Crystalline Structure Of Films Research Articles

Morphological instability, evolution, and scaling in strained epitaxial films: An amplitude-equation analysis of the phase-field-crystal model

Morphological properties of strained epitaxial films are examined through a mesoscopic approach developed to incorporate both the film crystalline structure and standard continuum theory. Film surface profiles and properties, such as surface energy, liquid-solid miscibility gap and interface thickness, are determined as a function of misfit strains and film elastic modulus. We analyze the stress-driven instability of film surface morphology that leads to the formation of strained islands. We find a universal scaling relationship between the island size and misfit strain which shows a crossover from the well-known continuum elasticity result at the weak strain to a behavior governed by a "perfect" lattice relaxation condition. The strain at which the crossover occurs is shown to be a function of liquid-solid interfacial thickness, and an asymmetry between tensile and compressive strains is observed. The film instability is found to be accompanied by mode coupling of the complex amplitudes of the surface morphological profile, a factor associated with the crystalline nature of the strained film but absent in conventional continuum theory.

Influence of substrate temperature and deposition rate on structural and mechanical properties of shape memory NiTi films

NiTi thin films deposited by DC magnetron sputtering of an alloy (Ni/Ti:45/55) target at different deposition rates and substrate temperatures were analyzed for their structure and mechanical properties. The crystalline structure, phase-transformation and mechanical response were characterized by X-ray diffraction (XRD), Differential Scanning Calorimetry (DSC) and Nanoindentation techniques, respectively. The films were deposited on silicon substrates maintained at temperatures in the range 300 to 500∘C and post-annealed at 600∘C for four hours to ensure film crystallinity. Films deposited at 300∘C and annealed for 600∘C have exhibited crystalline behavior with Austenite phase as the prominent phase. Deposition onto substrates held at higher deposition temperatures (400 and 500∘C) resulted in the co-existence of Austenite phase along with Martensite phase. The increase in deposition rates corresponding to increase in cathode current from 250 to 350 mA has also resulted in the appearance of Martensite phase as well as improvement in crystallinity. XRD analysis revealed that the crystalline film structure is strongly influenced by process parameters such as substrate temperature and deposition rate. DSC results indicate that the film deposited at 300∘C had its crystallization temperature at 445∘C in the first thermal cycle, which is further confirmed by stress temperature response. In the second thermal cycle the Austenite and Martensite transitions were observed at 75 and 60∘C respectively. However, the films deposited at 500∘C had the Austenite and Martensite transitions at 73 and 58∘C, respectively. Elastic modulus and hardness values increased from 93 to 145 GPa and 7.2 to 12.6 GPa, respectively, with increase in deposition rates. These results are explained on the basis of change in film composition and crystallization.

Property Modification of Copper Phthalocyanine Film by Nitrogen Dioxide Gas

Copper Phthalocyanine (CuPc) thin films have a potential to use as nitrogen dioxide (NO2) gas sensor therefore the study of property modification of CuPc thin film by NO2 gas can lead to a better understanding the sensor operation. CuPc thin films were prepared by thermal vacuum deposition on glass substrate and aluminum interdigitated electrodes. The film characteristics were investigated with and without NO2 gas. These characteristics indicated that the resistance of CuPc film decrease as the NO2 concentration increases. In order to achieve high efficient of sensing properties, the post-deposition film was also annealed. The surface modifications were investigated with different exposed time of NO2 gas. Their properties modifications before and after annealing were investigated on the film morphology and crystalline structure. The surface morphology of the film was examined by atomic force microscopy while the crystalline structure of the film was determined by x-ray diffraction. The electrical measurements of CuPc film exposed to NO2 gas were studied in order to investigate property modification of the CuPc film.

Optical Analysis of the Fine Crystalline Structure of Artificial Opal Films

Herein, we present a detailed analysis of the structure of artificial opal films. We demonstrate that, rather than the generally assumed face centered cubic lattice of spheres, opal films are better approximated by rhombohedral assemblies of distorted colloids. Detailed analysis of the optical response in a very wide spectral range (0.4 < or = a/lambda < or = 2, where a is the conventional lattice constant), as well as at perpendicular and off-normal directions, unambiguously shows that the interparticle distance coincides very approximately with the expected diameter only along directions contained in the same close-packed plane but differs significantly in directions oblique to the [111] one. A full description of the real and reciprocal lattices of actual opal films is provided, as well as of the photonic band structure of the proposed arrangement. The implications of this distortion in the optical response of the lattice are discussed.

