Epitaxial Thin Films Research Articles

Compositional Modification of Epitaxial Pb(Zr,Ti)O3 Thin Films for High‐Performance Piezoelectric Energy Harvesters

AbstractIn this study, Piezoelectric energy harvesters (PEHs) are fabricated based on epitaxial Pb(Zr,Ti)O3 (PZT) thin films deposited by an RF magnetron sputtering with varied Zr/[Zr+Ti] ratio. For a compatibility with micro‐electromechanical systems, the epitaxial PZT thin films are deposited on Si substrates (PZT/Si). The morphotropic phase boundary (MPB) are formed in the compositional range of 0.44 ≤ Zr/[Zr+Ti] ≤ 0.51 of the epitaxial PZT/Si, which is far broader than that of the bulk PZT. Meanwhile, effective transverse piezoelectric coefficient (|e31,f|) values are evaluated by the direct and converse piezoelectric effects using the unimorph cantilever method. Among the compositions, the rhombohedral‐dominant MPB (MPB‐R) of Zr/[Zr+Ti] = 0.51 exhibits a direct |e31,f| of 10.1 C m−2 and a relative permittivity (εr) of 285, leading to the maximum figure of merit of 40 GPa in this study. On the other hand, the maximum converse |e31,f| of 14.0 C m−2 is measured from the tetragonal‐dominant MPB (MPB‐T) of Zr/[Zr+Ti] = 0.44. At a resonance frequency, the MPB‐T presents a high output power density of 301.5 µW−1/(cm2 g2) under an acceleration of 3 m−1 s−2, which is very promising for the high‐performance PEH applications.

Effects of film thickness on crystal structure, surface topography, optical, and photoelectric properties of Ga2O3 thin film based solar blind photodetectors

β-Ga2O3 epitaxial thin films with different thicknesses were grown on Al2O3 (0001) substrates by the pulse laser deposition technology. The full width at half maximum of XRD rocking curve, grain size, surface roughness, and optical absorbance (200–300 nm) of the β-Ga2O3 films increase with increasing film thickness. In addition, with increasing film thickness, the adsorption edge presents a red-shift phenomenon, which is accompanied by a slight decrease in the optical band gap (). The metal-semiconductor-metal type photodetector made with the 401-nm β-Ga2O3 film shows the maximal photocurrent (1.06 μA @ 10 V), photo-to-dark current ratio (6.96 × 104), responsivity (90 mA W−1), and detectivity (0.5 × 1011 Jones) under 254-nm UV light illumination with a light intensity of 98.4 μW/cm2. Moreover, the 401-nm photodetector displays a small rise time of 0.111/0.561 s and a decay time of 0.046/0.047 s as well as a good stability and repeatability. These findings provide useful reference for the fabrication of β-Ga2O3 film based solar blind ultraviolet photodetectors.

Aluminum substitution in low damping epitaxial lithium ferrite films

Ferromagnetic insulators with ultra-low damping and tunable magnetic properties are of great interest for the study of spin wave-based phenomena as well as for applications in spin wave spintronics. We have developed epitaxial spinel structure ferrite thin films of Li0.5(AlxFe2.5−x)O4 (LAFO) with ultra-low magnetic damping. The degree of Al substitution tunes the saturation magnetization and in-plane magnetic anisotropy while maintaining excellent epitaxy and crystalline quality as well as low magnetic damping. All LAFO films, deposited on (001)-oriented MgAl2O4 (MAO), are under epitaxial compressive strain, giving rise to an easy magnetic axis in-plane and a hard out-of-plane axis. Element-specific x-ray magnetic circular dichroism measurements confirm that the magnetism originates from the majority of Fe3+ cations occupying the octahedral sites, which are displaced by increased Al3+ substitution. Broadband ferromagnetic resonance reveals the lowest Gilbert damping parameters of 5×10−4 for films 10–15 nm thick. Our films demonstrate the robustness of the low loss dynamic properties of Li0.5(AlxFe2.5−x)O4 that are promising for spin wave applications.

