Ultrathin Barrier Research Articles

PECVD SiNx passivation for AlGaN/GaN HFETs with ultra-thin AlGaN barrier

We have investigated the effects of a silicon nitride (SiNx) passivation process using plasma-enhanced chemical vapor deposition (PECVD) on a ultra-thin-barrier AlGaN/GaN heterostructure field-effect transistors (HFETs). The bulk charge characteristics of the PECVD SiNx films were dependent on the film deposition conditions, which strongly influenced the sheet resistance (Rsh) and flat-band voltage characteristics of AlGaN/GaN HFETs. The reduction in Rsh is a strong function of the amount of positive bulk charges in the SiNx passivation film. An optimized PECVD SiNx process was used to drastically decrease the Rsh from 45,450 Ω/sq to 732 Ω/sq. A Mo/Au Schottky-gate device fabricated with PECVD SiNx passivation exhibited a maximum drain current density of 172 mA/mm, quasi-normally-off operation, and breakdown voltage of > 1100 V

High Tunneling Magnetoresistance in Magnetic Tunnel Junctions with Subnanometer Thick Al2O3 Tunnel Barriers Fabricated Using Atomic Layer Deposition.

Pinhole-free and defect-free ultrathin dielectric tunnel barriers (TBs) are a key to obtaining high-tunneling magnetoresistance (TMR) and efficient switching in magnetic tunnel junctions (MTJs). Among others, atomic layer deposition (ALD) provides a unique approach for the fabrication of ultrathin TBs with several advantages including atomic-scale control over the TB thickness, conformal coating, and a low defect density. Motivated by this, this work explores the fabrication and characterization of spin-valve Fe/ALD-Al2O3/Fe MTJs with an ALD-Al2O3 TB thickness of 0.55 nm using in situ ALD. Remarkably, high TMR values of ∼77 and ∼90% have been obtained, respectively, at room temperature and at 100 K, which are comparable to the best reported values on MTJs having thermal AlOx TBs with optimized device structures. In situ scanning tunneling spectroscopy characterization of the ALD-Al2O3 TBs has revealed a higher TB height (Eb) of 1.33 ± 0.06 eV, in contrast to Eb ∼ 0.3-0.6 eV for their AlOx TB counterparts, indicative of significantly lower defect concentrations in the former. This first success of the MTJs with subnanometer thick ALD-Al2O3 TBs demonstrates the feasibility of in situ ALD for the fabrication of pinhole-free and low-defect ultrathin TBs for practical applications, and the performance could be further improved through device optimization.

Ultrathin barrier AlGaN/GaN hybrid‐anode‐diode with MOCVD in‐situ Si 3 N 4 ‐cap and LPCVD‐Si 3 N 4 bilayer passivation stack for dynamic characteristic improvement

A novel ultrathin barrier AlGaN/GaN hybrid-anode-diode (UTB-HAD) with in-situ Si 3 N 4 -cap passivation is experimentally demonstrated. The forward turn-on voltage ( V on ) of the UTB-HAD is determined by the intrinsic threshold voltage of the two-dimension electron gas (2DEG) channel, which can be precisely controlled by tailoring the as-grown AlGaN-barrier thickness. The typical V on as low as 0.48 V is obtained by using the UTB AlGaN/GaN with a barrier thickness of 4.9 nm. The MOCVD has grown in-situ Si 3 N 4 -cap and the LPCVD-Si 3 N 4 bilayer passivation stack is developed to effectively restore the 2DEG in the UTB AlGaN/GaN heterostructure and simultaneously improve the dynamic characteristics of the diode. The UTB-HAD and the novel passivation scheme are of great potential for power applications.

