Resulting Depth Profiles Research Articles

Laser doping mechanism of 4H-SiC by KrF excimer laser irradiation using SiNx thin films

In this study, nitrogen is doped into 4H-SiC by irradiating 4H-SiC with a SiNx thin film and a KrF excimer laser. The doping depth profile, crystal structure, electrical properties, and surface roughness results are analyzed to evaluate the excimer-laser doping mechanism. High-concentration doping is possible at a fluence of 2.5 J cm−2 and 10 shots, while maintaining the 4H-SiC crystal structure via solid-phase diffusion. However, changes in the 4H-SiC crystalline state are observed upon liquid-phase diffusion at a fluence of ≥2.8 J cm−2. At a fluence of 2.5 J cm−2 and 100 shots, nitrogen can be deeply diffused via solid-phase diffusion; however, an amorphous layer is formed on the surface and there is an increase in contact resistance.

Benchmark shielding calculations for fusion and accelerator-driven sub-critical systems

We conducted benchmark calculations of neutron-shielding experiments for fusion applications such as (1) Neutron spectra in iron shields with 14 MeV neutrons; and (2) Leakage neutron spectra from various sphere piles by 14 MeV neutrons, and for Accelerator-Driven Sub-Critical system (ADS) such as (3) Neutron energy spectra after transmission through iron and concrete shields using a quasi-monochromatic neutron source, (4) Thick target neutron yield produced by high-energy heavy ion incidence on a thick target, and (5) Induced radio activity in concrete exposed to secondary particles produced by heavy-ion incident reaction on an iron target by using the Particle and Heavy Ion Transport code System (PHITS). For case (1), the calculated neutron energy spectra using the nuclear data libraries, JENDL-4.0, agreed with the experimental data in iron shields. For the cases (2) of Al, Ti, Cr, and As piles, the neutron energy spectra calculated with JENDL-4.0 agreed with the experimental results well. For the case (3), the neutron spectra calculated with JENDL-4.0/HE reproduced the experimental data because the proton data library of7Li can produce source neutrons for proton incident reactions. For the case (4), calculated results for thick target neutron spectra produced by a 400 MeV per nucleon carbon incident reaction on lead reproduced the experimental data well. For the case (5) with a neutron source produced by the 200–400 MeV per nucleon heavy-ion incident reaction, the calculated results of the reaction rate depth profiles of197Au (n, γ)198Au reactions reproduced the experimental results within a factor of 2. Overall, PHITS with nuclear data libraries reproduced the experimental data sufficiently well. Thus, PHITS has a potential application in the design of advanced reactor systems, such as fusion and ADS facilities. This experimental data are also useful in validating nuclear reaction models in other Monte Carlo codes and evaluating nuclear data libraries for advanced reactor systems.

Effects of fiddler crab bioturbation on the geochemical migration and bioavailability of heavy metals in coastal wetlands

Fiddler crabs, found in coastal wetlands worldwide, function as ecosystem engineers. Their burrowing activity can significantly alter biogeochemistry at the local scale, however, the mobility of heavy metals (HMs) in burrow sediments remains unclear. Here, we used diffusive gradients in thin-film probes to obtain bioavailable Fe and HMs (Cu, Zn, Ni, Cd, Pb, Co, and Mo) in crab burrows from coastal wetlands (mudflats, salt marshes, and mangroves). The depth-profile results showed that most HMs were enriched at shallow and deep depths but deficient at middle depths. We highlighted that bioturbation improved oxic conditions, enhanced HM concentrations, and favored dissolved HM retention in burrow sediments, which served as a sink for overlying water HMs via burrow flushing but a potential source of particle HMs via enhanced resuspension. In deep anoxic layers, Fe(III) reduction drove the remobilization of HMs, except Cu and Mo, leading to the co-release of HMs with Fe. This Fe-HM coupling/decoupling was verified using enhanced two-dimensional high-resolution imaging, which revealed highly spatial heterogeneity of multiple HMs. Moreover, the hydrological conditions regulating bioturbation effects on HM behavior varied across different coastal wetlands. With coastal environmental changes, the key role of ubiquitous bioturbation in HM migration and bioavailability should be reconsidered.

