Metallic Arsenic Research Articles

Characteristics and risk assessment of heavy metals in groundwater at a typical smelter-contaminated site in Southwest China

To explore the characteristics and evaluate the risk of heavy metals in groundwater at a typical smelter-contaminated site, this study focuses on a representative a historical arsenic smelting plant in Southwest China, where the primary historical products were metallic arsenic (∼1000 tons/year) and arsenic trioxide (∼2000 ton/year). The results demonstrated As and Pb as the main pollutants in soil, and As and Cd as main pollutants in groundwater through soil profiling and quarterly groundwater analysis. The maximum As and Pb in the surface soil were 76800 and 2290 mg/kg, respectively, with As vertically infiltrating the deep gravel–sand layer (18–20 m). The groundwater pollution distribution progressively increased along flow direction, influenced by seasonal surface runoff and infiltration fluctuations. The groundwater pollutant concentrations during the dry season notably surpassed those during the wet season, with maximum As and Cd concentrations of 111.64 mg/L and 19.85 μg/L during the dry season, respectively. Furthermore, the analytic hierarchy process (AHP) was applied to evaluate the comprehensive risk of contaminated-site across pollution source load, regional groundwater intrinsic vulnerability, and evaluation of nearby sensitive receptors. The results revealed that the carcinogenic risk of lead in surface soil was moderate to high, while arsenic posed a high carcinogenic risk, contributing to an overall carcinogenic risk proportion of 89.6% in surface soil. Exposure through groundwater intake was identified as the primary pathway, with carcinogenic and noncarcinogenic risks exceeding those through skin contact. The final weights result demonstrated that the principal risk factors are the intrinsic arsenic load and protective target characteristics of regional groundwater at this site. This study provides a reference for comprehensive assessments of similarly contaminated industrial and smelting sites.

Sodium citrate enhancing electrodeposition of metallic arsenic from toxic trivalent arsenic and the mechanism understanding

Electrochemical conversion of hypertoxic trivalent arsenic to value-added metallic arsenic can not only contribute to pollution abatement, but also resources reutilization, therefore being widely explored. Electrochemical reduction of trivalent arsenic as a promising way is widely explored. However, the high efficiency conversion is retarded by the sluggish reduction kinetics of AsO33− and fierce evolution of side products of both H2 and toxic AsH3. Herein, by using the sodium citrate as the additive, the current efficiency for metal arsenic production is increased greatly from 60% to 91%, with the accompanied evolution of hypertoxic AsH3 being restrained from 0.15 Nm3/tAs to 0.022 Nm3/tAs, promising a high-efficiency and green process. The electrochemical tests and electrode surface characterizations as well as DFT calculations indicate that the added sodium citrate promotes both the diffusion of reactive AsO33− towards the cathode and its subsequent adsorption on the Ti cathode, contributing to smoother reduction for generating metal arsenic, with the evolution of toxic AsH3 being hindered at the same time. The results can provide new insights for the high-efficiency and greener conversion of hypertoxic trivalent arsenic to value-added metallic arsenic.

Thermal Conductivity of Buckled and Puckered Arsenene: An Equilibrium Molecular Dynamics (EMD) Simulation

This study aims to calculate the thermal conductivity of buckled and puckered arsene using Molecular Dynamics simulation through Large-scale Atomic/Molecular Massively Parallel Simulation (LAMMPS). Arsenene is the monolayer of arsenic, that belongs to one of its allotropes, the gray metallic arsenic. The temperature varied from 100K, 300K, 500K, and 700K with a fixed dimension of 10nm x 50nm. Also, the length of the material varied from 10nm, 20nm, and 30nm. 40nm and 50nm at a fixed temperature of 300 K with constant width of 10nm. Results show that the main contributor to the increase of thermal conductivity is due to the phonon means free path. As the length increases, the thermal conductivity also increases because of its long mean free path. Moreover, the decreasing thermal conductivity as to the increase in temperature is due to phonon mean free path shrinking, resulting in a reduction in thermal conductivity. At room temperature, the thermal conductivity of buckled and puckered arsenene is found to be 1.8 W/mK and 1.3 W/mK, respectively. This study will provide fresh insight into the thermal properties of buckled and puckered arsenene, which will be useful for nanoelectronics applications.

