Showing papers on "Biasing published in 2015"

Understanding the rate-dependent J–V hysteresis, slow time component, and aging in CH3NH3PbI3 perovskite solar cells: the role of a compensated electric field

Abstract: In this work we show that the rate-dependent hysteresis seen in current–voltage scans of CH3NH3PbI3 perovskite solar cells is related to a slow field-induced process that tends to cancel the electric field in the device at each applied bias voltage. It is attributed to the build-up of space charge close to the contacts, independent of illumination and most likely due to ionic displacement, which is enhanced when the device undergoes aging. This process can also lead to a reduction of the open-circuit voltage or the steady-state photocurrent and does not directly correlate with the development of the hysteresis if it is measured at a fixed voltage sweep rate.

An Atomically Layered InSe Avalanche Photodetector

Abstract: Atomically thin photodetectors based on 2D materials have attracted great interest due to their potential as highly energy-efficient integrated devices. However, photoinduced carrier generation in these media is relatively poor due to low optical absorption, limiting device performance. Current methods for overcoming this problem, such as reducing contact resistances or back gating, tend to increase dark current and suffer slow response times. Here, we realize the avalanche effect in a 2D material-based photodetector and show that avalanche multiplication can greatly enhance the device response of an ultrathin InSe-based photodetector. This is achieved by exploiting the large Schottky barrier formed between InSe and Al electrodes, enabling the application of a large bias voltage. Plasmonic enhancement of the photosensitivity, achieved by patterning arrays of Al nanodisks onto the InSe layer, further improves device efficiency. With an external quantum efficiency approaching 866%, a dark current in the pic...

Flexible MoS2 Field-Effect Transistors for Gate-Tunable Piezoresistive Strain Sensors.

Abstract: Atomically thin molybdenum disulfide (MoS2) is a promising two-dimensional semiconductor for high-performance flexible electronics, sensors, transducers, and energy conversion. Here, piezoresistive strain sensing with flexible MoS2 field-effect transistors (FETs) made from highly uniform large-area films is demonstrated. The origin of the piezoresistivity in MoS2 is the strain-induced band gap change, which is confirmed by optical reflection spectroscopy. In addition, the sensitivity to strain can be tuned by more than 1 order of magnitude by adjusting the Fermi level via gate biasing.

Electrical injection and detection of spin-polarized currents in topological insulator Bi2Te2Se

Abstract: Topological insulators (TIs) are an unusual phase of quantum matter with nontrivial spin-momentum locked topological surface states (TSS). The electrical detection of spin-momentum locking of the TSS in 3D TIs has been lacking till very recently. Many of the results are measured on samples with significant bulk conduction, such as metallic Bi2Se3, where it can be challenging to separate the surface and bulk contribution to the measured spin signal. Here, we report spin potentiometric measurements in thin flakes exfoliated from bulk insulating 3D TI Bi2Te2Se (BTS221) crystals, using two outside nonmagnetic (Au) contacts for driving a DC spin helical current and a middle ferromagnetic (FM)-Al2O3 tunneling contact for detecting spin polarization. The voltage measured by the FM electrode exhibits a hysteretic step-like change when sweeping an in-plane magnetic field between opposite directions along the easy axis of the FM contact to switch its magnetization. Importantly, the direction of this step-like voltage change can be reversed by reversing the direction of the DC current, and the amplitude of the change as measured by the difference in the detector voltage between opposite FM magnetization increases linearly with increasing bias current, consistent with the current-induced spin polarization of spin-momentum-locked TSS. Our work directly demonstrates the electrical injection and detection of spin polarization in TI and may enable utilization of spin-helical TSS for future applications in nanoelectronics and spintronics.

