High Barrier Height Research Articles

Achieving ultralow leakage current in Schottky-MIS cascode anode lateral field-effect diode based on AlGaN/GaN HEMT

In this paper, we design and fabricate a Schottky-metal-insulator-semiconductor (MIS) cascode anode GaN lateral field-effect diode (CA-LFED) to achieve ultralow reverse leakage current (ILEAK). The device based on AlGaN/GaN high-electron-mobility-transistor (HEMT) includes a normally-off MIS-controlled channel that is cascoded with a high barrier height Schottky contact. At reverse bias, the high electric-field is effectively prevented by the normally-off MIS-controlled channel edge. Together with the high barrier height Schottky contact, this feature significantly suppresses the ILEAK. Supported by the device fabrication, the CA-LFED with high breakdown voltage (BV) > 600 V shows an ultralow ILEAK of 3.6 × 10−9 A/mm as well as a low forward voltage drop (VF) of 2.2 V. The performance suggests that the CA-LFED can be a promising candidate for ultralow ILEAK and better VF-ILEAK trade-off GaN power diode applications.

Contact Resistance Reduction of MoS2-Based Field Effect Transistor Using Semimetallic TiS2 Contact Layer Deposited By Atomic Layer Deposition

Recently, two-dimensional transition metal dichalcogenides (2D TMDCs) have been researched as promising channel materials for filed effect transistors (FETs) owing to their high mobility at atomic-level thickness. As miniaturization and high performance of the electronic devices are required, however, high contact resistance at the interface between the metal electrode and 2D TMDC channel has become a major challenge.1 The high contact resistance at the metal-semiconductor interface originates from the uncontrollable high Schottky barrier height (SBH). Due to Fermi-level pinning induced by metal-induced gap state (MIGS) and defect-induced gap state (DIGS), the barrier height cannot be controlled by conventional Schottky-Mott rule, regardless of metal work function. Since MIGS occurs due to perturbation of metal wave function into the semiconductor, inserting semimetallic material as contact layer can be a solution to suppress the MIGS and further reduce the contact resistance.2 In this study, ALD TiS2 was used as semimetallic contact layer between the MoS2 channel and Ti/Au electrode for MoS2-based TFT. With the TiS2 contact layer, we could observe improvements in overall device performances, which is attributed to the semimetallic nature of TiS2. In addition, low temperature ALD TiS2 process contributed to the clean interface with vdW bonding between MoS2 and TiS2, resulting in suppression of DIGS, as well as MIGS. Owing to the ALD TiS2 contact, we could mitigate FLP and achieve metal-semiconductor interface with low Schottky barrier height, resulting in low contact resistance. References 1 K. Schauble et al., “Uncovering the effects of metal contacts on monolayer MoS2,” ACS Nano, vol. 14, no. 11, pp. 14798–14808, 2020, doi: 10.1021/acsnano.0c03515. 2 P. C. Shen et al., “Ultralow contact resistance between semimetal and monolayer semiconductors,” Nature, vol. 593, no. 7858, pp. 211–217, 2021, doi: 10.1038/s41586-021-03472-9.

Novel Al/CoFe/p-Si and Al/NiFe/p-Si MS-type photodiode for sensing

CoFe and NiFe are used in the construction of Si-based metal-semiconductor-type photodiodes. Thin film layers are sputtered onto thep-Si surface where Al metal contacts are deposited using the thermal evaporation technique. Film characteristics of the layers are investigated with respect to the crystalline structure and surface morphology. Their electrical and optical properties are investigated using dark and illuminated current-voltage measurements. When these two diodes are compared, Al/NiFe/p-Si exhibits better rectification properties than Al/CoFe/p-Si diode. There is also a high barrier height where these values for both diodes increase with illumination. According to the current-voltage analysis, the existence of an interlayer causes a deviation in diode ideality. In addition, the response to bias voltage, the derivation of electrical parameters, and the light sensitivity of diodes are evaluated using current-voltage measurements under different illumination intensities and also transient photosensitive characteristics.

