Strain In Thin Films Research Articles

Stabilizing skyrmions by nonuniform strain in ferromagnetic thin films without a magnetic field

Magnetic skyrmions with topologically protected spin textures have recently attracted much attention due to their fascinating properties and potential application in advanced spintronics devices. For ferromagnetic thin films, skyrmions usually appear only in the presence of magnetic field. The stabilization of skyrmions in the absence of magnetic field remains an important challenge. Here, using a real-space phase field model based on Ginzburg-Landau theory, we demonstrate that a nonuniform strain can stabilize skyrmions in the FeGa thin film without a magnetic field. The phase field simulations show that the FeGa thin film exhibits a metastable skyrmion phase in the absence of magnetic field when a nonuniform strain with a cosine profile is applied. It is found that the metastable skyrmions can be transformed into a helical phase if a localized magnetic field or pulse of spin-polarized current is applied, resulting in the coexistence of skyrmion and helical phases in the ferromagnetic thin films. Furthermore, the skyrmion and helical phases can remain dynamically stable during the motion driven by a spin-polarized current. The coexistence of skyrmion and helical phases in the ferromagnetic thin films without magnetic field has potential application in skyrmion-based spintronic devices.

Manipulating the Structural and Electronic Properties of Epitaxial SrCoO2.5 Thin Films by Tuning the Epitaxial Strain.

Structure determines material's functionality, and strain tunes the structure. Tuning the coherent epitaxial strain by varying the thickness of the films is a precise route to manipulate the functional properties in the low-dimensional oxide materials. Here, to explore the effects of the coherent epitaxial strain on the properties of SrCoO2.5 thin films, thickness-dependent evolutions of the structural properties and electronic structures were investigated by X-ray diffraction, Raman spectra, optical absorption spectra, scanning transmission electron microscopy (STEM), and first-principles calculations. By increasing the thickness of the SrCoO2.5 films, the c-axis lattice constant decreases, indicating the relaxation of the coherent epitaxial strain. The energy band gap increases and the Raman spectra undergo a substantial softening with the relaxation of the coherent epitaxial strain. From the STEM results, it can be concluded that the strain causes the variation of the oxygen content in the BM-SCO2.5 films, which results in the variation of band gaps with varying the strain. First-principles calculations show that strain-induced changes in bond lengths and angles of the octahedral CoO6 and tetrahedral CoO4 cannot explain the variation band gap. Our findings offer an alternative strategy to manipulate structural and electronic properties by tuning the coherent epitaxial strain in transition-metal oxide thin films.

Residual strain measurement of Cu2ZnSnS4 thin film using X-ray line broadening

ABSTRACTIn this study, the residual strain in Cu2ZnSnS4 (CZTS) photoabsorber for solar cell application was analyzed by X-ray line broadening. Metal precursor of CZTS was deposited on a glass substrate using the co-sputtering method with Cu, Zn, and Sn single targets at a substrate temperature of 100°C. Then, the metal precursor was sulfurized within a previously evacuated corning tube that at the present stage contained sulfur powder at an annealing temperature of 550°C for 10 min. The structural properties of a CZTS thin film were analyzed by performing X-ray diffraction. The main peaks, (112), (220), and (312) were clearly visible. The residual strain was determined by using a Williamson–Hall (W–H) plot, which is a very convenient tool for determining the residual strain of thin films. The slope of the graph indicates the residual strain of the CZTS thin film, and it is determined as 2.831 × 10− 4. Using the W–H plot, it was concluded that the residual stress of CZTS thin film is tensile.

