Work Function Modulation Research Articles

Conducting Polymer Based Gas Sensor Using PNIN- Gate All Around - Tunnel FET

In the present work, the n + source pocket PIN gate all around tunnel FET (PNIN-GAA-TFET) based gas sensor has been proposed. Various analyte gases such as hexane, methanol, isopropanol, dichloromethane, and chloroform have been examined for their sensitivity and stability. The sensing of various analyte gases relies on the fact of work function modulation of the conducting polymer (used as gate) on the exposure of the gas vapors. Subsequently, the work function modulation of the conducting polymer gate alters the characteristics of the device that is calibrated for examining the sensitivity of the proposed gas sensor. The shift in the subthreshold current after the exposure of the analyte gas has been used as a sensing metric. Moreover, the influence of ambient temperature, conduction path, i.e., the channel radius and the oxide thickness of gate on the sensitivity and the characteristics of the proposed gas sensors have also been examined. The employment of the proposed PNIN-GAA-TFET based gas sensor may fulfill the perpetual demands of chemical and pharmaceutical industries, biomedical diagnostics, environmental monitoring and automotive industries by offering a low power, highly sensitive and reliable analyte gas detection.

Impact of Different Gate Metals on the RF Performance of Gallium Oxide MOSFET

The performance of gallium oxide MOSFET is limited due to the issues associated with the requirement of good contact materials for gate electrode. The single metal layer at the gate electrode does not possess all the suitable material properties such as good adhesive behavior, high work function, and low specific resistance. In this work, the effect of different metals, to modulate the work function of depletion-type gallium oxide MOSFET is studied so as to improve its RF performance. The advantage of work function modulation and energy band aligned gate electrode shows low specific contact resistivity with a good ohmic contact. The key figure of merits (FOMs) for its RF performance investigation includes intrinsic capacitances (Cgs & Cgd), output conductance (gd), transconductance (gm), and cut-off frequency (fT). This work elucidates that the Ti/Au metal stack with high fT value found to be superior for RF applications.

Comparative analysis of silicon nano tube FET for switching applications using high K and work function modulation

Comparative analysis of silicon nano tube FET for switching applications using high K and work function modulation

Comparative analysis of silicon nano tube FET for switching applications using high K and work function modulation

In this paper we studied the impact of variation of dielectric constant and alternation of gate work function on silicon nanotube FET for low power and high speed switching applications. The silicon nano tubular structure offers better immunity towards short channel effects. Due to the gate engineered structure high K value structures possess high value of electron velocity. In this paper we have considered a silicon dioxide (SiO2), silicon nitride (Si3N4), hafnium oxide (HfO2), hafnium silicate (HfSiO4), tin oxide (SnO2) and titanium oxide (TO2) as a gate dielectric. Further by tuning the gate work function of the device we can able to achieve multiple threshold voltages and optimise the performance of the device. In this paper we discuss the impact of work function variation on ON-current, OFF-current and threshold voltage. From the analysis it has been found that HfO2 in SINTFET will be a superior alternative for future tubular FET devices and by tuning the gate work function nearby to 4.8 eV the silicon nano tube shows optimised better performance among all other values.

Work function modification of PEDOT:PSS by mixing with barium acetylacetonate.

Poly(3,4-ethylenedioxythiophene)polystyrene sulfonate (PEDOT:PSS) is often used as a hole injection and extractor for various organic electronic devices. This study investigated whether it is possible to n-dope PEDOT:PSS with barium acetylacetonate (Ba(acac)2) to change its work function so that to be more suitable for electron injection and extraction. Molecular dynamics simulations suggested that barium cations can interact with the aromatic rings of PEDOT and the negatively charged sulfonate in PSS. At high doping concentration, we found that PEDOT became dedoped and precipitated resulting in a clear solution after filtration. The absence of the absorption peak of PEDOT at 263 nm indicates the removal of PEDOT after filtration. The shift in O 1s to a lower binding energy as seen in X-ray photoelectron spectroscopy suggested that the polystyrene sulfonic acids are being ionized to form barium polystyrene sulfonate (Ba–PSS). By spin-coating the solution on top of indium tin oxide, the work function can be adjusted to as low as 3.6 eV. The ability of such a mixture to inject and extract electrons is demonstrated using 2,7-bis(diphenylphosphoryl)-9,9′-spirobifluorene as an electron transporting layer. We attributed the lowering of the work function as the result of the formation of an interfacial dipole as large as 1.37 eV at the ITO/Ba–PSS interface.

