Off-axis Sputtering Deposition Research Articles

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.

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.

Scalable fabrication of flexible thin-film batteries for smart lens applications

The smart lens system is considered one of the ultimate wearable electronics platform, with potential applications in visual-guide or health-monitoring system. However, its development has so far been limited by the development of suitable flexible batteries. Conventional flexible battery fabrication relies on laser-based lift-off techniques, which greatly hinder scalability of such batteries. Here, we design and demonstrate the flexible thin film batteries applied to contact lens form-factor, with direct fabrication on polymer substrates and single step low-temperature annealing. The battery utilizes olivine LiFePO4 thin film cathode, fabricated with 90° off-axis sputter deposition. This achieves unique nanoscale microstructure required for electrochemically active LiFePO4 thin films and effectively reduces the annealing temperature of LiFePO4 down to 400 °C for the first time. Equipped with lithium phosphorous oxynitride (LiPON) solid electrolyte and lithium metal anodes on polyimide substrates, the battery demonstrates the energy storage capacity of 35 μWh under wet condition. The storage capacity is sufficient to power glucose sensors embedded on the smart lens for up to 11.7 h. In addition, the high energy density of 70 μWh/cm2 flexible batteries may enable a diverse set of micro-scale devices, with scalable and CMOS-compatible fabrication processes.

Off-axis sputter deposition of ZnO films on c-sapphire substrates by utilizing nitrogen-mediated crystallization method

High-quality epitaxial ZnO films on c-plane sapphire substrates have been obtained by utilizing an off-axis sputtering configuration together with buffer layers prepared via nitrogen-mediated crystallization (NMC). The role of NMC buffer layers is to provide high density of nucleation site and, thus, to reduce the strain energy caused by the large lattice mismatch (18%) between ZnO and sapphire. The NMC buffer layers allow two-dimensional growth of subsequently grown ZnO films, being particularly enhanced by employing an off-axis sputtering configuration in which the substrate is positioned out of the high-energy particles, such as negative oxygen ions originating from the targets. As a result, ZnO films with smooth surfaces (root-mean-square roughness: 0.76 nm) and a high electron mobility of 88 cm2/V·s are fabricated. Photoluminescence spectra of the ZnO films show strong near-band-edge emission, and the intensity of the orange-red defect emission significantly decreases with increasing horizontal distance between the target and the substrate. From these results, we conclude that off-axis sputtering together with NMC buffer layers is a promising method for obtaining high-quality epitaxial ZnO films.

In-situ x-ray diffraction study of the growth of highly strained epitaxial BaTiO3 thin films

In-situ synchrotron x-ray diffraction was performed during the growth of BaTiO3 thin films on SrTiO3 substrates using both off-axis RF magnetron sputtering and pulsed laser deposition techniques. It was found that the films were ferroelectric during the growth process, and the presence or absence of a bottom SrRuO3 electrode played an important role in the growth of the films. Pulsed laser deposited films on SrRuO3 displayed an anomalously high tetragonality and unit volume, which may be connected to the previously predicted negative pressure phase of BaTiO3.

Biaxial texture development in aluminum nitride layers during off-axis sputter deposition

Polycrystalline aluminum nitride (AlN) layers were deposited by pulsed-dc reactive magnetron sputtering from a variable deposition angle α = 0°–84° in 5 mTorr pure N2 at room temperature. X-ray diffraction pole figure analyses show that layers deposited from a normal angle (α = 0°) exhibit fiber texture, with a random in-plane grain orientation and the c-axis tilted by 42° ± 2° off the substrate normal, yielding wurtzite AlN grains with the {101¯2} plane approximately parallel (±2°) to the substrate surface. However, as α is increased to 45°, two preferred in-plane grain orientations emerge, with populations I and II having the c-axis tilted toward and away from the deposition flux, by 53° ± 2° and 47° ± 1° off the substrate normal, respectively. Increasing α further to 65° and 84°, results in the development of a single population II with a 43° ± 1° tilt. This developing biaxial texture is attributed to a competitive growth mode under conditions where the adatom mobility is sufficient to cause intergrain mass transport, but insufficient for the thermodynamically favored low energy {0001} planes to align parallel to the layer surface. Consequently, AlN nuclei are initially randomly oriented and form a kinetically determined crystal habit exposing {0001} and {112¯0} facets. The expected direction of its highest growth rate is 49° ± 5° tilted relative to the c-axis, in good agreement with the 42°–53° measured tilt. The in-plane preferred orientation for α > 0° is well explained by the orientation dependence in the cross section of the asymmetric pyramidal nuclei to capture directional deposition flux. The observed tilt is ideal for shear mode electromechanical coupling, which is maximized at 48°.

