Thin film processing and multiferroic properties of Fe-BaTiO_3 hybrid composite

Multiferroic bi-layer Fe/BaTiO3 (BTO) thin films were successfully deposited on Pt(200)/MgO(100) substrates using ion beam sputter deposition (IBSD), and the mutiferroic properties were studied at room temperature. X-ray diffraction (XRD) analyses showed that BTO films were c-axis oriented and epitaxially grown on platinum coated MgO substrates, and (110) epitaxial Fe films were subsequently grown on (001) BTO films. Fe/BTO bi-layer films showed good ferroelectric and ferromagnetic properties at room temperature and the multiferroic coupling was observed, which should be attributed to the hybridization of Fe and Ti occurring at the ferromagnetic-ferroelectric interface.     

Stress and defect induced enhanced low field magnetoresistance and dielectric constant in La0.7Sr0.3MnO3 thin films

Colossal magnetoresistive manganite La_0.7Sr_0.3MnO₃ (LSMO) films were prepared by pulsed laser deposition on three different single crystal substrates using different deposition parameters. Characterizations of their surface morphologies, structural, magnetic and magneto-transport properties show that films on MgO single crystal substrates contain higher amount of structural defects compared to those on SrTiO₃ (STO) and NdGaO₃ (NGO) substrates. Low deposition rate and thicker films give rise to polycrystallinity and grain boundaries. The films on MgO substrate showed a broad paramagnetic (PM) to ferromagnetic (FM) transition accompanied with metal-insulator transition (MIT) much below their Curie temperature (T_C) indicating growth of strained structures due to large lattice mismatch (9%) between the substrate and the film. The deposited films on STO and NGO show least effect of substrate induced strain exhibiting sharper PM-FM transition and metallic behavior below T_C. The magnetoresistance (MR) measured with 300 mT field clearly shows two contributions, one due to grain boundary tunneling and the other due to colossal MR effect. The highest low field MR effect of 17% was achieved for the film on MgO with the highest thickness and surface roughness indicating the presence of grain boundary related defects. Also a high dielectric constant was observed for the same film at room temperature up to 100 kHz frequency. Coexistence of defect induced large low-field MR and abnormally high dielectric constant can give rise to different exciting applications.     

Electric and dielectric behavior of CaCu3Ti4O12-based thin films obtained by soft chemical method

CaCu₃Ti₄O₁₂ (stoichiometric) and Ca_1.1Cu_2.9Ti₄O₁₂ (non-stoichiometric) thin films have been prepared by the soft chemical method on Pt/Ti/SiO₂/Si substrates, and their electrical and dielectric properties have been compared as a function of the annealing temperature. The crystalline structure and the surface morphology of the films were markedly affected by the annealing temperature and excess calcium. The films show frequency-independent dielectric properties at room temperature which is similar to those properties obtained in single-crystal or epitaxial thin films. The room temperature dielectric constant of the 570-nm-thick CCTO thin films annealed at 600 °C at 10 kHz was found to be 124. The best non-ohmic behavior (α = 12.6) presented by the film with excess calcium annealed at 500 °C. Resistive hysteresis on the I-V curves was observed which indicates these films can be used in resistance random access memory (ReRAM).     

Synthesis-structure relations for reactive magnetron sputtered V2O5 films

V₂O₅ films were grown onto MgO (100) substrates by reactive magnetron sputtering between 26 °C to 300 °C to establish a detailed synthesis-structure relation. The effect of deposition temperature on structural characteristics and surface morphology was characterized using X-ray diffraction, Raman spectroscopy, atomic force microscopy and scanning and transmission electron microscopy. Films prepared at room temperature are amorphous while those deposited above 80 °C exhibit a polycrystalline structure with the orthorhombic symmetry of the V₂O₅ phase.     

Formation of oxidation-resistant Cu-Mg coatings on (001) Cu for oxide superconducting tapes

The formation of oxidation-resistant buffer layers on (001) oriented Cu for coated high-temperature superconducting tape applications was investigated. The approach employed Cu/Mg multilayer precursor films that were subsequently annealed to form either Mg-doped fcc Cu or intermetallic Cu₂ Mg. The precursor consisted of an Mg/Cu multilayer stack with 5 each of 25 nm thick Mg and 25 nm thick Cu layers which were grown at room temperature by sputter deposition. At annealing temperature of 400 °C, formation of the intermetallic Cu₂ Mg was observed. X-ray diffraction showed that the Cu₂ Mg (100) oriented grains were epitaxial with respect to the underlying Cu film, possessing a cube-on-cube orientation. In order to test oxidation resistance, CeO₂ films were deposited at elevated temperature on Ni/(Cu,Mg)/Cu/MgO structures. In case of the CeO₂ film on Ni/Cu/MgO, significant surface roughness due to the metal oxidation is observed. In contrast, no surface roughness is observed in the SEM images for the CeO₂/Ni/(Cu,Mg)/Cu/MgO structure.     

