Bias Resistance Research Articles

Characterize the switching performance of a superconducting nanowire cryotron for reading superconducting nanowire single photon detectors

Scalable superconducting nanowire single photon detector (SNSPDs) arrays require cryogenic digital circuits for multiplexing the output detection pulses. Among existing superconducting digital devices, superconducting nanowire cryotron (nTron) is a three-terminal device with an ultra-compact size, which is promising for large scale monolithic integration. In this report, in order to evaluate the potential and possibility of using nTrons for reading and digitizing SNSPD signals, we characterized the grey zone, speed, timing jitter and power dissipation of a proper designed nTron. With a DC bias on the gate, the nTron can be triggered by a few μA high and nanoseconds wide input signal, showing the nTron was capable of reading an SNSPD pulse at the same signal level. The timing jitter depended on the input signal level. For a 20 μA high and 5 ns wide input pulse, the timing jitter was 33.3 ps, while a typical SNSPD’s jitter was around 50 ps. With removing the serial inductors and operating it in an AC bias mode. The nTron was demonstrated to be operated at a clock frequency of 615.4 MHz, which was faster than the maximum counting rate of a typical SNSPD. In additional, with a 50 Ω bias resistor and biased at 17.6 μA, the nTron had a total power dissipation of 19.7 nW. Although RSFQ circuits are faster than nTrons, for reading SNSPD or other detector arrays that demands less operation speed, our results suggest a digital circuit made from nTrons could be another promising alternative.

Heat flux capacity measurement and improvement for the test of superconducting logic circuits in closed-cycle cryostats

With the current progress of superconducting integrated circuits, circuit complexities have reached more than 100,000 Josephson junctions. At the laboratory environment, circuit tests are generally being conducted at liquid Helium environments. With the commercialization of the superconducting integrated circuit-based systems and/or price increase of liquid Helium, utilization of the closed-cycle systems will be inevitable. Even though it is possible to implement closed-cycle systems with quite substantial excess cooling powers after all the wirings and peripheral circuits, one point not to be overlooked is the power density at the superconducting chips. We have observed that the circuits designed for operation at liquid Helium bath have much higher power densities that exceed the heat flux capacities of closed-cycle cryocoolers. We have measured that the heat flux capacities of closed-cycle systems are about 0.1 W/cm${}^{2}$ when the chip is in vacuum. Thus, if the power density in any of the bias resistors are higher than these values, it is difficult to keep the chip in superconducting state. With the positive feedback from a local hot spot under current bias, the chip rapidly heats up due to Joule heating. To increase the limit of power density, we propose an encapsulation method that increases the heat flux capacity of the system. With the encapsulation, the heat flux capacity of the system is increased to about 0.4 W/cm${}^{2}$.

Analysis of a Novel Circuit Arrangement to Suppress Crosstalk in 2-D Resistive Sensor Arrays

The multiplexed resistive arrays simplify the data collection with reduced connections and circuit complexity. However, the effect of crosstalk in these cases becomes inevitable. The crosstalk is caused due to the leakage currents in the multiplexer channels as well as the parasitic paths among the non-selected elements in the array. This paper analyses a circuit arrangement capable of suppressing crosstalk in 2D resistive sensor arrays. The proposed circuit employs transistors and diodes to modify simple voltage feedback commonly used for crosstalk reduction. The paper includes simulation results, approximate mathematical model for the “element under measurement” and other practical aspects of the circuit arrangement, such as the array size or the bias resistance. Finally, the circuit is compared (based on the simulation results) with two known circuit arrangement from the literature. The suggested circuit is clearly capable of eliminating crosstalk better than previously known methods. The analysis also reveals that the circuit is sensitive to temperature variations and must be modeled with temperature calibration factors for better model accuracy.

