Arsenic Passivation Research Articles

Time scale effect of magnetic ferric oxide modified biochar in-situ remediation of arsenic-contaminated paddy soil

The ferric oxide (Fe3O4)-impregnated magnetic mulberry branch biochar (MBC) was prepared, and a long-term incubation trial was conducted to assess its efficiency in the remediation of As-contaminated soil. The changes in soil physicochemical properties [pH, dissolved organic carbon (DOC), soil organic matter (SOM), and electrical conductivity (EC)], soil nutrient elements, soil enzymes, soil bacterial community structure, and biodiversity at four different doses of MBC (1 %, 3 %, 5 %, and 7 %, wt%) across three aging periods (30, 90, and 180 d) were revealed. The results showed that the residual state of As increased by 161.4 % after 180 day of incubation with 5 % MBC. Spearman correlation and random forest model analyses revealed that available nitrogen had the highest contribution to the arsenic passivation process in soil. The addition of 5 % MBC significantly altered the bacterial abundance and composition in the soil. Actinobacteria, Proteobacteria, Chloroflexi, Bacteroidetes, and Acidobacteriota were identified as the predominant bacteria at the phylum level in all treated groups. The visualizations of soil bacterial co-occurrence networks over different passivate periods indicated more cooperative mutualism and complex interactions from day 30–180. After MBC was added to the soil, the arsenic toxicity was reduced with increasing aging time through bacterial community succession and co-occurrence networks. Overall, MBC facilitated the conversion of available arsenic by influencing the physicochemical factors and microbial community structure of the soil, thus reducing arsenic toxicity. MBC is a promising material for remediating arsenic-contaminated paddy soils.

Predicting the efficiency of arsenic immobilization in soils by biochar using machine learning

Arsenic (As) pollution in soils is a pervasive environmental issue. Biochar immobilization offers a promising solution for addressing soil As contamination. The efficiency of biochar in immobilizing As in soils primarily hinges on the characteristics of both the soil and the biochar. However, the influence of a specific property on As immobilization varies among different studies, and the development and application of arsenic passivation materials based on biochar often rely on empirical knowledge. To enhance immobilization efficiency and reduce labor and time costs, a machine learning (ML) model was employed to predict As immobilization efficiency before biochar application. In this study, we collected a dataset comprising 182 data points on As immobilization efficiency from 17 publications to construct three ML models. The results demonstrated that the random forest (RF) model outperformed gradient boost regression tree and support vector regression models in predictive performance. Relative importance analysis and partial dependence plots based on the RF model were conducted to identify the most crucial factors influencing As immobilization. These findings highlighted the significant roles of biochar application time and biochar pH in As immobilization efficiency in soils. Furthermore, the study revealed that Fe-modified biochar exhibited a substantial improvement in As immobilization. These insights can facilitate targeted biochar property design and optimization of biochar application conditions to enhance As immobilization efficiency.

Goethite-enriched biochar mitigates soil emissions of CO2 during arsenic passivation: Effect and mechanisms

Iron-enriched biochar has excellent adsorption capacity for contaminants, which is a sustainable multifunctional potential material to mitigate environmental pollution. Both biochar and Fe (hydr)oxides are important materials for accumulating and preserving soil organic carbon. However, it is unclear that the potential of iron-enriched biochar for mitigating CO2 emission from contaminated soil. In this study, soil incubation and extraction experiments were conducted to determine the effects and mechanisms of goethite-enriched wheat straw biochars (GWBs) prepared at two pyrolysis temperatures (GWB450 and GWB600) on mitigation of CO2 emissions in arsenic (As) contaminated soil. The results showed that compared to the control, GWBs significantly reduced the soil available arsenic (As), decreased soil Eh, and promoted the water stability of soil aggregates (<1mm). GWBs also regulated bacterial community structures, and efficiently improved microbial carbon use efficiency (CUE) in contaminated soil, thereby significantly reducing CO2 emission by 37.2 %−40.9 % in 37 days. Spectroscopic and microscopic analysis indicated that microbial colonization widely occurred on the surfaces of the goethite-enriched biochar, and promoted the formation of organo-mineral layers, which resulted in the significant decrease in the CO2 emission. These findings enhance the understanding of the mechanisms of soil carbon sequestration induced by metal-enriched biochar and indicate a new strategy of biochar-based multifunctional soil amendment that can mitigate heavy metal pollution and reduce CO2 emission simultaneously in contaminated soils.

