Strain In Thin Films Research Articles

Control of Crystal Structure of BiFeO3 Epitaxial Thin Films by Adjusting Growth Conditions and Piezoelectric Properties

The effects of epitaxial strain on the crystal structure and piezoelectric properties of (100) BiFeO3 thin films were investigated. The epitaxial strain of BiFeO3 thin films grown by pulsed laser deposition was controlled by adjusting film thickness and growth temperature. From the results of X-ray diffraction reciprocal space mapping and in-plane piezoelectric force microscopy, it is found that the crystal structure of BiFeO3 thin films is a rhombohedral structure with tetragonal distortion, and that the extent of tetragonal distortion increases with decreasing film thickness and growth temperature. From the voltage-strain curves of the films, electric field strain is dominated by the piezoelectric effect in the low-voltage region, while the electrostrictive effect appears in the high-voltage region. It was found that piezoelectric strain increases with increasing extent of tetragonal distortion, which suggests that the piezoelectric properties of BiFeO3 thin films can be improved using a mixture of two types of structural distortion.

Epitaxial Ferroelectric Heterostructures with Nanocolumn‐Enhanced Dynamic Properties

AbstractThe possibility to tailor ferroelectricity by controlling epitaxial strain in thin films and heterostructures of complex metal oxides is well established. Here it is demonstrated that apart from this mechanism, 3D film growth during heteroepitaxy can be used to favor specific domain configurations that lead to step‐like polarization switching and a giant nonlinear dielectric response in sub‐switching ac electric fields. A combination of cube‐on‐cube epitaxial growth and the formation of columnar structures during pulsed laser deposition of Pb0.5Sr0.5TiO3 films on La0.5Sr0.5CoO3 bottom electrode layers and MgO (001) substrates stabilizes ferroelectric nanodomains with enhanced dynamic properties. In the Pb0.5Sr0.5TiO3 films, a‐ and c‐oriented epitaxial columns grow from the bottom to the top of the film leading to random polydomain architectures with strong associations between the ferroelectric domains and the nanocolumns. Polarization switching in the two domain populations is initiated at distinctive fields due to domain wall pinning on column boundaries. Moreover, piezoelectric coupling between ferroelectric domains leads to strong interdomain elastic interactions, which result in an enhanced Rayleigh‐type dielectric nonlinearity. The growth of epitaxial films with 3D columnar structures opens up new routes towards the engineering of enhanced ferroelectric and electromechanical functions in a broad class of complex oxide materials.

X-ray nanodiffraction reveals strain and microstructure evolution in nanocrystalline thin films

A novel synchrotron X-ray nanodiffraction approach for the quantitative characterization of strain and microstructure depth gradients in nanocrystalline thin films is introduced. Experiments were performed using monochromatic X-ray beams 100 and 250 nm in diameter on a 15 μm thick CrN thin film. The results reveal a correlation between applied deposition conditions and a complex residual strain depth profile with a step-like shape and superimposed low-amplitude oscillations. Microstructure analysis shows multiple grain nucleation sites, smooth texture transitions and oscillating grain size distributions.

Ultrafast photoinduced mechanical strain in epitaxial BiFeO3 thin films

We studied ultrafast dynamics and photoinduced mechanical strain of BiFeO3 thin films by dual-color transient reflectivity measurements (ΔR/R). Anisotropic photostriction in BiFeO3 is found to be mainly driven by the optical rectification effect. Results of the photostriction at various thicknesses show that the estimated sound velocity along [110] direction of BiFeO3 is 4.76 km/s.

Residual strain measurement of piezoelectric multilayers by spiral structure

A new structure is described to measure the residual strain of thin film of piezoelectric multi-layers. The spiral shaped structure consists of the four fixed-guided beams. Piezoelectric multilayers consisting of SiOx/Pt/PZT/Pt on SiNx substrate are used to evaluate the suggested structure. Finite element analysis predicts that the out-of-plane displacement of the spiral structure by residual stress depends linearly on the beam length, but there is little difference depending on the beam width. PZT is prepared by sol-gel method and multilayered spiral structures are released by microfabrication technique. Sensitivity analysis of the spiral structure with various layer stack shows that the high displacement of piezoelectric multilayers can be decreased by the application of SiOx layer with compressive stress over the piezoelectric multilayers.