P-GaN/n-IGZO self-powered ultraviolet photodetector with ultralow dark current and high sensitivity

Self-powered ultraviolet (UV) photodetector has attracted growing research interest for applications in biological and human related fields with a low power consumption. However, typical construction strategies of heterojunction require materials with high crystal quality to realize high device performance with low dark current, which normally necessitates costly thin film epitaxy process. In our study, a high sensitivity self-powered UV photodetector with an ultralow dark current has been proposed by constructing p-GaN/n-IGZO heterojunction via a facile room temperature sputtering of IGZO thin film on commercially available p-GaN/sapphire substrate. By implementing UV-Ozone surface treatment to introduce an ultrathin oxide layer in the junction interface, dark current of the photodetector can be as low as 0.45 pA, which leads to a high detectivity of 1.9 × 1013 Jones with a rise time of 0.92 ms and a decay time of 11.61 ms when operating in self-powered mode. Our work has put forward a facile and cost-effective route for developing highly sensitive UV detecting devices with a low dark current and a low power consumption, which paves the path to advances in green industry of UV optoelectronics.

Superior Energy Storage Performance in Antiferroelectric Epitaxial Thin Films via Structural Heterogeneity and Orientation Control

AbstractDielectric capacitors are desired for electronics and electrical power systems because of their fast charge–discharge speed and high‐power density. Nevertheless, dielectric capacitors typically exhibit lower energy densities in comparison to other energy storage systems like batteries or fuel cells. Among dielectrics, antiferroelectrics have shown great promise for high energy density because of their characteristic double hysteresis loops. However, current antiferroelectric capacitors still face challenges of low efficiency and low breakdown strength due to their large hysteresis, which is harmful to energy efficiency and reliability of the system. Herein, by engineering the nanoscale heterogeneity to mitigate hysteresis and controlling orientation to enhance the polarization, the exceptional energy storage performance of antiferroelectric (Pb0.97La0.02)(Zr0.55Sn0.45)O3 epitaxial thin films is demonstrated. Atomic‐resolution transmission electron microscopy and X‐ray reciprocal space mapping confirm the presence of nanoscale structural heterogeneity, characterized by fragmented antipolar nanodomains. These films exhibit remarkable energy densities, reaching up to ≈84.5 J cm−3, coupled with ultrahigh efficiencies of up to ≈98.5% and superior stability, maintaining efficiencies above 92% across a wide field range of ≈5 MV cm−1. Notably, these findings surpass the capabilities of previously reported dielectric materials, opening new avenues for advanced energy storage applications.

Electrically Tunable Spin Exchange Splitting in Graphene Hybrid Heterostructure

AbstractGraphene, with spin and valley degrees of freedom, fosters unexpected physical and chemical properties for the realization of next‐generation quantum devices. However, the spin‐symmetry of graphene is rather robustly protected, hampering manipulation of the spin degrees of freedom for the application of spintronic devices such as electric gate‐tunable spin filters. It is demonstrated that a hybrid heterostructure composed of graphene and LaCoO3 epitaxial thin film exhibits an electrically tunable spin‐exchange splitting. The large and adjustable spin‐exchange splitting of 155.9 – 306.5 meV is obtained by the characteristic shifts in both the spin‐symmetry‐broken quantum Hall states and the Shubnikov–de Haas oscillations. Strong hybridization‐induced charge transfer across the hybrid heterointerface has been identified for the observed spin exchange splitting. The substantial and facile controllability of the spin exchange splitting provides an opportunity for spintronics applications with the electrically‐tunable spin polarization in hybrid heterostructures.

Anisotropic Properties of Epitaxial Ferroelectric Lead-Free 0.5[Ba(Ti0.8Zr0.2)O3]-0.5(Ba0.7Ca0.3)TiO3 Films.

As the energy demand is expected to double over the next 30 years, there has been a major initiative towards advancing the technology of both energy harvesting and storage for renewable energy. In this work, we explore a subset class of dielectrics for energy storage since ferroelectrics offer a unique combination of characteristics needed for energy storage devices. We investigate ferroelectric lead-free 0.5[Ba(Ti0.8Zr0.2)O3]-0.5(Ba0.7Ca0.3)TiO3 epitaxial thin films with different crystallographic orientations grown by pulsed laser deposition. We focus our attention on the influence of the crystallographic orientation on the microstructure, ferroelectric, and dielectric properties. Our results indicate an enhancement of the polarization and strong anisotropy in the dielectric response for the (001)-oriented film. The enhanced ferroelectric, energy storage, and dielectric properties of the (001)-oriented film is explained by the coexistence of orthorhombic-tetragonal phase, where the disordered local structure is in its free energy minimum.

