Search

We report a systematic study on the structural and electronic properties of Bi2Te3-xSex topological insulator alloy grown by molecular beam epitaxy(MBE). A mixing ratio of Bi2Se3 to Bi2Te3 was controlled by varying the Bi:Te:Se flux ratio. X-ray diffraction and Raman spectroscopy measurements indicate the high crystalline quality for the as-grown Bi2Te3-xSex films. Substitution of Te by Se is also revealed from both analyses. The surfaces of the films exhibit terrace-like quintuple layers and their size of the characteristic triangular terraces decreases monotonically with increasing Se content. However, the triangular terrace structure gradually recovers as the Se content further increases. Most importantly, the angle-resolved photoemission spectroscopy results provide evidence of single-Dirac-cone like surface states in which Bi2Te3-xSex with Se/Te-substitution leads to tunable surface states. Our results demonstrate that by fine-tuned MBEgrowth conditions, Bi2Te3-xSex thin film alloys with tunable topological surface states can be obtained, providing an excellent platform for exploring the novel device applications based on this compound. [ABSTRACT FROM AUTHOR]

Excellent voltage linearity of metal-insulator-metal (MIM) capacitors is highly required for next generation radio frequency integration circuits. In this work, employing atomic layer deposition technique, we demonstrated how the voltage linearity of MIM capacitors was modulated by adding different thickness of SiO2 layer to the nano-stack of Al2O3/ZrO2. It was found that the quadratic voltage coefficient of capacitance (α) can be effectively reduced from 1279 to -- 75ppm/V² with increasing the thickness of SiO2 from zero to 4nm, which is more powerful than increasing the thickness of ZrO2 in the Al2O3/ZrO2 stack. This is attributed to counteraction between the positive α for Al2O3/ZrO2 and the negative one for SiO2 in the MIM capacitors with Al2O3/ZrO2/SiO2 stacks. Interestingly, voltage-polarity dependent conduction behaviors in the MIM capacitors were observed. For electron bottom-injection, the addition of SiO2 obviously suppressed the leakage current; however, it abnormally increased the leakage current for electron top-injection. These are ascribed to the co-existence of shallow and deep traps in ZrO2, and the former is in favor of the field-assisted tunnelling conduction and the latter contributes to the trap-assisted tunnelling process. The above findings will be beneficial to device design and process optimization for high performance MIM capacitors. [ABSTRACT FROM AUTHOR]

In this paper, by first principle calculations, we investigate systematically the band topology of a new half-Heusler family with composition of I(A)-III(A)-IV(A). The results clearly show that many of the I-III-IV half-Heusler compounds are in fact promising to be topological insulator candidates. The characteristic feature of these new topological insulators is the naturally strong band inversion strength (up to -2 eV) without containing heavy elements. Moreover, we found that both the band inversion strength and the bulk insulating gap of the compounds can be tailored through strain engineering, and therefore would be grown epitaxially in the form of thin films, and useful in spintronics and other applications. [ABSTRACT FROM AUTHOR]

We have studied in detail the influence of same-orbit and different-orbit hopping disorders in HgTe/CdTe quantum wells. Intriguingly, similar to the behavior of the on-site Anderson disorder, a phase transition from a topologically trivial phase to a topological phase is induced at a proper strength of the same-orbit hopping disorder. For different-orbit hopping disorder, however, the phase transition does not occur. The results have been analytically verified by using effective medium theory. A consistent conclusion can be obtained by comparing phase diagrams, conductance, and conductance fluctuations. In addition, the influence of Rashba spin-orbit interaction (RSOI) on the system has been studied for different types of disorder, and the RSOI shows different influence on topological phase at different disorders. The topological phase induced by same-orbit hopping disorder is more robust against the RSOI than that induced by on-site Anderson disorder. For different-orbit hopping disorder, no matter whether the RSOI is included or not, the phase transition does not occur. The results indicate, whether or not the topological Anderson insulator can be observed depends on a competition between the different types of the disorder as well as the strength of the RSOI in a system. [ABSTRACT FROM AUTHOR]

The Lifshitz-type semimetal-insulator transition, which is a transition of the electronic topology, has been considered as the most fundamental metal-insulator transition. Here, we present resistivity measurements under pressure in the vicinity of the quantum critical point of fcc Yb. We apply a previously suggested scaling for this type of transition and identify its universality class. Moreover, we observe an anomaly in the screening coefficient A of the T2 term in the resistivity at low temperatures in the metallic phase. We suggest an interpretation of this phenomenon as an effect of doping by Ca impurities unintentionally present in the Yb crystals. The observed behavior may very well be applicable to any doped system in the vicinity of such a transition. [ABSTRACT FROM AUTHOR]

On a nanosecond time scale, solid insulators abnormally fail in bulk rather than on surface, which is termed as the 'worm-hole' effect. By using a generator with adjustable output pulse width and dozens of organic glass (PMMA) and polystyrene (PS) samples, experiments to verify this effect are conducted. The results show that under short pulses of 10 ns, all the samples fail due to bulk breakdown, whereas when the pulse width is tuned to a long pulse of 7 μs, the samples fail as a result of surface flashover. The experimental results are interpreted by analyzing the conditions for the bulk breakdown and the surface flashover. It is found that under short pulses, the flashover threshold would be as high as the bulk breakdown strength (EBD) and the flashover time delay (td) would be longer than the pulse width (τ), both of which make the dielectrics' cumulative breakdown occur easily; whereas under long pulses, that Ef is much lower than EBD and td is smaller than τ is advantageous to the occurrence of the surface flashover. In addition, a general principle on solid insulation design under short pulse condition is proposed based on the experimental results and the theoretical analysis. [ABSTRACT FROM AUTHOR]

