High Bias Field Research Articles

Improved high voltage operation of amorphous In–Ga–Zn–O thin film transistor by carrier density enhancement

We report on improved high voltage operation of amorphous-In–Ga–Zn–O (a-IGZO) thin film transistors (TFTs) by increasing carrier density and distributing the high bias field over the length of the device which utilizes an off-set drain structure. By decreasing the O2 partial pressure during sputter deposition of IGZO, the channel carrier density of the high voltage a-IGZO TFT (HiVIT) was increased to ∼1018 cm−3. Which reduced channel resistance and therefore the voltage drop in the ungated offset region during the on-state. To further decrease the electric field in the offset region, we applied Ta capping and subsequent oxidation to locally increase the oxygen vacancy levels in the offset region thereby increasing local carrier density. The reduction of the drain field in the offset region from 1.90 V μm−1 to 1.46 V μm−1 at 200 V drain voltage, significantly improved the operational stability of the device by reducing high field degradation. At an extreme drain voltage of 500 V, the device showed an off-state current of ∼10−11 A and on-state current of ∼1.59 mA demonstrating that with further enhancements the HiVIT may be applicable to thin-film form, low leakage, high voltage control applications.

Low-electric-field-induced high electrocaloric properties via defect regulation in samarium-doped BaTiO3 ceramics

A small electrocaloric effect (ECE) is usually observed in aliovalent doped ferroelectric ceramics since decreased polarization is usually present. In this work, the high ECE is achieved in a barium titanate [BaTiO3 (BT)]-based ceramic by introducing defect polarization to enhance the ferroelectric polarization and polarization change. The defect dipoles consisting of donor ion and barium vacancy, or barium and oxygen vacancies were formed in samarium-doped (Ba1−1.5xSmx)TiO3 ceramics. The phase and defect structures, and dielectric and ferroelectric properties are analyzed to confirm the evolution of charge compensation states and defect structures, and the influences of defects on polarization rotation and ECE. An enhanced ferroelectric polarization and high internal bias field are present in the ceramic with a suitable defect dipole effect, leading to a high polarization change before/after applying an electric field, causing an enhanced ECE. Finally, a high electrocaloric temperature change of ∼1.11 K and a large electrocaloric strength of ∼0.037 K·cm kV−1 were achieved under a low electric field of 30 kV/cm, indicating that forming suitable defect dipoles can optimize the ECE. This work provides a significant paradigm for tuning ECE in ferroelectric materials via defect regulation.

Analytical solutions for resonant radiation performance of bending-mode magnetoelectric antennas

This paper proposes analytical solutions for the resonant radiation performance of bending-mode magnetoelectric (ME) antennas. The strain-mediated Converse ME (CME) coupling model of bending-mode ME antennas is first established by solving nonlinear constitutive equations and bending governing equations using the elastic mechanics method. Then, the calculated magnetic flux and electric displacement are employed to propose a resonant radiation field model based on the dipole method. The numerical results for the CME coefficient show a good agreement with the experimental data. It can be observed that the volume fraction ratio of the piezoelectric layer can control the CME coefficient and radiation efficiency with the same variation trend since it can determine the bending strain via changing the location of the neutral layer of the ME antennas, which also demonstrates the strain-mediated essence of the ME antennas. In addition, the volume fraction ratio can tune the resonant frequency within a wide range. The gain of the ME antenna is stable and higher than −168 dB with the volume fraction ratio ranging from 0.2 to 0.7. The tensile stress and compressive stress have the opposite effect on the resonant frequency at low and high bias magnetic fields. Meanwhile, the tensile (compressive) stress is beneficial for both the radiation and gain in the low (high) bias field region. This model may facilitate the understanding of the bending-mode radiation mechanism of ME antennas and provide a basis for designing asymmetric ME antennas.