Growth and stability of epitaxial zirconium diboride thin films on silicon (111) substrate

The epitaxial growth of boron rich hexagonal zirconium diborides (h-ZrB2+δ) thin films on Si(111) substrates using the magnetron co-sputtering technique with elemental zirconium and boron is reported. The effect of process temperature (700–900 °C) on the compositions and epitaxy quality was investigated. The chemical composition of the films was found to have a higher boron to zirconium ratio than the ideal stoichiometric AlB2-type ZrB2 and was observed to be sensitive to process temperature. Films deposited at 700 °C exhibited intense diffraction peaks along the growth direction corresponding to (000ℓ) of h-ZrB2 using both lab and synchrotron-based x-ray diffractograms. The thermal and compositional stability of the epitaxial h-ZrB2+δ film was further evaluated under a nitrogen-rich environment through isothermal annealing which showed a reduction in in-plane misorientation during thermal annealing. The relative stability of deviating compositions and the energetics of impurity incorporations were analyzed using density functional theory simulations, and the formation of native point defects or impurity incorporation in h-ZrB2 was found to be endothermic processes. Our experimental results showed that an epitaxial thin film of h-ZrB2+δ can be grown on Si(111) substrate using a magnetron co-sputtering technique at a relatively low processing temperature (700 °C) and has the potential to be used as a template for III-nitride growth on Si substrates.

Epitaxy and transport properties of alkali-earth palladate thin films

ABSTRACT Topological insulators and semimetals are an interesting class of materials for new electronic and optical applications owing to their characteristic electromagnetic responses originating from the spin-orbit coupled band structures. However, topological electronic structures are rare in oxide materials despite their chemical stability being preferable for applications. In this study, given the theoretical prediction of Dirac bands in CaPd3O4, we investigate the fabrication and transport properties of SrPd3O4 and CaPd3O4 thin films as candidates of oxide Dirac semimetals. We have found that these materials are epitaxially grown on MgO (100) substrate under limited growth conditions by pulsed laser deposition. The transport properties show a weak temperature dependence, suggestive of narrow-gap properties, although unintentionally doped holes hinder us from revealing the presence of the Dirac band. Our study establishes the basic thermodynamics of thin-film fabrication of these materials and will lead to interesting properties characteristic of topological band structure by modulating the electronic structure by, for example, chemical substitutions or pressure.

Guided anisotropic oxygen transport in vacancy ordered oxides

Anisotropic and efficient transport of ions under external stimuli governs the operation and failure mechanisms of energy-conversion systems and microelectronics devices. However, fundamental understanding of ion hopping processes is impeded by the lack of atomically precise materials and probes that allow for the monitoring and control at the appropriate time- and length- scales. In this work, using in-situ transmission electron microscopy, we directly show that oxygen ion migration in vacancy ordered, semiconducting SrFeO2.5 epitaxial thin films can be guided to proceed through two distinctly different diffusion pathways, each resulting in different polymorphs of SrFeO2.75 with different ground electronic properties before reaching a fully oxidized, metallic SrFeO3 phase. The diffusion steps and reaction intermediates are revealed by means of ab-initio calculations. The principles of controlling oxygen diffusion pathways and reaction intermediates demonstrated here may advance the rational design of structurally ordered oxides for tailored applications and provide insights for developing devices with multiple states of regulation.

