Effective Work Function Research Articles

Ab Initio Materials Modeling of Point Defects in a High-κ Metal Gate Stack of Scaled CMOS Devices: Variability Versus Engineering the Effective Work Function

Ab Initio Materials Modeling of Point Defects in a High-κ Metal Gate Stack of Scaled CMOS Devices: Variability Versus Engineering the Effective Work Function

Simple and efficient synthesis of [Ca24Al28O64]4+(4e−) electronic compounds and preliminary exploration of its electron emission as a cathode emitter

[Ca24Al28O64]4+(4e−) electronic compound (C12A7:e−) is a ceramic with extremely low work function, and it exhibits outstanding potential in cathode electron emission. However, the existing preparation methods for this material are cumbersome, time-consuming, and have limited electron density. In this paper, C12A7:e− were prepared using high-pressure hydrothermal reaction and plasma activation sintering. The free O2− in the cages of the sample achieved accelerated diffusion and transition under the high temperature, strong electric field, high-energy pulse current, and plasma activation. Meanwhile, a high concentration of electrons (2 × 1021 cm−3) was rapidly injected into the sample. In addition, the C12A7:e− blocks have fewer defects, such as pores, cracks, and impurities, and their relative density reaches 99.6 %. The electron emission density and effective work function of the C12A7:e− are 1.49 A/cm2 and 2.25 eV, respectively, at 1100 °C. This article provides a reference for preparing C12A7:e− and their applications in electronic emission.

Antimony-Platinum Modulated Contact Enabling Majority Carrier Polarity Selection on a Monolayer Tungsten Diselenide Channel.

We develop a novel metal contact approach using an antimony (Sb)-platinum (Pt) bilayer to mitigate Fermi-level pinning in 2D transition metal dichalcogenide channels. This strategy allows for control over the transport polarity in monolayer WSe2 devices. By adjustment of the Sb interfacial layer thickness from 10 to 30 nm, the effective work function of the contact/WSe2 interface can be tuned from 4.42 eV (p-type) to 4.19 eV (n-type), enabling selectable n-/p-FET operation in enhancement mode. The shift in effective work function is linked to Sb-Se bond formation and an emerging n-doping effect. This work demonstrates high-performance n- and p-FETs with a single WSe2 channel through Sb-Pt contact modulation. After oxide encapsulation, the maximum current density at |VD| = 1 V reaches 170 μA/μm for p-FET and 165 μA/μm for n-FET. This approach shows promise for cost-effective CMOS transistor applications using a single channel material and metal contact scheme.

The impact of S vacancies on the modulation of the work function and Schottky barrier at the Au/MoS2 interface

The S vacancy at metal/MoS2 interface plays a much important role than the semiconductor itself. In this work, the influence of different configurations of S vacancy concentrations on the effective work function and band structure of the Au/MoS2 interface has been investigated systematically using first-principles calculations. The study specifically explores the effective work function of the Au/MoS2 interface, the deviation of interface effects from the vacuum work function, and the dipole moment caused by interface charge transfer. The results reveal that the electronic work function of Au/MoS2 increases with the increase in S vacancy concentration, but the rate of increase tends to slow down with higher S concentrations. The variation in the effective work function of the Au/MoS2 interface may be attributed to the presence of S vacancies and the exposure of Mo atoms. S vacancies lead to a reduction in the Schottky barrier, resulting in increased leakage current. The Fermi pinning caused by S vacancy concentration and location is also observed. The results obtained in this study can serve as a theoretical foundation for applications in electronic devices that rely on metal/MoS2 contact.

