Thin Film Transfer Research Articles

Mechanism of Transverse Charge Transfer in Thin Films of Hexagonal Boron Nitride

A mechanism of transverse charge transfer through hexagonal boron nitride (h-BN) in a MIS structure has been studied. Experimental data for charge transfer have been analyzed in terms of different models of charge transfer in insulators. It has been shown that charge transfer in h-BN is described by the model of phonon-assisted tunneling between neutral traps. The thermal and optical energies of phonon-coupled traps in h-BN have been determined. Based on charge transfer measurements, XPS spectra, and the ab initio electronic structure of intrinsic defects in h-BN it has been found that boron–nitrogen divacancies are most probably responsible for charge transfer in h-BN and transfer is provided by electrons.

Identification of the ultra-short laser parameters during irradiation of thin metal films using the interval lattice Boltzmann method and evolutionary algorithm

The article presents the one-dimensional numerical modelling of heat transfer in thin metal films irradiated by ultra-short laser pulses. In the mathematical description the relaxation times and the boundary conditions for phonons and electrons are given as interval numbers. The direct problem has been solved by means of the interval lattice Boltzmann method using the rules of directed interval arithmetic, while at the stage of the inverse problem solution, the evolutionary algorithm is applied. On the stage of inverse problems investigations, the evolutionary algorithm coupled with the interval lattice Boltzmann method is used.

Mechanism of Transverse Charge Transfer in Thin Films of Hexagonal Boron Nitride

Mechanism of Transverse Charge Transfer in Thin Films of Hexagonal Boron Nitride

Electrostatics and Chemical Reactivity at the Air–Water Interface

It has been recently discovered that chemical reactions at aqueous interfaces can be orders of magnitude faster compared to conventional bulk phase reactions, but despite its wide-ranging implications, which extend from atmospheric to synthetic chemistry or technological applications, the phenomenon is still incompletely understood. The role of strong electric fields due to space asymmetry and the accumulation of ions at the interface has been claimed as a possible cause from some experiments, but the reorganization of the solvent around the reactive system should provide even greater additional electrostatic contributions that have not yet been analyzed. In this study, with the help of first-principles molecular dynamics simulations, we go deeper into this issue by a careful assessment of solvation electrostatics at the air-water interface. Our simulations confirm that electrostatic forces can indeed be a key factor in rate acceleration compared to bulk solution. Remarkably, the study reveals that the effect cannot simply be attributed to the magnitude of the local electric field and that the fluctuations of the full electrostatic potential resulting from unique dynamical behavior of the solvation shells at the interface must be accounted for. This finding paves the way for future applications of the phenomenon in organic synthesis, especially for charge transfer or redox reactions in thin films and microdroplets.

Quasi-2D SnO2 Thin Films for Gas Sensors: Chemoresistive Response and Temperature Effect on Adsorption of Analytes

We performed in silico calculations of electrical conductivity of quasi-2D SnO2 thin films with a (110) surface-prospect material for sensitive element of gas sensors. Electronic structure, charge transfer and chemoresistive response of quasi-2D SnO2 thin films during adsorption of alcohol molecules (ethanol, methanol, isopropanol and butanol) and ketones (acetone, cyclopentanone and cyclohexanone) were calculated. It was found that the electrical conductivity of quasi-2D SnO2 thin films decreases within 4-15% during adsorption of analytes. The influence of temperature on the concentration of analytes on the surface of quasi-2D SnO2 thin films was explored in dependence analyte's type.

Defect-mediated energy transfer in ZnO thin films doped with rare-earth ions

The rare-earth (RE) (Eu, Er, Nd) doped ZnO thin films were fabricated by a cost-effective chemical solution deposition method. The emission properties of ZnO:RE films were investigated under different excitation conditions, where the RE ions were excited either through direct pumping into the 4f energy levels of RE ions or through indirect excitation by energy transfer from the host material. It is demonstrated that under both excitation methods, the films showed strong emission from the RE ions at room temperature, which confirms the hypothesis that the RE ions can be effectively excited by the host material. Moreover, the influence of RE doping on the development of the crystalline structure of the ZnO thin film was studied. Only a small amount of REs was incorporated into the ZnO grains; most of the REs remained segregated at the grain boundaries, forming a thin oxide shell that strongly suppresses the sintering of the grains and reduces their size.

