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Showing papers in "Journal of Applied Physics in 2017"


Journal ArticleDOI
TL;DR: Hall thrusters as discussed by the authors are very efficient and competitive electric propulsion devices for satellites and are currently in use in a number of telecommunications and government spacecraft, with specific impulse values between 1000 and 3000's.
Abstract: Hall thrusters are very efficient and competitive electric propulsion devices for satellites and are currently in use in a number of telecommunications and government spacecraft. Their power spans from 100 W to 20 kW, with thrust between a few mN and 1 N and specific impulse values between 1000 and 3000 s. The basic idea of Hall thrusters consists in generating a large local electric field in a plasma by using a transverse magnetic field to reduce the electron conductivity. This electric field can extract positive ions from the plasma and accelerate them to high velocity without extracting grids, providing the thrust. These principles are simple in appearance but the physics of Hall thrusters is very intricate and non-linear because of the complex electron transport across the magnetic field and its coupling with the electric field and the neutral atom density. This paper describes the basic physics of Hall thrusters and gives a (non-exhaustive) summary of the research efforts that have been devoted to th...

326 citations


Journal ArticleDOI
TL;DR: In this paper, a step-by-step approach is described for reactive high power impulsive magnetron sputtering (R-HiPIMS), which is a coating technology for high and low sputter yield materials, respectively.
Abstract: High Power Impulse Magnetron Sputtering (HiPIMS) is a coating technology that combines magnetron sputtering with pulsed power concepts. By applying power in pulses of high amplitude and a relatively low duty cycle, large fractions of sputtered atoms and near-target gases are ionized. In contrast to conventional magnetron sputtering, HiPIMS is characterized by self-sputtering or repeated gas recycling for high and low sputter yield materials, respectively, and both for most intermediate materials. The dense plasma in front of the target has the dual function of sustaining the discharge and providing plasma-assistance to film growth, affecting the microstructure of growing films. Many technologically interesting thin films are compound films, which are composed of one or more metals and a reactive gas, most often oxygen or nitrogen. When reactive gas is added, non-trivial consequences arise for the system because the target may become “poisoned,” i.e., a compound layer forms on the target surface affecting the sputtering yield and the yield of secondary electron emission and thereby all other parameters. It is emphasized that the target state depends not only on the reactive gas' partial pressure (balanced via gas flow and pumping) but also on the ion flux to the target, which can be controlled by pulse parameters. This is a critical technological opportunity for reactive HiPIMS (R-HiPIMS). The scope of this tutorial is focused on plasma processes and mechanisms of operation and only briefly touches upon film properties. It introduces R-HiPIMS in a systematic, step-by-step approach by covering sputtering, magnetron sputtering, reactive magnetron sputtering, pulsed reactive magnetron sputtering, HiPIMS, and finally R-HiPIMS. The tutorial is concluded by considering variations of R-HiPIMS known as modulated pulsed power magnetron sputtering and deep-oscillation magnetron sputtering and combinations of R-HiPIMS with superimposed dc magnetron sputtering.

261 citations


Journal ArticleDOI
TL;DR: In this Perspective article, a few important examples are discussed: the development of methods to access nonlinear optical effects in the terahertz range; methods to probe nanoscale phenomena; and, the growing likelihood that teraHertz technologies will be a critical player in future wireless networks.
Abstract: The field of terahertz science and technology has been an active and thriving research area for several decades. However, the field has recently experienced an inflection point, as several exciting breakthroughs have enabled new opportunities for both fundamental and applied research. These events are reshaping the field, and will impact research directions for years to come. In this Perspective article, I discuss a few important examples: the development of methods to access nonlinear optical effects in the terahertz range; methods to probe nanoscale phenomena; and, the growing likelihood that terahertz technologies will be a critical player in future wireless networks. Here, a few examples of research in each of these areas are discussed, followed by some speculation about where these exciting breakthroughs may lead in the near future.

252 citations


Journal ArticleDOI
TL;DR: In this article, the authors used low temperature plasma discharges as sources of chemically reactive species that can be transported to interact with biological media, cells, and tissues and induce impactful biological effects.
Abstract: Low temperature plasmas have been used in various plasma processing applications for several decades. But it is only in the last thirty years or so that sources generating such plasmas at atmospheric pressure in reliable and stable ways have become more prevalent. First, in the late 1980s, the dielectric barrier discharge was used to generate relatively large volume diffuse plasmas at atmospheric pressure. Then, in the early 2000s, plasma jets that can launch cold plasma plumes in ambient air were developed. Extensive experimental and modeling work was carried out on both methods and much of the physics governing such sources was elucidated. Starting in the mid-1990s, low temperature plasma discharges have been used as sources of chemically reactive species that can be transported to interact with biological media, cells, and tissues and induce impactful biological effects. However, many of the biochemical pathways whereby plasma affects cells remain not well understood. This situation is changing rather ...

