Thin Native Oxide Research Articles

All-Soft Supercapacitors Based on Liquid Metal Electrodes with Integrated Functionalized Carbon Nanotubes.

Soft energy storage devices, such as supercapacitors, are an essential component for powering integrated soft microsystems. However, conventional supercapacitors are mainly manufactured using hard/brittle materials that easily crack and eventually delaminate from the current collector by mechanical deformation. Therefore, to realize all-soft supercapacitors, the electrodes should be soft, stretchable, and highly conductive without compromising the electrochemical performance. This paper presents all-soft supercapacitors for integrated soft microsystems based on gallium-indium liquid metal (eutectic gallium-indium alloy, EGaIn) electrodes with integrated functionalized carbon nanotubes (CNTs). Oxygen functional groups on the surface of the CNTs ensure strong adhesion between the functionalized CNTs and the thin native oxide layer on the surface of EGaIn, which enables delamination-free soft and stretchable electrodes even under mechanical deformation. The electrochemical performances of fabricated all-soft supercapacitors in a parallel-plate arrangement were investigated without and with applied mechanical deformation. The fabricated supercapacitors exhibit areal capacitances as high as 12.4 mF cm-2 and show nearly unchanged performance under 30% applied strain.Theymaintain >95% of their original capacitance after >4200 charging and discharging cycles with a periodic applied strain of 30%. Finally, fabricated supercapacitors have been successfully integrated with a commercial light-emitting diode to demonstrate an integrated soft microsystem.

Influence of a thin amorphous surface layer on de-channeling during aluminum implantation at different temperatures into 4H-SiC

Ion implantation is an important technique in semiconductor processing and has become a key technology for 4H-SiC devices. Today, aluminum (Al) implantations are routinely used for p-type contacts, p+-emitters, terminations and many other applications. However, in all crystalline materials, quite a few ions find a path along a crystal channel, so-called channeling, and these ions travel deep into the crystal. This paper reports on the channeling phenomenon during Al implantation into 4H-SiC, and in particular, the influence of a thin native oxide will be discussed in detail. The effects of thermal lattice vibrations for implantations performed at elevated temperatures will also be elucidated. 100 keV Al ions have been implanted along the [000-1] direction employing samples with 4° miscut. Before implantation, the samples have been aligned using the blocking pattern of backscattered protons. Secondary ion mass spectrometry has been used to record the Al depth distribution. To predict implantation profiles and improve understanding of the role of crystal structure, simulations were performed using the Monte-Carlo binary collision approximation code SIIMPL. Our results show that a thin surface layer of native oxide, less than 1 nm, has a decisive role for de-channeling of aligned implantations. Further, as expected, for implantations at elevated temperatures, a larger degree of de-channeling from major axes is present due to increased thermal vibrations and the penetration depth of channeled aluminum ions is reduced. The values for the mean-square atomic displacements at elevated temperatures have been extracted from experimental depth profiles in combination with simulations.

Combination of spectroscopic ellipsometry and spectroscopic reflectometry with including light scattering in the optical characterization of randomly rough silicon surfaces covered by native oxide layers

A combined method of spectroscopic ellipsometry and spectroscopic reflectometry is employed for the optical characterization of randomly rough surfaces of silicon single crystal covered with very thin native oxide layers within spectral range 197–1240 nm (1.0–6.3 eV). It is shown that random roughness of the samples of this system exhibits a wide interval of spatial frequencies so that the Rayleigh–Rice theory, scalar diffraction theory for coherent light and scalar diffraction theory with including light scattering must be used to achieve good fits of experimental data. The ellipsometric data are compatible with the Rayleigh–Rice theory within the entire spectral range while the reflectometric data must be processed separately within two sub-ranges using two different theoretical approaches. In one of the sub-ranges the combination of Rayleigh–Rice theory and scalar diffraction theory for coherent light is utilized for obtaining good fits of the corresponding experimental data of reflectance. Within the latter sub-range the scalar diffraction theory including the influence of light scattering on reflectance enables us to achieve good fits. Further, it is shown that using this combined method the values of basic roughness parameters, i.e. the rms values of heights, autocorrelation lengths and rms values of slopes of roughness irregularities, can be determined together with thickness values of the native oxide layers. A comparison of the results achieved optically with those determined by atomic force microscopy is performed. The assessment of efficiency of spectroscopic ellipsometry and spectroscopic reflectometry in characterizing random roughness with the wide interval of spatial frequencies is also presented.