The Degradation Mechanism of Mg2Si during Exploitation at High Temperature

Oxidized Mg2Si films are annealed in low vacuum at various annealing temperature, and the degradation mechanism of Mg2Si during exploitation at high temperature is investigated. The crystal structure, surface morphology, depth profile, Raman scattering, and electrical properties are measured by X‐ray diffraction (XRD), field emission scanning electron microscopy (FESEM), X‐ray photoelectron spectroscopy (XPS), Raman spectroscopy, and Hall effect system, respectively. XRD results show that MgO exists apparently in Mg2Si films, and as the annealing temperature increases, the content of MgO increases. The surfaces of films present distinct hexagon structures. The results of depth profiles show that the oxygen content decreases gradually from the surface to the subsurface. Raman spectroscopy analysis suggests that as the annealing temperature increases, the intensity of MgO peak enhances gradually. Hall measurement results indicate that the carrier concentration and mobility decrease significantly with the increase in annealing temperature. The films show n‐type at lower annealing temperature, while they present p‐type at higher annealing temperature, which indicates obvious conversions from n‐ to p‐type.

Microscale patterning of semiconductor c-Si by selective laser-heating induced KOH etching

Laser patterning has been used for the micro-scale fabrication of semiconductor devices like solar cells, photodetectors, LEDs, and also for modification of surfaces for wettability, reflected colors, initial bacterial adhesion, etc, due to its several advantages of patterning flexibility, spatial resolution, and mask-free operation over complex conventional lithography. Currently, laser-induced ablation is a promising patterning method in silicon solar cell fabrication; however, laser-induced defects, and thermal stresses remain a significant concern. In this paper, we demonstrate a laser ablation-free method for patterning c-Si based on the observation that the anisotropic etching of c-Si by KOH is highly temperature-dependent as the etching rate is about 100 times faster at 80 °C compared to room temperature. The laser heating-induced chemical etching (LHICE) of crystalline silicon can help alleviate such laser-induced damage by providing the necessary low temperature on the localized area(s) on the silicon substrate. We investigated the micro-second pulsed laser-assisted chemical etching of c-Si substrate for microscale patterning and showed that laser-induced damage could be eliminated as indicated by the minority carrier lifetime preservation. We also present results of the effect of laser processing parameters such as laser power, scan speed, and duty cycle on etching depth and surface morphology. The optical, surface morphology, depth profile, and LCPSim simulation results are also presented to optimize and understand the LHICE process. This versatile methodology of temperature-selective chemical etching could be applied to various thin-film and bulk materials used in diverse device fabrication.

Dark Reduction Drives Evasion of Mercury From the Ocean

Much of the surface water of the ocean is supersaturated in elemental mercury (Hg0) with respect to the atmosphere, leading to sea-to-air transfer or evasion. This flux is large, and nearly balances inputs from the atmosphere, rivers and hydrothermal vents. While the photochemical production of Hg0 from ionic and methylated mercury is reasonably well-studied and can produce Hg0 at fairly high rates, there is also abundant Hg0 in aphotic waters, indicating that other important formation pathways exist. Here, we present results of gross reduction rate measurements, depth profiles and diel cycling studies to argue that dark reduction of Hg2+ is also capable of sustaining Hg0 concentrations in the open ocean mixed layer. In locations where vertical mixing is deep enough relative to the vertical penetration of UV-B and photosynthetically active radiation (the principal forms of light involved in abiotic and biotic Hg photoreduction), dark reduction will contribute the majority of Hg0 produced in the surface ocean mixed layer. Our measurements and modeling suggest that these conditions are met nearly everywhere except at high latitudes during local summer. Furthermore, the residence time of Hg0 in the mixed layer with respect to evasion is longer than that of redox, a situation that allows dark reduction-oxidation to effectively set the steady-state ratio of Hg0 to Hg2+ in surface waters. The nature of these dark redox reactions in the ocean was not resolved by this study, but our experiments suggest a likely mechanism or mechanisms involving enzymes and/or important redox agents such as reactive oxygen species and manganese (III).