Subquantum-Well Influence on Carrier Dynamics in High Efficiency DUV Dislocation-Free AlGaN/AlGaN-Based Multiple Quantum Wells

We explore the effect of the subwell centers and related carrier dynamics mechanisms in dislocation-free DUV AlGaN/AlGaN multiple quantum wells (MQWs) homoepitaxially grown on an AlN substrate. Cross-sectional imaging and energy-dispersive X-ray compositional analyses using scanning transmission electron microscopy (STEM) reveal epitaxial layers of very high crystalline quality, as well as ultrathin Al-rich subquantum barrier and subwell layers at the interface between the wells and the barriers. Carrier dynamic analyses studied by power- and temperature-dependent time-resolved and time-integrated photoluminescence (PL) and PL excitation measurements, as well as numerical simulations, reveal the carrier repopulation mechanisms between the MQWs and subwell sites. This advanced analysis shows that the subwell/sub-barrier structure results in additional exciton localization centers, enhancing the internal quantum efficiency via staggered carrier repopulation into the MQWs to reach a maximum of ∼83% internal quantum efficiency, which remains high at high injected carrier densities in the droop region. Both experimental and numerical simulation results show that the slight efficiency droop can be due to Auger recombination, counteracted by a simultaneous increase in radiative recombination processes at high power density, demonstrating the role of the subwells/sub-barriers in efficiency enhancement.

Plasma-Enhanced Atomic Layer Deposition of Low Resistivity and Ultrathin Manganese Oxynitride Films with Excellent Resistance to Copper Diffusion

Low resistivity, high conformability, and ultrathin barriers against Cu diffusion have always been a critical challenge for fabrications of extremely large-scale integrated circuits. In this articl...

Tri-Gated Hybrid Anode AlGaN/GaN Power Diode With Intrinsic Low Turn-on Voltage and Ultralow Reverse Leakage Current

In this article, a novel tri-gated hybrid anode AlGaN/GaN power diode (TG-HAD) with intrinsic low turn-on voltage ( ${V}_{\mathrm{\scriptscriptstyle {ON}}}$ ) is demonstrated. A novel forward ${V}_{\mathrm{\scriptscriptstyle {ON}}}$ modulation technique based on an ultrathin barrier (UTB) AlGaN/GaN heterostructure is proposed. ${V}_{\mathrm{\scriptscriptstyle {ON}}}$ of the diode is determined by the intrinsic threshold voltage of the two-dimensional electron gas (2DEG) channel of the UTB heterostructure, which can be precisely controlled and which fundamentally features excellent uniformity by tailoring the AlGaN barrier thickness. On the other hand, compared with the planar GaN diodes, the proposed TG-HAD achieves significant reverse leakage current reduction originating from the effectively suppressed buffer leakage by the tri-gate design. The detailed device characteristics and the underlying mechanisms are investigated by TCAD simulation. The TG-HAD, together with the proposed turn-on voltage modulation technique, is promising for fabricating high performance large periphery devices for power applications.

Inhibition of tin whisker by electroplating ultra-thin Co-W amorphous barrier layer

Selecting suitable barrier layer material is one of the crucial issues for solder microbump in advanced interconnect metallization. In this work, the inhibiting effect of two barriers, Co-W and Ni, on tin whisker growth in high temperature and humidity environment (55 °C/85% RH) were studied. Low-cost electrodeposition was conducted to prepare the 200 nm amorphous Co-W barrier layer on Cu substrate. Compared with the Ni barrier layer, the amorphous Co-W barrier layer can effectively reduce the formation of intermetallic compounds during the storage process, suppressing the growth of tin whiskers and oxidative corrosion. Based on experimental results and inner stress calculation, the mitigation mechanisms of tin whisker growth within the two barrier layers were proposed, respectively, which are of great importance to further understand the behavior of tin whisker mitigation.

Ultrathin-Barrier AlGaN/GaN Hybrid-Anode-Diode With Optimized Barrier Thickness for Zero-Bias Microwave Mixer

