Work Function Of Material Research Articles

Conversion Method of Thermionic Emission Current to Voltage for High-Voltage Sources of Electrons

The stability of the electron thermionic emission current is one of the most important requirements for electron sources used, inter alia, in evaporators, production of rare gas excimers, and electron beam objects for high energy physics. In emission current control systems, a negative feedback signal, directly proportional to the emission current is transferred from the high-voltage anode circuit to the low-voltage cathode circuit. This technique, especially for high-voltage sources of electrons, requires the use of galvanic isolation. Alternatively, a method of converting the emission current to voltage in the cathode power supply circuit was proposed. It uses a linear cathode current intensity distribution and multiplicative-additive processing of two voltage signals, directly proportional to the values of cathode current intensity. The simulation results show that a relatively high conversion accuracy can be obtained for low values of the electron work function of the cathode material. The results of experimental tests of the dynamic parameters of the electron source and the steady-state Ie-V characteristic of the converter are presented. The implementation of the proposed Ie-V conversion method facilitates the design of the emission current controller, especially for high-voltage sources of electrons, because a negative feedback loop between the anode and cathode circuits is not required, all controller sub-components are at a common electrostatic potential.

Understanding and design of efficient carrier-selective contacts for solar cells

Numerous efforts have been devoted to either understanding or fabricating efficient carrier-selective contacts, since they play a significantly crucial role in high-performance solar cells. In this work, we demonstrate that the asymmetry between electron conductivity and hole conductivity is important but far not enough for efficient carrier-selective contacts. Taking electron-selective contacts as examples, cell performances are greatly affected by the electron conductivity, the hole conductivity, and the work function of electron-selective materials, together with conduction and valance band offset between absorbers and electron-selective layers. When designing efficient electron-selective contacts, all these factors should be considered to mitigate carrier recombination and downward bending of the electron quasi-Fermi level within and around the contacts and to ensure low interfacial resistance for electrons. Although the absorber is crystalline silicon in the study, we believe that our understanding and design guidelines are also applicable to solar cells based on other absorbers.

Electrical Performance Enhanced Doping Less Tunnel Field Effect Transistor by Metal Electrode Work Function Engineering

Tunnel field effect transistor (TFET) has been proposed as one of the promising alternative devices to MOSFET since it has low power consumption and excellent subthreshold swing (SS) characteristics of less than 60 mV / dec. However, conventional TFET fabrication requires ion implantation process to form p-i-n structure in active region, which induces a random distribution of dopant atoms. In the nm-scale device, even a small number of dopant atom dispersion can cause electrical characteristic variations such as threshold voltage (Vt)1, so alternative method for forming p-i-n region instead of doping process is needed.Doping-less TFET (DL-TFET) introduces charge plasma concept2 that controls carrier concentration of Si region by band alignment in MOS contact not by doping process, so random dopant fluctuation (RDF) issue can be solved. Since the active region carrier concentration is determined by the work function of the metal electrode, it is necessary to analyze the device performance by the metal work function condition. In this work, we manufactured doping-less TFET with adapting various metal work function to enhance electrical performance, and conducted CV measurement in MOS structure to confirm Si band bending and charge plasma by analyzing flat band voltage (VFB).DL-TFET was fabricated on SOI wafer and the overall structure is shown in Fig. 1. (a). In the case of the source region, the metal work function must be larger than that of Si to form the p-type active area, and drain region, on the contrary, must have smaller work function for n-type active region. Obviously, gate electrode work function has similar work function to intrinsic Si. For fabricating DL-TFET, we conducted reactive ion etch (RIE) process to define Si active layer and formed gate oxide (SiO2) by thermal oxidation. Then, we deposited gate metal and removed source / drain region oxide for contact hole. Spacer oxide was deposited using PECVD to isolate each metal electrode, finally we deposited source / drain metal. IV characteristics are shown in Fig. 1. (b), (c) when Pt / W / Al is applied at source / gate / drain area and we compared IV curve of various metal electrode and thermal conditions.In addition, to confirm whether the charge plasma was formed as intended, we fabricated MOS capacitor with 10 nm oxide thickness as shown in Fig. 1. (d). Through frequency sweep, we determined suitable bias frequency and conducted CV measurement (see Fig. 1. (e), (f), (g)). Band bending and charge plasma were confirmed by evaluating VFB shift depending on work function of each MOS gate material (see Fig. 1. (h)). Finally, we discussed the optimal metal electrode combination in terms of ambipolar current, off current, on current and SS. Key Words: Charge plasma, Doping-less TFET, Work function, CV measurement References Changho Shin, Jeong-Kyu Kim, Gwang-Sik Kim, Hyunjae Lee, Changhwan Shin, Jong-Kook Kim, Byung Jin Cho, and Hyun-Yong Yu, “Random Dopant Fluctuation-Induced Threshold Voltage Variation-Immune Ge FinFET With Metal–Interlayer–Semiconductor Source/Drain,” IEEE TRANSACTIONS ON ELECTRON DEVICES, vol. 63, No. 11, pp 4167-4172, Nov 2016. doi: 10.1109/TED.2016.2606511 Raymond J. E. Hueting, Bijoy Rajasekharan, Cora Salm, and d Jurriaan Schmitz, " The Charge Plasma P-N Diode" IEEE ELECTRON DEVICE LETTERS, vol. 29, issue 12, pp 1367-1369, Dec 2008. doi: 10.1109/LED.2008.2006864 Acknowledgment * This research was supported by Brain Korea 21 PLUS Program in 2021, the MOTIE (Ministry of Trade, Industry & Energy 20010836) and KSRC (Korea Semiconductor Research Consortium) support program for the development of the future semiconductor device. Figure 1