Jet electrochemical machining simulation of intersecting line removals with adjustable nozzle diameter by a finite area element grid

Jet electrochemical machining (Jet-ECM) is a machining process which makes flexible surface structuring of metallic materials with nearly no tool wear possible. Through a nozzle cathode with an inner diameter of e.g. 100 µm a continuous electrolyte jet is applied to the workpiece surface. When an external voltage is applied, an electric charge is transported between the nozzle cathode and the anodically polarized workpiece, which causes a material removal on the workpiece based on anodic metal dissolution. By moving the nozzle cathode in x and y directions, material is removed along the contour movement of the nozzle. In the previous studies multiphysical models based on the Finite-Element-Method (FEM) or Finite-Volume-Method (FVM) were presented to simulate this removal processes. It can be derived, that the simulation of the resulting material removal for intersecting line removals is resource intensive and complex in commercially available software. Furthermore, a simulation of intersecting line removals with adjustable nozzle diameters with FVM or FEM is challenging. As a new approach a model based on a finite area element grid was built up in Python. The model applies square sections to represent resulting depth profiles of a machined workpiece. The paper explains the physical design of the model for a simulation of intersecting line removals with adjustable nozzle diameter. Such removals are shown by the help of transient calculations. The resulting depth profiles for different nozzle adjustments are discussed.

HCl-H2SO4-H2O solution etching behavior of InAs (1 0 0) surface

Etch pits formed on (100)-oriented InAs surface by HCl-H2SO4-H2O solutions have been investigated by optical microscopy and atomic force microscopy (AFM) images. Depending on the composition of etchant, four types of etch pits are observed, which is attributed to different reaction rate of (111)A and (111)B planes by HCl and H2SO4. Excessive HCl results in long hexagonal shaped pits, whereas the addition of H2SO4 results in tetragonal pits. The direction of the composed pit sidewalls has been determined combining with the results of depth profiles on (110) and (11¯0) planes. The role of the HCl and H2SO4 in the over-all etching behavior of InAs and details about the geometrical relation between the sidewalls of the etch pits are discussed.

Effect of gas atmospheres on degradation of MgO thin film magnetic tunneling junctions by deionized water

The degradation of the MgO thin film barrier of magnetic tunneling junctions caused by the dissolution of the film in deionized water during manufacturing processes is critical to the performance of spintronic devices. In this work, the influence of gas atmospheres including oxygen (O2), nitrogen (N2) and carbon dioxide (CO2) on dissolution behavior of the film was investigated by using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and atomic absorption spectroscopy (AAS). XPS depth profile results reveal that deionized water reacts with the film and infiltrates into the deeper layer of the film resulting in the chemical transformation of MgO to Mg(OH)2. AAS results confirm that the films dissolve because Mg2+ ions are released into deionized water. Consequently, Mg concentration in deionized water increases. It is found that the concentration of Mg2+ ions in deionized water is highest in the dissolution of CO2-saturated deionized water and lowest in those of N2-saturated deionized water. Simultaneously, the release of Mg2+ causes the development of the coral-like clusters over their surface and a decrease of the film thickness. XPS depth profile and SEM cross-sectional results demonstrate that the film thickness slightly decreases in sample N2 and decreases considerably in sample CO2. This is due to the fact that CO2 gas reacts with water, reducing its pH and thus increasing of the dissolution rate, while N2 gas has no reaction with water. Finally, this work proposes a qualitative model based on chemical reaction to explain the dissolution behavior of the film.

Validated Analysis of Component Distribution Inside Perovskite Solar Cells and Its Utility in Unveiling Factors of Device Performance and Degradation.

Time-of-flight secondary-ion mass spectrometry (ToF-SIMS) has been used for gaining insights into perovskite solar cells (PSCs). However, the importance of selecting ion beam parameters to eliminate artifacts in the resulting depth profile is often overlooked. In this work, significant artifacts were identified with commonly applied sputter sources, i.e., an O2+ beam and an Ar-gas cluster ion beam (Ar-GCIB), which could lead to misinterpretation of the PSC structure. On the other hand, polyatomic C60+ and Ar+ ion beams were found to be able to produce depth profiles that properly reflect the distribution of the components. On the basis of this validated method, differences in component distribution, depending on the fabrication processes, were identified and discussed. The solvent-engineering process yielded a homogeneous film with higher device performance, but sequential deposition led to a perovskite layer sandwiched by methylammonium-deficient layers that impeded the performance. For device degradation, it was found that most components remained intact at their original position except for iodide. This result unambiguously indicated that iodide diffusion was one of the key factors governing the device lifetime. With the validated parameters provided, ToF-SIMS was demonstrated as a powerful tool to unveil the structure variation amid device performance and during degradation, which are crucial for the future development of PSCs.

