MgO Buffer Layer Research Articles

Growth and properties of highly orientated Sr 0.75Ba 0.25Nb 2O 6 thin films on silicon substrates with MgO or TiN buffer layers

Sr 0.75Ba 0.25Nb 2O 6 (SBN75) thin films were deposited on silicon substrate with MgO (100) or TiN (100) buffer layer by radio-frequency magnetron sputtering technique. X-ray diffraction confirmed that a 900 °C annealed SBN self-buffer layer introduced before SBN deposition can lead to the highly c-axis orientation of SBN75 thin film on MgO buffer layer. Energy-dispersive spectrometry analysis showed that the SBN75 films had target-film composition transfer and the TiN buffer layer was partially oxidized during SBN growth. The refractive index of SBN films on both MgO/Si and TiN/Si substrates was determined by fitting the measured reflectance curves with Sellmeier dispersion model in the visible region and the micro-structures were studied by scanning electron microscopy. In this paper, the conditions for SBN/MgO/Si treated as waveguide structure were also discussed.

Smooth surface, low electron concentration, and high mobility ZnO films on c-plane sapphire

ZnO thin films were grown on c-plane sapphire substrate using plasma-assisted molecular beam epitaxy. The thickness of MgO buffer layers was optimized for structural and electrical properties of the epi-ZnO films. It is found that with MgO buffer growth time of 60 s, the epi-ZnO film exhibited narrow X-ray diffraction peak as well as low surface roughness. In the meantime, the electron concentration reached a minimum of 2.03×10 16/cm 3 and high mobility of 169.4 cm 2/V S. These results demonstrate a route to grow good crystals together with excellent mobility and low residual electron concentration, which can ultimately satisfy the requirement for acceptor doping as well as device engineering.

Static and dynamic magnetic properties of epitaxial Co2FeAl Heusler alloy thin films

Structural and magnetic properties of epitaxial Co2FeAl Heusler alloy thin films were investigated. Films were deposited on single crystal MgO (001XS) substrates at room temperature, followed by an annealing process at 600 °C. MgO and Cr buffer layers were introduced in order to enhance crystalline quality, and improve magnetic properties. Structural analyses indicate that samples have grown in the B2 ordered epitaxial structure. VSM measures show that the MgO buffered sample displays a magnetization saturation of 1010 ± 30 emu/cm3, and Cr buffered sample displays a magnetization saturation of 1032 ± 40 emu/cm3. Damping factor was studied by strip-line ferromagnetic resonance measures. We observed a maximum value for the MgO buffered sample of about 8.5 × 10−3, and a minimum value of 3.8 × 10−3 for the Cr buffered one.

High tunable Ba<SUB align="right">0.5Sr<SUB align="right">0.5TiO<SUB align="right">3 thin film deposited on silicon substrate with MgO buffer layer

High tunable Ba 0.5 Sr 0.5 TiO 3 thin films were deposited by r.f. magnetron sputtering on Si substrate. MgO, as a buffer layer, was deposited by r.f. magnetron sputtering. An interdigital varactor was fabricated by micro-fabrication process and the dielectric properties were investigated. The results show that the crystallisation of Ba 0.5 Sr 0.5 TiO 3 was improved by the insertion of MgO buffer layer after the post annealing process. The MgO buffer layer enhances the oriental growth of BST along (100). Also, the tunability of the Ba 0.5 Sr 0.5 TiO 3 thin films was improved and the dielectric loss greatly decreased.

Heteroepitaxial film crystal silicon on Al2O3: new route to inexpensive crystal silicon photovoltaics

Crystal silicon (c-Si) film photovoltaics (PV) fabricated on inexpensive substrates could retain the desirable qualities of silicon wafer PV—including high efficiency and abundant environmentally-benign raw materials—at a fraction of the cost. We report two related advances toward film c-Si PV on inexpensive metal foils. First, we grow heteroepitaxial silicon solar cells on 2 kinds of single-crystal Al2O3 layers from silane gas, using the rapid and scalable hot-wire chemical vapor deposition technique. Second, we fabricate heteroepitaxial c-Si layers on large-grained, cube-textured NiW metal foils coated with Al2O3. In both experiments, the deposition temperature is held below 840 °C, compatible with low fabrication costs. The film c-Si solar cells are fabricated on both single-crystal sapphire wafer substrates and single-crystal γ-Al2O3-buffered SrTiO3 wafer substrates. We achieve ∼400 mV of open-circuit voltage despite crystallographic defects caused by lattice mismatch between the silicon and underlying substrate. With improved epitaxy and defect passivation, it is likely that the voltages can be improved further. On the inexpensive NiW metal foils, we grow MgO and γ-Al2O3 buffer layers before depositing silicon. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) confirm that the silicon layers are epitaxial and retain the ∼50 μm grain size and biaxial orientation of the foil substrate. With the addition of light-trapping, >15% film c-Si PV on metal foils is achievable.

