Auger electron spectroscopy and secondary ion mass spectrometry depth profiling with sample rotation

Recently, there has been a rapid increase in the application of multilayered structured materials, as opposed to bulk materials, in many areas of technological development. Accurate characterization of the structure and composition of advanced multilayers such as superlattices, quantum wells, contacts, and coatings is important for materials and device fabrication technology. Surface analysis techniques including Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy, and secondary ion mass spectrometry (SIMS) in conjunction with ion beam sputtering (sputter depth profiling) are at present the most widely used methods for characterization of modern multilayer thin film materials and devices. Ion-beam-induced surface topography, however, can limit depth resolution, and with SIMS, can also cause changes in the secondary ion yield. These changes are due to the high sensitivity of secondary ion yield to the local angle of incidence on sputter-roughened surfaces. Degradation of depth resolution and changes in secondary ion yields during sputter depth profiling have often limited studies of thin film interdiffusion, segregation, oxidation at interfaces, and impurity effects. Much theoretical and experimental work has been carried out to try to improve depth resolution including the use of low ion beam energy, high angle of incidence, and two ion guns. Recent studies of AES and SIMS with sample rotation have shown that depth resolution can be improved substantially and that constant secondary ion yields in SIMS can be achieved. We will first provide an overview of the studies made by various groups to improve depth resolution of metal multilayers using AES with rotation. Next we will review recent investigations of SIMS using sample rotation including studies of the effects of sample rotation on O₂⁺ ion-beam-induced topography, secondary ion yield, and the depth resolution of electronic, metallurgical and dielectric materials. The results presented demonstrate that SIMS with sample rotation provides constant secondary ion yield, and depth-independent depth resolution because sample rotation prevents ion-beam-induced roughness and reduces the effect of the inhomogeneity of low energy ion beams.     

Interaction of optical radiation with GaAs nanowhisker arrays

We have studied the reflectivity spectra of doped GaAs nanowhiskers (NWs) with various morphologies, which were grown by molecular-beam epitaxy. It is established that the character of the reflectivity spectra of NWs is determined by the shape of nanocrystals. NWs with droplike and pointlike vertices differently interact with electromagnetic radiation. Laser radiation produces a spectraly selective action on the NW array, which leads to a change in the NW height and induces “caking” of their vertices, thus modifying the reflectivity of the sample. This phenomenon can be used for the creation of local microstructures with preset characteristics in large NW arrays.     

Temperature variation of the morphology of nanocluster ensembles in the Ge/Si(100) system

The morphological characteristics of hut-cluster ensembles formed in a Ge/Si(100) heteroepitaxial system have been studied as functions of the substrate surface temperature by theoretical methods and by atomic force microscopy. As the temperature increases from 420 to 500°C, the lateral size of nanoclusters with a square base (grown at the same rate of 0.0345 ML/s to a total coverage of 6.2 ML) grows from 12 to 20 nm, while their number density on the substrate surface drops from 5.6×10¹⁰ to 1.5×10¹⁰ cm⁻². Predictions of a kinetic model are in sufficiently good agreement with the experimental data.     

Control over the parameters of InAs-GaAs quantum dot arrays in the Stranski-Krastanow growth mode

The effect of the growth temperature on the density, lateral size, and height of InAs-GaAs quantum dots (QD) has been studied by transmission electron microscopy. With the growth temperature increasing from 450 to 520°C, the density and height of QDs decrease, whereas their lateral size increases; i.e., the QDs are flattened. The blue shift of the photoluminescence line indicates decreasing QD volume. The observed behavior is in agreement with the thermodynamic model of QD formation. The effect of lowering the substrate temperature immediately after the formation of QDs on the QD parameters has been studied. On lowering the temperature, the lateral size of QDs decreases and their density increases; i.e., the parameters of QD arrays tend to acquire the equilibrium parameters corresponding at the temperature to which the cooling is done. The QD height rapidly increases with cooling and may exceed the equilibrium value for a finite time of cooling, which enables fabrication of QD arrays with a prescribed ratio between height and lateral size by choosing the time of cooling.     

