Characterization of strain relaxation of (0 0 1) oriented SrTiO3 thin films grown on LaAIO3 (1 1 0) by means of reciprocal space mapping using x-ray diffraction

The strain relaxation of SrTiO₃ r.f. magnetron sputter-deposited thin films on LaAlO₃ substrates have been studied by x-ray diffraction mapping. An investigation of different x-ray optics shows that a, so called, hybrid mirror monochromator in combination with a triple-bounce analyser crystal provides very good conditions for characterization of thin distorted films grown epitaxially onto substrates with high structural order. The in-plane and out-of-plane lattice parameters of the SrTiO₃ films could accurately be determined since the x-ray diffraction optics enabled the splitting of substrate peaks, caused by the twinning in the rhombohedral LaAlO₃ to be resolved and, provided film peak intensities are high enough, to precisely establish their positions. Films in the thickness range 9.3–144.0 nm were found to be partially relaxed, having a tetragonal distortion due to in-plane strain that was found to decrease with increasing film thickness, approaching an undistorted SrTiO₃ lattice parameter of 0.3927 nm. This value is 0.6% larger than the bulk indicating that the compositions of the films were slightly non-stoichiometric. The strain relaxation of the grown films was found to follow the general trend of a predicted strain–thickness relation based on energy density balance considerations regarding misfit dislocations and lattice strain.     

Ion beam assisted texturing of polycrystalline Y2O3 films deposited via electron-beam evaporation

Ion-beam assisted deposition of polycrystalline Y₂O₃ films using e-beam evaporation was investigated. For growth on non-crystalline substrates, low temperature growth yields randomly oriented polycrystalline material. At elevated temperature, surface energy anisotropy yields a (111) uniaxial texture. For film deposition with irradiation from an Ar ion beam, the out-of-plane texture remained (111) in orientation. The incident Ar ion beam induces an in-plane alignment of the Y₂O₃ films that is relatively broad. A six-fold symmetry in the out-of-plane X-ray diffraction phi-scans was observed for the (111) textured Y₂O₃ films, indicating a multi-variant in-plane texture and suggesting anisotropic damage along both the (110) and (100) projections. The lack of a sharp, single variant in-plane texture with ion beam irradiation is consistent with the relatively weak bond strength in Y₂O₃.     

Structure of epitaxial Mn-stabilized ZrO2 layers on yttria-stabilized zirconia single crystals prepared by sputtering

Epitaxial Mn-stabilized zirconia layers on yttria-stabilized ZrO₂ (YSZ) single crystals were prepared by sputtering a Mn_0.32Zr_0.68O₂ target. Substrates with (001), (110) and (111) orientation were used. Transmission electron microscopy cross sections showed a homogeneous thickness of the layers. Layers prepared at a substrate temperature of 600 °C grew epitaxially. X-ray diffraction showed that these layers are not exactly cubic. We found a slightly larger out-of-plane lattice parameter compared to the in-plane lattice parameter. This means the out-of-plane lattice misfit is lower than the in-plane lattice misfit, contrary to behaviour expected for epitaxially grown layers. This effect was largest for layers on YSZ(111) and small for layers on YSZ(001).     

Coercivity variation in exchange-coupled Fe/FePt bilayer with perpendicular magnetization

The soft/hard Fe/FePt film with perpendicular magnetization has been deposited on a glass substrate. The (001) oriented L1₀ FePt film was obtained when annealed by rapid thermal process at 800 °C and a Fe layer was deposited at room temperature with thicknesses of 2 nm to 20 nm. Controlling the Fe layer thickness allowed modification of the hysteresis loops from out-of-plane rigid magnet to in-plane exchange-spring like magnet due to the nanometer scale interface coupling. When the Fe layer thickness increased to 2 nm, the out-of-plane coercivity is reduced to 5.9 kOe but the remanence ratio (0.98) is still high. The Fe (2 nm)/FePt film shows perpendicular magnetization with linear in-plane hysteresis loop. The remanence ratio is reduced to 0.85 when the Fe layer thickness increased to 5 nm. When the Fe layer thickness was varied up to 10-20 nm, the in-plane hysteresis loop shows exchange-spring like behavior with two-step magnetization reversal processes. The films with perpendicular coercivity were moderated by the thickness of soft magnetic layer.     

