Work Function Variation Research Articles

A new design approach for enhancement of DC/RF characteristics with improved ambipolar conduction of charge plasma TFET: proposal, and optimization

This article presents a new device structure to suppress ambipolarity with enhanced electrostatic characteristics of charge plasma TFET (CP-TFET). Here, implantation of a metal angle (MA) of low workfunction inside the high-k dielectric (HfO $$_{2}$$ ) layer near source/channel interface gives excellent improvement in DC and RF characteristics of the proposed device. Deposition of MA is advantageous to increase abruptness of source/channel junction for reducing the tunneling barrier. Along with MA placement, the metal electrode, which is placed over the silicon wafer for inducing N $$^{+}$$ drain region, is divided into the two parts of low and high workfunctions. The workfunction of the part of metal electrode near the channel region is taken comparatively higher than the other part to restrict the tunneling of holes at drain/channel junction under negative bias ( $$-V_\mathrm{gs}$$ ) condition. Such concept induces asymmetrical concentration of charge carriers in the drain region, which widens the tunneling barrier at the drain/channel interface. Consequently, the proposed device shows better RF performance along with suppressed ambipolar conduction. Furthermore, reliability of conventional and proposed structures has been tested in terms of linearity. Simultaneously, the effect of workfunction and length variation of MA on the device characteristics is analyzed in optimization section of the article.

SKPM study on organic-inorganic perovskite materials

We report Atomic Force Microscopy (AFM) and Scanning Kelvin Probe Microscopy (SKPM) studies on the surface morphology and surface potential properties of CH3NH3PbI3, CH3NH3PbI3-xClx, CH3NH3PbI3-xBrx and CH3NH3PbBr3-xClx, respectively. For CH3NH3PbI3 rod structure, its surface potential is independent of the precursor concentration, suggesting a robust electronic feature. Surface potential studies of CH3NH3PbI3 particle reveal that the Fermi level within CH3NH3PbI3 is strongly influenced by the substrate. In the case of CH3NH3PbI3-xClx, its surface potential depends on precursor concentrations and we suspect that chlorine concentrated solutions might lead to more chlorine incorporation in the final products, thus lowering its Fermi level. Also, we studied the surface potentials of CH3NH3PbI3-xBrx and CH3NH3PbBr3-xClxwith specified halide ratios. The surface potential differences between different samples are related to their work function variations. These results are helpful to the understanding of the structural and electronic properties of perovskite materials.

Homogenization of AlxCoCrFeNi high-entropy alloys with improved corrosion resistance

The present work investigates the homogenization effect of 1250 °C heat treatment on the AlxCoCrFeNi high-entropy alloys (HEAs). The multi-phase microstructures with chemical segregations are inevitable with the increased Al content in the alloys, which cause work function variations and localized corrosion. After heat treatment, the homogenization effect revealed by the microstructure simplification and chemical-segregation reduction leads to the decreased work function variations and the improved corrosion resistance. Thermodynamic calculations that are reliable to predict the phase transformations of the AlxCoCrFeNi HEAs, indicates a further enhancement in corrosion resistance through annealing could be guided for many other HEAs systems.

Reduction of Variability in Junctionless and Inversion-Mode FinFETs by Stringer Gate Structure

Variabilities such as threshold voltage, on-current, and subthreshold swing due to random dopant fluctuation (RDF), work function variation (WFV), and line edge roughness (LER) are studied in junctionless (JL) stringer FinFETs and compared with inversion-mode devices using technology computer-aided design analysis. Compared with a conventional JL FinFET, the stringer gate JL FinFET shows suppressed variabilities due to RDF, WFV, and LER with better gate controllability and larger effective gate length, respectively.