Threshold behavior of the formation of nanometer islands in a Ge/Si(100) system in the presence of Sb

Atomic-force microscopy is used to study the behavior of an array of Ge islands formed by molecular-beam epitaxy on an Si (100) surface in the presence of an antimony flux incident on the surface. It is shown that, as the Sb flux increases to a certain critical level, the surface density of the islands increases; however, if this critical level is exceeded, nucleation of the islands is suppressed and mesoscopic small-height clusters are observed on the surface. This effect is explained qualitatively in the context of a kinetic model of the islands’ formation in heteroepitaxial systems mismatched with respect to their lattice parameters.     

Terahertz radiation generation in multilayer quantum-cascade heterostructures

I–V and radiative characteristics of multilayer quantum-cascade GaAs/AlGaAs heterostructures with a double metal waveguide are investigated. The I–V characteristics typical of the quantum-cascade lasers are seen. The observed threshold-intensity growth and narrow radiation spectrum are characteristic of laser generation. The measured radiation frequency was found to be about 3 THz, which coincides with the calculated value. Thus, fully domestic terahertz quantum-cascade lasers are demonstrated for the first time.     

The discovery of biomarkers for type 2 diabetic nephropathy by serum proteome analysis

Diabetic nephropathy (DN) is a serious kidney complication of diabetes, and constitutes the leading cause of end-stage renal disease. The earliest clinical evidence of DN is microalbuminuria, a term which refers to the appearance of small but abnormal amounts of albumin in the urine. However, screening methods for DN, such as biomarker assays, are yet to be developed for type 2 DN. In the present study, in an attempt to identify the biomarkers for initial diagnoses of type 2 DN, the protein profiles of human sera collected from 30 microalbuminuric type 2 diabetic patients were compared with those collected from 30 normoalbuminuric type 2 diabetic patients, via 2-DE. As a result, a total of 18 spots were determined to have different protein levels in the microalbuminuric patients. Twelve spots had lower protein levels of approximately 50%, and the other six had higher levels of approximately 100-300% as compared to the spots of normoalbuminuric patients. These spots were identified with ESI-Q-TOF (ESI-quadrupole-TOF) MS. Among the identified proteins, vitamin D-binding protein (DBP) and pigment epithelium-derived factor (PEDF) were verified by Western blotting. The results of this study indicate that the DBP may be employed as diagnostic and monitoring biomarkers of type 2 DN, contingent on further study into the matter.     

Cloning of orotidine-5′-phosphate decarboxylase (URA3) gene from sourdough yeast Candida milleri CBS 8195

The nucleotide sequence of the URA3 gene encoding orotidine-5′-phosphate decarboxylase (OMPDCase) in sourdough yeast Candida milleri CBS 8195 was determined by degenerate PCR and genome walking. Sequence analysis revealed the presence of an openreading frame of 810 bp, encoding 269 amino acid residue protein with the highest identity to the OMPDCase of the yeast Saccharomyces cerevisiae. Phylogenetic analysis of deduced amino acid sequence revealed that it shares a high degree of identity with other yeast OMPDCases. The cloned URA3 gene successfully complemented the ura3 mutation in S. cerevisiae, indicating that it encodes a functional OMPDCase in C. millieri CBS 8195.     

Hybrid AlGaAs/GaAs/AlGaAs nanowires with a quantum dot grown by molecular beam epitaxy on silicon

Data on the growth features and physical properties of GaAs inserts embedded in AlGaAs nanowires grown on Si(111) substrates by Au-assisted molecular beam epitaxy are presented. It is shown that by varying the growth parameters it is possible to form structures like quantum dots emitting in a wide wavelength range for both active and barrier regions. The technology proposed opens up new possibilities for the integration of direct-band III–V materials on silicon.     

Control of H2O generated during the CO2 hardening process in a casting mold

An additive reagent was introduced into a water glass binder system for enhancing the mechanical properties and dimension stability of a casting mold, and for improving the surface quality of a cast product. Two different processes with three different additive reagents were employed to investigate the relation between fracture strength of the mold and water (H₂O) existed in the mold. In processes I and II, the mold samples were coated with a water glass binder, and then dipped into different solutions with additive reagent after and before carbon dioxide (CO₂) hardening, respectively. The fracture strength of the mold was enhanced by reducing H₂O content in the mold, achieved by a hydrolysis reaction of additive reagents. In process I, the H₂O movement was restricted in the mold by the solid phase, converted from the water glass during CO₂ hardening. When employing process II, especially in the TEOS used as additive reagent, the fracture strength was significantly increased due to the effective reduction of H₂O content in the mold and the homogeneous generation of glass phase by a sol–gel reaction of the additives.