Thin Film Heterostructures Research Articles

Enhanced oxygen evolution reaction in flexoelectric thin-film heterostructures

Recently, the flexoelectric effect has triggered considerable interest in energy-related applications, such as flexo-actuation, flexo-photovoltaic, and flexo-catalysis, because of its ubiquitous feature allowing the creation of electric polarity, i.e., the flexoelectric polarization (Pflexo), in non-polar materials by strain gradient. Here, we show a flexoelectric strategy in electrocatalytic water splitting. Remarkably enhanced oxygen evolution reaction (OER) properties are achieved in strain-gradient LaFeO3 (LFO) thin-film heterostructures owing to the promotion of kinetic processes by Pflexo. The improved OER is demonstrated by increased current density of ∼300% in linear sweep voltammetry and lowered charge transfer resistance by two orders of magnitude in electrochemical impedance spectroscopy. These are ascribed to the flexoelectric-induced downward bending of the LFO band, as revealed by density functional theory calculations and band structure measurements. With Pflexo in the thin-film heterostructure catalysts, the adsorption of hydroxyl ions is strengthened on the polar LFO surface, and the transfer of electrons is accelerated from the reactants/key intermediates to the catalyst across the band-tilted LFO layer. These findings indicate the significance of flexoelectric effect in OER kinetics and open a new perspective for exploiting catalytic mechanisms and performances in water splitting.

Assemble of porous heterostructure thin film through CuS passivation for efficient electron transport in dye-sensitized solar cells

Three different modified solar cells have been passivated with copper sulfide (CuS) on a TiO2 electrode and manganese sulfide (γ‐MnS) hexagonal as photon absorbers. The MnS were prepared using (a-c) bis(N‐Piperl‐N‐p‐anisildithiocarbamato)Manganese(II) Complexes Mn[N-Piper‐N‐p‐Anisdtc] as (MnS_1), N‐p-anisidinyldithiocarbamato Mn[N‐p-anisdtc] as (MnS_2) and N‐piperidinyldithiocarbamato Mn[N‐piperdtc] as (MnS_3). The corresponding passivated films were denoted as CM-1, CM-2, and CM-3. The influence of passivation on the structural, optical, morphological, and photochemical properties of the prepared devices has been investigated. Raman spectra show that the combination of this heterostructure is triggered by the variation in particle size and surface effect, thus resulting in good electronic conductivity. The narrow band gaps could be attributed to good interaction between the passivative materials on the TiO2 surface. CM-2 cells, stability studies show that the cell is polarized and current flows due to electron migration across the electrolyte and interfaces at this steady state. The cyclic voltammetry (CV) curve for the CM-3 with the highest current density promotes the electrocatalytic activity of the assembled solar cell. The catalytic reactions are further confirmed by the interfacial electron lifetimes in the Bode plots and the impedance spectra. The current–voltage (J–V) analysis suggests that the electrons in the conduction band of TiO2/CuS recombine with the semiconductor quantum dots (QDs) and the iodolyte HI-30 electrolyte, resulting in 5.20–6.85% photo-conversions.

At the Limit of Interfacial Sharpness in Nanowire Axial Heterostructures.

As semiconductor devices approach dimensions at the atomic scale, controlling the compositional grading across heterointerfaces becomes paramount. Particularly in nanowire axial heterostructures, which are promising for a broad spectrum of nanotechnology applications, the achievement of sharp heterointerfaces has been challenging owing to peculiarities of the commonly used vapor-liquid-solid growth mode. Here, the grading of Al across GaAs/AlxGa1-xAs/GaAs heterostructures in self-catalyzed nanowires is studied, aiming at finding the limits of the interfacial sharpness for this technologically versatile material system. A pulsed growth mode ensures precise control of the growth mechanisms even at low temperatures, while a semiempirical thermodynamic model is derived to fit the experimental Al-content profiles and quantitatively describe the dependences of the interfacial sharpness on the growth temperature, the nanowire radius, and the Al content. Finally, symmetrical Al profiles with interfacial widths of 2-3 atomic planes, at the limit of the measurement accuracy, are obtained, outperforming even equivalent thin-film heterostructures. The proposed method enables the development of advanced heterostructure schemes for a more effective utilization of the nanowire platform; moreover, it is considered expandable to other material systems and nanostructure types.

