Thin Film Processing Research Articles

Microstructure and Its Heat Treatment Process of Thin Films Fabricated by Alternate Sputtering of (Bi,Pb)2Sr2CaCu2O x and Pb–Ca–Cu–O Targets on SrTiO3 Substrates

For obtaining microscopic knowledge toward a fabrication process of (Bi,Pb) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Sr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Ca <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">_x</i> (Bi,Pb-2223) single-phase material, we fabricated a multilayered film on a SrTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (001) substrate by a sputtering method using (Bi,Pb) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Sr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> CaCu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">_y</i> (Bi,Pb-2212) and Pb–Ca–Cu–O targets alternately at 650˚C. The as-grown film was an epitaxially grown multilayered film consisting of Bi,Pb-2212 and Pb–Ca–Cu–O layers. Coarse grains of impurity phases were also formed within the multilayered film, and these impurity phases were difficult to eliminate even after heat treatments. After 10 h heat treatment at 840°C with Bi,Pb-2223 pellets, the phase transition from Bi,Pb-2212 to Bi,Pb-2223 occurred in the multilayered film. The phase transition to Bi,Pb-2223 proceeded preferentially at the interface with the impurity phases and the Pb–Ca–Cu–O layers. These observation results suggest that the impurity phases promote three-dimensional atomic diffusion in the multilayered film to accelerate the phase transition to Bi,Pb-2213, as well as consume constituent elements, Ca, Cu and O. Even after the 100 h heat treatment at 840°C with Bi,Pb-2223 pellets, the phase transition from Bi,Pb-2212 to Bi,Pb-2223 did not proceed completely, and the fraction of the Bi,Pb-2223 phase in the whole superconducting phases was ∼50%. It is suggested that controlling the formation and microstructure of the impurity phases is a key to further increasing the fraction of the Bi,Pb-2223 phase.

Dependence of deposition bath temperature for p-electrodeposited-Cu2O onto n-TiO2/ZnO bilayer thin films

The synthesis of semiconductor materials is crucial to produce the best properties of heterojunction thin films solar cell. Titanium dioxide (TiO2) is coupled with zinc oxide (ZnO) to promote high transmittance due to low utilization of the solar spectrum. In this project, p-Cu2O as absorbing layer was deposited onto n-TiO2/ZnO bilayer thin film which acts as window layer in order to complete the device. Several bath temperatures of 30, 40, 50 and 60 °C were manipulated during Linear Sweep Voltammetry (LSV) measurement and electrodeposition process for p-Cu2O thin film. The LSV measurement was employed to determine the potential value and electrochemical properties of p-Cu2O thin film. Based on findings, the suitable deposition potential chosen is − 0.4 V vs Ag/AgCl. The p-Cu2O thin film deposited at bath temperature of 40 °C exhibited the highest diffraction peak intensity in the (111)-preferred plane orientation showing high crystallinity. The surface morphology of homogeneous and well-defined triangular shape with thickness of 4.93μm was observed. The estimated bandgap energy of 1.92 eV acquired indicated to p-Cu2O thin film. The n-TiO2/ZnO bilayer/p-Cu2O heterojunction thin film was successfully fabricated and showed significant electrical rectification properties. Here in, bath temperature influenced to the enhancement of heterointerface growth development in several properties.

Growth Process, Structure and Electronic Properties of Cr2GeC and Cr2-xMnxGeC Thin Films Prepared by Magnetron Sputtering

The growth and phase formation features, along with the influence of structure and morphology on the electronic, optical, and transport properties of Cr2GeC and Cr2-xMnxGeC MAX phase thin films synthesized by magnetron sputtering technique, were studied. It was found that the Cr:Ge:C atomic ratios most likely play the main role in the formation of a thin film of the MAX phase. A slight excess of carbon and manganese doping significantly improved the phase composition of the films. Cr2GeC films with a thicknesses exceeding 40 nm consisted of crystallites with well-developed facets, exhibiting metallic optical and transport properties. The hopping conduction observed in the Cr2-xMnxGeC film could be attributed to the columnar form of crystallites. Calculations based on a two-band model indicated high carrier concentrations N, P and mobility μ in the best-synthesized Cr2GeC film, suggesting transport properties close to single crystal material. The findings of this study can be utilized to enhance the growth technology of MAX phase thin films.