Effect of large work function modulation of MoS2 by controllable chlorine doping using a remote plasma

In this study, a simple and controllable chlorine doping method of MoS2 using a remote inductively coupled plasma (ICP) was studied and the effect of doping on the properties of MoS2 was investigated by adjusting the work function of MoS2.

Effect of Li Adsorption on Work Function Modulation of Bilayer α-Borophene: A Theoretical Study

Effect of Li Adsorption on Work Function Modulation of Bilayer <i>α</i>-Borophene: A Theoretical Study

Optical study of Au/Ti/Ni/Mg-nanoparticles decorated zinc oxide heterostructures: size effect induced modulation of work function

The development of the novel semiconductor heterogeneous functional materials is playing an ever more important role in materials and efficient devices with new properties and functions. In this work, heterostructures of Au/Ti/Ni decorated zinc oxide nanobelts were synthesized. The amount and diameter of metal nanoparticles were controlled by the sputtering time or the deposition time. The photoluminescence of metal/ZnO heterostructures has been investigated. Very strong enhancement of near-band-edge emission and simultaneous attenuation of visible emission were observed for ZnO nanobelts decorated with Au, Ti and Ni nanoparticles. But both the NBEUV and DL emission intensities of Ni/ZnO heterostructures decrease dramatically initially. A novel mechanism is proposed to explain observed kinds of experimental results based on characteristic of metal nanoparticles, LSPR, Purcell effects and work function change of nanosize induced. The Mg decorated zinc oxide nanobelts were synthesized for the first time. Then the entire mechanism was testified by the investigation on Mg/ZnO and Ni/ZnO heterostructures due to its much lower and higher work functions compared with that of ZnO nanobelts, respectively. The mechanism will facilitate the technical application of metal-nanoparticles decorated heterostructures, and be very useful for the creation of highly efficient optoelectronic devices.

Probing interfacial charge transfer dynamics in MoS2/TiO2 nanocomposites using scanning Kelvin probe for improved photocatalytic response

Layered Molybdenum disulfide (MoS2) is a promising co-catalyst for enhancing the photocatalytic response of titanium dioxide (TiO2). The modified interfacial charge carrier dynamics during work function (WF) modulation due to change in the surface electronic properties of MoS2/TiO2 nanocomposites (NC) plays a major role in enhancement of its photocatalytic activity (PCA). In the present work, MoS2/TiO2 NC with varying content of MoS2 is used for systematic alteration of surface electronic properties of MoS2/TiO2 NC. Scanning Kelvin probe microscopy (SKPM) and Raman spectroscopy are used to probe the interfacial charge transfer mechanism and its role in improving the electron-hole (e-h) pair separation during photocatalysis. The effect of WF modification on the photocatalytic degradation of Rhodamine B (RhB) is investigated by UV-Visible spectroscopy. The obtained results indicate that tailored WF modifies the kinetics involved in the photocatalysis by shifting the Fermi levels of MoS2 and TiO2 due to charge transfer across the interface. It is observed that an optimum level of MoS2 content in the NC based on their morphology and distribution induces better e-h separation leading to high PCA in MoS2/TiO2 NC. This study provides deeper insight into the mechanism of photocatalytic process of hybrid nanostructures.