Significant magnetoelectric enhancement of composite films of CoZnxFe2-xO4 particles and poly (vinylidene fluoride-trifluoroethylene) for AC magnetic sensors

Polymer-based magnetoelectric (ME) nanocomposites exhibit a low ME effect and high bias field at room temperature, which severely limits their application in flexible wearable sensors. To overcome these obstacles, Zn-doped magnetic nanoparticles of CoZnxFe2-xO4 (x = 0, 0.1, 0.2, and 0.3) were prepared by an auto-combustion method in this work. The obtained magnetostrictive curves imply that Zn element doping can improve greatly the piezomagnetic coefficient of magnetic nanoparticles, and among the tested materials, CoZn0.1Fe1.9O4 has the largest piezomagnetic coefficient. Composite films of poly(vinylidene fluoride-trifluoroethylene) and CoZn0.1Fe1.9O4 (P(VDF-TrFE)/CoZn0.1Fe1.9O4) were prepared by spin coating. The maximum ME voltage coefficient of P(VDF-TrFE)/CoZn0.1Fe1.9O4 composite film with a filler weight concentration of 10 wt% is 87.9 mV cm−1 Oe−1 with a bias field of 1050 Oe and a resonance frequency of 46 kHz, which is the highest value reported in the literature of 0–3 type polymer-based ME composite films at present. To evaluate the composite films for application in magnetic sensors, the ME output voltage was measured at a bias field of 1050 Oe with increasing AC magnetic field, demonstrating a good linear relationship with linearity of 0.999. These results indicate that the piezomagnetic coefficient of magnetic materials is an the important factor for the great enhancement of the ME voltage coefficient. This work provides a new approach for the synthesis of more effective polymer-based ME nanocomposites for potential applications in smart wearables.

Electric-field assisted ring-opening polymerization: On the kinetics and product properties of DGEBA/aniline model system

This work demonstrates the effect of a high dc bias field (up to 160 kV/cm) on the polyaddition reaction of the modeled epoxy resin system. Polymerization of bisphenol A diglycidyl ether (DGEBA) supported by high electric field results in obtaining epoxides with significantly increased molecular weight, which is seven-fold higher than the zero-field reference ( M n = 4.7 kg/mol). We also found a markedly reduced dispersity ( Ð = 1.52 at 0 kV/cm, Đ = 1.21 at 160 kV/cm). The molecular weight and dispersity of obtained polymers change almost linearly with the reaction time. By following polymerization kinetics in the real-time of the experiment, via dielectric spectroscopy, we observe slowing down the reaction progress at dc fields of E = 160 kV/cm, compared to the zero-field case, E = 0 kV/cm. The polymer materials obtained in the presence/absence of static electric fields exhibit distinctly different glass transition temperatures. Our study highlights a better control over the reaction and product properties attained by using only the magnitude of the applied field as the control variable. Thus, a high electric field could provide an alternative pathway in synthesizing and developing advanced polymer materials using simpler, “green”, less-expensive, and technologically demanding approaches.

The mechanism of heavy injection of electric charges to realize linear strain behavior of textured PMN–24%PT ceramics combined with Pt grains

Strain hysteresis is one of the important issues on piezo-ceramic actuators conducting fast and high-precision actuation. In this work, a new regulation mechanism was employed to resolve this issue using 0–3 type metal–ceramic composite. Pb(Mg1/3Nb2/3)76%Ti24%O3 <001> textured ceramics modified by fine Pt grains (Pt@<001>PMN–24%PT) were designed and achieved. Linear strain behavior with zero hysteresis was achieved by 1 vol%Pt@<001>PMN–24%PT composite ceramics. Element maps and TEM images indicated that there was no diffusion between Pt and PMN–24%PT. We explored the correlation between composite structure and strain hysteresis performance. Experimental results and simulation analysis indicated that widely-distributed conjunctions between platinum and PMN–24%PT grains helped to inject a great quantity of charges into PMN–24%PT oxide. The inner high bias field caused by heavy injection of electric charges could pre-tighten the ferroelectric domains of textured PMN–24%PT ceramics. Only when the domain structure has been pre-tightened, linear piezoelectric strain is available.

