Thin Film Heterostructures Research Articles

Investigation on ZnO nanorod array based ZnO/SnSe and ZnO/CdSe thin film heterostructures for photocatalytic degradation of methylene blue

ZnO/SnSe and ZnO/CdSe thin film photocatalysts were prepared by two steps via hydrothermal and thermal evaporation method for the photocatalytic degradation of methylene Blue (MB). UV/Vis spectrometer, XRD, SEM, EDX and PL were used to study the structural,morphological and optical characteristics of the deposited thin films. ZnO nanorods, ZnO/SnSe and ZnO/CdSe heterostructures were tested for the photocatalytic degradation of MB. Among these ZnO/SnSe heterostructure yielded a better performance as a photocatalyst. With the ZnO/SnSe thin film photocatalyst owing to its high charge separation efficiency, the best photocatalytic efficiency of 88% was attained in 150 min. This study confirms an improved photocatalytic efficiency of, ZnO/SnSe and ZnO/CdSe thin film heterostructures compared to ZnO nanorod arrays.

Rational design of oxide heterostructure InGaZnO/TiO2 for high-performance thin-film transistors

In this study, hetero-structured and self-passivated oxide thin film transistors (TFTs) were fabricated, wherein the channel comprised a N2O plasma (a-IGZO:N2O) treated a-IGZO front layer and TiO2 ultra-thin back layer. A high field-effect mobility (μFE) of 40.5 cm2/Vs and small sub-threshold swing (SS) of 250 mV/decade upon a SiO2/Si substrate were achieved using a rational design. Moreover, small threshold voltage shifts (ΔVth) of 0.5 (−0.7) and 0.8 (−0.9) V under gate bias and light illumination stress tests were obtained. Using Si3N4/HfO2 gate dielectrics, μFE and SS were enhanced to 53.6 cm2/Vs and 75 mV/decade, respectively. Experimental observations indicate that the electrons transfer from TiO2 to a-IGZO:N2O layer resulted in the accumulation of free carriers near a-IGZO:N2O/TiO2 interfaces. Additionally, an appropriate N2O treatment reduced the oxygen-related defects and bulk/interface trap density, while controlling the carrier density in the bilayer a-IGZO:N2O/TiO2 TFTs. Furthermore, the ultra-thin TiO2 layer acted as a surface passivation layer that reduced the leakage current and protected the channel from ambient influences. The proposed high-performance hetero-structured a-IGZO:N2O/TiO2 TFTs with superior stability are expected to provide a new and effective approach to obtaining high-performance metal oxide TFTs for application in thin film electronics.

Targeted Dehydration As a Route to Site-Selective Atomic Layer Deposition at TiO2 Defects

Crystallographic perfection in epitaxial thin film heterostructures can eliminate interface defects that dilute unique properties and reduce device performance. However, the requirement for epitaxial perfection greatly limits the selection of material candidates and deposition processes. Using selective interface reactions (SIRs), an atomic layer deposition (ALD)-based technique, we target transformation of undesirable defect sites at imperfect surfaces. Defects on the TiO2 surface affect the electronic properties, interfaces, and performance of optoelectronic devices that leverage TiO2 interfaces. We present first principles calculations to predict the difference in hydration/hydroxylation of pristine TiO2 terraces and minority atomic configurations (i.e. “defects”) including step edges and oxygen vacancies. We investigate hydroxylation differences through temperature dependent scanning tunneling microscopy (STM), and ultimately exploit these differences to selectively react ALD precursors at surface defect sites.

Theory-Guided Exploration of the Sr2Nb2O7 System for Increased Dielectric and Piezoelectric Properties and Synthesis of Vanadium-Alloyed Sr2Nb2O7

Ab initio methods provide a powerful tool in the search for novel polar materials. In particular, there has been a surge to identify lead-free piezoelectric materials to replace PbZr0.52Ti0.48O3. This study examines a computational strategy to identify increased piezoelectric and dielectric responses of alloy systems based on the linear interpolation of force constants, Born effective charges, and internal strain tensors from their end-point compounds. We choose the ferroelectric layered perovskite Sr2Nb2O7 as a parent structure and employ this alloying strategy for 19 potential cation substitutions, targeting thermodynamically metastable alloys with high piezoelectric response. From this screening, we identify Sr2Nb2–2xV2xO7 as a promising polar system. We conduct large-unit-cell calculations of Sr2Nb2–2xV2xO7 at x = 0.0625, 0.125 for multiple cation orderings and find a significant 184% enhanced piezoelectric response. The solid solution system is synthesized as single-crystalline thin-film heterostructures using pulsed-laser deposition, and an enhanced dielectric response is observed at x = 0.05 and at x = 0.1. We present the Sr2Nb2–2xV2xO7 alloy system designed through high-throughput computational screening methods with a large calculated piezoelectric response and experimentally verified increased dielectric response. Our methodology is provided as a high-throughput screening tool for novel materials with enhanced polarizability and alloy systems with potential morphotropic phase boundaries.

