Integration of epitaxial permalloy on Si (1 0 0) through domain matching epitaxy paradigm

This paper addresses epitaxial integration of magnetic materials with Si (1 0 0) based solid state devices. Epitaxial Ni_82.5Fe_17.5 (permalloy, Py) thin films have been synthesized by pulsed laser deposition (PLD) on Si (1 0 0) using MgO/TiN as a template buffer. This epitaxial growth of these large lattice misfit systems was achieved through domain matching epitaxy (DME). The in-plane XRD pattern and selective area electron diffraction (SAED) results clearly indicate cube-on-cube epitaxial alignment. The bright field TEM image of Py/MgO/TiN/Si (1 0 0) heterostructure infers a Py layer thickness of ∼30 nm, with a well aligned island (150–200 nm) structure that is consistent with Volmer–Weber type growth. Magnetization data collected at 4 K and 300 K indicates that the easy axis of the magnetization lies in the plane of the Py. In addition, we have observed an intrinsic positive exchange bias (PEB) field of ∼104 Oe, where the magnetic hysteresis loop is shifted toward the positive field axis under zero field cooling conditions.     

Microstructure and magnetic properties of (0 0 1)-oriented L10 FePt films: Role of Ag underlayer and Fe/Pt ratio

FePt multilayer films were deposited on Si(1 0 0) substrate with thermally grown SiO₂ film and sputtered Ag underlayer at room temperature by dc magnetron sputtering and subsequently annealing in vacuum. Experimental results suggest that proper thickness of Ag underlayer and slightly rich of Fe content can effectively induce the (0 0 1) texture of FePt films. A Fe_57.4Pt_42.6 thin film on the 8 nm Ag underlayer exhibits a large perpendicular coercivity of 7.6 kOe with magnetic remanence close to 1.     

Tuning exchange bias in epitaxial Ni/MgO/TiN heterostructures integrated on Si(1 0 0)

Epitaxial Ni thin films are integrated with tunneling barrier MgO on Si(1 0 0) substrate. During pulsed laser deposition, early island-like structure transformed into uniform thin film with increasing number of laser pulses. This led to transitions in exchange bias from positive to negative and back to positive, which is ascribed to morphology associated residual strain. The Ni island structure has a coercive field as high as 3 times of that of the continuous film. The current work holds a tremendous promise in the realization of magnetic devices integrated with the Si-platform.     

A molecular dynamics simulation of TiN film growth on TiN(0 0 1)

The growth of thin TiN films on the TiN(0 0 1) surface during reactive sputtering was simulated by molecular dynamics with the modified embedded-atom method potential. TiN₃ is found to be the smallest epitaxial island and the film grows via the layer mode. Vacancy concentration in the deposited films decreases with increasing the substrate temperature and kinetic energy of incident atoms, resulting from the enhancement of the thermal diffusion and kinetic energy assisted athermal diffusion. To get the stoichiometric TiN film, the N:Ti flux ratio should be larger than unity and be increased with higher incident energy due to the weak adsorption of atomic N on TiN(0 0 1).     

The epitaxial growth of (1 1 1) oriented monocrystalline Si film based on a 4:5 Si-to-SiC atomic lattice matching interface

Due to a huge lattice mismatch of about 20% theoretically existing between SiC and Si, it is difficult for growing monocrystalline Si/SiC heterojunction to realize the light control of SiC devices. However, based on a 4:5 Si-to-SiC atomic lattice matching interface structure, the monocrystalline Si films were epitaxially prepared on the 6H-SiC (0 0 0 1) substrate by hot-wall chemical vapor deposition in our work. The film was characterized by X-ray diffraction analysis with only (1 1 1) orientation occurring. The X-ray rocking curves illustrated good symmetry with a full width at half maximum of 0.4339° omega. A 4:5 Si-to-SiC atomic matching structure of the Si/6H-SiC interface clearly observed by the transmission electron microscope revealed the essence of growing the monocrystalline Si film on the SiC substrate.     

Probing interfacial properties of ferromagnetic/insulator bilayers with X-ray spectroscopies: Application to Fe,Co, Mn/MgO(0 0 1) interfaces

Electronic and magnetic properties of bcc Co, Fe and Mn(0 0 1) epitaxial monolayers in contact with a single-crystalline MgO(0 0 1) film were studied using X-ray photoemission spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) measurements. The XPS and XAS analysis clearly evidenced the weak hybridization between the MgO barrier and Fe or Co. On the contrary, a net oxidization of the Mn layer in contact with the MgO layer was observed. The magnetic properties were characterized by probing the XMCD signal of a unique atomic plane of transition metal in contact with MgO. The total magnetic moment per Co and Fe atoms were observed to increase compared to the bulk at the metal/oxide interface. Finally, Mn at the interface with MgO does not present any ferromagnetic behavior. This was assumed to be a consequence of the Mn oxidization.     

