Planar Thin Film Research Articles

Microwave Backscatter-Based Wireless Temperature Sensor Fabricated by an Alumina-Backed Au Slot Radiation Patch.

A wireless and passive temperature sensor operating up to 800 °C is proposed. The sensor is based on microwave backscatter RFID (radio frequency identification) technology. A thin-film planar structure and simple working principle make the sensor easy to operate under high temperature. In this paper, the proposed high temperature sensor was designed, fabricated, and characterized. Here the 99% alumina ceramic with a dimension of 40 mm × 40 mm × 1 mm was prepared in micromechanics for fabrication of the sensor substrate. The metallization of the Au slot patch was realized in magnetron sputtering with a slot width of 2 mm and a slot length of 32 mm. The measured resonant frequency of the sensor at 25 °C is 2.31 GHz. It was concluded that the resonant frequency decreases with the increase in the temperature in range of 25–800 °C. It was shown that the average sensor sensitivity is 101.94 kHz/°C.

Van der Waals epitaxial ZnTe thin film on single-crystalline graphene

Graphene template has long been promoted as a promising host to support van der Waals flexible electronics. However, van der Waals epitaxial growth of conventional semiconductors in planar thin film form on transferred graphene sheets is challenging because the nucleation rate of film species on graphene is significantly low due to the passive surface of graphene. In this work, we demonstrate the epitaxy of zinc-blende ZnTe thin film on single-crystalline graphene supported by an amorphous glass substrate. Given the amorphous nature and no obvious remote epitaxy effect of the glass substrate, this study clearly proves the van der Waals epitaxy of a 3D semiconductor thin film on graphene. X-ray pole figure analysis reveals the existence of two ZnTe epitaxial orientational domains on graphene, a strong X-ray intensity observed from the ZnTe [1¯1¯2] ǁ graphene [10] orientation domain, and a weaker intensity from the ZnTe [1¯1¯2] ǁ graphene [11] orientation domain. Furthermore, this study systematically investigates the optoelectronic properties of this epitaxial ZnTe film on graphene using temperature-dependent Raman spectroscopy, steady-state and time-resolved photoluminescence spectroscopy, and fabrication and characterization of a ZnTe-graphene photodetector. The research suggests an effective approach towards graphene-templated flexible electronics.

Size effect on melting temperatures of alumina nanocrystals: Molecular dynamics simulations and thermodynamic modeling

Molecular dynamics (MD) simulations and thermodynamic analysis are conducted to investigate size effect on melting point of alumina nanocrystals. Different geometries including spherical and cubic particles, planar thin films, and spherical shells are considered. The atomic interactions in MD simulations are captured using Vashishta et al.’s potential function. Thermophysical properties of concern such as the bulk melting point, latent heat, density, and surface free energy are calculated using MD simulations and fed as inputs to the thermodynamic models. Predictions of MD simulations are compared with those of thermodynamic melting models. For all cases, heterogeneous melting is observed, where nucleation of the melt phase occurred at the free surface and the melting front propagated into the interior regions of the crystal. Results suggest that the melting point drops sharply below a threshold particle size, which is a function of the geometry. The threshold particle size of spherical particles is greater than that of planar films. Melting point predictions are quite sensitive to the choice of thermodynamic model and thermophysical property values. To ascertain the effects of curvature and core size on shell melting point, melting points of planar semi-infinite films and spherical shells are calculated and compared. For particle sizes of concern to practical applications (∼100 nm or greater), shell melting point is nearly independent of core size and the semi-infinite planar film approximation is reasonable to estimate the melting point of alumina shells. In this size regime, melting points of 2–4 nm thick spherical oxide shells vary roughly in the range of 1800–2350 K, which is relatively close to the bulk melting point. Results of the present study do not indicate an enormous depression in the melting point, as the previous works suggest. This implies that the melting point depression of the oxide layer cannot fully describe the scatter in the measured ignition temperature of aluminum particles.

Ферромагнитный резонанс в квадратных решетках планарных магнитных элементов крестообразной формы

Abstract —Ferromagnetic resonance (FMR) in planar thin film structures comprised of magnetic cross-shaped elements arranged into square lattices has been studied by micromagnetic simulation. FMR spectra and spatial distributions of amplitudes of resonant oscillations were determined for vortex and antivortex states of magnetization in lattice crosses. Resonant modes have the shape of rotating magnetization distributions. Herewith, modes with the same configuration of magnetization but rotating in reverse directions are frequency split. The observed nonreciprocity of propagation of resonant spin oscillations is determined by the sign of magnetization distribution vorticity.