Effect of As Passivation on Vapor-Phase Epitaxial Growth of Ge on (211)Si as a Buffer Layer for CdTe Epitaxy

We report an investigation of epitaxial germanium grown by chemical vapor deposition (CVD) on arsenic-terminated (211)Si, which is the preferred substrate in the USA for fabrication of night-vision devices based on mercury cadmium telluride (MCT) grown by molecular-beam epitaxy (MBE). The films were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), cross-sectional transmission electron microscopy (XTEM), and x-ray diffraction (XRD). Arsenic passivation was found to be effective in preventing cross-contamination of unwanted residual species present inside the reactor chamber and also in prolonging the evolution of layer-by-layer growth of Ge for significantly more monolayers than on nonpassivated Si. The two-dimensional (2D) to three-dimensional (3D) transition resulted in Ge islands, the density and morphology of which showed a clear distinction between passivated and nonpassivated (211)Si. Finally, thick Ge layers (∼250 nm) were grown at 525°C and 675°C with and without As passivation, where the layers grown with As passivation resulted in higher crystal quality and smooth surface morphology.

Selective area metalorganic vapor-phase epitaxy of gallium arsenide on silicon

The selective area, metalorganic vapor-phase epitaxy of gallium arsenide on silicon substrates was investigated. Low-temperature arsenic passivation of the silicon surface was realized at 650 °C. A two-step growth method was used to deposit the GaAs films with an optimum nucleation temperature of 400 °C. Layers nucleated at 350 °C or below were found to be polycrystalline whereas those nucleated at 400 °C and above were single crystal. The X-ray diffraction full-width-at-half-maximum decreased from 890 to 775 arcsec for a 1.0-μm-thick GaAs layer grown on unpatterned versus patterned Si (1 0 0) wafers. The 77 K photoluminescence peak position was 1.489 and 1.498 eV for a 1.0-μm-thick GaAs layer on unpatterned and patterned substrates, respectively. Based on the photoluminescence peak shift, we find that selective area growth of GaAs on Si improves film crystallinity while reducing the in plane strain in the film by 35%.

CdZnTe heteroepitaxy on 3″ (112) Si: Interface, surface, and layer characteristics

Tellurium adsorption studies were made on clean and arsenic passivated (112) silicon surfaces. Quantitative surface coverage values for tellurium were determined by Auger electron spectroscopy. Saturation coverage of up to 1.2 monolayers of tellurium could be obtained on a clean (112) silicon surface. On an arsenic passivated (112) Si surface however, the tellurium saturation coverage was limited to only ∼0.3 monolayer. Analysis of the adsorption behavior suggested that tellurium and arsenic chemisorption occurs preferentially at step edges and on terraces, respectively. The study revealed that arsenic passivation led to a significant decrease in the sticking coefficient of tellurium and an increase in it’s surface mobility. A model describing zinc telluride nucleation on a (112) Si surface is proposed. Thin templates of ZnTe followed by Cd1−xZnxTe layers were deposited on (112) Si by molecular beam epitaxy (MBE). The characteristics of the MBE Cd1−xZnxTe layers were found to be sensitive to the initial ZnTe nucleation and Si surface preparation.