Room-temperature Bloch oscillations and interminiband Zener tunneling in a GaAs-based narrow-minigap superlattice

We investigate peculiar Bloch oscillations and interminiband Zener tunneling in a GaAs-based narrow-minigap superlattice up to room temperature, by using terahertz emission spectroscopy under dc bias electric fields. The Bloch oscillations observed previously with a π/2 phase shift at 10 K under relatively low bias fields are found to survive even at 300 K, where thermal energy kT exceeds the relevant minigap (k: Boltzmann constant, T: temperature). Furthermore, the interminiband Zener tunneling under high bias fields leads to a monocyclic terahertz signal with a temperature-dependent subsequent bumpy tail, indicating its occurrence at a few different occasions for Bloch oscillating electrons.

Robust quantum point contact via trench gate modulation

Quantum point contacts (QPC) are a primary component in mesoscopic physics and have come to serve various purposes in modern quantum devices. However, fabricating a QPC that operates robustly under extreme conditions, such as high bias or magnetic fields, still remains an important challenge. As a solution, we have analyzed the trench-gated QPC (t-QPC) that has a central gate in addition to the split-gate structure used in conventional QPCs (c-QPC). From simulation and modelling, we predicted that the t-QPC has larger and more even subband spacings over a wider range of transmission when compared to the c-QPC. After an experimental verification, the two QPCs were investigated in the quantum Hall regimes as well. At high fields, the maximally available conductance was achievable in the t-QPC due to the local carrier density modulation by the trench gate. Furthermore, the t-QPC presented less anomalies in its DC bias dependence, indicating a possible suppression of impurity effects.

Robust Fabrication of Hybrid Lead-Free Perovskite Pellets for Stable X-ray Detectors with Low Detection Limit.

X-ray detectors are widely utilized in medical diagnostics and nondestructive product inspection. Halide perovskites are recently demonstrated as excellent candidates for direct X-ray detection. However, it is still challenging to obtain high quality perovskites with millimeter-thick over a large area for high performance, stable X-ray detectors. Here, methylammonium bismuth iodide (MA3 Bi2 I9 ) polycrystalline pellets (PPs) are developed by a robust, cost effective, and scalable cold isostatic-pressing for fabricating X-ray detectors with low limit of detection (LoD) and superior operational stability. The MA3 Bi2 I9 -PPs possess a high resistivity of 2.28 × 1011 Ω cm and low dark carrier concentration of ≈107 cm-3 , and balanced mobility of ≈2 cm2 V-1 s-1 for electrons and holes. These merits enable a sensitivity of 563 μC Gyair -1 cm-2 , a detection efficiency of 28.8%, and an LoD of 9.3 nGyair s-1 for MA3 Bi2 I9 -PPs detectors, and the LoD is much lower than the dose rate required for X-ray diagnostics used currently (5.5 μGyair s-1 ). In addition, the MA3 Bi2 I9 -PPs detectors work stably under high working bias field up to 2000 V cm-1 after sensing an integrated dose >320 Gyair with continuous X-ray radiation, demonstrating its competitive advantage in practical application. These findings provide an approach to explore a new generation of low LoD, stable and green X-ray detectors based on MA3 Bi2 I9 -PPs.

Two-Step Poling for Improved Piezoelectric Properties in Lead-Free (Ba0.88Ca0.12)(Ti0.94Sn0.06)O3 Ceramic System

Pb-free (Ba0.88Ca0.12)(Ti0.94Sn0.06)O3 piezoelectric nanopowder was synthesized by solid-state reaction method. Piezoelectric properties were improved by two different approaches by optimizing poling conditions. Results indicate that poling near polymorphic phase boundary leads to the minimization of conduction current. However, low degree of poling observed indicates poor piezoelectricity. On the contrary, with Two-Step poling method, where the sample was sequentially subjected to low bias field at high temperature (step-1) and high bias field at low temperature (step-2), improved the piezoelectricity. Further, enhanced piezoelectric properties were obtained with optimized bias field of 1.25 kV/mm for step-1 with improved domain alignment.