Planarized Process for Single-Flux-Quantum Circuits With Self-Shunted Nb/Nb$_{x}$Si$_{1-x}$/Nb Josephson Junctions

We describe our process for fabricating single-flux-quantum circuits for applications in communications metrology, advanced computing, and quantum information. The process flow includes four superconducting metal layers, one junction layer, one palladium-gold bias resistor layer, and a contact pad layer. Planarization using chemical–mechanical polishing is performed on the two lower insulating layers. This process is based on Josephson junctions with niobium superconducting electrodes and self-shunted, amorphous niobium-doped silicon barriers on the insulating side near the metal-insulator transition. The barrier material is deposited by cosputtering silicon and niobium, with the junction properties controlled by the relative sputtering powers and the duration of the barrier deposition step. The profiles of the deposition rates across 3-in (76.2 mm) wafers give a corresponding radial profile of the critical current density $\boldsymbol{J_\mathrm{c}}$ that increases by $\boldsymbol{\sim }\text{46}$ % from the center to the edge of the wafer. To characterize our process, we have measured and show data for niobium metal quality and its magnetic penetration depth, as well as for wire and via inductances, and for via critical currents. The uniformity of the junction critical current $\boldsymbol{I_\mathrm{c}}$ across a wafer was measured on test chips with 2.7- ${\mu }$ m-diameter junctions. Within a 5 mm $\boldsymbol{\times }$ 5 mm area, the standard deviation of $\boldsymbol{I_\mathrm{c}}$ was $\boldsymbol{\sim }\text{2}$ % for chips near the center of the wafer and up to $\boldsymbol{\sim }\text{7}$ % for chips near the edge. We also describe process improvements and innovations under development aimed at increasing circuit density and high-speed performance.

Processing of AC-coupled n-in-p pixel detectors on MCz silicon using atomic layer deposited aluminium oxide

We report on the fabrication of capacitively (AC) coupled n+-in-p pixel detectors on magnetic Czochralski silicon substrates. In our devices, we employ a layer of aluminium oxide (Al2O3) grown by atomic layer deposition (ALD) as dielectric and field insulator, instead of the commonly used silicon dioxide (SiO2). As shown in earlier research, Al2O3 thin films exhibit high negative oxide charge, and can thus serve as a substitute for p-stop/p-spray insulation implants between pixels. In addition, they provide far higher capacitance densities than SiO2 due to their high dielectric constant, permitting more efficient capacitive coupling of pixels. Furthermore, metallic titanium nitride (TiN) bias resistors are presented as an alternative to punch-through or poly-Si resistors.Devices obtained by the above mentioned process are characterized by capacitance–voltage and current–voltage measurements, and by 2 MeV proton microprobe. Results show the expected high negative charge of the Al2O3 dielectric, uniform charge collection efficiency over large areas of pixels, and acceptable leakage current densities.

Single-Flux-Quantum Based Event-Driven Encoder for Large-Pixel Superconducting Nanowire Single-Photon Detector Array

We propose a single-flux-quantum (SFQ) based event-driven encoder for a large-pixel superconducting nanowire single-photon detector (SSPD) array with a row-column readout architecture. The designed SFQ encoder can acquire time-tagged address data with a high time resolution. To apply 32 × 32 pixel SSPD system with a row-column architecture, the SFQ encoder is equipped with two 32-channel input ports among which one is for positive pulses, whereas the other is for negative pulses. The number of Josephson junctions (JJs) in the designed circuit is 2780. The power dissipation and required total bias current are 0.21 mW and 170 mA, respectively, owing to the reductions in critical currents of the JJs and bias resistors. The circuit generates time-tagged address data by applying electrical pulses at room temperature not only in liquid helium but also in a 0.1-W Gifford-McMahon (GM) cryocooler.

