Showing 1–50 of 99 results for author: Ang, K

Ultralow p-type contact resistance for ultra-nanoscaled 2D-materials transistors

Authors: Ying Xiong, Tong Su, Qiang Li, Yee Sin Ang, Lain-Jong Li, L. K. Ang

Abstract: High contact resistance is one of the main bottlenecks for practical two-dimensional (2D) materials transistors, especially for p-type transistors and future 2D ultra-nanoscaled (sub-10 nm) FETs (PMOS + CMOS). We develop self-consistent contact resistance models for metal-2D semiconductor-metal devices to capture the essential interface physics for both vertical and edge configurations. Our calcul… ▽ More High contact resistance is one of the main bottlenecks for practical two-dimensional (2D) materials transistors, especially for p-type transistors and future 2D ultra-nanoscaled (sub-10 nm) FETs (PMOS + CMOS). We develop self-consistent contact resistance models for metal-2D semiconductor-metal devices to capture the essential interface physics for both vertical and edge configurations. Our calculations have been verified with various recent experiments of p-type and n-type contacts. For a given set of materials, the model determines the scaling of contact resistance over a wide range of device parameters including channel length (100s nm down to sub-10 nm), doping and mobility of the 2D materials, contact length of the electrodes, and applied voltages. These results identify the key factors in order to reduce the contact resistance for p-type 2D semiconductor WSe$_2$ towards the sub-10 nm channel length scale that are readily to be realized by future experiments. It is found that the effect of source-limited current saturation is the key challenge for down scaling 2D FET to sub-10 nm channel length. Two topological semi-metals as potential electrodes are proposed for 2D p-type semiconducting WSe$_2$ with our predicted contact resistance $R_c<$ 100 $Ω\; {\rm μm}$ approaching the quantum limit. Our model is also verified with the computational expensive full quantum atomistic model that is currently limited to a few nm scale. △ Less

Submitted 7 August, 2026; originally announced August 2026.

Scoping Review of AI, Metrology, and ESG in the Semiconductor Sector: Implications for Safe and Sustainable by Design (SSbD)

Authors: Karen Ang, Han-Teng Liao

Abstract: The semiconductor sector faces a dual transition: scaling manufacturing execution through Artificial Intelligence (AI) while satisfying stringent sustainability mandates, such as the EU Carbon Border Adjustment Mechanism (CBAM). This paper presents a scoping review of 1,465 documents indexed in Web of Science and Scopus, spanning AI-integrated metrology, supply chain ESG, and federated industrial… ▽ More The semiconductor sector faces a dual transition: scaling manufacturing execution through Artificial Intelligence (AI) while satisfying stringent sustainability mandates, such as the EU Carbon Border Adjustment Mechanism (CBAM). This paper presents a scoping review of 1,465 documents indexed in Web of Science and Scopus, spanning AI-integrated metrology, supply chain ESG, and federated industrial data spaces. Network analysis reveals a highly fragmented "core-periphery" knowledge structure, emphasizing a critical structural hole between AI-driven process optimization and downstream sustainability governance. To close these gaps, this study proposes a 6-layer Safe and Sustainable by Design (SSbD) architecture grounded in a System of Systems (SoS) paradigm. By establishing distinct "grid-to-core" and "standards-through-supply-chain" integration pathways, the proposed framework demonstrates how virtual metrology (VM), localized federated learning, and defensive RegTech mechanisms can build provenance-aware data fabrics. Ultimately, this architecture positions regulatory compliance as a driver for innovation, enabling secure, climate-neutral, and circular value chains in semiconductor manufacturing. △ Less

Submitted 25 July, 2026; originally announced July 2026.

Comments: to be published in the 32nd IEEE ICE/ITMC Conference (Porto, Portugal) proceedings

MSC Class: 90B06; 68M14; 90B50; 91B76; 68T42 ACM Class: H.3.5; K.4.4; I.2.11; H.4.1; K.6.5

Orchestrating the Twin Transition in Multinational Corporations: Technology Roadmapping for Green and Digital Global Business Services

Authors: Han-Teng Liao, Karen Ang

Abstract: Global Business Services (GBS) have emerged as a "living laboratory" for the Twin Transition of Green and Digital Transformation, as multinational corporations (MNCs) face increasing pressure to harmonize digital efficiency with environmental stewardship. Aiming to derive a socio-technical framework, this paper synthesizes Technology Roadmapping (TRM) with the International Telecommunication Union… ▽ More Global Business Services (GBS) have emerged as a "living laboratory" for the Twin Transition of Green and Digital Transformation, as multinational corporations (MNCs) face increasing pressure to harmonize digital efficiency with environmental stewardship. Aiming to derive a socio-technical framework, this paper synthesizes Technology Roadmapping (TRM) with the International Telecommunication Union (ITU) ICT-centric innovation ecosystem toolkit. A bibliometric analysis of research clusters reveals an evolutionary shift from basic process automation toward "Sustainable Intelligence," identifying the GBS unit as a central "operational airlock" that mediates between landscape pressures -- such as the EU's dual mandate and Carbon Border Adjustment Mechanisms -- and niche innovations in AI-native workflows. The study further maps these clusters onto a stakeholder engagement canvas, highlighting how resilient "Middle Power" hubs in Poland, Portugal, and Malaysia are bypassing the middle-income trap to provide a "third way" for global value chains amidst a bifurcated geopolitical cloud. The results offer a data-driven design approach for leaders and entrepreneurial support networks to orchestrate talent and supply chain flows, thereby enriching the conceptual understanding of Industry 5.0 and the role of GBS as a primary mechanism for navigating a volatile, multipolar digital economy. △ Less

Submitted 10 June, 2026; originally announced June 2026.

Comments: 9 pages, 6 figures

MSC Class: 90B50; 91D30; 91B55; 68P20; 90B06 ACM Class: K.6.0; K.4.4; H.4.1; H.1.1; J.1

From Stacks to Circuits: A Regenerative Socio-Technical Roadmap for AI Infrastructure within Planetary Boundaries

Authors: Han-Teng Liao, Karen Ang

Abstract: Current scaling trajectories for Generative AI, typified by linear supply-side "stacks," prioritize performance density while externalizing significant thermodynamic and material costs. As the "Twin Transition" of green and digital transformation accelerates, the industry faces technology gaps - including Scope 3 emissions and e-waste recycling - that impede sustainable scaling and lead to social… ▽ More Current scaling trajectories for Generative AI, typified by linear supply-side "stacks," prioritize performance density while externalizing significant thermodynamic and material costs. As the "Twin Transition" of green and digital transformation accelerates, the industry faces technology gaps - including Scope 3 emissions and e-waste recycling - that impede sustainable scaling and lead to social tensions. This study proposes a Regenerative Socio-Technical roadmap that repurposes the Sustainable Production and Consumption system map to reframe artificial intelligence infrastructure as a system-of-systems governed ultimately by planetary limits. By integrating the Institute of Electrical and Electronics Engineers International Roadmap for Devices and Systems (IEEE IRDS) sustainability considerations for semiconductor facilities, the study proposes a metabolic circuit framework that centers "Values and Needs" within production and consumption relationship loops. This study identifies critical gaps in current Nvidia-centric roadmaps and proposes a competing reference architecture. It demonstrates how a spontaneous order of resource parsimony and planetary accountability can provide an actionable pathway for regulatory compliance and industrial resilience in the digital circular economy. △ Less

Submitted 9 June, 2026; originally announced June 2026.

Comments: This document is a working paper and reflects the state of research as of May 2026. Comments are welcome and should be directed to the corresponding author at h.liao@ieee.org. This work is accepted for presentation at the 32nd IEEE ICE/ITMC Conference, Porto, Portugal

MSC Class: 93A14; 91B76; 93C95; 90B30; 90B06; 91B38; 37N40 ACM Class: C.2.4; D.2.11; K.4.1; K.4.2; I.2.1; I.2.m; K.6.4; K.6.5

Journal ref: 2026 IEEE International Conference on Engineering, Technology, and Innovation (ICE/ITMC), forthcoming 2026

Trustworthy Smart Fabs via Professional Proxies: Scaling Safe and Sustainable by Design (SSbD) through Industrial Data Spaces

Authors: Han-Teng Liao, Chang-Yi Kao, Karen Ang

Abstract: The convergence of the 2026 European Union Safe and Sustainable by Design (SSbD) framework, Corporate Sustainability Due Diligence Directive (CSDDD), and Carbon Border Adjustment Mechanism (CBAM) introduce a severe governance bottleneck for advanced semiconductor manufacturing facilities ("Smart Fabs"). Regulatory compliance demands have surpassed the capacity of manual corporate reporting, creati… ▽ More The convergence of the 2026 European Union Safe and Sustainable by Design (SSbD) framework, Corporate Sustainability Due Diligence Directive (CSDDD), and Carbon Border Adjustment Mechanism (CBAM) introduce a severe governance bottleneck for advanced semiconductor manufacturing facilities ("Smart Fabs"). Regulatory compliance demands have surpassed the capacity of manual corporate reporting, creating a direct conflict between multi-stakeholder transparency and corporate data privacy. This paper addresses this challenge by introducing a zero-trust socio-technical orchestration framework that operationalizes a six-layer SSbD reference architecture within trustworthy industrial data spaces. We propose a shift from reactive automation to autonomous governance through "Professional Proxies"-role-based agentic workflows executing within hardware-isolated trust zones. Structured as an interoperable network protocol stack, the framework coordinates an automated, five-step "relay race" between Facility, Process Engineering, and Finance proxy teams to align factory-floor yield models with macro-level sustainability mandates. By executing Virtual Metrology (VM) predictions and Federated Machine Learning (FML) inside hardware-rooted Trusted Execution Environments (TEEs), this architecture resolves the Data Sovereignty Paradox, demonstrating how fabs can export cryptographically signed compliance tokens via International Data Spaces (IDS) connectors without exposing proprietary process recipes. Ultimately, this framework provides technology managers with a verifiable, evidence-based pathway toward resilient, net-zero Industry 5.0 ecosystems. △ Less

Submitted 8 June, 2026; originally announced June 2026.

Comments: This work was accepted for presentation at the 32nd IEEE ICE/ITMC Conference, Porto, Portugal, 2026 but was subsequently withdrawn prior to publication due to submission volume limits. It is currently under consideration for publication elsewhere

MSC Class: 68M14; 68T42; 68U35; 90B30; 90C29; 91B55; 91B55 ACM Class: C.3; D.4.6; H.4.1; I.2.11; J.2; K.4.0; K.4.4

Chips in the Flatland : 2D Semiconductors for Future Computing Electronic

Authors: Narin Trakarnvanich, Mitra Sanchali, Tong Su, Haiyu Meng, Jing Lu, Kah-Wee Ang, Lain-Jong Li, Chit Siong Lau, Yee Sin Ang

Abstract: As transistor scaling approaches its fundamental physical limits in the Angstrom era, two-dimensional (2D) semiconductors have emerged as the promising channel material candidates for future computing. While the device physics of 2D semiconductors have been rigorously explored, translating these nanodevices into fully functional integrated circuits remains a largely uncharted frontier. This review… ▽ More As transistor scaling approaches its fundamental physical limits in the Angstrom era, two-dimensional (2D) semiconductors have emerged as the promising channel material candidates for future computing. While the device physics of 2D semiconductors have been rigorously explored, translating these nanodevices into fully functional integrated circuits remains a largely uncharted frontier. This review bridges the gap between material- and device-centric breakthroughs and circuit-level chip design in 2D semiconductors, a valley of death that has so far prevented translation of high-performance individual transistors into functional chips. We track the evolution of 2D semi-conductor field-effect transistors from basic Boolean logic families and standard cells to complex chip architectures, including recent milestones in RISC-V and monolithic CMOS microprocessors. Critically, we highlight the indispensable role of multiscale compact modeling, spanning semiclassical, quantum-hybrid and data-driven approaches, as the necessary link between device physics and the electronic design automation workflows for scalable chip development. By summarizing recent breakthroughs and identifying the bottlenecks in both fab and fabless trajectories of 2D semiconductors, this review shall provide insights that motivates the translation of proof-of-concept 2D transistors into fully functional computing chips, paving a way towards future Angstrom era computing technology empowered by 2D semiconductors. △ Less

Submitted 26 May, 2026; originally announced May 2026.