Photocatalytic Activity Enhancement of ZnO/CZTS Thin Film Heterostructures in Visible Region: Controlling Phase Pure Kesterite CZTS Thin Films and the Effect of the Number of Seed Layers on Photocatalytic Performance

Photocatalytic Activity Enhancement of ZnO/CZTS Thin Film Heterostructures in Visible Region: Controlling Phase Pure Kesterite CZTS Thin Films and the Effect of the Number of Seed Layers on Photocatalytic Performance

Experimental Study of the Proximity Effect in Thin-Film Heterostructures with the Superconducting Lead Layerof Variable Thickness

Experimental Study of the Proximity Effect in Thin-Film Heterostructures with the Superconducting Lead Layerof Variable Thickness

Impact of Stress and Material Porosity on Double Boost DC-DC Converter Technology to Enhance Field-Effect Heterotransistors and Heterodiodes Density

Currently, several critical issues in solid-state electronics, such as improving the performance, reliability and density of integrated circuits elements like diodes, field-effect transistors, and bipolar transistors are being intensively addressed. One effective approach to reduce the dimensions of integrated circuits elements is to manufacture them in thin film heterostructures with an optimal manufacturing regime. In this paper, we explore methods to increase the density of field-effect heterotransistors and heterodiodes in the framework of a double boost DC-DC converter. We consider manufacturing this converter in a heterostructure with a specific configuration, where several required areas of the heterostructure should be doped by diffusion or ion implantation. Subsequently, the dopant and radiation defects should be annealed using an optimized scheme. Based on this approach, we provide recommendations for determining the optimal annealing time to achieve the best compromise between increasing density of integrated circuits elements and minimizing local overheating during their operation. We also propose a method to reduce the mismatch-induced stress in the heterostructure. We present an analytical approach to analyze mass and heat transport in heterostructures during the manufacturing of integrated circuits, accounting for mismatch-induced stress. This approach allows for the consideration of spatial and temporal variations in the parameters of technological processes, as well as their nonlinearity.

Interface Design beyond Epitaxy: Oxide Heterostructures Comprising Symmetry-Forbidden Interfaces.

Epitaxial growth of thin-film heterostructures is generally considered the most successful procedure to obtain interfaces of excellent structural and electronic quality between 3D materials. However, these interfaces can only join material systems with crystal lattices of matching symmetries and lattice constants. This article presents a novel category of interfaces, the fabrication of which is membrane-based and does not require epitaxial growth. These interfaces therefore overcome the limitations imposed by epitaxy. Leveraging the additional degrees of freedom gained, atomically clean interfaces are demonstrated between threefold symmetric sapphire and fourfold symmetric SrTiO3. Atomic-resolution imaging reveals structurally well-defined interfaces with a novel moiré-type reconstruction.

Ionic Modulation of Ferromagnetic and Proximity‐Induced Magnetization in Co/Pd Heterostructures

AbstractDespite the rampant discovery of tunable magnetic properties using magneto‐ionic gating, e.g., magnetic anisotropy, exchange bias, and exchange interactions, there are few studies that give a quantitative understanding of the reversible and irreversible effects of ionic infiltration. In this study, in situ vibrating sample magnetometry, superconducting quantum interference device magnetometry, and X‐ray magnetic circular dichroism (XMCD) reveal the reversible and irreversible control of magnetization, anisotropy, proximity‐effects, spin, and orbital angular momenta. Pd/Co/Pd trilayers, loaded using solid‐state hydrogen‐ion gating, show a decrease in the saturation magnetization of Co, and an increase in the proximity‐induced moment of Pd. This results in little to no change in the net effective magnetization, yet, allows for the effective anisotropy to be reversibly controlled by 270 kJ m−3. The reversible control of the effective anisotropy is dominated by a reversible change in surface anisotropy, however, under repeated cycling, irreversible evolution occurs in the heterostructure. XMCD measurements indicate this is partly due to hydrogen‐induced modification of the spin and orbital angular momenta. Together, these measurements indicate that the origin of the reversible and irreversible effects of magneto‐ionic gating is interfacial and provides crucial insight to scale and optimize thin film heterostructures for enhanced longevity and faster response time.

