Microscopic study of electrical properties of CrSi<Subscript>2</Subscript> nanocrystals in silicon

Semiconducting CrSi₂ nanocrystallites (NCs) were grown by reactive deposition epitaxy of Cr onto n -type silicon and covered with a 50-nm epitaxial silicon cap. Two types of samples were investigated: in one of them, the NCs were localized near the deposition depth, and in the other they migrated near the surface. The electrical characteristics were investigated in Schottky junctions by current-voltage and capacitance-voltage measurements. Atomic force microscopy (AFM), conductive AFM and scanning probe capacitance microscopy (SCM) were applied to reveal morphology and local electrical properties. The scanning probe methods yielded specific information, and tapping-mode AFM has shown up to 13-nm-high large-area protrusions not seen in the contact-mode AFM. The electrical interaction of the vibrating scanning tip results in virtual deformation of the surface. SCM has revealed NCs deep below the surface not seen by AFM. The electrically active probe yielded significantly better spatial resolution than AFM. The conductive AFM measurements have shown that the Cr-related point defects near the surface are responsible for the leakage of the macroscopic Schottky junctions, and also that NCs near the surface are sensitive to the mechanical and electrical stress induced by the scanning probe.     

Nanoscale characterization of electrical transport at metal/3C-SiC interfaces

In this work, the transport properties of metal/3C-SiC interfaces were monitored employing a nanoscale characterization approach in combination with conventional electrical measurements. In particular, using conductive atomic force microscopy allowed demonstrating that the stacking fault is the most pervasive, electrically active extended defect at 3C-SiC(111) surfaces, and it can be electrically passivated by an ultraviolet irradiation treatment. For the Au/3C-SiC Schottky interface, a contact area dependence of the Schottky barrier height (ΦB) was found even after this passivation, indicating that there are still some electrically active defects at the interface. Improved electrical properties were observed in the case of the Pt/3C-SiC system. In this case, annealing at 500°C resulted in a reduction of the leakage current and an increase of the Schottky barrier height (from 0.77 to 1.12 eV). A structural analysis of the reaction zone carried out by transmission electron microscopy [TEM] and X-ray diffraction showed that the improved electrical properties can be attributed to a consumption of the surface layer of SiC due to silicide (Pt2Si) formation. The degradation of Schottky characteristics at higher temperatures (up to 900°C) could be ascribed to the out-diffusion and aggregation of carbon into clusters, observed by TEM analysis.     

Growth and fabrication of InAs/GaSb type II superlattice mid-wavelength infrared photodetectors

In this work, the transport properties of metal/3C-SiC interfaces were monitored employing a nanoscale characterization approach in combination with conventional electrical measurements. In particular, using conductive atomic force microscopy allowed demonstrating that the stacking fault is the most pervasive, electrically active extended defect at 3C-SiC(111) surfaces, and it can be electrically passivated by an ultraviolet irradiation treatment. For the Au/3C-SiC Schottky interface, a contact area dependence of the Schottky barrier height (Φ_B) was found even after this passivation, indicating that there are still some electrically active defects at the interface. Improved electrical properties were observed in the case of the Pt/3C-SiC system. In this case, annealing at 500°C resulted in a reduction of the leakage current and an increase of the Schottky barrier height (from 0.77 to 1.12 eV). A structural analysis of the reaction zone carried out by transmission electron microscopy [TEM] and X-ray diffraction showed that the improved electrical properties can be attributed to a consumption of the surface layer of SiC due to silicide (Pt₂Si) formation. The degradation of Schottky characteristics at higher temperatures (up to 900°C) could be ascribed to the out-diffusion and aggregation of carbon into clusters, observed by TEM analysis.     

