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Showing papers in "Journal of Applied Physics in 2010"


Journal ArticleDOI
TL;DR: The goal of this paper is to provide a comprehensive review of wireless sub-THz and THz communications and report on the reported advantages and challenges of using sub-terahertz andTHz waves as a means to transmit data wirelessly.
Abstract: According to Edholm’s law, the demand for point-to-point bandwidth in wireless short-range communications has doubled every 18 months over the last 25 years It can be predicted that data rates of around 5–10 Gb/s will be required in ten years In order to achieve 10 Gb/s data rates, the carrier frequencies need to be increased beyond 100 GHz Over the past ten years, several groups have considered the prospects of using sub-terahertz (THz) and THz waves (100–2000 GHz) as a means to transmit data wirelessly Some of the reported advantages of THz communications links are inherently higher bandwidth compared to millimeter wave links, less susceptibility to scintillation effects than infrared wireless links, and the ability to use THz links for secure communications Our goal of this paper is to provide a comprehensive review of wireless sub-THz and THz communications

991 citations


Journal ArticleDOI
TL;DR: In this article, the authors review recent progress in the understanding of effects of irradiation on various zero-dimensional and one-dimensional nanoscale systems, such as semiconductor and metal nanoclusters and nanowires, nanotubes, and fullerenes.
Abstract: A common misconception is that the irradiation of solids with energetic electrons and ions has exclusively detrimental effects on the properties of target materials. In addition to the well-known cases of doping of bulk semiconductors and ion beam nitriding of steels, recent experiments show that irradiation can also have beneficial effects on nanostructured systems. Electron or ion beams may serve as tools to synthesize nanoclusters and nanowires, change their morphology in a controllable manner, and tailor their mechanical, electronic, and even magnetic properties. Harnessing irradiation as a tool for modifying material properties at the nanoscale requires having the full microscopic picture of defect production and annealing in nanotargets. In this article, we review recent progress in the understanding of effects of irradiation on various zero-dimensional and one-dimensional nanoscale systems, such as semiconductor and metal nanoclusters and nanowires, nanotubes, and fullerenes. We also consider the t...

905 citations


Journal ArticleDOI
TL;DR: High aspect ratio (HAR) silicon etch is reviewed in this paper, including commonly used terms, history, main applications, different technological methods, critical challenges, and main theories of the technologies.
Abstract: High aspect ratio (HAR) silicon etch is reviewed, including commonly used terms, history, main applications, different technological methods, critical challenges, and main theories of the technologies. Chronologically, HAR silicon etch has been conducted using wet etch in solution, reactive ion etch (RIE) in low density plasma, single-step etch at cryogenic conditions in inductively coupled plasma (ICP) combined with RIE, time-multiplexed deep silicon etch in ICP-RIE configuration reactor, and single-step etch in high density plasma at room or near room temperature. Key specifications are HAR, high etch rate, good trench sidewall profile with smooth surface, low aspect ratio dependent etch, and low etch loading effects. Till now, time-multiplexed etch process is a popular industrial practice but the intrinsic scalloped profile of a time-multiplexed etch process, resulting from alternating between passivation and etch, poses a challenge. Previously, HAR silicon etch was an application associated primarily with microelectromechanical systems. In recent years, through-silicon-via (TSV) etch applications for three-dimensional integrated circuit stacking technology has spurred research and development of this enabling technology. This potential large scale application requires HAR etch with high and stable throughput, controllable profile and surface properties, and low costs.

598 citations


Journal ArticleDOI
TL;DR: In this paper, the electronic structure as well as the optical response of kesterite and stannite structures of Cu2ZnSnS4 and CoS4 were analyzed by a relativistic full-potential linearized augmented plane.
Abstract: The electronic structure as well as the optical response of kesterite and stannite structures of Cu2ZnSnS4 and Cu2ZnSnSe4 are analyzed by a relativistic full-potential linearized augmented plane wa ...