Insight into TaSi2 Nanostructures with Different Morphologies and Their Field Emission Properties

AbstractRecognizing the strong potential of cold cathodes for important commercial applications in fields such as electronics, there is a growing interest in the exploration of novel 1D nanomaterials. Among various cold cathode materials, TaSi2 is of great interest for its outstanding field emission performance. In this work, the Si‐TaSi2 eutectic composite with nano‐sized highly oriented TaSi2 fibers and semi‐coherent phase interfaces is prepared by the laser floating zone melting technique with a very high‐temperature gradient of 6000 K cm−1 at a solidification rate of 200 µm s−1. On the basis of directionally solidified Si‐TaSi2 eutectic composite, well‐aligned TaSi2 nanorod and nanotip arrays are fabricated by inductively coupling plasma (ICP) etching process and HNO3/HF wet etching process, respectively. The field emission measurements show that the field enhancement factor, turn‐on electric field, and effective work function are strongly affected by tip morphologies. The TaSi2 array with regular nanotip structure possesses the best field emission characteristic among all TaSi2 nanostructures, with a relatively low turn‐on field of 4.8 V µm−1 and a high current density of 733 µA cm−2. These findings preliminarily establish a clear relationship between the performance and structure of the array, providing technical guidance for the application of this material in electronic devices.

Direct evidence of low work function on SrVO3 cathode using thermionic electron emission microscopy and high-field ultraviolet photoemission spectroscopy

Perovskite SrVO3 has recently been proposed as a novel electron emission cathode material. Density functional theory (DFT) calculations suggest multiple low work function surfaces, and recent experimental efforts have consistently demonstrated effective work functions of ∼2.7 eV for polycrystalline samples, both results suggesting, but not directly confirming, that some fraction of even lower work function surface is present. In this work, thermionic electron emission microscopy (ThEEM) and high-field ultraviolet photoemission spectroscopy (UPS) are used to study the local work function distribution and measure the work function of a partially oriented- (110)-SrVO3 perovskite oxide cathode surface. Our results show direct evidence of low work function patches of about 2.0 eV on the cathode surface, with a corresponding onset of observable thermionic emission at 750 °C. We hypothesize that, in our ThEEM and UPS experiments, the high applied electric field suppresses the patch field effect, enabling the direct measurement of local work functions. This measured work function of 2.0 eV is comparable to the previous DFT-calculated work function values of the SrVO-terminated (110) SrVO3 surface (2.3 eV) and SrO-terminated (100) surface (1.9 eV). The measured 2.0 eV value is also much lower than the work function for the (001) LaB6 single crystal cathode (∼2.7 eV) and comparable to the effective work function of B-type dispenser cathodes (∼2.1 eV). If SrVO3 thermionic emitters can be engineered to access domains of this low 2.0 eV work function, they have the potential to significantly improve thermionic emitter-based technologies.

Prediction of Thermionic Energy Conversion Performance and Parametric Effects Using Genetic Algorithms to Fit Physics-Inspired Model Equations to Prototype Test Data

Abstract Thermionic converters have potential as an energy conversion technology for high-temperature space and terrestrial applications using concentrated solar, nuclear reaction, and combustion processes as the heat source. Recent studies have generated experimental performance data for narrow-gap thermionic energy conversion devices. This investigation explores the use of genetic algorithms to fit existing data with physics-inspired model equations. The resulting model equations can be used for performance prediction for system design optimization or to explore parametric effects on performance. The model equations incorporate Richardson’s law for current density, including both the saturated and Boltzmann regimes, with appropriate relations for power delivered to the external load. The transition regime is characterized using two separate models, each accounting for nonuniformity in emission surfaces and irregularities in the manufacturing process. The trained models enable performance prediction of small-gap thermionic energy conversion devices. In this study, data were fitted for two different prototype designs. The prototype test data and postulated values for the work functions and a transition regime parameter are substituted into physics-inspired model equations, yielding performance models with three adjustable constants. Optimized values of these constants are determined using a genetic algorithm to best fit the experimentally determined performance data for prototype thermionic conversion devices tested in earlier studies. This approach is demonstrated to fit the performance data to within 9%. This methodology also allows the user to back-infer the effective work function values, which were found in this study to be consistent with independent measurement.

