Work Function Modulation Research Articles

Work function modulation of AuCl 4− molecule adsorbed on graphene: A first-principles simulation

The work function modulations of AuCl 4 −–graphene system have been theoretically investigated. The calculated results of work function modulations are in agreement with the recent experiment [ACS Nano, 4(5) (2010) 2689]. However, the theoretical works are lack for revealing the mechanism of the work function modulation in AuCl 4 −–graphene system. Our simulation reveals the work function can be modulated by the adsorbed concentration. It should be originated from the Fermi level shifting and the emergence of charge redistribution. The behavior of charge redistribution can result in the enhancement of the surface dipole and further the modulation of the work function.

Fermi level shifting of TiO2 nanostructures during dense electronic excitation

Scanning Kelvin probe microscopy has been used to understand the modification of work function of TiO2 with swift heavy ion irradiation. The observed increase in contact potential difference (CPD) indicates a shift in Fermi level towards the valence band, which is due to the development of defects during the bombardment of high energy heavy ions. The change in CPD values on ion irradiation is attributed to electronic excitation induced defect concentration and surface roughness.

Effective Work Function Modulation of Graphene/Carbon Nanotube Composite Films As Transparent Cathodes for Organic Optoelectronics

In this study, we found that the work functions (Φ(w)) of solution-processable, functional graphene/carbon nanotube-based transparent conductors were readily manipulated, varying between 5.1 and 3.4 eV, depending on the nature of the doping alkali carbonate salt. We used the graphene-based electrodes possessing lower values of Φ(w) as cathodes in inverted-architecture polymer photovoltaic devices to effectively collect electrons, giving rise to an optimal power conversion efficiency of 1.27%.

Enhanced performance in polymer photovoltaic cells with chloroform treated indium tin oxide anode modification

Enhanced performance of a poly(3-hexylthiophene):(6,6)-phenyl C61 butyric acid methyl ester bulk heterojunction polymer photovoltaic cell is reported by modifying the indium tin oxide (ITO) anode with chloroform solution. Instead of the traditional UV-ozone treatment, the optimized chloroform modification on ITO anode can result in an enhancement in the power conversion efficiency of an identical device, originating from an increase in the photocurrent with negligible change in the open-circuit voltage. The performance enhancement is attributed to the work function modification of the ITO substrate through the surface incorporation of the chlorine, and thus improved charge collection efficiency.

Transport characteristics and surface potential distribution of electrically stressed TiO2 single crystals

We investigated transport and local electrical properties of Pt/TiO2 single crystal/Ti planar junctions with micron-sized gaps between the electrodes. Potential profiles, obtained by scanning Kelvin probe microscopy (SKPM), clearly showed that negative (positive) electrical stress to the Pt electrodes significantly reduced (hardly affected) the Pt/TiO2 contact resistance. In addition, the SKPM results revealed that the electrical stress caused modification of resistance and local work function of the TiO2 surface. All the results suggested that the electrical stress caused alteration of physical properties at the TiO2 surface between the electrodes as well as the Pt/TiO2 interface.

Odd–even modulation of electrode work function with self-assembled layer: Interplay of energy barrier and tunneling distance on charge injection in organic light-emitting diodes

In this study, a series of trifluoromethyl-terminated alkanethiols with different chain length (CF 3(CH 2) n SH, n = 5, 6, 10, 11, 12 and 13), were used to form self-assembled monolayers(SAMs) on silver and gold surfaces. The SAM modulates the metal work function as a result of the different orientation of the terminal dipolar CF 3 group due to different chain tilt of the SAM on Au and Ag surfaces, respectively. The SAM-modified Ag and Au were used as the anode in the fabrication of hole-only devices as well as top-emitting organic light-emitting diodes (TOLEDs) with different hole-transporting layers. The correlation between current density/luminous efficiency and the SAM chain length/work function were analyzed to provide information on charge balance. Thus with NPB as the hole-transporting layer, where little difference between the HOMO energy level and the work function of the SAM-modified electrode, the charge injection is modulated by the chain length (tunneling distance-dependent) of the SAM used for modification, decreasing with increasing chain length. With BPAPF as the HTL, where a barrier is present, the charge injection is modulated more by the work function (injection barrier-dependent), which shows an odd–even alternation.