Computer simulation of Fe epitaxial films on a Cu(100) substrate

The article addresses the problem of modeling thin epitaxial iron films on a Cu (100) substrate by calculating the surface potential. The purpose of the simulation is to determine the film structure. The model uses the paired Lennard-Jones potential to describe the interatomic interaction. We calculate the surface potential of the copper substrate using the first 10 surface atomic layers. The iron atoms in the first layer are placed in the minima of the surface potential. The surface potential of the monoatomic iron layer and the 9 copper layers underneath determine the position of the second iron layer. The position of atoms in the following layers is formed on the basis of similar calculations. Calculations show that the copper substrate stabilizes the fcc crystal lattice in the iron film at low temperatures. Bulk iron samples at low temperatures have a bcc crystal lattice. The period of the iron lattice along the substrate is equal to the period of the copper lattice. The crystal lattice period of iron perpendicular to the substrate has an intermediate value between the values for copper and for γ-ferrum. The interface effect causes additional vertical deformations in the first two layers of iron above the substrate.

Investigation of Optical functions, sub-bandgap transitions, and Urbach tail in the absorption spectra of Ga2O3 thin films deposited using mist-CVD

We report on the investigation of optical properties of epitaxial Ga2O3 thin films deposited at different temperatures using mist-Chemical Vapor Deposition on c-plane sapphire. The deposited films were of highly crystalline quality, with measured full width at half maxima (FWHM) of on-axis rocking curve scans as low as 62 arcsec for α-Ga2O3 sample grown at 500 °C. Pure phase κ-Ga2O3 was found to get stabilized as temperature increased to 650 °C. The optical properties of the as-deposited films were studied using absorbance, reflectance, and transmittance spectra. Various optical functions have been calculated, such as refractive index (n), extinction coefficient (k), real part of dielectric function (ϵreal), imaginary part of dielectric function (ϵimaginary) and high-frequency dielectric constant (ϵ∞). Furthermore, absorption spectra were modeled for sub-bandgap transitions and exponential onset near band-edge using Elliott–Toyozawa and Urbach–Martienssen models, respectively. Codi’s rule was used to estimate the value of the structural disorder parameter (χ) associated with Urbach’s tail. Variation of the extracted parameters, for instance, Urbach’s Energy (EU), phonon energy (hνp), steepness parameter (σ), electron–phonon interaction strength (Ee−phonon), Codi’s structural disorder parameter (χ), bandgap (Eg) and excitonic binding energy (Eexc) were studied against deposition temperature; these parameters were found to correlate with deposition temperature and hence the crystalline quality of the film. In light of the results, it was inferred that the exponential tail is inevitable at finite temperature because of the dominant Eg phonon mode present at ∼430cm−1 in α- or α- dominated Ga2O3 phase.

Electronic correlations in epitaxial CrN thin film

Chromium nitride (CrN) spurred enormous interest due to its coupled magnetostructural and unique metal-insulator transition. The underneath electronic structure of CrN remains elusive. Herein, the electronic structure of epitaxial CrN thin film has been explored by employing resonant photoemission spectroscopy (RPES) and X-ray absorption near edge spectroscopy study in combination with the first-principles calculations. The RPES study indicates the presence of a charge-transfer screened 3d ^n{underline{L}} (L: hole in the N-2p) and 3d ^{n-1} final-states in the valence band regime. The combined experimental electronic structure along with the orbital resolved electronic density of states from the first-principles calculations reveals the presence of Cr(3d)-N(2p) hybridized (3d ^n{underline{L}}) states between lower Hubbard (3d ^{n-1}) and upper Hubbard (3d ^{n+1}) bands with onsite Coulomb repulsion energy (U) and charge-transfer energy (Delta) estimated as approx 4.5 and 3.6 eV, respectively. It verifies the participation of ligand (N-2p) states in low energy charge fluctuations and provides concrete evidence for the charge-transfer (Delta<U) insulating nature of CrN thin film.

