Off-axis Sputtering Research Articles

Structure and ferroelectric photovoltaic effect modulation in the epitaxial BiFeO3/La0.5Sr0.5CO3 heterostructures

Square-shaped BiFeO3 (BFO) thin films were prepared on SrTiO3 (STO) substrates by off-axis magnetron sputtering using La0.5Sr0.5CoO3 (LSCO) as the bottom electrode, and both LSCO and BFO are perovskite structure. The BFO crystalline quality improves and the grain size becomes larger as the thickness H increases, and both have smaller mean square roughness. The effects of BFO thickness H, light intensity Ip and ambient temperature AT on the ferroelectric photovoltaic effect (FPE) for the Pt/BFO/LSCO heterostructures are significant. With the H increase, the open-circuit voltage Voc and short-circuit current Jsc increase linearly, with Voc up to 0.44 V. The short-circuit current modulation intensity ΔJsc follows the ΔJsc-300>ΔJsc-200>ΔJsc-100 rule. With the Ip increase, Pt/BFO(300nm)/LSCO' Jsc increases linearly, and Voc first increases and then tends to saturation, which satisfies Glass' law. With the AT increases, Voc is linearly decreasing and Jsc shows the law: decreasing→increasing→decreasing. The polarization-modulated FPE mechanism is investigated by analyzing the Pt/BFO/LSCO energy band structure, which is attributed to the coupling effect between the BFO depolarization field (Edp) and the built-in electric field at the interface (EPt/BFO & EBFO/LSCO), and the FPE mechanism is shifted from the interface barrier domination to the ferroelectric polarization domination as H increases.

A comparative study on the electrical properties of Ba0.6Sr0.4TiO3 film capacitors with different top electrodes

Au/Ba0.6Sr0.4TiO3 (BST)/La0.5Sr0.5CoO3 (LSCO) and Pt/BST/LSCO ferroelectric capacitors were successfully constructed on (001) LaAlO3 substrates via off-axis magnetron sputtering. X-ray diffraction (XRD) and Phi scan patterns confirmed that the BST film was epitaxial with an out-of-plane tetragonal phase. The ferroelectric and dielectric measurements reveal that, compared with the Pt/BST/LSCO capacitor, the Au/BST/LSCO capacitor exhibits a larger coercive field (∼139.7 kV/cm), smaller permanent polarization (∼2.94 μC/cm2), and lower tunability (∼65.22 %), which may be attributed to the higher difference in work function and weaker depolarization field screen effect of the top electrode, as well as smaller interfacial capacitance of the Au/BST interface than those of the Pt/BST interface. Therefore, based on series capacitor model and leakage behavior analysis, the thickness and dielectric constant of interfacial layer are quantitatively determined to be 3.52 nm and 12.13 for Pt/BST, and 4.42 nm and 5.82 for Au/BST, respectively. Our results pave a way for improving the dielectric performance in electrically tunable microwave devices.

The effects of crystal orientation on the ferroelectric and optoelectronic properties of Pt/Na0.5Bi0.5TiO3/La0.5Sr0.5CoO3 heterostructures

Due to their excellent ferroelectric, semiconducting, and optoelectronic properties, perovskite ferroelectric thin films have attracted increasing attention in the photovoltaic field. Using La0.5Sr0.5CoO3 (LSCO) as the bottom electrode, flake, strip and pyramidal Na0.5Bi0.5TiO3 (NBT) films were prepared on (001), (110) and (111) SrTiO3 (STO) substrates by off-axis magnetron sputtering and pulsed laser deposition, respectively. XRD and Phi scan results show that both LSCO and NBT are perovskite structures and satisfy the epitaxial growth along the substrate STO. NBT orientation has a significant effect on the ferroelectric properties of Pt/NBT/LSCO heterostructures. The remnant polarization 2Pr values for NBT(111), NBT(110) and NBT(001) are 87.5, 62.6 and 28.5 μC/cm2, respectively, and thus 2Pr(111)>2Pr(110)>2Pr(001). NBT(111) shows the best ferroelectric properties (Pr=41.3 μC/cm2), excellent fatigue-resisting (up to 1010 cycles) and retention performance (duration 104s). The 2Pr-T association 2 Pr=41.72+16.05*T0.35 for NBT(111) was obtained according to the curve fitting, it is found that the NBT orientation has a significant effect on the PV effect for PT/NBT/LSCO photovoltaics and the open-circuit voltage ∆Voc. The short-circuit current ∆Jsc of polarization regulate intensity follows the laws of ∆Voc-111>∆Voc-110>∆Voc-001 and ∆Jsc-111>∆Jsc-110>∆Jsc-001, the Jsc for Pt/NBT(111)/LSCO heterostructures increases linearly with the illumination Ip increasing, while the Voc decreases linearly. The ferroelectro-polarized modulation PV mechanism was investigated by analyzing band structure of the Pt/NBT/LSCO heterostructures, which is attributed to the coupling effect of the ferroelectric depolarization field and the interfacial Schottky barrier.

