MgO Buffer Layer Research Articles

Resputtering effect during MgO buffer layer deposition by magnetron sputtering for superconducting coated conductors

In this study, MgO thin films were deposited by radio-frequency magnetron sputtering. The film thickness in the deposition area directly facing the target center obviously decreased compared with that in other areas. This reduction in thickness could be attributed to the resputtering effect resulting from bombardment by energetic particles mainly comprising oxygen atoms and negative oxygen ions. The influences of deposition position and sputtering pressure on the deposition rate were investigated. Resputtering altered the orientation of the MgO film from (111) to (001) when the film was deposited on a single crystal yttria-stabilized zirconia substrate. The density distribution of energetic particles was calculated on the basis of the measured thicknesses of the MgO films deposited at different positions. The divergence angle of the energetic particle flux was estimated to be approximately 15°. The energetic particle flux might be similar to the assisting ion flux in the ion beam assisted deposition process and could affect the orientation of the MgO film growth.

Enhancement of lower critical field by reducing the thickness of epitaxial and polycrystalline MgB2 thin films

For potential applications in superconducting RF cavities, we have investigated the properties of polycrystalline MgB2 films, including the thickness dependence of the lower critical field Hc1. MgB2 thin films were fabricated by hybrid physical-chemical vapor deposition on (0001) SiC substrate either directly (for epitaxial films) or with a MgO buffer layer (for polycrystalline films). When the film thickness decreased from 300 nm to 100 nm, Hc1 at 5 K increased from around 600 Oe to 1880 Oe in epitaxial films and to 1520 Oe in polycrystalline films. The result is promising for using MgB2/MgO multilayers to enhance the vortex penetration field.

Structural characterization of epitaxial Mg2Si films grown on MgO and MgO-buffered Al2O3 substrates

(110)-oriented epitaxial Mg2Si films were grown on (100), (110), and (111) MgO single crystals by RF magnetron sputtering. Two, one, and three types of in-plane variants were observed for (100), (110), and (111) MgO single crystals, respectively. In addition, it was also demonstrated that epitaxial Mg2Si films can be grown on (001) Al2O3 substrates using an epitaxially grown (111) MgO buffer layer. Transmission electron microscopy studies revealed a clear interface between Mg2Si and the MgO buffer layer with an epitaxial relationship. This result indicates that Mg2Si films can be epitaxially grown on other substrates by using an epitaxial buffer layer of MgO.

Microstructure and magnetic properties of (0 01) textured L10 FePt films on amorphous glass substrate

L10 FePt thin films with an island-like morphology and magnetic perpendicular anisotropy were grown at low temperature (300<Tdep<375°C) by magnetron sputtering on Hoya glass substrates using a 30-nm thick Cr (200) underlayer. An MgO buffer layer with a thickness of 2nm was used to inhibit the diffusion from the Cr underlayer and promote the growth of (001) oriented L10 FePt films by inducing an in-plane lattice distortion. By varying the substrate temperature and the Ar sputter pressure (3.5<PAr<15mTorr) during the deposition, the degree of chemical order, the microstructure and the magnetic properties were tuned and the best properties in term of squareness ratio (Mr/Ms∼0.95) and coercive field (Hc∼14kOe) were observed for films deposited at Tdep=350°C and PAr=5mTorr, due to the appearance of a tensile strain, which favors the perpendicular anisotropy. The analysis of the angular dependence of remanent magnetization curves on the optimized sample suggests that the magnetization reversal is highly incoherent due to the inter-island interactions. Our results provide useful information on the low temperature growth of FePt films with perpendicular anisotropy onto glass substrates, which are relevant for a variety of technological applications, such as magnetic recording and spintronic devices.

