Surface Potential Fluctuations Research Articles

Electrochemical Characteristics and Corrosion Mechanisms of High-Strength Corrosion-Resistant Steel Reinforcement under Simulated Service Conditions

Long-term steel reinforcement corrosion greatly impacts reinforced concrete structures, particularly in marine and coastal settings. Concrete failure leads to human casualties, requiring extensive demolition and maintenance, which represents an inefficient use of energy and resources. This study utilizes microscopic observation, atomic force microscopy (SKPM), electrochemical experiments, and XPS analysis to investigate the corrosion behavior of 500CE and 500E under identical conditions. We compared 500E with 500CE, supplemented with 0.94% Cr, 0.46% Mo, 0.37% Ni, and 0.51% Cu through alloying element regulation to obtain a finer ferrite grain and lower pearlitic content. The results indicate that 500CE maintains a stable potential, whereas 500E exhibits larger grain sizes and significant surface potential fluctuations, which may predispose it to corrosion. In addition, despite its more uniform microstructure and stable electrochemical activity, 500E shows inferior corrosion resistance under prolonged exposure. The electrochemical corrosion rate of 500CE in both the pristine and passivated states and for various passivation durations is slower than that of 500E, indicating superior corrosion performance. Notably, there is a significant increase in the corrosion rate of 500E after 144 h of exposure. This study provides valuable insights into the chloride corrosion phenomena of low-alloy corrosion-resistant steel reinforcement in service, potentially enhancing the longevity of reinforced concrete structures.

Achieving high open-circuit voltage in efficient kesterite solar cells via lanthanide europium ion induced carrier lifetime enhancement

Short carrier lifetimes is a key challenge limiting the open-circuit voltage (VOC) and power conversion efficiency (PCE) of kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. In this work, for the first time, lanthanide europium (Eu) ions have been introduced into Ag-incorporated CZTSSe absorber layer, which can modify the selenization reaction pathway during the initial selenization process, leading to high-quality (Ag, Eu)-CZTSSe absorber with large compact crystal grains and benign surface potential fluctuations. This innovative absorber engineering can also optimize defects dynamics with less detrimental defects and defects-assisted non-radiative recombination. Thus, significantly enhanced minority carrier lifetimes from 0.97 to 3.15 ns can be achieved, representing one of the top values among various bulk and/or interface engineering treated CZTSSe. The champion CZTSSe thin-film solar cell exhibits an impressive VOC of 545.6 mV, accompanied with a stimulating increase in PCE from 10.68% to 13.30%. These findings underscore the potential of lanthanide cation doping to drive significant advancements in CZTSSe photovoltaic technology, and therefore promoting its further development and future applications.

Alleviating nanostructural phase impurities enhances the optoelectronic properties, device performance and stability of cesium-formamidinium metal-halide perovskites.

The technique of alloying FA+ with Cs+ is often used to promote structural stabilization of the desirable α-FAPbI3 phase in halide perovskite devices. However, the precise mechanisms by which these alloying approaches improve the optoelectronic quality and enhance the stability have remained elusive. In this study, we advance that understanding by investigating the effect of cationic alloying in CsxFA1-xPbI3 perovskite thin-films and solar-cell devices. Selected-area electron diffraction patterns combined with microwave conductivity measurements reveal that fine Cs+ tuning (Cs0.15FA0.85PbI3) leads to a minimization of stacking faults and an increase in the photoconductivity of the perovskite films. Ultra-sensitive external quantum efficiency, kelvin-probe force microscopy and photoluminescence quantum yield measurements demonstrate similar Urbach energy values, comparable surface potential fluctuations and marginal impact on radiative emission yields, respectively, irrespective of Cs content. Despite this, these nanoscopic defects appear to have a detrimental impact on inter-grains'/domains' carrier transport, as evidenced by conductive-atomic force microscopy and corroborated by drastically reduced solar cell performance. Importantly, encapsulated Cs0.15FA0.85PbI3 devices show robust operational stability retaining 85% of the initial steady-state power conversion efficiency for 1400 hours under continuous 1 sun illumination at 35 °C, in open-circuit conditions. Our findings provide nuance to the famous defect tolerance of halide perovskites while providing solid evidence about the detrimental impact of these subtle structural imperfections on the long-term operational stability.

Surface reaction for efficient and stable inverted perovskite solar cells.