Ab initiostudy of the factors affecting the ground state of rare-earth nickelates

We have used first-principles methods to investigate the factors that control the ground state of rare-earth nickelates, studying in detail the case of NdNiO${}_{3}$. Our results suggest a complex phase diagram, with the bulk compounds standing on the edge of various instabilities that can be triggered by both electronic (e.g., changes in the Coulomb repulsion) and structural (e.g., epitaxial mismatch) means. In particular, we reveal that several phase transitions can be induced by epitaxial strain in thin films and predict that a continuous transformation between insulating spin-density-wave- and metallic spin-spiral-like solutions occurs at moderate values of the in-plane mismatch. Our results provide a coherent picture of structural and electronic effects in nickelates and have implications for current experimental and theoretical work on these compounds.

Valence band effective-mass Hamiltonians for the group-III nitrides from quasiparticle self-consistentGWband structures

We present band gaps, electron effective masses, and valence band effective-mass Hamiltonian parameters as well as strain deformation potentials of the crystal field splittings for AlN, GaN, and InN obtained from quasiparticle self-consistent $GW$ calculations. Excellent agreement is obtained with experimental data for the crystal field and spin-orbit coupling splittings of bulk AlN and GaN. For InN, the discrepancy on the crystal field splitting is likely due to the residual strain in InN thin films from which that experimental value was extracted. We obtain a negative spin-orbit splitting for InN, which is plausible in view of the stronger negative contribution of In-4$d$ in InN than Ga-3$d$ in GaN. The inverse effective-mass parameters ${A}_{i}$ agree well with previous ${G}_{0}{W}_{0}$ calculations except for ${A}_{6}$. We find that the ${A}_{6}$ parameter describing the band dispersion in directions intermediate between parallel and perpendicular to the basal plane is not well described by the quasicubic approximation. Good agreement with the most reliable experimental data is obtained for hole effective-mass parameters in AlN and GaN, extracted from exciton binding energies and their fine structure. For InN and GaN, the spin-splittings of the bands in the plane due to spin-orbit coupling requires the inclusion of linear in k and spin terms.

Control of the charge/orbital ordering transition in epitaxial La7/8Sr1/8MnO3 thin films through compressive strain

The effect of compressive strain on charge/orbital ordering (COO) transition in epitaxial La7/8Sr1/8MnO3 (LSMO) thin films was investigated. The compressive strain of LSMO thin films grown on LaAlO3 substrates can be modified by varying the film thickness which was verified by x-ray diffraction and Raman scattering measurements. It was found that both the COO transition and magnetic transition of LSMO thin films could be suppressed by the compressive strain and their transition temperatures could be tuned by strain relaxation. The variation of COO transition with compressive strain can be explained by the strain-dependent lattice deformation by structural phase transition. These results indicate that the lattice degree of freedom plays a crucial role in the stabilization of COO phase in LSMO, and will be beneficial to the understanding of the microscopic origin of the COO phase in lightly doped manganites.

Spectroscopic ellipsometry as an optical probe of strain evolution in ferroelectric thin films

Heteroepitaxial strain in ferroelectric thin films is known to have a significant impact on both their low and high frequency dielectric properties. In this paper, we use ex-situ spectroscopic ellipsometry to study the strain evolution with film thickness, and strain relaxation in ferroelectric Ba0.5Sr0.5<TiO3 epitaxial films grown on single crystal substrates. For films grown on MgO substrates, a critical thickness for strain relaxation is observed. In addition, studies of Ba0.5Sr0.5<TiO3 films grown on different single crystal substrates reveal that the strain relaxation rate can be inferred from changes in the optical properties. Using this information, we show that the optical constants of Ba0.5Sr0.5<TiO3 can be readily tuned via strain engineering.

Interplay of point defects, biaxial strain, and thermal conductivity in homoepitaxial SrTiO3 thin films

Separating out effects of point defects and lattice strain on thermal conductivity is essential for improvement of thermoelectric properties of SrTiO3. We study relations between defects generated during deposition, induced lattice strain, and their impact on thermal conductivity κ in homoepitaxial SrTiO3 films prepared by ion-beam sputtering. Lowering the deposition temperature gives rise to lattice expansion by enhancement of point defect density which increases the hardness of the films. Due to a fully coherent substrate-film interface, the lattice misfit induces a large biaxial strain. However, we can show that the temperature dependence of κ is mainly sensitive on the defect concentration.

