Fall Time Research Articles

All-Monolithically Integrated Self-Scanning Addressable VCSEL Array for 3D Sensing

We propose an all-monolithically integrated self-scanning addressable vertical-cavity surface-emitting laser array for time-of-flight measurement. Some advantages of addressable VCSELs for ToF measurement applications include large reductions in power consumption, heat generation, multi-pass noise, and flare noise. In this paper, we discuss the characteristics of the proposed self-scanning addressable VCSEL array. All layers in the epilayer structure of the proposed VCSEL array were formed at once by metal-organic chemical vapor deposition. The device consists of an (Al)GaAs-based thyristor and a conventional top-emitting 940-nm oxide-confined VCSEL on an n-type GaAs substrate. The array contained 12 blocks (4 × 3) that have more than 40 emitters each. The device required only four signals from a field-programmable gate array to select the emitting blocks and one emission signal from a conventional VCSEL driver, even for arrays containing hundreds of blocks. The proposed module is capable of one-block sequential emission, parallel emission from several blocks, and emission from all blocks. The rise and fall times of the fabricated VCSEL array were observed to be 200 and 400 ps, respectively, for each emission mode. The influence of flare noise from an obstacle in front of the camera was dramatically reduced by avoiding emission to the obstacle.

Effects of nanosecond impulse and step excitation in pulsed electro acoustic measurements on signals for space charge determination in high-voltage electrical insulation

The presence of space charges in high-voltage insulation can influence the operational stresses of the dielectric materials; the topic is present in various, even distant domains such as power grids or transportation segments that are represented by electromobility, rail, more-electric aircraft, and space systems. In this article, the effects of nanosecond impulse and step excitation in the pulsed electro-acoustic measurements (PEA) of space charges in high-voltage electrical insulation have been investigated. Various parameters of a pulser (such as impulse width, rise time, and sensor thickness) have been investigated by using a designed programable pulser. The investigations have been performed on a polyethylene sample in one- or double-layer configurations. In the latter, an interlayer sprinkled charge was analyzed. The experimental results have shown the main PEA effect during the transition phase of the applied voltage. It was observed that the pulse transition has a critical importance for stimulating space-charge vibrations and generating elastic waves. Hence, the slope’s rise and fall times are crucial in pulser design. The pulser width also has an impact on a signal’s magnitude as well as on its dispersion and attenuation. The correspondence between a pulse and step excitation was presented. Some advantages are attributed to the step-based excitation in terms of fewer wave reflections and interferences as well as the avoidance of the wavelet cancelation effect for a very narrow pulse width. Hence, step excitation can result in the easier postprocessing and interpretation of individual peaks in the acquired PEA signal. Another advantage may be recognizing the space-charge polarity instead of the net result. Important recommendations refer to the tuning of pulser timing settings with respect to signal attenuation (especially in thicker specimens) and the adequate adjustment of the sensor’s thickness. The presented analysis and experimental results extend the insight into PEA measurement – especially with a focus on diagnostic applications.

Gel growth and characterization of Cs3Bi2Br9 perovskite single crystals for radiation detection

Metal halide perovskites have been sought for ionizing radiation detection due to their tunable bandgap, high quantum yield, high absorption coefficient, excellent charge transport properties, flexible chemistry synthesis and cost-effective manufacturing. Among the family of perovskites, bismuth-based halide perovskites have attracted a rapidly growing interest as possible alternatives to lead-based halide perovskites in the development of nontoxic perovskites for opto-electronic devices. Herein, bismuth-based inorganic perovskite Cs3Bi2Br9 single crystals were successfully grown using an innovative dual-diffusion gel growth technique for the first time. Silica gel has been developed as an enabling medium for gel growth of single crystals due to its transparency and easy control of nucleation sites. The UV-vis transmission spectrum was recorded using a light source of deuterium and halogen lamps and a Tauc plot was obtained, which gave an estimate of the bandgap energy, 2.54 eV. Silver electrodes were used on the top surface and the bottom surface of Cs3Bi2Br9 single crystals for material characterization and detector tests. Current-voltage (I-V) measurements gave a room-temperature resistivity of 1.79×1011 Ω•cm. The Cs3Bi2Br9 single crystals were then tested for X-ray response using ON\OFF testing which revealed attractive responsiveness for X-ray photons (rise and fall time: ≈0.3 s ). Using the net current (IXray−Idark), which can be extracted from the X-ray response measurements at varied applied voltages, a modified Hecht fitting was applied to estimate the mobility-lifetime product of electrons, μτ=5.12×10−4 ±6.70×10−5 cm2/V for a Ag/Cs3Bi2Br9/Ag device. This study shows that our innovative crystal growth method, enabled by the unique gel growth process, can be used as an appealing technique to grow functional crystals for opto-electronic devices. Meanwhile, Cs3Bi2Br9 has shown great potential as a promising candidate for X-ray detection applications. The efforts in this work will serve as a metric for growing halide perovskites in the gel for opto-electronic devices.

