Film-substrate Interface Research Articles

Time-resolved piezoelectric response in relaxor ferroelectric (Pb0.88La0.12)(Zr0.52Ti0.48)O3 thin films

The domain switching dynamics in a relaxor ferroelectric lanthanum-modified lead zirconate titanate thin film with 12 mol. % La was investigated by time-resolved x-ray diffraction. While most frequently epitaxial thin films are investigated, the present work reports results on a polycrystalline thin film. Asymmetric butterfly loops of the strain as a function of the applied electric field evidenced a built-in electric field oriented toward the thin film–substrate interface. The piezoelectric coefficient d33 (in the film reference frame) diminishes with the increasing frequency of an applied AC electric field. From the strain transient during the application of positive-up negative-down voltage pulse sequences with frequencies of up to 100 kHz, characteristic times of the order of 100–200 ns were determined for these relaxor ferroelectric thin films. While switching times ranging from the picosecond to the millisecond range are reported in the literature for different materials, these characteristic switching times are comparable to epitaxial lead zirconate titanate thin films of various compositions despite the polycrystallinity of the present thin film. However, the evidenced built-in electric field significantly influences the switching behavior for different polarities.

Tunable optical and plasmonic response of Au nanoparticles embedded in Ta-doped TiO2 transparent conducting films

Localized surface plasmon resonances (LSPRs) are fascinating optical phenomena occurring in metal nanostructures, like gold nanoparticles (NPs). Plasmonic excitation can be tailored efficiently in the visible range by acting on size, shape, and NP surroundings, whereas carrier density is fixed, thus restricting the LSPR modulation. Transparent conductive oxides (TCOs), on the other hand, are gaining increasing interest for their transparency, charge carrier tunability, and plasmonic features in the infrared. The combination of these two materials into a metal-TCO nanocomposite can give access to unique electrical and optical characteristics, to be tailored in view of the desired optoelectronic application. In this paper, Au NPs and Ta-doped ${\mathrm{TiO}}_{2}$ TCO films have been merged with the aim to master the Au plasmon resonance by acting on the dielectric properties of the surrounding TCO. Morphology, structure, and electrical properties have been investigated as well to understand the optical response of the nanosystems. The role of the embedding geometry has been explored, revealing that the largest LSPR shift (550--760 nm) occurs when the NPs are sandwiched in the middle of the film and not at the bottom of the film (substrate-film interface). Ta doping in the TCO has been varied (5--10% at. and bare ${\mathrm{TiO}}_{2}$) to induce a permittivity change of the matrix. As a result, Au LSPR is clearly blueshifted when decreasing the dielectric permittivity at higher Ta content in the sandwich configuration. Despite the nonoptimal electrical performance caused by defectivity of the films, Au-$\mathrm{Ta}:\mathrm{Ti}{\mathrm{O}}_{2}$ multifunctional nanocomposites are promising candidates for their optical behavior as highly tunable plasmonic conductive metamaterials for advanced light management.

Oxygen diffusion in different thicknesses La-=SUB=-0.8-=/SUB=-Sr-=SUB=-0.2-=/SUB=-MnO-=SUB=-3-delta-=/SUB=- films on NdGaO-=SUB=-3-=/SUB=- substrates

The results of investigation by the indirect method of step-by-step varying the oxygen content in the series of epitaxial La0.8Sr0.2MnO3-delta films on single-crystal NdGaO3 substrates are presented. Using numerical simulation we have revealed that the oxygen diffusion coefficient significantly decreases along the film thickness in the direction from the outer surface to the film-substrate interface under conditions of "compressive" mechanical stresses caused by the mismatch of the in-plane crystalline parameters of the film and substrate materials. In films of d~12-75 nm thickness, the effect is manifested in the fact that the value of the diffusion coefficient in the vicinity of the outer surface of the films also decreases significantly as the thickness of the films decreases. The questions of the applicability of the indirect method for evaluating the oxygen content in thin epitaxial films, as well as other manifestations of effects caused by mechanical stresses are discussed. Keywords: doped manganites, oxygen index, heat treatment, mechanical stress, the oxygen diffusion coefficient.