Investigation of dual work function metal (DWFM) gate stacks with ALD TaAlN and TaAlC for multi threshold voltages (VTHs) engineering in MOS device integration

This study investigates methods to modulate the effective work function (EWF) and control the multiple threshold voltages (VTHs) of metal-oxide-semiconductor (MOS) devices by applying metal electrodes with different work function materials (WFMs), including a single layer of n-type (TaAlC) and p-type (TaAlN) as well as their stacked structures, through atomic layer deposition (ALD). Dual-Work-Function-Metal (DWFM) gate is a crucial aspect of confirming the feasibility of metal gate stacks in MOS devices for a complementary metal-oxide-semiconductor (CMOS) integration scheme. The gate stack structure of p-Si/IL (Inter layer)/HfO2/DWFM (p/n-type compatible metal stack)/W exhibits good adhesion and even chemical composition distribution, as evidenced by transmission electron microscope (TEM) and energy dispersive spectrometer (EDS) analysis. By fabricating MOS capacitors using this WFM structure and extracting EWF from the measured capacitance-voltage (C–V) characteristics, EWF values of 4.62 eV and 4.99 eV were identified both before and after annealing using forming gas, indicating pWFM-like properties. These properties are attributed to the strengthened Al–O bonds and reduced CO peaks seen in x-ray photoelectron spectroscopy (XPS) analysis. The use of different Ta precursor from Tris(diethylamido)(tert-butylimido)tantalum(V) (TBTDET) as well as TaCl5 resulted in the increase of the EWF following the unique p-WFM like feature. As candidates for WFM electrode in metal-oxide-semiconductor field effect transistor (MOSFET) with the DWFMs (n/p stacks) were evaluated, resulting in a VTH of 0.84 V that places in the middle of the VTH shift range from 0.6 V (nWFM) to 1.16 V (pWFM), approximately. The DWFMs allow for modulation of the EWF, which in turn reduces ambiguity in addressing replacement metal gate (RMG) challenges and opens up new avenues for achieving multi-VTH solutions in CMOS integration schemes.

Electrical properties and band alignments of Sb2Te3/Si heterojunctions, low-barrier Sb2Te3/n-Si and high-barrier Sb2Te3/p-Si junctions

We investigated the electrical junction properties of the layered Sb2Te3 film formed on Si substrates. The current−voltage characteristics of the Sb2Te3/n-Si heterojunction showed an ohmic properties, whereas the Sb2Te3/p-Si heterojunction exhibited rectifying properties with a high barrier height of 0.77 eV. The capacitance−voltage characteristics of MOS capacitors with the Sb2Te3 electrode indicated an effective work function of 4.44 eV for the Sb2Te3 film. These findings suggest that the Sb2Te3/Si heterostructure possesses a low conduction band offset, as inferred from the temperature dependence of the current−voltage characteristics of the Sb2Te3/n-Si.

Time dependence of SrVO3 thermionic electron emission properties

Single phase, polycrystalline perovskite oxide SrVO3, with its intrinsic low effective work function and facile synthesis process, is a promising thermionic electron emitter cathode candidate, in which previous works have shown evidence of an effective work function as low as 2.3 eV. In this work, we study the vacuum activation process of SrVO3 and find that it has promising emission stability over 15 days of continuous high temperature operation. We find that SrVO3 shows surface Sr and O segregation during its operation, which we hypothesize is needed to create a positive surface dipole, leading to a low effective work function. Emission repeatability from cyclic heating and cooling suggests the promising stability of the low effective work function surface, and additional observations of drift-free emission during 1 h of continuous emission testing at high temperature further demonstrate its excellent performance stability. This assessment of the emission stability over time and the interplay of evolving surface chemistry with emission behavior are necessary for understanding how best to prepare, process, and operate SrVO3 cathodes.

