Fast Fall Time Research Articles

Mesoscopic Perovskite Light-Emitting Diodes.

Solution-processed hybrid bromide perovskite light-emitting-diodes (PLEDs) represent an attractive alternative technology that would allow overcoming the well-known severe efficiency drop in the green spectrum related to conventional LEDs technologies. In this work, we report on the development and characterization of PLEDs fabricated using, for the first time, a mesostructured layout. Stability of PLEDs is a critical issue; remarkably, mesostructured PLEDs devices tested in ambient conditions and without encapsulation showed a lifetime well-above what previously reported with a planar heterojunction layout. Moreover, mesostructured PLEDs measured under full operative conditions showed a remarkably narrow emission spectrum, even lower than what is typically obtained by nitride- or phosphide-based green LEDs. A dynamic analysis has shown fast rise and fall times, demonstrating the suitability of PLEDs for display applications. Combined electrical and advanced structural analyses (Raman, XPS depth profiling, and ToF-SIMS 3D analysis) have been performed to elucidate the degradation mechanism, the results of which are mainly related to the degradation of the hole-transporting material (HTM) and to the perovskite-HTM interface.

Theoretical Analysis and Improvement on Pulse Generator Using BJTs as Switches

Bipolar junction transistors (BJTs) have been widely studied and used in nanosecond high-voltage pulse generators due to the advantages of fast switching speed and high repetitive frequency. Usually the Marx-type circuit is used to raise the peak value of output pulses. In this paper, a traditional BJT-Marx circuit using charging resistors is analyzed theoretically and experimentally. Based on the results, the structure of a circuit is changed to decrease the isolation voltage across the resistors, and diodes are used to replace charging resistors to block capacitors discharging to resistors. Experimental results showed that the three Marx generators proposed in this paper output pulses with higher voltage amplitude, and the last improved circuit outputs pulses with faster fall times at a higher efficiency.

Room-Temperature Solution-Processed NiOx:PbI2 Nanocomposite Structures for Realizing High-Performance Perovskite Photodetectors.

While methylammonium lead iodide (MAPbI3) with interesting properties, such as a direct band gap, high and well-balanced electron/hole mobilities, as well as long electron/hole diffusion length, is a potential candidate to become the light absorbers in photodetectors, the challenges for realizing efficient perovskite photodetectors are to suppress dark current, to increase linear dynamic range, and to achieve high specific detectivity and fast response speed. Here, we demonstrate NiOx:PbI2 nanocomposite structures, which can offer dual roles of functioning as an efficient hole extraction layer and favoring the formation of high-quality MAPbI3 to address these challenges. We introduce a room-temperature solution process to form the NiOx:PbI2 nanocomposite structures. The nanocomposite structures facilitate the growth of the compact and ordered MAPbI3 crystalline films, which is essential for efficient photodetectors. Furthermore, the nanocomposite structures work as an effective hole extraction layer, which provides a large electron injection barrier and favorable hole extraction as well as passivates the surface of the perovskite, leading to suppressed dark current and enhanced photocurrent. By optimizing the NiOx:PbI2 nanocomposite structures, a low dark current density of 2 × 10(-10) A/cm(2) at -200 mV and a large linear dynamic range of 112 dB are achieved. Meanwhile, a high responsivity in the visible spectral range of 450-750 nm, a large measured specific detectivity approaching 10(13) Jones, and a fast fall time of 168 ns are demonstrated. The high-performance perovskite photodetectors demonstrated here offer a promising candidate for low-cost and high-performance near-ultraviolet-visible photodetection.

Thermal UV treatment on SU-8 polymer for integrated optics

We report a simple and low-cost method to fabricate SU-8-based polymer waveguide devices. The influence of hard-baking temperature on SU-8 polymer treated with or without UV radiation was investigated in detail. Based on these properties, the straight type, Y branch type, March-Zehnder (M-Z) type and 1 × 4 splitter waveguides were successfully fabricated. And a polymeric thermal-optic (TO) switch with M-Z type waveguide was also fabricated by this method. The device exhibits low power consumption of less than 6.4 mW, fast rise time of about 149.3 µs and fast fall time of about 139.3 µs. The experimental results showed that this method can bypass the need for complex lithography and provide high resolution and fine waveguide quality desired for TO switches as well as other planar lightwave devices.

