Thin Film Fabrication Techniques Research Articles

Development of “Mist CVD”, Functional Thin Film Fabrication Techniques Utilizing Mist Flow

Development of “Mist CVD”, Functional Thin Film Fabrication Techniques Utilizing Mist Flow

3D LiMn2O4 Electrodes for High Rate Thin-Film Microbatteries

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

Magnetic and superconducting properties of a heavy-fermion CeCoIn5 epitaxial film probed by nuclear quadrupole resonance

Since the progress in the fabrication techniques of thin-films of exotic materials such as strongly correlated heavy-fermion compounds, microscopic studies of the magnetic and electronic properties inside the films have been needed. Herein, we report the first observation of 115In nuclear quadrupole resonance (NQR) in an epitaxial film of the heavy-fermion superconductor CeCoIn5, for which the microscopic field gradient within the unit cell as well as magnetic and superconducting properties at zero field are evaluated. We find that the nuclear spin-lattice relaxation rate in the film is in excellent agreement with that of bulk crystals, whereas the NQR spectra show noticeable shifts and significant broadening indicating a change in the electric-field distribution inside the film. The analysis implies a displacement of In layers in the film, which however does not affect the magnetic fluctuations and superconducting pairing. This implies that inhomogeneity of the electronic field gradient in the film sample causes no pair breaking effect.

Optical security and authentication using nanoscale and thin-film structures

Authentication of encoded information is a popular current trend in optical security. Recent research has proposed the production of secure unclonable ID tags and devices with the use of nanoscale encoding and thin-film deposition fabrication techniques, which are nearly impossible to counterfeit but can be verified using optics and photonics instruments. Present procedures in optical encryption provide secure access to the information, and these techniques are improving daily. Nevertheless, a rightful recipient with access to the decryption key may not be able to validate the authenticity of the message. In other words, there is no simple way to check whether the information has been counterfeited. Metallic nanoparticles may be used in the fabrication process because they provide distinctive polarimetric signatures that can be used for validation. The data is encoded in the optical domain, which can be verified using physical properties with speckle analysis or ellipsometry. Signals obtained from fake and genuine samples are complex and can be difficult to distinguish. For this reason, machine-learning classification algorithms are required in order to determine the authenticity of the encoded data and verify the security of unclonable nanoparticle encoded or thin-film-based ID tags. In this paper, we review recent research on optical validation of messages, ID tags, and codes using nanostructures, thin films, and 3D optical codes. We analyze several case scenarios where optically encoded devices have to be authenticated. Validation requires the combined use of a variety of multi-disciplinary approaches in optical and statistical techniques, and for this reason, the first five sections of this paper are organized as a tutorial.

Large-area thin film deposition technologies for fabricating hybrid perovskite solar cells