Magnetic properties and their dependence on deposition parameters of Co/Al2O3 multilayers grown by pulsed laser deposition

Co/Al₂O₃ multilayered thin films were grown on Si (111) substrates by pulsed laser deposition (PLD) at temperatures from room temperature (RT) to 600 °C. The Co/Al₂O₃ multilayered thin film grown at RT contains continuous cobalt layers in alumina matrices, with no evidence of island formation. On the other hand, cobalt showed a tendency to form islands in alumina matrices for growth temperatures in the range of 300–600 °C. All the Co/Al₂O₃ multilayered thin films showed ferromagnetic behavior up to RT. It was observed that variations in the deposition parameters can significantly influence the magnetic properties of Co/Al₂O₃ multilayers. Depending on the temperature and pulse rate, RT coercivities in the 50–300 Oe range were observed. Films deposited at 600 °C using a laser pulse rate of 10 Hz exhibited a decrease of coercivity with increasing measurement temperature. On the other hand, films deposited at 600 °C using a reduced pulse rate of 2 Hz demonstrated an “anomalous” relationship between low-temperature coercivity and temperature. In these films, coercivity exhibited a weak tendency to increase with temperature. Squareness (M_r/M_s) of the hysteresis loops and its dependence on the temperature was also shown to be strongly affected by the deposition parameters. These observations have been rationalized on the basis of two competing magnetic anisotropies that act along different directions in the material.     

Structural and optical properties of epitaxial ZnO thin films on 4H–SiC (0 0 0 1) substrates prepared by pulsed laser deposition

Epitaxially grown ZnO thin films on 4H–SiC (0 0 0 1) substrates were prepared by using a pulsed laser deposition (PLD) technique at various substrate temperatures from room temperature to 600 °C. The crystallinity, in-plane relationship, surface morphology and optical properties of the ZnO films were investigated by X-ray diffraction (XRD), atomic force microscopy (AFM) and photoluminescence (PL) measurements, respectively. XRD analysis showed that highly c-axis oriented ZnO films were grown epitaxially on 4H–SiC (0 0 0 1) with no lattice rotation at all substrate temperatures, unlike on other hexagonal-structured substrates, due to the very small lattice mismatch between ZnO and 4H–SiC of ～5.49%. Further characterization showed that the substrate temperature has a great influence on the properties of the ZnO films on 4H–SiC substrates. The crystalline quality of the films was improved, and surfaces became denser and smoother as the substrate temperature increased. The temperature-dependent PL measurements revealed the strong near-band-edge (NBE) ultraviolet (UV) emission and the weak deep-level (DL) blue-green band emission at a substrate temperature of 400 °C.     

Influence of laser treatment on the electrical properties of plasma-enhanced-atomic-layer-deposited TiO2 thin films

TiO₂ dielectric films with 38 nm thickness were grown on Si (100) substrates at 200°C by plasma-enhancedatomic-layer deposition. Laser-irradiated TiO₂ films maintained an amorphous phase similar to as-grown films and showed an increase in permittivity and leakage current density with increasing laser powers and the number of laser shots at constant laser power. Laser-irradiation of TiO₂ films at room temperature produced oxygen vacancies at the film surface and new Ti³⁻ valences. The electrons and space charges produced through the defect chemistry increased the leakage current density and permittivity in laser-irradiated TiO₂ films, respectively. The dielectric and electrical properties of the laser-irradiated TiO₂ films were completely recovered to correspond with those of as-grown films by post-annealing at 300°C for 5 min in O₂ ambient.     

