Vertical Devices Research Articles

Dark Current Suppression in Two‐dimensional Histamine Lead Iodine Perovskite Single Crystal for X‐ray Detection and Imaging

AbstractLead halide perovskites have emerged as attractive X‐ray detector materials, owing to properties such as strong X‐ray stopping power, excellent carrier transport, and high sensitivity. Additionally, they can be easily prepared by using solution‐based synthesis approaches. However, traditional 3D (three‐dimensional) perovskites X‐ray detectors have shown limited application due to high dark currents generated under bias voltage as a result of strong ion migration. In this work, an X‐ray detector with a vertical structure device is demonstrated using 2D (two‐dimensional) histamine lead halide perovskite single crystal HAPbI4 (HPI, HA = histamine). Due to the dielectric screening effect of diamine and the vertical structure of the HPI device, the fabricated detector shows a sensitivity of 7737 µC Gyair−1 cm−2 under a bias voltage of 30 V. Furthermore, the detector shows a sensitivity of 293 µC Gyair−1 cm−2 and detection limit of 51.38 nGyair s−1 without bias voltage, wherein the dark current is almost completely suppressed. All of these properties indicate the X‐ray detection device is a promising candidate for next‐generation optoelectronic applications.

Nitrogen-doped Ga2O3 current blocking layer using MOCVD homoepitaxy for high-voltage and low-leakage Ga2O3 vertical device fabrication

In this Letter, a high-quality and high-resistivity nitrogen (N)-doped Ga2O3 current blocking layer (CBL) is grown utilizing metal-organic chemical vapor deposition homoepitaxial technology. By using nitrous oxide (N2O) as oxygen source for Ga2O3 growth and N source for doping and controlling the growth temperature, the grown CBL can effectively achieve high (∼1019 cm−3) or low (∼1017 cm−3) N doping concentrations, as well as high crystal quality. Furthermore, the electrical properties of the developed CBL are verified at the device level, which shows that the device using the CBL can withstand bidirectional voltages exceeding 3.5 kV with very low leakage (≤1 × 10−4 A/cm2). This work can pave the way for the realization of high-voltage and low-leakage Ga2O3 vertical devices, especially metal-oxide-semiconductor field effect transistors.

Giant Bulk Photovoltaic Power Generation in 2D AgBiP2Se6 Crystals

AbstractWith the theoretical high power conversion efficiency (PCE) exceeding the thermodynamic Schockley–Queisser (S–Q) limit, the bulk photovoltaic (BPV) effect, which is manifested in acentric crystals across the entire visual spectrum, holds great potential for next‐generation light‐harvesting devices. Here the abnormally giant second harmonic generation (SHG) activity and BPV response in 2D AgBiP2Se6 crystals with broken inversion symmetry is demonstrated, which can be remarkably enhanced with the thickness downscaling. Based on the vertical device sandwiched with graphene electrodes, the wideband light absorption of 2D AgBiP2Se6 crystals with Eg of 1.49 eV enables giant BPV response in the entire visible spectrum, and efficient utilization of solar energy contributes to large short‐current (≈330 mA cm−2), high PCE (≈0.13%) and EQE (≈12.5%) under 532 nm light illumination. Furthermore, benefitting from the local ion redistribution driven by an in‐plane electric field, the BPV photocurrent generation is effectively enhanced 3 times, yielding an extremely high PCE of 0.18%, according high among the reported 2D acentric crystals. The study reintroduces AgBiP2Se6 to the 2D acentric crystal family and lays the groundwork to develop BPV devices for light‐harvesting applications.