Epitaxial growth and stoichiometry control of pyrochlore Nd2Ru2O7 thin films

We report on the epitaxial growth of pyrochlore NRO thin films utilizing a reactive off-axis sputtering technique. The growth process employed Ru and Nd metal targets with a repetitive substrate temperature sequence. X-ray diffraction, magnetic susceptibility, and spectroscopic ellipsometry measurements confirm that the structural, magnetic, and electronic properties of near-stoichiometric NRO films match those of the bulk material. Our spin-polarized density functional calculations based on Nd2Ru2O7 structural parameters accurately describe the optical spectra and assign Hubbard bands to the interband transitions observed above the optical band gap of 0.2 eV. By utilizing the technique's capability to adjust the degree of ruthenium deficiency, we investigated Nd2Ru2O7 films across a wide range of stoichiometric variations. Decreasing the Ru/Nd ratio results in lattice expansion, an increase in the optical band gap, and the suppression of Ru 4d intersite optical transitions. Additionally, this adjustment facilitates the elimination of minor inclusions of a ferromagnetic NdOx impurity phase, influencing the magnetic properties of stoichiometric films at low temperatures. The successful growth of Nd2Ru2O7 films opens up promising opportunities for designing and exploring strain- and light-induced states in pyrochlore ruthenates. Published by the American Physical Society 2024

Structural and resistive properties of Pt/BiFeO3/La0.5Sr0.5CoO3 heterostructure for storage application

BiFeO3 has promissing application in non-volatile ferroelectric memory, but the dominant mechanism has not been unified. Here, Pt/BiFeO3 (100 nm)/La0.5Sr0.5CoO3 (Pt/BFO/LSCO) heterostructures have been fabricated on (001) SrTiO3 (STO) single crystal substrates by off-axis magnetron sputtering. Both XRD and TEM analyses confirm that the (001) oriented BFO/LSCO heterostructures have been epitaxially grown on STO. In addition, XRD, TEM along with XPS analyses further indicate that the BFO film (001) presents epitaxial perovskite structure. On the basis of electric field regulation of Pt/BFO and BFO/LSCO interface barrier, the switching interface state is controlled by charge filling and trapping in interface potential well, which is responsible for I–V resistance and G-V hysteretic curve. Based on the fitting results of the conduction mechanism of I–V characteristic curve, it proves that the space charges limiting current play a dominant role, and the filling and trapping of interface traps are considered as the main resistance mechanisms. Moreover, ultraviolet irradiation can modulate the I–V resistance curve but does not change its conduction mechanism. This work provides experimental data for the practical application of integrated BFO resistive memories.

Tunable anomalous Hall effect and Curie temperature in perpendicular magnetic anisotropic lanthanum doped NiCo2O4 film

Perpendicular magnetic anisotropic NiCo2O4 (NCO) films show volatile physical properties due to the complicated ion occupation/valence disproportion, making it feasible to be manipulated. In this work, the tunable anomalous Hall effect is observed in high-quality lanthanum (La) doped NCO films grown by the off-axis RF sputtering technique. It reveals that the longitudinal resistance (Rxx) significantly increases as the growth of La doping level, while the Curie temperature (TC) decreases with the rising of La doping level. A universal scaling law between the longitudinal conductance (σxx) and the anomalous Hall conductance (σxy), including the terms of dirty scaling mechanism, intrinsic contribution, and side jump, is proposed to explain the observed anomalous Hall effect. Additionally, an intrinsic linear relation between σxx and TC is revealed. The evolution of anomalous Hall resistance (RA), TC, and Rxx is closely related to the complex valence state/occupation of Ni ions induced by La doping. This work provides a strategy to manipulate the physical properties of NCO film, making it a potential material for spintronic applications.

