Session S45: Oxides Films, Coatings, and Related Applications

Low loss amorphous Ta2O5 coatings grown by reactive sputtering for dielectric mirrors used for gravitational wave detection

The ability of LIGO and others in the gravitational wave community to detect astronomical events relies on the quality of optical mirrors used in interferometers. A lot of effort is devoted to reduce optical absorption and mechanical loss in the layers that make up the mirror coatings. Amorphous tantala is of interest as the high index of refraction layer but contributes the most to overall mechanical loss. The mechanisms that lead to mechanical loss must be understood in order to minimize the losses.

Amorphous tantala 500nm films are deposited using reactive sputtering of a tantalum target where growth temperatures are varied from room temperature to 600C. Thermally activated and tunneling mechanisms both contribute to the overall mechanical loss which can be measured through internal friction techniques. The thermally activated are measured at room temperature using Gentle Nodal Suspension and the tunneling are measured at temperatures below 10K using Double Paddle Oscillators. These results provide a deeper understanding of the energy dissipation in amorphous tantala due to both tunneling and thermally activated loss mechanisms.   

Understanding the role of carbon in active trap centre formation in porous alumina for ion beam dosimetry

In recent days, due to increased use of hadron therapy for cancer and tumor treatment, precise online dose monitoring is an important issue for safety purpose. Regarding hadron therapy, recently carbon ion beam with high Linear Energy Transfer (LET) is found to be more effective than the photon beams. Among several known TL/OSL oxides phosphors, C-doped alumina (Al₂O₃) is favorable for radiation dosimetry, especially in medical field due to its tissue equivalent in terms of radiation absorption, simple glow curve, and high sensitivity. A facile approach to improve thermoluminescence sensitivity of electrochemically anodized porous Al₂O₃ (AAO) is presented by introducing carbon ions for ion beam dosimetry. Initially, ion implantation technique has been carried out for Carbon doping in AAO in controlled manner. HAADF-STEM, EDS mapping, SEM studies reveal the evolution of a porous structure followed by the carbon distribution upto 200nm. However, the evolution of optically active F⁺ centres with increasing ion fluence has been examined by photoluminescence investigation at room temperature and thermoluminescence (TL) measurement while the chemical nature of such defect centres has been extracted by depth dependent XPS analysis.   

Interface-engineered hole doping in Sr2IrO4/LaNiO3 heterostructure

The relativistic Mott insulator Sr2IrO4 is known for the Jeff = 1/2 Mott insulating state which closely resembles the electronic structure of parent compounds of superconducting cuprates. Here, by means of interface engineering of the Sr2IrO4/LaNiO3 heterostructure, the hole-doped phase of Sr2IrO4 has been realized with a markedly higher doping level close to 10%. X-ray absorption studies at Ni L2 edge confirmed that 5d electrons from Ir sites are transferred onto Ni sites leading to the formation of a high spin Ni2+ state. Despite the large amount of doping, Sr2IrO4/LaNiO3 heterostructure shows a non-metallic behavior but a narrower band gap compared to the bulk Sr2IrO4. This implies strong electronic reconstruction at the interfaces. These findings highlight a powerful utility of interfaces to realize emerging electronic states of the Ruddlesden-Popper phases of Ir-based oxides.   

Over 100-THz Bandwidth Selective-Difference-Frequency Generation at LaAlO3/SrTiO3 Nanojunctions

The ability to combine continuously-tunable narrow-band terahertz (THz) generation that can access both far-infrared and mid-infrared regimes, with nanometer-scale spatial resolution, holds great potential for uncovering the underlying light-matter interactions as well as realizing selective control of rotational or vibrational resonances in nanoscale objects. Here, we report selective difference frequency generation with over 100 THz bandwidth through femtosecond optical pulse shaping. The THz emission is generated at nanoscale junctions at the interface of LaAlO₃/SrTiO₃ defined by conductive atomic force microscope lithography, with the potential to perform THz spectroscopy on individual nanoparticles or molecules. Numerical simulation of the time-domain signal helps to identify different contributing components for the THz generation. These results transform the LaAlO₃/SrTiO₃ interface into a promising platform for integrated lab-on-chip devices.   