High-performance irreversibility field and flux pinning force density in BaHfO3-doped GdBa2Cu3Oy tape prepared by pulsed laser deposition

The high irreversibility field Bi and strong flux pinning force were obtained for BaHfO3-doped GdBa2Cu3Oy tapes on a metallic Hastelloy substrate with an ion beam-assisted deposition MgO buffer layer. At 77.3 K, Bi of 15.8 T and the maximum flux pinning force density of 23.5 GN/m3 for B ∥ c were achieved for the 1.5 vol % BaHfO3-doped GdBa2Cu3Oy tape, which are the world's highest recorded values for REBa2Cu3Oy-coated conductors on metallic substrates (RE = rare earth). From the flux pinning analysis, the c-axis correlated pinning by well-aligned BaHfO3 nanorods plays an important role in achieving the high irreversibility field and flux pinning force density performance.

Perpendicular Magnetic Anisotropy of Co2FexMn1−xSi Heusler Alloy Ultra-Thin Films

Film thickness, composition, and buffer layer dependences of perpendicular magnetic anisotropy (PMA) were investigated for ultrathin Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Si Heusler alloy films. The perpendicular magnetization was achieved when the thickness of the Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> MnSi on the Pd buffer layer was 0.6 and 0.8 nm. For Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> FeSi, only a 0.6 nm thick film exhibited perpendicular magnetization. Effective uniaxial magnetic anisotropy energy (Keffu ) was 1-2 × 106 erg/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> . From the buffer layer dependence, it was revealed that perpendicular magnetization of Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> FeSi films was not achieved for a MgO or Cr buffer layer. These results imply that contributions from the Pd/Heusler alloy interface plays an important role in the PMA of ultrathin Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Si Heusler alloy films.

Tunnel Magnetoresistance and Spin-Transfer-Torque Switching in PolycrystallineCo2FeAlFull-Heusler-Alloy Magnetic Tunnel Junctions on AmorphousSi/SiO2Substrates

We studied polycrystalline B2-type Co2FeAl (CFA) full-Heusler alloy based magnetic tunnel junctions (MTJs) fabricated on a Si/SiO2 amorphous substrate. Polycrystalline CFA films with a (001) orientation, a high B2 ordering, and a flat surface were achieved using a MgO buffer layer. A tunnel magnetoresistance (TMR) ratio up to 175% was obtained for an MTJ with a CFA/MgO/CoFe structure on a 7.5-nm-thick MgO buffer. Spin-transfer torque induced magnetization switching was achieved in the MTJs with a 2-nm-thick polycrystalline CFA film as a switching layer. Using a thermal activation model, the intrinsic critical current density (Jc0) was determined to be 8.2 x 10^6 A/cm^2, which is lower than 2.9 x 10^7 A/cm^2, the value for epitaxial CFA-MTJs [Appl. Phys. Lett. 100, 182403 (2012)]. We found that the Gilbert damping constant evaluated using ferromagnetic resonance measurements for the polycrystalline CFA film was ~0.015 and was almost independent of the CFA thickness (2~18 nm). The low Jc0 for the polycrystalline MTJ was mainly attributed to the low damping of the CFA layer compared with the value in the epitaxial one (~0.04).

Crystalline Al2O3 on buffered soda-lime glass by e-beam

We report for the first time the successful deposition of a continuous crystalline Al2O3 film on ordinary soda-lime glass via an extremely highly textured MgO buffer layer, using the ordinary electron-beam evaporation method. As a result, a highly crystalline thin-film of silicon may now be deposited on inexpensive Al2O3 buffered soda-lime glass for cost-effective photovoltaic and other electronic applications. In addition, our results have made possible a new kind of inexpensive and light-weight “sapphire glass” that can be used for display covers for smartphones etc.

Effect of Thickness and Annealing Temperature on Magnetic Properties of Ultrathin γ-Fe2O3 Films Grown on Silicon Substrate

The effect of thickness and annealing temperature on magnetic properties of ultrathin γ-Fe2O3 films with MgO buffer layer grown on silicon substrate is investigated. The saturation magnetization and coercive force of samples at room temperature increase with increasing of annealing temperature, and decrease as annealing temperature is above 873 K (600 °C). The saturation magnetization of samples decreases with increasing of the thickness of γ-Fe2O3 at room temperature. The samples with 3 to 4 nm thick γ-Fe2O3 annealed at 873 K (600 °C) show saturation magnetization of about 400 emu/cm3, which is close to the bulk value of ~390 emu/cm3 within the error range.