In this article, the hybrid-anode-diode (HAD) is demonstrated for a zero-bias microwave mixer. In order to achieve a strong nonlinearity of the device at zero bias (i.e., 0 V), a novel technique based on the ultrathin-barrier (UTB) AlGaN/GaN heterostructure is proposed to precisely modulate the turn-on voltage ( ${V}_{{\text {T}}}$ ) of the HAD and yet the nonlinearity at 0 V. In order to determine the optimized AlGaN-barrier thickness and, thus, facilitate the device fabrication, the UTB-HAD is first designed by the TCAD simulation to achieve a high sensitivity at 0 V. In the fabricated UTB-HAD, the zero-bias curvature coefficient ( ${\gamma }$ ) of 20.4 and $10.2\,\,{V}^{\,-{1}}$ is, respectively, measured at room temperature (RT) and 200 °C, which indicates a high zero-bias sensitivity obtained in the fabricated device. The UTB-HAD for a zero-bias microwave mixer is characterized and exhibited proper functionality from RT up to 200 °C. The novel device structure together with the nonlinearity modulation technology demonstrated in this article is of great potential for high-sensitivity and high-temperature zero-bias microwave applications.

Scalable photonic sources using two-dimensional lead halide perovskite superlattices

Miniaturized photonic sources based on semiconducting two-dimensional (2D) materials offer new technological opportunities beyond the modern III-V platforms. For example, the quantum-confined 2D electronic structure aligns the exciton transition dipole moment parallel to the surface plane, thereby outcoupling more light to air which gives rise to high-efficiency quantum optics and electroluminescent devices. It requires scalable materials and processes to create the decoupled multi-quantum-well superlattices, in which individual 2D material layers are isolated by atomically thin quantum barriers. Here, we report decoupled multi-quantum-well superlattices comprised of the colloidal quantum wells of lead halide perovskites, with unprecedentedly ultrathin quantum barriers that screen interlayer interactions within the range of 6.5 Å. Crystallographic and 2D k-space spectroscopic analysis reveals that the transition dipole moment orientation of bright excitons in the superlattices is predominantly in-plane and independent of stacking layer and quantum barrier thickness, confirming interlayer decoupling.

Solution‐Processed, Photo‐Patternable Fluorinated Sol–Gel Hybrid Materials as a Bio‐Fluidic Barrier for Flexible Electronic Systems

AbstractReports have recently been published on ultrathin biofluid barriers, which enable the long‐term measurement of biological signals and exhibit conformability on nonlinear surfaces such as skin and organs. However, inorganic‐ and organic‐based barriers have process incompatibility and high water permeability, respectively. Siloxane‐ (inorganic) based fluorinated epoxy (organic) hybrid materials (FEH) are demonstrated for bio‐fluidic barrier and the biocompatibility and barrier performance for flexible electronic systems as solution‐processed oxide thin‐film transistors (TFTs) on 1.2 µm thick polyimide (PI) thin film substrate is confirmed. FEH thin film can be patterned as small as 10 µm through conventional photolithography. The fabricated solution‐processed indium oxide TFTs with FEH barriers exhibit durable performance over 16 h with no dramatic change of transfer characteristics in phosphate‐buffered saline (PBS) environment. Furthermore, to realize FEH barriers for flexible systems, the solution‐processed indium oxide TFTs with FEH barriers on ultrathin PI substrate are demonstrated subjected to compression test and successfully measure the electrical properties with no irreversible degradation during 1000 cycles of mechanical testing in PBS.

Ultrathin permselective membranes: the latent way for efficient gas separation.

Membrane gas separation has attracted the attention of chemical engineers for the selective separation of gases. Among the different types of membranes used, ultrathin membranes are recognized to break the trade-off between selectivity and permeance to provide ultimate separation. Such success has been associated with the ultrathin nature of the selective layer as well as their defect-free structure. These membrane features can be obtained from specific membrane preparation procedures used, in which the intrinsic properties of different nanostructured materials (e.g., polymers, zeolites, covalent–organic frameworks, metal–organic frameworks, and graphene and its derivatives) also play a crucial role. It is likely that such a concept of membranes will be explored in the coming years. Therefore, the goal of this review study is to give the latest insights into the use of ultrathin selective barriers, highlighting and describing the primary membrane preparation protocols applied, such as atomic layer deposition, in situ crystal formation, interfacial polymerization, Langmuir–Blodgett technique, facile filtration process, and gutter layer formation, to mention just a few. For this, the most recent approaches are addressed, with particular emphasis on the most relevant results in separating gas molecules. A brief overview of the fundamentals for the application of the techniques is given. Finally, by reviewing the ongoing development works, the concluding remarks and future trends are also provided.