Physical Delithiation of Epitaxial LiCoO2 Battery Cathodes as a Platform for Surface Electronic Structure Investigation.

We report a novel delithiation process for epitaxial thin films of LiCoO2(001) cathodes using only physical methods, based on ion sputtering and annealing cycles. Preferential Li sputtering followed by annealing produces a surface layer with a Li molar fraction in the range 0.5 < x < 1, characterized by good crystalline quality. This delithiation procedure allows the unambiguous identification of the effects of Li extraction without chemical byproducts and experimental complications caused by electrolyte interaction with the LiCoO2 surface. An analysis by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) provides a detailed description of the delithiation process and the role of O and Co atoms in charge compensation. We observe the simultaneous formation of Co4+ ions and of holes localized near O atoms upon Li removal, while the surface shows a (2 × 1) reconstruction. The delithiation method described here can be applied to other crystalline battery elements and provide information on their properties that is otherwise difficult to obtain.

Atomic layer deposition of calcium fluoride for barrier coating applications

We report on the development of an atomic layer deposition (ALD) process for calcium fluoride thin films using bis(N,N-di-i-propylformamidinato)calcium(II) and anhydrous HF as reactants. Deposition rates were observed to be linear versus the number of ALD cycles performed, varying between 0.4 and 0.3 Å/cycle at substrate temperatures ranging from 175 to 250 °C, respectively. The optical properties of the resulting nanocrystalline CaF2 films were characterized by spectroscopic ellipsometry and show good transparency down to a UV wavelength of 193 nm, and good agreement with reference refractive index parameters. One motivation for the development of this ALD process is for chemical barrier applications in fluorine-containing chemistries. The robustness of the coating was tested by reactive ion etching of CaF2 films deposited on silicon in an SF6 plasma, yielding an Si:CaF2 etch ratio of greater than 2000:1 at a plasma power of 300 W.

Direct laser writing of 3D electrodes on flexible substrates

This report describes a 3D microelectrode array integrated on a thin-film flexible cable for neural recording in small animals. The fabrication process combines traditional silicon thin-film processing techniques and direct laser writing of 3D structures at micron resolution via two-photon lithography. Direct laser-writing of 3D-printed electrodes has been described before, but this report is the first to provide a method for producing high-aspect-ratio structures. One prototype, a 16-channel array with 300 µm pitch, demonstrates successful electrophysiological signal capture from bird and mouse brains. Additional devices include 90 µm pitch arrays, biomimetic mosquito needles that penetrate through the dura of birds, and porous electrodes with enhanced surface area. The rapid 3D printing and wafer-scale methods described here will enable efficient device fabrication and new studies examining the relationship between electrode geometry and electrode performance. Applications include small animal models, nerve interfaces, retinal implants, and other devices requiring compact, high-density 3D electrodes.

Latent and controllable doping of stimuli-activated molecular dopants for flexible and printable organic thermoelectric generators

Conjugated polymers (CPs) are a promising class of materials for organic thermoelectric generators (OTEGs); however, achieving high electrical conductivity through molecular doping while maintaining compatibility with thin-film printing processes remains a huge challenge. In this paper, we present a novel doping strategy using stimuli-activated molecular dopants (SAMDs) based on photoacid generators (PAGs) that can be activated by light of a specific wavelength. We demonstrate that this approach can effectively control the doping efficiency and optoelectronic properties of CP-PAG-blended thin films, resulting in a wide range of electrical conductivities. Our selected PAG molecules enabled efficient printing of the CP-PAG mixed solution and yielded a high thermoelectric figure of merit. To elucidate the mechanism behind this high thermoelectric performance, we systematically investigated the morphologies, microstructures, and energy structures of the PAG-doped CP thin films and performed various comparative tests. We also demonstrate the feasibility of using SAMDs to print flexible OTEG modules on thin polyimide substrates. We believe that our work represents a significant step toward the development of efficient, scalable, and sustainable thermoelectric devices for power generation and waste heat recovery, and highlights the advantages of PAG-based SAMDs for printable organic thermoelectrics.