Fast Response Ultra-violet Photodetectors Based on Sol Gel Derived Ga-doped ZnO

Ultraviolet photoconductivity in Gallium doped ZnO (GZO) thin film synthesized by sol-gel technique is investigated. Response characteristics of Pure ZnO thin film and GZO thin film UV photodetector biased at 5V for UV radiation of λ = 365nm and intensity = 24μwatt/cm2 have been studied. GZO UV photodetector is found to exhibit a high photocoductive gain (K = 9.32×102) with fast rise and fall time (Tr = 3 s and Tf = 9 s) in comparison to pure ZnO thin film based photodetector with K = 4.9×101, Tr = 6 s and Tf = 18 s. Formation of some neutral atoms at the grain boundaries of GZO thin film due to heavy doping of Gallium in ZnO lattice is the key factor for lowering of dark current (Ioff ∼ 0.11μA). Upon UV illumination, the release of trapped electrons from surface defects or adsorbed oxygen results in an enhanced photocurrent (Ion = 0.1mA) and hence higher value of K is obtained for GZO thin film based photodetector.

低抖动快前沿重复频率高压脉冲触发源研制

低抖动快前沿重复频率高压脉冲触发源研制

Influence of the applied voltage shape on the barrier discharge operation modes in helium

For the investigation of a barrier discharge (BD) operated in helium, a discharge cell configuration was used which allows an electrical characterization and simultaneous measurements of volume processes as well as the interaction of the BD with a dielectric surface. The emission development in the volume has been recorded spatio-temporally and spectrally resolved by the established cross-correlation spectroscopy (CCS) operating in the PPG mode. The phase resolved measurement (over one discharge period) of surface charges deposited on a BSO crystal was realized by the utilization of the electro-optic Pockels effect in combination with a high-speed camera. Depending on the gap distance, the dielectrics, and the shape of feeding voltage, the BD can operate either in the diffuse Townsend-like or glow-like mode. The emission is localized near the anode and cathode for the Townsend-like mode and glow-like mode, respectively. In small gaps (distance about 1 mm), a sinusoidal feeding voltage leads to the unusual Townsend-like mode in helium. But, a square wave voltage shape induces the glow-like mode, probably due to the fast rise and fall time of the applied voltage slopes. A sawtooth voltage generates both discharge modes over one period. The presented results show clearly the correlation of the emission development with electrical measurements, and deposited surface charge and transported charges.

Improved Switching for Magneto-Optic Fiber-Based Technologies

A new method for magnetic field generation is presented that enhances the reliability of fiber-based interferometric switches. Suitable switching technologies have been proposed and implemented, but magneto-optic (MO) materials have shown promise in realizing all-optical switching with reasonable power, loss, and switching speed. However, bottlenecks in field generation drivers have limited the performance of MO devices in fiber-based switches. Improving the design of the field generation system will enhance this technology, ultimately realizing practical and reliable solutions. Conventional drivers in MO switching devices utilize one coil with a low number of turns to quickly magnetize the MO material. This, in effect, leads to a fast optical rise time. However, the optical fall time is determined by the response of the MO material and the decay rate of any trapped magnetic energy in the coil, which can be sluggish. The proposed design achieves fast optical fall time by better controlling the demagnetization of the MO material. In this paper, the design of the magnetic field generation system is presented and compared to other recently proposed systems, the electrical system is characterized, and the reliability of the optical output is investigated.

Response-time improvement of a 2×2 thermo-optic switch with polymer/silica hybrid waveguide

A polymer/silica hybrid 2×2 directional coupler (DC) Mach–Zehnder interferometer (MZI) thermo-optic (TO) switch is designed and carefully fabricated. Because of larger thermal conductivity relative to the polymer, silica is utilized as bottom cladding for accelerating heat release. The fabricated TO switch with polymer/silica hybrid structure exhibits low power consumption, less than 7.2mW, fast rise time of about 106μs and fast fall time of about 93μs. This response time is reduced by 40% compared to that of the TO switch with polymer waveguide.

Nanosecond square high voltage pulse generator for electro-optic switch

A scalable square high voltage pulse generator, which has the properties of fast rise time, fast fall time, powerful driving capability, and long lifetime, is presented in this paper by utilizing solid state circuitry. A totem-pole topology is designed to supply a powerful driving capability for the electro-optic (EO) crystal which is of capacitive load. Power MOSFETs are configured in series to sustain high voltage, and proper driving circuits are introduced for the specific MOSFETs configurations. A 3000 V pulse generator with ~49.04 ns rise time and ~10.40 ns fall time of the output waveform is presented. This kind of generator is desirable for electro-optic switch. However, it is not specific to EO switch and may have broad applications where high voltage fast switching is required.

