Oxygen Contamination Research Articles

Growth Rate and Temperature Dependence of Oxygen Incorporation in Si1-XGex Thin Films

A series of Si1-xGex heterostructures were grown at different substrate temperatures using molecular beam epitaxy (MBE) on Si (100) substrates, where the composition x was varied in a “staircase” pattern by stepping down the germanium flux between constant composition layers while maintaining a constant silicon flux, and vice versa. The silicon flux was supplied with an e-beam evaporator, while the germanium flux was provided with an effusion cell. The germanium composition and oxygen concentration of these staircase structures were then measured using secondary ion mass spectroscopy (SIMS). From the SIMS profiles, it is evident that the oxygen concentration is inversely proportional to and linear with growth rate, regardless of how that rate is changed (either fixing silicon and varying germanium or vice versa). This implies that the dominant source of oxygen contamination in a clean growth system does not originate from either the silicon or germanium, in which case the concentration would be exponential with individual source rate or source flux, but instead comes from the residual background in the MBE chamber. Further, the oxygen concentration retains its linearity in growth rate when changing substrate growth temperature, and shows an exponential dependence in substrate growth temperature for a given rate, where the oxygen concentration increases at lower growth temperature.

Oxygen depth profiling by resonant backscattering and glow discharge optical emission spectroscopy of Ti-6Al-4V alloy oxidized by ion implantation and plasma based treatment

Oxygen depth profiling by means of 16O(α,α)16O backscattering and glow discharge optical emission spectroscopy (GDOES) was investigated in two different sets of Ti-6Al-4V samples. The first set was made of Ti-6Al-4V samples implanted at room temperature and 550°C with 50 and 150keV O+ ions at fluences ranging from 1.5×1017 to 6.0×1017ions/cm2. The second set consisted of Ti-6Al-4V samples treated at 550°C for 7h and 24h under low pressure (8Pa) oxygen, eventually with RF plasma activation. These results are part of a wider investigation on Ti-6Al-4V motivated by recent publications which have shown that an oxide layer can enhance hydrogen absorption and can then promote Ti-6Al-4V alloys as efficient hydrogen storage materials.The results obtained by the two characterization techniques were compared and discussed, enabling to adjust the dependence to the oxygen concentration of the sputtering rates to be used in the time-to-depth transformation required in GDOES analysis. Considering the low thickness of oxidized alloy, usual procedures employed in GDOES depth calculation were indeed not adapted. Once calibrated thanks to the resonant RBS, GDOES can then be easily employed as fast characterization of oxidized and/or hydrogenated surface of Ti-6Al-4V.The obtained results show that the oxygen content into the surface oxidized layer slightly increases in samples implanted at higher fluence and higher temperature. However the overall oxidized layer thickness (<200nm) remains within the projected ion depth range and is not significantly increased by thermal diffusion at 550°C. Taken into account the initial oxide layer, the incorporated oxygen quantity mainly corresponds to the implanted fluence but it can be slightly higher with 550°C implantation, indicating a slight additional oxidation by residual oxygen or surface contamination. The oxygen penetrations and contents in samples oxidized by thermally activated diffusion treatments were more pronounced compared to implanted samples, especially with plasma assistance. A highly oxidized layer (>50 at.%) of about 100nm was obtained after 7h of treatment. X-ray diffraction revealed the presence of rutile TiO2 phase only after a long duration (24h) thermally assisted treatment whereas oxygen incorporation mainly results in a c-axis expanded hexagonal titanium phase in all the other samples.

Development of Low Cost PM Ti Alloys by Thermomechanical Processing of Powder Blends

This research focuses on the development of low cost powder metallurgy (PM) Ti alloys suitable for application in PM thermomechanical processing with mechanical properties comparable to those of wrought Ti6Al4V alloy. The alloy systems studied are Ti3Al2V, Ti5Fe and Ti3.2Fe1Cr0.6Ni0.1Mo (Ti5SS). The alloy mixtures were produced by blending Ti HDH powders with Al40V, 316SS master alloy powders or elemental Fe powder. The blended powders were further consolidated using various methods: high vacuum sintering (HVS), induction sintering (IS), powder compact forging (PCF) and powder compact extrusion (PCE). It is found that, PM Ti3Al2V and Ti5Fe alloy processed by PCE or PCF followed by recrystallization annealing (RA) achieved tensile properties comparable with wrought Ti6Al4V alloy. Tensile properties such as yield strength (YS) of 910MPa, UTS of 1010MPa and 15% elongation to fracture for Ti3Al2V alloy are reported. Ti5Fe alloy gives YS and UTS of 870MPa and 968MPa respectively, combined with 20.3% elongation to fracture. The tensile results are related to the microstructure developed during the consolidation processes. The oxygen contamination as a result of the high temperature processing is also reported.