Nonlinear Magnetic Orbital Hall Effect Induced by Spin-Orbit Coupling

Authors: Hui Wang, Huiying Liu, Yanfeng Ge, Xukun Feng, Jiaojiao Zhu, Jin Cao, Cong Xiao, Shengyuan A. Yang, Lay Kee Ang

Abstract: Electrical readout of 180$^\circ$ switching in strictly compensated collinear antiferromagnets remains a major challenge in antiferromagnetic spintronics. Electrical writing of perpendicularly magnetized ferromagnets by out-of-plane orbital torque remains an important challenge in orbitronics. In this work, we propose a second-order nonlinear magnetic orbital Hall effect in the source antiferromag… ▽ More Electrical readout of 180$^\circ$ switching in strictly compensated collinear antiferromagnets remains a major challenge in antiferromagnetic spintronics. Electrical writing of perpendicularly magnetized ferromagnets by out-of-plane orbital torque remains an important challenge in orbitronics. In this work, we propose a second-order nonlinear magnetic orbital Hall effect in the source antiferromagnet as a simultaneous recipe for both difficulties. This orbitronics effect is induced by spin-orbit coupling and is odd in the Néel vector, thus is a unique effect that integrates both functionalities via electric control of the Néel vector in the source antiferromagnet. Our first-principles calculations in CuMnAs predict significant non-perturbative orbital effects from spin-orbit coupling, with a orbital Berry-curvature dipole mechanism. These findings unveil new possibilities opened by topological antiferromagnetic orbitronics. △ Less

Submitted 2 April, 2026; originally announced April 2026.

System Design of the Ultra Mobility Vehicle: A Driving, Balancing, and Jumping Bicycle Robot

Authors: Benjamin Bokser, Daniel Gonzalez, Aaron Preston, Alex Bahner, Annika Wollschläger, Arianna Ilvonen, Asa Eckert-Erdheim, Ashwin Khadke, Bilal Hammoud, Dean Molinaro, Fabian Jenelten, Henry Mayne, Howie Choset, Igor Bogoslavskyi, Itic Tinman, James Tigue, Jan Preisig, Kaiyu Zheng, Kenny Sharma, Kim Ang, Laura Lee, Liana Margolese, Nicole Lin, Oscar Frias, Paul Drews , et al. (17 additional authors not shown)

Abstract: Trials cyclists and mountain bike riders can hop, jump, balance, and drive on one or both wheels. This versatility allows them to achieve speed and energy-efficiency on smooth terrain and agility over rough terrain. Inspired by these athletes, we present the design and control of a robotic platform, Ultra Mobility Vehicle (UMV), which combines a bicycle and a reaction mass to move dynamically with… ▽ More Trials cyclists and mountain bike riders can hop, jump, balance, and drive on one or both wheels. This versatility allows them to achieve speed and energy-efficiency on smooth terrain and agility over rough terrain. Inspired by these athletes, we present the design and control of a robotic platform, Ultra Mobility Vehicle (UMV), which combines a bicycle and a reaction mass to move dynamically with minimal actuated degrees of freedom. We employ a simulation-driven design optimization process to synthesize a spatial linkage topology with a focus on vertical jump height and momentum-based balancing on a single wheel contact. Using a constrained Reinforcement Learning (RL) framework, we demonstrate zero-shot transfer of diverse athletic behaviors, including track-stands, jumps, wheelies, rear wheel hopping, and front flips. This 23.5 kg robot is capable of high speeds (8 m/s) and jumping on and over large obstacles (1 m tall, or 130% of the robot's nominal height). △ Less

Submitted 17 March, 2026; v1 submitted 25 February, 2026; originally announced February 2026.

Comments: 17 Pages, 11 figures, 3 movies, 2 tables

Nonperturbative Magnetic Orbital Hall Effect in Altermagnets

Authors: Xukun Feng, Jin Cao, Lay Kee Ang, Shengyuan A. Yang, Cong Xiao, X. C. Xie

Abstract: Recent studies on altermagnets have focused considerable attention on nonrelativistic effects that persist in the absence of spin-orbit coupling (SOC). As a result, the relative importance of various phenomena in altermagnets has commonly been judged by their dependence on SOC. Here, we challenge this common wisdom by uncovering the magnetic orbital Hall effect, which is nonperturbative in SOC str… ▽ More Recent studies on altermagnets have focused considerable attention on nonrelativistic effects that persist in the absence of spin-orbit coupling (SOC). As a result, the relative importance of various phenomena in altermagnets has commonly been judged by their dependence on SOC. Here, we challenge this common wisdom by uncovering the magnetic orbital Hall effect, which is nonperturbative in SOC strength. We establish the symmetry properties of this effect, demonstrating that it is strictly forbidden in conventional collinear antiferromagnets yet universally allowed in all ten spin-Laue classes of collinear altermagnets. Counterintuitively, although SOC-induced, it reaches giant magnitudes in altermagnets-comparable to or even exceeding the nonrelativistic spin Hall effect. Moreover, altermagnetic symmetry enables unconventional collinear-polarized orbital currents, allowing field-free manipulation of perpendicular magnetization. Our first-principles calculations predict strong room-temperature responses in the experimentally established altermagnets CrSb and FeSb2. These findings reveal the previously overlooked potential of altermagnetic orbitronics and broaden the horizons for altermagnets in high-performance magnetic memory applications. △ Less

Submitted 21 May, 2026; v1 submitted 22 February, 2026; originally announced February 2026.

Out-of-Plane Nonlinear Orbital Hall Torque

Authors: Hui Wang, Xukun Feng, Jin Cao, Huiying Liu, Weibo Gao, Cong Xiao, Shengyuan A. Yang, Lay Kee Ang

Abstract: Despite recent advances in orbitronics, generating out-of-plane orbital torques essential for field-free deterministic switching of perpendicular magnetization remains a key challenge. Here, we propose a strategy to produce such unconventional torques across broad classes of materials, by leveraging the nonlinear orbital Hall effect. We demonstrate that this nonlinear orbital response is dramatica… ▽ More Despite recent advances in orbitronics, generating out-of-plane orbital torques essential for field-free deterministic switching of perpendicular magnetization remains a key challenge. Here, we propose a strategy to produce such unconventional torques across broad classes of materials, by leveraging the nonlinear orbital Hall effect. We demonstrate that this nonlinear orbital response is dramatically amplified by topological band degeneracies, where it overwhelmingly dominates the spin response even in systems with strong spin-orbit coupling. These features are confirmed via a quantitative investigation of representative topological metals RhSi, YPtBi, and PbTaSe$_2$, by combining our theory with first-principles calculations. The resulting orbital torques substantially surpass those from linear mechanisms reported thus far. These findings propel the research of orbital transport into the nonlinear regime, broaden the scope of orbital source materials, and establish a new pathway towards high-performance orbitronic devices. △ Less

Submitted 13 November, 2025; originally announced November 2025.

Human or AI? Comparing Design Thinking Assessments by Teaching Assistants and Bots

Authors: Sumbul Khan, Wei Ting Liow, Lay Kee Ang

Abstract: As design thinking education grows in secondary and tertiary contexts, educators face the challenge of evaluating creative artefacts that combine visual and textual elements. Traditional rubric-based assessment is laborious, time-consuming, and inconsistent due to reliance on Teaching Assistants (TA) in large, multi-section cohorts. This paper presents an exploratory study investigating the reliab… ▽ More As design thinking education grows in secondary and tertiary contexts, educators face the challenge of evaluating creative artefacts that combine visual and textual elements. Traditional rubric-based assessment is laborious, time-consuming, and inconsistent due to reliance on Teaching Assistants (TA) in large, multi-section cohorts. This paper presents an exploratory study investigating the reliability and perceived accuracy of AI-assisted assessment compared to TA-assisted assessment in evaluating student posters in design thinking education. Two activities were conducted with 33 Ministry of Education (MOE) Singapore school teachers to (1) compare AI-generated scores with TA grading across three key dimensions: empathy and user understanding, identification of pain points and opportunities, and visual communication, and (2) examine teacher preferences for AI-assigned, TA-assigned, and hybrid scores. Results showed low statistical agreement between instructor and AI scores for empathy and pain points, with slightly higher alignment for visual communication. Teachers preferred TA-assigned scores in six of ten samples. Qualitative feedback highlighted the potential of AI for formative feedback, consistency, and student self-reflection, but raised concerns about its limitations in capturing contextual nuance and creative insight. The study underscores the need for hybrid assessment models that integrate computational efficiency with human insights. This research contributes to the evolving conversation on responsible AI adoption in creative disciplines, emphasizing the balance between automation and human judgment for scalable and pedagogically sound assessment. △ Less

Submitted 11 December, 2025; v1 submitted 17 October, 2025; originally announced October 2025.

Comments: to be published in IEEE TALE 2025

Co-Designing Interdisciplinary Design Projects with AI

Authors: Wei Ting Liow, Sumbul Khan, Lay Kee Ang

Abstract: Creating interdisciplinary design projects is time-consuming and cognitively demanding for teachers, requiring curriculum alignment, cross-subject integration, and careful sequencing. International research reports increasing teacher use of AI alongside persistent workload pressures, underscoring the need for planning support. This paper presents the Interdisciplinary Design Project Planner (IDPpl… ▽ More Creating interdisciplinary design projects is time-consuming and cognitively demanding for teachers, requiring curriculum alignment, cross-subject integration, and careful sequencing. International research reports increasing teacher use of AI alongside persistent workload pressures, underscoring the need for planning support. This paper presents the Interdisciplinary Design Project Planner (IDPplanner), a GPT-based planning assistant grounded in Design Innovation principles, alignment with Singapore secondary school syllabuses, and 21st-century competencies. In a within-subject, counterbalanced workshop with 33 in-service teachers, participants produced two versions of the same project: manual and AI-assisted, followed by self- and peer-evaluations using a six-dimensional rubric. The AI-assisted version received higher scores for Curriculum Alignment, Design Thinking Application, and Coherence and Flow, with a marginal advantage for Assessment Strategies. Teacher reflections indicated that AI-assisted planning improved structure, sequencing, and idea generation, while contextualization to local syllabuses, class profiles, and student needs remained teacher-led. Contributions include a purpose-built planning tool that organizes ideas into a ten-component flow with ready-to-adapt prompts, templates, and assessment suggestions; an empirical, rubric-based comparison of planning quality; and evidence that AI can function as a pedagogical planning partner. Recommendations emphasize hybrid teacher-AI workflows to enhance curriculum alignment and reduce planning complexity, and design suggestions for developers to strengthen contextual customization, iterative design support, and localized rubrics. Although instantiated with a Singapore-based curriculum, the planning flow and rubric are framework-agnostic and can be parameterized for other systems. △ Less

Submitted 17 October, 2025; originally announced October 2025.