ZnO Thin Films of High Crystalline Quality Deposited on Sapphire and GaN Substrates by High Temperature Sputtering

ABSTRACTSingle orientation ZnO (00.2) films were deposited by means of high temperature high vacuum reactive magnetron sputtering onto Al2O3 (0001) and GaN (0001) substrates. In order to obtain films of high crystalline quality a novel approach to ZnO sputter deposition was employed, adapting the practice used in MBE technology, of using a MgO buffer layer deposited on sapphire at a high-temperature followed by a ZnO nucleation layer deposited at low temperature. ZnO films were also grown on epitaxial GaN/Al2O3 substrates where the GaN layer was treated as the buffer layer. Following the deposition, all samples were annealed ex-situ in an O2 flow at 800°C. The obtained ZnO films have a lattice constant c equal to 5.2036 Å and 5.214 Å for the films deposited on Al2O3 and GaN substrates, respectively. Secondary ion mass spectroscopy depth profiles, scanning and transmission electron microscopy cross sectional images and atomic force microscope were used to characterize the structural properties of the films. Electrical properties were assessed using Hall effect measurement. Photoluminescence spectra were also taken.

Influence of MgO and ZrO2 buffer layers on dielectric properties of Ba(Zr0.20Ti0.80)O3 thin films prepared by sol–gel processing

Ba(Zr0.20Ti0.80)O3 (BZT) thin films are deposited on Pt(111)/Ti/SiO2/Si, MgO and ZrO2 buffered Pt(111)/Ti/SiO2/Si substrates by a sol–gel process. The BZT thin films directly grown on Pt(111)/Ti/SiO2/Si substrates exhibit highly (111) preferred orientation, while the films deposited on Pt(111)/Ti/SiO2/Si substrates with MgO and ZrO2 buffer layers show highly (110) preferred orientation. At 100kHz, dielectric constants are 417, 311 and 321 for the BZT thin films grown on Pt(111)/Ti/SiO2/Si, MgO and ZrO2 buffered Pt(111)/Ti/SiO2/Si substrates, respectively. The difference in dielectric properties of three BZT films can be attributed to the series capacitance effect, interface conditions and their orientations.

Properties of ZnO grown on sapphire with different buffer layers

The present difficulties of ZnO in device applications urge deeper understanding of the growth kinetics in fabrication of high quality ZnO. To investigate the influence of buffer layers on the over-grown epitaxial ZnO layers, high-temperature (HT)-ZnO films with different buffer layers were grown on sapphire by molecular beam epitaxy, characterized by atomic force microscopy, photoluminescence and X-ray diffraction. Hexagonal MgO islands were observed on the low-temperature (LT)-MgO buffer layers. To improve the crystal quality, it is of great importance to optimize the buffer thickness and growth temperatures. The crystal quality of HT-ZnO grown on thinner MgO buffer layer (20nm) is better than that on thicker MgO buffer layer (120nm). And the surface and relative intensity of near band edge emission of sample with varied temperature grown ZnO buffer is better than other samples.

Study of the integrated growth of dielectric films on GaN semiconductor substrates

Typical perovskite oxides SrTiO₃ (STO) and PbZr₀.₅₂Ti₀.₄₈O₃ (PZT) were fabricated on GaN semiconductor substrates by pulsed laser deposition. STO and PZT films were deposited on bare GaN, TiO₂, and MgO-buffered GaN. The effects of TiO₂ and MgO buffer-layers on the orientations and electric properties of the perovskite films were systematically studied. The crystalline properties were in situ monitored by reflective high energy electron diffraction and ex situ characterized by X-ray diffraction. It was found that the epitaxial temperature of STO and PZT was reduced by inserting a buffer layer. (111)-oriented films were obtained on bare and TiO₂-buffered GaN. However, the orientations of the perovskite films were changed to be (110) when deposited on MgO buffer layer. Furthermore, PZT films deposited on MgO- and TiO₂-buffered GaN show better electric performance compared with these films directly deposited on GaN. These results show that perovskite oxide could be epitaxially grown on GaN semiconductor substrates by inserting a proper buffer layer.

Thermoelectric Properties of C-Axis-Oriented Ca<sub>3</sub>Co<sub>4</sub>O<sub>9+δ</sub> Films Grown on MgO-Buffered Si (100) by PLD Technique

High-quality c-axis-oriented Ca3Co4O9+δ thin films have been grown directly on Si (100) wafers with inserting MgO buffer layers by pulsed-laser deposition (PLD). X-ray diffraction and scan electron microscopy show good crystallinity of the Ca3Co4O9+δ films. The resistivity and Seebeck coefficient of the Ca3Co4O9+δ thin films on Si (100) substrates are 9.8 mΩcm and 189 μV/K at the temperature of 500K, respectively, comparable to the single-crystal samples. This advance demonstrates the possibility of integrating the cobaltate-based high thermoelectric materials with the current state-of-the-art silicon technology for thermoelectricity-on-a-chip applications.

