Thin Film Heterostructures Research Articles

Ionic Gating for Tuning Electronic and Magnetic Properties

The energy-efficient manipulation of the properties of functional materials is of great interest from both a scientific and an applied perspective. The application of electric fields is one of the most widely used methods to induce significant changes in the properties of materials, such as their structural, transport, magnetic, and optical properties. This article presents an overview of recent research on the manipulation of the electronic and magnetic properties of various material systems via electrolyte-based ionic gating. Oxides, magnetic thin-film heterostructures, and van der Waals 2D layers are discussed as exemplary systems. The detailed mechanisms through which ionic gating can induce significant changes in material properties, including their crystal and electronic structure and their electrical, optical, and magnetic properties, are summarized. Current and potential future functional devices enabled by such ionic control mechanisms are also briefly summarized, especially with respect to the emerging field of neuromorphic computing. Finally, a brief outlook and some key challenges are presented.

Schottky Barrier Control of Self-Polarization for a Colossal Ferroelectric Resistive Switching.

Controlling the domain evolution is critical both for optimizing ferroelectric properties and for designing functional electronic devices. Here we report an approach of using the Schottky barrier formed at the metal/ferroelectric interface to tailor the self-polarization states of a model ferroelectric thin film heterostructure system SrRuO3/(Bi,Sm)FeO3. Upon complementary investigations of the piezoresponse force microscopy, electric transport measurements, X-ray photoelectron/absorption spectra, and theoretical studies, we demonstrate that Sm doping changes the concentration and spatial distribution of oxygen vacancies with the tunable host Fermi level which modulates the SrRuO3/(Bi,Sm)FeO3 Schottky barrier and the depolarization field, leading to the evolution of the system from a single domain of downward polarization to polydomain states. Accompanied by such modulation on self-polarization, we further tailor the symmetry of the resistive switching behaviors and achieve a colossal on/off ratio of ∼1.1 × 106 in the corresponding SrRuO3/BiFeO3/Pt ferroelectric diodes (FDs). In addition, the present FD also exhibits a fast operation speed of ∼30 ns with a potential for sub-nanosecond and an ultralow writing current density of ∼132 A/cm2. Our studies provide a way for engineering self-polarization and reveal its strong link to the device performance, facilitating FDs as a competitive memristor candidate used for neuromorphic computing.

Dipole behavior in thin film heterostructure composed of Er-doped SnO2 and GaAs: Influence of polarization bias, temperature and stray light

The heterostructure system GaAs/SnO2 is built by resistive evaporation of Er-doped SnO2 powder on the top of GaAs semi-insulating substrate. The SnO2 powder comes from the drying of SnO2 sol-gel solution. The possible formation of dipoles in this heterostructure is investigated by the thermally stimulated depolarization current (TSDC) technique, and the possibilities for dipole formation are explored, such as the EL2 defect in the GaAs side, oxygen vacancies and Er ions in the SnO2 layer. The dipole relaxation activation energies are found in the range 0.2 eV to 0.3 eV, in good agreement with the ionization energies of these defects. The main TSDC bands: 213 meV with peak of −298 pA, for positive bias, and 218 meV with peak of 128 pA for negative bias, are modified by stray (room) light, which may correspond to the second ionization level of oxygen vacancies in SnO2, which are excited by the room lights and do not return to the original orientation.

Photocarrier transport of ferroelectric photovoltaic thin films detected by the magnetic dynamics of adjacent ferromagnetic layers

We have observed photocarrier transport behaviors in ${\mathrm{BiFeO}}_{3}/{\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ (BFO/LSMO) heterostructures by using time-resolved synchrotron x-ray magnetic circular dichroism in reflectivity. The magnetization of LSMO layers was used as a probe of photo-induced carrier dynamics in the photovoltaic BFO layers. During the photo-induced demagnetization process, the decay time of LSMO ($x$=0.2) magnetization strongly depends on the ferroelectric polarization direction of the BFO layer. The variation of decay time should be attributed to the different sign of accumulated photocarriers at the BFO/LSMO interface induced by the photovoltaic effect of the BFO layer. The photocarriers can reach the BFO/LSMO interface and influence the magnetization distribution in the LSMO layers within the timescale of $\ensuremath{\sim}100$ ps. Our results provide a novel strategy to investigate carrier dynamics and mechanisms of optical control of magnetization in thin film heterostructures.

