Μm Thick Samples Research Articles

Thermally stimulated current and differential scanning calorimetry spectroscopy for the study of polymer nanocomposites

The thermally stimulated discharge current (TSC) and differential scanning calorimetry (DSC) spectroscopy have been recorded in 25 μm thick samples of pristine polycarbonate (PC) and zinc oxide nano particle-filled polycarbonate. Polycarbonate (PC)/zinc oxide (ZnO) nanocomposites of different mass ratio (e.g., 1, 3, and 5%) were prepared by sol–gel method, followed by film casting. The glass transition temperature of nanocomposite samples increases with increase in concentration of ZnO nano fillers. It is due to the strong interaction between inorganic and organic components. The TSC peaks of nanocomposite and pristine PC indicate the multiple relaxation process. It has been observed that the magnitude of TSC decreases with increase in concentration of nanofillers. The TSC characteristics of 5% filled nanocomposites shows exponential increase of current at higher temperature region. This increase in current is caused by formation of charge-transfer complex between inorganic phase (e.g., ZnO) and organic phase (e.g., PC). Thus, the nano material like zinc oxide transfers the charge carriers from inorganic phase to organic phase rapidly and resultant current increases exponentially. This current is known as leakage current or breakdown current. TSC peak height is observed as a function of the polarizing field. The height of TSC peak increases as the field increases in pristine PC, while TSC peak height is suppressed in nanocomposite samples. This indicates the amount of space charge is smaller in the nanocomposites with a proper addition of ZnO nano fillers than in the pristine PC.

Quadratic electro-optic effect in the nonconjugated conductive polymer iodine-doped poly( [formula omitted]-pinene) measured at longer wavelengths including [formula omitted

The quadratic electro-optic effect in the nonconjugated conductive polymer iodine-doped poly( β -pinene), measured at longer wavelengths including 1.55 μm, is reported. The field-induced birefringence technique was used. A modulation depth of 0.1% was observed in a 1 μm thick sample for an applied field of 1 V/μm at 1.55 μm. The corresponding Kerr coefficient obtained is 1.6×10 −10 m/V 2. The Kerr coefficients measured at 790–810 nm are also very large. This exceptionally large (largest known) optical nonlinearity has been attributed to the subnanometer size metallic quantum dots formed in this polymer upon doping with iodine and the optimized film quality recently achieved.

Photovoltaic characterization of Copper–Indium–Gallium Sulfide (CIGS2) solar cells for lower absorber thicknesses

Chalcopyrites are important contenders among thin-film solar cells due to their direct band gap and higher absorption coefficient. Copper–Indium–Gallium Sulfide (CIGS2) is a chalcopyrite material with a near-optimum band gap of ~ 1.5 eV. Record efficiency of 11.99% has been achieved on a 2.7 μm CIGS2 film prepared by sulfurization at the Florida Solar Energy Center (FSEC) PV Materials Lab. In this work, photovoltaic performance analysis has been carried out for a 1.5 μm absorber prepared under similar conditions as that of a 2.7 μm thick absorber sample. It was observed that there is an increase in diode factor and reverse saturation current density when the absorber thickness was decreased. The diode factor increased from 1.69 to 2.18 and reverse saturation current density increased from 1.04 × 10 − 10 mA/cm 2 to 1.78 × 10 − 8 mA/cm 2. This can be attributed to a decrease in the grain size when the absorber thickness is decreased. It was also observed that there is an improvement in the shunt resistance. Improvement in shunt resistance can be attributed to optimized value of i:ZnO for lower absorber thickness and less shunting paths due to a smoother absorber.