Quantum size effects in Ag thin films grown on the fivefold surface of the icosahedral Al-Cu-Fe quasicrystal: Influence of the growth temperature

We have studied the growth and electronic structure of Ag thin films on the fivefold surface of the icosahedral (i)-Al-Cu-Fe quasicrystal using scanning tunneling microscopy, low energy electron diffraction (LEED), ultraviolet photoemission spectroscopy, and density functional theory. Upon deposition at 400 K, Ag islands grow to form crystallites with a preferred thickness for a given coverage. LEED patterns reveal five rotational domains of Ag crystallites with (111) orientation for coverages larger than approximately seven monolayers. Quantum well states are observed in the photoemission spectra of Ag/i-Al-Cu-Fe ranging from 5 to 35 monolayers, indicating electron confinement within the film thickness and, thus, confirming electronic growth of Ag thin films on quasicrystalline surfaces. Electronic structure calculations have been performed to discuss the possible origins of the confinement at the film-substrate interface.

Resonant precession of magnetization and precession—induced DC voltages in FeGaB thin films

Measurements of frequency dependent ferromagnetic resonance and spin pumping driven dc voltage (V dc) are reported for amorphous films of Fe78Ga13B9 alloy to address the phenomenon of self-induced inverse spin Hall effect (ISHE) in plain films of metallic ferromagnets (FMs). The V dc signal, which is anti-symmetric on field reversal, comprises of symmetric and asymmetric Lorentzians centered around the resonance field. Dominant role of thin film size effects is seen in setting the magnitude of static magnetization, V dc and dynamics of magnetization precession in thinner films (⩽8 nm). The film thickness dependence of magnetization parameters indicates the presence of a magnetically disordered region at the film—substrate interface, which may promote preferential flow of spins generated by the precessing magnetization towards the substrate. However, the V dc signal also draws contributions from rectification effects of a ≈0.4% anisotropic magnetoresistance and a large (≈54 nΩ m) anomalous Hall resistivity (AHR) of these films which ride over the effect of spin–orbit coupling driven spin-to-charge conversion near the film—substrate interface. We have addressed these data in the framework of the existing theories of electrodynamics of a ferromagnetic film subjected to radio-frequency field in a coplanar waveguide geometry. Our estimation of the self-induced ISHE for the sample with 54 nΩ m AHR shows that it may contribute significantly (≈90%) to the measured symmetric voltage. This study is expected to be very useful for fully understanding the spin pumping induced dc voltages in metallic FMs with disordered interfaces and large anomalous Hall effect.

Spalling-Induced Liftoff and Transfer of Electronic Films Using a van der Waals Release Layer.

Heterogeneous integration strategies are increasingly being employed to achieve more compact and capable electronics systems for multiple applications including space, electric vehicles, and wearable and medical devices. To enable new integration strategies, the growth and transfer of thin electronic films and devices, including III-nitrides, metal oxides, and 2D materials, using 2D boron nitride (BN)-on-sapphire templates are demonstrated. The van der Waals (vdW) BN layer, in this case, acts as a preferred mechanical release layer for precise separation at the substrate-film interface and leaves a smooth surface suitable for vdW bonding. A tensilely stressed Ni layer sputtered on top of the film induces controlled spalling fracture that propagates at the BN/sapphire interface. By incorporating controlled spalling, the process yield and sensitivity are greatly improved, owed to the greater fracture energy provided by the stressed metal layer relative to a soft tape or rubber stamp. With stress playing a critical role in this process, the influence of residual stress on detrimental cracking and bowing is investigated. Additionally, a back-end selected area lift-off technique is developed which allows for isolation and transfer of individual devices or arbitrary shapes.

