Performance Limit and Design Strategy of Black Phosphorus Field-Effect Transistors

Recently, a novel two-dimensional (2D) semiconductor of few-layer black phosphorus (BP) or phosphorene has been explored extensively for future electronic device applications. BP field-effect transistors (FETs) exhibited promising device characteristics such as high field-effect mobility (μ_eff > 1,000 cm²/V-s), large on current (Ion ~300 μA/μm), and large on-off current ratio (Ion/Ioff > 10⁸). In principle, the performance of BP FETs can be further improved by scaling the device, but their performance limit has not been explored particularly for multilayer BP FETs. In addition, most BP devices were studied individually without performing optimization in material or device parameters, and therefore, comprehensive design strategies for different target applications are currently absent. In this thesis, performance limit and design strategy of phosphorene FETs will be discussed by means of self-consistent atomistic quantum transport simulations using non-equilibrium Green’s function (NEGF) formalism. First, the scaling limit of bilayer BP FET is investigated. It is shown that, while the scaling of gate dielectric monotonically enhances the overall performance of bilayer BP FETs, channel length can only be scaled down to ~8 nm due to significant short-channel effects. Bilayer phosphorene FETs are benchmarked against bilayer MoS₂ and WSe₂ FETs along with a monolayer phosphorene device, which reveals that bilayer phosphorene FETs have favorable switching characteristics over other similar 2D bilayer semiconductor devices, making both monolayer and bilayer phosphorene attractive for future switching applications. In general, thickness or the number of layers in 2D semiconductors is a key parameter to determine the material’s electronic properties and the overall device performance of 2D material electronics. Therefore, the impact of having different number of phosphorene layers on the transistor performance is investigated next, considering two specific target applications of high-performance and low-power devices. Our results suggest that, for high-performance applications, monolayer phosphorene should be utilized in conventional FET structure since it can provide the equally large on current as other multilayer phosphorenes (Ion > 1 mA/μm) without showing a penalty of relatively lower density of states, along with favorableness for steep switching and large immunity to gate-induced drain leakage. On the other hand, more comprehensive approach is required for investigating low-power devices, where operating voltage, doping concentration, and channel length should be carefully engineered along with the thickness of phosphorene in the tunnel FET (TFET) structure to achieve ultra-low leakage current without sacrificing on current significantly. Our extensive simulation results revealed that either bilayer or trilayer phosphorene can provide the best performance in TFETs with the maximum Ion/Ioff of ~2×10¹¹ and the subthreshold swing as low as 13 mV/dec. In addition, our comparative study of phosphorene-based conventional FET and TFET clearly shows the feasibility and the limitation of each device for different target applications, providing irreplaceable insights into the design strategy of phosphorene FETs that can be also extended to other similar layered material electronic devices

Assessment of High-Frequency Performance Limit of Black Phosphorus Field-Effect Transistors

© 2017 IEEE.Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.Recently gigahertz frequencies have been reported with black phosphorus (BP) field-effect transistors (FETs), yet the high-frequency performance limit has remained unexplored. Here we project the frequency limit of BP FETs based on rigorous atomistic quantum transport simulations and the small-signal circuit model. Our self-consistent non-equilibrium Green’s function (NEGF) simulation results show that semiconducting BP FETs exhibit clear saturation behaviors with the drain voltage, unlike zero-bandgap graphene devices, leading to >10 THz frequencies for both intrinsic cutoff frequency (fT) and unity power gain frequency (fmax). To develop keen insight into practical devices, we discuss the optimization of fT and fmax by varying various device parameters such as channel length (Lch), oxide thickness, device width, gate resistance, contact resistance and parasitic capacitance. Although extrinsic fT and fmax can be significantly affected by the contact resistance and parasitic capacitance, they can remain near THz frequency range (fT = 900 GHz; fmax = 1.2 THz) through proper engineering, particularly with an aggressive channel length scaling (Lch ≈ 10 nm). Our benchmark against the experimental data indicates that there still exists large room for optimization in fabrication, suggesting further advancement of high-frequency performance of state-of-the-art BP FETs for the future analogue and radio-frequency applications.NSERC RGPIN-05920-2014 and STPGP 478974-1

