Atomic Depth Research Articles

Glow discharge-optical emission spectroscopy for in situ analysis of surfaces in plasmas

In situ, real-time characterization of surfaces exposed to plasmas is of great interest. Common chemical analysis methods such as x-ray photoelectron spectroscopy, Auger electron spectroscopy, and secondary ion mass spectroscopy cannot be used. Here, we discuss the use of glow discharge-optical emission spectroscopy for this purpose. A small coupon piece (aluminum coated with yttria in this study) was mounted on an rf-biased electrode and inserted into an opening in the reactor wall. Silicon or SiO2 substrates on a separately rf-biased electrode were etched in an inductively coupled plasma (ICP) of Cl2/Ar/O2 or C4F8/O2, respectively. Pulsed bias was applied to sputter the surface of the coupon piece in the wall at the edge of the ICP, either after etching in an Ar ICP or during etching in the Cl2/Ar/O2 ICP. Optical emission from the region above the coupon surface was collected and spectrally resolved. The difference in intensity between the coupon bias on and off conditions was used to determine what species were present on the surface. A quantification method for converting emission intensities into atomic composition depth profiles is presented.

Hot Exoplanet Atmospheres Resolved with Transit Spectroscopy (HEARTS)

Context.WASP-121 b is a hot Jupiter that was recently found to possess rich emission (day side) and transmission (limb) spectra, suggestive of the presence of a multitude of chemical species in the atmosphere.Aims.We survey the transmission spectrum of WASP-121 b for line-absorption by metals and molecules at high spectral resolution and elaborate on existing interpretations of the optical transmission spectrum observed with theHubbleSpace Telescope (HST).Methods.We applied the cross-correlation technique and direct differential spectroscopy to search for sodium and other neutral and ionised atoms, TiO, VO, and SH in high-resolution transit spectra obtained with the HARPS spectrograph. We injected models assuming chemical and hydrostatic equilibrium with a varying temperature and composition to enable model comparison, and employed two bootstrap methods to test the robustness of our detections.Results.We detect neutral Mg, Na, Ca, Cr, Fe, Ni, and V, which we predict exists in equilibrium with a significant quantity of VO, supporting earlier observations by HST/WFC3. Non-detections of Ti and TiO support the hypothesis that Ti is depleted via a cold-trap mechanism, as has been proposed in the literature. Atomic line depths are under-predicted by hydrostatic models by a factor of 1.5 to 8, confirming recent findings that the atmosphere is extended. We predict the existence of significant concentrations of gas-phase TiO2, VO2, and TiS, which could be important absorbers at optical and near-IR wavelengths in hot Jupiter atmospheres. However, accurate line-list data are not currently available for them. We find no evidence for absorption by SH and find that inflated atomic lines can plausibly explain the slope of the transmission spectrum observed in the near-ultraviolet with HST. The Na ID lines are significantly broadened (FWHM~50 to 70 km s−1) and show a difference in their respective depths of ~15 scale heights, which is not expected from isothermal hydrostatic theory. If this asymmetry is of astrophysical origin, it may indicate that Na Iforms an optically thin envelope, reminiscent of the Na Icloud surrounding Jupiter, or that it is hydrodynamically outflowing.

Complementary physicochemical analysis by ellipsometry and Auger spectroscopy of nano-sized protective coating layers

Protective SiC-rich nano layer was created by ion beam mixing of Si/C multilayers. The transformation of the Si and C layers into a homogeneous SiC layer was analyzed using complementary depth profiling by spectroscopic ellipsometry (SE) and Auger electron spectroscopy (AES). The distribution of elements and their chemical states was revealed by AES, whereas SE measured the accurate thicknesses, density, crystallinity, and the distribution of different phases. The optical properties of the created SiC layer determined by SE were almost identical to that of ion implantation-amorphized SiC. Amorphous Si and void formations were revealed by SE. The void profile determined by SE correlated well with the Xe distribution measured by AES. The complementary capabilities of SE and AES for a detailed chemical investigation were pointed out, such as the atomic (AES), structural (AES and SE) and non-destructive depth profiling (SE) features. It was shown that besides the synergism that the two methods reveal, SE is capable of a quick, sensitive and non-destructive testing of such layer structures and materials.