Bulk Single Crystals and Physical Properties of Rutile GeO2 for High‐Power Electronics and Deep‐Ultraviolet Optoelectronics

The top‐seeded solution growth for rutile GeO2 single crystals using alkali carbonates or fluorides as flux is applied. Structural data of obtained single crystals confirm the rutile phase with a = b = 4.3966 Å and c = 2.8612 Å. The crystals with diameter of 5–15 mm are either undoped or intentionally doped with Sb5+, Sn4+, Al3+, Ga3+, and F− ions. It is found that Sb5+ is a very efficient n‐type donor enabling free electron concentration even above 1020 cm−3; thus, Sb‐doped GeO2 is a potential substrate for vertical power devices. In contrast, crystals doped with Al and Ga do not show p‐type conductivity suggested by the theory. The onset of the absorption occurs at 5.0 and 5.5 eV perpendicular and parallel to the c‐axis, respectively. Rutile GeO2 shows a very intense photoluminescence peaking at 420 nm (blue) and 520 nm (green). Raman spectra show narrow lines, in particular at high phonon energy (B1g, 170 cm−1). Prepared wafers show FWHM values of rocking curves below 30 arcsec and polishing is achieved down to RMS roughness of 0.15 nm. Transmission electron microscopy images do not show point or extended structural defects with uniform Sb distribution.

III–V heterostructure tunnel field-effect transistor operation at different temperature regimes

Tunnel field-effect transistors (TFETs) are a potential alternative to MOSFETs for low-temperature electronics. We provide an in-depth experimental characterization of TFETs analyzing the fundamental physical behavior at different temperature regimes. TFET characteristics from 13 to 300 K both in forward and reverse bias are discussed by employing a variation in InAs/InGaAsSb/GaSb heterojunction vertical nanowire devices. Evaluation of the TFET Negative Differential Resistance (NDR) characteristics at different temperatures is established as a technique to probe the dopant incorporation. It is observed that the temperature dependence of the Fermi degeneracy and Fermi-Dirac distribution largely influences the transistor performance at each operating temperature. Our investigation reveals that the TFETs demonstrate lower subthreshold swing than the physical limit of MOSFETs above 125 K. For low-temperature applications, the devices can be operated down to a low operating bias of 0.1 V, while for high temperature, a larger bias of 0.3 V is preferred.

200 cm2/Vs electron mobility and controlled low 1015 cm−3 Si doping in (010) β -Ga2O3 epitaxial drift layers

We report on metalorganic chemical vapor deposition (MOCVD) growth of controllably Si-doped 4.5 μm thick β-Ga2O3 films with electron concentrations in the 1015 cm−3 range and record-high room temperature Hall electron mobilities of up to 200 cm2/Vs, reaching the predicted theoretical maximum room temperature phonon scattering-limited mobility value for β-Ga2O3. Growth of the homoepitaxial films was performed on Fe-doped (010) β-Ga2O3 substrates at a growth rate of 1.9 μm/h using TEGa as the Gallium precursor. To probe the background electron concentration, an unintentionally doped film was grown with a Hall concentration of 3.43 × 1015 cm−3 and Hall mobility of 196 cm2/Vs. Growth of intentionally Si-doped films was accomplished by fixing all growth conditions and varying only the silane flow, with controllable Hall electron concentrations ranging from 4.38 × 1015 to 8.30 × 1015 cm−3 and exceptional Hall mobilities ranging from 194 to 200 cm2/Vs demonstrated. C-V measurements showed a flat charge profile with the ND+–NA− values correlating well with the Hall-measured electron concentration in the films. SIMS measurements showed the silicon atomic concentration matched the Hall electron concentration with carbon and hydrogen below detection limit in the films. The Hall, C-V, and SIMS data indicate the growth of high-quality 4.5 μm thick β-Ga2O3 films and controllable doping into the mid 1015 cm−3 range. These results demonstrate MOCVD growth of electronics grade record-high mobility, low carrier density, and thick β-Ga2O3 drift layers for next-generation vertical β-Ga2O3 power devices.

