DC Bias Voltage Research Articles

Temperature Stable Piezoelectric Imprint of Epitaxial Grown PZT for Zero-Bias Driving MEMS Actuator Operation.

In piezoelectric transducer applications, it is common to use a unipolar operation signal to avoid switching of the polarisation and the resulting nonlinearities of micro-electromechanical systems. However, semi-bipolar or bipolar operation signals have the advantages of less leakage current, lower power consumption and no additional need of a DC−DC converter for low AC driving voltages. This study investigates the potential of using piezoelectric layers with an imprint for stable bipolar operation on the basis of epitaxially grown lead zirconate titanate cantilevers with electrodes made of a metal and metal oxide stack. Due to the manufacturing process, the samples exhibit high crystallinity, rectangular shaped hysteresis and a high piezoelectric response. Furthermore, the piezoelectric layers have an imprint, indicating a strong built-in field, which shifts the polarisation versus electric field hysteresis. To obtain the stability of the imprint, laser doppler vibrometry and switching current measurements were performed at different temperatures, yielding a stable imprinted electric field of −1.83 MV/m up to at least 100 °C. The deflection of the cantilevers was measured with a constant AC driving voltage while varying the DC bias voltage to examine the influence of the imprint under operation, revealing that the same high deflection and low nonlinearities, quantified by the total harmonic distortion, can be maintained down to low bias voltages compared to unipolar operation. These findings demonstrate that a piezoelectric layer with a strong imprint makes it possible to operate with low DC or even zero DC bias, while still providing strong piezoelectric response and linear behaviour.

Tunable four-port MIMO/self-multiplexing THz graphene patch antenna with high isolation

A tunable terahertz four-port multiple-input-multiple-output (MIMO) graphene-based microstrip patch antenna with self-multiplexing ability is designed and numerically studied. Insertion of cross-slot in the graphene patch improved the isolation level to the order of 50\(-\)70 dB in the four-port MIMO mode operation. Four separate biasing pads are used to obtain independent frequency tunability, which gives rise to MIMO and/or self-diplexing, self-triplexing, and self-quadruplexing operation, by using different combinations of dc bias voltages. The MIMO and self-multiplexing actions are attained while using common ground plane and continuous graphene patch. The MIMO performance parameters like envelope correlation coefficient (ECC), diversity gain, mean effective gain, port-to-port isolation, etc. are found to be within the acceptable limits. A four-port equivalent circuit model of the MIMO antenna is presented to provide insight into the radiation mechanism and to validate the simulation results. The designed four port antenna is found to provide ECC in the order of 0.0073 and isolation in the range of 35–43 dB while it is operated in the self-diplexing with two-port MIMO mode operation.

Ultrathin Piezoelectric Resonators Based on Graphene and Free-Standing Single-Crystal BaTiO3.

Suspended piezoelectric thin films are key elements enabling high-frequency filtering in telecommunication devices. To meet the requirements of next-generation electronics, it is essential to reduce device thickness for reaching higher resonance frequencies. Here, the high-quality mechanical and electrical properties of graphene electrodes are combined with the strong piezoelectric performance of the free-standing complex oxide, BaTiO3 (BTO), to create ultrathin piezoelectric resonators. It is demonstrated that the device can be brought into mechanical resonance by piezoelectric actuation. By sweeping the DC bias voltage on the top graphene electrode, the BTO membrane is switched between the two poled ferroelectric states. Remarkably, ferroelectric hysteresis is also observed in the resonance frequency, magnitude and Q-factor of the first membrane mode. In the bulk acoustic mode, the device vibrates at 233 GHz. This work demonstrates the potential of combining van der Waals materials with complex oxides for next-generation electronics, which not only opens up opportunities for increasing filter frequencies, but also enables reconfiguration by poling, via ferroelectric memory effect.