215 citations


Journal ArticleDOI
TL;DR: In this paper, the authors performed electrical characterizations of Nb2O5 doped 0.65BF-0.35BaTiO3 (0.65BiFeO3 − 0.35BT) ceramics over broad temperature and frequency ranges through dielectric spectroscopy and ac conductivity measurements, and the experimental results were well fitted based on a Maxwell-Wagner (MW) interfacial polarization model.
Abstract: Electrical characterizations of Nb2O5 doped 0.65BiFeO3–0.35BaTiO3 (0.65BF–0.35BT) ceramic were carried out over broad temperature and frequency ranges through dielectric spectroscopy, impedance spectroscopy, and ac conductivity measurements. The dielectric constant and loss tangent are drastically reduced with introducing Nb2O5 into the 0.65BF–0.35BT system. Two dielectric anomalies are detected in the temperature regions of 100 °C ≤ T ≤ 280 °C and 350 °C ≤ T ≤ 480 °C, and the Curie temperature (TC) was confirmed in higher temperature region. A dielectric relaxation with large dielectric constants was detected near the TC. This dielectric relaxation becomes even stronger with the gradual increase in the Nb2O5 content. Impedance spectroscopy results clearly show the contributions of grains and grain boundaries in the frequency range of 100 Hz ≤ f ≤ 1 MHz, and the relaxation processes for grains and grain boundaries are non-Debye-type. The grain boundaries are more resistive than that of the grains, revealing the inhomogeneity in samples. The experimental results are well fitted based on a Maxwell-Wagner (MW) interfacial polarization model below 100 kHz, and the MW interfacial polarization effect becomes more and more obvious with the increase in the Nb2O5 content. The increase in dielectric constant is possibly related to space charge polarization, which is caused by charges accumulated at the interface between the grain and grain boundaries. Frequency dependence of the ac conductivity confirms the MW interfacial polarization effect below 100 kHz.

176 citations


Journal ArticleDOI
TL;DR: In this paper, lead-free (1−x)(0.8Bi0.5Na0.3-0.2SrTiO3)-xNaNbO3 ceramics were fabricated by a conventional sintering route with pure perovskite phase via XRD analysis and the Raman spectrum was exploited to give an insight into the variation of local structural evolution.
Abstract: Lead-free (1−x)(0.8Bi0.5Na0.5TiO3-0.2SrTiO3)-xNaNbO3 (x = 0–0.1, abbreviated as BNT-ST-xNN) ceramics were fabricated by a conventional sintering route with pure perovskite phase via XRD analysis. Raman spectrum was exploited in order to give an insight into the variation of local structural evolution. All compositions exhibited an obvious evolution of dielectric relaxation behaviors. Dielectric and ferroelectric properties clarified that a crossover from nonergodic to ergodic relaxor properties was obtained with the addition of NN content. A relatively large energy storage density was obtained WRec ∼ 0.74 J/cm3 at 7 kV/mm for x = 0.05 at room temperature. Particularly, the energy storage properties exhibited temperature (25–160 °C) and frequency stability (0.1–20 Hz) with WRec around 0.6 J/cm3 at 6 kV/mm for x = 0.05 within the ergodic region. Pulsed discharging current waveforms were measured under different electric fields to detect the energy storage density and discharging speed behavior. An illustrat...

166 citations


Journal ArticleDOI
TL;DR: In this article, the effects of recombination at the interfaces and grain boundaries as lifetime and doping of the CdTe layer change were analyzed for maximizing open-circuit voltage and efficiency.
Abstract: CdTe devices have reached efficiencies of 22% due to continuing improvements in bulk material properties, including minority carrier lifetime. Device modeling has helped to guide these device improvements by quantifying the impacts of material properties and different device designs on device performance. One of the barriers to truly predictive device modeling is the interdependence of these material properties. For example, interfaces become more critical as bulk properties, particularly, hole density and carrier lifetime, increase. We present device-modeling analyses that describe the effects of recombination at the interfaces and grain boundaries as lifetime and doping of the CdTe layer change. The doping and lifetime should be priorities for maximizing open-circuit voltage (Voc) and efficiency improvements. However, interface and grain boundary recombination become bottlenecks for device performance at increased lifetime and doping levels. This work quantifies and discusses these emerging challenges f...

