Thin Film Processing Research Articles

Comparative Study of High-Temperature Annealed and RTA Process β-Ga2O3 Thin Film by Sol–Gel Process

As a wide energy band gap semiconductor, a Ga2O3 thin film was prepared by the sol–gel process with different annealing processes. Since Ga2O3 is a type of metal oxide structure, an oxygen annealing process can be considered to remove oxygen defects. An effective oxygen annealing process can help form a β-Ga2O3 structure with reduced defects. In this study, different types of annealing effects for β-Ga2O3 were investigated and compared. An electric furnace process using thermal effect characteristics of and an Rapid Thermal Annealing (RTA) process applied with an infrared radiation light source were compared. Two and 4 h thermal annealing processes were conducted at 900 °C in the furnace. Meanwhile, to study the optical annealing effects, 2 h furnace at 900 °C + 15 min in rapid thermal annealing and only 15 min in rapid thermal annealing effects were compared, respectively. Through increasing the thermal annealing temperature and time, β-Ga2O3 can be formed even though a sol–gel process was employed in this experiment. An annealing temperature of at least 900 °C was required to form β-Ga2O3 thin film. Moreover, by introducing an RTA process just after the spinning process of thin film, a β-Ga2O3 thin film was formed on the sapphire substrates. Compared with the electric furnace process applied for 2 h, the RTA process performed in 15 min has a relatively short process time and results in similar structural and optical characteristics of a thin film. From the X-ray diffraction patterns and UV spectrometer analysis, optically annealed β-Ga2O3 thin films on the sapphire substrate showed a highly crystalized structure with a wide energy band gap of 4.8 eV.

Acceptor Modulation Strategies for Improving the Electron Transport in High-Performance Organic Field-Effect Transistors.

High-performance ambipolar and electronic type semiconducting polymers are essential for fabricating various organic optoelectronic devices and complementary circuits. This review summarizes the strategies of improving the electron transport of semiconducting polymers via acceptor modulation strategies, which include the use of single, dual, triple, multiple, and all acceptors as well as the fusion of multiple identical acceptors to obtain new heterocyclic acceptors. To further improve the electron transport of semiconducting polymers, the introduction of strong electron-withdrawing groups can enhance the electron-withdrawing ability of donors and acceptors, thereby facilitating electron injection and suppressing hole accumulation. In addition, the relationships between the molecular structure, frontier molecular orbital energy levels, thin film morphology, microstructure, processing conditions, and device performances are also comprehensively discussed. Finally, the challenges encountered in this research area are proposed and the future outlook is presented.

Electrical and Optical Properties of Aluminum-Doped Zinc Oxide Films on a Glass Substrate Prepared by a Toroidal Magnetron Sputtering System at Low Temperature

Aluminum-doped zinc oxide (AZO) thin films were prepared on a glass substrate by employing a toroidal magnetron sputtering system to enhance both resistivity and transparency. The glass substrate temperature was sustained below 50 °C during the deposition of 200 nm thick AZO without an additional heating system for the substrate, and no pre- or postannealing process was carried out. This scenario is quite desirable to reduce damage to the thermally sensitive substrate during the conventional AZO thin film process; nonetheless, significant crystallinity was observed in the XRD analysis due to energy transfer by sputtered ions. The electrical resistivity was approximately 8.3×10−4 Ωcm, and the average transmittance was 90.43%, which were achieved under optimized conditions, with the ratio between the O2 and Ar gas flow rates varied under a specific applied DC power.

