© 2023 by the authors.In this study, we report the use of a radiofrequency plasma-assisted chemical vapor deposition (RF-CVD) system with a hollow cathode geometry to hydrogenate anatase TiO2 thin films. The goal was to create black TiO2 films with improved light absorption capabilities. The initial TiO2 was developed through magnetron sputtering, and this study specifically investigated the impact of hollow cathode hydrogen plasma (HCHP) treatment duration on the crucial characteristics of the resulting black TiO2 films. The HCHP treatment effectively created in-bandgap states in the TiO2 structure, leading to enhanced light absorption and improved conductivity. Morphological analysis showed a 24% surface area increase after 15 min of treatment. Wettability and surface energy results displayed nonlinear behavior, highlighting the influence of morphology on hydrophilicity improvement. The anatase TiO2 phase remained consistent, as confirmed by diffractograms. Raman analysis revealed structural alterations and induced lattice defects. Treated samples exhibited outstanding photodegradation performance, removing over 45% of methylene blue dye compared to ~25% by the pristine TiO2 film. The study emphasized the significant impact of 15-min hydrogenation on the HCHP treatment. The research provided valuable insights into the role of hydrogenation time using the HCHP treatment route on anatase TiO2 thin films and demonstrated the potential of the produced black TiO2 thin films for photocatalytic applications.

© 2023 Universidade Federal de Sao Carlos. All rights reserved.This work reports on the properties of GaN films grown by reactive magnetron sputtering onto glass substrate kept at relatively low temperature (400°C), using different RF power applied to the Ga target. Their structural, morphological, vibrational and optical properties were characterized by X-ray diffraction, atomic force and scanning electron microscopies, Raman spectroscopy and UV-vis spectrophotometry. The films have wurtzite phase with strong preferential orientation in the c-axis direction. Moreover, two clear contributions to the (0002) diffraction peak could be found, indicating the presence of two different morphologies, which were discussed in terms of the formation of an intermediate layer between the substrate and a dominating columnar-like microstructured film.

© 2023 Elsevier Inc. All rights reserved.Solar cells and photovoltaic devices are based overall on metal oxides and heterostructures. This is a technology in advance, with remarkable interest due to its low impact on the environment in comparison to the most used forms of energy conversion. Additionally, the oxides are most abundant, easy, and less expensive to obtain compared to other materials for such applications. Although the metal oxide–based photovoltaic devices show, usually, low conversion efficiency, some studies have shown capable of obtaining PCE higher than 10% or 20% using enhanced heterostructures. This chapter presented a review of the state of the art of metal oxide heterostructures applied mainly in photovoltaic devices and solar cells. A special focus is given to studies related to some of the most applied materials, such as ZnO, ZnO:Al, (AZO), and TiO2, and to heterostructures based on metallic oxides of copper, zinc, magnesium, vanadium, etc.

© 2022 Universidade Federal de Sao Carlos. All rights reserved.This work reports the properties of GaN films grown onto c-Si (100) at relatively low substrate temperature (400°C) by reactive magnetron sputtering. The study depicts the effect of working pressure and RF power on the GaN film structural, vibrational and optical properties characterized by X-ray diffraction, atomic force and scanning electron microscopies, Raman spectroscopy and spectroscopic ellipsometry. Unusual low pressure deposition condition (0.40 Pa) was achieved by using a separated argon inlet directed to the Ga target surface, resulting in improved crystalline quality of the films. In this condition, the preferential crystalline orientation, the surface morphology and the optical gap of the GaN films show a strong dependence on the RF power applied to the Ga target, where low RF power (30-60 W) was responsible for increasing the c-axis orientation and the optical gap, while higher RF power (75-90 W) decreased the overall crystal quality and increased the surface roughness.

© 2021 Author(s).The influence of the oxygen gas supply on the stoichiometry, structure, and orientation texture of polycrystalline cobalt oxide films was investigated in this study. The films were grown by direct current reactive magnetron sputtering using a metallic Co target and different O2 inlet flow rates (0.5-5.0 SCCM). The deposition power (80 W), the argon gas flow (40 SCCM), and the total working pressure (0.67 Pa) were kept constant during depositions. The results evidence a strong influence of the oxygen flow over the film's stoichiometry and structure, where low oxygen flows (<2.0 SCCM) favor the formation of the rock salt CoO phase while higher oxygen flows (>2.5 SCCM) favor the spinel Co3O4 phase formation. The coexistence of monoxide and tetraoxide phases is only observed for the 2.5 SCCM oxygen flow condition. Strain effects related to the oxygen partial pressure are also observed and discussed. Computer simulations of the reactive sputtering growth supported the analysis of the film properties and its correlation to the oxygen partial pressure.

