Strain In Thin Films Research Articles

Gamma-induced stress, strain and p-type doping in MBE-grown thin film MoTe2.

A thorough examination of the stability of a 2D MoTe2 thin film exposed to high-dose gamma radiation (γ) is addressed in this study. This study compares the film before and after irradiation (10-600 kGy dosage) to report the impact of γ radiation on the surface morphology, work function, tensile strain and charge redistribution in the MoTe2 thin film. The radiation damage to the film is monitored by optical micrographs (OM) and with atomic force microscopy (AFM) and conceptualized by thermal strain in the film. Raman spectroscopy has been used to analyze the shifting and to address the strain that arises due to irradiation in its vibrational mode. Kelvin probe force microscopy (KPFM) has been performed to evaluate the work function of the film, which increases by 0.14 eV for the 600 kGy γ-irradiated sample, implying shifting of the Fermi-level to the valence band of the spectrum and thus it results in p-type doping in the film. Owing to the reduced atomic mass and high energy of tellurium atoms, γ-irradiation causes tellurium vacancies, which lead to the formation of dangling bonds at unoccupied sites. When oxygen is adsorbed at these reactive spots, a charge-transfer mechanism takes place. This mechanism involves the transfer of electrons from the thin MoTe2 film to the adsorbed oxygen, forming oxides and causing p-type doping. Furthermore, p-doping is verified by the valence band shifting by 1.27 eV in 600 kGy in γ-irradiated samples in X-ray photoelectron spectroscopy. This comprehensive study shows how γ irradiation affects the chemical and physical characteristics of the MoTe2 thin film. Consequently, it shows that if devices integrating MoTe2 thin films are meant to be used in high-dose radiation conditions, the adsorbate concentrations, radiation shielding and required lifetimes must be carefully evaluated.

Correlation of structural and magnetic properties of RFeO3 ( R=Dy,Lu )

In orthoferrites the rare-earth (R) ion has a big impact on structural and magnetic properties; in particular, the ionic size influences the octahedral tilt and the R3+−Fe3+ interaction modifies properties like the spin reorientation. Growth-induced strain in thin films is another means to modify materials properties since the sign of strain affects the bond length and therefore directly the orbital interaction. Our study focuses on epitaxially grown (010)-oriented DyFeO3 and LuFeO3 thin films, thereby investigating the impact of compressive lattice strain on the magnetically active Dy3+ and magnetically inactive Lu3+ compared to uniaxially strained single-crystal DyFeO3. The DyFeO3 films exhibits a shift of more than 20 K in spin-reorientation temperatures, maintain the antiferromagnetic Γ4 phase of the Fe lattice below the spin reorientation, and show double-step hysteresis loops for both in-plane directions between 5 and 390 K. This is the signature of an Fe-spin-induced ferromagnetic Dy3+ lattice above the Néel temperature of the Dy. The observed shift in the film spin reorientation temperatures vs lattice strain is in good agreement with isostatic single-crystal neutron diffraction experiments with a rate of 2 K/kbar. Published by the American Physical Society 2024

(Invited) Optical Excitations and Growth in Layered Organic Metal Halide Semiconductors

Hybrid organic metal halides, such as CH3NH3PbI3, have garnered attention because they are earth-abundant, solution-processable materials that can be used to form solar cells with high power conversion efficiency (>20%). There are significant questions about the unusual electronic properties of this class of materials because of the coupling between excitations and the lattice. We will present our work investigating the optoelectronic properties of layered organic metal halide systems and the relationship to structure and growth conditions. We will discuss the nature of optical excitations in layered compounds of the class R2PbX4 where R is an organic cation and X is a halide. These systems show signatures of unusually strong optical frequency magnetic dipole transitions that can be understood through self-trapped exciton formation. We will then discuss how mechanical strain during growth influences broad photoluminescence in a model layered system based on the large cation ethylammonium (EA) with the structure (EA)2(EA)n-1PbnBr3n+1. In this system the emission is strongly impacted by growth-induced strain in thin films quantified using X-ray scattering. Our results suggest that broad emission, relevant for LEDs, can be tuned in thin films. Recent work on how growth impacts the behavior of low dimensional system will also be presented.