Effects of H<Superscript>+</Superscript> Ion Implantation and Annealing on the Properties of CuIn<Subscript>0.75</Subscript>Ga<Subscript>0.25</Subscript>Se<Subscript>2</Subscript> Thin Films

In this paper the effects of post-deposition annealing followed by hydrogen ion-implantation on the properties of CuIn_0.75Ga_0.25Se₂ thin films have been investigated. The samples were grown by flash evaporation onto glass substrates heated at temperature between room temperature and 200 °C. Selected samples were subsequently processed under several sets of conditions, including vacuum, selenium, inert (argon) and forming gas (a 9:1 mixture of N₂:H₂) followed by hydrogen ion-implantation. A high-resolution near-infrared photoacoustic spectrometer (PAS) of the gas-microphone type was used for room temperature analysis of non-radiative defect levels in the as-grown, annealed and hydrogen implanted thin films. The absorption coefficient has been derived from the PA spectra to determine the gap energy and to establish the activation energies for several defect-related energy levels. The changes observed in the PA spectra following annealing and ion-implantation has been directly correlated with the compositional and structural properties of the samples.     

Micro/nanomechanical properties of aluminum-doped zinc oxide films prepared by radio frequency magnetron sputtering

Aluminum-doped zinc oxide (ZnO) was grown on glass substrates by using RF magnetron sputtering. In order to investigate the effect of growth temperature on the mechanical properties of Al-doped ZnO films, the temperature of the substrates during deposition was controlled at room temperature (R.T.), 150 °C, and 300 °C. The crystal structure and topography of the deposited films were investigated by X-ray diffraction (XRD) and Atomic Force Microscopy (AFM). The mechanical properties of films were measured by using nanoindentation and micro-reciprocating pin-on-plate tester to characterize the hardness, modulus, and tribological behavior. The tribological behavior of silicon (100) wafer was also obtained to compare with that of the Al-doped ZnO. It was found that Al-doped ZnO films with (002) oriented plane was favored at high growth temperature. The mechanical properties of the films were significantly affected by growth temperature. The film grown at room temperature showed a relatively low friction coefficient of 0.25 and high wear resistance.     

Magnetooptical and magnetic properties of Fe/ZnTe/Fe nanoheterostructures: Manifestation of interlayer exchange coupling

Magnetooptical and magnetic properties of Fe/ZnTe bilayers and Fe/ZnTe/Fe sandwiches with a variable thickness of the semiconductor layer t _ZnTe = 6–24 Å prepared by magnetron sputtering on Si(100) substrates at room temperature have been studied. In the sandwiches, two ranges of thicknesses, t _ZnTe = 8–12 and 20–23 Å, were found at which the magnitude of the magnetorefractive effect (MRE) approximately twofold exceeds the magnitude of the MRE in the bilayers. In the field dependences of the transverse Kerr effect at ZnTe thicknesses corresponding to the greatest values of the MRE, two features have been found in the form of kinks (steps) which are absent in the case of bilayers. At the same thicknesses of the semiconductor layer, the magnetic measurements of trilayers revealed a sharp decrease in the imaginary component of the ac susceptibility with increasing temperature from 10 to 300 K. A conclusion is drawn on the appearance of a weak interlayer exchange coupling of the antiferromagnetic type in the Fe/ZnTe/Fe system with increasing temperature.     

Preparation and dielectric properties of compositionally graded （Ba, Sr）TiO3 thin film by sol-gel technique

Compositional graded BaxSr1-xTiO3 （x=0.6, 0.7, 0.8, 0.9, 1.0） （BST） thin films （less than 400 nm） were fabricated on Si and Pt/Ti/SiO2/Si substrates by sol-gel technique. A special heating treatment was employed to form the uniform composition gradients at 700 ℃. The microstructures of the films were studied by means of X-ray diffraction, atomic force microscope and field emission scanning electron microscopy. The results show that the films have uniform and crack-free surface morphology with perovskite structure phase. The small signal dielectric constant （εr） and dielectric loss （tanδ） are found to be 335 and 0.045 at room temperature and 200 kHz. The dielectric properties change significantly with applied dc bias, and the graded thin film show high tunability of 42.3% at an applied field of 250 kV/cm. All the results indicate that the graded BST thin films prepared by sol-gel technique have a promising candidate for microelectronic device.     