Architectured Cu–TNTZ Bilayered Coatings Showing Bacterial Inactivation under Indoor Light and Controllable Copper Release: Effect of the Microstructure on Copper Diffusion

A Ti–23Nb–0.7Ta–2Zr–1.2O alloy (at %), called “gum metal”, was deposited by direct-current magnetron sputtering (DCMS) on an under layer of copper. By varying the working pressure during the deposition, columnar TNTZ (Ti–Nb–Ta–Zr) nanoarchitectures were obtained. At low working pressures, the upper layer was dense with a coarse surface (Ra = 12 nm) with a maximum height of 163 nm; however, the other samples prepared at high working pressures showed columnar architectures with voids and an average roughness of 4 nm. The prepared coatings were characterized using atomic force microscopy (AFM) for surface topography, energy dispersive X-ray spectroscopy (EDX) for atomic mapping, scanning electron microscopy (SEM) for cross-section imaging, contact angle measurements for hydrophilic/hydrophobic balance of the prepared surfaces, and X-ray diffraction (XRD) for the crystallographic structures of the prepared coatings. The morphology and the density of the prepared coatings were seen to influence the hydrophilic properties of the surface. The antibacterial activity of the prepared coatings was tested in the dark and under low-intensity indoor light. Bacterial inactivation was seen to happen in the dark from samples presenting columnar nanoarchitectures. This was attributed to the diffusion of copper ions from the under layer. To verify the copper release from the prepared samples, an inductively coupled plasma mass spectrometer (ICP-MS) was used. Additionally, the atomic depth profiling of the elements was carried out by X-ray photoelectron spectroscopy (XPS) for the as-prepared samples and for the samples used for bacterial inactivation. The low amount of copper in the bulk of the TNTZ upper layer justifies its diffusion to the surface. Recycling of the antibacterial activity was also investigated and revealed a stable activity over cycles.

Aberration corrected spin polarized low energy electron microscope

Spin Polarized Low Energy Electron Microscopy (SPLEEM) is a powerful tool to reveal the magnetic structure of ferromagnetic surfaces on the atomic depth scale level[1-3]. With aberration corrected LEEM and a high brightness spin polarized electron gun, high spatial resolution will provide more details for ultra-thin ferromagnetic film studies. This study reports the first realization of aberration corrected SPLEEM (AC-SPLEEM). The performance of the setup was tested on ferromagnetic Fe nanoscale islands on a W(110) single crystal, with spatial resolution of 3.3 nm in spin asymmetry images.

The diffusion behaviors at the Cu-Al solid-liquid interface: A molecular dynamics study

The Cu-Al composite material possesses a large potential value in practical application due to its excellent properties. Whereas the Cu/Al interface is an inevitable issue to be solved both experimentally and theoretically. Thus, the effects of temperature, pressure on atomic interdiffusion along the direction perpendicular to the Cu(1 1 0)/Al solid-liquid interface have been investigated by using a molecular dynamics (MD) method with the embedded atomic method (EAM) potentials. The results indicate that the diffusion coefficients of both Cu and Al atoms layers, as well as the diffusion depth of Al atoms, satisfy Arrhenius formula with the system temperature. The diffusion depth of Cu atoms presents a linear relationship with system temperatures. While the thickness of diffusion layer presents a parabolic relation with diffusion time, which is also in good agreement with the experimental results. In addition, we show that the diffusion rate of Cu atoms diffusion in Al liquid is more than 5 times that of Al atoms diffusion in Cu layers, and sequentially the alloy layers are mainly formed by the diffusion of Cu atoms in the Al liquid. Finally, the intermetallic in alloy layers is mainly identified as θ-Al2Cu once solidification by annealing. These findings have useful in designing Cu/Al interface in practices.

X-ray scattering by the fused silica surface etched by low-energy Ar ions.

Anomalously high x-ray scattering at a wavelength of 0.154 nm by super-polished substrates of fused silica, which were etched by the argon ions with the energy of 300 eV, is detected. The scattering intensity increases monotonically with increasing of the etching depth. The effect is explained by the scattering on the volume inhomogeneities with the lateral size greater than 0.5 μm of the subsurface "damaged" layer. The concentration of volume inhomogeneities increases with the increase of the fluence of argon ions, but the concentration of implanted argon atoms in the layer quickly reaches the maximum value and then begins a trend of going down. The thickness of the "damaged" layer is approximately equal to the penetration depth of the Ar atoms and can be directly determined from the x-ray specular reflection. It is shown that the presence of volume inhomogeneities of the subsurface "damaged" layer does not affect the geometric roughness of the surface. The observed effect imposes limitations on the usage of grazing incidence x-ray optics without reflective coatings and of the diffuse x-ray scattering (DXRS) method for studying the substrate roughness. A new method that potentially enables to evaluate the applicability of the DXRS method in practice is proposed.