Natural and Anthropic Sources of Arsenic in the Groundwater and Soils of the Mekong Delta

Human exposure to arsenic (As) is primarily through drinking water and food ingestion. Arsenic is naturally present in the environment and has been known as “the king of poisons” since the Middle Ages. It is mutagenic, teratogenic, and carcinogenic and approximately 70% comes from ingested food and 29% from water. Once ingested, arsenic can bio-accumulate in the human body or be excreted. Arsenic in groundwater is a main source of As in humans and the two arsenicals most abundant in water are arsenite (+3 oxidation state) and arsenate (+5 oxidation state). In order of toxicity from the most toxic to least toxic are arsines, arsenites, arsenoxides, arsenates, pentavalent arsenicals, Arsenic compounds, and metallic arsenic. Arsenic accumulates in the body when ingested in small doses. It often takes decades before physical symptoms of As poisoning show. While As is element normally found in the human body, it is highly toxic in excess amounts. The lethal dose for rates is 48 μg/L which translates to 125 mg for a middle-aged male. The maximum safe limit for As ingestion for an average Vietnamese middle-aged male is 220 μg per day. This lethal dosage puts As in a highly toxic category in food and toxicology. Most of the As in the Mekong Delta groundwater is from natural alluvial sediment sources. Other anthropic sources include the burial of millions of Vietnamese with elevated As levels since 1962, industrial sources, smelting by-products, water treatment plants, sewage and wastewater treatment discharges into waterways have added to the Mekong Delta As levels in the soil and groundwater. However, Agent Blue, the As-based herbicide, used during the Vietnam War, did contribute a significant amount (over 1,132,400 kg of manufactured (anthropic) As) to Southern Vietnam landscape. The As spikes and levels in the Mekong Delta soils and groundwater need restoration. The uptake of trace amounts of As in rice is indeed a critical food security and human health issue and requires mitigation.

Electrochemical codeposition of arsenic from acidic copper sulfate baths: The implications for sustainable copper electrometallurgy

• The main source of arsenic contamination is codeposition during the nucleation stage. • Convection and concentrated Cu(II) electrolytes mitigate arsenic contamination. • The reduction of As(III) to metallic As does not occur readily. • Fast removal of As(III) possible by copper-arsenic codeposition. Copper producers face increased demand associated with increasing complexity in feedstock composition, including high amounts of impurity metals. In this work, linear sweep voltammetry was used to study the electrodeposition behavior of copper and arsenic, define strategies for the production of grade A copper, and the removal of arsenic from complex electrolytes. Our results show that the copper concentration is a key parameter to control in the electrodeposition process. The continuous deposition of arsenic from the electrolyte requires copper in solution (≤10 g L −1 Cu(II) for 2 g L −1 As(III)) to form copper arsenides . The deposition of metallic arsenic does not occur readily. Conversely, the use of a concentrated Cu(II) solution (e.g. 40 g L −1 ) resulted in grade A copper from an electrolyte with a maximum of 2 g L −1 As(III) under galvanostatic control at a current density of – 42 mA cm −2 . Time-of-Flight Secondary Ion Mass Spectrometry depth profile measurements on copper deposits revealed that arsenic contamination was entirely concentrated near the substrate side of the deposit and progressively decreased further into the deposit. The codeposition of arsenic occurred along with the initial copper nucleation, when the electrochemical potential for electrodepostion under galvanostatic control is temporarily lower. These findings provide important insights for future sustainable copper electrodeposition technologies from complex feedstocks.

