Bias Sweep Research Articles

Amperometric Sensing of NOx with Cyclic Voltammetry.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Amperometric Sensing of NO x with Cyclic Voltammetry

Cyclic voltammetry (CV) techniques were used to measure the concentration of nitric oxide (NO) and nitrogen dioxide in air using an amperometric gas sensor. A porous gold membrane working electrode in sulfuric acid electrolyte was used with a counter electrode of bare Pt wire and quasi-reference electrode of electrodeposited Pt on a Pt wire. The anodic peak current was proportional to the concentration of NO at various bias sweep rates. The limit of detection was about 4.8 ppm for NO in air or 31 ppm for in air for the CV methods. A parameter related to the diffusion coefficient and membrane permeability can be determined from CV measurements and was at the high range, cm2 s−1, of reported values of diffusion constants. Simple models of combined effects of gas and liquid mass-transport and electrokinetic current limiting mechanisms complicate explanation of observed currents with porous gas electrodes. © 2003 The Electrochemical Society. All rights reserved.

Capacitance study of defects induced in heavily damaged region formed in hydrogen implanted Si

Capacitance–voltage ( C– V) characteristics at high frequency (1 MHz) as a function of temperature ( T) were measured to study defects created by hydrogen implantation followed by high temperature annealing. The initial material used in this study was p-type Czochralski Si (Cz-Si) implanted with 130 keV H 2 + at a high dose of 4×10 16 cm −2. The post-implantation annealing was carried out in argon ambient at 450 °C per 10 h, i.e. at the temperature of thermal donor creating in Si. The presence of the deep-level defects manifest themselves through several unusual features in C– V( T) characteristics. A flat region, sudden jump of capacitance and hysteresis effect in C– V data were observed during sweeping bias voltage. Our experiments by filling and emptying deep-level defects with carriers before the voltage sweep also revealed unusual hysteresis effects in C– V curves. The results obtained can be readily understood if we assume that deep-level defects in the depletion region are present in very high concentration, the defect spatial distribution is inhomogeneous as usual in implanted samples, and the defects control the electrical characteristics. We also conclude that a relatively thick p-type layer is formed during implantation at the surface of the n-type Si that is formed as a result of annealing.

Design of adiabatic logic for a quantum CNOT gate

We examine the realization of a quantum CNOT gate by adiabatic operations. The principles of such systems and their analysis are briefly discussed and a model consisting of two weakly coupled double-potential well qubits is studied numerically. Regions of the parameter space with suitable well-defined sets of wavefunctions are found, in which then an adiabatic sweep of an external bias produces the switching behavior of CNOT. Results are presented on the adiabatic condition and the identification with the parameters of a flux-coupled two-SQUID system is given. For typical parameters adiabatic times in the nanosecond regime are obtained.

Microstructure and electrical properties of Ln 2Ti 2O 7 (Ln=La, Nd)

The effects of microstructure on the electrical properties of Ln 2Ti 2O 7 (Ln=La, Nd) films were investigated by depositing on Pt/Ti/Si and Y 2O 3/Si substrates using the sol–gel and spin-coating method. The films were crystallized at 800 °C in O 2 for 30 min in a tube furnace by direct insertion. According to the A-site cation, the different tendencies in ferroelectric properties between films and single crystals were observed. The ferroelectric parameters are: Pr=4.4 μC/cm 2, Ec=131 kV/cm for Nd 2Ti 2O 7 film; and Pr=1.8 μC/cm 2, Ec=75.28 kV/cm for La 2Ti 2O 7 film, respectively. C-V characteristics of Ln 2Ti 2O 7/Y 2O 3/Si structure were measured to investigate the ferroelectric memory effects at 1 MHz with a bias sweep rate of 0.18 V/s. The values of the memory window were 2.7 and 2.3 V for Nd 2Ti 2O 7 and La 2Ti 2O 7 films with 7 V of applied voltage, respectively.

