture shear strain, we also present an analytical theory based on the kinetic analysis. We show that wrinkling behavior of graphene under shear deformation can be significant. We compute the amplitude to wavelength ratio of wrinkles using molecular dynamics and compare it with existing theory. Our results indicate that graphene can be a promising mechanical material under shear deformation.
TL;DR: In this article, a proof-of-concept demonstration of negative capacitance effect in a nanoscale ferroelectric-dielectric heterostructure was presented. But the authors did not consider the effect of temperature on the performance of a bilayer of Pb(Zr0.2Ti0.8)O3 and dielectric SrTiO3.
Abstract: We report a proof-of-concept demonstration of negative capacitance effect in a nanoscale ferroelectric-dielectric heterostructure. In a bilayer of ferroelectric Pb(Zr0.2Ti0.8)O3 and dielectric SrTiO3, the composite capacitance was observed to be larger than the constituent SrTiO3 capacitance, indicating an effective negative capacitance of the constituent Pb(Zr0.2Ti0.8)O3 layer. Temperature is shown to be an effective tuning parameter for the ferroelectric negative capacitance and the degree of capacitance enhancement in the heterostructure. Landau’s mean field theory based calculations show qualitative agreement with observed effects. This work underpins the possibility that by replacing gate oxides by ferroelectrics in nanoscale transistors, the sub threshold slope can be lowered below the classical limit (60 mV/decade).
TL;DR: In this paper, the use of metamaterials was proposed to enhance the evanescent wave coupling and improve the transfer efficiency of a wireless power transfer system based on coupled resonators.
Abstract: In this letter, we propose the use of metamaterials to enhance the evanescent wave coupling and improve the transfer efficiency of a wireless power transfer system based on coupled resonators. A magnetic metamaterial is designed and built for a wireless power transfer system. We show with measurement results that the power transfer efficiency of the system can be improved significantly by the metamaterial. We also show that the fabricated system can be used to transfer power wirelessly to a 40 W light bulb.
TL;DR: In this paper, the photocatalytic characteristics of graphene oxide (GO) nanostructures synthesized by modified Hummer's method were investigated by measuring reduction rate of resazurin (RZ) into resorufin (RF) as a function of UV irradiation time.
Abstract: The photocatalytic characteristics of graphene oxide (GO) nanostructures synthesized by modified Hummer’s method were investigated by measuring reduction rate of resazurin (RZ) into resorufin (RF) as a function of UV irradiation time. The progress of the photocatalytic reaction was monitored by change in color from blue (RZ) into pink (RF) followed by absorption spectra. It exhibited excellent photocatalytic activity, leading to the reduction of RZ in UV irradiation. The fitting of absorbance maximum versus time suggests that the reduction of RZ follow the pseudo first-order reaction kinetics. These results indicate that GO have great potential for use as a photocatalyst.
TL;DR: In this paper, phase transitions in ferroelectric silicon doped hafnium oxide (FE-Si:HfO2) were investigated by temperature dependent polarization and x-ray diffraction measurements.
Abstract: We investigated phase transitions in ferroelectric silicon doped hafnium oxide (FE-Si:HfO2) by temperature dependent polarization and x-ray diffraction measurements. If heated under mechanical confinement, the orthorhombic ferroelectric phase reversibly transforms into a phase with antiferroelectric behavior. Without confinement, a transformation into a monoclinic/tetragonal phase mixture is observed during cooling. These results suggest the existence of a common higher symmetry parent phase to the orthorhombic and monoclinic phases, while transformation between these phases appears to be inhibited by an energy barrier.