Selective carbon dioxide electroreduction to ethylene and ethanol by core-shell copper/cuprous oxide

Developing an effective catalyst to reduce carbon dioxide (CO2) to value-added C2+ products is a key challenge for CO2 utilization. Here we report a core-shell copper with native oxide (Cu@Cu2O) catalyst featuring a thin native oxide layer on the surface grown under ambient conditions. The Cu(I) oxide species on Cu@Cu2O surface remained relatively stable against reduction under CO2 electroreduction condition, and the synergism between surface Cu+ and Cu0 in Cu@Cu2O contributed to boosting its efficiency and selectivity toward C2 products. This Cu@Cu2O catalyst achieved an excellent selectivity (∼50% at −1.0 V vs reversible hydrogen electrode) for C2 (ethylene and ethanol) production.

Visualization of three different phases in a multiphase steel by scanning electron microscopy at 1 eV landing energy

In this study, we investigated an observation technique by super low energy scanning electron microscopy (SLESEM) at below 5 eV and its contrast mechanism for analyzing complex microstructures of a multiphase steel consisting of ferrite, martensite and austenite. With SLESEM at 1 eV, the three phases were observed as different brightness levels, ferrite as the darkest contrast, martensite as the second brightest and austenite as the brightest. These contrasts disappeared at 2 eV or higher. Similar contrasts and phenomena were also observed in the results of low energy electron microscopy (LEEM). According to the energy dependences of the LEEM intensities of the three phases, the threshold energies of the transition from electron reflection to surface impact were determined to be 0.00 eV, 0.15 eV and 0.39 eV for ferrite, martensite and austenite, respectively. These differences in thresholds indicate that the potentials on the surfaces of each phase are different, which is considered to result in the different brightness of each phase. This potential differences are probably due to the contact potentials generated when phases with different work functions contact each other. Although the sample is covered by a thin native oxide film (several nm thickness), the potentials can affect the incident electrons through the oxide film.

Investigating the mechanical properties of GeSn nanowires.

Germanium tin (GeSn) has been proposed as a promising material for electronic and optical applications due to the formation of a direct band-gap at a Sn content >7 at%. Furthermore, the ability to manipulate the properties of GeSn at the nanoscale will further permit the realisation of advanced mechanical devices. Here we report for the first time the mechanical properties of GeSn nanowires (7.1-9.7 at% Sn) and assess their suitability as nanoelectromechanical (NEM) switches. Electron microscopy analysis showed the nanowires to be single crystalline, with surfaces covered by a thin native amorphous oxide layer. Mechanical resonance and bending tests at different boundary conditions were used to obtain size-dependent Young's moduli and to relate the mechanical characteristics of the alloy nanowires to geometry and Sn incorporation. The mechanical properties of the GeSn nanowires make them highly promising for applications in next generation NEM devices.

Oxidation limited thermal boundary conductance at metal-graphene interface

Thermal management is a substantial challenge in high-power-density micro- and nanoelectronic devices, and the thermal resistance at the interfaces in these devices is a major bottleneck to heat removal. Graphene has emerged as a potential candidate for next generation nanoelectronic devices because of its exceptional transport properties; however, the thermal interaction between graphene and other materials such as metals is not completely understood. Here we report thermal boundary conductance (TBC) measurements at metal-graphene-metal (M-G-M) interfaces at room temperature using time-domain thermoreflectance. The metals used in this study represent two classes based on the type of bonding formed with graphene. Ti and Ni form chemisorbed interfaces (strong bonding) with graphene and high TBC is expected while Au forms physisorbed interfaces (weak bonding). The measured TBC at M-G-M interfaces showed little variation (∼30 MW/m2-K) and was similar to metal-graphene-SiO2 interfaces, contrary to high TBC predicted by previous simulation studies. X-ray photoelectron spectroscopy was used to estimate thickness of the native oxide layer of bottom Ti (2.8 nm) and Ni (2.5 nm) layers. The conductance of these thin native oxide layer was much greater than the overall TBC but prevented formation of chemisorbed interfaces between graphene and metal for Ti and Ni cases leading to significantly lower TBC and highlighting an important consideration for practical applications.