A frequency and bandwidth tunable metamaterial absorber in x-band

Abstract: Smart control is an attracting and important function for modern electromagnetic wave absorber. This paper presents the design, fabrication, and measurement of a frequency and bandwidth tunable metamaterial absorber (MA) in X-band. The unit cell of the MA consists of a microstrip resonator loaded with the varactors. Simulation and measurement results show that by tuning the bias voltage on the varactors, the peak absorption frequency can be tuned by 0.44 GHz with the peak absorption greater than 95%. Field and circuit model analysis is conducted to reveal the working mode and predict the absorbing frequency. After that, by specially designing the bias circuit so as to adjust the bias voltage on neighboring unit cells separately, dual resonance and absorption peaks occur, and the overall absorption bandwidth can thus be tuned conveniently by controlling the difference of the two resonance frequencies. The center absorbing frequency can also be tuned. Simulation and experiment results show that the 75% absorption (−6 dB reflection) bandwidth can be tuned from 0.40 GHz to 0.74 GHz, which is a two-fold tuning range. This work is believed to improve the state-of-the-art smart metamaterial absorber.

A 33 MHz 70 dB-SNR Super-Source-Follower-Based Low-Pass Analog Filter

Abstract: In this paper, a 4th-order low-pass continuous-time analog filter is presented, that is implemented with the cascade of two efficient and compact biquadratic cells, realized using the Super-Source-Follower topology. The biquadratic cell uses only two capacitors and four transistors: two transistors for the signal processing and two transistors as current sources for biasing purpose. The 4th-order filter prototype has been integrated in 0.18 µm CMOS technology. For a 33 MHz cut-off frequency, the filter performs 18 dBm-IIP3 for two tones at 2 MHz and 3 MHz, with total current of 770 µA from a single 1.8 V supply voltage.

Effects of Applied Bias and High Field Stress on the Radiation Response of GaN/AlGaN HEMTs

Abstract: The sensitivity of GaN/AlGaN HEMTs to 1.8 MeV proton irradiation is greatly enhanced by biasing the devices during irradiation and/or applying high field stress before irradiation. The resulting defect energy distributions are evaluated after irradiation and/or high field stress via low-frequency noise measurements. Significant increases are observed in acceptor densities for defects with ~ 0.2 and ~ 0.7 eV energy levels. These defects appear to dominate the degradation in threshold voltage and transconductance for these devices. Density functional theory (DFT) calculations show that N vacancy-related defects in GaN and hydrogenated O N complexes in AlGaN are strong candidates for the defects with ~ 0.2 eV energy levels in these devices. We also present evidence that the previously unidentified ~ 0.7 eV defect in GaN is a N anti-site defect (N Ga).

Bias Control for Stacked Transistor Configuration

Abstract: Various methods and circuital arrangements for biasing one or more gates of stacked transistors of an amplifier are presented, where the amplifier can be an envelope tracking amplifier. Circuital arrangements to generate reference gate-to-source voltages for biasing of the gates of the transistors of the stack are also presented. Particular biasing for a case of an input transistor of the stack is also presented.

Short-circuit evaluation and overcurrent protection for SiC power MOSFETs

Abstract: In this paper, the short-circuit (SC) performance of two different SiC MOSFETs is experimentally investigated for different input voltages, biasing voltages and case temperatures. The measurement results are compared to simulations, and a good agreement is achieved. For fault handling, two different overcurrent protection (OCP) circuits are designed and applied to the SiC MOSFETs. The desaturation method is successfully tested with a hardware solution substituting the blanking time delay. The second method is based on sensing the voltage drop across the parasitic inductance at the source pin. The experimental and simulation results show that both OCP methods have the capability to detect a short circuit condition in the SiC MOSFET within safe SC time avoiding device failure.

Highly sensitive fast-response UV photodiode fabricated from rutile TiO2 nanorod array on silicon substrate

Abstract: An ultraviolet photodiode based on rutile TiO2 nanorods, which were grown on p-type Si substrate seeded with a TiO2 layer, was synthesized by radiofrequency reactive magnetron sputtering. Chemical bath deposition was performed to grow rutile TiO2 nanorods. X-ray diffraction and field emission-scanning electron microscopy were conducted to determine the structural and optical properties of the sample. The synthesized TiO2 nanorods exhibited tetragonal rutile structure. The device showed 3.79 × 102 sensitivity when it was exposed to 325 nm light (1.6 mW/cm) at 5 V bias voltage. In addition, the internal gain of the photosensor was 4.792 and the photoresponse peak was 460 mA/W. The photocurrent was 6.09 × 10−4 A. The response and recovery times of the PD were 50.8 and 57.8 ms, respectively, upon illumination of a pulsed UV light (325 nm, 1.6 mW/cm2) at 5 V bias voltage.