© 2021, The Author(s), under exclusive licence to Springer-Verlag GmbH, DE part of Springer Nature.Electroactive Co3O4 films were deposited by reactive magnetron sputtering (RMS) onto an assembly composed of a thin TiO2 layer over a commercial indium-doped tin oxide (ITO) conductor electrode, forming an ITO/TiO2/Co3O4 electrocatalytic platform. The platform was tested as a non-enzymatic device for glucose electrooxidation. The characterization of the electroactive TiO2/Co3O4 heterojunction was carried out by x-ray diffraction (XRD), Raman spectroscopy, field-emission scanning electron microscope (FE-SEM), and energy dispersive spectroscopy (EDS) techniques. It was shown that the Co3O4 top layer is homogeneous and free from undesirable secondary phases. The electrochemical measurements, characterization and performance, were carried out by cyclic voltammetry (CV), chronoamperometry, and electrochemical impedance spectroscopy (EIS). The cyclic voltammogram shows the linear dependence between the anodic and cathodic current peak of the redox process at the electrode surface, showing the electrochemical activity of the Co3+/Co4+ redox pair, as well as good reversibility and efficiency of charge transfer. From the chronoamperometric curves, two electrochemical parameters were estimated, the diffusion coefficient (D) and catalytic rate constant (kobs) of glucose with the values of 1.2 × 10−6 cm2 s−1 and 2.8 × 106 cm3 mol−1 s−1, respectively. The ITO/TiO2/Co3O4 heterojunction electrode showed an acceptable linear range from 10 to 1000 μM glucose concentration with a molar sensitivity (S) of 30.0 μA cm−2 mM−1 and a detection limit (LOD) of 1.42 μM (S/N = 3). These electrochemical results show the higher electroactivity of TiO2/Co3O4 heterojunction electrode than that of bare Co3O4 electrode and other literature results.

© 2020The increase in the biointegration speed of titanium alloys is an important factor in the recovery and quality of life after an implant. The coating of these materials with thin films using plasma technologies is a viable alternative that can change the surface properties without changing the bulk properties. In this work, Diamond-like Carbon films doped with silver nanoparticles were deposited on the surface of Ti6Al4V alloys using a conjugate reactor, which uses Plasma Enhanced Chemical Vapor Deposition technique associated with a silver hollow cathode. The flow of argon was varied (from 20 to 80 sccm) to evaluate its influence on surface roughness and biointegration. Secondary Ion Mass Spectrometry depth profile showed the effectiveness of the hollow cathode to form a silver concentration gradient from the substrate up to the film surface, which is desirable in biomedical applications. Atomic Force Microscopy detected that increasing argon flow from 20 to 80 sccm produced a more acicular relief and promoted an increase in sp3 hybridization, which characterizes films with better adhesion and mechanical resistance, as well as biomedical applications in which the material is subjected to load-bearing and wear. These results indicated the possibility of tuning the film roughness according to its biomedical application. The results of in vivo tests suggested that silver doping in Diamond-like Carbon films promoted faster biointegration than non-doped Diamond-like Carbon films and indicated the potential for their applicability in medical prosthetic materials.

© 2021 Universidade Federal de Sao Carlos. All rights reserved.Every year, billions of dollars are invested in research and development for space applications, including new systems, new technologies, and new materials. DLC (Diamond-Like Carbon) is a promising material for use in these applications, but its use faces a technological barrier, since it is severely etched by atomic oxygen and ozone. In this study, SiOx-DLC thin films were deposited as a top layer of diamond-like carbon (DLC) films on Ti-6Al-4V substrates to increase resistance against corrosion by atomic oxygen and ozone as well as meet the requirements for use in Low Earth Orbit (LEO) satellites. The corrosion resistance of the films was evaluated using oxygen plasma, and the tribological and mechanical properties were investigated. The SiOx-DLC top layer reduced the corrosion rate two orders of magnitude and increased the critical load from 16.2 ± 1.5 N to 18.4 ± 0.4 N.

© 2020 Elsevier B.V.The demand for the development of more efficient, low emission, and high-performance aircraft have required new methods to obtain lighter materials, with higher temperature resistance and chemical stability. For these purposes, Environmental Barrier Coatings (EBC) are largely studied. EBC's are commonly obtained through Atmospheric Plasma Spray, where the process parameters, as well as the precursor characteristics, and its interaction with the plasma jet influence the properties of the deposited material. In this context, this work aims to deposit EBC on C/C composites through the high-velocity plasma spray (HVPS) using a new concept of hybrid SiO2 + ZrO2 precursor. For this, a solid load of 7% yttria-stabilized zirconia (7YSZ) is mixed in a Si(OH4) solution with concentrations of 10, 20, and 30 g/L. From Raman and XRD analyses, ZrSiO4 and SiC were identified, its formations occur due to the reactions of SiO2 with ZrO2 (7YSZ) and the SiO2 precursor with C/C substrate. Finally, we demonstrate the versatility of the HVPS process using a hybrid precursor, capable of producing micro/nanostructured EBC coatings.