Work Function Variations in Twisted Graphene Layers

By combining optical imaging, Raman spectroscopy, kelvin probe force microscopy (KFPM), and photoemission electron microscopy (PEEM), we show that graphene’s layer orientation, as well as layer thickness, measurably changes the surface potential (Φ). Detailed mapping of variable-thickness, rotationally-faulted graphene films allows us to correlate Φ with specific morphological features. Using KPFM and PEEM we measure ΔΦ up to 39 mV for layers with different twist angles, while ΔΦ ranges from 36–129 mV for different layer thicknesses. The surface potential between different twist angles or layer thicknesses is measured at the KPFM instrument resolution of ≤ 200 nm. The PEEM measured work function of 4.4 eV for graphene is consistent with doping levels on the order of 1012cm−2. We find that Φ scales linearly with Raman G-peak wavenumber shift (slope = 22.2 mV/cm−1) for all layers and twist angles, which is consistent with doping-dependent changes to graphene’s Fermi energy in the ‘high’ doping limit. Our results here emphasize that layer orientation is equally important as layer thickness when designing multilayer two-dimensional systems where surface potential is considered.

Band Engineering via Sn‐doping of Zinc Oxide Electron Transport Materials for Perovskite Solar Cells

AbstractThe performance of planar perovskite solar cells (PSCs) is quite dependent on the interfacial conditions and then the interfacial band engineering is very important not only for the effective improvement of power conversion efficiency (PCE) but also for the better understanding of the charge transfer in the cells. In this report, the band engineering of the ZnO based electron transport layer (ETL) in PSCs was studied by modulating Sn‐doping level (0 ≤ x ≤ 0.2). A V‐like variation of work function (Wf) as function of Sn‐doping level in Zn1‐xSnxO1+x films was realized from 4.23 to 4.39 eV. As a result, the photovoltaic performance of PSCs with the Zn1‐xSnxO1+x ETLs was adjusted and the V‐like tendencies of photovoltaic parameters of devices, such as open‐circuit voltage (VOC) and the short‐circuit current (JSC), were found. The maximum values of VOC and JSC were achieved as 1.04 V and 20.68 mA cm−2 with an ETL of Zn1‐xSnxO1+x, respectively, which corresponds to a highest PCE of 14.12% for ZnO‐based pervoskite solar cells with a large fill factor of 65.62.

A 3D statistical simulation study of titanium metal gate WFV on electrical parameters in n-channel Ge step-FinFET

This paper highlights the impact of work function variability (WFV) of titanium metal gate grain sizes on the electrical parameters in a Ge step-FinFET. The statistical variability on threshold voltage (σVT), on current (σIon), and off current (σIoff) are explored for wide range of device dimensions at several average grain sizes using 3D TCAD simulator. We also reported the shape of distribution of electrical parameters at various device dimensions and grain sizes. Results exhibit that σVT decreases for increase in either channel length or fin width. With increased channel length, decreased σIon and σIoff values are observed. The same parameter values are increased with the increase in fin width. Additionally, the impact of ratio of grain size to square root of gate area (RGG) on σVT is reported and compared with existing both experimental and Monte Carlo (MC) simulations. The value of σVT vs. RGG is studied and it follows the same trends with existing MC simulations. At a particular value of RGG, a close agreement is found between TCAD simulation data and experimental data.

Modeling quantum yield, emittance, and surface roughness effects from metallic photocathodes

Detailed measurements of momentum distributions of emitted electrons have allowed the investigation of the thermal limit of the transverse emittance from metal photocathodes. Furthermore, recent developments in material design and growth have resulted in photocathodes that can deliver high quantum efficiency and are sufficiently robust to use in high electric field gradient photoinjectors and free electron lasers. The growth process usually produces photoemissive material layers with rough surface profiles that lead to transverse accelerating fields and possible work function variations, resulting in emittance growth. To better understand the effects of temperature, density of states, and surface roughness on the properties of emitted electrons, we have developed realistic three-dimensional models for photocathode materials with grated surface structures. They include general modeling of electron excitation due to photon absorption, charge transport, and emission from flat and rough metallic surfaces. The models also include image charge and field enhancement effects. We report results from simulations with flat and rough surfaces to investigate how electron scattering, controlled roughness, work function variation, and field enhancement affect emission properties. Comparison of simulation results with measurements of the quantum yield and transverse emittance from flat Sb emission surfaces shows the importance of including efficient modeling of photon absorption, temperature effects, and the material density of states to achieve agreement with the experimental data.