Selective Separation of Arsenic from Lead Smelter Flue Dust by Alkaline Pressure Oxidative Leaching

This study investigated the feasibility of using an alkaline pressure oxidative leaching process to treat lead smelter flue dust containing extremely high levels of arsenic with the aim of achieving the selective separation of arsenic. The effects of different parameters including NaOH concentration, oxygen partial pressure, liquid-to-solid ratio, temperature, and time for the extraction of arsenic were investigated based on thermodynamic calculation. The results indicated that the leaching efficiency of arsenic reached 95.6% under the optimized leaching conditions: 80 g/L of NaOH concentration, 1.0 MPa of oxygen partial pressure, 8 mL/g of liquid-to-solid ratio, 120 °C of temperature, 2.0 h of time. Meanwhile, the leaching efficiencies of antimony, cadmium, indium and lead were less than 4.0%, basically achieving the selective separation of arsenic from lead smelter flue dust. More than 99.0% of arsenic was converted into calcium arsenate product and thus separated from the leach solution by a causticization process with CaO after other metal impurities were removed from the solution with the addition of Na2S. The optimized causticization conditions were established as: 4.0 of the mole ratio of calcium to arsenic, temperature of 80 °C, reaction time of 2.0 h. The resulting product of calcium arsenate may be used for producing metallic arsenic.

Removal of aqueous-phase Pb(II), Cd(II), As(III), and As(V) by nanoscale zero-valent iron supported on exhausted coffee grounds

Nanoscale zero-valent iron (NZVI) is recognized as an excellent adsorbent for metallic contaminants. Nevertheless, NZVI itself tends to agglomerate, so that its performance deterioriates without supporting materials. The use of exhausted coffee grounds as a supporting material for NZVI is expected to resolve this problem and provide the social benefits of waste minimization and resource recycling. In this study, NZVI was supported on exhausted coffee grounds (NZVI-Coffee ground) to enhance its dispersion. The aims of this study were to characterize NZVI-Coffee ground with a focus on atomic dispersion, evaluate NZVI-Coffee ground as an adsorbent for typical metallic contaminants and arsenic, and assess the effects of solution chemistry on the adsorption process. In order to achieve these goals, characterization, adsorption kinetics, adsorption equilibrium, and the effects of pH and temperature on adsorption were studied. Pb(II), Cd(II), As(III), and As(V) were selected as target contaminants. The characterization study showed that atomic dispersion was enhanced four-fold by supporting NZVI on coffee grounds. The enhanced dispersion resulted in rapid kinetic characteristics and large adsorption capacity. The optimum pH for adsorption of Pb(II) and Cd(II) was 4–6, and that for As(III) and As(V) was 2–4. The pH effect can be explained by surface protonation/deprotonation and adsorbate speciation. Only the adsorption of Pb(II) was an exothermic process; those of other species were endothermic. In every tested case, the adsorption process was spontaneous. According to the results, NZVI-Coffee ground is an effective adsorbent for the removal of aqueous phase Pb(II), Cd(II), As(III), and As(V).

The Salave Mine, a Variscan intrusion-related gold deposit (IRGD) in the NW of Spain: Geological context, hydrothermal alterations and ore features