Optimizing Forward Drop and Reverse Leakage Trade‐Off in AlGaN/GaN Lateral Diode with Schottky‐Metal‐Insulator‐Semiconductor Cascode Anode

Herein, AlGaN/GaN lateral diode with the Schottky‐Metal‐Insulator‐Semiconductor (MIS) cascode anode (CALD) to optimize forward voltage drop (VF) and reverse leakage current (ILEAK) trade‐off is proposed. In the CALD device, a normally‐on Ni/HfO2/AlGaN MIS‐controlled channel as well as a lateral Ni/GaN Schottky contact are cascoded at the anode. The designed normally‐on MIS‐controlled channel has high electron concentration at forward bias and prevents high electric‐field at reverse bias, which ensure both low VF and low ILEAK. Meanwhile, the Ni/GaN Schottky contact with a high Schottky barrier height further suppresses the ILEAK. As a result, the fabricated CALD demonstrates an optimized VF–ILEAK trade‐off relationship, including a low VF of 1.6 V and a low ILEAK of 5.2 × 10−8 A mm−1 (at reverse voltage of 400 V), together with a high breakdown voltage (BV) >700 V. These high performances suggest that the CALD can be a promising candidate for GaN power diode applications requiring a better VF–ILEAK trade‐off.

Strategy to enhance the performance of spin field effect transistors-insert effective intermediate layer graphene

Abstract Novel spin field effect transistors (FETs) with metal contacts are designed to reduce the high Schottky barrier height (SBH) due to Fermi pinning, reducing energy consumption and increasing their performance. Herein, we effectively enhance the conductivity (106 orders of magnitude) and current threshold of the FETs by introducing interlayer graphene in the contact interface between the semiconductor blue phosphorus and the metal, thereby reducing the interlayer resistance. Electronic structure analysis shows that Blue Phosphorus–Graphene–Cu modulates the lowest SBH, yielding a larger FETs conductance compared to other metal systems. The spin injection further enhances the efficiency of FETs as rectifiers (enhanced 13%). This theoretical work provides rational guidance for realizing innovations in next-generation high-performance transistor technology, demonstrating the inherent potential of the regulatory mechanism.

Ohmic Contact Resistance in SiC Diodes with Ti and NiSi P<sup>+</sup> Contacts

Ohmic contacts play a major role in the signal transfer between the semiconductor device and the external circuitry. One of the main technological issues to develop high-performance SiC-based devices is the control of metal/SiC contact properties to fabricate low resistance and high stability SiC Ohmic contacts to p-type SiC. This is mostly due to intrinsic SiC characteristics like large work function, low dopant activation for p-type materials and low hole mobility. These limits are even more emphasized in SiC JBS or MPS diodes, where Schottky and Ohmic contacts on the P doped regions embedded in the active area to improve surge ruggedness are usually formed by using the same metallization process. This naturally results either in a high Schottky barrier height in the Schottky contact with consequent increase of the conduction loss at low currents or in a poorly conductive Ohmic contact, leading to reduced IFSM capability. Therefore, the optimization and control of the process parameters like for example the P+ doping concentration peak underneath the metallization layer and the annealing process temperatures is crucial to obtain a good Ohmic contact and enhance the device´s robustness against surge current.

High-Performance Ultraviolet Photodetectors Based on Nanoporous GaN with a Ga2O3 Single-Crystal Layer.

The utilization of a nanoporous (NP) GaN fabricated by electrochemical etching has been demonstrated to be effective in the fabrication of a high-performance ultraviolet (UV) photodetector (PD). However, the NP-GaN PD typically exhibits a low light-dark current ratio and slow light response speed. In this study, we present three types of UV PDs based on an unetched GaN, NP-GaN distributed Bragg reflector (DBR), and NP-GaN-DBR with a Ga2O3 single-crystal film (Ga2O3/NP-GaN-DBR). The unetched GaN PD does not exhibit a significant photoresponse. Compared to the NP-GaN-DBR PD device, the Ga2O3/NP-GaN-DBR PD demonstrates a larger light-dark current ratio (6.14 × 103) and higher specific detectivity (8.9 × 1010 Jones) under 365 nm at 5 V bias due to its lower dark current (3.0 × 10-10 A). This reduction in the dark current can be attributed to the insertion of the insulating Ga2O3 between the metal and the NP-GaN-DBR, which provides a thicker barrier thickness and higher barrier height. Additionally, the Ga2O3/NP-GaN-DBR PD device exhibits shorter rise/decay times (0.33/0.23 s) than the NP-GaN-DBR PD, indicating that the growth of a Ga2O3 layer on the DBR effectively reduces the trap density within the NP-GaN DBR structure. Although the device with a Ga2O3 layer presents low photoresponsivity (0.1 A/W), it should be feasible to use Ga2O3 as a dielectric layer based on the above-mentioned reasons.