Lattice Boltzmann method with effective correction of phonon properties for nano/microscale heat transfer

The significant self-heating effect severely restricts the performance and reliability of nano-electronic devices. Accordingly, it is very important to understand the process and mechanism of nano/microscale heat transfer for thermal management and thermal design of devices. In this work, we propose a new Lattice Boltzmann Method (LBM) scheme with effective correction of phonon mean free path (MFP) and relaxation time to study phonon heat transfer in silicon thin films and silicon medium with defects, where the correction factor is dependent on the lattice structure of LBM. The transformation analysis of phonon transfer mechanism at different scales shows that the size effect of cross-plane thermal conductivity is more remarkable than that of in-plane thermal conductivity. And the thermal conductivity of silicon medium with defects decreases exponentially as defect density increases. The proposed new LBM scheme can generate more accurate results than the traditional ones in the heat conduction simulations of different nano/microscale structures.

Flexible fabric-based GaAs thin-film solar cell for wearable energy harvesting applications

GaAs photovoltaic (PV) cells have been extensively studied for flexible energy harvesting devices due to their merits such as thin-film feasibility, flexibility, and high-efficiency. However, GaAs-based thin-film PV cells have a limitation for the applications in wearable platform since they are not compatible with fabric carrier. To handle this problem, we report thin-film transfer technique that involves a sacrificial layer with a double-layer structure, an Au–Au bonding technique, a Cr/Au bilayer to induce metal stress, and an etchant for fast epitaxial lift-off (ELO). In addition, a polyimide layer attached underneath the fabric substrate not only protects the fragile epi layer but also accelerates the lateral etching via spontaneous bending. The application of these techniques enables the successful transfer of GaAs thin-film PV epi structures onto fabric platform without degrading crystal quality. Fabric-based GaAs PV cells are fabricated via the standard PV cell fabrication process. The platform expansion of GaAs thin-film techniques has great potential for large-scale commercialization. • III-V epi-layers onto fabric substrate can be realized without degrading crystal quality. • The GaAs solar cells are directly integrated with fabric platform. • High efficiencies over 17.49% can be achieved on fabric platform. • Fabric-based GaAs solar cells present the stable output power with high flexibility.

Advanced Epitaxial Lift‐Off and Transfer Procedure for the Fabrication of High‐Quality Functional Oxide Membranes

AbstractIn the past 5 years, the transfer of epitaxial oxide thin films has drawn a renewed interest in the scientific community. The major challenge in this technology is to minimize the appearance of extended bulk defects such as plastic deformations, cracks, and delamination, which are induced by the transfer process to a new host substrate. In this work, a procedure for the transfer of epitaxial oxide films where a rigid bond to the final host holder is obtained via a metallic Au/Ag bonding layer is presented. Here, the transfer of SrRuO3 (SRO) and SrRuO3/SrTiO3 (STO) epitaxial films grown on a water‐soluble Sr3Al2O6 sacrificial layer is reported. These epitaxial films are grown on a STO substrate and transferred onto a Si host substrate. Roughness values lower than 1nm are observed for the transferred SRO membranes. Cross‐section analysis shows straight interfaces without plastic deformation of the membranes. X‐ray diffraction rocking‐curve analysis evidences that mechanical damage is minimized and the membranes remain close to their initial quality. This procedure represents an important step forward in the development of advanced technologies for membrane transfer of epitaxial oxides and superstructures.

Variation of magnetism in two-dimensional MnS2 thin films

Atomically thin layers with variable magnetism are highly desirable for next-generation data storage and spintronic devices. Variations of magnetism in two-dimensional (2D) materials have inspired 2D device designs that mimic their functions. In this work, the electronic structure and charge transfer of 2D MnS2 thin films consisting of two monolayers can be changed between two different structures. Variation of magnetism between two different antiferromagnetic (AFM) configurations are realized in the MnS2 thin films. The calculated magnetic anisotropy shows that the easy axes of the thin films are perpendicular to the xy-plane. The underlying variation of magnetism in the air-stable, semiconductive, and facile 2D MnS2 system is revealed. Variations in the magnetism of the 2D thin films are determined from charge transfer between the Mn and S as induced by changes in the stacking orders of the nonmagnetic S atoms. Our propose-designed MnS2 thin films enable emerging 2D magnetism and exotic physical phenomena for nonvolatile spintronic devices that operate without external strains or, electric or magnetic fields.