Refined Synthesis and Crystal Growth of Pb2P2Se6 for Hard Radiation Detectors

The refined synthesis and optimized crystal growth of high quality Pb2P2Se6 single crystals are reported. Improved experimental procedures were implemented to reduce the oxygen contamination and improve the stoichiometry of the single crystal samples. The impact of oxygen contamination and the nature of the stoichiometry deviation in the Pb2P2Se6 system were studied by first-principles density functional theory (DFT) electronic structure calculations as well as experimental methods. The DFT calculations indicated that the presence of interstitial oxygen atoms (Oint) leads to the formation of a deep level located near the middle of the gap, as well as a shallow acceptor level near the valence band maximum. In addition, total energy calculations of the heat of formation of Pb2P2Se6 suggest that the region of thermodynamic stability is sufficiently wide. By refining the preparative procedures, high quality Pb2P2Se6 single crystal samples were reproducibly obtained. These Pb2P2Se6 single crystals exhibited ex...

Effects of oxygen contamination on monolayer GeSe: A computational study

Natural oxidation is a common degradation mechanism of both mechanical and electronic properties for most of the new two-dimensional materials. From another perspective, controlled oxidation is an option to tune material properties, expanding possibilities for real-world applications. Understanding the electronic structure modifications induced by oxidation is highly desirable for new materials like monolayer GeSe, which is a new candidate for near-infrared photodetectors. By means of first-principles calculations, we study the influence of oxygen defects on the structure and electronic properties of the single layer GeSe. Our calculations show that the oxidation is an exothermic process, and it is nucleated in the germanium sites. The oxidation can cause severe local deformations on the monolayer GeSe structure and introduces a deep state in the bandgap or a shallow state near the conduction band edge. Furthermore, the oxidation increases the bandgap by up to 23 %, and may induce direct to indirect bandgap transitions. These results suggest that the natural or intentionally induced monolayer GeSe oxidation can be a source of new optoelectronic properties, adding another important building block to the two-dimensional layered materials.

Electron Enhanced Growth of Crystalline Gallium Nitride Thin Films at Room Temperature and 100 °C Using Sequential Surface Reactions.

Low energy electrons may provide mechanisms to enhance thin film growth at low temperatures. As a proof of concept, this work demonstrated the deposition of gallium nitride (GaN) films over areas of ∼5 cm2 at room temperature and 100 °C using electrons with a low energy of 50 eV from an electron flood gun. The GaN films were deposited on Si(111) wafers using a cycle of reactions similar to the sequence employed for GaN atomic layer deposition (ALD). Trimethylgallium (Ga(CH3)3, TMG), hydrogen (H) radicals and ammonia (NH3) were employed as the reactants with electron exposures included in the reaction cycle after the TMG/H and NH3 exposures. A number of ex situ techniques were then employed to analyze the GaN films. Spectroscopic ellipsometry measurements revealed that the GaN films grew linearly with the number of reaction cycles. Linear growth rates of up to 1.3 Å/ cycle were obtained from the surface areas receiving the highest electron fluxes. Grazing incidence X-ray diffraction analysis revealed polycrystalline GaN films with the wurtzite crystal structure. Transmission electron microscopy (TEM) images showed crystalline grains with diameters between 2 and 10 nm depending on the growth temperature. X-ray photoelectron spectroscopy depth-profiling displayed no oxygen contamination when the GaN films were capped with Al prior to atmospheric exposure. However, the carbon concentrations in the GaN films were 10-35 at. %. The mechanism for the low temperature GaN growth is believed to result from the electron stimulated desorption (ESD) of hydrogen. Hydrogen ESD yields dangling bonds that facilitate Ga-N bond formation. Mass spectrometry measurements performed concurrently with the reaction cycles revealed increases in the pressure of H2 and various GaN etch products during the electron beam exposures. The amount of H2 and GaN etch products increased with electron beam energy from 25 to 200 eV. These results indicate that the GaN growth occurs with competing GaN etching during the reaction cycles.

Calorimetric study of peculiar hydrogenation behavior of nanocrystalline TiFe

Thermodynamic analysis of hydrogen interaction with TiFe nanocrystalline intermetallic compound produced by mechanical alloying has been carried out using microcalorimetric titration technique. The boundary of the phase regions and H2 absorption/desorption reaction enthalpies have been defined in comparison with conventional as-cast alloy. It has been concluded that specific microstructure and oxygen contamination determine the system features within the α solid solution region and play a key role in formation of β2 and γ, but does not affect the thermodynamic parameters of the main β → α transformation.