Effect of deposition regime transition on the properties of Al:ZnO transparent conducting oxide layer by radio frequency magnetron sputtering system

High-quality zinc oxide thin films doped with aluminium adatoms have effectively been fabricated on pristine soda-lime silica glass substrates via radio frequency magnetron sputtering system. The deposition temperature was varied to explore the impact of deposition regime transition of as-deposited Al:ZnO thin films on their performance as transparent conducting oxide layers. In particular, the depositions were conducted at room temperature, 100 °C, 200 °C, and 300 °C, allowing for a comprehensive assessment of the resulting films. The Raman spectra depicted the modulation of Raman bands in correlation with the deposition regime transition, illustrating the impact of thermal induction on various properties of the as-deposited aluminium-doped zinc oxide thin films. Atomic force microscopy reveals the transformation from nearly spherical to elongated shape structure was obtained as the deposition process shifted from kinetic limited to thermodynamic limited regimes. The phase analysis and grazing incident of x-ray diffractometer disclose a single crystal orientation has been achieved at thermodynamic limited regime. However, two different crystal planes were predominant comparing between the surface and structural of as-deposited aluminium-doped zinc oxide thin films. It is also evident that a highly transparent with low lattice strain and better carrier concentrations of as-deposited aluminium-doped zinc oxide thin films were realized at thermodynamic limited regimes.

Advancements and challenges in strained group-IV-based optoelectronic materials stressed by ion beam treatment

In this perspective article, we discuss the application of ion implantation to manipulate strain (by either neutralizing or inducing compressive or tensile states) in suspended thin films. Emphasizing the pressing need for a high-mobility silicon-compatible transistor or a direct bandgap group-IV semiconductor that is compatible with complementary metal–oxide–semiconductor technology, we underscore the distinctive features of different methods of ion beam-induced alteration of material morphology. The article examines the precautions needed during experimental procedures and data analysis and explores routes for potential scalable adoption by the semiconductor industry. Finally, we briefly discuss how this highly controllable strain-inducing technique can facilitate enhanced manipulation of impurity-based spin quantum bits (qubits).

Flexo-photovoltaic effect in strained bismuth telluride thin films without substrate bending under light irradiation

Bismuth telluride (Bi2Te3) is gaining increasing attention as a material that exhibits flexo-photovoltaic effect in the infrared region, in addition to conventional thermoelectric materials and topological insulators. However, the flexo-photovoltaic effect is observed only when strains are applied to the Bi2Te3 thin films by bending the substrates. To enhance the versatility of the flexo-photovoltaic effect, in this study, Bi2Te3 thin films were sputtered onto a flexible substrate under a pre-curved condition, and strains were induced when the films were flattened. The crystal orientation and strains in the Bi2Te3 thin films were affected by the pre-curved conditions. The Bi2Te3 thin film deposited using a concave pre-curved condition exhibited an output voltage increase of approximately 15 % compared with the film deposited under a flat condition. This discovery suggests that strained Bi2Te3 thin films can efficiently produce an output voltage under light irradiation without requiring a p–n junction, making them suitable for various photodetector applications.

A self-referenced interferometer for in situ cryogenic wafer curvature measurements.

A self-referenced interferometer to measure time-varying curvature in mechanically unstable environments is needed in many applications. One application that demands this measurement technique with fast data acquisition, 2D sensitivity, and insensitivity to vibration is the measurement of thermal strain in thin films in operational environments. The diverging beam interferometer described here demonstrates an angular sensitivity to the local curvature using interferograms captured by a CMOS camera. Two-dimensional Fourier analysis is used to extract curvature changes. The interferometer demonstrates an experimental sensitivity to curvature changes on the order of 10-4 m-1 and is used to measure thermal stresses in a cryogenic environment of a polycrystalline titanium nitride thin film on a silicon wafer that exhibits anisotropic curvature.

Significant effects of epitaxial strain on the nonlinear transport properties in Ca2RuO4 thin films with the current-driven transition

Control of the nonlinear transport properties in strongly correlated materials with metal–insulator transitions has become an important task in the development of next-generation computing elements. Here, we demonstrate that the nonlinear transport properties in the Mott insulator Ca2RuO4, which are caused by the current-driven transition with a nonthermal-type mechanism, can be significantly affected by epitaxial strain in thin films. Ca2RuO4 epitaxial thin films were grown on various single-crystal substrates by the solid-phase epitaxy method under nonvacuum conditions, and the effects of epitaxial strain on the transport properties were investigated. While nonlinear transport properties with negative differential resistance behavior were observed in [001]-oriented Ca2RuO4/LaAlO3 (001) thin films, the current transport nonlinearity was critically diminished in the current–voltage characteristics of Ca2RuO4/NdCaAlO4 (100) thin films with the [110] orientation. Through structural characterization of the thin films, a possible correlation between the strain states and nonlinear transport properties was discussed.