Charge retention characteristics in a metal–insulator–semiconductor capacitor containing Ge nanocrystals

We investigate the charge retention characteristics in a metal–insulator–semiconductor capacitor containing Ge nanocrystals (∼3 nm in diameter) using capacitance–voltage measurements with various gate bias sweep rates. The metal–insulator–semiconductor capacitor is fabricated by pulsed laser deposition and rapid thermal oxidation. The capacitance–voltage curve shows the strong hysteresis indicating charging/discharging of carriers during the gate bias sweeping. The stored charge densities, evaluated from the flat band voltage shifts, show the non-dispersive carrier relaxation, which is a characteristic property for nanocrystals arranged in a layer. In addition, we observe an interesting capacitance ledge during forward bias sweeping. From the trap distribution determination by the Terman method, the capacitance ledge is due to an energetically localized trap at 0.33 eV + Ev. The energetic position of the trap is in agreement with the ground-state hole energy level in the Ge nanocrystals. This suggests the possibility that the trap level is related to the fact that the stored charges initially tunnel out at the ground-state hole energy level.

Ferroelectric-gate field effect transistors using Nd 2Ti 2O 7/Y 2O 3/Si structures

Ferroelectric field effect devices offer the possibility of non-volatile active memory elements. In the metal–ferroelectric–semiconductor field effect transistor, it is important for a ferroelectric material to have a low dielectric constant to enable the application of sufficient electric field to a ferroelectric film. In this work, we report the fabrication and characterization of a ferroelectric-gate field effect transistor using Nd 2Ti 2O 7(NTO)/Y 2O 3/Si structures. The crystalline property of the film as a function of annealing temperature was characterized by X-ray diffraction. C– V characteristics were measured to investigate the ferroelectric memory effects at 1 MHz with a bias sweep rate of 0.18 V/s. As a result, the memory windows were in the range of 0.98–3.31 V when the applied voltage varied from 3 to 9 V. Specifically, we confirmed the ferroelectricity of crystallized NTO films.

The influence of deep traps on transient current–voltage characteristics of organic light-emitting diodes

Abstract Transient current–voltage (I–V) characteristics of organic light-emitting diodes made from both conjugated polymers and low molecular-weight materials show hysteresis effects in the reverse bias regime depending on the direction and speed of the bias sweep. This behaviour is quantitatively investigated here for the example of devices based on N,N′-diphenyl-N,N′-bis(1-naphtyl)-1,1′-biphenyl-4,4′-diamine with Ca and indium–tin oxide as electrodes. To clarify the origin of this peculiarity numerical simulations have been carried out supposing the existence of deep acceptor-like trap states. Typical trends are shown by systematically varying parameters such as measuring conditions, trap characteristics, basic doping level, mobility and injection conditions. Based on the simulated potential and concentration profiles it is shown that the hysteresis of the I–V characteristics is caused by recharging of deep traps for holes. It occurs only if the reverse steady-state current is lower than the trap recharging current and if both currents have different bias dependencies. The origin for the large time needed for the traps to relax into the equilibrium state is clarified. In accordance with the high barrier for the holes at the cathode the calculated reverse current is much smaller than the measured one. Using a new analytical expression for the Schottky diode I–V characteristics for a low-doped thin film device, it is shown qualitatively that in real devices a leakage current should dominate for reverse bias.

Floating Potentials in the Vicinity of Biased Flush-Mounted Probes

An array of surface-mounted probe triplets in the Alcator C-Mod divertor region [1, 2] demonstrates the influence of a biased probe tip on local plasma potential. For the experiments described here, each poloidally aligned triplet contains one probe which measures the local I-V characteristic with a bias sweep and two passive probes which sample the floating potential. The floating potential at the two passive tips varies as a function of the bias at the swept tip. This dependence is shown to vary according to the local plasma parameters. Variations in both the amplitude and width of the floating potential disturbance on electron temperature and density are resolved and compared with different mechanisms of cross-field current transport.

Fabrication and characterization of Nd2Ti2O7 for ferroelectric field effect transistor

Ferroelectric field effect devices offer the possibility of nonvolatile active memory elements. In the metal-ferroelectric-insulator-semiconductor field effect transistor, it is important for a ferroelectric material to have a low dielectric constant to enable the application of sufficient electric field to a ferroelectric film. In this work, Nd2Ti2O? (NTO) films on CeO2/Si (100) were attained by sol-gel process. The crystalline property of the film according to annealing temperature was characterized by X-ray diffraction. C-V characteristics were measured to investigate the ferroelectric memory effects by HP4284A LCR meter at 1MHz with a bias sweep rate of 0.18V/s. As a result, the crystalline of NTO thin film on CeO2/Si(100) was developed from 700°C and perfect crystalline phase was obtained at 1000 °C. The memory windows were in the range of 0.2 to 2.5 V when the applied voltage varied from 3 to 9 V. And, the fact that C-V loops were caused by the ferroelectricity of NTO was confirmed by P-E characterization of Au/NTO/Pt structure.