554 citations


Journal ArticleDOI
TL;DR: In this paper, the authors combine modern silicon microtechnology with advanced deposition methods to fabricate nanocrystalline-diamond lenses for third and fourth-generation synchrotron sources.
Abstract: Diamond has unique properties which make it the ideal material for use in synchrotron instrumentation. X-ray optics made of diamond are almost transparent, they possess strength, and are subject to very low thermal expansion; therefore they will be able to withstand the powerful beams generated by fourth-generation light sources without compromising brilliance. For this reason, several groups are attempting fabrication of refractive lenses and zone plates made of diamond. Lithography and, in general, microfabrication technology, are the ultimate tools for the innovation of synchrotron focusing optics. We propose to combine modern silicon microtechnology with advanced deposition methods to fabricate nanocrystalline-diamond lenses for third- and fourth-generation synchrotron sources. The fabrication method is described here and microfocusing synchrotron tests are illustrated.

400 citations


Journal ArticleDOI
TL;DR: In this paper, the distributions of lithium and stress in a LiCoO2 particle are calculated and the energy release rates are then calculated for the particle containing preexisting cracks.
Abstract: During charging or discharging of a lithium-ion battery, lithium is extracted from one electrode and inserted into the other. This extraction-insertion reaction causes the electrodes to deform. An electrode is often composed of small active particles in a matrix. If the battery is charged at a rate faster than lithium can homogenize in an active particle by diffusion, the inhomogeneous distribution of lithium results in stresses that may cause the particle to fracture. The distributions of lithium and stress in a LiCoO2 particle are calculated. The energy release rates are then calculated for the particle containing preexisting cracks. These calculations predict the critical rate of charging and size of the particle, below which fracture is averted.

382 citations


Journal ArticleDOI
TL;DR: In this paper, first-principles calculations were applied to twelve metal-graphene interfaces and investigated the detailed interface atomic and electronic structures of physisorption and chemisorsorption interfaces, showing that external electric field can be used to modulate graphene energy-levels and the corresponding control of doping or energy range of hybridization.
Abstract: Metal-graphene contact is a key interface in graphene-based device applications, and it is known that two types of interfaces are formed between metal and graphene. In this paper, we apply first-principles calculations to twelve metal-graphene interfaces and investigate the detailed interface atomic and electronic structures of physisorption and chemisorption interfaces. For physisorption interfaces (Ag, Al, Cu, Cd, Ir, Pt, and Au), Fermi level pinning and Pauli-exclusion-induced energy-level shifts are shown to be two primary factors determining graphene’s doping types and densities. For chemisorption interfaces (Ni, Co, Ru, Pd, and Ti), the combination of Pauli-exclusion-induced energy-level shifts and hybridized states’ repulsive interactions lead to a band gap opening with metallic gap states. For practical applications, we show that external electric field can be used to modulate graphene’s energy-levels and the corresponding control of doping or energy range of hybridization.

362 citations


Journal ArticleDOI
TL;DR: In this article, a combination of in situ X-ray diffractometry and x-ray fluorescence (XRF) at a synchrotron light source allowed identifying phases, which tend to decompose and evaporate a Sn-containing compound.
Abstract: In this paper the Sn loss from thin films of the material system Cu–Zn–Sn–S and the subsystems Cu–Sn–S and Sn–S in high vacuum is investigated. A combination of in situ x-ray diffractometry and x-ray fluorescence (XRF) at a synchrotron light source allowed identifying phases, which tend to decompose and evaporate a Sn-containing compound. On the basis of the XRF results a quantification of the Sn loss from the films during annealing experiments is presented. It can be shown that the evaporation rate from the different phases decreases according to the order SnS→Cu2SnS3→Cu4SnS4→Cu2ZnSnS4. The phase SnS is assigned as the evaporating compound. The influence of an additional inert gas component on the Sn loss and on the formation of Cu2ZnSnS4 thin films is discussed.