Performance enhancement through optimization of metal oxide electron transport layer in hybrid solar cell

This work deals with analysis and optimization of PEDOT:PSS-Si-SnO2 hybrid solar cell performance. The effect of interface defect and temperature on such device are analysed and calibrated with the experimental work in the literature. The performance of such solar cell is boosted using SnO2 as back side electron bypass layer and considerable optimization is done. The outcome of this work indicates suitable range for electron affinity of such bypass layer within 3.9 eV to 4.7 eV. Beyond these, the interface recombination current limits the open circuit voltage and efficiency is degraded. The energetic position of the interface defect state at the Si-SnO2 also affects the efficiency in such device. Moreover, allowed work function of back contact material for optimum performance in such device finds in the range 4 eV to 4.7 eV.

Anomalous First Breakdown Behavior for HFO1234ze(E)

Environmentally friendly gases have been widely investigated to replace sulphur hexafluoride (SF6) in compressed gas insulated equipment for the power industry. This work presents the experimental investigations on the gas stability of HFO1234ze(E) (hereinafter referred to as C <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> F <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf> ) where a unique first breakdown voltage behavior is found for AC, DC and lightning impulse (LI) voltages. A splatter of dark colored soot is observed on the plane electrode after the first breakdown that is followed by consecutive LI breakdowns chopped on the front. The breakdown characteristic stabilizes after the first voltage withstand and the 50% breakdown voltage (U <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</inf> ) is calculated using the subsequent stabilized data series. This first breakdown behavior of C <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> F <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf> is consistently observed for different tested electrode materials and configurations. The voltage difference between the first breakdown and U <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</inf> varies significantly due to the work function of the electrode material and electric field uniformity. For AC breakdowns with comparatively higher discharge energy, the data series is more consistent after the first breakdown but with significant level of soot formation indicating that this is the main influencing factor for the first breakdown effect. Soot is identified as fluorinated carbon through X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses.

Modeling of Double-Gate Field-Effect Diodes for Analog Applications

As technological trends impose device dimension limitations, exploring other devices apart from metal oxide semiconductor field effect transistor, due to short-channel effects, has become inevitable. Higher ION/IOFF ratios and faster switching rates are quintessential characteristics for nanoelectronics. A field effect diode (FED) is one such device that exhibits greater drive current and low leakages when compared to silicon on insulator technology. Also, the FED fabrication process is compatible with complimentrary metal oxide semiconductor technology. Recently, several approaches have been proposed to tackle the low OFF current problem in FED such as Modified—FED and Side—Contacted FED. The FED structure proposed here is also believed to exhibit lesser short-channel effects, smaller OFF-state current, and higher ON-state current than a regular FED structure. We have explored the possible effects of material variation and work function engineering in our structure. Comprehensive analysis of the proposed device has also been done on some figures of merit such as subthreshold slope, energy-delay product, intrinsic gate delay, etc. thereby making this device suitable for electrostatic-discharge protection. Our numerical investigation promises FED’s efficacy in future logic applications in the nanometre regime.

Spectroscopic insight of low energy electron emission from diamond surfaces

Low work function materials are desirable in many applications such as electron emission and photocatalysis. We have studied low energy electron emission from low work function hydrogen terminated diamond surfaces via electron spectroscopy to gain insight into the mechanisms involved during electron excitation and emission. Electron emission was found to be dominated by electrons within the band gap energy region, allowed due to the negative electron affinity diamond surface, while sub-bandgap illumination was able to significantly increase emission current. Substantial upward surface band bending greater than 2 eV was observed for the diamond samples, which affect electron accumulation at the surfaces. Intra-bandgap states are shown to strongly influence electron emission behavior, which can have great implications for various energy conversion devices.