Probing the energy landscape of the lipid interactions of the serotonin1A receptor

G protein-coupled receptors (GPCRs) are lipid-regulated transmembrane proteins that play a central role in cell signaling and pharmacology. Although the role of membrane lipids in GPCR function is well established, the underlying GPCR-lipid interactions have not been thermodynamically characterized due to the complexity of these interactions. In this work, we estimate the energetics and dynamics of lipid association from coarse-grain simulations of the serotonin1A receptor embedded in a complex membrane. We show that lipids bind to the receptor with varying energetics of 1–4 kT, and timescales of 1–10 μs. The most favorable energetics and longest residence times are observed for cholesterol, glycosphingolipid GM1, phosphatidylethanolamine (PE) and phosphatidylserine (PS) lipids. Multi-exponential fitting of the contact probability suggests distinct dynamic regimes, corresponding to ps, ns and μs timescales, that we correlate with the annular, intermediate and non-annular lipid sites. The timescales of lipid binding correspond to high barrier heights, despite their relatively weaker energetics. Our results highlight that GPCR-lipid interactions are driven by both thermodynamic interactions and the dynamical features of lipid binding.

Over 600 V Lateral AlN-on-AlN Schottky Barrier Diodes with Ultra-Low Ideality Factor

This letter reports the demonstration of lateral AlN Schottky barrier diodes (SBDs) on single-crystal AlN substrates by metalorganic CVD (MOCVD) with an ultra-low ideality factor (η) of 1.65, a high Schottky barrier height of 1.94 eV, a breakdown voltage (BV) of 640 V, and a record high normalized BV by the anode-to-cathode distance. The device current was dominated by thermionic emission, while most previously reported AlN SBDs suffered from defect-induced current with higher η (>4). This work represents a significant step towards high-performance ultra-wide bandgap AlN-based high-voltage and high-power devices.

Reduced graphene oxide decorated CuO@NiO nanocomposite and their improved photosensitive activity for photodetection applications

In our proposed work, pure rGO and CuO@NiO/rGO (CNR) nanocomposites (NCs) with different ratios of CuO@NiO:rGO (90:10, 75:25 and 50:50) were synthesized and explored by XRD, SEM with EDX, TEM, XPS, UV–visible, PL and utilized to measure the current – voltage characteristics under dark and illumination conditions. TEM analysis indicates ultra-thin nanosheets of rGO with lengths of several tens of micrometer and CuO@NiO particle sizes is ∼50 nm. XPS analysis showed the presence of element and chemical states for Cu, Ni, O, and C in CNR-(50:50) sample. The energy gap (Eg) of pure rGO is 3.99eV whereas the CNR for 90:10, 75:25 and 50:50 NCs energy gap (Eg) values are viz., 1.43eV, 1.41eV and 1.39eV. The diode characteristic of CNR-(50:50)/n-Si exhibit lower ideality factor (n = 1.96), higher barrier height (ΦB = 0.80eV) under illumination condition. The CNR-(50:50)/n-Si photodiode exhibits higher photosensitivity (PS), responsivity (R), External Quantum Efficiency (EQE) and detectivity (D*) values of 956.45 %, 656.29 mA/W, 367.6/1 % and 5.72 × 1012 (Jones) respectively.