Perovskite solar cells (PSCs) with an inverted structure (often referred to as the p-i-n architecture) are attractive for future commercialization owing to their easily scalable fabrication, reliable operation and compatibility with a wide range of perovskite-based tandem device architectures1,2. However, the power conversion efficiency (PCE) of p-i-n PSCs falls behind that of n-i-p (or normal) structure counterparts3-6. This large performance gap could undermine efforts to adopt p-i-n architectures, despite their other advantages. Given the remarkable advances in perovskite bulk materials optimization over the past decade, interface engineering has become the most important strategy to push PSC performance to its limit7,8. Here we report a reactive surface engineering approach based on a simple post-growth treatment of 3-(aminomethyl)pyridine (3-APy) on top of a perovskite thin film. First, the 3-APy molecule selectively reacts with surface formamidinium ions, reducing perovskite surface roughness and surface potential fluctuations associated with surface steps and terraces. Second, the reaction product on the perovskite surface decreases the formation energy of charged iodine vacancies, leading to effective n-type doping with a reduced work function in the surface region. With this reactive surface engineering, the resulting p-i-n PSCs obtained a PCE of over 25 per cent, along with retaining 87 per cent of the initial PCE after over 2,400 hours of 1-sun operation at about 55 degrees Celsius in air.

Local capacitance-voltage profiling and deep level transient spectroscopy of SiO2/SiC interfaces by scanning nonlinear dielectric microscopy

We extend our local capacitance-voltage profiling method for the real-space nanoscale investigation of the SiO2/SiC interface, which is still an issue in the application of SiC to power electronics devices. Our technique is based on time-resolved scanning nonlinear dielectric microscopy and can be used for collecting local capacitance-voltage and its voltage-derivative profiles at each measurement point. This technique can also be combined with local deep level transient spectroscopy for simultaneously imaging the spatial fluctuations of interface defect density in real-space. Here we use the proposed technique to measure SiC wafers with a thermal oxide layer and investigate the effect of interface nitridation. From the data volume obtained by our technique, various images are extracted and their correlation is analyzed to characterize and discuss the spatial fluctuations and hysteresis of the local capacitance-voltage profiles. As expected, the nitridation of the SiO2/SiC interface after thermal oxidation is effective for the reduction of hysteresis and fluctuations in local capacitance-voltage profiles. However, significant spatial fluctuations remain even after the nitridation treatment, which suggests the SiO2/SiC interface has intrinsic non-uniformity generating high surface potential fluctuations.

Surface Potential Fluctuations of SiO<sub>2</sub>/SiC Interfaces Investigated by Local Capacitance-Voltage Profiling Based on Time-Resolved Scanning Nonlinear Dielectric Microscopy

We investigate surface potential fluctuations on SiO2/SiC interfaces by local capacitance-voltage profiling based on time-resolved scanning nonlinear dielectric microscopy. As experimental indicators of surface potential fluctuations, we measured the spatial fluctuations of local capacitance-voltage and its first derivative profiles through the detection of the voltages at the infection points of the profiles. We show that, even for a sample with a nitrided interface with low interface defect density, the fluctuations of the measured voltages are much higher than the thermal energy at room temperature. This indicates the existence of high potential fluctuations, which can give the significant impacts on the carrier transport at the SiO2/SiC interface of SiC metal-oxide-semiconductor field effect transistors.

An ultrathin amorphous defective co-doped hematite passivation layer derived via an in situ electrochemical method for durable photoelectrochemical water oxidation

Owing to the passivation of surface states, low surface-potential fluctuations, and low charge-transfer resistance, an in situ electrochemically passivated photoanode shows higher photoelectrochemical performance and outstanding stability for ∼100 h.

Tailoring Growth Kinetics toward a Size-Dependent Work Function of Ge Nanocrystals Synthesized by Inert Gas Condensation

