Search Results (16)

Ultra-thin PTCDI-C8 films are vapor-deposited under ultra-high vacuum (UHV) conditions onto surfaces of p- or n-doped GaN(0001) samples. The X-ray photoelectron spectroscopy (XPS) results reveal a lack of strong chemical interaction between the PTCDI-C8 molecule and the substrate. Changes in the electronic structure of the substrate or the adsorbed molecules due to adsorption are not noticed at the XPS spectra. Work function changes have been measured as a function of the film thickness. The position of the HOMO level for films of thicknesses 3.2–5.5 nm has been determined. Energy diagrams of the interface between p- and n-type GaN(0001) substates and the PTCDI-C8 films are proposed. The fundamental molecular building blocks of the PTCDI-C8 films on GaN(0001), assembled by self-organization, have been identified. They are rows of PTCDI-C8 molecules stacked in “stand-up” positions in reference to the substrate, supported by the π–π bonds which are formed between the molecular cores of the molecules and monomolecular layers constituted by rows which are tilted in reference to the layer plane. The layers are epitaxially oriented. The epitaxial relation between the rows and the crystallographic directions of the substrate are determined. A model of the PTCDI-C8 film’s growth on GaN(0001) substrate is proposed. The 3D islands of PTCDI-C8 molecules formed on the substrate surface during film deposition are thermodynamically unstable. The Volmer–Weber type of growth observed here is a kinetic effect. Rewetting processes are noticeable after film aging at room temperature or annealing at up to 100 °C. Full article

Ultrathin broadband absorbers with high efficiency, wide angular tolerance, and low fabrication cost are in demand for various applications. Here, we present an angle-insensitive ultrathin (<150 nm) broadband absorber with an average 96.88% (experiment) absorptivity in the whole visible range by utilizing a simple dielectric–semiconductor–lossy metal triple-layer film structure. The excellent broadband absorption performance of the device results from the combined action of the enhanced absorptions in the semiconductor and lossy metal layers exploiting strong interference effects and can be maintained over a wide viewing angle up to ±60°. Benefiting from the lossy metal providing additional absorption, our design reduces the requirement for the semiconductor’s material dispersion and has great flexibility in the material selection of the metal layer. Additionally, the lithography-free nature of the proposed broadband visible absorber provides a high-throughput fabrication convenience, thus holding great potential for its large-area applications in various fields. Full article

Ultra-thin diamond membranes, diamanes, are one of the most intriguing quasi-2D films, combining unique mechanical, electronic and optical properties. At present, diamanes have been obtained from bi- or few-layer graphene in AA- and AB-stacking by full hydrogenation or fluorination. Here, we study the thermal conductivity of diamanes obtained from bi-layer graphene with twist angle $\theta$ between layers forming a Moiré pattern. The combination of DFT calculations and machine learning interatomic potentials makes it possible to perform calculations of the lattice thermal conductivity of such diamanes with twist angles $\theta$ of 13.2${}^{\circ }$, 21.8${}^{\circ }$ and 27.8${}^{\circ }$ using the solution of the phonon Boltzmann transport equation. Obtained results show that Moiré diamanes exhibit a wide variety of thermal properties depending on the twist angle, namely a sharp decrease in thermal conductivity from high for “untwisted” diamanes to ultra-low values when the twist angle tends to 30${}^{\circ }$, especially for hydrogenated Moiré diamanes. This effect is associated with high anharmonicity and scattering of phonons related to a strong symmetry breaking of the atomic structure of Moiré diamanes compared with untwisted ones. Full article

The MgO nanolayer effect on the microstructure and magnetic characterizations added into Fe/Pt stacked films directly deposited onto MgO (001) single-crystal substrates at the reduced temperature of 380 °C using electron-beam technology was investigated in this present work. The nanograin isolation and exchange decoupling for the FePt–MgO system is attributed to the magnetic FePt isolated grains that originate from MgO atoms with a spreading behavior mostly along grain boundaries owing to its weaker surface energy than that of a single Fe or Pt element. The grain and domain size decreased when the MgO nanolayer was applied due to the interpenetration of MgO and created a strain-energy variation at the MgO/FePt interface. Measuring angular-dependent coercivity indicates a general trend of a domain-wall motion, and changes to the rotation of the reverse-domain model occurred as the MgO nanolayers were added into FePt films. The intergrain interaction is confirmed by the Kelly–Henkel plot, which shows that there is strong intergrain exchange coupling (positive δM type) between neighboring grains in the continuous Fe/Pt stacked films without MgO nanolayers. In addition, a negative δM type occurred when the Fe/Pt stacked films were added into MgO nanolayers, showing that the MgO nanolayer can be applied to adjust the force of intergrain exchange coupling between the adjacent FePt nanograins, and the addition of MgO nanolayers change into magnetic decoupling; thus, there was a formed dipole interaction in our claimed FePt–MgO composite structure of stacked ultrathin films at a reduced temperature of 380 °C. Full article