High voltage pulser with a fast fall-time for plasma immersion ion implantation

A novel high voltage (HV) modulator that offers a short fall time to minimize sputtering effects and allow more precise control of the incident ion fluence in plasma immersion ion implantation is described. The use of 36 insulated-gate bipolar transistors in the 30 kV hard-tube pulser reduces the HV fall time to 3.5 μs, compared to a fall time of 80 μs if a pull-down resister is used. The voltage balance is achieved by a voltage-balancing resistor, clamped capacitance, and the synchronization of drive signals. Compared to the traditional method employing a pull-down resister or an additional hard tube, our design consumes less power and is more economical and reliable.

Effect of wakefields on the propagation of a short laser pulse in a plasma channel

The effect of electric wakefields (generated inside the pulse) on the propagation of a short, asymmetric laser pulse in a plasma channel is analyzed. A nonlinear wave equation governing the propagation dynamics of a moderately intense laser pulse in a plasma channel along with equations describing the generation of wakefields is set up. The simultaneous evolution describing the pulse length and spot size in the presence of wakefields is derived using a variational technique. Using numerical methods the evolution of spot size, pulse length and intensity is graphically analyzed. The frequency chirp induced due to the combined effects of group velocity dispersion and self-phase modulation is also obtained. A comparative study of the propagation characteristics of two pulses, one having a fast rise and slow fall time while the other with a slow rise and fast fall time, has been presented.

Mixed-mode interfacial adhesive strength of a thin film on an anisotropic substrate

The mixed-mode interfacial adhesion strength between a gold (Au) thin film and an anisotropic passivated silicon (Si) substrate is measured using laser-induced stress wave loading. Test specimens are prepared by bonding a fused silica (FS) prism to the back side of a 〈1 0 0〉 Si substrate with a thin silicon nitride (Si x N y ) passivation layer deposited on the top surface. A high-amplitude stress wave is developed by pulsed laser ablation of a sacrificial absorbing layer on one of the lateral surfaces of the FS prism. Due to the negative non-linear elastic properties of the FS, the compressive stress wave evolves into a decompression shock with fast fall time. Careful selection of the incident angle between the pulse and the FS/Si interface generates a mode-converted shear wave in refraction, subjecting the Si x N y /Au thin film interface to dynamic mixed-mode loading, sufficient to cause interfacial fracture. A detailed analysis of the anisotropic wave propagation combined with interferometric measurements of surface displacements enables calculation of the interfacial stresses developed under mixed-mode loading. The mixed-mode interfacial strength is compared to the interfacial strength measured under purely tensile loading.

Carrier capture at the SiO2–Si interface: A physical model

The carrier (electron and hole) capture cross section of defects at the SiO2–Si interface is measured using trap-fill time in gigahertz charge-pumping experiment. The ability to use a highly asymmetric rectangular wave with very fast rise and fall times in our charge-pumping experiment greatly simplified data interpretation. Trap-fill times of less than or equal to 0.7ns are found for both electrons and holes. The corresponding capture cross section of 10−15cm2 or larger for both electron and hole cannot be explained by the usual multiphonon emission mechanism. A modified cascade capture model involving the strained Si–Si bonds around the Pb center is proposed to explain the large capture cross sections.

Microparticle flow generation by a ferroelectric plasma source

The paper presents the experimental results of the investigation of microparticle emission from a Ba–Ti-based ferroelectric sample subjected to a driving pulse with a slow rise time and a fast fall time. It was found that the formation of incomplete discharges on the surface of the ferroelectric is accompanied by the emission of an intense positively charged microparticle flow. This microparticle flow (density, velocity distribution, momentum and average microparticle charge) was studied using electrical and optical diagnostics at different distances from the front surface of the ferroelectric sample and at several time delays with respect to the fast fall of the driving pulse. It was shown that microparticles are positively charged at about 6×10−15 C; they have an average size of about 5 μm, a density of about 7×104 cm−2 and an average velocity of about 6×103 cm s−2. The mechanism of the formation of these microparticles and the application of microparticle flows as a propellant for a small thruster are ...