Nanoconstriction microscopy of the giant magnetoresistance in cobalt/copper spin valves

We use nanometer-sized point contacts to a Co/Cu spin valve to study the giant magnetoresistance (GMR) of only a few Co domains. The measured data show strong device-to-device differences of the GMR curve, which we attribute to the absence of averaging over many domains. The GMR ratio decreases with increasing bias current. For one particular device, this is accompanied by the development of two distinct GMR plateaus, the plateau level depending on bias polarity and sweep direction of the magnetic field. We attribute the observed behavior to current-induced changes of the magnetization, involving spin transfer due to incoherent emission of magnons and self-field effects.

Energy level quantization in underdamped niobium Josephson junctions [Nb/AlO/sub x//Nb

Macroscopic quantum effects in Josephson systems have recently attracted interest in the scientific community both for the physics involved and in view of applications. Here we present experiments on the presence of energy level quantization at temperatures above the classical-quantum regime crossover temperature in different Josephson junctions. This has been possible by extending the measurements of the escape rate out of the zero-voltage state at higher sweeping bias rate (dI/dt up to 100 A/sec) in order to induce nonstationary conditions in the energy potential describing the junction dynamics. Our measurements allow us to obtain as fitting parameter the "effective" resistance to be used in the RSJ model which is compatible with the subgap resistance at very low voltage. The very low dissipation level obtained at low temperatures renders our results quite interesting in view of applications in the quantum limit.

Macroscopic Quantum Tunneling in Josephson Junctions and Squids

We report experiments on energy level quantization in Nb/AlOx/Nb Josephson junctions while measurements on SQUIDs are in progress. Data on the presence of energy level quantization are shown at temperatures above the classical-quantum regime crossover temperature. This has been possible by extending the measurements of the rate of the escape from the zero-voltage state at higher sweep bias rate in order to induce non-stationary conditions in the energy potential describing the junction dynamics. Data are compatible with extremely low "effective" dissipation in our system. Theoretical predictions on quantum effects related to the presence of non-stationary conditions in the potential associated to a rf SQUID are reported in view of new possible experiments.

Improvement of the Electrical Properties of YMnO3 Thin Films in A Metal/Ferroelectric/Si Structure

AbstractHighly (0001)-oriented films of YMnO3 were grown on n-type Si(100) substrates by a chemical solution deposition with a modified precursor solution. Spin-coated films were crystallized by rapid thermal annealing at 650 °C, and showed improved structural and electrical properties. Capacitance-voltage (C-V) measurements at 1 MHz showed a counterclockwise hysteresis, with a memory window of 1.9 V at 9 V, due to ferroelectric polarization, and a dielectric constant of 25. The effects of mobile ionic charge and effective interface charge in the C-V measurements were found to be small by investigating the bias sweep rate dependence and flat-band voltage shift, respectively. The interface trap density near the Si midgap was obtained to be about 1.3 × 1011 cm−2eV−1 through conductance measurements. Current-voltage characteristics showed a leakage current density of 16 nA/cm2 at 3 V. An asymmetric polarization-voltage (P-V) hysteresis curve became symmetric one with a remanent polarization value of 0.1 μ C/cm2 under He-Ne laser illumination. The depolarization field in the ferroelectric film and charge compensation by the light-generated minority carriers may be responsible for the observed P-V characteristics. The low temperature fabrication of this YMnO3 film showed good structural and electrical properties for application to nonvolatile ferroelectric memory devices.