Modulation of PtSi Work Function by Alloying with Low Work Function Metal

In order to reduce PtSi Schottky barrier height (SBH) for electron, we investigated modulation of PtSi work function by alloying with low work function metal, such as Hf (3.9eV) and Yb (2.7eV). Pt (10-20nm)/Hf, Yb (0-10nm)/n-Si(100) stacked structures were in-situ deposited at room temperature by RF magnetron sputtering method. In case of PtxHf1-xSi formed at 400°C/60min annealing in N2, SBH for electron was reduced from 0.85eV to 0.53eV with Hf thickness without increase of sheet resistance. Yb incorporation also affected the SBH modulation, however, the sheet resistance increased with increase of Yb thickness.

Tuning the gas sensing performance of single PEDOT nanowire devices

This paper reports the synthesis and dopant dependent electrical and sensing properties of single poly(ethylenedioxythiophene) (PEDOT) nanowire sensors. Dopant type (i.e. polystyrenesulfonate (PSS(-)) and perchlorate (ClO(4)(-))) and solvent (i.e. acetonitrile and 1 : 1 water-acetonitrile mixture) were adjusted to change the conjugation length and hydrophilicity of nanowires which resulted in change of the electrical properties and sensing performance. Temperature dependent coefficient of resistance (TCR) indicated that the electrical properties are greatly dependent on dopants and electrolyte where greater disorder was found in PSS(-) doped PEDOT nanowires compared to ClO(4)(-) doped nanowires. Upon exposure to different analytes including water vapor and volatile organic compounds, these nanowire devices displayed substantially different sensing characteristics. ClO(4)(-) doped PEDOT nanowires from an acetonitrile bath show superior sensing responses toward less electronegative analytes and followed a power law dependence on the analyte concentration at high partial pressures. These tunable sensing properties were attributed to variation in the conjugation lengths, dopant type and concentration of the wires which may be attributed to two distinct sensing mechanisms: swelling within the bulk of the nanowire and work function modulation of Schottky barrier junction between nanowire and electrodes.

Work function modulation of PtSi by alloying with Yb

Work function modulation of PtSi by alloying with Yb to achieve ultra-low contact resistance for advanced CMOS was investigated. PtxYbySi was formed by depositing Pt(6-18nm)/Yb(2-14nm)/n-Si(100) stacked structure followed by 400-800°C/1-30min silicidation in N2 ambient. It was found that barrier height for electron(ΦBn) was decreased as the silicidation temperature and time increased, and ΦBn was reduced to 0.52eV by depositing Pt(6nm)/Yb(14nm) followed by 800°C/30min silicidation. It was found that Yb diffusion toword the silicide/Si interface was enhanced, compared to the sample formed by 500-600°C silicidation, which leads to further decrease of ΦBn. The estimated effective work function of PtSi decreased from 4.92eV to 4.57eV.

Field induced shape and work function modification for the ZrO/W(100) Schottky cathode

The origin of the frequently observed long and short term current drifts after a change in the extractor voltage for the ZrO/W(100) Schottky cathode has been investigated. It has been found that a reversible, field dependent change in the equilibrium work function and shape of the cathode end form occurs at the typical operating temperature of 1800 K. The shape change results in three distinct geometric end form shapes. Although these end forms have been observed previously [L. W. Swanson and G. A. Schwind, in Handbook of Charged Particle Optics, 2nd ed., edited by J. Orloff (CRC, New York, 2008), Chap. 1, p. 1; S. Fujita, T. R. Wells, W. Ushio, H. Sato, and M. M. El-Gomati, J. Microsc. 235, 215 (2010); M. S. Bronsgeest and P. Kruit, J. Vac. Sci. Technol. B 27, 2524 (2009)], the purpose of this study is to elucidate the operating conditions under which these changes occur and the associated changes in emission properties. Emitters in the radii range 200–900 nm were investigated over the typical current density operating range employed in most electron probe forming systems. A correlation between work function and electric field has been found, which can be explained by a reversible, field induced change in the ZrO equilibrium coverage in the high field region of the emitter, thereby causing a change in the work function. It is believed that these field induced changes in the equilibrium work function and emitter shape at 1800 K explain the short and long term drifts observed after a change in extractor voltage.

Effective Work Function Modulation by Aluminum Ion Implantation on Hf-Based High- $k$/Metal Gate pMOSFET

The impact of aluminum (Al) implantation into TiN/HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> / SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> on the effective work function is investigated. Al implanted through poly-Si cannot attain sufficient flatband voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FB</sub> ) shift unless at higher implantation energy. Al implanted through TiN at 1.2 keV with a dose of 5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> raised the V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FB</sub> to about 250 mV compared with a nonimplanted gate stack. Moreover, the V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FB</sub> shift can be up to about 800 mV at 2 keV with the same dose level accompanied with slightly equivalent oxide thickness penalty and gate leakage current degradation. Optimized process window to control Al diffusion depth was essential to minimize these impacts.