PtSe2 Field-Effect Transistors: New Opportunities for Electronic Devices

© 2017 IEEE.Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.PtSe2, a new family of transition metal dichalcogenides, has been explored for electronic device applications using density functional theory (DFT) and non-equilibrium Green’s function (NEGF) within the third nearest neighbor tight-binding approximation. Interestingly, despite its small effective mass (me* as low as 0.21m0; m0 being electron rest mass), PtSe2 has large density of states (DOS) due to its unique six-valley conduction band within the first Brillouin zone, unlike MoX2 family. This has direct impacts on the device characteristics of PtSe2 field-effect transistors, resulting in superior on-state performance (30% higher on current and transconductance) as compared to the MoSe2 counterpart. Our simulation shows that PtSe2 device with a channel longer than 15 nm exhibits near-ideal subthreshold swing, and sub-100 mV/V of drain-induced barrier lowering can be achieved with an aggressively scaled gate oxide, demonstrating new opportunities for electronic devices with novel PtSe2.NSERC Discovery NSERC Strategic Project WIN Nanofellowshi

Performance Limit Projection of Germanane Field-Effect Transistors

© 2017 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other users, including reprinting/ republishing this material for advertising or promotional purposes, creating new collective works for resale or redistribution to servers or lists, or reuse of any copyrighted components of this work in other works.Here we explore the performance limit of monolayer germanane (GeH) field-effect transistors (FETs). We first plotted an electronic band structure of GeH using density functional theory (DFT) and then tight-binding parameters were extracted. Device characteristics of GeH FETs are investigated using rigorous self-consistent atomistic quantum transport simulations within tight-binding approximations. Our simulation results indicate that GeH FETs can exhibit exceptional on-state device characteristics such as high Ion (>2 mA/μm) and large gm (~7 mS/μm) with VDD = 0.5 V due to the very light effective mass of GeH (0.07m0), while maintaining excellent switching characteristics (SS ~64 mV/dec). We have also performed a scaling study by varying the channel length, and it turned out that GeH FET can be scaled down to ~14 nm channel without facing significant short channel effects but it may suffer from large leakage current at the channel length shorter than 10 nm. Finally, we have benchmarked GeH FET against MoS2 counterpart, exhibiting better suitability of GeH device for high-performance applications compared to MoS2 transistors.NSERC Discovery Grant || RGPIN-05920-2014 NSERC Strategic Project Grant || STPGP 478974-1

Potential of Core-Collapse Supernova Neutrino Detection at JUNO

JUNO is an underground neutrino observatory under construction in Jiangmen, China. It uses 20kton liquid scintillator as target, which enables it to detect supernova burst neutrinos of a large statistics for the next galactic core-collapse supernova (CCSN) and also pre-supernova neutrinos from the nearby CCSN progenitors. All flavors of supernova burst neutrinos can be detected by JUNO via several interaction channels, including inverse beta decay, elastic scattering on electron and proton, interactions on C12 nuclei, etc. This retains the possibility for JUNO to reconstruct the energy spectra of supernova burst neutrinos of all flavors. The real time monitoring systems based on FPGA and DAQ are under development in JUNO, which allow prompt alert and trigger-less data acquisition of CCSN events. The alert performances of both monitoring systems have been thoroughly studied using simulations. Moreover, once a CCSN is tagged, the system can give fast characterizations, such as directionality and light curve

Detection of the Diffuse Supernova Neutrino Background with JUNO

As an underground multi-purpose neutrino detector with 20 kton liquid scintillator, Jiangmen Underground Neutrino Observatory (JUNO) is competitive with and complementary to the water-Cherenkov detectors on the search for the diffuse supernova neutrino background (DSNB). Typical supernova models predict 2-4 events per year within the optimal observation window in the JUNO detector. The dominant background is from the neutral-current (NC) interaction of atmospheric neutrinos with 12C nuclei, which surpasses the DSNB by more than one order of magnitude. We evaluated the systematic uncertainty of NC background from the spread of a variety of data-driven models and further developed a method to determine NC background within 15\% with {\it{in}} {\it{situ}} measurements after ten years of running. Besides, the NC-like backgrounds can be effectively suppressed by the intrinsic pulse-shape discrimination (PSD) capabilities of liquid scintillators. In this talk, I will present in detail the improvements on NC background uncertainty evaluation, PSD discriminator development, and finally, the potential of DSNB sensitivity in JUNO