360 citations


Journal ArticleDOI
TL;DR: In this paper, trap densities at high-permittivity (k) dielectric/III-V semiconductor interfaces and their distribution in the semiconductor band gap are compared.
Abstract: Methods to extract trap densities at high-permittivity (k) dielectric/III-V semiconductor interfaces and their distribution in the semiconductor band gap are compared. The conductance method, the Berglund intergral, the Castagne–Vapaille (high-low frequency), and Terman methods are applied to admittance measurements from metal oxide semiconductor capacitors (MOSCAPs) with high-k/In0.53Ga0.47As interfaces with different interface trap densities. The results are discussed in the context of the specifics of the In0.53Ga0.47As band structure. The influence of different conduction band approximations for determining the ideal capacitance-voltage (CV) characteristics and those of the MOSCAP parameters on the extracted interface trap density are investigated. The origins of discrepancies in the interface trap densities determined from the different methods are discussed. Commonly observed features in the CV characteristics of high-k/In0.53Ga0.47As interfaces are interpreted and guidelines are developed to obtain...

349 citations


Journal ArticleDOI
TL;DR: The domain morphology and crystal structure of lead-free piezoelectric ceramics were systematically studied with transmission electron microscopy for compositions x=0.04 through 0.11.
Abstract: The domain morphology and crystal structure of (1−x)(Bi1/2Na1/2)TiO3xBaTiO3 lead-free piezoelectric ceramics were systematically studied with transmission electron microscopy for compositions x=0.04 through 0.11. It was found that the ceramics with compositions x<0.06 display a R3c symmetry with ferroelectric domains of ∼100 nm forming complex structures at room temperature. Only nanodomains with faint contrast were observed in the compositions of 0.07≤x≤0.09. The presence of weak 1/2 (ooe) superlattice diffraction spots and absence of 1/2 (ooo) ones (o stands for odd and e stands for even miller indices) seem to suggest a P4bm symmetry at room temperature. The morphotropic phase boundary composition x=0.06 showed mixed R3c and P4bm phases. Large lamellar ferroelectric domains with P4mm symmetry were found to dominate in the ceramic of x=0.11. The observed domain structure correlates extremely well with the frequency dispersion of dielectric constant at room temperature and a new concept “relaxor antifer...

340 citations


Journal ArticleDOI
TL;DR: In this paper, the variation in fracture strength of graphene with temperature, strain rate, and crack length using molecular dynamics (MD) simulations, kinetic analysis of fracture with a nonlinear elastic relation, and the quantized fracture mechanics theory.
Abstract: We investigate the variation in fracture strength of graphene with temperature, strain rate, and crack length using molecular dynamics (MD) simulations, kinetic analysis of fracture with a nonlinear elastic relation, and the quantized fracture mechanics theory. Young’s modulus does not vary significantly with temperature until about 1200 K, beyond which the material becomes softer. Temperature plays a more important role in determining the fracture strength of graphene. Our studies suggest that graphene can be a strong material even, when subjected to variations in temperature, strain rate, and cracks.

Journal ArticleDOI
TL;DR: In this paper, the formation of nearly wavelength-sized laser-induced periodic surface structures (LIPSS) on single-crystalline silicon upon irradiation with single (N=1) and multiple (N≤1000) linearly polarized femtosecond (fs) laser pulses (pulse duration τ=130 fs, central wavelength λ=800 nm) in air is studied experimentally.
Abstract: The formation of nearly wavelength-sized laser-induced periodic surface structures (LIPSS) on single-crystalline silicon upon irradiation with single (N=1) and multiple (N≤1000) linearly polarized femtosecond (fs) laser pulses (pulse duration τ=130 fs, central wavelength λ=800 nm) in air is studied experimentally. Scanning electron microscopy (SEM) and optical microscopy are used for imaging of the ablated surface morphologies, both revealing LIPSS with periodicities close to the laser wavelength and an orientation always perpendicular to the polarization of the fs-laser beam. It is experimentally demonstrated that these LIPSS can be formed in silicon upon irradiation by single fs-laser pulses—a result that is additionally supported by a recent theoretical model. Two-dimensional Fourier transforms of the SEM images allow the detailed analysis of the distribution of the spatial frequencies of the LIPSS and indicate, at a fixed peak fluence, a monotonous decrease in their mean spatial period between ∼770 nm...