Modelling of Indoor Air Quality and Thermal Comfort in Passive Buildings Subjected to External Warm Climate Conditions

Air renewal rate is an important parameter for both indoor air quality and thermal comfort. However, to improve indoor thermal comfort, the air renewal rate to be used, in general, will depend on the outdoor air temperature values. This article presents the modelling of indoor air quality and thermal comfort for occupants of a passive building subject to a climate with warm conditions. The ventilation and shading strategies implemented for the interior spaces are then considered, as well as the use of an underground space for storing cooled air. The indoor air quality is evaluated using the carbon dioxide concentration, and thermal comfort is evaluated using the Predicted Mean Vote index. The geometry of the passive building, with complex topology, is generated using a numerical model. The simulation is performed by Building Thermal Response software, considering the building’s geometry and materials, ventilation, and occupancy, among others. The building studied is a circular auditorium. The auditorium is divided into four semi-circular auditoriums and a central circular space, with vertical glazed windows and horizontal shading devices on its entire outer surface. Typical summer conditions existing in a Mediterranean-type environment were considered. In this work, two cases were simulated: in Case 1, the occupation is verified in the central space and the four semi-circular auditoriums and all spaces are considered as one; in Case 2, the occupation is verified only in each semi-circular auditorium and each one works independently. For both cases, three strategies were applied: A, without shading and geothermal devices; B, with a geothermal device and without a shading device; and C, with both shading and geothermal devices. The airflow rate contributes to improving indoor air quality throughout the day and thermal comfort for occupants, especially in the morning. The geothermal and shading devices improve the thermal comfort level, mainly in the afternoon.

High drain field impact ionization transistors as ideal switches

Impact ionization effect has been demonstrated in transistors to enable sub-60 mV dec−1 subthreshold swing. However, traditionally, impact ionization in silicon devices requires a high operation voltage due to limited electrical field near the device drain, contradicting the low energy operation purpose. Here, we report a vertical subthreshold swing device composed of a graphene/silicon heterojunction drain and a silicon channel. This structure creates a low voltage avalanche impact ionization phenomenon and leads to steep switching of the silicon-based device. Experimental measurements reveal a small average subthreshold swing of 16 µV dec−1 over 6 decades of drain current and nearly hysteresis-free, and the operating voltage at which a vertical subthreshold swing occurs can be as low as 0.4 V at room temperature. Furthermore, a complementary silicon-based logic inverter is experimentally demonstrated to reach a voltage gain of 311 at a supply voltage of 2 V.

A Study on the Channel Holes’ Diameter Effects of High-Performance Vertical-Channel Flash Memory Cells

Abstract High-performance vertical-channel flash (HVF) memory cells were fabricated on the single crystalline Si (c-Si) sidewalls of the cylindrical deep wells in c-Si substrate. To investigate the diameter effects of the cylindrical deep wells, namely channel holes, on HVF cells, the channel holes with different diameters, ranging from 65 nm to 260 nm, were made. Memory gate stacks of SiO2/ Al2O3/HfO2/Al2O3/TiN/W were formed by ozone oxidation and then ALD with the deposition thicknesses of 1/5/7/8/2/150 nm, respectively. For the devices with their diameters equal to or greater than 150 nm, their electrical properties, such as Vt, SS, DIBL, and program/erase characteristics, are close. As expected, DIBL and SS become better as the diameter increasing due to better gate control with larger diameter. However, large changes were occurred for the devices with the diameters of 90 nm and 65 nm. A simple model based on cylinder bulk for vertical flash memory devices was presented to obtain an approximate analytical solution for depletion-width and explain our experimental data. For the devices with the diameters of 150 nm, the high On/Off current ratio of 107 and relatively large memory window of 4.5 V were achieved. However, programming/erasing efficiency were degraded with hole diameter decreasing.