Control of Gate Metal Effective Work Functions and Interface Layer Thickness by Designing Interface Thermodynamics Based on Heteroatom Incorporation into High-k HfO2 Gate Dielectrics

We have proposed a new approach for controlling both gate metal effective work functions and interface layer thickness at the same time based on the incorporation of hetero atoms into HfO2 gate dielectrics. We investigated C atom incorporation into HfO2 by the first principles calculations. The results indicate that additional C atoms in HfO2 can reduce an interfacial SiO2 layer, which also causes modification of a gate metal effective work function. Redesigning interface thermodynamics by incorporating hetero atoms can be a crucial recipe for controlling both gate metal work functions and thickness of interfacial SiO2 layers in high-k gate stacks

Coupling Epitaxy, Chemical Bonding, and Work Function at the Local Scale in Transition Metal-Supported Graphene

Resonance tunneling spectroscopy and density functional theory calculations are employed to explore local variations in the electronic surface potential of a single graphene layer grown on Rh(111). A work function modulation of 220 meV is experimentally measured, indicating that the chemical bonding strength varies significantly across the supercell of the Moiré pattern formed when graphene is bonded to Rh(111). In combination with high-resolution images, which provide precise knowledge of the local atomic registry at the carbon-metal interface, we identify experimentally, and confirm theoretically, the atomic configuration of maximum chemical bonding to the substrate. Our observations are at odds with reported trends for other transition metal substrates. We explain why this is the case by considering the various factors that contribute to the bonding at the graphene/metal interface.

Theoretical Investigation of the Binding Process of Corannulene on a Cu(111) Surface†

DFT-GGA calculations, enhanced to include effects of dispersion, are used to investigate the adsorption process of corannulene on a Cu(111) surface. In accord with experiments, we consider the dynamics of corannulene approaching the surface in a tilted fashion, concave side-up, enabling interactions between one of the six-membered rings and the surface over a 3-fold hollow site. Electronic structure analyses, including projected density of states and detection of work function modification, are used to aid in the understanding of the specific nature of the interaction between the corannulene and the metal surface in the complex system. Results show substantial charge rearrangement at the interface, the net effect being a large interface dipole that, added to the intrinsic molecular dipole, causes a significant decrease of the surface work function. Despite the charge rearrangement, no appreciable charge transfer occurs, and the general orbital structure of the individual components is retained. The analysis suggests that the adsorption of corannulene on Cu(111) is not a chemisorption process. Increased packing of corannulene on the surface leads to progressively smaller adsorption-induced interface dipoles, due to the depolarizing field created by the molecules.

A study on selective etching for elevated PtSi salicide process and work function modulation of PtSi alloying with Hf

AbstractThe selective etching process for elevated self‐aligned silicide (salicide) utilizing PtSi has been investigated. We have developed a novel selective etching process utilizing dilute aqua regia followed by etching with dilute HF. It was found that the residual Pt‐rich silicide layers on the sidewall were successfully removed. We also investigated work function modulation of PtSi alloyed with Hf. The barrier height for electrons of PtSi was reduced approximately 0.1 eV for PtxHf1‐xSi formed by the silicidation of Pt (17 nm)/Hf (4 nm)/Si (100) stacked layer structures. © 2010 Wiley Periodicals, Inc. Electron Comm Jpn, 93(8): 32–37, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecj.10215

Potential Energy Landscape for Hot Electrons in Periodically Nanostructured Graphene

We explore the spatial variations of the unoccupied electronic states of graphene epitaxially grown on Ru(0001) and observed three unexpected features: the first graphene image state is split in energy; unlike all other image states, the split state does not follow the local work function modulation, and a new interfacial state at +3 eV appears on some areas of the surface. First-principles calculations explain the observations and permit us to conclude that the system behaves as a self-organized periodic array of quantum dots.

