Vertical Devices Research Articles

(Invited) Wide Bandgap Semiconductor Based Devices for Digital and Industrial Applications

Wide bandgap (WBG) semiconductor devices based on SiC, GaN and Ga2O3 provide advantages for many strategic initiatives in various industrial sectors, such as data centers, electric vehicles, renewable energy systems like solar and wind power inverters. Innovations in WBG technologies would enable new paths on energy efficiency for pushing towards sustainable perspectives. For instance, higher efficiency and lower power dissipation are always desirable for digital economy/society and power electronic applications owing to the energy saving at the end with the goal to reduce CO2 emissions.Here we will emphasize our ongoing WBG semiconductor related research/development and innovation activities, especially on GaN technology owing its rapid/huge demanding in the market nowadays. Thus, the GaN material growth, device processing/packaging and characterization/verification will be elaborated in detail in this talk.In recent years an epitaxy technique to grow large diameter GaN-on-Si wafers by coalesced nanowire has been established at RISE ProNano Lab. The GaN nanowires are epitaxially grown and then merged into a planar epitaxial film with reduced dislocation density as compared to commercially available substrates. It allows for fabrication of thick GaN layers that required for high-voltage vertical devices on Si. A 4.1-µm-thick GaN device layer on a GaN-on-Si template has been grown successfully. It has a threading dislocation density on the order 108 cm-2 and a root mean square roughness of < 0.4 nm. This result has laid the foundation for scaling to the thicker (10-12 µm) crack-free GaN drift layer on lager wafer diameters up to 200 mm.RISE has long term experience working on GaN based High Electron Mobility Transistors (HEMTs). Since 2019 we have worked on high performance and cost-competitive power HEMTs on buffer-free GaN-on-SiC wafers in frame of EU UltimateGaN project. It has focused not only on verification of the high device performance but also on its yield for revealing its competitiveness in the cost-sensitive power electronics market. The fabricated HEMTs demonstrated the expected threshold voltage, good linearity with gate length and width, Ion/Ioff ratio of 1010 as well as high output current up to 16 A with multiple fingers. In such wafer level device fabrication, different types of multi-finger HEMTs could be included with designed electrodes for flip-chip soldering them into package modules. The power module was designed and produced in house by ceramic additive manufacturing (AM), also known as three-dimensional (3D) printing. The package design has aimed to address the stringent thermal and electrical requirements of these types of HEMTs for power electronic applications. Importantly the ceramic AM package design also enables the incorporation of intricate 3D features into the package structure for achieving increased electrical isolation distance between the source and drain contact pads of the HEMT in the package.In addition, SiC and Ga2O3 based sensors/devices will be briefed in this talk, which will provide a glance for design and fabrication of the hybrid WBG components for various greener applications.

Experimental measurement of stiffness coefficient of high-temperature graphite pebble fuel elements in helium at high temperatures

Graphite material plays an important role in nuclear reactors especially the high-temperature gas-cooled reactors (HTGRs) by its outstanding comprehensive nuclear properties. The structural integrity of graphite pebble fuel elements is the first barrier to core safety under any circumstances. The correct knowledge of the stiffness coefficient of the graphite pebble fuel element inside the reactor's core is significant to ensure the valid design and inherent safety. In this research, a vertical extrusion device was set up to measure the stiffness coefficient of the graphite pebble fuel element by the Institute of Nuclear and New Energy Technology (INET) of Tsinghua University in China. The stiffness coefficient equations of graphite pebble fuel elements at different temperatures are given (in a helium atmosphere). The result first provides the data on the high-temperature stiffness coefficient of pebbles in helium gas. The result will be helpful for the engineering safety analysis of pebble-bed nuclear reactors.