On-axis sputtering fabrication of Tm3Fe5O12 film with perpendicular magnetic anisotropy

Thulium iron garnet, Tm3Fe5O12 with perpendicular magnetic anisotropy is fabricated using an on-axis sputtering technique followed by annealing, whereas previous reports have used unusual off-axis geometries. Stoichiometric Tm3Fe5O12 is obtained after the modification of the deposition conditions involving the position of the substrate relative to the cathode, which affects both the chemical and structural properties. The effective perpendicular magnetic anisotropy of 8.6 kJ/m3 is well in line with the results of previous studies using pulse laser deposition and off-axis sputtering. A maze domain pattern is observed, and the domain-wall energy is evaluated as 0.69 mJ/m2.

Dielectric and ferroelectric photovoltaic properties of epitaxial BiFeO3 film with La0.5Sr0.5CoO3 bottom electrode

In order to deeply investigate the dielectric and ferroelectric photovoltaic properties of BiFeO3 (BFO) thin films, the Pt/BFO/La[Formula: see text]Sr[Formula: see text]CoO3 (LSCO) heterostructure ferroelectric capacitor grown on (001) SrTiO3 (STO) substrate is successfully fabricated by off-axis magnetron sputtering. The X-ray diffraction (XRD) and Phi scanning results show that the STO-based BFO and LSCO films are both (001) epitaxial structure. Although the BFO ferroelectric film exhibits an obvious dielectric dispersion phenomenon, a remarkable ferroelectric photovoltaic performance is obtained with open circuit voltage ([Formula: see text]) of 0.38[Formula: see text]V and short-circuit current ([Formula: see text]) of 0.23[Formula: see text]mA/cm2, respectively. With increasing test temperatures, [Formula: see text] decreases slowly and then rapidly, while [Formula: see text] increases rapidly and then decreases. At a critical temperature of 80∘C, the BFO ferroelectric film exhibits a faster photovoltaic response, in which the values of [Formula: see text] and [Formula: see text] are about 0.19[Formula: see text]V and 0.28[Formula: see text]mA/cm2, respectively. Moreover, the energy band analysis indicates that a large work function difference ([Formula: see text][Formula: see text]eV) between LSCO and Pt leads to a strong built-in electric field in the BFO film, which is beneficial to separate the photo-generated carriers and improve ferroelectric photovoltaic effect. In all, this study not only shows that BFO is an excellent and environmentally friendly photovoltaic candidate material, but also provides a practical strategy for improving the performance of BFO ferroelectric photovoltaic devices.

RF-sputtered Z-cut electro-optic barium titanate modulator on silicon photonic platform

Epitaxial BaTiO3 integrated on Si or Si-on-insulator using off-axis radio frequency sputtering is a promising material platform for building electro-optic modulators based on the Pockels effect. Barium titanate thin films with c-axis orientation have been epitaxially integrated on silicon-on-insulator wafers. They exhibit excellent structural quality with Pockels coefficient (r33) > 130 pm/V and propagation loss <2 dB/cm. Our results show that off-axis sputtered BaTiO3 films yield electro-optic modulation similar to that of high-quality films grown by molecular beam epitaxy and that the material is suitable for implementation of low-power Mach–Zehnder interferometer electro-optic modulators integrated on silicon in a Z-cut configuration.

Optoelectronic Properties of Ferroelectric Composites of Bi3.25La0.75Ti3O12 (BLT) and Co-Doped BLT Thin Films Modified by FeCo-Doped BLT

Driven by the growing demand for renewable and clean energy, the photovoltaic effect of various solar cells and materials was investigated for the conversion of light energy into electricity. We modified the Bi3.25La0.75Ti3O12 (BLT) and Co-doped BLT (Co–BLT) composites with Fe and Co-doped BLT (FeCo–BLT) films to narrow the bandgap and increase visible light absorption, thereby improving the efficiency of the photovoltaic reaction. In this study, BLT and Co–BLT thin films were fabricated by off-axis sputtering and then modified with FeCo–BLT thin films to produce dual-ferroelectric, thin-film composite materials that improved the photovoltaic power generation performance. Photoelectric test results showed that the modified double-ferroelectric, thin-film composites had superior optoelectronic properties. The current density was significantly enhanced by modifying the BLT films with doped Fe and Co. Therefore, this modification improved the efficiency of ferroelectric thin-film photovoltaic reactions.

