Thin Film Deposition Techniques Research Articles

Correlating surface stoichiometry and termination in SrTiO3 films grown by hybrid molecular beam epitaxy

Hybrid oxide molecular beam epitaxy (hMBE), a thin-film deposition technique in which transition metal cations are delivered using a metal-organic precursor, has emerged as the state-of-the-art approach to the synthesis of electronic-grade complex oxide films with a stoichiometric growth window. However, numerous questions remain regarding the chemical mechanisms of the growth process and the surface properties of the resulting films. To examine these properties, thin film SrTiO3 (STO) was prepared by hMBE using a titanium tetraisopropoxide (TTIP) precursor for Ti delivery and an elemental Sr source on annealed STO and Nb-doped STO substrates with varying TTIP:Sr flux ratios to examine the conditions for the reported stoichiometric growth window. The films were transferred in vacuo to an x-ray photoelectron spectroscopy system to study the surface elemental composition. Samples were examined using x-ray diffraction to compare our surface sensitive results with previously reported measurements of the bulk of the films in the literature. Ex situ studies by atomic force microscopy, scanning tunneling microscopy, and low-energy electron microscopy confirmed the presence of surface reconstructions and an Ehrlich–Schwoebel barrier consistent with A-site SrO termination. We find that a surface exhibiting a mixture of SrO and TiO2 termination or a full SrO termination is necessary to obtain stoichiometric adsorption-controlled growth. These results indicate that surface Sr is necessary to maintain the chemical equilibrium for stoichiometric growth during the hMBE process, which is important for the design of future interfacial systems using this technique.

Alleviating halide perovskite surface defects

Alleviating halide perovskite surface defects

Microwave Irradiation Assisted Synthesis of Gallium Aluminum Oxide Using Metal Acetylacetonate Precursors, Peculiar Observations and Safety Guidelines for Young Scientists

Microwave-assisted nano-particle synthesis has seen a boom in recent decade. Different variants are being tested all over the world, be it microwave-irradiation-assisted solvothermal synthesis, thin-film deposition using microwave plasma-assisted CVD, microwave-assisted combustion synthesis, or mist CVD. This technology induces self-catalysis, faster reaction time, less energy and is considered safer from other purely chemical variants. Gallium-Aluminum Oxide is an oxide of vital importance. Gallium Oxide in its most stable form, β-Ga2O3, has a band-gap of ∼4.8 eV and cannot be used for deep ultra-violet (UV) applications (i.e., below 250 nm). Aluminum oxide is a wide band-gap material. The ionic radius of Aluminum ions is similar to that of Gallium ions. Therefore, according to Hume-Rothery rule, Aluminum ion can substitute Gallium in its lattice structure and vice-versa. This gives the possibility of band-gap engineering from 4.6 eV (band-gap, β-Gallium Oxide) to 8.8 eV (band-gap, α-Aluminum Oxide), extending the application of Gallium Oxide to deep UV applications. In present work, three different compositions of Gallium-Aluminum Oxide thin films (varied by mass) are deposited, using microwave-irradiation-assisted solvothermal thin-film deposition technique and metal acetylacetonate precursors. A stoichiometric ratio of ∼5:1 is obtained for 90:100 variation. For 50:50 and 10:90, the stoichiometric ratio remained close to 5:5 and 1:9, respectively, showing good agreement with the Gallium to Aluminum ratio in precursor solution. The investigation is done by using energy dispersive X-Ray analysis. A particle variation from 5 nm to 500 nm was obtained as calculated from Field Emission Scanning Electron Microscopy (FESEM) results. In second part of the work, the corresponding author wants to strongly argue that “Microwave in kitchen is not same as microwave in lab” and report some of the observations from lab to pave a way towards a truly Green or safer technology.