This paper reports an investigation of the structural, chemical and electrical properties of ultra-thin (5 nm) aluminum nitride (AlN) films grown by plasma enhanced atomic layer deposition (PE-ALD) on gallium nitride (GaN). A uniform and conformal coverage of the GaN substrate was demonstrated by morphological analyses of as-deposited AlN films. Transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) analyses showed a sharp epitaxial interface with GaN for the first AlN atomic layers, while a deviation from the perfect wurtzite stacking and oxygen contamination were detected in the upper part of the film. This epitaxial interface resulted in the formation of a two-dimensional electron gas (2DEG) with a sheet charge density n_s ≈ 1.45 × 10¹² cm⁻², revealed by Hg-probe capacitance–voltage (C–V) analyses. Nanoscale resolution current mapping and current–voltage (I–V) measurements by conductive atomic force microscopy (C-AFM) showed a highly homogeneous current transport through the 5 nm AlN barrier, while a uniform flat-band voltage (V_FB ≈ 0.3 V) for the AlN/GaN heterostructure was demonstrated by scanning capacitance microscopy (SCM). Electron transport through the AlN film was shown to follow the Fowler–Nordheim (FN) tunneling mechanism with an average barrier height of <Φ_B> = 2.08 eV, in good agreement with the expected AlN/GaN conduction band offset. Full article

We study how to enhance the transverse magneto-optical Kerr effect (TMOKE) of ultra-thin magnetic dielectric films through the excitation of strong magnetic resonances on metasurface with a metal nanowire array stacked above a metal substrate with an ultra-thin magnetic dielectric film spacer. The plasmonic hybridizations between the Au nanowires and substrate result in magnetic resonances. The periodic arrangement of the Au nanowires can excite propagating surface plasmon polaritons (SPPs) on the metal surface. When the SPPs and the magnetic resonances hybridize, they can strongly couple to form two strong magnetic resonances, which are explained by a coupled oscillator model. Importantly, benefitting from the strong magnetic resonances, we can achieve a large TMOKE signal up to 26% in the ultra-thin magnetic dielectric film with a thickness of only 30 nm, which may find potential applications in nanophotonics, magnonics, and spintronics. Full article

A novel, high throughput method to characterize the chemistry of ultra-thin diamond-like carbon films is discussed. The method uses surface sensitive SEM/EDX to provide substrate-specific, semi-quantitative silicon nitride/DLC stack composition of protective films extensively used in the hard disk drives industry and at Angstrom-level. SEM/EDX output is correlated to TEM to provide direct, gauge-capable film thickness information using multiple regression models that make predictions based on film constituents. The best model uses the N/Si ratio in the films, instead of separate Si and N contributions. Topography of substrate/film after undergoing wear is correlatively and compositionally described based on chemical changes detected via the SEM/EDX method without the need for tedious cross-sectional workflows. Wear track regions of the substrate have a film depleted of carbon, as well as Si and N in the most severe cases, also revealing iron oxide formation. Analysis of film composition variations around industry-level thicknesses reveals a complex interplay between oxygen, silicon and nitrogen, which has been reflected mathematically in the regression models, as well as used to provide valuable insights into the as-deposited physics of the film. Full article

This study investigated the ultrathin Cu(002) capping nano-island effects on the magnetic characterizations and microstructure of epitaxial FePt(001) films directly fabricated on MgO(001) substrates at the relatively low temperature of 300 °C via electron-beam deposition. The enhancement of the coercivity is attributed to the lowered exchange coupling of FePt magnetic grains that begun from Cu atom behavior of spreading in many directions mainly along grain boundaries due to its lower surface energy than that of pure Fe or Pt. The measurement of angular-dependent coercivity shows a tendency of a domain-wall motion shift toward the rotation of the reverse-domain type upon the thickness of the Cu capping nano-island layer atop the FePt films. The intergranular interaction was clarified by the Kelly–Henkel plot, which indicated that there was strong exchange coupling (positive δM) between neighboring grains in the FePt continuous films without Cu capping nano-islands. On the other hand, a negative δM value was gained when the FePt films were capped with a Cu(002) single layer, indicating that the Cu capping layer can be used to control the strength of intergrain exchange coupling between the adjacent FePt grains and thicker Cu(002) capping nano-islands toward magnetic isolation; thus, there was an existence of dipole interaction in our designed Cu/FePt composite structure of stacked films. Full article