It is shown that, in addition to the diffusion and broken neutrality drift (BND) modes well-known for insulators and very lightly doped semiconductors, the quasineutral drift (QND) mode is possible. The transition from the BND to QND mode is accompanied by the appearance of a portion with a very sharp current rise in the current-voltage characteristic. This effect is observed in a new type of semiconductor detectors (CIDs, Current Injected Detectors) of high-intensity neutron and proton radiation, suggested, in particular, for Large Hadron Collider. The effect is unambiguously attributed now to the presence of radiation-induced deep centers in a semiconductor. It is shown, however, in this paper that the effect of a very sharp rise in current upon a slight increase in voltage is even possible when there are no deep centers. An equation adequately describing the possible transport modes in intrinsic semiconductors and insulators is derived. The results of an analytical study are confirmed by an adequate simulation. [ABSTRACT FROM AUTHOR]

Electrical properties of detonation diamond nanoparticles (NDs) with individual diameters of ∼5 nm are important for many applications. Although diamond is an insulator, it is known that hydrogen-terminated bulk diamond becomes conductive when exposed to water. We show that heating ND in hydrogen gas at 600-900 °C resulted in a remarkable decrease in resistivity from 107 to 105 Ω cm, while the resistivity was essentially unchanged after treatment at 400 °C and lower temperatures. Fourier Transform Infrared Spectroscopy and X-ray photoelectron spectroscopy (XPS) studies revealed that hydrogenation of ND occurs at 600-900 °C, suggesting that the decrease in resistivity is based on transfer doping at the hydrogenated ND surface. Oxidation of the hydrogenated sample at 300 °C recovers resistivity to its original value. The resistivity of treated ND as a function of the O/C atomic ratio showed a transition from resistive (O/C ratio > 0.033) to conductive (O/C ratio < 0.033) state. This is consistent with the idea that the change in the resistivity is caused by the shift of the valence band maximum to above the Fermi level due to the dipole of the C-H bonds leading to transfer doping. [ABSTRACT FROM AUTHOR]

This study analyzes the scalability of nitride-based nanowire high electron mobility transistors. A fully self-consistent 3D model that solves drift-diffusion and Poisson equations is employed to investigate the short-channel effect (SCE). The scaling ability of nanowire transistors with a gate-length (Lg) of 100 nm to 20 nm is examined. The current-voltage curve, current gain, subthreshold swing, and drain-induced barrier lowering results are presented. The results show that the restrained SCE is provided by the excellent gate control of the nanowire transistor. Various wire widths and gate insulators are tested to minimize the SCE for the device with a 20 nm Lg. [ABSTRACT FROM AUTHOR]

We show that a bilayer graphene ribbon deposited above a ferromagnetic insulator can behave as a spin-filtering device. The ferromagnetic material induces exchange splitting in the graphene ribbon, and due to the Fano antiresonances occurring in the transmission of the graphene ribbon as a function of ribbon length and energy, it is possible to obtain a net spin current. This happens when an antiresonance for one spin channel coincides with a maximum transmission for the opposite spin. We propose these structures as a means to obtain spin-polarized currents and spin filters in graphene-based systems. [ABSTRACT FROM AUTHOR]

In this work we study the cross-linked poly(vinyl alcohol)/poly(3-hexylthiophene) interfacial properties of an organic field effect transistor. We use cross-linked poly(vinyl alcohol) prepared with different ammonium dichromate:poly(vinyl alcohol) proportions, ranging from 0% to 35%, as insulator. Using admittance spectroscopy, we show that the interfacial properties change when the ammonium dichromate concentration is altered. The interfacial properties and the better insulation are responsible for the improvement of the device performance in these organic field effect transistors, achieving best performance in the blend with ammonium dichromate:poly(vinyl alcohol) proportion of 0.25:1. [ABSTRACT FROM AUTHOR]

One of the main issues in the characterization of silicon on insulator (SOI) substrates is to determine the quality of the film-buried oxide interface. This interface quality is strongly connected to the carrier lifetime in the silicon film. In this paper, we extend the well-known pseudo-MOSFET characterization technique for SOI wafers to the extraction of carrier lifetime. The experiment consists in comparing the drain current measured under dark and under laser shining. A model is proposed to evaluate the surface recombination velocity in SOI. Experimental results are validated by numerical simulations. [ABSTRACT FROM AUTHOR]

We report on the effect of exposure to atmospheric conditions on the THz conductivity of thin films of the topological insulator Bi2Se3. We find (1) two contributions of mobile charge carriers to the THz conductivity immediately after growth and (2) the spectral weight of the smaller of these decays significantly over a period of several days as the film is exposed to ambient conditions, while the other remains relatively constant. We associate the former with a bulk response and the latter with the surface. The surface response exhibits the expected robustness of the carriers from 2D topological surface states. We find no evidence for a third spectral feature derived from topologically trivial surface states. [ABSTRACT FROM AUTHOR]

