Metal Fermi Level Research Articles

Pristine and oxidised Ag-nanoparticles on free-standing graphene as explored by X-ray photoelectron and Auger spectroscopy

Silver nanoparticles with a mean diameter of 15 nm have been grown on free-standing graphene and explored by soft X-ray synchrotron radiation using the ESCAmicroscopy beamline at Elettra. In particular, the 3d core orbitals and valence band (4d/5 s) of pristine and plasma-oxidised Ag-nanoparticles have been investigated by X-ray photoelectron spectroscopy (XPS), X-ray induced valence band photoemission, and MNN Auger spectroscopy. The X-ray spectra of the pristine Ag-nanoparticles, i.e. XPS, Auger and UPS spectra, are very similar to these of the metallic Ag-bulk film while the spectra of the oxidised Ag-nanoparticles exhibit distinct differences with respect to the oxidised Ag-bulk film. This is inferred from a larger valence band width, an extended valence band photoemission intensity nearly up to the Fermi level of the bulk metal, a smaller two-hole Coulomb correlation energy and a rather bulk-like Auger spectrum. We interpret this as to result from a shell-like composition of the Ag-nanoparticles with a thin passivating oxide layer and an inner metallic core. Moreover, the 3d photoemission peak of the oxidised nanoparticles is shifted to higher binding energy with respect to the bulk oxide peak. This is attributed not only to an XPS chemical shift but also to an additional final state Coulomb charging according to the electrostatic liquid drop model. Comparing the core binding energy shift of Ag-nanoparticles with the electrostatic potential of a spherical compact particle and a flat nanoislands a morphology transformation of the oxidised nanoparticles from 3D to 2D is obvious in the size range 5–15 nm.

Low-Frequency Noise Characteristics in Multi-Layer WSe₂ Field Effect Transistors with Different Contact Metals.

In this work, we analyze characteristics of Ohmic, Schottky forward and reverse contact through a low-frequency noise (LFN) measurement, combining two types of metals (Pd and Au) as the source and drain (S/D) contacts that enable p-type properties in multi-layer WSe₂ field effect transistors (FETs). The LFN is one of the significant factors liming the performance of nano-scale devices such as TMDCs FETs having large surface-to-volume ratio. In addition, the LFN analysis, which relates to the device reliability, can help identify sensitive areas for current transport and evaluate the analog circuit applicability. Theoretically, the multi-layer WSe₂ has reasonable electron affinity and bandgap that can make p-channel FET using the metal with a relatively high work-function. However, it is experimentally confirmed that Schottky contact characteristics are exhibited in the multi-layer WSe₂ FETs with various metals except Pd due to the metal Fermi level pinning phenomenon. Mobility (μeff, ~87.5 cm²/V·s), one of the electrical performance extracted from fabricated devices with Pd as S/D electrodes shows a great difference from that (~0.572 cm²/V·s) of devices with Au as S/D electrodes. The measured electrical characteristics show that a Schottky contact is formed at an interface between Au and WSe₂ causing the higher LFN of the FETs than that of device with Pd as S/D electrodes. This characteristic is also verified by confirming the reduction of LFN due to the decreased effect of the Schottky property as the drain bias is increased.

Contact Engineering High-Performance n-Type MoTe2 Transistors.

Semiconducting MoTe2 is one of the few two-dimensional (2D) materials with a moderate band gap, similar to silicon. However, this material remains underexplored for 2D electronics due to ambient instability and predominantly p-type Fermi level pinning at contacts. Here, we demonstrate unipolar n-type MoTe2 transistors with the highest performance to date, including high saturation current (>400 μA/μm at 80 K and >200 μA/μm at 300 K) and relatively low contact resistance (1.2 to 2 kΩ·μm from 80 to 300 K), achieved with Ag contacts and AlOx encapsulation. We also investigate other contact metals (Sc, Ti, Cr, Au, Ni, Pt), extracting their Schottky barrier heights using an analytic subthreshold model. High-resolution X-ray photoelectron spectroscopy reveals that interfacial metal-Te compounds dominate the contact resistance. Among the metals studied, Sc has the lowest work function but is the most reactive, which we counter by inserting monolayer hexagonal boron nitride between MoTe2 and Sc. These metal-insulator-semiconductor (MIS) contacts partly depin the metal Fermi level and lead to the smallest Schottky barrier for electron injection. Overall, this work improves our understanding of n-type contacts to 2D materials, an important advance for low-power electronics.