A Simulation Study of SiGe Shell Channel CFET for Sub-2-nm Technology Nodes

In this article, a SiGe shell complementary FET (CFET) composed of an inversion mode (IM) nFET and a junctionless accumulation mode pFET under 2-nm (N2) and 1.5-nm (N1.5) technology nodes was studied for the characteristics of a single device and the CMOS logic circuit using 3-D numerical simulation. A comparison study between Si shell nanosheet and SiGe shell CFET with a Ge mole fraction in the range of 0.5–0.9. Compared with the Si shell nanosheet (NS), the SiGe shell CFET improves the ON-state current, transconductance, transition frequency, and intrinsic delay up to 29%, 32%, 31%, and 21%, respectively. Due to the mobility improvement in SiGe shell CFET, the fall time, rise time, propagation delay, and maximum operating frequency of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text {Si}_{{0.5}}\text {Ge}_{{0.5}}$ </tex-math></inline-formula> shell CFET are also improved up to 2%, 43%, 3%, and 3%, respectively, compared with their Si shell counterparts. This SiGe shell CFET has a lower parasitic RC, a smaller layout area, and better analog and digital performance at the sub-2-nm technology node. This makes it a promising architecture for future high-performance CMOS logic applications.

Feasibility of Integrating Bimetallic Au-Ag Non-Alloys Nanoparticles Embedded in Reduced Graphene Oxide Photodetector

To coordinate the resonant wavelength of the plasmonic nanoparticles (NPs), the emission band of the reduced graphene oxide (rGO) photodetector at the NIR-region is crucial for the optimal plasmon-enhanced luminescence in the device. In contrast to monometallic NPs, where limits the dimensions and extended resonant wavelength, we integrated an Au-Ag bimetallic NPs (BMNPs) to enable resonance tuning at the longer wavelength at the excitation source of 785 nm. These features showed an increase in radiative recombination rates as well as the quantum yield efficiency of the device. The BMNPs were produced from the dewetting process of 600 °C and 500 °C, both at 1 min after the deposition thickness layer of Au (8 nm) and Ag (10 nm) on the Si substrate using the electron-beam evaporation process. Our BMNPs-rGO photodetector exhibited the responsivity of 2.25 · A W−1, Jones of specific detectivity of 2.45×1011Jones, and external quantum efficiency (EQE) of 356%. The rise time and fall time for the photodetector were 32 ns and 186 ns, respectively. This work provided an essential information to enable the versatile plasmon-enhanced application in 2-dimensional (2D) material optoelectronic devices.

A Novel Ultrafast Turn-Off Failure Detection Method of Integrated Gate Commutated Thyristor for VSC Application

Integrated gate commutated thyristor (IGCT) has prospects in voltage source converter application. However, it has no desaturation characteristic in conducting state like insulated gate bipolar transistor, for which fast turn- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> failure detection for blocking converter in deadtime is of great significance. In this letter, an ultrafast IGCT turn- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> failure detection method is proposed. First, turn- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> process and failure mechanism of IGCT is analyzed. It is clarified that turn-off capacitors of gate driver will continue to discharge in turn- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> failure, thus generating a voltage drop on turn- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off mosfet</small> , which can be the criterion of failure. Then prototype, threshold and timing sequence are designed. Analog circuit including amplifier, hysteretic comparator and reference circuit is used for fast and accurate measurement of voltage on <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mosfet</small> . The failure gate current and fall time of turn- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> trail current is measured in pulse test. Finally, experiments are carried out to verify the feasibility of the proposed scheme. The results show that the proposed scheme can achieve reliable turn- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> failure detection within 19 microseconds. Neither false detection nor missed detection occurred. Proposed method has great accuracy and rapidity, and is convenient in implementation. It will greatly decrease the extreme failure rate of IGCT-based converters.