Fabrication of monolayer h-BN/LaB6 heterostructure thin film with low work function and high chemical stability

To expand the applications of low work function materials, high chemical stability is necessary to prevent the surface from unfavorable reactions. We developed a 20-nm-thick lanthanum hexaboride (LaB6) thin film covered by a monolayer hexagonal boron nitride (h-BN), which exhibits not only low work function but also high chemical stability. Results demonstrated that the h-BN/LaB6 heterostructure can be formed by vacuum annealing a nitrogen-doped LaB6 thin film. The formation process was investigated using Auger electron spectroscopy (AES), high-resolution electron energy loss spectroscopy (HREELS), and X-ray absorption near edge structure (XANES) spectroscopy. We have elucidated that the doped nitrogen atoms and boron atoms in the film thermally diffuse into the surface during annealing, thereby forming the monolayer h-BN on the surface. Work function was measured using scanning tunneling microscopy (STM). From a practical perspective, chemical stability was evaluated with a heating temperature necessary for restoring the low work function state after air exposure. The work function was comparable to that of the clean LaB6(100) surface. Moreover, it recovered at a much lower temperature than the cleaning temperature of the LaB6(100) surface. We anticipate that this material development will facilitate implementation of the low work function material.

CVD-Grown Monolayer MoS2 and GaN Thin Film Heterostructure for a Self-Powered and Bidirectional Photodetector with an Extended Active Spectrum.

Photodetector technology has evolved significantly over the years with the emergence of new active materials. However, there remain trade-offs between spectral sensitivity, operating energy, and, more recently, an ability to harbor additional features such as persistent photoconductivity and bidirectional photocurrents for new emerging application areas such as switchable light imaging and filter-less color discrimination. Here, we demonstrate a self-powered bidirectional photodetector based on molybdenum disulfide/gallium nitride (MoS2/GaN) epitaxial heterostructure. This fabricated detector exhibits self-powered functionality and achieves detection in two discrete wavelength bands: ultraviolet and visible. Notably, it attains a peak responsivity of 631 mAW-1 at a bias of 0V. The device's response to illumination at these two wavelengths is governed by distinct mechanisms, activated under applied bias conditions, thereby inducing a reversal in the polarity of the photocurrent. This work underscores the feasibility of self-powered and bidirectional photocurrent detection but also opens new vistas for technological advancements for future optoelectronic, neuromorphic, and sensing applications.

Ferromagnetic resonance measurement with frequency modulation down to 2 K.

Ferromagnetic resonance (FMR) spectroscopy is a powerful technique to study the precessional dynamics of magnetization in thin film heterostructures. It provides valuable information about the mechanisms of exchange bias, spin angular momentum transfer across interfaces, and excitation of magnons. A key desirable feature of FMR spectrometers is the capability to study magnetization dynamics over a wide phase space of temperature (T), frequency (f), and magnetic field (B). The design, fabrication, and testing of such a spectrometer, which uses frequency modulation techniques for improved detection of microwave absorption, reduces heat load in the cryostat and allows simultaneous measurements of inverse spin Hall effect (ISHE) induced dc voltages, is described in this paper. The apparatus is based on a 2-port transmitted microwave signal measurement using a grounded co-planar waveguide. The input radio frequency (RF) signal, frequency modulated at a tunable f-band, excites spin precession in the sample, and the attenuated RF signal is measured phase sensitively. The sample stage, inserted in the bore of a superconducting solenoid, allows magnetic field and temperature variability of 0 to ±5T and 2-310K, respectively. We demonstrate the working of this Cryo-FMR and ISHE spectrometer on thin films of Ni80Fe20 and Fe60Co20B20 over a wide T, B, and f phase space.

Two-Dimensional Tunneling Memtransistor with Thin-Film Heterostructure for Low-Power Logic-in-Memory Complementary Metal-Oxide Semiconductor.

