High-resolution Medium Research Articles

Structure of the SiNx/GaAs (110) interface modified with ultrathin Si and sulfur passivation

The SiNx/GaAs (110) interface structure of the metal-insulator-semiconductor (MIS) capacitors modified with ultrathin Si or Si/Ge (5 to 15 Å) heterolayers and sulfur passivation were studied by capacitance-voltage (CV) measurements, high resolution medium energy ion scattering and extended x-ray absorption fine structure with synchrotron radiation. The results showed that Si/S multilayer passivation can improve the interface properties of the MIS capacitors made on n-type GaAs (110). The element depth profiling in a subnanometer scale and the atomic structure measurements showed that a Si or Si/Ge heterolayer on sulfur passivated GaAs (110) can substantially limit Ga(As) precipitation at the interface but cannot completely suppress the incorporation of Ga(As) into the Si heterolayer. These results can be used to interpret the observed CV characteristics of the capacitors and to understand the limitation of the passivation approach.

THERMAL BEHAVIOR OF THE CLEAN Ni(111) SURFACE

High resolution medium energy ion scattering has been used to study the interlayer spacing and atomic vibrations at the Ni(111) surface in the temperature range of 430–1100 K. The surface was not found to exhibit significant relaxation; it remains nearly bulk-terminated for all temperatures studied. The amplitudes of the atomic vibrations at T <800 K are enhanced by 30–40% with respect to their bulk values. This enhancement increases drastically at higher temperatures, so that the surface amplitudes become twice as large as the bulk amplitudes at 1100 K. We did not observe changes in the surface-structural properties near the Curie point.

An ion scattering study of the interaction of oxygen with Si(111): surface roughening and oxide growth

High-resolution medium energy ion scattering (MEIS) has been used to examine the initial oxidation of Si(111) at elevated temperatures (870–1170 K) and low oxygen pressures (5 × 10 −8–10 −5 Torr). These oxidation parameters cover three different parts of the ( p,T) phase space: the oxide growth regime (passive oxidation), the surface etching regime (active oxidation), and a transition (“roughening”) regime. We show that in the passive oxidation regime, a non-stoichiometric oxide grows initially only in the surface layer. This stage is followed by “bulk” (3D) oxidation. Our results demonstrate that under certain conditions, the initial oxidation may be accompanied by roughening of the surface. During active oxidation, Si leaves the surface as SiO. Vacancies are formed stochastically, and the vacancies can form vacancy islands. At relatively low temperatures, the vacancy island formation occurs much faster than step flow can smooth the surface, leading to a roughened surface. Finally, our experiments also support the existence of a transition regime between “passive” and “active” oxidation. We obtain direct evidence for the presence of oxygen on the surface in this regime. In addition, both silicon and oxygen spectra show clear evidence of roughening. Our interpretation, consistent with that of others, is that the surface is partially covered by passive surface oxide islands with the remaining area free of oxygen. Continuous etching of the bare silicon areas results in the surface developing a vertical roughness as high as 20–30 Å.

An isotopic labeling study of the growth of thin oxide films on Si(100)

The mechanism of thin (<8 nm) oxide growth on Si(100) has been studied by high-resolution medium energy ion scattering in combination with oxygen isotope substitution in the T=800–900 °C and 0.1–1 Torr oxygen pressure regime. Isotopic labeling experiments demonstrate that the Deal–Grove model breaks down for these films. In addition to the traditional oxidation reaction at the Si/SiO2 interface, two other spatially specific reactions take place during thermal oxidation: an exchange reaction at the oxide surface and an oxidation reaction in the near-interfacial region.

The initial stages of the oxidation of Al(111). II

Employing high energy resolution medium energy ion scattering we have identified the direction in which adsorbed oxygen sits relative to the surface Al atoms on the Al(111) surface. Using energy differences associated with the deflection of the ions by oxygen atoms after scattering from the surface layer Al (multiple scattering) the oxygen site has been identified as a surface three-fold hollow site directly above third layer Al atoms and 0.58 Å above the surface.

Preparations of liposomal fluconazole and their in vitro antifungal activity.

Fluconazole was successfully incorporated into multilamellar (MLV) and large unilamellar liposomes (LUV). Both MLV and LUV were stable up to 72 h in saline but were less stable in the high-resolution medium. The MLV-entrapped fluconazole was found to be four-fold more active than LUV-entrapped fluconazole against Candida pseudotropicalis and over six-fold more active against C. albicans. The MLV-fluconazole was one-fold less active than free fluconazole in terms of its endpoints (MIC value). However, when compared with free fluconazole, MLV-fluconazole was one-fold more active against two strains of C. albicans and equally active against C. kefyr and C. parapsilosis. In an incubation time-dependent assay against C. tropicalis, MLV-Fluconazole was one-fold more active after 16 h incubation and two-fold less active than fluconazole after 24 or 36 h post-incubation. Our results demonstrate the usefulness of liposomal formulation of the water-soluble azole, fluconazole, in the limited in vitro assay method used.

