Gunther Ziegler

Single crystal growth of SiC substrate material for blue light emitting diodes

The fabrication of 6H-SiC ingot single crystals with up to 20-mm diam and 24-mm length is described. Crystal growth was realized with a modified Lely method using a suitable seed crystal. The growth temperature was 2200°C to obtain preferential growth of the 6H-modification with 2.9-eV bandgap useful for blue light emitting silicon carbide diodes. It is the first time that 6H-SiC ingot crystals yielding substrates for the industrial production of devices based on this compound were obtained. With controlled Al doping of the crystal it was possible to fabricate blue light diodes with the highest quantum efficiency reported so far.

Silicon carbide blue light emitting diodes with improved external quantum efficiency

The most relevant features in the fabrication and room temperature operation of blue light emitting 6H‐SiC:Al,N diodes are reported. Epilayers are grown either on opaque Acheson substrates or on semi‐ to fully transparent substrate platelets obtained from larger ingots grown in our laboratory. The p‐n junction is obtained by overcompensating Al acceptors with N donors. Electroluminescent light, which is mainly due to N‐donor‐Al‐acceptor pair recombination is generated in the n region of the diode. The emission spectrum is saturated to about 90% at normal driving conditions, the hue of the diodes being slightly dependent on the current density. Optimization of diode preparation and design, especially the use of transparent substrates, yields external quantum efficiencies of 1.1×10−4 for unencapsulated samples, nearly triplicating the best quantum efficiency values for SiC:Al,N diodes reported so far.

Influence of strain on Nb3Sn multifilament conductors

In Nb3Sn multifilament conductors, the superconducting component is subjected to compressive stress. This affects both the superconducting critical temperature and the critical current. After removal of the matrix by a chemical treatment, the degradation of the critical temperature is partly reversed and the critical current increases. With the aid of a model, which takes into account the plastic behavior of the components, the strains resulting from thermal shrinkage are calculated. The resulting degradation of Tc and the measured Tc values are in fairly good agreement. The strain‐dependent change of critical current is solely caused by a change of the upper critical field; the pinning force function remained almost unaltered.

A new degradation phenomenon in blue light emitting silicon carbide diodes

In 6H-SiC blue emitting diodes prepared by sawing epitaxial wafers the development of greenish striations in the luminescing layer is observed together with a decrease in external quantum efficiency. It is proposed that this degradation is due to the development of numerous stacking faults leading to an intermediate (cubic?) state of lower band gap than the 6H modification.

Herstellung von V3Ga- und Nb3Sn- multifilamentleitern hoher stromtragfähigkeit mit hilfe chemischer transportreaktionen☆

Abstract Composite wires containing vanadium or niobium filaments in a copper or bronze matrix can be charged with gallium or tin by the aid of chemical transport reactions. The reactions take place in a closed reaction chamber heated isothermally. Chemical transport is initiated by traces of water or iodine or hydrogenchloride. In short samples of the conductors appropriately annealed overall current densities up to 2·10 5 A/cm 2 (4.2 K; 5 T) were attained. Under certain experimental conditions the direction of the transport can be reversed. Then the greater part of the tin is extracted from its copper solution.