163 citations


Journal ArticleDOI
TL;DR: In this paper, a broadband microwave cross-polarization-conversion (CPC) metasurface is designed, simulated, fabricated, and tested on an FR4 dielectric substrate backed by a metallic ground plane.
Abstract: A broadband microwave cross-polarization-conversion (CPC) metasurface is designed, simulated, fabricated, and tested. The metasurface consists of coupled split-ring-resonators (SRRs) with two splits in each SRR and is designed on an FR4 dielectric substrate backed by a metallic ground plane. An efficient CPC, both for normal as well as for oblique incidence, is achieved with 3 dB fractional bandwidth of 73% from 5 to 10.8 GHz. This wideband polarization conversion results from multiple plasmonic resonances occurring at three neighboring frequencies. Owing to the sub-wavelength unit cell size and symmetric structure of the coupled SRRs, the response of the metasurface is independent of the polarization and incidence angle of the incoming wave, which makes it a potential candidate for many practical applications. The proposed design is validated both numerically and experimentally. Experimental results are found to be in good agreement with simulations.

147 citations


Journal ArticleDOI
TL;DR: In this paper, the relationship between the Rabi frequency (ΩRF) related to the applied electric field and Autler-Townes (AT) splitting, when performing atom-based radiofrequency (RF) electric (E) field strength measurements using Rydberg states and electromagnetically induced transparency (EIT) in an atomic vapor, was investigated.
Abstract: We investigate the relationship between the Rabi frequency (ΩRF, related to the applied electric field) and Autler-Townes (AT) splitting, when performing atom-based radio-frequency (RF) electric (E) field strength measurements using Rydberg states and electromagnetically induced transparency (EIT) in an atomic vapor. The AT splitting satisfies, under certain conditions, a well-defined linear relationship with the applied RF field amplitude. The EIT/AT-based E-field measurement approach derived from these principles is currently being investigated by several groups around the world as a means to develop a new SI-traceable RF E-field measurement technique. We establish conditions under which the measured AT-splitting is an approximately linear function of the RF electric field. A quantitative description of systematic deviations from the linear relationship is key to exploiting EIT/AT-based atomic-vapor spectroscopy for SI-traceable field measurement. We show that the linear relationship is valid and can be...

141 citations


Journal ArticleDOI
TL;DR: In this paper, the dopant-induced ferroelectric HfO2 formation has been systematically investigated by using cation (Sc, Y, Nb, Al, Si, Ge, and Zr) and anion (N) dopants.
Abstract: The dopant-induced ferroelectric HfO2 formation has been systematically investigated by using cation (Sc, Y, Nb, Al, Si, Ge, and Zr) and anion (N) dopants Both differences and similarities are discussed among various dopants by focusing on two major factors, the oxygen vacancy (Vo) and the dopant ionic size First, the doping concentration dependence of the remanent polarization in 27 (±2) nm HfO2 films is quantitatively estimated Then, by comparing the polarization result with the structural transformation in doped HfO2, the pathway of the dopant-induced HfO2 phase transition is discussed among monoclinic, ferroelectric orthorhombic, tetragonal, and cubic phases Finally, it is addressed that a dopant species independent phase transition route may exist in HfO2 owing to the same kinetic transition process, in which the ferroelectric phase seems to be at an intermediate state between tetragonal and monoclinic phases

140 citations


Journal ArticleDOI
TL;DR: In this paper, the authors report on the thin film resistivity of several platinum-group metals (Ru, Pd, Ir, and Pt) based on experimentally determined mean linear distances between grain boundaries as well as ab initio calculations of the electron mean free path.
Abstract: We report on the thin film resistivity of several platinum-group metals (Ru, Pd, Ir, and Pt). Platinum-group thin films show comparable or lower resistivities than Cu for film thicknesses below about 5 nm due to a weaker thickness dependence of the resistivity. Based on experimentally determined mean linear distances between grain boundaries as well as ab initio calculations of the electron mean free path, the data for Ru, Ir, and Cu were modeled within the semiclassical Mayadas–Shatzkes model [Phys. Rev. B 1, 1382 (1970)] to assess the combined contributions of surface and grain boundary scattering to the resistivity. For Ru, the modeling results indicated that surface scattering was strongly dependent on the surrounding material with nearly specular scattering at interfaces with SiO2 or air but with diffuse scattering at interfaces with TaN. The dependence of the thin film resistivity on the mean free path is also discussed within the Mayadas–Shatzkes model in consideration of the experimental findings.