Repeatable change in electrical resistance of Si surface by mechanical and electrical nanoprocessing

The properties of mechanically and electrically processed silicon surfaces were evaluated by atomic force microscopy (AFM). Silicon specimens were processed using an electrically conductive diamond tip with and without vibration. After the electrical processing, protuberances were generated and the electric current through the silicon surface decreased because of local anodic oxidation. Grooves were formed by mechanical processing without vibration, and the electric current increased. In contrast, mechanical processing with vibration caused the surface to protuberate and the electrical resistance increased similar to that observed for electrical processing. With sequential processing, the local oxide layer formed by electrical processing can be removed by mechanical processing using the same tip without vibration. Although the electrical resistance is decreased by the mechanical processing without vibration, additional electrical processing on the mechanically processed area further increases the electrical resistance of the surface.     

Space charge spectroscopy of diamond

Capacitance–voltage (CV) and deep level transient spectroscopy (DLTS) experiments (thermally and optically excited) are applied to explore various properties of boron-doped type IIb high pressure high temperature diamond and of homoepitaxially grown boron doped CVD diamond: (a) properties of Al Schottky contacts; (b) acceptor densities and hydrogen boron interactions; and (c) energy distributions and densities of compensating defects. Two distinct excitation energies at 0.9 and 1.25 eV are detected by DLTS and optically excited DLTS. Carbon implantation experiments reveal that the defect with optical excitation energy of 1.25 eV is the positively charged vacancy. Annealing measurements show that in the temperature regime of 525–600 K a significant part of the vacancy density is annealed which is attributed to recombination of carbon interstitial with vacancies.     

Analysis of Thermal Effects on Electrical Characterization of AlGaN/GaN/Si FAT-HEMTs

In this paper we report on the characteristics of Schottky contact behavior of AlGaN/GaN High Electron Mobility Transistors (HEMTs) on a Si Substrate. A variety of electrical techniques such as capacitance-voltage (C-V) and deep level transient spectroscopy (DLTS) measurements have been used to characterize the diode. The behavior of the ideality factor n, the effective barrier height Φ_b, and the series resistance R_S is studied with temperature. C-V measurements successively sweeping up and down the voltage from 0 V to 12 V(absolute value) have demonstrated a hysteresis phenomenon which is more pronounced when the temperature increases. This parasitic effect can be attributed to the presence of traps activated in the Schottky diode. The related deep levels defects which are responsible for parasitic effects, were characterized and extracted by the Deep Level Transient Spectroscopy (DLTS) technique that has been used in previous studies. The identification of these traps showed a correlation between DLTS and C-V measurements.     

Low-damage direct patterning of silicon oxide mask by mechanical processing

To realize the nanofabrication of silicon surfaces using atomic force microscopy (AFM), we investigated the etching of mechanically processed oxide masks using potassium hydroxide (KOH) solution. The dependence of the KOH solution etching rate on the load and scanning density of the mechanical pre-processing was evaluated. Particular load ranges were found to increase the etching rate, and the silicon etching rate also increased with removal of the natural oxide layer by diamond tip sliding. In contrast, the local oxide pattern formed (due to mechanochemical reaction of the silicon) by tip sliding at higher load was found to have higher etching resistance than that of unprocessed areas. The profile changes caused by the etching of the mechanically pre-processed areas with the KOH solution were also investigated. First, protuberances were processed by diamond tip sliding at lower and higher stresses than that of the shearing strength. Mechanical processing at low load and scanning density to remove the natural oxide layer was then performed. The KOH solution selectively etched the low load and scanning density processed area first and then etched the unprocessed silicon area. In contrast, the protuberances pre-processed at higher load were hardly etched. The etching resistance of plastic deformed layers was decreased, and their etching rate was increased because of surface damage induced by the pre-processing. These results show that etching depth can be controlled by controlling the etching time through natural oxide layer removal and mechanochemical oxide layer formation. These oxide layer removal and formation processes can be exploited to realize low-damage mask patterns.     