Comments: to be published in IEEE TALE 2025

Building Bigraphs of the real world

Authors: Kang Rong Roy Ang

Abstract: This report proposes a formal specification for organising all buildings, streets and administrative areas in the world into a hierarchical space-partitioning tree using data from OpenStreetMap. This hierarchical structure is encoded into a bigraph, serving as a digital twin of the world and capturing complete street connectivity. It presents a tool implemented in OCaml (source code at https://git… ▽ More This report proposes a formal specification for organising all buildings, streets and administrative areas in the world into a hierarchical space-partitioning tree using data from OpenStreetMap. This hierarchical structure is encoded into a bigraph, serving as a digital twin of the world and capturing complete street connectivity. It presents a tool implemented in OCaml (source code at https://github.com/royangkr/bigraph-of-the-world ) that constructs bigraphs for regions from any part of the world. In addition, it contributes algorithmic improvements to open-source bigraph-building tools that enable them to efficiently construct and transform extremely large bigraphs, achieving up to a 97x speedup among other gains. △ Less

Submitted 2 July, 2025; originally announced August 2025.

Comments: Submitted in partial fulfilment of the requirements for Part II of the Computer Science Tripos at the University of Cambridge

Computational Design of Two-Dimensional MoSi$_2$N$_4$ Family Field-Effect Transistor for Future Ångström-Scale CMOS Technology Nodes

Authors: Che Chen Tho, Zongmeng Yang, Shibo Fang, Shiying Guo, Liemao Cao, Chit Siong Lau, Fei Liu, Shengli Zhang, Jing Lu, L. K. Ang, Lain-Jong Li, Yee Sin Ang

Abstract: Advancing complementary metal-oxide-semiconductor (CMOS) technology into the sub-1-nm angström-scale technology nodes is expected to involve alternative semiconductor channel materials, as silicon transistors encounter severe performance degradation at physical gate lengths below 10 nm. Two-dimensional (2D) semiconductors have emerged as strong candidates for overcoming short-channel effects due t… ▽ More Advancing complementary metal-oxide-semiconductor (CMOS) technology into the sub-1-nm angström-scale technology nodes is expected to involve alternative semiconductor channel materials, as silicon transistors encounter severe performance degradation at physical gate lengths below 10 nm. Two-dimensional (2D) semiconductors have emerged as strong candidates for overcoming short-channel effects due to their atomically thin bodies, which inherently suppress electrostatic leakage and improve gate control in aggressively scaled field-effect transistors (FETs). Among the growing library of 2D materials, the MoSi$_2$N$_4$ family -- a synthetic septuple-layered materials -- has attracted increasing attention for its remarkable ambient stability, suitable bandgaps, and favorable carrier transport characteristics, making it a promising platform for next-generation transistors. While experimental realization of sub-10-nm 2D FETs remains technologically demanding, computational device simulation using first-principles density functional theory combined with nonequilibrium Green's function transport simulations provide a powerful and cost-effective route for exploring the performance limits and optimal design of ultrascaled FET. This review consolidates the current progress in the computational design of MoSi$_2$N$_4$ family FETs. We review the physical properties of MoSi$_2$N$_4$ that makes them compelling candidates for transistor applications, as well as the simulated device performance and optimization strategy of MoSi$_2$N$_4$ family FETs. Finally, we identify key challenges and research gaps, and outline future directions that could accelerate the practical deployment of MoSi$_2$N$_4$ family FET in the angström-scale CMOS era. △ Less

Submitted 26 June, 2025; originally announced June 2025.

Comments: 27 pages, 11 figures

Pressure-Driven Metallicity in Ångström-Thickness 2D Bismuth and Layer-Selective Ohmic Contact to MoS2

Authors: Shuhua Wang, Shibo Fang, Qiang Li, Yunliang Yue, Zongmeng Yang, Xiaotian Sun, Jing Lu, Chit Siong Lau, L. K. Ang, Lain-Jong Li, Yee Sin Ang

Abstract: Recent fabrication of two-dimensional (2D) metallic bismuth (Bi) via van der Waals (vdW) squeezing method opens a new avenue to ultrascaling metallic materials into the ångström-thickness regime [Nature 639, 354 (2025)]. However, freestanding 2D Bi is typically known to exhibit a semiconducting phase [Nature 617, 67 (2023), Phys. Rev. Lett. 131, 236801 (2023)], which contradicts with the experimen… ▽ More Recent fabrication of two-dimensional (2D) metallic bismuth (Bi) via van der Waals (vdW) squeezing method opens a new avenue to ultrascaling metallic materials into the ångström-thickness regime [Nature 639, 354 (2025)]. However, freestanding 2D Bi is typically known to exhibit a semiconducting phase [Nature 617, 67 (2023), Phys. Rev. Lett. 131, 236801 (2023)], which contradicts with the experimentally observed metallicity in vdW-squeezed 2D Bi. Here we show that such discrepancy originates from the pressure-induced buckled-to-flat structural transition in 2D Bi, which changes the electronic structure from semiconducting to metallic phases. Based on the experimentally fabricated MoS2-Bi-MoS2 trilayer heterostructure, we demonstrate the concept of layer-selective Ohmic contact in which one MoS2 layer forms Ohmic contact to the sandwiched Bi monolayer while the opposite MoS2 layer exhibits a Schottky barrier. The Ohmic contact can be switched between the two sandwiching MoS2 monolayers by changing the polarity of an external gate field, thus enabling charge to be spatially injected into different MoS2 layers. The layer-selective Ohmic contact proposed here represents a layertronic generalization of metal/semiconductor contact, paving a way towards layertronic device application. △ Less

Submitted 25 June, 2025; v1 submitted 5 June, 2025; originally announced June 2025.

Comments: 14 pages, 5 figures

Unconventional tunnel magnetoresistance scaling with altermagnets

Authors: Zongmeng Yang, Xingyue Yang, Jianhua Wang, Qiang Li, Rui Peng, Ching Hua Lee, Lay Kee Ang, Jing Lu, Yee Sin Ang, Shibo Fang

Abstract: In conventional magnetic tunnel junctions (MTJs), the tunnel magnetoresistance (TMR) typically increases with barrier thickness as electron transmission in the antiparallel configuration decays faster than that of the parallel configuration. In this work, we reveal an anomalous scaling effect in altermagnetic tunnel junctions (AMTJs), where the TMR decreases anomalously with an increasing barrier… ▽ More In conventional magnetic tunnel junctions (MTJs), the tunnel magnetoresistance (TMR) typically increases with barrier thickness as electron transmission in the antiparallel configuration decays faster than that of the parallel configuration. In this work, we reveal an anomalous scaling effect in altermagnetic tunnel junctions (AMTJs), where the TMR decreases anomalously with an increasing barrier thickness. The anomalous scaling originates from the overlapping spin-split branches forming a transmission path that cannot be suppressed in the antiparallel state. Such phenomenon is explained by a double-barrier model and is further demonstrated using ab initio quantum transport simulations in 2D V2Te2O/Cr2Se2O/V2Te2O and V2Te2O/ZnSe/V2Te2O AMTJs. Our work identifies a peculiar unexpected transport characteristic of AMTJ, providing a fundamental limit on AMTJ device design and illustrating the potential optimal design of AMTJ at the ultrascaled monolayer limit. △ Less

Submitted 15 September, 2025; v1 submitted 22 May, 2025; originally announced May 2025.

An Automated Blackbox Noncompliance Checker for QUIC Server Implementations

Authors: Kian Kai Ang, Guy Farrelly, Cheryl Pope, Damith C. Ranasinghe

Abstract: We develop QUICtester, an automated approach for uncovering non-compliant behaviors in the ratified QUIC protocol implementations (RFC 9000/9001). QUICtester leverages active automata learning to abstract the behavior of a QUIC implementation into a finite state machine (FSM) representation. Unlike prior noncompliance checking methods, to help uncover state dependencies on event timing, QUICtester… ▽ More We develop QUICtester, an automated approach for uncovering non-compliant behaviors in the ratified QUIC protocol implementations (RFC 9000/9001). QUICtester leverages active automata learning to abstract the behavior of a QUIC implementation into a finite state machine (FSM) representation. Unlike prior noncompliance checking methods, to help uncover state dependencies on event timing, QUICtester introduces the idea of state learning with event timing variations, adopting both valid and invalid input configurations, and combinations of security and transport layer parameters during learning. We use pairwise differential analysis of learned behaviour models of tested QUIC implementations to identify non-compliance instances as behaviour deviations in a property-agnostic way. This exploits the existence of the many different QUIC implementations, removing the need for validated, formal models. The diverse implementations act as cross-checking test oracles to discover non-compliance. We used QUICtester to analyze analyze 186 learned models from 19 QUIC implementations under the five security settings and discovered 55 implementation errors. Significantly, the tool uncovered a QUIC specification ambiguity resulting in an easily exploitable DoS vulnerability, led to 5 CVE assignments from developers, and two bug bounties thus far. △ Less

Submitted 19 May, 2025; originally announced May 2025.

Comments: Accepted to ASIA CCS 2025. For Code & PoCs, see https://github.com/QUICTester

QUIC-Fuzz: An Effective Greybox Fuzzer For The QUIC Protocol

Authors: Kian Kai Ang, Damith C. Ranasinghe

Abstract: Network applications are routinely under attack. We consider the problem of developing an effective and efficient fuzzer for the recently ratified QUIC network protocol to uncover security vulnerabilities. QUIC offers a unified transport layer for low latency, reliable transport streams that is inherently secure, ultimately representing a complex protocol design characterised by new features and c… ▽ More Network applications are routinely under attack. We consider the problem of developing an effective and efficient fuzzer for the recently ratified QUIC network protocol to uncover security vulnerabilities. QUIC offers a unified transport layer for low latency, reliable transport streams that is inherently secure, ultimately representing a complex protocol design characterised by new features and capabilities for the Internet. Fuzzing a secure transport layer protocol is not trivial. The interactive, strict, rule-based, asynchronous nature of communications with a target, the stateful nature of interactions, security mechanisms to protect communications (such as integrity checks and encryption), and inherent overheads (such as target initialisation) challenge generic network protocol fuzzers. We discuss and address the challenges pertinent to fuzzing transport layer protocols (like QUIC), developing mechanisms that enable fast, effective fuzz testing of QUIC implementations to build a prototype grey-box mutation-based fuzzer; QUIC-Fuzz. We test 6, well-maintained server-side implementations, including from Google and Alibaba with QUIC-Fuzz. The results demonstrate the fuzzer is both highly effective and generalisable. Our testing uncovered 10 new security vulnerabilities, precipitating 2 CVE assignments thus far. In code coverage, QUIC-Fuzz outperforms other existing state-of-the-art network protocol fuzzers (Fuzztruction-Net, ChatAFL, and ALFNet) with up to an 84% increase in code coverage where QUIC-Fuzz outperformed statistically significantly across all targets and with a majority of bugs only discoverable by QUIC-Fuzz. We open-source QUIC-Fuzz on GitHub. △ Less

Submitted 25 March, 2025; originally announced March 2025.