Journal ArticleDOI
TL;DR: In this paper, the growth of high quality epitaxial beta-gallium oxide (β-Ga2O3) using a compound source by molecular beam epitaxy has been demonstrated on c-plane sapphire (Al2O 3) substrates.
Abstract: The growth of high quality epitaxial beta-gallium oxide (β-Ga2O3) using a compound source by molecular beam epitaxy has been demonstrated on c-plane sapphire (Al2O3) substrates. The compound source provides oxidized gallium molecules in addition to oxygen when heated from an iridium crucible in a high temperature effusion cell enabling a lower heat of formation for the growth of Ga2O3, resulting in a more efficient growth process. This source also enabled the growth of crystalline β-Ga2O3 without the need for additional oxygen. The influence of the substrate temperatures on the crystal structure and quality, chemical bonding, surface morphology, and optical properties has been systematically evaluated by x-ray diffraction, scanning transmission electron microscopy, x-ray photoelectron spectroscopy, atomic force microscopy, spectroscopic ellipsometry, and UV-vis spectroscopy. Under optimized growth conditions, all films exhibited pure 2¯01 oriented β-Ga2O3 thin films with six-fold rotational symmetry when grown on a sapphire substrate. The thin films demonstrated significant absorption in the deep-ultraviolet (UV) region with an optical bandgap around 5.0 eV and a refractive index of 1.9. A deep-UV photodetector fabricated on the high quality β-Ga2O3 thin film exhibits high resistance and small dark current (4.25 nA) with expected photoresponse for 254 nm UV light irradiation suggesting that the material grown using the compound source is a potential candidate for deep-ultraviolet photodetectors.

Journal ArticleDOI
TL;DR: In this article, a semi-coarse version of the Wannier-Fourier interpolation method was proposed for short-range non-polar optical phonon (EPI) elements in order to ease the computational requirement in FBMC simulation.
Abstract: We investigate the high-field transport in monoclinic β-Ga2O3 using a combination of ab initio calculations and full band Monte Carlo (FBMC) simulation. Scattering rate calculation and the final state selection in the FBMC simulation use complete wave-vector (both electron and phonon) and crystal direction dependent electron phonon interaction (EPI) elements. We propose and implement a semi-coarse version of the Wannier-Fourier interpolation method [Giustino et al., Phys. Rev. B 76, 165108 (2007)] for short-range non-polar optical phonon (EPI) elements in order to ease the computational requirement in FBMC simulation. During the interpolation of the EPI, the inverse Fourier sum over the real-space electronic grids is done on a coarse mesh while the unitary rotations are done on a fine mesh. This paper reports the high field transport in monoclinic β-Ga2O3 with deep insight into the contribution of electron-phonon interactions and velocity-field characteristics for electric fields ranging up to 450 kV/cm i...

Journal ArticleDOI
TL;DR: In this article, an efficient two-dimensional T-A formulation based approach is proposed to calculate the electromagnetic characteristics of tape stacks and coils made of second generation high temperature superconductors.
Abstract: An efficient two dimensional T-A formulation based approach is proposed to calculate the electromagnetic characteristics of tape stacks and coils made of second generation high temperature superconductors. In the approach, a thin strip approximation of the superconductor is used in which the superconducting layer is modeled as a 1-dimensional domain. The formulation is mainly based on the calculation of the current vector potential T in the superconductor layer and the calculation of the magnetic vector potential A in the whole space, which are coupled together in the model. Compared with previous T-based models, the proposed model is innovative in terms of magnetic vector potential A solving, which is achieved by using the differential method, instead of the integral method. To validate the T-A formulation model, it is used to simulate racetrack coils made of second generation high temperature superconducting (2G HTS) tape, and the results are compared with the experimentally obtained data on the AC loss. The results show that the T-A formulation is accurate and efficient in calculating 2G HTS coils, including magnetic field distribution, current density distribution, and AC loss. Finally, the proposed model is used for simulating a 2000 turn coil to demonstrate its effectiveness and efficiency in simulating large-scale 2G HTS coils.