Bias sweep rate effects on quasi-static capacitance of MOS capacitors

MOS capacitance measurements are very fundamental characterization methods for MOS and FET structures. This paper discusses the effects of a finite bias sweep rate on quasi-static and high-frequency (HF) capacitance-voltage (C-V) measurements. As typically measured, a finite sweep rate causes the transition region from inversion to depletion of the quasistatic C-V curve to be shifted by several tenths of a volt along the bias voltage axis. The physical origin of this shift as well as a model to account for the effect is discussed. In order to understand quasi-static MOS C-V measurements and to extract fundamental parameters such as substrate doping density and polysilicon depletion effects from C-V measurements, these bias sweep rate effects must be understood and taken into account.

Resonant macroscopic quantum tunneling in SQUID systems.

A detailed theoretical analysis of the resonant macroscopic quantum tunneling in superconducting quantum interference device systems is presented. Our approach allows us to include the effect of both temperature and sweeping rate of the external flux, and to study the phenomenon both in quasistationary and nonstationary conditions, which can be induced by a fast sweep of the external bias. Moreover we compare our theory with the experimental data of Rouse, Han, and Lukens [Phys. Rev. Lett. {bold 75}, 1614 (1995)] referring to the quasistationary case, while other observable effects are predicted in the nonstationary case. {copyright} {ital 1996 The American Physical Society.}

Ferroelectric memory FET with Ir/IrO2 electrodes

Abstract We proposed a MFMIS structure having a floating gate as a bottom electrode between a ferroelectric thin film and the gate SiO2. Conventional gate SiO2 can be used and ferroelectric thin films can be grown on bottom electrodes which have a good matching with the ferroelectric materials due to adopt the MFMIS structure. Ir and IrO2 on poly-Si were used as floating gate. When a IrO2 layer was formed between PZT and poly-Si, a high-quality PZT thin film was obtained and the PZT films show no fatigue up to 1012 cycles of switching pulses. From the ID-VG characteristics measurement for 1·2 μm P-ch MFMIS FET, the shift in Vth or the memory window for a bias sweep of ±15V was about 3·3V. The difference of ID-VD curves which corresponded to ID-VG characteristics were found between before and after a programming pulse was applied.

Active silicon x-ray filter for measuring electron temperature

Silicon diodes are commonly used for x-ray measurements in the soft-x-ray region between a few hundred eV and 20 keV. Recent work by Cho et al. [J. Appl. Phys. 72, 8 (1992)] has shown that the charge collecting region in an underbiased silicon detector is the depletion depth plus some contribution from a region near the depleted region due to charge diffusion. The depletion depth can be fully characterized as a function of the applied bias voltage and is roughly proportional to the square root of the bias voltage. We propose a technique to exploit this effect to use the silicon within the detector as an actively controlled x-ray filter. With reasonable silicon manufacturing methods, a silicon diode detector can be constructed in which the sensitivity of the collected charge to the impinging photon energy spectrum can be changed dynamically in the visible to above the 20-keV range. This type of detector could be used to measure the electron temperature in, for example, a tokamak plasma by sweeping the applied bias voltage during a plasma discharge. The detector samples different parts of the energy spectrum during the bias sweep, and the data collected contains enough information to determine the electron temperature. Benefits and limitations of this technique are discussed along with comparisons to similar methods for measuring electron temperature and other applications of an active silicon x-ray filter.

Observation of current spikes in thin oxide MOS capacitor

Dielectric thin films O.ljiAcnr2 current spikes have been reported recently in MOS capacitors. The authors attribute this observation to current transients, arising from the staircase sweep of the gate bias, and explain it in terras of the charging and discharging of the silicon space charge layer, and recombination of electrons and holes near the interface.

Modeling of metal-ferroelectric-semiconductor field effect transistor subthreshold current

Abstract A subthreshold current model for metal-ferroelectric-semiconductor field effect transistor (MFSFET) is derived from a new analytical ferroelectric hysteresis model,1.2 the semiconductor surface model,3 the classical diffusion current model,4 as well as the Maxwell equations. The model predicates the shift of MFSFET subthreshold current between forward sweep and backward sweep of gate bias. The simulation results show the effects of coercive voltage, the remanent polarization, the saturation polarization, and the free interface charge between ferroelectric and semiconductor. The external field effect on the saturation polarization (better known as ferroelectric space charge effect) is also modeled. The conventional method to evaluate the interface trapped charge for metal-oxide-semiconductor system is also proposed for MFS system based on the ideal subthreshold current model and its experimental data.