Interfacial electronic structure modulated magnetic properties in Ta/CoFeB/Ta multilayers

Thinfilm heterostructure Ta/CoFeB/TaOx can exhibit chiral magnetic textures such as skyrmions as well as spin-orbit torque induced zero field magnetization switching, which offers great promise for next-generation information storage technologies. Here, we provide a strong correlation between the structural and magnetic properties in as-deposited and annealed Ta (1 nm)/ CoFeB (1.5 nm)/ TaOx (1 nm) trilayer and Ta (1 nm)/ CoFeB (1.5 nm)/ Ta (t)/ CoFeB (1.5 nm)/ TaOx (1 nm)] multilayered stack with t = 0.5, 1 nm. Interestingly, x-ray reflectivity results indicate improvement in the interface quality post annealing up to 300 °C. While the thinner Ta spacer layer (0.5 nm) in the stack causes the formation of smaller interface width, the thicker Ta spacer (1 nm) results in relatively larger interface width. Annealing results in the enhancement of saturation magnetization (Ms) and uniaxial magnetic anisotropy (UMA) and the extent of variation is mostly determined by thickness of the spacer Ta layer. Magneto-optical Kerr effect measurement suggests the existence of strong two-fold UMA in the film stack with Ta spacer layer 0.5 nm, however, its strength diminishes as the Ta spacer layer thickness is increased to 1.0 nm. Furthermore, the in-depth chemical/electronic investigations using x-ray photoelectron spectroscopy technique reveal the formation of additional chemical states such as TaB and TaOB at the interfaces. Remarkably, Boron diffusion and likely oxygen migration from the Fe-oxide/Co-oxide in CoFeB to neighboring Ta layers improves the metallic nature of Fe and Co due to annealing. Note that the thicker Ta spacer has a significant impact on the chemical transformation at the interface in comparison to that of thinner Ta spacer layer. Overall, in these Ta/CoFeB multilayered stacks, the thickness of the Ta spacer layer and annealing temperature play a crucial role in controlling interface quality that eventually governs the magnetic characteristics. These results provide new insight into the interfacial electronic structure modifications which are essential for controlling DMI and stabilization of skyrmions in Ta/CoFeB multilayers for future spintronic device applications.

Tunable coupling in magnetic thin film heterostructures with a magnetic phase transition

The magnetic properties of permalloy-based trilayers of the form Py0.8Cu0.2/Py0.4Cu0.6/Py/IrMn were studied as the spacer layer undergoes a paramagnetic to ferromagnetic phase transition. We find the coupling between the free Py0.8Cu0.2 layer and the exchange bias pinned Py to be strongly temperature-dependent: there is negligible coupling above the Curie temperature of the Py0.4Cu0.6 spacer layer, strong ferromagnetic coupling below that temperature, and a tunable coupling between these extremes. Polarized neutron reflectometry was used to measure the depth profile of the magnetic order in the system, allowing us to correlate the order parameter with the coupling strength. The thickness dependence shows that these are interface effects with an inverse relationship to thickness, and that there is a magnetic proximity effect that enhances the Curie temperature of the spacer layer with characteristic length scale of about 7 nm. As a demonstration of potential functionality of such a system, the structure is shown to spontaneously flip from the antiparallel to parallel magnetic configuration once the spacer layer has developed long-range magnetic order.