Experimental research on vertical straw cleaning and soil tillage device based on Soil-Straw composite model

Strip tillage is a necessary technical means to solve blockage problems during the no-tillage sowing process under the condition of total straw return to the field. However, the large amount of straw on the surface, thick stems, and slow decomposition make it difficult for the traditional straw wheel structure to effectively clean the straw out of the operating area. Based on the distribution of straw and soil characteristics on the no-tillage surface, a Soil-Straw composite model is established using the discrete element method (DEM) for undisturbed soil under straw coverage conditions. The relative error of its bonding parameters is 7.9 %, which can accurately predict the mechanical behavior of soil contact components. A vertical rotation-based straw strip cleaning and soil tillage method is proposed, and three vertical straw strip cleaning and soil tillage devices combining spring teeth (ST) and rake teeth (RT) were designed. The reasonable operating parameters are determined through trajectory analysis, with a forward velocity of 1.67 m/s and a rotational velocity of 300 r/min. Based on the Soil-Straw composite model, the combination of three ST and RT types is studied using DEM-MFBD coupled simulation technology. Regarding the straw cleaning rate, the straw cleaning rates of STb-RTb and STc-RTc are 96.66 % and 96.99 %, respectively, meeting the operational requirements. The field test results showed that the straw cleaning and soil fragmentation rates of the STc-RTc combination were 97.7 % and 96.4 %. The bench test and field test verification show that the DEM-MFBD coupled simulation technology based on the Soil-Straw model can predict the operating quality and deformation stress of the machine. This study provides a new solution for a wide range of straw cleaning in straw-covered areas.

Step Graded Floating Island Embedded Drift Design Engineering for High-Performance Vertical Power Devices

Step Graded Floating Island Embedded Drift Design Engineering for High-Performance Vertical Power Devices

Effect of Sn doping on low-temperature growth of Ge epilayers on half-metallic Co2FeSi

In this study, we demonstrate the low-temperature growth of a Ge layer via Sn doping in a ferromagnet/Ge/Co-based Heusler alloy multilayer on a Si(111) substrate having a vertically stacked structure. The continuous doping of 5% Sn into the Ge interlayer enables the epitaxial growth of the Ge layer on the Co-based Heusler alloy, even at ∼110 °C, and reduces the surface roughness. Elemental mapping shows that low-temperature growth via Sn doping suppresses interdiffusion at the Ge/Co-based Heusler alloy interface. The magnetic properties of the all-epitaxial CoFe/Ge/Co-based Heusler alloy structure are improved. For vertical spin-valve devices with Sn doping, the room-temperature spin signal is enhanced. These results indicate that the chemically abrupt Ge/Co-based Heusler alloy interface enhances the magnitude of the room-temperature spin signal owing to Sn-induced low-temperature growth.

Definition of parameters of loading modes of the test bench for tractor cabin’s vibration isolators

BACKGROUND: During the operation, tractor cabin’s vibration isolators are under the simultaneous load vertical, longitudinal- and lateral-angular vibrations of the cabin. These vibrations have different amplitudes, frequencies and time-domain behavior. It is possible to simulate comprehensive operational loads on vibration isolators only with bench testing. AIMS: Within this study, the trial to define parameters of loading modes for bench testing of tractor cabin’s vibrational isolators and to define corresponding parameters of drives of the bench loading devices was made. METHODS: This paper presents explanation and the results of calculation of loading modes parameters for testing of tractor cabin’s vibration isolator at the bench capable of producing loads from vertical, longitudinal- and lateral-angular vibrations of a tractor cabin simultaneously, as well as the results of calculation of corresponding parameters of drives of this bench. The procedure of preparation for testing was developed. This procedure includes estimation of values of operational loads amplitudes and frequencies, the range of change of necessary velocities and torques of electric drives of the bench loading devices and identification of adequate drives with required properties, determination of behavior of force and kinematic excitation reproduced by the test bench. Exemplary calculation of parameters of vertical loading at the considered bench was performed for the AKSS-400M rubber-metal vibration isolator used in cabin suspension of the K-708.4 wheeled tractor. A number of assumptions were made in the calculation. Nominal values of velocity and torque of the electric motor drive of the vertical loading device of the bench were obtained as the results. Thus, the electric motor with the reduction gear made by TRANSTECNO was selected. Characteristics of force and kinematic excitations reproduced by the bench using electric motor frequency control were built. RESULTS: Analysis of graphs helped to define acceptable range of frequencies reproduced by the bench for operational loads in vertical direction limited by abilities of the selected electric drive. CONCLUSIONS: The results of calculation of electric drive parameters can be used in design of the considered bench for testing of tractor cabin’s vibration isolators, particularly in the development of loading devices (cam eccentrics, pushers, return springs).