Diffusion Barrier Prediction of Graphene and Boron Nitride for Copper Interconnects by Deep Learning

The continuous scaling-down size of interconnects should be accompanied with ultra-thin diffusion barrier layers, which is used to suppress Cu diffusion into the dielectrics. Unfortunately, conventional barrier layers with thicknesses less than 4 nm fail to perform well. With the advent of 2D layered materials, graphene and hexagonal boron nitride have been proposed as alternative Cu diffusion barriers with thicknesses of ≈1 nm. However, defects such as vacancies may evolve into a Cu diffusion path, which is a challenging problem in design of diffusion barrier layers. The energy barrier of Cu atom diffused through a di-vacancy defect in graphene and hexagonal boron nitride is calculated by density functional theory. It is found that graphene offers higher energy barrier to Cu than hexagonal boron nitride. The higher energy barrier is attributed to the stronger interaction between Cu and C atoms in graphene as shown by charge density difference and Bader’s charge. Furthermore, we use the energy barriers of different vacancy structures and generate a dataset that will be used for machine learning. Our trained convolutional neural network is used to predict the energy barrier of Cu migration through randomly configured defected graphene and hexagonal boron nitride with $R^{2}$ of >99% for $4 \times 4$ supercell. These results provide guides on choosing between 2D materials as barrier layers, and applying deep learning to predict the 2D barrier performance.

Low-temperature separation of helium-helion mixture

Abstract The research is devoted to the problem of designing materials with an adjustable property of permeability. The obtained tool for property regulation allows achieving hyper-selectivity in relation to separation of helium isotope mixtures, as well as some other gas mixtures. The reasearch is theoretical in nature; however, it suggests a clear direction of activity for experimenters. The result obtained is valid for ultrathin barriers of any form. As a result, a new exact solution of the Schrödinger equation of wave dynamics, which is valid for the case of two-barrier systems, is found. This solution allows for comprehensive consideration of the process of wave passage through a barrier and identification of the causes leading to super-permeability of individual components.

Characteristics of an Amorphous Carbon Layer as a Diffusion Barrier for an Advanced Copper Interconnect.

The size of the advanced Cu interconnects has been significantly reduced, reaching the current 7.0 nm node technology and below. With the relentless scaling-down of microelectronic devices, the advanced Cu interconnects thus requires an ultrathin and reliable diffusion barrier layer to prevent Cu diffusion into the surrounding dielectric. In this paper, amorphous carbon (a-C) layers of 0.75-2.5 nm thickness have been studied for use as copper diffusion barriers. The barrier performance and thermal stability of the a-C layers were evaluated by annealing Cu/SiO2/Si metal-oxide-semiconductor (MOS) samples with and without an a-C diffusion barrier at 400 °C for 10 h. Microstructure and elemental analysis performed by transmission electron microscopy (TEM) and secondary ion mass spectroscopy showed that no Cu diffusion into the SiO2 layer occurred in the presence of the a-C barrier layer. However, current density-electric field and capacitance-voltage measurements showed that 0.75 and 2.5 nm thick a-C barriers behave differently because of different microstructures being formed in each thickness after annealing. The presence of the 0.75 nm thick a-C barrier layer considerably improved the reliability of the fabricated MOS samples. In contrast, the reliability of MOS samples with a 2.5 nm thick a-C barrier was degraded by sp2 clustering and microstructural change from amorphous phase to nanocrystalline state during annealing. These results were confirmed by Raman spectroscopy, X-ray photoelectron spectroscopy and TEM analysis. This study provides evidence that an 0.75 nm thick a-C layer is a reliable diffusion barrier.