Star Formation Laws and Efficiencies across 80 Nearby Galaxies

We measure empirical relationships between the local star formation rate (SFR) and properties of the star-forming molecular gas on 1.5 kpc scales across 80 nearby galaxies. These relationships, commonly referred to as “star formation laws,” aim at predicting the local SFR surface density from various combinations of molecular gas surface density, galactic orbital time, molecular cloud free fall time, and the interstellar medium dynamical equilibrium pressure. Leveraging a multiwavelength database built for the Physics at High Angular Resolution in Nearby Galaxies (PHANGS) survey, we measure these quantities consistently across all galaxies and quantify systematic uncertainties stemming from choices of SFR calibrations and the CO-to-H2 conversion factors. The star formation laws we examine show 0.3–0.4 dex of intrinsic scatter, among which the molecular Kennicutt–Schmidt relation shows a ∼10% larger scatter than the other three. The slope of this relation ranges β ≈ 0.9–1.2, implying that the molecular gas depletion time remains roughly constant across the environments probed in our sample. The other relations have shallower slopes (β ≈ 0.6–1.0), suggesting that the star formation efficiency per orbital time, the star formation efficiency per free fall time, and the pressure-to-SFR surface density ratio (i.e., the feedback yield) vary systematically with local molecular gas and SFR surface densities. Last but not least, the shapes of the star formation laws depend sensitively on methodological choices. Different choices of SFR calibrations can introduce systematic uncertainties of at least 10%–15% in the star formation law slopes and 0.15–0.25 dex in their normalization, while the CO-to-H2 conversion factors can additionally produce uncertainties of 20%–25% for the slope and 0.10–0.20 dex for the normalization.

Excellent Optoelectronic Properties and Low Contact Resistance of Graphene/MoS2 Heterostructure Optoelectronic Devices: First-Principles Calculation and Experimental Verification

Two-dimensional (2D) materials have enormous applications and are widely studied in the field of electronics and optoelectronic devices. The electrical contacts between the 2D materials and the electrodes seriously affect the performance of the 2D material optoelectronic devices. Exploring the electronic structure and transport behavior at the interface between 2D materials and electrodes and the underlying physical causes will contribute to the development of 2D material integrated circuits and optoelectronic fields. In this study, the charge transport barriers at the Au/MoS2 and graphene/MoS2 interfaces were investigated from both the Schottky and tunneling barriers through theoretical calculations, which demonstrated that the graphene/MoS2 interface has not only a very low Schottky barrier but also a low tunneling barrier compared to the Au/MoS2 interface. Then, three types of optoelectronic devices, Au/MoS2/Au, graphene/MoS2/graphene, and graphene/MoS2/Au field-effect transistors (FETs), were fabricated. Compared with the Au/MoS2/Au device, the graphene/MoS2/graphene device shows an excellent optical response (R = 654 mA W–1 at 532 nm laser with a power density of 4.8 mW cm–2) and ultrafast response time (rise time 9.8 ms, 12.8 ms fall time). The graphene/MoS2/Au device exhibits excellent rectification behavior and optoelectronic response, with almost no current and photocurrent gain and cutoff state under reverse bias, excellent optoelectronic response under positive bias (responsivity R = 293.4 mA W–1 at a power density of 4.8 mW cm–2 for a 532 nm wavelength laser), and excellent response time (rise time 12.55 ms, fall time 15.44 ms). Our studies are expected to bring opportunities for highly sensitive, high-speed, and energy-efficient photodetectors for comprehensive applications.

Two-dimensional (2D) α-In2Se3/Ta2NiSe5 heterojunction photodetector with high sensitivity and fast response in a wide spectral range

As a typical representative of group III-VI two-dimensional (2D) layered semiconductors, α-In2Se3 with layer-dependent direct bandgap (1.39–1.45 eV) has attracted great attention in optoelectronics. At present, various α-In2Se3 based photodetectors have been developed, which show ultrahigh responsivity and detectivity at visible (VIS) wavelengths. However, despite the high sensitivity, the response time is quite long, and the realization of broadband detection from VIS to infra-red (IR) wavelengths is also challenging for these α-In2Se3 photodetectors. In this paper, we introduced a narrow and direct bandgap 2D semiconductor Ta2NiSe5 and made an α-In2Se3/Ta2NiSe5 heterojunction photodetector, which achieved a wide response wavelength of 405–1550 nm with high performance. Originating from the n-type nature of the two semiconductors and the particular type-I energy band alignment, the potential barriers of holes at the α-In2Se3/Ta2NiSe5 interface is much higher than that of electrons, which leads to an ultra-high responsivity. At 520 nm illumination, the photoresponsivity is 12 A/W and 533 A/W under a self-driven state and a slight bias voltage of 0.1 V, respectively. The highest detectivity is over 8.2 × 1013 Jones. Furthermore, the rise and fall time is shortly at 25 and 423 μs, respectively. We believe that the high-sensitive, broadband, and fast photodetector has great potential in the emerging field of 2D optoelectronics.