Uniaxial strain modulation of the skyrmion phase transition in ferromagnetic thin films

Mechanical control of magnetic order through strain is an important and promising approach in the development of advanced spintronics devices. Here, using a real-space phase-field model based on Ginzburg-Landau theory, we explore the uniaxial strain modulation of topological phase transition in ferromagnetic thin films. In the phase-field model, a magnetoelastic coupling between magnetization and strain, rather than the strain-induced anisotropic Dzyaloshinskii-Moriya interaction, is introduced to describe the strain-modulated anisotropic deformation and topological phase transition of skyrmions. The phase-field simulations show that a uniaxial tensile strain in the ferromagnetic thin films can not only change the wavelength and propagation direction of the spin spiral in helical phase but also induce topological phase transitions of skyrmions. Under specific magnetic fields, the ferromagnetic thin film exhibits the ferromagnetic-to-skyrmion and skyrmion-to-helical phase transitions serially when the uniaxial tensile strain increases, which are different from those induced by uniaxial stress in bulk ferromagnets. The present work not only provides a magnetoelastic coupling mechanism on the uniaxial strain control of topological magnetic structures but also suggests an effective way to tune the electromagnetic properties of ferromagnetic thin films for a new generation of spintronic devices based on skyrmions.

Goldstone-like phonon modes in a (111)-strained perovskite

Goldstone modes are massless particles resulting from spontaneous symmetry breaking. Although such modes are found in elementary particle physics as well as in condensed matter systems like superfluid helium, superconductors and magnons - structural Goldstone modes are rare. Epitaxial strain in thin films can induce structures and properties not accessible in bulk and has been intensively studied for (001)-oriented perovskite oxides. Here we predict Goldstone-like phonon modes in (111)-strained SrMnO3 by first-principles calculations. Under compressive strain the coupling between two in-plane rotational instabilities give rise to a Mexican hat shaped energy surface characteristic of a Goldstone mode. Conversely, large tensile strain induces in-plane polar instabilities with no directional preference, giving rise to a continuous polar ground state. Such phonon modes with U(1) symmetry could emulate structural condensed matter Higgs modes. The mass of this Higgs boson, given by the shape of the Mexican hat energy surface, can be tuned by strain through proper choice of substrate.

Tailoring Lattice Strain and Ferroelectric Polarization of Epitaxial BaTiO3 Thin Films on Si(001)

Ferroelectric BaTiO3 films with large polarization have been integrated with Si(001) by pulsed laser deposition. High quality c-oriented epitaxial films are obtained in a substrate temperature range of about 300 °C wide. The deposition temperature critically affects the growth kinetics and thermodynamics balance, resulting on a high impact in the strain of the BaTiO3 polar axis, which can exceed 2% in films thicker than 100 nm. The ferroelectric polarization scales with the strain and therefore deposition temperature can be used as an efficient tool to tailor ferroelectric polarization. The developed strategy overcomes the main limitations of the conventional strain engineering methodologies based on substrate selection: it can be applied to films on specific substrates including Si(001) and perovskites, and it is not restricted to ultrathin films.

Low-energy ion beam modified surface property and mechanism of high temperature superconductor YBa2Cu3O7- thin film

The interaction between ion beam and solid target is widely used in material modification. For the high temperature superconducting thin film modification, however, earlier experiments show that the samples are accompanied by the degradation in superconducting properties due to the structural damage of materials. In order to improve surface morphologies and superconducting properties of YBa2Cu3O7- (YBCO) thin films, we introduce a new ion beam structure modification (ISM) method. Although the ion bombardment time parameter effect is not clear, the related mechanism should be clarified. In this paper, the bombardment processes with duration times of 8 min, 10 min and 12 min are investigated in a vacuum chamber with an Ar+ Kaufman ion source, and the direction between the incident ion beam and the normal of sample is fixed at a certain angle. Surface morphologies and the microstructures of YBCO samples are characterized by scanning electron micrographs and X-ray diffraction patterns, respectively. In the respect of superconducting properties, the critical current density Jc is measured by Jc-scanning test. The results indicate that the needle-like a-axis grains and pores disappear gradually with the increase of the ion bombardment time. In order to characterize the effects of ion beam bombardment time on the internal strain in YBCO thin films, the relationship between the full width at half maximum and the Bragg diffraction angle of YBCO (00l) peak is studied by the William-Hall equation. The results show that the internal strain in YBCO thin film increases with increasing the ion beam bombardment time. At the same time, the critical current density Jc value of the sample after ISM processing increases, which is more than 2.2 times higher than that of the initial sample. The main reason for the increases of critical current density Jc in YBCO thin film is due to the drastic shrink of CuO bond caused by the increasing internal strain. Based on the bond contraction pair theory, the shrink of CuO bond improves the energy to break Cooper-pairs, and then increases the current carrying capacity of high temperature superconducting YBCO thin film, especially in copper-oxygen (CuO2) plane. The ISM process might be a useful method of markedly improving the surface morphology, meanwhile, the critical current density Jc value also increases in high temperature superconducting YBCO thin film.