Bias-selectable dual-band mid-/long-wavelength infrared photodetectors based on InAs/InAs1−xSbx type-II superlattices

Abstract: A high performance bias-selectable mid-/long-wavelength infrared photodetector based on InAs/InAs1−xSbx type-II superlattices on GaSb substrate has been demonstrated. The mid- and long-wavelength channels' 50% cut-off wavelengths were ∼5.1 and ∼9.5 μm at 77 K. The mid-wavelength channel exhibited a quantum efficiency of 45% at 100 mV bias voltage under front-side illumination and without any anti-reflection coating. With a dark current density of 1 × 10−7 A/cm2 under 100 mV applied bias, the mid-wavelength channel exhibited a specific detectivity of 8.2 × 1012 cm· Hz/W at 77 K. The long-wavelength channel exhibited a quantum efficiency of 40%, a dark current density of 5.7 × 10−4 A/cm2 under −150 mV applied bias at 77 K, providing a specific detectivity value of 1.64 × 1011 cm· Hz/W.

Control of the Metal-Insulator Transition in VO2 Epitaxial Film by Modifying Carrier Density

Abstract: External controlling the phase transition behavior of vanadium dioxide is important to realize its practical applications as energy-efficient electronic devices. Because of its relatively high phase transition temperature of 68 degrees C, the central challenge for VO2-based electronics, lies in finding an energy efficient way, to modulate the phase transition in a reversible and reproducible manner. In this work, we report an experimental realization of p-n heterojunctions by growing VO2 film on p-type GaN substrate. By adding the bias voltage on the p-n junction, the metal-insulator transition behavior of VO2 film can be changed continuously. It is demonstrated that the phase transition of VO2 film is closely associated with the carrier distribution within the space charge region, which can be directly controlled by the bias voltage. Our findings offer novel opportunities for modulating the phase transition of VO2 film in a reversible way as well as extending the concept of electric-field modulation on other phase transition materials.

Carrier transport at the metal–MoS2 interface

Abstract: This study illustrates the nature of electronic transport and its transition from one mechanism to another between a metal electrode and MoS2 channel interface in a field effect transistor (FET) device. Interestingly, measurements of the contact resistance (Rc) as a function of temperature indicate a transition in the carrier transport across the energy barrier from thermionic emission at a high temperature to tunneling at a low temperature. Furthermore, at a low temperature, the nature of the tunneling behavior is ascertained by the current–voltage dependency that helps us feature direct tunneling at a low bias and Fowler–Nordheim tunneling at a high bias for a Pd–MoS2 contact due to the effective barrier shape modulation by biasing. In contrast, only direct tunneling is observed for a Cr–MoS2 contact over the entire applied bias range. In addition, simple analytical calculations were carried out to extract Rc at the gating range, and the results are consistent with the experimental data. Our results describe the transition in carrier transport mechanisms across a metal–MoS2 interface, and this information provides guidance for the design of future flexible, transparent electronic devices based on 2-dimensional materials.

Suppression of dark current in germanium-tin on silicon p-i-n photodiode by a silicon surface passivation technique.

Abstract: We demonstrate that a complementary metal-oxide-semiconductor (CMOS) compatible silicon (Si) surface passivation technique effectively suppress the dark current originating from the mesa sidewall of the Ge0.95Sn0.05 on Si (Ge0.95Sn0.05/Si) p-i-n photodiode. Current-voltage (I-V) characteristics show that the sidewall surface passivation technique could reduce the surface leakage current density (Jsurf) of the photodiode by ~100 times. A low dark current density (Jdark) of 0.073 A/cm2 at a bias voltage of −1 V is achieved, which is among the lowest reported values for Ge1-xSnx/Si p-i-n photodiodes. Temperature-dependent I-V measurement is performed for the Si-passivated and non-passivated photodiodes, from which the activation energies of dark current are extracted to be 0.304 eV and 0.142 eV, respectively. In addition, the optical responsivity of the Ge0.95Sn0.05/Si p-i-n photodiodes to light signals with wavelengths ranging from 1510 nm to 1877 nm is reported.

