Preparation and characterization of delafossite CuCrO2 film on flexible substrate

Delafossite CuCrO₂ film was successfully prepared on a flexible plastic substrate using a heat transfer process. The metallic acetate of the first layer decomposed to form a carbonized layer for use as a release layer. The thin film was deposited on the first layer by chemical solution deposition and heat treated to form a CuCrO₂ film. After that, the delafossite CuCrO₂ film was transferred from a Si substrate to a polymethyl methacrylate (PMMA) substrate. The CuCrO₂/PMMA film structure, morphology, and optical properties were studied by different characterization techniques. X-ray diffraction analysis and Raman spectrum analysis confirmed that the delafossite CuCrO₂ film was transferred onto the PMMA substrate. XPS analysis showed that the metallic acetate was annealed in N₂ to form a carbonized layer. The resistivity of the CuCrO₂/PMMA was gauged by a four-point probe method, and the visible light transmittance was approximately 58%.     

Patterns of conducting polypyrrole with tunable morphologies

An effective and reliable method was established to fabricate ordered arrays of conducting polypyrrole (PPy) with tunable morphologies. A polydimethylsiloxane (PDMS) stamp was used to lift up the top layer of silica spheres with a closed-packed hexagonal arrangement on a silicon wafer. The lattice spacing of the silica sphere array on the PDMS stamp could be tuned by controlling the swelling of the stamp. Afterwards, the silica spheres were transferred onto a poly(vinyl alcohol) (PVA) film which was pre-spin-coated on an ITO glass. The silica spheres were etched away with hydrofluoric (HF) acid, and the resulting porous PVA film maintained an identical ordered array but of holes. Electrochemical polymerization of pyrrole monomer was carried out with the ITO glass as a working electrode, on which was the PVA film with ordered holes. Different charge densities applied led to PPy patterns with different morphologies; a possible mechanism was addressed.     

Ex situ and in situ characterization studies of spin-coated TiO2 film electrodes for the electrochemical ozone production process

An electrode composed of silicon/titanium oxide/platinum/titanium dioxide (Si/TiO_X/Pt/TiO₂) was fabricated by spin-coating TiO₂ multilayers on a Si/TiO_X/Pt substrate and was used in electrochemical ozone production (EOP). EOP was realized when the Si/TiO_X/Pt substrate was completely covered with the TiO₂ film and a current efficiency of 7% was achieved at a low current density of 26.7 mA cm⁻² in 0.01 M HClO₄ at 15 °C. The TiO₂ film was found to be of an anatase-type TiO₂ and that to comprise aperture structures from the X-ray diffraction (XRD) and transmission electron microscopy (TEM) observations. Moreover, the fabricated TiO₂ film was found to be an n-type semiconductor by photoelectrochemical measurements. The high efficiency at a low current density of EOP on the TiO₂ n-type semiconductor was explained to result from the electron transfer through the TiO₂/HClO₄ interface as tunneling current. When the tunneling current passes through a depletion layer of TiO₂, the electrode potential is necessarily high enough to facilitate EOP.     

Nucleation on resistive substrates: Analysis of effect on film uniformity

When plating onto resistive substrates, potential drop in the electrode can cause non-uniformities in the current density distribution. Multiple studies of this so-called “terminal effect” have been conducted, assuming a layer-by-layer growth mechanism, as would apply, for example, to Cu deposition onto a thin Cu seed layer. However, when depositing Cu onto a material such as Ta or Ru, electrodeposition may occur by three-dimensional nucleation followed by growth. In such cases, a reduction in substrate resistance may not be realized prior to coalescence of the deposited film. Simulations show that nucleation can have a very significant impact on film-thickness uniformity. Results show a linear and significant increase in non-uniformity with the coalescence thickness of the depositing film. Simulations are extended to account for spatial variations of coalescence length. Implications of the model to wafer scale plating of Cu for interconnect applications are discussed.     