Accessing externally induced spatially-resolved strain in GaAs thin-film solar cells by electroluminescence imaging

Externally induced strain effect was investigated in GaAs thin-film solar cells by electroluminescence (EL) imaging method. The strained regions demonstrated not only enhancement of local EL intensity but also redshift of the EL emission peaks. The observations were explained well by a gradient quantum-well (GQW) model correlated with tensile strain in the GaAs thin films. External biaxial strain was considered in this model which was accordingly obtained from the energy shift of the EL spectra. Finally, the spatially resolved strain in GaAs thin-film solar cells was obtained from the EL images by relating the EL emission intensity to the shift of the EL spectra. This work provides a potential standard method for diagnosis of externally induced strain in thin-film solar cells by EL measurements.

Epitaxial integration of CoFe2O4 thin films on Si (001) surfaces using TiN buffer layers

Epitaxial cobalt ferrite thin films with strong in-plane magnetic anisotropy have been grown on Si (001) substrates using a TiN buffer layer. The epitaxial films have been grown by ion beam sputtering using either metallic, CoFe2, or ceramic, CoFe2O4, targets. X-ray diffraction (XRD) and Rutherford spectrometry (RBS) in random and channeling configuration have been used to determine the epitaxial relationship CoFe2O4 [100]/TiN [100]/Si [100]. Mössbauer spectroscopy, in combination with XRD and RBS, has been used to determine the composition and structure of the cobalt ferrite thin films. The TiN buffer layer induces a compressive strain in the cobalt ferrite thin films giving rise to an in-plane magnetic anisotropy. The degree of in-plane anisotropy depends on the lattice mismatch between CoFe2O4 and TiN, which is larger for CoFe2O4 thin films grown on the reactive sputtering process with ceramic targets.

Investigation of mechanical properties of CVD grown titanium silicon nitride thin films under reduced atmosphere

Titanium silicon nitride (TiSiN) thin films were deposited by thermal chemical vapour deposition using TiO2 + Si3N4 powder with different H2 flow rates. Morphological, structural, and mechanical properties of deposited TiSiN films were characterized using different techniques by SEM, XRD, Raman, and nano-indentation. SEM images reveal that surface roughness of TiSiN thin films decreased with increasing of H2 flow rate. The Raman spectroscopy indicated that the intensity of acoustic phonon mode decreases, whereas intensity of optical phonon mode increases with increasing of H2 flow rate. The maximum hardness, Young’s modulus, and yield strength of the TiSiN films are 18.23, 185.26, and 83.2 GPa, respectively. The crystallite size and lattice strain of TiSiN thin films vary 2.08–4.43 nm and 0.02–0.055, respectively, for different H2 flow rates. The quantitative and qualitative analyses of TiSiN thin were carried out using the Origin 9.0 software.