Effects of Laser Surface Melting on Ti–30Nb–2Sn Sintered Alloy

Laser surface melting (LSM) is used to simultaneously melt a thin layer and quench Ti–30Nb–2Sn substrate produced by powder metallurgy route. Results show LSM is useful to eliminate non‐desirable open porosity and to reduce even more the Young Modulus. The best microstructure is achieved below the melting pool where high peak temperature combined with high cooling rate during laser processing is present. LSM retains beta titanium without traces of martensite or grain boundary alpha due to oxygen contamination.

STM study of the preparation of clean Ta(110) and the subsequent growth of two-dimensional Fe islands

This report deals with the preparation of a clean Ta(110) surface, investigated by means of scanning tunneling microscopy/spectroscopy as well as by low-energy electron diffraction and Auger electron spectroscopy. The surface initially exhibits a surface reconstruction induced by oxygen contamination. This reconstruction can be removed by annealing at high temperatures under ultrahigh vacuum conditions. The reconstruction-free surface reveals a surface resonance at a bias voltage of about −500mV. The stages of the transformation are presented and discussed. In a next step, Fe islands were grown on top of Ta(110) and investigated subsequently. An intermixing regime was identified for annealing temperatures of (550–590)K.

Structural and chemical analysis of annealed plasma-enhanced atomic layer deposition aluminum nitride films

Plasma-enhanced atomic layer deposition was utilized to grow aluminum nitride (AlN) films on Si from trimethylaluminum and N2:H2 plasma at 200 °C. Thermal treatments were then applied on the films which caused changes in their chemical composition and nanostructure. These changes were observed to manifest in the refractive indices and densities of the films. The AlN films were identified to contain light element impurities, namely, H, C, and excess N due to nonideal precursor reactions. Oxygen contamination was also identified in the films. Many of the embedded impurities became volatile in the elevated annealing temperatures. Most notably, high amounts of H were observed to desorb from the AlN films. Furthermore, dinitrogen triple bonds were identified with infrared spectroscopy in the films. The triple bonds broke after annealing at 1000 °C for 1 h which likely caused enhanced hydrolysis of the films. The nanostructure of the films was identified to be amorphous in the as-deposited state and to become nanocrystalline after 1 h of annealing at 1000 °C.

Thermal etching of silver: Influence of rolling defects

Silver is well known to be thermally etched in an oxygen-rich atmosphere and has been extensively studied in the laboratory to understand thermal etching and to limit its effect when this material is used as a catalyst. Yet, in many industrial applications the surface of rolled silver sheets is used without particular surface preparation. Here, it is shown by combining FIB-tomography, FIB-SIMS and analytical SEM that the kinetics of thermal etch pitting are significantly faster on rolled Ag surfaces than on polished surfaces. This occurs due to range of interacting phenomena including (i) the reaction of subsurface carbon-contamination with dissolved oxygen to form pores that grow to intersect the surface, (ii) surface reconstruction around corrosion pits and surface scratches, and (iii) sublimation at low pressure and high temperature. A method to identify subsurface pores is developed to show that the pores have {111} and {100} internal facets and may be filled with a gas coming from the chemical reaction of oxygen and carbon contamination.

Surfaces and interfaces governing the OMVPE growth of APD-free GaP on AsH3-cleaned vicinal Si(100)

Direct growth of GaP on Si enables the integration of III–V and Si optoelectronic devices for a wide variety of applications, and has therefore been the subject of much research for many decades. Most of this effort has been directed toward overcoming the two main technical challenges: 1) removing atmospheric oxygen and carbon contamination and 2) establishing the conditions needed for “APD-free” III–V epitaxy (i.e., without antiphase domains). We have developed an OMVPE process for APD-free GaP growth on Si which overcomes both challenges by using AsH3 to clean and prepare the Si surface in situ at a relatively low temperature. This process is based upon a very brief “AsH3-cleaning” step which simultaneously removes atmospheric contamination (thereby eliminating the need for Si regrowth) and creates a single-domain As-terminated Si surface. Here we discuss the key process steps using results from a suite of analysis tools.