Journal ArticleDOI
TL;DR: Al-doped ZnO (AZO) films of ∼100nm thickness with various Al doping were prepared at 150°C by atomic layer deposition on quartz substrates as mentioned in this paper.
Abstract: Al-doped ZnO (AZO) films of ∼100 nm thickness with various Al doping were prepared at 150 °C by atomic layer deposition on quartz substrates. At low Al doping, the films were strongly textured along the [100] direction, while at higher Al doping the films remained amorphous. Atomic force microscopy results showed that Al–O cycles when inserted in a ZnO film, corresponding to a few atomic percent Al, could remarkably reduce the surface roughness of the films. Hall measurements revealed a maximum mobility of 17.7 cm2/V s. Film resistivity reached a minima of 4.4×10−3 Ω cm whereas the carrier concentration reached a maxima of 1.7×1020 cm−3, at 3 at. % Al. The band gap of AZO films varied from 3.23 eV for undoped ZnO films to 3.73 eV for AZO films with 24.6 at. % Al. Optical transmittance over 80% was obtained in the visible region. The detrimental impact of increased Al resulting in decreased conductivity due to doping past 3.0 at. % is evident in the x-ray diffraction data, as an abrupt increase in the opti...

Journal ArticleDOI
TL;DR: This paper shows that the type of crosslinks can markedly affect the stress-relaxation behavior of the gels, and implications of these observations are discussed.
Abstract: Long-chained polymers in alginate hydrogels can form networks by either ionic or covalent crosslinks. This paper shows that the type of crosslinks can markedly affect the stress-relaxation behavior of the gels. In gels with only ionic crosslinks, stress relaxes mainly through breaking and subsequent reforming of the ionic crosslinks, and the time scale of the relaxation is independent of the size of the sample. By contrast, in gels with only covalent crosslinks, stress relaxes mainly through migration of water, and the relaxation slows down as the size of the sample increases. Implications of these observations are discussed.

Journal ArticleDOI
TL;DR: In this paper, the authors showed that transformer oil-based nanofluids with conductive nanoparticle suspensions have substantially higher positive voltage breakdown levels with slower positive streamer velocities than that of pure transformer oil.
Abstract: Transformer oil-based nanofluids with conductive nanoparticle suspensions defy conventional wisdom as past experimental work showed that such nanofluids have substantially higher positive voltage breakdown levels with slower positive streamer velocities than that of pure transformer oil. This paradoxical superior electrical breakdown performance compared to that of pure oil is due to the electron charging of the nanoparticles to convert fast electrons from field ionization to slow negatively charged nanoparticle charge carriers with effective mobility reduction by a factor of about 1×105. The charging dynamics of a nanoparticle in transformer oil with both infinite and finite conductivities shows that this electron trapping is the cause of the decrease in positive streamer velocity, resulting in higher electrical breakdown strength. Analysis derives the electric field in the vicinity of the nanoparticles, electron trajectories on electric field lines that charge nanoparticles, and expressions for the char...

Journal ArticleDOI
TL;DR: In this article, the authors reviewed the recent progress in this field by extending the scope to various types of two-dimensional carbon nanostructures including graphene and free-standing carbon nanowalls/nanosheets.
Abstract: Since its discovery in less than five years ago, graphene has become one of the hottest frontiers in materials science and condensed matter physics, as evidenced by the exponential increase in number of publications in this field Several reviews have already been published on this topic, focusing on single and multilayer graphene sheets Here, we review the recent progresses in this field by extending the scope to various types of two-dimensional carbon nanostructures including graphene and free-standing carbon nanowalls/nanosheets After a brief overview of the electronic properties of graphene, we focus on the synthesis, characterization and potential applications of these carbon nanostructures