Highly improved photo-sensing ability of In2S3 thin films through cerium doping

In this work, the influence of rare earth doping on the efficiency of photodetection in In2S3 thin films coated on glass substrates through a nebulizer-assisted spray pyrolysis technique was studied. The In2S3 films were doped with rare earth cerium ions by varying their weight percentage from 0 to 3%. The changes in the structural, morphological, optical, and light sensing capabilities of Ce-doped In2S3 thin films were thoroughly studied using appropriate characterization techniques. X-ray diffraction confirmed the formation of cubic crystallites with a highest crystallite size of 37 nm for 2% Ce doped In2S3 sample. The compositional studies indicate the presence of indium, sulfur, and cerium alone in the prepared films confirming its purity. The optical band gap from Tauc's plot was found to lie in the range of 2.85 –2.77 eV. The band gap is found to decrease with the increase in the replacement of In3+ ions with Ce3+ ions. The optical analysis showed an increased absorption in the ultraviolet region. The sample doped with 2% of cerium is found to have a maximum value of responsivity 0.752 A/W, detectivity 4.53 × 1010 Jones and an EQE of 243%. The rise and fall time of the same sample recorded 0.3 s and 0.4 s respectively. The work paves way to the fabrication of an efficient In2S3 thin film-based UV photo detector doped with cerium at a low cost.

Vertical‐type 3D/Quasi‐2D n‐p Heterojunction Perovskite Photodetector

AbstractThis research demonstrates a state‐of‐the‐art vertical‐transport photodetector with an n‐type 3D MAPbI3/p‐type quasi‐2D (Q‐2D) BA2MA2Pb3I10 perovskite heterojunction. This structure introduces a ≈0.6 V built‐in electric field at the n‐p junction that greatly improves the characteristics of the perovskite photodetector, and the presence of Q‐2D perovskite on the surface improves the life. The electrical polarities of the 3D and the Q‐2D perovskite layers are simply controlled by self‐constituent doping, making clearly defined n‐p characteristics. Doctor‐blade coating is used to fabricate the photodetector with a large area. The Q‐2D materials with highly oriented (040) Q‐2D (n = 2,3) planes are near the surface, and the (111) preferred planes mixed with high index Q‐2D materials (n = 4,5) are found near the 3D/Q‐2D interface. The stacking and interface are beneficial for carrier extraction and transport, yielding an external quantum efficiency of 77.9%, a carrier lifetime long as 295.7 ns, and a responsibility of 0.41 A W−1. A low dark current density of 6.2 × 10−7 mA cm−2 and a high detectivity of 2.82 × 1013 Jones are obtained. Rise time and fall time are fast as 1.33 and 10.1 µs, respectively. The results show the application potential of 3D/Q‐2D n‐p junction perovskite photodetectors.

High-performance photodetector based on semi-encompassed CH3NH3PbCl3–ZnO microwire heterojunction with alterable spectral response

Recently, heterojunctions consisting of hybrid organic-inorganic lead (Pb) halide perovskites and other semiconductors have drawn increasing attention for the potential application in photodetectors due to their exceptional performance. However, their performance is usually limited by the relatively low crystalline quality of perovskites, and the response spectra of the devices are difficult to adjust according to the practical requirement. Here, high quality CH3NH3PbCl3 micro-sized crystals have been successfully fabricated on one side of individual ZnO microwire to form heterojunction photodetector by a two-step crystallization method. The heterojunction device presents a low dark current (60 nA at −6 V) along with a rapid response speed (rise time of <20 μs and fall time of ∼500 μs). More interestingly, the modulation of the response spectra and the responsivity can be realized by operating the device under front or back illumination due to the self-filtering properties. Our findings provide a promising method for combining perovskites with other inorganic materials to form high-performance heterojunction photodetectors.