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.

Magnetic and electric properties of magnetron-sputtered YBCO/LSMO and LSMO/YBCO double layers

Magnetic and electric properties of epitaxially grown YBa2Cu3O7−x(YBCO)/La0.7Sr0.3MnO3(LSMO) and LSMO/YBCO structures prepared by off-axis and faced-target magnetron sputtering deposition on SrTiO3 [100] substrates without intermediate buffer are investigated. The possibility is demonstrated to fabricate soft (with low coercive force Hc=25Oe) films, independent of the temperature used, and ‘tuned-soft’ (with coercive force Hc up to 200Oe), the latter being achieved by heating substrate non-uniformly during deposition. A novel technique is realised for substrate heating during deposition, whereby the substrate is separated from the heater by thin (0.2mm in diameter) quartz fibres, which allow several deposition cycles for one sample in different vacuum installations. At 77K the superconducting film has Tc∼86K and Jc∼0.7×106A/cm2 as a bottom layer, and Tc∼88K and Jc∼1.7×106A/cm2 as a top layer (the optimal stand-alone YBCO films have Tc∼90K and Jc∼(2.5–5)×106A/cm2 at 77K). The magnetic measurements of the in-plane magnetic moment by SQUID magnetometer gave M=0.6–1.2Am2 at H=200Oe. The results demonstrate the possibility of depositing double-layer combinations of YBCO/LSMO films with reproducible high crystalline quality, without an intermediate buffer for spintronics applications.

Electrostatic modulation of the electronic properties of Nb-doped SrTiO3 superconducting films

We have performed ferroelectric field effect experiments using an epitaxial heterostructure composed of ferroelectric Pb(Zr0.2Ti0.8)O3 and superconducting Nb-doped SrTiO3. The films were prepared on (001) SrTiO3 substrates by off-axis radio-frequency magnetron sputtering and pulsed-laser deposition. By switching the polarization field of the 500-Å-thick Pb(Zr0.2Ti0.8)O3 layer, a large change of about 30% in resistivity and a 20% shift of Tc (ΔTc∼0.05 K) were induced in the 400-Å-thick epitaxial Nb-doped SrTiO3 layer. The relationship between Tc and the electrostatically modulated average carrier concentration can be mapped onto the phase diagram of chemically doped SrTiO3.

X-Ray Reflectivity Study of Interdiffusion at YBa2Cu3O7-x and Metal Interfaces

We present a study of interdiffusion at interfaces of YBa2Cu3O7-x (YBCO) and metals, Au, Ag, and Pb by using X-ray reflectivity. YBCO thin films were epitaxially grown by off-axis sputter deposition and coevaporation, with the c-axis perpendicular to the SrTiO3 substrate surfaces. The capping layers were subsequently deposited on the YBCO film in situ and ex situ near room temperature. Glancing incident X-ray reflectivity was employed to investigate the surfaces and their buried interfaces. We find that interdiffusion at the interfaces of Au/YBCO and Ag/YBCO is negligible. However, a large interdiffusion zone, ∼ 60 Å, is present at the Pb/YBCO interface and the lead films grown, both in situ and ex situ, were entirely oxidized. We do not observe any diffraction peaks from the Pb/YBCO films. The diffraction peaks are found up to (007) from the YBCO films of the Au/YBCO and Ag/YBCO films. This implies that the loss of crystalline structure in a ∼ 300-Å-thick YBCO film underneath Pb is caused by interdiffusion.