A facile and combined methodology to fabricate sputtered thin film micro-patterns for heater/sensor applications utilizing CO2 laser-cut masks

ABSTRACT We propose a facile and combined methodology to develop magnetron sputtered various metals and alloy (e.g. tungsten, tin and nichrome), and metal oxide (indium tin oxide) thin film-based micro-patterns deposited on flexible and rigid substrates utilizing reusable patterned mask. CO2 laser-cut glass-based mask is fabricated and assembled with the substrates for pattern printing by sputtered thin film deposition technique. Fabrication of mask is adopted with the rapid CO2 laser-cut process aided with computer-aided design software. The laser-cut glass-based mask can easily fix on any type of substrates and any resistive materials deposition can be carried out. Currently, the developed thin film micro-patterns show good uniformity, dense morphology and without sign of any discontinuity. Finally, a successful functional characterization of a typical nichrome line-heater also confirms the feasibility and proof of concept of the present proposed approach that is much facile, less time-consuming and economical.

Observation of exceptional points in helically structured thin films.

Exceptional points (EPs) in the polarization space were observed in reflection on helically structured thin films. These films have form anisotropy at the nanoscale introduced through dynamic control of crystalline growth geometry by changing the orientation of the substrate with respect to the impinging vapor. They are simpler alternatives to metasurfaces, because they can be produced at low cost using conventional thin-film deposition techniques. The EPs were experimentally confirmed by eigenstate swapping on a closed circuit surrounding them and were predicted by numerical calculations. Reflective surfaces operating at an EP could be used to make ultrasensitive sensors.

Nanovesicle Formulation Enhances Anti-inflammatory Property and Safe Use of Piroxicam.

Enhanced utilization of certain drugs may be possible through the development of alternative delivery forms. It has been observed that NSAIDs have adverse gastrointestinal tract effects such as irritation and ulceration during anti-inflammatory therapy. This challenge may be overcome through nano topical formulations. This study aimed to explore the potentials of a transdermal nanovesicular formulation for safe and enhanced delivery of piroxicam (PRX), a poorly water-soluble NSAID. Preformulation studies were conducted using DSC and FTIR. Ethosomal nanovesicular carrier (ENVC) was prepared by thin-film deposition technique using Phospholipon® 90 H (P90H) and ethanol and then converted into gel form. The formulation was characterized using a commercial PRX gel as control. Permeation studies were conducted using rat skin and Franz diffusion cell. Samples were assayed spectrophotometrically, and the obtained data was analyzed by ANOVA using GraphPad Prism software. The preformulation studies showed compatibility between PRX and P90H. Spherical vesicles of mean size 343.1 ± 5.9 nm, and polydispersity index 0.510 were produced, which remained stable for over 2 years. The optimized formulation (PE30) exhibited pseudoplastic flow, indicating good consistency. The rate of permeation increased with time in the following order: PE30 > Commercial, with significant difference (p< 0.05). It also showed higher inhibition of inflammation (71.92 ± 9.67%) than the reference (64.12 ± 7.92%). ENVC gel of PRX was formulated. It showed potentials for enhanced transdermal delivery and anti-inflammatory activity relative to the reference. This may be further developed as a safe alternative to the oral form.