Electronic band alignment in AlGaN/GaN high electron-mobility transistors investigated using scanning photocurrent microscopy

The band alignment in AlGaN/GaN HEMT devices was investigated using scanning photocurrent microscopy (SPCM) with UV and visible light sources. The polarity of the SPCM on the conduction channel exhibited a switching behavior from p-type to n-type response as the gate bias was increased. The flat-band voltage, which was higher than the DC turn-on voltage, indicates that an ohmic metallic contact was formed for electron transport. We obtained the band offset between the conduction band and the metal Fermi level, which yielded a value of 2.1 eV on average.

Point defect concentrations in surface layers of binary oxides

The dependence of point defect concentrations in the surface layer (the outer most layer of the bulk) of binary oxides, on oxygen partial pressure, P(O2), and acceptor concentration, Ab, is evaluated. With one calibration point the defect concentration, at constant temperature, under any P(O2) and Ab can be determined. The method of calculation is demonstrated on binary oxides which exhibit oxygen vacancies, electrons and holes. A relation between the dependence on P(O2) and Ab of concentrations of point defects in the surface layer, and the dependence of the concentrations of the corresponding defects deep in the neutral bulk, is presented. Two types of surface layers are distinguished according to their electron states. One that forms a band which is partially populated by electrons, leading to a two dimensional metallic surface and Fermi level pinning (FLP) and a second, insulating one and no FLP. Four characteristic cases are identified: FLP or the absence of FLP in the surface layer combined with the presence or absence of significant chemisorption. It is shown that the dependence on P(O2) and Ab of the defect concentrations in the surface layer may be the same as that of the corresponding defects deep in the neutral bulk. The difference is only in the magnitude. The conditions for that to take place are an insulating surface and low concentration of chemisorbed particles. The latter conditions are quite common, as the surface of oxides is mostly not metallic, oxygen chemisorption is low at elevated temperature and for acceptor doped, p-type oxides, chemisorption is also low at low temperature. Different relations are derived under other conditions. In particular, for metallic surface, the oxygen vacancy concentration in the surface layer follows only one dependence, P(O2)−1/2, irrespective of the dependence on P(O2) and Ab of the bulk vacancy concentration and irrespective of chemisorption. The electrical potential distribution is derived for the four cases.

Ultrafast strong-field photoelectron emission due to two-color laser fields

Electron emission from solids driven by two-color lasers provides great flexibility for the control of electron dynamics in ultrashort spatiotemporal scales due to the interference effect. Here, we construct an analytical model for the highly nonlinear photoelectron emission from a metal surface illuminated by two-color laser fields, by solving the time-dependent Schr\odinger equation. The exact solution is valid for arbitrary harmonic orders, laser intensities, phase difference between two lasers, and metal work function and Fermi level. We find two-color lasers can strongly modulate both the electron energy spectra and the emission current up to 99%. Using the same input parameters, our theoretical prediction for the photoemission current modulation depth (93.9%) is almost identical to the experimental result (94%) in [M. F\orster et al., Phys. Rev. Lett. 117, 217601 (2016)].

Determination of Carrier Polarity in Fowler-Nordheim Tunneling and Evidence of Fermi Level Pinning at the Hexagonal Boron Nitride/Metal Interface.

Hexagonal boron nitride (h-BN) is an important insulating substrate for two-dimensional (2D) heterostructure devices and possesses high dielectric strength comparable to SiO2. Here, we report two clear differences in their physical properties. The first one is the occurrence of Fermi level pinning at the metal/h-BN interface, unlike that at the metal/SiO2 interface. The second one is that the carrier of Fowler-Nordheim (F-N) tunneling through h-BN is a hole, which is opposite to an electron in the case of SiO2. These unique characteristics are verified by I- V measurements in the graphene/h-BN/metal heterostructure device with the aid of a numerical simulation, where the barrier height of graphene can be modulated by a back gate voltage owing to its low density of states. Furthermore, from a systematic investigation using a variety of metals, it is confirmed that the hole F-N tunneling current is a general characteristic because the Fermi levels of metals are pinned in the small energy range around ∼3.5 eV from the top of the conduction band of h-BN, with a pinning factor of 0.30. The accurate energy band alignment at the h-BN/metal interface provides practical knowledge for 2D heterostructure devices.