Gravity-related transport process in off-axis sputtering deposition

In the reactive off-axis sputtering deposition of ZnO films, three transport regimes from a ballistic motion to a diffusive drift of the ejected species were observed when the growth pressures were varied from 5 to 150mTorr. The distribution of the film thickness at different substrate locations was measured. When the substrate surface was parallel to the gravity vector, a film thickness variation along the direction of gravity was detected in the region of diffusive transport deposition. This phenomenon depends strongly on the growth pressures while only slightly on the growth temperatures. Based on experimental results, a gravity-related motion of the sputtered species is proposed in film growth under relative low growth pressures.

Gravity-related transport in reactive off-axis sputtering deposition?

An experiment was designed to investigate the transport characterization of reactive off- axis sputtering deposition. Three transport regions were observed when the growth pressures vary from 5 mtorr to 150 mtorr. A new gravity-related phenomenon was revealed in film growth at relatively high growth pressures. This effect is related to the collision process or thermalization of transport species. This study also suggests a design for the substrate holder orientation in the off-axis sputtering system.

Effects of target-to-substrate angle on off-axis sputter deposition and EPR studies of near-surface magnetic properties of YBCO thin films

We have determined the dependence of target-to-substrate angle on the elemental concentration of c-axis YBCO thin films. Away from the standard off-axis position, energy distributions of sputtered elements vary spatially within the sputter plume due to the angular dependence of thermalization. Standard materials characterization techniques and angle-dependent Rutherford Backscattering Spectrometry (RBS) demonstrate that films grown away from the standard off-axis geometry produce bulk Y(123) with modified surface morphology and deposition rate. Several thin film planar tunneling experiments are consistent with a broken-time-reversal symmetry (BTRS) state. To compliment tunneling measurements, we measure electron paramagnetic resonance (EPR) of the near-surface region of YBCO thin films. Preliminary data are consistent with the spontaneous formation of magnetic moments at low temperature.

Polarity effects of substrate surface in homoepitaxial ZnO film growth

Epitaxial ZnO films were grown on two polar surfaces (O-surface and Zn-surface) of (0 0 0 1) ZnO single-crystal substrates using off-axis magnetron sputtering deposition. It was found that the two polar ZnO surfaces have different surface structure and morphology, which strongly influence epitaxial film growth. An interesting result shows that high-temperature annealing of ZnO single crystals will improve the surface structure on the O-terminated surface rather than the opposite surface. The morphology and structure of homoepitaxial films grown on the ZnO substrates were different from heteroepitaxial films grown on the Al 2O 3. The measurements of low-angle incident X-ray diffraction and atomic force microscopy of ZnO films indicate that the O-terminated surface is better for ZnO epitaxial film growth.

Surface texture and structure of ZnO films synthesized by off-axis sputtering deposition

Morphology and structure of ZnO films deposited on (0001) sapphire and glass substrates by off-axis sputtering were investigated at various temperatures and pressures. All films show highly textured structures on glass substrates and epitaxial growth on sapphire substrates. The full width at half-maximum of theta rocking curves for epitaxial films is less than 0.5°. In textured films, it rises to several degrees. The trend of surface textures in films grown at low pressures is similar to those grown at high temperatures. A morphology transition from large well-defined hexagonal grains to flat surface was observed at a pressure of 50 mtorr and temperature of 550 °C. The experiment results are explained by the transport behavior of depositing species.

Pressure effects in ZnO films using off-axis sputtering deposition

ZnO films are deposited on (0 0 0 1) sapphire and quartz substrates using the off-axis reactive magnetron sputtering deposition. Based on film thickness measurements, three transport regions of sputtered particles are observed when films are deposited in the pressure regions of 5–150 mTorr. X-ray diffraction, scanning probes microscopy, and electrical measurements are also used to characterize these films. The full-width at half-maximum of θ-rocking curves for epitaxial films is less than 0.5°. In textured films, it rises to several degrees. The epitaxial films deposited at high pressure reveal a flat surface with some hexagonal facets. The density of hexagonal facets decreases when the growth pressure is reduced. The resistivity of these epitaxial films also depends on the growth pressures. A relationship between the pressure effects and film properties are discussed.