With the demand for high-performance and miniaturized semiconductor devices continuously rising, the development of innovative tunneling transistors via efficient stacking methods using two-dimensional (2D) building blocks has paramount importance in the electronic industry. Hence, 2D semiconductors with atomically thin geometries hold significant promise for advancements in electronics. In this study, we introduced tunneling memtransistors with a thin-film heterostructure composed of 2D semiconducting MoS2 and WSe2. Devices with the dual function of tuning and memory operation were realized by the gate-regulated modulation of the barrier height at the heterojunction and manipulation of intrinsic defects within the exfoliated nanoflakes using solution processes. Further, our investigation revealed extensive edge defects and four distinct defect types, namely monoselenium vacancies, diselenium vacancies, tungsten vacancies, and tungsten adatoms, in the interior of electrochemically exfoliated WSe2 nanoflakes. Additionally, we constructed complementary metal-oxide semiconductor-based logic-in-memory devices with a small static power in the range of picowatts using the developed tunneling memtransistors, demonstrating a promising approach for next-generation low-power nanoelectronics.

E-beam synthesized fast-switching TiO2/SnO2 type-II heterostructure photodetector

A fast-switching TiO2/SnO2 heterostructure thin-film (TF) photodetector synthesized by electron beam evaporation technique is analyzed in this study. The substrate utilized is n-type silicon (Si), while gold (Au) is employed as the top electrode. To assess sample morphology and confirm elemental composition, field emission scanning electron microscopy (FESEM), energy dispersive x-ray spectroscopy (EDS), and chemical mapping were conducted. Structural characteristics were determined using X-ray diffraction (XRD) analysis. The XRD analysis confirmed the presence of various phases of TiO2 (anatase and rutile) and SnO2 (rutile). UV-Vis spectroscopy revealed multiple absorption peaks, at 447 nm, 495 nm, 560 nm, and 673 nm, within the visible spectrum. The device demonstrates high detectivity (D∗) of 1.737×109 Jones and a low noise equivalent power (NEP) of 0.765×10−10W. Evaluation of the device’s switching response through current-time characteristic (I-T) analysis indicates rapid switching with a rise time and fall time of 0.33 s and 0.36 s, respectively.

Interfacial strain induced giant magnetoresistance and magnetodielectric effects in multiferroic BCZT/LSMO thin film heterostructures

Layered thin films of the ferroelectric perovskite Ba0.85Ca0.15Ti0.9Zr0.1O3 (BCZT) and the ferromagnetic half-metal La0.80Sr0.20MnO3 (LSMO) are well-known multiferroic systems that show promise for spintronic applications. In this work, the structure–property relationships are explored in novel BCZT/LSMO thin film heterostructures with optimized ferroic properties. Epitaxial BCZT/LSMO thin film heterostructures are grown by varying the lattice mismatch strains on single crystal LaAlO3 (LAO) (100) and MgO (100) substrates using the pulsed laser deposition technique. The epitaxial strain in the films gives rise to a tetragonal distortion of the BCZT and LSMO unit cells and significantly affects their magnetotransport and magnetodielectric properties. The BCZT/LSMO/LAO heterostructure exhibits a colossal magnetoresistance effect due to a large out-of-plane tensile strain, which induces enhanced carrier hopping in the LSMO layer as compared to the BCZT/LSMO/MgO film. The larger tetragonal distortion of the BCZT unit cell in BCZT/LSMO/MgO contributes to higher dielectric permittivity, with a greater dielectric maxima temperature and freezing temperature. Magnetodielectric measurements reveal a hitherto unobserved giant magnetodielectric effect in the BCZT/LSMO/MgO film, attributed to a large in-plane strain, which induces interfacial polarization distortion at the interfacial layer. Overall, this work elucidates the unique strain and charge-mediated cross-coupled phenomena of magnetic and electric orders in multiferroic thin film heterostructures, which are critical for their technological applications.

