High-quality Interface Research Articles

Formation and Effect of Deposited Thin TiO2 Layer With Compressive Strain and Oxygen Vacancies on GaAs (001) Substrate

The integration of metal oxides and GaAs semiconductors is quite attractive for its potential applications, but interfacial diffusion and lattice mismatch usually cause huge challenges toward achieving high-performance electronic devices. In this article, we reported a thin layer of epitaxial TiO2 (110) on a GaAs (001) substrate with significant compressive strain, lattice distortion, and oxygen vacancies, where the oxygen vacancies proved to be the critical factor to induce the compressive strain and lattice distortion. In this case, the lattice mismatches between this compressed TiO2 (110) and GaAs (001) surface were calculated to be as small as 1.3 and 0.24% along the [110] and [001] orientations of TiO2, respectively. Further, no Ga-oxides or As-oxides were found at the interface, indicating that the TiO2 layer inhibited the diffusion of Ga and As atoms effectively. In summary, TiO2 film can be grown epitaxially on GaAs (001) substrates with non-negligible compressive strain, lattice distortion, oxygen vacancies, and a high-quality interface. This study also provides an approach to integrate different functional oxides on TiO2-buffered GaAs for various GaAs-based electronic devices with higher reliability and performance.

High-performance photodetectors based on Schottky junctions formed by vertical 2D-3D-2D graphene sandwich nanocavity and germanium substrate

The inefficient light absorption and gapless features of two-dimensional graphene (2D-graphene) severely impact quantum yield and electron-hole recombination, limiting its suitability in optoelectronics. Herein, a bottom 2D-graphene directly on the Ge substrate by chemical vapor deposition (CVD), a middle 3D-graphene is synthesized in-situ using 2D-graphene as the buffer layer via plasma-enhanced chemical vapor deposition (PECVD). A top 2D-graphene is transferred to 2D/3D-graphene/Ge by thermal release tape (TRT) technology to form a vertical 2D/3D/2D graphene sandwich cavity on the Ge substrate. The bottom 2D-graphene layer can not only perform as an interfacial layer for the in-situ preparation of 3D-graphene but also increase the electrical conductivity of the interface and advance the character of the Schottky junction formed between 3D-graphene/Ge, thereby improving the photon detection. The fabricated photodetector exhibits outstanding characteristics at a 1550 nm wavelength, with a high responsivity of 1.4 A/W and detectivity of 1.1 × 1014 Jones. This is due to the enhanced light absorption of the sandwich cavity and a high-quality interface layer (bottom 2D-graphene). The results reveal that this vertical 2D/3D/2D graphene sandwich architecture will facilitate the development of graphene-based NIR detection.

Observation of Strong Interlayer Couplings in WS2/MoS2 Heterostructures via Low-Frequency Raman Spectroscopy.

Van der Waals (vdW) heterostructures based on two-dimensional (2D) transition metal dichalcogenides (TMDCs), particularly WS2/MoS2 heterostructures with type-II band alignments, are considered as ideal candidates for future functional optoelectronic applications owing to their efficient exciton dissociation and fast charge transfers. These physical properties of vdW heterostructures are mainly influenced by the interlayer coupling occurring at the interface. However, a comprehensive understanding of the interlayer coupling in vdW heterostructures is still lacking. Here, we present a detailed analysis of the low-frequency (LF) Raman modes, which are sensitive to interlayer coupling, in bilayers of MoS2, WS2, and WS2/MoS2 heterostructures directly grown using chemical vapor deposition to avoid undesirable interfacial contamination and stacking mismatch effects between the monolayers. We clearly observe two distinguishable LF Raman modes, the interlayer in-plane shear and out-of-plane layer-breathing modes, which are dependent on the twisting angles and interface quality between the monolayers, in all the 2D bilayered structures, including the vdW heterostructure. In contrast, LF modes are not observed in the MoS2 and WS2 monolayers. These results indicate that our directly grown 2D bilayered TMDCs with a favorable stacking configuration and high-quality interface can induce strong interlayer couplings, leading to LF Raman modes.