Organic-inorganic hybrid perovskite solar cells have attracted tremendous research interests due to their amazing light to electric power conversion efficiencies (PCEs). In the past four years, the PCE of perovskite solar cells has significantly increased up to 22.1%, which outperforms several other types of third-generation solar cells and becomes the most promising candidate to compete with the traditional silicon-based solar cells. This result is mainly owing to some excellent properties of the hybrid perovskite active layer, such as a high absorption coefficient, an appropriate band gap and a long carrier diffusion length. Nevertheless, most of the reported high efficiencies were only achieved with very small active areas in the range of 0.03 to 0.2cm2, which is likely to cause measurement errors. What is worse, the material utilization ratio is only 1% during film deposition, hindering the industrial production of perovskite solar cells in the future. Therefore, a large-scale production process has become a big challenge to realize the purpose of commercial applications. To date, the best certified PCE of 19.6% has been obtained with an active area exceeding 1cm2. The most important aspect is fabricating large-area uniform, pinhole-free and large crystal grain perovskite thin films for scaling up high PCE perovskite solar cells. To realize the purpose of large-area, high-quality perovskite film fabrication, a range of thin film fabrication techniques have been proposed, including spin-coating, vacuum flash-assisted solution process (VASP), doctor blading, slot-die coating, inkjet printing, spray coating, vapor assisted deposition and soft-cover deposition. This review aims at giving an overview of these thin film deposition techniques for the processing of perovskite thin film fabrication. By comparing the film quality and material utilization ratio corresponding to different film deposition methods, the studies on perovskite thin film fabrication techniques are summarized. Among these techniques, spin-coating has been widely used in laboratory for perovskite film fabrication, but the material utilization ratio is usually less than 1%, hindering the fabrication of perovskite film with large-area. Spin-coating combined with VASP exhibits an amazing result for high-quality perovskite film fabrication, but it still has a low material utilization ratio in spin-coating process. Other techniques, like doctor blading, slot-die coating and inkjet printing are suitable for large-area perovskite film fabrication with high material utilization ratio. Spray coating and soft-cover deposition also have shown great potential for large-area perovskite film fabrication. The PCEs of perovskite solar cells have reached 13% and 17% with an active area exceeding 1cm2, respectively. Particularly, a high-quality perovskite film with a large-area of 51cm2 was obtained by soft-cover deposition, which made this technique more promising for the development of perovskite solar cells in the future. Based on this review, we suggest that VASP is a suitable technique for depositing high-quality large-area perovskite film. It would be a promising technique to combine VSAP with other techniques, such as doctor blading, inkjet printing, etc., to enhance material utilization ratio for large-scale perovskite solar cells. In addition, some simpler and lower-cost techniques should be developed for the fabrication of perovskite solar cells in the future.

Crystal Isomers of ScFeO3

In inorganic compounds, “crystal isomers”, which can exist in metastable phases, are obtained by various solution-processing techniques, high-pressure syntheses, as well as physical and chemical thin film fabrication techniques. The metastable phase depends on the processing, allowing the hierarchy of the Gibbs free energy to be controlled in a phase at a given temperature. In this study, we successfully stabilize five metastable phases, four phases of ScFeO3 and one Sc0.48Fe1.52O3, prepared from one ScFeO3 target by the pulsed laser deposition technique. The crystal structures are identified by X-ray diffraction and high-angle annular dark field-scanning transmitted electron microscopy measurements. The relationship between the crystal structure of the film and the substrate is κ-Al2O3-type Sc0.48Fe1.52O3 on SrTiO3(111), spinel-type ScFeO3 on MgO(001), corundum-type ScFeO3 on Fe2O3/Al2O3(0001) and NdCaAlO4(001), YMnO3-type ScFeO3 on Al2O3(0001), and bixbyite-type ScFeO3 on YSZ(001). Four of these structu...

Power density optimization for micro thermoelectric generators

Microfabricated thermoelectric generators (μTEGs) can harvest modest temperature differences to provide reliable solid-state electricity for low-power electronics, sensors in distributed networks, and biomedical devices. While past work on μTEGs has focused on fabrication and demonstration, here we derive and explore comprehensive design guidelines for optimizing power output. A new closed-form thermoelectric device model agrees well with the traditional iterative approach. When thermoelectric leg length is limited by thin-film fabrication techniques, a very low (<10%) active thermoelectric fill fraction is required to optimize device power output, requiring careful selection of filler material. Parasitic resistance due to electrical interconnects is significant when a small number of thermocouples is used, and this loss can be reduced by increasing the number of thermocouples while decreasing the cross-sectional area of the legs to maintain the same fill fraction. Finally, a discussion of the “incompleteness of ZT” shows that different combinations of thermal conductivity, electrical conductivity, and Seebeck coefficient resulting in the same ZT will result in different device performance and optimization decisions. For μTEGs, we show it is best to increase Seebeck coefficient, followed by decreasing thermal conductivity for short leg lengths and increasing electrical conductivity for long leg lengths.