Enhanced Solar-Driven Applications of ZnO@Ag Patchy Nanoparticles

ZnO@Ag patchy nanostructures were demonstrated to be efficient and stable photocatalysts for the photodegradation of organic contaminants in aqueous solutions. The photoinduced charge transfer from the conduction band of ZnO toward the Fermi level of the noble metal was favored and exploited to enhance the photocatalytic efficiency of ZnO, with a mechanism based on hole stabilization. Naked ZnO and ZnO@Ag patchy nanostructures were demonstrated to degrade methylene blue, a model compound, in aqueous solution under 370–800 nm light irradiation (100 mW cm–2); in particular, the introduction of silver nanoparticles allowed one to increment twice the constant rate of the reaction when fitted as pseudo-first-order kinetics. Furthermore, the degradation of 2,4-dichlorophenol under direct sunlight irradiation was studied. The photo-oxidation catalyzed by patchy nanostructures was noticeably increased. In fact, the observed half time (t1/2) was reduced by almost 4 times in comparison with the value observed for b...

Energy level alignment at the interface of cadmium sulphide single crystal and phthalocyanines: The role of the crystal surface states

Ultraviolet photoelectron spectroscopy was used to study energy levels of (0001) and (0001_) surfaces of bare CdS crystal terminated with Cd and S atoms, respectively, as well as energy level alignment at hybrid interfaces of CdS crystal and donor zinc phthalocyanine (PcZn) or acceptor fluoro-substituted zinc phtalocyanine (F-PcZn) layers, respectively. The data allowed us to distinguish a slight difference between ionization potential of the Cd- and S-terminated facets of the crystal and different contribution of these surfaces to formation of the interfacial dipole at the hybrid interfaces. Ionization potential for the S-terminated surface was slightly higher as compared to the Cd-terminated one for the same crystal, and a more positive dipole was always created on the S-terminated surface independent of whether the donor or acceptor phthalocyanine was used. The results showed that PcZn and F-PcZn render a dual effect at the CdS surface. First, the molecules created a dipole at the interface, however, the sign of this dipole was opposite for PcZn and F-PcZn, respectively. Second, the both molecules contributed to the formation of a depletion layer near the crystal surface. The role of the surface states of the crystal in the above effects has been elucidated. Interaction of PcZn and CdS surface was associated with the pinning of the upper occupied level of the surface states to the positive charge transfer state corresponding to the oxidized HOMO level of the molecule at the interface, whereas interaction of F-PcZn and CdS surface with the pinning of the upper occupied level of the surface states to the negative charge transfer corresponding to the reduced LUMO level of the molecule at the interface, i.e., similar to what is observed for the metal-organic interfaces, where an upper occupied level of semiconductor surface states in our case plays the role of Fermi level in metal.

Measuring the Surface Photovoltage of a Schottky Barrier under Intense Light Conditions: Zn/p-Si(100) by Laser Time-Resolved Extreme Ultraviolet Photoelectron Spectroscopy

A metal–semiconductor heterojunction is investigated by Auger and photoelectron spectroscopy to characterize the structural and electronic properties of the metallic film and to obtain the time-resolved electronic response induced by femtosecond laser excitation of the semiconductor material. The 3.5 monolayer (ML) Zn films deposited on p-type Si(100) at liquid nitrogen temperature grows in a layer-by-layer fashion. Electronic structure measurements by extreme ultraviolet (XUV) photoelectron spectroscopy indicate that the films are metallic in nature, creating a Schottky barrier at the 3.5 ML Zn/p-Si(100) interface. Utilizing a 35 fs, 800 nm pump pulse at a pump intensity of (2.5–6) × 109 W/cm2 to excite the Si and a time-delayed extreme ultraviolet pulse to probe the Zn, we observed large transient surface photovoltage shifts of 0.3–2.2 eV at carrier densities of (1.5–4.5) × 1020 cm–3. Three shifts are determined the Zn 3d core level, the photoemission onset, and the metallic Fermi level. The photovoltag...