Self-organized homo-epitaxial growth of (001) vanadium assisted by oxygen surface reconstruction

In this paper the effect of oxygen on the vanadium homoepitaxial growth process is analyzed by using Auger spectroscopy, electron diffraction and scanning tunneling microscopy. As the oxygen induced 1×5 surface structure got a lattice spacing 6% different from the pure V lattice, relaxation is observed by electron diffraction during the growth. The average in-plane lattice spacing is thus shown to be proportional to the oxygen surface concentration. The surface lattice relaxation is observed to exponentially vary with the number of deposited atomic planes. A kinetic model is proposed and allows us to explain these observations. Furthermore, it helps us to distinguish two regimes depending on growth temperature. At high temperature, the oxygen surface concentration during growth is due to oxygen upward diffusion from the underneath V layer. For lower temperature however, this upward diffusion is not efficient and another source of oxygen contamination is evidenced. When the oxygen surface concentration is sufficient, a spectacular self-organization is observed at the surface by surface microscopy. Ribbons shape islands are observed and are tentatively explained as a consequence of oxygen surface concentration and stress induced by the surface reconstruction.

The influence of oxygen contamination on the thermal stability and hardness of nanocrystalline Ni–W alloys

Nanocrystalline Ni–W alloys are reported in the literature to be stabilized against high temperature grain growth by W-segregation at the grain boundaries. However, alternative thermal stability mechanisms have been insufficiently investigated, especially in the presence of impurities. This study explored the influence of oxygen impurities on the thermal stability and mechanical properties of electrodeposited Ni-23at%W with aberration-corrected scanning transmission electron microscopy (STEM) and nanoindentation hardness testing. The primary finding of this study was that nanoscale oxides were of sufficient size and volume fraction to inhibit grain growth. The oxide particles were predominantly located on grain boundaries and triple points, which strongly suggests that a particle drag mechanism was active during annealing. In addition, W-segregation was observed at the oxide/Ni(W) interfaces rather than the presumed Ni(W) grain boundaries, further supporting the argument that alternative mechanisms are responsible for thermal stability in these alloys. Lastly, alloys with nanoscale oxides exhibited a higher hardness compared to similar alloys without oxides, suggesting that the particles are widely advantageous. Overall, this work demonstrates that impurity oxide particles can limit grain growth, and alternative mechanisms may be responsible for Ni–W thermal stability.

Effects of silicon contents on the characteristics of Zr–Ti–Si–W thin film metallic glasses

In comparison with the conventional crystalline alloy films, the amorphous thin film metallic glasses (TFMGs) have been reported to exhibit unique properties, such as high strength, improved fatigue property and good corrosion resistance. Among Zr-based TFMGs, the Zr–Ti–Si–W TFMGs have not yet been explored extensively. In this work, quaternary Zr–Ti–Si–W TFMGs were fabricated on Si wafer and AISI 316L stainless steel substrates by a co-sputtering system. The target power of Si target was adjusted to achieve TFMGs with Si contents ranging from 14.3 to 19.7at.%. For the low oxygen contained Zr–Ti–Si–W TFMGs, the hardness and corrosion resistance increased with increasing Si content. The highest hardness of 8.1GPa was obtained for the amorphous coating containing the second highest Si content and low oxygen concentration. Highly-dosed oxygen contamination, up to around 9 to 10at.%, would make the amorphous coating exhibit larger granular surface and columnar cross-sectional microstructure. The high oxygen content also showed detrimental effect to the hardness and the corrosion resistance of coating, due to pronounced surface crack defects. However, similar hardness and corrosion resistance enhancement by the increasing Si concentration were also observed for the high oxygen contained Zr–Ti–Si–W TFMGs. Nevertheless, sufficient adhesion property, good corrosion resistance and acceptable biocompatibility were achieved for five amorphous Zr–Ti–Si–W coatings regardless their oxygen concentration.

Roles of fluorine and annealing on optical and structural properties of Nd:YF3 phosphor

The optical and structural properties of Nd:YF3 phosphors, synthesized by precipitation reaction using ethylene glycol solvent, were investigated. The Y:F molar ratio of precursors, where NH4F was employed as the fluorine source, was varied to prevent oxygen contamination during nucleation of the particles and to improve the photoluminescence efficiency. Structural investigations were carried out by means of X-ray diffraction, Fourier transform infrared spectroscopy and energy dispersive X-ray techniques. Samples prepared using precursors in stoichiometric proportion presented orthorhombic YF3 structure. The use of fluorine source in excess promoted formation of NH4Y2F7 crystalline phase, which was decomposed into YF3 after annealing at 400°C for 1h. Emission spectra of samples thermally treated enhanced by over 50 times compared with the as prepared. The results obtained show the role of fluorine excess associated with thermal annealing to obtain YF3 with improved features.

