Zur Epitaxie von Galliumnitrid auf nichtstöchiometrischem Spinell im System GaCl/NH3/He

The crystalline perfection and the epitaxial relationships of GaN layers prepared by the reaction of GaCl and NH₃ in He carrier gas on {111} and {100} spinel substrates have been determined by RHEED measurements. The epitaxial relationships were found to be The best crystalline perfection could be achieved at growth temperatures of 1000…︁1050°C and growth rates more than 1 μm/min. – The growth direction 〈101 1〉 has been found to be more suitable for rapid growth than the direction 〈0001〉.     

RHEED Studies during Surface Cleaning of Silicon for MBE Substrates

Silicon wafers were prepared for MBE growth by chemical treatment and temperature annealing under UHV conditions. The (7 × 7) and (2 × 1) reconstruction of {111} and {100}-oriented Si surfaces were obtained only after high temperature annealing for few minutes at 1200 °C. The reconstruction effects were found to be stable for a long time even if the pressure in the UHV chamber increased to 10⁻⁵ Pa.     

Stress,Strain, and Symmetry of Pseudomorphically Grown Epitaxial Layers

The complete stress and strain tensors and the point symmetry of pseudomorphically grown epitaxial layers are calculated from the epitaxial relationship and the elastic constants of the deposit material. The angle misalignment of crystallographic directions in the deposit due to the strain is also calculated and results are given for the epitaxial systems CaF₂/Si(100) and CaF₂/Si(111).     

Zur Heteroepitaxie von AIBIIIC2VI-Verbindungen – Interpretation von RHEED-Diagrammen

Semiconductors of the A^IB^IIIC₂^VI type, crystallizing in the chalcopyrite structure, grow epitaxially with their {112}-planes on monocrystalline substrates with a three-fold symmetry of faces. It is shown that application of the RHEED technique permits definitively to decide what kind of epitaxial overgrowth takes place, supposing the atomic scattering factors of the I and III atoms are sufficiently different and/or the c/a axis ratio differs markedly from 2. CuInSe₂/GaAs and CuGaSe₂/GaAs show one-dimensional epitaxy     

Rheology of plastic fats

Summary A plastic material is obtained by rapid cooling of a solution of glyceryl tristearate in paraffin oil. The fat crystals produced form a three-dimensional network structure, which is mainly responsible for the rheological properties of the material.The properties have been investigated by means of creep measurements at deformations not exceeding a few percent, and in a concentric cylinder viscometer at very low shear rates.Results of measurements are discussed in terms of the properties of the three-dimensional network of fat crystals.

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Zusammenfassung Eine plastische Substanz wird durch rasches Abkühlen einer Lösung von Glyceryltristearat in Paraffinöl erhalten. Es entstehen dabei zusammenhängende Fettkristalle, welche eine feste Struktur bilden. Die Theologischen Eigenschaften des Materials werden hauptsächlich durch diese vernetzte Struktur der festen Teilchen bestimmt.Die Messung der Eigenschaften erfolgte mittels Kriechversuchen bei sehr kleinen Deformationen und im Rotationsviskosimeter bei sehr niedrigem Geschwindigkeitsgefälle. Die Ergebnisse dieser Messungen werden an Hand von den Eigenschaften der vernetzten Struktur der Fettkristalle diskutiert.

     

Interaction forces between condensed bodies in contact

Interaction forces at very small distances have mainly been investigated by extrapolation from larger distances, and by wetting and contact angle studies. The main result is a satisfactory understanding of the contribution of Van der Waals forces. Other types of forces, in particular those that may arise at distances of atomic dimensions, are not understood. The paper attempts to investigate the “other” forces by combining evidence from contact angle studies with results of direct measurements of the force of adhesion between a small, spherical particle and a flat substrate.     

Effect of Pr-Ca substitution on the transport and magnetic behavior of LaMnO3 perovskite

The effect of simultaneous substitution of a fluctuating cation and a divalent cation in LaMnO₃ perovskite modifies the properties of the material to exhibit large valence colossal magnetoresistance (CMR) effect. A good example of these properties is (La_1−2x Pr _x Ca _x )MnO₃ (LPCMO) type CMR material. In this communication it is reported that, with the increase in x (for x=0.1, 0.15, 0.2), the T _c varies between 100 and 120 K with improvisation in metal-insulator transition. Interestingly, resistance increases with x from few hundred ohms to few kilo ohms with corresponding decrease in the unit cell volume. The results of the studies using X-ray diffraction (XRD), electrical resistivity, magnetoresistance and ac susceptibility measurements on LPCMO samples for understanding the structural, transport and magnetic properties are discussed in detail.