Influence of the properties of p-a-Si:H layer and the work function of front contact on silicon heterojunction solar cell performance

Heterojunction c-Si and hydrogenated amorphous silicon (a-Si:H) devices with intrinsic a-Si:H thin layer, known as HIT solar cells, are promising alternatives to conventional c-Si solar cells for the possibility of higher open circuit voltage, low temperature processing and lower costs. We have studied the influence of the properties of p-a-Si:H layer and the work function of the front electrode material on the performance parameters of n-c-Si based HIT solar cells using numerical simulation. The findings are further analyzed using energy band diagrams and carrier concentration profiles at the heterointerface of a-Si:H/n-c-Si to reveal the underlying charge transport mechanisms.

Modelling of Multigate MOSFET Short Channel Structure for Low Power Application

The paper proposes a multi-gate structure for MOSFET and focuses on studying the structure with variations in parameters such as channel length, dielectric material, source material, and gate work function. By ensuring small device dimensions, the proposed structure has been simulated using the Silvaco-TCAD tool along with analysis of short-channel effects. The proposed structure can find its usage in low power and high-frequency applications due to high switching rates. The structure also possesses high transconductance at low gate voltages, increasing intrinsic gain and proving useful in RF devices. As the device is nano-scaled, it can prove worthy for fast switching applications with low losses.

Floating Filler (FF) in an Indium Gallium Zinc Oxide (IGZO) Channel Improves the Erase Performance of Vertical Channel NAND Flash with a Cell-on-Peri (COP) Structure

In this study, we developed a V-NAND with an improved IGZO-P type (IP) floating filler (FF) structure based on an IGZO channel verified in previous studies and demonstrated that it has a very fast erase speed through device simulation. The proposed FF structure can supply holes generated through the Gate-Induced Drain Leakage (GIDL) phenomenon in the upper polysilicon string select line (SSL) channel to the IGZO channel through a P-type filler, and the structure proposed by this operation shows a very fast erase speed of 4 μs. A fast erase speed was achieved because the filler adjacent to the IGZO channel, like IP structures in previous studies, functioned as a path through which electrons emitted from the charge storage layer moved easily, rather than simply supplying holes. This assumption was confirmed by assessing the change in electron density of the channel during the erase operation. Next, we investigated the optimum conditions for leakage current reduction through various condition changes of the lower ground select line (GSL) gate in the proposed structure. We confirmed that the leakage current of the proposed structure can be minimized by changing the number of lower GSL gates, changing the length of the GSL channel, and/or changing the work function of the GSL gate material. We obtained a leakage current of 10−17 A when the GSL channel was 480 nm long with six GSL gates, each with a length of 40 nm. The work function of the gates was 4.96 eV.

Schottky contact formation by an insulator: Lithium fluoride on silicon

The low work function material lithium fluoride (LiF) facilitates electron-selective contacts to n-type silicon and is frequently used in dopant-free heterocontacts for silicon solar cells. Our photoelectron spectroscopy (PES) data show that LiF deposition on n-Si leads, indeed, to Fermi-level crossing of the conduction band minimum. Furthermore, PES reveals intrinsic surface band bending on hydrogen-terminated p-Si(100). LiF deposition on p-Si leads to a rigid shift of the Si core-levels and the vacuum-level pointing to ideal Schottky contact formation. This is further supported by the open-circuit voltage of Al/LiF/p-Si/Al solar cells, which corresponds to the photoelectron spectroscopy (PES)-measured band bending magnitude.

Design of a hollow cathode thruster: concepts, parameter study and initial test results

Two electric propulsion concepts have been developed at Technische Universität Dresden as spin-off devices of regular hollow cathodes and initial testing has been conducted. Both devices represent millinewton thrusters that take advantage of thermionic electron emission using the low work function materials C12A7, LaB6, and thoriated tungsten in different design configurations. The first concept represents an electrothermal thruster which generates thrust by expanding and accelerating a heated propellant in a nozzle. Initial thrust measurement tests were carried out which showed thrust levels well above cold gas thrust, but low thrust efficiencies. The influence of different geometric parameters on the discharge properties and the performance is investigated and presented. The second thruster concept is a novel electromagnetic device in which charge carriers in a plasma discharge are accelerated by an applied magnetic field that is orthogonally oriented to the discharge current. Initial tests with C12A7 were not successful, but the functionality of the concept was shown by thrust measurements using a thoriated tungsten wire as an electron emitter.