Electrodeposited NiFe2O4/Cu2O heterostructure thin films with enhanced photocurrent generation

In comparison with single-phase materials, heterostructures have been known for superior water splitting applications. In this study, Cu2O and NiFe2O4 are chosen to fabricate thin film heterostructures. Cu2O is electrodeposited at 60 °C for 5 min on ITO-coated glass substrates using three-electrode system. After deposition, the phase formation is confirmed using powder x-ray diffraction studies. The NiFe2O4 (NFO) thin films are deposited using RF sputtering method at room temperature for 2 h on Cu2O/ITO substrates to obtain NFO/Cu2O/ITO Type-II heterostructure. The scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) cross-sectional images show that the thickness of NFO layer is 120 nm and Cu2O layer is 1.5 µm. The photocurrent density of Cu2O on ITO is 0.08 ± 0.002 mA/cm2, and it increased to 0.12 ± 0.002 mA/cm2 after adding NFO layer on Cu2O film due to Type-II heterojunction formation.

Evidence of magnetoelectronic electromagnon-mediated transport in flexoelectronic heterostructures

The superposition of atomic vibrations and flexoelectronic effect gives rise to a cross correlation between free charge carriers and temporal magnetic moment of phonons in conducting heterostructures under an applied strain gradient. The resulting dynamical coupling is expected to give rise to quasiparticle excitations called magnetoelectronic electromagnons that carry electronic charge and temporal magnetic moment. Here, we report experimental evidence of magnetoelectronic electromagnons in the freestanding degenerately doped $p$-Si based heterostructure thin film samples. These quasiparticle excitations give rise to long-distance ($>100 \ensuremath{\mu}\mathrm{m}$) spin transport, demonstrated using spatially modulated transverse magnetothermoelectric and nonlocal resistance measurements. The magnetoelectronic electromagnons are nonreciprocal and give rise to large magnetochiral anisotropy $(0.352\phantom{\rule{0.16em}{0ex}}{\mathrm{A}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{T}}^{\ensuremath{-}1})$ that diminishes at lower temperatures. The superposition of nonreciprocal magnetoelectronic electromagnons gives rise to longitudinal and transverse modulations in charge carrier density, spin density, and magnetic moment, demonstrated using the Hall effect and edge dependent magnetoresistance measurements, which can also be called inhomogeneous magnetoelectronic multiferroic effect. These quasiparticle excitations are analogous to photons where time dependent polarization and temporal magnetic moment replace electric and magnetic field, respectively, and most likely topological because they manifest topological Nernst effect. Hence, the magnetoelectronic electromagnon can potentially give rise to quantum interference and entanglement effects in conducting solid state systems at room temperature in addition to efficient spin transport.

Creation and investigation of thin-film heterostructures based on Fe/Nb

Fe/Nb-based thin-film heterostructures prepared under various conditions are studied. Here, Fe/Nb and Fe/Al2O3/Nb structures prepared on MgO and Al2O3 single-crystal substrates at various substrate temperatures during deposition were studied. According to the results of our studies, the structures prepared at high substrate temperatures (600 K) on MgO demonstrate the most stable and, most importantly, reproducible transport properties of the samples.

Enhanced solar-driven water splitting by ZnO/CdTe heterostructure thin films-based photocatalysts

Solar-driven hydrogen production by water splitting using a photocatalyst is considered the most effective approach to produce hydrogen. Hydrogen is the most suitable renewable energy source. The efficiency of hydrogen production is still low. The efficiency of hydrogen production through photocatalysis can be enhanced by preparing a suitable and efficient photocatalyst. In this work, ZnO thin films were deposited on CdTe thin films at 600 °C, 650 °C, and 700 °C temperatures to form ZnO/CdTe heterostructure thin films by chemical vapor deposition (CVD) as photoelectrodes for water splitting. The photoelectrochemical properties showed that ZnO/CdTe heterostructure thin films have better photocurrent response compared to pristine ZnO and CdTe thin films. EIS results showed that the charge transfer at the electrode-electrolyte interface for ZnO/CdTe heterostructure thin films is much better than that of the pristine ZnO film. The ZnO/CdTe-700 °C heterostructure thin film has a 112-fold higher rate of photocatalytic hydrogen generation than pure ZnO.