Epitaxial growth of GaN on β-Ga2O3 via RF plasma nitridation

The lack of suitable p-type dopant for β-Ga2O3 remains a hurdle for vertical power device applications. Epitaxy of GaN on Ga2O3 substrates was demonstrated as an alternative. (–201)-oriented β-Ga2O3 was converted into (0001)-oriented hexagonal GaN via nitrogen plasma in a plasma-assisted molecular beam epitaxy chamber, as verified by XRD and RHEED. The resulting nitridated GaN layers were characterized by TEM, x-ray reflectivity, and AFM to relate the nitridation conditions to crystallinity, layer thickness, and surface roughness. The crystallinity of subsequently grown epitaxial GaN films was quantified via XRD rocking curves and related to the nitridation layer properties across varying nitridation conditions. Specifically, the effect of the grain size and nitridation layer thickness was investigated to determine their role in threading screw dislocation management.

Study on the influence law of flooding phenomenon in the killing operation of empty wells

Flooding in the process of well killing in empty wells results in a waste of kill fluid and increases the time of well killing, which is not conducive to its smooth operation. To address this problem, a large-size vertical visualization experimental device was designed and constructed, falling experiments were performed on kill fluid under different apparent gas velocities, apparent liquid velocities, and kill-fluid viscosities, and the gas–liquid two-phase migration state at the injection point during the process of flooding was analyzed in this study. Commonly used critical gas velocity models of flooding were compared, considering the influence of fluid properties and rheological parameters on the critical gas velocity. A combination of dimensionless numbers was used to fit the experimental data, and a model of the critical gas velocity of flooding was established. The comparison results showed that the prediction accuracy of the relationship in this study was higher than that of the traditional relationship, with an average error of 9.89%. The influence of different parameters on the critical gas velocity of flooding was analyzed, and the key parameters for well killing in empty wells were optimized, providing a theoretical basis for empty well killing operations.

Bio‐Voltage Diffusive Memristor from CVD Grown WSe2 as Artificial Nociceptor

AbstractMemristors have emerged as a promising candidate to mimic the human behavior and thus unlocking the potential for bio‐inspired computing advancement. However, these devices operate at a voltages which are still far from the energy‐efficient biological counterpart, which uses an action potential of 50–120 mV to process the information. Here, a diffusive memristor is reported from synthetic WSe2 fabricated in Ag/WSe2/Au vertical device geometry. The devices operate at bio‐voltages of 40–80 mV with Ion/Ioff ratio of 106 and steep switching turn ON and OFF slopes of 0.77 and 0.88 mV per decade, respectively. The power consumption in standby mode and power per set transition are found to be 10 fW and 64 pW, respectively. Further, the diffusive memristors are utilized to emulate the nociceptor, a special receptor for sensory neurons that selectively responds to noxious stimuli. Nociceptor in turn imparts a warning signal to the central nervous system which then triggers the motor response to take precautionary actions to prevent the body from injury. The key features of a nociceptor including “threshold”, “relaxation”, “no‐adaptation” and “sensitization” are demonstrated using artificial nociceptors. These illustrations imply the feasibility of developing low‐power diffusive memristors for bio‐inspired computing, humanoid robots, and electronic skins.

Analysis of Power Losses and Experimental Method for Determining Resistance in Electric Elevators

To be able to carry out a high-quality analysis of the operation and design of elevator systems with a driving pulley, it is necessary to determine the size of the losses that occur in the drive mechanism and elevator guide rails. The paper defines an experimental procedure for determining the losses and the efficiency coefficient of elevator facilities depending on the relative load of the cabin in operational conditions. The experiment was carried out on a passenger elevator with a rated load of 320 kg and a lifting height of 28 m. An analysis of the resistance, i.e., the source of power losses in vertical lifting devices, was performed, the most significant of which occur and are especially pronounced in worm gear reducers and on the guiding system of the cabin and counterweight, where the type of guiding elements also has an influence. A particular problem is expressed due to the changing state of the contact surfaces (between the guide rails and the guide shoes) during exploitation and the eccentric loading of the cabin. Also, the paper analysed the obtained measurement results to determine the dependence of the efficiency coefficient of the facility concerning the relative load of the cabin.