The Salave Gold deposit is located at the northern end of the Oscos Gold Belt, which forms part of the Asturian-Leonese Zone of the Variscan Massif. The Gold Belt and the Salave deposit are genetically related to the emplacement of post-kinematic granitoids, which were largely controlled by Variscan fracture systems. In the Salave area, three main igneous complexes, Porcia, Salave and Represas, outcrop in an approximately E-W direction. Gold deposits are only found in the intense hydrothermal alterations affecting the western part of the Salave outcrop, just where the Mondoñedo thrust reaches the Cantabrian Sea. The geometry of the mineralized body is approximately tabular and is roughly parallel to the Mondoñedo thrust. A network of fractures controlled the gold ore. It consisted of sub-horizontal and sub-vertical fractures, the latter acting as feeders. Along the fracture network an intense albitization replaced the granodiorite and partially filled the open fractures. Farther from the fracture system the granodiorite rock is affected by sericitic-propylitic alteration halo. The most external alteration halo is the chloritic alteration. The transition from one alteration to another is gradual and irregular. The albitized areas and some areas slightly beyond them are then intensely sericitized (advanced sericitic alteration), sulfurized and carbonatized along the network of fractures, and becomes the richest areas in gold. The sulfurized areas are rich in arsenopyrite, pyrite and stibnite, always over 10% and reaching as much as 40% in volume. All the altered rocks mentioned have associated late carbonates as veins or micro-veins, disseminated or filling open spaces.The isocon diagrams (using Zr and Nb as immobile elements) and representative analytical data show that in chloritic alteration there is a small gain of mass, in propylitic alteration the gain is negligible, in albitic there is no gain or loss, in less intense advanced sericitic there is a small loss of mass and in the intense advanced sericitic the loss of mass is significant. As far as the metallic elements, sulfur, arsenic and antimony, are concerned, significant amounts of S, Au, As, Sb and Bi were consistently added to the original rock, mainly during advanced sericitic alterations. Ag is added during albitization to advanced sericitic alteration. Pb, Cu, W, Mo and U are more erratic but appear to have been added from the beginning, together with some Bi, S and As, and to have remained quite constant during all the alterations as if they had been introduced previously to gold mineralization.Four mineral associations have been described at Salave, Cu-Fe-Zn-S, Mo-Bi-S-(W), S-As-Fe-(Au) and S-Sb-Au-(Pb-Bi-Ag-Cu-Zn). The two gold related ones, are post granodiorite consolidation. Visible gold (or electrum) is scarce. Invisible gold at Salave was detected using microprobe analyses. It accounts for most of the Salave gold. The invisible gold in arsenopyrite is in solid solution, while in pyrite a small amount appears in solid solution, but most is probably in the form of microinclusions. Invisible gold is higher in arsenopyrite than in pyrite. The Salave gold deposit is a fracture-controlled deposit whose characteristics, such as stable isotope data, fluid inclusions and geochemistry, indicate a strong igneous influence, albeit partially influenced by meteoric waters and host-rocks. Thus, the deposit should be considered as a reduced intrusion-related gold deposit.

Diffusion of In and Ga in TiN/HfO2/InGaAs nanofilms

The structure of TiN/HfO2 nanofilms grown on InxGa1−xAs substrates was assessed through angular-resolved x-ray photoelectron spectroscopy. The quantitative analysis made possible determining the thickness and composition of the various layers constituting the nanofilms treated at different temperatures (un-annealed, 500 °C/120 s and 700 °C/10 s). The TiN layer is crucial to prevent oxygen desorption from the dielectric during annealing. Small amounts of oxidized gallium and metallic arsenic are located at the HfO2/InGaAs interface. The thickness of the HfO2 layer remains stable under the thermal treatments. Annealing affects the In 3d5/2 and Ga 3p signals. From the angular dependence of the peak intensities and the detailed knowledge of the structure of the films, it was possible to determine that annealing causes In and Ga out-diffusion into the metallic layer, and also to quantify the amount of transported matter. This, along with density functional theory calculations, allowed for an estimation of the activation energy of the diffusion of indium through HfO2.

Sodium arsenite induced reactive oxygen species generation, nuclear factor (erythroid‐2 related) factor 2 activation, heme oxygenase‐1 expression, and glutathione elevation in Chang human hepatocytes

Liver is one of the major target organs of arsenic toxicity and carcinogenesis. Nuclear factor (erythroid-2 related) factor 2 (Nrf2) is a redox-sensitive transcription factor, regulating critically cellular defense responses against the toxic metallic arsenic in many cell types and tissues. This study was conducted to evaluate the hepato-cellular Nrf2 and Nrf2-regulated antioxidant reactions of sodium arsenite exposure in Chang human hepatocytes. Nrf2 and heme oxygenase-1 (HO-1) protein levels were detected by Western blot, and Nrf2-regulated HO-1 mRNA expressions were determined using semiquantitative RT-PCR by 0∼50 μmol/L of sodium arsenite exposure for 2, 6, 12, and 24 h. We also observed the changes of intracellular reactive oxygen species (ROS) and total cellular glutathione (GSH) by flow cytometry and spectrophotometry, respectively. Our results showed that intracellular ROS were both dose- and time-dependent induced by inorganic arsenic; Cellular Nrf2 protein levels increased rapidly after 2 h of exposure, elevated significantly at 6 h, and reached the maximum at 12 h. The endogenous Nrf2-regulated downstream HO-1 mRNA and protein were also induced dramatically and lasted for as long as 24 h. In addition, intracellular GSH levels elevated in consistent with Nrf2 activation. Our findings here suggest that inorganic arsenic alters cellular redox balance in hepatocytes to trigger Nrf2-regulated antioxidant responses promptly, which may represent an adaptive cell defense mechanism against inorganic arsenic induced liver injuries and hepatoxicity.