Formation of quaternary [formula omitted] epilayers driven by thermally induced interdiffusion between spinel [formula omitted] epilayer and [formula omitted] substrate

Zinc aluminogallate, Zn(AlxGa1−x)2O4 (ZAGO), a single-phase spinel structure, offers considerable potential for high-performance electronic devices due to its expansive compositional miscibility range between aluminum (Al) and gallium (Ga). Direct growth of high-quality ZAGO epilayers however remains problematic due to the high volatility of zinc (Zn). This work highlights a novel synthesis process for high-quality epitaxial quaternary ZAGO thin films on sapphire substrates, achieved through thermal annealing of a ZnGa2O4 (ZGO) epilayer on sapphire in an ambient air setting. In-situ annealing x-ray diffraction measurements show that the incorporation of Al in the ZGO epilayer commenced at 850 °C. The Al content (x) in ZAGO epilayer gradually increased up to around 0.45 as the annealing temperature was raised to 1100 °C, which was confirmed by transmission electron microscopy (TEM) and energy dispersive x-ray spectroscopy. X-ray rocking curve measurement revealed a small full width at half maximum value of 0.72 °, indicating the crystal quality preservation of the ZAGO epilayer with a high Al content. However, an epitaxial intermediate β–(AlxGa1−x)2O3 layer (β - AGO) was formed between the ZAGO and sapphire substrate. This is believed to be a consequence of the interdiffusion of Al and Ga between the ZGO thin film and sapphire substrate. Using density functional theory, the substitution cost of Ga in sapphire was determined to be about 0.5 eV lower than substitution cost of Al in ZGO. Motivated by this energetically favorable substitution, a formation mechanism of the ZAGO and AGO layers was proposed. Spectroscopic ellipsometry studies revealed an increase in total thickness of the film from 105.07 nm (ZGO) to 147.97 nm (ZAGO/AGO) after annealing to 1100 °C, which were corroborated using TEM. Furthermore, an observed increase in the direct (indirect) optical bandgap from 5.06 eV (4.7 eV) to 5.72 eV (5.45 eV) with an increasing Al content in the ZAGO layer further underpins the formation of a quaternary ZAGO alloy with a tunable composition.

Leakage current characteristics of epitaxial Bi5Ti3FeO15 thin films affected by distribution of 90° domain walls with high conductivity

We investigated leakage current characteristics affected by crystallinity and domain wall currents of epitaxial BTFO thin films on Nb-doped SrTiO3 substrates. The c-oriented BTFO thin film and BTFO thin films with a mixture of a-oriented and c-oriented crystallinity were prepared by controlling the substrate and PLD deposition rate. The c-oriented BTFO thin film exhibited the best leakage current characteristics because the Bi2O2 layers were placed perpendicular to the c-axis to reduce leakage currents. The BTFO thin films with a mixture of a-oriented and c-oriented crystallinity exhibited improved ferroelectric properties and piezoelectric properties compared to the c-oriented BTFO thin film. The current domains of the BTFO thin films corresponding to the ferroelectric domain structures were observed by a conducting atomic force microscope, and it was observed that leakage currents were formed around the domain walls. In particular, the largest leakage currents are formed at the boundaries of c-oriented domains and a-oriented domains, and these domain boundaries confirmed that BTFO thin films with a mixture of a-oriented and c-oriented crystallinity were responsible for the largest leakage currents.

NiO thin film with an extremely high index (7 overline{1 } 4) on r-plane sapphire substrate

Thin film epitaxy is essential for state-of-the-art semiconductor applications. The combination of different substrates and deposition methods leads to various crystallographic orientation relationships between thin films and substrates, which have a decisive impact on thin film performance. By utilizing pulsed laser deposition, we discovered a very high-index (7 overline{1 } 4)–oriented NiO thin film when deposited on r-plane text{(10}overline{1}text{2)} sapphire substrates. The in-plane epitaxial relations are {[13overline{1 }]}_{text{NiO}}text{||[1}overline {2}{text{10]}}_{text{Sapphire}} and {[overline{1 }12]}_{text{NiO}}text{||[10}overline{11}{text{]}}_{text{Sapphire}}, and the lattice mismatch is 3.2% and 0.4% along two directions, respectively. Exchange bias studies by the deposition of Co on NiO (111) and NiO (7 overline{1 } 4) show different behaviors, which may be associated with the spin density and alignment on the surface.Graphical abstract

The microstructural evolution of sputtered ZnO epitaxial films to stress-relaxed nanorods