Selective area regrowth of n-GaAs with reduced interface carrier depletion using arsenic passivation

Selective area molecular beam epitaxy (MBE) of n-GaAs through a Si 3N 4 mask has been explored. An arsenic capping layer was deposited in the MBE growth chamber prior to mask definition, in order to protect the epilayer surface during ex situ processing. After in situ thermal decapping and high-temperature (680°C) regrowth at a low growth rate (0.5 μm/h), the samples were examined with scanning electron and atomic force microscopy. The data unveil selective regrowth with lateral definition on a micrometer length scale and GaAs surfaces with a smooth mirror-like finish. Electrolytic carrier profiling and secondary ion mass spectrometry measurements of the homoepitaxial regrowth interface show a significant reduction of the interface potential barrier, from V bi=0.45 V to V bi=0.07 V, and reduced carbon impurity concentrations, compared to regrowth on non-passivated GaAs epilayers.

Step structure of arsenic-terminated vicinal Ge (100)

Germanium (100) crystals, 9° off-axis towards the [011] were exposed to 2.0 Torr of tertiarybutylarsine and 99.0 Torr of hydrogen at 650 °C, then heated to between 450 and 600 °C in vacuum or H2. The resulting surfaces consist of narrow dimer-terminated terraces, with (1×2) and (2×1) domains, that are separated by steps between one and eight atomic layers in height. The distribution of (1×2) and (2×1) domains changes with temperature, exhibiting a pronounced maximum in the (1×2) fraction at 510 °C. These results suggest that the arsenic passivation of germanium is a critical step in gallium arsenide heteroepitaxy.

Patterned regrowth of n-GaAs by molecular beam epitaxy using arsenic passivation

Patterned regrowth of n-GaAs through a Si3N4 mask prepared on molecular beam epitaxy (MBE) grown n-GaAs layers has been explored. An arsenic capping layer was deposited in the MBE growth chamber prior to mask definition, in order to protect the GaAs surface during ex situ processing. Removal of this cap from the lithographically defined mask area was accomplished by hydrogen plasma etching prior to Si3N4 deposition. After in situ thermal desorption of the As cap and phase selective regrowth, the samples were inspected with scanning electron and atomic force microscopy to establish the growth selectivity and examine the surface morphology. Electrolytic carrier profiling and secondary ion mass spectrometry measurements were carried out to characterize the growth-interrupted GaAs–GaAs interface. We find that the surface morphology and defect density of regrown GaAs epilayers are comparable to those of continuously grown epilayers of the same overall thickness. Structuring of the arsenic cap with hydrogen pla...

Epitaxial growth of ZnS on bare and arsenic-passivated vicinal Si(100) surfaces

We report a detailed study of molecular beam epitaxial growth of ZnS films on bare and arsenic-passivated vicinal Si(100) surfaces. This study elucidates the initiation of microtwinning and stacking-fault defects on double-stepped substrate surfaces. The study also sheds light on the function of arsenic passivation in reducing crystal defects in ZnS epitaxial layers. Three substrate surfaces, Si(100) 2×1, Si(100):As 2×1, and Si(100):As 1×2, were used for the ZnS epitaxial growth studies. Adsorption experiments were performed to demonstrate the chemical passivation effect of an arsenic overlayer. Reflection high-energy electron diffraction was used to study growth modes and the epitaxial relationship of the ZnS layers to the substrates. Transmission electron microscopy was used to study the crystal-defect structures. Secondary ion mass spectroscopy was used to determine the chemical profiles of the heteroepitaxial interfaces of ZnS layers grown on arsenic-passivated surfaces. One of the main results demonstrated by this work is that thin ZnS films can be grown epitaxially with much better crystal quality on As-passivated Si surfaces than on bare Si surfaces.

Molecular Beam Epitaxy Of Si/ZnS/Si(100) Heterostructures For Silicon-Based Quantum Devices

ABSTRACTSpecial methods were developed to grow Si/ZnS/Si heterostructures for the purpose of developing silicon-based quantum devices. An arsenic passivation technique was used on silicon surfaces for the growth of ZnS. The effectiveness of this surface passivation was demonstrated by the improved crystal quality and decreased heterostructural interface-state density. A two-step growth method was developed for the epitaxial growth of silicon on ZnS surfaces. In this method the normal silicon epitaxial growth was preceded by a solid phase epitaxy process. Single crystal films of silicon were successfully grown on ZnS surfaces. Our discussion addresses two key issues: crystal defects and the density of interface states. The results of RHEED (reflection high energy electron diffraction), C-V (capacitance voltage), and SIMS (secondary ion mass spectroscopy) measurements are presented.