DFT+U study of sulfur hexafluoride decomposition components adsorbed on ceria (110) surface

The detection of sulfur hexafluoride (SF6) decomposition components is significant for monitoring the conditions of SF6-gas-insulated equipment. In present study, the adsorption of SF6 and its decomposition components (H2S, SO2, SOF2, and SO2F2) on ceria (110) was investigated by performing calculations based on density functional theory (DFT). Adsorption energy, adsorption configuration, charge transfer, density of states, potential energy and work function for molecular adsorption were adopted to evaluate the performance of ceria (110) on sensing SF6 decomposition components. Among all the gas molecules, H2S, SO2 and SOF2 were chemically adsorbed on ceria surface with the adsorption energy following the order: SOF2 > SO2 > H2S, while SO2F2 and SF6 were physically adsorbed on the substrate. Besides, the amount of charge transfer and work function modification of H2S, SO2 and SOF2 molecule’s adsorption were larger than those of SO2F2 and SF6. These calculation results demonstrated ceria (110) showed prominent sensitivity to H2S, SO2 and SOF2 while less activity to SF6, which means sensor based on ceria is promising for detecting H2S, SO2 and SOF2 in SF6 gas background, and thus has the potential to online monitor the working condition of SF6-gas-insulated equipment.

Photo-Rechargeable Perovskite Cell for Direct Solar Energy Conversion and Storage

Perovskite solar cells (PSCs) are remarkably efficient. Recent years have witnessed a surge in PSC researches in both material development and structure optimization. In a PSC, a lead halide perovskite absorber is typically sandwiched between layers of hole and electron transport materials (i.e., HTM and ETM, respectively) to facilitate the extraction of photoexcited charges, but this may also create issues of energy level mismatch and interfacial degradation. A critical challenge for both HTM and ETM materials is the ineffective charge collection because of their incompatible work functions with the perovskite, which causes significant efficiency loss. In addition, the electron mobility of an ETM doesn’t typically match with the hole mobility in an HTM, leading to interfacial charge accumulation, hysteresis and degradation. There have been extensive efforts in developing materials via work-function modulation and interfacial treatment; however, they are expensive and challenging, and may not be compatible with the solution processing of the perovskite. In this work, we demonstrated replacing HTM and ETM with two pairs of nonaqueous redox couples to extract charges from the perovskite-electrolyte interfaces through photoelectrochemical or electrochemical reactions. These redox couples, dissolved in a low dielectric constant solvent, are not only to fully encapsulate and stabilize the perovskite, but also offer easy fine-tuning of their electrochemical properties to match with the perovskite. Most importantly, the nonaqueous redox couples serve as efficient energy media and solar energy can be directly converted/stored into chemical energy at the point of solar energy generation without the intermediate step such as electricity. Recent progress will be discussed and the intricate interplay between the perovskite layer morphology and redox couples will be presented.

The electronic properties of hydrogenated Janus MoSSe monolayer: a first principles investigation

The geometry, stability and electronic properties of hydrogenated Janus MoSSe monolayer (MoSSe-Hx) (x = 0–16) were investigated by first-principles calculations. The most stable position for H adsorption on the surface of Janus MoSSe monolayer was studied. Energetically, Sets is the most favorable and stable site for H adsorption. Janus MoSSe-Hx is predicted to be stable because its formation energy is lower than graphane and H2S, and the adsorption energy is lower than H atoms adsorbed on graphene and MoS2. The analysis of the electronic density distribution and bond population also show that Janus MoSSe-Hx forms stable structure. Then, the variations of electronic structure of Janus MoSSe-Hx were investigated. With the increase of the adsorbed numbers of H atoms, the density of state (DOS) of Janus MoSSe monolayer changed. It can be found that the DOS of Janus MoSSe-H2 has additional peaks in the band gap, which are introduced from H 1s doping states. With the increase of H atoms adsorbed, there are more H 1s doping states in the band gap. Correspondingly, the band gap of the Janus MoSSe-Hx monolayer gradually decreases. The band gap of pristine Janus MoSSe is 1.47 eV, and the band gap of 2 H adsorbed on Janus MoSSe is 0.31 eV. When 16 H atoms are adsorbed on Se side of Janus MoSSe, the band gap is 0.23 eV. Also, the work function of the Janus MoSSe-Hx monolayer was observed to decrease as the number of adsorbed H atoms increased. After hydrogenation process, the Se atoms transfer electrons to H atoms and Mo atoms, Mo atoms neutralize part of electrons, which leads to the weakening of the electric field pointing outward from Mo atoms to vacuum in the Janus MoSSe. As a result, the work function of the pristine Janus MoSSe is 5.24 eV, and the work function of Janus MoSSe-Hx(x = 16) decreases to 3.70 eV. Therefore, our results provide a new method to realize band gap and work function modulation of Janus MoSSe by simple hydrogenation method. Our results suggest that the hydrogenated Janus MoSSe monolayer has potential applications in designing novel electronic and optical device, or photocatalyst such as water splitting with the intrinsic built-in electronic field.

