Multilayers For Use Research Articles

Optimization of a stacked multilayer for use in radiative cooling window

Optimization of a stacked multilayer for use in radiative cooling window

Reflective solar blind filter based on dielectric multilayer

We report on the design and fabrication of LaF3/MgF2 multilayer for use as a reflective solar blind filter. The deposited filter has the average reflectance of 36.5% in 240–280nm, and it has the average reflectance of 1.8% in 281–760nm. Reflectance suppression ratio between in-band and out-of-band is 20.3:1.

Ni thickness influence on magnetic properties (Co/Ni/Co/Pt) multilayers with perpendicular magnetic anisotropy

We present a study of perpendicular magnetic anisotropy in [Co/Ni(t)/Co/Pt]×8 multilayers for use as free layers in magnetic tunnel junctions (MTJ) and spin valves. The thickness t of the Ni sub-layer was varied and the resulting magnetic properties were investigated. As determined from magnetic force microscopy and magnetometry measurements, all multilayers exhibited a perpendicular magnetic anisotropy with an increase of saturation magnetization with thickness t. From the temperature dependence of the magnetization, well described by a Bloch law, we find that the spin-wave stiffness constant of the [Co/Ni(t)/Co/Pt]×8 multilayers is larger compared to (Co/Ni) multilayers. These multilayers could be the basis for spintronic devices where the reduction of total Pt content could help to reduce the damping constant while keeping the magnetic anisotropy energy relatively high. These are conflicting requirements needed for high performance magnetic memory devices.

Critical Temperature Tuning of Ti/TiN Multilayer Films Suitable for Low Temperature Detectors

We present our current progress on the design and test of Ti/TiN multilayer for use in kinetic inductance detectors. Sensors based on sub-stoichiometric TiN film are commonly used in several applications. However, it is difficult to control the targeted critical temperature \(T_c\), to maintain precise control of the nitrogen incorporation process and to obtain a production uniformity. To avoid these problems we investigated multilayer Ti/TiN films that show a high uniformity coupled with high quality factor, kinetic inductance and inertness of TiN. These features are ideal to realize superconductive microresonator detectors for astronomical instruments application but also for the field of neutrino physics. Using pure Ti and stoichiometric TiN, we developed and tested different multilayer configurations, in terms of number of Ti/TiN layers and in terms of different interlayer thicknesses. The target was to reach a critical temperature \(T_c\) around \((1{\div } 1.5)\) K in order to have a low energy gap and slower recombination time (i.e. low generation–recombination noise). The results prove that the superconductive transition can be tuned in the \((0.5{\div } 4.6)\) K temperature range by properly choosing the Ti thickness in the \((0{\div } 15)\) nm range, and the TiN thickness in the \((5{\div } 100)\) nm range.

Optical, electrical, and structural properties of ZrON/Ag/ZrON multilayer transparent conductor for organic photovoltaics application

We investigated the electrical and optical properties of chemically stable nitride-based ZrON/Ag/ZrON multilayers for use in organic photovoltaics. As the ZrON layer thickness increases, the carrier concentration smoothly decreases and the transmission window region becomes wider and gradually shifts toward the lower energy-side. The optical bandgap also decreases with decreasing carrier concentration. The ZrON/Ag/ZrON samples exhibit somewhat lower transmittance in the region of ∼400–800nm than the ITO/Ag/ITO samples. The aging test shows that the resistivity of the ZrON/Ag/ZrON samples remains almost the same even after aging for 1month in air. It is shown that the ZrON/Ag interface is more chemically stable than the ITO/Ag interface. Simulation results exhibit that the ZnPc:C60 bulk hetero-junction organic solar cells fabricated with the ZrON(30nm)/Ag/ZrON(70nm) layers give a short circuit current of 9.54mA/cm2, which is comparable to that (10.54mA/cm2) of the cells with 90-nm-thick ITO electrodes. The ZrON/Ag/ZrON samples experienced a large shift in the absorption edge. This behavior is explained in terms of the material characteristics of the ZrON layers and the higher refractive index of the ZrON/Ag/ZrON samples.