Nickel electroless deposition process on chemically pretreated Si(100) wafers in aqueous alkaline solution

Pretreating Si wafer surfaces with hydrochloric acid and hydrogen peroxide mixture (HPM) or ethanol was found to enhance the reactivity of chemical Ni deposition on Si(100) wafers in a simple bath of NiSO₄-(NH₄)₂SO₄ at pH 9.0. This phenomenon was identified as the acceleration of anodic reaction involved in chemical Ni deposition reaction on Si wafer surfaces, namely oxidation process of surface Si. Just after immersing into the alkaline bath, a reactive surface where oxidation reaction of Si was accelerated was formed on Si wafer surface with HPM or ethanol pretreatment. On the reactive surface, uniform and glossy Ni deposition film was obtained. In order to clarify the effects of HPM- or ethanol-pretreatment on anodic reaction, the pretreated Si(100) wafers were immersed into aqueous alkaline solution excluding NiSO₄, resulting acceleration of Si oxidation compared to the unpretreated Si(100) surface. The progress of surface reactivity was also clarified by open circuit potentials (OCP), XPS, and ex-situ ATR FTIR. Moreover, by using this pretreatment, selective deposition onto nano-patterned Si substrate was performed. A two-dimensional array of fine Ni dots (diameter ca. 80 nm) was successfully fabricated.     

Effect of rapid thermal annealing on the structural and electrical properties of TiO2 thin films prepared by plasma enhanced CVD

Titanium oxide thin films were deposited on p-type Si(100), SiO₂/Si, and Pt/Si substrates by plasma enhanced chemical vapor deposition using high purity Ti(O-i-C₃H₇)₄ and oxygen. As-deposited amorphous TiO₂ thin films were treated by rapid thermal annealing (RTA) in oxygen ambient, and the effects of RTA conditions on the structural and electrical properties of TiO₂ films were studied in terms of crystallinity, microstructure, current leakage, and dielectric constant. The dominant crystalline structures after 600 and 800 ‡C annealing were an anatase phase for the TiO₂ film on SiO₂/Si and a rutile phase for the film on a Pt/Si substrate. The dielectric constant of the as-grown and annealed TiO₂ thin films increased depending on the substrate in the order of Si, SiO₂/Si, and Pt/ Si. The SiO₂ thin layer was effective in preventing the formation of titanium silicide at the interface and current leakage of the film. TEM photographs showed an additional growth of SiO_x from oxygen supplied from both SiO₂ and TiO₂ films when the films were annealed at 1000 ‡C in an oxygen ambient. Intensity analysis of Raman peaks also indicated that optimizing the oxygen concentration and the annealing time is critical for growing a TiO₂ film having high dielectric and low current leakage characteristics.     

Fine-patterning of sol-gel derived PZT film by a novel lift-off process using solution-processed metal oxide as a sacrificial layer

Sub-5 µm pattern of sol-gel derived lead-zirconium-titanate (PZT) film with a thickness of 80–390 nm was successfully prepared on Pt(111)/TiO_x/SiO₂/Si (100) substrate by a novel lift-off process using solution-processed metal oxides as a sacrificial layer. The process is simply divided into three steps: In-Zn-O (IZO) sacrificial layer spin-coating and patterning, PZT film formation followed by lift-off process. The results suggested that the IZO layer is effective in preventing PZT crystallization because of its thermal stability during PZT post-annealing, and its barrier-effects between the spin-coated PZT precursor and the Pt/TiO_x substrate. Consequently, the micro-pattern of lift-off PZT exhibited better properties than that formed by wet-etching. In particular, the lift-off PZT films possessed better ferroelectric properties, higher break-down voltage, and more well-defined shape than those of films patterned by conventional wet-etching. This lift-off process shows great promise for highly integrated devices due to its fine pattern-ability.     

Micro-patterning of PZT thick film by lift-off using ZnO as a sacrificial layer

Micro-pattern of 8.2-μm-thick PZT films was prepared on Pt/Ti/SiO₂/Si (1 0 0) substrate wafer by combining composite sol–gel and a novel lift-off using ZnO as a sacrificial layer. The processes include ZnO sacrificial layer deposition and patterning, PZT film preparation, and final lift-off. The results reveal the micro-pattern was better than that formed by wet etching, the PZT thick films patterned by lift-off possessed similar dielectric characters, better ferroelectric properties, and higher breakdown voltage than those of films patterned by wet etching. The lift-off is suitable for micro-patterning of PZT thick films.     