Journal ArticleDOI
TL;DR: In this article, the effects of many process variables and alloy properties on the structure and properties of additively manufactured parts using four dimensionless numbers were examined using a well-tested three-dimensional transient heat transfer and fluid flow model.
Abstract: The effects of many process variables and alloy properties on the structure and properties of additively manufactured parts are examined using four dimensionless numbers. The structure and properties of components made from 316 Stainless steel, Ti-6Al-4V, and Inconel 718 powders for various dimensionless heat inputs, Peclet numbers, Marangoni numbers, and Fourier numbers are studied. Temperature fields, cooling rates, solidification parameters, lack of fusion defects, and thermal strains are examined using a well-tested three-dimensional transient heat transfer and fluid flow model. The results show that lack of fusion defects in the fabricated parts can be minimized by strengthening interlayer bonding using high values of dimensionless heat input. The formation of harmful intermetallics such as laves phases in Inconel 718 can be suppressed using low heat input that results in a small molten pool, a steep temperature gradient, and a fast cooling rate. Improved interlayer bonding can be achieved at high Ma...

Journal ArticleDOI
TL;DR: In this article, an optically transparent metamaterial with broadband absorption is presented theoretically and demonstrated experimentally, which comprises of structures made of resistive films of indium-tinoxide and exhibits over 10 dB absorption in the frequency range of 6.06-14.66 GHz.
Abstract: In this paper, an optically transparent metamaterial with broadband absorption is presented theoretically and demonstrated experimentally. The design comprises of structures made of resistive films of indium-tin-oxide and the metamaterial exhibits over 10 dB absorption in the frequency range of 6.06–14.66 GHz. The novelty of the structure lies in its large absorption bandwidth along with a reduced thickness and optical transparency compared to broadband absorbers reported earlier. Besides, the proposed design is polarization-insensitive and gives rise to angular independent absorption for both transverse electric and transverse magnetic polarizations. The absorption mechanism in the structure has been studied by deriving an equivalent circuit model as well as analyzing several design parameters. Finally, a prototype of the proposed structure has been fabricated and measured, which shows good agreement with the simulated results.

Journal ArticleDOI
Abstract: The Acoustic Black Hole (ABH) effect can be used to effectively reduce structural vibrations by trapping flexural waves in a thin-walled structure with a power-law thickness variation In the present study, we used a wavelet-decomposed energy method to investigate an Euler-Bernoulli beam embedded with multiple ABHs Broadband transmission attenuation bands at relatively low frequencies are observed in a beam containing only a few ABH elements To explain the underlying phenomena, an infinite structure with periodic ABH elements is analyzed Numerical results show that the periodic boundary conditions in terms of displacement and rotational slope of a unit cell, based on the finite model, are sufficient to describe the band structures, without requiring full treatment of the entire infinite structure This provides an efficient and flexible means to predict, and eventually optimize, the band structure based on a single element Meanwhile, the ABH-induced locally resonant band gaps coincide with the attenua

Journal ArticleDOI
TL;DR: In this article, a metasurface acting as a linear polarization rotator, that can efficiently convert linearly polarized electromagnetic waves to cross polarized waves within an ultra wide frequency band and with a broad incident angle, is proposed.
Abstract: In this work, a metasurface acting as a linear polarization rotator, that can efficiently convert linearly polarized electromagnetic waves to cross polarized waves within an ultra wide frequency band and with a broad incident angle, is proposed. Based on the electric and magnetic resonant features of the unit cell, composed by a double-head arrow, a cut-wire, and two short V-shaped wire structures, three resonances, which lead to the bandwidth expansion of cross-polarization reflections, are generated. The simulation results show that an average polarization conversion ratio of 90% from 17.3 GHz to 42.2 GHz can be achieved. Furthermore, the designed metasurface exhibits polarization insensitivity within a broad incident angle, from 0° to 50°. The experiments conducted on the fabricated metasurface are in good agreement with the simulations. The proposed metasurface can find potential applications in reflector antennas, imaging systems, and remote sensors operating at microwave frequencies.

Journal ArticleDOI
TL;DR: In this article, an optimized GaAs buffer layer with thermal cycle annealing and InGaAs/GaAs dislocation filter layers has a threading dislocation density of 7.2 cm−2, which is a factor of 40 lower than an unoptimized buffer layer.
Abstract: We report a systematic study of high quality GaAs growths on on-axis (001) GaP/Si substrates using molecular beam epitaxy. Various types of dislocation filter layers and growth temperatures of initial GaAs layer were investigated to reduce the threading dislocation densities in GaAs on GaP/Si. Electron channeling contrast imaging techniques revealed that an optimized GaAs buffer layer with thermal cycle annealing and InGaAs/GaAs dislocation filter layers has a threading dislocation density of 7.2 × 106 cm−2, which is a factor of 40 lower than an unoptimized GaAs buffer. The root-mean-square surface roughness was greatly decreased from 7.8 nm to 2.9 nm after the optimization process. A strong enhancement in photoluminescence intensity indicates that the optimized GaAs template grown on on-axis (001) GaP/Si substrates is a promising virtual substrate for Si-based optoelectronic devices.