Full SnO2 double-layer dye-sensitized solar cells: Slowly increasing phenomenon of power conversion efficiency

Dye-sensitized solar cells (DSCs) have been proven as effective photovoltaic devices for low-cost and large-scale solar energy conversion applications. Fast electron transport, large specific surface area, and slow interfacial electron recombination are indispensable features for an efficient photoelectrode. In this work, we report full SnO2 double-layer DSCs. SnO2 nanoparticles as electron transport layer and large SnO2 particles as scattering layer (SL) are prepared by the hydrothermal method, respectively. Ultrathin Al2O3 barrier layer on the surface of SnO2 film is formed by immersing in Al (NO3)3 solutions and annealed at atmosphere in turn to retard back transfer of electrons to the electrolyte or to the oxidized dye molecules. Four kind cells with SnO2, SnO2/SL, SnO2/Al2O3 and SnO2/SL/Al2O3 film as photoanodes were fabricated, respectively, and their power conversion efficiency (PCE) was continuously tested for 60 days. The open-circuit voltage (Voc), fill factor (FF) and PCE of four kind cells are gradually improved, while short-circuit current (Jsc) is slightly reduced during testing. As a result, the PCE of the SnO2 based DSC increased from 1.36% (0 day) to 3.5% (20 day), and the PCE of the SnO2/SL/Al2O3 based DSC increased from 2.53% (0 day) to 4.57% (20 day). This novel slow increasing phenomenon of PCE observed in SnO2 DSCs is different from that of TiO2 and ZnO. This kind of structure of DSCs and the slowly increasing phenomenon have far-reaching significance for understanding the working mechanism and increasing efficiency.

Sandwich-structure transferable free-form OLEDs for wearable and disposable skin wound photomedicine

Free-form optoelectronic devices can provide hyper-connectivity over space and time. However, most conformable optoelectronic devices can only be fabricated on flat polymeric materials using low-temperature processes, limiting their application and forms. This paper presents free-form optoelectronic devices that are not dependent on the shape or material. For medical applications, the transferable OLED (10 μm) is formed in a sandwich structure with an ultra-thin transferable barrier (4.8 μm). The results showed that the fabricated sandwich-structure transferable OLED (STOLED) exhibit the same high-efficiency performance on cylindrical-shaped materials and on materials such as textile and paper. Because the neutral axis is freely adjustable using the sandwich structure, the textile-based OLED achieved both folding reliability and washing reliability, as well as a long operating life (>150 h). When keratinocytes were irradiated with red STOLED light, cell proliferation and cell migration increased by 26 and 32%, respectively. In the skin equivalent model, the epidermis thickness was increased by 39%; additionally, in organ culture, not only was the skin area increased by 14%, but also, re-epithelialization was highly induced. Based on the results, the STOLED is expected to be applicable in various wearable and disposable photomedical devices.

In brief

PAGE 1288 Researchers from the College of Automotive Engineering, Seoul, have developed a driver model for use in driver-assisted system technologies. The model is based on a generative adversarial network (GAN) and performs better than a rule-based model when replicating human driving behaviour. PAGE 1265 Chinese researchers present a novel symmetrical antenna design using a chessboard grid hollow substrate (CGHS). Scattering performance of the circularly polarised antenna is enhanced and the device operates between 7 and 20 GHz. Reduction of radar cross section (RCS) at the operating bandwidth is realised by the CGHS. Comparison of different scenarios comparing human, GAN and rule-based behaviour. PAGE 1279 Fei Zhou and colleagues present an anisotropy-based image smoothing method, designed to remove unwanted texture from images. The method was found to compare well with recent similar methodologies. In particular, the methodology presented here did a better job of preserving image edges compared to other methods. Structure of the proposed antenna. PAGE 1303 A collaboration of Chinese researchers presents an AlGaN/GaN ultra-thin barrier hybrid-anode-diode (UTB-HAD) which has been tailored to allow zero-bias microwave detection. Computer simulations found the optimal thickness of the AlGaN barrier to be ∼5 nm. A high-voltage sensitivity of 2.7 mV/μW at zero bias was found. Illustration of structural edge (solid) and textural edge (dashed) parts of an image. PAGE 1297 Dr Hidekazu Tsuchida presents a new modulation and detection scheme for frequency modulated continuous wave light detection and ranging (FMCW-LiDAR) systems. The proposed scheme is immune to nonlinear chirp and relative accuracies below ±2 mm/s and ±1 mm for velocity and distance measurements, respectively, are reported. Ultra-thin barrier hybrid-anode-diode (UTB-HAD) device proposed by the group. Experimental schematic for velocity and distance measurements.