Comparison of work function variation between FinFET and 3D stacked nanowire FET devices for 6-T SRAM reliability

In this work, work-function variation (WFV) on 5 nm node gate-all-around (GAA) silicon 3D stacked nanowire FET (NWFET) and FinFET devices are studied for 6-T SRAM cells through 3D technology computer-aided design (TCAD) simulation. The NWFET devices have strong immunity for the unprecedented short channel effects (SCEs) compared with the FinFET devices owing to increased gate controllability. However, due to the narrow gate area, the single NWFET is more vulnerable to WFV effects than FinFET devices. Our results show that the WFV effects on single NWFETs are larger than the FinFETs by 45–55%. In the case of standard SRAM bit cells (high density: 111 bit cell), the variation of read stability (read static noise margin) on single NWFETs are larger than the FinFETs by 65–75%. Therefore, to improve the performance and having immunity to WFV effects, it is important to analyze the degree of variability in 3D stacked device architectures without area penalty. Moreover, we investigated the WFV effects for an accurate guideline with regard to grain size (GS) and channel area of 3D stacked NWFET in 6-T SRAM bit cells.

Investigation of Work Function Variation Induced by Metal Gate and Process Variation Effect in 3D Stacked Nanowire FET Devices

Investigation of Work Function Variation Induced by Metal Gate and Process Variation Effect in 3D Stacked Nanowire FET Devices

A new design approach for enhancement of DC/RF performance with improved ambipolar conduction of dopingless TFET

In this article, we propose a new device structure to suppress ambipolar conduction with improved DC/RF performance of dopingless TFET (DL-TFET). Here gate underlapping technique is applied near drain side to suppress ambipolarity, which results improved Analog/RF performance of the device. In addition to gate underlapping technique, a novel initiative has also been taken by placing a metal angle (MA) in the oxide layer near source/channel interface to get excellent DC characteristics of the proposed device. Placement of MA is helpful to increase abruptness at source/channel interface for higher tunneling rate of charge carriers. Length variation of gate underlapping has been performed in the section of device optimization for reducing ambipolarity of the device. Simultaneously, variation in workfunction and position of metal angle is also analysed in this section to ease the fabrication complexity.

Low pressure hydrogen sensing based on carbon nanotube field emission: Mechanism of atomic adsorption induced work function effects

The mechanism of the innovative adsorption based field emission hydrogen sensing technique is investigated from experimental and first-principles investigations. Hydrogen pressure from 10−3 to 10−7 Pa range could be detected. High carbon nanotube (CNT) crystallinity plays key role for the reduction of the effective work function from the dissociated hydrogen adsorption, leading to the field emission current increase. For CNT emitters with high dangling-bond (DB) type of defects/structures, the dissociated H adsorption occurs preferentially on the DB site with negligible work function variation. Joule heating & temperature rising is a core factor to assist the dissociated hydrogen adsorption with enhanced sensing effect. For work function sensitive CNT devices, operations under hydrogen & high temperature conditions may produce performance stability issue.

Investigation of metal-gate work-function variability in FinFET structures and implications for SRAM cell design

In sub-20 nm CMOS technology nodes, the parameter variability has become a main hurdle during the scaling of devices. Recently, the use of metal gate stacks in nano-scale FinFET structure has a significant impact on the intrinsic parameter fluctuations due to the granular nature of metals. A 14 nm FinFET structure has been considered to study the impact of metal-gate work-function variability by using the 3-D device and mixed mode circuit simulation. The present work investigates the impact of work function variability (WFV) on electrical characteristics of various possible FinFET architectures followed by the regression model. Further, the investigation has been extended to study the stability performance of 6-T SRAM cell under the influence of WFV. It is observed that work function variation may result in considerable performance degradation in device as well as SRAM operation in nano domain.