Modelling the Growth and Etch of Thin Film Metal Oxides

HfO2 and ZrO2 are two high-k materials that are crucial in semiconductor devices. Atomic level control of material processing is required for fabrication of thin films of these materials at nanoscale device sizes. Atomic layer deposition (ALD) and thermal atomic layer etching (ALE) allow fabrication of ultra-thin films for semiconductor device processing. ALD is a well-known metal oxide thin film deposition technique in which metal and oxygen containing precursors are added sequentially to the reactor to ensure self-limiting precursor adsorption and reaction to enable a high level of control over film thickness. Thermal ALE, which is a relatively modern technique, uses self-limiting fluorination (using hydrogen fluoride exposure) and subsequent ligand exchange reactions at elevated temperatures to remove up to a monolayer of the metal oxide material. This modern approach for controlled etching is the reverse of ALD and removes only the fluorinated layer.Given that it is difficult to investigate ALD and ALE reactions directly using experimental techniques, first-principles-based atomic-level simulations using density functional theory (DFT) can give deep insights into the precursor chemistry and the reactions that drive the deposition and etch of different materials. This contribution presents first principles density functional theory modelling to examine the growth and etch of thin films of HfO2 and ZrO2. Concerning the ALD reaction, the adsorption mechanism of precursors TDMAHf and TDMAZr are studied at the bare and hydroxylated surfaces of HfO2 and ZrO2 respectively. Stable OH coverages of 0.50 ML and 0.63 ML on the surfaces of HfO2 and ZrO2 were found respectively. Ligand loss from the metal precursor can be achieved by protonation from the OH terminated surface of both metal oxides. Concerning the ALE reaction, HF exposures on the surfaces of HfO2 and ZrO2 are studied. HF coverages ranging from 1.0 ± 0.3 to 17.0 ± 0.3 HF/nm2 are investigated and a mixture of molecularly and dissociatively adsorbed HF molecules is present at higher coverages. Theoretical etch rates of -0.61 ± 0.02 Å /cycle for HfO2 and -0.57 ± 0.02 Å /cycle ZrO2 were calculated using maximum coverages of 7.0 ± 0.3 and 6.5 ± 0.3 M-F bonds/nm2 respectively (M = Hf, Zr).

Ultrathin Diamond Nanofilms-Development, Challenges, and Applications.

Diamond is a highly attractive material for ample applications in material science, engineering, chemistry, and biology because of its favorable properties. The advent of conductive diamond coatings and the steady demand for miniaturization in a plethora of economic and scientific fields resulted in the impetus for interdisciplinary research to develop intricate deposition techniques for thin (≤1000nm) and ultra-thin (≤100nm) diamond films on non-diamond substrates. By virtue of the lowered thickness, diamond coatings feature high optical transparency in UV-IR range. Combined with their semi-conductivity and mechanical robustness, they are promising candidates for solar cells, optical devices, transparent electrodes, and photochemical applications. In this review, the difficulty of (ultra-thin) diamond film development and production, introduction of important stepping stones for thin diamond synthesis, and summarization of the main nucleation procedures for diamond film synthesis are elucidated. Thereafter, applications of thin diamond coatings are highlighted with a focus on applications relying on ultrathin diamond coatings, and the excellent properties of the diamond exploited in said applications are discussed, thus guiding the reader and enabling the reader to quickly get acquainted with the research field of ultrathin diamond coatings.

Solvent Engineering as a Vehicle for High Quality Thin Films of Perovskites and Their Device Fabrication.

Organic-inorganic halide perovskite solar cells (PSCs) have shown a significant growth in power conversion efficiencies (PCEs) during last decade. Progress in device architecture and high-quality perovskite film fabrication has led to an incredible efficiency over 25% in close to a decade. Developments in solution-based thin film deposition techniques for perovskite layer preparation in PSCs provide low cost and ease of process for their manufacturing, making them a potential contender in future solar energy harvesting technologies. From small area single solar cells to large area perovskite solar modules, solvents play crucial roles in thin film quality and therefore, the device performance and stability. A comprehensive overview of solvent engineering toward achieving the highest qualities for perovskite light absorbing layers with various compositions and based on different fabrication processes is provided in this review. The mechanisms indicating the essential roles a solvent, or a solvent mixture can play to improve the crystallinity, uniformity, coverage and surface roughness of the perovskite films, are discussed. Finally, the role of solvent engineering in transferring from small area laboratory scale PSC fabrication to large area perovskite film deposition processes is explored.

Sensitive Materials and Coating Technologies for Surface Acoustic Wave Sensors

Since their development, surface acoustic wave (SAW) devices have attracted much research attention due to their unique functional characteristics, which make them appropriate for the detection of chemical species. The scientific community has directed its efforts toward the development and integration of new materials as sensing elements in SAW sensor technology with a large area of applications, such as for example the detection of volatile organic compounds, warfare chemicals, or food spoilage, just to name a few. Thin films play an important role and are essential as recognition elements in sensor structures due to their wide range of capabilities. In addition, other requisites are the development and application of new thin film deposition techniques as well as the possibility to tune the size and properties of the materials. This review article surveys the latest progress in engineered complex materials, i.e., polymers or functionalized carbonaceous materials, for applications as recognizing elements in miniaturized SAW sensors. It starts with an overview of chemoselective polymers and the synthesis of functionalized carbon nanotubes and graphene, which is followed by surveys of various coating technologies and routes for SAW sensors. Different coating techniques for SAW sensors are highlighted, which provides new approaches and perspective to meet the challenges of sensitive and selective gas sensing.