Studies of carrier transport mechanism in NiO/Ag/NiO transparent conducting electrode/ZnO metal-semiconductor-metal photodetectors

Oxide/metal/oxide multilayers as a transparent conducting electrode (TCE) have been developed to replace metals due to their high transparency and low sheet resistance. Nickel oxide (NiO) film with a high work function was used as an oxide to form NiO/Ag/NiO (NAN) TCE, therefore a high barrier height between NAN/zinc oxide (ZnO) interface. In the study, NAN TCE was deposited on ZnO surface to fabricate metal-semiconductor-metal (MSM) photodetectors (PDs) and study its carrier transport mechanism. The NAN TCE has a low sheet resistance of 6.5 Ω/sq. and transmittance more than 40% in a 300−1000 nm wavelength range. Such issues result in the figure-of-merit is higher (2.3 × 10−4 Ω−1) than that (2.5 × 10−7 Ω−1) of pure single NiO thin film. As compared to the conventional Au/ZnO MSM-PDs, the NAN/ZnO MSM-PDs demonstrates a lower leakage current as a result of Ni atoms diffusing into ZnO and passivating the defects. Due to the high work function of NiO, the NAN/ZnO interface exhibits a barrier height as high as 0.91 eV. The Au/ZnO MSM-PDs reveals only one carrier conduction of ohmic due to the electrons tunnel form Au into ZnO through the surface defects. In contrast, two distinct carrier transport mechanisms were observed in the NAN/ZnO MSM-PDs. At low-voltage for V⩽0.64 V , ohmic conduction dominates and the electrons inject from NAN to ZnO, trapped by the defect states of ZnO. At high-voltage for V⩾0.64 V , the trapped electrons acquire enough energy and emit from trap to conduction band, entering Poole–Frankel emission transport.

Low turn-on voltage and 2.3 kV β -Ga2O3 heterojunction barrier Schottky diodes with Mo anode

In this work, we propose combining a low work function anode metal and junction barrier Schottky structure to simultaneously achieve low turn-on voltage (Von) and high breakdown voltage (BV), which alleviates the dilemma that high BV requires high Schottky barrier height (SBH) and high Von. Molybdenum (Mo) is used to serve as the anode metal to reduce the SBH and facilitate fast turn-on to achieve a low Von. To resolve the low SBH related low BV issue, a p-NiO/n-Ga2O3-based heterojunction structure is used to enhance β-Ga2O3 sidewall depletion during the reverse state to improve the BV. With such a design, a low Von = 0.64 V(@1A/cm2) and a high BV = 2.34 kV as well as a specific on-resistance (Ron,sp) of 5.3 mΩ cm2 are demonstrated on a 10 μm-drift layer with a doping concentration of 1.5 × 1016 cm−3. β-Ga2O3 JBS diodes with low Von = 0.64 V and a power figure of merit of 1.03 GW/cm2 show great potential for future high-voltage and high-efficiency power electronics.

Monoethanolamine assisted CO2 hydrogenation to methanol – A computational study

Carbon dioxide emission to the atmosphere has to be reduced which can be done by utilizing CO2 in the synthesis of added value products. At the same time this will lead to a process which can help to store renewable energy. The use of hydrogen produced from the electrolysis of water in carbon dioxide reduction to methanol could be an efficient way to store energy and to convert CO2 into an added value product. The synthesis of methanol from CO2 is usually performed catalytically in gas phase. Many scientists nowadays expressed interest in testing the feasibility of the reaction also in aqueous phase. In this direction, monoethanolamine (MEA) can be used as a solvent for capturing and trapping carbon dioxide in an aqueous phase. In this work, the hydrogenation of (2-hydroxyethyl) carbamic acid (HO-(CH2)2-NH-COOH) which is one of the produced species during the capture process has been investigated by using computational tools. The uncatalyzed and catalyzed-like hydrogenation mechanisms leading to methanol (and MEA+water) as a product has been studied by using high level ab initio calculations in aqueous phase. The mechanisms have been described at the molecular level to provide a deeper understanding of the processes. The calculations indicate that the highest barrier height in the catalyzed-like process is only 114.67 kJ/mol, which is 227.71 kJ/mol lower than the corresponding step in the uncatalyzed mechanism. Furthermore, the energy storage efficiency of the catalyzed-like process is 96.68 %, which is 7.5 times more efficient than the uncatalyzed mechanism.