Simulation Analysis and Energy-Saving Techniques for ERSFQ Circuits

Energy-efficient rapid single flux quantum (ERSFQ) circuits have become a viable alternative for the implementation of superconducting circuits due to a large amount of static power consumption in RSFQ circuits. ERSFQ circuits are built upon the popular RSFQ logic circuits by replacing the power-dissipating resistor bias network with a bias network consisting of active devices. In this paper, a simulation study of ERSFQ biasing scheme is carried out by building simulation test benches for both synchronous and asynchronous ERSFQ circuits. A study is carried out to present the optimum value of biasing inductance, influence of the feeding Josephson transmission line (FJTL) and the effect of its size, the effect of the feeding clock frequency, and the effect of the circuit operating frequency. An innovative clock-choking mechanism using magnetic Josephson junctions is also proposed for the FJTL in the case of no logic circuit activity for a current-recycling circuit block, which would help in eliminating the dynamic power consumed due to the switching of bias junctions in a logic circuit. Simulation results for current recycling ERSFQ circuits are presented along with a strategy for implementing large superconducting circuits.

A Study on Corrosion Resistance of Electroplated Gold over Nanocrystalline Ni-W for Contact Applications

Highly conductive metal with least corrosion resistance is the most prominent requirement of contact industries. Although deposition of gold over Ni sulfamate is a standard interface for contact and connector applications, the performance of gold over Ni sulfamate has faced several problems such as wear durability, porosity of gold layers and biased corrosion resistance. The deposition of nanocrystalline Ni-W (nc Ni-W) between gold and Ni sulfamate improves the mechanical and surface properties of interfaces. In this work two different types of samples were processed and compared in terms of corrosion. The first sample (Au/NiSO4) was prepared using electroplating of gold, Ni sulfamate and Ni strike over base material of copper alloy with thicknesses of 0.9 µm, 1.9 µm and 0.5 µm respectively. In the second sample (Au/nc Ni-W/NiSO4), all layers of electroplating were same except the addition of 1.8 µm of nanocrystalline Ni-W before deposition of gold. Both samples were passed through the environments of (a) 85% relative humidity at 85oC for 24 hours, (b) 85% relative humidity at 85oC for 96 hours and (c) nitric acid vapors for 60 minutes. The effect on electrical, mechanical and materials properties of samples were investigated before and after passing through these environments using Force Deflection Resistance (FDR), Life-cycle test (LCT), Scanning electron microscopy (SEM), Energy dispersive spectroscopy (EDS) and X-Ray CT scanning. Although Au/NiSO4 is a well-known interface for connector applications [1], the deposition of nc Ni-W between gold and NiSO4 reduces the rate of corrosion because nc Ni-W has a weak tendency for grain boundary segregation and building broader range of accessible grain sizes [2]. There is formation of grain colonies in Ni-W deposit which controls crack propagation and corrosion [3]. The performance of two different interfaces of Au/NiSO4 and Au/nc Ni-W/ NiSO4 against corrosion was tested after electroplating on spring probes. Spring probe is one of the product of Smiths Interconnect which is used for different applications like defense, medical, rail, semiconductor, space etc. [4]. In FDR, the spring force, coefficient of friction and contact resistance was measured under a hysteresis of compression and extension for a travel distance of 0.14 inch. In the case of Au/NiSO4, the difference in compression and extension force at 0.14 inch was 6.1 oz. In the case of Au/nc Ni-W/ NiSO4, this difference was 4.6 oz which means that there is less friction with the deposition of nanocrystalline Ni-W underneath gold. Moreover, the difference in contact resistances was lower in second sample which means that the addition of nc Ni-W enhances the interface of gold and NiSO4. After treatment in humidity from 24 hrs to 96 hrs the difference in compression and extension force at 0.14 inch was increased by 7 oz in first sample but in second sample this difference was only increased by 3.6 oz which means that there is less increase in friction due to the presence of nc Ni-W. In LCT the resistance was measured at a test current of 0.025 amps under a stroke length of 0.09 inch for 10,000 life cycles with a cycle rate of 4 cycles/sec. The rate of increase in resistance with number of cycles was higher in the case of Au/NiSO4 than Au/nc Ni-W/ NiSO4. In first sample, the sticky coefficient was increased from 0 to 25 % after treatment in humidity for 24 hrs and it further increased to 37.5% after keeping in humidity for 96 hrs. The wear off rate and friction reduction of Au/nc Ni-W/ NiSO4 helped in maintaining the sticky coefficient to be constant even after kept in humidity for 96 hrs. The failure of the probes were tested from front and cross section SEM, point and line scan EDS and X-Ray CT scanning. SEM was used to observe mechanical marks and microstructure analysis which explained the reasoning of low wear rate in the case of Au/NiSO4. The failure of the probes was also confirmed from X-Ray CT Scanning in the probes during compression of spring. The complete set of experimental results will be added in the full version of the paper.