ZrCuAlNi thin film metallic glass grown by high power impulse and direct current magnetron sputtering

Abstract High-power impulse magnetron sputtering (HiPIMS) is a thin film deposition technique that combines the advantages of energetic deposition methods with magnetron sputtering. HiPIMS has so far mostly been utilized for the growth of crystalline coatings. Here we offer a study devoted to metallic glasses, in which we compare Zr60Cu25Al10Ni5 (target composition) thin films deposited by conventional direct current magnetron sputtering (DC) and HiPIMS. Film microstructure is strongly dependent on the choice of the sputtering method. The DC layers show a columnar structure with intra-columnar porosity, which provides a pathway for oxygen diffusion into the film. In contrast, HiPIMS films are column-free and possess about 4% higher density, as revealed by X-ray reflectivity. Electron diffraction reveals a decrease in average atomic spacing for the latter film of about 12%. These differences in film properties and morphology can be attributed to an increase in ad-atom mobility during HiPIMS caused by an increase in ion energy and flux of the film-forming species enabling a more efficient energy and momentum transfer to the growing film surface. The relative contribution of metallic and hence film forming ions to the overall ion flux of the DC plasma compared to the HiPIMS plasma is 13% and 96%, respectively. Additionally, a substrate bias causes the ionized film forming species during HiPIMS to arrive close to the substrate normal reducing shadowing effects. Different microstructures have a direct effect on the average roughness values, which for DC and HiPIMS films are 1.4 nm and 0.2 nm, respectively. The indentation hardness H and Young's modulus E are higher for the HiPIMS sample, at 9.2 ± 0.3 GPa and 131.6 ± 3.6 GPa, respectively. The increase in hardness for the HiPIMS sample as compared to the DC sample (~35%) can be attributed to higher film density, compressive (HiPIMS) as opposed to tensile (DC) stress and the lack of a columnar structure.

Laser-patterned Si/TiN/Ge anode for stable Si based Li-ion microbatteries

Progress in micro/nano-electro-mechanical system (M/NEMS) and microelectronics technologies urgently require the synchronous development of the micro/nano energy storage systems. Herein, a novel patterned Si/TiN/Ge electrode is fabricated on the Si substrate surface by the facile femtosecond-laser direct writing and the microelectronic thin film deposition techniques. The surface-engraved Si/TiN/Ge electrode displays superior cyclability and rate capability when evaluated as anode for lithium ion microbatteries (LIMBs). The patterned TiN interlayer can act as an effective buffer layer to optimize the Li ions storage in the Si substrate, and the introduction of the Ge nanocomposites would further enhance the areal capacity, thus eventually bring the improved electrochemical performance. Moreover, the Li ions storage behaviours are experimentally investigated from the post morphologies of all the cycled electrodes, and theoretically studied by the COMSOL Multiphysics simulations and the first principles calculations. These results would shed light on the new strategy to design and exploit other high-performance Si based LIMBs.

Chimeric liposomes incorporating functional copolymers: preparation and pH/thermo-responsive behaviour in aqueous solutions

The purpose of this study is to prepare stimuli-responsive chimeric liposomes (i.e. hybrid polymer-lipid liposomes) containing functional copolymers and conduct aqueous solution studies in order to determine their properties and potential as drug-delivery nanocarriers. Two random copolymers, composed of the hydrophilic, pH and thermo-responsive 2-(dimethyl amino) ethyl methacrylate (DMAEMA) monomer and the hydrophobic stearyl methacrylate (SMA) monomer, were synthesized via free-radical polymerization and molecularly characterized using SEC, FTIR, and 1H-NMR. The synthesis was followed by aqueous solution studies, utilising dynamic light scattering (DLS) in order to determine their stimuli responsive self-assembly properties. The preparation of chimeric liposomes was mediated by thin film deposition and hydration, followed by aqueous solution studies via DLS, ζ-potential and fluorescence spectroscopy. The drug-loading studies include curcumin loading via a thin film deposition and hydration technique, while aqueous solution properties of the drug-loaded chimeric liposomes were determined utilizing DLS, and UV–Vis spectroscopy.