Characterization of In‐Plane Piezoelectric Strain of Ferroelectric Thin Films by the Magnetoelectric Coupling Effect

Piezoelectric response of the ferroelectric/piezoelectric thin films is crucial for actuation of micro‐electromechanical systems (MEMS) devices as well as strain‐mediated electronic devices. Among many piezoelectric modes, longitudinal and transverse piezostrain is the most widely used. Various methods have been well established for quantitative characterization of the longitudinal piezostrain, such as laser interferometer and piezoelectric force microscopy. However, it is still a great challenge to characterize transverse piezostrain. Herein, a new method is established to characterize the transverse (in‐plane) piezostrain of ferroelectric/piezoelectric thin films by the inverse magnetoelectric (ME) coupling effect. A ferromagnetic thin film is patterned onto the ferroelectric thin film to form an artificial multiferroic heterostructure. The transverse piezostrain can transfer from the ferroelectric layer to the ferromagnetic layer and shift its ferromagnetic resonance field through the ME coupling effect. By comparing with a control sample of “bulk piezoceramic/ferromagnetic thin film” whose in‐plane piezostrain can be easily measured, the in‐plane piezostrain and piezoelectric coefficient of the ferroelectric thin films are then estimated. The in‐plane piezoelectric coefficient d31 and piezoelectric strain S31 of Pb(Zr,Ti)O3 (PZT) thin films are measured by this method. This new method will promote to fully understand the piezoelectric properties of the ferroelectric/piezoelectric thin films.

High-performance pulse light stable perovskite indoor photovoltaics

Perovskite solar cells offer great potential as a sustainable power source for distributed electronic devices that operate indoors. However, the impact of advanced lighting technology, especially the widely used pulse width modulation (PWM) technology, on perovskite photovoltaics has been ignored. Herein, for the first time in photovoltaics, we find that the light impact emitted by the PWM lighting system caused dynamic strain in perovskite thin films, induced phase separation, and accelerated the generation of metallic lead (Pb0) defects, leading to irreversible degradation of the cell performance after 27 h (T80). To address this issue, formamidinium triiodide (FAI3) is chosen to treat the surface of the perovskite and release residual stress, resulting in reduced lattice deformation during dynamic strain processes. Meanwhile, it suppresses harmful Pb0 defects and reduces Voc loss at low light intensity. The champion device achieves impressive power conversion efficiency (PCE) of 35.14% and retains 99.5% of the initial PCE after continuous strobe light soaking for 2160 h.

Investigation of gradient band gap in Cu2ZnSnS4 thin films with residual strain

This study investigates the change in the band gap of Cu2ZnSnS4 (CZTS) thin films under a gradual change in the residual strain of the thin films. Two CZTS thin film samples are fabricated at different initial substrate temperatures before annealing. X-ray diffraction results are obtained at various incidence angles, and the residual strains at the incidence angles are obtained from the Williamson–Hall plot. The larger the mean residual strain of the CZTS thin films, the greater are the redshift and full width at half maximum of the main Raman peak position. Results of Tauc plot analysis show that the CZTS thin film samples exhibit a gradient band gap. The sample with a wider distribution of the residual strain shows a greater band gap variation.

Temporal and spatial tracking of ultrafast light-induced strain and polarization modulation in a ferroelectric thin film.

Ultrashort light pulses induce rapid deformations of crystalline lattices. In ferroelectrics, lattice deformations couple directly to the polarization, which opens the perspective to modulate the electric polarization on an ultrafast time scale. Here, we report on the temporal and spatial tracking of strain and polar modulation in a single-domain BiFeO3 thin film by ultrashort light pulses. To map the light-induced deformation of the BiFeO3 unit cell, we perform time-resolved optical reflectivity and time-resolved x-ray diffraction. We show that an optical femtosecond laser pulse generates not only longitudinal but also shear strains. The longitudinal strain peaks at a large amplitude of 0.6%. The access of both the longitudinal and shear strains enables to quantitatively reconstruct the ultrafast deformation of the unit cell and to infer the corresponding reorientation of the ferroelectric polarization direction in space and time. Our findings open new perspectives for ultrafast manipulation of strain-coupled ferroic orders.