The future applications of hyperbolic metamaterials demand stacks of materials with alternative ultra-thin conductive/dielectric films with good homogeneity of the thickness and reduced roughness level. In this work, the technology of pulsed-DC magnetron sputtering of aluminum was optimized using the Taguchi method in order to fabricate Al films with improved roughness level. The performed structural characterization proved the smaller Al domains and better homogeneity of the surface. The optimized process was used to fabricate a multilayer structure of Al/HfO_x as the metamaterial media. The fabricated structures were optically characterized in the UV/VIS range. The presented findings demonstrated the tunability effect of the effective reflectance of the examined stacks. The presented results are promising for the future application of multilayer structures in novel photonic devices based on hyperbolic metamaterials. Full article

Silver-based low-emissivity (low-E) coatings are applied on architectural glazing to cost-effectively reduce heat losses, as they generally consist of dielectric/Ag/dielectric multilayer stacks, where the thin Ag layer reflects long- wavelength infrared (IR), while the dielectric layers both protect the Ag and act as an anti-reflective barrier. The architecture of the multilayer stack influences its mechanical properties and it is strongly dependent on the residual stress distribution in the stack. Residual stress evaluation by combining focused ion beam (FIB) milling and digital image correlation (DIC), using the micro-ring core configuration (FIB-DIC), offers micron-scale lateral resolution and provides information about the residual stress variation with depth, i.e., it allows depth profiling for both equibiaxial and non-equibiaxial stress distributions and hence can be effectively used to characterize low-E coatings. In this work, we propose an innovative approach to improve the depth resolution and surface sensitivity for residual stress depth profiling in the case of ultra-thin as-deposited and post-deposition annealed Si₃N₄/Ag/ZnO low-E coatings, by considering different fractions of area for DIC strain analysis and accordingly developing a unique influence function to maintain the sensitivity of the technique at is maximum during the calculation. Residual stress measurements performed using this novel FIB-DIC approach revealed that the individual Si₃N₄/ZnO layers in the multilayer stack are under different amounts of compressive stresses. The magnitude and orientation of these stresses changes significantly after heat treatment and provides a clear explanation for the observed differences in terms of scratch critical load. The results show that the proposed FIB-DIC combined-areas approach is a unique method for accurately probing non-equibiaxial residual stresses with nano-scale resolution in thin films, including multilayers. Full article

In this paper, a reconfigurable sensing platform based on an asymmetrical metal-insulator-metal stacked structure integrating an indium tin oxide (ITO) ultrathin film is proposed and investigated numerically. The epsilon-near-zero (ENZ) mode and antisymmetric mode can be resonantly excited, generating near-perfect absorption of over 99.7% at 1144 and 1404 nm, respectively. The absorptivity for the ENZ mode can be modulated from 90.2% to 98.0% by varying the ENZ wavelength of ITO by applying different voltages. To obtain a highly sensitive biosensor, we show that the proposed structure has a full-width at half-maximum (FWHM) of 8.65 nm and a figure-of-merit (FOM) of 24.7 with a sensitivity of 213.3 nm/RI (refractive index) for the glucose solution. Our proposed device has potential for developing tunable biosensors for real-time health monitoring. Full article

Nanoplasmonic waveguides utilizing surface plasmon polaritons (SPPs) propagation have been investigated for more than 15 years and are now well understood. Many researchers make their efforts to find the best ways of using light and overcoming the speed limit of integrated circuits by means of SPPs. Here, we introduce the simulation results and fabrication technology of dielectric-metal-dielectric long-range nanoplasmonic waveguides, which consists of a multilayer stack based on ultrathin noble metals in between alumina thin films. Various waveguide topologies are simulated to optimize all the geometric and multilayer stack parameters. We demonstrate the calculated propagation length of L_prop = 0.27 mm at the 785 nm wavelength for the Al₂O₃/Ag/Al₂O₃ waveguides. In addition, we numerically show the possibility to eliminate signal cross-talks (less than 0.01%) between two crossed waveguides. One of the key technology issues of such waveguides’ nanofabrication is a dry, low-damage-etching of a multilayer stack with extremely sensitive ultrathin metals. In this paper, we propose the fabrication process flow, which provides both dry etching of Al₂O₃/Au(Ag)/Al₂O₃ waveguides nanostructures with high aspect ratios and non-damage ultrathin metal films patterning. We believe that the proposed design and fabrication process flow provides new opportunities in next-generation photonic interconnects, plasmonic nanocircuitry, quantum optics and biosensors. Full article