Study on the accumulated impurities at the epilayer/substrate interface and their influence on the leakage current of metal-semiconductor-field effect transistors

Carrier impurities accumulated at the interface of a molecular beam epitaxial (MBE) grown GaAs layer-substrate have been studied in connection with substrate surface preparation just prior to MBE growth. Although the accumulated impurity density of silicon (higher than 7 × 10 17 cm -3) at the interface is the same for both H 2SO 4- and HF-treated samples, the oxygen level is much higher for the H 2SO 4-treated (higher than 1 × 10 19 cm -3) than for the HF-treated sample (less than 2 × 10 18 cm -1). High density of carriers (higher than 7 × 10 17 cm -3 is found to be accumulated at the interface only for the HF-treated sample. Further, absorbances at 1080 and 1260 cm -1 in FTIR spectra, which correspond to Si-O bonds, are observed only for the H 2SO 4-treated sample. The accumulated carrier concentration obtained in the HF-treated sample is thought to originate from the accumulated silicon which is not inactivated by making Si-O bonds. The accumulated impurities depend upon the initial surface condition of GaAs substrate before growth. The arsenic passivation layer has a good effect on reducing the accumulated impurities, which causes the carrier accumulation.

Arsenic passivation of MBE grown GaAs(100): structural and electronic properties of the decapped surfaces

The passivation properties and thermal stability of As on GaAs(100) were studied under ultra-high vacuum (UHV) conditions for As caps of different thicknesses and storage times in air. The As desorption was monitored by mass spectrometry (QMS) and the substrate surface quality by means of Auger electron spectroscopy (AES), electron diffraction (LEED) and Raman spectroscopy (RS). Additional desorption experiments were performed in an oven in N2 atmosphere. These samples were then investigated by spatially resolved Raman spectroscopy under atmospheric conditions.The Raman spectra of the air exposed samples show that the cap consists of an amorphous As layer with small As2O3 crystallites on top. AES investigations reveal in addition strong carbon contaminations. Arsenic desorption in UHV takes place in two steps, a first one at around 160°C which can be attributed to the desorption of the As2O3 crystallites and a second one at around 350°C due to the desorption of the amorphous As layer. From 360°C on with further increasing temperature (1 × 1) (2 × 4) and (4 × 1) reconstructions are observed by LEED. The band bendings at the surfaces as determined by Raman scattering were found to be not systematically related to the differently reconstructed surfaces. Residual carbon contaminations were found after Arsenic desorption in many cases, whereas the surfaces are always free of oxygen contaminations.

Arsenic passivation of silicon by photo-assisted metalorganic vapor-phase epitaxy

Earlier work by Bringans et al. [R. D. Bringans and M. A. Olmstead, J. Vac. Sci. Technol. B 7, 1232 (1989); R. I. C. Uhrberg, R. D. Bringans, M. A. Olmstead, R. Z. Bachrach, and J. E. Northrup, Phys. Rev. B 35, 3945 (1987)] and by Woolf [D. A. Woolf, D. I. Westwood, and R. H. Williams, Semicond. Sci. Technol. 4, 1127 (1989)] has shown that molecular‐beam epitaxy deposition of arsenic may be used to provide a capping and passivation layer on clean silicon surfaces both to protect the clean surface and to allow the transport of samples between laboratories for characterization purposes. In the present work we have shown that it is also possible to carry out such capping by means of photoassisted metalorganic vapor phase epitaxy using excimer laser radiation to decompose arsine. Suitable control of the laser beam intensity allows the deposition of films varying in thickness from one monolayer to several hundreds of angstroms.