Negative bias stress stable PtOx/InGaZnOx Schottky barrier diodes optimized by oxygen annealing

In this work, bottom-Schottky-structure InGaZnOx (IGZO) Schottky barrier diodes (SBDs) with sputtered PtOx anodes were fabricated and annealed in oxygen at different temperatures. Critical parameters and negative bias stress (NBS) stability of SBDs with different annealing temperatures are investigated. With the annealing temperature increases, the barrier height and rectification ratio of the SBDs exhibited a rising-then-declining trend, while the ideality factor slightly increased until 200 °C. The SBDs show up overall reliability except for a leakage current rising trend under light, which can be attributed to free electron generation from the ionized oxygen vacancy. Among all the SBDs, the 175 °C annealed ones exhibited the best overall performance, including a high barrier height of 0.89 eV, an ideality factor of 1.14, and a large rectification ratio of over 108. Compared to the initial SBDs, the annealed ones showed up great improvement in NBS stability except for the 200 °C annealed ones, which was permanently degraded and not able to recover to original states. According to experimental result analysis and IGZO material characteristics, a stability model based on the subgap trap transition from VO2+ to VO and new VO2+ creation was proposed, which applies to both the short-term and long-term NBS tests. The results above demonstrate that oxygen annealing at appropriate temperature is an effective method to improve both device performance and NBS stability for PtOx–IGZO SBDs.

Electrical properties and band alignments of Sb2Te3/Si heterojunctions, low-barrier Sb2Te3/n-Si and high-barrier Sb2Te3/p-Si junctions

We investigated the electrical junction properties of the layered Sb2Te3 film formed on Si substrates. The current−voltage characteristics of the Sb2Te3/n-Si heterojunction showed an ohmic properties, whereas the Sb2Te3/p-Si heterojunction exhibited rectifying properties with a high barrier height of 0.77 eV. The capacitance−voltage characteristics of MOS capacitors with the Sb2Te3 electrode indicated an effective work function of 4.44 eV for the Sb2Te3 film. These findings suggest that the Sb2Te3/Si heterostructure possesses a low conduction band offset, as inferred from the temperature dependence of the current−voltage characteristics of the Sb2Te3/n-Si.

Dark Current Reduction and Performance Improvements in Graphene/Silicon Heterojunction Photodetectors Obtained Using a Non-Stoichiometric HfOx Thin Oxide Layer.

Graphene/silicon heterojunction photodetectors suffer from a high dark current due to the high surface states and low barrier height at the interface, which limits their application. In this study, we introduce an HfOx interfacial layer via magnetron sputtering to address this issue. With this new structure, the dark current is reduced by six times under a bias voltage of -2 V. Under 460 nm illumination, the responsivity is 0.228A/W, the detectivity is 1.15 × 1011 cmHz1/2W-1, and the noise equivalent power is 8.75 × 10-5 pW/Hz1/2, demonstrating an excellent weak light detection capability. Additionally, the oxygen vacancies in the HfOx interfacial layer provide a conductive channel for charge carriers, resulting in a 2.03-fold increase in photocurrent and an external quantum efficiency of 76.5%. The photodetector maintains good photoresponse ability at a low bias voltage. This work showcases the outstanding performance of HfOx films as interfacial layer materials and provides a new solution for high-performance photodetectors, as well as a new path to improve the photovoltaic conversion efficiency of solar cells.

Investigation on Contact Properties of 2D van der Waals Semimetallic 1T-TiS2/MoS2 Heterojunctions.

Two-dimensional transition metal dichalcogenides (2D TMDCs) are considered promising alternatives to Si as channel materials because of the possibility of retaining their superior electronic transport properties even at atomic body thicknesses. However, the realization of high-performance 2D TMDC field-effect transistors remains a challenge owing to Fermi-level pinning (FLP) caused by gap states and the inherent high Schottky barrier height (SBH) within the metal contact and channel layer. This study demonstrates that high-quality van der Waals (vdW) heterojunction-based contacts can be formed by depositing semimetallic TiS2 onto monolayer (ML) MoS2. After confirming the successful formation of a TiS2/ML MoS2 heterojunction, the contact properties of vdW semimetal TiS2 were thoroughly investigated. With clean interfaces of the TiS2/ML MoS2 heterojunctions, atomic-layer-deposited TiS2 can induce gap-state saturation and suppress FLP. Consequently, compared with conventional evaporated metal electrodes, the TiS2/ML MoS2 heterojunctions exhibit a lower SBH of 8.54 meV and better contact properties. This, in turn, substantially improves the overall performance of the device, including its on-current, subthreshold swing, and threshold voltage. Furthermore, we believe that our proposed strategy for vdW-based contact formation will contribute to the development of 2D materials used in next-generation electronics.