Influence of Surface Structures on Quality of CdTe(100) Thin Films Grown on GaAs(100) Substrates**Supported by the National Key Research and Development Program of China under Grant Nos 2017YFB0405704 and 2017YFA0305400, the 1000-Young Talent Program of China, and the Shanghai Sailing Program under Grant No 17YF1429200.

We report the epitaxial growth of single-crystalline CdTe(100) thin films on GaAs(100) substrates using molecular beam epitaxy. By controlling the substrate pre-heated temperature with adjustable Te flux, three different reconstructed surfaces are realized, and their influence on the subsequent CdTe growth is investigated. More importantly, we find that both the presence of a thin native oxide layer and the formation of Ga-As-Te bonds at the interface enable the growth along the (100) orientation and help to reduce the threading dislocations and other defects. Our results provide new opportunities for compound semiconductor heterogeneous growth via interfacial engineering.

High Responsivity and Detectivity Graphene-Silicon Majority Carrier Tunneling Photodiodes with a Thin Native Oxide Layer

A photocurrent amplifier operable at low bias voltages with high responsivity and detectivity is highly demanding for various optoelectronic applications. This study shows majority carrier graphene-native oxide-silicon (GOS) photocurrent amplifiers complying with the demands. The photocurrent amplification is primarily attributed to the photoinduced Schottky barrier height (SBH) lowering for majority carriers. The unavoidably formed thin native oxide layer between graphene and silicon during the wet graphene transfer process plays significant roles in lowering of the dark leakage current as well as photoinduced SBH lowering. As a result, the photocurrent to dark current ratio is as high as ∼12.7 at the optical power density of 1.45 mW cm–2. These GOS devices show a high responsivity of 5.5 AW–1 at an optical power (458 nm in wavelength) of 15 μWcm–2, which corresponds to ∼1400% quantum efficiency. Further the response speed is as fast as a few ten-microseconds. Thus, these GOS majority carrier photodiodes...

Elucidating Electro-Hydrodynamics at a Liquid Metal Oxide-Electrolyte Interface Via Electrochemical Impedance Spectroscopy

This work characterizes the behavior of a eutectic alloy of gallium and indium (75% Ga, 25% In, by weight, ‘EGaIn’) in response to electric fields. The metal is a liquid at room temperature (M.P., 15.5 °C) and exhibits low toxicity. These fluidic metals may be injected into microfluidic systems, fibers, and capillary networks to form soft electronic devices that are soft and compliant. Once injected, the metal remains in its place because of the adhesive nature of its thin native oxide. Preventing the oxide adhesion within microchannels enables reversible actuation of EGaIn. Actuating liquid metals may be useful for soft actuators, reconfigurable optical displays, frequency tunable antennas, and other opto-fluidic technologies. In this work, we utilize low voltages (<2 V) to reversibly move droplets of EGaIn through microchannels. Pre-wetting the channels with an aqueous solution prior to injecting the metal prevents oxide adhesion; the water forms an interfacial ‘slip-layer’ the metal and channel wall. Thereafter, an applied electric field (~10-20 V/m) actuates the liquid metal by establishing a gradient of surface tension; this effect is known as continuous electrowetting (CEW). Although CEW has been utilized before with mercury, which is toxic, the adhesive nature of the Ga oxide complicates CEW behavior. This work utilizes optical microscopy, cyclic voltammetry, and electrochemical impedance spectroscopy (EIS) to characterize electro-hydrodynamics of CEW with EGaIn under a variety of conditions. Specifically, we compare electro-hydrodynamic behavior of EGaIn with and without the presence of an oxide ‘skin’. In addition, we elucidate the influence of material properties of the electrolyte on the metal-electrolyte interface. An equivalent circuit model from EIS experiments is proposed to understand the metal-electrolyte interface and to control the movement of the liquid metal within microfluidic systems.

Electroless Deposition and Ignition Properties of Si/Fe2O3 Core/Shell Nanothermites.