High-performance metal mesh/graphene hybrid films using prime-location and metal-doped graphene

We introduce high-performance metal mesh/graphene hybrid transparent conductive layers (TCLs) using prime-location and metal-doped graphene in near-ultraviolet light-emitting diodes (NUV LEDs). Despite the transparency and sheet resistance values being similar for hybrid TCLs, there were huge differences in the NUV LEDs’ electrical and optical properties depending on the location of the graphene layer. We achieved better physical stability and current spreading when the graphene layer was located beneath the metal mesh, in direct contact with the p-GaN layer. We further improved the contact properties by adding a very thin Au mesh between the thick Ag mesh and the graphene layer to produce a dual-layered metal mesh. The Au mesh effectively doped the graphene layer to create a p-type electrode. Using Raman spectra, work function variations, and the transfer length method (TLM), we verified the effect of doping the graphene layer after depositing a very thin metal layer on the graphene layers. From our results, we suggest that the nature of the contact is an important criterion for improving the electrical and optical performance of hybrid TCLs, and the method of doping graphene layers provides new opportunities for solving contact issues in other semiconductor devices.

Electronic structures of 1-ML C84/Ag(111): Energy level alignment and work function variation

The electronic structures of fullerene/metal interface are critical to the performance of devices based on fullerene in molecular electronics and organic electronics. Herein, we investigate the electronic structures at the interface between C84 and Ag(111) by photoelectron spectroscopy and soft X-ray absorption spectroscopy techniques. It is observed that C84 monolayer on Ag(111) surface (1-ML C84/Ag(111)) has metallic nature. A charge transfer from substrate to the unoccupied states of C84 is determined to be 1.3 electrons per molecule. However, the work function of 1-ML C84 (4.72 eV) is observed slightly larger than that of the clean Ag(111) substrate (4.50 eV). A bidirectional charge transfer model is introduced to understand the work function variation of the fullerene/metal system. In addition to the charge transfer from substrate to the adsorbate's unoccupied states, there exists non-negligible back charge transfer from fullerene occupied molecular orbital to the metal substrate through interfacial hybridization. The Fermi level will be pinned at ∼4.72 eV for C84 monolayer on coinage metal substrate.

Stress dependence of the suspended graphene work function: Vacuum Kelvin probe force microscopy and density functional theory

We report on work function measurements on graphene, which is exfoliated over a predefined array of wells in silicon oxide, by Kelvin probe force microscopy operating in a vacuum. The obtained graphene sealed microchambers can support large pressure differences, providing controllable stretching of the nearly impermeable graphene membranes. These measurements allow detecting variations of the work function induced by the mechanical stresses in the suspended graphene where the work function varies linearly with the strain and changes by 62 ± 2 meV for 1 percent of strain. Our related ab initio calculations result in a work function variation that is a factor of 1.4 larger than the experimental value. The limited discrepancy between the theory and the experiment can be accounted for by a charge transfer from the unstrained to the strained graphene regions.

Impact of Equivalent Oxide Thickness on Threshold Voltage Variation Induced by Work-Function Variation in Multigate Devices

Using 3-D technology computer aided design simulation, we investigated the impact of equivalent oxide thickness (EOT) on threshold voltage ( ${V}_{{\text {TH}}}$ ) variation induced by work-function variation (WFV) in multigate devices. The WFV-induced ${V}_{{\text {TH}}}$ variation in multigate devices does not significantly vary with the dielectric constant of the gate dielectric material, but increases with decreasing physical oxide thickness ( ${T}_{{\text {OX}}}$ ). As ${T}_{{\text {OX}}}$ becomes thinner, electric field tends to be locally concentrated, causing a large variation of electrostatic potential in channel. The slope of the ratio of average grain size to gate area (RGG) plot is observed with various oxide thicknesses. It is confirmed that we can alleviate the WFV-induced ${V}_{{\text {TH}}}$ variation without significant performance degradation if gate dielectric layer becomes thicker with appropriately adopted higher-k engineering. In addition, the impact of EOT (including interface layer) on the WFV-induced ${V}_{{\text {TH}}}$ variation is studied.