Variance reduction estimation for return models with jumps using gamma asymmetric kernels

Abstract This paper discusses Nadaraya-Watson estimators for the unknown coefficients in second-order diffusion model with jumps constructed with Gamma asymmetric kernels. Compared with existing nonparametric estimators constructed with Gaussian symmetric kernels, local constant smoothing using Gamma asymmetric kernels possesses some extra advantages such as boundary bias correction, variance reduction and resistance to sparse design points, which is validated through theoretical details and finite sample simulation study. Under the regular conditions, the weak consistency and the asymptotic normality of these estimators are presented. Finally, the statistical advantages of the nonparametric estimators are depicted through 5-minute high-frequency data from Shenzhen Stock Exchange in China.

High-performance RF Switch in 0.13 μm RF SOI process

A high-performance single-pole single-throw (SPST) RF switch for mobile phone RF front-end modules (FEMs) was designed and characterized in a 0.13 μm partially depleted silicon-on-insulator (PD SOI) process. In this paper, the traditional series-shunt configuration design was improved by introducing a suitably large DC bias resistor and leakage-preventing PMOS, together with the floating body technique. The performance of the RF switch is greatly improved. Furthermore, a new Ron × Coff testing method is also proposed. The size of this SPST RF switch is 0.2 mm2. This switch can be widely used for present 4G and forthcoming 5G mobile phone FEMs.

Metal–Insulator–Metal Diodes: A Potential High Frequency Rectifier for Rectenna Application

Metal–insulator–metal (MIM) diodes are among the most promising candidates for applications in the high frequency regime. Owing to the tunneling dominant current conduction mechanism, they facilitate femtosecond fast switching time, which has drawn great research attention for many potential high-speed applications and especially as a rectifier in rectenna based energy harvesting. Since its advent in the early 1960s, a lot of development has occurred in various aspects of design, fabrication and characterization of MIM diodes for rectenna applications. In this work, a detailed study on MIM diodes is conducted emphasizing the advancements in design and fabrication of MIM diodes and future challenges from the point of view of their application in rectennas. In addition, the fabrication and characterization of a graphene (Gr) based Al/AlOx/Gr MIM diode are also presented herein, exhibiting highly asymmetric current–voltage characteristics with large current density and a good degree of nonlinearity. An asymmetricity exceeding 2500 and the corresponding current density up to ∼ 1 A/cm2 were obtained at a voltage bias of 1 V. The peak nonlinearity was ∼ 3.8, whereas the zero bias resistance was as low as ∼ 600 Ω. These performance metrics are highly desirable for rectification operation and hence the as-fabricated Al/AlOx/Gr MIM diode holds great promise for its potential use as a rectifying element in rectennas.

