Work Function Variation Research Articles

Is it effective to harvest visible light by decreasing the band gap of photocatalytic materials?

In situ variations in the electron work function and photo-current of TiO2 nanotubes demonstrate that long-wavelength illumination only has a minor effect on the excitation of electrons in the nanotubes after being exposed to short-wavelength light or when the short-wavelength light coexisted, indicating that the solar spectrum may not be utilized as efficiently as expected by extending the absorption spectrum of the photocatalytic material to visible light range with decreased band gaps.

Engineering the work function of armchair graphene nanoribbons using strain and functional species: a first principles study

First principles density functional theory calculations were performed to study the effects of strain, edge passivation, and surface functional species on the structural and electronic properties of armchair graphene nanoribbons (AGNRs), with a particular focus on the work function. The work function was found to increase with uniaxial tensile strain and decrease with compression. The variation of the work function under strain is primarily due to the shift of the Fermi energy with strain. In addition, the relationship between the work function variation and the core level shift with strain is discussed. Distinct trends of the core level shift under tensile and compressive strain were discovered. For AGNRs with the edge carbon atoms passivated by oxygen, the work function is higher than for nanoribbons with the edge passivated by hydrogen under a moderate strain. The difference between the work functions in these two edge passivations is enlarged (reduced) under a sufficient tensile (compressive) strain. This has been correlated to a direct–indirect bandgap transition for tensile strains of about 4% and to a structural transformation for large compressive strains at about − 12%. Furthermore, the effect of the surface species decoration, such as H, F, or OH with different covering density, was investigated. It was found that the work function varies with the type and coverage of surface functional species. Decoration with F and OH increases the work function while H decreases it. The surface functional species were decorated on either one side or both sides of AGNRs. The difference in the work functions between one-sided and two-sided decorations was found to be relatively small, which may suggest an introduced surface dipole plays a minor role.

Suppression of Within-Device Variability in Intrinsic Channel Tri-Gate Silicon Nanowire Metal–Oxide–Semiconductor Field-Effect Transistors

In this paper, the variabilities of threshold voltage (VTH), drain-induced barrier lowering (DIBL), and current onset voltage (COV) in intrinsic channel silicon nanowire metal–oxide–semiconductor field-effect transistors (MOSFETs) were evaluated and compared with those of conventional bulk and fully depleted (FD) silicon-on-insulator (SOI) MOSFETs. The random component of variability is extracted by a “within-device” variability method to exclude the systematic component. It is found that the within-device variabilities of DIBL and COV as well as VTH are extremely small in intrinsic channel nanowire MOSFETs owing to the non-intentionally doped channel and small gate workfunction variability. The intrinsic channel nanowire MOSFET is promising for a future scaled device structure in terms of not only the short channel effect suppression but also the variability suppression.

阴极氧化对逸出功的影响

To explore how the degree of cathode oxidation influences the ignites characteristics of ring He-Ne laser,photoelectric retarding potential method is used to investigate the variation of the work function with the cathode oxidation.As the cold cathode of discharge device,aluminum′s work function and the work function variation before and after oxidation are measured under the high vacuum conditions by Лукирский spherical photo electronic measure equipment and parallel electrodes optic-electronic discharge device.The experimental results show that work function of aluminum cathode in the natural oxidation is 2.60±0.2 eV.The mass percent of oxygen on aluminum cathode surface is 3.2%.At different times of oxidation process,the work function of aluminum cathode decreases with the growth of the oxidation time.The reasons that work function measurements of aluminum cathode may be in 2.5～4.3 eV are explained.The conclusion from the experimental results is the oxidation time is longer,the smaller the work function of the material,the more conducive to the ignition of the ring He-Ne laser within a certain range.Cathode oxidation also affect the ignition voltage and the cathode potential drop of gas discharge process.So the choice of the optimal oxidation time ought to consider these two factors.