Two-dimensional analytical model for fully depleted SOI MOSFETs with vertical trapezoid doping including effects of the interface trapped charges

The two-dimensional (2D) potential distribution for vertical trapezoidal doping thin-body fully depleted (FD) silicon-on-insulator (SOI) devices is derived by adopting the evanescent mode analysis method, in which the 2D effects in gate oxide region, channel region and buried oxide region are taken into account. Moreover, the effects of interface trapped charge are considered. Using this potential model, the subthreshold performance of the device including subthreshold current, and subthreshold swing under various conditions have been studied. The result shows that the analytical model is good agreement with the simulated results. Therefore, it provides a feasible way of developing new 2D models for vertical trapezoidal doping thin-body FD SOI devices. Besides offering the physical insight into device physics, the analytical model provides the basic designing guidance for vertical trapezoidal doping thin-body FD SOI devices.

Mixed-flow pump cavitation characteristics extraction based on power spectrum density through pressure pulsation signal analysis

Mixed-flow pump is an important machine for transporting fluids, which has the advantages of both centrifugal pump and axial flow pump, but it is prone to cavitation, endangering efficiency and safe operation. A 20 m × 8.5 m high-precision hydraulic mechanical stand and a vertical mixed-flow pumping station device were built for the study of cavitation signals. The detection of pressure pulsation signal is a direct measurement of the cavitation region, and the decay of information is less. In this study, the time domain and frequency domain characteristics of water pressure pulsation signal are analyzed by using fast Fourier transform method. The power spectrum density (PSD) is used to extract the characteristics of each cavitation stage, especially the full cavitation stage with random signal characteristics. The results show that PSD method reflects more information and is more sensitive than traditional time–frequency domain analysis. The characteristic power spectrum (PS) value of the cavitation of the mixed-flow pump machinery can be given through the feature extraction of the PS, which plays a great role in the effective identification and prevention of cavitation.

Numerical Analysis of Friction-Filling Performance of Friction-Type Vertical Disc Precision Seed-Metering Device Based on EDEM

A seed-metering device is a key component for precision seeding and the core component of precision seed-metering devices. Nowadays, high-speed seeding is a trend in the development of seed-metering devices, but the filling performance of mechanical seed-metering devices decreases under the condition of high speed. Therefore, this paper explores a controllable method to improve the filling force of seeds, thereby increasing the limit operation speed of the existing mechanical seed-metering devices, so as to achieve high-speed seeding. The friction-filling method of friction vertical disc precision seed-metering devices was numerically simulated using the DEM. In this paper, the relationship between the relevant parameters and seed-filling force was confirmed via comparing theoretical formulas. The friction-filling method was studied via numerical simulation and experimental verification. This research demonstrated that during the process of friction filling, the pressure on the side wall of the tube increased with the e exponent with the change in the position of the particles. When the friction coefficient between the particles and the side wall is less than the friction coefficient between the particles, the e exponent increases. A surge occurs when the ratio of the square tube side length to the particle radius is n3+2(n=1,2,3).

Pelvis-Sacrum-Lumbar Spine Injury Characteristics From Underbody Blast Loading.

