Strain In Thin Films Research Articles

Measurement of Shear Strengths of Cu Films Using Precise Chip Forming.

The mechanical properties of thin films are under-researched because of the challenges associated with conventional experimental methods. We demonstrate a technique for determining the intrinsic shear strength and strain of thin films using a nano-cutting technique based on an orthogonal cutting model with precise control of the cutting system. In this study, electroplated Cu films with thicknesses of 1.5 μm and 5 μm and a sputtered Cu film with a thickness of 130 nm were fabricated to evaluate the mechanical strength. Experiments revealed a shear strength of approximately 310 MPa with a shear strain of 1.57 for the electroplated Cu film and a shear strength of 389 MPa with a shear strain of 2.03 for the sputtered Cu film. In addition, X-ray diffraction analysis was performed to correlate the experimental results.

Breaking of Thermopower-Conductivity Trade-Off in LaTiO3 Film around Mott Insulator to Metal Transition.

Introducing artificial strain in epitaxial thin films is an effective strategy to alter electronic structures of transition metal oxides (TMOs) and to induce novel phenomena and functionalities not realized in bulk crystals. This study reports a breaking of the conventional trade‐off relation in thermopower (S)–conductivity (σ) and demonstrates a 2 orders of magnitude enhancement of power factor (PF) in compressively strained LaTiO3 (LTO) films. By varying substrates and reducing film thickness down to 4 nm, the out‐of‐plane to the in‐plane lattice parameter ratio is controlled from 0.992 (tensile strain) to 1.034 (compressive strain). This tuning induces the electronic structure change from a Mott insulator to a metal and leads to a 103‐fold increase in σ up to 2920 S cm−1. Concomitantly, the sign of S inverts from positive to negative, and both σ and S increase and break the trade‐off relation between them in the n‐type region. As a result, the PF (=S 2 σ) is significantly enhanced to 300 µW m− 1K−2, which is 102 times larger than that of bulk LTO. Present results propose epitaxial strain as a means to finely tune strongly correlated TMOs close to their Mott transition, and thus to harness the hidden large thermoelectric PF.

Strain-Controlled Spin Transition in Heterostructured Metal-Organic Framework Thin Film.

Metal-organic framework (MOF) thin films have recently attracted much attention as a new platform for surface/interface research, where unconventional structural and physical properties emerge. Among the many MOFs as candidates for fabrication of thin films, Hofmann-type MOFs {Fe(pz)[M(CN)4]} [pz = pyrazine; M = Ni (Nipz), M = Pt (Ptpz)] are attractive, because they undergo spin transitions with concomitant structural changes. Here, we demonstrate the first example of a strain-controlled spin transition in heterostructured MOF thin films. The spin transition temperature of Ptpz can be controlled in the temperature range of 300-380 K by fabricating a nanometer-sized heterostructured thin film with a Nipz buffer layer, where the smaller lattice of Nipz causes epitaxial compressive strain to the Ptpz layer. The fabricated heterostructured thin film exhibited a remarkable increase in spin transition temperature with a dynamic structural transformation, confirmed by variable-temperature (VT) X-ray diffraction and VT Raman spectroscopy. By verifying interfacial strain in a heterostructured thin film, we can rationally control the characteristics of MOFs-not only spin transition but also various physical properties such as gas storage, catalysis, sensing, proton conductivity, and electrical properties, among others.

The combined use of wrinkling and cracking for the characterization of the mechanical properties of the laser-fabricated nanometer-thick amorphous carbon films

Direct laser writing carbonization has been recently developed to enable facile fabrication of patternable nanometer-thick two-dimensional amorphous carbon films, which may find promising applications in high-performance sensing, energy storage, catalysis, biomaterials etc. Given its nanometer-thick character and brittleness in nature, it is quite challenging to evaluate and characterize the mechanical properties of this novel material. With assistance of a sacrificial-layer transfer approach for sample preparation, herein, we rely on wrinkling metrology as a viable tool for characterizing the elastic modulus of the laser-fabricated nanometer thick amorphous carbon films. Moreover, upon combining the wrinkling metrology with channel cracking behavior investigation, the strength-related mechanical properties, including fracture toughness, fracture strength and fracture strain of the carbon thin films of varied thickness have been evaluated comprehensively. Within the experimental conditions being studied, the elastic modulus, fracture toughness, fracture strength and fracture strain of the laser-fabricated nanometer-thick carbon films respectively fall in a range of 23.1–34.9 GPa, 16.3–151.7 J/m2, 0.5–1.4 GPa, and 2.0–4.5%. The results and methodology presented in this study demonstrate the usefulness of combining wrinkling metrology and cracking technique as a simple but generally applicable approach for comprehensive characterization and evaluation of the mechanical properties of different types of brittle and nanometer-thick thin films.