Comments: Accepted to ESORICS 2025. For QUIC-Fuzz code, see https://github.com/QUICTester/QUIC-Fuzz

Journal ref: European Symposium on Research in Computer Security (ESORICS), 2025

Intrinsic Dynamic Generation of Spin Polarization by Time-Varying Electric Field

Authors: Xukun Feng, Jin Cao, Zhi-Fan Zhang, Lay Kee Ang, Shen Lai, Hua Jiang, Cong Xiao, Shengyuan A. Yang

Abstract: Electric control of spin in insulators is desired for low-consumption and ultrafast spintronics, but the underlying mechanism remains largely unexplored. Here, we propose an intrinsic effect of dynamic spin generation driven by time-varying electric field. In the intraband response regime, it can be nicely formulated as a Berry curvature effect and leads to two phenomena that are forbidden in the… ▽ More Electric control of spin in insulators is desired for low-consumption and ultrafast spintronics, but the underlying mechanism remains largely unexplored. Here, we propose an intrinsic effect of dynamic spin generation driven by time-varying electric field. In the intraband response regime, it can be nicely formulated as a Berry curvature effect and leads to two phenomena that are forbidden in the $dc$ limit: linear spin generation in nonmagnetic insulators and intrinsic N{é}el spin-orbit torque in $\mathcal{PT}$-symmetric antiferromagnetic insulators. These phenomena are driven by the time derivative of field rather than the field itself, and have a quantum origin in the first-order dynamic anomalous spin polarizability. Combined with first-principles calculations, we predict sizable effects driven by terahertz field in nonmagnetic monolayer Bi and in antiferromagnetic even-layer MnBi$_2$Te$_4$, which can be detected in experiment. △ Less

Submitted 15 September, 2024; originally announced September 2024.

Journal ref: Phys. Rev. Lett. 135, 106301 (2025)

Strongly correlated multi-electron bunches from interaction with quantum light

Authors: Suraj Kumar, Jeremy Lim, Nicholas Rivera, Wesley Wong, Yee Sin Ang, Lay Kee Ang, Liang Jie Wong

Abstract: Strongly correlated electron systems are a cornerstone of modern physics, being responsible for groundbreaking phenomena from superconducting magnets to quantum computing. In most cases, correlations in electrons arise exclusively due to Coulomb interactions. In this work, we reveal that free electrons interacting simultaneously with a light field can become highly correlated via mechanisms beyond… ▽ More Strongly correlated electron systems are a cornerstone of modern physics, being responsible for groundbreaking phenomena from superconducting magnets to quantum computing. In most cases, correlations in electrons arise exclusively due to Coulomb interactions. In this work, we reveal that free electrons interacting simultaneously with a light field can become highly correlated via mechanisms beyond Coulomb interactions. In the case of two electrons, the resulting Pearson correlation coefficient (PCC) for the joint probability distribution of the output electron energies is enhanced over 13 orders of magnitude compared to that of electrons interacting with the light field in succession (one after another). These highly correlated electrons are the result of momentum and energy exchange between the participating electrons via the external quantum light field. Our findings pave the way to the creation and control of highly correlated free electrons for applications including quantum information and ultra-fast imaging. △ Less

Submitted 13 May, 2024; v1 submitted 23 April, 2024; originally announced April 2024.

Comments: 3 figures for Main Text, 4 figures for Supplementary Materials, Supplementary is available at end of Main Text figures

Quantum Metric Nonlinear Spin-Orbit Torque Enhanced by Topological Bands

Authors: Xukun Feng, Weikang Wu, Hui Wang, Weibo Gao, Lay Kee Ang, Y. X. Zhao, Cong Xiao, Shengyuan A. Yang

Abstract: Effects manifesting quantum geometry have been a focus of physics research. Here, we reveal that quantum metric plays a crucial role in nonlinear electric spin response, leading to a quantum metric spin-orbit torque. We argue that enhanced quantum metric can occur at band (anti)crossings, so the nonlinear torque could be amplified in topological metals with nodal features close to Fermi level. By… ▽ More Effects manifesting quantum geometry have been a focus of physics research. Here, we reveal that quantum metric plays a crucial role in nonlinear electric spin response, leading to a quantum metric spin-orbit torque. We argue that enhanced quantum metric can occur at band (anti)crossings, so the nonlinear torque could be amplified in topological metals with nodal features close to Fermi level. By applying our theory to magnetic Kane-Mele model and monolayer CrSBr, which feature nodal lines and Weyl points, we demonstrate that the quantum metric torque dominates the response, and its magnitude is significantly enhanced by topological band structures, which even surpasses the previously reported linear torques and is sufficient to drive magnetic switching by itself. △ Less

Submitted 1 February, 2024; originally announced February 2024.

Journal ref: Materials Today Quantum 6 (2025) 100040

Revised Fowler-Dubridge model for photoelectron emission from two-dimensional materials

Authors: Luo Yi, Ang Y. S., Ang L. K

Abstract: We revise the Fowler-Dubridge (FB) model for photoelectron emission from two-dimensional (2D) materials to include the effects of reduced dimensionality, non-parabolic and anisotropic energy dispersion of 2D materials. Two different directions of electron emission are studied, namely vertical emission from the surface and lateral emission from the edge. Our analytical model reveals a universal tem… ▽ More We revise the Fowler-Dubridge (FB) model for photoelectron emission from two-dimensional (2D) materials to include the effects of reduced dimensionality, non-parabolic and anisotropic energy dispersion of 2D materials. Two different directions of electron emission are studied, namely vertical emission from the surface and lateral emission from the edge. Our analytical model reveals a universal temperature scaling of T\b{eta} with \b{eta} = 1 and \b{eta} = 3/2, respectively, for the surface and edge emission over a wide class of 2D materials, which are distinct from the traditional scaling of \b{eta} = 2 originally derived for the traditional bulk materials. Our comparison shows good agreement to two experiments of photo-electron emitted from graphene for both surface and edge emission. Our calculations also show the photoelectron emission is more pronounced than the coexisting thermionic emission for materials with low temperature and Fermi energy. This model provides helpful guidance in choosing proper combinations of light intensity, temperature range and type of 2D materials for the design of photoemitters, photodetectors and other optoelectronics △ Less

Submitted 11 September, 2023; originally announced September 2023.

Comments: 6 pages, 4 figures

Over-Barrier Photoelectron Emission with Rashba Spin-Orbit Coupling

Authors: Bi Hong Tiang, Yee Sin Ang, L. K. Ang

Abstract: We develop a theoretical model to calculate the quantum efficiency (QE) of photoelectron emission from materials with Rashba spin-orbit coupling (RSOC) effect. In the low temperature limit, an analytical scaling between QE and the RSOC strength is obtained as QE $\propto (\hbarω-W)^2+2E_R(\hbar ω-W) -E_R^2/3$, where $\hbarω$, $W$ and $E_R$ are the incident photon energy, work function and the RSOC… ▽ More We develop a theoretical model to calculate the quantum efficiency (QE) of photoelectron emission from materials with Rashba spin-orbit coupling (RSOC) effect. In the low temperature limit, an analytical scaling between QE and the RSOC strength is obtained as QE $\propto (\hbarω-W)^2+2E_R(\hbar ω-W) -E_R^2/3$, where $\hbarω$, $W$ and $E_R$ are the incident photon energy, work function and the RSOC parameter respectively. Intriguingly, the RSOC effect substantially improves the QE for strong RSOC materials. For example, the QE of Bi$_2$Se$_3$ and Bi/Si(111) increases, by 149\% and 122\%, respectively due to the presence of strong RSOC. By fitting to the photoelectron emission characteristics, the analytical scaling law can be employed to extract the RSOC strength, thus offering a useful tool to characterize the RSOC effect in materials. Importantly, when the traditional Fowler-Dubridge model is used, the extracted results may substantially deviate from the actual values by $\sim90\%$, thus highlighting the importance of employing our model to analyse the photoelectron emission especially for materials with strong RSOC. These findings provide a theoretical foundation for the design of photoemitters using Rashba spintronic materials. △ Less

Submitted 2 August, 2023; originally announced August 2023.

Comments: 6 pages, 3 figures

Analytical model of space charge current for a cylindrical porous trap-limited dielectric

Authors: Samra Kanwal, Chun Yun Kee, L. K. Ang

Abstract: In this study, analytical models for space charge limited current (SCLC) transport in a porous (or disordered) trap-limited dielectric are derived for both planar and cylindrical configuration. By considering the porous solid as a fractional object characterized by a parameter a less than 1, we formulate its fractional capacitance and determine the SCLC transport by using the transit time approach… ▽ More In this study, analytical models for space charge limited current (SCLC) transport in a porous (or disordered) trap-limited dielectric are derived for both planar and cylindrical configuration. By considering the porous solid as a fractional object characterized by a parameter a less than 1, we formulate its fractional capacitance and determine the SCLC transport by using the transit time approach. At a equal to 1, it will recover the well-known Mott Gurney (MG) law and Mark Helfrich (MH) law for trap-free and trap-limited cases, respectively. For cylindrical geometry, our findings show an analytical form that is not available from the traditional methods. We anticipate the proposed analytical model will serve as a useful tool for characterizing the current-voltage measurements in SCLC transport in dielectric breakdown and organic electronics, where spatial porosity of the materials is inevitable. The introduced fractional parameter a extracted from such characterization can facilitate the quantitative determination of the relationship between spatial porosity and charge mobility. △ Less

Submitted 30 July, 2023; originally announced July 2023.

Designer Edge States in Fractional Polarization Insulators

Authors: Wei Jie Chan, Peihao Fu, L. K. Ang, Yee Sin Ang

Abstract: We theoretically investigated the topological-protected edge states (TESs) in an anisotropic honeycomb lattice with mirror and chiral symmetries, characterized by an alternative topological invariant - fractional polarization (FP), rather than the conventional Chern number. This system termed an FP insulator is a potential platform for edge-state engineering due to its disconnected TESs. These dis… ▽ More We theoretically investigated the topological-protected edge states (TESs) in an anisotropic honeycomb lattice with mirror and chiral symmetries, characterized by an alternative topological invariant - fractional polarization (FP), rather than the conventional Chern number. This system termed an FP insulator is a potential platform for edge-state engineering due to its disconnected TESs. These disconnected and robust TESs are susceptible to perturbative chiral symmetry-breaking terms which can generate various patterns including the vanishing helical, spin-polarized, and chiral TESs. Moreover, helical and chiral TES can be achieved by the finite size effect, not possible from the aforementioned terms alone. The demonstration of these various TES in an FP-insulator offers an alternative route in designing reconfigurable two-dimensional nanoelectronic devices. △ Less

Submitted 13 June, 2023; originally announced June 2023.