Maximization of photocatalytic activity of Bi2S3/TiO2/Au ternary heterostructures by proper epitaxy formation and plasmonic sensitization

Ternary Bi2S3/TiO2/Au heterostructure has been successfully synthesized by decorationg Au nanoparticls onto binary Bi2S3/TiO2 heterostructure. Structural analysis suggests that the heterostructure consists of orthorhombic Bi2S3, tetragonal anatase TiO2 and face centered cubic Au. The appearance of extra Raman peak at 123cm−1 and the modification of Raman peaks of Bi2S3 and TiO2 phases indicates a significant molecular interaction during growth of TiO2 nanocrystals (NCs) on the surface of Bi2S3 nanorods. Self assembly of nanorods triggered by oleic acid results in urchin like morphology of pure Bi2S3. The {011} facet of TiO2 nanocrystals grow epiaxially on {013} facet of Bi2S3 nanorods. Periodic arrangement of (111) plane of Au and (011) plane of Bi2S3 favors epitaxial growth in Bi2S3/TiO2/Au heterostructure. Unique design of the ternary heterostructure makes a preferable pathway for the photogenerated electrons by suppressing the electron-hole recombination. The observation of decreased electron spin resonance signal intensity based on spin label technique confirms the electron transfer from photoexcited heterostructure. Favorable energetic positions of conduction and valence band edges of semiconductors (Bi2S3 and TiO2) and Fermi level of metal (Au) are the key factors to enhance the photo catalytic activity of ternary heterostructure. Plasmonic phenomenon of metal (Au) nanoparticles by changing their size and shape was also investigated to remove organic pollutants from waste water.

Insights into the interactions of biomolecules with small gold clusters: a theoretical study from a DFTB perspective

In this study, we highlight the benefits and limits of using density-functional tight-binding (DFTB) with respect to standard density functional theory (DFT). The DFTB method is found to reliably reproduce the DFT adsorption energies and geometries. Overall, our results suggest that DFTB is a good reference method to set the correct chemical states and the initial geometries of proline interacting with gold cluster. The frontier orbitals energies suggest that in proline–gold cluster complex, the proline molecule can only act as an electron donor and the gold cluster as acceptor. The analysis of the electronic properties for arginine, arginine dipeptide, and TAT peptide which underline the role on the interaction of the s-like H states with the d-like Au states produces both bonding and anti-bonding occupied orbitals, and the process is well described by a model for the interaction of localized orbitals with narrow-band dispersive electron states. The bonding orbitals well below the metal Fermi level contribute to the adsorption of arginine, arginine dipeptide, and TAT peptide on gold.

Energy Level Alignment at Hybridized Organic–Metal Interfaces: The Role of Many-Electron Effects

Hybridized molecule/metal interfaces are ubiquitous in molecular and organic devices. The energy level alignment (ELA) of frontier molecular levels relative to the metal Fermi level (EF) is critical to the conductance and functionality of these devices. However, a clear understanding of the ELA that includes many-electron self-energy effects is lacking. Here, we investigate the many-electron effects on the ELA using state-of-the-art, benchmark GW calculations on prototypical chemisorbed molecules on Au(111), in eleven different geometries. The GW ELA is in good agreement with photoemission for monolayers of benzene-diamine on Au(111). We find that in addition to static image charge screening, the frontier levels in most of these geometries are renormalized by additional screening from substrate-mediated intermolecular Coulomb interactions. For weakly chemisorbed systems, such as amines and pyridines on Au, this additional level renormalization (~1.5 eV) comes solely from static screened exchange energy, allowing us to suggest computationally more tractable schemes to predict the ELA at such interfaces. However, for more strongly chemisorbed thiolate layers, dynamical effects are present. Our ab initio results constitute an important step towards the understanding and manipulation of functional molecular/organic systems for both fundamental studies and applications.

Temperature dependent electron transport in oligo (3-methylthiophene) derivative molecular devices

Addressable molecular junctions based on a thiol-functionalized oligo (3-methylthiophene) derivative incorporated with pyridine unit were fabricated using polymer stamp-printing method. The junction charge transport was measured from 95 to 299 K under both dark and light conditions. Asymmetric current–voltage characteristics with temperature, light, and bias-direction dependence were observed. Simmons tunneling combined with hopping play dominant roles at low bias under both dark and light. The thermionic and Poole–Frankel emissions are also found to affect the charge transport in some cases. Based on the density functional theory in conjunction with the non-equilibrium Green's function method, we investigated the transport properties of a series of single-molecule junctions with different molecular structures, in which negative differential resistance was observed. Density functional theory calculation and transmission spectrum show that the tunneling barrier may depend on the relative energetic position of the frontier molecular orbitals with respect to the metal Fermi level under the effects of bias and temperature.