Porous Semiconductor Micropatterns Formed on Focussed Ion Beam Implants

In this work we report a principle that allows one to write visible light emitting silicon patterns of arbitrary shape down to the sub-micrometer scale. We demonstrate that porous Si growth can electrochemically be initiated preferentially at surface defects created in an n-type Si substrate by Si⁺+ ion bombardment. Using a focused ion beam (FIB) as a source of ions, arbitrary defect patterns can be written into a substrate. The growth of light emitting porous silicon is then selectively achieved by an electrochemical treatment which triggers Si dissolution only at these defect sites. The selectivity of the electrochemical dissolution reaction can be attributed to a facilitated Schottky barrier breakdown at the implanted surface defects which leads to the desired pore formation in confined surface areas.     

Atomic force microscope study of the rejection of colloids by membrane pores

An atomic force microscope (AFM) in conjunction with the colloid probe technique has been used to study the electrical double layer interactions between a 0.75 μm silica sphere and a polymeric microfiltration track etch Cyclopore membrane (nominally 1 μm) in aqueous solutions. The silica colloid probe was used to image the membrane surface (using the double layer mode) at different imaging forces in high purity water and at constant imaging force in sodium chloride solutions of different ionic strengths at pH 8. Force-distance measurements show clearly how the sphere detects the membrane surface. Quality of images produced from scanning the 0.75 μm silica particle across the surface deteriorates with increasing distance between the silica sphere and membrane surface. Such images were compared with those obtained from scanning a sharp silicon nitride tip over the membrane surface.     

Robustness of 4H-SiC 1200 V Schottky diodes under high electrostatic discharge like human body model stresses: An in-depth failure analysis

The aim of this study is to propose a complete failure analysis of silicon carbide (SiC) junction barrier Schottky (JBS) diodes under high electrostatic discharge (ESD) human body model (HBM)-like stresses addressing the limit of robustness for this new generation of high power devices. The Physics of Failure is fully investigated, first by analyzing electrical measurements which are relevant to physical integrity and interface states using parameters predicted by thermionic emission (TE) theory. Secondly Optical Beam Induced Resistance CHange (OBIRCH) is used for the localization of surface defects. Finally, Focused Ion Beam (FIB) cuts are performed and Transmission Electron Microscopy (TEM) analyses are carried out to characterize the structural and elemental composition modifications. With the results, correlations can be made between electrical and physical degradations, leading to reliable hypotheses about the root cause of the weaknesses of these devices when subjected to this kind of stress.     

Local structural changes due to the electric field-induced migration of oxygen vacancies at Fe-doped SrTiO3 interfaces

We report on our study of dc voltage-induced structural changes at reduced and oxidized Fe-doped SrTiO₃ (Fe:STO) electrode interfaces using second harmonic generation (SHG) together with photoluminescence (PL) method. We show that oxygen vacancy defects play a critical role in determining the local electrical and structural properties of interfacial depletion regions at Schottky junctions. The SHG results show that the dc electric field causes oxygen ions and vacancies to displace toward the anode and cathode in the low field regime, respectively. This process forms electrostrictive distortions within local interfacial depletion regions which are described by Fe:Ti-O bond stretching and bending. Differences in the EFISHG responses from the oxidized and reduced crystal interfaces are explained according to local oxygen vacancy concentrations and dynamics and their effects on the Schottky barrier heights and depletion region widths at each interface. These results are further supported by our PL measurements. Oxygen ion migration toward the Fe:STO surface leads to enhanced fluorescence intensities from in-gap acceptor states. We demonstrate that SHG and PL measurements are well-suited for understanding and resolving the underlying causes of dielectric breakdown processes and device failure brought on by dc electric field and ionic defect migrations in perovskite-type electroceramics.     

