Nitric-phosphoric acid etching effects on the surface chemical composition of CdTe thin film

Nitric-phosphoric (NP) acid etching has been regarded as one of the most effective methods for the formation of low resistance back contact with the metal electrode in CdTe based photovoltaic cells. We studied CdTe back surfaces and the changes with time of exposure to NP acid with x-ray photoelectron spectroscopy (XPS), and atomic force microscopy. Strong etching dependence on the back surface chemical composition, and surface roughness, was observed. In order to study the effect of the NP acid etching on surface degradation, the sample was left in open ambient condition for three weeks and XPS measurement in combination with ion sputtering was performed on unetched and highly etched parts. The difference in depth profiles of the NP acid etched and unetched CdTe surface has been discussed.     

Effect of back-contact barrier on thin-film CdTe solar cells

The presence of a back-contact barrier affects the current–voltage characteristics of thin-film CdS/CdTe/metal solar cells primarily by impeding hole transport, a current-limiting effect commonly referred to as “rollover.” In this work, the CdS/CdTe solar cell with a CdTe/metal back-contact barrier is modeled by two opposite polarity diodes in series. Analytic simulations are fitted to the measured current–voltage curve, the voltage distribution between the two diodes is shown under different conditions, and the back-contact barrier height is extracted. Room-temperature barrier heights exceeding 0.5 eV will generally result in significant fill-factor reduction.     

Ion etching methods for depth profiling of complex three-dimensional samples in combination with scanning Auger electron microscopy

Ion etching of surfaces combined with detection of secondary events (particles or radiation emitted) are used for depth profiling of samples with interesting features at-, near-, or somewhat below the surface. These methods are destructive and relatively slow, and compete with non-destructive methods like Rutherford backscattering spectroscopy, energy-dispersive X-ray spectroscopy in the scanning electron microscope or angle-resolved photoemission spectroscopy, which are non-destructive and relatively faster methods. In this work we have concentrated on the use of noble gas ion sputtering with low-energy beams in combination with electron excited Auger electron spectroscopy and imaging for analysis of nanostructured and microstructured samples. No attempt will be made here to justify this method over the other methods, as their relative merits depend on the nature of the sample and the problem at hand. We have thus chosen to study samples and problems for which this technique would be obvious to use. This work is also aimed at providing practical standards and guidelines (“metrology”) for the use of the technique in the context of industrial nanotechnology. The use of Auger electron spectroscopy instead of photoemission spectroscopy is preferred for laterally non-uniform samples due to the presently better resolution capabilities of electron beams and narrower information depths of typical Auger electron transitions. The use of Auger electrons for concentration sampling, and low-energy beams of noble gas ions for sputtering, reduces the adverse influence of atomic mixing. In this report two systems are intensively studied with sputter profiling in combination with Auger electron spectroscopy and scanning electron imaging: a hard disk and a surface of a stainless-steel sample.     

Atomic force microscopy imaging to measure precipitate volume fraction in nickel-based superalloys

In nickel-based superalloys, quantitative analysis of scanning electron microscopy images fails in providing accurate microstructural data, whereas more efficient techniques are very time-consuming. As an alternative approach, we propose to perform quantitative analysis of atomic force microscopy images of polished/etched surfaces (quantitative microprofilometry). This permits the measurement of microstructural parameters and the depth of etching, which is the main source of measurement bias. Thus, nonbiased estimations can be obtained by extrapolation of the measurements up to zero etching depth. In this article, we used this approach to estimate the volume fraction of γ′ precipitates in a nickel-based superalloy single crystal. Atomic force microscopy images of samples etched for different times show definition, homogeneity, and contrast high enough to perform image analysis. The result after extrapolation is in very good agreement with volume fraction values available from published reports.     