Image-charge detection of the Rydberg transition of electrons on superfluid helium confined in a microchannel structure

The image-charge detection provides a new direct method for the detection of the Rydberg transition of electrons trapped on the surface of liquid helium. The interest in this method is motivated by the possibility to accomplish the spin state readout for a single trapped electron, thus opening a new pathway towards using electron spins on liquid helium for quantum computing. Here, we report on the image-charge detection of the Rydberg transition in a many-electron system confined in an array of 20 μm wide and 4 μm deep channels filled with superfluid helium. Such detection is made possible because of a significant enhancement of the image-charge signal due to close proximity of trapped electrons to the electrodes embedded in the microchannel structure. The transition frequency of electrons in the range of 400–500 GHz is highly controllable by the dc bias voltages applied to the device and is in a good agreement with our calculations. This work demonstrates that microchannel structures provide a suitable platform for electron manipulation and their quantum state detection, with a feasibility of scaling the detection method to a single electron.

On mechanisms influencing etching/polymerization balance in multi-component fluorocarbon gas mixtures

In this work, we performed both experimental and model-based study of plasma parameters, steady-state gas phase compositions and heterogeneous process kinetics in CF4 + C4F8 + Ar and CF4 + CHF3 + Ar gas mixtures under the condition of 13.56 MHz inductively-coupled plasma. As constant input parameters we used gas pressure (6 mTorr) and input power (700 W) while variable ones were fluorocarbon component ratios and bias power. It was found that the substitution of CF4 for any second fluorocarbon gas a) results in rather weak effects on electrons- and ions-related plasma characteristics (especially in the case of C4F8); and b) causes sufficient changes in kinetics of neutral species that accelerate the formation of polymerizing radicals and suppress the F atoms density (especially in the case of CHF3). The latter is due to the effective decay of F atoms in the CHFx + F → CFx + HF reaction family. On the contrary, the variation of bias power does not influence kinetics of plasma active species, but changes only the ion bombardment intensity (and thus, the ion-induced reaction kinetics) through the negative dc bias voltage. The phenomenological (based on the gas-phase-related tracing parameters) analysis of heterogeneous process kinetics indicated that the fluorocarbon gas ratio represents the more effective tool to adjust etching characteristics, polymerization rate and the fluorocarbon film thickness. The last two values were found to be always higher in C4F8 - containing plasmas.

On origin of resistive and capacitive contributions to impedance of memory states in Cu/TiO2/Pt RRAM devices by impedance spectroscopy

The resistive switching phenomenon is attributed to local formation and rupture of conductive filaments (CF), however some aspects of the switching mechanism still need wider consensus. The origin of resistance contribution to low resistance state (LRS) and high resistance state (HRS) is although well known, the origin of capacitive contribution to the memory state of resistive random access memory device (RRAM) is ambiguous. Here we report impedance spectroscopic studies carried on Cu/TiO2/Pt devices to address the origin of resistive and capacitive contribution, where the effect of DC bias voltage, reset stop voltage and set compliance current on the impedance of RRAM memory states were investigated in detail. The studies revealed that the capacitive contribution was dominated by the surrounding TiO2 bulk and contrary to existing knowledge, gap formed owing to ruptured CF has negligible role. These studies also reaffirmed the origin of the resistive contribution to the HRS and LRS state, which was dictated by the ruptured gap and connected CF, respectively. Results of this study may be significant towards improving the switching time and operating frequency performance of futuristic memory, RF switch and neuromorphic computing applications of RRAM devices.

Effects of Carbon Doping and DC Bias Voltage on Microstructure and Mechanical Properties of AlCrCN Films Synthesized via HiPIMS.