In Situ X-ray Measurements to Follow the Crystallization of BaTiO3 Thin Films during RF-Magnetron Sputter Deposition

Here, we report on adding an important dimension to the fundamental understanding of the evolution of the thin film micro structure evolution. Thin films have gained broad attention in their applications for electro-optical devices, solar-cell technology, as well storage devices. Deep insights into fundamental functionalities can be realized via studying crystallization microstructure and formation processes of polycrystalline or epitaxial thin films. Besides the fundamental aspects, it is industrially important to minimize cost which intrinsically requires lower energy consumption at increasing performance which requires new approaches to thin film growth in general. Here, we present a state of the art sputtering technique that allows for time-resolved in situ studies of such thin film growth with a special focus on the crystallization via small angle scattering and X-ray diffraction. Focusing on the crystallization of the example material of BaTiO3, we demonstrate how a prototypical thin film forms and how detailed all phases of the structural evolution can be identified. The technique is shaped to enable a versatile approach for understanding and ultimately controlling a broad variety of growth processes, and more over it demonstrate how to in situ investigate the influence of single high temperature sputtering parameters on the film quality. It is shown that the whole evolution from nucleation, diffusion adsorption and grain growth to the crystallization can be observed during all stages of thin film growth as well as quantitatively as qualitatively. This can be used to optimize thin-film quality, efficiency and performance.

Three-wavelength white organic light-emitting diodes on silicon for high luminance and color gamut microdisplays

Organic light-emitting diode (OLED) microdisplays have attracted considerable attention owing to their low power consumption, rapid response, high contrast ratio, and self-luminescence. To expand the use of OLED microdisplays, high luminance and vivid color are required. Herein, we developed a high-luminance, low-voltage, three-wavelength, top-emitting white OLED (WOLED) using a silicon substrate for complementary metal–oxide–semiconductor-based, full-color microdisplay applications. Optimal thickness of the emitting layer and thickness and position of the interlayer were determined to improve device performance. The device exhibited luminance of ∼4,200 cd/m2 at 3.3 V, and the maximum luminance reached 170,000 cd/m2 at 11.4 V. In addition, red, green, and blue color filters were deposited onto the WOLED after the thin-film encapsulation process to verify feasibility of the device for full-color microdisplay applications. The color gamut of the developed WOLED with color filters was 136% compared to sRGB. The maximum luminance and color gamut of the developed device is among the highest values obtained to date for OLED microdisplays.

Intermediate Phase-Free Process for Methylammonium Lead Iodide Thin Film for High-Efficiency Perovskite Solar Cells.

Solvent engineering by Lewis‐base solvent and anti‐solvent is well known for forming uniform and stable perovskite thin films. The perovskite phase crystallizes from an intermediate Lewis‐adduct upon annealing‐induced crystallization. Herein, it is explored the effects of trimethyl phosphate (TMP), as a novel aprotic Lewis‐base solvent with a low donor number for the perovskite film formation and photovoltaic characteristics of perovskite solar cells (PSCs). As compared to dimethylsulfoxide (DMSO) or dimethylformamide (DMF), the usage of TMP directly crystallizes the perovskite phase, i.e., reduces the intermediate phase to a negligible degree, right after the spin‐coating, owing to the high miscibility of TMP with the anti‐solvent and weak bonding in the Lewis adduct. Interestingly, the PSCs based on methylammonium lead iodide (MAPbI3) derived from TMP/DMF‐mixed solvent exhibit a higher average power conversion efficiency of 19.68% (the best: 20.02%) with a smaller hysteresis in the current‐voltage curve, compared to the PSCs that are fabricated using DMSO/DMF‐mixed (19.14%) or DMF‐only (18.55%) solvents. The superior photovoltaic properties are attributed to the lower defect density of the TMP/DMF‐derived perovskite film. The results indicate that a high‐performance PSC can be achieved by combining a weak Lewis base with a well‐established solvent engineering process.

Non-linear convective flow of the thin film nanofluid over an inclined stretching surface