High energy storage efficiency and temperature stability realized in the Ba0.3Sr0.7Zr0.18Ti0.82O3/LaNiO3 heterostructure thin films directly deposited on the conductive Si substrate

A high-performance energy storage capacitor would not display high energy storage density (Ue) but high energy storage efficiency (η). However, during raising the electric field (E) or temperature (T), the η in dielectric thin films often shows an obvious degradation process. The degradation process is closely related to the change of the charge carrier transport mechanism as a function of E or T. Compared with the electric breakdown strength, the relationship between η and the charge carrier transport mechanisms in the dielectric thin films was less concerning. In this work, by increasing the trigger E and T of the space charge limited current mechanism, the beginning of the η degradation process was delayed into higher E and T. Consequently, a high Ue (∼56.6 J/cm3) and η (∼93.9%) were achieved simultaneously in Ba0.3Sr0.7Zr0.18Ti0.82O3/LaNiO3 heterostructure thin films directly deposited on low-cost conductive silicon wafers. Moreover, the temperature stability was greatly improved. This result would also offer a promising approach to improve the electrical properties of the dielectric oxide thin films in a harsh environment of high temperatures by redistributing oxygen vacancies through the heterostructure interface.

Nanostructured La0.75Sr0.25Cr0.5Mn0.5O3-Ce0.8Sm0.2O2 Heterointerfaces as All-Ceramic Functional Layers for Solid Oxide Fuel Cell Applications.

The use of nanostructured interfaces and advanced functional materials opens up a new playground in the field of solid oxide fuel cells. In this work, we present two all-ceramic thin-film heterostructures based on samarium-doped ceria and lanthanum strontium chromite manganite as promising functional layers for electrode application. The films were fabricated by pulsed laser deposition as bilayers or self-assembled intermixed nanocomposites. The microstructural characterization confirmed the formation of dense, well-differentiated, phases and highlighted the presence of strong cation intermixing in the case of the nanocomposite. The electrochemical properties—solid/gas reactivity and in-plane conductivity—are strongly improved for both heterostructures with respect to the single-phase constituents under anodic conditions (up to fivefold decrease of area-specific resistance and 3 orders of magnitude increase of in-plane conductivity with respect to reference single-phase materials). A remarkable electrochemical activity was also observed for the nanocomposite under an oxidizing atmosphere, with no significant decrease in performance after 400 h of thermal aging. This work shows how the implementation of nanostructuring strategies not only can be used to tune the properties of functional films but also results in a synergistic enhancement of the electrochemical performance, surpassing the parent materials and opening the field for the fabrication of high-performance nanostructured functional layers for application in solid oxide fuel cells and symmetric systems.

Weakly pinned skyrmion liquid in a magnetic heterostructure

Magnetic skyrmions are topological quasiparticles that can be formed in systems with particular magnetic interactions. Here, the authors study dense arrays of skyrmions in a thin film heterostructure, showing that the skyrmions form a liquid, the properties of which depend strongly on the size and density of the skyrmions. The presence of pinning sites alters the behavior of the skyrmion liquid compared to typical liquids, but these pinning sites are shown to have a narrow energy distribution compatible with proposed applications.

Local Epitaxial Templating Effects in Ferroelectric and Antiferroelectric ZrO2.

Nanoscale polycrystalline thin-film heterostructures are central to microelectronics, for example, metals used as interconnects and high-K oxides used in dynamic random-access memories (DRAMs). The polycrystalline microstructure and overall functional response therein are often dominated by the underlying substrate or layer, which, however, is poorly understood due to the difficulty of characterizing microstructural correlations at a statistically meaningful scale. Here, an automated, high-throughput method, based on the nanobeam electron diffraction technique, is introduced to investigate orientational relations and correlations between crystallinity of materials in polycrystalline heterostructures over a length scale of microns, containing several hundred individual grains. This technique is employed to perform an atomic-scale investigation of the prevalent near-coincident site epitaxy in nanocrystalline ZrO2 heterostructures, the workhorse system in DRAM technology. The power of this analysis is demonstrated by answering a puzzling question: why does polycrystalline ZrO2 transform dramatically from being antiferroelectric on polycrystalline TiN/Si to ferroelectric on amorphous SiO2/Si?