The defects responsible for the permanent component observed in negative bias temperature (NBT) stressed metal-oxide-semiconductor field effect transistors with an oxynitride gate insulator were investigated by using isochronal annealing experiments, spin dependent recombination (SDR), and spin dependent tunneling (SDT) technique. Two defects were found in the permanent component after light NBT stresses; interface states (Dit) and fixed positive charges (Dpc), which are closely related. The data support a model where hydrogen emitted from interfacial Si-H bonds by NBT stresses reacts with Si-X-Si structures (X = oxygen or nitrogen) in the gate insulator, which leaves silicon dangling bonds (Dit) and leads to the generation of Si-X+H-Si (overcoordinated oxygen or nitrogen, Dpc). Heavy NBT stresses simultaneously accelerate the formation and generation of new defects, which act as additional Dit and Dpc. Moreover, these defects cause stress-induced leakage current. Concerning their origin, defects similar to K- and E′γ-centers were detected by using SDR and SDT. They are unrelated to hydrogen and can be formed through the breaking of Si-X bonds. On the basis of these results, we propose a model for the generation and recovery behavior of defects and present a comparison with the previous studies. [ABSTRACT FROM AUTHOR]

We discuss the role of back-gate biasing on effective field (Eeff) and its impact on the mobility in advanced ultrathin semiconductor-on-insulator metal-oxide-semiconductor field-effect-transistors using the multibranch mobility analysis. This technique is enabled using the integral definition for Eeff, which is the accurate method to determine Eeff even in volume-conduction regime. For further clarification, the difference between Eeff estimated by this integral definition and by the conventional formula is examined for our target devices. From the multibranch mobility analysis, we show that in devices with thin-buried oxide and ground plane doping, the reduction in Eeff due to back-gate biasing is large, leading to a significant mobility enhancement for both N-channel and P-channel devices. Furthermore, we show that the mobility enhancement effect via back-gate biasing in P-channel devices is larger than that in N-channel devices. This enhancement is attributed to the increase in the mobility under the volume-conduction regime. The mobility dependence with respect to Eeff reveals that strong volume conduction is achieved in P-channel devices, leading to a reduction of the surface scattering events at both interfaces, thereby increasing the mobility. This advantage adds to the already reported increase in the population of the light-hole bands. [ABSTRACT FROM AUTHOR]

Room temperature photoluminescence (PL) was observed along 50 μm long Ge strips on insulator on bulk Si substrates fabricated by rapid melt growth. The PL peaks evidently exhibited a redshift from the origin to the end of the Ge strip because of the shrinkage of the direct bandgap of Ge. Moreover, PL intensities increased along the direction of lateral epitaxial growth primarily because of the decrease in the energy difference between the direct and indirect gaps of Ge. The change in the Ge band structure, which facilitated changes in PL peaks and intensities, was found to have resulted from the variation of tensile strain ratios and Si fractions along Ge strips. Furthermore, the PL intensity at the end of the strip was one magnitude higher than that of bulk Ge, which indicates the high quality of Ge-on-insulator structures. [ABSTRACT FROM AUTHOR]

We study theoretically the conductance properties of a ferromagnetic insulator/d-wave superconductor (FI/d-wave SC) junction and a normal metal/ferromagnetic insulator/d-wave superconductor (NM/FI/d-wave SC) junction formed on the surface of a three dimensional topological insulator, where the direction of the magnetization m in ferromagnetic region is perpendicular to the surface. We find that with increasing the magnetization of the ferromagnetic region, a conductance peak to conductance dip conversion occurs, at a bias voltage depending on the d-wave superconducting orientation angular. We find also that the gate voltage applied across the ferromagnetic region strongly affects the influence of the magnetization on the zero bias conductance of the NM/FI/d-wave SC junction. [ABSTRACT FROM AUTHOR]

The current density-voltage (J-V) characteristics of Ag/DNA/p-InP metal-insulator-semiconductor (MIS) structures have been investigated in room temperature. We have observed that the Ag/DNA/p-InP structure shows an excellent rectifying behavior and that this structure increases the barrier height (φb0). The main electrical parameters of these structures, such as ideality factor (n), barrier height, and average series resistance values were found to be 1.087, 0.726 eV, and 66.92 Ω. This value of n was attributed to the presence of an interfacial insulator layer at the Ag/p-InP interface and the density of interface states (Nss) localized at the InP/DNA interface. The values of Nss localized at the InP/DNA interface were found at 0.675-Ev in the 1.38 × 1012 eV-1 cm-2. [ABSTRACT FROM AUTHOR]

Correlated oxides that exhibit metal-insulator phase transitions are emerging as potential candidates for switching devices. One such material is SmNiO3, which has a transition temperature above room temperature (∼400 K in bulk crystals). In this work, we study temperature- and bias-dependent conduction mechanisms in epitaxial and polycrystalline SmNiO3 thin films. In both cases, at low electric field we observe thermally assisted hopping conduction through defect states with activation energies of ∼170 meV and ∼270 meV, respectively. At high electric field the conduction transitions to a space-charge limited regime controlled by an exponential trap distribution. The power law exponents are ∼3 in epitaxial films and ∼8-14 in polycrystalline films. The trap decay parameter in epitaxial films does not have the expected 1/T temperature dependence, which may be a signature of bandgap narrowing at high temperature because of the insulator-to-metal transition. The larger activation energy and power law dependency in polycrystalline films are consistent with additional defect density from extraneous phases. In polycrystalline films, current-voltage data measured perpendicular to the film surface are rectifying because of asymmetry in electrode work functions with a ratio of 104 at ± 1 V. We find that whereas the space-charge limited conduction for positive bias is bulk limited, the negative bias conduction is injection limited. [ABSTRACT FROM AUTHOR]