This work compares the hardness and adhesion properties of AlCrN and AlCrCN hard coatings synthesized via HiPIMS using Al70Cr30 and Cr targets. The hardness and adhesion properties of AlCrCN films were optimized by performing deposition under various C2H2 flow rates (5, 8, 10, 13, 15, or 20 sccm) and DC bias voltages (−40, −60, −80, −100, or −120 V). EPMA results clearly indicated that the carbon content was increased from 1.9 to 12.2 at.% with increasing C2H2 flow rate from 5 to 20 sccm. XPS results confirmed a various content of chemical bonds (Cr-N, C-N, sp2, and sp3) with various C2H2 flow rate. Grain and columnar refinement in AlCrCN were derived from XRD, TEM, and SAED results. The higher hardness (28.6 GPa) and Young’s modulus (358 GPa) were obtained using an C2H2 flow rate of 5 sccm and a bias voltage of −60 V. Both of which subsequently decreased to 13.5 GPa and 212 GPa, respectively. This can be attributed to the C-N bond inhibiting the development of metal-N bonds. Increasing the bias voltage to −120 V increased the hardness to 32.9 GPa and the Young’s modulus to 372 GPa. Note that the application of bias voltage to enhance hardness should also be applicable to carbon-doped AlCrN films as well. All samples presented good adhesion characteristics (class 1; ISO26443:2008-06).

Modeling and Characterizing of an Enhanced Practical Terahertz Photoconductive Antenna

Optical generation can be divided into photoconductive generation and optical rectification. Moreover, the generation of photoconductive is more efficient than optical rectification when standard laser oscillator systems are used. At photoconductive antenna (PCA), the efficiency and terahertz (THz) power generated are affected by different parameters (e.g. incident power of laser pump, dielectric properties and the applied DC bias voltage difference). In this research, four proposed designs of PCAs are presented and compared. Furthermore, microfabrication aspects and concerns have been considered in this study, which are rarely presented in the literature. Therefore, effects of different parameters are shown in this work such as adding adhesion layer in between electrodes and the substrate, varying of gap properties and dipole length. Output frequency, photocurrent and the total effective energy of different antenna models at different values of laser power have been studied and compared.

Scaling of Time-Dependent Tunneling Current in Terahertz Scanning Tunneling Microscopes

Terahertz scanning tunneling microscopy (THz-STM) has enabled spatiotemporal imaging with femtosecond temporal resolution and angstrom spatial resolution. In THz-STMs, the junction bias is modulated by coupling THz pulses, which results in transient voltage bias and extremely high transient tunneling currents. In order to have efficient imaging of the sample surface, it is important to understand the nonlinear tunneling current response and its parametric dependence. In this work, we theoretically investigate the basic scaling of rectified electrons in a THz-STM junction. We use a self-consistent quantum model that includes both space-charge potential and exchange-correlation potential, which were ignored in previous studies. Since THz-STMs are operated at high transient voltage in field emission regime, the effects of exchange-correlation potential become crucial. We validate our calculation with recently reported experimental data and investigate the rectification property of the tip-sample junction for different parameters. We find that the time-dependent tunneling current and the electron transport can be manipulated by varying the dc bias voltage (polarity, amplitude), incident THz field (polarity, shape, peak amplitude), work functions of STM tip and sample---especially their difference $\mathrm{\ensuremath{\Delta}}W$, and the tip-sample separation. Our study provides an important framework that can be used in the future to characterize, control, and improve THz-induced currents and probing techniques at the nanometer scale over subpicosecond time periods.

Electrically tunable liquid crystal phase shifter with excellent phase shift capability per wavelength based on opposed coplanar waveguide