Aging and degradation of transparent copper iodide thin film electrodes for functional electronic devices

Transparent CuJ copper iodide thin-film electrodes are widely used in various optoelectronic devices, such as solar cells, electroluminescent displays, touch screens, multifunctional photoconverters, etc. Their advantages are high transparency, because CuJ absorbs light in a limited range of wavelengths, which allows them to transmit visible light, making them ideal for transparent devices, low cost, because copper iodide is an inexpensive material, making it economically viable for mass production, simplicity fabrication, as CuJ can be easily deposited on various substrates using simple film deposition techniques such as vacuum deposition and magnetron sputtering. However, transparent CuJ type thin-film electrodes are subject to aging and degradation, which can reduce their performance and service life. The main factors affecting the aging and degradation of CuJ are the processes of oxidation, diffusion, degradation under the influence of light, and humidity. Despite numerous studies in the direction of obtaining high-quality thin films of copper iodide, today there is a need for a detailed study of the processes of their aging and degradation of properties. Therefore, in this article, we investigated the degradation processes of transparent upper copper iodide electrodes of thin-film multicomponent semiconductor heterostructures. This article shows the features of the use of film copper iodide in multilayer thin-film semiconductor structures. The influence of atmospheric conditions and lighting on the values of the rates of degradation of electrical resistance of copper iodide in semiconductor thin-film structures was studied. It has been found that atmospheric exposure to oxygen, moisture and other factors increases the electrical resistance of the copper iodide top electrode, and the use of Elastosil sealant does not lead to appreciable improvement. Heterostructures in vacuum are minimally affected by external factors, and therefore a significant increase in their CuJ upper electrode resistance values indicates the predominant role of aging of copper iodide films, which is not related to atmospheric conditions. It was established that natural sunlight significantly increases the degradation of copper iodide films, which should be taken into account when forming and using the corresponding thin film structures.

Prominent ferroelectric properties in multilayered TiO2 Mn-doped BiFeO3 spin-coated thin films

The study investigates the effects of doping on the leakage current and ferroelectric properties of bismuth ferrite (BiFeO3) thin film-based heterostructures. The spin coating method has been employed to deposit thin films of BiFeO3 and heterostructures of TiO2/BiFeO3 and TiO2/BiFe0.95Mn0.05FeO3 on fluorine-doped tin oxide substrates. The ferroelectric behavior of the TiO2/BiFe0.95Mn0.05FeO3 films exhibited better ferroelectric hysteresis loop compared to the other prepared films. The ferroelectric behavior reveals the TiO2/BiFe0.95Mn0.05FeO3 heterostructure showed a significantly lower leakage current density compared to both the BiFeO3 thin film and TiO2/BiFeO3 heterostructure, and the TiO2/BiFe0.95Mn0.05FeO3 heterostructures are likely suitable for solar cell applications. Highlights FTO/TiO2/BiFe0.95Mn0.05FeO3/Au heterostructure thin films prepared by spin coating method. Structure and optical properties of TiO2/BiFe0.95Mn0.05FeO3 heterostructure thin films were investigated. TiO2/BiFe0.95Mn0.05FeO3 heterostructure thin films showed a low optical energy band gap of 2.25 eV. Enhanced ferroelectric property in TiO2 thin film composited with BiFe0.95Mn0.05FeO3 thin films.

Perspectives and progress on wurtzite ferroelectrics: Synthesis, characterization, theory, and device applications

Wurtzite ferroelectrics are an emerging material class that expands the functionality and application space of wide bandgap semiconductors. Promising physical properties of binary wurtzite semiconductors include a large, reorientable spontaneous polarization, direct band gaps that span from the infrared to ultraviolet, large thermal conductivities and acoustic wave velocities, high mobility electron and hole channels, and low optical losses. The ability to reverse the polarization in ternary wurtzite semiconductors at room temperature enables memory and analog type functionality and quasi-phase matching in optical devices and boosts the ecosystem of wurtzite semiconductors, provided the appropriate combination of properties can be achieved for any given application. In this article, advances in the design, synthesis, and characterization of wurtzite ferroelectric materials and devices are discussed. Highlights include: the direct and quantitative observation of polarization reversal of ∼135 μC/cm2 charge in Al1−xBxN via electron microscopy, Al1−xBxN ferroelectric domain patterns poled down to 400 nm in width via scanning probe microscopy, and full polarization retention after over 1000 h of 200 °C baking and a 2× enhancement relative to ZnO in the nonlinear optical response of Zn1−xMgxO. The main tradeoffs, challenges, and opportunities in thin film deposition, heterostructure design and characterization, and device fabrication are overviewed.