Interaction of gold, palladium and Au-Pd alloy deposits with oxidized Si(100) substrates

The growth of thin gold, palladium and Au-Pd alloy deposits from the vapour phase on Si(100) wafer material covered with thermally grown or “native” oxide layers was studied by means of Auger electron spectroscopy and reflection high energy electron diffraction as well as by scanning electron microscopy and Auger sputter profiling. The mean thickness of the metal and alloy deposits ranged from 3 to 15 nm. The temperature dependence of the growth modes and crystallographic texture was investigated in the range from room temperature to 620°C. Post- deposition annealing experiments were carried out at temperatures of up to 620°C for up to 60 h. Palladium was found to penetrate through “native” oxide interfaces, probably at inhomogeneities, and palladium silicide formation was observed starting at temperatures as low as 220°C. Almost no diffusion of gold through the “native” oxide was detected. In the case of alloy deposits, segregation led to an enrichment of gold at the surface while considerable amounts of palladium diffused into the silicon substrate to form silicide. Almost no diffusion of gold and palladium through thermally grown oxide interfaces of thickness 7 nm could be detected under our experimental conditions. However, indications for a diffusive Pd-SiO₂ interaction were found by an enrichment of palladium and oxygen near the alloy- oxide interface in annealed deposits even on this type of oxide.     

The release of 5-fluorouracil from microspheres of poly(epsilon-caprolactone-co-ethylene oxide)

The purpose of this study was to evaluate the in vitro release of 5-fluorouracil from microspheres prepared using a novel triblock copolymer of ε-caprolactone and ethylene oxide as the encapsulating material. Microspheres of poly(ε-caprolactone-co-ethylene oxide) were prepared by employing the “hot-melt” method of microencapsulation. Microspheres were sized using sieve analysis and scanning electron microscopy (SEM). Release studies were performed using a custom-made rotating paddle dissolution apparatus. Copolymer microspheres, fabricated by the hot melt method were shown by electron microscopy to have smooth, nonporous surfaces. Drug-loaded microspheres were found to have a broad distribution of sizes, which was thought to be a consequence of the wide range of crystal sizes of the encapsulated unmilled drug. Nonlinear release kinetics were observed from microspheres in the size fraction 75-250 μm, with a pronounced “burst release” associated with the presence of drug at the surface of the microspheres. A specific delineation of the drug release mechanism was not possible due to rapid gelation, swelling, and subsequent dissolution of the microspheres that occurred on hydration. This work describes the preparation of microspheres that swell rapidly and coalesce together on hydration, accompanied by rapid drug release and copolymer dissolution over a 2-hr period.     

AlxGa−xAs/GaAs heterostructure characterization by wet chemical etching

Selective wet chemical etching of the Al_xGa_1−xAs/GaAs system has been applied to heterostructure characterization. Samples of LPE grown AlGaAs/GaAs laser double-heterostructures and separate confinement heterostructures as well as antiresonant reflecting optical waveguides heterostructures were treated with “I2 solution” (I₂:KI:H₂O) and hydrochloric acid. These compounds selectively etch the ternary Al_xGa_1−xAs layers, but with different “threshold composition” x_th values (the x value is that above which the etching rate of a given compound increases sharply). Selectively etched samples have been examined by SEM. The experimental dependence of etching rate on the x value for I2 solution has been derived. From this dependence, the x composition of any ternary layer can be estimated simply. Observations were made of the “microscopic” properties of the heterostructure, such as the smoothness of the interfaces and the uniformity of layers. All imperfections resulting from the growth process, such as interface perturbations or compositional nonuniformity of layers, are clearly seen. An additional advantage of this etching technique is its simplicity. It allows quick examination of grown heterostructure for the selection of wafers for further processing.     

A computational approach to determining the depth of surface flaws by the ACPD technique

The boundary element method is used to analyze the ACPD (alternating current potential drop) field perturbed by a semi-elliptical surface flaw in a metallic sheet. The depth of the penetration of the alternating current is assumed to be small as compared to the crack depth. This allows us to adopt the “unfolding” technique developed by Dover et al. [ASTM STP 722, 401–427 (1981)], which reduces the problem to a two-dimensional surface Laplacian field. This surface potential field is determined for different surface cracks with crack aspect ratios ranging from 0.2 to 2. To facilitate the use of these results, an empirical formula is developed based on the numerical results. A simple inverse analysis methodology is also developed for determining the crack depth for semi-elliptical surface flaws of known surface length with measured potential drop readings.     

Electron swarming in nanostructures

Some applications of the quantum field chemistry concept for computer simulations of the condensed state nanostructural processes are proposed and discussed. New concept of strong-correlated electron subsystem confined inside closed compact basin of physical space – an electron “swarm” – is used for extension of the quantum topology definitions. The approach gives some new treatments not only for atoms and an electron environment of bonds, but also for a new quantum topology of lodges, which play a significant role in nano-sized processes of matter. Every lodge is closed in 3D-space with a special zero-gradient density boundary and has N-quantized electric charge. The N-electron lodge can be vacant or occupied by strong-correlated N-electron swarm. Occupied N-lodges can represent not only nanocrystals, but also molecules and “corpuscular” electrons. A simple model of g-dimensional electron swarming wells arranged through interatomic regions inside nanosystem interfaces and channels in materials is calculated and discussed.     

