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A method was described for determining the thickness of epitaxical thin films common to electronic materials. The equations were developed based on the kinematical theory of X-ray diffraction and effects of both primary and secondary extinctions were considered. As an example of the applications of this method, thickness measurement of AlGaAs thin films on GaAs was demonstrated. These films were grown by molecular beam epitaxy. The integrated reflected intensities from the film and the substrate were obtained by the X-ray double crystal diffractometer. An excellent agreement was obtained between the results from X-ray measurements and RHEED oscillation data.
In this study, the equations based on x-ray diffraction theory were developed to determine the thickness of multiple layer thin films. The kinematical expression of the integrated reflected intensity from the substrate and films was corrected for the primary and secondary extinction effects assuming a mosaic crystal model. As an example of the application of the method, thicknesses of a double heterostructure system, namely AlAs/AIGaAs/GaAs, were determined. Good agreement was obtained between the results from the x-ray measurement and scanning electron microscopy data demonstrating high precision of this technique.
Epitaxial AIN thin films grown on sapphire, silicon and silicon carbide substrates were studied using x-ray double crystal diffractometry and transmission electron microscopy to compare the structure, residual stress and defect concentration in these thin films. The AIN thin films was found to have a wurtzite type of structure with a small distortion in lattice parameters which results in a small residual stress of the order of 109 dynes/cm2 in the film. The strain due to lattice parameter mismatch between the substrate and film is too small to account for the residual stress present. The calculated stress from the difference in thermal expansion coefficients between the film and substrate agrees well with the experimental values. Both the x-ray and transmission electron microscopy measurements indicate a low defect density in the AIN thin film grown on 6H-SiC substrate which could be attributed to the small difference in lattice parameters between AIN and 6H-SiC. The defect density in the AIN thin film grown on other substrates were considerably higher. This is the first report of the successful growth of single crystal AIN thin films with such a low concentration of defect density.
Let n be a finite affine plane with n points on a line. We denote by G(II) the graph whose vertices are all points and lines of II, with two vertices adjacent if and only if one is a point, the other is a line, and the point and line are incident. Let L(11) denote the line graph of G(II), i.e., the vertices of L(II) are the edges of G(II), and two vertices of L(II) are adjacent if the corresponding edges of G(II) are adjacent. It is clear that L(II) is a regular, connected graph with n2(n + 1) vertices and valence 2n — 1.
Research demonstrates the importance of nutrition for early brain development. Few studies have examined the effectiveness of multiple micronutrient powders (MNP) on child development. This study examined the impacts of home fortification with MNP on motor and mental development, executive function and memory of children living in Bihar. This two-arm cluster-randomised effectiveness trial selected seventy health sub-centres to receive either MNP and nutrition counselling (intervention) or nutrition counselling alone (control) for 12 months. Front-line health workers delivered the intervention to all households in study communities with a child aged 6–18 months. Data were collected using cross-sectional surveys at baseline and endline by selecting households from intervention (baseline, n 2184; endline, n 2170) and control (baseline, n 2176; endline, n 2122) communities using a two-stage cluster-randomised sampling strategy. Children in the intervention group had a significantly larger improvement from baseline to endline compared with those in the control group on scores for motor and mental development (Cohen’s d, motor=0·12; 95 % CI 0·03, 0·22; mental=0·15; 95 % CI 0·06, 0·25). Greater impacts of MNP on motor and mental development were observed in children from households with higher stimulation scores at baseline compared with those with lower stimulation (Cohen’s d, motor=0·20 v. 0·09; mental=0·22 v. 0·14; Pinteraction<0·05). No significant treatment differences were seen for executive function or memory. Home fortification with MNP through the existing health infrastructure in Bihar was effective in improving motor and mental development and should be considered in combination with other child development interventions such as stimulation.