A Fully Printed Ultra-Thin Charge Amplifier for On-Skin Biosignal Measurements

In this contribution, we propose a fully printed charge amplifier for on-skin biosignal measurements. The amplifier is fabricated on an ultra-thin parylene substrate and consists of organic transistors, integrated bias and feedback resistors, and a feedback capacitor. The fabrication process utilizes inkjet-printed Ag ink for source, drain, gate, and capacitor electrode metallization as well as for the interconnects between the amplifier elements. Dispensed polystyrene, 2,7-dihexyl-dithieno[2,3-d;2’,3’-d’]benzo[1,2-b;4,5-b’]dithiophene (PS:DTBDT-C6), is used as the transistor channel material, dispensed poly(3-hexylthiophene) (P3HT) as the high-resistivity material for the printed resistors, and parylene as the capacitor dielectric. A pass band optimized for pulse-wave measurement (60 mHz to 36 Hz) is achieved with a maximum charge amplification of 1.6 V/nC. To demonstrate the potential of the proposed printed amplifier, a radial arterial pulsewave signal recorded with a printed piezoelectric poly(vinylidenefluoride-co-trifluoroethylene) (PVDF-TrFE) sensor was fed to it and the output was analyzed to quantify the similarity of the pulse-wave features calculated from the original signal and the amplifier output. The amplified signal contains all the essential features of a pulse wave, such as both systolic waves, the dicrotic notch, and diastolic wave, which enable the accurate derivation of the clinically relevant indices utilized in the evaluation of vascular health.

Modifiability of the history dependence of force through chronic eccentric and concentric biased resistance training.

The increase and decrease in steady-state isometric force following active muscle lengthening and shortening are referred to as residual force enhancement (RFE) and force depression (FD), respectively. The RFE and FD states are associated with decreased (activation reduction; AR) and increased (activation increase; AI) neuromuscular activity, respectively. Although the mechanisms have been discussed over the last 60 years, no studies have systematically investigated the modifiability of RFE and FD with training. The purpose of the present study was to determine whether RFE and FD could be modulated through eccentric and concentric biased resistance training. Fifteen healthy young adult men (age: 24 ± 2 yr, weight: 77 ± 8 kg, height: 178 ± 5 cm) underwent 4 wk of isokinetic dorsiflexion training, in which one leg was trained eccentrically (-25°/s) and the other concentrically (+25°/s) over a 50° ankle excursion. Maximal and submaximal (40% maximum voluntary contraction) steady-state isometric torque and EMG values following active lengthening and shortening were compared to purely isometric values at the same joint angles and torque levels. Residual torque enhancement (rTE) decreased by ~36% after eccentric training ( P < 0.05) and increased by ~89% after concentric training ( P < 0.05), whereas residual torque depression (rTD), AR, AI, and optimal angles for torque production were not significantly altered by resistance training ( P ≥ 0.05). It appears that rTE, but not rTD, for the human ankle dorsiflexors is differentially modifiable through contraction type-dependent resistance training. NEW & NOTEWORTHY The history dependence of force production is a property of muscle unexplained by current cross bridge and sliding filament theories. Whether a muscle is actively lengthened (residual force enhancement; RFE) or shortened (force depression) to a given length, the isometric force should be equal to a purely isometric contraction-but it is not! In this study we show that eccentric training decreased RFE, whereas concentric training increased RFE and converted all nonresponders (i.e., not exhibiting RFE) into responders.

Effect of C and O dopant atoms on the electronic properties of black phosphorus nanotubes

The electronic structures of the C and O-doped black phosphorene are investigated theoretically by using first-principles density functional theory (DFT). The results show that they are all metallic. And the black phosphorus nanotubes (BPNTs) rolled from the pure, C-doped and O-doped black phosphorene with different radii and chiralities are also studied. C and O atoms substitute for P atoms enable the armchair BPNT to have a metallic feature with several bands cross the Fermi level. Surprisingly, the number of the bands cross the Fermi level just equals the number of dopant atoms. But for the zigzag BPNT, when the O atoms decrease to 4, no band crosses the Fermi level and at which a minimal energy gap appears. Furthermore, the transport properties of two-probe devices fabricated by the pure A24 and Z34 BPNTs together with the C-doped A24 BPNTs are investigated theoretically. These calculated results show that the electronic transport properties of the A24 BPNT can be significant changed by C dopant atom and vary with the number of the doped atoms. The current-voltage curves of these BPNTs with C atoms substitute for P atoms present sinusoidal characteristics under low bias and negative differential resistance (NDR) properties. This study provides an effective route for tuning the electronic structures of black phosphorene and BPNTs and thus the transport properties of their electronic devices. And the unique properties of these black phosphorene and BPNTs also offer an inspiration for their subsequent experiments.

