Off-axis Radio Frequency Magnetron Sputtering Research Articles

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.

In-situ growth of high-quality epitaxial BiFeO3 thin film via off-axis RF magnetron sputtering

This work demonstrates an in-situ heteroepitaxial growth of BiFeO3 thin film on the SrRuO3-buffered (100) SrTiO3 substrate via off-axis radio-frequency (RF) magnetron sputtering. The coexistence of a dominant rhombohedral-like phase and a small amount of tetragonal-like phase of BiFeO3 thin film with a good (100) orientation was revealed. Furthermore, the fabricated BiFeO3 thin film exhibited the enhanced electrical properties, e.g. a reduced leakage current (J < 3.2 × 10−3 A/cm2) and a low loss (tanδ < 0.087), as well as a large intrinsic polarization of 2Pr ∼ 124 μC/cm2 with a coercive field of 2Ec ∼ 408 kV/cm. This finding can provide a simple and feasible in-situ sputtering technique for the high-quality thin films.

Ferroelectric domains in epitaxial PbxSr1−xTiO3 thin films investigated using X-ray diffraction and piezoresponse force microscopy

We present a detailed study of compressively strained PbxSr1−xTiO3 thin films grown by off-axis radio frequency magnetron sputtering on (001)-oriented Nb-doped SrTiO3 substrates. Film tetragonality and the ferroelectric critical temperatures are measured for samples of different composition and thickness and compared with a phenomenological Landau-Devonshire model. 180∘ ferroelectric domains are observed using both X-ray diffraction and piezoresponse force microscopy and domain sizes obtained by the two techniques are compared and discussed.

In situ X-ray diffraction and the evolution of polarization during the growth of ferroelectric superlattices.

In epitaxially strained ferroelectric thin films and superlattices, the ferroelectric transition temperature can lie above the growth temperature. Ferroelectric polarization and domains should then evolve during the growth of a sample, and electrostatic boundary conditions may play an important role. In this work, ferroelectric domains, surface termination, average lattice parameter and bilayer thickness are simultaneously monitored using in situ synchrotron X-ray diffraction during the growth of BaTiO3/SrTiO3 superlattices on SrTiO3 substrates by off-axis radio frequency magnetron sputtering. The technique used allows for scan times substantially faster than the growth of a single layer of material. Effects of electric boundary conditions are investigated by growing the same superlattice alternatively on SrTiO3 substrates and 20 nm SrRuO3 thin films on SrTiO3 substrates. These experiments provide important insights into the formation and evolution of ferroelectric domains when the sample is ferroelectric during the growth process.

Biosensor for human IgE detection using shear-mode FBAR devices.

Film bulk acoustic resonators (FBARs) have been evaluated for use as biosensors because of their high sensitivity and small size. This study fabricated a novel human IgE biosensor using shear-mode FBAR devices with c-axis 23°-tilted AlN thin films. Off-axis radio frequency (RF) magnetron sputtering method was used for deposition of c-axis 23°-tilted AlN thin films. The deposition parameters were adopted as working pressure of 5 mTorr, substrate temperature of 300°C, sputtering power of 250 W, and 50 mm distance between off-axis and on-axis. The characteristics of the AlN thin films were investigated by X-ray diffraction and scanning electron microscopy. The frequency response was measured with an HP8720 network analyzer with a CASCADE probe station. The X-ray diffraction revealed (002) preferred wurtzite structure, and the cross-sectional image showed columnar structure with 23°-tilted AlN thin films. In the biosensor, an Au/Cr layer in the FBAR backside cavity was used as the detection layer and the Au surface was modified using self-assembly monolayers (SAMs) method. Then, the antigen and antibody were coated on biosensor through their high specificity property. Finally, the shear-mode FBAR device with kt2 of 3.18% was obtained, and the average sensitivity for human IgE detection of about 1.425 × 105 cm2/g was achieved.

Orientation-dependent piezoelectric properties in lead-free epitaxial 0.5BaZr0.2Ti0.8O3-0.5Ba0.7Ca0.3TiO3 thin films

Orientation-engineered 0.5BaZr0.2Ti0.8O3-0.5Ba0.7Ca0.3TiO3 (BZT-BCT) thin films were deposited on La0.7Sr0.3MnO3-coated SrTiO3 single-crystalline (001), (110), and (111) substrates by off-axis radio-frequency magnetron sputtering. X-ray diffraction confirmed a highly epitaxial growth of all the as-deposited films. It is believed the strong orientation dependence of ferroelectric and piezoelectric properties on the films is attributed to the relative alignment of crystallites and spontaneous polarization vector. The optimal ferroelectric response lies in the [001] direction, whereas a comparatively large effective piezoelectric coefficient d33,eff of 100.1 ± 5 pm/V was attained in [111] BZT-BCT thin film, suggesting its potential application for high-performance lead-free piezoelectric devices.