Combat-related injuries from improvised explosive devices occur commonly to the lower extremity and spine. As the underbody blast impact loading traverses from the seat to pelvis to spine, energy transfer occurs through deformations of the combined pelvis-sacrum-lumbar spine complex, and the time factor plays a role in injury to any of these components. Previous studies have largely ignored the role of the time variable in injuries, injury mechanisms, and warfighter tolerance. The objective of this study is to relate the time or temporal factor using a multi-component, pelvis-sacrum-lumbar spinal column complex model. Intact pelvis-sacrum-spine specimens from pre-screened unembalmed human cadavers were prepared by fixing at the superior end of the lumbar spine, pelvis and abdominal contents were simulated, and a weight was added to the cranial end of the fixation to account for torso effective mass. Prepared specimens were placed on the platform of a custom vertical accelerator device and aligned in a seated soldier posture. An accelerometer was attached to the seat platen of the device to record the time duration to peak velocity. Radiographs and computed tomography images were used to document and associate injuries with time duration. The mean age, stature, weight, body mass index, and bone density of 12 male specimens were as follows: 65 ± 11 years, 1.8 ± 0.01 m, 83 ± 13 kg, 27 ± 5.0 kg/m2, and 114 ± 21 mg/cc. They were equally divided into short, medium, and long time durations: 4.8 ± 0.5, 16.3 ± 7.3, and 34.5 ± 7.5 ms. Most severe injuries associated with the short time duration were to pelvis, although they were to spine for the long time duration. With adequate time for the underbody blast loading to traverse the pelvis-sacrum-spine complex, distal structures are spared while proximal/spine structures sustain severe/unstable injuries. The time factor may have implications in seat and/or seat structure design in future military vehicles to advance warfighter safety.

Research on vertical GaN devices based on gradient Al components

A groove-gate power device with a linearly gradient Al composition P-type AlGaN superjunction (abbreviated as LG-SJCAVET) is proposed, which uses polarized P-type AlGaN material instead of the traditional P-type GaN buried layer, avoiding the technical bottleneck of achieving high-aspect-ratio P-type doping in GaN materials. Simulation results show that, under the same structural parameters, the breakdown voltage of LG-SJCAVET reaches 2954V, and the on-resistance is 1.669 mΩ⋅cm2, which is due to no current flowing through the P-pillar, resulting in an increase in on-resistance, while affecting the DC characteristics of the device. The power figure of merit of the device is 5.27 GW/cm2, which is 67.11% and 27.38% higher than that of the traditional device and P-type GaN buried layer device, respectively. Therefore, the LG-SJCAVET device solves the problems of the high technical difficulty of the traditional P-type GaN buried layer process, difficulty in activating acceptor impurities, and poor thermal stability of the device, exhibiting superior breakdown voltage characteristics.

Improved oxygen ion migration efficiency and resistive switching properties in SrFeOx memristor with vertical superlattice-like structure

The resistive switching (RS) behaviour between high and low resistance states in SrFeOx memristors originates from topological phase transition (TPT) induced by the migration of oxygen ions. Accelerating the migration of oxygen ions in the anisotropic crystal structure of SrFeO2.5 is of great importance for improving the RS properties and promoting the application for this TPT memristor. In this study, SrFeOx TPT memristors with different superlattice-like (SLL) structures were fabricated and the prepared devices with a vertical-type SIL structure display an ultra-low operation voltage of only 0.6 V, excellent cycling stability, data retention, and endurance performance. Moreover, the microstructures of vertical and horizontal SLL SrFeO2.5 were characterized with X-ray diffraction and transmission electron microscope. Furthermore, the differences in oxygen ion migration rate in the two structures were also tested by in-situ X-ray photoelectron spectroscopy. Finally, the optical band gaps and oxygen ion migration barriers of the two structures were calculated using first principles. All the characterizations and simulation results prove that the vertical type SrFeOx shows a higher oxygen ion migration efficiency. More encouragingly, the vertical SLL devices exhibit up to 96 % recognition accuracy for the Mixed National Institute of Standards and Technology image recognition task.