Hole-initiated melting process of thin films.

We perform numerical and experimental studies on the melting process of thin films initiated by a small hole in the continuum regime. The presence of a nontrivial capillary surface, namely the liquid/air interface, leads to a few counterintuitive results: (1) The melting point is elevated if the film surface is partially wettable, even with a small contact angle. (2) For a film that is finite in size, melting may prefer to start from the outer boundary rather than a hole inside. (3) More complex melting scenarios may arise, including morphology transitions and the "de facto" melting point being a range instead of a single value. These are verified by experiments on melting alkane films between silica and air. This work continues a series of investigations on the capillary aspects of melting. Both our model and analysis approach can be easily generalized to other systems.

Solution-Processed 2D Transition Metal Dichalcogenides: Materials to CMOS Electronics

ConspectusTwo-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) have demonstrated exceptional potential as materials for future complementary metal-oxide-semiconductor (CMOS) technology. This is primarily because of their atomic thickness and excellent electrical and mechanical properties. With advancements in fabrication technology, electronic devices based on 2D TMD materials have rapidly progressed from isolated units for scientific experimentation to integrated circuits with practical applications. Among the different production methods, the solution-processing of 2D TMD nanomaterial dispersions offers the distinct advantages of low-temperature processing and cost-effective manufacturing for large-scale flexible and wearable electronics. A wide range of 2D nanoflake inks with versatile electronic properties can be assembled into atomic-thick thin films with dangling-bond-free van der Waals interfaces between adjacent nanoflakes. Furthermore, direct printing techniques can easily integrate multifunctional devices, such as n-type and p-type transistors, into CMOS devices and more complex integrated circuits. Despite these benefits and previous accomplishments, the field of solution-processed CMOS electronics using 2D TMD semiconducting materials is in its early stages of development and requires further research. One of the current challenges is the production of scalable and high-purity 2D semiconductor mono- and few layers with large lateral sizes and narrow thickness distribution. The field-effect mobility of solution-processed 2D TMD transistors remains lower than that of the transistors manufactured using mechanical exfoliation and chemical vapor deposition methods. In particular, limited research has been conducted on solution-processed p-type 2D TMD transistors. As a result, solution-processed CMOS devices using n-type and p-type 2D TMD transistors are scarce. In this Account, we provide an overview of the recent progress in the field of solution-processed CMOS electronics employing 2D TMD materials. First, we introduce the basic liquid exfoliation methods, such as sonication-assisted exfoliation and molecular intercalation methods, that are commonly utilized to prepare 2D TMD dispersions. In addition, we discuss the production of monolayer 2D materials, which serve as the building blocks for fabricating atomic-thick thin films. Subsequently, we review the typical techniques for depositing 2D inks, including spin coating, drop casting, and inkjet printing. Furthermore, we outline the thin-film patterning process for each technique, which is crucial for integrating multifunctional materials in CMOS devices. Subsequently, we focus on the recent advancements in solution-processed 2D TMD transistors. Furthermore, we explore the various factors that can improve the performance of the devices with regard to charge transport and charge traps. Afterward, we highlight notable applications of solution-processed CMOS technology, such as logic circuits and ring oscillators. Finally, we provide an overview of the challenges and opportunities in the development of solution-processed 2D materials and the integration of multifunctional devices for the advancement of CMOS electronics. This Account aims to provide a comprehensive guide for readers, offering both a broad overview and an in-depth insight into solution-processed 2D material-based electronics, covering a wide range of topics from the preparation of 2D TMD ink to device fabrication and CMOS applications. Therefore, this Account is expected to drive further progress and advancements in this field and promote the realization of practical applications.