Measuring Temperature Dependence of Critical Cracking Strain of Thin Films for Organic Light Emitting Diode

Measuring Temperature Dependence of Critical Cracking Strain of Thin Films for Organic Light Emitting Diode

Investigation of the effects of misfit strain on barium strontium titanate thin films deposited on base metal substrates by a modified phenomenological model

The Landau-Devonshire phenomenological model, which has been utilized to investigate epitaxial barium strontium titanate (BST) thin films, was modified to investigate the effects of misfit strain on the dielectric properties of polycrystalline BST thin films deposited on base metal substrates. The modification considers the relaxation of lattice misfit stress resulting from the formation of in-plane misfit dislocations. The modified lattice misfit strain was calculated by referring to the ferroelectric critical grain size. Moreover, the misfit strain and dielectric properties of BST thin films with different structures and substrates were investigated by the models. It was found that the measured dielectric constant and tunability of BST thin films on different metal substrates overall agreed with the computed data. In addition, the good agreement was also observed for sandwich-like structural BST thin films deposited on LNO buffered stainless steel plates. Our results indicated that the modified L-D models might be utilized to predict dielectric properties of polycrystalline BST thin films for varied substrates and multilayer structures.

The fabrication of layer‐by‐layer mode LaCoO3 film by pulsed laser deposition

The relationship between strain and growth conditions in LaCoO3 thin film was obtained to control the magnetic‐electric characteristics. The LaCoO3 thin films on the SrTiO3 substrates have been achieved by the pulsed laser deposition method, and the reflection high‐energy electron diffraction method (RHEED) was applied to monitor the growth process in situ; the layer‐by‐layer growth mode was discovered. The X‐ray diffraction and atomic force microscopy were applied to the phase analysis, and the layer thickness and the layer‐by‐layer growth mode were uncovered. Compared with the 100‐nm LaCoO3 thin films, the strain in the layer‐by‐layer ultra thin film was more controllable. The enhanced magnetic properties of the layer‐by‐layer mode ultra‐thin films could be tested in future work.

Substrate induced tuning of compressive strain and phonon modes in large area MoS2 and WS2 van der Waals epitaxial thin films

Large area MoS2 and WS2 van der Waals epitaxial thin films with control over number of layers including monolayer is grown by pulsed laser deposition utilizing slower growth kinetics. The films grown on c-plane sapphire show stiffening of A1g and E12g phonon modes with decreasing number of layers for both MoS2 and WS2. The observed stiffening translate into the compressive strain of 0.52% & 0.53% with accompanying increase in fundamental direct band gap to 1.74 and 1.68eV for monolayer MoS2 and WS2, respectively. The strain decays with the number of layers. HRTEM imaging directly reveals the nature of atomic registry of van der Waals layers with the substrate and the associated compressive strain. The results demonstrate a practical route to stabilize and engineer strain for this class of material over large area device fabrication.

Inelastic relaxation in Gd-doped ceria films: Micro-Raman spectroscopy

Micro-Raman spectroscopy and X-ray diffraction were used to study time-dependent structural changes in sputtered Ce1-xGdxO2-x/2 films. The F2g peak position was measured at five well-defined film locations; X-ray diffraction assessed in-plane compressive strain and unit cell volume. For x=0.05, at all locations monitored post-annealing, the F2g frequency continued to increase during two weeks and for x=0.10, at 4 of 5 locations during six weeks. This time-dependent behavior is attributed to relaxation of local elastic dipolar strain fields. Raman spectroscopy can properly evaluate strain in thin films of these materials only when mechanical properties, defect structure, temporal and thermal history are considered.

In situ monitoring of photostriction in chalcogenide glass film using fiber Bragg grating sensors

ABSTRACTThe reversible photostriction (photomechanical strain) in Ge35S65 chalcogenide thin film deposited by a solvent casting method has been monitored using a fiber Bragg grating (FBG) sensor. The shift in Bragg wavelength is used as a probing parameter to quantitatively measure the photoinduced strain arising because of structural modifications in these films under illumination. Exposure to band gap light (405 nm) and above band gap light (302 and 254 nm) leads to a reversible photostriction effect of the order of 100 µε. The present study shows that FBG sensors can be used to effectively measure the optomechanical actuation in chalcogenide films caused by the reversible photostriction effect in the visible and ultraviolet wavelength region.