A tunable liquid crystal phase shifter based on opposed coplanar waveguide (OCPW) with excellent phase shift capability per wavelength is proposed. The proposed OCPW not only has good transmission characteristics and a quasi-TEM operating mode similar to the CPW, but also avoids the metalized via and complicated bias circuit in designing the phase shifter. Consequently, the phase shifter is easy to fabricate and miniaturize. The main structure of the phase shifter includes two functional components. One is the transition section, in which the flaring grounds and gradient stubs are utilized to realize the impedance and wave-number matching to the OCPW phase shift part; the other is a tunable phase shift section, which is composed of the OCPW centerline and ground with staggered periodic stubs stretching out. To realize tunable phase shifting by applying different DC bias voltages, a DC bias network capable of blocking RF signal is elaborately designed. An equivalent circuit model of the phase shift part is established and represented using the ABCD matrix, so that the relation between the related parameters and the phase shift per wavelength can be easily obtained to guide the design. The measurement results demonstrate that the proposed phase shifter achieves a total of 300° phase shifting, as high as 289°/λ0 phase shift per wavelength, and a good figure-of-merit of 59°/dB at 14.5 GHz. The phase shifter features the reflection coefficient less than −10 dB and the transmission coefficient bigger than −6 dB from 9.3 to 15 GHz.

Ultrathin tin sulfide field-effect transistors with subthreshold slope below 60 mV/decade

In this paper, we present for the first time a field-effect-transistor (FET) having a 10 nm thick tin sulfide (SnS) channel fabricated at the wafer scale with high reproducibility. SnS-based FETs are in on-state for increasing positive back-gate voltages up to 6 V, whereas the off-state is attained for negative back-gate voltages not exceeding −6 V, the on/off ratio being in the range 102–103 depending on FET dimensions. The SnS FETs show a subthreshold slope (SS) below 60 mV/decade thanks to the in-plane ferroelectricity of SnS and attaining a minimum value SS = 21 mV/decade. Moreover, the low SS values can be explained by the existence of a negative value of the capacitance of the SnS thin film up to 10 GHz (for any DC bias voltage between 1 and 5 V), with the minimum value being −12.87 pF at 0.1 GHz.

On the equivalence between mass perturbation and DC voltage bias in coupled MEMS resonators: Theoretical and experimental investigation

In this work, we consider MEMS devices made of mechanically coupled microbeams under electric actuation. We conduct an experimental study to identify the occurrence of veering and then investigate its dynamic response for different electric actuations. A slight change in the DC voltage bias from the veering point is observed to affect significantly the frequency response. Indeed, jump to large orbits occurred when perturbing the applied DC voltage while operating near the cyclic-fold bifurcation point. We also develop and validate a mathematical model to simulate the response of the device. The model showed similarities in the softening effect of the DC voltage bias and an added mass when matching their induced shift in the natural frequency. As such, one can discern the inherent nonlinear effects of DC voltage bias on coupled resonators and exploit them for mass sensing applications without going over the hassle of mass deposition, which requires the deployment of complex processes. We also investigate different mass detection mechanisms. We show the potential of mode localization and the significant and abrupt jumps in the deflection of the coupled microbeams due to mass perturbation to enhance the sensitivity of MEMS mass sensors.

Design and Performance Investigation of a Temperature Compensated Transmitter With GaN HEMTs for Phased-Array Applications

This article presents a temperature compensated transmitter for phased-array applications. The high-temperature effects on each individual transmitter building block are studied and compensated to mitigate the performance variations. Furthermore, a new active, tunable, and variable gain phase shifter VGPS) architecture is proposed, which features low-performance variation with temperature and high gain without compromising the phase shift range. A temperature sensor is designed to adaptively adjust the critical dc bias voltages to minimize the variation of the active devices. A class- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{F}^{-1}$ </tex-math></inline-formula> power amplifier PA) is designed, as the last stage in the transmitter chain, and its performance variation is compensated through the VGPS stage by adaptively tuning its characteristics. Gallium-nitride (GaN)-on-silicon-carbide (SiC) high-electron-mobility transistors (HEMTs) are adopted as the active devices and a voltage-controlled capacitor (varactor) used in the VGPS stage. The transmitter is prototyped on a piece of Rogers 4003C substrate, and the system performance is measured. The transmitter has the minimum and the maximum small-signal gain of 5.2 and 31.5 dB, respectively, with the maximum variation of < 2.3 dB across the temperature range and the frequency range of 2–2.1 GHz. The insertion phase variation also remains below 6.5° across the phase shift, temperature, and frequency range. The maximum output power and power added efficiency (PAE) are measured to be 33 dBm and 58%, respectively, at the maximum ambient temperature of 220 °C. The measurement results also show that the transmitter is able to transmit 64-QAM signals with the data rate of 120 Mb/s at the maximum ambient temperature of 220 °C.