Si incorporation in Ti1 − xSixN films grown on TiN(001) and (001)-faceted TiN(111) columns

Thin films consisting of TiN nanocrystallites encapsulated in a fully percolated SiNy tissue phase are archetypes for hard and superhard nanocomposites. Here, we investigate metastable SiNy solid solubility in TiN and probe the effects of surface segregation during the growth of TiSiN films onto substrates that are either flat TiN(001)/MgO(001) epitaxial buffer layers or TiN(001) facets of length 1–5nm terminating epitaxial TiN(111) nanocolumns, separated by voids, deposited on epitaxial TiN(111)/MgO(111) buffer layers. Using reactive magnetron sputter deposition, the TiSiN layers were grown at 550°C and the TiN buffer layers at 900°C. On TiN(001), the films are NaCl-structure single-phase metastable Ti1−xSixN(001) with N/(Ti+Si)=1 and 0≤x≤0.19. These alloys remain single-crystalline to critical thicknesses hc ranging from 100±30nm with x=0.13 to 40±10nm with x=0.19. At thicknesses h>hc, the epitaxial growth front breaks down locally to form V-shaped polycrystalline columns with an underdense feather-like nanostructure. In contrast, the voided epitaxial TiN(111) columnar surfaces, as well as the TiN(001) facets, act as sinks for SiNy. For Ti1−xSixN layers with global average composition values 〈x〉=0.16, the local x value in the middle of Ti1−xSixN columns increases from 0.08 for columns with radius r⋍2nm to x=0.14 with r⋍4nm. The average out-of-plane lattice parameter of epitaxial nanocolumns encapsulated in SiNy decreases monotonically with increasing Si fraction 〈x〉, indicating the formation of metastable (Ti,Si)N solid solutions under growth conditions similar to those of superhard nanocomposites for which the faceted surfaces of nanograins also provide sinks for SiNy.

Antiferromagnetic interlayer exchange coupling in epitaxial Fe/MgO/Fe trilayers with MgO barriers as thin as single monolayers

Comparative studies of the magnetic properties of epitaxial Fe/MgO/Fe trilayers, which were directly grown on an MgO(001) crystal and a homoepitaxial MgO buffer layer, were performed to examine the interlayer exchange coupling as a function of the MgO spacer thickness. Measurements of the magneto-optic Kerr effect at room temperature demonstrated that there was a strong antiferromagnetic coupling (AFC) between the Fe layers, which was mediated by the insulating MgO barrier. A measurable AFC was found in a range of MgO thickness (dMgO) from a single monolayer (2 Å) to 6 Å, which had a maximum strength of −1.15 ergs/cm2 at dMgO = 2.7 Å for the Fe/MgO/Fe trilayers that were prepared on the buffered sample and was notably weaker for the trilayers that were prepared directly on the MgO(001).

Polarity characterization by anomalous x-ray dispersion of ZnO films and GaN lateral polar structures

We demonstrate the use of anomalous x-ray scattering of constituent cations at their absorption edge, in a conventional Bragg-Brentano diffractometer, to measure absolutely and quantitatively the polar orientation and polarity fraction of unipolar and mixed polar wurtzitic crystals. In one set of experiments, the gradual transition between c+ and c− polarity of epitaxial ZnO films on sapphire as a function of MgO buffer layer thickness is monitored quantitatively, while in a second experiment, we map the polarity of a lateral polar homojunction in GaN. The dispersion measurements are compared with piezoforce microscopy images, and we demonstrate how x-ray dispersion and scanning probe methods can provide complementary information that can discriminate between polarity fractions at a material surface and polarity fractions averaged over the film bulk.