A comparison was made of 6H-SiC surfaces etched with H2, C2H4/H2, and HCl/H2, and the resulting crystal quality of epitaxial GaN films deposited on these substrates. To remove the many fine scratches and to smooth the rough surfaces typical of commercial SiC substrates, the Si-face 6H-SiC substrates were etched in H2, C2H4/H2, and HCl/H2 at 145 °C. GaN was subsequently deposited on these etched surfaces after first depositing a low temperature GaN buffer layer via metalorganic chemical vapor deposition (MOCVD). The surface morphologies after etching and after GaN deposition were characterized by atomic force microscopy and Normaski differential interference contrast microscopy, while the crystal quality of the GaN films was assessed by double crystal x-ray rocking curves and x-ray topography. 6H-SiC substrate surfaces were improved in terms of the removal of scratches and the reduction of surface roughness, and both surface morphology and crystal quality of the subsequently deposited GaN films were enhanced. However, the dislocation density was not decreased by the surface etching. The best GaN film was produced by etching the substrate in pure H2 for 40 minutes before growth. Recommendations for the optimum substrate treatment are made.
Boron was incorporated into GaN in order to determine its limits of solubility, its ability of reducing the lattice constant mismatch with 6H-SiC, as well as its effects on the structural and optical properties of GaN epilayers. BxGa1−xN films were deposited on 6H-SiC (0001) substrates at 950 °C by low pressure MOVPE using diborane, trimethylgallium, and ammonia as precursors. A single phase alloy with x=0.015 was successfully produced at a gas reactant B/Ga ratio of 0.005. Phase separation into pure GaN and BxGa1−xN alloy with x=0.30 was deposited for a B/Ga reactant ratio of 0.01. This is the highest B fraction of the wurtzite structure alloy ever reported. For B/Ga ratio ≥ 0.02, no BxGa1−xN was formed, and the solid solution contained two phases: wurtzite GaN and BN based on the results of Auger and x-ray diffraction. The band edge emission of BxGa1−xN varied from 3.451 eV for x=0 with FWHM of 39.2 meV to 3.465 eV for x=0.015 with FWHM of 35.1 meV. The narrower FWHM indicated that the quality of GaN epilayer was improved with small amount of boron incorporation.
High resolution Schottky barrier detectors for alpha particles have been fabricated on 20 μm n-type 4H-SiC epitaxial layers. Schottky barrier contact structure was accomplished by deposition of 10 nm nickel on the Si face of the epilayers. The detectors were characterized for structural, electrical, and spectroscopic properties. Scanning electron microscopy and Nomarski optical microscopy revealed a micropipe density lower than 1 cm-2. The current-voltage (I-V) characteristics of the device exhibited very low leakage current of the order of 6.5 pA at an operating bias of 90 V. C-V measurements revealed a typical effective doping concentrations of 2.4 × 1014 cm-3 in these epilayers. The detectors were evaluated for alpha particles detection using a 241Am source. An energy resolution of ∼0.98% for 5.48 MeV alpha particles was observed. The separate contribution of charge carrier drift and diffusion to the total charge collection efficiency has been calculated in these detectors following a drift-diffusion model. Detailed electronic noise analysis in terms of equivalent noise charge (ENC) was carried out to study the effect of various noise components that contribute to the total electronic noise in the detection system. Effect of shaping time, presence of source and bias on the ENC has been studied in details.
Cd0.9Zn0.1Te (CZT) single crystal has been grown using a tellurium solvent method. Two CZT crystals have been chosen from two different locations of the grown ingot. The two crystals were characterized using infrared transmission (IR) imaging and radiation detectors in planar geometry were fabricated on them. Current-voltage characteristics (I-V) revealed a resistivity of ∼8.6×1010 Ω−cm for detector A (6.9×6.9×4.8 mm3) and 6.7×1010 Ω−cm for detector B (11.5×11.7×2.6 mm3). IR imaging showed a lesser concentration of Te inclusions/precipitates in detector A. The transport properties viz., electron drift-mobility and electron mobility-lifetime product were measured using alpha spectroscopy in these detectors in a planar configuration. Detector A showed better charge transport properties compared to detector B. The superior transport properties of crystal A were reflected in the spectroscopic properties of the detectors. Gamma pulse height measurements using a 241Am isotope revealed an energy resolution of ∼5 % for detector A and ∼7 % for detector B. A digital spectrometer and a biparametric correction scheme was incorporated to recover the pulse height spectrum of high energy gamma rays (137Cs source) from the effect of poor hole movement.