High-T/sub c/ SNS edge junctions with integrated YBa/sub 2/Cu/sub 3/O/sub x/ groundplanes

We have fabricated high-T/sub c/ SNS weak links in an edge geometry with integrated YBa/sub 2/Cu/sub 3/O/sub x/ (YBCO) groundplanes and SrTiO/sub 3/ insulators, using a process which incorporates six epitaxial layers, including a Co-doped-YBCO normal-metal interlayer. The SNS edge junctions were produced using films deposited by both off-axis sputtering and pulsed laser deposition. These devices exhibit tight J/sub c/ spreads and high I/sub c/R/sub n/ products in a current density regime well-suited for SFQ circuit fabrication. We also describe results on SNS junctions fabricated in a novel "slot" geometry, designed to reduce junction and interconnect inductances.

Growth of NdBa/sub 2/Cu/sub 3/O/sub 7-δ/ thin films and fabrication of directly coupled dc SQUID magnetometer

Thin films of NdBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// were fabricated by off-axis rf sputtering and pulsed laser deposition (PLD) methods. The deposition parameters for thin film fabrication, such as gas pressure, substrate temperature, annealing conditions, and rf power or laser energy density were studied. Highly c-axis oriented thin films with the maximum T/sub c/ (R=0) of 87 K were obtained at the optimum deposition conditions of rf sputtering. It, however, was more difficult to optimize the PLD conditions. The directly coupled dc SQUIDs were fabricated on SrTiO/sub 3/ bi-crystal substrates using single layer NdBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// film grown by PLD method.

CMR films structure as a function of growth and processing

Among the family encompassed by the complex metal oxides are the colossal magneto-resistive perovskites with potential applications in advanced high density magnetic data storage and sensor devices based on single or multilayers of these materials fabricated by vapor phase deposition methods. Realization of this potential requires solving basic thin film fabrication problems coupled to understanding the resulting structure-properties relationships. The specific objectives addressed in this study were to determine the dependence of growth and transport properties: (1) reproducibility on the pulsed laser deposition (PLD) target, (2) on sputter deposition chamber gas and substrate, and (3) on a post-deposition anneal in a reducing atmosphere. Toward these ends, we have grown La 0.67Ca 0.33MnO 3 films on perovskite substrates by PLD and RF off-axis sputter deposition from stoichiometric targets at temperatures from 400 to 900°C. The PLD films were grown from a new commercial target and post-deposition annealed in O 2 to reproduce, for comparison, the conditions used previously for the films grown with an in-house fabricated target. The sputter deposited films were grown in both ArO 2 mixed and Ar-only atmospheres, some grown simultaneously on different substrates; one ArO 2 grown film was post-annealed in a reducing atmosphere. Microstructures were studied by scanning probe microscopies and temperature-dependent transport properties by 4-point resistance measurements. Previously we reported on characterization of the T substrate( T s)-dependent microstructure of PLD as-deposited and post-annealed films grown from an in-house fabricated target. Annealed films grown from the new commercial target exhibited improved consistency in T-dependent transport and magnetization properties, independent of growth temperature. None of the as-grown sputter deposited films grown in the ArO 2 mixture exhibited a metallic transition while those grown in Ar-only at T s > 800°C had a range of transition temperatures between 110 and 180 K. The variation in substrate-dependent structure demonstrated again the sensitivity of growth mechanism to template. Finally, the 500°C film grown in the ArO 2 mixture, went from insulating at all T's to exhibiting a 215°C metallic transition after the reducing anneal treatment.

Superconductor–normal–superconductor step-edge junctions with Au barriers

YBa2Cu3O7−δ superconductor–normal–superconductor junctions in step-edge geometry are fabricated by an in situ process using off-axis sputter deposition. Gold is used as the normal metal interlayer. A special inhibiting layer is introduced to ensure proper separation of the superconducting electrodes. The temperature dependence of the critical current Ic is nearly linear. The junctions show high normal resistances up to RN=10 Ω and high IcRN products up to 8.8 mV at 4.2 K. I–V characteristics exhibit a large excess current Iex over the whole temperature range, even for electrically narrow junctions. The power dependence of Shapiro step heights shows characteristic deviations from the resistively and capacitively shunted junction model. Calculations using the time-dependent Ginzburg–Landau equations are carried out to model the measured I–V characteristics with and without microwave irradiation. A point-contact-like superconductor–constriction–superconductor model is suggested to explain the excess current and the measured temperature dependence of Ic and Iex. Time-dependent Ginzburg–Landau equations are used to account for the observed dynamic characteristics.