© 2020 Universidade Federal de Sao Carlos. All rights reserved.This work reports how the solar conversion efficiency of dye-sensitized solar cells (DSSCs) depends on the crystalline structure of both the compact TiO2 blocking layer (BL) and homoepitaxially grown porous TiO2 mesoporous structure. The films were grown by simultaneous sputtering of titanium targets by DC magnetron and by high-power impulse magnetron systems (HiPIMS). The deposition conditions were managed to produce in situ dense BLs and porous TiO2 films. The only variable was the polarization of the BLs (0 to -200V). The polarization caused phase transformations from pure anatase phase through rutile-anatase mixed phases to rutile phase. The polarization results in decreasing intensity of the anatase (101) peak of the porous layers. The latter promptly decreased linearly the value of the short-current Isc and exponentially the open-circuit voltage Voc of the cells. Another inference is the surface energy of the BLs, which follows an exponential decay as a function of the film polarization. XPS study of the Ti 2p3/2 – Ti 2p1/2 doublet reveals an appearance of a shake-up satellite, whose area exponentially decreases as the polarization potential rises. This phenomenon is discussed and related with other physical aspects of the homo-epitaxially grown films with different texture. The anatase phase content and its purity predefined by the experimental conditions determine the quality of the DSSC, as well as other components such as the dye type, the electrolyte, and the electrode materials.

© 2020 by the authors. Licensee MDPI, Basel, Switzerland.Black TiO2 materials have been quite widely explored due to their large solar absorption and superior photocatalytic activity. In this paper, the blackening process of titanium dioxide (TiO2) thin film using the hollow cathode hydrogen plasma (HCHP) technique is reported. First, pristine anatase TiO2 films were grown by magnetron sputtering onto silicon and cover glass substrates and then annealed at 450◦C for 2 h. Then, the as-grown TiO2 films were treated with HCHP for 15 min. The physical, chemical and morphological properties of the films were analyzed by profilometry, X-ray diffraction (XRD), UV-Vis spectrophotometry, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) techniques. Electrical and photocatalytic measurements were performed by four-point probe and methylene blue UV degradation assays, respectively. The results showed that the black TiO2 film is highly absorbent in the UV-visible region, has low electrical resistance and greater surface area compared to the non-treated TiO2 film. These properties of black TiO2 film, as well as its performance as a photocatalytic agent, were investigated, indicating the superior quality of this material in thin film form and the promising potential of the HCHP treatment to produce hydrogenated TiO2 in short process time.

© 2019 Elsevier Ltd and Techna Group S.r.l.Life expectancy has been growing, and more people are developing bone diseases such as arthritis and osteoporosis. Degenerative pathologies, injuries, and trauma can damage the bone tissues, requiring treatments that facilitate its repair, replacement, or regeneration. In this context, many materials have been developed to match this demand. Bioglasses and ceramics are promising inorganic materials to produce scaffolds for bone regeneration due to their attractive properties, such as biocompatibility, osteoinduction, and osteoconduction, besides their similarity with bone composition. Although their established advantages, these materials present limitations such as inadequate mechanical properties and fast degradation rate. Research work has been widely carried out to develop bioglasses, silicate, and phosphate calcium ceramics scaffolds with appropriated properties to enlarge their applications in bioengineering. Different fabrication techniques have also been evaluated. Incorporating other materials or particles, such as polymers, oxides and metal particles into the scaffolds has shown beneficial effects in mechanical strength and bone production stimulation. In this review, we provide an overview concerning the recent advances in developing calcium phosphates, calcium silicates, bioglasses, and composites scaffolds for bone regeneration in medical and dental applications.