A spark gap (SG) triggered by a semi-insulating GaAs photoconductive semiconductor switch (PCSS) is presented. Currents as high as 5.6 kA have been generated using the combined switch, which is excited by a laser pulse with energy of 1.8 mJ and under a bias of 4 kV. Based on the transferred-electron effect and gas streamer theory, the breakover characteristics of the combined switch are analyzed. The photoexcited carrier density in the PCSS is calculated. The calculation and analysis indicate that the PCSS breakover is caused by nucleation of the photoactivated avalanching charge domain. It is shown that the high output current is generated by the discharge of a high-energy gas streamer induced by the strong local electric field distortion or by overvoltage of the SG resulting from quenching of the avalanching domain, and periodic oscillation of the current is caused by interaction between the gas streamer and the charge domain. The cycle of the current oscillation is determined by the rise time of the triggering electric pulse generated by the PCSS, the pulse transmission time between the PCSS and the SG, and the streamer transit time in the SG. [ABSTRACT FROM AUTHOR]

We investigate theoretically the surface states of three-dimensional topological insulator cylinder nanowires analytically and numerically. In contrast to the conventional semiconductor cylinder nanowires, these surface states exhibit unique massless Dirac dispersion and interesting transport properties. We find that the persistent charge current and persistent spin current, i.e., the Aharonov-Bohm oscillation, can be induced by the driven magnetic flux. The amplitude of persistent charge current shows an oscillating behavior with increasing the electron density. [ABSTRACT FROM AUTHOR]

We studied evanescent wave coupling behavior between low index-contrast upper-level waveguides and thin-film Si and Ge photodetectors on SOI and germanium-on-insulator (GOI) substrates, respectively. We present a simple and intuitive leaky-mode phase-matching model using a ray-optics approach to determine the conditions for efficient coupling, both in 2D and 3D structures. It is shown that the presence of leaky modes that are phase-matched between the waveguide and the Si or Ge photodetector layer is the key condition for efficient coupling. Our approach was compared to other methods, such as finite-difference time domain (FDTD)/beam propagation method (BPM) and mode analysis. We report that, depending on the way a waveguide photodetector device is designed, waveguide-to-photodetector coupling efficiency may or may not be critically sensitive to design parameters, such as the photodetector layer thickness. As an example, the stark contrast of coupling behavior in the two most popular Ge photodetector structures integrated with Si rib waveguide versus channel waveguide is shown. The device design factors and the trends that affect such coupling sensitivity are identified and explained in the paper. [ABSTRACT FROM AUTHOR]

The mechanism of triggering the metal-insulator transition (MIT) by an external electric field in small scale VO2 structures has been attributed to excess carrier injection, rather than Joule heating. This is in part due to the fact that the delay time for MIT to be induced by Joule heating seems much longer than what is observed. However, modeling the resistivity as a function of temperature, explicitly considering phase coexistence of metallic and insulating states near the MIT, and considering thermal dissipation in realistic structures, we demonstrate that Joule heating can exhibit a self-accelerating, avalanche-like behavior, in which the time scale for thermally driven breakdown can be in the nanosecond regime if the device is small enough. This model matches experimental results of our micrometer scale device quite well. Over-threshold voltages, temperature, and size effects on switching delay time and threshold voltage are discussed. [ABSTRACT FROM AUTHOR]

A comparative study of the effect of Fe and Ni doping on the bismuth based perovskite La0.6Bi0.4MnO3.1, a projected spintronics magnetic semiconductor has been carried out. The doped systems show an expressive change in magnetic ordering temperature. However, the shifts in ferromagnetic transition (TC) of these doped phases are in opposite direction with respect to the parent phase TC of 115 K. The Ni-doped phase shows an increase in TC ∼200 K, whereas the Fe-doped phase exhibits a downward shift to TC∼95 K. Moreover, the Fe-doped is hard-type whereas the Ni-doped compound is soft-type ferromagnet. It is observed that the materials are semiconducting in the ferromagnetic phase with activation energies of 77 & 82 meV for Fe & Ni-doped phases, respectively. In the presence of external magnetic field of 7 T, they exhibit minor changes in the resistivity behaviors and the maximum isothermal magnetoresistance is around -20% at 125 K for the Ni-phase. The results are explained on the basis of electronic phase separation and competing ferromagnetic and antiferromagnetic interactions between the various mixed valence cations. [ABSTRACT FROM AUTHOR]

In this review, we try to derive a comprehensive understanding of the transport mechanisms in three dimensional ensembles of Si quantum dots (QDs) that are embedded in an insulating matrix. This understanding is based on our systematic electrical measurements as a function of the density of Si nanocrystallites as well as on a critical examination of the available literature. We conclude that in ensembles of low density QDs, the conduction is controlled by quantum confinement and Coulomb blockade effects while in the high density regime, the system behaves as a simple disordered semiconductor. In between these extremes, the transport is determined by the clustering of the QDs. In view of the clustering, two types of transitions in the electrical and optical properties of the system are identified. In order to understand them, we introduce the concept of 'touching.' The application of this concept enables us to suggest that the first transition is a local carrier deconfinement transition, at which the concentration of the non 'touching' QDs reaches its maximum, and that the other transition is associated with the onset of percolation in a continuous disordered network of 'touching' QDs. It is hoped that our conclusions for the entire possible density range will provide guidance for the discussion and understanding of the transport in ensembles of semiconductor QDs in general and in ensembles of Si and Ge QDs in particular. [ABSTRACT FROM AUTHOR]