Journal ArticleDOI
TL;DR: In this article, it was shown that dilute composites with VO2 nanoparticles embedded in hosts with properties mimicking glass or polymer can yield significantly decreased luminous absorption jointly with much enhanced transmittance modulation of solar energy.
Abstract: VO2-based films are thermochromic and show infrared reflectance above a “critical” temperature in the vicinity of room temperature. Implementations on energy efficient windows have been discussed for decades but have been severely curtailed since the luminous absorptance is undesirably large and the solar energy transmittance modulation is too small. Here we show by calculations based on effective medium theory that dilute composites with VO2 nanoparticles embedded in hosts with properties mimicking glass or polymer can yield significantly decreased luminous absorption jointly with much enhanced transmittance modulation of solar energy. These results demonstrate that VO2-based nanothermochromics opens new avenues toward energy efficient fenestration.

Journal ArticleDOI
TL;DR: In this article, the authors studied the polarization-electric-field hysteresis, the dielectric permittivity dispersion, the piezoelectric properties, the electric-field-induced strain, and the interrelations between these properties for bismuth ferrite (BiFeO3) ceramics.
Abstract: We have studied the polarization-electric-field hysteresis, the dielectric permittivity dispersion, the piezoelectric properties, the electric-field-induced strain, and the interrelations between these properties for bismuth ferrite (BiFeO3) ceramics. The results indicate that the domain-wall movement in BiFeO3 is strongly inhibited by charged defects, most probably acceptor-oxygen-vacancy defect pairs. The domain-wall mobility can be considerably increased by preventing the defects from migrating into their stable configuration; this can be achieved by thermal quenching from above the Curie temperature, which freezes the disordered defect state. Similarly, Bi2O3 loss during annealing at high temperatures contributes to depinning of the domain walls and an increase in the remanent polarization. The possible defects causing the pinning effect are analyzed and discussed. A weakening of the contacts between the grains in the ceramics and crack propagation were observed during poling with constant field at 100 kV/cm. This is probably caused by an electrically induced strain associated with ferroelastic domain reversal. A relatively large piezoelectric d33 constant of 44 pC/N was obtained by “cyclic poling,” in which the electric field was released after each applied cycle with the purpose to relax the mechanical stresses and minimize the problem of cracking.

Journal ArticleDOI
TL;DR: In this paper, the photothermal induced resonance (PTIR) was used to measure the local infrared absorption spectrum of a sample shined with a tunable infrared laser pulse, and detected the induced photothermal expansion with the tip of an atomic force microscope (AFM).
Abstract: We present a theoretical investigation of the physics involved in a recently developed spectromicroscopy technique, called photothermal induced resonance (PTIR). With this technique, one measures the local infrared absorption spectrum of a sample shined with a tunable infrared laser pulse, and detects the induced photothermal expansion with the tip of an atomic force microscope (AFM). Simple physical assumptions allow us to describe analytically the heating and expansion of the sample, the excitation of the vibration modes of the AFM cantilever, and the detected signal. We show that the signal depends on the thermal expansion velocity rather than on the absolute displacement of the tip, and we investigate the influence of the laser pulse length. Eventually, we express the PTIR signal in terms of relevant parameters, and prove its proportionality to the sample absorbance. This analytical approach complement our experimental results and validates the PTIR method as a technique of choice for infrared spectro...

Journal ArticleDOI
TL;DR: In this article, the optical UC efficiency of trivalent erbium doped sodium yttrium fluoride (NaYF4:20%Er3+ +) was investigated for silicon solar cells.
Abstract: Upconversion (UC) of subband-gap photons is a promising possibility to enhance solar cell efficiency by making also the subband-gap photons useful. For this application, we investigate the material system of trivalent erbium doped sodium yttrium fluoride (NaYF4:20%Er3+), which shows efficient UC suitable for silicon solar cells. We determine the optical UC efficiency by calibrated photoluminescence measurements. Because these data are free from any influence of losses associated with the application of the upconverter to the solar cell, the obtained values constitute the upper limit that can be achieved with an optimized device. Subsequently, we compare the results of the optical measurements with the results obtained by using solar cells as detectors on which the upconverter material is applied. We find an optical UC quantum efficiency of 5.1% at a monochromatic irradiance of 1880 W m−2 (0.27 cm2 W−1) at 1523 nm. The device of silicon solar cell and applied upconverter showed an external quantum efficien...