Proximity-coupled Ti/TiN multilayers for use in kinetic inductance detectors

We apply the superconducting proximity effect in TiN/Ti multi-layer films to tune the critical temperature, TC, to within 10 mK with high uniformity (less than 15 mK spread) across a 75 mm wafer. Reproducible TC’s are obtained from 0.8 to 2.5 K. These films had high resistivities, >100μΩ cm, and internal quality factors for resonators in the GHz range, on the order of 100 k and higher. Trilayers of both TiN/Ti/TiN and thicker superlattice films were prepared, demonstrating a well controlled process for films over a wide thickness range. Detectors were fabricated and shown to have single photon resolution at 1550 nm. The high uniformity and controllability coupled with the high quality factor, kinetic inductance, and inertness of TiN make these films ideal for use in frequency multiplexed kinetic inductance detectors and potentially other applications such as nanowire detectors, transition edge sensors, and associated quantum information applications.

Mo/Si and MoSi2/Si nanostructures for multilayer Laue lens

To develop a multilayer Laue lens (MLL), we fabricated depth-graded Mo/Si and MoSi2/Si multilayers with each boundary according to the Fresnel zone configuration. The multilayers were deposited by magnetron sputtering. From the result of SEM image analysis of the multilayer cross sections, MoSi2/Si multilayer had smaller layer-thickness errors than Mo/Si multilayer. In addition, from the result of the focusing test by using 20-keV X-rays, the measured beam size of MoSi2/Si MLL had a small blurring from the diffraction limited beam size. These results suggest that MoSi2/Si multilayer is better suited than Mo/Si multilayer for use as an MLL in hard x-ray nanofocusing.

Improved Photoconduction Effects of Nanometer-Sized Silicon Dot Multilayers

We have been developing a photoconductive film consisting of a nanometer-sized Si dot multilayer for use as the target of a highly sensitive and functional image sensor. The nanometer-sized Si dot multilayer (100 nm in thickness) was deposited by sequentially forming self-assembled Si dots and carrying out their subsequent thermal oxidation. The correlation between the fabrication condition and the photoconductive properties was investigated for fabricated Si nanodot multilayer samples under a UV light incidence. It was shown that the quantum efficiency can significantly be improved by increasing the spatial density of Si nanodots and passivating interfacial dangling bonds with hydrogen termination. Under appropriate processing parameters, a quantum efficiency higher than 50% can be obtained even at low bias voltages below 1 V. The Si nanodot multilayer is potentially useful as a highly sensitive image sensing target compatible with low-voltage operation.

Multilayer phase-only diffraction gratings: Fabrication and application to extreme ultraviolet optics

The use of phase-only diffractive devices has long played an important role in advanced optical systems in varying fields. Such devices include gratings, diffractive and holographic optical elements, diffractive lenses, and phase-shift masks for advanced lithography. Extending such devices to the increasingly important regime of extreme ultraviolet (EUV) wavelengths, however, is not trivial. Here, the authors present an effective fabrication and etch process enabling high-resolution patterning of Mo∕Si multilayers for use in EUV phase devices, providing another method for fabrication of high numerical aperture diffractive devices or high-resolution EUV phase shift masks.

Utilizing ultrathin DNA/poly-lysine multilayer films to create liquid/liquid interfaces:spectroscopic characterization, interfacial reactions and nanoparticle adsorption

Alternating electrostatic multilayer adsorption of poly-L-lysine (pLys) and DNA is used tocreate well-defined biopolymer multilayers for use as an ultrathin aqueous phase inliquid–liquid interfacial measurements. The molecular structure and thickness of thepolyelectrolyte multilayers are determined using a combination of polarizationmodulation FT-IR reflection-absorption spectroscopy (PM-FTIRRAS) and FT-surfaceplasmon resonance (FT-SPR) thickness measurements. Electroactive species such asferri/ferrocyanide ions can be incorporated into the DNA/pLys polyelectrolytemultilayers. The ion transport activity of these electroactive films when in contact with1,2-dichoroethane is verified by electrochemical measurements. Micron-sized patterns ofthese multilayers are created by either photopatterning, vapour-deposited spot patterningor microfluidic stencil processing, and are used in conjunction with fluorescence andsurface plasmon resonance imaging (SPRI) to monitor (i) the intercalation ofdye molecules into DNA/pLys ultrathin films, (ii) the electrostatic adsorption ofgold nanoparticles onto DNA/pLys multilayers and (iii) the spatially controlledincorporation and reaction of enzymes into patterned biopolymer multilayers.