Characterization of Al xGa 1− xAs/GaAs heterostructures for single quantum wells grown by a solid arsenic MOCVD system

This work presents the results of the growth and characterization of Al x Ga 1− x As/GaAs multilayer structures obtained in a metallic-arsenic-based-MOCVD system. The main goal is to explore the ability of the growth system to grow high quality multilayer structures like quantum wells. The use of metallic arsenic could introduce important differences in the growth process due to the absence of the hydride group V precursor (AsH 3), which manifests in the electrical and optical characteristics of both GaAs and Al x Ga 1− x As layers. The characterization of these epilayers and structures was performed using low-temperature photoluminescence, Hall effect measurements, X-ray diffraction, Raman spectroscopy, secondary ion mass spectroscopy (SIMS) and Atomic Force Microscopy (AFM).

Growth and Interface Evolution of HfO2 Films on GaAs(100) Surfaces

The initial film growth (2–100 cycles) and the interface evolution of thin films on GaAs surfaces were investigated for an atomic layer deposition chemistry that utilizes tetrakis(ethylmethyl) amino hafnium and at . Starting surfaces include native oxide and HF or -etched substrates. X-ray photoelectron spectroscopy shows that deposition on native oxide GaAs surfaces results in the gradual consumption of the arsenic and gallium oxides. Arsenic oxides are easier to remove, leaving some metallic arsenic–arsenic suboxide at the interface. The removal of the gallium oxides is slower, and some residual and are detected after 100 process cycles. High resolution transmission electron microscopy confirms the presence of an almost sharp interface for the 100 cycle (12 nm) film and indicates that the as-deposited film is polycrystalline. The depositions on either HF or -etched substrates result in a sharp interface with very little residual gallium oxide and arsenic suboxide present. Rutherford backscattering spectroscopy shows that steady-state growth comparable to that achieved on is reached after ALD cycles for all GaAs surfaces; however, high initial surface activity is detected for the etched surfaces.

Sn 4As 3 revisited: Solvothermal synthesis and crystal and electronic structure

A facile one pot method of synthesis of tin arsenide Sn 4As 3 starting from metallic tin and elemental arsenic under mild solvothermal conditions in ethylenediamine in the presence of ammonium chloride is offered. The dissolving of the tin metal in ethylenediamine and the role of NH 4Cl are discussed. The crystal structure of Sn 4As 3 has been re-determined. It is shown to crystallize in the trigonal non-centrosymmetric space group R3 m, ( a=4.089(1) Å, c=36.059(6) Å, Z=3), which differs from the previously reported centrosymmetric structure ( R 3 ¯ m ) . The crystal structure of Sn 4As 3 consists of alternating layers of arsenic and tin atoms that are combined into seven-layer blocks and build up along the c-axis. The major structural feature is the short tin–tin distances (3.24 Å) between the adjacent blocks. The analysis of the density of states and band structure reveals that Sn 4As 3 should have metallic properties, which is in line with the previously reported experimental observations. Analysis of chemical bonding employing the electron localization function shows that only for the shortest Sn–As contacts the bonding is pairwise, while four-center bonds are formed between arsenic and tin atoms at relatively long distances (>2.85 Å). Moreover, each tin atom holds an electron lone-pair.