Study of thermal recrystallisation in Si implanted by 0.4‐MeV heavy ions

A Si crystal layer on SiO2/Si was implanted using 0.4‐MeV Kr+, Ag+, and Au+ at ion fluences of 0.5 × 1015 to 5.0 × 1015 cm−2. Subsequent annealing was performed at temperatures of 450° and 800° for 1 hour. The structural modification in a Si crystal influences ion beam channelling phenomena; therefore, implanted and annealed samples were investigated by Rutherford backscattering spectrometry under channelling (RBS‐C) conditions using an incident beam of 2‐MeV He+ from a 3‐MV Tandetron in random or in aligned directions. The depth profiles of the implanted atoms and the dislocated Si atom depth profiles in the Si layer were extracted directly from the RBS measurement. The damage accumulation and changes in the crystallographic structure before and after annealing were studied by X‐ray diffraction (XRD) analysis. Lattice parameters in modified silicon layers determined by XRD were discussed in connection to RBS‐C findings showing the crystalline structure modification depending on ion implantation and annealing parameters.

Application of the ion mixing method for doping near surface layers of the silicon single crystals

Penetration of alien atoms (Be, Al, Ni, Mo) into Si, diamond monocrystals substrates was investigated under Ar+ ion bombardment of samples having thermally evaporated films of 30–50 nm. Sputtering was carried out using a wide energy spectrum beam of Ar+ ions with mean energy 9.4 keV to dose D = 1×1016–1019 ion/cm2. Implanted atom distribution in the targets was measured by Rutherford backscattering spectrometry (RBS) of H+ and He+ ions with start energy of 1.6 MeV as well as secondary ion mass-spectrometry (SIMS).During the bombardment, the penetration depth of Ar atoms increases with dose linearly. This depth is more than 3–20 times deeper than the projected range of bombarding ions and recoil atoms. This is a “ion mixing” process.The analysis shows that the experimental data for foreign atoms penetration depth are similar to the data calculated for atom migration through the interstitial site in a field of internal (lateral) compressive stresses created in the near-surface layer of the substrate as a result of implantation. Under these experimental conditions atom ratio ri/rm (ri - radius of dopant atom, rm - radius of substrate atom) can play a principal determining role. Show that maximum penetration depth of the film atoms in the substrates may be determine by “isotropic model” under ion beam (with wide energy spectrum – polyenergy) irradiation of the “film–substrate” systems too.

Molecular Simulations of Sputtering Preparation and Transformation of Surface Properties of Au/Cu Alloy Coatings Under Different Incident Energies

The surface properties of coatings during deposition are strongly influenced by temperature, particle fluxes, and compositions. In addition, the precursor incident energy also affects the surface properties of coatings during sputtering. The atomistic processes associated with the microstructure of coatings and the surface morphological evolution during sputtering are difficult to observe. Thus, in the present study, molecular dynamics simulation was employed to investigate the surface properties of Au/Cu alloy coatings (Cu substrate sputtering by Au atoms) with different incident energies (0.15 eV, 0.3 eV, 0.6 eV). Subsequently, the sputtering depth of the Au atoms, the particle distribution of the Au/Cu alloy coating system, the radial distribution function of particles in the coatings, the mean square displacement of the Cu atoms in the substrate, and the roughness of the coatings were analyzed. Results showed that the crystal structure and the sputtering depth of Au atoms were hardly influenced by the incident energy, and the incident energy had little impact on the motion of deep-lying atoms in the substrate. However, higher incident energy resulted in higher surface temperature of coatings, and more Au atoms existed in the coherent interface. Moreover, it strengthened the motion of Cu atoms and reduced the surface roughness. Therefore, the crystal structure of coatings and the motions of deep-lying atoms in the substrate are not influenced by the incident energy. However, the increase in incident energy will enhance the combination of coatings and the base while optimizing the surface structure.