Electrode Equivalence for Use in Bioimpedance Spectroscopy Assessment of Lymphedema.

Bioimpedance spectroscopy (BIS) is commonly used in the assessment and monitoring of lymphedema. This study investigated electrodes as a source of variability that could impact the accuracy of BIS in the clinic and determined if Ag/AgCl electrocardiograph (ECG) electrodes could be used as an alternative to instrument-specific electrodes. Two types of Ag/AgCl electrodes were studied: instrument-specific bioimpedance electrodes (bioimpedance) and single tab ECG electrodes (cardiac). Six areas of investigation were addressed: intrinsic electrode resistance; electrode age; drive electrode position; electrode width/surface area; concordance between cardiac and bioimpedance electrodes; and mixing electrode types and batches. Participants included women (n = 26) and men (n = 8), both with (n = 4) and without lymphedema (n = 30). Resistance (R0) of the limbs was measured and used to calculate interlimb BIS ratios. Intrinsic electrode resistance varied between batches (p ≤ 0.001), with cardiac electrodes recording higher resistance. Electrode age had no impact on limb resistance (p = 0.85). Drive electrode position biased limb resistance (0.1%-2.3%) and electrode size/surface area had a small (≤1%), but significant effect on limb resistance (p ≤ 0.001). However, calculation of interlimb BIS ratios negated the impact of these as well as any effect of mixing electrode batches and types (p = 0.15-0.96). Electrode type had no impact on arm and leg resistance, or interlimb BIS ratios (p = 0.173-0.289). Calculation of interlimb BIS ratios improves accuracy of clinical BIS. Ag/AgCl cardiac electrodes can be used as an alternative to device-specific electrodes to measure limb resistance.

Strength and Step Activity After Eccentric Resistance Training in Those With Incomplete Spinal Cord Injuries.

Background: Individuals with spinal cord injuries (SCIs) often experience general weakness in the lower extremities that undermines daily step activity. Objective: To investigate the efficacy of eccentrically biased resistance training on lower extremity strength and physical activity of individuals with spinal injuries. Methods: Individuals with long-standing incomplete SCIs (N = 11) capable of completing a 10-meter walk assessment were included. All participants who completed the familiarization period finished the training. Individuals trained two times per week for 12 weeks on a lower body eccentric resistance training machine. It was hypothesized that the outcome variables (eccentric strength, isometric strength, and daily step physical activity) would improve as a result of the training intervention. Results: Eccentric strength [F(1.27, 12.71) = 8.42, MSE = 1738.35, H-F p = .009] and isometric strength [F(1.97, 19.77) = 7.10, MSE = 11.29, H-F p = .005] improved as a result of the training while daily step activity remained unchanged [F(2.00, 18.00) = 2.73, MSE = 216,836.78, H-F p = .092]. Conclusions: Eccentric resistance training improves eccentric and isometric strength. These physiological adaptations may translate to improved gait mechanics, but further study is required to identify this potential crossover effect.

Material influence in newly proposed ferroelectric energy harvesters

Recently, a novel method for mechanical energy harvesting has been proposed, which is based on stress-induced polarization switching in ferroelectric materials. Compared with the traditional piezoelectric energy harvesters, a huge improvement in the output energy has already been theoretically demonstrated. In this article, the influence of different materials on the energy-harvesting performance associated with this new strategy is further studied. The state-of-the-art phase-field model is adopted to investigate the nonlinear hysteretic energy-harvesting process in two nanoscale ferroelectric energy harvesters, which are respectively based on two typical ferroelectric materials—single-crystal BaTiO3 and PbTiO3. In both cases, the effects of the bias voltage and bias resistance are carefully investigated and the optimum values are obtained. Later, the energy-harvesting process and energy flow details in both harvesters working at the optimum conditions are presented and carefully compared in the context of real applications. Furthermore, the energy-harvesting performance of a BaTiO3-based nanoscale piezoelectric energy harvester with equivalent material size is additionally simulated with the finite element method and compared with the corresponding results of the ferroelectric energy harvesters, where obvious advantages associated with the new strategy are demonstrated.