© 2019 Elsevier B.V. The aim of this study was to tailor the deposition parameters of magnetron sputtering to synthetize tantalum oxide (Ta x O y ) films onto commercially pure titanium (cpTi) surface. The structural and optical properties, morphology, roughness, elemental chemical composition and surface energy were assessed. The impact of Ta x O y films on initial Streptococcus sanguinis adhesion was investigated. The morphology and spreading of pre-osteoblastic (MC3T3-E1) cells on a crystalline tantalum oxide film were evaluated. Ta x O y films with estimated thickness of 600 nm and different structures (amorphous or crystalline) were produced depending on the various oxygen flow rates and parameters used. X-ray diffraction analysis revealed that the 8 O 2 sccm (600 °C/400 W) group showed crystallization corresponding to the β-Ta 2 O 5 phase. Optical analysis showed that the 4 O 2 sccm (200 °C 300 W) to 8 O 2 sccm (600 °C 300 W) groups and 10 O 2 sccm (200 °C 300 W) group presented regular and large-amplitude interference oscillations, suggesting high optical homogeneity of the films. The crystalline β-Ta 2 O 5 coating showed higher roughness and surface energy values than the other groups (P <.05) and was biocompatible. Compared with cpTi, the amorphous and crystalline tantalum oxide films did not increase bacterial adhesion (P >.05). By tailoring the deposition parameters, we synthetized a crystalline β-Ta 2 O 5 coating that improved titanium surface properties and positively affected cell spreading and morphology, making it a promising surface treatment for titanium-based implants.

© 2018 Elsevier B.V.Amorphous carbon thin films were grown by magnetron sputtering on crystalline Si and fluorine-doped tin oxide (FTO) substrates and annealed at 600 °C in vacuum. Subsequently, the films were subjected to SF6 plasma nanotexturing processes using different secondary gases such as H2, O2, and Ar. The samples were characterized by profilometry, atomic force microscopy, Raman spectroscopy, FTIR spectroscopy, and contact angle measurement. The samples have shown nanostructured surface patterns that strongly depended on the secondary plasma gas used (H2, O2, or Ar); the films subjected to the SF6 + Ar plasma nanotexturing presented the highest surface roughness. Investigation of the atomic structure indicated that the annealing process is responsible for a significant increase in sp2 hybridization of C. The contact angle measurement results have revealed the hydrophilic to hydrophobic transition owing to the annealing process. In contrast, super hydrophilic behavior was observed after the plasma nanotexturing processes. The increase in both the surface roughness and hydrophilicity of these nanotextured carbon thin films are highly desirable characteristics for their application as counter-electrodes in dye-sensitized solar cells and batteries.

© 2018 Author(s).The influence of the reactive magnetron sputtering deposition power on determining the stoichiometry and structure of cobalt oxide polycrystalline films is investigated using experimental and simulated data. Direct current discharges with powers in the 80 - 240 W range are tested using a metallic Co target and an Ar + O 2 plasma. X-ray diffraction results show that lower deposition powers favor the spinel C o 3 O 4 phase, while higher powers produce films presenting the rocksalt CoO phase. Computer simulations indicate that lower power processes occur in the poisoned target regime, while higher power depositions favor the metallic target regime. Consistent with the simulations, oxygen optical emissions (O I = 777.3 m) from the plasma show a significant decrease while the cobalt emissions (e.g., the C o I = 340.5 nm line) are significantly increased when the deposition power is increased. The results show that the film stoichiometry and structure are directly related to the deposition power, at constant O 2 flow.

© 2018, Springer-Verlag GmbH Germany, part of Springer Nature.The authors regret an error in the Experimental section of the published article:

© 2017, Springer-Verlag GmbH Germany.Plasma-treated carbon thin films are investigated as counter electrodes for dye-sensitized solar cells. The films were grown onto fluorine-doped tin oxide (FTO) substrates by magnetron sputtering using pure graphite target and argon atmosphere and subsequently annealed at 600 °C for 30 min in vacuum. These films were then submitted to a plasma texturing process in a reactive ion etching reactor using three different gas combinations: sulfur hexafluoride/argon (SF6 + Ar), sulfur hexafluoride/hydrogen (SF6 + H2), and sulfur hexafluoride/oxygen (SF6 + O2). The morphology and structure of the obtained films were characterized by scanning electron microscopy and Raman spectroscopy. Cyclic voltammetry technique allowed accessing the improvements in their catalytic properties, while the photocurrent-voltage curves under simulated solar illumination AM 1.5G (100 mW/cm2) evaluated the performance of the respective assembled solar cells. The results show that photovoltaic performance is significantly affected by the different plasma texturing conditions used. The carbon counter electrode obtained after SF6 + O2 plasma texturing achieved the best power conversion efficiency of 2.23%, which is comparable to the 2.31% obtained using the commercial platinum counter electrode.

© 2017 Elsevier B.V.The effect of an external magnetic field on electrical impedance was measured on tris-(8-hydroxyquinoline) aluminum (Alq3) based OLEDs at different temperatures. Magnetic field effects (MFEs) were responsible for significant changes on the real and imaginary components of the impedance, and for the intensification of the negative capacitance (NC) effect. The observed MFEs do not present a strong temperature dependence. Simulations via equivalent circuits and numerical solutions of Boltzmann transport equations in a drift-diffusion approximation and employing small sinusoidal signal analysis indicate that such effects are consistent with an enhancement of the carrier mobilities and a quenching of the recombination rates. Such changes lead to reduced resistance and more intense NC effects on the device. The results were interpreted in terms of the currently accepted OMAR models: electron-hole pair model, triplet-polaron reaction mechanism and bipolaron model.