Experimental study on energy and hydraulic performance of the axial flow pumping device with bell-shaped inlet channel

Abstract Due to the unique inlet design of the axial flow pump device with a bell shaped inlet channel, there is currently insufficient quantitative research on its hydraulic characteristics. This study investigates the hydraulic behavior of a vertical axial flow pumping device equipped with a bell-shaped inlet channel, analyzing various blade angles. Energy, cavitation, runaway, and pressure pulsation characteristics were assessed through meticulous testing on a high-precision test bench. Experimental findings indicate that altering the blade angle of the vertical axial flow pumping device with a bell-shaped inlet channel can influence its overall efficiency to some extent. Moreover, as the pump’s blade angle increases incrementally, the device’s cavitation performance gradually diminishes. Pressure pulsation tests reveal a relatively minor amplitude of pressure pulsation at the impeller outlet, contrasting with a more pronounced amplitude observed at the guide vane outlet compared to the pump outlet. These research results provide valuable insights for the design and optimization of actual pumping stations.

Study on flow characteristics and structural properties of vertical axial pump with low guide vane height

Abstract With the full construction of the South to North Water Diversion Project and the renovation of pumping stations, vertical axial flow pump devices have been widely used in various pumping station projects. This article uses the streamlined method to design the impeller and guide vanes of a vertical axial flow pump device. To analyze the hydraulic and structural characteristics, this paper uses CFD numerical simulation and fluid-structure interaction calculation methods. And the trend of output power change under different flow conditions was divided into three stages, elucidating the reasons for the sudden change in output power under flow conditions of 0.8Q-1.0Q. This article focuses on analyzing the internal flow state of the designed lower height guide vane segment to explore the problem of low guide vane height that is prone to occur during the design process. According to the research results, it is found that the lower height guide vane can also play a good role in stabilizing flow and recovering circulation under certain operating conditions. This article also studied the structural characteristics of the impeller and guide vanes, explaining the stress and strain distribution under design flow conditions.

High-Performance Broadband Photodetectors Combining Perovskite and Organic Bulk Heterojunction Bifunctional Layers

Perovskite can be used to prepare high-performance photodetectors due to its excellent optical properties. However, the detection range of perovskite photodetectors is mostly limited to the visible light range, restricting their further development and application. In recent years, combining perovskite with organic bulk heterojunctions to prepare photodetectors with broadband detection capability has proven to be an effective strategy. Through this approach, the response spectrum of the photodetector can be flexibly regulated, and organic compounds can improve the perovskite film quality by passivating defects and inhibit the penetration of water molecules in the air, thereby improving the device performance and stability. In this work, we propose and demonstrate the feasibility of combining MAPbI3 perovskite with PTB7-Th:COTIC-4F to prepare high-performance photodetectors with wide spectral response characteristics. With the assistance of an organic bulk heterojunction, the defects of perovskite crystals are effectively passivated, and the detection spectrum of the device is successfully extended to about 1100 nm. As a result, the responsivity achieved is 0.58 A/W, 1.19 A/W, and 1.41 A/W under laser illumination of 532 nm, 808 nm, and 980 nm, with the power density of 5 μW/cm2 at the bias voltage of −0.5 V, respectively, which is one of the best performances among vertical device structures of this type. Moreover, the stability of the final hybrid film has been greatly improved. This work provides a new approach to the preparation of high-performance and broadband perovskite photodetectors.

A Novel Isolation Approach for GaN-Based Power Integrated Devices.

This paper introduces a novel technology for the monolithic integration of GaN-based vertical and lateral devices. This approach is groundbreaking as it facilitates the drive of high-power GaN vertical switching devices through lateral GaN HEMTs with minimal losses and enhanced stability. A significant challenge in this technology is ensuring electrical isolation between the two types of devices. We propose a new isolation method designed to prevent any degradation of the lateral transistor's performance. Specifically, high voltage applied to the drain of the vertical GaN power FinFET can adversely affect the lateral GaN HEMT's performance, leading to a shift in the threshold voltage and potentially compromising device stability and driver performance. To address this issue, we introduce a highly doped n+ GaN layer positioned between the epitaxial layers of the two devices. This approach is validated using the TCAD-Sentaurus simulator, demonstrating that the n+ GaN layer effectively blocks the vertical electric field and prevents any depletion or enhancement of the 2D electron gas (2DEG) in the lateral GaN HEMT. To our knowledge, this represents the first publication of such an innovative isolation strategy between vertical and lateral GaN devices.