Cause of Death and Predictors of All-Cause Mortality in Anticoagulated Patients With Nonvalvular Atrial Fibrillation : Data From ROCKET AF

M. Kaste on työryhmän ROCKET AF Steering Comm jäsen.Background-Atrial fibrillation is associated with higher mortality. Identification of causes of death and contemporary risk factors for all-cause mortality may guide interventions. Methods and Results-In the Rivaroxaban Once Daily Oral Direct Factor Xa Inhibition Compared with Vitamin K Antagonism for Prevention of Stroke and Embolism Trial in Atrial Fibrillation (ROCKET AF) study, patients with nonvalvular atrial fibrillation were randomized to rivaroxaban or dose-adjusted warfarin. Cox proportional hazards regression with backward elimination identified factors at randomization that were independently associated with all-cause mortality in the 14 171 participants in the intention-to-treat population. The median age was 73 years, and the mean CHADS(2) score was 3.5. Over 1.9 years of median follow-up, 1214 (8.6%) patients died. Kaplan-Meier mortality rates were 4.2% at 1 year and 8.9% at 2 years. The majority of classified deaths (1081) were cardiovascular (72%), whereas only 6% were nonhemorrhagic stroke or systemic embolism. No significant difference in all-cause mortality was observed between the rivaroxaban and warfarin arms (P=0.15). Heart failure (hazard ratio 1.51, 95% CI 1.33-1.70, P= 75 years (hazard ratio 1.69, 95% CI 1.51-1.90, P Conclusions-In a large population of patients anticoagulated for nonvalvular atrial fibrillation, approximate to 7 in 10 deaths were cardiovascular, whereasPeer reviewe

Real-time Monitoring for the Next Core-Collapse Supernova in JUNO

Core-collapse supernova (CCSN) is one of the most energetic astrophysical events in the Universe. The early and prompt detection of neutrinos before (pre-SN) and during the SN burst is a unique opportunity to realize the multi-messenger observation of the CCSN events. In this work, we describe the monitoring concept and present the sensitivity of the system to the pre-SN and SN neutrinos at the Jiangmen Underground Neutrino Observatory (JUNO), which is a 20 kton liquid scintillator detector under construction in South China. The real-time monitoring system is designed with both the prompt monitors on the electronic board and online monitors at the data acquisition stage, in order to ensure both the alert speed and alert coverage of progenitor stars. By assuming a false alert rate of 1 per year, this monitoring system can be sensitive to the pre-SN neutrinos up to the distance of about 1.6 (0.9) kpc and SN neutrinos up to about 370 (360) kpc for a progenitor mass of 30$M_{\odot}$ for the case of normal (inverted) mass ordering. The pointing ability of the CCSN is evaluated by using the accumulated event anisotropy of the inverse beta decay interactions from pre-SN or SN neutrinos, which, along with the early alert, can play important roles for the followup multi-messenger observations of the next Galactic or nearby extragalactic CCSN.Comment: 24 pages, 9 figure

Theoretical Investigation of Contact Effects on the Performance of 2D-Material Nanotransistors