© 2016 Elsevier B.V. All rights reserved.Since Grätzel and O'Regan started in 1991, dye-sensitized solar cells (DSSC) have been extensively studied around the world. In addition to increasing efficiency, their characteristics such as low cost materials and inexpensive manufacturing processes are attractive for organic solar cells. Several parts of DSSC devices are being researched such as semiconductor engineering, low cost counter electrodes, electrolytes, and dyes. In this work, platinum (Pt) thin films were deposited by sputtering technique to produce counter electrodes for DSSC. The films were characterized by profilometry, elipsometry, four-point probe sheet resistance, spectrophotometry, and electrochemical impedance spectroscopy. The electrode response was also compared to that built from a commercial platinum solution. The results allow us to determine the minimum Pt film thickness necessary to achieve a relevant reduction of the sheet resistance and charge transfer resistance, which preserve a significant electrode transparency. The 22 nm and 24.8 nm thick films combined low charge transfer resistance and good transparency. The 122 nm Pt film presented the lowest charge transfer resistance.

© 2015 Elsevier B.V.This paper presents a study about deposition parameters with tribological and mechanical properties of a-SiC:H films deposited by PECVD on titanium alloy (Ti-6Al-4V). HMDSO and TMS were used as silicon, carbon, and hydrogen precursors. The deposition temperature and pressure ranged from 400 to 600. °C and from 0.5 to 3.0. Torr, respectively. The chemical composition and structural properties of the contained samples were analyzed by XRD, FT-IR, and Raman spectroscopy. The mechanical and tribological properties were evaluated by scratching, nanohardness, friction, and wear tests. The results showed that the films presented amorphous structures, and those obtained with 500. °C and 3.0. Torr presented high adhesion and tribological performance. The films grown by using only HMDSO as precursor at 500. °C showed the highest deposition rate, higher hardness, good adhesion, and less friction coefficient values.

© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.This paper investigates the geometrical, electronic, and optical properties of a Ga24N24H46 nanoparticle using Density Functional Theory (DFT). The results show that this nanoparticle maintains geometrical parameters very similar to those of the GaN crystal, although it was noticed that the bond length along the direction [0001] of the Ga24N24H46 nanoparticle is smaller than those of the base of the tetrahedron, which is the opposite of what occurs in the crystal. The bandgap of the passivated nanoparticle calculated with DFT is greater than that of the crystal, while an estimate for the hydrogen-free Ga24N24 structure shows a much lower bandgap, in accordance with the literature. The simulation of the optical absorption spectra via Time-Dependent DFT allowed the association of the spatial shape of electronic orbitals with particular transition energies. The highest occupied (HOMO) and lowest unoccupied (LUMO) electronic levels are located on the (0001) and (000-1) surfaces of the particle, respectively, showing that the passivation of GaN nanoparticles should maintain its known photocatalytic activity, and that transition probability between those surface states is relatively low as compared to the HOMO-4 and LUMO transitions at 4.16eV. Results are compared with the available experimental data.

© 2015 Wroclaw University of Technology.Dynamical mechanical analysis yields information about the mechanical properties of a material as a function of deforming factors, such as temperature, oscillating stress and strain amplitudes. GaAs and Mn-doped GaAs at varied levels, used in making electronic devices, suffer from damage due to changes in environmental temperatures. This is a defective factor experienced during winter and summer seasons. Hence, there was a need to establish the best amount of manganese to be doped in GaAs so as to obtain a mechanically stable spin injector material to make electronic devices. Mechanical properties of Ga1-xMnxAs spin injector were studied in relation to temperatures above room temperature (25 °C). Here, creep compliance, Young's moduli and creep recovery for all studied samples with different manganese doping levels (MDLs) were determined using DMA 2980 Instrument from TA instruments Inc. The study was conducted using displace-recover programme on DMA creep mode with a single cantilever clamp. The samples were prepared using RF sputtering techniques. From the creep compliance study it was found that MDL of 10 % was appropriate at 30 °C and 40 °C. The data obtained can be useful to the spintronic and electronic device engineers in designing the appropriate devices to use at 30 °C and above or equal to 40 °C.