Optimized Thin Film Processing of Sodium Mixed Oxy-Sulfide-Nitride Glassy Solid Electrolytes for All-Solid-State Batteries

Na4P2S7–6xO4.62xN0.92x (NaPSON) glassy solid electrolytes (GSEs) were prepared and tested for their electrochemical properties and processability into thin films. The x = 0.2 composition (NaPSON-2) was found to be highly conducting, non-crystallizable, largely stable against Na-metal, and supports symmetric cell cycling up to >100 μA cm–2 without shorting. For these reasons, it was processed into thin films drawn to 50 μm and tested in symmetric and asymmetric cells. Measurements of the sodium ion conductivity using symmetric cells demonstrated that the conductivity of NaPSON-2 was unchanged by film forming. Galvanostatic cycling at 5 μA cm–2 of a 1.3 mm thick disc of NaPSON-2 showed stability over 450 h, while cycling a 50 μm thin film of NaPSON-2 showed a very slow growth in the resistance. Cyclic voltammetry and X-ray photoelectron spectroscopy of the NaPSON-2 thin film GSE revealed that it did not react with Na-metal at its surface, but rather in the bulk of the film, showing S, Na2S, and Na3P reaction products. The source of the surface stability was determined to be the preferential segregation of trigonally coordinated nitrogen. These low-cost and easily processed NaPSON GSEs provide a system of materials that could provide for significantly lower cost higher energy density grid-scale batteries.

Optically modified second harmonic generation in silicon oxynitride thin films via local layer heating

Strong second harmonic generation (SHG) in silicon nitride has been extensively studied—among others, in terms of laser-induced SHG enhancement in Si3N4 waveguides. This enhancement has been ascribed to the all-optical poling induced by the coherent photogalvanic effect. Yet, an analogous process for Si3N4 thin films has not been reported. Our article reports on the observation of laser-induced threefold SHG enhancement in Si3N4 thin films. The observed enhancement has many features similar to all-optical poling, such as highly nonlinear power dependence, cumulative effect, or connection to the Si3N4–Si interface. However, identical experiments for low-oxygen silicon oxynitride thin films lead to complex behavior, including laser-induced SHG reduction. Following a thorough experimental study, including the effects of repetition rate or pulse length, the observed results were ascribed to heat-induced SHG variation. In addition to revealing a new mechanism of laser-induced SHG variation, our results also provide a means to identify this mechanism.

Growth mechanisms of interfacial carbon layers at the epitaxial Al2O3(0001)/Cu(111) interface as application for epitaxial film lift-off

The growth of interfacial carbon (IFC) layers at the epitaxial sapphire/metal interface could unlock the transfer process for large-scale epitaxial thin films, such as graphene. To assess the feasibility of the application of IFC, understanding of the growth mechanisms of IFC layers at the epitaxial metal/sapphire interface is required. In this study, atmospheric pressure chemical vapor deposition is used to directly synthesize wafer-scale multilayer carbon at the interface between the Al2O3(0001) substrate and epitaxial Cu(111) layer. Characterization by Raman spectroscopy, AFM, and XRD, indicates the IFC layers exhibit low defect density and sp2 bonding character. Furthermore, IFC as thick as 100 nm could be synthesized. The effect of the partial pressure of reaction gases is systematically investigated, and a growth mechanism is proposed based supported by experimental observations and simulation results. Finally, a mechanical lift-off of a Cu(111) film with the aid of the IFC layer is demonstrated as a proof-of-concept application.

A Quartz Crystal Microbalance (QCM) Study on the Formation of Aqueous κ-Carrageenan-Chitosan Composite Bilayers with NaCl and Graphene Oxide