A scanning Kelvin probe for synchrotron investigations: thein situdetection of radiation-induced potential changes

A wide range of high-performance X-ray surface/interface characterization techniques are implemented nowadays at every synchrotron radiation source. However, these techniques are not always `non-destructive' because possible beam-induced electronic or structural changes may occur during X-ray irradiation. As these changes may be at least partially reversible, an in situ technique is required for assessing their extent. Here the integration of a scanning Kelvin probe (SKP) set-up with a synchrotron hard X-ray interface scattering instrument for the in situ detection of work function variations resulting from X-ray irradiation is reported. First results, obtained on bare sapphire and sapphire covered by a room-temperature ionic liquid, are presented. In both cases a potential change was detected, which decayed and vanished after switching off the X-ray beam. This demonstrates the usefulness of a SKP for in situ monitoring of surface/interface potentials during X-ray materials characterization experiments.

Hydrogen Detection with Noble Metal-TiO2 Schottky Diodes

Fast detection of hydrogen at a wide concentration range is desired for many applications. We report detecting hydrogen using Au-TiO2-Ti and Ag-TiO2-Ti diodes. While hydrogen-Au and hydrogen-Ag interactions are very different, at a constant biasing voltage, the measured current in both diodes is highly sensitive to the partial pressure of hydrogen contamination in the surrounding atmosphere. Work function variations were investigated by connecting the I-V specifications to the energy barrier height established at the Au-TiO2 and Ag-TiO2 junctions. Electronic features of the devices were described based on the assumption of two different hydrogen-noble metal interactions: Hydrogen reduces Ag work function by reducing the adsorbed oxygen species from the silver surface, while Au work function is reduced by the same mechanism as well as the direct adsorption of hydrogen species to the gold surface. Both of these mechanisms result in hydrogen detection by Schottky barrier height reduction and current increase.

A Physical Model for Work-Function Variation in Ultra-Short Channel Metal-Gate MOSFETs

In this letter, an accurate physical model for work-function variation (WFV) relevant to ultrashort-channel (<; 32 nm) MOSFETs has been formulated that considers the work function and size of the individual grains in determining the local MOS band structure. The proposed model is shown to be much more accurate than the most recently published model. Additionally, using this new model, WFV effect in a 3-D device can be captured using 2-D device simulation, resulting in a significantly lower simulation time.

Linearity-Distortion Analysis of GME-TRC MOSFET for High Performance and Wireless Applications

In this present paper, a comprehensive drain current model incorporating the effects of channel length modulation has been presented for multi-layered gate material engineered trapezoidal recessed channel (MLGME-TRC) MOSFET and the expression for linearity performance metrics, i.e. higher order transconductance coefficients: gm<SUB>1</SUB>, gm<SUB>2</SUB>, gm<SUB>3</SUB>, and figure-of-merit (FOM) metrics; V<SUB>IP2</SUB>, V<SUB>IP3</SUB>, IIP3 and I-dB compression point, has been obtained. It is shown that, the incorporation of multi-layered architecture on gate material engineered trapezoidal recessed channel (GME-TRC) MOSFET leads to improved linearity performance in comparison to its conventional counterparts trapezoidal recessed channel (TRC) and rectangular recessed channel (RRC) MOSFETs, proving its efficiency for low-noise applications and future ULSI production. The impact of various structural parameters such as variation of work function, substrate doping and source/drain junction depth (X<SUB>j</SUB>) or negative junction depth (NJD) have been examined for GME- TRC MOSFET and compared its effectiveness with MLGME-TRC MOSFET. The results obtained from proposed model are verified with simulated and experimental results. A good agreement between the results is obtained, thus validating the model.

Thermionic field electron emission from graphite-based nanomaterials

Thermionic and field electron emissions governed by Richardson–Dushman and Fowler–Nordheim equations, respectively, normally coexist under typical experimental conditions. However, some carbon-based cathodes demonstrate deviations of temperature dependencies of the current–voltage characteristics from those predicted by these equations. The nature of deviations cannot be ascribed to insufficient variations of work function or surface topology, which result from temperature variations. In this work, the temperature dependencies of the electron emission characteristics were experimentally investigated for the nanographite film cathodes obtained by a chemical vapor deposition method. It was found that self-consistent explanation of the experimental observations of the thermionic and field electron emissions from the nanocarbon cathodes may be given by taking into account the temperature dependence of free carrier density.