Eu3+/Tb3+co-doped YBO3 three-dimensional (3D) microstructures have been hydrothermally prepared by adjusting solvent and the molar ratio of Y3+ to B (Y/B) at 180 °C. The whole process was carried out under alkaline conditions without the use of any surfactant or catalyst. Characterizations of the samples are carried out using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). The photoluminescence (PL) colors of YBO3 sample co-doped with Eu3+ and Tb3+ under ultraviolet excitation can be tuned from red, through yellow and green-yellow, to green by changing the relative doping concentrations of the two activator ions. These phosphors with multicolor emissions in the visible region be potentially used as labels for light-display systems, optoelectronic devices and biological molecules.
Four series of samples, prepared at 250° C by decomposition of a mixture of silane and argon in a radio frequency powered deposition systems (rf-PECVD), have been studied. The dilution rates were 1 %, 1.5 %, 5 % and 10 % of silane in argon and the total pressure was 0.5 Torr for the first series and 0.2 Torr for the others. Structural and transport properties of the materials have been studied as function of power density. Structural studies show the transition from purely amorphous material towards microcrystalline material with increasing rf power density. The transport parameters were measured in the as-deposited, light-soaked and annealed states and compared to those obtained on state of the art material. The best material obtained is clearly device grade material. This study shows that argon dilution allows to tailor the material for a given application.
The strain distribution within individual layers in Au/Ni multilayer systems was obtained by an iterative fitting of the experimental x-ray diffraction pattern with a kinematic model. The depth profile of strain in the modulation direction was obtained for these metallic multilayers with repeat periodicities ranging from 1.19 nm to 4.26 nm. It was found that the role of interfacial coherency and strengthening is of great importance in understanding the origin of the supermodulus effect in metallic multilayers.
A new x-ray diffraction method is developed to determine the full elastic strain tensor and its distribution about a strain center in single crystal materials. It is based on the recently developed Computer Aided Rocking Curve Analyzer and is particularly well suited for analysis of thin film structures common to electronic materials. This technique will be described in detail, and its application in measuring the non-uniform strains in InGaAsP epitaxial film on InP substrate will be presented. Also, possibility of using this method to measure the uniformity of film thickness will be discussed.
Planar strain in CaF2 and Ge/CaF2 films grown on (111) Si substrate has been measured by an x-ray double crystal diffraction technique using rocking curves. The films grown by a solid phase epitaxial approach using in situ rapid isothermal processing are found to have small tensile planar strain.
Heterostructures of SiC and AlN in either sequence, AlN on SiC or SiC on AlN, were grown on Si, Al2O3, and 6H-SiC substrates by (metalorganic) chemical vapor deposition (CVD). On Si substrates, a SiC layer was first grown by a two-step technique and an AlN layer was deposited subsequently. On other substrates, an AlN layer was first grown, followed by SiC deposition. Multi-layered structures (SiC/AlN/SiC) were also produced to demonstrate the ability of heteroepitaxy of SiC and AlN on each other.
AlN grown on 3C-SiC were highly oriented polycrystalline films. AlN films on 6H-SiC, SiC films on A1N/Al2O3, and SiC films on AlN/6H-SiC were single crystal. In the latter two cases, the SiC films were in hexagonal structure. These SiC films were smooth and specular in appearance and showed n-type conductivity.
A dynamical x-ray diffraction theory has been used to obtain microscopic strain profiles in thin Au/Ni multilayers. Depth profiles of strains in these multilayers, with repeat periodicities varying from 0.82 nm to 9.0 nm, are obtained by an iterative fitting of the calculated diffraction pattern with the experimental one. Interfacial coherency is found to play an important role in understanding the origin of the supermodulus effect in metallic multilayers.
Soft X-ray photoelectron spectromicroscopy has been employed to examine the microstructural chemistry of corrosion on thin films of AL-i%Cu-1%Si alloy. Cu-rich precipitates are formed after the film is annealed at around 320ºC Localized corrosion takes place around these precipitates, and circular microstructures have been observed. Energy distribution curves (EDCs) taken at specific points are analyzed and photoelectron images were obtained from different kinetic energies corresponding to the peaks in EDC. It was found that after corrosion the Al2Cu particles are revealed and distributed at the center of the corrosion structure with oxide as matrix, while the oxides form a ring-like band outside.