Synthesis and characterization of sol-gel derived TiO2 thin films: Effect of different pretreatment process

Titanium dioxide (TiO2) thin films have been successfully synthesized deposited on glass substrates by the sol-gel dip-coating method through different pretreating processes, including heated at 100, 500°C, via freeze drying, microwave heating for 10 min and subsequently annealed at 500°C for 2 h. The as-synthesized TiO2 films were characterized using X-ray diffraction (XRD), atomic force microscopy (AFM) and ultravioletvisible (UV-vis) absorption spectra analysis technology. The preparation of the precursor sols and TiO2 films were described in detail. Effects of 100, 500°C, freeze drying and microwave heating pretreatment on crystalline structure, surface morphology, roughness, particle size, optical property and electronic transition of TiO2 thin films have been primarily investigated. The XRD results demonstrate that the TiO2 films were well-crystallized and consisted of anatase TiO2 phase only with (101) plane. The average crystalline size is only about 15 nm at 100°C pretreatment and the absorption edge shifts to shorter wavelength comparing with that at 500°C, freeze drying and microwave heating pretreatment. Pretreatment process is important during the preparation of thin films and has obviously effect on the structure and optical property of TiO2 films due to the different heating mechanisms.

Preparation of cerium-doped titania macroporous films by a sol–gel spin coating using polypropylene glycol (PPG) as pore-creating agent: Effects of Ce ions, PPG and calcination on photocatalytic activity

Cerium-doped titania macroporous films (CTMs) were spin-coated on glass substrates from a controlled sol–gel solution using titanium alkoxide and polypropylene glycol (PPG) as precursor and pore-creating agent, respectively. The physicochemical properties of CTMs were characterized by scanning electron microscope (SEM), thermogravimetric and differential thermal analysis (TG-DTA), X-ray photoelectron spectroscopy (XPS), X-ray diffractometry (XRD) and BET surface area analysis. The photocatalytic activity of CTMs was evaluated by degradation of methylene blue (MB). The results showed the decomposition of PPG during calcinations was responsible for the generation of macroporous structure in the films. The Ce ions, PPG and calcination respectively influenced recombination of electron–hole pairs, surface area, hydroxyl concentration and crystalline structure of films, so that one of those factors could restrict the photocatalytic activity of films. Compared with others films, CTMs-1 showed the highest photocatalytic activity with 0.5 wt.% of Ce doping concentration and 2.0 g PPG additions at calcinating temperature of 550 °C. It was mainly attributed to the effects that both the optimum concentration of Ce ions doping and high surface area could promote the photocatalytic degradation by preventing recombination of electron–hole pairs and providing more MB molecules around TiO 2 and high concentration of hydroxyl.

Improved electrochromical properties of sol–gel WO 3 thin films by doping gold nanocrystals

In this investigation, the effect of gold nanocrystals on the electrochromical properties of sol–gel Au doped WO 3 thin films has been studied. The Au–WO 3 thin films were dip-coated on both glass and indium tin oxide coated conducting glass substrates with various gold concentrations of 0, 3.2 and 6.4 mol%. Optical properties of the samples were studied by UV–visible spectrophotometry in a range of 300–1100 nm. The optical density spectra of the films showed the formation of gold nanoparticles in the films. The optical bandgap energy of Au–WO 3 films decreased with increasing the Au concentration. Crystalline structure of the doped films was investigated by X-ray diffractometry, which indicated formation of gold nanocrystals in amorphous WO 3 thin films. X-ray photoelectron spectroscopy (XPS) was used to study the surface chemical composition of the samples. XPS analysis indicated the presence of gold in metallic state and the formation of stoichiometric WO 3. The electrochromic properties of the Au–WO 3 samples were also characterized using lithium-based electrolyte. It was found that doping of Au nanocrystals in WO 3 thin films improved the coloration time of the layer. In addition, it was shown that variation of Au concentration led to color change in the colored state of the Au–WO 3 thin films.