激光分子束外延BaTiO3/SrTiO3超晶格的晶格应变研究

采用脉冲激光分子束外延(PLMBE)方法,通过优化的工艺参数,在SrTiO3(100)单晶基片上外延结构为(8/8)的BaTiO3/SrTiO3超晶格薄膜.综合利用反射式高能电子衍射系统(RHEED)、高分辨率X射线衍射(HRXRD)以及高分辨率透射电镜选区电子衍射(SAED)技术,研究超晶格薄膜的晶格应变现象和规律.研究结果表明,在制备的BaTiO3/SrTiO3超晶格薄膜中,BaTiO3晶胞面外晶格增大,面内晶格减小;而SrTiO3晶胞面内及面外方向晶格都被拉伸,但面外晶格拉伸程度较大,SrTiO3晶胞产生了与BaTiO3晶胞方向一致的四方相转变.     

Effects of strain on ultrahigh-performance optoelectronics and growth behavior of high-quality indium tin oxide films on yttria-stabilized zirconia (001) substrates

In this study, in situ reflection high-energy electron diffraction patterns were used to observe the preparation of high-quality indium tin oxide (ITO) films on yttria-stabilized zirconia (001) substrates using laser molecular-beam epitaxy at different growth pressures and temperatures. X-ray diffraction images showed that increasing the growth temperature or decreasing the pressure diminished the out-of-plane lattice length of the film because of the increased lattice relaxation. Atomic force microscope images and scanning electron microscope showed some crack formations probably due to the release of the strain, causing an increase in the lattice length instead of decrease at the temperature range from 350 to 400 °C. High-resolution transmission electron microscopy showed that the interface between the ITO film and substrate was sharply defined, which indicates the high quality of the film. The sheet resistance of the film increased with increasing growth temperature or pressure, while the optical bandgap did not decrease monotonically because of the effects of strain in the film. Because of the strain, the ITO film initially grew as three-dimensional (3D) islands, like semiconductor quantum dots, and then changed to a quasi-two-dimensional form as the film thickness increased from 1 to 30 nm. The sheet resistance decreased rapidly as the film thickness increased from 20 to 150 nm, but the rate of decrease slowed as the film thickness increased from 150 to 500 nm. The film transmissivity was not significantly affected by the film thickness, growth temperature, or growth pressure.

     

Thickness dependence of strain and in-plane dielectric properties of highly (001) oriented (Ba,Sr)TiO3 thin films

Highly (001) oriented (Ba,Sr)TiO₃ thin films, grown on (001) LaAlO₃ substrates by pulsed laser deposition, exhibit strong variation of strain over the thickness range of 20-800 nm. The tensile elastic residual strain reaches a minimum value at a thickness of 250 nm, while the inhomogeneous strain decreases gradually with increasing film thickness. The 250-nm-thick film has the largest in-plane dielectric constant due to a smallest tensile elastic strain in the film and the largest in-plane tunability of 40% is achieved in the thickest film.     

Structure and magnetic properties of Fe epitaxial thin films prepared by UHV rf magnetron sputtering on GaAs single-crystal substrates

Fe thin films were prepared on GaAs single-crystal substrates of (100)_B3, (110)_B3, and (111)_B3 orientations by ultra high vacuum rf magnetron sputtering. The effects of substrate orientation and substrate temperature on the film growth, the structure, and the magnetic properties were investigated. On GaAs(100)_B3 substrates, Fe(100)_bcc single-crystal films are obtained at 300 °C, whereas Fe films consisting of bcc(100) and bcc(221) crystals epitaxially grow at room temperature (RT). Fe(110)_bcc and Fe(111)_bcc single-crystal films are respectively obtained on GaAs(110)_B3 and GaAs(111)_B3 substrates at RT-300 °C. The in-plane lattice spacings of these Fe epitaxial films are 0-9% larger than the out-of-plane lattice spacings due to accommodation of lattice mismatch between the films and the substrates. The film strain is decreased by employing an elevated substrate temperature of 300 °C. The in-plane magnetization properties are reflecting the magnetocrystalline anisotropy of bulk bcc-Fe crystal.     