Liquid phase deposition of mesoporous TiO2/DNA hybrid film: Characterization and photoelectrochemical investigation

A photoelectroactive TiO₂/DNA hybrid film was synthesized via the liquid phase deposition (LPD) process. Scanning electron microscopic (SEM) characterization showed that the compact TiO₂ film was changed to a mesoporous structure when DNA was present in the deposition solution, which might be the result of TiO₂ particles growing along the backbones of the double-helical structure of DNA molecules. Although UV absorption spectra and cyclic voltammograms indicated that the deposited TiO₂ on the substrate surface was decreased in the presence of DNA, an enhanced photocurrent response was observed. The electrochemical impedance and cyclic voltammetric measurements using K₃[Fe(CN)₆] as a redox probe suggested that the mesoporous film provided a relatively more efficient electron transfer interface, which could improve the photoelectron transfer rate from the semiconducting film to the electrode and reduce the recombination of photoelectrons and holes. This results in an enhanced photocurrent. Even after long-term and continuous UV irradiation, the mesoporous film exhibited a promoted photoelectrochemical response. The promoted photoelectrocatalytic degradation of methylene blue was obtained on the TiO₂/DNA composite film, which is consistent with the enhanced photocurrent, and this demonstrates that DNA behaved as a useful biomaterial for the synthesis of a photoelectroactive hybrid film with improved performance.     

Patterned growth of CdS by combined electrochemical atomic layer epitaxy and microcontact printing techniques

Ultrathin films of well-ordered CdS on Ag(1 1 1) can be attained by electrochemical atomic layer epitaxy (ECALE) technique, which is based on self-limiting reactions, such as underpotential deposition. The layer-by-layer electrodeposition on single crystal faces allows a precise control of film thickness and yields semiconducting nanostructures of high crystallinity. In this paper, the ECALE method has been used to deposit CdS on Ag(1 1 1) covered by patterns of hexadecanethiol, C₁₆SH. The patterned Ag(1 1 1) was obtained by using the microcontact printing technique of a polydimethylsiloxane (PDMS) stamp previously immersed in C₁₆SH. The stamp used in this work produced thiol strips 320 nm large, with an 850 nm pitch.The deposition of CdS takes only place on the uncovered silver substrate whereas it is prevented on the thiols monolayers, yielding CdS patterned according to the used stamp. The deposit has been characterized by electrochemical techniques and AFM measurements.     

Electroless silver deposition on Si(100) substrate based on the seed layer of silver itself

A new method for silver electroless deposition on Si(100) wafer, based on the silver itself as the seed layer, was developed. The seed layer was first deposited onto the etched wafer surface in an acidic solution of 0.005 mol l⁻¹ AgNO₃+0.06 mol l⁻¹ HF. Then the silver thin film was electrolessly deposited upon the seed layer in the electroless bath of AgNO₃+NH₃+acetic acid+NH₂NH₂ (pH 10.2). The NH₂NH₂ was taken as the reducing agent. The morphology of the seed layer and the silver film were characterized by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray analysis (EDX). The experimental results indicated that the seed layer showed excellent catalytic function for silver electroless deposition.     

Fabrication and electrochemical properties of carbon nanotube film electrodes

Carbon nanotube (CNT) film electrodes were fabricated by a novel process involving the electrostatic spray deposition (ESD) of a CNT solution. Acid treated CNTs were dispersed in an aqueous solvent through sonication and then the CNT solution was electrostatically sprayed onto a metallic substrate by the ESD method. The CNT film electrodes showed well-entangled and interconnected porous structures with good adherence to the substrate. A specific capacitance of 108 F/g was achieved for the electrodes in 1 M H₂SO₄. In addition, the CNT film electrode showed good high rate capability.     