Biomaterials and thin biofilms play a fundamental role in the medical, food and pharmaceutical industries. The Quartz crystal microbalance (QCM) measurement technique is one of the attractive techniques which have been used for monitoring the thin-film formation process. In the current work described here, κ-carrageenan and chitosan solutions were used for the layer-by-layer (LBL) deposition of polyelectrolyte multilayers (PEMs) on the gold surface of a 5 MHz AT-cut (thickness-shear mode) quartz crystal in a flow module. A sensitive QCM system was designed to detect 0.1 Hz differences in the resonance frequency (Δf) and 10−7 changes in the energy dissipation (ΔD) values, which are the measures of the deposited mass and the film rigidity, respectively. Negatively charged carrageenan and positively charged chitosan biomolecules in the solutions were used to build up sequential and very thin bilayers on the modified gold surface. The effects of NaCl and graphene oxide (GO) addition on the mass of the deposited films were also investigated. Intermolecular interactions between the biopolymer chains, Na+ cations and GO sheets were explained by interactions between molecules, such as electrostatic forces and hydrogen bondings.

High-Frequency Conductivity of Amorphous and Crystalline Sb2Te3 Thin Films

The results of study of charge transfer processes in thin amorphous and crystalline Sb2Te3 films in a wide range of frequencies and temperatures are presented. The frequency spectra of conductivity were obtained by the dielectric spectroscopy method. The authors analyzed the frequency dependences of the conductivity in the electric field and the temperature dependences of the exponent s. A transition from the classical correlated barrier hopping (CBH) to quantum mechanical tunneling (QMT) was observed at a certain temperature Tt. The CBH model allowed the authors to calculate the conductivity parameters of two phases. Two areas with different types of conductivity were revealed on the conduction spectra, and the activation energies of charge transfer processes for amorphous and crystalline films were determined. The following features were discovered: the difference in the temperatures of the change of the charge transfer mechanism and the transition from the semiconductor region to the metal region on the temperature dependence of conductivity. They can help to identify the amorphous phase in the quasi-binary chalcogenide Sb2Te3-GeTe system.

Towards low-temperature processing of lead-free BZT thin films for high-temperature energy storage performance

In this work, lead-free BaZr0.35Ti0.65O3 (BZT) thin films were grown on silicon by using pulsed laser deposition. The phase structures of the BZT thin films were controlled via the deposition temperature, and their effects on the breakdown strength and energy-storage performance were systematically investigated. Our results indicate that the BZT thin films deposited at a low temperature of 450 °C have a paraelectric phase and display the highest recoverable energy density of 121.2 J/cm3 and a medium energy efficiency of 88.2%. The ultrahigh energy storage achieved is mainly due to the remarkable improvement in the breakdown strength and the slimmer and slanted polarization hysteresis loop. Moreover, the BZT thin films demonstrate excellent thermal stability across a wide temperature measurement range (25–200 °C) and a superior fatigue endurance of up to 1010 switching cycles even under a high temperature measurement of 200 °C. This work provides a simple and effective way of improving the energy storage properties and stable performance of dielectric thin films for application in harsh environments.

Excimer laser processing of nanostructured SnO2 thin films and its impact on LPG sensing

KrF excimer laser (λ = 248 nm) has been employed to anneal nanocrystalline Tin oxide (SnO2) films deposited at 200 °C using spray pyrolysis. With the increase of irradiation laser fluence, the film crystallinity has been improved while the matrix stress decreased along all the crystal planes except (110) and (301), for an energy density of 150 mJ/cm2 as compared to non-irradiated films. The maximum liquid petroleum gas (LPG) response observed for a fluence 150 mJ/cm2 is 92.6% at 350 °C whereas a response is 4% at the onset sensing temperature of 100 °C. Moreover, the response and recovery times have also improved by the laser processing and the cross sensitivity for NH3 and NO2 gases are in a desirable range. FESEM images of irradiated films show polygonal shaped distributions of aggregated nanocrystalline particle-clusters with grain boundaries and nanovoids. XPS shows oxygen deficient lattice besides surface and/or pore-bound oxygen contributions. Raman analyses confirm the sub-stoichiometric SnO2-x phases and a high concentration of defects such as oxygen vacancies (OVs) on surface sites. Based on these observations, the gas sensing mechanism is discussed in brief.

Optoelectronic Devices of Large-Scale Transferred All-Inorganic Lead Halide Perovskite Thin Films.