Understanding the size-dependent electronic properties of germanium nanocrystals (Ge NCs) is of fundamental importance for improving the efficiency of optoelectronic devices based on such NCs. Here, Ge NCs with a tunable size were synthesized by magnetron-sputtering cluster-beam deposition, where the size of the as-deposited Ge NCs can be finely controlled between 6 and 36 nm by helium gas flow rates and variable magnetic field configurations above the target surface. Because the size of the as-deposited Ge NCs highly depends on the nucleation process inside the plasma region, a detailed comparison between these two process parameters on the size control was formulated from the perspective of the growth kinetic mechanism. Furthermore, the local surface potential of different-sized Ge NCs deposited on n-type silicon substrates was measured by Kelvin probe force microscopy. The surface potential fluctuation of n-type Si covered by Ge NCs shows a strong size-dependent relationship with the size of the Ge NCs, whereas the surface potential fluctuation increases when their size reduced. Because the surface potential fluctuation between the intrinsic Ge NCs and the n-type silicon substrate tends to get smaller as the NCs’ size decreases due to the quantum confinement effect, the number of charges transferred between the electronic bands will reduce as the size of Ge NCs decreases. The latter exactly explains the observed experimental results. Therefore, this work offers a perspective to understand the behavior of charge transfer, which plays an important role in the performance of optoelectronic devices.

Immunity to random fluctuations induced by interface trap variability in Si gate-all-around n-nanowire field-effect transistor devices

The impact of variations in the donor and acceptor interface trap distributions on the fluctuation characteristics of 7-nm-node Si gate-all around n-nanowire FET (n-NWFETs) is analyzed in a hardware-calibrated quantum-corrected three-dimensional (3D) drift–diffusion (DD) numerical simulation framework. Shifting the energy position of the peak in the acceptor trap density distribution (Dit) induces greater surface potential fluctuations and carrier mobility degradation compared with variation of the donor traps. It is found that single-charge traps (SCTs) and random interface traps (RITs) induce larger V $$_{\text {T}}$$ and drain-induced barrier lowering (DIBL) variations, along with charge neutrality level (CNL) variations induced by interface trap fluctuations. The Si n-NWFET shows better immunity to interface trap variability when the CNL is located between the midgap and the conduction-band edge. For future sub-7-nm high-performance NWFET logic devices, such interface trap variability will be one of the major sources of random fluctuations at the device level.

Insight into Al2O3/B-doped diamond interface states with high-temperature conductance method

We employed a high temperature conductance method to investigate the interface state properties of the Al2O3/B-doped diamond MOS structure by considering the surface potential fluctuation. Based on Gaussian approximation of Nicollian's model and Brew's graphical approach, we analyzed the frequency dependent characteristics of parallel conductance (Gp/ω–f) at various gate voltages and extracted the energy distribution of interface state density (Dit), capture cross section (σp), time constant (τit), and the standard deviation of surface potential fluctuation (σs). The Dit extracted by the conductance method exhibited good agreement with that by the high-low method, whereas there exist large errors when surface potential fluctuation was not considered by using the conductance method. The capture cross section extracted by the conductance method was on the order of 10−17 cm2. From the energy dependence of the interface state time constant, the hole capture and emission follow the Shockley–Read–Hall statistics. σs decreases with the energy position away from the valence band edge (Ev) of diamond, indicating that donor-like traps are distributed in the Ev side of diamond.

Bilayer tunneling field effect transistor with oxide-semiconductor and group-IV semiconductor hetero junction: Simulation analysis of electrical characteristics

Operation mechanisms and electrical characteristics of tunneling field-effect transistors (TFETs) employing a hetero tunneling junction by utilizing an n-type oxide-semiconductor (OS) and a p-type group-IV-semiconductor are comprehensibly analyzed. Gate-normal band-to-band tunneling (BTBT) has high potential for the superior TFET performance such as high on-state current and small sub-threshold swing (S.S.). Additionally, a hetero tunneling junction with type-II energy band alignment is promising to exponentially increase tunneling probability with keeping small off-state current. Therefore, in this study, we investigate the impact of key material and device parameters such as energy band alignment of source/channel regions and thickness of the OS channel layer or gate insulator based on technology computer aided design (TCAD) simulation. The gate-controlled uniform band bending along the source-drain direction realizes uniform BTBT in the entire region of the hetero tunneling junction. Also, the reduction of the tunneling barrier height, which is continuously controlled by the conduction band minimum of the OS-channel and the valence band maximum of the IV-source, is effective to increases on-state current and decrease S.S. value. On the other hand, the thicknesses of OS channel layer and gate insulator have strong influences on tunneling probability and threshold voltage. Therefore, the sub-threshold characteristics of TFETs are sensitive to non-uniformities in the tunneling junction such as channel thickness fluctuation and surface potential fluctuation at the metal-oxide-semiconductor (MOS) interfaces. These numerical analyses of the device operation are essentially important to understand the effects of key device parameters on the TFET performance and to realize the superior electrical performance.