Fermi-level depinning in Mo/Si junctions by insertion of amorphous Si-rich Mo silicide film formed via gas-phase reactions of MoF6 with SiH4

We demonstrate the reduction of the electron Schottky barrier height (SBH) to 0.48 eV at Mo/n-type Si junctions through the insertion of a semimetal Si-rich Mo silicide (MoSi n , n = 7.9) film. Raman scattering measurements elucidated the persistence of the same amorphous structure within the MoSi n film even when subjected to temperatures as high as 900 °C. This excellent thermal stability yielded a notable result: preservation of the SBH modulation effect even following annealing at 700 °C. Moreover, we investigated the capacitance−voltage characteristics of MOS capacitors, revealing that MoSi n film has a remarkably low effective work function, measuring 4.1 eV when deposited onto SiO2. The deposition of MoSi n film was accomplished with an excellent coverage by using MoF6 and SiH4 gases. Thus, MoSi n film is a promising contact material in advanced CMOS transistors.

Optoelectronic properties of the Sb/III-Sb interface induced by laser photooxidation

Laser photooxidation is used to form a surface antimony layer on p-type GaSb and GaAs0.06Sb0.94. The emerged Sb/III-Sb interface is studied by a combination of the Raman and photoluminescence spectroscopy supported by Kelvin probe microscopy and atomic force microscopy. The laser power density controls the thickness and structure of the Sb layer. Laser photooxidation thickens the amorphous Sb layer that emerged after native oxide formation. The thickening decreases the bandgap photoluminescence intensity and increases surface band bending. Further increase in the laser power density forms a multilayered antimonene phase with a rhombohedral structure. The crystalline Sb layer decreases the surface band bending due to changes in the work function. For the crystalline layer, the PL intensity further decreases. The effective work function model explains the experimental results. Finally, the study shows that it is possible to modify surface optoelectronic properties with a submicrometer lateral resolution.

Direct Observation of Contact Electrification Effects at Nanoscale Using Scanning Probe Microscopy

AbstractIn the last decade, contact electrification has gained attention for its potential in energy harvesting, addressing fundamental physics. Scanning probe microscopy (SPM) has evolved as a powerful platform, enabling the in situ characterization/manipulation of a sample's tribological/electrical properties at nanoscale. However, although both the sliding and tapping modes are available in the energy harvesting, the lateral sliding using contact mode SPM has predominantly been employed in triboelectric study. In this work, contact electrification on polydimethylsiloxane is investigated, using peak force tapping atomic force microscopy (PF‐AFM). As PF‐AFM quasi‐discretely offers vertical tapping motions of the probe with a regulated force amplitude, resembling a vertical‐type triboelectric nanogenerator, while minimizing lateral forces. The subsequent surface potential measurements reveal that the generated tribocharge is influenced by both the effective work function difference and energy dissipation at the interface. Furthermore, the accumulation of transferred charges is explored by measuring tip‐sample current during the PF‐AFM operation, showing the comparable results with the surface potential measurement. The results can be attributed to the contact potential difference assisted by the energy dissipation at the interface. This study offers an advanced opportunity to understand and study charge generation behavior based on surface properties without damaging the sample.

Interplay of Adsorption Geometry and Work Function Evolution at the TCNE/Cu(111) Interface.

The adsorption of organic electron acceptors on metal surfaces is a powerful way to change the effective work function of the substrate through the formation of charge-transfer-induced dipoles. The work function of the interfaces is hence controlled by the redistribution of charges upon adsorption of the organic layer, which depends not only on the electron affinity of the organic material but also on the adsorption geometry. As shown in this work, the latter dependence controls the work function also in the case of adsorbate layers exhibiting a mixture of various adsorption geometries. Based on a combined experimental (core-level and infrared spectroscopy) and theoretical (density functional theory) study for tetracyanoethylene (TCNE) on Cu(111), we find that TCNE adsorbs in at least three different orientations, depending on TCNE coverage. At low coverage, flat lying TCNE dominates, as it possesses the highest adsorption energy. At a higher coverage, additionally, two different standing orientations are found. This is accompanied by a large increase in the work function of almost 3 eV at full monolayer coverage. Our results suggest that the large increase in work function is mainly due to the surface dipole of the free CN groups of the standing molecules and less dependent on the charge-transfer dipole of the differently oriented and charged molecules. This, in turn, opens new opportunities to control the work function of interfaces, e.g., by synthetic modification of the adsorbates, which may allow one to alter the adsorption geometries of the molecules as well as their contributions to the interface dipoles and, hence, the work function.