Thermoluminescence assessment of 0.5, 1.0 and 4.0 µm thick HFCVD undoped diamond films

AbstractChemical vapor deposition (CVD) diamond has not found extensive application as a thermoluminescence (TL) dosimeter, mainly because its TL glow curve shape is not reproducible. A slight variation in the growing conditions may result in strong changes in the morphology, microstructure, and surface‐impurity‐related defects, considerably affecting the TL glow curve features. In order to study the main TL characteristics under controlled growing conditions, we present results on three 0.5, 1.0, and 4.0 µm thick hot filament CVD (HFCVD) diamond films grown on Si (100) substrates. The recorded TL glow curves were resolved into individual peaks by a home‐made deconvolution program and the kinetic parameters of the peaks were extracted. The best fits of the TL glow curves were obtained using four peaks, except for the 4.0 µm thick sample that required five TL components. All samples showed three high intensity TL peaks between 180–400 °C and a less intense one or two peaks in the 100–180 °C low‐temperature region. The low‐temperature components all obey second‐order kinetics while the high‐temperature ones obey first and 1.6 order kinetics. The samples showed similar TL peaks with comparable kinetics parameters. It is concluded that the manufacturing of high quality HFCVD diamond may require a further improvement in the growing process in order to exploit its excellent tissue‐equivalence properties in clinical and medical applications.

Effect of dentinal tubules and resin-based endodontic sealers on fracture properties of root dentin

Objective To investigate the role of dentinal tubules in the fracture properties of human root dentin and whether resin-filled dentinal tubules can enhance fracture resistance. Materials and methods Crack propagation in human root dentin was investigated in 200 μm thick longitudinal samples and examined by light and scanning electron microscopy. 30 maxillary premolar teeth were prepared for work of fracture ( Wf) test at different tubule orientations, one perpendicular and two parallel to dentinal tubules. Another 40 single canal premolars were randomly divided into four groups of 10 each: intact dentin, prepared but unobturated canal, canal obturated with epoxy rein (AH Plus™/gutta percha), or with UDMA resin sealer (Resilon ®/RealSeal ®). The samples were prepared for Wf test parallel to dentinal tubules. Wf was compared under ANOVA with statistical significance set at p < 0.05. Results Dentinal tubules influenced the path of cracks through dentin, with micro-cracks initiated in peritubular dentin of individual tubules ahead of the main crack tip. A significant difference ( p < 0.001) was found between Wf perpendicular to tubule direction (254.9 J/m 2) vs. parallel to tubule direction from inner to outer dentin (479.4 J/m 2). Neither canal preparation nor obturation using epoxy- or UDMA-based resins as sealer cements substantially influenced fracture properties of root dentin, despite extensive infiltration of dentinal tubules by both sealer cements.

Lateral spatial resolution of thermal lens microscopy during continuous scanning for nonstaining biofilm imaging

The possible application of continuous scanning thermal lens microscopy (TLM) as alternative online biofilm observation method is studied. As biofilm is a heterogeneous sample, the influence of spatially limited thermal flow at the sample heterogeneities and the biofilm-environment border has to be considered. The influence of the edges on the lateral resolution with respect to scanning velocity during continuous scanning TLM was therefore evaluated. Lateral scanning experiments on 100 nm thin gold stripes showed that the maximum scan speed can be predicted from a time constant of a lock-in amplifier and the beamwidth. Since three-dimensional mapping is needed to fully characterize the biofilm structure, depth scanning experiments with stained 4 μm thick polystyrene samples with the coaxial TLM setup were evaluated for signal width at full width at half maximum. Thus, a minimum step width for depth scanning of 10 μm for observation has been acquired. A three-dimensional image of unstained biofilm grown in a flow chamber was acquired using continuous scanning TLM.

Improvement of Pinning Force and Critical Current Density in Thick YBa 2 Cu 3 O 7−δ Films Grown on SrTiO 3 Substrates Decorated with LaNiO 3 Nanodots

We have investigated by DC magnetization measurements and frequency-dependent AC susceptibility the critical current density (Jc), pinning force (Fp) and pinning potential in thick (1.3–1.6 μm) YBa2Cu3O7−δ (YBCO) films grown by Pulsed Laser Deposition on SrTiO3 substrates decorated with LaNiO3 nanodots deposited by a few (5–15) laser pulses, in comparison with those of a 1 μm thick YBCO reference sample. Experiments show that the highest improvement of superconducting properties was achieved for films grown on substrates decorated with 10 laser pulses on the LaNiO3 target, which have, at 77.3 K, a Jc of 40–125% higher than in pure YBCO in fields between 1 and 2 T, and Fp increased by 40%. These results could be important for further improvement of current-carrying capability of coated conductors for in-field power applications.