Journal ArticleDOI
TL;DR: In this paper, the ionization energy and electron affinities of CsPbBr3 and the hole transport polymer polytriarylamine (PTAA) were measured using ultraviolet, X-ray, and inverse photo-emission spectroscopies.
Abstract: The inorganic lead halide perovskite CsPbBr3 promises similar solar cell efficiency to its hybrid organic-inorganic counterpart CH3NH3PbBr3 but shows greater stability. Here, we exploit this stability for the study of band alignment between perovskites and carrier selective interlayers. Using ultraviolet, X-ray, and inverse photoemission spectroscopies, we measure the ionization energy and electron affinities of CsPbBr3 and the hole transport polymer polytriarylamine (PTAA). We find that undoped PTAA introduces a barrier to hole extraction of 0.2–0.5 eV, due to band bending in the PTAA and/or a dipole at the interface. p-doping the PTAA eliminates this barrier, raising PTAA's highest occupied molecular orbital to 0.2 eV above the CsPbBr3 valence band maximum and improving hole transport. However, IPES reveals the presence of states below the PTAA lowest unoccupied molecular level. If present at the CsPbBr3/PTAA interface, these states may limit the polymer's efficacy at blocking electrons in solar cells w...

Journal ArticleDOI
TL;DR: In this paper, the authors developed thin-film microwave power attenuators operating from 1 to 10 GHz, which are built on a quartz substrate and use 75nm thick films of nichrome for dissipative components and 1 μm thick silver films as hot electron heat sinks.
Abstract: To reduce the level of thermally generated electrical noise transmitted to superconducting quantum devices operating at 20 mK, we have developed thin-film microwave power attenuators operating from 1 to 10 GHz. The 20 and 30 dB attenuators are built on a quartz substrate and use 75 nm thick films of nichrome for dissipative components and 1 μm thick silver films as hot electron heat sinks. The noise temperature of the attenuators was quantified by connecting the output to a 3D cavity containing a transmon qubit and extracting the dephasing rate of the qubit as a function of temperature and dissipated power P d in the attenuator. The minimum noise temperature T n of the output from the 20 dB attenuator was T n ≤ 53 mK for no additional applied power and T n ≈ 120 mK when dissipating 30 nW. In the limit of large dissipated power ( P d > 1 nW), we find T n ∝ P d 1 / 5.4, consistent with detailed thermal modeling of heat flow in the attenuators.

Journal ArticleDOI
TL;DR: In this article, the thermoelectric properties of orthorhombic group IV-VI monolayers AB (A,= Ge and Sn; B, = S and Se) are systematically investigated by the first-principles calculations and semiclassical Boltzmann transport theory.
Abstract: Two-dimensional (2D) materials may have potential applications in thermoelectric devices. In this work, the thermoelectric properties of orthorhombic group IV–VI monolayers AB (A = Ge and Sn; B = S and Se) are systematically investigated by the first-principles calculations and semiclassical Boltzmann transport theory. The spin-orbit coupling (SOC) is considered for their electron part, which produces observable effects on the power factor, especially for n-type doping. According to the calculated ZT, the four monolayers exhibit diverse anisotropic thermoelectric properties although they have a similar hinge-like crystal structure. The GeS along zigzag and armchair directions shows the strongest anisotropy, while SnS and SnSe show mostly isotropic efficiency of thermoelectric conversion. This can be explained by the strength of anisotropy of their respective power factor and electronic and lattice thermal conductivities. The calculated results show that the ZT between n- and p-type doping has little diffe...

Journal ArticleDOI
TL;DR: In this paper, the degradation and subsequent recovery of charge carrier lifetime upon light soaking at 75 °C observed in float-zone silicon wafers were performed. But, the degradation was only observed for p-type float zone silicon wafer passivated with passivation schemes involving silicon nitride layers.
Abstract: In this paper, we present new insight in the degradation and subsequent recovery of charge carrier lifetime upon light soaking at 75 °C observed in float-zone silicon wafers. Variations of doping type, dielectric passivation schemes and thermal treatments after layer deposition were performed. The degradation was only observed for p-type float-zone silicon wafers passivated with passivation schemes involving silicon nitride layers. An influence of thermal treatments after deposition was found. N-type wafers did not degrade independent of their passivation scheme. Room temperature re-passivation experiments showed the degradation to affect the wafer bulk, and photoluminescence studies demonstrated fine lateral striations of effective lifetime. We conclude that the degradation is caused by bulk defects that might be related to hydrogen complexes.