Biaxially textured Ag films by grazing ion beam assisted deposition

The effect of grazing incidence 4 keV Ar⁺ ion irradiation on the early stage of Ag thin film growth on amorphous Si was investigated. The double effect of axial and surface channeling resulted in grains oriented along the 〈110〉 axis in-plane, while the (111) out-of-plane texture was maintained. A slight average tilt of the (111) out-of-plane texture axis towards the ion beam direction is proposed to result from the difference between terrace and step edge sputtering yield. The observed tilt is consistent with a minimum erosion orientation of the surface profile.     

Texture and residual stress in FeMn/Ni80Fe20 multilayers

The texture and residual stress in FeMn/Ni₈₀Fe₂₀ multilayers were studied using conventional sin²ψ method. The results show that the FeMn and Ni₈₀Fe₂₀ layers are both (111) textured. The sin²ψ plots are nonlinear, which indicates a strong residual stress gradient through the depth of the sample. Fitting of the sin²ψ plots give the residual stress in the samples. The in-plane residual stress is tensile and decreases from the surface of the sample to the substrate. The out-of-plane residual stress cannot be neglected because of the strong texture. In the two samples with the same total thickness but different periods, the FeMn layers have the same texture and residuals stress; while for Ni₈₀Fe₂₀ layer, as the thickness of each period increases, the texture becomes stronger and the residual stress becomes larger. It appears that the in-plane residual stress increases as the texture becomes stronger.     

Three-dimensional linear elastic distributions of stress and strain energy density ahead of V-shaped notches in plates of arbitrary thickness

This paper presents an analytical solution, substantiated by extensive finite element calculations, for the stress field at a notch root in a plate of arbitrary thickness. The present approach builds on two recently developed analysis methods for the in-plane stresses at notch root under plane-stress or plane strain conditions, and the out-of-plane stresses at a three-dimensional notch root. The former solution (Filippi et al., 2002) considered the plane problem and gave the in-plane stress distributions in the vicinity of a V-shaped notch with a circular tip. The latter solution by Kotousov and Wang (2002a), which extended the generalized plane-strain theory by Kane and Mindlin to notches, provided an expression for the out-of-plane constraint factor based on some modified Bessel functions. By combining these two solutions, both valid under linear elastic conditions, closed form expressions are obtained for stresses and strain energy density in the neighborhood of the V-notch tip. To demonstrate the accuracy of the newly developed solutions, a significant number of fully three-dimensional finite element analyses have been performed to determine the influences of plate thickness, notch tip radius, and opening angle on the variability of stress distributions, out-of-plane stress constraint factor and strain energy density. The results of the comprehensive finite element calculations confirmed that the in-plane stress concentration factor has only a very weak variability with plate thickness, and that the present analytical solutions provide very satisfactory correlation for the out-of-plane stress concentration factor and the strain constraint factor.     

Impact of epitaxial strain on the ferromagnetic transition temperature of SrRuO3 thin films

Epitaxially strained SrRuO₃ films were grown on SrTiO₃, DyScO₃, and NdGaO₃ oxide substrates using liquid-delivery metal-organic chemical vapour deposition. Temperature dependent resistivity measurements showed a shift in the Curie temperature (T_C) of the ferromagnetic phase transition which is suggested to be caused by the incorporated elastic lattice strain in the films. T_C increased with tensile and decreased with compressive strain, which was inferred from high resolution x-ray diffraction measurements of the in-plane and out-of-plane lattice parameters.     

Growth and hydrogenation of epitaxial yttrium switchable mirrors on CaF2

Rutherford backscattering (RBS) ion channeling measurements and X-ray diffraction experiments are performed to study the epitaxial nature of as-deposited yttrium on CaF₂〈111〉 substrates and the effect of hydrogenation on the crystalline quality. The RBS and X-ray results clearly demonstrate the unique epitaxial relation between as-deposited films and the substrate, which is preserved upon loading with hydrogen. X-Ray diffraction reveals: (i) a remarkably large lattice expansion in the direction normal to the substrate, which decreases with increasing film thickness; and (ii) an in-plane compression of the lattice. This peculiar result is related to the difference in thermal expansion coefficients of film and substrate. RBS ion channeling measurements reveal a thickness dependence of the mismatch-induced stresses. As expected, the stresses relax with increasing distance from the film/substrate interface, but surprisingly, even with films as thick as 400 nm considerable dechanneling is still observed at the film surface. Film quality, i.e. the film/substrate mismatch as well as the induced stresses and their relaxation, are discussed in relation to atomic force microscopy (AFM) results on these epitaxial films.     