Activation energy and its fluctuations at grain boundaries of Er3+:BaTiO3 perovskite thin films: Effect of doping concentration and annealing temperature

An improved understanding of the grain growth kinetics in Erbium-doped Barium Titanate (BTE), has attracted great interest in its applications in photonic and electronic devices. To study the effects of doping and thermal treatment on the physical properties of films, the chemical surface, structural phase, grain size and fluctuations of the grain boundary of BTE films were analyzed and then their effects on the activation energy of the grain growth of the BTE films were revealed. BTE thin films were deposited on Si(100) substrates by electron beam physical vapor deposition (EB-PVD). XRD pattern analysis of the BTE films revealed a tetragonal crystal structure. Williamson–Hall analysis was used to confirm the tensile strain of the BTE thin films. The experimental results indicate that the Er doping in BaTiO3 show strong response by increasing the activation energy of the grain growth and increasing the grain size of the BTE films, while fluctuations of the surface roughness of the BTE films from 0.47 nm to 24.80 nm were also observed. Thin films: BTE2, BTE3, and BTE6 exhibit a grain growth exponent (n) of 5.71, 6.50, and 8.22 that respectively suggest a surface diffusion-controlled grain growth in these films. The activation energies for the grain growth came about 6.63, 6.95 and 19.7 kJ/mol for BTE2, BTE3 and BTE6, respectively, which were close to the values reported for the activation energy in BaTiO3. This was consistent with the high grain boundary density of the BTE films, which was responsible for the mechanism of surface diffusion and point defects. This behavior has been observed in case of BaTiO3, but for Erbium-doped barium titanate it has never been reported.

A study on strain and density in graphene-induced Bi2O3 thin film

A study on strain and density in graphene-induced Bi2O3 thin film

Tuning magnetic and optical properties through strain in epitaxial LaCrO3 thin films

We report on the effect of epitaxial strain on magnetic and optical properties of perovskite LaCrO3 (LCO) single crystal thin films. Epitaxial LCO thin films are grown by pulsed laser deposition on proper choice of substrates to impose different strain states. A combined experimental and theoretical approach is used to demonstrate the direct correlation between lattice-strain and functional properties. The magnetization results show that the lattice anisotropy plays a critical role in controlling the magnetic behavior of LCO films. The strain induced tetragonality in the film lattice strongly affects the optical transitions and charge transfer gap in LCO. This study opens possibilities to tailor the functional properties of LCO and related materials by strain engineering in epitaxial growth.

Twin-boundary engineering and thickness effect on strontium-barium niobate thin films on MgO substrate

The Sr0.5Ba0.5Nb2O6 (SBN50) films with different thicknesses synthesized by a high-current discharge rf-sputtering in an oxygen atmosphere. All synthesized films have the structure of tetragonal tungsten bronze. The dependence of the unit cell parameters of the film material demonstrates a size effect, the magnitude of which is comparable to the distortions resulting from the ferroelectric relaxor phase transformation of SBN crystals of the same composition. The twinning of the same type is observed in all films when the twinned film components are rotated relative to each other by 36.8 degrees around the fourth-order rotational axis. The orientation of the films relative to the substrate can be written as [001]SBN∥[001]MgO; [100]SBN+∥[310]MgO and [100]SBN−∥[31¯0]MgO, where the sign "+" and "–" marks the twin components. It is noteworthy that such an orientation of the film relative to the substrate corresponds to the maximum possible tensile stress of heteroepitaxial films acting from the MgO single crystal substrate. This type of twinning is not typical for SBN50 films on MgO grown by other synthesis methods. The appearance of this unusual type of twinning is associated with the peculiarities of synthesis. At the initial moment of sputtering, the substrate is additionally heated by the sputtered substance’s plasma. The substrate temperature can also be used in other synthesis methods to control twinning and strain in barium strontium niobate thin films.