Effect of Nonmagnetic Cap Layers for Nd–Fe–B Thin Films With Small Addition of Rare-Earth Element

Nd-Fe-B sintered magnets [1] exhibit a highest maximum energy products (BH) max among all permanent magnets, so they have been used for wide variety of applications. Particularly, their usage to traction motors for hybrid and electric vehicles (HEV's) have been increased recently and also will be increased in the future due to the consideration for the environmental issues. Since the Nd-Fe-B sintered magnets show a relatively low curie temperature of 315 0C, the substitution of heavy rare earth elements (HRE) such as Dy and Tb for Nd is essential for realizing a high thermal stability [2]. However, due to the scarce and localized natural resources of the HRE elements, Dy-free or less permanent magnet is strongly required. It is well known that the structure and the chemistry of the grain boundary phase between Nd 2 Fe 14 B phases play an important role to understand coercivity mechanism. Recent studies revealed that the magnetic coupling between the Nd 2 Fe 14 B main phases is thought to reduce by the grain boundary phase with Nd-rich and Cu-rich alloy phases after post-sinter annealing [3-4]. Furthermore, a lot of studies using a thin film fabrication technique have been done to control a structure and chemistry of the grain boundary phase [5]. However, coercivity mechanism has not yet been understood completely. Then, the elucidation of the coercivity mechanism is essential for reducing the amount of heavy rare earth elements from the Nd-Fe-B magnets. In this study, in order to see the effect of non-magnetic cap layers for Nd-Fe-B thin films, Nd-Fe-B/ Nd-Cu multilayer thin films with small addition of rare earth element have been prepared and their structure and magnetic properties have been investigated.

Metal–organic framework thin films with well-controlled growth directions confirmed by x-ray study

Metal–organic frameworks (MOFs) have attracted the attention of a variety of researchers because of their structural diversity and designability, and their varied physical properties based on their uniform microporosity. While MOFs are interesting as bulk materials, future applications in functional nanomaterials will require the use of MOFs as thin films, and to achieve this, several thin-film fabrication techniques have been developed. These techniques have provided rational design of a variety of MOF thin films; however, oriented crystal growth of a MOF thin film, which is mainly confirmed by X-ray diffraction, remains a challenge that should be addressed. In this article, we review thin-film fabrications and characterizations, and structural features of MOF thin films with perfect crystalline orientation.

Physics on development of open-air atmospheric pressure thin film fabrication technique using mist droplets: Control of precursor flow

Mist CVD, which is one of the functional thin-film fabrication methods carried out under open-air atmospheric pressure, has been developed for advanced control of precursor streams in order to achieve the fabrication of uniform thin films. In order to obtain a uniform stream of flow including mist droplets, collisional mixing has been considered, and a fine channel (FC) nozzle has been developed using this technology. In the FC, mist droplets are affected by the lift forces toward the direction of the substrate, the gas temperature of the entire area reaches a constant, and owing to the Leidenfrost effect, the evaporation time of a mist droplet is remarkably long as several hundred milliseconds and the migration of a mist droplet extends to a distance of several hundred millimeters. From these characteristic phenomena in the FC, the formation of uniform thin films on the entire surface has been achieved using the FC-type mist (FCM-) CVD system.

Comparative study of ZnTe thin films prepared using close space sublimation (CSS) and electron beam evaporation (EBE) thin film fabrication techniques for optoelectronic applications