Strong Fermi-Level Pinning at Metal/n-Si(001) Interface Ensured by Forming an Intact Schottky Contact with a Graphene Insertion Layer.

We report the systematic experimental studies demonstrating that a graphene layer inserted at metal/n-Si(001) interface is efficient to explore interface Fermi-level pinning effect. It is confirmed that an inserted graphene layer prevents atomic interdiffusion to form an atomically abrupt Schottky contact. The Schottky barriers of metal/graphene/n-Si(001) junctions show a very weak dependence on metal work-function, implying that the metal Fermi-level is almost completely pinned at charge neutrality level close to the valence band edge of Si. The atomically impermeable and electronically transparent properties of graphene can be used generally to form an intact Schottky contact for all semiconductors.

Study on Synergistic Mechanism of Inhibitor Mixture Based on Electron Transfer Behavior.

Mixing is an important method to improve the performance of surfactants due to their synergistic effect. The changes in bonding interaction and adsorption structure of IM and OP molecules before and after co-adsorbed on Fe(001) surface is calculated by DFTB+ method. It is found that mixture enable the inhibitor molecules with higher EHOMO donate more electrons while the inhibitor molecules with lower ELUMO accept more electrons, which strengthens the bonding interaction of both inhibitor agent and inhibitor additive with metal surface. Meanwhile, water molecules in the compact layer of double electric layer are repulsed and the charge transfer resistance during the corrosion process increases. Accordingly, the correlation between the frontier orbital (EHOMO and ELUMO of inhibitor molecules and the Fermi level of metal) and inhibition efficiency is determined. Finally, we propose a frontier orbital matching principle for the synergistic effect of inhibitors, which is verified by electrochemical experiments. This frontier orbital matching principle provides an effective quantum chemistry calculation method for the optimal selection of inhibitor mixture.

(Invited) Low Resistance Contacts to Nanoscale Semiconductor Devices

The scaling of silicon CMOS faces several obstacles necessitating the need to introduce materials such as germanium, III-Vs and 2D materials together with novel structures such as FinFET. As device scaling continues, parasitic source resistance largely dominated by contact resistance, is beginning to limit the device performance. Specific contact resistivity, ρc, of a metal-semiconductor (M/S) contact is dependent on the Schottky barrier height, ΦB, and the electrically active dopant density N at that interface. In M/S contacts the metal Fermi level is pinned at the charge neutrality level, ECNL, resulting in fixed electron and hole Schottky barrier heights ΦBN and ΦBP. High ΦB and low N results in high ρc, eclipsing the promise of intrinsic performance of scaled devices. To obtain low ρc it is essential to reduce ΦB and increase N. Fermi level pinning occurs because wave functions of electrons in the metal tail can decay into the semiconductor in the band gap, creating metal-induced gap states (MIGS). Charging of these states by the metal wavefunction creates a dipole charge causing the Fermi level to align to minimize the dipole charge towards zero, effectively pulling the EF at the interface towards ECNL. Historically the method to reduce ρc is by increasing N to > 1E20/cm3 thereby thinning the barrier, thus allowing more tunneling current. This method works well for n-Si and p-Ge which can be doped heavily. However, it is not very practical for n-Ge, p-Si, many III-Vs and 2D materials because of inability to dope them heavily. Second method is to create an additional dipole at the M-S interface to modulate ΦB by incorporating a dopant or a chalcogen close to the interface to modulate the dipole. Fluorine, sulfur and selenium have been shown to reduce ΦB in many semiconductors. Third method is by inserting a thin interlayer between the metal and semiconductor to depin the Fermi level and reduce ΦB. The metal electron wavefunction now decays in the interlayer resulting in fewer MIGS thus depinning the Fermi level, which now pins on the interlayer. Hence metal workfunction ΦM can now be used to tune the effective barrier height. For n-type ohmic contacts, metals with a low ΦM and interlayer with ~0 conduction band offset (ZnO, TiO2, SnO2) with semiconductor should result in a near zero ΦBN. For p-type ohmic contacts, metals with a high ΦM and insulator with ~0 valence band offset (NiO, CuAlO2) with semiconductor would be desirable. This method has been used to obtain low ΦB and thus low ρc in Si, Ge and III-V semiconductors and should prove to be very useful for 2D materials. Alternatively the interlayer can be chosen such that the metal pins near the band edge to get ~0 ΦB and it can be doped heavily to get thin barrier facilitating increased tunneling. SiGe source/drain in Si PMOS is a good example of this technique. Results of theoretical modeling and experimental work will be presented in this talk.