Highly textured Gd2Zr2O7 films grown on textured Ni-5 at.%W substrates by solution deposition route: Growth, texture evolution, and microstructure dependency

Growth, texture evolution and microstructure dependency of solution derived Gd₂Zr₂O₇ films deposited on textured Ni-5 at.%W substrates have been extensively studied. Influence of processing parameters, in particular annealing temperature and dwell time, as well as thickness effect on film texture and morphology are investigated in details. It is found that a rotated cube-on-cube epitaxy of Gd₂Zr₂O₇< 110>//NiW<100> in-plane texture forms as soon as the (004) out-plane texture appears, implying that epitaxial growth dominates the crystallization processes. Thermal energy plays an important role in minimizing the difference of interfacial energy along two directions in the anisotropic metallic substrate. Growth of Gd₂Zr₂O₇ films displays an ultrafast kinetics under optimized conditions. Independency of sharp epitaxial (004) and polycrystalline (222) orientation is revealed from further synchrotron diffraction studies. Fully covered films with a broad thickness range exhibit a high degree of biaxial orientation, similar surface morphology with crack free and nano-size grains microstructure, seemingly independent of neither heat treatment nor thickness. Particularly, we compared the porosity of the film surface and body according to surface or cross-sectional observation and Rutherford Backscattering Spectrometry analysis, pointing to inhomogeneous structure through film thickness, i.e., dense in the surface layer but porous in the body. This is attributed to trapped gas generated during either decomposition or crystallization in the thicker films. This work not only demonstrates a route for producing textured Gd₂Zr₂O₇ buffer layers with dense structure directly on technical substrates, but also provides some fundamental understandings related to chemical solution derived films grown on metallic substrates.     

Fiber texture of sputter deposited molybdenum films and structural zone model

The texture orientation of 2.5, 5, and 100 nm thick Mo films grown by sputter deposition on SiO₂ membranes at room temperature has been studied quantitatively using transmission electron microscopy. The sample tilt angle dependent diffraction patterns of these films revealed a fiber texture with the [110] out-of-plane direction for all film thicknesses. Films grown at different sputtering power and substrate–target distance showed a similar texture orientation. These results would imply that the growth was in zone II, according to the microstructure classification of the structural zone model. This is in contrast to the conventional assignment of zone I for Mo deposition at room temperature based on the low Mo homologous temperature of ∼0.1. It was found that texture dispersion was reduced as the thickness of the film increased. Possible mechanisms, including energetic particles bombardment during growth, which could affect the surface mobility and texture of the films, were discussed.     

Textures and structures of vapor deposits

The texture of vapor deposits (PVD and CVD) changes from the orientation that places the lowest energy lattice plane parallel to the substrate under the condition of low atom or ion concentration adjacent to the deposit, to the orientation that places the higher energy crystal planes parallel to the substrate as the atom or ion concentration adjacent to the deposit increases. However, in the early stage of deposition, the deposit-substrate interface energy and the surface energy constitute the most important energies of the system. Therefore, if the lattice match is established between the substrate and the deposit without generating much strain energy, the epitaxial growth takes place to reduce the interfacial energy. When the epitaxial growth does not take place, the surface energy is dominant in the early stage of deposition and the lowest energy crystal plane tends to be placed parallel to the substrate up to a critical thickness. The critical thickness depends on the deposition conditions. If the deposition condition does not favor placing the lowest energy crystal plane parallel to the substrate, the initial texture will change to that compatible with the deposition condition as the film thickness increases, and the texture turnover thickness will be short. The microstructure and surface topography of deposits are related to their textures.     