Spatial Confinement of the Optical Sensitizer to Realize a Thin Film Organic Photodetector with High Detectivity and Thermal Stability.

A thin film planar heterojunction organic photodetector (PHJ-OPD) is demonstrated. Different from a conventional sensitizer-doped photodetector, the limited spatial distribution of sensitizer in a PHJ-OPD enables significantly reduced thickness of the active layer without allowing the formation of unnecessary trap sites and electron percolation pathways. As a result, peak external quantum efficiency (EQE)of 120 700% and detectivity over 1013 Jones are demonstrated with thin active layer thickness of 150 nm, which can be a significant benefit for high-resolution image sensor application. Furthermore, the operating voltage can be decreased to -5 V while maintaining high detectivity over 1012 Jones. Remarkable thermal stability is also observed with minor change in detectivity for 2 h of continuous operation at 60 °C due to morphological robustness of PHJ. This work opens up a possibility of using a thin film PHJ-OPD as a key unit of high-resolution image sensor.

Enhanced photoelectrochemical water splitting performance using morphology-controlled BiVO4 with W doping.

Nanostructures exhibit numerous merits to improve the efficiency in solar-to-energy conversion. These include shortened carrier collection pathways, an increased volume ratio between depletion layer and bulk, enhanced light capture due to multiple light scattering in nanostructures, and a high surface area for photochemical conversion reactions. In this study, we describe the synthesis of morphology-controlled W-doped BiVO4 by simply tuning the solvent ratio in precursor solutions. Planar and porous W-doped BiVO4 thin films were prepared and compared. The porous film, which exhibits increased surface area and enhanced light absorption, has displayed enhanced charge separation and interfacial charge injection. Our quantitative analysis showed an enhancement of about 50% of the photoelectrochemical performance for the porous structure compared to the planar structure. This enhancement is attributed to improved light absorption (13% increase), charge separation (14% increase), and interfacial charge injection (20% increase).

A flexible electrostatic kinetic energy harvester based on electret films of electrospun nanofibers

This paper reports a paper-based electrostatic kinetic energy harvester (e-KEH) implementing multilayered electret films based on electrospun nanofibrous material. It is the first time that a fully flexible electret-based e-KEH is reported. The proposed electret, PVDF-PTFD nanofibrous covered by Parylene C, has a faster stabilization of surface potential than a planar thin film of Parylene C, and a higher stability of charge storage. With a maximum force of 0.5 N and a 3-layer electret, the device capacitance increases from 25 to 100 pF during a pressing operation. Working with the optimal resistive load of 16 MΩ, the device pressed manually delivers a peak instantaneous power up to 45.6 μW and an average energy of 54 nJ/stroke, corresponding to a peak instantaneous power density of 7.3 μW cm−2 and an average energy density of 8.6 nJ cm−2/stroke. Within 450 manual strokes, a 10 nF capacitor is charged up to 8.5 V by the prototype through a full-wave diode bridge. On a 1 μF capacitor, the energy delivery of 9.9 nJ/stroke has been obtained with a 10 Hz pressing movement excited by a vibrator with a maximum force of 0.5 N.

Cu Nanostructures for Enhanced Heat Transferin Micro Systems

Vertically aligned copper (Cu) nanorod arrays fabricated by glancing angle deposition (GLAD) technique was used to study the heat transfer by natural convection in micro systems. These nanorods were deposited on Cu thin film surface, which was coated on Si (100) wafer substrates. For comparison, planar Cu thin film samples were also produced by normal incidence deposition. This study focuses on investigating experimentally the heat transfer by natural convection from the sample(nanostructured plate) to the surrounding air. Newton's law of cooling has been utilized to calculate the free-convection heat transfer coefficient for Cu nanorods as well as Cu thin film. Compered to Cu thin film samples, it has been found that the heat transfer coefficient values of Cu nanorods are higher due to the enhanced heat transfer-surface area, which causes a significant reduction in thermal resistance. These nanostructured surfaces offer an effective method of device cooling such as for small electronic devices, micro-reactors and air conditioning.