Thermite, a composite of metal and metal oxide, finds wide applications in power and thermal generation systems that require high-energy density. Most of the researches on thermites have focused on using aluminum (Al) particles as the fuel. However, Al particles are sensitive to electrostatic discharge, friction, and mechanical impact, imposing a challenge for the safe handling and storage of Al-based thermites. Silicon (Si) is another attractive fuel for thermites because of its high-energy content, thin native oxide layer, and facile surface functionality. Several studies showed that the combustion properties of Si-based thermites are comparable to those of Al-based thermites. However, little is known about the ignition properties of Si-based thermites. In this work, we determined the reaction onset temperatures of mechanically mixed (MM) Si/Fe2O3 nanothermites and Si/Fe2O3 core/shell (CS) nanothermites using differential scanning calorimetry. The Si/Fe2O3 CS nanothermites were prepared by an electroless deposition method. We found that the Si/Fe2O3 CS nanoparticles (NPs) had a lower reaction onset temperature (∼550 °C) than the MM Si/Fe2O3 nanothermites (>650 °C). The onset temperature of the Si/Fe2O3 CS nanothermites is also insensitive to the size of the Si core NP. These results indicate that the interfacial contact quality between Si and Fe2O3 is the dominant factor for determining the ignition properties of thermites. Finally, the reaction onset temperature of the Si/Fe2O3 CS NPs is comparable to that of the commonly used Al-based nanothermites, suggesting that Si is an attractive fuel for thermites.

Plasmon-induced nanoscale quantised conductance filaments

Plasmon-induced phenomena have recently attracted considerable attention. At the same time, relatively little research has been conducted on electrochemistry mediated by plasmon excitations. Here we report plasmon-induced formation of nanoscale quantized conductance filaments within metal-insulator-metal heterostructures. Plasmon-enhanced electromagnetic fields in an array of gold nanodots provide a straightforward means of forming conductive CrOx bridges across a thin native chromium oxide barrier between the nanodots and an underlying metallic Cr layer. The existence of these nanoscale conducting filaments is verified by transmission electron microscopy and contact resistance measurements. Their conductance was interrogated optically, revealing quantised relative transmission of light through the heterostructures across a wavelength range of 1–12 μm. Such plasmon-induced electrochemical processes open up new possibilities for the development of scalable devices governed by light.

Size Dependence of Nanoscale Wear of Silicon Carbide.

Nanoscale, single-asperity wear of single-crystal silicon carbide (sc-SiC) and nanocrystalline silicon carbide (nc-SiC) is investigated using single-crystal diamond nanoindenter tips and nanocrystalline diamond atomic force microscopy (AFM) tips under dry conditions, and the wear behavior is compared to that of single-crystal silicon with both thin and thick native oxide layers. We discovered a transition in the relative wear resistance of the SiC samples compared to that of Si as a function of contact size. With larger nanoindenter tips (tip radius ≈ 370 nm), the wear resistances of both sc-SiC and nc-SiC are higher than that of Si. This result is expected from the Archard's equation because SiC is harder than Si. However, with the smaller AFM tips (tip radius ≈ 20 nm), the wear resistances of sc-SiC and nc-SiC are lower than that of Si, despite the fact that the contact pressures are comparable to those applied with the nanoindenter tips, and the plastic zones are well-developed in both sets of wear experiments. We attribute the decrease in the relative wear resistance of SiC compared to that of Si to a transition from a wear regime dominated by the materials' resistance to plastic deformation (i.e., hardness) to a regime dominated by the materials' resistance to interfacial shear. This conclusion is supported by our AFM studies of wearless friction, which reveal that the interfacial shear strength of SiC is higher than that of Si. The contributions of surface roughness and surface chemistry to differences in interfacial shear strength are also discussed.