Efficient visible-light-driven degradation of tetracycline by a 2D/2D rGO-Bi2WO6 heterostructure

Constructing heterostructures has been a simple yet effective strategy for improving the photocatalytic performance of individual semiconductor photocatalysts. However, the poor quality of the contacted interface coupled with the narrow and overlapping light absorption scope between heterocomponents limits potential improvement. Herein, a 2D/2D rGO-Bi2WO6 heterostructure with face-to-face compact contact interface and UV to NIR light absorption ability was synthesized to overcome the aforementioned limitations. The as-prepared 2 wt%-rGO-Bi2WO6 with a high contact interface quality exhibits the highest kinetic rate of (5.53 ± 0.75) × 10−2 L mg−1 min−1 toward tetracycline (TC) degradation, which is 2.4 times higher than that of pristine Bi2WO6 and 2.1 times higher than that of the 2 wt%-rGO-Bi2WO6 composite with a poor interface quality. Moreover, approximately 30% of TC can be mineralized with a 2 wt%-rGO-Bi2WO6 presented system after 120 min. The subsequent Escherichia coli culture and liquid chromatography–mass spectrometry were employed to detect the biotoxicity variation of degradation intermediates and the possible transformation pathways of TC, respectively. Finally, the reactive species trapping results indicate that photogenerated holes and superoxide radical anions play dominant roles during the TC degradation process. This work provides a facile and effective method to fabricate an efficient heterojunction photocatalyst for pollutant degradation.

A Ferroelectric p–i–n Heterostructure for Highly Enhanced Short‐Circuit Current Density and Self‐Powered Photodetection

Abstract2D‐layered materials and van der Waals heterostructures (vdWHs) have attracted intense interests in optoelectronic applications. However, the performance of photovoltaic effect based on vdWHs is still unsatisfactory, which is subjected to many factors, including settled small built‐in potentials and low light absorptions. Here, a pronounced photovoltaic effect is reported in MoTe2/(C4H9NH3)2(CH3NH3)2Pb3I10/α‐In2Se3 ferroelectric p–i–n vdWHs. An all‐dry transfer method held in an inert environment is utilized to ensure a good materials’ stability and high interfacial quality. The short‐circuit current density can increase three orders to 38 mA cm−2 by introducing perovskite; hence, the device works as a sensitive self‐powered photodetector with a photo on/off ratio of 2.4 × 105, a detectivity of 1.2 × 1012 Jones, and response times of 730 µs/620 µs. Moreover, the short‐circuit current density increases more than 25 times by enhancing the built‐in potential through programming the ferroelectric polarization of α‐In2Se3. In total, short‐circuit current density as high as 468 mA cm−2 is reached, which is one‐to‐two orders higher than that of typical lateral vdWHs. This work demonstrates the potential of high‐performance optoelectronic devices based on ferroelectric van der Waals p–i–n heterostructures.

Boosting room-temperature NO2 detection via in-situ interfacial engineering on Ag2S/SnS2 heterostructures

The effective detection of hazardous gases has become extremely necessary for the ecological environment and public health. Interfacial engineering plays an indispensable role in the development of innovative materials with exceptional properties, thus triggering a new revolution in the realization of high-performance gas sensing. Herein, the rational designed Ag2S/SnS2 heterostructures were synthesized via a facile in-situ cation-exchange method. The coshared S atoms derived from in-situ interfacial engineering enable intimate atomic-level contact and strong electron coupling between SnS2 and Ag2S, which efficiently assist interfacial charge redistribution and transport as confirmed theoretically and experimentally. Benefiting from the high-quality interface of the heterostructures, the resultant Ag2S/SnS2 sensor delivered an ultrahigh response (286%) together with short response/recovery time (17 s/38 s) to 1 ppm NO2. The sensor also demonstrated superior sensing selectivity and reliable repeatability at room-temperature. Such excellent sensing performance could be synergistically ascribed to the junction effect and interfacial engineering of Ag2S/SnS2 heterostructures, which not only modulates the electronic properties of SnS2 but also provides abundant adsorption sites for gas sensing. This study offers guidance for engineering heterostructures with high-quality interface, which might stimulate the exploitation of other novel materials and widen their potential applications.