Comments: 9 pages, 4 figures

Universal Model of Optical-Field Electron Tunneling from Two-Dimensional Materials

Authors: Yi Luo, Yee Sin Ang, L. K. Ang

Abstract: We develop analytical models of optical-field electron tunneling from the edge and surface of two-dimensional (2D) materials, including the effects of reduced dimensionality, non-parabolic energy dispersion, band anisotropy, quasi-time dependent tunneling and emission dynamics indueced by the laser field. We discover a universal scaling between the tunneling current density $J$ and the laser elect… ▽ More We develop analytical models of optical-field electron tunneling from the edge and surface of two-dimensional (2D) materials, including the effects of reduced dimensionality, non-parabolic energy dispersion, band anisotropy, quasi-time dependent tunneling and emission dynamics indueced by the laser field. We discover a universal scaling between the tunneling current density $J$ and the laser electric field $F$: In($J/|F|^β)\propto1/|F|$ with $β= 3 / 2$ in the edge emission and $β= 1$ in the vertical surface emission, which both are distinctive from the traditional Fowler-Nordheim (FN) model of $β= 2$. The current density exhibits an unexpected high-field saturation effect due to the reduced dimensionality of 2D materials, which is completely different from the space-charge saturation commonly observed in traditional bulk materials. Our results reveal the dc bias as an efficient method in modulating the optical-field tunneling sub-optical-cycle emission characteristics. Importantly, our model is in excellent agreement with a recent experiment on graphene. Our findings offer a theoretical foundation for the understanding of optical-field tunneling emission from the 2D material system, which is useful for the development of 2D-material based optoelectronics and vacuum nanoelectronics. △ Less

Submitted 20 March, 2023; originally announced March 2023.

Comments: 8 pages, 2 figures, 2 tables

Quantum Shot Noise Signatures of Two-Dimensional Semi-Dirac System

Authors: Wei Jie Chan, L. K. Ang, Yee Sin Ang

Abstract: Two-dimensional ($2$D) semi-Dirac systems, such as $2$D black phosphorus and arsenene, can exhibit a rich topological phase transition between insulating, semi-Dirac, and band inversion phases when subjected to an external modulation. How these phase transitions manifest within the quantum transport and shot noise signatures remain an open question thus far. Here, we show that the Fano factor conv… ▽ More Two-dimensional ($2$D) semi-Dirac systems, such as $2$D black phosphorus and arsenene, can exhibit a rich topological phase transition between insulating, semi-Dirac, and band inversion phases when subjected to an external modulation. How these phase transitions manifest within the quantum transport and shot noise signatures remain an open question thus far. Here, we show that the Fano factor converges to the universal $F\approx0.179$ at the semi-Dirac phase, and transits between the sub-Poissonian ($F\approx1/3$) and the Poissonian shot noise ($F\approx1$) limit at the band inversion and the insulating phase, respectively. Furthermore, the conductance of $2$D semi-Dirac system converges to the contrasting limit of $G/G_0 \rightarrow 1/d$ and $G/G_0 \rightarrow0$ at the band inversion and the insulating phases, respectively. The quantum tunneling spectra exhibits a peculiar coexistence of massless and massive Dirac quasiparticles in the band inversion regime, thus providing a versatile sandbox to study the tunneling behavior of various Dirac quasiparticles. These findings reveal the rich interplay between band topology and quantum transport signatures, which may serve as smoking gun signatures for the experimental studies of semi-Dirac systems near topological phase transition. △ Less

Submitted 8 March, 2023; originally announced March 2023.

Comments: Submitted to APL special issue, review in progress

Approaching intrinsic threshold breakdown voltage and ultra-high gain in graphite/InSe Schottky photodetector

Authors: Zhiyi Zhang, Bin Cheng, Jeremy Lim, Anyuan Gao, Lingyuan Lyu, Tianju Cao, Shuang Wang, Zhu-An Li, Qingyun Wu, L. K. Ang, Yee Sin Ang, Shi-Jun Liang, Feng Miao

Abstract: Realizing both ultra-low breakdown voltage and ultra-high gain has been one of the major challenges in the development of high-performance avalanche photodetector. Here, we report that an ultra-high avalanche gain of 3*10^5 can be realized in the graphite/InSe Schottky photodetector at a breakdown voltage down to 5.5 V. Remarkably, the threshold breakdown voltage can be further reduced down to 1.8… ▽ More Realizing both ultra-low breakdown voltage and ultra-high gain has been one of the major challenges in the development of high-performance avalanche photodetector. Here, we report that an ultra-high avalanche gain of 3*10^5 can be realized in the graphite/InSe Schottky photodetector at a breakdown voltage down to 5.5 V. Remarkably, the threshold breakdown voltage can be further reduced down to 1.8 V by raising the operating temperature, approaching the theoretical limit of 1.5E_g/e with E_g the band gap of semiconductor. We develop a two-dimensional impact ionization model and uncover that observation of high gain at low breakdown voltage arises from reduced dimensionality of electron-phonon (e-ph) scattering in the layered InSe flake. Our findings open up a promising avenue for developing novel weak-light detectors with low energy consumption and high sensitivity. △ Less

Submitted 11 November, 2022; originally announced November 2022.

Lanthanum Oxyhalide Monolayers: An Exceptional Dielectric Companion to Two-Dimensional Semiconductors

Authors: Zhuoling Jiang, Tong Su, Cherq Chua, L. K. Ang, Chun Zhang, Liemao Cao, Yee Sin Ang

Abstract: Two-dimensional (2D) layered dielectrics offers a compelling route towards the design of next-generation ultimately compact nanoelectronics. Motivated by recent high-throughput computational prediction of LaO$X$ ($X$ = Br, Cl) as an exceptional 2D dielectrics that significantly outperforms HfO$_2$ even in the monolyaer limit, we investigate the interface properties between LaOX and the archetypal… ▽ More Two-dimensional (2D) layered dielectrics offers a compelling route towards the design of next-generation ultimately compact nanoelectronics. Motivated by recent high-throughput computational prediction of LaO$X$ ($X$ = Br, Cl) as an exceptional 2D dielectrics that significantly outperforms HfO$_2$ even in the monolyaer limit, we investigate the interface properties between LaOX and the archetypal 2D semiconductors of monolayer transition metal dichacolgenides (TMDCs) $M$S$_2$ ($M$ = Mo, W) using first-principle density functional theory simulations. We show that LaO$X$ monolayers interacts weakly with $M$S$_2$ via van der Waals forces with negligible hybridization and interfacial charge transfer, thus conveniently preserving the electronic properties of 2D TMDCs upon contact formation. The conduction and valance band offsets of the interfaces exhibit a sizable value ranging from 0.7 to 1.4 eV, suggesting the capability of LaO$X$ as a gate dielectric materials. Based on Murphy-Good electron emission model, we demonstrate that LaOCl/MoS$_2$ is a versatile dielectric/semiconductor combinations that are compatible to both NMOS and PMOS applications with leakage current lower than $10^{-7}$ Acm$^{-2}$, while LaO$X$/WS$_2$ is generally compatible with PMOS application. The presence of an interfacial tunneling potential barrier at the van der Waals gap further provide an additional mechanism to suppress the leakage current. Our findings reveal the role LaO$X$ as an excellent dielectric companion to 2D TMDC and shall provide useful insights for leveraging the dielectric strength of LaO$X$ in the design of high-performance 2D nanodevices. △ Less

Submitted 2 November, 2022; v1 submitted 31 October, 2022; originally announced October 2022.

Comments: 10 pages, 4 Figures, 3 Tables

Linear Optical Modulators for Prospective Communications at the 2 μm Waveband

Authors: Jia Xu Brian Sia, Xiang Li, X. Guo, Jiawei Wang, Wanjun Wang, Zhongliang Qiao, Callum G. Littlejohns, Chongyang Liu, Kian Siong Ang, Graham T. Reed, Hong Wang

Abstract: The 2 μm waveband is an area that could have significant technological consequences, with applications ranging from spectroscopy, LIDAR and free-space communications. The development of the thulium-doped fiber amplifier, hollow-core photonic bandgap fiber and 2 μm GaSb-based diode lasers has highlighted the ability of the waveband in alleviating the fiber capacity crisis in the incumbent communica… ▽ More The 2 μm waveband is an area that could have significant technological consequences, with applications ranging from spectroscopy, LIDAR and free-space communications. The development of the thulium-doped fiber amplifier, hollow-core photonic bandgap fiber and 2 μm GaSb-based diode lasers has highlighted the ability of the waveband in alleviating the fiber capacity crisis in the incumbent communication infrastructure. The above has initiated vibrant development in the silicon photonic-space at 2 μm, where the area is capable of enabling highly-integrated photonic circuits, and potentially at low-cost and high-volumes. However, as of now, modulator linearity at 2 μm has not been addressed. The metric, as characterized by spurious free dynamic range is imperative for numerous applications such as RF photonic links for 5G and digital analog transmission in coherent communications. The development of linear optical modulators will be crucial in bringing these applications to the 2 μm. In view of that, this work is the first to address modulator linearity at the 2 μm, where the ring-assisted Mach-Zehnder modulator is developed, indicating spurious free dynamic range as high as 95 dB.Hz^2/3. It is found that that modulator spurious free dynamic range has a strong dependence on modulator bias voltage where it impacts the linearity of the transfer function in which the input RF signal is applied upon. The demonstrated modulator indicates favorable performance within silicon photonic modulators developed at 2 μm with bandwidth exceeding 17.5 GHz and modulation efficiency ranging from 0.70 to 1.25 V.cm. △ Less

Submitted 8 October, 2022; originally announced October 2022.

Comments: 9 pages, 11 figures

The effects of point defect type, location, and density on the Schottky barrier height of Au/MoS2 hetero-junction: A first-principles study

Authors: Viacheslav Sorkin, Hangbo Zhou, Zhi Gen Yu, Kah-Wee Ang, Yong-Wei Zhang

Abstract: Using DFT calculations, we investigate the effects of the type, location, and density of point defects in monolayer MoS2 on electronic structures and Schottky barrier heights (SBH) of Au/MoS2 heterojunction. Three types of point defects in monolayer MoS2, that is, S monovacancy, S divacancy and MoS (Mo substitution at S site) antisite defects, are considered. The following findings are revealed: (… ▽ More Using DFT calculations, we investigate the effects of the type, location, and density of point defects in monolayer MoS2 on electronic structures and Schottky barrier heights (SBH) of Au/MoS2 heterojunction. Three types of point defects in monolayer MoS2, that is, S monovacancy, S divacancy and MoS (Mo substitution at S site) antisite defects, are considered. The following findings are revealed: (1) The SBH for the monolayer MoS2 with defects is universally higher than that for its defect-free counterpart. (2) S divacancy and MoS antisite defects increase the SBH to a larger extent than S monovacancy. (3) A defect located in the inner sublayer of MoS2, which is adjacent to Au substrate, increases the SBH to a larger extent than that in the outer sublayer of MoS2. (4) An increase in defect density increases the SBH. These findings indicate a large variation of SBH with the defect type, location, and concentration. We also compare our results with previously experimentally measured SBH for Au/MoS2 contact and postulate possible reasons for the large differences among existing experimental measurements and between experimental measurements and theoretical predictions. The findings and insights revealed here may provide practice guidelines for modulation and optimization of SBH in Au/MoS2 and similar heterojunctions via defect engineering. △ Less

Submitted 2 August, 2022; originally announced August 2022.