Aluminum bond pad corrosion of wirebond packages

Abstract Corrosion of aluminum bond pads in wirebonded packages can lead to device reliability issues under the harsh conditions in automotive applications. Corroded Al (aluminum) surface forms a mud-crack type appearance with a relatively very thick aluminum oxide/hydroxide (&amp;gt; 0.1μm) compared to thin protective native aluminum oxide (&amp;lt; 7nm). Various factors contributing to the formation of mud-crack type corrosion are identified. In all cases of mud-crack corrosion observed for packaged devices, Cl (chlorine) was detected on the corroded Al pad. The corrosive impact of high levels of Cl on the Al pad resulting in thick aluminum oxide growth will be discussed. The concentration levels of Cl to form mud-crack corrosion is far beyond the level of ionizable Cl typically present in packaging materials (such as mold compound or organic die attach) so the source of high levels of Cl is due to foreign contamination. The effect of Cl on mud-crack corrosion depends on other factors including pH and applied bias voltage. Corrosion susceptibility is found to be significantly reduced, even in the presence of high levels of Cl, when the pH &amp;gt; 6 and close to the neutral value of a pH of 7. It was also found that Cl is not essential to form mud-crack corrosion. Conditions leading to the corrosion in the absence of Cl are presented. F (fluorine) is a known contaminant in semiconductor processing and its influence on Al pad corrosion is examined. The differences on the impact of F vs. Cl on mud-crack corrosion will be discussed. Corrosion was observed irrespective of the wirebonds in the package (gold (Au) or copper (Cu)). However, the type of bi-metallic contact influences the extent of corrosion. The impact of bi-metallic contact on the mud-crack corrosion will be presented.

Investigation on the non-ideal behaviour of Au/n-InP Schottky diodes by the simulation of I–V–T and C–V–T characteristics

We report the simulation of current–voltage (I–V) and capacitance–voltage (C–V) characteristics of Au/n-InP Schottky contact in the temperature range of 200–400 K by steps of 20 K with and without interface states, traps and tunnelling current. The simulation was carried out using Atlas-Silvaco-Tcad device simulator. The simulation is performed using the appropriate physical models to explain the behaviour of the physical phenomena of the Schottky diode. The simulation results of I–V–T and C–V–T without native oxide, tunnelling current and traps are far away from experimental data. Thus, native oxide, tunnelling current and traps should be considered. Our results strongly suggest that the spatial inhomogeneities at the MS interface did not affect the I–V and C–V characteristics in the simulated temperature range. In addition, the extracted parameters, zero-bias barrier height , capacitance barrier height and the ideality factor were found to be strongly temperature-dependent due to the deviation of the transport mechanism from thermionic emission current. Moreover, the obtained results lead to a coherent explanation of the discrepancy between and . Good agreement of simulated I–V–T and C–V–T results with existing experimental data are obtained by considering a thin native oxide layer at the interface, traps and tunnelling current.

Photosensitive MIS structures with ultrathin dielectric based on CdXHg1‐XTe (x ∼ 0.4)

AbstractThe work is aimed to studying a possibility of CdHgTe surface passivation and stabilization by tunnel‐thin (∼ 2 nm) native oxide formed by glow discharge plasma oxidation. Kinetics of MCT plasma treatment in two regimes has been obtained. Investigation of the stabilization effect during vacuum annealing and platinum deposition has been performed. It has been established that presence of a native oxide layer on MCT surface results in elimination of mercury out diffusion during the processes. MIS structures with platinum contacts and thin native oxide have been made. They have rectifying behaviour in contrast with structures without the native oxide layer. The barrier height and the detectivity have been determined from the capacitance‐voltage and current‐voltage dependences. The detectivity value of 4×1011cm.Hz1/2W‐1 (at 78 K) determined by dark current shot noise has been reached. (© 2016 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Magnetization reversal of an individual exchange-biased permalloy nanotube

We investigate the magnetization reversal mechanism in an individual permalloy (Py) nanotube (NT) using a hybrid magnetometer consisting of a nanometer-scale SQUID (nanoSQUID) and a cantilever torque sensor. The Py NT is affixed to the tip of a Si cantilever and positioned in order to optimally couple its stray flux into a Nb nanoSQUID. We are thus able to measure both the NT's volume magnetization by dynamic cantilever magnetometry and its stray flux using the nanoSQUID. We observe a training effect and temperature dependence in the magnetic hysteresis, suggesting an exchange bias. We find a low blocking temperature $T_B = 18 \pm 2$ K, indicating the presence of a thin antiferromagnetic native oxide, as confirmed by X-ray absorption spectroscopy on similar samples. Furthermore, we measure changes in the shape of the magnetic hysteresis as a function of temperature and increased training. These observations show that the presence of a thin exchange-coupled native oxide modifies the magnetization reversal process at low temperatures. Complementary information obtained via cantilever and nanoSQUID magnetometry allows us to conclude that, in the absence of exchange coupling, this reversal process is nucleated at the NT's ends and propagates along its length as predicted by theory.