Journal ArticleDOI
TL;DR: In this article, the electrical and optical properties of oxygen vacancies (VO), zinc vacancies (VZn), hydrogenated VZn, and isolated dangling bonds in ZnO using hybrid functional calculations were investigated.
Abstract: We investigate the electrical and optical properties of oxygen vacancies (VO), zinc vacancies (VZn), hydrogenated VZn, and isolated dangling bonds in ZnO using hybrid functional calculations. While the formation energy of VO is high in n-type ZnO, indicating that this center is unlikely to form, our results for optical absorption signals associated with VO are consistent with those observed in irradiated samples, and give rise to emission with a peak at less than 1 eV. Under realistic growth conditions, we find that VZn is the lowest-energy native defect in n-type ZnO, acting as an acceptor that is likely to compensate donor doping. Turning to optical transitions, we first examine NO as a case study, since N-related transitions have been identified in experiments on ZnO. We also examine how hydrogen, often unintentionally present in ZnO, forms stable complexes with VZn and modifies its optical properties. Compared with isolated VZn, VZn-H complexes have charge-state transition levels lower in the band gap...

Journal ArticleDOI
TL;DR: In this article, Barium titanate (BaTiO3) nanoparticles are introduced into the polyimide matrix, giving rise to enhanced dielectric permittivity (6.8), energy density (5.2 ) and discharge efficiency (86.7%) at the room temperature.
Abstract: Polyimide (PI) films with extremely high breakdown strength (451 kV/mm), energy density (5.2 J/cm3), and discharge efficiency (86.7%) at the room temperature are fabricated by simple solution casting method. Barium titanate (BaTiO3) nanoparticles are introduced into PI matrix, giving rise to enhanced dielectric permittivity (6.8) and low dielectric loss (0.012). Dielectric and energy storage performances of the BTO/PI nanocomposites are thoroughly investigated up to 200 °C. The breakdown strengths of both pure PI and BTO/PI nanocomposites decrease dramatically with the increase in the temperature, owing to the low thermal conductivity of PI and the consequent thermal runaway caused by the accumulation of Joule heat under high electric field. Despite of the enhanced electric displacement, BTO/PI nanocomposites exhibit low discharge energy density due to the substantially compromised breakdown strength induced by the BTO nanoparticles.

Journal ArticleDOI
TL;DR: In this paper, the authors investigate the physical mechanisms responsible for dielectric breakdown in amorphous (a)-SiO2 using a multi-scale approach where the energetic parameters derived from a microscopic mechanism are used to predict the macroscopic degradation parameters of BD, i.e., time-dependent dieectric breakdown (TDDB) statistics, and its voltage dependence.
Abstract: Despite extensive experimental and theoretical studies, the atomistic mechanisms responsible for dielectric breakdown (BD) in amorphous (a)-SiO2 are still poorly understood. A number of qualitative physical models and mathematical formulations have been proposed over the years to explain experimentally observable statistical trends. However, these models do not provide clear insight into the physical origins of the BD process. Here, we investigate the physical mechanisms responsible for dielectric breakdown in a-SiO2 using a multi-scale approach where the energetic parameters derived from a microscopic mechanism are used to predict the macroscopic degradation parameters of BD, i.e., time-dependent dielectric breakdown (TDDB) statistics, and its voltage dependence. Using this modeling framework, we demonstrate that trapping of two electrons at intrinsic structural precursors in a-SiO2 is responsible for a significant reduction of the activation energy for Si-O bond breaking. This results in a lower barrier...