Fabrication of long-length ion beam-assisted deposited MgO templates for YBCO-coated conductors

The biaxially textured ion beam-assisted deposited (IBAD)-MgO templates were successfully prepared on polycrystalline Hastelloy metal substrate with reel-to-reel system for YB2Cu3O7−δ (YBCO)-coated conductors. By the solution deposition planarization technique, amorphous Y2O3 films were coated on untreated Hastelloy substrate as the bed layer to obtain smooth, dense, and crack-free surface for subsequent IBAD-MgO deposition. The 50 m long IBAD-MgO and homo-epitaxial (epi)-MgO buffer layers deposited on Y2O3 films exhibit excellent crystallographic consistency along the scope with full width half maximum (FWHM) values of (110) ΔΦ and (200) Δω in the range of 5.5°–6.0° and 3.0°–3.5°, respectively. To match the lattice constant of YBCO material, LaMnO3 cap layer was fabricated on IBAD-MgO templates by radio frequency (rf) magnetron sputtering with the FWHM values of in-plane and out-of-plane of 6.8° and 3.7°, respectively, indicating excellent biaxial texture.

Role of reaction atmosphere in preparation of potassium tantalate through sol–gel method

Potassium tantalate (KT) thin films and powders of both K2Ta2O6 (KT pyrochlore) and KTaO3 (KT perovskite) structures were prepared by means of chemical solution deposition method using Si(111) with ZnO and MgO buffer layers as a substrate. The influence of reaction atmosphere on reaction pathway and phase composition for both KT powders, and KT thin films has been studied mainly by means of powder diffraction and infrared spectroscopy. When an oxygen flow instead of static air atmosphere has been used the process of pyrolysis in oxygen runs over much narrower temperature interval (200–300 °C), relatively to air atmosphere (200–600 °C) and almost no (in case of powders), or no (in case of thin films) pyrochlore intermediate phase has been detected in comparison with treatment in air, where the pyrochlore phase is stable at temperatures 500–600 °C (powders). KT perovskite phase starts to crystallize at temperatures 50° and 150 °C lower compared to air atmosphere in case of powders and thin films, respectively. Microstructure formed by near-columnar grains and small grains of equiaxed shape was observed in films treated in oxygen and air atmosphere, respectively.

Structure of high-T c superconducting tape of the second generation with high current-carrying capacity

Commercial high-Tc superconducting tape produced by Superpower Inc. (the U.S.A.) is studied by scanning and transmission electron microscopy. The superconductor structure is shown to consist of a superconducting layer of average composition GdBa1.5Cu2.5–3.0Oy coated by a silver layer and buffer layers of LaMnO3, MgO,Y2O3, and Al2O3 deposited on a metallic tape of nickel alloy (Ni-Cr-Mo-Fe-W-Co-Mn). The superconducting layer is formed by the superconducting oxide GdBa2Cu3Oy containing lamellar inclusions Gd2O3.

Low-Dielectric-Loss Barium Strontium Titanate Thin Films with MgO Buffer Layer for Tunable Microwave Devices

The dielectric and microwave properties of Ba0.6Sr0.4TiO3 (BST60) thin films with a MgO buffer layer deposited on Al2O3 substrates were investigated. Insertion of the MgO buffer layer is demonstrated to be an effective approach to fabricate low-dielectric-loss BST thin films. x-Ray pattern analysis indicates that the thin films exhibit good crystalline quality with a pure perovskite phase and that insertion of the MgO buffer layer does not change the crystal structure of BST. The nonlinear dielectric properties of the BST films were measured by using an interdigital capacitor (IDC). At room temperature, the tunability of the BST films with a MgO buffer layer was 24.1% at a frequency of 1 MHz with an applied electric field of 80 kV/cm. The dielectric loss of the BST thin films is only 0.005 to 0.007 in the frequency range from 20 Hz to 2 MHz, the same as for BST films prepared on single-crystal MgO substrates. The microwave dielectric properties of the BST thin films were also measured by a vector network analyzer from 50 MHz to 10 GHz.