Large area Silicon-energy filters for ion implantation

In this work we present the first time to our knowledge a large area Si-energy filter for ion implantation based on a 150 mm SOI Wafer Flow Process. The filter consists of a microstructured 125 mm diameter Si-membrane with 6.5 μm thickness supported by the remaining silicon wafer ring. These filters are of large importance in the cost effective doping of SiC for high power device fabrication. We will present in this work the Si-energy filter fabrication process, characterization results of single process steps and SIMS depth profiles after ion implantation which are in good accordance to the calculated simulation model. Additionally the impact of defects on the substrate is discussed in this work and mandatory steps are described to reduce the amount of holes in the finished membrane.

Calibration of NMR porosity to estimate permeability in carbonate reservoirs

Understanding the performance of carbon dioxide (CO2) stored in geologic reservoirs requires some knowledge of the subsurface heterogeneity, where the most reliable data is extracted from wellbore measurements. This advantage is especially true for carbonate reservoirs were reactions between supercritical CO2, the reservoir brine, and carbonate minerals create new storage capacity and enhanced permeability. Techniques like NMR well logging hold promise for estimating subsurface permeability, because the total porosity and pore size can be calculated from response of 1H NMR signal. Resulting depth profiles of porosity and permeability from downhole logs are useful input for reservoir simulations and can be used to design injection protocols and estimate storage capacity.We conducted a detailed characterization study to quantify the NMR response to matrix pore space, the connectivity to larger vugs and fractures, and the resulting permeability of core samples from distinct carbonate formations. The cores used in study span 5 orders in permeability, 4 orders in mean T2, and 2%–42% in porosity. The goal of the study was to use independent measures of pore volumes, surface relaxivity, fluid-accessible surface areas, and permeability to develop carbonate permeability models.It was not possible to develop a single model that reproduced permeability for different carbonate formations, because phenomena specific to the reservoirs could not be extracted even with the state-of-the-art characterization techniques used in this study. Estimates of permeability require calibration. Best estimates for a given formation were achieved by using a simple form of the Kozeny equation (Eq. 6):

Investigating PID Shunting in Polycrystalline CIGS Devices via Multi-Scale, Multi-Technique Characterization

We investigated potential-induced degradation (PID) in CuIn1-xGaxSe2 (CIGS) mini-modules stressed in the laboratory. Small cores were removed from the modules and were subjected to analysis. We completed a proof-of-concept correlative study relating cathodoluminescence to sodium content via time-of-flight secondary-ion mass spectrometry imaging. By comparing one-dimensional depth profile results and three-dimensional tomography results on stressed and unstressed CIGS mini-modules, we can see that PID in CIGS results from sodium migration through absorber, most likely via grain boundaries. Potassium concentration distributions show little change when adding a voltage bias to a temperature and humidity stress. This suggests doping with other large alkali ions, such as cesium and rubidium, rather than sodium can increase the PID resistance of CIGS modules.

Effect of energy per atom (E/n) on the Ar gas cluster ion beam (Ar-GCIB) and O2+ cosputter process.

Gas cluster ion beam (GCIB) is a promising technique for preserving molecular structures during ion sputtering and successfully profiling biological and soft materials. However, although GCIB yields lower damage accumulation compared with C60+ and monoatomic ion beams, the inevitable alteration of the chemical structure can introduce artifacts into the resulting depth profile. To enhance the ionization yield and further mask damage, a low-energy O2+ (200-500 V) cosputter can be applied. While the energy per atom (E/n) of GCIB is known to be an important factor influencing the sputter process, the manner through which E/n affects the GCIB-O2+ cosputter process remains unclear. In this study, poly(ethylene terephthalate) (PET) was used as a model material to investigate the sputter process of 10-20 kV Ar1000-4000+ (E/n = 2.5-20 eV per atom) with and without O2+ cosputter at different energies and currents. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) with Bi32+ as the primary ion was used to examine surfaces sputtered at different fluences. The sputter craters were also measured by alpha-step and atomic force microscopy in quantitative imaging mode. The SIMS results showed that the steady-state cannot be obtained with E/n values of less than 5 eV per atom due to damage accumulation using single GCIB sputtering. With a moderate E/n value of 5-15 eV per atom, the steady-state can be obtained, but the ∼50% decay in intensity indicated that damage cannot be masked completely despite the higher sputter yield. Furthermore, the surface Young's modulus decreased with increasing E/n, suggesting that depolymerization occurred. At an E/n value of 20 eV per atom, a failed profile was obtained with rapidly decreased sputter rate and secondary ion intensity due to the ion-induced crosslink. With O2+ cosputtering and a moderate E/n value, the oxidized species generated by O2+ enhanced the ionization yield, which led to a higher ion intensity at steady-state in general. Because higher kinetic energy or current density of O2+ led to a larger interaction volume and more structural damage that suppressed molecular ion intensity, the enhancement from O2+ was most apparent with low-energy-high-current (200 V, 80 μA cm-2) or high-energy-low-current (500 V, 5 μA cm-2) O2+ cosputtering with 0.5 μA cm-2 GCIBs. In these cases, little or no intensity drop was observed at the steady-state.