Evaluation of Polymer Materials for MEMS Microphones

Polymers are attractive materials for the fabrication of micro-electro-mechanical system (MEMS) microphones because of their low Young’s moduli, generally lower processing temperatures than silicon, and simple fabrication; such microphones are sensitive and economical. However, an earlier polymer diaphragm revealed lower sensitivity than expected. In addition, the viscoelastic properties of polymer diaphragm have been considered to degrade the sensing performance due to a high-level mechanical noise. Here, we investigated the possibility of the polymer material as a MEMS microphone in terms of stiffness and damping. Stiffness was analyzed and experimentally verified. The obtained polymer diaphragm damping coefficient was compared with the coefficient of a silicon MEMS microphone to explore whether performance was degraded by material damping. All samples were fabricated using a thin film transfer method. SU-8 was mainly used but Polymethyl methacrylate (PMMA) was also selected for comparing the material damping effect. The measured residual stress of an SU-8 diaphragm was 18 MPa; the measured flexibility was similar to the state-of-art of a silicon-based MEMS microphone, and it was proven that there is room for further improvement through process optimization. The damping effect induced by viscoelasticity was very small, compared with the effect of air damping; indeed, the intrinsic damping effect of the polymer on device performance was negligible. [2021-0213]

Thin-Film Transistors from Electrochemically Exfoliated In2Se3 Nanosheets.

The wafer-scale fabrication of two-dimensional (2D) semiconductor thin films is the key to the preparation of large-area electronic devices. Although chemical vapor deposition (CVD) solves this problem to a certain extent, complex processes are required to realize the transfer of thin films from the growth substrate to the device substrate, not to mention its harsh reaction conditions. The solution-based synthesis and assembly of 2D semiconductors could realize the large-scale preparation of 2D semiconductor thin films economically. In this work, indium selenide (In2Se3) nanosheets with uniform sizes and thicknesses were prepared by the electrochemical intercalation of quaternary ammonium ions into bulk crystals. Layer-by-layer (LbL) assembly was used to fabricate scalable and uniform In2Se3 thin films by coordinating In2Se3 with poly(diallyldimethylammonium chloride) (PDDA). Field-effect transistors (FETs) made from a single In2Se3 flake and In2Se3 thin films showed mobilities of 12.8 cm2·V−1·s−1 and 0.4 cm2·V−1·s−1, respectively, and on/off ratios of >103. The solution self-assembled In2Se3 thin films enriches the research on wafer-scale 2D semiconductor thin films for electronics and optoelectronics and has broad prospects in high-performance and large-area flexible electronics.

Freestanding Perovskite Oxide Films: Synthesis, Challenges, and Properties

AbstractIn this review paper, recent progress in the fabrication, transfer, and fundamental physical properties of freestanding oxide perovskite thin films is discussed. First, the main strategies for the synthesis and transfer of freestanding perovskite thin films are analyzed. In this initial section, particular attention is devoted to the use of water‐soluble (Ca,Sr,Ba)3Al2O6 thin films as sacrificial layers, one of the most promising techniques for the fabrication of perovskite membranes. The main functionalities that have been observed in freestanding perovskite thin films are then reviewed. In doing so, the authors begin by describing the emergence of new phenomena in ultrathin perovskite membranes when released from the substrate. They then move on to a summary of the functional properties that are observed in freestanding perovskite membranes under the application of strain. Indeed, freestanding thin films offer the unique possibility to actively control the strain state far beyond what can be observed with traditional methods, allowing the investigation of the profound interplay between structural and electronic properties in oxides. Overall, this review highlights the potential of oxide‐based freestanding thin films to become the preferred platform for the study of novel functionalities in perovskite oxide materials.

Lie symmetry analysis of heat transfer in a liquid film over an unsteady stretching surface with viscous dissipation and external magnetic field

Fluid flow and heat transfer in thin films over unsteady stretching sheets are studied and analyzed by employing similarity transformations and analytical or numerical solution schemes. We derive Lie point symmetries for system of partial differential equations describing the flow and heat transfer in a thin liquid film on an unsteady stretching surface with viscous dissipation in the presence of external magnetic field. We deduce a 5-dimensional Lie point symmetry algebra. We use a couple of the admitted Lie point symmetry generators to construct new similarity transformations for the considered model, through associated invariants. These transformations enable reductions of the model to nonlinear ordinary differential equations. We construct analytic solutions for this system using the Homotopy analysis method to study the considered MHD flow and heat transfer. These results are presented in the form of tables and figures to reflect variations in fluid velocity, film thickness and heat transfer with the magnetic parameter, Prandtl number, Eckert number and unsteadiness parameter. We show that these variations in the velocity and temperature profiles are different from those reported earlier for the flow and heat transfer within a thin film under the effects of viscous dissipation and external magnetic field.