Influence of aluminium incorporation on the structure of ZrN films deposited at low temperatures

We have studied the influence of Al incorporation in the crystalline structure of ZrN thin films deposited by dc magnetron sputtering at low temperature. The amount of aluminium in the films depends directly on the power applied to the aluminium cathode during the deposition. Energy dispersive x-ray analysis and x-ray diffraction (XRD) were used to obtain the chemical composition and crystalline structure of the films, respectively. When Al atoms are incorporated into the ZrN coatings, the strong ZrN (2 0 0) orientation is modified by a combination of other ones such as ZrN (1 1 1), Zr3N4 (2 1 1) and hexagonal AlN (1 0 0) as detected from the XRD spectra for high aluminium concentrations. Fourier-transform infrared spectroscopy allowed us to identify oxides and nitrides, ZrO, AlO and AlN, incorporated into the deposited films. The effect of a bias voltage applied to the substrate has also been investigated and related to the changes in the microstructure and in the nanohardness values of the ZrAlN films.

Effect of thickness of ALD grown TiO 2 films on photoelectrocatalysis

Atomic layer deposited TiO 2 films were used for photoelectrocatalysis for the first time. Films with different thicknesses were grown and comprehensively characterized by structural and photoelectrochemical techniques. Methylene blue was used as a model substance in photocatalytic and photoelectrocatalytic degradation studies. In both cases, degradation rate was observed to saturate above certain film thickness. The reason for saturation could not be explained by the electrical parameters alone, and it was suggested that film morphology and crystalline structure are of major importance in this case.

Influence of the aluminum incorporation on the structure of sputtered ZrN x films deposited at low temperatures

We have studied the influence of the Al incorporation in the crystalline structure of ZrN thin films deposited by dc magnetron sputtering processes at low temperature. The incorporation of the aluminum in the films depends directly on the Ar/N 2 ratio in the gas mixture and the power applied to the aluminum cathode during the deposition. The chemical composition and the crystalline structure of the films were analyzed by Energy Dispersive X-ray (EDX) spectroscopy and X-ray Diffraction (XRD), respectively. When Al atoms are incorporated into the ZrN coatings, the strong ZrN (200) preferred orientation is modified to a combination of phases related to (111) ZrN with a contribution of cubic (111) AlN and possibly (211) Zr 3N 4, which are detected by XRD for high aluminum concentrations. Fourier Transform Infrared (FTIR)spectroscopy allowed us to complete the identification of the nitrides and oxides incorporated into the deposited films. The effect of a bias voltage applied to the substrate has also been investigated and related to the changes in the microstructure and in the nanohardness values of the ZrAlN films.

Transport and gas sensing properties of In 2O 3 nanocrystalline thick films: A Hall effect based approach

Undoped nanosized In 2O 3 with n-type conduction was produced in both polymorphic forms (cubic and rhombohedral) and deposited by screen-printing as thick films. These films show high sensitivity to low O 3 concentration levels. They have been investigated by four point conductance and Hall effect measurements under sensor operating conditions (elevated temperature and ozone exposure). The effective values of the charge carrier concentration and mobility have been calculated from the experimental records using the recipe for the single crystals. The response to O 3 is discussed in the frame of the standard models for gas sensors. The observed deviations from the model are explained in connection with the film crystalline structure and microscopic parameters spread.

Dye-sensitized solar cells based on oriented ZnO nanowire-covered TiO 2 nanoparticle composite film electrodes

The oriented ZnO nanowire-covered TiO 2 nanoparticle composite film electrodes were fabricated by screen-printed TiO 2 nanoparticle layer on conducting glass and low-temperature hydrothermal growth of ZnO nanowires. The film morphology, composition and crystalline structure were confirmed by field-emission scanning electron microscopy, energy dispersive X-ray spectra and X-ray diffraction patterns respectively. Dye-sensitized solar cells based on the composite electrode gained short-circuit current density of 8.04 mA/cm 2, open-circuit photovoltage of 0.67 V, fill factor of 0.40, and overall conversion efficiency of 2.15%.

Crystalline structure of solution-processed pentacene thin films

We report on the crystalline structure of solution-processed pentacene thin films, prepared without using any precursors or derivatives. In these films, two types of surface structure were observed: flat domains and rough domains. The flat domains exhibited higher field effect mobilities than the rough domains and were composed of two-dimensional plate-like domains of several hundred micrometers containing large crystallites. The in-plane structures of the solution-processed thin films were analyzed by grazing-incidence X-ray diffraction (GIXD). The in-plane two-dimensional mapping of the GIXD patterns reveals that the solution-processed pentacene thin films have a crystal structure similar to that reported by Campbell et al. [Acta Crystallogr. 15 (1962) 289].