Epitaxial growth of cubic Mg0.45Zn0.55O thin films on SrTiO3 (001) substrate with MgO buffer layer

Cubic Mg0.45Zn0.55O thin films with MgO buffer layer were epitaxially grown on SrTiO3 substrate by radio frequency oxygen plasma assisted molecular beam epitaxy. X-ray diffraction measurements indicate high quality single crystal films of (200) orientation. The growth follows cubic-on-cubic alignment with epitaxial relationship of Mg0.45Zn0.55O (001)[100]//MgO (001)[100]//SrTiO3 (001)[100]. Atomic force microscopy measurements reveal a smooth surface with the rms roughness of 0.36nm in an area of 5×5μm2. The band gap of the film is calculated to be 4.8eV and in the solar blind range. Since the MgO buffer layer can suppress the sub-band-gap photoresponse from the SrTiO3 substrate and does not absorb moisture like the bare MgO substrate, the Mg0.45Zn0.55O thin films with MgO buffer layer grown on SrTiO3 substrate have promising applications in solar blind ultraviolet photodetectors.

Growth of high quality Zn0.9Mg0.1O films on c-plane sapphire substrates by plasma-assisted molecular beam epitaxy

We report the growth and characterization of single-crystalline, epitaxial Zn0.9Mg0.1O films on c-plane sapphire substrates with MgO buffer layer by plasma-assisted molecular beam epitaxy. The quality of the Zn0.9Mg0.1O epilayers is evidenced by a Hall mobility of more than 60cm2V−1s−1 at room temperature and an X-ray diffraction (XRD) rocking curve full-width at half-maximum of 47arcsec for the (0002) reflection. A screw dislocation density of 4×106cm−2 is estimated by XRD. Transmission electron microscopy revealed that the thickness of the MgO buffer layer is about 3.5nm, and a highly c-axis oriented Zn0.9Mg0.1O (0002) reflection corresponding to the wurtzite-phase is observed. Alloying with Mg is found to widen the bandgap energy of the ZnO, and luminescence is observed at 3.502eV at low temperature.

Reduction of the deposition temperature of high quality EuO films on Yttria Stabilized Zirconia by incorporating an MgO buffer layer

High quality stoichiometric EuO ferromagnetic thin films have been grown by Molecular Beam Epitaxy (MBE) on MgO coated-Yttria Stabilized Zirconia (YSZ) (100) substrates. The proof is made that introducing an MgO buffer layer, that avoid oxygen transfer from YSZ to EuO, allows the preparation of high quality stoichiometric EuO films at reduced deposition temperature compared with films directly deposited onto YSZ, maintaining similar Eu flux and oxygen partial pressure. Structure and texture were characterized by X-ray diffraction showing out-of plane and in-plane ordering for films deposited onto MgO buffer layers. The crystallographic quality was corroborated by a Curie temperature around 69K and a magnetization moment close or equal to 6.49·10−23J/T (7 μB), corresponding to bulk EuO single crystal values.

Evaluation of optical parameters and characterization of ultrasonically sprayed MgO films by spectroscopic ellipsometry

MgO is a chemically stable buffer layer and a good candidate with its unique optical and structural characteristics such as low refractive index and good lattice matching for the deposition of technologically important materials. In this work, we have produced MgO films by a low cost ultrasonic spray pyrolysis technique and tried to improve their characteristics by thermal annealing. MgO films have been obtained on glass substrates at 325±5°C and annealed at two different temperatures as 500°C and 600°C for 2h. Elemental analyses have been made by energy-dispersive X-ray spectroscopy. Thicknesses, refractive indices and extinction coefficient values have been determined by spectroscopic ellipsometry technique using Cauchy–Urbach model. Band gap values of the films have been calculated using the absorbance spectra and optical method. X-ray diffraction patterns have been used to investigate the structural properties and to calculate lattice parameters. Atomic force microscope images have been taken to see the effect of thermal annealing on surface morphology. Electrical resistivity values of the films have been determined using a four-point probe set-up. As a result, annealing temperature has a strong effect on the mentioned properties and a low cost ultrasonic spray pyrolysis technique allows the production of new and alternative MgO buffer layers for technological applications.