Journal ArticleDOI
TL;DR: In this article, a series of CIGS thin film solar cells with differently prepared heterojunctions has been investigated by admittance spectroscopy, capacitancevoltage (CV) profiling, and temperature dependent current-voltage measurements, and the results strongly contradict the common assignment of the N1 response to a donor defect at or close to the heterointerface.
Abstract: A series of Cu(In,Ga)Se2 (CIGS) thin film solar cells with differently prepared heterojunctions has been investigated by admittance spectroscopy, capacitance-voltage (CV) profiling, and temperature dependent current-voltage (IVT) measurements. The devices with different CdS buffer layer thicknesses, with an In2S3 buffer or with a Schottky barrier junction, all show the characteristic admittance step at shallow energies between 40 and 160 meV, which has often been referred to as the N1 defect. No correlation between the buffer layer thickness and the capacitance step is found. IVT measurements show that the dielectric relaxation frequency of charge carriers in the CdS layers is smaller than the N1-resonance frequency at low temperatures where the N1 step in admittance is observed. These results strongly contradict the common assignment of the N1 response to a donor defect at or close to the heterointerface. In contrast, an explanation for the N1 response is proposed, which relates the admittance step to a non-Ohmic back-contact acting as a second junction in the device. The model, which is substantiated with numerical device simulations, allows a unified explanation of characteristic admittance, CV, and IVT features commonly observed in CIGS solar cells.

Journal ArticleDOI
TL;DR: In this paper, the synthesis of highly stable exfoliated graphene based nanofluids with water and ethylene glycol as base fluids with out any surfactant and subsequent studies on their thermal and electrical conductivities.
Abstract: We report for the first time, the synthesis of highly stable exfoliated graphene based nanofluids with water and ethylene glycol as base fluids with out any surfactant and the subsequent studies on their thermal and electrical conductivities. Graphene was synthesized by thermal exfoliation of graphene oxide at 1050 °C in Ar atmosphere. The as-synthesized graphene has been suitably functionalized and further dispersed it in the base fluids without any surfactant. Thermal and electrical conductivities of these nanofluids were measured for varying volume fractions and at different temperatures. An enhancement in thermal conductivity by about 14% has been achieved at 25 °C with deionized water (DI) as base fluid at a very low volume fraction of 0.056% which increases to about 64% at 50 °C. Electrical conductivity measurements for these nanofluids indicate an enormous enhancement at 25 °C for a volume fraction of 0.03%in DI water.

Journal ArticleDOI
TL;DR: In this article, the experimental conditions necessary for the glow regime of NRP discharges have been determined, with the notable result that there exists a minimum and maximum gap distance for its existence at a given ambient gas temperature.
Abstract: In atmospheric pressure air preheated from 300 to 1000 K, the nanosecond repetitively pulsed (NRP) method has been used to generate corona, glow, and spark discharges. Experiments have been performed to determine the parameter space (applied voltage, pulse repetition frequency, ambient gas temperature, and interelectrode gap distance) of each discharge regime. In particular, the experimental conditions necessary for the glow regime of NRP discharges have been determined, with the notable result that there exists a minimum and maximum gap distance for its existence at a given ambient gas temperature. The minimum gap distance increases with decreasing gas temperature, whereas the maximum does not vary appreciably. To explain the experimental results, an analytical model is developed to explain the corona-to-glow (C-G) and glow-to-spark (G-S) transitions. The C-G transition is analyzed in terms of the avalanche-to-streamer transition and the breakdown field during the conduction phase following the establish...