Microstructure and transport properties of [0 0 1]-tilt bicrystal grain boundaries in iron pnictide superconductor,cobalt-doped BaFe2As2

Relationships between microstructure and transport properties of bicrystal grain boundary (BGB) junctions were studied in cobalt-doped BaFe₂As₂ (BaFe₂As₂:Co) epitaxial films grown on [0 0 1]-tilt bicrystal substrates of MgO and (La, Sr)(Al, Ta)O₃ with misorientation angles θ_GB = 3–45°. The θ_GB of BaFe₂As₂:Co BGBs were exactly transferred from those of the bicrystal substrates. No segregation of impurities was detected at the BGB junction interfaces, and the chemical compositions of the BGBs were uniform and the same as those in the bulk film regions. A transition from a strongly-coupled GB behavior to a weak-link behavior was observed in current density–voltage characteristics under self-field around θ_GB ～ 9°. The critical current density decreased from (1.2–1.6) × 10⁶ A/cm² of the intragrain transport to (0.7–1.1) × 10⁵ A/cm² of θ_GB = 45° because supercurrent becomes more governed by Josephson current with increasing θ_GB.     

Epitaxial La2 / 3Sr1 / 3MnO3 thin films with unconventional magnetic and electric properties near the Curie temperature

We used Pulsed Laser Deposition (PLD) in oxidizing environment to epitaxially grow optimally doped manganite La_2 / 3Sr_1 / 3MnO₃ (LSMO) thin films over a (001) oriented SrTiO₃ substrate. Synthesized samples show good room temperature magnetic properties accompanied by a peculiar extension of the metallic conduction regime to temperatures higher than the Curie point.In this paper we present a study of the dependence of transport and magnetic properties of LSMO thin films on the oxygen pressure during PLD growth. We show how interaction of the growing films with O₂ molecules is fundamental for a correct synthesis and in which way it is possible to adjust PLD experimental parameters in order to tune LSMO thin film properties.The persistence of the metallic conduction regime above the Curie temperature indicates some minor changes of the electronic structure near the Fermi level, which is responsible for the half-metallic behavior of LSMO at low temperature. This feature is rather intriguing from the technological point of view, as it could pave the way to the increase of operating temperature of devices based on LSMO.     

Epitaxial growth of MgFe2O4 (111) thin films on sapphire (0001) substrate

Pulsed laser deposition technique is used to fabricate thin films of MgFe₂O₄ on sapphire substrate. X-ray diffraction patterns show that films are epitaxial along (???) direction on c-axis oriented sapphire. The observation of six fold symmetry in phi scan for the film suggests the presence of twinned in-plane alignments. The presence of magnetic hysteresis loop at room temperature indicates the ferromagnetic behavior of the film. The coercive field of 742 Oe and remnant magnetization of 151 emu/cm² is observed at room temperature. The coercive field and remnant magnetization decrease with increase in temperature.     

Recombination luminescence and trap levels in undoped and Al-doped ZnO thin films on quartz and GaSe (0 0 0 1) substrates

Photoluminescence spectra of ZnO and ZnO:Al (1.00, 2.00 and 5.00 at.%) films on GaSe (0 0 0 1) lamellas and amorphous quartz substrates, obtained by annealing, at 700 K, of undoped and Al-doped metal films, are investigated. For all samples, the nonequilibrium charge carriers recombine by radiative band-to-band transitions with energy of 3.27 eV, via recombination levels created by the monoionized oxygen atoms, forming the impurity band laying in the region 2.00 ? 2.70 eV. Al doping induces an additional recombination level at 1.13 eV above the top of the valence band of ZnO films on GaSe substrates. As a result of thermal diffusion of Zn and Al into the GaSe interface layer from ZnO:Al/GaSe heterojunction, electron trap levels located at 0.22 eV and 0.26 eV below the conduction band edge of GaSe, as well as a deep recombination level, responsible for the luminescent emission in the region 1.10 ? 1.40 eV, are created.     