Comparative study of the native oxide on 316L stainless steel by XPS and ToF-SIMS

The very thin native oxide film on stainless steel, of the order of 2 nm, is known to be readily modified by immersion in aqueous media. In this paper, X-ray photoelectron spectroscopy (XPS) and time of flight secondary ions mass spectrometry are employed to investigate the nature of the air-formed film and modification after water emersion. The film is described in terms of oxide, hydroxide, and water content. The preferential dissolution of iron is shown to occur on immersion. It is shown that a water absorbed layer and a hydroxide layer are present above the oxide-like passive film. The concentrations of water and hydroxide appear to be higher in the case of exposure to water. A secure method for the peak fitting of Fe2p and Cr2p XPS spectra of such films on their metallic substrates is described. The importance of XPS survey spectra is underlined and the feasibility of C60+ SIMS depth profiling of a thin oxide layer is shown.

Al2O3 Coatings Deposited By ALD on Al Alloys

The key question that was addressed in this work is the following: would a uniform surface layer of aluminium oxide on aluminium alloys be protective against corrosion despite the presence of intermetallic Al-Cu particles? To answer this question, well defined aluminium oxide coatings were deposited by atomic layer deposition (ALD) on the surface of a model binary aluminium-copper alloy (Al-Cu5%wt.) in which copper-rich particles were produced by ageing. The corrosion resistance of uncoated, mechanically polished aluminium alloy samples covered by a native oxide layer was compared with the one of aluminium alloy samples protected by a 20 nm thick Al2O3 layer deposited by Atomic Layer Deposition (ALD). Detailed surface analysis was performed by X-ray Photoelectron Spectroscopy (XPS), Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and Scanning Electron Microscopy (SEM) on the samples before and after immersion in 0.01M NaCl + 0.3 % vol. H2O2 (pH=6.2). The polished Al-Cu alloy with a thin native oxide layer (around 5 nm, as measured by XPS), exhibits a copper enrichment at the oxide/Al alloy interface, clearly evidenced by ToF-SIMS depth profiling. The ToF-SIMS chemical images and EDX analysis confirmed the presence of copper-rich intermetallics (Al2Cu) in the interfacial oxide/substrate region. A short immersion (~7 min) of this alloy in a near neutral NaCl solution lead to severe surface modifications: formation of mixed aluminium-copper oxide layer rich in CuO and Cu(OH)2 (as shown by XPS), and formation of an inhomogeneous, thick aluminium oxide deposit (as shown by SEM and ToF-SIMS imaging). Localized corrosion in the vicinity of the intermetallic particles (well known as trenching around particles) is clearlyobserved by SEM and ToF-SIMS imaging. A significant improvement on corrosion resistance was observed after deposition of the 20 nm-thick Al2O3 ALD coating. Despite the presence of intermetallics at the oxide/substrate interface (observed in the SEM) no significant corrosion occurred in the chloride solution, the Al2Cu particles remained intact, and no trenching around them was observed. In conclusion, the formation of a good quality alumina layer on the Al-5%Cu alloy is beneficial for its corrosion resistance. The coating can be deposited by ALD without prior removal of the intermetallic particles.

Titanium Oxide: A Bioactive Factor in Osteoblast Differentiation.

Titanium and titanium alloys are currently accepted as the gold standard in dental applications. Their excellent biocompatibility has been attributed to the inert titanium surface through the formation of a thin native oxide which has been correlated to the excellent corrosion resistance of this material in body fluids. Whether this titanium oxide layer is essential to the outstanding biocompatibility of titanium surfaces in orthopedic biomaterial applications is still a moot point. To study this critical aspect further, human fetal osteoblasts were cultured on thermally oxidized and microarc oxidized (MAO) surfaces and cell differentiation, a key indicator in bone tissue growth, was quantified by measuring the expression of alkaline phosphatase (ALP) using a commercial assay kit. Cell attachment was similar on all the oxidized surfaces although ALP expression was highest on the oxidized titanium alloy surfaces. Untreated titanium alloy surfaces showed a distinctly lower degree of ALP activity. This indicates that titanium oxide clearly upregulates ALP expression in human fetal osteoblasts and may be a key bioactive factor that causes the excellent biocompatibility of titanium alloys. This result may make it imperative to incorporate titanium oxide in all hard tissue applications involving titanium and other alloys.