Fabrication of UV photodetector based on GaN/ Psi heterojunction using pulse laser deposition method: Effect of different laser wavelengths

In this study, GaN/pSi heterojunction photodetectors were fabricated via pulsed laser deposition method (PLD). Photoelectrochemical etching method (PECE) with laser aid prepared a pSi substrate. Different laser wavelengths were tested on GaN and pSi films' structural, spectroscopic, electrical, and performance characteristics. XRD showed that GaN/pSi heterojunction photodetectors were polycrystalline and hexagonal. PL measurements showed two emission peaks attributable to GaN films (374, 364, 359 nm) and the pSi substrate (730, 729, 729 nm). Shorter laser wavelengths raised the GaN/pSi heterojunction's optical energy gap from 3.271 to 3.3415 eV. FESEM pictures showed that the average size of the manufactured samples was 51.88, 36.12, and 22.76 nm for spherical nanoparticles and a cauliflower-like shape. Atomic force microscopy (AFM) results show the root mean square of the surface roughness of prepared samples to be 4.83, 8.87, and 14.51 nm, respectively, as the laser wavelength became shorter with full coverage of all pores of prepared pSi. GaN/pSi heterojunction photodetectors had rectification in their dark I–V characteristics, and the heterojunction made at 355 nm had the best junction characteristics. The spectral responsivity of GaN/pSi photodetectors shows that the maximum responsivity was 29.01 A/W at 370 nm for a photodetector prepared at 355 nm. High detectivity and external quantum efficiency were 8.02×10+12 Jones and 359.73% at 370 nm, respectively. The photodetector fabricated using 355 nm showed a fast rise time of 328μsec and a fall time of 617μsec.

High-performance Al-doped ZnO flexible ultraviolet photodetector via piezo-phototronic effect

Al-doped ZnO (AZO) flexible ultraviolet (UV) photodetectors were fabricated on polyethylene terephthalate substrates by radio frequency magnetron sputtering technique at room temperature. The single-layer AZO photodetector has a high photocurrent/dark current ratio and exhibits excellent photoresponse performance under UV illumination. When the tensile strain increases from 0 to 0.33, the photocurrent gradually increases, and the sensitivity and linear dynamic range increase by 10 times and 1.5 times, respectively. Under 23.5 mW/cm2 UV illumination at 4 V bias, the rise time and fall time are 0.2 and 0.3 s, respectively, showing that the AZO flexible UV photodetector has good reproducibility and stability. The energy band diagrams before and after applying tensile strain are analyzed to further study the interface modulation behavior. The results reveal that the piezo-phototronic effect has an important influence on the performance optimization and modulation of flexible UV photodetectors.

Properties and alcohol sensing applications of quasi-2D (PEA)2(MA)3Sb2Br9 thin films

We fabricated an alcohol detector based on (PEA)2(CH3NH3)3Sb2Br9 ((PEA)2MA3Sb2Br9) lead-free perovskite-like films. The XRD pattern revealed that the (PEA)2MA3Sb2Br9 lead-free perovskite-like films exhibited a quasi-2D structure. The optimal current response ratios are 74 and 84 for 5 and 15% alcohol solutions, respectively. When the amount of PEABr decreases in the films, the conductivity of the sample in ambient alcohol with a high alcohol concentration solution increases. The alcohol was dissolved into water and carbon dioxide due to the catalyst effect of the quasi-2D (PEA)2MA3Sb2Br9 thin film. The rise and fall times for the alcohol detector were 1.85 and 0.7 s, respectively, indicating that the detector was suitable.

Heterogeneous integration of a GaN-based photonic integrated circuit with an Si-based transimpedance amplifier.

The heterogeneous integration of a GaN-based photonic integrated circuit (PIC) and an Si-based transimpedance amplifier (TIA) is demonstrated in this work. The monolithic GaN PIC, fabricated from a GaN-on-Si light-emitting diode (LED) wafer, comprises LEDs whose optical outputs are coupled to photodetectors (PD) through suspended waveguides. The PIC chip is mounted onto a printed circuit board together with a TIA chip and two filter chip capacitors, occupying a compact footprint. The components are interconnected directly using wire-bonds to minimize signal delays and attenuation. The integrated system achieves rise and fall times of 2.21 and 2.10 ns, respectively, a transmission delay of 3.54 ns, and a bandwidth exceeding 390 MHz. Transmission of a pseudorandom binary sequence-3 (PRBS-3) signal across the integrated system is also demonstrated at the data transmission rate of 280 Mbit/s with a clearly resolved open eye diagram.