PARTICLE-SIZE DISTRIBUTION INFLUENCE IN HIGH-SPEED EROSION OF ALUMINIUM

The paper discusses the influence of the abrasive particle-size distribution on typical high speed abrasive-waterjet erosion parameters. The size distributions of the used abrasive particles are modelled by a Rosin-Rammler-Sperling (RRSB) grain-size distribution containing the distribution parameters D and n. Both parameters are independently varied to characterise different particle-size distributions. Aluminium specimens are eroded by abrasive-waterjets at velocities of 320 m/s, and the erosion depth, depth distribution, and the surface roughness are measured. The depth distribution and the surface roughness are very sensitive to the particle-size distribution parameters, whereas the average erosion depth is not influenced significantly. These results offer the possibility to select an “optimum” grain-size distribution for maximum surface quality at fixed kinematics conditions.     

Improvement of Heater Surface Flatness and Uniformity of TiN Film Deposition

The deposition rate and the thickness uniformity of chemical vapor deposition (CVD) titanium nitride films depend on wafer temperatures. The heater surface conditions, such as flatness, roughness, and surface imperfections, can greatly affect heat transfer efficiency from the heater surface to the wafer, and the process performance. Because heater surface imperfections or “hot spots” that caused poor uniformity had to be eliminated, the origin of “hot spots” was identified by a detailed study of heater surface profiles. A better visualization of “hot spots” could be obtained by comparing wafer-chucking patterns with deposition patterns. Thus, the time needed to locate “hot spots” could be shortened. The manufacturing process was revised to prevent “hot spots” and improve heater performance.     

Structure and formation energy of steps on the GaAs(001)-2 × 4 surface

The energies of various steps on the As-terminated GaAs(001)-2 × 4 surface are evaluated using a novel, approximate method of “linear combination of structural motifs”. It is based on the observation that previous total energy minimizations of semiconductor surfaces produced invariably equilibrium structures made of the same recurring local structural motifs, e.g. tetrahedral fourfold Ga, pyramidal threefold As, etc. Furthermore, such surface structures were found to obey consistently the octet rules as applied to the local motifs. We thus express the total energy of a given semiconductor surface as a sum of (i) the energies _M of the local structural motifs appearing in the surface under consideration and (ii) an electrostatic term representing the Madelung energy of point charges resulting from application of the octet rule. The motif energies are derived from a set of pseudopotential total energy calculations for flat GaAs(001) surfaces and for point defects in bulk GaAs. This set of parameters suffices to reproduce the energies of other (001) surfaces, calculated using the same pseudopotential total energy approach. Application to GaAs(001)-2 × 4 surfaces with steps reveals the following. (i) “Primitive steps”, defined solely according to their geometries (i.e. step heights, widths and orientations) are often unstable. (ii) Additional, non-geometric factors beyond step geometries such as addition of surface adatoms, creation of vacancies and atomic rebonding at step edges are important to lower step energies. So is step-step interaction. (iii) The formation of steps is generally endothermic. (iv) The formation of steps with edges parallel to the direction of surface As dimers (A steps) is energetically favored over the formation of steps whose edges are perpendicular to the As dimers (B steps).     

Hypervelocity impact of rod projectiles with L/D from 1 to 32

Beside a short remark on the “hydrodynamic theory of rod projectiles”, the paper deals with the terminal ballistic behaviour of cylindrical projectiles against semi-infinite targets. Experimental data of EMI, completed by results of some other authors, are presented. Crater parameters like depth, diameter and volume and their dependence on projectile velocity (up to 5000 m/s), projectile and target material properties, as well as L/D-ratios (1–32), will be discussed. Mainly the projectile materials steel and tungsten sinter-alloys are considered. Target materials are mild steel and high strength steel, an Al-alloy and a tungsten sinter-alloy. The results show that the influence of material density on the crater dimensions is considerably greater than the influence of strength. The L/D ratio determines the velocity dependence of crater depth, diameter and volume. At high velocities in the hydrodynamic regime, the crater depth of short cylinders (L/D 1) is approximately proportional to v_p^2/3 (V_p=projectile velocity). With increasing L/D-ratio, the slope of the penetration curves decreases and converges for rods (L/D 1) versus a saturation, i. e. becomes nearly independent on v_p. A consequence of this saturation is the existence of a so-called “tangent velocity”, above which an optimal increase of efficiency is only realized by increasing the projectile mass and not the velocity. Furthermore, ballistic limits of real targets like single plates and symmetric double plates meteorite bumper shield) are taken into account. The expected better performance of “segmented rods” is also discussed.     