Bulk high-entropy hexaborides

For the first time, a group of CaB6-typed cubic rare earth high-entropy hexaborides have been successfully fabricated into dense bulk pellets (>98.5 % in relative densities). The specimens are prepared from elemental precursors via in-situ metal-boron reactive spark plasma sintering. The sintered bulk pellets are determined to be single-phase without any detectable oxides or other secondary phases. The homogenous elemental distributions have been confirmed at both microscale and nanoscale. The Vickers microhardness are measured to be 16−18 GPa at a standard indentation load of 9.8 N. The nanoindentation hardness and Young’s moduli have been measured to be 19−22 GPa and 190−250 GPa, respectively, by nanoindentation test using a maximum load of 500 mN. The material work functions are determined to be 3.7–4.0 eV by ultraviolet photoelectron spectroscopy characterizations, which are significantly higher than that of LaB6.

Impact of auxiliary gate work function on boosting electrical performance of a gate-all-around field effect transistor with emphasis on the scaling behavior

To recuperate subthreshold characteristics and short channel endurance of gate-all-around field effect transistors (GAA-FETs) by scaling, we incorporate an auxiliary gate with an appropriate work function close to the source side of a conventional circular GAA(CGAA) transistor in our simulation study. The CGAA electrical characteristics have been calibrated against a 22 nm IBM sample. Based on our simulation results, utilizing an auxiliary gate with higher work function material in the proposed structure (AuxG-GAAFET) leads electron energy sub-bands to be more split out. This reduces electron concentration in the channel and thus the leakage current is decreased along with subthreshold slope (SS). Furthermore, the proposed AuxG-GAAFET shows better ION/IOFF current ratio, lower threshold voltage roll-off, better subthreshold slope (SS) and better drain induced barrier lowering (DIBL) factor caused by scaling. For this, utilizing an appropriate auxiliary gate can be among promising methods for deep transistor scaling in the future.

Simulation and analysis of ZnO- based extended-gate gate-stack junctionless NWFET for hydrogen gas detection

The sensitivity of the hydrogen gas detector can be improved by using the extended-gate technique in nanowire FET. A ZnO-based extended-gate gate-stack junctionless nanowire field-effect transistor (EG-GS-JLNWFET) hydrogen gas device is considered for the first time in this work. Catalytic gate metal, palladium (Pd) on high-K dielectric reduces the overall work function; the concept is further used in sensor design. The presence of gate stacking and extended-gate structure together improve the gate controllability, which results in an improved current ratio. Sensitivity parameters, $${S_{{I_{{{\text{ON}}}} }} }$$ and $${S_{{I_{{{\text{OFF}}}} }} }$$ are considered for performance analysis. Results are compared with JLNWFET. The gate-stack junctionless nanowire field- effect transistor (GS-JLNWFET) shows enhancement compared to both the sensitivities in the proposed device. When compared to JLNWFET-based sensor, EG-GS-JLNWFET shows 2.59% and 24.85% improvement, respectively, in the sensitivity with respect to IOFF and ION for a change of 160 meV in the work function of palladium (Pd). Leakage in any device must be as low as possible, as it lowers the standby power losses in the device. In this work, the proposed design has lowered the leakage as compared to the rest of the two structures. The results also show an augmentation in sensitivity with the increase in gate material work function

Petal-like N, O-codoped carbon nanosheets as Mott–Schottky nanoreactors in electrodes of Zn-Air batteries and supercapacitors

Heteroatoms-doped carbon nanosheets showed excellent electrocatalytic activity in oxygen reduction reaction (ORR). Herein, monoatomic Fe–N–C sites anchored on petal-like N, O-codoped carbon nanosheets surface were prepared using hemin chloride and were selected as Mott-Schottky nanoreactors in electrodes of Zn-air batteries and supercapacitors. Electron transport and mass transfer at the heterojunction interface between the nanosheets and Fe–N–C active sites were investigated thoroughly. Results indicated that not only petal-like nanosheets with abundant mesopores and high specific surface area could provide fast electrolytes diffusion path, but also the monoatomic Fe–N–C as Mott–Schottky nanoreactors was conducive to the electron transfer. The NGA-900@HC towards ORR under alkaline conditions showed a half-wave (E1/2) and onset potential (Eonset) of 0.85 and 1.16 V, respectively, which were better than those of Pt/C catalyst (vs. RHE) under a kinetic current density (JK) of 58.7 mA cm−2 at 0.55 V with the rotation rate of 1600 rpm and a limiting current density (JL) of 5.5 mA cm−2. This work provided an in-depth understanding of the correlation between ORR activity and work functions of doped mesoporous carbon materials.