Fabrication and electron emission of [Ca24Al24.08Zr3.92O64]4+(4e−) electrides

Abstract[Ca24Al24.08Zr3.92O64]4+(4e−) electrides are ceramic materials with low work functions that can store high‐density free electrons. So, it has great application potential in electron emission. In this paper, The mixed powders of CaO, Al2O3, ZrO2, and Al in a certain stoichiometric ratio were used as precursors, and [Ca24Al24.08Zr3.92O64]4+(4e−) blocks were fabricated through in‐situ reduction. The synthetic block samples have fewer pores, cracks, and impurities, and the blocks' relative density reaches 99.6%. At the same time, the blocks’ cages were injected with high‐density electrons during the sintering, and the electron density reached 2 × 1021 cm−3. This makes the sample realize the transition from insulation to conductor. In addition, the block's emission current and effective work function is 1.75 A/cm2 and 2.25 eV, respectively, at 1100°C. The research in this paper provides a new way for the low‐cost and industrial production of high‐quality [Ca24Al24.08Zr3.92O64]4+(4e−).

Prediction of the Effective Work Function of Aspirin and Paracetamol Crystals by Density Functional Theory-A First-Principles Study.

Crystals of active pharmaceutical ingredients (API) are prone to triboelectric charging due to their dielectric nature. This characteristic, coupled with their typically low density and often large aspect ratio, poses significant challenges in the manufacturing process. The pharmaceutical industry frequently encounters issues during the secondary processing of APIs, such as particle adhesion to walls, clump formation, unreliable flow, and the need for careful handling to mitigate the risk of fire and explosions. These challenges are further intensified by the limited availability of powder quantities for testing, particularly in the early stages of drug development. Therefore, it is highly desirable to develop predictive tools that can assess the triboelectric propensity of APIs. In this study, Density Functional Theory calculations are employed to predict the effective work function of different facets of aspirin and paracetamol crystals, both in a vacuum and in the presence of water molecules on their surfaces. The calculations reveal significant variations in the work function across different facets and materials. Moreover, the adsorption of water molecules induces a shift in the work function. These findings underscore the considerable impact of distinct surface terminations and the presence of molecular water on the calculated effective work function of pharmaceuticals. Consequently, this approach offers a valuable predictive tool for determining the triboelectric propensity of APIs.

Exploring hafnium oxide's potential for passivating contacts for silicon solar cells

We investigate the potential of ultra-thin HfO2 films grown by atomic layer deposition for passivating contacts to silicon focusing on variations in film thickness and post-deposition annealing temperature. A peak in passivation quality – as assessed by carrier lifetime measurements – is reported for 2.2 nm thick films annealed at 475 °C, for which a surface recombination velocity <1 cm/s is determined. For films <2.2 nm thick, there is a marked decrease in passivation quality. X-ray diffraction highlights a change from crystallised monoclinic to amorphous HfO2 as film thickness decreases from 12 nm to 2.2 nm. Kelvin probe results indicate that as-deposited 2.2–12 nm films have similar effective work functions, although the work function of 1 nm films is considerably lower. Upon post-deposition annealing in vacuum, all films exhibit a reduction in effective work function at temperatures coincident with the onset of passivation in air-annealed samples. An initial investigation into the contact resistivity in a passivating contact structure utilizing HfO2 reveals a strong post-deposition annealing temperature dependence, with the lowest resistance achieved below 375 °C, followed by a decrease in performance as temperature increases towards the optimal temperature for passivation (475 °C). Limitations of the contact structure used are discussed.