Effect of substrate and growth method on vanadium dioxide thin films by RF magnetron sputtering: Vanadium metal oxidation vs reactive sputtering

Vanadium dioxide (VO2) undergoes a metal–insulator phase transition at ∼70 °C and has attracted substantial interest for potential applications in electronics, including those in neuromorphic computing. The vanadium–oxygen system has a rather complicated phase diagram, and controlling the stoichiometry and the phase of thin films of vanadium oxides is a well-known challenge. We explore the novel combination of two methods of VO2 thin film deposition using off-axis RF magnetron sputtering on (100)- and (111)-oriented yttria-stabilized zirconia (YSZ) substrates: reactive sputtering of vanadium in an oxygen environment and sputtering of vanadium metal followed by oxidation to VO2. Interestingly, the reactive sputtering process on both substrate orientations yields the metastable semiconducting VO2 (B) phase, which is structurally stabilized by the YSZ surface. The metal sputtering and oxidation process on YSZ produces mainly the equilibrium monoclinic (or M1) phase of VO2 that exhibits a metal–insulator transition. Using this method, we obtained thin films of (010)-textured polycrystalline VO2 (M1) that show a metal–insulator transition with an on/off ratio larger than 1000.

Spin polarized STM imaging of nanoscale Néel skyrmions in an SrIrO3/SrRuO3 perovskite bilayer

Oxide heterostructures are an emerging materials platform for engineering the size, stability and motion of magnetic skyrmions in next-generation devices. Here we use off-axis sputtering to grow epitaxial thin films, comprising just two unit cells of SrIrO3 on ten unit cells of the oxide ferromagnet, SrRuO3. Spin polarized scanning tunneling microscopy reveals isolated skyrmions and directly establishes their Néel character, attributed to non-collinear exchange interactions at the interface. Statistical analysis reveals a distribution of sizes and shapes, with an average diameter of 3 nm that is among the smallest reported to date in any system. Skyrmions were only observed in SrIrO3-covered regions of the film, but were otherwise independent of SrIrO3 grain area and thickness. To help explain these observations, we performed density functional theory calculations, which suggest SrIrO3 acquires an induced magnetic moment in proximity to SrRuO3 and thus may host columnar spin textures stabilized by the interface.

Robust perpendicular magnetic anisotropy in off-axis sputtered europium iron garnet (EuIG) thin films

The rare earth iron garnet (REIG), Eu3Fe5O12 (EuIG) has emerged as a promising material for ferrimagnetic insulator based spintronics. Earlier studies have shown that perpendicular magnetic anisotropy (PMA) in EuIG and other REIGs can be achieved by magnetostriction and misfit strain at the film-substrate interface. Typically, the strain relaxes significantly with thickness, limiting the thickness of films that can be grown using this strategy. Here we show PMA that does not reduce substantially with thickness in epitaxial EuIG films grown by off-axis sputtering; The PMA remains robust in films > 100 nm thick. Epitaxially induced strain with minimal relaxation is confirmed using high resolution symmetric and asymmetric x-ray diffraction reciprocal space maps.

Single-crystal epitaxial europium iron garnet films with strain-induced perpendicular magnetic anisotropy: Structural, strain, magnetic, and spin transport properties