Strain induced ionic conductivity enhancement in epitaxial Ce0.9Gd0.1O2−δ thin films

Strained epitaxial Ce0.9Gd0.1O2−δ (CGO) thin films are deposited on MgO(001) substrates with SrTiO3 (STO) buffer layers. The strain in CGO epitaxial thin films is induced and controlled by varying the thickness of STO buffer layers. The induced strain is found to significantly enhance the in-plane ionic conductivity in CGO epitaxial thin films. The ionic conductivity is found to increase with decrease in buffer layer thickness. The tailored ionic conductivity enhancement is explained in terms of close relationships among epitaxy, strain, and ionic conductivity.

Nano- and microstructured gold tubes for surface-enhanced Raman scattering by vapor-induced strain of thin films

We have demonstrated a new fabrication methodology for gold nano- and microtubes with regular cylinders by vapor-induced strain of porous thin films, which exhibit highly sensitive SERS signals for Rhodamine 6G detection. The strain originates from the temperature difference in the top and bottom of the gold film, which is brought about by the vapor of the Se powder, because Selenium has a low melting point. Around its melting point, Se appears in the vapor state, while the gold nanofilms remain in the solid phase. By exploiting such vapor-driven self-rolling of the gold film, shrinkage or collapse of the tube wall by capillary forces can be effectively avoided. Moreover, the diameter and the length of the obtained gold tubes can be well controlled by changing the Se powder evaporation temperature and the thickness of the gold film. The wall number of the tubes varied with the evaporation time and speed of the Se powder. Attributed to the larger surface area of the porous structures, the tubes exhibit SERS signals nearly 20 times larger than those of the gold film. This means that the detection limit of gold film is enhanced by over one order of magnitude.

Ripple-modulated electronic structure of a 3D topological insulator

Three-dimensional topological insulators host linearly dispersing states with unique properties and a strong potential for applications. An important ingredient in realizing some of the more exotic states in topological insulators is the ability to manipulate local electronic properties. Direct analogy to the Dirac material graphene suggests that a possible avenue for controlling local properties is via a controlled structural deformation such as the formation of ripples. However, the influence of such ripples on topological insulators is yet to be explored. Here we use scanning tunnelling microscopy to determine the effects of one-dimensional buckling on the electronic properties of Bi(2)Te(3.) By tracking spatial variations of the interference patterns generated by the Dirac electrons we show that buckling imposes a periodic potential, which locally modulates the surface-state dispersion. This suggests that forming one- and two-dimensional ripples is a viable method for creating nanoscale potential landscapes that can be used to control the properties of Dirac electrons in topological insulators.

Dielectric Enhancement Effect in (Ba0.7Sr0.3TiO3/Ba0.5Sr0.5TiO3)n (n = 1,2,3,4) Multilayered Thin Films Deposited by Chemical Solution Deposition Method

Multilayered Ba0.7Sr0.3TiO3/Ba0.5Sr0.5TiO3 (BST0.7/0.5)n (n = 1, 2, 3 and 4, respectively) thin films with different layered periods are prepared on Pt/TiO2/SiO2/Si substrates by chemical solution deposition method. The microstructure and dielectric properties of the films are investigated systematically. Obvious dielectric enhancement occurs in (BST0.7/0.5)n multilayer comparing with Ba0.5Sr0.5TiO3 and Ba0.7Sr0.3TiO3 monolayered thin films due to Maxwell-Wagner effect. On the other hand, strain and grain sizes of the multilayered thin films can also affect the dielectric properties. Specially, the (BST0.7/0.5)2 shows the best dielectric properties with dielectric constant of 557, loss tangent of 0.025 and tunability of 46% at 200 KHz, which is beneficial in applications.

Measuring Residual Stress on the Nanometer Scale - Novel Tools for Fundamental and Applied Research

ABSTRACTIn the present paper, strain measurements based on second harmonic generation (SHG) and electron backscatter diffraction (EBSD) is demonstrated via two illustrative applications. While SHG gains access to strains in buried interfaces, EBSD can be used to measure strains in crystalline thin films with high spatial resolution on the order of tens of nanometers and high surface sensitivity. In addition, target preparation using low-voltage ion beam milling is demonstrated, gaining access to unstrained sample positions in strained silicon on insulator (sSOI) systems which are necessary for common “pattern-shift” methodologies.