Effects of Annealing Temperature and Nitrogen Content on Effective Work Function of Tungsten Nitride

By varying annealing temperature (300°C–400°C) and nitrogen content (x = 0.33, 0.56) in the WNx films, the effective work function of WNx can modulate from 4.29 to 4.91 eV. WNx is deposited on SiO2/Si stacks by reactive sputtering. The effective work function increases with increasing annealing temperature and decreases with increasing nitrogen content. The effective work function modulation is due to the dipole formation at the WNx/SiO2 interface. Oxygen diffuses from SiO2 into WNx during annealing and forms the WNxOy near the interface. The dipole with the direction from WNxOy to WNx is formed because the group electronegativity of WNxOy near the WNx/SiO2 interface is larger than that of the WNx. The WNx with high nitrogen content can alleviate the oxidation and reduce dipole formation.

Work function modulation of electrodes contacted to molybdenum disulfide using an attached metal pad

The transport properties of electronic devices fabricated using two-dimensional materials are severely affected by the Schottky barrier at the contact of an electrode. The Schottky barrier height exhibits a strong correlation with the work function of the electrode. We observed rectifying current–voltage characteristics for a back-gated field effect transistor with a channel of molybdenum disulfide and Al electrodes, where one of the electrodes is attached to a Au pad. This result is explained in terms of the increase in the effective work function of the Al electrode attached to the Au pad. The dependence of a photocurrent on the bias voltage exhibited an opposite tendency to the current–voltage characteristics; this is also attributed to the work function modulation of the electrode, thus resulting in the variation in the Schottky barrier height.

Impact of Work-Function Modulation and Hetero Gate Engineering on Linearity and RF Performance of Charge Plasma TFET

Impact of Work-Function Modulation and Hetero Gate Engineering on Linearity and RF Performance of Charge Plasma TFET

Effects of Interfacial Layers on the Open Circuit Voltage of Polymer/Fullerene Bulk Heterojunction Devices Studied by Charge Extraction Techniques.

Interfacial layers are frequently used in organic solar cells performing various functions, including blocking surface recombination, improving selectivity of charge carrier extraction, modification of the work function of the contact materials, and enhancing light absorption within the photoactive layer through an optical cavity effect. The aim of this work is to investigate the origin of performance enhancement of bulk heterojunction solar cells using various electron and hole interfacial layers, with a particular focus on separating the contributions of work function modification and reduced recombination to the improvement of the open circuit voltage ( Voc). Solar cells using poly[ N-9'-hepta-decanyl-2,7-carbazole- alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)]:[6,6]-phenyl C70-butyric acid methyl ester (1:4) active layers were prepared with a combination of polymeric, metal oxide, and polyelectrolyte electron and/or hole interfacial layers. Four device structures with (i) no interfacial layers (reference); (ii) only hole; (iii) only electron; (iv) both electron and hole interfacial layers were fabricated and compared using current-voltage, transient photovoltage, and charge extraction measurements. The voltage gains (Δ Voc) at matched charge density attributed to work function modification (Δ Voch or Δ Voce) are distinguished from the increase in Voc arising from increased charge carrier density. At the hole contact, Δ Voch was 0.21 V by using a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) hole interfacial layer, whereas Δ Voce was 0.29 V on the electron contact using a polyethoxylate imine-TiO x interfacial layer compared to reference devices. The electron lifetime also improved by orders of magnitude with the use of either electron or hole contact layers, contributing to a further 0.35-0.38 V increase in the open circuit voltage (Δ Vocrec) because of increased charge density. The increased charge carrier lifetime is proposed to originate from the larger spatial separation of the electrons and holes in the device because of the increased internal field. Using both an electron and a hole interfacial layer did not significantly increase the charge carrier lifetime compared to single interfacial layer devices; therefore, the Voc did not increase significantly. The findings presented clarify the role of interfacial layers in organic solar cells and provide new insights into using time-resolved charge extraction techniques to understand the influence of interfacial layers on the open circuit voltage.