Thermal stability of MgO film on Si grown by atomic layer deposition using Mg(EtCp)2 and H2O

MgO films were deposited on Si via atomic layer deposition (ALD) using Mg(EtCp)₂ and H₂O precursors and their thermal stability was examined as a function of the post-deposition annealing (PDA) temperature. The characteristic self-limiting behavior of the ALD process was confirmed by changing several parameters, such as precursor pulsing times, deposition temperature, and number of cycles. The exceptional resulting step coverage was verified on a patterned wafer with a high aspect ratio. The band gap and dielectric constant of the as-deposited ALD-MgO film were extracted to be approximately 7.5 eV and 8.4, respectively, and were stable up to the PDA temperature of 700 °C. However, considerable outward diffusion of the underlying Si atoms toward MgO started to occur above 700 °C, and most of the MgO film was converted to an amorphous Mg-silicate phase at 900 °C with a thin layer of remaining MgO on top.     

Additively patterned ferroelectric thin films with vertical sidewalls

The functional properties of electroceramic thin films can be degraded by subtractive patterning techniques used for microelectromechanical (MEMS) applications. This work explores an alternative deposition technique, where lead zirconate titanate (PZT) liquid precursors are printed onto substrates in a desired geometry from stamp wells (rather than stamp protrusions). Printing from wells significantly increased sidewall angles (from ~1 to >35 degrees) relative to printing solutions from stamp protrusions. Arrays of PZT features were printed, characterized, and compared to continuous PZT thin films of similar thickness. Three‐hundred‐nanometer‐thick printed PZT features exhibit a permittivity of 730 and a loss tangent of 0.022. The features showed remanent polarizations of 26 μC/cm², and coercive fields of 95 kV/cm. The piezoelectric response of the features produced an e_31,f of ?5.2 C/m². This technique was also used to print directly atop prepatterned substrates. Optimization of printing parameters yielded patterned films with 90° sidewalls. Lateral feature sizes ranged from hundreds of micrometers down to one micrometer. In addition, several device designs were prepatterned onto silicon on insulator (SOI) wafers (Si/SiO₂/Si with thicknesses of 0.35/1/500 μm). The top patterned silicon was released from the underlying material, and PZT was directly printed and crystallized on the free‐standing structures.     

Fluorescence spectra of poly(di-n-hexylsilane)/TiO2 nanoparticle hybrid film

The interaction between poly(di-n-hexylsilane) (PDHS) and TiO₂ nanoparticle was studied based on the temperature dependence of the fluorescence of a PDHS/TiO₂ nanoparticle hybrid film. The polysilane is a suitable probe to investigate a guest polymer-host matrix interaction because the photophysical properties of polysilanes remarkably depend on the conformation of the σ-conjugated Si-Si chain. The PDHS/TiO₂ nanoparticle hybrid film showed a fluorescence band assigned to a disordered structure even at 80 K whereas only the fluorescence band of an ordered structure was observed for the PDHS film at 80 K. The disordering of the Si-Si main chain was explained by the perturbation of the n-hexyl side chain in the neighborhood of the TiO₂ nanosurface. The non-radiative deactivation of the excited state via the disorder-induced local potential minima was suggested by the temperature dependence of the fluorescence intensities of the disordered and ordered structures in the temperature region from 80 to 160 K.     

Nucleation and growth behavior of aluminum nitride film using thermal atomic layer deposition

Aluminum nitride (AlN) film possesses wide band gap energy (~6.2 eV) and a high dielectric constant (~9.2), and is resilient to thermal and chemical stimuli. It also exhibits several functionalities, such as piezoelectricity and pyroelectricity. Therefore, AlN film has been used for electronic and optoelectronic devices and micro-electromechanical systems (MEMSs). Among the various methods of AlN thin film growth, atomic layer deposition (ALD) can control film thickness at the nanoscale. Uniform and conformal film growth is possible at temperatures lower than that of chemical vapor deposition or molecular beam epitaxy. Because the ALD process relies on surface chemical reactions, it shows substrate dependency. To control film uniformity from the beginning, an understanding of nucleation and growth behavior on the substrate is necessary. Therefore, the nature of nucleation and growth behaviors on different substrates is investigated. In this study, AlN films are grown on bare Si and TiN substrates at 295–342 °C by thermal ALD using trimethyl aluminum (TMA) and ammonia. Facile nucleation and linear growth on the TiN substrate, and substrate-inhibited nucleation on the Si substrate, are observed. NH₃ pretreatment may enhance the growth rate at the nucleation stage. Therefore, the dissociation of NH₃ on the substrate is crucial to making uniform nuclei for the subsequent growth of AlN film.     