Saúde ambiental e condições de balneabilidade em coleção hídrica do médio rio doce (MG)

O objetivo deste trabalho foi avaliar a saude ambiental de cinco lagoas:da Prata, do Pau, Vermelha, Silvana e Nova, em uma regiao do Medio RioDoce no Estado de Minas Gerais, Brasil. Foram avaliadas a composicao e adistribuicao de macroinvertebrados bentonicos relacionados com a presencade parâmetros biologicos (grupo coliforme), fisicos (temperatura) e quimicos(oxigenio dissolvido ― OD, condutividade, pH, potencial de reducao) emcada uma das lagoas mencionadas. As amostragens de macroinvertebradosforam realizadas entre dezembro de 2007 e janeiro de 2009. Os taxonsidentificados foram analisados a partir de indices de diversidade Shannon-Wiener (H’), uniformidade Pielou (e), Biological Monitoring Working Party(BMWP), Average Score Per Taxon (ASPT), indice biotico de familia (IBF) eanalise de componentes principais (ACP). Fatores como a abundância dasfamilias Thiaridae e Physidae, OD, o potencial de reducao, a pluviosidade, apresenca de macrofitas aquaticas e a contaminacao da agua por coliformesforam os parâmetros que mais influenciaram na distribuicao e composicaoda assembleia de macroinvertebrados e nas condicoes de balneabilidade daslagoas estudadas.

Reduction of moisture and airborne molecular contamination on the purge system of 450 mm Front Opening Unified Pod

The Front Opening Unified Pod (FOUP) is used for wafer storage and transport to prevent contamination from internal and external exposure. The FOUP may be contaminated by gases from manufacturing process or by the chemical compounds emitted from the stored wafers in the FOUP. Moisture, oxygen, and Airborne Molecular Contaminations (AMCs) are common sources of many defects and failures of chips. Moisture and Hydrogen Fluoride (HF) are always the most common forms of AMC by virtue of their existence in chemicals. In the past, techniques for removal of these contaminants have used various methods such as purging by nitrogen or compressed dry air (CDA). As the volume of 450 mm FOUP is up to 0.7060 m3, about two and half folds of a 300 mm FOUP, and considering the purge efficiency of purge gas, optimization of FOUP purge technology is urgently needed. In this study, we proposed an alternative method, which combines vacuum technology and purge technology (thereafter referred as “vacuum purge” system). This study aims to evaluate our newly proposed vacuum purge system against the conventional purge system. The Cavity ring-down spectroscopy (CRDS) was used as monitoring system for HF concentration. Results show that with initial Relative Humidity (RH) value of 50% and HF concentration of 9 ppb in the FOUP, RH and HF concentration can be reduced to less than 5% of the initial RH or HF concentration by vacuum purge system in 3 min and 40 s, respectively. For conventional purge system, it took more than 30 min and 80 s to reach that level. For conventional purge system, RH and HF concentration were first-order linearly related to the inversed purge flow rates. Increasing inversed purge flow rate significantly increase purge time.

Fully CMOS-compatible titanium nitride nanoantennas

CMOS-compatible fabrication of plasmonic materials and devices will accelerate the development of integrated nanophotonics for information processing applications. Using low-temperature plasma-enhanced atomic layer deposition (PEALD), we develop a recipe for fully CMOS-compatible titanium nitride (TiN) that is plasmonic in the visible and near infrared. Films are grown on silicon, silicon dioxide, and epitaxially on magnesium oxide substrates. By optimizing the plasma exposure per growth cycle during PEALD, carbon and oxygen contamination are reduced, lowering undesirable loss. We use electron beam lithography to pattern TiN nanopillars with varying diameters on silicon in large-area arrays. In the first reported single-particle measurements on plasmonic TiN, we demonstrate size-tunable darkfield scattering spectroscopy in the visible and near infrared regimes. The optical properties of this CMOS-compatible material, combined with its high melting temperature and mechanical durability, comprise a step towards fully CMOS-integrated nanophotonic information processing.

Electrical, luminescent, and deep trap properties of Si doped n-GaN grown by pendeo epitaxy

Electrical and luminescent properties and deep trap spectra of Si doped GaN films grown by maskless epitaxial lateral overgrowth (MELO) are reported. The dislocation density in the wing region of the structure was 106 cm−2, while in the seed region it was 108 cm−2. The major electron traps present had activation energy of 0.56 eV and concentrations in the high 1015 cm−3 range. A comparison of diffusion length values and 0.56 eV trap concentration in MELO GaN and epitaxial lateral overgrowth (ELOG) GaN showed a good correlation, suggesting these traps could be effective in carrier recombination. The doped MELO films were more uniform in their electrical properties than either ELOG films or undoped MELO films. We also discuss the differences in deep trap spectra and luminescence spectra of low-dislocation-density MELO, ELOG, and bulk n-GaN samples grown by hydride vapor phase epitaxy. It is suggested that the observed differences could be caused by the differences in oxygen and carbon contamination levels.