Large Anomalous Frequency Shift in Perpendicular Standing Spin Wave Modes in BiYIG Films Induced by Thin Metallic Overlayers.

Interface-driven effects on magnon dynamics are studied in magnetic insulator-metal bilayers using Brillouin light scattering. It is found that the Damon-Eshbach modes exhibit a significant frequency shift due to interfacial anisotropy generated by thin metallic overlayers. In addition, an unexpectedly large shift in the perpendicular standing spin wave mode frequencies is also observed, which cannot be explained by anisotropy-induced mode stiffening or surface pinning. Rather, it is suggested that additional confinement may result from spin pumping at the insulator-metal interface, which results in a locally overdamped interface region. These results uncover previously unidentified interface-driven changes in magnetization dynamics that may be exploited to locally control and modulate magnonic properties in thin-film heterostructures.

Solar cells based on CdTe thin films (Part II)

This paper discusses the use of semiconductor solar cells based on thin-film cadmium telluride (CdTe) in modern energy production. The advantages and disadvantages of using CdTe thin-film solar cells are analyzed, and arguments are presented in favor of the implementation of mass production technologies for CdTe solar modules, which can compete with silicon analogs in terms of compromise between efficiency and cost. The physical and chemical properties of the binary Cd-Te system are described, and the relationship between the physical, chemical, electrical, and optical properties of CdTe is analyzed, making it attractive for use in thin-film solar cells. Special attention is given to the investigation of photovoltaic properties, which are important parameters for determining photoconductivity, and the advantages and disadvantages of CdTe film photovoltaic properties are discussed. CdTe thin-film heterostructures (HSs), which are important components of modern solar cells, are considered, and their main advantages and disadvantages are described. It is argued that simple methods of manufacturing and forming HSs, which do not require complex and expensive equipment, are an important advantage of CdTe-based solar cell technology.

Nd:YAG infrared laser as a viable alternative to excimer laser: YBCO case study

We report on the growth and characterization of epitaxial YBa_{2}Cu_{3}O_{7-delta } (YBCO) complex oxide thin films and related heterostructures exclusively by Pulsed Laser Deposition (PLD) and using first harmonic Nd:Y_{3}Al_{5}O_{12} (Nd:YAG) pulsed laser source (lambda = 1064 nm). High-quality epitaxial YBCO thin film heterostructures display superconducting properties with transition temperature sim 80 K. Compared with the excimer lasers, when using Nd:YAG lasers, the optimal growth conditions are achieved at a large target-to-substrate distance d. These results clearly demonstrate the potential use of the first harmonic Nd:YAG laser source as an alternative to the excimer lasers for the PLD thin film community. Its compactness as well as the absence of any safety issues related to poisonous gas represent a major breakthrough in the deposition of complex multi-element compounds in form of thin films.

AlN/Si 기판 상에 성장된 바나듐 이산화물 박막 기반 전자 소자에서 광열 효과로 제어되는 고반복률 전류 스위칭

In this study, an electronic device based on a vanadium dioxide (VO₂) heterostructure thin film grown on an aluminum nitride(AlN)/silicon(Si) substrate was fabricated to realize high-repetition-rate current switching controlled by the photothermal effect. In the conventional VO₂/sapphire(Al₂O₃) device, the current switching controlled by the photothermal effect takes a long time for the cooling process to return the device to the off-state. This problem made high-repetition-rate switching impossible. On the other hand, it was assumed that the VO₂/AlN/Si device would be suitable for high-repetition-rate switching because it has a higher critical temperature and superior thermal conductivity than the VO₂/Al₂O₃ device. And, we demonstrated the photothermally-controlled current switching with a repetition rate of up to 60㎐ while maintaining a high on-state current of 50㎃ using an optimized experimental setup and anewly fabricated device. The switching response time was also improved to 1ms level.