Influencing the structural, vibrational and electronic properties of pulsed laser deposited PrNi0.95Cu0.05O3 thin films by tuning epitaxial strain

The variation of epitaxial strain in thin films via thickness modification is an established way for influencing the structural and transport properties in low dimensional mott materials. To understand the effect of strain on physical properties, we have prepared a series of PrNi0.95Cu0.05O3 thin films on single crystal LaAlO3 (001) substrate, with varied thickness (5 - 25 nm) using pulsed laser deposition technique. The crystallographic structure of these films is highly oriented towards the (001) axis. The c-axis lattice constant and the unit cell volume decreases with increasing thickness, and therefore, the epitaxial strain tends to partially relax. This partial relaxation of the strain causes a substantial softening of the Raman modes. Temperature-dependent Raman spectra are analyzed to estimate two key parameters, i.e., Grüneisen parameter and first-order temperature coefficient. Power-dependent Raman Spectroscopy and the above-mentioned two parameters have been used to estimate the thermal conductivity of the 25 nm thin film. Balkanski model for three phonon processes has been used to understand the anharmonic contribution in the system, which scales with thickness. The theoretical fits to the temperature-dependent resistivity plots suggest that the films exhibit non-Fermi liquid behavior.

Self-Organized Ferroelastic Superdomains with Enhanced Piezoresponse in (101)-Oriented Pb(Zr0.2,Ti0.8)O3 Thin Films

Thin film engineering, utilizing proper control of crystalline orientation, strain, thickness, and defect density in thin films, has offered tremendous degrees of freedom for researchers to modify the physical properties, phase stability, and domain architectures in a wide spectrum of functional materials. This work unveils a naturally formed superdomain architecture arising from the low-orientation-symmetry-induced energetically degenerate state in a (101)-oriented tetragonal Pb(Zr,Ti)O3 epitaxial film. Different from conventional (101)-oriented/(110)-oriented domains found in (101)-oriented tetragonal Pb(Zr,Ti)O3 epitaxial heterostructures, these superdomains are composed of (101)-oriented domains with an alternative arrangement of opposite out-of-plane polarizations. Additionally, the conjunction between these superdomains has been found as another variant of the 90° domain wall that can exist in the low-symmetric preferred crystallographic orientation. The superdomains simultaneously exhibit an atypical piezoresponse hysteresis loop with additional metastable states, corresponding to the multistate ferroelastic domain switching process. This work gives distinct insight into the correlation between the domain pattern, lattice symmetry, and polarity switching, offering an effective way for exploring and engineering ultrafine domain features in tetragonal ferroelectrics.

Engineering polar vortex from topologically trivial domain architecture

Topologically nontrivial polar structures are not only attractive for high-density data storage, but also for ultralow power microelectronics thanks to their exotic negative capacitance. The vast majority of polar structures emerging naturally in ferroelectrics, however, are topologically trivial, and there are enormous interests in artificially engineered polar structures possessing nontrivial topology. Here we demonstrate reconstruction of topologically trivial strip-like domain architecture into arrays of polar vortex in (PbTiO3)10/(SrTiO3)10 superlattice, accomplished by fabricating a cross-sectional lamella from the superlattice film. Using a combination of techniques for polarization mapping, atomic imaging, and three-dimensional structure visualization supported by phase field simulations, we reveal that the reconstruction relieves biaxial epitaxial strain in thin film into a uniaxial one in lamella, changing the subtle electrostatic and elastostatic energetics and providing the driving force for the polar vortex formation. The work establishes a realistic strategy for engineering polar topologies in otherwise ordinary ferroelectric superlattices.

Anisotropic chemical expansion due to oxygen vacancies in perovskite films

In scientifically intriguing and technologically important multifunctional ABO3 perovskite oxides, oxygen vacancies are most common defects. They cause lattice expansion and can alter the key functional properties. Here, it is demonstrated that contrary to weak isotropic expansion in bulk samples, oxygen vacancies produce strong anisotropic strain in epitaxial thin films. This anisotropic chemical strain is explained by preferential orientation of elastic dipoles of the vacancies. Elastic interaction of the dipoles with substrate-imposed misfit strain is suggested to define the dipolar orientation. Such elastic behavior of oxygen vacancies is anticipated to be general for perovskite films and have critical impacts on the film synthesis and response functions.