Study on quasi-orthogonal machining of elastomer pad by single-point diamond tool

Chemical mechanical polishing (CMP) process has been a popular wafer and thin film planarization process for semiconductor fabrication. In CMP process, a diamond dresser with well-distributed diamond grits is usually applied for regenerating the pad surface topography to maintain the pad polishing capability. This paper describes the fundamentally quasi-orthogonal diamond dressing process by pyramid single-point diamond tools at different grit angles under the fixed down pressure and slow dressing speed for elastomer pad conditioning. Experiments of single-point diamond dressing by both face direction dressing (FDD) and edge direction dressing (EDD) have been performed to investigate the normal force profile and pad surface topography. Experimental results show that FDD generates a higher quality of pad surface with lesser plowing volume and relatively stable pad cutting rate (PCR). Moreover, diamond grit with grit angle of 90° has been found to be most suitable while shifting between EDD and FDD during actual diamond dressing process. Results of this study can be applied to diamond dresserv design and optimization of the pad surface topography uniformity in diamond dressing process for CMP of integrated circuit (IC) production.

On-Chip Magnetic Nanoparticle Manipulation and Trapping for Biomedical Applications

Biomedical applications and point-of-use devices often rely on nanoparticle concentration strategies. In this paper, on-chip magnetic trapping and manipulation of 300–500 nm superparamagnetic nanoparticles are achieved using thin-film planar circular copper coils. The trap configuration combining a central coil surrounded by four other coils, all connected in series, successfully enables simultaneous attraction and repulsion, enhancing concentration of magnetic nanoparticle (MNP). The simultaneous attraction and repulsion has advantages over static permanent magnets, where unidirectional actuation allows either attraction or repulsion. Because the distance between traps and the MNP is large (hundred micrometer), large electric currents up to 500 mA are needed to achieve fields of the order of hundreds of microtesla, which requires thermal management. Joule heating is effectively tackled by resorting to a novel cooling scheme comprising triangular microchannels milled onto copper substrates. Experimental results combining thermal surface mapping, magnetic field mapping, and optical (fluorescence microscope) observations could validate the trapping efficacy of Estapor microspheres functionalized with BODIPY 515 fluorophores.

Study on light-capture performance of silicon thin-film hollow waveguide solar cells

Preliminary studies have found that hollow waveguide solar cell is expected to be able to make full use of sunlight and enhance the application level of existing planar silicon thin film solar cell. However, it is still an urgent problem to quantitatively evaluate the light injection capability of this type of hollow waveguide solar cell. A silicon thin-film hollow waveguide solar cell capable of capturing more light was proposed based on transfer-matrix simulations of reflectivity of a traditional multilayered silicon solar cell. The light capture of the waveguide solar cell was evaluated using Ray model under different light illumination conditions. The numerical results agree with the experimental results. The waveguide layers, hollow core size, waveguide length and the maximum light launching angle influence the light capture performance of the waveguide cell. A single-junction silicon thin-film hollow waveguide solar cell is able to achieve the energy absorption efficiency of a multi-junction planar silicon solar cell.

Epitaxial growth of GaSb on V-grooved Si (001) substrates with an ultrathin GaAs stress relaxing layer

We report epitaxial growth of GaSb nano-ridge structures and planar thin films on V-groove patterned Si (001) substrates by leveraging the aspect ratio trapping technique. GaSb was deposited on {111} Si facets of the V-shaped trenches using metal-organic chemical vapor deposition with a 7 nm GaAs growth initiation layer. Transmission electron microscopy analysis reveals the critical role of the GaAs layer in providing a U-shaped surface for subsequent GaSb epitaxy. A network of misfit dislocations was uncovered at the GaSb/GaAs hetero-interface. We studied the evolution of the lattice relaxation as the growth progresses from closely pitched GaSb ridges to coalesced thin films using x-ray diffraction. The omega rocking curve full-width-at-half-maximum of the resultant GaSb thin film is among the lowest values reported by molecular beam epitaxy, substantiating the effectiveness of the defect necking mechanism. These results thus present promising opportunities for the heterogeneous integration of devices based on 6.1 Å family compound semiconductors.

Preparation of high-quality planar FeRh thin films for in situ TEM investigations

The preparation of a planar FeRh thin film using a focused ion beam (FIB) secondary electron microscope (SEM) for the purpose of in situ transmission electron microscopy (TEM) is presented. A custom SEM stub with 45° faces allows for the transfer and milling of the sample on a TEM heating chip, whilst Fresnel imaging within the TEM revealed the presence of the magnetic domain walls, confirming the quality of the FIB-prepared sample.