Work Function Engineering of 2D Materials: The Role of Polar Edge Reconstructions.

2D materials have attracted tremendous interest as functional materials because of their diverse and tunable properties, especially at their edges. A material's work function is a critical parameter in many electronic devices; however, a fundamental understanding and a path toward large alterations of the work function in 2D materials still remain elusive. Here, we report the first evidence for anisotropy of the work function in 2D MoS2 from first-principles calculations. We also demonstrate large work-function tunability (in the range of 3.45-6.29 eV) choosing the 2H phase of MoS2 as a model system by sampling various edge configurations. We furthermore reveal the origin of this work function anisotropy and tunability by extending the existing work function relation to the local dipole moment at surfaces of 3D materials to those at edges in 2D materials. We then use machine-learning approaches to correlate work function with edge structures. These results pave the way for intrinsic edge engineering for electronic and catalytic applications.

Analytical and Compact Modeling Analysis of a SiGe Hetero-Material Vertical L-Shaped TFET

This paper deals with the development of a novel 2-D analytical modeling of heterojunction vertical L-shaped tunnel FET for characterisation of surface potential and drain current. The compact analysis includes the properties of dual modulation effect with the numerous efforts to predict the characteristics of current and to discuss the method of device improvement. The dual modulation effect is used to regulate the biasing voltage at both the junction of source and drain of surface potential to determine the tunneling width. A 2-D Poisson equation is solved for the proposed model by using parabolic approximation method with constant electric field which are used to determine the effect of SiGe mole-fraction, gate-drain biasing potential, dielectric constant of gate oxide, drain doping concentration, metal gate work-function and different binary compound material on the device surface potential. Most importantly, a new channel surface potential expression is derived that can forecast the effect of drain and gate biasing. The derived model results are compared with those of simulated results in order to evaluate the validity of electrical parameter model.

Investigating the feasibility of energy harvesting using material work functions

Electronic devices, with low power demand, can be energized using vibration energy harvesters which gather and transform energy from mechanical vibrations. This investigation looks at the feasibility of a method of energy harvesting from mechanical vibrations using the naturally occurring charging phenomenon within a system of two bodies which possess different work functions. In this work, a brief review of similar technologies, namely piezoelectric, electromagnetic and electrostatic energy harvesters is first given. This is followed by the development of a theoretical model and an investigation of different Work Function Energy Harvester (WFEH) operation modes, with conclusions on a possible optimum mode of operation. The design of an experiment to test the developed theory is then presented followed by some preliminary results. Generally it is found that a WFEH has potential for use in energy harvesting applications with the possibility of giving equal or better output power when compared to traditional electrostatic harvesters.

Modification of Schottky barrier height using an inorganic compound interface layer for various contact metals in the metal/p-Si device structure

In this study, lithium fluoride (LiF) was employed as an interface material by thermal evaporation technique and its effect of on basic device parameters such as Schottky barrier height and ideality factor was examined for different contact metals. In the device fabrication process, the aluminum (Al) metal was used only as the ohmic contact for all devices while Al, gold (Au), chromium (Cr) and nickel (Ni) were employed as Schottky contacts. The reference device structures were also fabricated without the LiF interfacial layers to better understand the effect of the interfacial layer. Thus, the metal/p-Si/Al and metal/LiF/p-Si/Al Schottky type device structures were obtained by using p type silicon (Si) wafer and Al, Au, Cr, Ni for top metal contacts to investigate the effect of these metals on the device parameters. The electrical measurements of the reference samples and Au/LiF/p-Si/Al, Al/LiF/p-Si/Al, Cr/LiF/p-Si/Al, Ni/LiF/p-Si/Al Schottky type diode structures were characterized by current–voltage (I–V) measurements at the room temperature and dark. Basic diode parameters such as ideality factor (n) and barrier height (Φb) of eight Schottky devices were calculated by the thermionic emission (TE) theory, Cheung and Norde functions at room temperature. The results revealed that the LiF interface material caused an increase in the barrier height (BH) compared to the reference device. This increase in the BH values of the devices in accordance with their reference samples was different for Al, Au, Cr, and Ni metals. In the calculation made by TE theory, the LiF interface increased the BH of the Al/LiF/p-Si device from 0.69 to 0.78, while increasing the BH of the Ni/ LiF/p-Si device from 0.57 to 0.65. The obtained results highlighted that the work function of Schottky metals and interface material are effective in modulating the BH of the device. In addition, I–V measurements of the AI/LiF/p-Si/Al device were taken under light and the results obtained showed that this device can be used in photodiode applications.