Ferroelectric phase transitions induced by a strain gradient

In perovskite oxide ferroelectrics, gradients of lattice strain are known to induce nanoscale topological structures, leading to novel or enhanced functionality. Here, we experimentally detect and theoretically analyze thickness distribution of structural properties in epitaxial ${\mathrm{Pb}}_{0.5}{\mathrm{Sr}}_{0.5}\mathrm{Ti}{\mathrm{O}}_{3}$ films grown on (001) $\mathrm{SrTi}{\mathrm{O}}_{3}$ substrates. We show that the relaxation of substrate-imposed stress produces a strain gradient, which leads to the formation of distinct ferroelectric phases as a function of distance from the film-substrate interface within the same film. Charge carriers trapped at phase boundaries stabilize the induced phases and manifest themselves under electric field. Crosstalk between the phases, where polarization may rotate in one phase and invert in the other one, opens perspectives for advanced ferroelectric thin film devices.

Dynamic Tilting of Ferroelectric Domain Walls Caused by Optically Induced Electronic Screening.

Optical excitation perturbs the balance of phenomena selecting the tilt orientation of domain walls within ferroelectric thin films. The high carrier density induced in a low-strain BaTiO_{3} thin film by an above-band-gap ultrafast optical pulse changes the tilt angle that 90° a/c domain walls form with respect to the substrate-film interface. The dynamics of the changes are apparent in time-resolved synchrotron x-ray scattering studies of the domain diffuse scattering. Tilting occurs at 298K, a temperature at which the a/b and a/c domain phases coexist but is absent at 343K in the better ordered single-phase a/c regime. Phase coexistence at 298K leads to increased domain-wall charge density, and thus a larger screening effect than in the single-phase regime. The screening mechanism points to new directions for the manipulation of nanoscale ferroelectricity.

Direct numerical simulations of three-dimensional surface instability patterns in thin film-compliant substrate structures

A comprehensive numerical study of three-dimensional surface instability patterns is presented. The formation of wrinkles is a consequence of deformation instability when a thin film, bonded to a compliant substrate, is subject to in-plane compressive loading. We apply a recently developed computational approach to directly simulate complex surface wrinkling from pre-instability to post-instability in a straightforward manner, covering the entire biaxial loading spectrum from pure uniaxial to pure equi-biaxial compression. The simulations use embedded imperfections with perturbed material properties at the film-substrate interface. This approach not only triggers the first bifurcation mode but also activates subsequent post-buckling states, thus capable of predicting the temporal evolution of wrinkle patterns in one simulation run. The state of biaxiality is found to influence the surface pattern significantly, and each bifurcation mode can be traced back to certain abrupt changes in the overall load–displacement response. Our systematic study reveals how the loading condition dictates the formation of various instability modes including one-dimensional (1D) sinusoidal wrinkles, herringbone, labyrinth, and checkerboard.

Constrained sintering of alumina micro-ring films on stiff and compliant substrates: Constriction or dilation?

The sintering behavior of ring-shaped alumina films on different types of substrates (polished sapphire and platinum coated sapphire) has been investigated. Variation of substrate material, layer thickness and heating schedule led to different interface properties which were quantified using the interfacial friction parameter as a measure of the slip distance of the free edge. This interfacial friction implies the compliance of the film-substrate interface, thus allowing the quantification of the ability of the free edges to slide along the substrate. Ring structures were made with outer radii of 100 –500 µm and the ratios of inner to outer radii were 0.1 (broad ring segment with small inner hole) and 0.4 (narrow ring segment with big inner hole). During sintering, the outer film edge was found to recede in all systems, whereas the behavior of the free edge located at the inner hole was strongly dependent on the interfacial friction. When sintering of alumina micro-rings were carried out on sapphire substrate, a high interfacial friction resulted in the positive displacement of the inner edge, causing the inner hole to open. Sapphire substrates coated with platinum offered a lower interfacial friction resulting in the opening of the hole in a more distinct manner as compared to the uncoated sapphire substrates. As the thickness of the film also affects the interfacial friction, a thicker film (27 µm) on platinum coated substrate had very low friction. This resulted in a negative displacement of the inner ring leading to closing of the hole. These effects are in qualitative agreement with the predicted analytical model for constrained sintering of annular ceramic films. A combination of film thickness, substrate material and aspect ratio of the ring either leading to constriction or dilation of the annular alumina film were carefully investigated.