Annealing effect on properties of transparent and conducting ZnO thin films

This work presents the effect of postdeposition annealing on the structural, electrical and optical properties of undoped ZnO (zinc oxide) thin films, prepared by radio-frequency sputtering method. Two samples, 0.17 and 0.32 µm-thick, were annealed in vacuum from room temperature to 350 °C while another 0.32 µm-thick sample was annealed in air at 300 °C for 1 h. X-ray diffraction analysis revealed that all the films had a c-axis orientation of the wurtzite structure normal to the substrate. Electrical measurements showed that the resistivity of samples annealed in vacuum decreased gradually with the increase of annealing temperature. For the 0.32 µm-thick sample, the gradual decrease of the resistivity was essentially due to a gradual increase in the mobility. On the other hand, the resistivity of the sample annealed in air increased strongly. The average transmission within the visible wavelength region for all films was higher than 80%. The band gap of samples annealed in vacuum increased whereas the band gap of the one annealed in air decreased. The main changes observed in all samples of this study were explained in terms of the effect of oxygen chemisorption and microstructural properties.

Free-standing zinc-blende (cubic) GaN layers and substrates

It has been demonstrated that it is possible to grow by molecular beam epitaxy bulk free-standing zinc-blende (cubic) GaN layers. Such layers have potential applications as lattice matched substrates for growth of non-polar cubic GaN device structures of improved performance. We present the data from characterisation measurements that confirm the cubic nature of the GaN crystals and show that the fraction of the hexagonal material is not more than about 10% in the best 50 μm thick free-standing GaN samples. Our research is aimed at increasing the size of the free-standing cubic GaN layers in order to make this technology suitable for the commercial production of substrates and we have now demonstrated the growth of 100 μm thick, 2-in diameter GaN substrates.

MOVPE growth of blue InxGa1–xN/GaN LEDs on 150 mm Si(001)

AbstractWe report on the implementation of GaN‐based, blue light emitting diodes on 150 mm Si(001) substrates grown by metalorganic vapor phase epitaxy. The 2.8 µm thick samples are completely crack‐free and the optically active layers consist of fivefold InxGa1–xN/GaN multiple quantum wells. The homogeneity of the light emission across the whole sample is determined by photoluminescence wafer mapping exhibiting an averaged peak wavelength of 456 nm at room temperature and a standard deviation of less than 1%. A bright blue light emission is obtained by an electrical excitation with a turn‐on voltage of below 2.8 V. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Temperature-dependent Hall effect studies of ZnO thin films grown by metalorganic chemical vapour deposition

The electrical properties of zinc oxide (ZnO) thin films of various thicknesses (0.3–4.4 µm) grown by metalorganic chemical vapour deposition on glass substrates have been studied by using temperature-dependent Hall-effect (TDH) measurements in the 18–300 K range. The high quality of the layers has been confirmed with x-ray diffraction, transmission electron microscopy, scanning electron microscopy and photoluminescence techniques. TDH measurements indicate the presence of a degenerate layer which significantly influences the low-temperature data. It is found that the measured mobility generally increases with increasing layer thickness, reaching a value of 120 cm2 V−1 s−1 at room temperature for the 4.4 µm thick sample. The lateral grain size of the layers is also found to increase with thickness indicating a clear correlation between the size of the surface grains and the electrical properties of corresponding films. Theoretical fits to the Hall data suggest that the bulk conduction of the layers is dominated by a weakly compensated donor with activation energy in the 33–41 meV range and concentration of the order of 1017 cm−3, as well as a total acceptor concentration of mid-1015 cm−3. Grain boundary scattering is found to be an important limiting factor of the mobility throughout the temperature range considered.