Suppressed Out-of-Plane Polarizability of Free Excitons in Monolayer WSe2.

Monolayer semiconductors are atomically thin quantum wells with strong confinement of electrons in atwo-dimensional (2D) plane. Here, we experimentally study the out-of-plane polarizability of excitons in hBN-encapsulated monolayer WSe2 in strong electric fields of up to 1.6 V/nm (16 MV/cm). We monitor free exciton photoluminescence peaks with increasing electric fields at a constant carrier density, carefully compensating for unintentional photodoping in our double-gated device at 4 K. We show that the Stark shift is smaller than 0.4 meV despite the large electric fields applied, yielding an upper limit of polarizability α z to be ∼10-11 Dm/V. Such a small polarizability, which is nearly two orders of magnitude smaller than the previously reported value for MoS2, indicates strong atomic confinement of electrons in this 2D system and highlights the unusual robustness of free excitons against surface potential fluctuations.

Experimental evidence of the existence of multiple charged states at Al2O3/GaN interfaces

The electrical properties of metal-oxide-semiconductor (MOS) capacitors with Al2O3/GaN interfaces formed by atomic layer deposition at various deposition temperatures (Td) were systematically investigated through comparison with the interface properties of a Si MOS capacitor. Although interface trap densities (Dit) for the GaN MOS capacitors are almost the same as for the Si MOS capacitor, surface potential fluctuation (σs) for the GaN MOS capacitors formed at various Td are much larger than for the Si MOS capacitor. Comparison between a theoretical calculation related to σs and the measured results clarified that the energy dependences of σs in the bandgap of GaN cannot be explained by electron trapping to the interface traps with the single acceptor nature (0/−). We found that the result is experimental evidence of the existence of multiple charge states at the Al2O3/GaN interfaces. The existence of multiple charge states suggests that it may be difficult to obtain good carrier transport in future GaN devices with a MOS structure.

Impacts of Al2O3/GaN interface properties on the screening effect and carrier mobility in an inversion layer

The impact of Al2O3/GaN interface properties with a large surface potential fluctuation (σs) and acceptor nature of interface traps on the screening effect of interface charges due to inversion carriers in a metal-oxide-semiconductor field-effect transistor (MOSFET) were systematically investigated. Also, the impact of the large σs on the inversion layer mobility (μeff) in the MOSFET was investigated using a model with one-dimensional surface potential fluctuation. It was found that charges of the interface traps at the Al2O3/GaN interfaces are not screened enough by the inversion carriers due to the acceptor nature of the traps, and that μeff strongly depends on the σs values. Consequently, we found that a large σs of around 8 for the Al2O3/GaN interface significantly degrades the GaN MOSFET performance.

Electrostatically actuated encased cantilevers.

Background: Encased cantilevers are novel force sensors that overcome major limitations of liquid scanning probe microscopy. By trapping air inside an encasement around the cantilever, they provide low damping and maintain high resonance frequencies for exquisitely low tip–sample interaction forces even when immersed in a viscous fluid. Quantitative measurements of stiffness, energy dissipation and tip–sample interactions using dynamic force sensors remain challenging due to spurious resonances of the system.Results: We demonstrate for the first time electrostatic actuation with a built-in electrode. Solely actuating the cantilever results in a frequency response free of spurious peaks. We analyze static, harmonic, and sub-harmonic actuation modes. Sub-harmonic mode results in stable amplitudes unaffected by potential offsets or fluctuations of the electrical surface potential. We present a simple plate capacitor model to describe the electrostatic actuation. The predicted deflection and amplitudes match experimental results within a few percent. Consequently, target amplitudes can be set by the drive voltage without requiring calibration of optical lever sensitivity. Furthermore, the excitation bandwidth outperforms most other excitation methods.Conclusion: Compatible with any instrument using optical beam deflection detection electrostatic actuation in encased cantilevers combines ultra-low force noise with clean and stable excitation well-suited for quantitative measurements in liquid, compatible with air, or vacuum environments.