Thickness-Dependent Nonlinear Electrical Conductivity of Few-Layer Muscovite Mica

We report on the direct current (dc) current-voltage ($I$-$V$) characteristics of few-layer muscovite mica (MuM) flakes exfoliated and transferred onto $\mathrm{Si}\mathrm{O}$${}_{2}$/$\mathrm{Si}$ substrate, under different substrate dc bias voltages. Contrary to usual observations in conventional two-dimensional systems, we observe an increase in the in-plane electrical conductivity with a reducing thickness of MuM flakes. At a given voltage, the electrical conductivity of approximately five-layered MuM flake (${T}_{3}$) is 3 orders of magnitude larger than that in approximately ten-layered MuM flake (${T}_{2}$). The $I$-$V$ characteristics are used to analyze the mechanism of conduction. The model-based analysis reveals the hopping-conduction mechanism to be dominant as compared to the Poole-Frenkel effect. The thickness-dependent work function is measured using Kelvin probe force microscopy for a MuM flake on $\mathrm{Si}$ substrate. Assuming that the measured work function is correlated with the Fermi level, we report an upward movement of the Fermi level, toward the conduction band with the reducing thickness of MuM flakes, indicating an increase in the conduction-band carrier density. The observed increase in conductivity in ${T}_{3}$ when compared to ${T}_{2}$ may be attributed to surface doping due to the increased contribution from ${\mathrm{K}}^{+}$ ions and lattice relaxation. Our results show that there is a possibility of using few-layer mica as a wide-band-gap semiconductor and that it can open up different avenues for two-dimensional electronic devices.

Parametric amplification of a resonant MEMS mirror with all-piezoelectric excitation

We report a resonant torsional micro-mirror with all-piezoelectric driving and tunable spring stiffness. Stiffness modulation finds two practical applications. First is tuning of the resonance frequency, achieved by applying DC bias voltage to the stiffness modulating structures. A tuning rate was found to be 0.95 Hz V−1 with up to 20 Hz of usable frequency range. Second, when direct excitation of the torsional mode is combined with the harmonic modulation of the spring stiffness, an optical scan angle is shown to be increased by more than 4° through 2:1 degenerate parametric amplification. By varying the phase of the parametric pump with respect to the direct excitation, the Q-factor is tuned between 617 and 898, corresponding to the minimum and maximum parametric gain factors of 0.84× and 1.21×, respectively, achieved at a nominal unpumped optical scan angle of 16.3°. Increasing the pump amplitude shows a moderate increase in the amplifier's gain with clear saturation at 1.43× in the superthreshold pumping regime, indicating a presence of the third order stiffness nonlinearity. The results show potential to apply parametric amplification to future piezo-micro-electro-mechanical-system actuators for large frequency and large-stroke mechanical response achieved at ambient pressure.

Experimental Generation of Reconfigurable Ultra-Wideband Signal Based on the Pulse Difference by Using Optical Polarization Synthesis

A reconfigurable optical ultra-wideband (UWB) pulse generation scheme based on the pulse difference by using polarization synthesis is proposed and demonstrated experimentally. The Gaussian pulses with an appropriate time delay are modulated via the two sub-MZMs of the DPol-MZM, and the optically carried signals are combined via orthogonal polarization multiplexing, avoiding the optical interference. By setting the DC bias voltages of the sub-MZMs and the time delay, a pair of polarity-reversed Gaussian monocycle, doublet, triplet and quadruplet signals are generated with the central frequency of 3.37, 4.79, 4.5, 5.05 GHz and the fractional bandwidths of 166%, 129%, 124%, 89% accordingly. The signals of pulse shape modulation, binary phase shift keying modulation and on-off keying modulation are generated with the experimental system.