Photorefractive Effect of Ferroelectric Liquid Crystals with Application of a Biased Alternating Electric Field

The photorefractive effect of a ferroelectric liquid crystal (FLC) was investigated with application of a biased alternating electric field. A two-beam coupling gain coefficient of 96 cm−1 was obtained for a 10 µm-thick sample. The gain coefficient obtained was found to be much larger than those obtained with application of a DC electric field.

Preparation, characterization, and heat resistance studies of a holographic photopolymer based on SU-8 epoxy resin

A holographic photopolymer based on SU-8 epoxy resin as the binder and trimethylolpropane ethoxylate triacrylate as the monomer with an iodonium salt photoinitiator was studied. A diffraction efficiency of 93% and a refractive index modulation (Delta n) of 1.3 x 10(-2) were obtained from a 16 microm thick sample with write beam intensities of 50 mW/cm(2) at a wavelength of 514.5 nm, an exposure time of 60 s, and a postbaking at 65 degrees C for 1 h. The material exhibits good heat resistance in the range from 40 degrees C to 160 degrees C and long projected life.

Effect of microarc oxidised layer thickness on plain fatigue and fretting fatigue behaviour of Al–Mg–Si alloy

The objective of this work was to investigate the performance of microarc oxide coatings of two different thicknesses (40 and 100 μm) on Al–Mg–Si alloy samples under plain fatigue and fretting fatigue loadings. Tensile residual stress present in the substrate of 40 μm thick coated samples induced early crack initiation in the substrate and so their plain fatigue lives were shorter than those of untreated specimens. Presence of more pores and tensile surface residual stress in 100 μm thick coated samples caused early crack initiation at the surface leading to their inferior plain fatigue lives compared with 40 μm thick coated samples. While the differences between the lives of coated and uncoated specimens were significant under plain fatigue loading, this was not the case under fretting fatigue loading. This may be attributed to relatively higher surface hardness of coated specimens. The performance of 40 μm thick coated samples was better than that of 100 μm thick coated specimens under both plain fatigue and fretting fatigue loadings.

Surface modifications of crystalline silicon created by high intensity 1064 nm picosecond Nd:YAG laser pulses

A study of silicon modification induced by a high intensity picosecond Nd:YAG laser, emitting at 1064 nm, is presented. It is shown that laser intensities in the range of 5 × 10 10–0.7 × 10 12 W cm −2 drastically modified the silicon surface. The main modifications and effects can be considered as the appearance of a crater, hydrodynamic/deposition features, plasma, etc. The highest intensity of ∼0.7 × 10 12 W cm −2 leads to the burning through a 500 μm thick sample. At these intensities, the surface morphology exhibits the transpiring of the explosive boiling/phase explosion (EB) in the interaction area. The picosecond Nd:YAG laser-silicon interaction was typically accompanied by massive ejection of target material in the surrounding environment. The threshold for the explosive boiling/phase explosion (TEB) was estimated to be in the interval 1.0 × 10 10 W cm −2 < TEB ≤ 3.8 × 10 10 W cm −2.

A New Benchmark for TiO2 Nanotube Array Growth by Anodization

We report on the anodic formation of a self-standing 720 μm thick TiO2 nanotubular membrane by complete consumption of a 250 μm thick titanium foil sample. By employing double sided electrochemical oxidation of titanium in an electrolyte comprised of water, NH4F, and ethylene glycol, we obtain two highly ordered, hexagonal close-packed titania nanotube arrays 360 μm in length that are separated by a thin compact oxide layer; the individual nanotubes in each array have an aspect ratio of ∼2200. The potentiostatic anodization of titanium in an ethylene glycol, NH4F, and water electrolyte dramatically increases the rate of nanotube array growth to approximately 15 μm/h, representing a growth rate ∼750−6000% greater than that seen, respectively, in other polar organic or aqueous based electrolytes previously used to form TiO2 nanotube arrays. We consider the effects of electrolyte composition, applied potential, and anodization duration on the length and diameter of the resulting nanotubes in terms of a growth rate model, with results suggesting that reduced hydroxyl ion injection from the electrolyte, which enables faster high field ionic conduction through the barrier layer, is responsible for the high nanotube growth rates achieved. Furthermore, as reported herein for the first time, we are able to make self-standing TiO2 nanotube array films ranging in thickness from 50 to 360 μm.