Observation of the Superconducting Proximity Effect from Surface States in SmB_{6}/YB_{6} Thin Film Heterostructures via Terahertz Spectroscopy.

The ac conduction of epitaxially grown SmB_{6} thin films and superconducting heterostructures of SmB_{6}/YB_{6} are investigated via time-domain terahertz spectroscopy. A two-channel model of thickness-dependent bulk states and thickness-independent surface states accurately describes the measured conductance of bare SmB_{6} thin films, demonstrating the presence of surface states in SmB_{6}. While the observed reductions in the simultaneously measured superconducting gap, transition temperature, and superfluid density of SmB_{6}/YB_{6} heterostructures relative to bare YB_{6} indicate the penetration of proximity-induced superconductivity into the SmB_{6} overlayer; the corresponding SmB_{6}-thickness independence between different heterostructures indicates that the induced superconductivity is predominantly confined to the interface surface state of the SmB_{6}. This study demonstrates the ability of terahertz spectroscopy to probe proximity-induced superconductivity at an interface buried within a heterostructure, and our results show that SmB_{6} behaves as a predominantly insulating bulk surrounded by conducting surface states in both the normal and induced-superconducting states in both terahertz and dc responses, which is consistent with the topological Kondo insulator picture.

Acid-modified CNT/Zinc Oxide nanowires based high performance broadband photodetector

In this study, the authors have reported the impact of post-treatment via exposure to acid on single walled carbon nanotubes (SWNTs) thin film (TF) based SWNT/ZnO Nanowire (NW) broad band photodetector. The ZnO NWs were deposited on SWNT (with and without acid-treated) using a simple catalytic free process called glancing angle deposition (GLAD) technique. Acid-treated SWNT samples warranted the growth of high quality ZnO NWs over them. On fabricating photodetectors with the acid-treated ZnO NW/SWNT TF heterostructure (HS) gave better device performance as compared to the as-deposited ZnO NW/SWNT TF HS (without acid-treatment) sample. The acid-treated device showed a large responsivity (85.45 A/W), specific detectivity (0.859 × 1012 Jones) and with a low noise equivalent power of 3.9101 pW values. Moreover, the oxygen adsorption–desorption mechanism in SWNTs impacted the electrical resistance of the nanotubes which affected nanotube conductivity. The acid-treatment favoured relatively faster charge separation at the ZnO NW/SWNT heterojunction thus providing a fast device response (trise = 0.11 s, tfall = 0.39 s at + 5 V). The fabricated acid-treated device showed good broad band detection (250 nm-750 nm) which was explained with respect to the optical absorption profile of the sample.

Comparing spin injection in Fe75Co25/Bi2Te3 at GHz and optical excitations

Spin-to-charge conversion (S2CC) processes in thin-film heterostructures have attracted much attention in recent years. Here, we describe the S2CC in a 3D topological insulator Bi2Te3 interfaced with an epitaxial film of Fe75Co25. The quantification of spin-to-charge conversion is made with two complementary techniques: ferromagnetic resonance based inverse spin Hall effect (ISHE) at GHz frequencies and femtosecond light-pulse induced emission of terahertz (THz) radiation. The role of spin rectification due to extrinsic effects like anisotropic magnetoresistance (AMR) and planar Hall effects (PHE) is pronounced at the GHz timescale, whereas the THz measurements do not show any detectible signal, which could be attributed to AMR or PHE. This result may be due to (i) homodyne rectification at GHz, which is absent in THz measurements and (ii) laser-induced thermal spin current generation and magnetic dipole radiation in THz measurements, which is completely absent in GHz range. The converted charge current has been analyzed using the spin diffusion model for the ISHE. We note that regardless of the differences in timescales, the spin diffusion length in the two cases is comparable. Our results aid in understanding the role of spin pumping timescales in the generation of ISHE signals.