Structural characterization of La 0.9Ba 0.1MnO 3/Y-ZrO 2 film by X-ray diffraction

Perovskite manganite La 0.9Ba 0.1MnO 3(LBMO) films were deposited on (0 0 1)-oriented single crystal yttria-stabilized zirconia (YSZ) substrate by 90° off-axis radio frequency magnetron sputtering. The film thickness ranged from 10 nm to 100 nm. Grazing incidence X-ray diffraction technique and high resolution X-ray diffraction were applied to characterize the structure of LBMO films. The LBMO film mainly consisted of (0 0 1)-orientated grain as well as weakly textured (1 1 0)-orientated grain. The results indicated that an amorphous layer with thickness of about 4 nm was formed at the LBMO/YSZ interface. The strain in LBMO film was small and averaged to be about –0.14%. The strain in the film was not lattice mismatch-induced strain but residual strain due to the difference in thermal expansion coefficient between film and substrate.

Dielectric properties of composition spread SiO 2–Al 2O 3 mixed phase thin films deposited at room temperature by off-axis RF magnetron sputtering

The dielectric properties of composition spread SiO 2–Al 2O 3 thin films deposited by off-axis radio-frequency magnetron sputtering at room temperature were explored to obtain optimized compositions, which have low dielectric constants and losses. The specific points (compositions) showing superior dielectric properties of low dielectric constants (8.13 and 9.12) and losses (tanδ ∼0.02) at 1 MHz were found in area of the distance of 25.0 mm (Al 2Si 3O 8) and 42 mm (Al 2.4Si 3O 8) apart from SiO 2 target side in 75 mm × 25 mm sized Pt/Ti/SiO 2/Si(1 0 0) substrates, respectively. The specific thin films were amorphous phase and the compositions were Al 2Si 3O 8 ( k ∼8.13) and Al 2.4Si 3O 8 ( k ∼9.12).

Epitaxial growth and piezoelectric characterization of the (1−x)BiScO3−(x)PbTiO3 ultrathin film

We studied ferroelectric properties of morphotropic phase boundary (1−x)BiScO3−(x)PbTiO3 (BSPT, x = 0.64) epitaxial thin films on SrTiO3 (001) and Nb:SrTiO3 (001). BSPT thin films with various thicknesses were deposited using off-axis radio frequency magnetron sputtering. By analyzing x-ray data of BSPT thin films with various thicknesses, we confirmed that films thinner than ∼22 nm were in a strained state. Films thicker than ∼22 nm were in a relaxed state because of the strain relaxation mechanism caused by misfit dislocation formation. Clear piezoresponses and polarization reversal phenomena can be observed in the ultrathin limit down to 8 nm through Piezo Force Microscope experiments. The piezoresponse data as a function of thickness correlates with the structural modification of thin films.

ZnO as a buffer layer for growth of BiFeO3 thin films

Multiferroic BiFeO3 thin film was grown on the ZnO-buffered Pt/TiO2/SiO2/Si(100) substrate by off-axis radio frequency magnetron sputtering, where the ZnO buffer layer gave rise to a strong (110) texture for the BiFeO3 thin film. The resulting BiFeO3/ZnO thin film exhibits diode-like and resistive hysteresis behavior, in which the resistive hysteresis and rectifying ratio are dependent on the applied voltage and temperature. The resistive switching behavior of the BiFeO3/ZnO thin film is shown to relate to the trap-controlled space charge limited conduction and interface-limited Fowler–Nordheim tunneling, while the polarization reversal takes place in the BiFeO3 layer of the heterostructure. The BiFeO3/ZnO thin film is also demonstrated with a higher remanent polarization (2Pr∼153.6 μC/cm2), a much lower dielectric loss (tan δ∼0.012), and a better fatigue endurance as compared to those of the BiFeO3 thin film without a ZnO buffer layer, where the much reduced leakage is largely responsible for the enhanced ferroelectric behavior. The ZnO as a buffer layer for BiFeO3 significantly changes the dielectric relaxation and conduction mechanisms, when the dielectric relaxation and electrical conduction are governed by the thermal excitation of carriers from the second-ionization and short-range motion of oxygen vacancies, respectively, while the relaxation process remains the same over the entire temperature range of 20 to 200 °C investigated in the present study.