Comments: 20 pages, 8 figures

Cataloguing MoSi$_2$N$_4$ and WSi$_2$N$_4$ van der Waals Heterostructures: An Exceptional Material Platform for Excitonic Solar Cell Applications

Authors: Che Chen Tho, Chenjiang Yu, Qin Tang, Qianqian Wang, Tong Su, Zhuoer Feng, Qingyun Wu, C. V. Nguyen, Wee-Liat Ong, Shi-Jun Liang, San-Dong Guo, Liemao Cao, Shengli Zhang, Shengyuan A. Yang, Lay Kee Ang, Guangzhao Wang, Yee Sin Ang

Abstract: Two-dimensional (2D) materials van der Waals heterostructures (vdWHs) provides a revolutionary route towards high-performance solar energy conversion devices beyond the conventional silicon-based pn junction solar cells. Despite tremendous research progress accomplished in recent years, the searches of vdWHs with exceptional excitonic solar cell conversion efficiency and optical properties remain… ▽ More Two-dimensional (2D) materials van der Waals heterostructures (vdWHs) provides a revolutionary route towards high-performance solar energy conversion devices beyond the conventional silicon-based pn junction solar cells. Despite tremendous research progress accomplished in recent years, the searches of vdWHs with exceptional excitonic solar cell conversion efficiency and optical properties remain an open theoretical and experimental quest. Here we show that the vdWH family composed of MoSi$_2$N$_4$ and WSi$_2$N$_4$ monolayers provides a compelling material platform for developing high-performance ultrathin excitonic solar cells and photonics devices. Using first-principle calculations, we construct and classify 51 types of MoSi$_2$N$_4$ and WSi$_2$N$_4$-based [(Mo,W)Si$_2$N$_4$] vdWHs composed of various metallic, semimetallic, semiconducting, insulating and topological 2D materials. Intriguingly, MoSi$_2$N$_4$/(InSe, WSe$_2$) are identified as Type-II vdWHs with exceptional excitonic solar cell power conversion efficiency reaching well over 20%, which are competitive to state-of-art silicon solar cells. The (Mo,W)Si$_2$N$_4$ vdWH family exhibits strong optical absorption in both the visible and ultraviolet regimes. Exceedingly large peak ultraviolet absorptions over 40%, approaching the maximum absorption limit of a free-standing 2D material, can be achieved in (Mo,W)Si$_2$N$_4$/$α_2$-(Mo,W)Ge$_2$P$_4$ vdWHs. Our findings unravel the enormous potential of (Mo,W)Si$_2$N$_4$ vdWHs in designing ultimately compact excitonic solar cell device technology. △ Less

Submitted 4 July, 2022; v1 submitted 23 June, 2022; originally announced June 2022.

Comments: 30 pages, 7 figures

Stack operation of tensor networks

Authors: Tianning Zhang, Tianqi Chen, Erping Li, Bo Yang, L. K. Ang

Abstract: The tensor network, as a facterization of tensors, aims at performing the operations that are common for normal tensors, such as addition, contraction and stacking. However, due to its non-unique network structure, only the tensor network contraction is so far well defined. In this paper, we propose a mathematically rigorous definition for the tensor network stack approach, that compress a large a… ▽ More The tensor network, as a facterization of tensors, aims at performing the operations that are common for normal tensors, such as addition, contraction and stacking. However, due to its non-unique network structure, only the tensor network contraction is so far well defined. In this paper, we propose a mathematically rigorous definition for the tensor network stack approach, that compress a large amount of tensor networks into a single one without changing their structures and configurations. We illustrate the main ideas with the matrix product states based machine learning as an example. Our results are compared with the for loop and the efficient coding method on both CPU and GPU. △ Less

Submitted 24 May, 2022; v1 submitted 28 March, 2022; originally announced March 2022.

Comments: 9 pages, 10 figures, close to the online published version, for the code on Github, see this https://github.com/veya2ztn/Stack_of_Tensor_Network

Journal ref: Front. Phys. 10:906399 (2022)

Multiferroic van der Waals heterostructure FeCl$_2$/Sc$_2$CO$_2$: Nonvolatile electrically switchable electronic and spintronic properties

Authors: Liemao Cao, Xiaohui Deng, Guanghui Zhou, Shi-Jun Liang, Chuong V. Nguyen, L. K. Ang, Yee Sin Ang

Abstract: Multiferroic van der Waals (vdW) heterostrucutres offers an exciting route towards novel nanoelectronics and spintronics device technology. Here we investigate the electronic and transport properties of multiferroic vdW heterostructure composed of ferromagnetic FeCl$_2$ monolayer and ferroelectric Sc$_2$CO$_2$ monolayer using first-principles density functional theory and quantum transport simulat… ▽ More Multiferroic van der Waals (vdW) heterostrucutres offers an exciting route towards novel nanoelectronics and spintronics device technology. Here we investigate the electronic and transport properties of multiferroic vdW heterostructure composed of ferromagnetic FeCl$_2$ monolayer and ferroelectric Sc$_2$CO$_2$ monolayer using first-principles density functional theory and quantum transport simulations. We show that FeCl$_2$/Sc$_2$CO$_2$ heterostructure can be reversibly switched from semiconducting to half-metallic behavior by electrically modulating the ferroelectric polarization states of Sc$_2$CO$_2$. Intriguingly, the half-metallic phase exhibits a Type-III broken gap band alignment, which can be beneficial for tunnelling field-effect transistor application. We perform a quantum transport simulation, based on a \emph{proof-of-concept} two-terminal nanodevice, to demonstrate all-electric-controlled valving effects uniquely enabled by the nonvolatile ferroelectric switching of the heterostructure. These findings unravels the potential of FeCl$_2$/Sc$_2$CO$_2$ vdW heterostructures as a building block for designing a next generation of ultimately compact information processing, data storage and spintronics devices. △ Less

Submitted 29 March, 2022; originally announced March 2022.

Comments: 9 pages, 5 figures, accepted for publication in Physical Review B (2022)

Modeling of electric double layer at solid-liquid interface with spatial complexity

Authors: Cherq Chua, Chun Yun Kee, L. K. Ang, Yee Sin Ang

Abstract: Electrical double layer (EDL) is formed when an electrode is in contact with an electrolyte solution, and is widely used in biophysics, electrochemistry, polymer solution and energy storage. Poisson-Boltzmann (PB) coupled equations provides the foundational framework for modeling electrical potential and charge distribution at EDL. In this work, based on fractional calculus, we reformulate the PB… ▽ More Electrical double layer (EDL) is formed when an electrode is in contact with an electrolyte solution, and is widely used in biophysics, electrochemistry, polymer solution and energy storage. Poisson-Boltzmann (PB) coupled equations provides the foundational framework for modeling electrical potential and charge distribution at EDL. In this work, based on fractional calculus, we reformulate the PB equations (with and without steric effects) by introducing a phenomenal parameter $D$ (with a value between 0 and 1) to account for the spatial complexity due to impurities in EDL. The electrical potential and ion charge distribution for different $D$ are investigated. At $D$ = 1, the model recover the classical findings of ideal EDL. The electrical potential decays slowly at $D <$1, thus suggesting a wider region of saturated layer under fixed surface potential in the presence of spatial complexity. The fractional-space generalized model developed here provides a useful tool to account for spatial complexity effects which are not captured in the classic full-dimensional models. △ Less

Submitted 1 March, 2022; originally announced March 2022.

Comments: 7 pages 5 figures

SUTD-PRCM Dataset and Neural Architecture Search Approach for Complex Metasurface Design

Authors: Tianning Zhang, Yee Sin Ang, Erping Li, Chun Yun Kee, L. K. Ang

Abstract: Metasurfaces have received a lot of attentions recently due to their versatile capability in manipulating electromagnetic wave. Advanced designs to satisfy multiple objectives with non-linear constraints have motivated researchers in using machine learning (ML) techniques like deep learning (DL) for accelerated design of metasurfaces. For metasurfaces, it is difficult to make quantitative comparis… ▽ More Metasurfaces have received a lot of attentions recently due to their versatile capability in manipulating electromagnetic wave. Advanced designs to satisfy multiple objectives with non-linear constraints have motivated researchers in using machine learning (ML) techniques like deep learning (DL) for accelerated design of metasurfaces. For metasurfaces, it is difficult to make quantitative comparisons between different ML models without having a common and yet complex dataset used in many disciplines like image classification. Many studies were directed to a relatively constrained datasets that are limited to specified patterns or shapes in metasurfaces. In this paper, we present our SUTD polarized reflection of complex metasurfaces (SUTD-PRCM) dataset, which contains approximately 260,000 samples of complex metasurfaces created from electromagnetic simulation, and it has been used to benchmark our DL models. The metasurface patterns are divided into different classes to facilitate different degree of complexity, which involves identifying and exploiting the relationship between the patterns and the electromagnetic responses that can be compared in using different DL models. With the release of this SUTD-PRCM dataset, we hope that it will be useful for benchmarking existing or future DL models developed in the ML community. We also propose a classification problem that is less encountered and apply neural architecture search to have a preliminary understanding of potential modification to the neural architecture that will improve the prediction by DL models. Our finding shows that convolution stacking is not the dominant element of the neural architecture anymore, which implies that low-level features are preferred over the traditional deep hierarchical high-level features thus explains why deep convolutional neural network based models are not performing well in our dataset. △ Less

Submitted 24 February, 2022; originally announced March 2022.

Comments: 20 pages, 7 figures, 2 tables

Tunable electronic properties and band alignments of MoSi$_2$N$_4$/GaN and MoSi$_2$N$_4$/ZnO van der Waals heterostructures

Authors: Jin Quan Ng, Qingyun Wu, L. K. Ang, Yee Sin Ang

Abstract: Van de Waals heterostructures (VDWH) is an emerging strategy to engineer the electronic properties of two-dimensional (2D) material systems. Motivated by the recent discovery of MoSi$_2$N$_4$ - a synthetic septuple-layered 2D semiconductor with exceptional mechanical and electronic properties, we investigate the synergy of \ce{MoSi2N4} with wide band gap (WBG) 2D monolayers of GaN and ZnO using fi… ▽ More Van de Waals heterostructures (VDWH) is an emerging strategy to engineer the electronic properties of two-dimensional (2D) material systems. Motivated by the recent discovery of MoSi$_2$N$_4$ - a synthetic septuple-layered 2D semiconductor with exceptional mechanical and electronic properties, we investigate the synergy of \ce{MoSi2N4} with wide band gap (WBG) 2D monolayers of GaN and ZnO using first-principle calculations. We find that MoSi$_2$N$_4$/GaN is a direct band gap Type-I VDWH while MoSi$_2$N$_4$/ZnO is an indirect band gap Type-II VDWH. Intriguingly, by applying an electric field or mechanical strain along the out-of-plane direction, the band structures of MoSi$_2$N$_4$/GaN and MoSi$_2$N$_4$/ZnO can be substantially modified, exhibiting rich transitional behaviors, such as the Type-I-to-Type-II band alignment and the direct-to-indirect band gap transitions. These findings reveal the potentials of MoSi$_2$N$_4$-based WBG VDWH as a tunable hybrid materials with enormous design flexibility in ultracompact optoelectronic applications. △ Less

Submitted 24 February, 2022; v1 submitted 29 December, 2021; originally announced December 2021.