Two-dimensional (2D) materials have attracted significant attention for electronic device applications since the first graphene transistor was demonstrated in 2004. Various 2D materials not only exhibit excellent carrier mobility and suitable bandgap, but also provide great opportunities for flexible and transparent device applications. However, the fabrication of high-performance 2D transistors is limited by various factors, such as unintentional doping, defects, and poor contact properties. In this study, some of the promising 2D materials, such as black phosphorus (BP) and molybdenum disulfide (MoS₂), and their electronic devices are studied. In particular, the contact effects on the performance of 2D material nanoscale transistors are explored by means of theory. Simulation methods for ohmic and Schottky contact in 2D-material field-effect transistors (FETs) are discussed in detail. Simulation settings in non-equilibrium Green’s function (NEGF) and boundary conditions in Poisson’s equation are specified. A quantum transport simulator is built to explore the performance of those devices with different types of contacts. First, the effects of contact resistance (Rc) on the high-frequency performance limit of BP FETs are studied using self-consistent quantum simulations. A detailed comparison between intrinsic and extrinsic cut-off frequency (fT) and unity power gain frequency (fmax) is made. Unlike zero-bandgap graphene devices, semiconducting BP FETs exhibit clear saturation behaviors, which is critical for high fmax. It is shown that near THz frequency range of fT and fmax are highly promising for high-frequency applications, which is possible with an aggressive channel length scaling (Lch ≂ 10 nm) along with state-of-the-art fabrication techniques for low Rc. Our benchmarking against the experimental data indicates that there still exists large room for optimization of Rc. Based on the recent temperature studies of 2D-material FETs, two different trends can be observed. We propose a model based on the effective mass approximation to explain the low-temperature current-voltage measurements of multilayer MoS₂ thin-film-transistors (TFTs). Our model suggests that the different temperature responses with Schottky and ohmic contacts result from various aspects of contacts, such as Schottky barrier height and barrier thickness. We also investigated the distinct device-to-device low-temperature responses in multilayer MoS₂ TFTs. Our comprehensive study provides a systematic scheme for the analysis of the contact properties in 2D material-based FETs. Recently two-dimensional transition metal dichalcogenides (TMDs) lateral heterojunction field-effect transistors (FETs) have been demonstrated experimentally, in which metallic TMDs were used for the source/drain. We systematically investigate the contact property and device performance of monolayer 1T/1T’-2H MoS₂, MoSe₂, and MoTe₂ FETs. Schottky barrier (SB) heights are extracted from density functional theory calculations, and non-equilibrium Green’s function (NEGF) transport simulations have been performed to study device characteristics. Our simulation results reveal that ON and OFF-state characteristics of these devices are limited due to the inherent Schottky barrier. We optimize the performance of TMD lateral heterojunction SBFETs by using two different approaches: improving the electrostatic control by scaling equivalent oxide thickness and gate underlap and by moderately doping the gate underlap region. Our comprehensive study reveals that 1T’-2H MoTe₂ SBFET shows the highest ON current (~1 mA/µm) among the three with a reasonably small subthreshold swing (80 mV/dec) if properly scaled, while 1T-2H MoS₂ SBFET exhibits the highest Ion/Ioff (~10⁷) when Ohmic contact is established with moderate doping in the gate underlap region. This study not only provides physical insight into the electronic devices based on novel TMD heterostructures but also suggests engineering practice for device performance optimization in experiments. We also investigate the geometric effect of contact in 2D heterojunctions. The electron transport through the interface has been simulated with the top-contact and side-contact 1T-2H MoS₂. We studied the five potential stacking modes in top-contact MoS₂ junctions. The accurate maximally localized wannier functions and Schottky barrier height in top and side contact junctions have been extracted for conductance calculation. The conductance comparison shows side contact is better than top contact in the 1T-2H MoS₂ heterojunction. The oscillations of conductance are observed with different 1T2H overlap lengths with a top contact. Also, it is compared with the strong conductance oscillation in the semiconducting mono-bi-monolayer black phosphorus (BP) heterojunctions. The current flow pattern of the 1T-2H MoS₂ junction shows that the majority of current transitions from 1T layer to 2H layer happen at the edge. We further modify the weak van der Waals interactions at the edge, suggesting a potential engineering method to achieve a better contact property in metal-semiconductor top-contact junctions. This study may help us better understand metal-semiconductor junctions in 2D materials. Lastly, future works are suggested. The device simulations of top contact and side contact MoS₂ FETs are the next work to compare their device performance. There are huge numbers of novel systems in van der Waals 2D material heterojunctions. We can achieve tunneling FETs by engineering the band alignment between 2D materials with different bandgaps. In addition, with the developed simulator, vertical tunneling junctions can be investigated in layered 2D material systems