Review MXenes as a new type of nanomaterial for environmental applications in the photocatalytic degradation of water pollutants

Two-dimensional titanium carbide (MXene) with an adjustable bandgap (0.92–1.75eV), excellent structural stability, high conductivity and hydrophilicity has always been a hotspot in the field of environmental photocatalysis. However, the rapid recombination of light-excited carriers of a single photocatalytic material decreases quantum efficiency and photocatalytic performance. The modification of MXene could overcome these problems to improve photocatalytic properties. Among various improvement strategies, the composition of MXene heterostructure and Schottky junction is an effective and straightforward strategy for adjusting electronic structure and accelerating photocatalytic performance. This review aims to design typical, cost-effective heterojunctions and Schottky junctions and their progress, mechanisms, and trends in environmental organic pollutants' degradation. This review detailed the heterogeneous catalytic mechanism of MXene-based photocatalysts for the degradation of organic pollutants. It is discussed the way to improve the photocatalytic performance of MXene by constructing heterojunction and Schottky junction. The surface properties, catalyst performance and pollution management of various MXene-based catalysts were compared, and then some dilemmas and application strategies of MXene development were analyzed in depth. This review can open up ideas for new approaches and provide valuable clues for designing MXene as a cocatalyst to develop more effective photocatalysts for practical application in environmental pollution management.     

Current-Voltage characteristics in macroporous silicon/SiOx/SnO2:F heterojunctions

ABSTRACT: We study the electrical characteristics of Macroporous Silicon/Transparent Conductor Oxide junctions obtained by the deposition of fluorine doped SnO2 onto macroporous silicon thin films, using the spray pyrolysis technique. Macroporous silicon was prepared by the electrochemical anodization of a silicon wafer to produce pores sizes ranging between 0.9 to 1.2 mum diameter. Scanning electronic microscopy was performed to confirm the pore filling and surface coverage. The transport of charge carriers through the interface was studied by measuring the current-voltage curves in dark and under illumination. In the best configuration we obtain a modest open circuit voltage of about 70 mV and a short circuit current of 3.5 mA/cm2 at illumination of 110 mW/cm2. In order to analyze the effects of the illumination on the electrical properties of the junction, we proposed a model of two opposing diodes, each one associated with an independent current source. We obtain a good accordance between experimental data and the model. The current-voltage curves on illumination conditions are well fitted with the same parameters obtained in dark where only the photocurrent intensities in the diodes are free parameters.     

Surface electronic properties on boron doped (111) CVD homoepitaxial diamond films after oxidation treatments

Surface electronic properties on oxidized boron (B) doped (111) homoepitaxial diamond films are investigated by Hall effect measurements and Schottky junction characterizations. Surface electronic properties on (111) diamond strongly depend on annealing treatments after wet-chemical oxidation, whereas for those on (001) diamond no change due to annealing can be detected. Hall effect results show that a p-type surface conductive layer (SCL) exists on (111) diamond surface in air after wet-chemical oxidation followed by annealing in Ar atmosphere (WO–AN) above 300 °C, but does not if only wet-chemical oxidation or air-oxidation is applied. This SCL disappears at annealing temperature above 350 °C in air. Schottky junction characteristics suggest that the Fermi level is unpinned at the (111) surface after WO–AN. Surface electronic characteristics on (111) diamond after WO–AN are similar to those generated by hydrogen termination.     

Junction investigation of graphene/silicon Schottky diodes

ABSTRACT: Here we present a facile technique for the large-scale production of few-layer graphene flakes. The as-sonicated, supernatant, and sediment of the graphene product were respectively sprayed onto different types of silicon wafers. It was found that all devices exhibited current rectification properties, and the supernatant graphene devices have the best performance. Schottky junctions formed between graphene flakes and silicon n-type substrates exhibit good photovoltaic conversion efficiency while graphene/p-Si devices have poor light harvesting capability.     