To enhance the surface properties of solids the mechanism of thin films is frequently used. Penetration, degradation, stiffness, illumination, diffusion, absorption, and electric performance are all characteristics of a bulk substance medium that a thin film can improve. In nanotechnology, thin film processing can be extremely useful. Therefore, the time-dependent nonlinearly convective stream of thin film nanoliquid over an inclined stretchable sheet with magnetic effect is investigated in current work. The features of mass and heat transport processes are explained using important factors like thermophoresis and Brownian movement. Nonlinear partial differential equations are obtained to model the time-dependent liquid film flow over an inclined surface, which are then turned into couple ordinary differential equations utilizing appropriate alterations. The results of the computation of the model problem are collected using an analytical approach Homotopy Analysis Method and presented the final finding numerically and graphically. During the flow assessment, the impact of individual flow factors such as magnetic, Brownian, and thermophoresis parameters on regular profiles (temperature, velocity, and concentration) are analyzed and found to be quite remarkable. Furthermore, the consequence of M and Nt factors on the velocity, concentration and thermal distribution leads to diminishing conduct. On the other hand, the thermal profile of the liquid film rises in response to the thermophoresis factor. The % wise variation in the skin friction, Nusselt number and Sherwood number versus physical parameters has been obtained and discussed.

Aqueous solution-deposited aluminum-gallium-oxide alloy gate dielectrics for low voltage fully oxide thin film transistors

Different to conventional high-κ gate dielectric fabrication that usually generates porosity and pinhole sites when evaporating solvents or impurities in the thin-film formation process, herein, we report a simple aqueous route to deposit aluminum-gallium-oxide (AGO) alloy gate dielectrics. Compared to GaOx dielectric, higher performance and aqueous solution-processed low voltage fully oxide thin film transistors (TFTs) are achieved based on the AGO dielectric films. The solution-processed IZO(300 °C)/AGO TFT with optimal performance shows a good charge carrier saturation mobility of 55.4 cm2 V−1 s−1, an on/off current ratio of ∼104, threshold voltage of 0.1 V, and a low operation voltage of 5 V. Our study represents a significant step toward the development of low-cost, easy-control, and large-area oxide electronics.

Thin film processing of multiferroic BiFeO3: From sophistication to simplicity. A review

The obtaining of BiFeO3 in the form of a thin film represents a critical issue for its application in electronic devices since this is the required geometry for the integration of this material in microelectronic circuits. There are several techniques for this purpose, from those that use a gas or plasma phase to transport the precursors to the substrate, which would be included within the PVD or CVD techniques (for its acronym Physical Vapor Deposition and Chemical Vapor Deposition) to those that use a phase liquid for this transport, which would be included under the CSD (Chemical Vapor Deposition) techniques. However, among the large number of published papers, there is much controversy about which of them would be most suitable. It has been clearly demonstrated that all of them have the sufficient capacity to produce uniform and homogeneous thin films in thickness throughout the entire substrate, and although in many articles pure BiFeO3 films have been obtained with an exploitable functional response, others have reported the typical drawbacks that usually entails the obtaining this material: appearance of secondary phases, high leakages or a poor functional response. Nevertheless, there is a common aspect in the specialized literature that seems to be in agreement: the first group of techniques require very sophisticated equipment that involves high energy consumption in terms of temperature and pressure (vaccum), while the techniques that are based on solutions are characterized by their higher simplicity. In this context, the purpose of the present review is to summarize the main aspects of each technique in the obtaining of BiFeO3 thin films, from those that are more sophisticated to the simplest and environmentally benevolent ones, in order to provide an easier understanding of them.

Performance Improvement of Sb Phase Change Thin Film by Y Doping

Yttrium-doped Sb phase change thin films were prepared by magnetron sputtering and the in-situ resistance temperature measurement system was used to study the process of phase change thin films. With the addition of yttrium atoms, the crystallization temperature of Sb thin film increases, indicating improvement of their thermal stability and data retention ability. Furthermore, yttrium-doped Sb thin films have higher crystalline and amorphous resistance, which is conducive to the phase transition by Joule heating under electric pulse current. The optical band gap energy increases after yttrium doping, which means lower conductivity and higher conduction activation energy. X-ray diffraction shows that the doping of yttrium atoms can inhibit the grain growth and refine the grain size. X-ray photoelectron spectroscopy was used to examine the chemical state of the element, indicating the formation of a new Y-Sb bond, which verifies the improvement of stability. According to atomic force microscopy images, the yttrium-doped Sb material exhibits a surface morphology with less roughness, implying better interfacial properties and reliability. In general, yttrium-doped Sb thin films have better development in the application of phase change memory.