Hysteresis-free and μs-switching of D/E-modes Ga2O3 hetero-junction FETs with the BV2/Ron,sp of 0.74/0.28 GW/cm2

In this Letter, we report on establishing high performance hysteresis-free and μs-switching depletion/enhancement-mode (D/E-mode) β-Ga2O3 heterojunction (HJ) field effect transistors (FETs) with the state-of-art power figure-of-merit (P-FOM). By optimizing the p-NiOx/n-Ga2O3 interface and n-Ga2O3 recess technology, a positive threshold voltage (VT) as well as a low subthreshold slope can be substantially achieved. The trade-off between the on-resistance (Ron,sp) and breakdown voltage (BV) is improved by incorporation of T-shaped NiOx, resulting in the Ron,sp of 6.24/13.75 mΩ cm2 and the breakdown voltage (BV) of 2145/1977 V for D/E-mode devices and yielding the P-FOM = BV2/Ron,sp to be 0.74/0.28 GW/cm2. To the best of all the authors' knowledge, those P-FOMs are the highest ones among all published lateral Ga2O3 FETs. Benefited from the high-quality interface, a negligible hysteresis of 4 mV and μs-switching can be essentially achieved, showing the great promise of Ga2O3 HJ-FETs for future high-power, high-efficiency, and high-speed power electronics.

Scalable Moir\xe9 Lattice with Oriented TMD Monolayers

Moiré lattice in artificially stacked monolayers of two-dimensional (2D) materials effectively modulates the electronic structures of materials, which is widely highlighted. Formation of the electronic Moiré superlattice promises the prospect of uniformity among different moiré cells across the lattice, enabling a new platform for novel properties, such as unconventional superconductivity, and scalable quantum emitters. Recently, epitaxial growth of the monolayer transition metal dichalcogenide (TMD) is achieved on the sapphire substrate by chemical vapor deposition (CVD) to realize scalable growth of highly-oriented monolayers. However, fabrication of the scalable Moiré lattice remains challenging due to the lack of essential manipulation of the well-aligned monolayers for clean interface quality and precise twisting angle control. Here, scalable and highly-oriented monolayers of TMD are realized on the sapphire substrates by using the customized CVD process. Controlled growth of the epitaxial monolayers is achieved by promoting the rotation of the nuclei-like domains in the initial growth stage, enabling aligned domains for further grain growth in the steady-state stage. A full coverage and distribution of the highly-oriented domains are verified by second-harmonic generation (SHG) microscopy. By developing the method for clean monolayer manipulation, hetero-stacked bilayer (epi-WS2/epi-MoS2) is fabricated with the specific angular alignment of the two major oriented monolayers at the edge direction of 0°/ ± 60°. On account of the optimization for scalable Moiré lattice with a high-quality interface, the observation of interlayer exciton at low temperature illustrates the feasibility of scalable Moiré superlattice based on the oriented monolayers.

Demonstration of High Interface Quality AlGaN/GaN MIS-HEMT with Fully Wet Recess and MOCVD Grown AlN Dielectric

In this study, the results indicate that a method combining fully-recessed wet etching and regrown channel by MOCVD is capable of obtaining high quality interface in GaN MIS-HEMT. A low Vth hysterisis GaN MIS-HEMT of 0.3V is demonstrated in this work. The GaN MIS-HEMT has a Vth of-1.5 V, a high Id,max of 771mA/mm and a RON of 13.5 Ω·mm. The wet etching shows good uniformity while the MOCVD grown AlN enhances the maximum drain current. The concept provides new insights to gate recess fabrication and MOCVD grown high quality dielectrics.