Unveiling the polar properties on barium bismuthate perovskite thin films with distinct Ba/Bi ratios

This research aims to investigate the domain structure of BaBiO3 (80:20), BaBiO3 (50:50), and BaBiO3 (30:70) thin films obtained by the polymeric precursor method. For better sample designations, the following codes were employed: (80:20) (BBO82), (50:50) (BBO55), and (30:70) (BBO37). The piezo-ferroelectric coupled behavior observed for barium bismuthates thin films was elucidated in terms of bismuth and oxygen vacancies as well as grain morphology, which can be controlled by stoichiometric changes (Ba/Bi ratios). The films crystallize in a rhombohedral BaBiO3 structure with an R-3R space group without any deleterious secondary phase. Additionally, the hysteretic strain, dielectric, ferroelectric, and piezoelectric properties of BaBiO3 thin films are strongly dependent on the film texture. Sample BBO55 showed a distinguished preferred orientation along the (341) plane with a low associated coercive field (66.87 kV/cm), free of imprint and with no polarizations gap compared to samples BBO37 and BBO82. Sample BBO55 presented the highest piezoelectric (d33-eff ≈ 45.7 pm/V) and ferroelectric (Pr≈19.87 μC/cm2) responses. This behavior can be ascribed to a highly textured film, which generates less strain throughout the crystal lattice, facilitating the polarization switching mechanism and improving piezo/ferroelectricity. These findings are significant for developing novel, high-performance, lead-free piezoelectric materials to be applied in different electronic components such as magnetic field sensors, switches, actuators, and other types of memory devices.

Strain and atomic stacking of bismuth thin film in its quasi-van der Waals epitaxy on (111) Si substrate.

We report on the structural properties of Bi thin films grown on (111) Si substrates with a thickness of 22-30 BL. HRXRD and EBSD measurements show that these Bi films are mainly composed of twinning grains in the (0003) direction. The grain size can be as large as tens of microns. From a double-peak (01[Formula: see text]4) φ-scan, we found two pairs of twinning phases coexisting with a rotation angle of ~ 3.6°. We proposed a coincidence site lattice model based on preferential close-packed sites for Bi atoms on Si (111) surface to explain the coexistence of the rotation phases in the quasi-van der Waals epitaxy. From the measured lattice constants c and a of our samples, along with the data from the literature, we derived a c-a relation: (c-c0) = - 2.038(a-a0), where c0 and a0 are the values of bulk Bi. The normalized position of the second basis atom in the unit cell x, in these strained Bi films is found very close to that of bulk Bi, indicating that the strain does not disturb the Peierls distortion of the lattice. The fixed ratio of bilayer thickness to lattice constant c, reveals that the elastic properties of covalent-bonded bilayer dominate those of Bi crystal.

Relationship between oxidation, stresses, morphology, local resistivity, and optical properties of TiO2, Gd2O3, Er2O3, SiO2 thin films on SiC

The relationship between internal mechanical stresses, surface morphology, nanoscale electrical properties, and optical characteristics in TiO2, Gd2O3, Er2O3, and SiO2 thin films on SiC substrates was investigated. The oxide films were synthesized using the rapid thermal annealing and analyzed through scanning spreading resistance microscopy, photoluminescence, and absorption spectroscopy. Tensile stresses were found in the films, they are attributed to thermal and lattice mismatch, oxidation, and grain boundaries. These stresses influence on surface morphology, resistivity variations, and photoluminescence intensity. Surface roughness and grain structure were found to correlate with variations in resistivity, which were attributed to conductive pathways along grain boundaries and possible metallic phases. Photoluminescence intensity was also observed to correlate with estimated lattice mismatch strain. Gd2O3/SiC exhibited the fewest defects, while Er2O3 and TiO2 showed more, with Er2O3 being the most mismatched and roughest. The results indicate that internal strains in oxide thin films on SiC substrates can influence on surface morphology, leading to formation of defects and spatial inhomogeneity. These fluctuations in local conductivity and luminescence center density have significant implications for dielectric and optical applications. The study provides insights for future processing refinements to mitigate internal strains and enhance the performance of oxide thin films in semiconductor and optical technologies.

Tuning Ionic Conductivity in Fluorite Gd-Doped CeO2-Bixbyite RE2O3 (RE = Y and Sm) Multilayer Thin Films by Controlling Interfacial Strain.

