Ludwig Pohl

Physical properties of nematic phenylcyclohexanes, a new class of low melting liquid crystals with positive dielectric anisotropy

Abstract Those isotropic and anisotropic molecular properties of p-disubstituted phenylcyclohexane which govern the electrooptical behaviour of these molecules are described and compared to the properties of known liquid crystals.

Low‐(Delta)n‐twisted nematic cell with improved optical properties

It is demonstrated by model calculations and experimental results, that the range of viewing angles of a twisted nematic (TN) display can be considerably improved by using a liquid‐crystal material with a low refractive index anisotropy Dn<0.1.

Nematic liquid crystals with positive dielectric and negative diamagnetic anisotropy

Abstract p -Alkyl- p ′-cyanocyclohexylcyclohexanes CCH represent a new class of thermotropic nematic liquid crystals which no longer exhibit UV absorption abobe 200 nm, are distinguised by an optical anisotropy of less than 0.1, and in contrast to all previously known “dielectric positive”, thermotropic nematic liquid crystals possess a negative diamagnetic anisotropy.

Growth of indium phosphide by metalorganic vapor phase epitaxy using dimethyl (3‐dimethylaminopropyl) indium as a new indium source

Dimethyl (3‐dimethylaminopropyl) indium was synthesized and used for the first time as an indium source for the growth of InP layers by metalorganic vapor phase epitaxy at atmospheric pressure. This compound is liquid at room temperature with a vapor pressure of 30–40 Pa at 30 °C, which enables its use at low source temperatures. The layers were grown at a bubbler temperature of 30 °C. The growth temperature was varied between 580 and 660 °C. Hall measurements revealed good electrical data with carrier mobilities up to 49 900 cm2 /V s at 77 K. Temperature‐dependent photoluminescence experiments confirmed these results and indicated that zinc was the main residual acceptor impurity.

Liquid crystals for active matrix displays

Abstract Active matrix liquid crystal displays belong to one of the fastest growing fields in display research and development. Several pocket TVs using this technology are already available commercially. High quality displays for TV and instrumentation require an understanding and optimization of liquid crystal material parameters. We have, therefore, investigated the influence of the elastic, dielectric and optic properties on the electrical resistance of the liquid crystal as well as on the response times and the viewing angle dependence of active matrix displays. Based on correlations derived from our investigations, we have developed new liquid crystals which have suitable properties and a very high electrical resistivity.

Novel liquid precursors for the growth of InP and GaInAs epitaxial layers by MOVPE

Abstract The two novel liquid In precursors TMIn-HN i Pr 2 and (3-dimethylaminopropyl)dimethylindium (DADI) were synthesized and their behaviour in the MOVPE growth of InP studied. Layers with low temperature mobilities in excess of 100,000 cm 2 /V·s could be grown. When growing GaInAs, some side reactions between TMIn-HN i Pr 2 and AsH 3 were observed, which did not significantly affect the layer quality. DADI could be used in combination with tertiarybutylphosphine resulting in acceptable InP quality at moderate growth temperatures.

The LP-MOVPE of GaInAs with saturated group III precursors

Abstract This contribution presents the results of growth kinetic studies using trimethylindium-diisopropylamine-adduct ((CH3)3InNHi(C3H7) 2 (TMIAd)) as an indium precursor and dimethylaminopropyl-galla-cyclohexan ((CH2)5Ga(CH2)3N(CH3)2 (APGH)) as Ga source for the LP-MOVPE (low pressure metalorganic vapour phase epitaxy) of GaInAs/InP. For comparison, experiments using the standard sources trimethylindium ((CH3)3In (TMI)) and trimethylgallium ((CH3)3Ga (TMG)) were also carried out. It is shown that GaAs and InP layers with properties comparable to that grown with the standard sources can be grown with these saturated Ga and In compounds. In the case of GaInAs/InP, the dependence of the solid composition on the variation of the V/III ratio demonstrates that the use of the coordinatively saturated group III precursors leads to a reduction of undesired prereactions between the group III compounds and AsH3. This is in contrast to the standard group III sources. In order to achieve lattice-matched growth, quite different Ga/In ratios are necessary because of the different diffusion constants of the group III precursors. The electrical data of the GaInAs/InP layers grown with both saturated or standard group III sources do not show significant differences. This fact, together with the improved growth process, indicates the advantages of the novel group III precursors.

Multiplexible Liquid Crystalline Broad Range Systems

Abstract A sharp steepness, a low temperature dependence of the threshold, and a fast response time at low temperatures are preconditions for the multiplexibility of liquid crystals suitable for outdoor application. The steepness is efficiently improved if K 3/K 1 ≤ 1. A temperature independent steepness and threshold can be realized if K 3/K 1, and ΣK i/Δe become constant. Response time is proportional to γ1/K 1. Since viscous properties change by 1 to 2 of the tenth power more than elastic constants with temperature, a fast response behavior at low temperature is possible only by reducing the temperature dependence of the viscosity. Taking into account these preconditions, it is possible to develop nematic systems multiplexible up to 1:8 from -20°C to +60°C without temperature compensation of the driving voltage or with temperature compensation up to 1;16, having a total response time less than 1 s even below -20°C.

Metalorganic Molecular Beam Epitaxy of AlGaAs Using APAH

To take advantage of the benefits of gaseous precursors for the growth of AlGaAs, a reduction of the carbon and oxygen uptake from the sources is necessary. Therefore, new precursors have been developed recently to overcome these problems. An entirely new approach is the use of an intramolecular saturated precursor, where the coordinative saturation against oxygen is realised by means of a double ring structure. 1-(3-dimethylaminopropyl)-1-ala-cyclohexan (APAH) is a representative of this class of precursors. In metalorganic molecular beam epitaxy (MOMBE) the suitability of APAH for the growth of AlxGa1-xAs layers (0≤x≤1) was investigated in combination with TEGa and arsine. For the grown layers morphology and crystallinity are found to be excellent, whereas the background carrier concentrations are in the range of p=1017 to 1019 cm-3. SIMS measurements clearly identify carbon as the main acceptor. Other than carbon, nitrogen is also incorporated from APAH. Despite this, APAH is useful for particular heterostructure applications as demonstrated by quantum well structures with excellent interfaces. Therefore, in MOMBE of AlGaAs, APAH is a suitable Al precursor for applications where p-type doping of 1017 cm-3 or above is required or acceptable (e.g., in HBT).

Cyclosilan-d18 und 3-Trimethylsilyl-tetradeutero—natriumpropionat, zwei neue Protonenkernresonanzstandards für hohe Temperaturen und wäßrige Lösungen

The properties of useful internal 1H-NMR reference compounds are discussed. Two new standards for measurements at high temperatures and in aqueous solutions are proposed. They are characterized by a high boiling point, thermal and chemical stability and chemical shift as better reference compounds than those used up today.ZusammenfassungEs werden die Bedingungen aufgezeigt, die innere Standards für Protonenkernresonanzmessungen erfüllen müssen. Den bisher verwendeten Bezugssubstanzen für Messungen bei hohen Temperaturen und in wäßrigen Lösungen werden zwei neue Verbindungen gegenübergestellt, die sich unter anderem durch ihren hohen Siedepunkt, ihre thermische und chemische Stabilität und ihre chemische Verschiebung als universeller anwendbare Nullpunktsubstanzen auszeichnen.