Acoustic and atomic force microscopy characterization of microbubbles with varying shell chemistry

Applications of microbubbles (MBs) in diagnostic and therapeutic interventions critically depend on their stability and scattering properties. The shell chemistry of MBs defines these properties. We investigated the effects of shell chemistry on the size, abundance, acoustic response, and mechanical properties of MBs by varying the poly(ethylene glycol) (PEG) molar ratio (0 to 100%) in a two-lipid (DPPC and DPPE-PEG2000) component shell formulation. Increasing PEG concentration from 0% to 10% resulted in an increase in the number of MBs by at least 10-fold, with adverse effects upon further increases. Microbubbles made with 5–10% PEG generated the strongest fundamental as well as nonlinear (subharmonic and second harmonic) components at the excitation frequency of 2.25 MHz. We used interfacial rheological models to determine the mechanical properties of MB shells as functions of PEG concentration using experimentally measured attenuation values. We also employed atomic force microscopy (AFM) to perform thin planar film characterization of the shells. The correlation between the AFM measurements of film properties and the acoustic responses of the corresponding coated MBs will be discussed.

3D LiMn2O4 Electrodes for High Rate Thin-Film Microbatteries

3D thin-film microbatteries are attracting a lot of interest due to their fast charging capability and higher capacity provided by the surface area enhancement. The thin-film deposition techniques (e.g. RF-sputtering and CVD) typically used in case of planar thin-film microbatteries cannot be used for the 3D microbatteries. Indeed, for the latter, the thin-film needs to be conformally coated over high aspect ratio microstructured substrates which cannot be done using line-of-sight sputtering techniques. In this paper, we report on a new conformal thin-film fabrication technique for the preparation of 3D LiMn2O4 (LMO) thin-film electrodes. LiMn2O4 is an interesting cathode material for Li-ion batteries due to its high capacity and low volume change during the Li-ion insertion/extraction process. Moreover, the cubic spinel structure of LMO helps reaching a fast charging which is one of the key requirements for 3D thin-film batteries. Our novel fabrication technique is based on the electrochemical deposition of MnO2 thin-film and its conversion upon thermal treatment to LiMn2O4 using a Li2CO3 precursor. Using this fabrication technique up to 350 nm conformal and continuous LMO films were prepared on different current collector materials both on planar substrates and on Si pillar arrays as 3D substrates providing an area enhanced of 21x (see SEM images in figure 1 (a, b, and b’)). Elastic recoil detection (ERD) analysis show that the fabricated films indeed have a stoichiometry close to the theoretical stoichiometry of LiMn2O4 and with a homogeneous element distribution throughout the layer. The prepared planar and 3D LMO thin-film are electrochemically active with comparable rate performance. The full theoretical capacity is reached at 0.1 C and 55% of the capacity is still retained at high C-rates of 10C (see figure 1 (c)). As expected from the area enhancement, the capacity for the 3D LMO electrode is 21 times that of planar LMO electrode. Finally, a functional solid-state Li-ion battery was demonstrated using our 3D LMO thin-film cathode and a Li-foil anode using a nanocomposite solid electrolyte (SCE). Figure 1

Improved Reproducibility and Intercalation Control of Efficient Planar Inorganic Perovskite Solar Cells by Simple Alternate Vacuum Deposition of PbI2 and CsI.

Vacuum deposition is a simple and controllable approach that aims to form higher-quality perovskite films compared with those formed using solution-based deposition processes. Herein, we demonstrate a novel method to promote the intercalation control of inorganic cesium lead iodide (CsPbI3) perovskite thin films via alternate vacuum deposition. We also investigated the effect of layer-by-layer deposition of PbI2/CsI to fabricate efficient planar heterojunction CsPbI3 thin films and solar cells. This procedure is comparatively simple when compared with commonly used coevaporation techniques; further, precise intercalation control of the CsPbI3 thin films can be achieved by increasing the number of layers in the layer-by-layer deposition of PbI2/CsI. The best control and the highest reproducibility were achieved for the deposition of four double layers owing to the precise intercalation control during the deposition of the CsPbI3 thin film. A power conversion efficiency of 6.79% was obtained via alternating vacuum deposition of two double layers with a short-circuit current density (Jsc) of 12.06 mA/cm2, an open-circuit voltage (Voc) of 0.79 V, and a fill factor (FF) of 0.72. Our results suggest a route for inorganic precursors to be used for efficient perovskite solar cells via alternating vacuum deposition.