Annealing effect on the metal gate effective work function modulation for the Al/TiN/SiO 2/p-Si structure

In this work, the thermal annealing effect on the metal gate effective work function (EWF) modulation for the Al/TiN/SiO 2/p-Si(1 0 0) structure was investigated. Compared with the sample of TiN/SiO 2/p-Si(1 0 0) structure, for the sample additionally capped with Al the flat band voltage has a very obvious shift as large as 0.54 V to the negative direction after forming gas annealing. It is also revealed that the thermal budget can effectively influence both the EWF of the gate electrode and the thickness of the gate dielectric layer when a post annealing at 600 °C with different soak times was applied to the samples with Al cap. Material characterization indicates that the diffusion of Al and the formation of Al oxide during annealing should be responsible for all the phenomena. The interface trap density Dit calculated from the high-frequency C– V and the laser-assisted high-frequency C– V curves show that the introduction of Al does not cause reliability problem in the Al/TiN/SiO 2/p-Si structure.

Threshold Voltage Tunability of p-Channel Metal Oxide Semiconductor Field-Effect Transistor with Ternary HfxMoyNz Metal Gate and Gd2O3 High-k Gate Dielectric

p-channel metal oxide semiconductor field-effect transistor (pMOSFET) devices with a ternary HfxMoyNz metal gate and a Gd2O3 high-k gate dielectric have been demonstrated for the first time. The nitrogen-concentration-control method is a simple and cost-effective technique for metal work-function modulation. HfxMoyNz thin films were cosputtered from pure hafnium (Hf) and molybdenum (Mo) targets in nitrogen (N2) and argon (Ar) mixtures. The HfxMoyNz thin films have low resistivity and high thermal stability up to 950 °C. The threshold voltage (Vth) of the HfxMoyNz/Gd2O3 pMOSFET can be tuned from -0.6 to -0.08 V by controlling the nitrogen flow ratio. Moreover, there is little negative bias temperature instability (NBTI) degradation of the HfxMoyNz/Gd2O3 pMOSFET device. Compared with the HfxMoyNz/SiO2 pMOSFET, the HfxMoyNz/Gd2O3 pMOSFET has a small threshold voltage modulation owing to the extrinsic Fermi level effect at the HfxMoyNz and Gd2O3 interface. A physical model has been proposed to explain the extrinsic Fermi level pinning effect of the HfxMoyNz/Gd2O3 pMOSFET device.

De Novo Designed Coiled-Coil Proteins with Variable Conformations as Components of Molecular Electronic Devices

Conformational changes of proteins are widely used in nature for controlling cellular functions, including ligand binding, oligomerization, and catalysis. Despite the fact that different proteins and artificial peptides have been utilized as electron-transfer mediators in electronic devices, the unique propensity of proteins to switch between different conformations has not been used as a mechanism to control device properties and performance. Toward this aim, we have designed and prepared new dimeric coiled-coil proteins that adopt different conformations due to parallel or antiparallel relative orientations of their monomers. We show here that controlling the conformation of these proteins attached as monolayers to gold, which dictates the direction and magnitude of the molecular dipole relative to the surface, results in quantitative modulation of the gold work function. Furthermore, charge transport through the proteins as molecular bridges is controlled by the different protein conformations, producing either rectifying or ohmic-like behavior.

Fabrication of advanced La-incorporated Hf-silicate gate dielectrics using physical-vapor-deposition-based in situ method and its effective work function modulation of metal/high-k stacks

Lanthanum (La) incorporation into Hf-silicate high-permittivity (high-k) gate dielectrics was conducted using a physical-vapor-deposition (PVD)-based in situ method. PVD-grown metal Hf, La, and Hf–La alloys on base SiO2 oxides received in situ annealing to form high-quality HfLaSiO dielectrics, and subsequent deposition of metal gate electrodes was carried out to fabricate advanced metal/high-k gate stacks without breaking vacuum. The in situ method was found to precisely control La content and its depth profile and to tune the effective work function of metal/high-k stacks. Remarkable leakage current reduction of almost seven orders of magnitude compared with conventional poly-Si/SiO2 stacks and excellent interface properties comparable to an ideal SiO2/Si interface were also achieved at an equivalent oxide thickness of around 1.0 nm. Our x-ray photoelectron spectroscopy analysis revealed that, as previously suggested, effective work function modulation due to La incorporation is attributed to the interface dipole (or localized sheet charge) at the bottom high-k/SiO2 interface, which is crucially dependent on the La content at the interface. Moreover, it was found that high-temperature annealing causing interface oxide growth leads to redistribution of La atoms and forms the uppermost La-silicate layer at the metal/high-k interface by releasing the dipole moment at the bottom high-k/SiO2 interface. Based on these physical and electrical characterizations, the advantages and process guidelines for La-incorporated dielectrics were discussed in detail.