Toolbox for atomic layer deposition process development on high surface area powders.

Atomic layer deposition (ALD) is an industrially applied technique for thin film deposition. The vast majority of processes target flat substrates rather than powders. For ALD on powders, new processes are needed, as different reaction conditions are required. Here, two setups are described in detail, which enhance the ALD process development for powders. The first setup described is capable of directly measuring the vapor pressure of a given precursor by a capacitance diaphragm gauge. Promising precursors can be pre-selected, and suitable precursor saturation temperatures can be determined. The second setup consists of four parallel reactors with individual temperature zones to screen the optimal ALD temperature window in a time efficient way. Identifying the precursor saturation temperature beforehand and subsequently performing the first ALD half cycle in the parallel setup at four different reactor temperatures simultaneously will drastically reduce process development times. Validation of both setups is shown for the well-known ALD precursors, trimethylaluminum to deposit aluminum oxide and diethyl zinc to deposit zinc oxide, both on amorphous silica powder.

Self-regulated growth of [111]-oriented perovskite oxide films using hybrid molecular beam epitaxy

Exotic material properties and topological nontrivial surface states have been theoretically predicted to emerge in [111]-oriented perovskite layers. The realization of such [111]-oriented perovskite superlattices has been found challenging, and even the growth of perovskite oxide films along this crystallographic direction has been proven as a formidable task, attributed to the highly polar character of the perovskite (111) surface. Successful epitaxial growth along this direction has so far been limited to thin film deposition techniques involving a relatively high kinetic energy, specifically pulsed laser deposition and sputtering. Here, we report on the self-regulated growth of [111]-oriented high-quality SrVO3 by hybrid molecular beam epitaxy. The favorable growth kinetics available for the growth of perovskite oxides by hybrid molecular beam epitaxy on non-polar surfaces was also present for the growth of [111]-oriented films, resulting in high-quality SrVO3(111) thin films with residual resistivity ratios exceeding 20. The ability to grow high-quality perovskite oxides along energetically unfavorable crystallographic directions using hybrid molecular beam epitaxy opens up opportunities to study the transport properties of topological nontrivial and correlated electron systems.

Ferroelectricity in bulk hafnia.

Ferroelectricity in bulk crystals of hafnium oxide demonstrates that these properties are not limited to films prepared by thin-film deposition techniques.

Optical in-situ temperature management for high-quality ZnO molecular beam epitaxy

Temperature management of the substrate is of great importance for molecular beam epitaxy as well as for other thin film deposition techniques. In this work, we compare different in-situ methods for simultaneous measurement of substrate temperature and growth rate for wide band semiconductors such as ZnO and GaN. The different analysis techniques investigated are emissivity-corrected pyrometry as well as band gap thermometry and reflectivity-based growth-rate determination. Combining in-situ and ex-situ measurements with simulation data, we demonstrate characteristic behaviors of common substrate mounting techniques for Al2O3 and ZnO substrates. Moreover, we investigate the performance of reflectometry-based growth rate determination during ZnO homoepitaxy.