ANFIS-based Models for Coating Quality Prediction for Thin-Film Deposition Processes

Thin-film deposition processes have gained much popularity due to their unique capability to enhance the physical and chemical properties of various materials. Identification of the best parametric combination for a deposition process to achieve desired coating quality is often considered to be challenging due to the involvement of a large number of input process parameters and conflicting responses. This study discusses the development of adaptive neuro-fuzzy inference system-based models for the prediction of quality measures of two thin-film deposition processes, i.e., SiCN thin-film coating using thermal chemical vapor deposition (CVD) process and Ni–Cr alloy thin-film coating using direct current magnetron sputtering process. The predicted response values obtained from the developed models are validated and compared based on actual experimental results which exhibit a very close match between both the values. The corresponding surface plots obtained from the developed models illustrate the effect of each process parameter on the considered responses. These plots will help the operator in selecting the best parametric mix to achieve enhanced coating quality. Also, analysis of variance results identifies the importance of each process parameter in the determination of response values. The proposed approach can be applied to various deposition processes for modeling and prediction of observed response values. It will also assist as an operator in selecting the best parametric mix for achieving desired response values.

Solution processing of chalcogenide glasses: A facile path towards functional integration

Chalcogenide Glasses (ChGs) are technologically promising materials widely used in different optical, electronic and optoelectronic applications. Solution processing of ChGs has provided a pathway for less complex thin film processing of these materials and their integration into flexible devices. This paper reviews the basic science behind the solution processing of ChGs, properties of solution processed ChG films and recent functional applications of solution processed ChGs. Solution processing of ChGs also provides the facile method of incorporation of nanostructures into ChG matrix which integrate the functional properties of nanostructures and ChGs in a single solution or thin film. This paper also reviews the research work undertaken on the incorporation of nanostructures in ChGs. • This paper presents a review of synthesis, properties and applications of Chalcogenide glass (ChG) films obtained by solution processing. • It discusses the chemistry involved in the processing of glasses, which helps in understanding the control over stoichiometry of ChG. • This review also provides insights on some recent application of solution processed chalcogenide glasses. • A review on the research work done on the incorporation of nanostructures in ChGs is also presented.

Real-Time Observation of Surface Chemical Reactions Proceeding in the Depth Direction by Wavelength-Dispersive Soft X-ray Absorption Spectroscopy.

It has long been a challenging task to observe surface chemical reactions proceeding in the depth direction without stopping the reaction (i.e., real-time observation), with subnanometer depth resolution. X-ray absorption spectroscopy (XAS) in the soft X-ray region is one of the most crucial methods to analyze the chemical states of light elements; however, simultaneous time- and depth-resolved XAS measurements during the reaction have not been realized before. Herein, we show the real-time observation of the time-evolving oxidation process of a Co thin film with a fluorescence-yield wavelength-dispersive XAS method, which also has subnanometer depth resolution. The series of Co L-edge XAS spectra show that the oxide component increases from the surface to deeper regions as the reaction proceeds. The progress of oxidation in the depth direction was interpreted by the reaction with O2 gas at the surface and the interlayer reaction by oxygen migration.

Terahertz Rectennas on Flexible Substrates Based on One-Dimensional Metal–Insulator–Graphene Diodes

Flexible energy harvesting devices fabricated in scalable thin-film processes are important components in the field of wearable electronics and the Internet of Things. We present a flexible rectenna based on a one-dimensional junction metal-insulator-graphene diode, which offers low-noise power detection at terahertz (THz) frequencies. The rectennas are fabricated on a flexible polyimide film in a scalable process by photolithography using graphene grown by chemical vapor deposition. A one-dimensional junction area reduces the junction capacitance and enables operation in the D-band (110 - 170 GHz). The rectenna on polyimide shows a maximum voltage responsivity of 80 V/W at 167 GHz in free space measurements and minimum noise equivalent power of 80 pW/$\sqrt{\text{Hz}}$.