Electrically Tunable Ferrite Bandpass Filter in X-Band With Wide Tunability

An electrically tunable bandpass filter is designed using ferrite inductor as a tuning element. The device is made electrically tunable by winding copper coils around ferrite substrate. Ferrite substrate is made of low-temperature co-fire ceramic compatible ferrite tape (ESL 40011). The $50~\Omega $ microstrip line is printed on the ferrite substrate in the form of a rectangular loop. The area of the loop determines the center frequency of the filter. This structure results in a closed-loop magnetic field in the ferrite substrate when bias is applied thereby reducing the losses and improving efficiency. Center frequency of bandpass filter is around 9.6 GHz. Tunability of the filter is around 11%. Bias current is varied from 50 to 350 mA. This corresponds to an effective magnetic field varying from 15 to 80 Oe. Center frequency of the bandpass filter shifted from 9.6 GHz to around 10.8 GHz due to the application of bias field. Due to the reduction in the slope of bias curve (hysteresis curve) of the ferrite material with an increase in the bias field, the tunability of the device saturates at high bias fields. The coils are wound around the ferrite material to provide a dc bias in the azimuthal direction (in the plane of substrate). High degree of tunability has been achieved from very low bias currents by this design. Insertion loss of the filter varied from 2 to 2.6 dB which is primarily due to the losses in the ferrite material. This filter can be used for high-power applications due to the ferrite material used for fabrication. High-power measurements show stability up to 2 W of input power. Beyond 2 W, this filter can be used for pulsed power applications. Low-field losses in the ferrite material can be avoided by choosing an operating frequency above $4\pi \text{M}_{\mathbf {s}}$ which is about 9 GHz for this material.

Phase shift of terahertz Bloch oscillations induced by interminiband mixing in a biased semiconductor superlattice

We investigate terahertz waveforms emitted from a GaAs-based superlattice with interminiband mixing under various dc bias electric fields, injecting electrons into the conduction first miniband with femtosecond optical pulses. Bloch oscillations are observed under relatively low bias fields; surprisingly, the oscillation phase is found to shift by π/2 from those reported previously for Wannier–Stark ladder states in nearly isolated minibands. The oscillatory feature of terahertz emission is gradually destroyed under higher bias fields. We show that the observed π/2 phase shift can be ascribed to significant delocalization of electron wavefunctions driven by interminiband mixing.

Dipolar field-induced asymmetric magnetization hysteresis of immobile superparamagnetic nanoclusters

Abstract Despite the critical contribution of dipolar interparticle interactions to the hysteretic magnetization curve of superparamagnetic nanoclusters, the quantitative analysis of dipolar field remains a major challenge at room temperature. To investigate such demagnetizing effect due to a close core-to-core distance within a nanocluster, we evaluate the symmetry of ac minor magnetization hysteresis in the presence of static magnetic field. While the nanoclusters are immobilized in a gelatin matrix, we record their dynamic magnetizations for a given 100-kHz sinusoidal magnetic field perpendicular to the static field and further correlate the changes in the coercive fields and remanences with the dipolar field and its demagnetization ratio, respectively. In this work, we also show that the dipolar field-induced shift of the magnetization loop is size-dependent. Under high static bias field, larger core size proportionally enhances the resulting dipolar field. Meanwhile, the cluster size appears to be influential to the dipolar magnetism triggered at low bias field. This study conclusively highlights the distinguishable dynamics between randomly oriented and collaterally aligned magnetic moments.