Strain engineering on structural, optoelectronic and photocatalytic properties of BP, BAs and BSb monolayers

Recently, hydrogen energy gains a lot of attention for researcher due to its clean resource and easily production in photocatalytic process. Two dimension materials have superior chemical and physical properties compared to their bulk counterparts, having promising advantages in photocatalytic process. We perform density functional theory calculation and investigated the structural, optoelectronic and photocatalytic properties of BP, BAs and BSb monolayer. The calculated band structure shows that all these monolayers have direct band nature. A direct to indirect band gap transition is observed for 8% and 10% compressive strain for BSb monolayer. The band gap values show increase for compressive strain while decrease monotonically under tensile strain. The partial density of states confirms the different atoms contribution in valence and conduction bands. Effective masses and work function are also calculated and found to be vary under compressive and tensile strain. A blue shift in the excitonic peaks for compressive strain, while a red shift is observed for tensile strain. Photocatalytic response shows that BP (for unstrained and strain) and BAs (for 4%, 6% and 10% compressive strain) are good candidates for full water splitting. Furthermore, the thermal stability confirms that, these systems are stable under both tensile and compressive strain.

First-principles calculations of the electronic, optical properties and quantum capacitance of Mn doped Sc2CO2 monolayer under biaxial strain

The electronic structures and quantum capacitance of Mn atom doped Sc2CO2 under biaxial strain are investigated by first-principles calculations. The results indicate that pristine Sc2CO2 monolayer is nonmagnetic, while Mn doping makes the system have the magnetic character, which is mainly from Mn and C atoms neighboring to Mn atom. The total magnetic moment under strain ranges from 2.16 μB to 2.69 μB. The local magnetic moment of Mn atom under strains ranges from 2.59 μB to 4.33 μB, and the variation in local magnetic moment has the largest value of 1.84 μB. Spin-up band structures under strain exhibit the characteristic of the indirect semiconductor, and have the largest band gap of 0.3 eV at −2 % strain. The spin-down band structures under strain show the character of direct semiconductor and the largest band gap is 1.83 eV at 2 % strain. Sc2CO2-Mn under strains except +5 % are suitable for cathode material, while Sc2CO2-Mn with +5 % strain changes to cathode material in ionic/organic system. Effective mass, work function, and optical properties are further explored.

Numerical and experimental investigation of thermal regimes of thermionic cathodes of arc plasma torches

The modelling method based on decoupling the simulation of the cathodic part of the arc (the cathode and the near-cathode non-equilibrium plasma layer) from the simulation of the arc on the whole has been extended to cathodes of arc plasma torches, consisting of an insert with a conical tip, made of pure or doped tungsten, and a surrounding water-cooled copper holder. The method was validated by comparison with the experiment, performed on a 200 A DC arc in atmospheric-pressure argon. Standard work function of polycrystalline tungsten of 4.54 eV was used for modelling of pure-tungsten insert and a good agreement with the experiment was found with respect to both the insert tip shape and the temperature distribution in the tip, recorded in the stable operation mode. There are no unambiguous data on the work function for arc cathodes made of doped tungsten, although in situ measurements of the effective work function of cathodes of high-pressure arc discharges provide useful hints. On the other hand, the experiments reported in this work show that the tip temperatures of inserts made of tungsten doped with of thorium, or lanthanum, or yttrium, recorded during the stable-mode operation at the arc current of 200 A, vary in a rather narrow range 3100–3200 K. This suggests that the work functions of doped tungsten inserts, operated in the stable mode, are close to each other as well. Indeed, the results of modelling with the same value of the work function of 3 eV give a reasonably good agreement with the experiment in all three cases.