β-Ga2O3 Schottky Barrier Diode with Ion Beam Sputter-Deposited Semi-Insulating Layer

Vertical Schottky barrier diodes based on an ion beam sputter (IBS)-deposited β-Ga2O3 film on a single-crystalline (2¯01) unintentionally doped (UID) β-Ga2O3 with a Ni contact were developed. To form ohmic Ti/Ni contacts, the IBS-Ga2O3/UID β-Ga2O3 structures were wet-etched, and an indium tin oxide (ITO) intermediate semiconductor layer (ISL) was deposited on the opposite surface of the UID β-Ga2O3. The IBS-deposited Ga2O3 layer was polycrystalline and semi-insulating. Low leakage currents, rectification ratios of 3.9 × 108 arb. un. and 3.4 × 106 arb. un., ideality factors of 1.43 and 1.24, Schottky barrier heights of 1.80 eV and 1.67 eV as well as breakdown voltages of 134 V and 180 V were achieved for diodes without and with ITO-ISL, respectively. The surface area of the IBS-Ga2O3 film acted as a thin dielectric layer and, together with the preliminary wet etching, provided low leakage currents and relatively high Schottky barrier heights. Diodes with a Schottky barrier based on a Ni/IBS-deposited Ga2O3 film contact were demonstrated for the first time.

Interception performance of intake structures with a flow diversion barrier under supercritical flow conditions

Intake structures are commonly used in drainage systems to redirect surface rainfall runoff into underground tunnels. The Hong Kong West Drainage Tunnel (HKWDT) system is designed to capture stormwater from steep upland catchments during periods of heavy rainfall, concurrently maintaining a minimal environmental flow downstream during dry weather. However, during heavy rainfall events, excess rainfall water is often directed into the drainage system, presenting a significant challenge to the urban drainage system. Undistorted Froude scale physical models and computational fluid dynamics (CFD) models are developed to investigate the flow performance. In this study, effects of various design parameters on water diversion performance are examined across a wide range of inflow conditions. Steeper channel bottom slopes leading to a larger proportion of water being directed downstream. Extending the length of the bottom rack effectively reduces adhering flow. Introducing a transverse barrier generates a hydraulic jump, which increases flow diversion into the LFC. With higher barrier heights, the hydraulic jump becomes stronger, facilitating greater water diversion into the downstream sewage system. The results indicate potential solutions to these challenges and provide valuable insights into optimizing flow diversion performance in intake structures, particularly in areas characterized by steep terrains and varying flow conditions.

Evaluation of ferroelectricity in a distorted wurtzite-type structure of Sc-doped LiGaO2.

Since the discovery of ferroelectricity in a wurtzite-type structure, this structural type has gathered much attention as a next-generation ferroelectric material due to its high polarization value combined with its high breakdown strength. However, the main targets of wurtzite-type ferroelectrics have been limited thus far to simple nitride/oxide compounds. The investigation of new ferroelectric materials with wurtzite-type structures is important for understanding ferroelectricity in such structures. We therefore focus on β-LiGaO2 in this study. Although AlN and ZnO possess well-known wurtzite-type structures (P63mc), β-LiGaO2 has a distorted wurtzite-type structure (Pna21), and there are no reports of ferroelectricity in LiGaO2. In this study, we have revealed that LiGaO2 exhibits relatively high barrier height energy for polarization switching, however, Sc doping effectively reduces that energy. Then, we conducted thin film preparation and evaluation for Sc-doped LiGaO2 to observe its ferroelectric properties. We successfully observed ferroelectric behavior by using piezoresponse force microscopy measurements for LiGa0.8Sc0.2O2/SrRuO3/(111)SrTiO3.