Morphological anisotropy and mosaicity in epitaxially grown VO2 films

Epitaxially grown films of VO₂ are one of the most promising materials for oxide electronics due to a very convenient temperature and remarkable controllability of the insulator-to-metal phase transition (IMT) by doping, strain, photo excitations or applying an electric field. Scaling the IMT-based devices down to submicron size is a technological challenge because of the high variability of the resulted functional characteristics from device to device. Our work focuses on the structural and morphological origins of such variations.
We used X-ray nano diffraction mapping to study the evolution of mosaicity, strain and phase coexistence in 70 nm thick VO₂ film (r-cut Al₂O₃) during thermal cycling and annealing. We found high anisotropy in the grain and crack network structure which remarkably resonates with anisotropy of transport properties. We were able to distinguish a persistent morphological pattern due to the film manufacturing from transient features introduced by thermal cycling. Our results manifest the important role of nanomorphology in the performance of VO₂-based nanodevices. We suggest that it can also be considered as a new degree of freedom to control functional properties in oxide electronics.   

Preparation and X-Ray Diffraction Study of Strongly Oriented Thin Films with Potential Magnetoelectric Effect

Thin films of M, Y and Z hexagonal ferrites with a potential of magnetoelectric (ME) effect were prepared by chemical solution deposition method and processing parameters were tested and optimized. Several substrates were used, and different substrate/seeding layer/ferrite layer architectures were proposed to get strong preferred grain orientation. The films were studied by X-ray diffraction (XRD), AFM and EBSD. Lattice parameters, out-of-plane and in-plane grain orientations, crystallite sizes, microstrains and residual stresses were obtained by different XRD symmetric and asymmetric scans and their combinations.
New Y-ferrite phases were prepared with the composition BaSrZnCoFe₁₁(Me)O₂₂ (Me = Al, Ga, In, Sc). For Me = Al, Ga the magnetic structure is of non-colinear ferrimagnetic type with unspecified helical magnetic structure.
ME Z-type ferrite Sr₃Co₂Fe₂₄O₄₁ and Ba_xSr_3-xCo₂Fe₂₄O₄₁ thin films were prepared and characterized for the first time. Three present M, S and Z phases showed well out-of-plane and in-plane mutual orientations.   

Magnetic Scattering studies of rare earth thin film spiral antiferromagnetics

Understanding complex magnetic interactions by studying mesoscale magnetism creates and enhances technological applications. Holmium (Ho) is a rare-earth magnetic element with high magnetic moment per atom, and in bulk exhibits exotic magnetic properties such as a spiral antiferromagnetic state, the formation of chiral domains, or Spin-Slips.
In our study using X-ray magnetic resonant scattering we successfully detected the formation of antiferromagnetic spiral domains in thin film Ho by measuring magnetic satellite peaks around out-of-plane Ho Bragg peaks. Layers of Ho down to 100 nm are epitaxially grown on differently oriented MgO substrates (100), (110), (111) with an additional W seed layer using magnetron sputtering. Temperature dependent measurements of the transitions between ferromagnetic, antiferromagnetic, and paramagnetic state reveal a hysteresis in both the magnetic satellite peak and the thermal expansion of Ho. Furthermore, the width and amplitude of the hysteresis also depend on the Ho layer thickness, and even more pronounced, on the orientation of the MgO substrate. This shows a strong bottom interface dependency between the W layer and the structure and magnetic behavior of Ho, and, in general, the influence of the atomic structure on magnetism itself.   

SrTiO3 surfaces prepared by HCl etching

In molecular beam epitaxy synthesis of crystalline heterostructures, the quality of the substrate surface is crucial. The substrate not only provides a structural template for the deposited film, but for many new materials which are just a few atomic layers thick, the interaction with the substrate determines the emergent electronic properties. SrTiO₃ substrates are widely used for growth of complex oxides and superconducting FeSe, but reproducible surface preparation remains challenging. The standard recipe employs a buffered hydrofluoric acid etch, but it has been shown that this step results in significant fluorine replacement of oxygen near the surface of SrTiO₃ [1]. Here we characterize halogen doping from use of an alternative etchant, hydrochloric acid.

[1] S.A. Chambers, T.C. Droubay, C. Capan, and G.Y. Sun, Surface Science 606, 554-558 (2012).   