Epitaxial growth of topological insulator Bi2Se3 film on Si(111) with atomically sharp interface

Atomically sharp epitaxial growth of Bi₂Se₃ films is achieved on Si(111) substrate with molecular beam epitaxy. Two-step growth process is found to be a key to achieve interfacial-layer-free epitaxial Bi₂Se₃ films on Si substrates. With a single-step high temperature growth, second phase clusters are formed at an early stage. On the other hand, with low temperature growth, the film tends to be disordered even in the absence of a second phase. With a low temperature initial growth followed by a high temperature growth, second-phase-free atomically sharp interface is obtained between Bi₂Se₃ and Si substrate, as verified by reflection high energy electron diffraction (RHEED), transmission electron microscopy (TEM) and X-ray diffraction. The lattice constant of Bi₂Se₃ is observed to relax to its bulk value during the first quintuple layer according to RHEED analysis, implying the absence of strain from the substrate. TEM shows a fully epitaxial structure of Bi₂Se₃ film down to the first quintuple layer without any second phase or an amorphous layer.     

Effects of MgO and MgO/Pd seed-layers on perpendicular magnetic anisotropy of CoPd thin films

We studied the effects of MgO and MgO/Pd seed-layers on perpendicular magnetic anisotropy in co-sputtered CoPd films. CoPd films with the MgO seed-layer showed perpendicular magnetic properties that were superior to those with another after annealing. The loop squareness was unity, indicating strong perpendicular magnetic anisotropy, when the MgO seed-layer was thicker than 2 nm. We observed that the out-of-plane CoPd (111) texture was strongly developed, as well as the in-plane tensile stress in the CoPd films. The magnetoelastic anisotropy coming from a negative magnetostriction λ₁₁₁ under the in-plane tensile stress dominating over other anisotropies is likely responsible for creating such strong perpendicular magnetic anisotropy. In the case of the MgO/Pd seed-layer, the CoPd films showed mixed anisotropy having both in-plane and out-of-plane magnetic anisotropy components after annealing. The appearance of the strong (100) texture of the CoPd films with the MgO/Pd seed-layer is believed to have caused the decrease in the perpendicular magnetic anisotropy that originated from the magnetoelastic anisotropy due to the additional contribution from the positive magnetostriction λ₁₀₀ but less contribution from the negative magnetostriction λ₁₁₁ when the CoPd films are under in-plane tensile stress.     

Morphology and texture evolution of nanostructured CaF2 films on amorphous substrates under oblique incidence flux

The morphology and biaxial texture of vacuum evaporated CaF(2) films on amorphous substrates as a function of vapour incident angle, substrate temperature and film thickness were investigated by scanning electron microscopy, x-ray pole figure and reflection high energy electron diffraction surface pole figure analyses. Results show that an anomalous [220] out-of-plane texture was preferred in CaF(2) films deposited on Si substrates at < 200?°C with normal vapour incidence. With an increase of the vapour incident angle, the out-of-plane orientation changed from [220] to [111] at a substrate temperature of 100?°C. In films deposited with normal vapour incidence, the out-of-plane orientation changed from [220] at 100?°C to [111] at 400?°C. In films deposited with an oblique vapour incidence at 100?°C, the texture changed from random at small thickness (5 nm) to biaxial at larger thickness (20 nm or more). Using first principles density functional theory calculation, it was shown that [220] texture formation is a consequence of energetically favourable adsorption of CaF(2) molecules onto the CaF(2)(110) facet.     

Fabrication of biaxially textured magnesium oxide thin films by ion-beam-assisted deposition

Biaxially textured MgO thin films were grown by ion-beam-assisted deposition. The film growth parameters of film thickness, ion-to-atom arrival ratio (r-value), ion beam angle, and ion beam voltage were studied. Film characterization was performed by X-ray diffraction, pole figure analysis, and atomic force microscopy (AFM). Full-width half-maximum (FWHM) of MgO (220) ?-scans and MgO (002) ω-scans, respectively, were used to evaluate in-plane and out-of-plane film texture. MgO (220) ?-scan FWHM of 3.2° and MgO (002) ω-scan FWHM of 1.2° was achieved on amorphous Si₃N₄-coated Si substrates using a 1500-V ion source oriented at 45° to the substrate normal and an r-value of 0.90. Depositions on metallic substrates yielded MgO (220) ?-scan FWHM values of 5.2° and MgO (002) ω-scan FWHM of 2.5°. Root-mean-square surface roughness of these films as measured by AFM was ≈2.3 nm.