Resistive Hysteresis and Diodelike Behavior of BiFeO[sub 3]/ZnO Heterostructure

Bilayered BiFeO 3 /ZnO thin films, which were grown on the SrRuO 3 -buffered MgO(100) substrate by off-axis radio frequency magnetron sputtering, are investigated for their current―voltage relationships at varying voltages and temperatures. A resistive hysteresis and a diodelike behavior are observed for the bilayered BiFeO 3 /ZnO thin films. The resistive hysteresis and the rectification ratio are enhanced with increasing electrical field and temperature. The observed behavior arises from the interfacial depletion layer and ferroelectric switching, where the charge coupling between the semiconducting ZnO and the ferroelectric BiFeO 3 layers occurs.

BiFeO 3 thin films of (1 1 1)-orientation deposited on SrRuO 3 buffered Pt/TiO 2/SiO 2/Si(1 0 0) substrates

BiFeO 3 (BFO) thin films of varying degrees of (1 1 1) orientation were successfully grown on SrRuO 3-buffered Pt/TiO 2/SiO 2/Si(1 0 0) substrates by off-axis radio-frequency magnetron sputtering. They demonstrate much enhanced ferroelectric behavior, including a much enhanced remnant polarization (2 P r ∼ 197.1 μC cm −2 at 1 kHz) measured by positive-up negative-down (PUND), at an optimized deposition temperature of 590 °C. The effects of film deposition temperature on the degree of (1 1 1) orientation, film texture, ferroelectric behavior, leakage current and fatigue endurance of the BFO thin films were systematically investigated. While the degree of (1 1 1) orientation is optimized at 590 °C, the defect concentration in the film increases steadily with increasing deposition temperature, as demonstrated by the dependence of leakage behavior on the deposition temperature. The polarization behavior is shown to strongly depend on the degree of (1 1 1) orientation for the BFO thin film. Oxygen vacancies are shown to involve in the conduction and dielectric relaxation of the BFO thin films deposited at different temperatures, as demonstrated by their dielectric and conduction behavior as a function of both temperature (in the range 294–514 K) and frequency (in the range 10 −1–10 6 Hz).

Studies on strain relaxation and in-plane orientation of La0.7Ca0.3MnO3 film by grazing incidence X-ray diffraction

Abstract Perovskite manganite La 0.7 Ca 0.3 MnO 3 (LCMO) films were deposited on (0 0 1)-oriented single crystal MgO and SrTiO 3 (STO) substrates by 90° off-axis radio frequency magnetron sputtering. The thickness of LCMO/STO film was 50 nm. The thickness of LCMO/MgO films ranged from 5 nm to 200 nm. LCMO films were well (0 0 1)-oriented grown with high crystalline quality. Grazing incidence X-ray diffraction technique and normal X-ray diffraction were applied to characterize the in-plane and out-of-plane lattice strains and strain relaxation. The in-plane growth orientations of LCMO films with respect to substrate surface were also studied. The results indicated that the lattice strain of LCMO films began to relax when the film thickness is less than 5 nm. The mechanism for strain relaxation in LCMO films is perhaps different from that for tetragonal distortion. The in-plane growth orientation of LCMO/MgO film studied by grazing incidence X-ray diffraction technique is consistent with that from transmission electron microscopy.

Multiferroic behaviour and orientation dependence of lead-free (1 − x)BiFeO3–x(Bi0.50Na0.50)TiO3 thin films

Lead-free multiferroic (1 − x)BiFeO3–x(Bi0.50Na0.50)TiO3 (BFOBNTx) thin films were deposited by off-axis radio frequency magnetron sputtering on SrRuO3 buffered SrTiO3 (1 1 1), SrTiO3 (1 0 1), SrTiO3 (1 0 0) and LaAlO3 (0 0 1) substrates. Their ferroelectric behaviour is strongly dependent on the film composition and orientation. 0.95BiFeO3–0.05(Bi0.50Na0.50) TiO3 (BFOBNT5) exhibits notably better ferroelectric behaviour and fatigue endurance than the films of other compositions. The largest spontaneous polarization occurs in its (1 1 1) direction, where a remanent polarization of 2Pr ∼ 198.3 µC cm−2 is measured, while the polarizations in other directions are consistent with the projections from the (1 1 1) direction.

Phase transitions and electrical behavior of lead-free (K0.50Na0.50)NbO3 thin film

Lead-free (K0.50Na0.50)NbO3 (KNN) thin films with a high degree of (100) preferred orientation were deposited on the SrRuO3-buffered SrTiO3(100) substrate by off-axis radio frequency magnetron sputtering. They possess lower phase transition temperatures (To-t∼120 °C and Tc∼310 °C), as compared to those of KNN bulk ceramic (To-t∼190 °C and Tc∼400 °C). They also demonstrate enhanced ferroelectric behavior (e.g., 2Pr=24.1 μc/cm2) and fatigue endurance, together with a lower dielectric loss (tan δ∼0.017) and a lower leakage current, as compared to the bulk ceramic counterpart. Oxygen vacancies are shown to be involved in the conduction of the KNN thin film.