This paper reports results obtained on exchange-biased IrMn/Al2O3/Co films deposited by magnetron sputtering, where the thickness of the non-magnetic insulator layer, tAl2O3, was varied. Ferromagnetic resonance and static magnetization measurements were used to study the exchange interaction between the antiferromagnet (IrMn) and ferromagnet (Co) layers. X-ray diffractometry and x-ray reflectometry as well as high-resolution transmission electron microscopy were employed for structural characterization of the films. It was found that the IrMn/Co exchange coupling decreases very abruptly with tAl2O3 being the exponential decay length at least four times smaller than the values previously reported for exchange-bias systems. Such a rapid suppression of the coupling is explained in terms of the prompt loss of the direct contact between the antiferromagnet and the ferromagnet with tAl2O3. [ABSTRACT FROM AUTHOR]

Polar [formula] (1×1), semipolar GaN(1013) surfaces prepared in NH3 vapor, and their disordered counterparts are investigated by reflection electron energy loss spectroscopy (REELS) and low-energy electron diffraction. The REELS spectra are measured in a range of polar angles at electron kinetic energies of 200 and 1000 eV. The electron energy loss function is determined from the REELS within the framework of the semiclassical approach. Good agreement between experimental and theoretical functions is achieved at all angles for the disordered GaN surfaces and for the ordered surfaces measured at a kinetic energy of 1000 eV. The agreement is worse for the crystals measured at 200 eV, which is explained by the coherent scattering contributions at low energies. The optical constants of the GaN surfaces are derived from the computed dielectric functions: the optical properties of the (0001) and (1013) surfaces are similar, except for differences in bandgap values, which may be due to observed steps on the (1013) surface. The surface optical properties of a disordered GaN surface are found to be different from the GaN crystals. There are pronounced changes in the electronic band structure for disordered GaN due to the preferential sputtering of nitrogen. [ABSTRACT FROM AUTHOR]

Thin films of Pr0.5Ca0.5MnO3 were fabricated on (001) oriented SrLaAlO4, NdGaO3, and SrTiO3 substrates using a hybrid solution route and spin coating techniques. Good crystalline and epitaxial quality of the films was confirmed with X-ray diffraction and transmission electron microscopy studies. Strain in the film grown on NdGaO3 substrate did not relax during annealing process and the film exhibited charge-ordered insulator phase at low temperatures even with magnetic fields up to 9 T. However, the films on SrLaAlO4 and SrTiO3 substrates (with partially relaxed compressive and tensile strain, respectively) displayed melting of the charge-ordered phase with applied magnetic fields of less than 5 T. The results suggest that strain-relaxation rather than only the type of strain plays an important role in lowering critical melting magnetic fields in these films. [ABSTRACT FROM AUTHOR]

We report the highest pressure under which a transparent insulator has been observed. The refractive index of ramp-compressed lithium fluoride (LiF) is measured up to a pressure of 800 GPa and is observed to maintain its linear dependence on density. An effective single-oscillator model infers that the bandgap monotonically closes with increasing density, indicating that metallization of LiF should occur at pressures above 4000 GPa, and that LiF should remain transparent at extremely high pressures. [ABSTRACT FROM AUTHOR]

Atomic layer deposited (ALD) Pb(Zr,Ti)Ox (PZT) ultra-thin films were synthesized on an ALD Al2O3 insulation layer on 4H-SiC substrate for metal-ferroelectric-insulator-semiconductor (MFIS) device applications. The as-deposited PZT was amorphous but crystallized into a perovskite polycrystalline structure with a preferred [002] orientation upon rapid thermal annealing (RTA) at 950 °C. The capacitance-voltage and current-voltage characteristics of the MFIS devices indicate carrier injection to the film induced by polarization and Fowler-Nordheim (FN) tunneling when electric field was high. The polarization-voltage measurements exhibited reasonable remanent and saturation polarization and a coercive electrical field comparable to that reported for bulk PZT. The piezoresponse force microscope measurements confirmed the polarization, coercive, and retention properties of ultra-thin ALD PZT films. [ABSTRACT FROM AUTHOR]

We present a comprehensive investigation of the low-field hole mobility in strained Ge and III-V (GaAs, GaSb, InSb, and In1-xGaxAs) p-channel inversion layers with both SiO2 and high-κ insulators. The valence (sub)band structure of Ge and III-V channels, relaxed and under biaxial strain (tensile and compressive) is calculated using an efficient self-consistent method based on the six-band k·p perturbation theory. The hole mobility is then computed using the Kubo-Greenwood formalism accounting for nonpolar hole-phonon scattering (acoustic and optical), surface roughness scattering, polar phonon scattering (III-Vs only), alloy scattering (alloys only) and remote phonon scattering, accounting for multisubband dielectric screening. As expected, we find that Ge and III-V semiconductors exhibit a mobility significantly larger than the 'universal' Si mobility. This is true for MOS systems with either SiO2 or high-κ insulators, although the latter ones are found to degrade the hole mobility compared to SiO2 due to scattering with interfacial optical phonons. In addition, III-Vs are more sensitive to the interfacial optical phonons than Ge due to the existence of the substrate polar phonons. Strain-especially biaxial tensile stress for Ge and biaxial compressive stress for III-Vs (except for GaAs)-is found to have a significant beneficial effect with both SiO2 and HfO2. Among strained p-channels, InSb exhibits the largest mobility enhancement. In0.7Ga0.3As also exhibits an increased hole mobility compared to Si, although the enhancement is not as large. Finally, our theoretical results are favorably compared with available experimental data for a relaxed Ge p-channel with a HfO2 insulator. [ABSTRACT FROM AUTHOR]