Comments: 8 pages, 4 figures

Quantum interference between fundamentally different processes is enabled by shaped input wavefunctions

Authors: J. Lim, Y. S. Ang, L. K. Ang, L. J. Wong

Abstract: We present a general framework for quantum interference (QI) between multiple, fundamentally different processes. Our framework reveals the importance of shaped input wavefunctions in enabling QI, and predicts unprecedented interactions between free electrons, bound electrons, and photons: (i) the vanishing of the zero-loss peak by destructive QI when a shaped electron wavepacket couples to light,… ▽ More We present a general framework for quantum interference (QI) between multiple, fundamentally different processes. Our framework reveals the importance of shaped input wavefunctions in enabling QI, and predicts unprecedented interactions between free electrons, bound electrons, and photons: (i) the vanishing of the zero-loss peak by destructive QI when a shaped electron wavepacket couples to light, under conditions where the electron's zero-loss peak otherwise dominates; (ii) QI between free electron and atomic (bound electron) spontaneous emission processes, which can be significant even when the free electron and atom are far apart, breaking the common notion that electron and atom must be close by to significantly affect each other's processes. Our work shows that emerging quantum waveshaping techniques unlock the door to greater versatility in light-matter interactions and other quantum processes in general. △ Less

Submitted 10 August, 2022; v1 submitted 26 November, 2021; originally announced November 2021.

Comments: 11 pages, 3 figures

Deep learning-based design of broadband GHz complex and random metasurfaces

Authors: Tianning Zhang, Chun Yun Kee, Yee Sin Ang, L. K. Ang

Abstract: We are interested to explore the limit in using deep learning (DL) to study the electromagnetic response for complex and random metasurfaces, without any specific applications in mind. For simplicity, we focus on a simple pure reflection problem of a broadband electromagnetic (EM) plane wave incident normally on such complex metasurfaces in the frequency regime of 2 to 12 GHz. In doing so, we crea… ▽ More We are interested to explore the limit in using deep learning (DL) to study the electromagnetic response for complex and random metasurfaces, without any specific applications in mind. For simplicity, we focus on a simple pure reflection problem of a broadband electromagnetic (EM) plane wave incident normally on such complex metasurfaces in the frequency regime of 2 to 12 GHz. In doing so, we create a deep learning (DL) based framework called metasurface design deep convolutional neural network (MSDCNN) for both the forward and inverse design of three different classes of complex metasurfaces: (a) Arbitrary connecting polygons, (b) Basic pattern combination, and (c) Fully random binary patterns. The performance of each metasurface is evaluated and cross-benchmarked. Dependent on the type of complex metasurfaces, sample size, and DL algorithms used, MSDCNN is able to provide good agreements and can be a faster design tool for complex metasurfaces as compared to the traditional full-wave electromagnetic simulation methods. However, no single universal deep convolutional neural network (DCNN) model can work well for all metasurface classes based on detailed statistical analysis (such as mean, variance, kurtosis, mean squared error). Our findings report important information on the advantages and limitation of current DL models in designing these ultimately complex metasurfaces. △ Less

Submitted 16 September, 2021; originally announced October 2021.

Comments: 32 pages, 14 figures, 1 table. Accepted in APL Photonics (2021)

Propagation-induced limits to high harmonic generation in 3D Dirac semimetals

Authors: Jeremy Lim, Yee Sin Ang, Lay Kee Ang, Liang Jie Wong

Abstract: 3D Dirac semimetals (DSMs) are promising materials for terahertz high harmonic generation (HHG). We show that 3D DSMs' high nonlinearity opens up a regime of nonlinear optics where extreme subwavelength current density features develop within nanoscale propagation distances of the driving field. Our results reveal orders-of-magnitude enhancement in HHG intensity with thicker 3D DSM films, and show… ▽ More 3D Dirac semimetals (DSMs) are promising materials for terahertz high harmonic generation (HHG). We show that 3D DSMs' high nonlinearity opens up a regime of nonlinear optics where extreme subwavelength current density features develop within nanoscale propagation distances of the driving field. Our results reveal orders-of-magnitude enhancement in HHG intensity with thicker 3D DSM films, and show that these subwavelength features fundamentally limit HHG enhancement beyond an optimal film thickness. This decrease in HHG intensity beyond the optimal thickness constitutes an effective propagation-induced dephasing. Our findings highlight the importance of propagation dynamics in nanofilms of extreme optical nonlinearity. △ Less

Submitted 17 June, 2021; originally announced June 2021.

Comments: 6 pages, 3 figures

Absence of Space-Charge-Limited Current in Unconventional Field Emission

Authors: Cherq Chua, Chun Yun Kee, Yee Sin Ang, L. K. Ang

Abstract: For field emission (FE), it is widely expected that its emitting current density $J$ will become space-charge-limited current (SCLC) due the built-up of charge in-transit within a gap spacing $D$ biased at sufficiently large voltage $V$. In this paper, we reveal a peculiar finding in which this expected two-stage transition (from FE to SCLC) is no longer valid for FE not obeying the traditional Fo… ▽ More For field emission (FE), it is widely expected that its emitting current density $J$ will become space-charge-limited current (SCLC) due the built-up of charge in-transit within a gap spacing $D$ biased at sufficiently large voltage $V$. In this paper, we reveal a peculiar finding in which this expected two-stage transition (from FE to SCLC) is no longer valid for FE not obeying the traditional Fowler-Nordheim (FN) law. %Such effect arises when the non-FN based emitters fails to inject sufficient %charge current at high voltage to sustain the SCLC. By employing a generalized FN scaling of $\ln\left(J/V^k\right) \propto - 1/V$, we show the existence of a \emph{critical exponent} $k_c \equiv 3/2$ where unusual behaviours occur for $k < k_c$: (a) Only FE at small $D$ (no transition to SCLC even at infinitely large $V$), and (b) Three-stage transition from FE first to SCLC then back to FE at large $D$. For any $k > k_c$, the conventional two-stage transition from FE to SCLC will always occur for all $D$, which also includes the conventional FN law at $k$ = 2. Using various unconventional FE models with $k \neq 2$, we specifically demonstrate these peculiar transitions. Under a normalized model, our findings uncover the rich interplay between the source-limited FE and bulk-limited SCLC over a wide range of operating conditions. △ Less

Submitted 21 May, 2021; originally announced May 2021.

Comments: 6 pages, 3 figures

Thermal-Field Electron Emission from Three-Dimensional Topological Semimetals

Authors: Wei Jie Chan, Yee Sin Ang, L. K. Ang

Abstract: A model is constructed to describe the thermal-field emission of electrons from a three-dimensional ($3$D) topological semimetal hosting Dirac/Weyl node(s). The traditional thermal-field electron emission model is generalised to accommodate the $3$D non-parabolic energy band structures in the topological Dirac/Weyl semimetals, such as cadmium arsenide (\ch{Cd3As2}), sodium bismuthide (\ch{Na3Bi}),… ▽ More A model is constructed to describe the thermal-field emission of electrons from a three-dimensional ($3$D) topological semimetal hosting Dirac/Weyl node(s). The traditional thermal-field electron emission model is generalised to accommodate the $3$D non-parabolic energy band structures in the topological Dirac/Weyl semimetals, such as cadmium arsenide (\ch{Cd3As2}), sodium bismuthide (\ch{Na3Bi}), tantalum arsenide (\ch{TaAs}) and tantalum phosphide (\ch{TaP}). Due to the unique Dirac cone band structure, an unusual dual-peak feature is observed in the total energy distribution (TED) spectrum. This non-trivial dual-peak feature, absent from traditional materials, plays a critical role in manipulating the TED spectrum and the magnitude of the emission current. At zero temperature limit, a new scaling law for pure field emission is derived and it is different from the well-known Fowler-Nordheim (FN) law. This model expands the recent understandings of electron emission studied for the Dirac $2$D materials into the $3$D regime, and thus offers a theoretical foundation for the exploration in using topological semimetals as novel electrodes. △ Less

Submitted 20 May, 2021; originally announced May 2021.

Comments: 10 pages, 4 figures

Refining Targeted Syntactic Evaluation of Language Models

Authors: Benjamin Newman, Kai-Siang Ang, Julia Gong, John Hewitt

Abstract: Targeted syntactic evaluation of subject-verb number agreement in English (TSE) evaluates language models' syntactic knowledge using hand-crafted minimal pairs of sentences that differ only in the main verb's conjugation. The method evaluates whether language models rate each grammatical sentence as more likely than its ungrammatical counterpart. We identify two distinct goals for TSE. First, eval… ▽ More Targeted syntactic evaluation of subject-verb number agreement in English (TSE) evaluates language models' syntactic knowledge using hand-crafted minimal pairs of sentences that differ only in the main verb's conjugation. The method evaluates whether language models rate each grammatical sentence as more likely than its ungrammatical counterpart. We identify two distinct goals for TSE. First, evaluating the systematicity of a language model's syntactic knowledge: given a sentence, can it conjugate arbitrary verbs correctly? Second, evaluating a model's likely behavior: given a sentence, does the model concentrate its probability mass on correctly conjugated verbs, even if only on a subset of the possible verbs? We argue that current implementations of TSE do not directly capture either of these goals, and propose new metrics to capture each goal separately. Under our metrics, we find that TSE overestimates systematicity of language models, but that models score up to 40% better on verbs that they predict are likely in context. △ Less

Submitted 19 April, 2021; originally announced April 2021.

Comments: 14 pages, 5 figures, 3 tables. To appear at NAACL 2021

ACM Class: I.2.7

FBCNet: A Multi-view Convolutional Neural Network for Brain-Computer Interface

Authors: Ravikiran Mane, Effie Chew, Karen Chua, Kai Keng Ang, Neethu Robinson, A. P. Vinod, Seong-Whan Lee, Cuntai Guan

Abstract: Lack of adequate training samples and noisy high-dimensional features are key challenges faced by Motor Imagery (MI) decoding algorithms for electroencephalogram (EEG) based Brain-Computer Interface (BCI). To address these challenges, inspired from neuro-physiological signatures of MI, this paper proposes a novel Filter-Bank Convolutional Network (FBCNet) for MI classification. FBCNet employs a mu… ▽ More Lack of adequate training samples and noisy high-dimensional features are key challenges faced by Motor Imagery (MI) decoding algorithms for electroencephalogram (EEG) based Brain-Computer Interface (BCI). To address these challenges, inspired from neuro-physiological signatures of MI, this paper proposes a novel Filter-Bank Convolutional Network (FBCNet) for MI classification. FBCNet employs a multi-view data representation followed by spatial filtering to extract spectro-spatially discriminative features. This multistage approach enables efficient training of the network even when limited training data is available. More significantly, in FBCNet, we propose a novel Variance layer that effectively aggregates the EEG time-domain information. With this design, we compare FBCNet with state-of-the-art (SOTA) BCI algorithm on four MI datasets: The BCI competition IV dataset 2a (BCIC-IV-2a), the OpenBMI dataset, and two large datasets from chronic stroke patients. The results show that, by achieving 76.20% 4-class classification accuracy, FBCNet sets a new SOTA for BCIC-IV-2a dataset. On the other three datasets, FBCNet yields up to 8% higher binary classification accuracies. Additionally, using explainable AI techniques we present one of the first reports about the differences in discriminative EEG features between healthy subjects and stroke patients. Also, the FBCNet source code is available at https://github.com/ravikiran-mane/FBCNet. △ Less

Submitted 17 March, 2021; originally announced April 2021.