The spin injector part of spintronic FET and diodes suffers from fatigue due to rising heat on the depletion layer. In this study the stiffness of Ga1-xMnxAs spin injector in terms of storage modulus with respect to a varying temperature, 45 C≤T≤70 C was determined. It was observed that the storage modulus for MDLs (Manganese Doping Levels) of 0%, 1% and 10% decreased with increase in temperature while that with MDLs of 20% and 50% increase with increase in temperature. MDLs of 20% and 50% appear not to allow for damping but MDLs ≤20% allow damping at temperature range of 45 C≤T≤70 C. The magnitude of storage moduli of GaAs is smaller than that for ferromagnetic Ga1-xMnxAs systems. The loss moduli for GaAs were found to reduce with increase in temperature. Its magnitude of reducing gradient is smaller than Ga1-xMnxAs systems. The two temperature extremes show a general reduction in loss moduli for different MDLs at the study temperature range. From damping factor analysis, damping factors for ferromagnetic Ga1-xMnxAs was found to increase with decrease in MDLs contrary to GaAs which recorded the largest damping factor at 45 C≤T≤70 C. Hence, MDL of 20% shows little damping followed by 50% while MDL of 0% has the most damping in an increasing trend with temperature. © 2013 Elsevier Ltd.

The physics of electrically detected magnetic resonance (EDMR) quadrature spectra is investigated. An equivalent circuit model is proposed in order to retrieve crucial information in a variety of different situations. This model allows the discrimination and determination of spectroscopic parameters associated to distinct resonant spin lines responsible for the total signal. The model considers not just the electrical response of the sample but also features of the measuring circuit and their influence on the resulting spectral lines. As a consequence, from our model, it is possible to separate different regimes, which depend basically on the modulation frequency and the RC constant of the circuit. In what is called the high frequency regime, it is shown that the sign of the signal can be determined. Recent EDMR spectra from Alq 3 based organic light emitting diodes, as well as from a-Si:H reported in the literature, were successfully fitted by the model. Accurate values of g-factor and linewidth of the resonant lines were obtained. © 2014 AIP Publishing LLC.

The evolution of the optical branch in the Raman spectra of (Ga,Mn)N:Mg epitaxial layers as a function of the Mn and Mg concentrations, reveals the interplay between the two dopants. We demonstrate that the various Mn-Mg-induced vibrational modes can be understood in the picture of functional Mn-Mg k complexes formed when substitutional Mn cations are bound to k substitutional Mg through nitrogen atoms, the number of ligands k being driven by the ratio between the Mg and the Mn concentrations. © 2013 © 2013 Author(s).

The combined effects of substrate temperature, substrate orientation, and energetic particle impingement on the structure of GaN films grown by reactive radio-frequency magnetron sputtering are investigated. Monte-Carlo based simulations are employed to analyze the energies of the species generated in the plasma and colliding with the growing surface. Polycrystalline films grown at temperatures ranging from 500 to 1000 °C clearly showed a dependence of orientation texture and surface morphology on substrate orientation (c- and a-plane sapphire) in which the (0001) GaN planes were parallel to the substrate surface. A large increase in interplanar spacing associated with the increase in both a- and c-parameters of the hexagonal lattice and a redshift of the optical bandgap were observed at substrate temperatures higher than 600 °C. The results showed that the tensile stresses produced during the film's growth in high-temperature deposition ranges were much larger than the expected compressive stresses caused by the difference in the thermal expansion coefficients of the film and substrate in the cool-down process after the film growth. The best films were deposited at 500 °C, 30 W and 600 °C, 45 W, which corresponds to conditions where the out diffusion from the film is low. Under these conditions the benefits of the temperature increase because of the decrease in defect density are greater than the problems caused by the strongly strained lattice that occurr at higher temperatures. The results are useful to the analysis of the growth conditions of GaN films by reactive sputtering. © 2013 AIP Publishing LLC.

The understanding and control of ferromagnetism in diluted magnetic semiconducting oxides (DMO) is a special challenge in solid-state physics and materials science due to its impact in magneto-optical devices and spintronics. Several studies and mechanisms have been proposed to explain intrinsic ferromagnetism in DMO compounds since the theoretical prediction of room-temperature ferromagnetism. However, genuine and intrinsic ferromagnetism in 3d-transition metal-doped n-type ZnO semiconductors is still a controversial issue. Furthermore, for DMO nanoparticles, some special physical and chemical effects may also play a role. In this contribution, structural and magnetic properties of sonochemically prepared cobalt-doped ZnO nanoparticles were investigated. A set of ZnO samples was prepared varying cobalt molar concentration and time of ultrasonic exposure. The obtained results showed that single phase samples can be obtained by the sonochemical method. However, cobalt nanoclusters can be detected depending on synthesis conditions. Magnetic measurements indicated a possible ferromagnetic response, associated to defects and cobalt substitutions at the zinc site by cobalt. However, ferromagnetism is depleted at higher magnetic fields. Also, an antiferromagnetic response is detected due to cobalt oxide cluster at high cobalt molar concentrations. © 2012 Springer Science+Business Media, LLC.