Noise Model of Capacitive and Textile Capacitive Noncontact Electrodes for Bioelectric Applications.

In this paper, a comprehensive model for the electronic noise properties and frequency-dependent responses of printed circuit board (PCB)-based as well as textile noncontact capacitive electrodes is presented. For bioelectric diagnostics, noncontact capacitive electrodes provide an interesting alternative to classical galvanically coupled electrodes, since such a low-cost diagnostic system can be applied without preparation time and in mobile wireless environments. For even higher user comfort, textile capacitive electrodes are preferable. This paper provides a thorough study of the influence of the electrical components of capacitive electrodes and textile capacitive electrodes, as well as their surface area and circumferences on the resulting noise properties of the electrode by independently measuring the corresponding noise spectra. Consequently, the equivalent noise model is developed. The most important noise source is the high input bias resistance, which, in combination with the involved capacitance, results in an apparent $1/f^2$-power noise spectrum. By comparing the noise measurements with the noise model of the electrode, we conclude that the surface of the electrode contributes to an additional $1/f$ -power noise in the noise spectrum. We also found that the highest possible coupling capacitance is most favorable for low-noise behavior. Therefore, electrodes with electrically conducting fabric surfaces are investigated. Due to this, it is possible to enlarge the surface of the electrode and maintain a small distance between the body and the surface of the electrode. We show that with the use of textile capacitive electrodes, it is possible to reduce the noise characteristics considerably. Our findings in this paper provide a necessary source for further optimization of capacitive electrodes for bioelectric measurement applications.

Temperature effects in contacts between a metal and a semiconductor nanowire near the degenerate doping

We have investigated the nonlinear conductance in diffusion-doped Si:GaAs nanowires contacted by patterned metal films in a wide range of temperatures T. The wire resistance RW and the zero bias resistance RC, dominated by the contacts, exhibit very different responses to temperature changes. While RW shows almost no dependence on T, RC varies by several orders of magnitude as the devices are cooled from room temperature to T = 5 K. We develop a model that employs a sharp donor level very low in the GaAs conduction band and show that our observations are consistent with the model predictions. We then demonstrate that such measurements can be used to estimate carrier properties in nanostructured semiconductors and obtain an estimate for ND, the doping density in our samples. We also discuss the effects of surface states and dielectric confinement on carrier density in semiconductor nanowires.

The financial crisis, acquisition premiums and the moderating effect of CEO power

The recent financial crisis brought upon a period of increased information uncertainty for firms and market agents and in the context of mergers and acquisitions, increased information asymmetry between bidders and targets. This led to an overall increase in average acquisition premiums. However, the final realized premium can be moderated through CEO characteristics in dealing with such issues as uncertainty and asymmetry. Bidder CEOs can reduce these premiums through their expertise, networks and control: powerful CEOs will tend to pay smaller premiums than weaker CEOs because they are better equipped to deal with the increased information uncertainty and asymmetry, either through risk-averse behavior or better assessment of target quality. Our results based on a six-year sample of S&P500 firms engaged in M&A activity, and centered on the onset of the recent financial crisis, largely support our predictions. While earlier research suggested that CEO power could lead to higher premiums, our study points to the opposite conclusion: during times of financial crisis, CEO power effectively constrains premiums. This extends our understanding of why bidder CEOs overpay beyond the role of bidder anchoring bias, hubris, and target resistance by considering the role of macro-environmental conditions and power.