A ferroelectriclike dielectric response was observed in a field-effect transistor using oriented bis(ethylenedithio)tetrathiafulvalene-tetracyanoquinodimethane crystals. Phase transitions at 285 and 320 K were clearly observed in the temperature dependence of field-effect electron mobility. The phase transition at 320 K corresponds to the metal-insulator transition previously reported in a bulk crystal. On the other hand, the field-effect electron and hole mobilities exhibited an abrupt increase at 285 K, which had not been discovered by other physical measurements in the bulk crystal and is nonetheless sufficiently stable and reproducible. In addition, the abrupt increase in carrier mobilities was clearly correlated with the decrease in the dielectric response. The temperature variation in difference hysteresis curves demonstrated the feature of ferroelectric transition. [ABSTRACT FROM AUTHOR]

Charge accumulation on a solid dielectric surface is one of the critical concerns for the design and optimization of the insulation system in a high-voltage power equipment, since it will lead to the overstress of electrical insulation. Therefore, it is important to obtain the charge density distribution on a solid dielectric surface with high accuracy. The acquisition of surface charge for insulators requires multipoint potential measurements to establish the inverse calculation for the determination of an unknown charge distribution. Up to now, extensive studies have been conducted on this subject; nevertheless, the methods are either too complicated and time consuming, or only applicable for specific arrangements, or with poor accuracy. In this paper, the problem is divided into two categories, i.e., shift-variant system and shift-invariant system, and the basic principle of an improved inversion algorithm is interpreted to solve the problem. The 2-D Fourier transform and Wiener filter techniques are employed in the algorithm for shift-invariant system thus the relationship between potential and charge density can be processed in spatial frequency domain, which tremendously simplifies the conventional procedure. The accuracy and resolution of the algorithm are discussed in detail with the aid of numerical examples. In the end, experiments are conducted and the effectiveness of the algorithm is verified. [ABSTRACT FROM AUTHOR]

Nanomaterials with layered structures, with their intriguing properties, are of great research interest nowadays. As one of the primary two-dimensional nanomaterials, the hexagonal boron nitride nanosheet (BNNS, also called white graphene), which is an analogue of graphene, possesses various attractive properties, such as high intrinsic thermal conductivity, excellent chemical and thermal stability, and electrical insulation properties. After being discovered, it has been one of the most intensively studied two-dimensional non-carbon nanomaterials and has been applied in a wide range of applications. To support the exploration of applications of BNNSs, exfoliation, as one of the most promising approaches to realize large-scale production of BNNSs, has been intensively investigated. In this review, methods to yield BNNSs by exfoliation will be summarized and compared with other potential fabrication methods of BNNSs. In addition, the future prospects of the exfoliation of h-BN will also be discussed. [ABSTRACT FROM AUTHOR]

A topological insulator is a material with an insulating interior but time-reversal symmetry-protected conducting edge states. Since its prediction and discovery almost a decade ago, such a symmetry-protected topological phase has been explored beyond electronic systems in the realm of photonics. Electrons are spin-1/2 particles, whereas photons are spin-1 particles. The distinct spin difference between these two kinds of particles means that their corresponding symmetry is fundamentally different. It is well understood that an electronic topological insulator is protected by the electron's spin-1/2 (fermionic) time-reversal symmetry Tf²=-1. However, the same protection does not exist under normal circumstances for a photonic topological insulator, due to photon's spin-1 (bosonic) time-reversal symmetry Tb²=1. In this work, we report a design of photonic topological insulator using the Tellegen magnetoelectric coupling as the photonic pseudospin orbit interaction for left and right circularly polarized helical spin states. The Tellegen magnetoelectric coupling breaks bosonic time-reversal symmetry but instead gives rise to a conserved artificial fermionic-like-pseudo time-reversal symmetry, Tp (Tp#178;=-1), due to the electromagnetic duality. Surprisingly, we find that, in this system, the helical edge states are, in fact, protected by this fermionic-like pseudo time-reversal symmetry Tp rather than by the bosonic time-reversal symmetry Tb. This remarkable finding is expected to pave a new path to understanding the symmetry protection mechanism for topological phases of other fundamental particles and to searching for novel implementations for topological insulators. [ABSTRACT FROM AUTHOR]

The fractional derivative method is considered as a possible formalism for the non-destructive insulation diagnosis by polarization and depolarization current measurements. The underlying motive is based on non-Debye relaxation processes clearly observed in long-time relaxation experiments with oil–paper, electrolytic and ultra-capacitors, transformers and other systems. Modes of relaxation at different time scales depend on charging prehistory and are sensitive to the state of a system. Fractional formalism provides useful parameters characterizing the state of the insulation system, which can be extracted from polarization–depolarization current measurements. Adequacy of the technique is examined by comparison of theoretical and experimental results for oil–paper, and ultra-capacitors. [ABSTRACT FROM AUTHOR]

Copyright of Technical Electrodynamics / Tekhnichna Elektrodynamika is the property of National Academy of Sciences of Ukraine, Institute of Electrodynamics and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Ab initio calculations have predicted a transition between topological insulators and band insulators in superlattices consisting of Sb2Te3 and GeTe, when the periodicity is varied. We examine the amplitude of the weak antilocalization effect to confirm the transition. In agreement with the predictions, robust surface states are present for (Sb2Te3)1(GeTe)2 but absent when the content of the well-known topological insulator Sb2Te3 is increased as (Sb2Te3)4(GeTe)2, manifesting that the electronic coupling in the superlattices affects the emergence of the helical surface states nontrivially. [ABSTRACT FROM AUTHOR]