Electric and Magnetic Properties of Multiferroic BiFeO$_{3}$ and YMnO$_{3}$ Thin Films

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We grew rhombohedrally distorted BiFeO<formula formulatype="inline"> <tex Notation="TeX">$_{3}$</tex></formula> and hexagonal YMnO<formula formulatype="inline"> <tex Notation="TeX">$_{3}$</tex></formula> thin films on Pt/TiO<formula formulatype="inline"> <tex Notation="TeX">$_{2}$</tex></formula>/SiO<formula formulatype="inline"> <tex Notation="TeX">$_{2}$</tex></formula>/Si substrates via RF magnetron sputtering technique in a pure oxygen atmosphere. BiFeO<formula formulatype="inline"> <tex Notation="TeX">$_{3}$</tex></formula> and YMnO<formula formulatype="inline"> <tex Notation="TeX">$_{3}$</tex></formula> targets were self-made by the usual solid-state reaction method. We investigated the effects of deposition temperature upon crystalline structure, surface morphology, magnetization, and electrical polarization of BiFeO<formula formulatype="inline"><tex Notation="TeX">$_{3}$</tex> </formula> and YMnO<formula formulatype="inline"><tex Notation="TeX">$_{3}$</tex> </formula> thin films. The crystalline structure was studied by X-ray diffraction, and the topography of film surface was analyzed by atomic force microscopy. We also conducted measurements of ferroelectric and ferromagnetic hysteresis loops to study the electrical and magnetic behavior of the samples. Polarization, as a function of electric field in capacitor structures based on our BiFeO<formula formulatype="inline"><tex Notation="TeX">$_{3}$</tex></formula> films, shows hysteretic behavior with a coercive field of 54 kV/cm and a remanent polarization of 21 <formula formulatype="inline"><tex Notation="TeX">$\mu$</tex></formula>C/cm<formula formulatype="inline"><tex Notation="TeX">$^{2}$</tex></formula>; whereas, YMnO<formula formulatype="inline"><tex Notation="TeX">$_{3}$</tex></formula> films show hysteretic behavior with coercive field of 2.4 kV/cm, remanent polarization of 1.2 <formula formulatype="inline"><tex Notation="TeX">$\mu$</tex></formula>C/cm<formula formulatype="inline"><tex Notation="TeX">$^{2}$</tex></formula>, and saturation polarization of 3.5 <formula formulatype="inline"><tex Notation="TeX">$\mu$</tex> </formula>C/cm<formula formulatype="inline"><tex Notation="TeX">$^{2}$</tex> </formula>. Magnetization measurements of the BiFeO<formula formulatype="inline"> <tex Notation="TeX">$_{3}$</tex></formula> films evidence weak ferromagnetism, that can be related to the presence of a small quantity of ferromagnetic impurities. </para>

Crystalline Structure of HfZrO Thin Films and ZrO2 / HfO2 bi-Layers Grown by AVD for MOS Applications

The downscaling in CMOS transistors requires the introduction of new materials with a higher dielectric constant. The electrical properties of HfxZryOz films and ZrO2 / HfO2 bi-layers grown by Atomic Vapor Deposition were analyzed for their integration as gate oxides. While crystallinity of those layers was found to be dependent on their zirconium concentration, the electrical performance was shown to depend mainly on the growth temperature of the high-k films.

PZT thin film preparation by pulsed DC magnetron sputtering

Pulsed DC magnetron sputtering was used in this study to prepare lead zirconate titanate (Pb(Zr x Ti 1− x )O 3, PZT) thin films. A single metallic target was used for the deposition onto a Pt/Ti/SiO 2/Si substrate and parameters such as: pulse frequency, duty cycle, O 2/Ar flow ratio controlled so as to analyze the effect of the parameters on thin film deposition rate, crystalline structure and morphology. After the deposition, the thin film was annealed in a rapid thermal annealing (RTA) furnace. The experimental results showed that, when the pulse frequency was in the range of 10 kHz–100 kHz, along with the lowering of frequency and the oxygen argon flow rate ratio, the deposition rate gradually increased and the formation of PZT thin film perovskite phase was enhanced; however, if the oxygen argon flow rate ratio was too high, it caused the PZT thin film to generate a pyrochlore phase. However, when the duty cycle was in the range of 95%–75%, the highest deposition rate and better perovskite phase could be obtained in the range of 75%–80%.