Secondary ion mass spectrometry investigation of carbon grain formation in boron nitride epitaxial layers with atomic depth resolution

A refined SIMS procedure allows reaching atomic resolution and characterization of each layer in van der Waals structures separately.

Depth sensitive imaging of graphene with an atomic resolution microscope

The three-dimensional information of atomic positions is required to determine the atomic structure of materials. However, images obtained using high-resolution transmission electron microscopes are generally two-dimensional projections of three-dimensional structures. The depth resolution of general microscopes is still on the order of nanometers, and a smaller depth of field is required to realize atomic depth-resolution imaging. Here, we propose highly depth-sensitive imaging using a low-voltage atomic-resolution transmission electron microscope equipped with a higher order geometrical aberration corrector and a monochromator. A long wavelength of low energy electrons and a large acceptance angle of the microscope allow for a shallow depth of field and atomic-level depth sensitivity. We demonstrate that the depth resolution and depth precision can allow for ångström and sub-ångström levels, respectively. Applying this highly depth-sensitive microscope, the depth deviation of monolayer graphene with dislocations is detected as the difference of defocus. The buckling structures of dislocation dipole and tripole are directly observed using a single image. Combining the proposed depth-sensitive microscope with through-focal imaging will allow for the analysis of various low-dimensional materials in three dimensions with atomic depth resolution.

Surface mechanical coating of Cu plate by Cu Al powders

ABSTRACTThrough Surface Mechanical Coating (SMC), a plate of pure Cu was successfully coated using of Al-50-wt% Cu during mechanical alloying. Effect of milling time and heat treatment on formation of coating at 1050 °C was investigated. Compositional and microstructural study of the coating and its properties were conducted by means of X-Ray Diffraction (XRD), Micro-focus XRD, Scanning Electron Microscopic (SEM), Electron Probe Micro-Analyzer (EPMA), Transition Electron Microscopic (TEM), and Vickers Microhardness test. Analyses results showed that Al4Cu9, Cu(Al), and AlCu3 phases were formed. The results also showed that heating causes the composite coating created by SMC to be replaced with moderate concentrations of Cu and Al. Through heat treatment, coating hardness will be reduced to even half, while thickness has a twofold increase. As milling time rises, atomic diffusion depth for heated samples will be raised also. TEM investigations approved the presence of amorphous phase and 10nm grain size.

Optimal photon generation from spontaneous Raman processes in cold atoms

Spontaneous Raman processes in cold atoms have been widely used in the past decade for generating single photons. Here, we present a method to optimise their efficiencies for given atomic coherences and optical depths. We give a simple and complete recipe that can be used in present-day experiments, attaining near-optimal single photon emission.

Proposal for a three-dimensional magnetic measurement method with nanometer-scale depth resolution

We propose a magnetic measurement method based on combining depth sectioning and electron magnetic circular dichroism in scanning transmission electron microscopy. Electron vortex beams with large convergence angles, as those achievable in current state-of-the-art aberration correctors, could produce atomic lateral resolution and depth resolution below 2~nm.

Evaluation of sputtering induced surface roughness development of Ni/Cu multilayers thin films by Time-of-Flight Secondary Ion Mass Spectrometry depth profiling with different energies O2+ ion bombardment

As a follow up on a previous paper on the quantitative evaluation of sputtering induced surface roughness and its influence on Auger electron spectroscopy depth profiling of polycrystalline Ni/Cu multilayers thin films, the ion sputtering induced surface roughness of these multilayered Ni/Cu polycrystalline thin films under low energy (0.5, 1.0 and 2.0 keV) O2+ ion bombardment and 1.0 keV Cs+ ion sputtering were investigated with Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and Atomic Force Microscopy (AFM). The lowest energy (0.5 keV) O2+ ion sputtering gave the best depth resolution for the different energy ion sputtering. Furthermore the 1.0 keV Cs+ ion sputtering led to a better resolution than the O2+ ion sputtering. The ToF-SIMS depth resolution was also better than the AES depth profiling with a 2.0 keV Ar+ ion sputtering. The Mixing-Roughness-Information depth (MRI) model used for fitting of the ToF-SIMS depth profiles accounts for the broadening upon experimental depth profiling owing to the effects of atomic mixing, surface roughness and information depth. The MRI fitted roughness parameters obtained from the ToF-SIMS depth profiling are in good agreement with the root-mean-square roughness obtained from the AFM topography scans of the crater bottom. Ion sputtering induced roughness and depth resolution during ToF-SIMS depth profiling of Ni/Cu multilayers were quantitatively evaluated according to the MRI fitting parameters.