The annealing effect on work function variation of WNxCy films deposited by remote plasma atomic layer deposition

Tungsten‐nitrogen‐carbide (WNxCy) thin films were investigated as the metal gate of complementary metal‐oxide‐semiconductor (CMOS) devices. WNxCy thin films were deposited by employing the remote plasma atomic layer deposition (RPALD) using a bis(tert‐butylimido) bis (dimethylamido) tungsten (BTBMW) precursor and hydrogen plasma as a reactant. The growth rate of the WNxCy films was about 0.12 nm/cycle. X‐ray diffraction (XRD) analysis indicated that the films consisted of a mixture of tungsten carbide and tungsten nitride phases. The atomic force microscope (AFM) analysis further confirmed that the WNxCy film surfaces deposited by RPALD were smooth. In addition, the chemical bonding state analysis showed that the WNxCy films consisted of WN, WC, and WO phases. To measure the work function of the WNxCy film, a MOSCAP (metal oxide semiconductor capacitor) stack was fabricated and the flat band voltage was measured by current–voltage (C–V) measurements. A WNxCy work function value of 4.91 eV was suitable for p‐MOS and the work function of the WNxCy films varied depending on the annealing treatment, and was higher than the work function of the as‐deposited WNxCy film.

Si and Ge step-FinFETs: Work function variability, optimization and electrical parameters

We present two different step-FinFETs under the consideration that fin material is made of either Si or Ge, named as Si step-FinFET and Ge step-FinFET. A comparative simulation study among conventional FinFET (C-FinFET), and proposed step-FinFETs is presented. It is observed that Ge step-FinFET offers better Ion/Ioff ratio, with a lower intrinsic delay. Parametric analysis shows that Si step-FinFET is more resistant to subthreshold swing (SS), drain induced barrier lowering (DIBL), threshold voltage (VT) roll off and Ge step-FinFET has higher Ion/Ioff ratio, lower intrinsic delay at various channel length and oxide thickness. In presence of gate metal work function variations (WFV), a comparison of electrical parameters between C-FinFET, Si step-FinFET, and Ge step-FinFET has also been studied. We found that Si step-FinFET gives lesser variation in threshold voltage (σVT), lesser variation in subthreshold swing (σSS), and higher variation in current ratio (σ(Ion/Ioff)) than C-FinFET. σVT, σSS, and σ(Ion/Ioff) are compared for Ge step-FinFET and Si step-FinFET.

Fluctuation Sensitivity Map: A Novel Technique to Characterise and Predict Device Behaviour Under Metal Grain Work-Function Variability Effects

A new technique developed for the analysis of intrinsic sources of variability affecting the performance of semiconductor devices is presented. It is based on the creation of a fluctuation sensitivity map (FSM), which supplies spatial information about the source of variability affecting the device performance and reliability, providing useful advice in the development of fluctuation-resistant device architectures. We have applied the FSM to metal grain work-function variations (MGWVs), since they are one of the major contributors to device variability. This technique is computationally very efficient because, once the original FSM is created, it can be used to predict the MGWV for different metal gates or grain sizes (GSs). Two state-of-the-art devices were used as test-models: a 10.7-nm gate length Si FinFET and 10.4-nm gate length In0.53Ga0.47As FinFET. The cross-sectional shape (triangular, rectangular, or bullet), the metal used in the gate (TiN or WN), and the GS (10, 7, and 5 nm) have been used as test scenarios for this technique.