Location of the mainshock and aftershock sequences of the <italic>M</italic>6.9 Mainling earthquake, Tibet

We measured the location of the mainshock of the November 18, 2017, earthquake ( M 6.9) that occurred in Mainling County, Nyingchi City, Tibet, by using arrival-time data recorded by temporary seismic stations deployed on the Namche Barwa syntaxis and surrounding regions. The seismic stations offered good coverage of the hypocenter region and provided a good opportunity to precisely locate the mainshock and aftershock sequences. The Mainling earthquake was located at 29.87° N, 95.05° E, and the focal depth is 12 km. We investigated the focal mechanism of the mainshock by applying the W phase source inversion algorithm from the broad-band records of global seismic networks accessed from the IRIS data center. First, we resampled the original waveform to 1 SPS and then removed the mean, trend, and instrument response of the waveform, filtering it with a bandwidth of 150–70 s. After inversion, we obtained a solution with M w = 6.45 and a best double couple given by strike/dip/rake=319.9°/61.7°/101.0°. This result suggested that the seismogenic fault of the Mainling earthquake was characterized by a high-angle and NE-dipping thrust. We determined the location of the aftershock sequences by applying Geiger and double difference (DD) algorithms. Due to the lack of the local-scale tomographic model and reliable Poisson’s ratio data in the hypocentral region, we used only the P-wave arrival times to locate the aftershock sequences. We determined absolute locations for 148 aftershocks with high quality P-wave arrivals recorded by 14 temporary seismic stations and estimated the locations using the 1-D velocity model, in which the P-wave velocity was 6 km/s in the upper crust, 6.6 km/s in the lower crust, and 8.1 km/s in the uppermost mantle. The location errors were 0.6 km in the horizontal direction with an average residual of 0.07 s. In addition, we used the DD location algorithm in an attempt to minimize uncertainties in the aftershock locations. It was confirmed that the DD location algorithm greatly improved the relative location accuracy of events that are close to one another compared to the traditional Geiger location method. After DD location, the location errors of the aftershock sequences are 0.23, 0.26, and 1.43 km in the north-south, east-west, and vertical directions, respectively. The results indicated that the aftershocks were distributed along both sides (NW and SE directions) of the mainshock, which spread approximately 40 km in length and 10 km in width. Results of the focal depth profiles suggested that the aftershocks were located at depths above 15 and 10 km in the NW and SE directions of the mainshock, respectively. The aftershocks were mainly located at two sub-parallel faults. In the first two hours, the M 6.9 Mainling earthquake and aftershocks were mainly distributed at fault F1, and after that the aftershocks were distributed at fault F2. This suggested that the Xixingla fault zone is composed of a series of high-angle and NE-dipping faults, and the M 6.9 Mainling earthquake occurred at the south most one. The M 6.9 Mainling earthquake triggered seismic activities of a sub-parallel fault located to the northeast. The occurrence of the M 6.9 Mainling earthquake indicates that the Namche Barwa syntaxis is still in a high tectonic compression state at present. The Namche Barwa terrane continues moving towards the NE direction, leading the shortening deformation in the interior of the terrane. It is possible that a large earthquake may occur at Motuo and Mainling faults in the future along both sides of the Namche Barwa syntaxis.