Ultraviolet–green photoluminescence of β-Ga2O3 films deposited on MgAl6O10 (1 0 0) substrate

β-Ga₂O₃ films were grown on double-side polished MgAl₆O₁₀ (1 0 0) substrate by metal organic chemical vapor deposition (MOCVD) at 600, 650 and 700 °C. The refractive index dispersive behaviors of Ga₂O₃ films have the typical shape of the normal dispersion curve. Photoluminescence (PL) spectra measured at room temperature revealed that all the films exhibited intense ultraviolet (UV)–green emission from 300 to 650 nm. A minor deep UV emission around 275 nm (∼4.51 eV) was observed for the sample prepared at 700 °C. The intensity of the emission increased markedly when measured at low temperature. The corresponding PL mechanisms were discussed in detail and a schematic diagram was proposed.     

Crystallographic behavior of FeS2 films formed on different substrates

FeS₂ polycrystalline films were prepared on amorphous glass, monocrystalline Si (1 0 0), polycrystalline Al and microcrystalline TiO₂ film substrates by sulfuration annealing of magnetron sputtered iron films. The crystal microstructure and orientation distribution of the films were investigated. The FeS₂ films formed on Si (1 0 0) and glass substrates have relatively fine and uniform grains and small lattice distortion at the interface between the film and substrate but insignificant preferred orientation. The FeS₂ films formed on Al or TiO₂ substrates have relatively inhomogeneous microstructure, large lattice distortion at the interface and a (2 0 0) or (2 2 0) preferred orientation. High strain energy at the interface should be responsible for the preferred orientation and inhomogeneous microstructure in the films.     

Atomic oxygen adsorption on the silicon-doped hafnium carbide (0 0 1) surface from first principles

To improve the oxidation resistance of Hafnium carbide (HfC) surface, we investigated the adsorption of atomic oxygen on the silicon–doped HfC (0 0 1) surface by first principles. The O/HfC (0 0 1) system was also calculated for comparison. The (√2 × √2) R45° supercell was constructed to calculate the adsorption. In calculations, we treated the exchange and correlation potential with the revised version of the Perdew–Burke–Ernzerhof generalized-gradient approximation (GGA-RPBE). Our data demonstrate that the preference adsorption site for oxygen atom is the 4–fold hollow site on the silicon–doped HfC (0 0 1) surface. The oxygen on the silicon–doped surface receives more charges from silicon atoms than that in O/HfC (0 0 1) from carbon atoms. The Si–O bonds exhibit ionic and covalent characteristics, while the C–O bond exhibits primarily covalence. And the covalence of Si–Hf bonds is stronger than that of C–Hf bonds. The strong Si–O and Si–Hf bonds indicate the strong interactions of oxygen with the silicon–doped surface. The strong interactions can explain the possibility of improving the oxidation resistance of HfC surface via doping silicon.     

Electromechanical properties of [0 0 1], [0 1 1] and [1 1 1] oriented Pb(Mg1/3Nb2/3)O3–0.32PbTiO3 crystals under uniaxial stress

The electric field and the stress induced strain and polarization responses of [0 0 1], [0 1 1] and [1 1 1] oriented Pb(Mg_1/3Nb_2/3)O₃–0.32PbTiO₃ (PMN–0.32PT) relaxor ferroelectric single crystals have been systematically investigated by experimental study. The responses of [0 0 1] oriented single crystal to stress cycle and electric field are explained by polarization rotation and phase transformations mechanism. However, the responses of [0 1 1] and [1 1 1] oriented single crystal should be explained by domain switching. The differences of strain and polarization between the minimum and maximal values of electric field in [0 0 1] orientation firstly increase then decrease with enhancing of compressive stress.     

First principles study of interface structure and electronic property of Au/SrTiO3(0 0 1)

First principles method is carried out to study the interface structure and electronic property of nine probable Au/SrTiO₃(0 0 1) models. It is revealed that besides O or Ti vacancies having to exist in the interface, the interface energy and bonding nature are strongly dependent on the atomic configurations at interface. Such interfacial configurations in which each Au atom in the upper Au layer bonds with each O atom in the TiO₂-terminated SrTiO₃(0 0 1) under layer would be preferable to be formed. From the analysis of the electronic structure of the defective Au/SrTiO₃(0 0 1) interfaces, the interactions between Au and various vacancies on SrTiO₃(0 0 1) interface are very different.     