The magnetic characteristics of Ga1-xMnxN nanocrystalline films (x = 0.08 and x = 0.18), grown by reactive sputtering onto amorphous silica substrates (a-SiO2), are shown. Further than the dominant paramagnetic-like behaviour, both field- and temperature-dependent magnetization curves presented some particular features indicating the presence of secondary magnetic phases. A simple and qualitative analysis based on the Brillouin function assisted the interpretation of these secondary magnetic contributions, which were tentatively attributed to antiferromagnetic and ferromagnetic phases. © 2012 Elsevier Masson SAS. All rights reserved.

Ga1-xMnxN (0≤x≤0.18) films grown onto amorphous silica substrate by reactive sputtering are characterised by high resolution transmission electron microscopy, energy dispersive spectroscopy, and energy filtered transmission electron microscopy. The electron transmission images and the electron diffraction patterns evidence the presence, at the substratefilm interface, of a few tens of nm thick intermediate layer with a high density of non-oriented nanocrystals (NCs). This intermediate layer represents the nucleation site for the subsequent growth of a compact Ga1-xMn xN columnar nanostructure, whose thickness (600900 nm) is only limited by the deposition time. The columnar region shows a fibre texture with the c axis of the wurtzite nanocrystals corresponding to the column axis, both disposed perpendicular to the film surface. The thickness of the initial NC-rich layer and the coalescence of the nanocolumns are found to have a systematic dependence on the Mn concentration. No evidence of Mn segregation or of Mn rich phases is observed even for the samples with the highest Mn concentration. The correlation between the observed film microstructure and the reactive sputtering deposition parameters is discussed. © 2011 Elsevier B.V. All rights reserved.

Periodic slab calculations based on density functional theory were performed at the B3LYP level to gain insight into the surfaces of wurtzite GaN nanostructures. The (101̄0) and (112̄0) GaN surfaces are the most thermodynamically stable surfaces, the energy of the former being slightly smaller than that of the latter. The thermodynamic stability associated with the equilibrium shape of nanowires was determined using the calculated values. Doping with Mn further decreases the surface energy of (101̄0) and (112̄0). The minimum surface energy of Ga1 - xMnxN (0.04 ≤ x ≤ 0.17) is found at x ∼ 0.08, for (101̄0) and (112̄0) slab models. Substitution of Ga with Mn in different positions relative to the surface shows that the total energy increases as the Mn atoms move from the surface layer to the interior sites of the slabs. Mn doping is also responsible for decreases in the band gap energy: the minimum calculated band gap in the Ga1 - xMnxN (101̄0) slab was found at x ∼ 0.17, whereas the (112̄0) surface presented the corresponding minimum at x ∼ 0.04. The magnetic moments associated with Mn were observed to increase as the ion positions moved closer to the surfaces. © 2011 Elsevier B.V. All rights reserved.

A computational method to simulate the changes in the electronic structure of Ga1-xMnxN was performed in order to improve the understanding of the indirect contribution of Mn atoms. This periodic quantum-mechanical method is based on density functional theory at B3LYP level. The electronic structures are compared with experimental data of the absorption edge of the GaMnN. It was observed that the indirect influence of Mn through the structural parameters can account for the main part of the band gap variation for materials in the diluted regime (x<0.08), and is still significant for higher compositions (x∼0.18).

The effect of manganese on the vibrational properties of Ga 1-xMnxN (0≤x≤0.18) films has been investigated by Raman scattering using 488.0 and 632.8 nm photon excitations. The first-order transverse and longitudinal optical GaN vibrational bands were observed in the whole composition range using both excitations, while the corresponding overtones, as well as a prominent peak located in 1238 cm-1 (153.5 meV) were only observed in the Mn-containing films under 488.0 nm excitation. We propose that the peak observed at 1238 cm-1 is due to resonant Mn local vibrational modes, the excitation process being related to electronic transitions involving the Mn acceptor band. © 2008 IOP Publishing Ltd.

We have focused on the optical absorption edge of nanocrystalline Ga 1-xMnxN (0.00≤x≤0.18) films deposited by reactive RF magnetron sputtering. The films obtained are nanocrystalline with grain sizes of about 25 nm, having wurtzite structure and strong orientation texture in the c-axis direction. The optical characterizations of the absorption edges were obtained in the 190-2600 nm spectral range. The increase of the Mn content causes an increase of the absorption coefficient which can be clearly noticed at low energies, and a quasi-linear decrease of the optical gap. Broad absorption bands observed around ∼1.3 and ∼2.2 eV were associated with transitions between the Mn acceptor level and the valence and conduction bands, respectively. The observed changes in the optical properties due to the Mn incorporation observed in these nanocrystalline films are similar to those reported for ferromagnetic GaMnN single-crystal films. © 2008 IOP Publishing Ltd.