(Invited) Low Resistance Contacts to Nanoscale Semiconductor Devices

Low Resistance Contacts to Nanoscale Semiconductor Devices Krishna C. Saraswat and Gautam Shine Department of Electrical Engineering, Stanford University, Stanford, CA, 94305 As device scaling continues, parasitic resistance largely dominated by contact resistance, is beginning to limit the device performance. Specific contact resistivity, ρc, of a metal-semiconductor (M/S) contact is dependent on the Schottky barrier height, ΦB, and the electrically active dopant density N at that interface. In M/S contacts the metal Fermi level is pinned at the charge neutrality level, ECNL due to metal induced gap states (MIGS), resulting in fixed electron and hole Schottky barrier heights. High ΦB and low N results in high ρc, eclipsing the promise of intrinsic performance of scaled devices. To obtain low ρc it is essential to reduce ΦB and increase N. In this paper we review the problem caused by Fermi level pinning and various methods to overcome this problem and thus obtain low contact resistance. These methods include heavy doping to reduce barrier thickness, Fermi level depinning with interfacial layers, dipole formation at the interface, and heterostructures for barrier lowering.

On the widths of Stokes lines in Raman scattering from molecules adsorbed at metal surfaces and in molecular conduction junctions.

Within a generic model we analyze the Stokes linewidth in surface enhanced Raman scattering (SERS) from molecules embedded as bridges in molecular junctions. We identify four main contributions to the off-resonant Stokes signal and show that under zero voltage bias (a situation pertaining also to standard SERS experiments) and at low bias junctions only one of these contributions is pronounced. The linewidth of this component is determined by the molecular vibrational relaxation rate, which is dominated by interactions with the essentially bosonic thermal environment when the relevant molecular electronic energy is far from the metal(s) Fermi energy(ies). It increases when the molecular electronic level is close to the metal Fermi level so that an additional vibrational relaxation channel due to electron-hole (eh) exciton in the molecule opens. Other contributions to the Raman signal, of considerably broader linewidths, can become important at larger junction bias.

Charge injection in thin dielectric layers by atomic force microscopy: influence of geometry and material work function of the AFM tip on the injection process

Charge injection and retention in thin dielectric layers remain critical issues for the reliability of many electronic devices because of their association with a large number of failure mechanisms. To overcome this drawback, a deep understanding of the mechanisms leading to charge injection close to the injection area is needed. Even though the charge injection is extensively studied and reported in the literature to characterize the charge storage capability of dielectric materials, questions about charge injection mechanisms when using atomic force microscopy (AFM) remain open. In this paper, a thorough study of charge injection by using AFM in thin plasma-processed amorphous silicon oxynitride layers with properties close to that of thermal silica layers is presented. The study considers the impact of applied voltage polarity, work function of the AFM tip coating and tip curvature radius. A simple theoretical model was developed and used to analyze the obtained experimental results. The electric field distribution is computed as a function of tip geometry. The obtained experimental results highlight that after injection in the dielectric layer the charge lateral spreading is mainly controlled by the radial electric field component independently of the carrier polarity. The injected charge density is influenced by the nature of electrode metal coating (work function) and its geometry (tip curvature radius). The electron injection is mainly ruled by the Schottky injection barrier through the field electron emission mechanism enhanced by thermionic electron emission. The hole injection mechanism seems to differ from the electron one depending on the work function of the metal coating. Based on the performed analysis, it is suggested that for hole injection by AFM, pinning of the metal Fermi level with the metal-induced gap states in the studied silicon oxynitride layers starts playing a role in the injection mechanisms.