Low density of interface trap states and temperature dependence study of Ga2O3 Schottky barrier diode with p-NiOx termination

This work acquires a vertical β-Ga2O3 Schottky barrier diode (SBD) with the advanced termination structure of p-type NiOx and n-type β-Ga2O3 heterojunctions and coupled field plate structures to alleviate the crowding electric field. A Ga2O3 SBD delivers an average breakdown voltage of 1860 V and a specific on-resistance of 3.12 mΩ cm2, yielding a state-of-the-art direct-current Baliga's power figure of merit of 1.11 GW/cm2 at an anode area of 2.83 × 10−5 cm2. In addition, the Ga2O3 SBD with the same fabrication process at a large area of 1.21 × 10−2 cm2 also presents a high forward current of 7.13 A, a breakdown voltage of 1260 V, and a power figure-of-merit of 235 MW/cm2. According to dynamic pulse switching and capacitance-frequency characteristics, an optimized p-NiOx/Ga2O3 interface with a maximum trap density of 4.13 × 1010 eV−1 cm−2 is delivered. Moreover, based on the forward current-voltage measurement at various temperatures, the physics behind a forward conduction mechanism is illustrated. Ga2O3 SBDs with p-NiOx/n-Ga2O3 heterojunction termination, field plate, high power figure of merit, and high quality interface as well as suppressed resistance increase after dynamic pulse switching, verifying their great promise for future high power applications.

A zero-dimensional/two-dimensional Ag–Ag2S–CdS plasmonic nanohybrid for rapid photodegradation of organic pollutant by solar light

Herein, the two synthesis strategies are employed for rational design of 0D/2DAg-Ag2S–CdS heterojunctions towards photocatalytic degradation of methyl orange (MO) under simulated solar light. As the first strategy, a ternary Ag–Ag2S–CdS nanosheet (NS) heterojunction was fabricated via combined cation exchange and photo-reduction (CEPR) method (Ag–Ag2S–CdS/CEPR). The second strategy employed coprecipitation (CP) method (Ag–Ag2S–CdS/CP). Strikingly, SEM, TEM and HR-TEM images are manifested the first strategy is beneficial for retaining the original thickness (20.2 nm) of CdS NSs with a dominant formation of metallic Ag, whereas the second strategy increases the thickness (33.4 nm) of CdS NSs with a dominant formation of Ag2S. The Ag–Ag2S–CdS/CEPR exhibited 1.8-fold and 3.5-fold enhancement in photocatalytic activities as compared to those of Ag–Ag2S–CdS/CP and bare CdS NSs, respectively. This enhanced photocatalytic activity could be ascribed to fact that the first strategy produces a high-quality interface with intimate contact between the Ag–Ag2S–CdS heterojunctions, resulting in enhanced separation of photo-excited charge carriers, extended light absorption, and enriched active-sites. Furthermore, the degradation efficiency of Ag–Ag2S–CdS/CEPR was significantly reduced to ∼5% in the presence of BQ (•O2− scavenger), indicating that •O2− is the major active species that can decompose MO dye under simulated solar light.

Hysteresis-free Ga2O3 solar-blind phototransistor modulated from photoconduction to photogating effect

High tunability of photoresponse characteristics under work conditions is desired for a single solar-blind photodetector to be applied in multifarious fields. Three-terminal metal–oxide–semiconductor field-effect phototransistors have shown excellent controllability of performance, but the hysteresis issue impedes their stable operation. In this work, the metal–semiconductor field-effect phototransistor based on the exfoliated Ga2O3 microflake and graphene thin film is demonstrated. The high-quality quasi-van der Waals interface between Ga2O3 and graphene eliminates the hysteresis issue and generates a subthreshold swing as low as 69.4 mV/dec. By regulating gate voltage (Vg), the dominated mechanism of photocurrent generation in the device can be tuned continuously from the fast photoconduction effect to photogating effect with high photogain. Accordingly, the responsivity, dark current, detectivity, rejection ratio, and decay time of the device can be well adjusted by the Vg. At Vg = −1 V and a source to drain voltage of 2 V, the device shows excellent performance with a responsivity of 2.82 × 103 A/W, a rejection ratio of 5.88 × 105, and a detectivity of 2.67 × 1015 Jones under 254 nm illumination. This work shows the possibility of realizing highly tunable solar-blind photodetectors to meet the requirements for different application fields by introducing gate voltage modulation.

