Off-Axis Texture and Crystallographic Accommodation in Multicomponent Nitride Thin Films Deposited by Pulsed Magnetron Sputtering

The study of texture in thin films is one of the possibilities to obtain information on the fundamental physical processes which govern the thin film growth. Near the substrate, the crystalline orientation of the coating layer will usually be influenced by the surface state of the substrate (grain orientation and dimensions, roughness etc.). However, during the film deposition, the texture of the coating can evolve. This evolution is a particular interesting phenomenon in order to unravel the atomic movements occurring during the deposition. To gain an overall picture for the evolution of texture and development of microstructure with film thickness and substrate material in nanostructured nitride films, a variety of analytical x-ray diffraction (XRD) techniques like θ–2θ scan, pole figure, and residual stress by sin2ψ method were utilized. The coatings of Ti1-xAlxN from different target compositions were deposited onto various substrates (WC-Co, glass, Si(100)). Based on the results obtained, Ti0.5Al0.5N films on WC-Co and glass reveal that surface energy minimization at low thickness leads to the development of (002) orientation. On the other hand, the competitive growth promotes the growth of (111) planes parallel to film surface at higher thickness. However, despite the prediction of growth models, the (002) grains are not completely overgrown by (111) grains at higher thickness. Rather, the (002) grains still constitute the surface, but are tilted away from the substrate normal showing substantial in-plane alignment to allow the (111) planes remain parallel to film surface. Conversely, films on Si had a dominant (002) texture nearly parallel to the surface for all samples, not changing with thickness. This indicates that the surface and interface energy anisotropies provide the driving force for texture development. For films on WC-Co, the stress along (002) which was compressive at low thickness decreases with gradual tilt of (002) and changes to tensile at higher thickness. Tilting of (002) is to minimize the overall energy of the system as the (111) planes develop with thickness store very high compressive stress. On the contrary, the stress state in (002) grains on Si remained compressive through the film thickness. Morphological and roughness observation by SEM and AFM reveal that films on WC-Co and glass display higher roughness than on Si. This confirms that the development of orientation is accompanied by an increase in surface roughness of the film. In contrast, cross- sectional TEM observations reveal a smooth surface region, and the SAED patterns and HR-TEM image support the crystallographic results regarding the texture formation for films on Si. Film hardness was measured by nanoindentation and a correlation between (111) texture, stress, hardness and the type of substrate is obtained. However, for Ti0.67Al0.33N films, the texture growth mechanism is similar to Ti0.5Al0.5N, but the final dominant orientation at higher thickness is (113) and not (111) as expected. Using θ–2θ scans and pole figures, we establish that the preferential orientation changes with film thickness from low index (002) across (111) to high index (113) plane. We have found that to minimize the total energy of the system, the orientation of the growing film switches from (111) to (113) with preferentially less density that causes a decrease in hardness and stress formed in the (111)-oriented grains. Different mechanisms which could explain the crossover of (002), (111), and (113) orientations through film thickness are discussed. Development of the (113) orientation is examined with respect to stress, morphology and mechanical properties. In order to obtain further information on the crystallographic accommodation and to resolve whether the incident flux angle α plays any role in this off-axis texture formation, we have deposited series of Ti0.5Al0.5N films under various incident angles from 0° to 60° on Si(100) substrates at room temperature. We show that both in-plane and out-of-plane crystallographic orientations respond strongly to the deposition angle. For α = 0°, the pole figures display a (111) and (002) mixed out-of-plane orientation with random in-plane alignment. In contrast, under oblique angle deposition (OAD), inclined textures are observed with the (111) direction moving toward the incident flux direction and the (002) moving away, showing substantial in- plane alignment. This observation suggests that TiAlN crystals prefer to grow with the (002) direction perpendicular to the substrate while maintaining the minimization of the surface free energy by maximizing the (111) surface area toward the incident flux. The in-plane texture, which is randomly oriented at normal incidence, gives rise to two preferred orientations under oblique angles – one along the direction of flux and other away from the deposition source. The biaxial texture results from a competition among texture mechanisms related to surface mobilities of adatoms, geometrical and directional effects. The surface and cross-section of the films were observed by SEM. OAD films develop a kind of smooth tiles of a roof structure, with no faceted crystallites. The columns of these films were tilted toward the direction of incident flux. The dependence of (111) texture tilt angle and column angle β on the incidence flux angle α is evaluated using four well-known models. Transmission electron microscopy (TEM) study reveals a voided, intercolumnar structure with oblique growth toward the flux direction. The selected area diffraction pattern (SAED) pattern supports the pole figure observations. Measurements of the nanoindentation test were performed in order to discuss the change of mechanical properties as a function of incident flux angle. Furthermore, when the substrate temperature is elevated from room temperature to 400°C and 650°C, due to the enhanced mobility of the adatoms, the surface diffusion length overrules the self- shadowing effects in traditional OAD at ambient temperature. The mobility affects the angle of columns, off-axis angles of (111) and (002) texture as well as the microstructure and mechanical properties of the coatings. We also demonstrate that the biaxial alignment and inclination of columns is independent of film composition