Plastic deformation of ZnO thin films through edge and screw dislocation movements

AbstractColumnar wurtzite grains were formed in sputtered ZnO thin films deposited on a plastic polyethylene terephthalate substrate. Selected‐area diffraction patterns reveal that the columnar grains in the sputtered films present two preferred growth planes, namely, the basal (0002) and prismatic (100) growth planes. The diffraction patterns obtained also confirm that the microstructure of sputtered indium tin oxide thin films is amorphous in nature. Tensile tests indicate that the fracture strain of the ZnO thin film occurs between 1.73% and 2.14%, while the fracture strain of the indium tin oxide thin film occurs between 0.24% and 0.67%. Thus, the fracture toughness of the sputtered ZnO thin film is greater than that of the sputtered indium tin oxide thin film. High‐resolution transmission electron microscopic images demonstrate that edge and screw dislocations could be identified in the sputtered ZnO thin films. Moreover, edge and screw dislocation movements may, respectively, be observed in the basal‐ and prismatic‐oriented ZnO columnar grains of the sputtered ZnO thin films. Our results indicate that movements of the edge and screw dislocations in the basal‐ and prismatic‐oriented ZnO columnar grains account for the plastic deformation of the investigated ZnO thin films under tensile stress.

Phase field study on the effect of substrate elasticity on tip-force-induced domain switching in ferroelectric thin films

Tip-force-induced domain switching in ferroelectrics has recently attracted extensive interest as it provides an alternative switching strategy that might ease the problems brought by electrical switching. From the viewpoint of mechanics, substrate elasticity can largely modify the tip-induced deformation of ferroelectric thin films. However, so far, discussions on the influence of substrate elastic properties on such domain switching still remain exclusive. Here, a phase-field model is employed to study the influence of substrate stiffness on the domain switching in BaTiO3 (BTO) thin films, with the strain and stress distributions in BTO thin films and substrates solved by the finite element method. The results demonstrate that the substrate stiffness and loading modes (i.e., pressing and sliding) have a great influence on the symmetry of strain and stress distributions. The switched domain size is highly dependent on the substrate stiffness and loading modes. The switching is more efficient for thin films on a softer substrate. Moreover, the domain could be switched more effectively by the sliding mode under relatively large forces. Our study thus provides a strategy to increase the mechanical switching efficiency of ferroelectric thin films via tuning the substrate elasticity.

Epitaxial stabilisation of Ge1 x Sn x alloys

The thermodynamic stability of germanium tin alloys is investigated across the composition range by applying density functional theory together with the cluster expansion formalism (CE). It is known that GeSn alloys are immiscible and that non-equilibrium growth techniques are required to produce metastable films and nanostructures. Insight into the driving forces behind component segregation is gained by investigating the equilibrium thermodynamics of GeSn systems. The alloy free energy of mixing is computed by combining enthalpies from the CE with entropy terms for configurational and vibrational degrees of freedom. Volume deformations due to the large mismatch in ionic radii are readily found to be the key driving force for immiscibility at all temperatures of relevance. This leads to a study of epitaxial stabilisation by employing latticed matched substrates to favour growth of alloys with fractional compositions of x = 0, approximately x = 0.5 and x = 1. The reduction in the free energy of mixing due to epitaxial strain in thin films is quantified for each substrate leading to indicators for growth of kinetically stable films.

Tetragonally strained crystal structure and ferroelectric properties of epitaxial PbTiO3 thin films grown on single-crystal Rh substrates

To form tetragonally strained PbTiO3 (PTO) thin films, PTO thin films were deposited on single-crystal Rh substrates using pulsed laser deposition (PLD). Reducing the PLD deposition rate could further increase the tetragonal strain of the PTO thin films, improve the crystallinity due to the layer-by-layer growth of the films, and improve their ferroelectric and piezoelectric properties. From the time-dependent ferroelectric switching current curves of the PTO capacitors, it was confirmed that the switching speed of the ferroelectric polarization increases as the crystallinity of the thin film improves. In particular, it was confirmed that the band gap could be increased due to the tetragonal strains formed by the lattice misfit between PTO and Rh, which supports the good leakage current characteristics of the PTO films with relatively large tetragonal strains.

Effect of Cu incorporation on the crystallinity, lattice strain, morphology and electrical properties of nanostructured ZnS-PVA thin films

Cu incorporated ZnS-PVA thin films at different concentration of Cu (0.5, 1, 5, 10 and 15% by volume) have been synthesized by solvent casting followed by thermolysis technique. X- Ray diffraction (XRD) studies revealed that the as-prepared thin films are nanocrystalline and possesses cubic phase with preferred orientation along (111) plane. The XRD peak intensity of the films decreases significantly at high Cu concentration. The lattice strain estimated through Williamson-Hall plot, increases with increase of Cu concentration. Scanning Electron Microscope images indicate homogeneous and continuous surface morphology of 5% Cu incorporated ZnS-PVA thin films. However, at high percentage of Cu the surface roughness increases with the formation of elongated shaped particles of average length ~650 nm. Optical band gap decreases from 3.72 to 2.80 eV with increase of Cu concentration. The room temperature electrical resistivity of the films is of the order of 108 Ωcm.