Single-crystal europium iron garnet (EuIG) thin films epitaxially strain-grown on gadolinium gallium garnet (GGG)(100) substrates using off-axis sputtering have strain-induced perpendicular magnetic anisotropy (PMA). By varying the sputtering conditions, we have tuned the europium/iron (Eu/Fe) composition ratios in the films to tailor the film strains. The films exhibited an extremely smooth, particle-free surface with roughness as low as 0.1 nm as observed using atomic force microscopy. High-resolution x-ray diffraction analysis and reciprocal space maps showed in-plane epitaxial film growth, very smooth film/substrate interface, excellent film crystallinity with a small full width at half maximum of 0.012$^{\circ}$ in the rocking curve scans, and an in-plane compressive strain without relaxation. In addition, spherical aberration-corrected scanning transmission electron microscopy showed an atomically abrupt interface between the EuIG film and GGG. The measured squarish out-of-plane magnetization-field hysteresis loops by vibrating sample magnetometry in conjunction with the measurements from angle-dependent x-ray magnetic dichroism demonstrated the PMA in the films. We have tailored the magnetic properties of the EuIG thin films, including saturation magnetization ranging from 71.91 to 124.51 emu/c.c. (increase with the (Eu/Fe) ratios), coercive field from 27 to 157.64 Oe, and the strength of PMA field ($H_\bot$) increasing from 4.21 to 18.87 kOe with the in-plane compressive strain from -0.774 to -1.044%. We have also investigated spin transport in Pt/EuIG bi-layer structure and evaluated the real part of spin mixing conductance to be $3.48\times10^{14} {\Omega}^{-1}m^{-2}$. We demonstrated the current-induced magnetization switching with a low critical switching current density of $3.5\times10^6 A/cm^2$, showing excellent potential for low-dissipation spintronic devices.

Nanostructured Ge and GeSn films by high-pressure He plasma sputtering for high-capacity Li ion battery anodes

We fabricated nanostructured Ge and GeSn films using He radio-frequency magnetron plasma sputtering deposition. Monodisperse amorphous Ge and GeSn nanoparticles of 30–40 nm size were arranged without aggregation by off-axis sputtering deposition in the high He-gas-pressure range of 0.1 Torr. The Ge film porosity was over 30%. We tested the charge/discharge cycle performance of Li-ion batteries with nanostructured Ge and GeSn anodes. The Ge anode with a dispersed arrangement of nanoparticles showed a Li-storage capacity of 565 mAh/g after the 60th cycle. The capacity retention was markedly improved by the addition of 3 at% Sn in Ge anode. The GeSn anode (3 at% Sn) achieved a higher capacity of 1128 mAh/g after 60 cycles with 92% capacity retention. Precise control of the nano-morphology and electrical characteristics by a single step procedure using low temperature plasma is effective for stable cycling of high-capacity Ge anodes.

Wide range tunability of surface plasmon resonance of nanostructured Ag deposited by off-axis magnetron sputtering

In this work, nanostructured Ag showing size distribution dependent tunable surface plasmon resonance (SPR) are grown on glass substrate using off-axis DC magnetron sputtering at a sputtering power of 74 W. Argon gas pressure is set at 8.0 Pa and the deposition time is varied in the range 5–300 s. The structural, morphological, and optical properties are examined by using X-ray diffractometer (XRD), field emission scanning electron microscope (FESEM), and UV–Visible spectrophotometer. XRD patterns of all samples reveal characteristic (111) and (200) peaks of fcc Ag. Broad XRD peaks suggest formation of nanosized Ag crystallites with mean crystallite size of 3 nm. FESEM images depict near spherical Ag grains up to 120 s deposition time and elongated grains with greater substrate coverage with extended deposition time of 300 s. SPR features are well distinct in the optical absorption spectra which are highly sensitive to the deposition time. Within a short deposition time of 5–60 s, nanostructured Ag demonstrates wide range SPR tunability from 438 nm to 530 nm. It is observed that the variations in the size distribution as estimated from FESEM images are crucial to observe systematic red shift of SPR with increasing deposition time. Nanosized Ag produced by off-axis magnetron sputtering may be used as a vital component in plasmonic devices and sensors.

Thick BaTiO3 Epitaxial Films Integrated on Si by RF Sputtering for Electro-Optic Modulators in Si Photonics.