Defect distribution in ZnO thin films grown on a-plane sapphire substrates by catalytic-reaction-assisted chemical vapor deposition

This work examined the crystalline structure of a ZnO thin film grown on an a-plane sapphire substrate through a reaction between dimethylzinc and high-energy H2O molecules produced by the Pt-catalysed reaction of gaseous H2 and O2. The dislocation distribution in a ZnO film with a thickness of 5 μm grown on an a-plane sapphire substrate was observed using cross-sectional dual-beam transmission electron microscopy, and the dislocation density was evaluated by Ham’s method. A much higher dislocation density was found at the film-substrate interface than at the film surface. The influences of film thickness on the full width at half maximum of the X-ray diffraction rocking curve and on the electron mobility are both attributed to this localized high density of defects.

Drop Spreading and Confinement in Swelling-Driven Folding of Thin Films.

Surface instabilities are a versatile method for generating three-dimensional (3D) surface microstructure. When an elastomeric film weakly bonded to a substrate is swollen with solvent, buckle delamination and subsequent sliding of the film on the substrate lead to the formation of tall, self-contacting, and permanent folds. This paper explores the mechanics of fold development when such folding is induced by placing a drop on the surface of the film. We show that capillary effects can induce a strong coupling between folding and drop spreading: as folds develop, they wick the solvent toward the periphery of the drop, further propagating radially aligned folds. Accordingly, a solvent drop spreads far more on films that are weakly adhered to the substrate. As drop size reduces and folding becomes increasingly confined, debonding propagates along the perimeter of the wetted region, thus leading to corral-shaped fold patterns. On the other hand, as drop size increases and confinement effects weaken, isotropically oriented folds appear at a spacing that reduces as swelling increases. The spacing between the folds and the size of the corrals are both determined by the extent to which a single fold relieves compressive stress in its vicinity by sliding. We develop a model for folding which explicitly accounts for the fact that folds must initiate with near-zero volume under the buckle. The model shows that folds can appear even at very low swelling if there are large pre-existing debonded regions at the film-substrate interface.

Structural, optical, and morphological properties of chemically deposited CdSe thin films by ammonia-free precursor solution

We present a simple route for developing high-quality CdSe thin films at low temperature by means of the chemical bath deposition technique using an ammonia-free formulation. The effect of the concentration of the precursor solutions was evaluated through four formulations. The crystalline, optical, and morphological properties of the CdSe thin films were analyzed for the four deposition conditions. As-deposited CdSe films exhibited hexagonal phase and strong crystalline orientation along the [002] direction as observed by XRD analysis. The diffraction peak corresponding to the (002) crystalline planes shifts to higher two theta values in the patterns of the films deposited with increasing precursors concentration, showing the shrinkage of the crystalline lattice of up to 1.58%, as a product of the tensile stresses through the substrate-film interface. UV-Vis-NIR analysis revealed highly reflective films and good optical quality. The refractive index showed a maximum of 2.20–2.26 around 640 nm, while the extinction coefficient was zero at wavelengths lower than the absorption edge. The SEM and AFM analysis revealed the homogeneous and compact CdSe films surface with low average roughness. The energy band gap of the CdSe thin films varied in the range from 1.78 to 1.9 eV and the variation has been associated with the stresses induced in the film-substrate interface during the film growth. We found an opposite trend between the lattice constant (c) and the energy band gap of the CdSe thin films, which enable the adjustment of their optical properties through the concentration of the precursor solutions during the deposition process.