Effects of interfacial sulfidization and thermal annealing on the electrical properties of an atomic-layer-deposited Al2O3 gate dielectric on GaAs substrate

In this study we investigated the interfacial chemistry occurring between an atomic-layer-deposited Al2O3 high-k film and a GaAs substrate and the impact of sulfidization and thermal annealing on the properties of the resultant capacitor. We observed that sulfide passivation of the Al2O3∕GaAs structure improved the effect of Fermi level pinning on the electrical characteristics, thereby providing a higher oxide capacitance, smaller frequency dispersion, and reduced surface states, as well as decreased interfacial charge trapping and gate leakage currents. Photoemission analysis indicated that the (NH4)2S-treated GaAs improved the quality of the as-deposited Al2O3 thin film and preserved the stoichiometry of the dielectric during subsequent high-temperature annealing. This behavior was closely correlated to the diminution of GaAs native oxides and elemental arsenic defects and their unwanted diffusion. In addition, thermal processing under an O2 atmosphere, relative to that under N2, decreased the thickness of the Al2O3 gate dielectric and relieved the gate leakage degradation induced by metallic arsenic; as a result, superior dielectric reliability was attained. We discuss the underlying thermochemical reactions that account for these experimental observations.

Raman Studies of Carbon-Doped GaAs Layers Grown by a Metallic-Arsenic-Based Metalorganic Chemical Vapor Deposition System

High-quality p type GaAs epilayers were grown by metalorganic chemical vapor deposition using trimethylgallium and metallic arsenic as gallium and arsenic sources, respectively. The range of hole concentration analyzed goes from 1017 to 1019 cm−3, as measured by the Hall−van der Pauw method. For controlling the hole concentration, a mixture of hydrogen and nitrogen was used as the carrier gas. Raman scattering spectra show transversal optical mode at 270 cm−1 for low-doped samples and a longitudinal optical (LO) mode at 292 cm−1 produced by phonon-hole-plasmon coupling for high-doped samples. The relative intensity of the LO mode for the doped samples correlates very well with the hole concentration. As the carrier concentration increases, the LO phonon-plasmon-coupled mode increases. The corresponding decrease of the intensity of the LO mode is interpreted as the decrease in the depletion layer as the carrier concentration increases.

The flotation of metallic arsenic as a function of pH and pulp potential — A single mineral study

Arsenic is a penalty element in base metal gravity and flotation concentrates and during beneficiation efforts are often made to reduce its level in concentrates destined for smelting. In some Australian tin and tantalum circuits this arsenic can occur as elemental arsenic, lollingite, or arsenopyrite. A great deal is known about the flotation of arsenopyrite, but little is known about the flotation of lollingite and arsenic. This paper is concerned with the flotation of metallic arsenic with ethylxanthate as a function of pulp pH and pulp potential (Eh). A synthetic mixture of arsenic metal and quartz was used in all tests. Arsenic was found to be strongly floating (up to 95% recovery in 8 min flotation with Aerofroth 65 frother and 40 g/t of KEX) over the pH range 5–10. At more alkaline conditions, the recovery dropped off slowly with increasing pH. At pH 6, arsenic was strongly floating over the pulp potential range + 125 mV to + 275 mV vs. SHE (Standard Hydrogen Electrode) but exhibited an upper limiting threshold value of about + 375 mV vs. SHE. The flotation response dropped off slowly with more reducing conditions below about + 125 mV vs. SHE. Here the flotation kinetics were slow. At pH 10, arsenic was found to be strongly floating in the potential range − 300 mV vs. SHE up to about + 225 mV vs. SHE. Interestingly, no lower limiting potential in the reducing potential range tested (down to − 300 mV vs. SHE) was identified. The rate data indicated fast flotation kinetics at pH 10 at potentials less than about + 225 mV vs. SHE. At pH 6, little genuine flotation of metallic arsenic in the absence of collector was observed and it appears that metallic arsenic does not exhibit any significant natural flotability as do some other metals. Importantly, the results of this study show that metallic arsenic could potentially be readily removed from base metal concentrates by controlled potential flotation over a wide range of pH values by a simple reagent combination.

Electrochemical and spectroscopic study of arsenate removal from water using zero-valent iron media.