Interfacial mixing effect in a promising skyrmionic material: Ferrimagnetic Mn4N

Interfacial mixing of elements is a well-known phenomenon found in thin film deposition. For thin-film magnetic heterostructures, interfacial compositional inhomogeneities can have drastic effects on the resulting functionalities. As such, care must be taken to characterize the compositional and magnetic properties of thin films intended for device use. Recently, ferrimagnetic Mn4N thin films have drawn considerable interest due to exhibiting perpendicular magnetic anisotropy, high domain-wall mobility, and good thermal stability. In this study, we employed x-ray photoelectron spectroscopy (XPS) and polarized neutron reflectometry (PNR) measurements to investigate the interfaces of an epitaxially grown MgO/Mn4N/Pt trilayer deposited at 450 °C. XPS revealed the thickness of elemental mixing regions of near 5 nm at both interfaces. Using PNR, we found that these interfaces exhibit essentially zero net magnetization at room temperature. Despite the high-temperature deposition at 450 °C, the thickness of mixing regions is comparable to those observed in magnetic films deposited at room temperature. Micromagnetic simulations show that this interfacial mixing should not deter the robust formation of small skyrmions, consistent with a recent experiment. The results obtained are encouraging in terms of the potential of integrating thermally stable Mn4N into future spintronic devices.

Forward volume magnetoacoustic spin wave excitation with micron-scale spatial resolution

The interaction between surface acoustic waves (SAWs) and spin waves (SWs) in a piezoelectric-magnetic thin film heterostructure yields potential for the realization of novel microwave devices and applications in magnonics. In the present work, we characterize magnetoacoustic waves in three adjacent magnetic micro-stripes made from CoFe + Ga, CoFe, and CoFe + Pt with a single pair of tapered interdigital transducers (TIDTs). The magnetic micro-stripes were deposited by focused electron beam-induced deposition and focused ion beam-induced deposition direct-writing techniques. The transmission characteristics of the TIDTs are leveraged to selectively address the individual micro-stripes. Here, the external magnetic field is continuously rotated out of the plane of the magnetic thin film and the forward volume SW geometry is probed with the external magnetic field along the film normal. Our experimental findings are well explained by an extended phenomenological model based on a modified Landau–Lifshitz–Gilbert approach that considers SWs with nonzero wave vectors. Magnetoelastic excitation of forward volume SWs is possible because of the vertical shear strain ɛxz of the Rayleigh-type SAW.

Processing of SnO2/ZnO Heterostructure by the Sol-Gel Technique for Photocatalytic Applications

The construction of a SnO2 and ZnO thin film heterostructure is an efficient way to improve the photocatalytic properties of ZnO. However, the current techniques to produce it, for instance, chemical and physical vapor deposition are expensive and therefore not affordable for everyone. Several efforts have been made in order to obtain high quality thin films with lower cost using sol-gel base techniques. The aim of this work is to process high quality and low-cost SnO2 and ZnO thin film heterostructures supported on glass. The thin films were deposited by the spin coating method. The samples obtained were characterized by XRD, SEM, UV-Vis and FTIR. The thin film heterostructure exhibits homogeneous size nanoparticles (10~50 nm) that conform the surface and uniform submicronic thickness. High values of optical transmission >80% were measured for the heterostructure in the range (380-740 nm). The band gap of Eg 3.25 and 3.69 eV were obtained from Tauc’s plot.

AlScN‐on‐SiC Thin Film Micromachined Resonant Transducers Operating in High‐Temperature Environment up to 600 °C

AbstractThe experimental demonstration of aluminum scandium nitride (AlScN)‐on‐cubic silicon carbide (SiC) heterostructure thin film micromachined resonant transducers operating in a high‐temperature environment up to 600 °C is reported. Macroscopic and microscopic vibrations are investigated through a combination of ultrasensitive laser interferometry techniques and Raman spectroscopy. An average linear temperature coefficient of resonance frequency (TCf) of <1 ppm °C−1 within the temperature range from room temperature to 200 °C, and an average linear TCf of −16 ppm °C−1 between 200 and 600 °C, from the fundamental‐mode resonance of AlScN/SiC circular diaphragm resonator with a thickness of 1.9 µm and diameter of 250 µm, is obtained. Higher‐order modes exhibit much larger TCf, which make them strong candidates as high‐temperature‐tolerant temperature sensors or ultraviolet detectors. Raman spectroscopy indicates that the turning points of the peak positions of the longitudinal optical phonon modes of both 3C‐SiC and AlScN occur in almost the same temperature region where the turnover point of TCf is observed, suggesting that the microscopic vibrations in the crystal lattice and the macroscopic oscillation of the diaphragm are naturally mediated by the residual strain inside the materials at varying temperature.