A portable x-ray source with a nanostructured Pt-coated silicon field emission cathode for absorption imaging of low-Z materials

Abstract: We report the design, fabrication, and characterization of a portable x-ray generator for imaging of low-atomic number materials such as biological soft tissue. The system uses a self-aligned, gated, Pt-coated silicon field emitter cathode with two arrays of 62 500 nano-sharp tips arranged in a square grid with 10 μm emitter pitch, and a natural convection-cooled reflection anode composed of a Cu bar coated with a thin Mo film. Characterization of the field emitter array demonstrated continuous emission of 1 mA electron current (16 mA cm − 2) with >95% current transmission at a 150 V gate-emitter bias voltage for over 20 h with no degradation. The emission of the x-ray source was characterized across a range of anode bias voltages to maximize the fraction of photons from the characteristic K-shell peaks of the Mo film to produce a quasi-monochromatic photon beam, which enables capturing high-contrast images of low-atomic number materials. The x-ray source operating at the optimum anode bias voltage, i.e. 35 kV, was used to image ex vivo and nonorganic samples in x-ray fluoroscopic mode while varying the tube current; the images resolve feature sizes as small as ~160 µm.

Influences of bias voltage on the microstructures and tribological performances of Cr–C–N coatings in seawater

Abstract: CrCN coatings were deposited on single crystal silicon and 316L stainless steel substrates using the cathodic arc ion plating technique with different substrate bias voltages that ranged from − 10 to − 160 V. The relationships between substrate bias voltage and the coating characterization such as nanohardness, modulus, adhesion of the coatings and tribological behavior in ambient air, distilled water and seawater were investigated. Results showed that the hardness and modulus increased significantly with the increase of bias voltages from − 10 to − 160 V. Both the friction coefficients and wear rates of CrCN coatings in ambient air, distilled water and seawater decreased with the increase of bias voltages in the lower range, whereas, they were constant when the bias voltage increased to the high level. In addition, the friction coefficients and wear rates of CrCN coatings under aqueous solution were significantly lower than those under ambient air. Excellent friction and wear behaviors of the CrCN coatings deposited with bias voltages in the range of − 100 to − 130 V may be attributed to low roughness, excellent adhesion strength, high hardness and planar (2D) graphite-like structure.

Cu(In,Ga)Se2 superstrate solar cells: prospects and limitations

Abstract: Superstrate solar cells were prepared by thermal evaporation of Cu(In,Ga)Se2 onto ZnO coated glass substrates. For the first time, photo-conversion efficiencies above 11% were reached without the necessity of additional light soaking or forward biasing of the solar cell. This was achieved by modifying the deposition process as well as the sodium doping. Limitations of the superstrate device configuration and possible ways to overcome these were investigated by analyzing the hetero-interface with electron microscopy and X-ray photoemission spectroscopy measurements, combined with capacitance spectroscopy and device simulations. A device model was derived that explains how on the one hand the GaOx, which forms at the CIGSe/ZnO interface, reduces the interface recombination. On the other hand how it limits the efficiency by acting as an electron barrier at the hetero-interface presumably because of a high density of negatively charged acceptor states like CuGa. The addition of sodium enhances the p-type doping of the absorber but also increases the net doping within the GaOx. Hence, a trade-off between these two effects is required. The conversion efficiency was found to decrease over time, which can be explained in our model by field-induced diffusion of sodium cations out from the GaOx layer. The proposed device model is able to explain various effects frequently observed upon light soaking and forward biasing of superstrate devices. Copyright © 2014 John Wiley & Sons, Ltd.

Complex Mode Spectra of Graphene-Based Planar Structures for THz Applications

Abstract: In this work we analyze in detail the modal properties of a graphene-based planar waveguide (GPW) in the THz frequency range. The structure consists of a graphene sheet placed on top of a grounded dielectric slab. As is known, the surface conductivity of the graphene sheet can easily be tuned with a bias voltage via electric-field effect; we show here how such a bias affects the propagation features of both TM and TE modes supported by the GPW. An extensive dispersion analysis is performed for complex modes in both guided and radiative (leaky) regimes, considering also dielectric and metal losses as well as nonlocal effects in graphene. In particular, we focus on the behavior of the fundamental leaky modes since they exhibit quite interesting radiation features for suitable values of the bias. These results are very promising for the development of reconfig-urable leaky-wave Fabry-Perot cavity antennas based on graphene at THz frequencies.