Workfunction variation across surface of an H-terminated diamond film measured using Kelvin probe force microscopy

With the ability to image both topography and contact potential difference simultaneously, Kelvin probe force microscopy (KPM) is an effective tool for the electrical characterisation of diamond surfaces. In this work we measure variations in contact potential difference across the surfaces of boron-doped diamond films in order to investigate work function variations caused by surface features. Significantly, we demonstrate work function variations in excess of 300 mV across the surfaces of two differently prepared diamond films. Variations of this magnitude may have implications for the use of diamond in a number of electronic applications.

Analysis of Ultra-Thin-Body SOI Subthreshold SRAM Considering Line-Edge Roughness, Work Function Variation, and Temperature Sensitivity

This paper analyzes stability and variability of ultra-thin-body (UTB) SOI subthreshold SRAMs considering line-edge roughness (LER), work function variation (WFV), and temperature sensitivity. The intrinsic advantages of UTB SOI technology versus bulk CMOS technology with regard to the stability and variability of 6T SRAM cells for subthreshold operation are analyzed. Compared with LER, WFV causes comparable threshold voltage variation and much smaller subthreshold swing fluctuation, hence less impact on the UTB SOI subthreshold SRAMs. Even considering LER, the Lg=40 nm UTB SOI 6T subthreshold SRAM cells still provide sufficient margin (μRSNM/σRSNM >; 6 at Vdd=0.3 ~ 0.4 V) while the bulk subthreshold SRAMs with RDF fail to maintain adequate margin. Increasing temperature will increase the Vread, 0 and decrease RSNM because of the degraded subthreshold swing. The RSNM of UTB SOI subthreshold SRAMs show less temperature sensitivity compared with that of bulk subthreshold SRAMs. Due to larger body effect, the back-gating technique is more efficient for the Lg=40 nm and 25 nm UTB SOI subthreshold SRAMs compared with the bulk counterparts. By using lower threshold voltage devices with dual band-gap work functions, the Lg=25 nm UTB SOI subthreshold SRAMs show 31.9% reduction in σRSNM and 55% improvement in μRSNM/σRSNM.

Unveiling Molecular Adsorption Geometry in Cyclohexanethiolate Self-Assembled Monolayers with Local Barrier Height Imaging

The Au–S interface of thiol-based, self-assembled monolayers grown on Au(111) surfaces is far less understood than it should be. Local barrier height imaging, which has recently resolved work function variations in the buried interface of alkanethiolate monolayers, is now used to study self-assembled monolayers on Au(111) formed by cyclohexanethiol, a molecule with a flexible aliphatic ring structure. Multiple ordered phases are observed, consistent with its conformational flexibility which makes multiple interadsorbate interactions possible. In one particular phase, the appearance of two readily distinguishable features in local barrier height images implies at least two types of adsorption sites. Correlation of simultaneously acquired topographic and local barrier height data allows for a preliminary structural model that accounts for adsorption geometry.

Orientation dependent ionization potential ofIn2O3: a natural source for inhomogeneous barrier formation at electrode interfaces in organicelectronics

The ionization potentials of In2O3 films grown epitaxially by magnetron sputtering on Y-stabilizedZrO2 substrates with (100) and (111) surface orientation are determined using photoelectronspectroscopy. Epitaxial growth is verified using x-ray diffraction. The observedionization potentials, which directly affect the work functions, are in goodagreement with ab initio calculations using density functional theory. While the(111) surface exhibits a stable surface termination with an ionization potential of ∼ 7.0 eV, the surface termination and the ionization potential of the (100) surface depend strongly onthe oxygen chemical potential. With the given deposition conditions an ionization potential of ∼ 7.7 eV is obtained, which is attributed to a surface termination stabilized by oxygen dimers.This orientation dependence also explains the lower ionization potentials observed forIn2O3 compared toSn-doped In2O3 (ITO) (Klein et al 2009 Thin Solid Films 518 1197–203). Due to the orientation dependentionization potential, a polycrystalline ITO film will exhibit a laterally varying workfunction, which results in an inhomogeneous charge injection into organic semiconductorswhen used as electrode material. The variation of work function will become even morepronounced when oxygen plasma or UV–ozone treatments are performed, as anoxidation of the surface is only possible for the (100) surface. The influence of thedeposition technique on the formation of stable surface terminations is also discussed.