Novel Exploration of Flat-Band Voltage Manipulation by NPT for Advanced High-k/Metal-Gate CMOS Technology

Threshold voltage (VT) modulation is crucial for high performance (HP) and low power (LP) applications in the state-of-the-art FinFET and NanowireFET devices with gate-last HKMG integration scheme. Conventional metal-gate work function modulation normally requires a complicated film stack structure and a precise film thickness control in RMG (Replaced Metal Gate) process. As the physical gate length of RMG scaling down to below 14nm for sub-7nm technology node, the gap filling volume becomes rigorous and the VT tuning capability and turns inadequate for CMOS IC applications. This work presents a novel exploration of flat-band voltage (VFB) manipulation scheme with the nitrogen plasma treatment (NPT) in the high-k/metal-gate (HKMG) Metal-Oxide-Semiconductor Capacitor (MOSCAP) for the shrinking scaling of the further CMOS fabrication technology. HKMG formation is performed via ALD (Atomic Layer Deposition) and CVD (Chemical Vapor Deposition). The gate stacks of fabricated P- and N-MOSCAPs are ALD-HfO2(High-k layer, HK)/ALD-TiN(Capping layer)/ALD-TaN(Etch-stop layer, ESL)/CVD-TiN(Work-function layer, WFM)/ALD-W(Contacted metal) and ALD-HfO2(High-k layer, HK)/ALD-TiN(Capping layer)/ALD-TiAlC(Work-function layer, WFM)/ALD-TiN(Barrier layer)/ALD-W(Contacted metal), respectively. The critical NPT process is developed in chamber of EnduraTM (Applied Materials, Inc) and compatible to the mainstream gate-last HKMG technology. NPT process is applied in the first ALD-TiN capping layer both for P- and N-MOSCAPs. The developed NPT process on the first ALD-TiN capping layer demonstrates significant VFB modulation capability and causes a controllable effective work function of metal gate: 1) shifting to band center with increasing of the radio frequency powers both for P-/N-MOSCAPs; 2) shifting to opposite directions (P- to band edge, N- to band center) with reduced nitrogen flow ratio for P-/N-MOSCAPs; 3) shifting from -220mV to +60mV and from +130mV to +420mV for P-/N-MOSCAPs, respectively. Compared to the samples without NPT, the gate leakage current of both P- and N-MOSCAPs are improved with the NPT process. Based on the different gate stacks of P-/N-MOSCAPs and plasma effect, one unique physical model including N-vacancy/Al-trap effect is successfully proposed to illustrate the complicated shift behaviors both for HKMG P-/N-MOSCAPs. VFB shift for P-MOSCAP is mostly determined by N-vacancies, for the same layer of first capping layer(TiN) and work-function layer(TiN). The VFB shift for N-MOSCAP needs to consider both N-vacancies and Al-traps effect, for the difference between first capping layer(TiN) and the upper work-function layer(TiAlC). In conclusion, the technology of NPT in the first ALD-TiN capping layer of HKMG MOSCAPs for gate work function modulations is systematically investigated. Significant VFB shift values, such as -220mV/+130mV with different RF powers and +60mV/+420mV with different nitrogen flow ratios are achieved for P-/N-MOSCAPs, respectively. One simple theoretical model by employing the function of N-vacancy in ALD-TiN capping layer with the corresponding Al-trap effect by different WFM atoms in metal-gate is presented, which gives a unique physical explanation to the complex VFB manipulation behaviors with different device types and process conditions. The results demonstrate NPT process is one of the most promising technologies for the VT manipulation in future scaling CMOS technology nodes with less process cost. Figure 1