Fabrication and properties of ZnO/GaN heterostructure nanocolumnar thin film on Si (111) substrate

Zinc oxide thin films have been obtained on bare and GaN buffer layer decorated Si (111) substrates by pulsed laser deposition (PLD), respectively. GaN buffer layer was achieved by a two-step method. The structure, surface morphology, composition, and optical properties of these thin films were investigated by X-ray diffraction, field emission scanning electron microscopy, infrared absorption spectra, and photoluminiscence (PL) spectra, respectively. Scanning electron microscopy images indicate that the flower-like grains were presented on the surface of ZnO thin films grown on GaN/Si (111) substrate, while the ZnO thin films grown on Si (111) substrate show the morphology of inclination column. PL spectrum reveals that the ultraviolet emission efficiency of ZnO thin film on GaN buffer layer is high, and the defect emission of ZnO thin film derived from Zn_i and V_o is low. The results demonstrate that the existence of GaN buffer layer can greatly improve the ZnO thin film on the Si (111) substrate by PLD techniques.     

Aerosol assisted chemical vapour deposited (AACVD) of TiO2 thin film as compact layer for dye-sensitised solar cell

Compact TiO₂ has been introduced onto the surface of an indium tin oxide glass slide (ITO), using an aerosol-assisted chemical vapour deposition method. This serves as a blocking layer for a dye-sensitised solar cell (DSSC). The thickness of the compact TiO₂ could be controlled by deposition time. X-ray diffraction and Raman spectroscopy analyses reveal that the compact TiO₂ is made up of mixed anatase and rutile phases. The field emission scanning electron microscopy image displays a pyramidal morphology of the compact TiO₂. A layer of P25 paste was then smeared onto the compact TiO₂-modified ITO, using the doctor's blade method. A post-treatment procedure was applied to remove the contaminants from the prepared hybrid film, by immersing in a hydrochloric acid solution. The photoelectrochemical measurements and J–V characterisation of the hybrid film show an approximately fourfold increase in photocurrent density generation (114.22 µA/cm²), and approximately 25% enhancement of DSSC conversion efficiency (4.63%), compared to the acid-treated P25 paste alone (3.68%).     

TiO2-coated nanostructures for dye photo-degradation in water

The photocatalytic efficiency of a thin-film TiO₂-coated nanostructured template is studied by dye degradation in water. The nanostructured template was synthesized by metal-assisted wet etching of Si and used as substrate for the deposition of a thin film of TiO₂ (10 nm thick) by atomic layer deposition. A complete structural characterization was made by scanning and transmission electron microscopies. The significant photocatalytic performance was evaluated by the degradation of two dyes in water: methylene blue and methyl orange. The relevance of the reported results is discussed, opening the route toward the application of the synthesized nanostructured TiO₂ for water purification.     

Improved epitaxy of CsI(Tl) film on Si substrate buffered by graphene for X-ray detection

The growth of the CsI(Tl) scintillators deposited on glass substrates with proper wavelength matching the mainstream semiconductor circuits is crucial for X-ray imaging industry. But the coupling loss between the scintillators and silicon based photo sensors has limited the performance. So fabricating CsI(Tl) films on monocrystalline silicon is greatly important for next-generation high performance X-ray image devices. However, the lattice and thermal mismatch between CsI(Tl) and Si always deteriorates the morphology and structure, and thus degrades the property of the CsI(Tl) scintillation converters. Here, by utilizing crystalline graphene(Gr) as a buffer layer, the van der Waals epitaxial growth of CsI(Tl) film on Si(111) substrate is reported and its advanced applications are demonstrated. The introduced graphene buffer layer extremely frees the stress between CsI(Tl) and Si substrate. Therefore, the CsI(Tl) films show perfect micro-columnar and crystalline structures, and then, exhibit much higher light output and signal to noise ratio, and better modulation transfer functions and a bar pattern X-ray image compared to that on bare Si(111) substrate. This facile method not only lays the foundation for the epitaxy of the alkali halide crystals, including CsI(Tl), onto Si-based detectors and its applications in radiation detection, but also may broaden a new perspective of deposition of CsI(Tl) scintillators into flexible substrates.