1.7-kV vertical GaN p-n diode with triple-zone graded junction termination extension formed by ion-implantation

Edge termination has emerged as an important area in the design and realization of vertical GaN power electronic devices. While the material properties of GaN are promising for high-performance devices, in practice the breakdown voltage can be compromised by inadequate edge termination (ET). While solutions in other materials (e.g. Si, SiC) are well-known, these are challenging to implement in GaN due to inherent difficulties in p-type doping GaN. In this work, we report a etch-free triple-zone graded junction termination extension (JTE) for vertical GaN diodes formed by Nitrogen ion implantation. The triple-zone design offers lower peak fields for effective field control. In addition, the proposed triple-zone JTE is beneficial for increasing the fabrication process window and allowing for more variability in epitaxial wafer growth in terms of p-GaN doping and thickness while maintaining high breakdown voltage. The fabricated GaN p-n diodes with triple-zone JTE obtain a maximum breakdown voltage of 1.73 kV with specific on-resistance Ron of 0.4 mΩcm2. Temperature-dependent reverse characteristics show that the devices have a positive temperature coefficient of the breakdown voltage indicating an avalanche breakdown mechanism. These results suggest that vertical GaN p–n diodes with N-implanted triple-zone JTE are promising for power applications.

Graphene/aluminum oxide interfaces for nanoelectronic devices

In this work, we study theoretically and experimentally graphene/aluminum oxide interfaces as 0D/2D interfaces for quantum electronics as the nature of the interface is of paramount importance to understand the quantum transport mechanism. Indeed, the electronic transport is driven either by a channel arising from a strong hybridization at the interface, or by tunneling across a van der Waals interface, with very different electric characteristics. By combining electronic spectroscopy and scanning microscopy with density functional theory calculations, we show that the interface is of weak and van der Waals nature. Quantum transport measurements in a single electron transistor confirm this result. Our results provide a first insight into the interfacial properties van der Waals materials based single electron device, and the key role played by the control of the interface states. The weak van der Waals coupling reported is promising for single electron device, where the control of the environmental charges is known to be a key challenge towards applications. Moreover, the unique vertical device architecture, enabled by the dual role of graphene including its vertical electric field transparency, opens the doors for a new class of single electron devices with higher scaling capability and functionalities. This work paves the way to new atomic environment control in single electron device.

Vertical GaN MOSFET Power Devices.

Gallium nitride (GaN) possesses remarkable characteristics such as a wide bandgap, high critical electric field, robust antiradiation properties, and a high saturation velocity for high-power devices. These attributes position GaN as a pivotal material for the development of power devices. Among the various GaN-based devices, vertical GaN MOSFETs stand out for their numerous advantages over their silicon MOSFET counterparts. These advantages encompass high-power device applications. This review provides a concise overview of their significance and explores their distinctive architectures. Additionally, it delves into the advantages of vertical GaN MOSFETs and highlights their recent advancements. In conclusion, the review addresses methods to enhance the breakdown voltage of vertical GaN devices. This comprehensive perspective underscores the pivotal role of vertical GaN MOSFETs in the realm of power electronics and their continual progress.

Proposal for a practical MFC using vertical electrodes with improved air cathode function

In this study, we proposed a low-cost vertical electrode MFC device using Bokuju and smoked charcoal and evaluated its air cathode function by varying the cathode area. The vertical design of the device ensures contact between the cathode electrode and the air, easing the installation of the device and the replacement of the electrode. The output increase was also confirmed by the cathode, accompanied by the sterilizing effect of copper and reliable contact with air. Unfortunately, there was a slight improvement in air cathode function even when the area of the cathode exposed to air was wider than 100 mm2. However, when the device was inserted into a new soil of the same material after the first operation, the power density reached the same level as the first time. Thus, this device is nearing practical application by improving its long-term stability and increasing output power.