Transformers, modulators and other high voltage electrical equipment are traditionally filled with mineral oil, which serves as a coolant and dielectric insulator. A rising trend is observed globally towards the adoption of less-combustible, bio-degradable transformer liquids at ever increasing voltages and power ratings. This paper presents a study of the fire behavior of five different dielectric transformer coolants: mineral oil, silicone liquid, synthetic ester, and two natural esters. Two types of tests were performed: comparative small-scale tests with two types of pans in the Cone Calorimeter (ISO 5660-1), and intermediate-scale pool fire experiments under the hood of an oxygen consumption calorimeter. The data obtained in this study are used in a fire engineering analysis of the evacuation and smoke removal from the 5900 m gallery in the planned European Spallation Source in Lund, Sweden. The comparative results indicate a wide range of fire properties for the tested liquids. The heat release rates calculated from the Cone Calorimeter experiments are reasonably consistent with tabulated values, except for the silicone liquid. The latter forms a crust on the liquid surface which significantly impedes combustion. Heat losses from the burning surface to cooler liquid below and pan boundaries have a significant effect on the burning behavior. These scale-dependent phenomena imply that great care should be taken in using small-scale burning data in a fire engineering analysis. Additional work is needed to gain a better understanding of the relation between small-scale tests and large-scale tests, and between the behavior of these liquids in pool fire experiments and that in real fires. [ABSTRACT FROM AUTHOR]

A Microwave-Induction Heating (MIH) scheme is proposed for the poly(4-vinylphenol) (PVP) gate insulator cross-linking process to replace the traditional oven heating cross-linking process. The cross-linking time is significantly decreased from 1 h to 5 min by heating the metal below the PVP layer using microwave irradiation. The necessary microwave power was substantially reduced to about 50 W by decreasing the chamber pressure. The MIH scheme is a good candidate to replace traditional thermal heating for cross-linking of PVP as the gate insulator for organic thin-film-transistors. [ABSTRACT FROM AUTHOR]

Technology of light sensors, due to the wide range of applications, is a dynamically developing branch of both science and industry. This work presents concept of photodetectors based on a metal-ferroelectric-insulator-semiconductor, a structure which has not been thoroughly explored in the field of photodetectors. Functionality of the presented light sensor exploits the effects of photocapacitive phenomena, ferroelectric polarization, and charge trapping. This is accomplished by an interplay between polarization alignment, subsequent charge distribution, and charge trapping processes under given illumination condition and gate voltage. Change of capacitance serves as a read out parameter indicating the wavelength and intensity of the illuminating light. The operational principle of the proposed photocapacitive light sensor is demonstrated in terms of capacitance-voltage and capacitance-time characteristics of an Al/YMnO3/SiNx/p-Si structure exposed to green, red, and near infrared light. Obtained results are discussed in terms of optical properties of YMnO3 and SiNx layers contributing to the performance of photodetectors. Presented concept of light sensing might serve as the basis for the development of more advanced photodetectors. [ABSTRACT FROM AUTHOR]

A rising interest in DC systems at various voltage levels is still being observed. Such trends result in heightened requirements of insulation systems at DC stresses at various voltage levels, and also motivate investigations into partial discharges (PD), initiating degradation processes of insulation systems. The mechanism and PD forms at DC represent a different class that cannot be directly compared with AC mechanism. The PD mechanism at DC depends on many factors that determine the condition of the insulation system, including pressure. Thus, it is crucial to continue research on phenomena occurring at DC voltage where air at higher pressure might be an insulating medium for some power and power electronics equipment in the future. The PD measurement methodology is not normalised and the evaluation of measurement results requires a different approach than at AC. This study presents PD at DC in air at higher pressure in selected configurations of a modelled insulation system. For this purpose the time sequences of PD pulses were acquired in measurement system and clustered. Modern diagnostics and monitoring systems would utilise the set of patterns and features to classify the various forms of discharge. Distinctive behaviours of discharges at various pressures were observed. [ABSTRACT FROM AUTHOR]

We consider a real periodic Schrödinger operator and a physically relevant family of $${m \geq 1}$$ Bloch bands, separated by a gap from the rest of the spectrum, and we investigate the localization properties of the corresponding composite Wannier functions. To this aim, we show that in dimension $${d \leq 3}$$ , there exists a global frame consisting of smooth quasi-Bloch functions which are both periodic and time-reversal symmetric. Aiming to applications in computational physics, we provide a constructive algorithm to obtain such a Bloch frame. The construction yields the existence of a basis of composite Wannier functions which are real-valued and almost-exponentially localized. The proof of the main result exploits only the fundamental symmetries of the projector on the relevant bands, allowing applications, beyond the model specified above, to a broad range of gapped periodic quantum systems with a time-reversal symmetry of bosonic type. [ABSTRACT FROM AUTHOR]