The structural and vibrational properties of nanocrystalline Ga1-x Mnx N films deposited by reactive magnetron sputtering were analyzed in a wide composition range (0<x<0.18). The films were structurally characterized using x-ray diffraction with Rietveld refinement. The corresponding vibrational properties were investigated using micro-Raman and Fourier transform infrared spectroscopies. The films present a high crystallized fraction, crystallites having wurtzite structure, and high orientation texture with the c axis oriented perpendicular to the substrate surface. Rietveld analysis indicates that Mn atoms are incorporated substitutionally into Ga positions and show that the ionic character of cation- N bonds along the c axis is favored by the Mn incorporation. No evidence for Mn segregation or Mn rich phases was found in the composition range analyzed. Micro-Raman scattering spectra and infrared absorption experiments showed progressive changes with the increase of x and monotonic shifts of the GaN TO and LO peaks to lower frequencies. The structural and vibrational analyses are compared and the influence of Mn on the static and dynamic properties of the lattice is analyzed. © 2007 American Institute of Physics.

The growth of nanocrystalline Ga1-xMnxN (0.00≤x≤0.18) films grown by reactive RF-magnetron sputtering is focused here for the first time. The films were grown in a N2 atmosphere by co-sputtering technique using a Ga target covered with small pieces of Mn onto c-GaAs (1 0 0), c-Si (1 0 0) and amorphous SiO2 substrates maintained at 500 K. Scanning electron microscopy and X-ray diffraction (XRD) experiments did not show any evidence for Mn segregation within the studied composition range. EDX measurements show that the Mn concentration is increased monotonically with the fraction of the target area covered by Mn. The XRD characterization show that the films are nanocrystalline, the crystallites having mean grain sizes in the 15-19 nm range and wurtzite structure with preferential growth orientation along the c-axis direction. The lattice parameters of α-GaN (a and c) increase practically linearly with the increase of Mn incorporation. The changes in the structural properties of our films due to the Mn incorporation are similar to those that occur in ferromagnetic GaMnN single-crystal films. © 2006 Elsevier B.V. All rights reserved.

The structural and optical properties of nanocrystalline GaN and GaN:H films grown by RF-tnagnetron sputtering are focused here. The films were grown using a Ga target and a variety of deposition parameters (N2/H 2/Arflow rates, RF power, and substrate temperatures). Si (100) and fused silica substrates were used at relatively low temperatures (Ts ≤ 420K). The main effects resulting from the deposition parameters variations on the films properties were related to the presence of hydrogen in the plasma. The X-ray diffraction analysis indicates that the grain sizes (∼15nm) and the crystallized volume fraction significantly decrease when hydrogen is present in the plasma. The optical absorption experiments indicate that the hydrogenated films have absorption edges very similar to that of GaN single crystal films reported in the literature, while the non-hydrogenated samples present larger absorption tails encroaching into the gap energies.

We study the relationship between the optical gap and the optical-absorption tail breadth for the case of amorphous gallium arsenide (a-GaAs). In particular, we analyze the optical-absorption spectra corresponding to some recently prepared a-GaAs samples. The optical gap and the optical-absorption tail breadth corresponding to each sample is determined. Plotting the optical gap as a function of the corresponding optical-absorption tail breadth, we note that a trend, similar to that found for the cases of the hydrogenated amorphous silicon and hydrogenated amorphous germanium, is also found for the case of a-GaAs. The impact of alloying on the optical-absorption spectrum associated with a-GaAs is also briefly examined. © 2004 American Institute of Physics.

Both narrow and broad photoluminescence bands were observed in Ga 1-XAsX films prepared by flash evaporation of polycrystalline GaAs containing native C impurities. The observed narrow crystalline-like bands are similar to band-to-band and C acceptor impurity emissions in crystalline GaAs. The narrow bands are evidence that the As excess favors the PL active GaAs crystallite formation in films deposited onto silicon (100) substrate, even when the As excess is very large (X=0.84). This favoring is not observed in twin samples grown on silica glass substrates nor on Ga rich samples, indicating the important role of the combined effect of the As excess and Si substrate in the GaAs crystallite formation. The broad amorphous-like bands were observed in Ga rich and in moderately As rich samples. The photoluminescence emission is compared with the microstructure of the material as determined from the micro-Raman, absorption edge and reflectance measurements. The volume fraction of the crystallites formed is small and PL emission indicates that the crystallite electronic quality is much better than the ones formed heat treating films grown on silica glass substrates. © 2004 Elsevier B.V. All rights reserved.