Interfacial strain in heteroepitaxial oxide thin films is a powerful tool for discovering properties and recognizing the potential of materials performance. Particularly, facilitating ion conduction by interfacial strain in oxide multilayer thin films has always been seen to be a highly promising route to this goal. However, the effect of interfacial strain on ion transport properties is still controversial due to the difficulty in deconvoluting the strain contribution from other interfacial phenomena, such as space charge effects. Here, we show that interfacial strain can effectively tune the ionic conductivity by successfully growing multilayer thin films composed of an ionic conductor Gd-doped CeO2 (GDC) and an insulator RE2O3 (RE = Y and Sm). In contrast to compressively strained GDC-Y2O3 multilayer films, tensile strained GDC-Sm2O3 multilayer films demonstrate the enhanced ionic conductivity of GDC, which is attributed to the increased concentration of oxygen vacancies. In addition, we demonstrate that increasing the number of interfaces has no impact on the further enhancement of the ionic conductivity in GDC-Sm2O3 multilayer films. Our findings demonstrate the unambiguous role of interfacial strain on ion conduction of oxides and provide insights into the rational design of fast ion conductors through interface engineering.

The anomalous Hall effect controlled by residual epitaxial strain in antiferromagnetic Weyl semimetal Mn3Sn thin films grown by molecular beam epitaxy

The large anomalous Hall effect (AHE) in antiferromagnetic(AFM) Weyl semimetal Mn3Sn attracts intensive attentions in spintronics. Here, we report the structural property of high quality Mn3Sn thin film on insulator substrate MgO(110) by molecular beam epitaxy (MBE), and AHE in control of residual mismatch strain between Mn3Sn film and substrate. We are able to grow strain-free Mn3Sn(10 1¯ 0) films or alternatively strained Mn3Sn(11 2¯ 0) films via a three-step process. The strain-free Mn3Sn film has large anomalous Hall conductivity up to 30 Ω-1cm−1 at room temperature, which is comparable to bulk Mn3Sn. In contrast, AHE is switched off in strained Mn3Sn film due to piezomagnetic effect under a uniaxial compress strain of ∼2.0%. These findings provide a deeper understanding on AFM spintronic applications.

Residual Strain Evolution Induced by Crystallization Kinetics During Anti-Solvent Spin Coating in Organic-Inorganic Hybrid Perovskite.

Organic-inorganic hybrid perovskite (OIHP) polycrystalline thin films are attractive due to their outstanding photoelectronic properties. The anti-solvent spin coating method is the most widely used to synthesize these thin films, and the residual strain is inevitably originates and evolves during the process. However, this residual strain evolution induced by crystallization kinetics is still poorly understood. In this work, the in situ and ex situ synchrotron grazing-incidence wide-angle X-ray scattering (GIWAXS) are utilized to characterize the evolution and distribution of the residual strain in the OIHP polycrystalline thin film during the anti-solvent spin coating process. A mechanical model is established and the mechanism of the crystallization kinetics-induced residual strain evolution process is discussed. This work reveals a comprehensive understanding of the residual strain evolution during the anti-solvent spin coating process in the OIHP polycrystalline thin films and provides important guidelines for the residual strain-related strain engineering, morphology control, and performance enhancement.

Tuning of Structural and Morphological Characteristics of V2O5 Thin Films Using Low Energy 16 keV N + for Optical and Wetting Applications

Effect of nitrogen (N+) ion implantation on the morphological, structural, optical, and compositional properties of vanadium pentoxide (V2O5) thin films grown on glass substrates is studied. Surface morphology shows the formation of grains and the growth dynamics is governed by roughness (α) and growth (β) exponents. X-ray diffraction studies reveal that V2O5 exists in a hybrid form, with properties of both the orthorhombic and tetragonal phases. Ion implantation induces defects and strain in V2O5 thin films causing a reduction in crystalline properties and deformation in the β-phase with a corresponding change in crystallite size. Contact angle wetting properties are found to be co-related with fractal growth of the films under ion implantation. Oxygen vacancies and electron scattering/trapping centres are revealed to have increased after N+ implantation, leading to a smaller bandgap in the thin films. The benefits of decreasing the optical band-gap of V2O5 thin films for optical applications are outlined in the present work.

A stress relaxation strategy for preparing high-quality organic–inorganic perovskite thin films via a vapor–solid reaction

A post-vapor treatment was developed to alleviate compressive strains in perovskite thin films grown by vapor–solid reaction. Perovskite solar cells based on these films achieved an improved power conversion efficiency of over 22%.