Thermal stability of Co∕SiO2 multilayers for use in the soft x-ray region

The thermal stability of Co∕SiO2 multilayers was evaluated. Multilayer samples were deposited on Si substrates by means of the ion beam sputtering method and annealed at temperatures from 100 °C to 600 °C in a vacuum furnace. For the structural and optical evaluations, small-angle x-ray diffraction measurements, soft x-ray reflectivity measurement in the 1 keV energy region, and transmission electron microscopy observations were carried out. As the results, the Co∕SiO2 multilayers annealed up to 400 °C maintained the initial multilayer structure, and kept almost the same soft x-ray reflectivity as the as-deposited sample. A deterioration of the multilayer structure caused by the growth of cobalt grains was found on the samples annealed over 500 °C, and the soft x-ray reflectivity dropped in accordance with the deterioration of the multilayer structure.

Determination of the layered structure in Mo/Si multilayers by grazing incidence X-ray reflectometry

The formation of intermediate layers was investigated in Mo/Si multilayers produced by e-beam evaporation and ion-beam smoothening. A model of a four-layer step profile structure of the multilayer period was applied successfully to account for grazing incidence X-ray reflectivity of the structures. We have used a special method to determine the multilayer period composition, namely the use of a laterally graded multilayer of which only the thickness of the spacer layer was varied. The well-known modulation of the Bragg peak intensities of 0.154 nm X-rays was then used to determine the thickness and the composition of the silicides formed at the boundaries. The impact of the polishing conditions on the thickness and composition of the intermediate layers was investigated. The study allowed optimization of the composition of Mo/Si multilayer for use at near-normal incidence in the 12.5–15 nm wavelength area for Extreme UV Lithography.

Optical and mechanical characteristics of zinc sulphide-thorium fluoride mixed composition thin films for use in the near infrared region (1–10µm)

Mixed composition thin films of zinc sulphide-thorium fluoride have been deposited on glass and silicon substrates by thermal evaporation of mixtures of these materials in different proportions, from a single resistively heated source. The films are characterized for their optical properties (refractive index and extinction coefficient), mechanical properties (intrinsic stress), surface morphology and chemical composition. It is found that these films have tailorable refractive indices and low losses, and that films with certain compositions have low intrinsic stress and smooth surface morphology, making them suitable for incorporation in thin film multilayers for use in the near infrared region up to at least 10µm.

Relationship between strength and failure mode of ceramic multilayers

The mechanical strength of ceramic multilayers for use in solid oxide fuel cell stacks was examined. The multilayers consist of a substrate, made of yttria stabilised zirconia (YSZ), with a NiO/YSZ composite coating on both faces. Under some processing conditions the mechanical strength of the multilayers was similar to that of pure YSZ, while a much lower strength was found for other processing conditions. Inspections of the fracture surfaces revealed markedly different appearances, suggesting that a transition in failure mode had occurred. Fracture mechanics models for thin films on substrates were used to interpret the observed fracture modes, providing insight to the underlying failure mechanisms.

Multilayer-coated echelle gratings for soft x rays and extreme ultraviolet

Mechanically ruled or ion-etched blazed diffraction gratings can be coated with multilayer reflectors to increase their on-blaze efficiency in the soft x-ray and extreme ultraviolet (EUV) spectral regions. The quality of the groove facets produced by these conventional manufacturing techniques precludes their use in high spectral orders to achieve high resolving power, i.e., as echelles. However, coarse gratings fabricated in crystalline silicon by orientation-dependent etching have excellent groove profile and extremely smooth facets, which make them ideal for coating with multilayers for use as echelles at short wavelengths. Such a grating was coated with tungsten-carbon multilayers for use at soft x-ray wavelengths near the carbon K edge. It was evaluated using both a reflectometer based on a laser-produced plasma source and with synchrotron radiation. In the 50–220 Å wavelength region, diffraction orders up to the 61st were observed, allowing the grating performance to be compared with theoretical predictions. Such multilayer-coated echelles offer the possibility of extremely high spectral resolving power in the EUV and soft x-ray region.