Sliding Ferroelectricity Induced Ultrafast Switchable Photovoltaic Response in ε-InSe Layers.

2D sliding ferroelectric semiconductors have greatly expanded the ferroelectrics family with the flexibility of bandgap and material properties, which hold great promise for ultrathin device applications that combine ferroelectrics with optoelectronics. Besides the induced different resistance states for non-volatile memories, the switchable ferroelectric polarizations can also modulate the photogenerated carriers for potentially ultrafast optoelectronic devices. Here, it is demonstrated that the room temperature sliding ferroelectricity can be used for ultrafast switchable photovoltaic response in ε-InSe layers. By first-principles calculations and experimental characterizations, it is revealed that the ferroelectricity with out-of-plane (OOP) polarization only exists in even layer ε-InSe. The ferroelectricity is also demonstrated in ε-InSe-based vertical devices, which exhibit high on-off ratios (≈104) and non-volatile storage capabilities. Moreover, the OOP ferroelectricity enables an ultrafast (≈3ps) bulk photovoltaic response in the near-infrared band, rendering it a promising material for self-powered reconfigurable and ultrafast photodetector. This work reveals the essential role of ferroelectric polarization on the photogenerated carrier dynamics and paves the way for hybrid multifunctional ferroelectric and optoelectronic devices.

Optimization of non-spherical particle flow in vertical waste heat recovery devices: Analyzing structural configurations

Vertical waste heat recovery devices are emerging but are immature. The vertical waste heat recovery devices of circular, square, and optimized designs were analyzed for a full capacity of 8 tons of sintered ore. Using DEM theory, the effects of particle size distribution, velocity, and wall forces on void fraction distribution and particle flow were examined. The circular device demonstrated superior mixed particle flow compared to the square device, achieving integrated flow for 50 % of particles in the cone, in contrast to 26 % for the square device. An optimized circular device achieved integral flow for 90 % of particles, significantly enhancing overall flow and reducing wall adhesion, attributed to its improved internal design. Through insert and distributor design, the overall particle flow within the device improved, with almost no particles remaining on the walls. These findings underscore the potential for enhanced heat recovery efficiency through optimized internal structures in waste heat recovery devices.

Switching figure-of-merit, optimal design, and power loss limit of (ultra-) wide bandgap power devices: A perspective

Power semiconductor devices are utilized as solid-state switches in power electronics systems, and their overarching design target is to minimize the conduction and switching losses. However, the unipolar figure-of-merit (FOM) commonly used for power device optimization does not directly capture the switching loss. In this Perspective paper, we explore three interdependent open questions for unipolar power devices based on a variety of wide bandgap (WBG) and ultra-wide bandgap (UWBG) materials: (1) What is the appropriate switching FOM for device benchmarking and optimization? (2) What is the optimal drift layer design for the total loss minimization? (3) How does the device power loss compare between WBG and UWBG materials? This paper starts from an overview of switching FOMs proposed in the literature. We then dive into the drift region optimization in 1D vertical devices based on a hard-switching FOM. The punch-through design is found to be optimal for minimizing the hard-switching FOM, with reduced doping concentration and thickness compared to the conventional designs optimized for static FOM. Moreover, we analyze the minimal power loss density for target voltage and frequency, which provides an essential reference for developing device- and package-level thermal management. Overall, this paper underscores the importance of considering switching performance early in power device optimization and emphasizes the inevitable higher density of power loss in WBG and UWBG devices despite their superior performance. Knowledge gaps and research opportunities in the relevant field are also discussed.