Magnetohydrodynamic Experiments of Total Heat Flux Mitigation for Superorbital Earth Reentry

A set of experiments was conducted with the aim of evaluating total heat flux mitigation using magnetohydrodynamic flow control over ionized shock layers surrounding planetary entry vehicles. The University of Queensland’s X2 expansion tube was used to generate a flow condition representative of a Mars return trajectory. Spherical permanent magnets were fitted into a scaled test model based on NASA’s Stardust capsule. Thermocouples located at the stagnation point and shoulder of the test model, as well as thin-film heat transfer gauges located in the aft section, measured total heat flux for both magnetic and nonmagnetic experiments. High-speed imaging of the shock layer showed a significant increase in shock standoff when using the magnet, and polynomial fits were derived, describing standoff distance along the model’s forebody. Despite significant noise affecting the heat flux gauges signals for the magnetic experiments, a simple methodology allowed to compare heat flux during the useful test time. It was found that the applied magnetic field significantly reduces total heat flux at the stagnation point and shoulder of the test model, although heat flux to the aft section was found to significantly increase. Reasonable agreement with the numerical predictions was found for the aft section of the test model, but discrepancies were observed at the other locations.

Heat transfer analysis of Cu and Al2O3 dispersed in ethylene glycol as a base fluid over a stretchable permeable sheet of MHD thin-film flow

The process of thin films is commonly utilized to improve the surface characteristics of materials. A thin film helps to improve the absorption, depreciation, flexibility, lighting, transport, and electromagnetic efficiency of a bulk material medium. Thin-film treatment can be especially helpful in nanotechnology. As a result, the current study investigates the computational process of heat relocation analysis in a thin-film MHD flow embedded in hybrid nanoparticles, which combines the spherical copper and alumina dispersed in ethylene glycol as the conventional heat transfer Newtonian fluid model over a stretching sheet. Important elements such as thermophoresis and Brownian movement are used to explain the characteristics of heat and mass transfer analysis. Nonlinear higher differential equations (ODEs) were attained by transforming partial differential equations (PDEs) into governing equations when implementing the similarity transformation technique. The resulting nonlinear ODEs have been utilized by using the homotopy analysis method (HAM). The natures of the thin-film flow and heat transfer through the various values of the pertinent parameters: unsteadiness, nanoparticle volume fraction, thin-film thickness, magnetic interaction, and intensity suction/injection are deliberated. The approximate consequences for flow rate and temperature distributions and physical quantities in terms of local skin friction and Nusselt number were obtained and analyzed via graphs and tables. As a consequence, the suction has a more prodigious effect on the hybrid nanofluid than on the injection fluid for all the investigated parameters. It is worth acknowledging that the existence of the nanoparticles and MHD in the viscous hybrid nanofluid tends to enhance the temperature profile but decays the particle movement in the thin-film flow. It is perceived that the velocity and temperature fields decline with increasing unsteadiness, thin-film thickness, and suction/injection parameters. The novel part of the present work is to investigate the hybrid nanofluid including Cu–Al2O3 dispersed in Ethylene glycol as a base fluid in the presence of a magnetic field, which has not been investigated yet. So, in limiting cases the present work is validated with published work and found in excellent agreement as shown in Table 3.