Morphology and composition of GaN films grown by cyclic-pulsed laser deposition

We describe a detailed study of scanning electron microscopy (SEM) combined with energy dispersive spectroscopy (EDS) analysis to study composition and structure of 500 nm thick polycrystalline GaN samples. The films have been deposited by cyclic-pulsed laser deposition (cyclic-PLD) with a Nd:YAG nanosecond pulsed laser at 1064 nm. SEM pictures of the GaN layers revealed a structure composed of grains with typical dimensions of 200 nm. Coalescence of the grains was more evident for a 1 μm thick sample. EDS mapping of the GaN layer was performed for Ga, N, O, and Al and could be related with the corresponding SEM scan. Both EDS and XPS composition analyses pointed to a Ga rich (or N deficient) GaN layer.

Effects of thickness on the grain alignment and Jc properties of (Hg 0.8Re 0.2)Ba 2Ca 2Cu 3O x superconductor thick films

(Hg 0.8Re 0.2)Ba 2Ca 2Cu 3O x superconducting films of 1–80 μm thickness have been prepared on MgO(1 0 0) substrates using spraying process and post-Hg-vapor annealing. The effects of thickness on the grain alignment and J c properties have been investigated. The best T c and T zero were found to be 131.9 K and 129 K respectively, for 50 μm thick sample. Magnetic properties up to 6 T have been investigated. The calculated critical current density, J cmag, was found to be 4.8 × 10 5 A cm −2 at 4.2 K for 50 μm thick sample.

Influence of the ion–atom flux ratio on the mechanical properties of chromium nitride thin films

In this paper we report the mechanical properties of chromium nitride (CrN) thin films deposited at different levels of ion bombardment and their relationship with the microstructural parameters, such as grain size, preferred orientation and residual stress. The samples were deposited by unbalanced magnetron sputtering changing the substrate–target distance and the substrate bias, keeping other deposition condition fixed. The mechanical properties were obtained by nanoindentation performed on 1.8 μm thick samples. Under the different deposition conditions all of the CrN films were approximately stoichiometric, but clear variations in the microstructure were seen. The hardness was nearly constant at 24–27 GPa even when the grain size, residual stress and crystalline orientation changed. However, the elastic modulus showed a steady increase from 300 to 350 GPa, proportional to the variations in grain size and the residual stress level.

Homoepitaxial Growth and Characterization of 4H-SiC Epilayers by Low-Pressure Hot-Wall Chemical Vapor Deposition

Horizontal air-cooled low-pressure hot-wall CVD (LP-HWCVD) system is developed to get high quality 4H-SiC epilayers. Homoepitaxial growth of 4H-SiC on off-oriented Si-face (0001) 4H-SiC substrates purchased from Cree is performed at a typical temperature of 1500°C with a pressure of 40 Torr by using SiH4+C2H4+H2 gas system. The surface morphologies and structural and optical properties of 4H-SiC epilayers are characterized with Nomarski optical microscope, atomic force microscopy (AFM), x-ray diffraction, Raman scattering, and low temperature photoluminescence (LTPL). The background doping of 32 μm-thick sample has been reduced to 2-5×1015 cm-3. The FWHM of the rocking curve is 9-16 arcsec. Intentional N-doped and B-doped 4H-SiC epilayers are obtained by in-situ doping of NH3 and B2H6, respectively. Schottky barrier diodes with reverse blocking voltage of over 1000 V are achieved preliminarily.