New Formulation of Cardiff Behavioral Model Including DC Bias Voltage Dependence

A new mathematical formulation for the Cardiff nonlinear behavioral model is presented in this work which includes the dc bias voltages (drain and gate) into the model. It has been verified by modeling a GaN on SiC high electron mobility transistor (HEMT) at 3.5 GHz. For the case presented, interpolation of load–pull data has resulted in a more than 90% reduction in the density of the load–pull data required to generate a nonlinear behavioral model over a wide range of dc bias points.

Recent Developments in Quantum‐Circuit Refrigeration

AbstractThe recent progress in direct active cooling of the quantum‐electric degrees of freedom in engineered circuits, or quantum‐circuit refrigeration is reviewed. In 2017, the discovery of a quantum‐circuit refrigerator (QCR) based on photon‐assisted tunneling of quasiparticles through normal‐metal–insulator–superconductor junctions inspired a series of experimental studies demonstrating the following main properties: i) the direct‐current (dc) bias voltage of the junction can change the QCR‐induced damping rate of a superconducting microwave resonator by orders of magnitude and give rise to nontrivial Lamb shifts, ii) the damping rate can be controlled in nanosecond time scales, and ii) the dc bias can be replaced by a microwave excitation, the amplitude of which controls the induced damping rate. Theoretically, it is predicted that state‐of‐the‐art superconducting resonators and qubits can be reset with an infidelity lower than 10−4 in tens of nanoseconds using experimentally feasible parameters. A QCR‐equipped resonator has also been demonstrated as an incoherent photon source with an output temperature above 1 K yet operating at millikelvin. This source has been used to calibrate cryogenic amplification chains. In the future, the QCR may be experimentally used to quickly reset superconducting qubits, and hence assist in the great challenge of building a practical quantum computer.

Defect engineering in rare‐earth‐doped BaTiO3 ceramics: Route to high‐temperature stability of colossal permittivity

AbstractHigh‐performance colossal‐permittivity (CP) materials have huge potential applications in the miniaturization of electronic components and high‐energy storage applications. Here, we report CP behavior in rare‐earth Ln‐doped BaTiO3 (Ln = La, Ce, Pr, Nd, Sm, Gd, Dy, Ho, and Er) ceramics. CP (&gt;1 × 105) and low loss (&lt;5% at 1 kHz) were achieved. Additionally, all ceramic samples with excellent temperature stability over a wide temperature range (25–250°C). X‐ray photoelectron spectroscopy verified the existence of point defects (Ti3+ and ) in Ln‐doped BaTiO3 ceramic samples annealed in an N2 atmosphere. Electron paramagnetic resonance further demonstrated the existence of Ti3+. The coupling of point defects forms an electron‐pinned defect‐dipoles (EPDD) effect and induces strong hopping polarization. In addition, an internal barrier layer capacitance (IBLC) effect and a surface barrier layer capacitor (SBLC) effect are identified by impedance spectroscopy and DC bias voltage. The CP is attributed to the combined effect of EPDD, IBLC, and SBLC. Furthermore, the high‐temperature stability of CP is related to the strong coupling of defect‐dipole complexes.

High power high voltage bias-T for half wave resonators and radio frequency quadrupole couplers

High power high voltage bias-T units capable of delivering up to 100 kW CW RF power at 176MHz and up to 4kV DC were developed at the Soreq Nuclear Research Center for the Soreq Applied Research Accelerator Facility linac. Two separate bias-T units with different requirements were designed for the radio frequency quadrupole couplers and the half wave resonator couplers. The purpose of this bias-T is to prevent multipacting phenomena by application of a high voltage DC bias to inner conductors of RF couplers. Underlying design principles, indigenous development, and successful off-line and on-line tests results are presented.