Journal ArticleDOI
TL;DR: In this paper, a systematic review of long period fiber gratings (LPFGs) written by the CO2 laser irradiation technique is presented, and several pretreament and post-treatment techniques are proposed to enhance the efficiency of grating fabrications.
Abstract: This paper presents a systematic review of long period fiber gratings (LPFGs) written by the CO2 laser irradiation technique. First, various fabrication techniques based on CO2 laser irradiations are demonstrated to write LPFGs in different types of optical fibers such as conventional glass fibers, solid-core photonic crystal fibers, and air-core photonic bandgap fibers. Second, possible mechanisms, e.g., residual stress relaxation, glass structure changes, and physical deformation, of refractive index modulations in the CO2-laser-induced LPFGs are analyzed. Third, asymmetrical mode coupling, resulting from single-side laser irradiation, is discussed to understand unique optical properties of the CO2-laser-induced LPFGs. Fourthly, several pretreament and post-treatment techniques are proposed to enhance the efficiency of grating fabrications. Fifthly, sensing applications of the CO2-laser-induced LPFGs are investigated to develop various LPFG-based temperature, strain, bend, torsion, pressure, and biochemical sensors. Finally, communication applications of the CO2-laser-induced LPFGs are investigated to develop various LPFG-based band-rejection filters, gain equalizers, polarizers, and couplers.

Journal ArticleDOI
TL;DR: In this paper, the atomic and electronic properties of black phosphorus (BP) were studied via ab initio calculations, which revealed that the interlayer interaction in BP is Van der Waals Keesom force, which is critical to the formation of the layered structure.
Abstract: The atomic and electronic properties of black phosphorus (BP), which has been recently shown to have potential application as anode material for lithium ion batteries, are studied via ab initio calculations. The calculations reveal that the interlayer interaction in BP is Van der Waals Keesom force, which is critical to the formation of the layered structure. Interestingly, we also found that the small band gap of bulk BP (0.19 eV) when compared with that of single layer BP (0.75 eV) is partly because of the interlayer Van der Waals interaction in BP. The change in a materials band structure because of Van der Waals interaction is rarely reported in literature.

Journal ArticleDOI
TL;DR: In this paper, the impact of device size scaling on the light output, spectral shift, and self-heating of 400 nm InGaN light-emitting diodes (LEDs) was systematically investigated.
Abstract: We have systematically investigated the impact of device size scaling on the light output, spectral shift, and self-heating of 400 nm InGaN light-emitting diodes (LEDs). Devices with diameters in the range 20-300 µm have been studied. It is shown that smaller LED pixels can deliver higher power densities (despite the lower absolute output powers) and sustain higher current densities. Investigations of the electroluminescence characteristics of differently sized pixels against current density reveal that the spectral shift is dominated by blueshift at the low current density level and then by redshift at the high current density level, owing to the competition between the bandgap shrinkage caused by self-heating and band-filling effects. The redshift of the emission wavelength with increasing current density is much faster and larger for the bigger pixels, suggesting that the self-heating effect is also size dependent. This is further confirmed by the junction-temperature rise measured by the established spectral shift method. It is shown that the junction-temperature rise in smaller pixels is slower, which in turn explains why the smaller redshift of the emission wavelength with current density is present in smaller pixels. The measured size-dependent junction temperature is in reasonable agreement with finite element method simulation results.