(Invited) Benchmarking of Beyond the State-of-the-Art Vertical GaN Devices

In this paper theoretical benchmarking of semi-vertical and vertical gallium nitride (GaN) MOSFETs with rated voltage of 1.2 kV to 3.3 kV is performed against silicon carbide (SiC) devices. Limitations of the semi-vertical and vertical state-of-the-art GaN structures have been investigated by simulations. Specific design features and technology requirements for realization of high voltage vertical GaN MOSFETs are discussed and implemented in simulated structures. The main modifications to the structures are reduced cell pitch and introduction of electric field shielding implantation and thick oxide at the gate trench bottom and p-type implanted junction termination. The main findings are: (a) specific on-resistance of vertical GaN devices is 75% and 40% of that for 1.2 kV and 3.3 kV SiC MOSFETs, respectively, (b) semi-vertical GaN show no advantage over SiC MOSFETs for medium and high voltage devices over 1 kV, (c) vertical GaN has potential advantage for high and ultra-high voltage devices.

Electrical characteristics of in situ Mg-doped β-Ga2O3 current-blocking layer for vertical devices

The lack of p-type doping has impeded the development of vertical gallium oxide (Ga2O3) devices. Current blocking layers (CBLs) using implanted deep acceptors have been used to demonstrate vertical devices. This paper presents a pioneering demonstration of in situ Mg-doped β-Ga2O3 CBLs grown using metal–organic chemical vapor deposition. The Mg-doping density during growth was calibrated by quantitative secondary ion mass spectroscopy. Electrical test structures were designed with in situ Mg doped layers with various targeted Mg doping concentrations. The effectiveness of the CBL is characterized by using temperature-dependent current–voltage measurements using n-Mg-doped-n structures, providing crucial insight into the underlying mechanisms. Pulsed measurements show similar blocking characteristics as DC. To further validate the experimental results, a TCAD simulation is performed, and the electrically active effective doping is found to be dependent on the Mg-doping density, offering an alternate perspective on the optimization of CBL performance. Breakdown measurements show a peak 4 MV/cm field strength.

Development and analysis of thick GaN drift layers on 200 mm CTE-matched substrate for vertical device processing

This work reports the epitaxial growth of 8.5 µm-thick GaN layers on 200 mm engineered substrates with a polycrystalline AlN core (QST by QROMIS) for CMOS compatible processing of vertical GaN power devices. The epitaxial stack contains a 5 upmum thick drift layers with a Si doping density of 2 × 1016 cm−3 and total threading dislocation density of 4 × 108 cm−2. The thick drift layer requires fine-tuning of the epitaxial growth conditions to keep wafer bow under control and to avoid the formation of surface defects. Diode test structures processed with this epitaxial stack achieved hard breakdown voltages > 750 V, which is shown to be limited by impurity or metal diffusion from the contact metal stack into threading dislocations. Conductive Atomic Force Microscopy (cAFM) reveals some leakage contribution from mixed type dislocations, which have their core structure identified as the double 5/6 atom configuration by scanning transmission electron microscopy images. Modelling of the leakage conduction mechanism with one-dimensional hopping conduction shows good agreement with the experimental data, and the resulting fitting parameters are compared to similar findings on silicon substrates. The outcome of this work is important to understand the possibilities and limitations of vertical GaN devices fabricated on large diameter wafers.

Recognition of airfoil wake structure based on deep learning

Inspired by the perception ability of flows by organs of aquatic organisms, an identification method of airfoil wake structure based on deep learning is proposed. The wake structure of a NACA0012 airfoil at different Reynolds numbers and angles of attack collected by vertical soap film experimental device is used as the training object. A convolutional neural network is established to extract the features of wake structure of the airfoil. Bivariate identification, including the Reynolds number and angle of attack, is realized with high accuracy. Firstly, the variation law of airfoil wake structure is analyzed, and the loss function and the accuracy of verification set of three convolution neural networks with different structures are compared under the condition of small samples. The network with higher recognition ability is selected as the training model. Then, the data set is divided into training set, verification set and testing set with the ratio 3:1:1. The test set that does not participate in the training is only used for the evaluation, and the test results are expressed by a confusion matrix. The results show that the overall recognition accuracy of the model is above 92%.