Electroplated Functional Materials with 3D Nanostructures Defined by Advanced Optical Lithography and Their Emerging Applications

Electroplating has been favored to date as a surface treatment technology in various industries in the development of semiconductors, automobiles, ships, and steel due to its advantages of being a simple, solution-based process, with low cost and high throughput. Recently, classical electroplating has been reborn as an advanced manufacturing process for functional materials by combining it with unconventional optical three-dimensional (3D) nanofabrication techniques capable of generating polymer templates with high-resolution 3D periodic nanostructures. The bottom-up filling behavior of electroplating rising from a seed layer makes it possible to densely fill the nanoporous network of the template with heterogeneous inorganic materials. At this time, understanding and optimizing the process parameters (e.g., additive, current density, type of current waveform, etc.) of electroplating is critical for defect control. In addition, since electroplating is generally performed near room temperature, unlike other thin film deposition techniques, structural damage to the polymer template by heat during electroplating is almost negligible. Based on the excellent compatibility of electroplating and optical 3D nanofabrication, innovative functional materials with 3D periodic nanostructures targeting electrochemical or energy-related applications have been created. In this mini review, a strategy for producing functional materials with 3D periodic nanostructures through a templating process will be covered, and the recent cases of successful applications to electrodes for energy storage devices, electrocatalysts, and thermoelectric materials will be summarized. We will also discuss technical issues that need to be considered in the process to improve the quality of the resulting functional materials with 3D nanoarchitectures.

Redeposition-Free Deep Etching in Small KY(WO4)2 Samples.

KY(WO4)2 is a promising material for on-chip laser sources. Deep etching of small KY(WO4)2 samples in combination with various thin film deposition techniques is desirable for the manufacturing of such devices. There are, however, several difficulties that need to be overcome before deep etching of KY(WO4)2 can be realized in small samples in a reproducible manner. In this paper, we address the problems of (i) edge bead formation when using thick resist on small samples, (ii) sample damage during lithography mask touchdown, (iii) resist reticulation during prolonged argon-based inductively coupled plasma reactive ion etching (ICP-RIE), and (iv) redeposited material on the feature sidewalls. We demonstrate the etching of 6.5 µm deep features and the removal of redeposited material using a wet etch procedure. This process will enable the realization of waveguides both in ion-irradiated KY(WO4)2 as well as thin KY(WO4)2 membranes transferred onto glass substrate by bonding and subsequent polishing.

Interface Structure and Transport Properties of Thin-Film Solid-State Batteries with an Epitaxial Cathode

A problem of solid-state Li ion batteries is a relatively low ionic conductivity at the interface between electrodes and electrolytes that limits the charging/discharging rate. For designing the solid-state batteries, understanding the mechanism behind the low interface conductivity is crucial. Investigation of the atomic structure of the interfaces and its relation to the ionic conductivity would be helpful for this purpose. However, atomic-scale observation of the buried interfaces is often difficult in practical batteries which consist of grain aggregates. Recent progress in thin-film deposition techniques of electrodes and electrolyteshas opened a path to study the interface quantitatively. By using an all-in-vacuum battery fabrication system [1], we succeeded in fabricating a very low solid-electrolyte/electrode interface resistance in thin film batteries with an epitaxial cathode. The well-defined crystalline interfaces allow the analysis of atomic-scale structure and its transport properties. For the structure analysis, we used X-ray crystal truncation rod (CTR) scattering which enables the atomic-scale analysis in a non-destructive manner.The analysis of two interfaces will be presented: a solid-electrolyte/electrode interface and an electrode/current collector interface. In the former case, we studied Li(negative electrode)/Li3PO4(solid electrode)/LiCoO2(positive electrode) thin-film Li batteries with low and high (more than 10 times) interface resistances. It turned out that the low-resistance interface has an atomically well-ordered LiCoO2 surface and a disordered surface CoO2 layer can results in the high-resistance interface, demonstrating the crucial importance of the structural ordering in the interfacial layer for realizing a low-resistance interface [2]. In the second topic, we focused on electrode/current collector interface that can also affect the battery performance. We found a drastic improvement of the charging/discharging property by inserting 1.2 nm-thick band insulator LaAlO3 into the LiCoO2 (electrode)/Nb-doped SrTiO3 (current collector) interface. The CTR analysis revealed that a formation of electric dipole at the LaAlO3/Nb-doped SrTiO3 interface, which causes a energy band alignment preferable for the electronic conductance between the electrode and the current collector.