Effect of lanthanum silicate interface layer on the electrical characteristics of 4H-SiC metal-oxide-semiconductor capacitors

The effect of La-silicate interface layer (IL) on the electrical characteristics of 4H-SiC metal-oxide-semiconductor (MOS) capacitors with atomic-layer-deposited SiO2 (ALD-SiO2) gate dielectrics was investigated. In addition to a slight reduction in the interface state density (Dit), the surface potential fluctuation was greatly reduced due to the reduction in the fixed charges (Qfix) with La-silicate IL. Moreover, two orders of magnitude reduction in the oxide trap density in the ALD-SiO2 layer adjacent to the La-silicate IL was confirmed. Physical analysis revealed the reduction in carbon concentration and incorporation of La atoms adjacent to the La-silicate IL.

Impacts of oxidants in atomic layer deposition method on Al2O3/GaN interface properties

The electrical interface properties of GaN metal–oxide–semiconductor (MOS) capacitors with an Al2O3 gate insulator formed by atomic layer deposition method using three kinds of oxidants were investigated by the capacitance–voltage technique, Terman method, and conductance method. We found that O3 and the alternate supply of H2O and O3 (AS-HO) are effective for reducing the interface trap density (Dit) at the energy range of 0.15 to 0.30 eV taking from the conduction band minimum. On the other hand, we found that surface potential fluctuation (σs) induced by interface charges for the AS-HO oxidant is much larger than that for a Si MOS capacitor with a SiO2 layer formed by chemical vapor deposition despite the small Dit values for the AS-HO oxidant compared with the Si MOS capacitor. This means that the total charged center density including the fixed charge density, charged slow trap density, and charged interface trap density for the GaN MOS capacitor is higher than that for the Si MOS capacitor. Therefore, σs has to be reduced to improve the performances and reliability of GaN devices with the Al2O3/GaN interfaces.

Charge puddles in the bulk and on the surface of the topological insulator BiSbTeSe2 studied by scanning tunneling microscopy and optical spectroscopy

The peculiar surface transport properties of topological insulators often require tuning of the Fermi level close to the Dirac point. This, however, reduces the screening capabilities of the Dirac-like surface carriers and thus enhances the sensitivity to Coulomb disorder present in compensated materials. Coulomb disorder drives the formation of puddles, local accumulations of charge carriers both in the bulk and on the surface. In the topological insulator BiSbTeSe${}_{2}$, the coexistence of surface puddles and bulk puddles is revealed by scanning tunneling spectroscopy and optical spectroscopy. Quantitative analysis shows that bulk puddles contribute to the screening of surface potential fluctuations in this undoped compound. Understanding and controlling the puddles will be a key to realizing novel phenomena in 3D topological materials.

Understanding of frequency dispersion in C-V curves of metal-oxide-semiconductor capacitor with wide-bandgap semiconductor

Frequency dispersion of capacitance-voltage (C-V) characteristics of a GaN metal-oxide-semiconductor (MOS) capacitor was systematically investigated. A high frequency C-V curve without including capacitance associated with interface traps and negligibly small C-V hysteresis gave us an accurate relationship between the surface Fermi level and the gate voltage in the C-V curve. We found that obvious frequency dispersion was observed even at a gate bias condition of −1V, which has very long electron capturing time compared with time corresponding to inverse of the measurement frequencies. Furthermore, calculation based on an equivalent circuit of the MOS capacitor including surface potential fluctuation (σs) clarified that the frequency dispersion is owing to the large σs. This indicates that consideration of the impact of σs is quite important for accurate understanding of the C-V characteristics for wide-bandgap semiconductor.

Charge state stabilization of shallow nitrogen vacancy centers in diamond by oxygen surface modification

We investigated the charge state stability and coherence properties of near-surface single nitrogen vacancy (NV) centers in 12C-enriched diamond for potential use in nanoscale magnetic field sensing applications. The stability of charge states in negatively charged NV centers (NV−) was evaluated using one of the pulsed optically detected magnetic resonance measurements, Rabi oscillation measurements. During the accumulation of Rabi oscillations, an unstable shallow NV− was converted to a neutral state. As a result, the contrast of Rabi oscillations degraded, depending on charge state stability. We stabilized the NV− state of very shallow NV centers (∼2.6 ± 1.1 nm from the surface) created by 1.2 keV nitrogen ion implantation by diamond surface modification, UV/ozone exposure, and oxygen annealing. This improvement indicates that we can suppress the upward surface band bending and surface potential fluctuations through Fermi level pinning originating from oxygen-terminated diamond surfaces.