Strain in Vanadium Thin Film due Nanosecond Laser Treatment

Strain in Vanadium Thin Film due Nanosecond Laser Treatment A thin coating of vanadium was grown on a glass substrate using vacuum coating technique. Piezoelectric thickness monitor was used to measure the thickness of the deposited coatings in-situ. The vanadium layer was then treated with a nanosecond pulsed fiber laser source in ambient conditions, and this resulted in the surface cracking due to heat dissipation from the laser. The density of cracks was observed to increase in a direct proportion to the laser fluence. The increase in the density of the cracks was accompanied by the surface modification of the of the vanadium layer as observed in the high-resolution scanning electron microscope. At higher laser fluence there are pinholes forming on the surface of the laser, and this is due to the laser punching and taking place at those experimental conditions.

Role of grain size on redox induced compositional stresses in Pr doped ceria thin films.

In constrained geometries and in varying oxygen partial pressures and operating temperatures, exchange of oxygen ions between non-stoichiometric oxide thin films (for example, doped and undoped ceria systems) and the gas phase can lead to stresses. In this study, these compositional stresses were investigated in thin films of nanocrystalline 10% praseodymium doped ceria (PCO), as a function of average grain size. In situ wafer curvature measurements, along with High Temperature X-Ray Diffraction (HTXRD), were employed to measure stresses and strains, respectively on the PCO films during oxidation-reduction cycling, over the pO2 range of 10-1-10-5 atm at 750 °C. For relatively large grain sizes, the stress values agree well with the amount of expansion induced by oxygen non-stoichiometry (chemical expansion) predicted by a thin film defect equilibria model that was developed previously. The compositional stresses were found to increase with decreasing grain size. The origin of this effect, including the role of space charge effects near surfaces and interfaces are discussed in this paper. To our knowledge, this is the first time that such comparisons are reported by simultaneously employing high temperature in situ wafer curvature and HTXRD measurements on doped ceria systems.

Tuning the metal-insulator transition of vanadium dioxide thin films using a stretchable structure

Using the external mechanical strain to tune the metal-insulator transition (MIT) properties of VO2 thin films may break the limits of strain engineering methods via lattice mismatching. Herein, a method of transfer printing was used to achieve VO2 thin films on flexible and stretchable PDMS (polydimethylsiloxane) substrates. Finite elements analysis was performed to analyze the strain states imposed in the VO2 thin films and the electrical measurements were carried out to check the MIT behaviors. It is shown that a reliable tuning of strain and MIT behavior of VO2 thin films under the brittle fracture criteria can be achieved. The results have demonstrated the feasibility of tailoring the MIT behavior of VO2 thin films via controllable external strain and may help expand the application of VO2 thin films in flexible and stretchable devices.

Surface step terrace tuned microstructures and dielectric properties of highly epitaxial CaCu3Ti4O12 thin films on vicinal LaAlO3 substrates.

Controllable interfacial strain can manipulate the physical properties of epitaxial films and help understand the physical nature of the correlation between the properties and the atomic microstructures. By using a proper design of vicinal single-crystal substrate, the interface strain in epitaxial thin films can be well controlled by adjusting the miscut angle via a surface-step-terrace matching growth mode. Here, we demonstrate that LaAlO3 (LAO) substrates with various miscut angles of 1.0°, 2.75°, and 5.0° were used to tune the dielectric properties of epitaxial CaCu3Ti4O12 (CCTO) thin films. A model of coexistent compressive and tensile strained domains is proposed to understand the epitaxial nature. Our findings on the self-tuning of the compressive and tensile strained domain ratio along the interface depending on the miscut angle and the stress relaxation mechanism under this growth mode will open a new avenue to achieve CCTO films with high dielectric constant and low dielectric loss, which is critical for the design and integration of advanced heterostructures for high performance capacitance device applications.