Bandgap engineering of amorphous semiconductors for solar cell applications

AbstractDifferent bandgap engineering approaches are discussed with respect to their application in thin‐film solar cells based on hydrogenated amorphous silicon (a‐Si: H) and its alloys with Ge or C. After a survey of the different approaches reported so far in the literature, the main emphasis will lie on the application of a‐Si: H/a‐Si1‐xCx:H multilayers for use in the p‐doped window layer, or a graded bandgap a‐Si 1‐xCx:Hintrinsic layer with varying Ge content x. But also the possibility of using an a‐Si: H/a‐Si1‐xCx:H multilayer as the i‐layer will be addressed. Intrinsic films of both multilayers and graded bandgap material have been deposited in different series, varying electronic well and barrier widths in the first case, and the amount, form or local position of a change of the bandgap in the second case. Physical properties of both classes of materials have been investigated thoroughly. the results are discussed with respect to an application in amorphous thin‐film solar cells, also taking into account results from the literature about the implementation of these heterostructures in devices. Conclusions for the future perspectives of these approaches are drawn.

Extreme ultraviolet resist and mirror characterization: Studies with a laser plasma source

A monochromatized laser-produced plasma source of extreme ultraviolet (XUV) radiation has been used to study resists and reflective multilayers for use in projection x-ray lithography. We report the characterization of near-edge x-ray absorption fine structure (NEXAFS) spectra, exposure sensitivity, and contrast of selected polysilane resists at photon energies near 100 eV, where projection x-ray lithography is being developed. We find absorption resonance features in the NEXAFS spectra which we assign based on ab initio quantum chemical calculations to excitation into Si–Si and Si–C σ* orbitals. Using monochromatized XUV exposures on the Si–Si σ* resonance at 105 eV, followed by solvent dissolution development, we have measured the exposure sensitivity curves of these resists. We find sensitivities in the range of 600–3000 mJ/cm2 and contrasts in the range from 0.5–1.4, depending on the polysilane side chain. Exposure sensitivity measurements have also been performed below the edge at 92 eV where we find only minor differences from the results at 105 eV, which can be accounted for by a simple decrease in film absorption. The variation in Mo/Si multilayer mirror reflectance as a function of wavelength has also been measured for two mirrors having nearly identical layer periods, but different numbers of layer pairs (N). Increasing N from 20 to 40 is seen to increase the first order peak reflectance from 39%±2% to 50%±3% and to narrow the reflectance bandwidth from 7% to 5.6%. These results are in accord with calculations which use optical constants from the literature and a Debye–Waller interfacial roughness factor to account for interfacial reflectance losses.

Multilayer coated concave diffraction grating resolution and efficiency in the extreme ultraviolet

Recent advances in multilayer structures for the extreme ultraviolet now make it possible to construct diffraction gratings in the wavelength range below 350 A, which can be used for precision measurements using normal incidence spectrometers. We report results from two such gratings, one with conventionally ruled blazed facets and the other with a holographically ruled sinusoidal surface. Both gratings are 1 m in radius, 1200 lines/mm coated with a molybdenum-silicon multilayer for use in the 150 A wavelength region. A 1 m normal incidence spectrometer with a Garton flash tube source and a film detector was used to test the spectral resolution. Relative efficiency measurements were obtained by comparison to spectra produced by an osmium coated grating. Emission lines in the wavelength region of interest (characteristic of the source) are easily detected and well resolved with both multilayer coated gratings. Quantitative efficiency measurements were obtained using a Penning source coupled to a 1 m grazing incidence monochromator and an imaging photon counting detector.

Structural investigations of Langmuir-Blodgett films of 2-docosylamino-5-nitropyridine, a new type of non-centrosymmetric multilayer for use in non-linear optics

Langmuir-Blodgett films of 2-docosylamino-5-nitropyridine (DCANP) have been introduced as non-centrosymmetric multilayers which exhibit strong second harmonic generation (SHG). Both polarized UV-visible spectroscopy and SHG show preferential orientation with respect to the dipping direction. From polarized UV-visible spectroscopy of the monolayer on the water surface we identify the orientation as a result of the dipping process and not of shear forces occuring within the monolayer. From birefringence microscopy and electron diffraction of DCANP multilayers we conclude that the multilayers are composed of small crystal-like domains which are preferentially aligned but not homogeneously oriented.