Synthesis of TiO2 thin films using single molecular precursors by MOCVD method for dye-sensitized solar cells application and study on film growth mechanism

For dye-sensitized solar cells application, in this study, we have synthesized TiO₂ thin films at deposition temperature in the range of 300–750 °C by metalorganic chemical vapor deposition (MOCVD) method. Titanium(IV) isopropoxide, {TIP, Ti(OⁱPr)₄} and Bis(dimethylamido)titanium diisopropoxide, {BTDIP, (Me₂N)₂Ti(OⁱPr)₂} were used as single source precursors that contain Ti and O atoms in the same molecule, respectively. Crack-free, highly oriented TiO₂ polycrystalline thin films with anatase phase were deposited on Si(1 0 0) with TIP at temperature as low as 450 °C. XRD and TED data showed that below 500 °C, the TiO₂ thin films were dominantly grown in the [2 1 1] direction on Si(1 0 0), whereas with increasing the deposition temperature to 700 °C, the main film growth direction was changed to [2 0 0]. Above 700 °C, however, rutile phase TiO₂ thin films have only been obtained. In the case of BTDIP, on the other hand, only amorphous film was grown on Si(1 0 0) below 450 °C while a highly oriented anatase TiO₂ film in the [2 0 0] direction was obtained at 500 °C. With further increasing deposition temperatures over 600 °C, the main film growth direction shows a sequential change from rutile [1 0 1] to rutile [4 0 0], indicating a possibility of getting single crystalline TiO₂ film with rutile phase. This means that the precursor together with deposition temperature can be one of important parameters to influence film growth direction, crystallinity as well as crystal structure. To investigate the CVD mechanism of both precursors in detail, temperature dependence of growth rate was also carried out, and we then obtained different activation energy of deposition to be 77.9 and 55.4 kJ/mol for TIP and BTDIP, respectively. Also, we are tested some TiO₂ film synthesized with BTDIP precursor to apply dye-sensitized solar cell.     

Dehydrogenation properties of epitaxial (1 0 0) MgH2/TiH2 multilayers – A DFT study

The effect of structural changes in MgH₂ induced by contact with TiH₂ on the thermodynamics of hydrogen cycling has been studied using DFT calculations. Models of epitaxial (1 0 0) MgH₂/TiH₂ multilayers with different layer thicknesses and Mg:Ti ratios have been designed. The hydrogen desorption energy from the MgH₂ layer is found to depend on the thickness of the MgH₂ layers and the Mg:Ti ratio because the MgH₂ structure tends to change from fluorite to rutile with increasing MgH₂ thickness and Mg:Ti ratio. Three types of hydrogen can be distinguished: hydrogen in MgH₂ layer with desorption energy of 0.44 eV/H₂, hydrogen in MgH₂/TiH₂ interface with desorption energy of 1.19 eV/H₂, hydrogen in TiH₂ layer with desorption energy of 0.95 eV/H₂. The desorption energy of hydrogen desorption from the fluorite-like MgH₂ is 0.23 eV/H₂ lower than the bulk rutile MgH₂. The structural deformations as well as the interfacial energy differences between the partial hydrogenated and hydrogenated states have been found to be responsible for the tuned hydrogen desorption energy.     

Modulations in structural and ferroelectric properties due to tensile strain in BiFeO3 films on MgAl2O4 substrates induced by thermal-expansion

The effect of tensile strain on structural and ferroelectric properties of BiFeO₃ epitaxial films was investigated. The films grown by pulsed laser deposition on MgAl₂O₄ (0 0 1) substrates revealed monoclinic structure deviated from the bulk rhombohedral structure due to a tensile strain along the in-plane direction. The strain is induced by the difference in thermal expansion coefficients between the film and the substrate. A Poisson ratio is calculated from the in-plain and out-of-plain lattice constants at different temperatures measured by reciprocal space maps of X-ray diffraction. The small Poisson ratio compared to the bulk suggests a weaker elastic response at high temperature. The ferroelectric polarization of the tensile-strained film along the (0 0 1) is also decreased from the bulk value.     

Chemical etching of InP (1 0 0) wafer based on a volcanic mineral water

We have successfully demonstrated that a solution of spa water [Tamagawa Spa water (TaSW):H₂O₂ = 1:1] etches InP (1 0 0) wafer. The TaSW is a colorless acidic liquid of pH ∼1.1. It contains a considerable amount of positive ions, such as H⁺, Al³⁺, and Ca²⁺. The Cl⁻, HSO₄²⁻, and SO₄²⁻ ions are the main anions. The TaSW-etchant system provides shiny flat surfaces on the etched bottoms. The spa-etchant system has reproducible etching rates and does not erode photoresist masks. The etching kinetics is reaction-rate limited. The spa-etchant system is also found to etch GaAs (1 0 0) wafer, but the etched surface is considerably roughened.