We report the large-scale transfer process for monocrystalline CsPbBr3 thin films prepared by chemical vapor deposition (CVD) with excellent optical properties and stability. The transfer process is robust, simple, and effective, in which CsPbBr3 thin films could be transferred to several substrates and effectively avoid chemical or physical fabrication processes to damage the perovskite surface. Moreover, the transfer process endows CsPbBr3 and substrates with atomically clean and electronically flat interfaces. We utilize this transfer process to realize several optoelectronic devices, including a photonic laser with a threshold of 61 μJ/cm2, a photodetector with a responsivity of 2.4 A/W, and a transistor with a hole mobility of 11.47 cm2 V-1 s-1. High device performances mainly originate from low defects of high-quality single-crystal perovskite and seamless contact between CsPbBr3 and target substrates. The large-scale nondestructive transfer process provides promising opportunities for optoelectronic applications based on monocrystalline perovskites.

Dual Coil Patterned Ultra‐Thin Silicon Film Enable by Double‐Sided Process

AbstractDouble‐sided microfabrication process on an ultra‐thin silicon film has rarely been attempted due to the challenges in terms of the preparation and handling of a thin film in spite of its promising fabrication potentials. Such a process allows for doubling the thin film device density or providing dual functionalities for a thin film depending on whether the front and back sides of a thin film are processed identically or distinctively. Here, a novel double‐sided thin film processing strategy is introduced by realizing a dual coil patterned ultra‐thin silicon film that is working as an actuating or energy harvesting system. Experimentally, a dual coil patterned thin film enabled using the introduced approach shows remarkably enhanced device performance when compared with a single coil patterned counterpart. Furthermore, a multiphysics simulation model is developed and the resultant modeling data validate the experimentally measured performance enhancement. Finally, the structural durability of the thin film upon cyclic loading is tested and its diverse vibration modes are investigated.

Residual Strain Evolution Induced by Crystallization Kinetics During Anti-Solvent Spin Coating in Organic-Inorganic Hybrid Perovskite.

Organic-inorganic hybrid perovskite (OIHP) polycrystalline thin films are attractive due to their outstanding photoelectronic properties. The anti-solvent spin coating method is the most widely used to synthesize these thin films, and the residual strain is inevitably originates and evolves during the process. However, this residual strain evolution induced by crystallization kinetics is still poorly understood. In this work, the in situ and ex situ synchrotron grazing-incidence wide-angle X-ray scattering (GIWAXS) are utilized to characterize the evolution and distribution of the residual strain in the OIHP polycrystalline thin film during the anti-solvent spin coating process. A mechanical model is established and the mechanism of the crystallization kinetics-induced residual strain evolution process is discussed. This work reveals a comprehensive understanding of the residual strain evolution during the anti-solvent spin coating process in the OIHP polycrystalline thin films and provides important guidelines for the residual strain-related strain engineering, morphology control, and performance enhancement.

Study of device characteristics of intrinsic Josephson junction terahertz emitters related to annealing conditions of the crystals

We fabricated terahertz (THz) wave emitters from high-temperature superconductor Bi2Sr2CaCu2O8+δ (Bi2212) single crystals annealed under oxygen gas (O2) flow and nitrogen gas (N2) flow conditions. To better understand the annealing effects of the crystal for the device, we evaluated both device properties and a c-axis lattice constant using x-ray diffraction. Compared to the N2-annealed sample, the O2-annealed sample shows higher critical current in the current–voltage characteristics and no clear emission. In addition, multiple hysteresis loops were observed above 75 K. Based on the x-ray diffraction measurements, it is suggested that the presence of multiple hysteresis loops observed in the I–V characteristics of the O2-annealed sample is caused by the existence of layers that have varying levels of oxygen content along the c-axis direction of the crystal. The formation of these layers is attributed to the deposition process of metallic thin films during the device fabrication procedure. This result indicates that the Bi2212 crystal surface of the O2-annealed sample is more sensitive than that of the N2-annealed one. The information is useful for preparing the Bi2212 crystals for THz-wave emitting devices.