High Temperature Oxidation of Amorphous Zr-B-C-N Thin Films Grown by Electron Beam Evaporation

Boron carbonitride films have been shown to yield enhanced hardness and oxidation resistance, making them potentially attractive as protective coatings for high temperature applications. Most previous work has focused on thick films (>1μm) grown by magnetron sputtering. In this study, 200 nm thick ZrBCN films with a range of stoichiometries were grown on r-sapphire substrates at 850^oC using e-beam evaporation of ZrC and B sources in a nitrogen plasma. As-deposited films were found to be amorphous by XRD, and the nitrogen content was relatively constant independent of ZrC or B evaporation rates. XPS analysis of the films after air annealing over the range 500-800^oCshowed that B and N were entirely depleted at the surface, while ZrO₂ grains nucleated within the amorphous film matrix. XRD showed the formation of a tetragonal-ZrO₂ phase at 600^oC that became replaced by a monoclinic-ZrO₂ phase at 800^oC as the ZrO₂ grains grew in size. SEM analysis showed film delamination and cracking during high temperature oxidation to relieve film stress. The oxidation resistance of the 200 nm ZrBCN films is much less than reported for thicker sputtered films, indicating that the oxidation resistance mechanism is strongly dependent on film thickness.   

Reduction of Stress in Dual Ion Beam Sputtered Ta2O5 and SiO2 Optical Interference Coatings

We studied the mechanical and optical properties of tantalum pentoxide (Ta₂O₅) and silicon dioxide (SiO₂) films prepared by dual ion beam sputtering with the goal to realize thin films with a stress below 0.3 GPa. It is shown that the use of a low energy assist beam consisting of oxygen and argon species results in a Ta₂O₅ film with a stress below 100 MPa and a SiO₂ film with a stress below 250 MPa. Using photothermal common path interferometry (PCI), spectrophotometry, and ellipsometry, it is shown that these films are suitable for use in interference coatings.   

Properties of CCTO, a High Dielectric Constant Material

CaCu₃Ti₄O₁₂ (CCTO) shows an extremely high dielectric constant, which, in contrast to ferroelectrics like BaTiO₃, is nearly constant in a broad temperature range. Thus, this material has attracted tremendous attention due to its possible applications, e.g., for enhancing the performance of capacitive electronic elements. We used a simple Sol-Gel technique and established a novel composition of CCTO with high dielectric constant, and fabricated stable and reliable thin films. We have successfully incorporated silver nanoparticle in CCTO thin films. The conducting metal particles dispersed in a dielectric matrix are known to increase the effective dielectric constant of the medium. Based on these novel materials, we developed tunable multilayer capacitor with high dielectric constant.   

Influence of Assist Ion Bombardment on Mechanical Loss of Amorphous Tantala Thin Films for Gravitational Wave Interferometers

Thermal noise in high-reflectivity coatings is critical for improving the sensitivity of future gravitational wave detectors. Amorphous tantala (Ta₂O₅) thin film, used as high-index material, with lower mechanical loss is expected as one solution to further reduce the coating thermal noise. The material is also regarded as promising candidate for use in resistance random access memory as well as optical applications. In this work, amorphous tantala thin films have been deposited by reactive ion beam sputtering with assist Ar⁺ and Xe⁺ ion bombardment to investigate the effect of ion-induced surface diffusion on coating's mechanical and structural properties. The results show that the atomic structure of ion bombarded tantala thin films remain amorphous and their composition is mostly stoichiometric. The mechanical loss of ion-bombarded tantala coatings shows no significant improvement compared to non-bombarded samples. The coating loss angle of tantala is insensitive to ion dose or ion mass. An analysis based on surface diffusivity suggests that the ion-assisted surface diffusivity may be insignificantly modified under the current typical deposition conditions.   

TiO2 doped Ta2O5 coatings grown by biased target ion beam deposition for gravitational wave detectors

Advanced LIGO employs as end masses high reflectance mirrors of alternating layers of SiO₂ and Ta₂O₅ doped with 25% of TiO₂. Doping Ta₂O₅ with 25% TiO₂ decreased the mechanical loss which increased the sensitivity of the detector, but the physical reasons of this are still unknown. In this work we studied thin films of Ta₂O₅ doped with different TiO₂ concentrations grown by biased target ion beam deposition to evaluate the effect of doping in the mechanical loss and to search for high index materials with improved performance for the upcoming upgrade of LIGO and future detectors.
The deposition system consists of a low energy ion source and metallic targets individually pulsed biased. Control of each target bias allows for mixing Ta₂O₅ and TiO₂, with estimates of the doping obtained by x-ray photoelectron spectroscopy. Extensive characterization shows the films are nearly stoichiometric and dense, with an absorption loss at 1064 nm lower than 20 ppm. Mechanical loss was measured in as deposited coatings and after annealing. For all TiO₂ concentrations the mechanical loss is reduced after annealing, reaching a minimum for around 20% TiO₂ after annealing at 600 C comparable to state-of-the-art TiO₂ doped Ta₂O₅.