Mg0.58Zn0.42O Thin Films on MgO Substrates with MgO Buffer Layer

Cubic Mg0.58Zn0.42O thin films with (100) orientation were grown on cubic MgO substrates. The band gap of the alloy films corresponds to solar blind band. In the case that a MgO buffer layer was employed, the surface roughness was decreased from 38 to 1.6 nm under the same growth conditions. A metal−semiconductor−metal photodetector based on this MgZnO film was fabricated, which showed a low dark current of 0.16 pA and lower sub-bandgap photoresponse than the ones with rougher surface in our early reports.

Preparation and characterization of highly c-axis textured MgO buffer layer grown on Si(100) substrate by RF magnetron sputtering for use as growth template of ferroelectric thin film

Highly c-axis textured MgO thin films were grown directly on Si(100) substrates without any buffer layer by RF magnetron sputtering for use as growth template of ferroelectric film. We fixed the target-to-substrate spacing of 40 mm and then changed the substrate temperature, deposition pressure, and RF power to study the effect of deposition parameters on the growth of c-axis textured MgO thin films. The as-grown films were post-annealed by the rapid thermal annealing (RTA) and furnace annealing to improve the film quality. The experimental results show that the optimum deposition parameters are substrate temperature of 350 °C, oxygen pressure of 15 mTorr and RF power of 75 W. The full width at half maximum intensity (FWHM) of MgO(200) peak obtained from the XRD measurement was 0.8°, and it was further reduced to 0.5° and 0.27° after annealing by RTA and furnace, respectively. Highly c-axis textured PZT and BaTiO3 films could be obtained on this template. Hysteresis loops of the BaTiO3 films deposited on MgO(100) single crystalline substrate and MgO(200)/Si(100) template were measured for comparison. The results show that MgO/Si templates thus obtained are suitable for the synthesis of perovskite ferroelectric thin films.

Perpendicular Magnetic Anisotropy in CoFeB/Pd Bilayers

Perpendicular magnetic anisotropy is observed in ultrathin (~ 0.6 nm) amorphous Co40Fe40B20 when sputtered on an MgO (001) buffer layer and capped with Pd. The layers are superparamagnetic with a blocking temperature of ~ 230 K, below which they show an exponential temperature dependence of coercivity. Perpendicular magnetic anisotropy is observed in the as-deposited state and the mechanism is different from that of CoFeB/Pt, which requires postannealing. These ultrathin layers could be a model system for studies of electric field effects on magnetic anisotropy.

Fracture behavior of coated layer and its influence on critical current of DyBCO coated conductor

The coated conductors are attractive due to their high superconducting properties. As the conductors are subjected to mechanical and electromagnetic stresses in preparation and service, the study on the mechanical behavior and its influence on superconducting properties is demanded for application. In the present work, the DyBa2Cu3O7−δ coated conductor with MgO buffer layer deposited on the Hastelloy C-276 substrate by inclined substrate deposition, prepared at THEVA, was used as the sample. The relation of the deformation and fracture behavior to the change of critical current under applied tensile strain was investigated. The experimental result of tensile test and critical current measurement revealed that the change of critical current with increasing applied tensile strain is caused in two stages; micro-cracking of the coated layer in the fist stage and extensive multiple cracking of the coated layer induced by the discontinuous yielding of the substrate in the second stage.

Preparation and structural properties of YBCO films grown on GaN/ c-sapphire hexagonal substrate

Epitaxial YBCO thin films have been grown on hexagonal GaN/ c-sapphire substrates using DC magnetron sputtering and pulsed laser deposition. An MgO buffer layer has been inserted between the substrate and the YBCO film as a diffusion barrier. X-ray diffraction analysis indicates a c-axis oriented growth of the YBCO films. Φ-scan shows surprisingly twelve maxima. Transmission electron microscopy analyses confirm an epitaxial growth of the YBCO blocks with a superposition of three a– b YBCO planes rotated by 120° to each other. Auger electron spectroscopy and X-ray photoelectron spectroscopy reveal no surface contamination with Ga even if a maximum substrate temperature of 700 °C is applied.

Controlled growth of Zn-polar ZnO film on MgAl2O4 (1 1 1) substrate using MgO buffer layer

A pure rocksalt MgO buffer layer was used to modify the surface structure of MgAl2O4 (1 1 1) substrates to achieve growth of a Zn-polar ZnO film by radio frequency plasma-assisted molecular beam epitaxy. It is found that this pure rocksalt MgO buffer layer plays a crucial role in 30° rotation domain elimination, surface morphology improvement and Zn-polarity control of the ZnO film, as demonstrated by in situ reflection high-energy electron diffraction and ex situ transmission electron microscopy. Atomic force microscopy observation also illustrates a smooth surface for the ZnO film.