Solid Electrolyte Interphase and Nitrogen Doping Effect on Potassium Storage Mechanism in Graphite

The inferior cycling stability critically impedes the development of potassium-ion batteries (KIBs). The solid electrolyte interface (SEI) for nitrogen doped graphite foam (NGF) was studied in both KPF6 and KN(SO2F)2 (KFSI)-based organic electrolytes, aiming to unravel the SEI effect on K+ ion storage mechanism. Electrochemical characterizations disclose that the KN(SO2F)2-based cells deliver improved electrochemical performance in terms of reversibility and cycling stability, compared to KPF6 based cells. Experimental results including depth-profiling XPS and FTIR spectra, together with the theoretical calculations, reveal that (CH2OCO2K)2, C2H5OCO2K, KF and K2CO3 are the dominant components of SEI layers in both electrolytes. Particularly, the amount of K2CO3 in KPF6-based electrolyte is much more than that in KFSI-based electrolyte, resulting in inferior stability. Moreover, the depth-profile XPS results indicate that the SEI formed in KFSI based electrolyte is much more stable, compact and thinner than that in KPF6 based electrolyte. All these features, together, ensure good stability and high reversibility in KFSI-based electrolyte. The nitrogen doping effect on K+ ion storage was also explored. Enhanced electrochemical performance was identified upon increasing the nitrogen content, due to i) the enrichment of active sites for K+ ion storage and ii) the improved electronic conductivity. Moreover, the electrochemical performance is strongly dependent on N-doping types. Specifically, the pyridinic nitrogen dominates the reversible capacity, as identified by the presence of critical point at 4.29 at. %. In which, the atomic ratio of pyridinic N in NGF is 2.53 at. %, which is higher than that of 7.03 at. %, with only 2.16 at. % pyridinic N. The result is consistent with the theoretical study, which verifies that the charge transferred from K ions to NGF increases with the pyridinic nitrogen doping. Our results promote better understanding of K+ ion storage mechanism in graphite and provide invaluable guidance for optimized carbon-based electrode design for high-performance KIBs.

Appraisal of fiber-metal laminate panels reinforced with metal pins deposited by CMT welding

Adhesion between metal and composite is key to fiber-metal laminates (FML). CMT (Cold Metal Transfer) welding has been used for hybrid joints that couple metal pin anchoring with adhesive bonding, but this concept has not been extended to FML panels yet. The objective of this work was to employ metal pins deposited by CMT PIN in FML panels. Panels with different pin deposition patterns were compared with panels without pins in terms of impact energy dissipation and damage characterization, and damage tolerance by means of compression and buckling tests after impact. The pins did not make the FML panels more brittle and the change in deposition pattern did not change the capacity to dissipate impact energy. The pins increased the maximum permanent deformation of the panels after impact, especially if less spaced. The in-plane damage areas were larger in the panels with pins, corroborating the damage depth profile results and their connection with impact energy dissipation. The compression and buckling tests indicated that the pins have potential to improve damage tolerance of FML panels, since a less-catastrophic behavior was observed. Pin anchoring seems to retard propagation of debonding between metal sheets and composite and hold back delamination within the composite.

Deuterium permeation and retention behaviors in erbium oxide-iron multilayer coatings

Hydrogen isotope migration behaviors in ceramics-metal multilayer coatings have been elucidated for a further improvement of fueling system and radiological safety. Erbium oxide (Er2O3) coatings were fabricated by filtered vacuum arc deposition (VAD) on reduced activation ferritic/martensitic steel substrates. An iron (Fe) layer was fabricated by radio-frequency magnetron sputtering or covered with an Fe foil on the Er2O3 coating. An Er2O3-Fe-Er2O3 three-layer coating was also fabricated by the VAD on the Er2O3-Fe coating. The grain boundary diffusion model was constructed based on the results of grain structure analysis and deuterium depth profile for the Er2O3 single-layer coating from our previous works. The Er2O3-Fe two-layer coatings with different Fe layer thickness showed no significant difference in deuterium permeability. The Er2O3-Fe-Er2O3 three-layer coating showed a PRF of up to 104 due to a contribution of two diffusion barriers of the inner and outer Er2O3 layers. In addition, the three-layer coating had three times higher deuterium concentration than the Er2O3-Fe coating, although the concentration will be negligibly small from the perspective of tritium inventory in the fuel system.

Abnormal bipolar resistive switching behavior in carbon-iron composite films with different thicknesses

Carbon-iron composite films with different thicknesses (50, 100 and 150 nm) were deposited by DC magnetron sputtering method using a composite target (Fe: 4 at%, C: 96 at%). The resistive switching behaviors of Pt/Al/a-C:Fe/Au/Ti structures with different a-C:Fe film thicknesses are investigated. Abnormal bipolar resistive switching characteristics were observed in all Pt/Al/a-C:Fe/Au/Ti memory cells. Considering the endurance and retention properties, the RRAM cell with 100 nm thick a-C:Fe film as the dielectric layer showed the best performance with on/off-resistance ratio ∼10, and retention time >104 s. Based on the results of the X-ray photoelectron spectroscopy depth profile, the resistive switching behavior is attributed to mutative Al3+ fraction at the Al/a-C:Fe interface and the migration of Au ions between two electrodes.