VO<Subscript>2</Subscript> thin films: Defect mediation in room temperature ferromagnetic switching characteristics

Vanadium dioxide (VO₂) has tremendous potential in multifunctional device applications related to spintronics, switching, and magnetic recording. We have discovered that the room temperature ferromagnetism (RTFM) in undoped vanadium oxide epitaxial films can be switched on and off by altering the cooling ambient conditions which exhibit a sharp electrical transition at 341 K. By correlating the structural and ferromagnetic properties in VO₂, we envisage the potential for creation of novel multifunctional solid-state devices. High-quality epitaxial VO₂ thin films were grown on c-sapphire (0001) substrates, under different ambient conditions via the domain matching epitaxy paradigm. The observed RTFM has its origin in the valence charge defects with unpaired electrons in V⁺³ in VO₂ thin films, where the concentration of the defects could be varied with oxygen partial pressure. The VO₂ films-with a high ferro- to paramagnetic transition (Curie) temperature around 500 K estimated by fitting the magnetization data to the Bloch’s T^3/2 law, a saturated magnetization of 18 emu/cm³, and with a finite coercivity of 40 Oe at 300 K-can be useful for integrated smart sensors operable at room temperature.     

Structural and optical properties of electrodeposited ZnO thin films on conductive RuO2 oxides

ZnO thin films were grown on the 150 nm-thick RuO₂-coated SiO₂/Si substrates by electrochemical deposition in zinc nitrate aqueous solution with various electrolyte concentrations and deposition currents. Crystal orientation and surface structure of the electrodeposited ZnO thin films were characterized by X-ray diffraction (XRD) and scanning electron microscopy, respectively. The XRD results show the as-electrodeposited ZnO thin films on the RuO₂/SiO₂/Si substrates have mixed crystallographic orientations. The higher electrolyte concentration results in the ZnO thin films with a higher degree of c-axis orientation. Moreover, the use of an ultra-thin 5 nm-thick ZnO buffer layer on the RuO₂/SiO₂/Si substrate markedly improves the degree of preferential c-axis orientation of the electrodeposited ZnO crystalline. The subsequent annealing in vacuum at a low temperature of 300 °C reduces the possible hydrate species in the electrodeposited films. The electrodeposited ZnO thin films on the 5 nm-thick ZnO buffered RuO₂/SiO₂/Si substrates grown in 0.02 M electrolyte at −1.5 mA with a subsequent annealing in vacuum at 300 °C had the best structural and optical properties. The UV to visible emission intensity ratio of the film can reach 7.62.     

Influence of substrate temperature on the preparation of GaP thin films

Gallium phosphide (GaP) was prepared by treating specpure gallium with AnalaR grade Zn₃P₂ in an argon atmosphere. Thin films of GaP were evaporated onto glass substrates at various substrate temperatures under vacuum. Films evaporated onto substrates at and above 250°C were polycrystalline in nature. Films of various thicknesses were grown. The conductivity of these films in the dark and in white light is presented for the temperature range 100–300 K. Optical absorption spectra for films grown at substrate temperatures T_s = 90, 180 and 250°C were also recorded in the range 0.5–2.5 eV. All the features are attributed to the structural disorder which probably occurs during evaporation onto the glass substrates.     

A mixed-conduction model for oxide films on Fe, Cr and Fe-Cr alloys in high-temperature aqueous electrolytes—II. Adaptation and justification of the model

The aim of this two-part work is to propose a model for the corrosion mechanism of ferrous alloys in high-temperature aqueous environments. In this second part, the modifications to the mixed-conduction model (MCM) are discussed on the basis of experimental data presented in the first part for Fe, Cr and two Fe-Cr alloys (12 and 25 wt% Cr) in an aqueous solution at 200 °C. Application of the MCM to fit and predict experimental behaviour both at room temperature and at 200 °C is demonstrated. The major difference between the behaviour of films at room temperature and at 200 °C is that the mobility of ionic defects is much higher at the higher temperature. Estimates show that the ratio of the electronic and ionic diffusion coefficients (D_e/D_i) is of the order of 10⁵ at room temperature and ≈30 at 200 °C for pure Fe. Such a large difference explains the higher growth rate and thickness of films formed on Fe at the higher temperature. It is also in agreement with the higher defect content and lower field strengths in high-temperature films. The application of the MCM to Fe-Cr alloys indicates that the diffusion coefficient of major ionic current carriers is smaller for the alloys than for pure Fe. Alloying with Cr thus lowers the ionic mobility in the passive film on a ferrous alloy also at 200 °C.     