This study investigated the mechanisms involved in removing arsenate from drinking water supplies using zero-valent iron media. Batch experiments utilizing iron wires suspended in anaerobic arsenate solutions were performed to determine arsenate removal rates as a function of the arsenate solution concentration. Corrosion rates of the iron wires were determined as a function of elapsed time using Tafel analysis. The removal kinetics in the batch reactors were best described by a dual-rate model in which arsenate removal was pseudo-first-order at low concentrations and approached zero-order in the limit of high arsenate concentrations. The presence of arsenate decreased iron corrosion rates as compared to those in blank 3 mM CaSO4 background electrolyte solutions. However, constant corrosion rates were attained after approximately 10 days elapsed, indicating that the passivation processes had reached steady state. The cathodic Tafel slopes were the same in the arsenate and the blank electrolyte solutions. This indicates that water was the primary oxidant for iron corrosion and that arsenate did not directly oxidize the iron wires. The anodic Tafel slopes were greater in the arsenate solutions, indicating that arsenate formed complexes with iron corrosion products released at anodic sites on the iron surfaces. Ion chromatography analyses indicated that there was no measurable reduction of As(V) to As(III). X-ray absorption spectroscopy analyses indicated that all arsenic associated with the zero-valent iron surfaces was in the oxidation state. Interatomic arsenic-iron distances determined from EXAFS analyses were consistent with bidentate corner-sharing among arsenate tetrahedra and iron octahedra. Results from this study show that under conditions applicable to drinking water treatment, arsenate removal by zero-valent iron media involves surface complexation only and does not involve reduction to metallic arsenic.

Chemically selective adsorption of molecular oxygen on GaAs(100)c(2×8)

The chemisorption sites of molecular oxygen on the technologically important As-rich GaAs(100)c(2×8) surface were imaged with scanning tunneling microscopy (STM). The oxygen atoms insert into the arsenic–gallium backbonds and, subsequently, replace the arsenic atoms in the dimer rows. The displaced arsenic atoms aggregate in clusters of increasing size forming metallic arsenic. The strongly electronegative oxygen atoms are initially attracted by the larger electron density at the arsenic atoms, but the reaction of the oxygen atoms with the gallium atoms is thermodynamically favored. This leads to a 100% chemical selectivity for oxygen insertion into the As–Ga backbonds and subsequent chemisorption of the oxygen atoms into the arsenic sites.

Thermal stability of MISFET with low-temp molecular-beam epitaxy-grown GaAs and Al/sub 0.3/Ga/sub 0.7/As gate ins

GaAs and Al/sub 0.3/Ga/sub 0.7/As epilayers grown at LT by MBE were used as insulators in the fabrication of MISFET devices. Parametric changes were used to evaluate the thermal stability of MISFET, to identify failure mechanisms and validate the reliability of these devices. The LT-Al/sub 0.3/Ga/sub 0.7/As MISFET showed superior thermal stability. The degradation in the performance of MISFET with 1000 /spl Aring/ thick LT-GaAs gate insulator was worse than those of the MESFET. On the other hand, MISFET with 250 /spl Aring/ thick LT-GaAs gate insulators exhibited stable characteristics with thermal stressing, LF (low frequency) noise studies on the TLM structures of MISFET layers exhibited 1/f noise in the LT-Al/sub 0.3/Ga/sub 0.7/As samples and 250 /spl Aring/ LT-GaAs samples; whereas the 1000 /spl Aring/ thick LT-GaAs samples exhibited 1/f/sup 3/2/ noise, which was attributed to: (i) the thermal noise generated at the interface of the insulator, and (ii) the active layer due to the outdiffused metallic arsenic. Reverse gate-drain current degradation experiments were carried out at 120/spl deg/C, 160/spl deg/C, 200/spl deg/C, and 240/spl deg/C. Transconductance frequency dispersion studies were carried out before and after thermal stress on these MISFET. The transconductance of MISFET with 1000 /spl Aring/ LT-GaAs gate insulators was degraded by 40% at 100 kHz after thermal stress. The rest of the samples exhibited stable characteristics. These results indicate that composition changes had occurred at the interface in thicker LT-GaAs MISFET structures. Thinner LT-layers are ideal for achieving higher transconductance and better thermal stability without sacrificing the power capability of MISFET.

Arsenic-75 NMR: Knight shifts in arsenic metal

A NMR study of rhombohedral metallic arsenic has yielded values for the isotropic and axial components of the Knight shift of and .