In-situ strain effect on extrinsic transport of polycrystalline La0.7(Ca,Sr)0.3MnO3 films

The spin-polarised transport in ferromagnetic polycrystalline La0.7(Sr,Ca)0.3MnO3 films on piezoelectric substrate has been investigated. The systematic study involved in finding the effect of in-situ strain on extrinsic electrical transport of various thick polycrystalline La0.7(Sr,Ca)0.3MnO3 thin films. The in-situ strain in the manganite polycrystalline thin film is achieved by applying an electric field to the piezoelectric substrate 0.72 Pb(Mg1/3Nb2/3)O3–0.28 PbTiO3 (PMN-PT). A reversible strain of about 0.11% is acquired with an application of 10kV/cm to the piezoelectric substrate. A typical drop in resistance at low magnetic fields has been found in all the polycrystalline manganite films. The effect of reversible strain versus the resultant strain gauges was discussed in all the polycrystalline films. At low temperatures, the effect of strain on low-field magnetoresistance and high-field magnetoresistance was found to be negligible. Further, the results are compared with the transport in manganite films deposited on step edge junctions.

Effect of annealing on the structural, optical and electrical properties of ZnO thin films by spray pyrolysis

Zinc oxide thin films have been deposited on glass substrates at a substrate temperature of 673 K by spray pyrolysis. The samples are annealed in ambient atmosphere at various temperatures. The effect of annealing on structural, electrical, and optical properties of ZnO films has been investigated. X-ray diffraction patterns show that crystallinity of the ZnO films has been improved after annealing. The morphology of ZnO thin films is studied by atomic force microscopy. The tensile strain (compressive stress) is found to decrease with increase in annealing temperature which indicates the relaxation of tensile strain in ZnO thin films. A decrease in energy band gap is observed with increase of annealing temperature. The mechanism of blue-green luminescence of ZnO thin film has been analyzed. The resistivity is found to decrease with annealing temperature.

Modulation of strain, resistance, and capacitance of tantalum oxide film by converse piezoelectric effect

We deposited tantalum oxide film on a laminate structure composed of a Si substrate and a piezoelectric 0.72Pb(Mg1/3Nb2/3)O3–0.28PbTiO3 single crystal and achieved in situ modulation of the resistance and capacitance of the Ta2O5 film. The modulation arises from the induced lattice strain in the Ta2O5 film, which is induced by the electric-field-induced strain in the piezoelectric crystal. Under an external electric field of ∼2 kV/cm, the longitudinal gauge factor of the Ta2O5 film is ∼3300. The control of the strain using the converse piezoelectric effect may be further extended to tune the intrinsic strain of other oxide thin films.

Growth Mechanism and Strain Variation of GaN Material Grown on Patterned Sapphire Substrates With Various Pattern Designs

Growth mechanism and strain variation of GaN material grown on patterned sapphire substrates (PSS) with various pattern designs are investigated. Time evolutions of GaN epi-layer grown on trapezoid-shaped PSS (TPSS) and cone-shaped PSS (CPSS) are demonstrated by field emission scanning electron microscopy (SEM). From the transmission electron microscopy (TEM) observations, the threading dislocations (TDs) of CPSS is less than that of TPSS. Space-depended strain in GaN epi-layer grown on PSS is measured by micro-Raman spectroscopy. The magnitude of residual strain of 6-μm-period TPSS is Δσ ≈ 0.97 GPa and that of 6-μm -period CPSS is Δσ ≈ 0.17 GPa. On the other hand, a design of shorter period (3-μm-period PSS) could further release residual strain of epitaxial GaN thin film. Based on experimental results, it is suggested that the CPSS with short period has better epitaxial quality and less residual strain than TPSS.

Sizable electrocaloric effect in a wide temperature range tuned by tensile misfit strain in BaTiO3 thin films

The effects of misfit strain on the electrocaloric (EC) properties of BaTiO3 thin films arestudied by using the Landau-Devonshire thermodynamic theory. The “misfit-strain temperature” phase diagrams for different electric fields are constructed. It was found that the EC effect of a BaTiO3 thin film is strongly dependent on misfit strain. More interestingly, we found that BaTiO3 thin films with tensile misfit strain exhibit a sizable EC effect in a wider temperature range in comparison with those with compressive misfit strain.