Low-temperature electrical characteristics of nanocrystalline BaTiO3 thin films for cryogenic applications

Ferroelectric thin films are vital in a large number of applications due to their promising properties. Although ferroelectric films' electrical response is studied at high temperatures, their low-temperature behavior is not well understood. In the present study, the εr-V and I-V characteristics of BaTiO3 (BTO) thin films are explored at cryogenic temperatures. BTO thin films have systematically deposited different Ar/O2 ratios by RF sputtering and investigated their structural, microstructural, optical, and electrical properties. The microstructures of these films are well-packed with uniform grain growth and revealed lower rms roughness. The thickness of the films (~170 nm) is obtained by cross-sectional scanning electron microscope images and transmittance spectra. There is not much variation in the optical bandgap, which shows the full stoichiometry and similar thickness of the films. The εr-V response of the film measured at 387 K shows the asymmetry in dielectric constant maxima value. It signifies that there are movable ions and charge accumulation at the interface of film and electrodes. The dielectric response obtained as a temperature function exhibited a Tc of 387 K, indicating the phase transition from ferroelectric tetragonal phase to paraelectric cubic phase. The I-V characteristics of the film measured at 300 K revealed the leakage current of 10−6 Amp, whereas, at 43 K, it is 8.83 × 10−10 Amp. The slope of the I-V curve of the film measured between 1 and 5 V displayed the Ohmic nature. The slope of the film obtained at 6–10 V indicated the space charge limited conduction mechanism, which corresponds to discrete trap carriers. Therefore, trap carriers assisted discrete conduction mechanism is one of the leading factors controlling the conduction process in BTO oxide thin films. The acquired electrical response of the BTO films deposited at 50 Ar is promising for cryogenic devices.

All-Solid-State Thin Film μ-Batteries for Microelectronics.

Continuous advances in microelectronics and micro/nanoelectromechanical systems enable the use of microsized energy storage devices, namely solid‐state thin‐film μ‐batteries. Different from the current button batteries, the μ‐battery can directly be integrated on microchips forming a very compact “system on chip” since no liquid electrolyte is used in the μ‐battery. The all‐solid‐state battery (ASSB) that uses solid‐state electrolyte has become a research trend because of its high safety and increased capacity. The solid‐state thin‐film μ‐battery belongs to the family of ASSB but in a small format. However, a lot of scientific and technical issues and challenges are to be resolved before its real application, including the ionic conductivity of the solid‐state electrolyte, the electrical conductivity of the electrode, integration technologies, electrochemical‐induced strain, etc. To achieve this goal, understanding the processing of thin films and fundamentals of ion transfer in the solid‐state electrolytes and hence in the μ‐batteries becomes utmost important. This review therefore focuses on solid‐state ionics and provides inside of ion transportation in the solid state and effects of chemistry on electrochemical behaviors and proposes key technology for processing of the μ‐battery.

Nanoscale Self-Assembly of Poly(3-hexylthiophene) Assisted by a Low-Molecular-Weight Gelator toward Large-Scale Fabrication of Electrically Conductive Networks