Investigation on Variability in Metal-Gate Si Nanowire MOSFETs: Analysis of Variation Sources and Experimental Characterization

The characteristic variability in gate-all-around (GAA) Si nanowire (NW) metal-oxide-semiconductor field-effect transistors (SNWTs) is analyzed and experimentally investigated in this paper. First, the main variation sources in SNWTs are overviewed, with the detailed discussion on the specific sources of NW cross-sectional shape variation, random dopant fluctuation in NW source/drain extension regions and NW line-edge roughness (LER). Then, following the measurement-modeling approach, via calibrated statistical simulation that is based on the modified analytical model for GAA SNWTs with corrections of quantum effects and quasi-ballistic transport, the variability sources in SNWTs are experimentally extracted from the measured devices with 10-nm-diameter NW channels and TiN metal gate. The results indicate that NW radius variation and metal-gate work function variation dominate both the threshold voltage and on-current variations due to the ultrascaled dimensions and strong quantum effects of GAA NW structure. The NW LER also contributes, but relatively less, to the threshold voltage variation.

Recent Findings in Electrical Behavior of CMOS High-K Dielectric/Metal Gate Stacks

We first review the kinetics of the trapping/detrapping process involved in hysteresis phenomena of HfO2 films on a large temperature range (20K-500K). We show that, up to 400K, trapping and detrapping processes are temperature independent after correction of threshold voltage variation with temperature. In most cases, the capture and emission into or from available traps are mainly controlled by tunneling. Then, we address the variations of the effective metal work function and its relation with the different fabrication processes. Based on C-V and internal photoemission measurements, we show the existence of an interfacial voltage drop (i.e. dipole) at the high-k SiO2 interface, as well as of variations in metal work function with metal gate process. Finally, we present a complete study of the transport mechanisms throughout the SiO2/HfO2 gate stacks combining C-V, I-V and Transmission Electron Microscopy measurements, on several nMOS transistors with different interfacial layer and HfO2 thicknesses, over a large temperature range (80K to 400K).

Simulation Study of Intrinsic Parameter Fluctuations in Variable-Body-Factor Silicon-on-Thin-Box Metal Oxide Semiconductor Field Effect Transistors

The effects of intrinsic parameter fluctuations, including line-edge-roughness (LER), silicon-body thickness variation (STV) and work-function variation (WFV), in 20-nm-gate variable-γ silicon-on-thin-box (SOTB) metal oxide semiconductor field effect transistors (MOSFETs) have been investigated and compared with those of the conventional SOTB. Results show that the variable-γ SOTB offers not only an enhanced Ion but also a reduced Ion fluctuation with a small increase in the active-state Ioff fluctuation. The Vth-roll-off value in the variable-γ SOTB can be reduced by adopting a reverse-biased side gate to optimize the short channel effect, but the variability of the DIBL effect is enlarged. It is expected that a thinner silicon body can be used to reduce the dominant variability sources.