Electron beam generation in a diode having a ferroelectric plasma cathode controlled by optic fibers

An electron diode having a ferroelectric plasma cathode (FPS) controlled by optic fibers was operated at a repetition rate of 0.5 Hz during 1 hour, when an accelerating pulse of ~250 kV in amplitude and ~250 ns in duration is applied. The application of the fiber optic decoupling allows easy control and synchronization of the FPS operation with the firing of the HV generator. It was shown that the use of the FPS with fast fall time of the driving pulse allows reproducible generation of an electron beam having a current amplitude of ~1 kA and uniform cross-sectional current density distribution. It was shown also that in the case of the driving pulse ringing, the application of the accelerating pulse during the fall in the negative ringing leads to a plasma pre-filled mode of diode operation due to intense ion emission from the FPS. When the accelerating pulse is applied during the rise in the positive driving pulse ringing, one obtains diode operation with limited plasma emission ability. Only when the accelerating pulse is applied during the fall in the positive ringing of the driving pulse does one obtain diode operation in space-charge limited mode. This dependence of the diode operation on the timing of the accelerating pulse application with respect to the driving pulse is explained by processes related to the screening of ferroelectric bounded surface charges by the plasma charged particles.

ZnO-Sn Bilayer Ultraviolet (UV) Photon Detector with Improved Responsitivity

ABSTRACTThe influence of the integration of ultra-thin layer of different metals (Al, Cu, Sn, Te, Pb and Au) with the c-axis oriented ZnO thin film on the ultraviolet (UV) photoresponse is investigated. The transfer of electrons from the metal layer to the semiconductor at the interface compensate the surface states which otherwise takes electrons from the interior of ZnO layer and thereby increases the conductivity under UV illumination. The Sn/ZnO sample exhibits a responsivity of the order of 8.5 KV/W at a low UV intensity of 140 μW/cm2 (⎻ = 365 nm) with a fast rise and fall time of 105 and 400 ms respectively.

A 2.4 GHz CMOS sub-sampling mixer with integrated filtering

A sub-sampling mixer that incorporates sampling switches and hold capacitors into the parallel resonant LC load of an LNA is proposed. The noise figure of the proposed sub-sampling mixer is lower than that of a standard sampling circuit because the proposed mixer has narrow-band gain and input noise filtering properties. A novel level-shifting clock buffer with fast rise and fall times to drive the mixer sampling switches is presented. The mixer was fabricated in a 0.18 /spl mu/m CMOS process and measured results are presented for an RF input frequency of 2.42 GHz and a sampling frequency of 100 MHz. With a measured noise figure of 21.8 dB, the proposed circuit shows improved performance compared to other published sub-sampling mixers.

Toward a High-Speed Arbitrary Waveform Generation by a Novel Photonic Processing Structure

A new photonics-based method for high-speed arbitrary waveform generation is presented. It is based on generating high-speed short optical pulse samples and a grating and delay structure for amplitude weighting and time delaying the optical samples. In order to increase the number of samples, a novel delay-attenuation-coupling processing method is presented for realizing high resolution in waveform generation. Results are given for sawtooth waveform generation with fast fall time of 10% of period and waveform linearity deviation of 1%, demonstrating the high resolution and wide-band capabilities of this technique.

Tensile and mixed-mode strength of a thin film-substrate interface under laser induced pulse loading

Laser induced stress waves are used to characterize intrinsic interfacial strength of thin films under both tensile and mixed-mode conditions. A short-duration compressive pulse induced by pulsed-laser ablation of a sacrificial layer on one side of a substrate is allowed to impinge upon a thin test film on the opposite surface. Laser-interferometric measurements of test film displacement enable calculation of the stresses generated at the interface. The tensile stress at the onset of failure is taken to be the intrinsic tensile strength of the interface. Fused-silica substrates, with their negative nonlinear elasticity, cause the compressive stress wave generated by the pulse laser to evolve a decompression shock, critical for generation of the fast fall times needed for significant loading of surface film interfaces. By allowing the stress pulse to mode convert as it reflects from an oblique surface, a high amplitude shear wave with rapid fall time is generated and used to realize mixed-mode loading of thin film interfaces. We report intrinsic strengths of an aluminum/fused silica interface under both tensile and mixed-mode conditions. The failure mechanism under mixed-mode loading differs significantly from that observed under pure tensile loading, resulting in a higher interfacial strength for the mixed-mode case. Inferred strengths are found to be independent, as they should be, of experimental parameters.