Effect of culture media on in vitro susceptibility testing of fluconazole against some yeasts

Thirty clinical isolates, comprising six strains of Candida albicans, and four strains each of C. lusitaniae, C. parapsilosis, C. tropicalis, Cryptococcus neoformans, Torulopsis glabrata and Trichosporon beigelii were tested against fluconazole, using Sabouraud's dextrose (SD) broth and a high resolution (HR) medium (Pfizer Central Research, Inc.). The procedure was a standard tube (1 ml/tube) dilution, and C. albicans Y01 09 was included as a reference strain to monitor quality and reproducibility. Results indicated that the minimum inhibitory concentrations (MICs) for all isolates of C. albicans, C. lusitaniae, C. tropicalis, and Tr. beigelii were 100 micrograms ml-1 or greater in the SD medium. In the HR medium, however, the MICs for two isolates of C. albicans were 1.56 micrograms ml-1, in other four gave higher values (greater than 100 micrograms ml-1), and the results for C. lusitaniae and Tr. beigelii were in the range 1.56-3.12 micrograms ml-1. The MICs for C. tropicalis were unaffected (100 micrograms ml-1) by the medium used. All Cr. neoformans isolates yielded a uniform value (1.56 micrograms ml-1) in HR medium as compared to somewhat more variable results (MICs 0.39-1.56 micrograms ml-1) in SD broth. The MICs for T. glabrata in the SD and HR media were 3.12-12.5 and 6.25 micrograms ml-1, respectively. The data indicated that the HR medium is preferable for the in vitro susceptibility testing of C. albicans, C. lusitaniae and Tr. beigelii to fluconazole. The MICs for other yeasts were not affected by the culture medium. The reference C. albicans isolate yielded an MIC of 1.56 micrograms ml-1 throughout.

In- VitroComparative Activity of Fluconazole and Other Antifungal Agents AgainstBlastoschizomyces capitatus

Blastoschizomyces capitatus represents an emerging fungal pathogen in acute leukemia patients. The susceptibility to amphotericin B, 5-fluorocytosine, ketoconazole and fluconazole of nine clinical isolates was evaluated. A specific medium (high resolution medium) was used for testing fluconazole. This agent and 5-fluorocytosine were at least four- to eight-fold more active than amphotericin B and ketoconazole against all isolates but one.

Composition and structure of the native Si oxide by high depth resolution medium energy ion scatering

The structure and composition of the native Si oxide were studied using high depth resolution medium energy ion scattering (MEIS) spectrometry. The analysis revealed that the oxide is an amorphous material of thickness 20 Å. The results showed qualitatively that the interface between the oxide and the underlying structure consists of layers of Si atoms displaced from their normal lattice sites. The composition of the native Si oxide varies with depth. The region near the surface is more highly oxidized than that near the interface. However, based on the assumption that there are two nonregistered Si layers in the interface, the results showed that the oxide consists of a layer of stoichiometric SiO 2 of thickness ∼ 7 Å and a layer of suboxide (SiO x , x < 2) of thic ∼ 6 Å.

In vitro Susceptibility of 119 Yeast Isolates to Fluconazole, 5-Fluorocytosine, Amphotericin B and Ketoconazole

The in vitro activity of fluconazole was tested against 13 yeast species (119 strains) isolated from clinical specimens during a 3-month period. For comparative purposes, three other antifungal compounds (5-fluorocytosine, amphotericin B and ketoconazole) were also tested. The tests were carried out using a microautomated method previously developed in our laboratory. The method allowed us to determine the minimum inhibitory concentration (MIC) of the four antifungal drugs used. For each of the drugs we utilized different media. The MIC ranges (mg/l) of fluconazole were: 0.04-12.5 for Candida albicans, 0.19-6.25 for Candida parapsilosis, 12.5-50 for Candida krusei, 0.04-100 for Candida tropicalis, 0.04- greater than 100 for Candida glabrata, 0.09- greater than 25 for Cryptococcus neoformans, 0.09-0.78 for Saccharomyces cerevisiae, 6.25- greater than 100 for Trichosporon beigelii and 0.09-0.19 for Blastoschizomyces capitatus (Trichosporon capitatum). The MIC value (mg/l) was 0.39 for Candida guilliermondii and Candida lusitaniae, greater than 100 for Cryptococcus laurentii and 0.09 for the 3 isolates of Torulopsis candida. These results were obtained using the medium recommended for in vitro testing of fluconazole (high-resolution medium) by Pfizer UK.