X-ray photoelectron spectroscopy study of an exchange bias system on the basis of Co70Fe30/Mn83Ir17 interface

The consequences of 10 keV He+ ion bombardment on exchange biased Cu(50 nm)/Mn83Ir17(10 nm)/Co70Fe30(6.5 nm)/Ta(15 nm) thin film systems are studied by X-ray photoelectron spectroscopy. The modifications of the exchange bias field, the elemental depth concentration profiles, and the phase composition of the Mn83Ir17/Co70Fe30 interface zone have been investigated for selected doses. Measured atomic depth concentration profiles confirmed a broadening of the interface zone between ferromagnetic and antiferromagnetic layers upon bombardment; the width of the interface zone correlates with the relative change of the exchange bias field. The profiles of the Mn2p and Mn3s photoelectron peaks revealed the presence of a non-Mn83Ir17 phase containing quasiatomic manganese with mean local spin and magnetic moments greater than those in Mn within the Mn83Ir17 alloy in the interface zone. The magnitude of the exchange bias field correlates with the amount of manganese in this high-spin-Mn phase.

Atomic scale analysis of the corrosion characteristics of Cu-Li solid-liquid interfaces

The corrosion characters of Cu(001)-Li, Cu(110)-Li and Cu(111)-Li solid-liquid interfaces are investigated using molecular dynamics (MD) simulations. The interfacial structure and properties are studied through various quantitative characterizations. In the corrosion process, the interfacial alloying is observed via the analysis of fine-scale density profile. And the structure of the alloying layer still maintains the in-plane structure of corresponding Cu crystal plane. Because the interfacial alloying has changed the components of interfacial layers, the diffusion properties of atoms are different between the alloying layer and pure liquid Li layer. As a result, the alloying layers are more stable than pure liquid Li layers. In addition, the temperature effect on the interfacial structure and property suggests that the corrosion process is anisotropic, which is quantified by the penetration depth of Li atoms and the loss rate of Cu atoms. Specifically, the corrosion behavior is most easy to occur at the (110) interface, and most difficult at (111) interface. The anisotropy of the corrosion process is well explained in terms of the stability of Cu atoms on the substrate surface. As an important content of research, the concentration of dissolved Cu atoms in liquid Li is also measured by us.

Improved XPS analysis by visual inspection of the survey spectrum

In this paper, we have revisited a well‐known issue in quantitative X‐ray photoelectron spectroscopy (XPS) analysis: when quantitative XPS analysis is based solely on measured peak intensities, this can give rather confusing and misleading results. The problem is caused by peak intensity attenuation with photoelectron distance traveled in the sample and is an issue for samples where the atomic concentration varies substantially over the outermost ~5 nm. This is however the most significant depth range for many of the practical applications of nano‐structures which is today of increasing technological importance. We discuss 3 different methods to correct for the attenuation effect and their limitations. It is then pointed out how one can, by visually comparing observed peak shapes to a set of model spectra, often get a rough idea of the atom depth distribution in the surface region of the sample. This provides a practical tool to supplement the information obtained with XPS analysis based on peak intensities and chemical state information obtained from high‐resolution spectra.

Open circuit potential monitored digital photocorrosion of GaAs/AlGaAs quantum well microstructures

Nanostructuring of semiconductor wafers with an atomic level depth resolution is a challenging task, primarily due to the limited availability of instruments for in situ monitoring of such processes. Conventional digital etching relies on calibration procedures and cumbersome diagnostics applied between or at the end of etching cycles. We have developed a photoluminescence (PL) based process for monitoring in situ digital photocorrosion (DPC) of GaAs/AlGaAs microstructures at rates below 0.2 nm per cycle. In this communication, we demonstrate that DPC of GaAs/AlGaAs microstructures could be monitored with open circuit potential (OCP) measured between the photocorroding surface of a microstructure and an Ag/AgCl reference electrode installed in the sample chamber. The excellent correlation between the position of both PL and OCP maxima indicates that the DPC process could be monitored in situ for materials that do not necessarily exhibit measurable PL emission.