First-principles investigation of the adsorption of the 2,5-pyridine di-carboxylic acid onto the Cu(011) surface

First-principles calculations within the density functional theory (DFT) framework have been performed in order to investigate various conformations of the 2,5-pyridine di-carboxylic acid (PDCA) molecule adsorbed onto the Cu(011) surface. By means of DFT calculations the adsorption geometry, the bond formation and the electronic properties of PDCA molecule conformations on the Cu(011) surface have been studied. The most important structural property is the orientation of the COOH H atom which can point either toward the aromatic ring or toward the vacuum. This H atom position determines the possible reactions in which the adsorbed molecule can get involved and also has a significant impact on the value of the Cu-molecule system work function. Thus, we find that simply by changing the H atom orientation (from up to down) the Cu-molecule system work function can be varied with more than 2.5 eV. This is a significant result as a lot of effort is put nowadays in finding efficient ways for the in situ variation of the systems work function. Scanning tunneling microscopy (STM) images, reflexion absorption infrared vibrational spectra (RAIRS) as well as various thermodynamic properties (adsorption entropies, enthalpies) have also been investigated in order to get a better insight into the system studied and to provide support to possible experimental studies (STM or RAIRS experiments).

XPS and UPS in situ study of oxygen thermal desorption from nanocrystalline diamond surface oxidized by different process

The chemistry of oxygen bonding on the diamond surface is a rich area of surface science research. It is well known that different surface terminations lead to strong variation of the material work function. This effect in diamond assumes peculiar consequences. In fact the oxidized diamond surface is hydrophilic, due to the high work function it shows a positive electron affinity and it is non conductive. On the contrary hydrogenation completely changes the orientation of the surface dipoles, the surface becomes hydrophobic, the work function lowers leading to a negative electron affinity. In addition hydrogen induces subsurface carriers which render the diamond surface semiconducting. These distinctive electronic properties make the diamond surface very interesting for the fabrication of surface field effect transistors just playing with the oxygen/hydrogen chemistry. Hydrogenation is generally obtained during the diamond synthesis in plasma reactors. Differently, the diamond surface oxidation may be accomplished with different processes (wet chemistry, plasma, UV irradiation).The realization of electronic devices calls for a complete understanding of the carbon–oxygen interactions, their stability and their influence on the electronic properties of diamond. Aim of this work is to explore the properties of diamond surfaces oxidized with piranha mixture, with O 2 plasma and with UV irradiation in a pure O 2 atmosphere. Each of these oxidized surfaces were annealed in situ at different temperatures and analyzed with photoelectron spectroscopies. Decreases of the oxygen concentration obtained via thermal desorption are then correlated with variations of the electronic properties obtained from UPS analyses.

Work-Function-Tuned TiN Metal Gate FDSOI Transistors for Subthreshold Operation

The effective work function of a reactively sputtered TiN metal gate is shown to be tunable from 4.30 to 4.65 eV. The effective work function decreases with nitrogen flow during reactive sputter deposition. Nitrogen annealing increases the effective work function and reduces Dit. Thinner TiN improves the variation in effective work function and reduces gate dielectric charge. Doping of the polysilicon above the TiN metal gate with B or P has negligible effect on the effective work function. The work-function-tuned TiN is integrated into ultralow-power fully depleted silicon-on-insulator CMOS transistors optimized for subthreshold operation at 0.3 V. The following performance metrics are achieved: 64-80-mV/dec subthreshold swing, PMOS/NMOS on-current ratio near 1, 71% reduction in Cgd, and 55% reduction in Vt variation when compared with conventional transistors, although significant short-channel effects are observed.

Transistor Mismatch Properties in Deep-Submicrometer CMOS Technologies

Transistor mismatch data and analysis from poly/SiON and high-k/metal-gate (HKMG) bulk CMOS technologies are presented. It is found that the traditional mismatch figure of merit from the Pelgrom plot (A <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">VT</sub> ) continuously scales down as technology advances. Furthermore, the A <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">VT</sub> values for both nFET and pFET in the HKMG technology are significantly reduced from poly/SiON technologies. By normalizing the mismatch data against electrical oxide thickness (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">INV</sub> ) , threshold voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TH</sub> ), and effective work function, a direct comparison of the mismatch data from various technologies is made. The differences in nFET and pFET mismatch behaviors in both poly/SiON and HKMG technologies are discussed in detail. Correlation between transistor V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TH</sub> mismatch and flicker noise variation is observed in both poly/SiON and HKMG technologies. Finally, it is quantitatively demonstrated that effective work function variation does not generate significant V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TH</sub> variability in the present HKMG technology.