Stretchable Organic Light‐Emitting Devices with Invisible Orderly Wrinkles by using a Transfer‐Free Technique

AbstractStretchable organic light‐emitting devices (SOLEDs) based on orderly wrinkles have become a promising candidate for the new‐generation deformable displays because of their high‐optoelectronic performance and stretching stability. However, the complex thin film transfer process to form the wrinkles and the decreased display quality caused by the large‐period wrinkles with a few hundreds of microns are obstacles to their practical applications. Herein, a simple transfer‐free technique is designed to introduce the orderly wrinkles with a much smaller period of 70 µm into the SOLEDs, which is the smallest value reported to date for the orderly wrinkles integrated in the SOLEDs. The small‐period wrinkles are invisible by naked eyes and beneficial to the applications of the SOLEDs in high‐quality stretchable displays. The electroluminescence performance of the SOLED with 20% stretchability are comparable to those of the rigid OLEDs on glass substrates. After 2000 cycles of stretching, the luminance of the device remained at 90% of its initial value. Meantime, the transfer‐free technique is compatible with the flexible encapsulation process. The luminance of the encapsulated SOLEDs exhibit negligible decline after 600‐min continuous operation under ambient condition. This work provides an effective solution for achieving high‐performance SOLEDs by a simple process.

Scotch-Tape Surface Wrinkling Based Thin-Film Material Properties Extraction

Abstract This paper presents a new simple method to measure thin film material properties using scotch-tape surface wrinkling. Metal thin films have been deposited on polymer substrates by e-beam evaporation. After patterned by photolithography and wet etching, long and narrow thin metal layers have been transferred by peel-off onto the scotch tape. The effect of velocity of scotch tape peel-off on the metal thin film transfer from polymer substrates to the scotch tape has been investigated. After transfer, metal thin film wrinkling patterns have been established on the scotch tape due to mechanical properties mismatch between the two materials. The wrinkling patterns have been characterized in term of amplitude and wavelength and they are compared with FEM (Finite Element Method) buckling simulation results for material properties extraction. By consequence, elastic moduli of 300 nm-thick gold film and a multilayer of 30 nm Ti on 300 nm Au have been found 147 GPa and 885 GPa based on the measured wavelength of the wrinkling films.

Magnetohydrodynamics Boundary Layer Flow of Hybrid Nanofluid in a Thin-Film Over an Unsteady Stretching Permeable Sheet

This analysis explored the computational process of heat transfer analysis in a thin-film MHD flow embedded in the hybrid nanoparticles, which combine the spherical copper and alumina dispersed in ethylene glycol as the conventional heat transfer Newtonian fluid model over a stretching sheet. The nonlinear ordinary differential equations (ODEs) was attained by transforming partial differential equation (PDEs) as governing equations when implementing the similarity transformations technique. The resulting nonlinear ODEs have been utilized by using the Keller box method. The natures of the thin-film flow and heat transfer through the various values of the pertinent parameters: unsteadiness, nanoparticle volume fraction, thin-film thickness, magnetic interaction and intensity suction/injection are deliberated. The approximate results for velocity and temperature distributions and physical quantities in terms of local skin friction and Nusselt number have been obtained and analyzed via graphs and tables. As a consequence, the suction expresses a more prodigious effect on the hybrid nanofluid rather than injection fluid for all the investigation parameters. It is worth acknowledging that the existence of the nanoparticles and MHD in the viscous hybrid nanofluid tends to enhance the temperature profile but decay the particle movement in the thin-film flow. It is perceived that the velocity and temperature profiles decline for the growth of the unsteadiness, thin-film thickness and suction/injection parameters.

Controlled spalling of (100)-oriented GaAs with a nanoimprint lithography interlayer for thin-film layer transfer without facet formation

Controlled spalling is a potentially low-cost, facile layer transfer technique that can be used for a variety of thin-film device applications and can enable reuse of expensive, single-crystal wafer substrates. However, controlled spalling of (100)-oriented GaAs substrates, commonly used in III-V device growth, suffers from the need for fracture surface repreparation before the wafer can be reused due to the formation of large triangular facets. In this work, we demonstrate facet suppression in (100)-GaAs through the use of buried nanoimprint lithography (NIL) patterned layers that guide the spalling fracture along more favorable paths than the facet-forming path. We show the fracture interaction with three NIL patterned interlayers that have varying aspect ratio, arrangement, and feature shapes and demonstrate the constraint of the fracture path to within <200 nm in height compared to the 5–10 µm facet height observed with controlled spalling of (100) GaAs without a buried interlayer. Our observations show that patterns with high fill factor and continuity in the spall direction as well as the presence of voids in the overgrowth above the features are all beneficial to facet suppression during controlled spalling of thin films from (100) GaAs substrates. Our results can be used in conjunction with overgrowth optimization to further improve this technique for low-cost layer transfer and wafer reuse with (100)-oriented GaAs substrates.