Emergent and Tunable Topological Surface States in Complementary Sb/Bi2Te3 and Bi2Te3/Sb Thin-Film Heterostructures.

Epitaxial thin-film heterostructures offer a versatile platform for realizing topological surface states (TSSs) that may be emergent and/or tunable by tailoring the atomic layering in the heterostructures. Here, as an experimental demonstration, Sb and Bi2Te3 thin films with closely matched in-plane lattice constants are chosen to form two complementary heterostructures: Sb overlayers on Bi2Te3 (Sb/Bi2Te3) and Bi2Te3 overlayers on Sb (Bi2Te3/Sb), with the overlayer thickness as a tuning parameter. In the bulk form, Sb (a semimetal) and Bi2Te3 (an insulator) both host TSSs with the same topological order but substantially different decay lengths and dispersions, whereas ultrathin Sb and Bi2Te3 films by themselves are fully gapped trivial insulators. Angle-resolved photoemission band mappings, aided by theoretical calculations, confirm the formation of emergent TSSs in both heterostructures. The energy position of the topological Dirac point varies as a function of overlayer thickness, but the variation is non-monotonic, indicating nontrivial effects in the formation of topological heterostructure systems. The results illustrate the rich physics of engineered composite topological systems that may be exploited for nanoscale spintronics applications.

Magnetic-field–induced Zeeman excitation mode in paramagnetic NdGaO3single-crystal probed by magneto-terahertz spectroscopy

The magnetic field control of quantum matter enables to study various exotic phenomena which emerge in complex oxide materials. The thin film heterostructures of these systems, potentially relevant in oxide electronics, require a great deal of care in distinguishing the properties of the film from those of the substrate underneath. Here, we unravel a low-energy excitation mode in NdGaO3 crystals which is an important constituent of oxide thin film heterostructures. Employing terahertz (THz) time-domain spectroscopy with control of the external magnetic field, an excitation mode resonating at a frequency of 0.2THz was observed along two key (001) and (110) crystallographic orientations. The mode develops and strengthens with an application of increasing magnetic field and weakens with increasing temperature; all attributes conforming to the predictions for the energetics of Zeeman splitting and intra-level transition within Kramer's doublet ground state. A dominance of the magnetic field control of this mode along (110) compared to that along (001) orientation also further unveils a structural control. NdGaO3crystals are widely used as substrates in oxide thin film heterostructures; hence, while realizing ultrafast control of magnetic order, specifically in oxide thin films, the Kramer crystal field splitting mode should be carefully separated from ordered spin resonance mode.

Proximity-Induced Magnetism in a Topological Insulator/Half-Metallic Ferromagnetic Thin Film Heterostructure

Topological insulator (TI) Bi2Se3 thin films were prepared on half-metallic ferromagnetic La0.7Sr0.3MnO3 thin film by magnetron sputtering, forming a TI/FM heterostructure. The conductivity of Bi2Se3was modified by La0.7Sr0.3MnO3 at high- and low-temperature regions via different mechanisms, which could be explained by the short-range interactions and long-range interaction between ferromagnetic insulator and Bi2Se3 due to the proximity effect. Magnetic and transport measurements prove that the ferromagnetic phase and extra magnetic moment are induced in Bi2Se3 films. The weak anti-localized (WAL) effect was suppressed in Bi2Se3 films, accounting for the magnetism of La0.7Sr0.3MnO3 layers. This work clarifies the special behavior in Bi2Se3/La0.7Sr0.3MnO3 heterojunctions, which provides an effective way to study the magnetic proximity effect of the ferromagnetic phase in topological insulators.