Directed Irradiation Synthesis as an Advanced Plasma Technology for Surface Modification to Activate Porous and “as-received” Titanium Surfaces

For the design of smart titanium implants, it is essential to balance the surface properties without any detrimental effect on the bulk properties of the material. Therefore, in this study, an irradiation-driven surface modification called directed irradiation synthesis (DIS) has been developed to nanopattern porousand“as-received”c.p. Tisur faces with the aim of improving cellular viability. Nano features were developed using singly-charged argon ions at 0.5 and 1.0 keV energies, incident angles from 0◦ to 75◦ degrees, and fluences up to 5.0×1017 cm−2. Irradiated surfaces were evaluated by scanning electron microscopy, atomic force microscopy and contact angle, observing an increased hydrophilicity (a contact angle reduction of 73.4% and 49.3%) and a higher roughness on both surfaces except for higher incident angles, which showed the smoothest surface. In-vitro studies demonstrated the biocompatibility of directed irradiation synthesis (DIS) reaching 84% and 87% cell viability levels at 1 and 7 days respectively, and a lower percentage of damaged DNA in tail compared to the control c.p. Ti. All these results confirm the potential of the DIS technique to modify complex surfaces at the nanoscale level promoting their biological performance.Department of Defense (Spain) contract W81XWH-11-2-0067Ministry of Economy and Competitiveness of Spain grant MAT2015-71284-

Removal of Methylene Blue from Water Using Magnetic GTL-Derived Biosolids: Study of Adsorption Isotherms and Kinetic Models

Global waste production is significantly rising with the increase in population. Efforts are being made to utilize waste in meaningful ways and increase its economic value. This research makes one such effort by utilizing gas-to-liquid (GTL)-derived biosolids, a significant waste produced from the wastewater treatment process. To understand the surface properties, the biosolid waste (BS) that is activated directly using potassium carbonate, labelled as KBS, has been characterized using scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDS), X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), and Brunauer–Emmett–Teller (BET). The characterization shows that the surface area of BS increased from 0.010 to 156 m2/g upon activation. The EDS and XPS results show an increase in the metal content after activation (especially iron); additionally, XRD revealed the presence of magnetite and potassium iron oxide upon activation. Furthermore, the magnetic field was recorded to be 0.1 mT using a tesla meter. The magnetic properties present in the activated carbon show potential for pollutant removal. Adsorption studies of methylene blue using KBS show a maximum adsorption capacity of 59.27 mg/g; the adsorption process is rapid and reaches equilibrium after 9 h. Modelling using seven different isotherm and kinetic models reveals the best fit for the Langmuir-Freundlich and Diffusion-chemisorptionmodels, respectively. Additional thermodynamic calculations conclude the adsorption system to be exothermic, spontaneous, and favoring physisorption

Removal of Methylene Blue from Water Using Magnetic GTL-Derived Biosolids: Study of Adsorption Isotherms and Kinetic Models

Global waste production is significantly rising with the increase in population. Efforts are being made to utilize waste in meaningful ways and increase its economic value. This research makes one such effort by utilizing gas-to-liquid (GTL)-derived biosolids, a significant waste produced from the wastewater treatment process. To understand the surface properties, the biosolid waste (BS) that is activated directly using potassium carbonate, labelled as KBS, has been characterized using scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDS), X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), and Brunauer–Emmett–Teller (BET). The characterization shows that the surface area of BS increased from 0.010 to 156 m2/g upon activation. The EDS and XPS results show an increase in the metal content after activation (especially iron); additionally, XRD revealed the presence of magnetite and potassium iron oxide upon activation. Furthermore, the magnetic field was recorded to be 0.1 mT using a tesla meter. The magnetic properties present in the activated carbon show potential for pollutant removal. Adsorption studies of methylene blue using KBS show a maximum adsorption capacity of 59.27 mg/g; the adsorption process is rapid and reaches equilibrium after 9 h. Modelling using seven different isotherm and kinetic models reveals the best fit for the Langmuir-Freundlich and Diffusion-chemisorptionmodels, respectively. Additional thermodynamic calculations conclude the adsorption system to be exothermic, spontaneous, and favoring physisorption

Nanotextured porous titanium scaffolds by argon ion irradiation: Toward conformal nanopatterning and improved implant osseointegration