Study of the Long-Range Exchange Coupling in Nd-Fe-B/Ti/Fe Multilayered Structure

The exchange coupling between two ferromagnetic thin films, one with magnetically hard and the other with soft phases, separated by a thin non-magnetic layer, is studied. Nd-Fe-B/Ti/Fe thin film heterostructures were fabricated using DC magnetron sputtering on Si substrates, which were heated in situ at 650 °C using a house-built vacuum-compatible heater. The effect of the thickness of the Ti buffer layer and the annealing temperature on the formation of various phases of Nd-Fe-B was investigated. The effect of the thickness of the non-magnetic Ti spacer layer on the exchange coupling strength between the hard phase Nd-Fe-B ferromagnetic thin layer and the soft phase transition metal Fe layer was experimentally investigated. Hysteresis loops of multilayer thin films indicate an antiferromagnetic coupling was observed when the thickness of the spacer layer was 2 nm. This is within the range of an antiferromagnetic coupling calculation based on RKKY theory predictions.

Tuneable Vertical Hysteresis Loop Shift in Exchange Coupled La0.67Sr0.33MnO3‐SrRuO3 Bilayer

AbstractHarnessing extra degrees of freedom at the heterostructure interface is of crucial importance to bring additional functionalities in modern spintronic devices. Here, a vertical hysteresis loop shift (vertical bias) is demonstrated in an exchange biased system of ferromagnetic thin film heterostructure of La0.67Sr0.33MnO3 (10 nm)‐SrRuO3 (SRO) (20 nm), after field cooling with ±1 T below 100 K close to the Curie temperature (TC) ≈125 K of SRO and loop sweeping under ±1 T field. Besides, a positive exchange bias (HEB) is also observed below TC ≈125 K showing a maximum ≈11 mT at 2 K. The vertical shift is modeled closely using micromagnetic simulations and the layers’ thickness dependency is demonstrated. The reason for the shift is attributed to the simultaneous role of the interfacial antiferromagnetic interaction and the hard anisotropy of SRO against the Zeeman energy. Finally, from the experimental and simulation results, a generalized model of controllable and tunable vertical shift is proposed applicable for other material systems possessing glassy phases, uncompensated/canted spins, absent interfacial exchange coupling, etc., and hence can be informative for the use of vertical shift in future spintronic devices.

Effects of interfacial buffering layer on imprint and domain switching dynamics in Pb(Zr,Ti)O3 thin-film heterostructures

Interfacial engineering is important for ferroelectric thin-film heterostructures because of the modulation of boundary conditions of the spontaneous polarizations and their switching behaviors, which are essential for ferroelectric electronics. In this work, we study the effects of interfacial buffering layer, 5-nm-thick SrTiO3 (STO), on the imprint and domain switching of epitaxial Pt/Pb(Zr,Ti)O3/SrRuO3 (SRO) thin-film heterostructures and capacitors. By buffering the ultrathin SrTiO3 layer at the Pb(Zr,Ti)O3 surface, the imprint effect can be dramatically alleviated as observed in the piezoresponse force microscopy (PFM)-measured domain structures and polarization–electric field hysteresis loops in thin-film capacitors. However, when the SrTiO3 layer is buffered at the Pb(Zr,Ti)O3/SrRuO3 interface, the imprint effect is slightly increased. These phenomena are explained based on the band alignments among the Pt and SrRuO3 electrodes and the Pb(Zr,Ti)O3 layer associated with the existence of oxygen vacancies in the SrTiO3 layer. With the reduction of imprint effect, the domain switching dynamics are also improved in the SrTiO3-buffered Pb(Zr,Ti)O3 capacitor, in which the switching activation field is decreased by about 45.3% in comparison with that of the pristine capacitor. These results facilitate the design and optimization of ferroelectric devices with the improvements in domain configurations, switching behaviors and band alignments.