Ultrafast light-induced THz switching in exchange-biased Fe/Pt spintronic heterostructure

The ultrafast optical control of magnetization in spintronic structures enables one to access to the high-speed information processing, approaching the realm of terahertz (THz). Femtosecond visible/near-infrared laser-driven ferromagnetic/nonmagnetic metallic spintronic heterostructures-based THz emitters combine the aspects from the ultrafast photo-induced dynamics and spin-charge inter-conversion mechanisms through the generation of THz electromagnetic pulses. In this Letter, we demonstrate photoexcitation density-dependent induced exchange-bias tunability and THz switching in an annealed Fe/Pt thin-film heterostructure, which otherwise is a widely used conventional spintronic THz emitter. By combining the exchange-bias effect along with THz emission, the photo-induced THz switching is observed without any applied magnetic field. These results pave the way for an all-optical ultrafast mechanism to exchange-bias tuning in spintronic devices for high-density storage, read/write magnetic memory applications.

A strategy to improve the performance of TiO2 nanotube array film photocatalysts by magnetron-sputtered amorphous BiFeO3

Amorphous BiFeO3/TiO2 nanotube array thin films were prepared on indium tin oxide glass using an anodic oxidation technique coupled with magnetron sputtering. The morphology, structure, optical properties, photoelectrochemical properties, and photocatalytic activity of heterostructured nanotube array thin films with different morphologies of BiFeO3 were studied. HRTEM observations show the interface between amorphous BiFeO3 and anatase TiO2. The amorphous BiFeO3/TiO2 nanotube array thin films show enhanced visible-light absorption and outstanding separation and transfer of photoinduced charges. The BFO3-TNA sample shows excellent photocatalytic activity. By using it, 97.6% of chromium (VI) can be removed in 60 min under visible-light illumination. This work will provide a feasible route to further enhance the photocatalytic activities of visible-light responsive TiO2 nanotube array thin films as photocatalysts for rapidly removing chromium (VI) from water.

Heterostructure Silicon Solar Cells with Enhanced Power Conversion Efficiency Based on Si x /Ni3+ Self-Doped NiO x Passivating Contact.

Developing efficient crystalline silicon/wide-band gap metal-oxide thin-film heterostructure junction-based crystalline silicon (c-Si) solar cells has been an attractive alternative to the silicon thin film-based counterparts. Herein, nickel oxide thin films are introduced as the hole-selective layer for c-Si solar cells and prepared using the reactive sputtering technique with the target of metallic nickel. An optimal Ni3+ self-doped NiOx film is obtained by tuning the reactive oxygen atmosphere to construct the optimized c-Si/NiOx heterostructure band alignment. A thin SiOx interlayer was further introduced to reduce the defect of the c-Si/NiOx interface with the UV–ozone (UVO) treatment. The constructed p-type c-Si/SiOx/NiOx/Ag solar cell exhibits an increase in the open voltage from 586 to 611 mV and achieves a 19.2% conversion efficiency.

THz emission from Fe/Pt spintronic emitters with L10-FePt alloyed interface

SummaryRecent developments in nanomagnetism and spintronics have enabled the use of ultrafast spin physics for terahertz (THz) emission. Spintronic THz emitters, consisting of ferromagnetic (FM)/non-magnetic (NM) thin film heterostructures, have demonstrated impressive properties for the use in THz spectroscopy and have great potential in scientific and industrial applications. In this work, we focus on the impact of the FM/NM interface on the THz emission by investigating Fe/Pt bilayers with engineered interfaces. In particular, we intentionally modify the Fe/Pt interface by inserting an ordered L10-FePt alloy interlayer. Subsequently, we establish that a Fe/L10-FePt (2 nm)/Pt configuration is significantly superior to a Fe/Pt bilayer structure, regarding THz emission amplitude. The latter depends on the extent of alloying on either side of the interface. The unique trilayer structure opens new perspectives in terms of material choices for the next generation of spintronic THz emitters.

Axially Bound Magnetic Skyrmions: Glueing Topological Strings Across an Interface.

A major challenge in topological magnetism lies in the three-dimensional (3D) exploration of their magnetic textures. A recent focus has been the question of how 2D skyrmion sheets vertically stack to form distinct types of 3D topological strings. Being able to manipulate the vertical coupling should therefore provide a route to the engineering of topological states. Here, we present a new type of axially bound magnetic skyrmion string state in which the strings in two distinct materials are glued together across their interface. With quasi-tomographic resonant elastic X-ray scattering, the 3D skyrmion profiles before and after their binding across the interface were unambiguously determined and compared. Their attractive binding is accompanied by repulsive twisting; i.e., the coupled skyrmions mutually affect each other via a compensating twisting. This state exists in chiral magnet–magnetic thin film heterostructures, providing a new arena for the engineering of 3D topological phases.