Stress shielding and osseointegration are two main challenges in bone regeneration, which have been targeted successfully by chemical and physical surface modification methods. Direct irradiation synthesis (DIS) is an energetic ion irradiation method that generates self-organized nanopatterns conformal to the surface of materials with complex geometries (e.g., pores on a material surface). This work exposes porous titanium samples to energetic argon ions generating nanopatterning between and inside pores. The unique porous architected titanium (Ti) structure is achieved by mixing Ti powder with given amounts of spacer NaCl particles (vol % equal to 30%, 40%, 50%, 60%, and 70%), compacted and sintered, and combined with DIS to generate a porous Ti with bone-like mechanical properties and hierarchical topography to enhance Ti osseointegration. The porosity percentages range between 25% and 30% using 30 vol % NaCl space-holder (SH) volume percentages to porosity rates of 63%–68% with SH volume of 70 vol % NaCl. Stable and reproducible nanopatterning on the flat surface between pores, inside pits, and along the internal pore walls are achieved, for the first time on any porous biomaterial. Nanoscale features were observed in the form of nanowalls and nanopeaks of lengths between 100 and 500 nm, thicknesses of 35-nm and heights between 100 and 200 nm on average. Bulk mechanical properties that mimic bone-like structures were observed along with increased wettability (by reducing contact values). Nano features were cell biocompatible and enhanced in vitro pre-osteoblast differentiation and mineralization. Higher alkaline phosphatase levels and increased calcium deposits were observed on irradiated 50 vol % NaCl samples at 7 and 14 days. After 24 h, nanopatterned porous samples decreased the number of attached macrophages and the formation of foreign body giant cells, confirming nanoscale tunability of M1–M2 immuno-activation with enhanced osseointegration

Impact of the Oxygen Content on the Optoelectronic Properties of the Indium-Tin-Oxide Based Transparent Electrodes for Silicon Heterojunction Solar Cells

Amorphous/crystalline silicon heterojunction (SHJ) solar cells technology is attracting tremendous attention in recent years due to its potential to achieve high power conversion efficiencies at low fabrication temperatures and using few process steps. However, the commercial mass production of this technology is still somehow restricted so far, which is mainly due to the high sensitivity of the SHJ solar cell parameters to the growth conditions. A significant distinctness between the SHJ configuration and the standard silicon wafer solar cell is the current collection scheme. Indeed, as the SHJ silicon wafer solar cell is limited by the low lateral conductivity of the thin-film-silicon layers used to form the contact, a transparent conductive oxide (TCO) is systematically employed to improve the carrier transport properties, whilst also acting as an antireflective coating (ARC) for the front side. From the variety of TCOs, indium tin oxide (ITO) is the most frequently used. In this work, we investigate the properties of ITO thin films grown by DC magnetron sputtering using different oxygen to total flow ratios [r(O-2) = O-2/(Ar+O-2)] ranging from 0.01 (1%) to 0.08 (8 %). Hall effect measurements together with optical spectrometry were carried out and were found to be drastically affected by the oxygen content. Furthermore, time of flight-secondary ion mass spectrometry (TOF-SIMS) was used to determine the depth profiling of indium, oxygen, tin, silicon, phosphorous, and hydrogen throughout the ITO and silicon layers forming the solar cell. Finally, silicon heterojunction devices were fabricated and the associated photovoltaic performance were evaluated as a function of the r(O-2) into the ITO electrodes. Lower oxygen flow ratio was found to yield the best performance which is attributed to lower parasitic resistive losses

Impact of the Oxygen Flow during the Magnetron Sputtering Deposition on the Indium Tin Oxide thin films for Silicon Heterojunction Solar Cell

We report on the optoelectronic properties of ITO layers deposited by DC sputtering, using different oxygen to total flow ratios [r(O-2) = O-2/Ar, ranging from 1% to 8%], for silicon heterojunction (SHJ) solar cell application. The depth profiling of the various elements throughout the thicknesses and interfaces of the ITOs and thin films forming the SHJ device was determined by time-of-flight secondary ion mass spectrometry. Finally, the photovoltaic performance of the fabricated SHJ cells was evaluated with respect to the r(O-2) into the ITO layers. Lower r(O-2) was found to yield the best PV performance which is attributed to lower parasitic resistive losses

Directed Irradiation Synthesis as an Advanced Plasma Technology for Surface Modification to Activate Porous and “as-received” Titanium Surfaces

For the design of smart titanium implants, it is essential to balance the surface properties without any detrimental effect on the bulk properties of the material. Therefore, in this study, an irradiation-driven surface modification called directed irradiation synthesis (DIS) has been developed to nanopattern porous and “as-received” c.p. Ti surfaces with the aim of improving cellular viability. Nanofeatures were developed using singly-charged argon ions at 0.5 and 1.0 keV energies, incident angles from 0° to 75° degrees, and fluences up to 5.0 × 1017 cm−2. Irradiated surfaces were evaluated by scanning electron microscopy, atomic force microscopy and contact angle, observing an increased hydrophilicity (a contact angle reduction of 73.4% and 49.3%) and a higher roughness on both surfaces except for higher incident angles, which showed the smoothest surface. In-vitro studies demonstrated the biocompatibility of directed irradiation synthesis (DIS) reaching 84% and 87% cell viability levels at 1 and 7 days respectively, and a lower percentage of damaged DNA in tail compared to the control c.p. Ti. All these results confirm the potential of the DIS technique to modify complex surfaces at the nanoscale level promoting their biological performance