ZnTe thin films were deposited onto soda lime glass substrates by the close space sublimation (CSS) and electron beam evaporation (EBE) techniques under vacuum (as deposited, annealed at 300 °C and Ag doped for 10 min). Structural, morphological, optical and electrical properties of ZnTe films were studied before and after doping. The ZnTe thin films fabricated by EBE technique showed improvement in microstructure, morphology, crystalline orientation and physical parameters such as porosity and density as compared to CSS fabricated thin films. The highest density with very small porosity is observed in EBE fabricated films. The improvement in electrical parameters such as resistivity, mobility, sheet concentration, type of semiconductor (p or n type), bulk concentration and magnetoresistance were observed in EBE fabricated thin films. The highest decrease in electrical resistivity (8·34×102 Ω cm) was observed by incorporating Ag dopant via EBE technique. The main purpose of this was a comparative study between EBE and CSS techniques and an improvement in the electrical and optical properties such as electrical conductivity, mobility, sheet resistance/concentration and the band gap of the fabricated thin films. The doping of Ag was performed to observe their effects on morphology, structure, electrical and photovoltaic properties of the as deposited thin films. It was observed that electrical and optical properties of the fabricated films enhanced with dopant and annealing temperature.

Responsive Multidomain Free-Standing Films of Gold Nanoparticles Assembled by DNA-Directed Layer-by-Layer Approach

Responsive free-standing films of gold nanoparticles are fabricated by a new approach combining the programmable DNA-directed self-assembly and the layer-by-layer (LbL) thin film fabrication technique. This approach allows for the assembly of multidomain nanoparticle films with each domain possessing distinct properties in response to external stimuli, which is essential for the formation of dynamic nanostructures. Large area free-standing films of DNA-modified gold particles are fabricated by the selective melting of a sacrificial nanoparticle domain, taking advantage of the unique sharp melting transition of DNA-modified gold nanoparticles. Furthermore, we show that released multidomain films can be designed to further split into multiple intact daughter films in a precisely controlled manner, demonstrating that this new approach provides a powerful means to fabricate free-standing nanoparticle films that are capable of programmable transformation.

Spectrum-splitting photovoltaic system using transmission holographic lenses

The optical efficiency of a holographic spectrum-splitting optical system with transmission holographic lenses is investigated. Spectrum-splitting is a promising approach to improve the efficiency of photovoltaic (PV) systems. By removing the lattice-matching constraints, it is possible to utilize low-cost thin-film PV materials and fabrication techniques. Transmission holograms are fabricated with the recording of the interference patterns of two or more coherent beams. It is also possible to use converging construction wavefronts to record holographic gratings that are matched to the focusing beam from the primary concentrator optics. Experimental holograms are fabricated in dichromated gelatin, and high diffraction efficiency is obtained. A single holographic lens is used to divide a broad spectrum into two types of PV cells. The position and orientation of the PV cells are chosen to match the dispersion properties of the holographic lens. The optical transfer efficiency of the holographic lens is measured to be ∼90% at the peak with fast transitions between the high diffraction efficiency and the high transmission spectral regions. With a GaAs solar cell and a 2.1-eV bandgap solar cell, the system efficiency is 31.0% under one-sun which is improved by 11.9% over the best single PV cell. The achievable system efficiency with the prototype filter is 96% compared to that of the ideal system.

Enhanced efficiency of light-trapping nanoantenna arrays for thin-film solar cells

We suggest a novel concept of efficient light-trapping structures for thin-film solar cells based on arrays of planar nanoantennas operating far from plasmonic resonances. The operation principle of our structures relies on the excitation of chessboard-like collective modes of the nanoantenna arrays with the field localized between the neighboring metal elements. We demonstrated theoretically substantial enhancement of solar-cell short-circuit current by the designed light-trapping structure in the whole spectrum range of the solar-cell operation compared to conventional structures employing anti-reflecting coating. Our approach provides a general background for a design of different types of efficient broadband light-trapping structures for thin-film solar-cell technologically compatible with large-area thin-film fabrication techniques.

Integration of Microfabricated Capacitive Bridge and Thermistor on the Alumina Substrates

This paper describes a thin film fabrication technique of planar metallic microstructure with geometries between 10 and 20μm on 96% alumina substrate. This technique integrates a microfabricated capacitive bridge and thermistor on the same substrate employing metal and vacuum deposition. The procedure showed good resolution using a substrate with medium grain size between 3-5μm and average peak to valley surface roughness (Ra) 0.1μm. The film adhesion was tested by soldering metallic wires on the capacitor electrodes. These structures can be used for thin film characterization. We also deposited an oxide thin film (TiO2:WO3) by using polymeric precursors method.