Broadband planar multilayered absorbers tuned by VO2 phase transition

The metal-insulator transition makes vanadium dioxide an attractive material for developing reconfigurable optoelectronic components. Here we report on dynamically tunable broadband absorbers consisting of planar multilayered thin films. By thermally triggering the phase transition of vanadium dioxide, the effective impedance of multilayered structures is tuned in or out of the condition of impedance matching to free-space, leading to switchable broadband absorptions. Two types of absorbers are designed and demonstrated by using either the insulating or metallic state of vanadium dioxide at the impedance matched condition. The planar multilayered absorbers exhibit tunable absorption bands over the wavelength ranges of 5–9.3 μm and 3.9–8.2 μm, respectively. A large modulation depth up to 88% is measured. The demonstrated broadband absorbance tunability is of potential interest for reconfigurable bolometric sensing, camouflaging, and modulation of mid-infrared lights.

Stabilising Oxide Core—Platinum Shell Catalysts for the Oxygen Reduction Reaction

Thin film planar models of core-shell catalysts for the oxygen reduction reaction in PEM fuel cells, have been synthesised with tin doped titania as the support for platinum. High-throughput methods have been used to investigate the effect that the level of tin doping in the titania core supporting material and the degree of crystallisation have on the loading of platinum at which bulk platinum like oxygen reduction behaviour is achieved. At high effective coverages of platinum, the oxygen reduction activity is always similar to that of a polycrystalline platinum electrode. This suggests that a continuous platinum thin film can always be formed at higher loadings. As the loading of platinum is decreased on all of the support materials studied, the activity eventually decreases, corresponding to effective platinum coverages that form nucleated particles on the support. The effective platinum coverage at which a continuous layer of platinum is formed, and exhibits the high activity, depends strongly on the support. The critical effective coverage, below which the oxygen reduction activity decreases, is ca. 5ML on anatase, and slightly higher on the amorphous phase of titania. Doping the titania with tin 10at.%>Sn>0% initially results in an increase in this critical effective coverage. At higher tin concentrations this critical coverage reduces again, and for supports with 23at.%<Sn<40at.%, only ca. 1.6ML is required. Therefore, these catalysts have the highest mass activity. Stability cycling of these catalysts also shows that they are the more stable supported systems. It is also shown that the doped titania support material itself is stable in a sulphuric acid environment at 80°C for Sn<at 28%. These results suggest that active and stable titania supported catalysts can be formed on tin doped supports with concentrations in the range 28at.%>Sn>23at.%.

Comparison of Axial Wire-Wound and Planar Thin-Film SQUID-Based Gradiometers for Super-Paramagnetic Relaxometry Measurements

Superparamagnetic relaxometry (SPMR) is a new technology used for detection and localization of cells tagged with super-paramagnetic nanoparticles. SPMR is one of the most promising methods for early cancer diagnosis with the potential for submillimeter size tumor resolution. SQUID-based gradiometers are very suitable for such sensitive measurements. Presently, most commonly used SPMR instruments consist of axial wire-wound gradiometers connected to SQUID-sensors placed inside tight superconducting shields. This configuration avoids flux trapping by the Josephson junctions during magnetization. A thin-film planar gradiometer integrated with a SQUID on a single chip traps flux, if it picks up even small portion of a magnetizing field. Thermal cycling is very slow, therefore it is not practical. Special measures must be applied such as newly developed methods of SQUIDs ac de-fluxing. Planar thin-film gradiometers have advantages over axial wire-wound gradiometers because of their much higher balance level, higher reliability, and reproducibility in production, and significant simplification of multichannel systems design. In this paper, we compare experimental results received using axial wire-wound and planar thin-film gradiometers. This comparison is based on SPMR measurements conducted using the seven-channel axial second-order gradiometer system and two systems with different geometries of the thin-film first-order planar gradiometer arrays. We discuss advantages and drawbacks of the two types of gradiometers.

AC Defluxing of SQUIDs

Magnetic flux trapping is a very serious problem in SQUID-based instrumentation and superconducting electronics. A critical consideration for avoiding flux trapping is a prevention of SQUID sensors or circuits from being exposed to magnetic fields. Unfortunately, this measure cannot be applied, if sensors must be exposed to a strong magnetic field or transients. For example, unavoidable exposures take place in ultralow field magnetic resonance imaging and superparamagnetic relaxation measurements using unshielded thin-film planar SQUID sensors. SQUID sensors stop working after being exposed to magnetic fields of only a few mT in strength. Earlier, we reported successful results of defluxing of planar LTS SQUID-gradiometers using strong decaying ac magnetic pulses. In this paper, we present further experimental results and discuss a possible mechanism to explain the observed defluxing effects. The new ac defluxing technique is much faster and dissipates much less energy than a conventional thermal cycling.