Arsenic passivation of Si and Ge surfaces

Abstract This article reviews the passivating effect of arsenic monolayers on various silicon and germanium surfaces. It has been shown that such monolayers dramatically reduce the reactivity of the initial surfaces. The atomic structure and the nature of the bonding between the As layer and the various Si and Ge surfaces are described and the reasons for the passivation effects are discussed. Results of experimental and theoretical investigations of As-terminated Si and Ge surfaces are reviewed and compared with the same surfaces terminated with other atomic layers. Finally, the effect of As-termination on interface formation and heteroepitaxy is considered. In particular, the initial stage of GaAs growth on Si, which is dominated by the As-passivation of the Si substrate, is discussed.

Arsenic passivation of MOMBE grown GaAs surfaces

A process has been developed by which after metal organic molecular beam epitaxy (MOMBE) a freshly grown GaAs(100) surface is passivated by an arsenic overlayer. This As capping allows the transfer of a MOMBE sample through atmosphere into another UHV system where after desorption of the passivating layer a clean GaAs surface is prepared for surface studies. Detailed investigations show that the passivating film is homogeneous and consists of amorphous arsenic. After desorption of the As film at 610 K. the MOMBE grown GaAs surface exhibits a (1 × 1) LEED patterns, which changes into a c(6 × 4) pattern after annealing to 770 K, thus indicating an As deficient GaAs(100) surface.

Arsenic passivation of InP surface for metal-insulator-semiconductor devices based on both ultra-high vacuum technique and chemical procedure

Two passivation treatments of InP surface are presented: (i) thermal treatment under an arsenic partial pressure by molecular-beam epitaxy technique, and (ii) a new As-based aqueous chemical treatment. Both result in similarly improved electronic and physicochemical properties of the InP surface. The fast interface state density is reduced, the drift phenomena are practically suppressed and the thermal stability is greatly enhanced.

Arsenic passivation of GaAs and pyrolytic decomposition of organotin compounds for i n s i t u selected area doping of GaAs molecular beam epitaxy

Development of in situ selected area doping techniques would add a powerful new dimension to GaAs MBE technology. We report here a study of an in situ selected area doping procedure based on the following steps: (i) MBE growth of GaAs on a clean GaAs (001) substrate, (ii) adsorption of an As passivation cap which would be patterned for selected area doping, (iii) adsorption of an organotin compound, (iv) thermal desorption of the organotin compound along with the As cap in the passivated areas and with pyrolytic decomposition of the organotin compound in the unpassivated regions, and (v) regrowth of GaAs with selected area Sn incorporation into the regrowth layer. In this study we evaluated steps (iii)–(v). Steps (iii) and (iv) were investigated by means of in situ Auger electron spectroscopy, and step (v) by electrolytic C–V profiling of the carrier concentration. The organotin compounds in this study were tetrabutyltin and dibutyltin dibromide.

Molecular-beam-epitaxy GaAs regrowth with clean interfaces by arsenic passivation

Clean interfaces between GaAs layers grown by molecular-beam epitaxy (MBE) have been achieved by deposition of arsenic on the surface of the first layer grown, before removing it from the vacuum. Secondary ion mass spectroscopy, deep level transient spectroscopy, and capacitance-voltage carrier profiling show that this arsenic passivation procedure results in no measurable impurity incorporation, no deep levels, and no perturbation of carrier profiles in either n-type or p-type GaAs growth/regrowth interfaces after exposure to air. This is in contrast to growth over nonpassivated etched surfaces which showed large perturbations in free carrier profiles, deep level densities, and impurity incorporation at the interface. This demonstrates that nearly ideal growth/regrowth interfaces are achievable in MBE if impurity incorporation and nonstoichiometry are controlled.

Summary Abstract: MBE GaAs regrowth with clean interfaces by arsenic passivation

Summary Abstract: MBE GaAs regrowth with clean interfaces by arsenic passivation