Thermally activated polarization dynamics under the effects of lattice mismatch strain and external stress in ferroelectric film

The strain, inevitably existing in ferroelectric films and devices, is an important factor for determining their polarization properties and performances. It requires an understanding of the effects of strain on polarization dynamics of ferroelectric film. Considering the strain effect, an analytic thermally activated electric polarization-switching model was developed for the study of the polarization properties of ferroelectric films. Using this model, we successfully simulated the time dependent polarization reversal, the coercive field, the polarization switching time, and the polarization hysteresis loops as a function of the lattice mismatch, external applied stress, applied electric field, and temperature. Theoretical predications of the model are in good agreement to experimental observations and they reinforce the possibility of using sophisticated thin film fabrication techniques such as lattice mismatch strain to engineer ferroelectric thin films structures and devices with controlled properties. Our study offers clear analytical relations that can be easily used in future studies of ferroelectrics.

Status and perspectives in thin films and patterning of spin crossover compounds

Spin crossover compounds are a class of functional materials able to switch their spin state upon external stimuli. They were proposed as potential candidates for several technological applications that require highly controlled thin films and patterns. Here we present a critical overview of the most important approaches for thin film growth and patterning of spin-crossover compounds, giving special attention to Fe(II) based molecules, which are the most studied materials. We present both conventional approaches to thin film growth (Langmuir-Blodgett, constructive chemical approach, spin coating, drop casting and vacuum sublimation) and patterning (combined top-down/bottom-up method, soft and unconventional lithography). We critically discuss the application of thin film growth and fabrication techniques highlighting the most critical aspects and the perspectives opened by the recent progress.

Quantum dot phosphors and their application to inorganic electroluminescence device

The colloidal semiconductor quantum dots (QDs), as represented by colloidal CdSe QDs, synthesized via a chemical solution route that is termed the hot-soap synthesis are excellent phosphors and are expected to be used in various light emission devices, because of their adjustable emission energy utilizing the quantum size effect and high luminosity. We have recently developed less-toxic and cadmium-free colloidal QDs and a QD-activated electroluminescent (EL) device. In this article, we describe the recent development of the ternary I–III–VI2 and ZnO colloidal QDs, respectively, for the phosphors used with visible and ultraviolet lights. A thin film fabrication technique from the colloidal QDs and its application to the fabrication of a CdSe QD-activated all-inorganic EL device are introduced.

Microwave ZnO Thin-Film Transistors

We have developed ZnO thin-film transistor design and fabrication techniques to demonstrate microwave frequency operation with 2-mum gate length devices produced on GaAs substrates. Using SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gate insulator and pulsed laser deposited ZnO active layers, a drain-current ON/OFF ratio of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> , a drain-current density of 400 mA/mm, a field-effect mobility of 110 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V ldr s, and a subthreshold gate voltage swing of 109 m\/dec were achieved. Devices with Ti-gate metal had current and power gain cutoff frequencies of 500 and 400 MHz, respectively.

Manufacturing technology for the fabrication a discrete actuator using a magnetic shape memory alloy

To demonstrate the feasibility of combining thin-film fabrication techniques and discrete MSM stripes, a hybrid actuator system was designed. The actuator system consists of four thin-film stators and two discrete MSM stripes mounted in a row. For creating an actuator motion, one of the MSM stripes has to be excited by a pair of stators to cause variant switching. This results in an elongation plus a compression of the second stripe and vice versa. The technologies required for fabricating the thin-film stator are sputter deposition, PECVD, electroplating, etching, and photolithography. This paper describes the fabrication process for a thin-film stator used for the MSM micro actuator system. It also presents technology study results indicating the feasibility of the planned fabrication sequence.