Microscopic Mechanisms of Local Interfacial Resistive Switching in LaMnO3+δ

Manganite perovskites exhibit promising resistive switching properties, for which the understanding of the related microscopic physicochemical changes taking place is still rather scarce. In this work the resistance of a LaMnO3+δ thin film has been locally tuned within a range of 2 orders of magnitude using conductive atomic force microscopy. With the use of X-ray photoemission electron microscopy it has been possible to simultaneously unravel composition and work function modification related to changes in the LaMnO3+δ resistance state. The resistance change is found to be triggered by oxygen ions drifting to the surface, where they remain adsorbed. Concomitant to this oxygen displacement, the Mn oxidation state is reduced from +3.6 to +3.1, while the work function decreases by 0.28 eV. We discuss the effect of these physicochemical modifications on the conduction mechanism, which is in agreement with a space-charge-limited conduction (SCLC) mechanism where the current is restrained by the density of tra...

Interface Behaviour and Work Function Modification of Self-Assembled Monolayers on Sn-Doped In2O3

The modification of the work function of Sn-doped In2O3 (ITO) by vacuum adsorption of 4-(Dimethylamino)benzoic acid (4-DMABA) has been studied using in situ photoelectron spectroscopy. Adsorption of 4-DMABA is self-limited with an approximate thickness of a single monolayer. The lowest work function obtained is 2.82 ± 0.1 eV, enabling electron injection into many organic materials. In order to identify a potential influence of the ITO substrate surface on the final work function, different ITO surface orientations and treatments have been applied. Despite the expected differences in substrate work function and chemical bonding of 4-DMABA to the substrate, no influence of substrate surface orientation is identified. The resulting work function of ITO/4-DMABA substrates can be described by a constant ionization potential of the adsorbed 4-DMABA of 5.00 ± 0.08 eV, a constant band alignment between ITO and 4-DMABA and a varying Fermi energy in the ITO substrate. This corresponds to the behaviour of a conventional semiconductor heterostructure and deviates from the vacuum level alignment of interfaces between organic compounds. The difference is likely related to a stronger chemical bonding at the ITO/4-DMABA interface compared to the van der Waals bonding at interfaces between organic compounds.

Unusual Fermi‐Level Pinning and Ohmic Contact at Monolayer Bi2O2Se–Metal Interface

AbstractVery recently, high‐mobility and air‐stable 2D semiconductor Bi2O2Se has been discovered and believed to be a promising channel candidate for the next‐generation field‐effect transistor (FET). High‐performance few‐layer Bi2O2Se FETs have been realized due to the existence of ohmic contact between few‐layer Bi2O2Se and the metal electrodes. However, monolayer (ML) Bi2O2Se FET exhibits poor device performance owing to lack of good contact between ML Bi2O2Se and the metal electrode. This work simulates the ML Bi2O2Se Schottky barrier field‐effect transistors with a sequence of common electrodes (Au, Pd, Pt, Ag, Sc, and Ti) for the first time by using ab initio quantum transport simulations. For Ag, Au, and Pd electrodes, a lateral n‐type Schottky contact is formed with similar Schottky barrier heights of 0.43–0.52 eV due to a strong usual Fermi‐level pinning (FLP) to the band gap of ML Bi2O2Se. Remarkably, Pt, Sc, and Ti electrodes lead to a desirable lateral n‐type ohmic contact because of an unusual FLP above the ML Bi2O2Se conduction band as a result of electrode work function modification at the interface. Therefore, high performance is anticipated for ML Bi2O2Se devices with these low‐resistance ohmic contacts.