Thick epitaxial BaTiO3 films ranging from 120 nm to 1 μm were grown by off-axis RF magnetron sputtering on SrTiO3-templated silicon-on-insulator (SOI) substrates for use in electro-optic applications, where such large thicknesses are necessary. The films are of high quality, rivaling those grown by molecular beam epitaxy (MBE) in crystalline quality, but can be grown 10 times faster. Extraction of lattice parameters from geometric phase analysis of atomic-resolution scanning transmission electron microscopy images revealed how the in-plane and out-of-plane lattice spacings of sputtered BaTiO3 changes as a function of layer position within a thick film. Our results indicate that compared to molecular beam epitaxy, sputtered films retain their out-of-plane polarization (c-axis) orientation for larger thicknesses. We also find an unusual re-transition from in-plane polarization (a-axis) to out-of-plane polarization (c-axis), along with an anomalous lattice expansion, near the surface. We also studied a method of achieving 100% a-axis-oriented films using a two-step process involving amorphous growth and recrystallization of a seed layer followed by normal high temperature growth. While this method is successful in achieving full a-axis orientation even at low thicknesses, the resulting film has a large number of voids and misoriented grains. Electro-optic measurement using a transmission setup of a sputtered BTO film grown using the optimized conditions yields an effective Pockels coefficient as high as 183 pm/V. A Mach-Zehnder modulator fabricated on such films exhibits phase shifting with an equivalent Pockels coefficient of 157 pm/V. These results demonstrate that sputtered BTO thick films can be used for integrated electro-optic modulators for Si photonics.

Ferromagnetic insulating epitaxially strained La2NiMnO6 thin films grown by sputter deposition

The field of oxide spintronics can strongly benefit from the establishment of ferromagnetic insulators with near room-temperature Curie temperature. Here, we investigate the structural, electronic, and magnetic properties of epitaxially strained thin films of the double perovskite La2NiMnO6 (LNMO) grown by off-axis radio-frequency magnetron sputtering. We find that the films retain insulating behavior and a bulk-like Curie temperature in the order of 280 K independently of the epitaxial strain conditions. These results suggest a prospective implementation of LNMO films in multi-layer device architectures where a high-temperature ferromagnetic insulating state is a prerequisite.

Epitaxial Growth of SrMnO3 Films on SrTiO3 Substrates by Off-Axis Angle Radio Frequency Magnetron Sputtering

ABSTRACTIn this study, high-quality SrMnO3 (001) film was grown on a SrTiO3(001) substrate using a 4 N purity target by 90° off-axis RF magnetron sputtering. In this study, the influence of the plasma center and surrounding positions on the growth of the SrMnO3 film was investigated under different sputtering temperatures and working pressure conditions. The results revealed that the SrMnO3 film exhibited the best crystallinity at the sputtering center under a high working pressure (135 mTorr), high substrate temperature (750 °C), and Ar to O ratio of 1:1. After investigating the relationship between the growth rate and the working pressure through the Alpha step, the film was grown at a lower sputtering rate of 2 nm/min. X-ray diffraction confirmed that the SrMnO3 film was epitaxially grown on the SrTiO3 substrate with an orientation relationship of SrMnO3(001)//SrTiO3(001). The growth state of SrMnO3 on the crystal SrTiO3 substrate was investigated by scanning electron microscopy (SEM); the results revealed that the surface was smooth and compact. In addition, the atomic force microscopy results were consistent with the SEM result; the results revealed that the surface of the film was atomically flat, and the atomic level flatness was 0.906 nm. Furthermore, the contact angle measurement results revealed that the film and substrate surface energy were almost similar and exhibited similar adhesion and internal stress. In addition, energy-dispersive X-ray spectroscopy analysis revealed that the atomic composition ratio in the high-temperature sputtered SrMnO3 film was consistent with the stoichiometric ratio of SrMnO3. The scope and results of this study will lay a foundation on the further research of the performance of SrMnO3 film.

Improved thin film growth using Slow Kinetics Intermittent Sputtering

Radio-frequency off-axis magnetron sputtering is a well established technique to produce high quality epitaxial thin films of complex oxides. It has been successfully used for over two decades to grow thin films, superlattices and even solid solutions. The main drawback is the common lack of in situ monitoring of the growth, which can significantly slow down the optimisation of the many growth parameters. However, once the optimal parameters are found, they are usually very stable in time, leading to consistently high quality thin films. One of the main growth parameters is the growth temperature, with typical optimal ranges as narrow as 20 °C. Here, using the prototypical ferroelectric PbTiO3 as a model system, we show that by periodically interrupting the deposition process to allow the deposited material to relax, we can significantly increase the temperature range over which we obtain atomically flat surfaces to more than 50 °C. Moreover, the overall crystalline quality is greatly improved, as shown by X-ray diffraction. Finally, we demonstrate the applicability of this method to other materials.