Pulsed laser-induced dewetting and thermal dewetting of Ag thin films for the fabrication of Ag nanoparticles

Two different dewetting methods, namely pulsed laser-induced dewetting (PLiD)—a liquid-state dewetting process and thermal dewetting (TD)—a solid-state dewetting process, have been systematically explored for Ag thin films (1.9–19.8 nm) on Si substrates for the fabrication of Ag nanoparticles (NPs) and the understanding of dewetting mechanisms. The effect of laser fluence and irradiation time in PLiD and temperature and duration in TD were investigated. A comparison of the produced Ag NP size distributions using the two methods of PLiD and TD has shown that both produce Ag NPs of similar size with better size uniformity for thinner films (<6 nm), whereas TD produced bigger Ag NPs for thicker films (≥8–10 nm) as compared to PLiD. As the film thickness increases, the Ag NP size distributions from both PLiD and TD show a deviation from the unimodal distributions, leading to a bimodal distribution. The PLiD process is governed by the mechanism of nucleation and growth of holes due to the formation of many nano-islands from the Volmer−Weber growth of thin films during the sputtering process. The investigation of thickness-dependent NP size in TD leads to the understanding of void initiation due to pore nucleation at the film-substrate interface. Furthermore, the linear dependence of NP size on thickness in TD provides direct evidence of fingering instability, which leads to the branched growth of voids.

Superconducting Sr2RuO4 Thin Films without Out-of-Phase Boundaries by Higher-Order Ruddlesden-Popper Intergrowth.

Ruddlesden-Popper (RP) phases (An+1BnO3n+1, n = 1, 2,···) have attracted intensive research with diverse functionalities for device applications. However, the realization of a high-quality RP-phase film is hindered by the formation of out-of-phase boundaries (OPBs) that occur at terrace edges, originating from lattice mismatch in the c-axis direction with the A'B'O3 (n = ∞) substrate. Here, using strontium ruthenate RP-phase Sr2RuO4 (n = 1) as a model system, an experimental approach for suppressing OPBs was developed. By tuning the growth parameters, the Sr3Ru2O7 (n = 2) phase was formed in a controlled manner near the film-substrate interface. This higher-order RP-phase then blocked the subsequent formation of OPBs, resulting in nearly defect-free Sr2RuO4 layer at the upper region of the film. Consequently, the Sr2RuO4 thin films exhibited superconductivity up to 1.15 K, which is the highest among Sr2RuO4 films grown by pulsed laser deposition. This work paves the way for synthesizing pristine RP-phase heterostructures and exploring their unique physical properties.

Insights into the nucleation, grain growth and phase transformation behaviours of sputtered metastable β-W films

Metastable phase in tungsten film is of great interests in recent years due to its giant spin Hall effects, however, little information has been known on its nucleation, growth and phase transformation. In this paper, a 900 nm-thick tungsten film with double-layer structure (α-W underlayer and β-W above it) was produced on SiO2/Si substrate by high vacuum magnetron sputtering at room temperature. The structural properties of β-W were systemically investigated by X-ray diffraction, transmission electron microscopy, thermodynamic calculation, first-principle and phase-field simulations. It is found that the β-W nucleation is energetically favoured on the SiO2 surface compared to the α-W one. As the film thickening proceeds, β-W[211] turns to be preferred direction of growth owing to the elastic strain energy minimization, which is verified by phase-field simulations. Moreover, the β→α phase transformation takes place near the film-substrate interface while the rest of the film keeps the β-W phase, leading to a double-layer structure. This localized phase transition is induced by lower Gibbs free energy of α-W phase at larger grain sizes, which can be confirmed by thermodynamic calculation. Further in-situ heating TEM analysis of the as-deposited film reveals that the β→α phase transformation is fulfilled by α/β interface propagation rather than local atomic rearrangements. Our findings offer valuable insights into the intrinsic properties of metastable phase in tungsten.

Ferromagnetic phase nucleation and its growth evolution in FeRh thin films

The process of the nucleation and evolution of the ferromagnetic phase in the thin Fe49Rh51 film on a MgO substrate near the phase transition from the antiferromagnetic (AFM) to the ferromagnetic (FM) state was comprehensively characterized by means of measurements of the temperature, time and magnetic field dependences of the magnetization. The observed relaxation processes of magnetization indicate the nucleation of the FM phase on the film surface and its further growth towards the film-substrate interface upon heating or applying the external magnetic field. The confirmation and verification of the FM phase growth evolution were carried out by the methods of vibrational magnetometry and magnetic force microscopy. A phenomenological model described the evolution of the FM phase growth in thin films based on the Bean-Rodbell and Kolmogorov-Johnson-Mehl-Avrami (KJMA) models is proposed. The article presents a new approach for interpreting Temperature First Order Reversal Curve (TFORC) diagrams based on comparative analysis of the coercivity and squareness coefficient temperature dependences and the peak position on the diagram.