Tunnelling magnetoresistance and spin filtration effect in functionalized graphene sheet with CrO2 as electrode: An ab-initio study

Spin-dependent transport behavior of functionalized graphene sheet with CrO2 as half-metallic magnet (HMM) electrodes are explored using DFT (density functional theory) in a combination with non-equilibrium Green’s function formalism (NEGF). Functionalized graphene sheet with Hydrogen and Fluorine atoms are examined as ultra-thin tunnel barriers in the magnetic tunnel junction devices. Computations based on the DFT for functionalization of graphene sheet with H and F atoms show opening of the band gap in the graphene sheet. A band gap opening of 3.53 eV, 3.16 eV, and 2.64 eV have been observed when Graphene sheet is functionalized with H atoms on both sides, F atoms on both sides, and both F and H atoms on each side of the graphene sheet respectively. The results of all the three functionalized graphene sheet configurations have been compared. On comparison of the results for the three configurations, it is observed that a large value of tunneling magnetoresistance (TMR) around 344 × 103% is obtained for the H-Graphene-F device configuration. This suggests that the H-Graphene-F device configuration acts as a perfect ultra-thin insulating barrier for the spin-based devices. Negative differential resistance (NDR) behavior have been observed for F-Graphene-F and H-Graphene-F device configurations in parallel magnetization (PM) case. All the three configurations show perfect spin filtration efficiency of around 100%. Applied voltage dependent transmission spectrum have been plotted for all the three device configurations to validate the current-voltage behaviour. These unique and interesting nature of all the three devices suggest their applications in spintronics domain.

Planar anisotropic Shubnikov-de-Haas oscillations of two-dimensional electron gas in AlN/GaN heterostructure

Two-dimensional electron gas (2DEG) buried in ultrathin barrier AlN/GaN heterostructures is the key to exploit high-speed and high-power devices in the aspect of modern semiconductor electronics. Here, we report Shubnikov-de-Haas oscillations of the 2DEG in an AlN/GaN heterostructure with planar anisotropy along [11-20] and [1-100] axes. The effective mass extracted from oscillations exhibits an evident disparity, as (0.19 ± 0.02)me along the [11-20] axis and (0.24 ± 0.02)me along the [1-100] axis. Meanwhile, the quantum scattering time is obviously different along the aforementioned directions, with 0.08 vs 0.26 ps for the first subband and 0.19 vs 0.27 ps for the second subband. Both the effective mass and the quantum scattering time contribute to the anisotropy of the quantum mobility, which are 750 and 1907 cm2/V s for E1 and 1760 and 1980 cm2/V s for E2 along [11-20] and [1-100] axes, respectively. These parameters are obviously crucial in designing devices using AlN/GaN heterostructures.

Rational Design of Ultrathin Gas Barrier Layer via Reconstruction of Hexagonal Boron Nitride Nanoflakes to Enhance the Chemical Stability of Proton Exchange Membrane Fuel Cells.

Hexagonal boron nitride (hBN) has great potential as a promising gas barrier layer in proton exchange membrane fuel cells (PEMFCs) as it shows high proton conductivity as well as excellent gas-blocking capability. However, structural defects and mechanical damage during the transfer of the hBN layer and membrane swelling have limited the application of hBN sheets to PEMFCs. Here, an ultrathin gas barrier layer is successfully fabricated on a proton exchange membrane via reconstruction of mechanically exfoliated hBN nanoflakes using a direct spin-coating process. The hBN-coated layer effectively suppresses the gas crossover and inhibits the formation of reactive oxygen radicals in the electrodes without reducing the proton conductivity of the membrane. It is also demonstrated that the structural advantages of hBN-coated gas barrier layers promise high performance of a unit cell even after a open-circuit voltage (OCV) hold test for 100 h. Furthermore, through in-depth postmortem analyses, a time-dependent degradation mechanism of membrane electrode assembly under the OCV condition is rationally proposed.