Double-sided friction stir spot welding of ultra-high strength C-Mn-Si martensitic steel by adjustable probes

High-quality joints of ultra-high strength C-Si-Mn quenched and tempered martensitic steel were successfully fabricated by double-sided friction stir spot welding (FSSW) with adjustable probes. We found that the mechanical properties of the welded joints increased with the rotation speed, due to the increase of hard phase and strengthened welding interface. In the surroundings of the shoulder/probe interface, a large amount of martensite was formed resulting from the higher austenitizing temperature and the higher cooling rate. In addition, severe material flow vertical to the welding interface and strong relative slip along the interface conspicuously favored the fragmentation and dispersion of the oxides, and the drive force for grain boundary migration introduced by severe material flow further promoted the dispersion of the oxides. Consequently, a high-quality welding interface was fabricated, and a high-strength welded joint with a stable plug failure was obtained. Furthermore, we pointed out that soundly welded high strength interface of ultra-high strength steel can be obtained by higher welding temperature and introducing strong material flow vertical to the interface.

In‐Situ Generated CsPbBr3 Nanocrystals on O‐Defective WO3 for Photocatalytic CO2 Reduction

Metal halide perovskite (MHP) nanocrystals (NCs) have shown promising application in photocatalytic CO2 reduction, but their activities are still largely restrained by severe charge recombination and narrow solar spectrum response. Assembly of heterojunctions can be beneficial to the charge separation in MHPs while the assembly process usually brings native interfacial defects, impeding efficient charge separation between two materials. Herein, an in-situ generation strategy was developed to prepare CsPbBr3 /WO3 heterojunction, using WO3 nanosheets (NSs) as growing substrate for the growth of CsPbBr3 NCs. The developed CsPbBr3 /WO3 heterojunction exhibited a high-quality interface, greatly facilitating charge transfer between two semiconductors. The hybrid photocatalyst displayed an excellent activity toward CO2 reduction, which was about 7-fold higher than pristine CsPbBr3 NCs and 3.5-fold higher than their assembled counterparts. The experimental results and theoretical simulations revealed that a Z-scheme mechanism with a favorable internal electric field was responsible for the good performance of CsPbBr3 /WO3 heterojunction. By using O-defective WO3 NSs as a near-infrared (NIR) light absorber, the CsPbBr3 /WO3 heterojunction could harvest NIR light and showed an impressive activity toward CO2 reduction. This work demonstrates a new strategy to design MHP-based heterojunctions by synergistically considering the interface quality, charge transfer mode, interfacial electric field, and light response range between two semiconductors.

Mixed Cohesive Zone Modeling of Interface Debonding between Propellant and Insulation

High-quality interface between propellant and insulation is the strict requirement difficult to quantify in solid rocket motor. In this study, the mixed mode delamination process of propellant and insulation interface is considered in double cantilever sandwich beam (DCSB) and single lap-joint (SLJ) test. The PPR cohesive zone model (CZM) and bilinear CZM in ABAQUS are introduced in this mixed fracture progress. In order to implement the PPR model in ABAQUS, user subroutine user element (UEL) is programmed for the novel CZM. Two simple pure mode I and mode II fracture problems are designed to check the accuracy of the UEL, and the result of verification is excellent. DCSB and SLJ test and their corresponding results are used again in the same inverse analysis with the two typical effective displacement-based and potential-based CZM. Base on the results, a series discussion and some conclusions are made. The debonding progress of the propellant and insulation interface in DCSB and SLJ test are mixed mode. The PPR CZM is prior in simulation than the bilinear CZM in ABAQUS because the PPR CZM is much more flexible with changeable traction-separation shape. The real normal and tangential displacement at damage initiation shows the unreasonable change in bilinear CZM in ABAQUS under mixed mode fracture. The PPR CZM and bilinear CZM in ABAQUS are all thickness-dependent model. The real initial stiffness and the critical displacement in the bilinear CZM and the real maximum traction in PPR model are dependent on the thickness of cohesive element. The different thickness dependence of the two model is caused by the implementation method.