(100) Oriented Sputter-deposited Pt Thin Films on MgO/Fe Composite Bilayers using SiO2/Si substrates With Addition of O2 During Pt Sputtering

AbstractPerovskite type oxide thin films have attracted a lot of attention, because they are essential materials which will be used for various electric devices such as ferroelectric random access memory (FeRAM) and tunable filter devices. When the materials are used for such capacitive devices, bottom electrode layers for oxide films are very important, since they significantly affect the crystallinity of the oxide films. Platinum (Pt) is one of the well known bottom electrode materials used for the oxide thin films. Pt provides also better nucleation sites for such perovskite materials due to small lattice misfit. Since dielectric properties of ferroelectric films are originated from the displacement of ions in a crystal along the c-axis direction, c-axis oriented ferroelectric thin films are required to attain better dielectric properties. (100) oriented Pt layers are required to attain c-axis oriented perovskite type ferroelectric films. In our previous report, we succeeded in preparing (100)-oriented Pt thin films with thickness of 20 nm on SiO2/Si substrate at substrate temperature Ts above 400 °C using MgO (100) buffer layers which deposited on Fe (100) seed layers. However, the growth of Pt(111) texture appeared when the thickness was increased from 20 nm to 100 nm, since (100) texture has relatively higher surface energy than (111) closely packed texture for Pt surface. It suggested that surface energy of the films changed during the deposition. In order to keep the surface energy, addition of O2 gas was performed during Pt deposition. Pt thin films with (100) preferred orientation with thickness above 100 nm were deposited on the (100) oriented MgO layer prepared on very thin seed Fe layers deposited on SiO2/Si substrates at Ts of 500 °C by facing-targets sputtering. It was also succeeded to attain (100) oriented perovskite oxide layer when they were deposited on the Pt(100)/Mg(100)/Fe/SiO2/Si underlayer.

스퍼터링법으로 증착된 산화아연 박막의 구조적 성질에 대한 산화마그네슘 완충층의 효과 연구

The effect of MgO buffer layer on the structural properties of sputter-grown ZnO thin film was investigated. Sapphire (0001) and Si (100) substrate were used for the growth and MgO buffer layer was inserted between ZnO thin film and the substrate. X-ray diffraction pattern indicated that enhanced crystallinity in the ZnO thin film grown was achieved by inserting very thin MgO buffer layer, regardless of the substrate type. The strain in the ZnO thin film could also be controlled by the insertion of the MgO buffer layer, and tendency of the strain was strongly dependent on the substrate type.

Determination of refractive indices and thicknesses of Ba0.7Sr0.3TiO3 films with and without MgO (001) buffer layer on silicon substrate

Ba 0.7 Sr 0.3 TiO 3 (BST30) thin films, deposited on single crystal Si (100) substrates with or without a MgO(001) buffer layer by the sol-gel method, have been investigated for their optical properties with a visible reflection spectrophotometer. An improved simplex numerical optimization method based on the reflectance spectra fitting is utilized to obtain the refractive indices and thicknesses of the thin films. Two models for the refractive index variations of the films (with or without a MgO buffer layer) together with a resemble double-layer structure are used to describe the films. The crystallization and microstructure of the films are characterized by the field emission scanning electron microscopy, which indicates that the MgO(001) buffer layer can significantly hold up the interdiffusion between the BST30 layer and the Si substrate, acuminate the interfaces, and improve the crystallinity of the BST thin film deposited on it. Furthermore, due to the large difference between the refractive index of the BST30 thin film and that of the MgO(001) buffer layer, which results in a strong beam confinement, a desirable waveguide structure could be constructed.

Epitaxial Growth and Magnetic Properties of Half-Metallic Fe$_{3}$O$_{4}$ on Si(100) Using MgO Buffer Layer

The growth and magnetic properties of epitaxial magnetite Fe3O 4 on Si(100) using MgO buffer layer have been studied by reflection high-energy electron diffraction, X-ray photoelectron spectroscopy, and magneto-optical Kerr effect. The epitaxial Fe 3O 4 films were prepared by in situ post growth annealing of ultrathin epitaxial Fe films at 220degC in an oxygen partial pressure of 8 times 10-4 mbar. The epitaxial relationship was found to be Fe3 O4 (100)lang001rang//MgO(100)lang001rang//Si(100)lang001rang with the MgO film grown cube-on-cube orientation with the Si substrate. While the Fe unit cell rotates 45deg to match that of the MgO buffer layer, it rotates 45deg back upon oxidation. A low saturation field has been observed indicating low density of antiphase boundaries.