Electrical characteristics of CNFETs with asymmetric gating structures

We demonstrated a straightforward technique, including device structures and associated fabrication processes to produce carbon nanotube field-effect transistors (CNFETs) which have different gate oxide thickness at two terminal junctions. Since a thinner oxide layer induces stronger gate field at one junction terminal than a thicker one, robust unipolar CNFETs were carried out. The blockage of one type of carriers is due to a Schottky barrier at the terminal junction with the thicker bottom oxide. The high ON/OFF ratios, up to 10⁴, in such asymmetric gating devices show that the gate field is equally effective as the devices with uniformly thin oxide layer, and the ensuring unipolar behavior is much feasible toward nanotubes based electronics.     

On the Growth and Microstructure of Carbon Nanotubes Grown by Thermal Chemical Vapor Deposition

Carbon nanotubes (CNTs) were deposited on various substrates namely untreated silicon and quartz, Fe-deposited silicon and quartz, HF-treated silicon, silicon nitride-deposited silicon, copper foil, and stainless steel mesh using thermal chemical vapor deposition technique. The optimum parameters for the growth and the microstructure of the synthesized CNTs on these substrates are described. The results show that the growth of CNTs is strongly influenced by the substrate used. Vertically aligned multi-walled CNTs were found on quartz, Fe-deposited silicon and quartz, untreated silicon, and on silicon nitride-deposited silicon substrates. On the other hand, spaghetti-type growth was observed on stainless steel mesh, and no CNT growth was observed on HF-treated silicon and copper. Silicon nitride-deposited silicon substrate proved to be a promising substrate for long vertically aligned CNTs of length 110–130 μm. We present a possible growth mechanism for vertically aligned and spaghetti-type growth of CNTs based on these results.     

The current image of single SnO2 nanobelt nanodevice studied by conductive atomic force microscopy

A single SnO₂ nanobelt was assembled on a pair of Au electrodes by electric-field assembly method. The electronic transport property of single SnO₂ nanobelt was studied by conductive atomic force microscopy (C-AFM). Back-to-back Schottky barrier-type junctions were created between AFM tip/SnO₂ nanobelt/Au electrode which can be concluded from the I-V curve. The current images of single SnO₂ nanobelt nanodevices were also studied by C-AFM techniques, which showed stripes patterns on the nanobelt surface. The current images of the nanobelt devices correlate the microscopy with separate transport properties measurement together.     

Investigation of schottky barriers at Pd-ZnO junctions in varistors

This is the first comprehensive study of the electrical behaviour, namely the I–V characteristics, of electrode-grain junctions in Pr-based ZnO varistor ceramics with Pd electrodes. These junctions have been investigated by a micro 4-point probe setup on the microstructural scale. A mean Schottky barrier height of 0.47 ± 0.03 eV was found. The reverse current through the junctions could be described by a model based on an interfacial layer. Furthermore, crystal orientations and polarities of grains with respect to the electrode layers were determined by electron back scatter diffraction and analysing etching patterns to check a possible influence on the barrier height. But within the set of experiments no correlation between grain orientation and barrier height could be found.     

Lithium salts as leachable corrosion inhibitors and potential replacement for hexavalent chromium in organic coatings for the protection of aluminum alloys

Lithium salts are being investigated as leachable corrosion inhibitor and potential replacement for hexavalent chromium in organic coatings. Model coatings loaded with lithium carbonate or lithium oxalate demonstrated active corrosion inhibition and the formation of a protective layer in a damaged area during neutral salt spray exposure. The present paper provides an abridged overview of the initial studies into this novel inhibitor technology for the active corrosion protection of aluminum alloys. Coating defects were investigated by microscopic techniques before and after exposure to corrosive conditions. Scanning electron microscopy analysis of cross-sections of the coating defect area demonstrated that the protective layer comprises a typical three-layered structure, which included a dense layer near the alloy surface, a porous middle layer, and a flake-shaped out layer. Potentiodynamic polarization measurements obtained with a microcapillary cell positioned in the coating defect area and electrochemical impedance spectroscopy confirmed the corrosion protective properties of these protective layers. The long-term corrosion inhibition of the lithium-based coating technology was tested in industrial coating systems.