Journal ArticleDOI
TL;DR: The results revealed that the intrinsic (reversible) contribution plays a dominant role in the high piezoelectric activity for PMN-PT crystals, and the extrinsic contribution was found to be less than 5% for the compositions away from R-M(C) and M(C)-T phase boundaries, due to a stable domain engineered structure.
Abstract: The piezoelectric response of [001] poled domain engineered (1−x)Pb(Mg1/3Nb2/3)O3−xPbTiO3 (PMN-PT) crystals was investigated as a function of composition and phase using Rayleigh analysis. The results revealed that the intrinsic (reversible) contribution plays a dominant role in the high piezoelectric activity for PMN-PT crystals. The intrinsic piezoelectric response of the monoclinic (MC) PMN−xPT crystals, 0.31≤x≤0.35, exhibited peak values for compositions close to R-MC and MC-T phase boundaries, however, being less than 2000 pC/N. In the rhombohedral phase region, x≤0.30, the intrinsic piezoelectric response was found to increase as the composition approached the rhombohedral-monoclinic (R-MC) phase boundary. The maximum piezoelectric response was observed in rhombohedral PMN-0.30PT crystals, being on the order of 2500 pC/N. This ultrahigh piezoelectric response was determined to be related to the high shear piezoelectric activity of single domain state, corresponding to an ease in polarization rotation, for compositions close to a morphotropic phase boundary (MPB). The role of monoclinic phase is only to form a MPB with R phase, but not directly contribute to the ultrahigh piezoelectric activity in rhombohedral PMN-0.30PT crystals. The extrinsic contribution to piezoelectric activity was found to be less than 5% for the compositions away from R-MC and MC-T phase boundaries, due to a stable domain engineered structure. As the composition approached MPBs, the extrinsic contribution increased slightly (<10%), due to the enhanced motion of phase boundaries.

Journal ArticleDOI
TL;DR: In this article, the authors propose and experimentally validate a first-principles based model for the nonlinear piezoelectric response of an electroelastic energy harvester.
Abstract: We propose and experimentally validate a first-principles based model for the nonlinear piezoelectric response of an electroelastic energy harvester The analysis herein highlights the importance of modeling inherent piezoelectric nonlinearities that are not limited to higher order elastic effects but also include nonlinear coupling to a power harvesting circuit Furthermore, a nonlinear damping mechanism is shown to accurately restrict the amplitude and bandwidth of the frequency response The linear piezoelectric modeling framework widely accepted for theoretical investigations is demonstrated to be a weak presumption for near-resonant excitation amplitudes as low as 05 g in a prefabricated bimorph whose oscillation amplitudes remain geometrically linear for the full range of experimental tests performed (never exceeding 025% of the cantilever overhang length) Nonlinear coefficients are identified via a nonlinear least-squares optimization algorithm that utilizes an approximate analytic solution obta

Journal ArticleDOI
TL;DR: In this paper, the authors have developed models allowing a direct comparison between the single-gate, double-gate and gate-all-around configuration at high drain voltage, when the drain-voltage dependence is negligible.
Abstract: Tunnel field-effect transistors (TFETs) are potential successors of metal-oxide-semiconductor FETs because scaling the supply voltage below 1 V is possible due to the absence of a subthreshold-swing limit of 60 mV/decade. The modeling of the TFET performance, however, is still preliminary. We have developed models allowing a direct comparison between the single-gate, double-gate, and gate-all-around configuration at high drain voltage, when the drain-voltage dependence is negligible, and we provide improved insight in the TFET physics. The dependence of the tunnel current on device parameters is analyzed, in particular, the scaling with gate-dielectric thickness, channel thickness, and dielectric constants of gate dielectric and channel material. We show that scaling the gate-dielectric thickness improves the TFET performance more than scaling the channel thickness and that improvements are often overestimated. There is qualitative agreement between our model and our experimental data.

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the band structure of tensile-strained germanium using a 30 band k⋅p formalism and obtained that the crossover from indirect to direct band gap occurs for a tensile in-plane strain of 1.9%.
Abstract: We have investigated the band structure of tensile-strained germanium using a 30 band k⋅p formalism. This multiband formalism allows to simultaneously describe the valence and conduction bands, including the L, Δ, and Γ valleys. We calculate the energy band variation as a function of strain and obtain that the crossover from indirect to direct band gap occurs for a tensile in-plane strain of 1.9%. The effective masses of density of states are deduced from the calculated conduction and valence band density of states. Significant deviations are observed as compared to the effective masses of density of states values of unstrained bulk germanium. We finally calculate the optical gain that can be achieved with tensile-strained bulk germanium. An optical gain larger than 3000 cm−1 is predicted for a carrier density of 1×1018 cm−3 and a 3% in-plane biaxial strain. This optical gain is larger than the one of GaAs calculated with the same formalism and is much larger than the experimental free-carrier absorption ...