The navy railgun prototype propels its projectile with Lorentz force and is capable of delivering $\sim $ Mach 7 speed for $\sim 15$ -kg projectiles. The source of the Lorentz force is a gigawatt-class pulsed power supply (PPS) that delivers $\sim 1$ MA current with $\sim 10$ kV voltage into the railgun load. The energy efficiency of the railgun is usually less than 40%: a considerable amount of energy is released as heat, and a small amount as electromagnetic energy. The electromagnetic radiation of the railgun, although mostly shielded by its metallic cover before firing, may leak out at the final stage when the projectile leaves the barrel and can be a potential electromagnetic compatibility issue for onboard electronics. We analyze the electromagnetic (EM) radiation coming from the rails and armature of the railgun with commercial software using the multilevel fast multipole method or higher order basis functions in the method of moment and evaluate the effects on electromagnetic radiation by adding one key component at a time in four models. Three field probes are set up 10 m from the muzzle, and four near fields are plotted on top of the muzzle. The near-field intensities and far-field radiation patterns at the final firing stage are presented and the frequency response is simulated. We found that most far-field patterns are directed toward the breech direction due to impedance mismatch between the PPS and railgun, and $E$ -field probes show stronger intensity on the $z$ -axis. Outer metal casing helps concentrate the electromagnetic energy inside the gun, and dielectric material absorbs the field within. The $E$ -field inside the gun is well below the breakdown threshold of common dielectric insulators. Strong radiation may occur if the railgun voltage spectrum contains a large megahertz component. [ABSTRACT FROM PUBLISHER]

The Mott insulator in correlated electron systems arises from classical Coulomb repulsion between carriers to provide a powerful force for electron localization. Turning such an insulator into a metal, the so-called Mott transition, is commonly achieved by "bandwidth" control or "band filling." However, both mechanisms deviate from the original concept of Mott, which attributes such a transition to the screening of Coulomb potential and associated lattice contraction. Here, we report a pressure-induced isostructural Mott transition in cubic perovskite PbCrO3. At the transition pressure of ~3 GPa, PbCrO3 exhibits significant collapse in both lattice volume and Coulomb potential. Concurrent with the collapse, it transforms from a hybrid multiferroic insulator to a metal. For the first time to our knowledge, these findings validate the scenario conceived by Mott. Close to the Mott criticality at ~300 K, fluctuations of the lattice and charge give rise to elastic anomalies and Laudau critical behaviors resembling the classic liquid-gas transition. The anomalously large lattice volume and Coulomb potential in the low-pressure insulating phase are largely associated with the ferroelectric distortion, which is substantially suppressed at high pressures, leading to the first-order phase transition without symmetry breaking. [ABSTRACT FROM AUTHOR]

The Mott insulator in correlated electron systems arises from classical Coulomb repulsion between carriers to provide a powerful force for electron localization. Turning such an insulator into a metal, the so-called Mott transition, is commonly achieved by "bandwidth" control or "band filling." However, both mechanisms deviate from the original concept of Mott, which attributes such a transition to the screening of Coulomb potential and associated lattice contraction. Here, we report a pressure-induced isostructural Mott transition in cubic perovskite PbCrO3. At the transition pressure of ~3 GPa, PbCrO3 exhibits significant collapse in both lattice volume and Coulomb potential. Concurrent with the collapse, it transforms from a hybrid multiferroic insulator to a metal. For the first time to our knowledge, these findings validate the scenario conceived by Mott. Close to the Mott criticality at ~300 K, fluctuations of the lattice and charge give rise to elastic anomalies and Laudau critical behaviors resembling the classic liquid-gas transition. The anomalously large lattice volume and Coulomb potential in the low-pressure insulating phase are largely associated with the ferroelectric distortion, which is substantially suppressed at high pressures, leading to the first-order phase transition without symmetry breaking. [ABSTRACT FROM AUTHOR]

Oxide-based metal–insulator–metal structures are of special interest for future resistive random-access memories. In such cells, redox processes on the nanoscale occur during resistive switching, which are initiated by the reversible movement of native donors, such as oxygen vacancies. The formation of these filaments is mainly attributed to an enhanced oxygen diffusion due to Joule heating in an electric field or due to electrical breakdown. Here, the development of a dendrite-like structure, which is induced by an avalanche discharge between the top electrode and the Ta2O5-x layer, is presented, which occurs instead of a local breakdown between top and bottom electrode. The dendrite-like structure evolves primarily at structures with a pronounced interface adsorbate layer. Furthermore, local conductive atomic force microscopy reveals that the entire dendrite region becomes conductive. Via spectromicroscopy it is demonstrated that the subsequent switching is caused by a valence change between Ta4+ and Ta5+, which takes place over the entire former Pt/Ta2O5-x interface of the dendrite-like structure. [ABSTRACT FROM AUTHOR]

The results of investigations into the formation process of polysilicon elements in the presence of a complex relief in the form of silicon islands in a silicon-on-insulator (SOI) structure are presented. The optimal production parameters of the process are found and their determining physicochemical mechanisms are described. A novel organization concept of vacuum-plasma etching of polysilicon elements in the presence of a complex relief is proposed. Reactive-ion etching of polysilicon elements during the formation of the structures of circuits with elevated radiation resistance, such as SOI, is developed and optimized. [ABSTRACT FROM AUTHOR]

We determine conditions on the filling of electrons in a crystalline lattice to obtain the equivalent of a band insulator--a gapped insulator with neither symmetry breaking nor fractionalized excitations. We allow for strong interactions, which precludes a free particle description. Previous approaches that extend the Lieb-Schultz-Mattis argument invoked spin conservation in an essential way and cannot be applied to the physically interesting case of spin-orbit coupled systems. Here we introduce two approaches: The first one is an entanglement-based scheme, and the second one studies the system on an appropriate flat "Bieberbach" manifold to obtain the filling conditions for all 230 space groups. These approaches assume only time reversal rather than spin rotation invariance. The results depend crucially on whether the crystal symmetry is symmorphic. Our results clarify when one may infer the existence of an exotic ground state based on the absence of order, and we point out applications to experimentally realized materials. Extensions to new situations involving purely spin models are also mentioned. [ABSTRACT FROM AUTHOR]