A new toroidal metamaterial phase shifter

AbstractIn this paper, the design of a rectangular waveguide metamaterial (MTM) phase shifter will be introduced in X band. The main disadvantages of the class of waveguide MTM phase shifters include high insertion loss (about 10 dB) and high DC magnetic bias field requirement (6‐7 kOe), that treated in the proposed phase shifter scheme by using toroidal shape ferrite core in conjunction with metalized parallel microstrip lines and implementing the multi‐section matching network, that leads to 3.2 dB insertion loss and a simple bias mechanism such as conventional toroidal phase shifter without consisting any high reluctance air‐gap, which could be implemented by no need of any feather coil system or higher current values. The results of simultaneous ENG and MNG design procedure for 288° differential phase shift with the 2 cm ferrite core length, shows 4 times compactness of proposed phase shifter in comparison with conventional toroidal phase shifter.

Numerical investigation of saturation behaviors in photoconductive antennas under high bias conditions

A photoconductive antenna (PCA) is modeled using a higher order solution derived from the Boltzmann’s equation to accurately predict anomalous behaviors in this system. In the case concerning the application of high bias fields, photoexcited carriers gain sufficient energies, so that they become hot carriers. Moreover, screening fields in PCAs are numerically investigated in the first picoseconds subsequent to the excitation using hot-carriers effects. To explore another exotic phenomenon in a PCA, its driving photocurrent is decomposed into DC and THz components. Then, the saturation behavior of the THz component peak and the sublinear dependence of the DC component with respect to an externally applied bias are numerically investigated. Furthermore, depending on the accumulation of photoexcited carriers, velocity overshoot phenomenon may occur in the driving photocurrent against the applied bias. These phenomena cannot be predicted by the conventional drift-diffusion model. To simplify the analysis of PCAs, a time-dependent equivalent circuit model is proposed for describing the associated operational principles. Finally, using a coherent detection scheme and the circuit model, it will be shown that owing to the screening fields, the THz power saturates with respect to the applied laser power.

Synthesis of Amorphous InSb Nanowires and a Study of the Effects of Laser Radiation and Thermal Annealing on Nanowire Crystallinity.

Although various synthesis and characterization strategies have been employed for the synthesis of crystalline nanowires, there is very little work done on development of low-dimensional amorphous semiconductors. This paper presents a simple strategy to grow amorphous (a-) nanowires (NWs) in a chemical vapor deposition (CVD) system. The NWs were grown on substrate coated with indium film and the lack of crystallinity in the as-grown stoichiometric NWs was ascertained by Raman spectroscopy and electron transport measurements. A model proposed to explain the amorphous NW growth mechanism takes into account the fact that NW growth was carried out at the high temperature ramp-up rate of 75 C/min. This high rate is believed to affect the growth kinematics and determine the arrangement of atoms in the growing NW. Raman spectrum of the as-grown sample shows a broad peak around 155 cm, indicative of the presence of high density of homopolar - bonds in the amorphous matrix. It was also found that high intensity laser light induces localized crystallization of the NW, most likely due to radiation-stimulated diffusion of defects in a-. The nonlinear trend of the current-voltage characteristics for individually contacted a- NWs was analyzed to prove that the non-linearity is not induced by Schottky contacts. At high bias fields, space charge limited conduction was the proposed electron transport mechanism. Post-growth annealing of the as-grown a- NWs was found to be very effective in causing the NWs to undergo a phase transition from amorphous to crystalline.

Role of hydrogen, oxygen and composite formation in relaxational polarization of hafnium oxide and aluminum oxide