Epitaxial ZnO thin films and nanorods were grown on c-sapphire substrate by reactive sputtering of Zn in Ar-O2 atmosphere at substrate temperatures ranging from near room temperature to 700 °C. Scanning electron microscopy showed that with increase in substrate temperature, the morphology transformed from columnar films to vertically aligned ZnO nanorods. High resolution x-ray diffraction revealed improvement in crystalline and epitaxial quality with increase in substrate temperature, along with the decrease of edge dislocation density from 5 × 1012 cm−2 to 8 × 1010 cm−2. The films grown near room temperature showed large hydrostatic strain (∼6 × 10−3) and compressive intrinsic biaxial stress (-0.8 GPa), which decreased substantially with increase in substrate temperature, due to the desorption of excess oxygen and reduction in edge dislocation density. Above the substrate temperature of 500 °C, the intrinsic biaxial stress reversed from compressive to mildly tensile in the case of nanorods, due to the intrinsic tensile stress, originating from crystallite coalescence. Raman measurements correlate well with the changes in biaxial stress and confirm the improvements in crystallinity and epitaxial quality of nanorods grown at higher temperatures. Room temperature photoluminescence of the ZnO films showed weak near-band-edge emission, which enhanced drastically in the case of nanorods due to their superior microstructure and epitaxial quality.

Interfacial layer effect on the enhancement of gilbert damping in RF magnetron sputtered Y3Fe5O12/Gd3Ga5O12 thin films

We report the magnetization dynamics study in Y3Fe5O12 (YIG) film grown by rf magnetron sputtering techniques. The growth-induced interfacial layer is believed to affect the top YIG layer, interrupting the magnetic properties associated with it drastically. The structural data confirmed the epitaxial and strained growth of YIG on a highly matched GGG (100) substrate. The frequency variation of the FMR signal yields various important parameters, such as the line width (ΔH), effective magnetization (4πMeff), and damping constant (α) at 300 K. The significant increase in 4πMeff to 2187.848 Oe reflects the presence of non-collinear or antiparallel arrangement of magnetic moment in YIG thin-film. Apart from this value of α was reported to be 1.5 × 10−3 which is substantially higher than the values reported in the literature for such higher-quality epitaxial thin films. We anticipate that a detailed analysis of these findings will provide excellent speculation in the field of magnonics, advanced spintronics and terahertz frequency-related applications.

Influence of Lattice Mismatch on Structural and Functional Properties of Epitaxial Ba0.7Sr0.3TiO3 Thin Films.

Substrate-induced strains can significantly influence the structural properties of epitaxial thin films. In ferroelectrics, this might lead to significant changes in the functional properties due to the strong electromechanical coupling in those materials. To study this in more detail, epitaxial Ba0.7Sr0.3TiO3 films, which have a perovskite structure and a structural phase transition close to room temperature, were grown with different thicknesses on REScO3 (RE-rare earth element) substrates having a smaller lattice mismatch compared to SrTiO3. A fully strained SrRuO3 bottom electrode and Pt top contacts were used to achieve a capacitor-like architecture. Different X-ray diffraction techniques were applied to study the microstructure of the films. Epitaxial films with a higher crystalline quality were obtained on scandates in comparison to SrTiO3, whereas the strain state of the functional layer was strongly dependent on the chosen substrate and the thickness. Differences in permittivity and a non-linear polarization behavior were observed at higher temperatures, suggesting that ferroelectricity is supressed under tensile strain conditions in contrast to compressive strain for our measurement configuration, while a similar reentrant relaxor-like behavior was found in all studied layers below 0°C.

Energy storage and multiferroic properties of La-doped epitaxial BiFeO3 thin films according to La doping concentration

La-doped epitaxial BiFeO3 (LBFO) thin films with La doping concentrations of 0, 10, 20, 25, and 30 % were deposited by pulsed laser deposition on Nb-doped single-crystal SrTiO3 substrates. The LBFO thin films doped with a La doping concentration of 10 % showed the best ferroelectric properties due to a large c/a ratio of about 1.04. As the concentration of La increased, the ferroelectric polarizations of the LBFO films decreased while the ferromagnetic properties steadily improved. In particular, the LBFO thin films tended to increase energy storage efficiency as the concentration of La increased, and the LBFO thin films with a La doping concentration of 30 % showed a high energy storage density of about 30.7 J/cm3.