Comments: This work has been submitted to the IEEE for possible publication

Designing a Concentrated High-Efficiency Thermionic Solar Cell Enabled by Graphene Collector

Authors: Xin Zhang, Xiaohang Chen, Jinchan Chen, Lay Kee Ang, Yee Sin Ang

Abstract: We propose a concentrated thermionic emission solar cell design, which demonstrates a high solar-to-electricity energy conversion efficiency larger than 10\% under 600 sun, by harnessing the exceptional electrical, thermal and radiative properties of the graphene as a collector electrode. By constructing an analytical model that explicitly takes into account the non-Richardson behavior of the ther… ▽ More We propose a concentrated thermionic emission solar cell design, which demonstrates a high solar-to-electricity energy conversion efficiency larger than 10\% under 600 sun, by harnessing the exceptional electrical, thermal and radiative properties of the graphene as a collector electrode. By constructing an analytical model that explicitly takes into account the non-Richardson behavior of the thermionic emission current from graphene, space charge effect in vacuum gap, and the various irreversible energy losses within the subcomponents, we perform a detailed characterization on the conversion efficiency limit and electrical power output characteristics of the proposed system. We systematically model and compare the energy conversion efficiency of various configurations of graphene-graphene and graphene-diamond and diamond-diamond thermionic emitter, and show that utilizing diamond films as an emitter and graphene as a collector offers the highest maximum efficiency, thus revealing the important role of graphene in achieving high-performance thermionic emission solar cell. A maximum efficiency of 12.8\% under 800 sun has been revealed, which is significantly higher than several existing solid-state solar cell designs, such as the solar-driven thermoelectric and thermophotovoltaic converters. Our work thus opens up new avenues to advance the efficiency limit of thermionic solar energy conversion and the development of next-generation novel-nanomaterial-based solar energy harvesting technology. △ Less

Submitted 5 February, 2021; originally announced February 2021.

Comments: 12 pages, 7 figures, 1 table

Quantum Transport in Two-Dimensional WS$_2$ with High-Efficiency Carrier Injection Through Indium Alloy Contacts

Authors: Chit Siong Lau, Jing Yee Chee, Yee Sin Ang, Shi Wun Tong, Liemao Cao, Zi-En Ooi, Tong Wang, Lay Kee Ang, Yan Wang, Manish Chhowalla, Kuan Eng Johnson Goh

Abstract: Two-dimensional transition metal dichalcogenides (TMDCs) have properties attractive for optoelectronic and quantum applications. A crucial element for devices is the metal-semiconductor interface. However, high contact resistances have hindered progress. Quantum transport studies are scant as low-quality contacts are intractable at cryogenic temperatures. Here, temperature-dependent transfer lengt… ▽ More Two-dimensional transition metal dichalcogenides (TMDCs) have properties attractive for optoelectronic and quantum applications. A crucial element for devices is the metal-semiconductor interface. However, high contact resistances have hindered progress. Quantum transport studies are scant as low-quality contacts are intractable at cryogenic temperatures. Here, temperature-dependent transfer length measurements are performed on chemical vapour deposition grown single-layer and bilayer WS$_2$ devices with indium alloy contacts. The devices exhibit low contact resistances and Schottky barrier heights (\sim10 k$Ω$\si{\micro\metre} at 3 K and 1.7 meV). Efficient carrier injection enables high carrier mobilities ($\sim$190 cm$^2$V$^{-1}$s$^{-1}$) and observation of resonant tunnelling. Density functional theory calculations provide insights into quantum transport and properties of the WS$_2$-indium interface. Our results reveal significant advances towards high-performance WS$_2$ devices using indium alloy contacts. △ Less

Submitted 4 February, 2021; originally announced February 2021.

Comments: 5 Figures

Journal ref: ACS Nano 2020, 14, 10, 13700-13708

Semiconductor-to-metal transition in bilayer MoSi$_2$N$_4$ and WSi$_2$N$_4$ with strain and electric field

Authors: Qingyun Wu, Liemao Cao, Yee Sin Ang, Lay Kee Ang

Abstract: With exceptional electrical and mechanical properties and at the same time air-stability, layered MoSi2N4 has recently draw great attention. However, band structure engineering via strain and electric field, which is vital for practical applications, has not yet been explored. In this work, we show that the biaxial strain and external electric field are effective ways for the band gap engineering… ▽ More With exceptional electrical and mechanical properties and at the same time air-stability, layered MoSi2N4 has recently draw great attention. However, band structure engineering via strain and electric field, which is vital for practical applications, has not yet been explored. In this work, we show that the biaxial strain and external electric field are effective ways for the band gap engineering of bilayer MoSi$_2$N$_4$ and WSi$_2$N$_4$. It is found that strain can lead to indirect band gap to direct band gap transition. On the other hand, electric field can result in semiconductor to metal transition. Our study provides insights into the band structure engineering of bilayer MoSi$_2$N$_4$ and WSi$_2$N$_4$ and would pave the way for its future nanoelectronics and optoelectronics applications. △ Less

Submitted 16 January, 2021; originally announced January 2021.

Comments: 5 pages, 3 figures

Designing 24-hour Electrical Power Generator: Thermoradiative Device for Harvesting Energy from Sun and Outer Space

Authors: Xin Zhang, Guofeng Yang, Mengqi Yan, Lay Kee Ang, Yee Sin Ang

Abstract: Energy harvesting from sun and outer space using thermoradiative devices (TRD), despite being promising renewable energy sources, are limited only to daytime and nighttime period, respectively. A system with 24-hour continuous energy generation remains an open question thus far. Here, we propose a TRD-based power generator that harvests solar energy via concentrated solar irradiation during daytim… ▽ More Energy harvesting from sun and outer space using thermoradiative devices (TRD), despite being promising renewable energy sources, are limited only to daytime and nighttime period, respectively. A system with 24-hour continuous energy generation remains an open question thus far. Here, we propose a TRD-based power generator that harvests solar energy via concentrated solar irradiation during daytime and via thermal infrared emission towards the outer space at nighttime, thus achieving the much sought-after 24-hour electrical power generation. We develop a rigorous thermodynamical model to investigate the performance characteristics, parametric optimum design, and the role of various energy loss mechanisms. Our model predicts that the TRD-based system yields a peak efficiency of 12.6\% at daytime and a maximum power density of 10.8 Wm$^{-2}$ at nighttime, thus significantly outperforming the state-of-art record-setting thermoelectric generator. These findings reveal the potential of TRD towards 24-hour electricity generation and future renewable energy technology. △ Less

Submitted 14 January, 2021; originally announced January 2021.

Comments: 6 pages, 3 figures

Designing Efficient Metal Contacts to Two-Dimensional Semiconductors MoSi$_2$N$_4$ and WSi$_2$N$_4$ Monolayers

Authors: Qianqian Wang, Liemao Cao, Shi-Jun Liang, Weikang Wu, Guangzhao Wang, Ching Hua Lee, Wee Liat Ong, Hui Ying Yang, Lay Kee Ang, Shengyuan A. Yang, Yee Sin Ang

Abstract: Metal contacts to two-dimensional (2D) semiconductors are ubiquitous in modern electronic and optoelectronic devices. Such contacts are, however, often plagued by strong Fermi level pinning (FLP) effect which reduces the tunability of the Schottky barrier height (SBH) and degrades the performance of 2D-semiconductor-based devices. In this work, we show that monolayer MoSi$_2$N$_4$ and WSi$_2$N… ▽ More Metal contacts to two-dimensional (2D) semiconductors are ubiquitous in modern electronic and optoelectronic devices. Such contacts are, however, often plagued by strong Fermi level pinning (FLP) effect which reduces the tunability of the Schottky barrier height (SBH) and degrades the performance of 2D-semiconductor-based devices. In this work, we show that monolayer MoSi$_2$N$_4$ and WSi$_2$N$_4$ - a recently synthesized 2D material class with exceptional mechanical and electronic properties - exhibit strongly suppressed FLP and wide-range tunable SBH when contacted by metals. An exceptionally large SBH slope parameter of S=0.7 is obtained, which outperform the vast majority of other 2D semiconductors. Such surprising behavior arises from the unique morphology of MoSi$_2$N$_4$ and WSi$_2$N$_4$. The outlying Si-N layer forms a native atomic layer that protects the semiconducting inner-core from the perturbance of metal contacts, thus suppressing the FLP. Our findings reveal the potential of MoSi$_2$N$_4$ and WSi$_2$N$_4$ monolayers as a novel 2D material platform for designing high-performance and energy-efficient 2D nanodevices. △ Less

Submitted 23 December, 2020; v1 submitted 14 December, 2020; originally announced December 2020.

Comments: 28 pages, 5 figures

Two-dimensional van der Waals electrical contact to monolayer MoSi$_2$N$_4$

Authors: Liemao Cao, Guanghui Zhou, Qianqian Wang, L. K. Ang, Yee Sin Ang

Abstract: Two-dimensional (2D) MoSi$_2$N$_4$ monolayer is an emerging class of air-stable 2D semiconductor possessing exceptional electrical and mechanical properties. Despite intensive recent research efforts devoted to uncover the material properties of MoSi$_2$N$_4$, the physics of electrical contacts to MoSi$_2$N$_4$ remains largely unexplored thus far. In this work, we study the van der Waals heterostr… ▽ More Two-dimensional (2D) MoSi$_2$N$_4$ monolayer is an emerging class of air-stable 2D semiconductor possessing exceptional electrical and mechanical properties. Despite intensive recent research efforts devoted to uncover the material properties of MoSi$_2$N$_4$, the physics of electrical contacts to MoSi$_2$N$_4$ remains largely unexplored thus far. In this work, we study the van der Waals heterostructures composed of MoSi$_2$N$_4$ contacted by graphene and NbS$_2$ monolayers using first-principle density functional theory calculations. We show that the MoSi$_2$N$_4$/NbS$_2$ contact exhibits an ultralow Schottky barrier height (SBH), which is beneficial for nanoelectronics applications. For MoSi$_2$N$_4$/graphene contact, the SBH can be modulated via interlayer distance or via external electric fields, thus opening up an opportunity for reconfigurable and tunable nanoelectronic devices. Our findings provide insights on the physics of 2D electrical contact to MoSi$_2$N$_4$, and shall offer a critical first step towards the design of high-performance electrical contacts to MoSi$_2$N$_4$-based 2D nanodevices. △ Less

Submitted 16 December, 2020; v1 submitted 12 October, 2020; originally announced October 2020.

Comments: 5 pages, 5 figures (accepted version, to appear in Applied Physics Letters)