Composition and structure of the native Si oxide by high depth resolution medium energy ion scattering

Composition and structure of the native Si oxide by high depth resolution medium energy ion scattering

Radiation damage in silicon (001) due to low energy (60–510 eV) argon ion bombardment

Low energy Ar ion bombardment is increasingly used as a surface preparation prior to thin film deposition, and in the etching of semiconductor materials for modern microelectronic devices. A study of the radiation damage and gas build-up effects associated with low energy (60–510 eV) Ar+ bombardment of Si(001) was carried out using mass-analysed monoenergetic Ar+ beams in conjuction with in situ high depth resolution (~ 3 Å) medium energy ion scattering (MEIS). The bombardment and the analysis were performed under ultra-high vacuum (UHV) conditions. Ion bombardment damage was observed to increase linearly with the ion energy for doses ⩽ 1016cm−2. Significant annealing of the damage for 510 eV, Ar+ 5 × 1015cm−2 was observed in the bombardment temperature range between 300 and 673 K. The FWHM of the damaged layer agreed reasonably with calculated values obtained using the IMPETUS, TRIM and MARLOWE codes.

Collaborative investigation of variables in susceptibility testing of yeasts

A multicenter study was performed to evaluate the effect of medium, incubation time (24 and 48 h), and temperature (30 and 35 degrees C) on intra- and interlaboratory variations in MICs of flucytosine, amphotericin B, and ketoconazole for yeasts. Testing was performed on coded isolates of Candida species (11 strains) and Cryptococcus neoformans (2 strains) by using a standard macrodilution protocol 11 laboratories. Four chemically defined media buffered to pH 7.0 with morpholinepropanesulfonic acid were evaluated, including buffered yeast nitrogen base, synthetic amino acid medium-fungal, RPMI 1640 medium, and high-resolution antifungal assay medium. Intralaboratory variability was less than or equal to fourfold for 97% of the replicate sets of data. The highest level of interlaboratory agreement, irrespective of antifungal agent or incubation conditions, was observed with RPMI 1640 medium. Intralaboratory variability was less than or equal to fourfold for 93% of the determinations with ketoconazole and 100% with flucytosine tested in RPMI 1640 medium at 35 degrees C for 24 h. Variability in amphotericin B results was less than or equal to fourfold for 81% of the determinations in RPMI 1640 medium at 35 degrees C for 48 h. The rank order of MICs within each antifungal test group was similar among the various laboratories and was generally in agreement with the reference rank order regardless of the test medium that we used.

Low temperature material reaction at the Ti/Si(111) interface

High resolution medium energy ion scattering has been used to study material reactions at the Ti/Si(111) interface at temperatures which are anomalously low (∼250–400 °C) relative to normal silicide growth temperatures (≥600 °C). The depth-dependence of compositional mixing is inconsistent with the simple interfacial silicide formation process established for many metal/silicon systems. The results suggest that mechanisms of material reaction other than interfacial silicide compound formation dominate the interfacial reaction at low temperatures, such as grain boundary diffusion leading to localized silicide formation at grain boundaries.

Interdiffusion at the polyimide–Cu interface

Interdiffusion at the polymide-Cu interface has been studied with high resolution medium energy ion scattering (MEIS) and cross section transmission electron microscopy (TEM). Cu was evaported on polyimide in UHV and studied in situ with ion scattering. In the temperature range from 293 to 593 K Cu is found to diffuse into polyimide and form small spherical particles. The size of these spheres and the depth at which they occur increase with increasing temperature. Once the Cu spheres have been formed they become relatively immobile. It is argued that the kinetics of Cu-polyimide interdiffusion is determined by atomic diffusion of Cu. Chemical effects do not appear to play an important role.

Developed Polaroid film as a high-resolution recording medium for high-power pulsed laser radiation

When an unexposed but developed high-speed Polaroid film is subjected to intense pulsed laser radiation the photographic layer is evaporated, leaving a light spot at the place of exposure. This fact is often used as a means of establishing the presence of a high-power pulsed laser beam. This note reports on a quantitative investigation of the properties of high-speed Polaroid film with respect to energy (or power) and spatial resolution when subjected to Nd:YAG ( lambda =1.06 mu m) pulsed laser radiation. The results show that this type of film serves as a suitable recording medium with good energy resolution properties. Spatial frequencies higher than 200 lines mm-1 have been resolved.

Determination of Proteins in Biological Fluids by Electroimmunodiffusion and Two-Directional Immunoelectrophoresis

Abstract We have used electroimmunodiffusion and two-directional immunoelectrophoresis to measure proteins in biological fluids. Electroimmunodiffusion is based on the electrophoresis of antigens in antibody-containing agarose under standardized conditions. Pre-albumin, albumin, α1antitrypsin, α2-macroglobulin, IgG, IgA, IgM, IgD, and other proteins have been determined in our laboratory with a precision of ±5%. The lower limit for quantitation is 5 µg/ml. For immunoglobulin quantitation, treatment of the samples with potassium cyanate (carbamylation) before electrophoresis improves sensitivity and accuracy. Two-directional immunoelectrophoresis is based on electrophoretic separation of proteins in agarose or other high-resolution media followed by electrophoresis at right angles to the first direction, in a gel in which an appropriate antiserum has been incorporated. Qualitative and semiquantitative changes in proteins in abnormal specimens can be assessed. The two techniques combined provide a very sensitive tool for the evaluation of protein changes in biologic fluids. A simple apparatus has been designed in which a number of specimens can be analyzed simultaneously by either method.