Metal-insulator-metal MIM devices of Al 2 O 3 were annealed to various temperatures, 366, 411 and 455 K. Their capacitance measurements were conducted at room temperature. It is illustrated that the annealing process reduces the polarization nonlinearity in high DC bias fields. This way the nonlinearity can be decreased up to 75% of its initial value. Al 2 O 3 was also prepared under O 2 atmosphere. This leads to a drastic suppression of the previously observed electrode polarization for the low frequency region and for high temperatures in the case of samples without O 2 treatments. With O 2 treatments also the nonlinearity decreases. Due to the electrode polarization the relaxational polarization of Al 2 O 3 above 300 K could not be interpreted by the asymmetric double well potential (ADWP) model. It is shown that the capacitance behavior of oxygen treated samples can be interpreted in a wide temperature range by our model. Furthermore, it is found that the charge injection in the case of HfO2 and the loss are much stronger than for Al 2 O 3 Preparing a composite of the two oxides this charge injection as well as the loss was highly suppressed. The composite yields a higher permittivity than that for Al2O3. The capacitance measurements are also conducted for Al 2 O 3 MIM samples consisting of Pd and Au as electrodes after exposing them to hydrogen gas. These and the annealing experiments exhibit an important role of hydrogen in contributing to the relaxational polarization of metal oxides.

Study of exchange bias and memory effect in core-shell CoCr2O4 nanoparticles

CoCr2O4 of 50 nm particles having core-shell structure demonstrate two different phenomena such as memory effect and exchange bias depending on temperature. The memory effect is responsible for spin glass behavior in the shell and exchange bias is due to core-shell interactions. From magnetization measurement, we observe a long range ferrimagnetic (FiM) order in core and the spin glass behavior at the shell. The spin glass nature is confirmed from memory effect experiment performed using zero field cool (ZFC) protocol below blocking temperature (TB) i.e. 80 K. While the strength of memory effect decreases with decrease in temperature and vanishes at 30 K, the exchange bias increases with decrease in temperature down to 50 K and shows two orders of magnitude more than the bulk at lowest temperature. The high exchange bias field obtained in CoCr2O4 nanoparticles is not only due to the contribution of interface between collinear and non-collinear spin alignment, but also due to spiral ordering below 30 K. While the exchange interaction and strong interface anisotropy results in an exchange bias field (HEB), the training effect have the contribution from the interface as well as from the core shell interaction depending on the particle size.

Exchange bias effect in Ni50Mn35In15/BiFeO3 heterostructure thin film

In this study, we studied the exchange bias (EB) effect in Ni50Mn35In15 and Ni50Mn35In15/BiFeO3 heterostructure thin films deposited onto Pt/Ti/SiO2/Si substrate using dc/rf magnetron sputtering. In pure Ni50Mn35In15 film, the shift of the hysteresis loop from the origin up to 110 Oe was observed at 10 K due to coexistence of FM-AFM interface. On the other hand, the shift of the hysteresis loop was significantly enhanced (480 Oe) in Ni50Mn35In15/BiFeO3 heterostructure thin film at 10 K in field cooled. Further, a high exchange bias field of 80 Oe was found at room temperature in Ni50Mn35In15/BiFeO3 heterostructure thin film. The observed exchange bias field (HE) in this heterostructure thin film was attributed to the presence of a pinned and uncompensated spins in the antiferromagnetic at the interface, and induced by the interface exchange coupling between Ni50Mn35In15 and BiFeO3. This behaviour is an additional property for the Ni50Mn35In15/BiFeO3 heterostructure thin film to be used in various other magnetic memory devise applications.

Size Effect on Interlayer Coupling and Magnetoresistance Oscillation of Magnetic Tunnel Junction Embedded With Iron Nanoparticles

Here, we investigate the size effect of perpendicular-anisotropic double-barrier magnetic tunnel junction (MTJ) devices embedded with iron nanoparticles. A sputtering system in conjunction with the postannealing process is employed to prepare the sheet film and standard lithography techniques followed by the ion etching technique are used to fabricate the micrometer to submicrometer MTJ devices. A strong ferromagnetic coupling is observed as we reduce the size of the device to submicrometer scale, which is due to the reduction of magnetostatic energy of the device. Furthermore, a magnetoresistance (MR) oscillation is observed at room temperature while reducing the size of the device. MR peaks at low bias fields are believed to have magnon contributions, whereas the peaks observed at higher bias fields are responsible for phonon-assisted tunneling. Zero-bias anomalies are also observed and are more prominent in antiparallel states of the devices.