High-performance methylammonium-free ideal-band-gap perovskite solar cells

Summary The development of mixed tin-lead (Sn-Pb)-based perovskite solar cells (PSCs) with low band gap (1.2–1.4 eV) has become critical not only for pushing single-junction devices toward the maximum efficiency given by the Shockley-Queisser limit, but also for enabling all-perovskite tandem devices beyond this limit. However, achieving high power-conversion efficiency (PCE) and long-term device operation stability simultaneously remains a significant challenge for Sn-Pb-based PSCs. Here, we demonstrate near ideal-band-gap (∼1.34 eV) methylammonium-free Sn-Pb-based PSCs with high efficiency (∼20%) and promising operational stability of maintaining >80% of initial PCE over 750 h under maximum-power-point tracking. The key to this success is the use of a SnCl2⋅3FACl complex additive that improves the microstructure and reduces the development of residual stress in the Sn-Pb perovskite thin films, which in turn enhances the efficiency and stability of the Sn-Pb-based ideal-band-gap PSCs.

Nanoscale light- and voltage-induced lattice strain in perovskite thin films.

We report on localized nonlinear lattice deformation and nanoscale structural rearrangement in methylammonium lead triiodide films triggered by the combined action of light and voltage. These effects, revealed by second harmonic piezoresponse force microscopy, are connected with organic cation motion, implicating localized cation migration as a key contributor to perovskite optoelectronic device instability under operating conditions.

Evaluation of Temperature and Germanium Concentration Dependence of EXAFS Oscillations in Si-Rich Silicon Germanium Thin Films

1. Background and purpose Silicon germanium (SiGe) has higher mobility and lower thermal conductivity than pure Si, and it is expected as a next-generation electronic and thermoelectric device material. Understanding of carrier scattering and phonon transport is important for controlling the thermal conductivity, which involves the thermal vibration of atoms, in electronic and thermoelectric devices. However, there has been no report evaluating the influence of the Ge closest ligand in SiGe thin film on thermal vibration. The dependence on Ge concentration and temperature has not been clarified yet. In this study, we evaluated the relationship between the local lattice vibration between Ge and the ligand in the SiGe thin film and the Ge concentration by XAFS (X-ray Absorption Fine Structure) measurement, and estimated the Einstein temperature (T E) that characterizes phonons. 2. Experimental method Si0.847Ge0.153 and Si0.703Ge0.297 were epitaxially grown on Si (001) substrates, and the thicknesses of these films were 33 and 38 nm, respectively [1]. We measured the XAFS of Ge-K absorption edge for the SiGe films by fluorescence XAFS measurement at BL14B2 in SPring-8. We also measured the Ge powder as a reference sample by transmission XAFS measurement. While the measurement, we controlled the sample temperature between 10 - 300 K or 300 - 600 K using a refrigerator or a heating stage, respectively. 3. Results and Discussion The Debye-Waller factor is expressed by Eq. (1) [2], and the amount of change in the Debye-Waller factor from the value at 10 K (ΔDWF) can be expressed by Eq. (2). Here, σ(T) is a Debye-Waller factor, T is an absolute temperature, A is a constant, and T E is the Einstein temperature. σ 2(T) = Acoth(T E / 2T), (1)Δσ 2(T) = A[coth(T E / 2T) – coth{T E / (2 × 10)}], (2)Figure 1 shows the relationship between the amount of change in the Debye-Waller factor (ΔDWF) based on the value at 10 K and temperature for Ge and SiGe. The solid and dashed lines exhibit the calculation using Eq.(1) with the obtained T E shown in Fig.2. From Fig. 1, the ΔDWF of SiGe is likely to change with temperature less than that of Ge, which suggests that the state of lattice vibration has been changed due to the alloy.It has been reported by Kosemura et al. that the coefficient of phonon sensitivity to strain in SiGe slightly change with Ge concentration[1], which indicates that the lattice strain in SiGe thin film affects phonons little. This is consistent with the fact that the two SiGe thin films with different Ge concentrations showed almost the same ΔDWFs as shown in Fig. 1.Figure 2 shows the relationship between T E and Ge concentration obtained from Eq. (2). From Fig. 2, it can be seen T E decreases as Ge concentration increases, then it is considered that the DWF in SiGe is more likely to be thermally excited as the Ge concentration increases. Moreover, since the estimated T Es depended on the Ge concentration, it was suggested that the thermal conductivity of SiGe thin film could be controlled by the Ge concentration.