Constrained Sintering of Alumina Micro-Ring Films on Stiff and Compliant Substrates: Constriction or Dilation?

The sintering behavior of ring-shaped alumina films on different types of substrates (polished sapphire and platinum coated sapphire) has been investigated. Variation of substrate material, layer thickness and heating schedule led to different interface properties which were quantified using the interfacial friction coefficient as a measure of the slip distance of the free edge. This interfacial friction implies the compliance of the film-substrate interface, thus allowing the quantification of the ability of the free edges to slide along the substrate. Ring structures were made with outer radii of 100 µm to 500 µm and the ratios of inner to outer radii were 0.1 (broad ring segment with small inner hole) and 0.4 (narrow ring segment with big inner hole). During sintering, the outer film edge was found to recede in all systems, whereas the behavior of the free edge located at the inner hole was strongly dependent on the interfacial friction coefficient. When sintering of alumina micro-rings were carried out on sapphire substrate, a high interfacial friction resulted in the positive displacement of the inner edge, causing the inner hole to open. Sapphire substrates coated with platinum offered a lower interfacial friction resulting in the opening of the hole in a more prominent way. As the thickness of the film also affects the interfacial friction, a thicker film (27 µm) on platinum coated substrate had very low friction. This resulted in a negative displacement of the inner ring leading to closing of the hole. These effects are in qualitative agreement with the predicted analytical model for constrained sintering of annular ceramic films. All the questions of whether an annular alumina film will constrict or dilate upon sintering are answered in the present work.

Interface-driven magnetic anisotropy of epitaxial [formula omitted] thin films

The effect of film-substrate interface on the magnetic anisotropy (MA) of epitaxial Fe4N thin films grown on single crystalline MgO (100) and MgO (111) substrates has been studied in this work. Both samples were grown simultaneously using a reactive dc magnetron sputtering process at a substrate temperature of 250°C. Single phase Fe4N samples were found to grow epitaxially on (100) and (111) oriented MgO substrates as confirmed from out-of-plane x-ray diffraction and in-plane ϕ-scan measurements. The MA of (100) and (111) oriented Fe4N samples was measured using longitudinal magneto-optical Kerr effect (MOKE). In spite of being grown simultaneously, the MA of both these samples was found to be different. Secondary ion mass spectroscopy and x-ray reflectivity measurements revealed the formation of a dense layer at the film-substrate interface. The MA of this dense layer along with the MA of Fe4N has a dramatic effect on magnetic reversal depending on the orientation of the substrate. Further, from the MOKE measurements, the difference in the magnetization switching processes was found to depend on the microstructure and orientation of Fe4N thin films.

Interface creation on a mixed-terminated perovskite surface

In the field of complex oxide heterostructures, understanding of the initial substrate surface can be critical to fundamental studies regarding the development of emergent properties at the film–substrate interface. For this reason, a considerable amount of effort has gone into the development of techniques to achieve surfaces with single termination for a variety of perovskite single crystals. However, a decisive understanding of how an interface is created when epitaxial growth occurs on a mixed terminated surface remains lacking. Employing in situ synchrotron X-ray scattering during thin film growth by molecular beam epitaxy, we investigate the initial stages of growth on (LaAlO3)(Sr2AlTaO6) (001) substrates with mixed termination. Using LaNiO3 as a model system, we find that the surface layers of the substrate restructure during deposition such that while a NiO2 monolayer is weakly bound, a LaO monolayer bonds strongly, effectively incorporating with (Al, Ta)O2 from the surface and forming a La(Al, Ta)O3 ultrathin film.