Self-powered (In,Ga)N-nanowire-based photodetector with fast response speed for under-seawater detection.

Due to the requirements of oceanography exploration and detection, self-powered photodetectors (PDs) with low-power consumption are essential for the next-generation optoelectronic applications. In this work, we successfully demonstrate a self-powered photoelectrochemical (PEC) PD in seawater based on the (In,Ga)N/GaN core-shell heterojunction nanowires. Compared to those of the PD in pure water, it is found that the upward and downward overshooting features of current can be the key reason contributing to the much faster response speed of the PD in seawater. Thanks to the enhanced response speed, the rise time of PD can be reduced more than 80%, and the fall time remains only 30% by applying in seawater instead of pure water. The key factors of generating these overshooting features should be the instantaneous temperature gradient, carrier accumulation and elimination on the semiconductor/electrolyte interfaces at the moments of light on and off. By the analysis of experimental results, the Na+ and Cl- ions are proposed to be the main factors affecting the PD behavior in seawater, which can enhance the conductivity and accelerate the oxidation-reduction reaction significantly. This work paves an effective way to develop the new self-powered PDs for the wide applications in under-seawater detection and communication.

Modeling Reset, Set, and Read Operations in Nanoscale Ge2Sb2Te5Phase‐Change Memory Devices Using Electric Field‐ and Temperature‐Dependent Material Properties

Herein, a finite element simulation framework for phase‐change memory devices that simultaneously solves for current continuity, electrothermal heating, and crystallization–amorphization dynamics using electrothermal models and dynamic material parameters that are functions of electric field and temperature is described. In this latest model, an electric field‐ and temperature‐dependent electrical conductivity model of stable amorphous Ge2Sb2Te5(GST) obtained from experiments performed on GST line cells to study Read, Reset, and Set operations of mushroom cells is incorporated. The effects of current polarity, heater height, Reset pulse rise and fall times, access device configuration, and ambient temperature are analyzed. The simulation results predict a 2x change in Reset current requirements with different current polarity due to thermoelectric effects. Heater height plays a significant role in thermal losses; ≈16% decrease in Reset current for 4x increase in the heater height is obtained. Increase in the ambient temperature results in a linear decrease in the Reset power required to achieve the same Reset/Set resistance contrast.

Towards the development of a self-powered vacuum ultraviolet photodetector based on calcium fluoride/gold interface

A calcium fluoride (CaF2) single crystal with asymmetric gold (Au) and aluminum (Al) electrodes is investigated as a photovoltaic sensor of high-energy vacuum ultraviolet (VUV) radiation. A photocurrent of at least 0.13nA is obtained at zero bias voltage, indicating that the sensor may be self-powered and can be operated without an external voltage source. The sensor is responsive only to wavelengths shorter than 130 nm, corresponding to photon energies greater than 9.5 eV. The 130-nm cutoff wavelength aligns with the transmission edge of the crystal, allowing it to screen out unwanted background UV and visible light without having to use filters. The 5.36 μs (fall time) time response of the sensor is similar to that of a commercial diamond UV sensor (5.05 μs). When operated in reverse- and forward-bias with an applied bias voltage ranging from –9 V to 9 V, the sensor exhibits rectification. In reverse-bias operation, increasing then decreasing the bias voltage leads to hysteresis which is attributed to surface defects at the CaF2/Au interface. As a self-powered filterless VUV sensor with a sufficiently fast response speed, the CaF2 sensor would be attractive for applications requiring high-power VUV light sources to be monitored.

A high‐voltage solid‐state Marx generator with adjustable pulse edges

AbstractMost reactors for non‐thermal plasma excitation are capacitive to pulse generators. The density and temperature of electrons in plasma are dominated by the discharging currents which are proportional to the pulse edges. However, adjusting the pulse edges is quite challenging since the switching speed and the inductance and capacitance in the discharging loops dominate them. This study proposes a drive circuit for solid‐state Marx generators with adjustable edges. The drive circuit controls the Miller plateau by adjusting the gate voltage amplitude to adjust both the rise time and fall time of pulses continuously and independently. The influence of the gate voltage on the Miller plateau and the rise time is studied. The feasibility is verified by both simulating and experimental results. An 8‐stage solid‐state Marx generator prototype was built, and 4.8‐kV pulses with a rise time in the range of 79 ns‐22.21 μs were obtained over capacitive loads. Adjusting pulse edges can control the current amplitudes in dielectric barrier discharge loads.