Nonreciprocal magnetoacoustic waves in synthetic antiferromagnets with Dzyaloshinskii-Moriya interaction

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 investigate the SAW-SW interaction in a Pt/Co(2 nm)/Ru(0.85 nm)/Co(4 nm)/Pt synthetic antiferromagnet (SAF) composed of two ferromagnetic layers with different thicknesses separated by a thin nonmagnetic Ru spacer layer. Because of the combined presence of interfacial Dzyaloshinskii--Moriya interaction (iDMI) and interlayer dipolar coupling fields, the optical SW mode shows a large nondegenerate dispersion relation for oppositely propagating SWs. Due to SAW-SW interaction, we observe nonreciprocal SAW transmission in the piezoelectric/SAF hybrid device. The equilibrium magnetization directions of both Co layers are manipulated by an external magnetic field to set a ferromagnetic, canted, or antiferromagnetic configuration. This has a strong impact on the SW dispersion, its nonreciprocity, and SAW-SW interaction. The experimental results are in agreement with a phenomenological SAW-SW interaction model, which considers the interlayer exchange coupling, iDMI, and interlayer dipolar coupling fields of the SWs.

Epitaxial growth of a β-Ga2O3 (−201)-oriented thin film on a threefold symmetrical SrTiO3 (111) substrate for heterogeneous integration

Epitaxial growth of a wide bandgap semiconductor β-Ga2O3 thin film with high crystal quality plays a decisive role in constructing optical and electronic devices. However, except for the native substrate, the scarcity of appropriate non-native substrates or the poor crystallization of the deposit in thin film growth severely limits the fabrication and applicability of the final heterostructures and devices. Here, by taking the consistent symmetry and closely matched atomic spacing between β-Ga2O3 (−201) and the cubic perovskite (111)-oriented plane of SrTiO3, we realize the epitaxial growth of single crystal β-Ga2O3 (−201) thin films on the SrTiO3 (111) substrate by the pulsed laser deposition method, as confirmed by wide-range reciprocal-space mapping and high-resolution scanning transmission electron microscopy. The fabricated β-Ga2O3 (−201) photodetector device on the SrTiO3 (111) substrate exhibits excellent ultraviolet optical detection performance with large on/off switching ratios and a fast response speed. Moreover, the β-Ga2O3/SrTiO3 (111) heterojunction shows type-II heterostructure characteristics for energy band alignment, which displays superior ability for electron–hole pairs separation with large conduction and small valance band offsets of 1.68 and 0.09 eV, respectively. The results offer us a new way to obtain high-quality β-Ga2O3 (−201) thin film heterostructures on cubic SrTiO3 (111) substrates and fabricate β-Ga2O3-based optical and electronic devices.

Observation of dielectric resonance and negative capacitance in 0.65Pb(Mg1/3Nb2/3)O3-0.35PbTiO3 textured thin films

Negative capacitance effect (NCE) in ferroelectric/dielectric thin film heterostructures has received a great deal of interest as it promises to bring down the operational voltage of field-effect transistors below the Boltzmann limit. Here, it has been observed in 0.65Pb(Mg1/3Nb2/3)O3-0.35PbTiO3 (PMN-PT)/SrRuO3 heterostructures grown on Pt/Ti/SiO2/Si substrate using pulsed laser deposition (PLD) technique. Phase pure PMN-PT thin film with (111) preferred orientation displayed excellent ferroelectric and dielectric properties with PS ∼36.80 μC/cm2, ϵr ∼1900 and tanδ ∼0.06 (at 1 kHz). Interestingly, dielectric resonance was realized around 3 MHz and fitted to Lorentz model. Its temperature variation beyond the ferroelectric phase transition showed clear signature of NCE after resonance frequency. Furthermore, the entire structure exhibited Fano-like resonance. The direct observation of NCE in relaxor ferroelectric, PMN-PT integrated on Si will aid in better understanding of the fundamental aspects of NCE which can trigger another technological revolution in semiconductor industry.