Journal ArticleDOI
TL;DR: In this article, the crystal structures and electrical properties of lead-free BiFeO3-BaTiO3 [(1-x)BFxBT] piezoelectric system are investigated as a function of BaTiO 3 concentration.
Abstract: The crystal structures and electrical properties of lead-free BiFeO3-BaTiO3 [(1-x)BFxBT] piezoelectric system are investigated as a function of BaTiO3 concentration. The well-saturated P-E hysteresis loop was observed in the 0.80BiFeO3–0.20BaTiO3 composition system, while a less hysteretic strain-electric field curve was exhibited by the 0.70BiFeO3–0.30BaTiO3 with a strain hysteresis of 16%, the value comparable to PZT-based piezoelectric ceramics. The crystal structures investigated under the synchrotron radiation X-ray diffraction exhibited a rhombohedral structure for BFBT system with x = 0.10–0.25 and a pseudo-cubic structure for BFBT system with x = 0.30–0.40. The structural phase diagram for the BiFeO3-BaTiO3 system is suggested based on the results of temperature-dependent synchrotron radiation X-ray diffraction measurement and investigated electrical properties.

Journal ArticleDOI
Abstract: Temperature dependent diffuse reflectance spectroscopy measurements were carried out on polycrystalline samples of BaTiO3 across the tetragonal to cubic structural phase transition temperature (TP). The values of various optical parameters such as band gap (Eg), Urbach energy (Eu), and Urbach focus (E0) were estimated in the temperature range of 300 K to 480 K. It was observed that with increasing temperature, Eg decreases and shows a sharp anomaly at TP. First principle studies were employed in order to understand the observed change in Eg due to the structural phase transition. Near TP, there exist two values of E0, suggesting the presence of electronic heterogeneity. Further, near TP, Eu shows metastability, i.e., the value of Eu at temperature T is not constant but is a function of time (t). Interestingly, it is observed that the ratio of Eu (t=0)/Eu (t = tm), almost remains constant at 300 K (pure tetragonal phase) and at 450 K (pure cubic phase), whereas this ratio decreases close to the transition temperature, which confirms the presence of electronic metastability in the pure BaTiO3. The time dependence of Eu, which also shows an influence of the observed metastability can be fitted with the stretched exponential function, suggesting the presence of a dynamic heterogeneous electronic disorder in the sample across TP. First principle studies suggest that the observed phase coexistence may be due to a very small difference between the total cohesive energy of the tetragonal and the cubic structure of BaTiO3. The present work implies that the optical studies may be a sensitive probe of disorder/heterogeneity in the sample.

Journal ArticleDOI
TL;DR: In this paper, the authors reported the highest responsivity for III-nitride metal Semiconductor metal solar-blind photodetectors on sapphire, with a visible rejection exceeding three orders of magnitude for front illumination.
Abstract: We report on the highest responsivity for III-nitride Metal Semiconductor Metal solar-blind photodetectors on sapphire. Devices on unintentionally doped AlGaN epilayers grown by Metal Organic Chemical Vapor Deposition exhibited sharp absorption cut-off in the range of 245–290 nm. Very high responsivity >5 A/W at 10 V bias was achieved with visible rejection exceeding three orders of magnitude for front illumination. Compared to the responsivity values reported in the literature for state-of-the-art solar-blind photodetectors, this work presents the highest values of responsivity at a given bias and up to sub-250 nm detection threshold. The high responsivity is attributed to an internal gain mechanism operating on these devices. The reverse-bias leakage current across these samples was found to be dominated by thermionic field emission at low biases and Poole-Frenkel emission from a deep trap level (0.7 eV from the conduction band-edge for Al0.50Ga0.50 N) at high biases.

Journal ArticleDOI
TL;DR: In this article, an as-grown undoped n-type β-Ga2O3 crystal was initially irradiated near room temperature with high-energy neutrons, which produced gallium vacancies (acceptors) and lowered the Fermi level, and self-trapped holes (i.e., small polarons) were then formed during subsequent irradiation at 77 K with x rays.
Abstract: We have experimentally observed self-trapped holes (STHs) in a β-Ga2O3 crystal using electron paramagnetic resonance (EPR). These STHs are an intrinsic defect in this wide-band-gap semiconductor and may serve as a significant deterrent to producing usable p-type material. In our study, an as-grown undoped n-type β-Ga2O3 crystal was initially irradiated near room temperature with high-energy neutrons. This produced gallium vacancies (acceptors) and lowered the Fermi level. The STHs (i.e., small polarons) were then formed during a subsequent irradiation at 77 K with x rays. Warming the crystal above 90 K destroyed the STHs. This low thermal stability is a strong indicator that the STH is the correct assignment for these new defects. The S = 1/2 EPR spectrum from the STHs is easily observed near 30 K. A holelike angular dependence of the g matrix (the principal values are 2.0026, 2.0072, and 2.0461) suggests that the defect's unpaired spin is localized on one oxygen ion in a nonbonding p orbital aligned near...