A Flow-Through Dissolution Approach to In Vivo/In Vitro Correlation of Adinazolam Release from Sustained Release Formulations

A Copley^TM fraction collector and a Disotest^TM flow-through system were coupled to provide an automatic discrete sampling flow-through dissolution system for use both in the “open-loop” and “closed-loop” mode. The system was used to investigate the release characteristics of adinazolam in sustained release formulations using a pH 1.2 simulated gastric fluid (without enzymes) dissolution medium (USP XXI). These experimental formulations are designed to provide relatively slow to rapid drug release. The dissolution effluent was analysed off-line by reverse phase HPLC to determine the adinazolam concentration at programmed timed intervals. The differential dissolution profiles produced when the system is used in the “open-loop” configuration are more discriminating in describing the release characteristics of the formulations according to the relative release rates than the “closed-loop” cumulative profiles. Using the characteristic dissolution time parameter from the Weibull function, a better correlation with in vivo bioavailability data was achieved for the data from the system in the “open-loop” mode than when it was used in the “closed-loop” mode. In the “open-loop” mode the Weibull function characteristic dissolution time parameter yielded the best quantitative correlation with a correlation coefficient of 0.92 compared to a value of 0.85 for the “closed-loop” configuration     

A microstructural study on the surface and interface of CdTe/CdS solar cells

The surface and interface properties of CdTe/CdS solar cells, including interfacial mixing, surface and interface geometrical morphology, CdTe grain size and preferential crystal orientation of CdTe layers were studied using Auger electron spectroscopy (AES) depth profiling, atomic force microscopy (AFM), Fourier transform infrared (FTIR) spectroscopy, optical reflectance (OR) and X-ray diffraction (XRD) techniques. The correlation between the surface and interface properties and CdTe/CdS solar cell performance was also investigated. AES depth profiling was used to analyse the interdiffusion between the CdTe and CdS layers. Atomic force microscopy (AFM) suggests that the interfacial geometrical morphology has a significant influence on the photovoltaic property of CdTe/CdS solar cells. Rough interfaces tend to increase the photovoltaic conversion efficiency of solar cells because of multiple reflections. X-ray diffraction shows that polycrystalline CdTe/CdS solar cells with higher efficiencies appear to be orientated with more (1 1 1) planes of CdTe parallel to the macrosurface, but CdTe single crystals with differently indexed surface planes show almost the same reflection behaviour. Further theoretical and experimental analyses are therefore needed to clarify this observation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of Particle Size on Deformation and Compaction Characteristics of Ascorbic acid and Potassium Chloride: Neat and Granulated Drug

Deformation and compaction characteristics of two soluble drugs, ascorbic acid and potassium chloride, were investigated. Five different particle size fractions of ascorbic acid with mean particle size (d₅₀) ranging from 30-300μm and four different particle size fractions of potassium chloride with d₅₀ ranging from 20-400 μm were selected in the study. The compaction behavior of the drug substances as neat drugs or as granulated drugs were evaluated on both a Carver press and an instrumented single-punch tablet press. The results clearly show that mean particle size of the drug substances plays an important role in their compactibility. Intrinsic compactibility of both drug substances was slightly improved with increased particle size. Granulations of the drugs with polyvinyl pyrrolidone significantly improved their compactibility. However, this effect was more pronounced in the drug substance with finer particle size. The Heckel plots indicate that deformation characteristics of both granulated drugs were related to their original mean particle sizes. The granulations prepared from the coarser particle size (d₅₀ 250 μm to 400 μm) underwent two stages of deformation, so-called “brittle fracture” and “plastic deformation”. While the granulations prepared from the finer particle size predoninantly underwent “plastic deformation”. The results indicated that the plastic deformation of both granulated drugs was progressively enhanced whilst fragmentation of particles was correspondingly reduced as the particle size of the drugs was decreased. Scanning electron photomicrographs indicated that the granulation process changed the surface morphology of the drug particles imparting more “microirregularities” or “defects”, thereby providing greater “interparticulate bonding” as compared with the neat drugs. Optimum particle size range of ascorbic acid and potassium chloride for satisfactory compactibility was found to be 30-40 μm and 20-40 μm, respectively. The present study demonstrates the importance of selecting the appropriate particle size of drug for the development of tablet dosage forms.     