Long-term packaging of miniaturized, flexible implantable medical devices is essential for the next generation of medical devices. Polymer materials that are biocompatible and flexible have attracted extensive interest for the packaging of implantable medical devices, however realizing these devices with long-term hermeticity up to several years remains a great challenge. Here, polyimide (PI) based hermetic encapsulation was greatly improved by atomic layer deposition (ALD) of a nanoscale-thin, biocompatible sandwich stack of HfO₂/Al₂O₃/HfO₂ (ALD-3) between two polyimide layers. A thin copper film covered with a PI/ALD-3/PI barrier maintained excellent electrochemical performance over 1028 days (2.8 years) during acceleration tests at 60 °C in phosphate buffered saline solution (PBS). This stability is equivalent to approximately 14 years at 37 °C. The coatings were monitored in situ through electrochemical impedance spectroscopy (EIS), were inspected by microscope, and were further analyzed using equivalent circuit modeling. The failure mode of ALD Al₂O₃, ALD-3, and PI soaking in PBS is discussed. Encapsulation using ultrathin ALD-3 combined with PI for the packaging of implantable medical devices is robust at the acceleration temperature condition for more than 2.8 years, showing that it has great potential as reliable packaging for long-term implantable devices. Full article

This paper details the design, fabrication, and characterization of highly uniform batch-fabricated sidewall etched vertical nanogap tunneling junctions for bio-sensing applications. The device consists of two vertically stacked gold electrodes separated by a partially etched sacrificial spacer layer of sputtered α-Si and Atomic Layer Deposited (ALD) SiO₂. A ~10 nm wide air-gap is formed along the sidewall by a controlled dry etch of the spacer. The thickness of the spacer layer can be tuned by adjusting the number of ALD cycles. The rigorous statistical characterization of the ultra-thin spacer films has also been performed. We fabricated nanogap electrodes under two design layouts with different overlap areas and spacer gaps, from ~4.0 nm to ~9.0 nm. Optical measurements reported an average non-uniformity of 0.46 nm (~8%) and 0.56 nm (~30%) in SiO₂ and α-Si film thickness respectively. Direct tunneling and Fowler–Nordheim tunneling measurements were done and the barrier potential of the spacer stack was determined to be ~3.5 eV. I–V measurements showed a maximum resistance of 46 × 10³ GΩ and the average dielectric breakdown field of the spacer stack was experimentally determined to be ~11 MV/cm. Full article

The FePt film above 10 nm critical lattice relaxation thickness was prepared and the ultrathin MgTiTaON layer was interleaved in between FePt film and the multilayer stack is FePt(6 nm)/[MgTiTaON(1 nm)/FePt(4 nm)]₂. Next, the FePt films were co-sputtered with (Ag, C) segregants during deposition and the layer stacks is FePt(6 nm)(Ag, C)(x vol %)/[MgTiTaON (1 nm)/FePt(4 nm)(Ag, C) (x vol %)]₂ (x = 0, 10, 20, 30, 40). After high temperature deposition at 470 °C, the granular FePt(Ag, C, MgTiTaON) film illustrated perpendicular magnetization and the out-of-plane coercivity (H_c) was increased with (Ag, C) segregants and the highest H_c is 18.3 kOe when x = 40. From cross-section images, the FePt layer are more continuous with 0 and 10 vol % (Ag, C) segregants and changed to an island structure when the (Ag, C) segregants increase to 20–40 vol %. The FePt grains were grown in separated islands in 20, 30 vol % (Ag, C) and changed to dense columnar-like morphology in 40 vol %. The second nucleated grains which contribute the in-plane magnetization are found in FePt (Ag, C) (40 vol %) film. The FePt islands are reached by inserting the ultrathin MgTiTaON layer and the island heights of FePt(Ag, C) (30, 40 vol %) are around 31–38 nm and the aspect ratios are 0.6–0.8. Full article