(Invited) Controlling Component Distribution in Electrodeposited Multilayer and Alloy Films

Many advanced materials are produced by electrodeposition nowadays which are mostly alloys or multilayers and contain at least two components. The electrodeposition of some elements and element combinations often requires the use of bath formulations different from the traditional aqueous-based solutions. For the investigation of the properties of such novel electrodeposited materials, it is important to be able to control the deposit composition over the sample volume. For alloy films, this requires ensuring a homogeneous composition profile both laterally and along the growth direction since otherwise the changes of a certain sample property upon varying the deposition conditions may be masked by composition inhomogeneities. For multilayers consisting of two alternating layers of different kind, in addition to the requirement of lateral uniformity, the control of a regular change of composition along the thickness is also necessary. Over the last two decades, our group was strongly involved in the research activity on the giant magnetoresistance (GMR) effect in electrodeposited (ED) magnetic/non-magnetic multilayer films [1]. During attempts towards a better understanding of layer formation in such multilayers and improving their GMR characteristics, we have identified some factors decisively influencing lateral composition variations and composition modulation in multilayers. We have also succeeded in suppressing the composition variation along the thickness in ED alloy films. In the present talk, the discussion of progress along these lines will cover the following three topics. A. Controlling lateral composition homogeneity of electrodeposits Lateral variations can occur due to the well-known edge-effects in the case of electrodeposition in a cell with lateral dimensions much larger than the cathode surface area. Also, the commonly used vertical cathode arrangement usually leads to composition and thickness variations along the cathode surface from the bottom to the top due to the buoyancy effect. The particular sensitivity of GMR to individual layer thicknesses was of great help in revealing macroscopic lateral inhomogeneities of ED multilayers due to both above described effects [2]. The use of a cell with a cathode completely filling the cross-section of the cell and the placement of the cathode with an upward-looking position at the bottom of the cell eliminates these drawbacks [3]. This way one can produce ED films which show a large degree of lateral homogeneity. B. Controlling composition modulation in multilayered electrodeposits It turned out from an analysis of possible two-pulse combinations for preparing magnetic/non-magnetic multilayers from a single bath that the best way is the use of a G/P pulse sequence in which a galvanostatic (G) pulse is used for the deposition of the less-noble magnetic metal whereas the more-noble non-magnetic metal is deposited under potentiostatic (P) control. The introduction of a method for establishing the electrochemically optimized potential for the deposition of the non-magnetic spacer layer on the basis of an analysis of current transients during multilayer deposition enabled us to control the composition modulation with a high accuracy. This implies also a proper control of constituent layer thicknesses which was a prerequisite for studying the true dependence of physical properties (e.g., GMR) on layer thicknesses not available beforehand. We have also made attempts to control multilayer formation by a partial anodic dissolution of a thick deposited non-magnetic layer in each period [4] or by intentionally applying a more positive potential for non-magnetic layer deposition than the optimized value and, thus, enabling a partial replacement of the magnetic layer atoms by non-magnetic atoms [5]. C. Controlling composition profile along the thickness in electrodeposited alloys In magnetic alloys such as permalloy (Ni-Fe), there is an inherent initial composition variation along the thickness when produced by d.c. plating. This occurs due to the interplay of the different deposition preference of the two components and the partial depletion of the solution near the cathode. The initial composition variation may lead to a deterioration of the magnetic properties which can be very pronounced at the small layer thicknesses required in applications striving for miniaturization. Based on depth-profile analysis results, a proper pulse-plating technique could be elaborated to ensure a composition homogeneity even in the initial first few hundred nanometer range [6]. 1. I. Bakonyi, L. Péter: Progr. Mater. Sci. 55 (2010) 107 2. I. Bakonyi et al., Phys. Rev. B 70(2004) 054427 3. V. Weihnacht et al., J. Electrochem. Soc. 150(2003) C507 4. N. Rajasekaran et al., J. Electrochem. Soc. 161(2014) D339 5. N. Rajasekaran et al., J. Electrochem. Soc. 162 (2015) D204 6. K. Neuróhr et al., Electrochim. Acta 103 (2013) 179