Effect of Cryogrinding on Physico-Chemical Properties of Drugs. II. Cortisone Acetate: Particles Sizes and Polymorphic Transition

The low temperature grinding (at 78 K) of cortisone acetate was carried out. From electron microscopy data, there were determined a form, mean linear sizes of particles and then specific surface was calculated. From X-ray data, a part of microcrystalline fraction in cryogrinded samples was calculated. As was indicated by X-ray data, the mechano-induced phase transition from “monoclinic” phase (form F_I) to “orthorhombic” one (form F_II) takes place as a result of I0-min grinding. The transition is confirmed by IR-spectroscopy results too. The local pressures induced by the mechanical stress seems to be the main cause of the phase transition observed.     

Imaging X-ray fluorescence spectroscopy: laboratory measurements

Following on from the proof-of principle measurements of Martin et al. (X-ray Spectrom. 28 (1999) 64) we further describe the development of an imaging X-ray fluorescence (IXRF) spectrometer with no moving parts. Our laboratory system is based on a microchannel plate (MCP) “lens”, a CCD X-ray detector with good sub-keV quantum efficiency and a conventional electron bombardment X-ray source. We have used this equipment to form images of a standard XRF target, demonstrating that “elemental maps” (images of the target in the characteristic X-rays of one particular element) may be formed with sub-millimetre resolution. In addition to fluorescent X-rays, we detected X-rays which had been Bragg reflected from the polycrystalline aluminium substrate of the target. It is possible that the resulting “Bragg images” may be exploited to measure spatially varying strain, manifested as lattice distortion, introduced, for example, by thin films deposited on the surface of a sample.     

Effect of nitrogen on diamond growth using unconventional gas mixtures

The influence of nitrogen on the growth of diamond using unconventional gas mixtures of CH₄---CO₂ by microwave plasma chemical vapor deposition was investigated. A clear improvement in the surface morphology and quality of the diamond films indicates the beneficial effect of adding nitrogen to CH₄-CO₂ gas mixtures. However, most interestingly, for lower methane concentration, the addition of small amounts of nitrogen resulted in the formation of isolated diamond particles possessing a vacant “cage-like” structure with completed {100} facets This result indicates that the continued addition of nitrogen gives rise to the deterioration of {111} facets and the retention of {100} facets. Analysis using Auger electron spectroscopy and secondary ion mass spectroscopy shows very low and uniform levels of nitrogen in the diamond films. Although the amount of atomic hydrogen in the ground state decreased and CN radicals increased with increasing amounts of added nitrogen, good-quality diamond films were deposited resulting from a larger amount of atomic oxygen and the decrease in the C₂ emissions in the gas phase under optimum conditions.     

Molecular beam epitaxy of CdTe and CdxHg1−xTe on InSb

The crystalline quality of InSb substrates and their transparency in the 8–12 μm IR window compare favourably with those of CdTe. We report the first results obtained for Cd_xHg_1−xTe (CMT) layers grown using molecular beam epitaxy over a buffer layer of CdTe on InSb. The (100)-oriented InSb wafer is chemically cleaned prior to thermal treatment which may be accompanied by ion beam cleaning; this process leads to a (2x4) reconstructed surface. Characterization of the surface using Auger spectroscopy, UV photoemission spectroscopy and X-ray photoelectron spectroscopy shows that it is free from oxygen and carbon. CdTe is grown at a substrate temperature of 523 K using a single CdTe cell. When 0.5 μm of CdTe has grown, the substrate temperature is lowered and the CMT is deposited epitaxially using three different cells for cadmium, mercury and tellurium. The layers exhibit X-ray rocking curves 6' wide and n-type conductivity with n = 3× 10¹⁵ cm⁻³ and μ = 40 000 cm² V⁻¹ s⁻¹. Measurement of the strain in each layer shows that CMT is totally relaxed on the CdTe which in turn grows coherently on the InSb.