Thickness Dependence of the Crystalline Structure and Hole Mobility in Thin Films of Low Molecular Weight Poly(3-hexylthiophene)

The morphology of thin films at the polymer-to-insulator interface is of great importance for OFET applications. In order to find a relation between the thickness dependence of structural order and the electrical parameters in low molecular weight (Mw ∼ 2.5 kDa) poly(3-hexylthiophene) (P3HT), we have performed grazing-incidence X-ray diffraction and field effect mobility measurements. The samples were prepared from solutions with different concentrations by spin-coating mainly onto HMDS-pretreated Si/SiO2 substrates, resulting in film thicknesses that vary between 10 and 200 nm. The X-ray diffraction curves display Bragg peaks of nanocrystallites diluted into an amorphous matrix where the orientational distribution of the crystallites changes significantly as a function of film thickness. The orientation of nanocrystals was found to be random for the thickest films. Reducing the film thickness, we found an increase in the alignment of the stacking direction of molecules along the surface normal. At same time the mean crystal size along the film normal decreases less than the decrease of film thickness. This is interpreted by a preferential pinning of nanocrystals at the film-to-insulator interface when the crystal size becomes in the order of the film thickness, i.e., below 25 nm. The model of pinning effect is supported by temperature-resolved X-ray measurements performed between room temperature and melting temperature. For films thicker than 25 nm the phase transition appears rather continuously with temperature, but it becomes sharp for thinner films. In contrast to X-ray measurements the field effect mobility is found to be constant within the whole investigated range. Our findings give evidence that the charge transport in low molecular weight P3HT is dominated by the ultrathin layer stabilized at the film-to-insulator interface. Despite the very uniform orientation of the crystallites within this layer, the field effect mobility remains low for all thicknesses. This is attributed to the presence of amorphous regions between highly crystalline domains, which ultimately limits the charge transport in the layer plane.

Structural comparison of GdF_3 films grown on CaF_2 (111) and SiO_2 substrates

Wide bandgap metal fluorides are the materials of choice for optical coating applications at 193 nm. Low loss and environmentally stable optics requires a mitigating fluoride film structure on a nanometer scale. To understand the growth mechanism of fluoride materials, GdF(3) films grown on CaF(2) (111) and SiO(2) substrates were investigated. Film inhomogeneity and surface roughness were modeled by fitting ellipsometric data with an effective medium approximation, indicating a correlation between film inhomogeneity and surface roughness. The modeled surface roughness was compared with the atomic force microscope measurement. Film inhomogeneity was correlated to the cone-shaped columnar structure revealed by cross-sectional images from a scanning electron microscope. The film crystalline structure was determined by x-ray diffraction measurement, suggesting a different growth mechanism of GdF(3) films on crystalline and amorphous substrates.

Behavior of Ti 0.5Al 0.5N thin film in nanoscale deformation with different loading rates

The nanostructured solid solution Ti 0.5Al 0.5N thin film (∼ 1.2 μm in thickness) was prepared on Si (100) substrates by reactive close-field unbalanced magnetron sputtering at a bias voltage of − 50 V in an Ar–N 2 gas mixture. The deformation behavior of the film was systematically investigated by nanoindentation measurements. Analyses of the obtained load–displacement curves showed no evidence of a “staircase” discontinuity in the specimen and the effect of the tip radius of the indenter on the behavior of elastic and plastic deformation can be determined. The critical shear stress, yielding strength and the size of plastic zone of the Ti 0.5Al 0.5N thin film under nanoindentation were evaluated by integrating the Hertzian stress analysis and the Johnson's cavity model. Results revealed that the new complete dislocation nucleation during indentation was not the prerequisite for the onset of plastic deformation, which met Von Mises yielding criteria. The relationship between the normalized plastic zone size and loading rates was also established. It was suggested that the ability of plastic deformation of the Ti 0.5Al 0.5N thin film under nanoindentation was sensitive to the loading rates, which is attributed to the metastable crystalline structure of thin film.

Pulsed laser deposited TiO 2 films: Tailoring optical properties

TiO 2 amorphous films have been pulsed laser deposited onto glass substrates. Film characterization by X-ray diffraction, atomic force microscopy and transmission spectroscopy was performed with the aim of extracting the information on the film crystalline structure, surface roughness and optical properties. Three methods for improving film optical performance have been employed, namely deposition at elevated temperatures, post-annealing in thermodynamically equilibrium conditions and rapid thermal annealing (RTA). The best characteristics were achieved in the case of the film subjected to RTA at 500 °C: refractive index n = 2.53, absorption coefficient α = 404 cm − 1 at λ = 550 nm and rms surface roughness as low as 0.6 nm. The results obtained were found to be one of the best published so far.