In situ isotope study of indium diffusion in InP/Al2O3 stacks

A high-quality interface of III–V/high-k dielectrics is critical to obtain high-performance devices. Indium out-diffusion in an InP-based stack is correlated with the density of interface states. Diffused In species are in the form of oxides, which invite the question about the diffusion of species through the high-k dielectric layer. For an InP/Al2O3 stack, O18 isotope tracing is carried out to investigate if the interface oxygen and indium atoms diffuse together or not. This work sheds light on the fundamental mechanism for III–V semiconductors' interface elemental diffusion as well as the interface passivation strategy.

Interfacial Engineering for Improved Stability of Flexible Perovskite Solar Cells

Because of unique optoelectronic properties, such as extended carrier lifetime, large absorption coefficient, high defect tolerance factor, low exciton binding energy, and ambipolar diffusion, metal halide perovskites display enormous potential for the next generation of photovoltaics. Flexible perovskite solar cells with low weight, high flexibility, and conformability have attracted attention for portable electronic products. The interface is crucial in perovskite solar cells for both photovoltaic efficiency and operational stability. A high-quality interface could be obtained through interface engineering. Here, we summarize the degradation mechanisms and review interfacial engineering with the emphasis on their effects in flexible perovskite solar cells. On the basis of recent research progress in flexible devices, current challenges, possible directions, and perspectives are discussed. This would be helpful to promote the commercialization of flexible perovskite solar cells.

Higher Defense Organization in Nepal

An efficient Higher Defense Organization (HDO) is mandatory for any nation to match their national security objectives based on their geostrategic environment, challenges, and historical experiences. The components of HDO need efficiency, synergy, and decision-making ability to complement national interests. Nepal is experiencing diverse national security challenges of the millennia. In Nepal’s context, there is a requirement to revisit higher defense management components and enhance their capability so we guarantee their high-quality interface with elements of national power to effectively secure national interest. In the article, the HDO will be studied to identify our weaknesses to draw a framework of understanding.

First demonstration of robust tri-gate β-Ga2O3 nano-membrane field-effect transistors

Nano-membrane tri-gate β-gallium oxide (β-Ga2O3) field-effect transistors (FETs) on SiO2/Si substrate fabricated via exfoliation have been demonstrated for the first time. By employing electron beam lithography, the minimum-sized features can be defined with the footprint channel width of 50 nm. For high-quality interface between β-Ga2O3 and gate dielectric, atomic layer-deposited 15 nm thick aluminum oxide (Al2O3) was utilized with tri-methyl-aluminum (TMA) self-cleaning surface treatment. The fabricated devices demonstrate extremely low subthreshold slope (SS) of 61 mV dec−1, high drain current (I DS) ON/OFF ratio of 1.5 × 109, and negligible transfer characteristic hysteresis. We also experimentally demonstrated robustness of these devices with current–voltage (I–V) characteristics measured at temperatures up to 400 °C.

Enhancing energy storage performances in an ultra-wide temperature range via interface engineering and thermal management for silicon-integrated dielectric capacitors

High-performance silicon-integrated dielectric thin film capacitors with superior thermal stability are strongly attractive for application in integrated circuits and electronic devices. Here, by combining interface engineering with thermal management, lead-free 0.85BaTiO3-0.15Bi(Mg0.5Zr0.5)O3 (BT-BMZ) dielectric thin film capacitors were integrated on Si, HfO2-buffered Si (HfO2/Si), and graphene-buffered HfO2/Si (G/HfO2/Si) substrates. Benefiting from not only high-quality interface between HfO2 and Si but also the heat dissipation effect of the graphene layer, an ultra-high energy storage density up to 102 J/cm3 with an efficiency of 74.57% was obtained in BT-BMZ/G/HfO2/Si at room temperature. More importantly, the optimized capacitor exhibited an ultra-stable energy density of 52.1 J/cm3 (±10%) with high efficiency (over 70%) in a wide temperature range of –100 to 175 °C, greatly broadening the working temperature in comparison to BT-BMZ/Si (–100 to 100 °C). The present research provides a scalable strategy to enhance energy storage performance of dielectric capacitors, especially at elevated temperatures.