Germanium-Tin on Si Avalanche Photodiode: Device Design and Technology Demonstration

Abstract: We report the demonstration of a Ge0.95Sn0.05 on silicon (Ge0.95Sn0.05/Si) avalanche photodiode (APD) having a separate-absorption-charge-multiplication structure, wherein a Ge0.95Sn0.05 layer and a Si layer function as an absorption layer and a multiplication layer, respectively. Material characterization was performed by atomic force microscopy, X-ray diffraction, and transmission electron microscopy. The dark current $I_{\rm dark}$ of the APD is dominated by the area-dependent bulk leakage rather than the surface leakage. The temperature dependence of breakdown voltage of the Ge0.95Sn0.05/Si APD was characterized and a thermal coefficient of 0.05% $\mathrm{K}^{\mathrm {-1}}$ was obtained, achieving a lower thermal sensitivity than the conventional III-V-based APDs. In the wavelength range of 1600–1630 nm, a responsivity of $\sim 1$ A/W (bias voltage ${V}_{\rm bias} = -9.8$ V) was achieved due to the internal avalanche gain.

Impact of the Oxide-Aperture Diameter on the Energy Efficiency, Bandwidth, and Temperature Stability of 980-nm VCSELs

Abstract: New oxide-confined 980-nm vertical-cavity surface-emitting lasers (VCSELs) with record temperature-stable small-signal bandwidths of 25.6 to 23.0 GHz at 25 to 85 °C are designed, fabricated, and characterized. Technology-based device parameters essential for system-level models of VCSEL-based short-reach and ultrashort-reach optical interconnects are extracted. These parameters include key intrinsic figures-of-merit, including the −3-dB modulation bandwidth, the bandwidth-to-electrical power ratio, and device input impedance, all as functions of temperature, oxide-aperture diameter, and desired range of bias current or current density. Further, the M-factor, relating the intrinsic VCSEL bandwidth to the error-free bit rate for a given external systems configuration and application, is introduced. Our present 980-nm VCSEL technology is capable of 40 Gb/s operation at 85 °C at a simultaneously low current density of 10 kA/cm2 with an energy of only 100 fJ per bit.

An ultrafast response grating structural ZnO photodetector with back-to-back Schottky barriers produced by hydrothermal growth

Abstract: An ultrafast response metal–semiconductor–metal type ZnO ultraviolet photodetector was fabricated by ultraviolet nanoimprint lithography (UV-NIL) with hydrothermal synthesis. The extremely fast response time was due to the Schottky barrier formation attributed to the control of the hydrothermal growth time and grating structure of ZnO, produced by a position-controlled pattering method of UV-NIL. With an on/off frequency of ultraviolet light of 2 kHz using an optical chopper, the device exhibits a rising time of 43 μs and a falling time of 54 μs at a low bias voltage (0.5 V) with a responsivity of 22.1 A W−1 in the active area of 5 × 5 μm2. In comparison to other fast response ZnO photodetectors, our device definitely uses easy and low-cost fabrication methods as well as exhibits high performance.

Influence of the negative bias in ion plating on the microstructural and tribological performances of Ti-Si-N coatings in seawater

Abstract: The TiSiN coatings were deposited on Ti 6 Al 4 V alloy by multi-arc ion plating with different substrate negative bias to improve the tribological properties of titanium alloy in seawater The TiSiN coatings have a coupled structure of amorphous Si 3 N 4 and nanocrystalline TiN The grain size decreases and the coatings become dense with the increase of the negative bias The hardness of TiSiN coatings increases from 39 GPa to 47 GPa with the increase of substrate negative bias from 10 V to 40 V The tribological tests reveal that the TiSiN coatings have a lower friction coefficient and higher wear rate in seawater compared with in atmosphere Moreover, the wear rates increase with the increase of bias voltage The TiSiN coating deposited at bias voltage of 10 V shows the best tribological properties in seawater