Achieving self-assembly of conjugated polymers is necessary to harness their charge transport properties in various applications, including field-effect transistors, sensors, and conductive gels for biomedical applications. Although many processes have been investigated, there are still opportunities for developing new strategies that can lead to materials with improved performances. Particularly, large-scale fabrication of three-dimensional conductive networks formed by the self-assembly of conjugated polymers and low-molecular-weight gelators (LMWGs), but with conjugated polymers at much lower quantity, would be advantageous. LMWGs can be selected from an extensive library of available systems and can be directed to self-assemble in various conditions. However, the simultaneous self-assembly of LWMGs and conjugated polymers is not fully understood. Here, we report a simple pathway for the self-assembly of poly(3-hexylthiophene) (P3HT), a conjugated polymer, in chloroform in the presence of di-Fmoc-l-lysine, an LMWG. Di-Fmoc-l-lysine was selected as the LMWG because it does not have significant interactions with P3HT. P3HT and di-Fmoc-l-lysine in chloroform form gels with decreasing temperature. UV–vis spectroscopy provides an insight into the photophysical response of the gelation process, revealing the self-assembly of P3HT in the gel network. The scattering experiments further capture the self-assembly of the P3HT network. The nanofibrillar microstructure has been captured using atomic force microscopy (AFM) for the gels without and with P3HT, where both P3HT and di-Fmoc-l-lysine form nanofibers independently. Both these nanofibers coexist and intermingle, displaying conductive domains in the dried films captured by conductive AFM. The conductive nanofibers form a percolated network in the dried samples, leading to bulk electrical conductivity similar to that of pristine P3HT films. This is achieved with only 20% P3HT content and the balance insulating di-Fmoc-l-lysine molecules. Our results provide a fundamental understanding of the self-assembly of P3HT in the presence of an LMWG, resulting in a conductive nanofibrillar network. Such knowledge can readily be implemented in other conjugated polymeric systems. The approach presented here has potential applications towards fabricating conductive gels for biomedical and sensor applications and large-scale processing of thin films for optoelectronic applications.

Thin Film Properties of Thermally Stable Protic Ionic Liquids

Abstract Ionic liquids (ILs) have aroused particular interest as potential lubricants because their thermal stabilities are quite superior to those of conventional lubricants, however, their thin film properties have not been investigated very much. Newly synthesized diazabicycloundecenium (DBU) and imidazolium (IM) sulfonates were evaluated as a potential molecularly thin film lubricant for carbon-coated media. The ILs have steady low friction coefficients even after a heat treatment at 250 °C at which those of the conventional lubricant, Z-DOL, steeply increased. A systematic analysis of the thin films was performed using FTIR-RAS and their thermal degradation kinetics were examined. The thermal degradation process of the IL thin films could be described by a zero-order kinetics mechanism. The calculated activation energies show that the ILs physiosorb on the surface and the thermal stabilities of the IL films are significantly lower than that of the bulk. The FTIR measurements indicated two friction reduction regimes for the ILs; i.e., the perfluoroalkylsulfonate molecules, which seem to be a thermally decomposed product from the IL molecules, still remained and oriented parallel to the surface in the temperature range of 200 °C or higher and reduced the friction.

Morphology dependent effective charge separation process in nanostructured MoS2 thin films for enhanced photodegradation behavior

2D-layered nanostructure-based advanced coatings are the most recent advancement in the field of advanced oxidation processes. Effective charge separation is one of the key parameters in enhancing the photodegradation ability of 2D-layered nanostructures. We report the cost-effective hydrothermal synthesis of MoS2 thin films on conductive ITO substrates with tunable morphology. Scanning electron microscopy studies manifested the modulation in the morphology of MoS2 thin films and confirmed the presence of spherical, thread, and sheet-like nanostructures under the evolution of precursor concentration. UV–visible absorption spectroscopy revealed the modulation in the optical absorption and effective bandgap narrowing with the changes in the surface morphology. The estimated bandgap value of MoS2 thin film samples varies from 1.82 to 1.69 eV. PL spectroscopy and electrochemical studies display the morphology-dependent charge separation behavior and confirm that the lowest recombination rate is attained by thread-like MoS2 nanostructures. Raman spectroscopy and x-ray photoelectron spectroscopy confirms the 2H phase of MoS2 thin films. MoS2 thin films with thread-like structures are found to be the most efficient for sunlight-induced photodegradation activity as compared to other MoS2 samples. Thread-like MoS2 structures containing MoS2 thin film with the highest charge transport properties decompose 85% of 5 µM methylene blue molecules and 90.4% of 5 µM rhodamine B molecule solution in 40 min and 80 min, respectively, under natural sunlight. The prominent charge separation effects on the enhanced photodegradation capabilities of MoS2 thin films due to distinct variations in morphology, which have not been reported till now, are explored precisely.

Alleviating halide perovskite surface defects

Alleviating halide perovskite surface defects