The fabrication and characterization ofPbTiO3 nanomesas realizedon nanostructured SrRuO3/SrTiO3 templates

We report the fabrication of PbTiO3 nanomesas down to 30 nm lateral size and 4 nm high on nanostructuredSrRuO3/SrTiO3 templates by off-axis radio frequency magnetron sputtering. The templates were prepared using atop-down lithography approach based on scanning tunneling microscopy. The growth rate of thePbTiO3 nanomesas was found to decrease with increasing growth temperature as well as withshrinking template size. Piezoresponse force microscopy measurements for thePbTiO3 nanomesas showed a strong increase in response with decreasing lateral size. A decrease ofthe coercive voltage was also observed for the same lateral size range. This laterallysize-dependent behavior is attributed to reduction of in-plane strain, when shrinking thenanomesa lateral dimensions.

Ferroelectric stripe domains in PbTiO3 thin films: Depolarization field and domain randomness

Observation of stripe domains in PbTiO3 thin films using standard x-ray diffraction analysis at room temperature is discussed. High-quality c-axis oriented thin films of varying thickness, from 6 to 210 unit cells, were grown on buffered NH4–HF etched SrTiO3(001) and Nb:SrTiO3(001) substrates using off-axis radio frequency magnetron sputtering. High-resolution linear Qx scans reveal a superstructure around the specular Bragg peaks, consistent with the presence of ferroelectric stripe domains. For thin samples, the stripe width is found to be proportional to the square root of the film thickness, with random in-plane orientation of domains. For films with a thickness of more than ∼100 unit cells, both monodomain samples and stripe domains were observed. We present evidence for the presence of a threshold depolarization field, above which there is a monotonically decreasing relationship between the domain width and the depolarization field. Furthermore, simulations show that random variations in size of the domains affect the separation of the diffuse scattering peaks from that of the specular reflection.

局所電界効果による希薄電子濃度超伝導の物性制御

We have performed local ferroelectric field effect experiments using an epitaxial heterostructure composed of ferroelectric Pb (Zr0.2Ti0.8)O3 and superconducting Nb doped SrTiO3, fabricated on an (100)SrTiO3 substrate by off-axis radio frequency magnetron sputtering and pulsed laser deposition. By switching the polarization field of the 50 nm thick Pb(Zr0.2Ti0.8)O3 layer, we induced a large change in resistivity and a shift of Tc in the 26 nm thick epitaxial Nb doped SrTiO3 layer. At 270 mK, superconducting on-off switch is realized. Ferroelectric domain array was poled in submicron scale by AFM (Atomic Force Microscope) tip to make a low Tc array in a high Tc superconducting path. A stepwise magnetoresistance was clearly observed in the array device around the Tc. This behavior suggests that 2D superconducting fluctuation induces phase slips and the low Tc array stabilizes them.

Characterization of crystalline Pb 0.92La 0.08Zr 0.4Ti 0.6O 3 thin films grown by off-axis radio frequency magnetron sputtering

In this study, we examine the growth conditions for Pb 0.92La 0.08Zr 0.4Ti 0.6O 3 thin films with a linear electro-optical response. Films grown on SrTiO 3(100) substrate show good crystalline quality and surface smoothness, if deposited within a narrow range of substrate temperatures around 530 °C. Such films had a (002) rocking curve full width at half maximum less than 0.2°, and a surface roughness less than 0.4 nm. X-ray photoelectron spectroscopy analysis confirmed the absence of surface impurity phases and revealed a single oxidation state for Pb 2+, Zr 4+, and Ti 4+cations. Good crystalline quality and a smooth film surface are required for the use of these materials in optical device applications.

POSSIBLE ABNORMAL MECHANISM FOR LATTICE STRAIN RELAXATION IN La0.7Ca0.3MnO3 THIN FILM

La 0.7 Ca 0.3 MnO 3(LCMO) thin films with the thickness of 50 nm were deposited on (001)-oriented single crystal SrTiO 3(STO), MgO and α- Al 2 O 3(ALO) by 90° off-axis radio frequency magnetron sputtering. Grazing incidence X-ray diffraction technique, associated with normal X-ray diffraction, was performed to measure the in-plane lattice parameter and investigate the lattice strain and strain relaxation in LCMO films. The results indicated that critical thickness of strain relaxation is very small, which may be related to large mismatch between film and substrate. The mechanism for strain relaxation in LCMO film is perhaps different from that for tetragonal distortion.