Amorphous hafnium oxide thin films for antireflection optical coatings

Hafnium oxide (HfO₂) thin films were grown on silicon and quartz substrates by radio frequency reactive magnetron sputtering at temperature < 52 °C. X-ray diffraction of the films showed no structure, suggesting that the films grown on the substrates are amorphous. The optical properties of these films have been investigated using spectroscopic ellipsometry with wavelength range 200-1400 nm and ultraviolet-visible spectrophotometer techniques. Also, the effects of annealing temperatures on the structure and optical properties of the amorphous HfO₂ (a-HfO₂) have been investigated. The films appeared to be monoclinic structure upon high temperature (1000 °C) annealing as confirmed by X-ray diffraction. The results show that the annealing temperature has a strong effect on the optical properties of a-HfO₂ films. The optical bandgap energy of the as-deposited films is found to be about 5.8 eV and it increases to 5.99 eV after the annealing in Ar gas at 1000 °C. The further study shows that the measurement of the optical properties of the amorphous films reveals a high transmissivity (82%-99%) and very low reflectivity (< 8%) in the visible and near-infrared regions at any angle of incidence. Thus, the amorphous structure yields HfO₂ film of significantly higher transparency than the polycrystalline (68%-83%) and monoclinic (78%-89%) structures. This means that the a-HfO₂ films could be a good candidate for antireflection (AR) optical coatings.     

Structural, optical and magnetic properties of Eu-doped ZnO films

Polycrystalline Zn_1−xEu_xO (x = 0, 0.02, 0.05) films were deposited on silicon (1 0 0) substrates by radio-frequency magnetron sputtering. The structural, optical and magnetic properties of the films were investigated. The results from both the X-ray diffraction and photoluminescence spectra reveal that Eu³⁺ ions successfully substitute for Zn²⁺ ions in the ZnO lattice. The magnetic field and temperature dependence of magnetization curves demonstrate that the Zn_0.95Eu_0.05O films are ferromagnetic at room temperature. No impurity phase was found in Eu-doped films with X-ray diffraction, Raman spectroscopy and zero-field-cooled measurements. The ferromagnetism is attributed to the intrinsic property of Eu-doped ZnO films and could be interpreted by the bound-magnetic-polaron model.     

Structural, electrical and mechanical properties of magnetron sputtered NiTi/PZT/TiOx thin film heterostructures

Shape memory alloy (NiTi) thin films coupled to ferroelectric lead zirconate titanate (PZT) produce an intelligent material capable of performing both sensing and actuating functions. In the present study, we report on the in-situ growth of NiTi/PZT/TiO_x heterostructure on Pt/Ti/SiO₂/Si substrates using magnetron sputtering technique. Deposition processing, microstructure, surface morphology, electrical properties and mechanical properties of these heterostructures were systematically investigated. The top NiTi films exhibit austenitic B2 structure with preferred (110) orientation. The varying thickness of NiTi films had a significant influence on properties of NiTi/PZT/TiO_x heterostructure. The bottom TiO_x layer was observed to favor the growth of perovskite PZT films with (100) orientation. Nanoindentation tests of these heterostructures were performed at room temperature. The mechanical hardness of the top NiTi layer of lower thickness was found to be highly influenced by underneath PZT layer. The heterostructure exhibited an interesting martensite to austenite phase transformation and polarization-electric field hysteresis behavior with remanent polarization (P_r) and the coercive field (E_r) of 17.1 μC/cm² and 69.6 kV/cm, respectively. These heterostructures having a layer of SMA material coupled to a ferroelectric material with underneath TiO_x layer are of immense technological importance for MEMS devices.     

Structural, optical and electrical properties of N-doped ZnO thin films prepared by thermal oxidation of pulsed filtered cathodic vacuum arc deposited ZnxNy films

In this study, N-doped ZnO thin films were fabricated by oxidation of Zn_xN_y films. The Zn_xN_y thin films were deposited on glass substrates by pulsed filtered cathodic vacuum arc deposition (PFCVAD) using metallic zinc wire (99.999%) as a cathode target in pure nitrogen plasma. The influence of oxidation temperature, on the electrical, structural and optical properties of N-doped ZnO films was investigated. P-type conduction was achieved for the N-doped ZnO obtained at 450 °C by oxidation of Zn_xN_y, with a resistivity of 16.1 Ω cm, hole concentration of 2.03 × 10¹⁶ cm⁻³ and Hall mobility of 19 cm²/V s. X-ray photoelectron spectroscopy (XPS) analysis confirmed the incorporation of N into the ZnO films. X-ray diffraction (XRD) pattern showed that the films as-deposited and oxidized at 350 °C were amorphous. However, the oxidized films in air atmosphere at 450-550 °C were polycrystalline without preferential orientation. In room temperature photoluminescence (PL) spectra, an ultraviolet (UV) peak was seen for all the samples. In addition, a broad deep level emission was observed.