Dietmar Theis

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.

Cross-sectional transmission electron microscopy of electroluminescent thin films fabricated by various deposition methods

Abstract For the assessment of device performance the microstructure of electroluminescent ZnS : Mn films was investigated by TEM of thin cross sections. Results on ZnS : Mn films prepared by thermal or e-beam evaporation (EBE), by atomic layer epitaxy (ALE) and by sputtering are presented and discussed. EBE films always exhibit a very fine grained layer of initial growth upon which — depending on the substrate temperature during deposition — columnar or conical grains grow, determined by the direction of material flow. ALE films show no fine grained region and exhibit comparatively large columnar grains, most of which extend from the bottom to the top of the film. In spite of large microstructural differences the overall electrooptic performance (threshold behaviour, brightness—voltage characteristics, and attainable brightness) of EBE and ALE samples is comparable. The efficiency, however, seems to be higher in ALE samples.

Microstructure and light emission of ac thin‐film electroluminescent devices

The microstructure of ac‐thin film electroluminescent devices was studied by transmission electron microscopy (TEM) of cross‐sectional specimens, and was correlated to electro‐optical characteristics of the devices. The cross sections reveal the microstructure of the BaTiO3/ZnS:Mn/BaTiO3 layer structure as a function of film depth. The rf‐sputtered BaTiO3 films are amorphous; the electron‐beam evaporated ZnS films are polycrystalline. The first‐to‐grow region of the ZnS layers always exhibits a very small grain size. With increasing film thickness the growth of larger columnar grains is observed. At higher substrate temperatures the grains have a more conical shape. The Mn‐concentration, measured by x‐ray microanalysis on the cross‐sectional specimens in a scanning TEM, shows an overall increase with growing ZnS film thickness, because the ZnS was evaporated from a single ZnS:Mn source. Annealing at 550 °C improves the electro‐optic characteristics of the samples considerably, without changes in microstructure being observable. Annealing at 850 °C induces strong recrystallization. The initial fine‐grained region disappears and the grains extend from the bottom to the top of the ZnS film.

Application of thin film electroluminescent devices

Abstract The main application for electroluminescent devices is considered to be in alphanumeric readout devices with several hundred to several tens of thousand picture elements. The use of high-field electroluminescence in less complex application fields like domestic appliances or vehicle dashboards may also attain considerable commercial importance in the future. In this paper, however, specifications for matrix addressed alphanumeric readouts are given and compared to the features of efficient electroluminescent layer structures. It is concluded that the present a.c. addressed thin films with high contrast and good lifetime data are well suited for application as electro-optic transducers for flat, solid state, medium-sized displays.

Turn-On Principles of the MOS-GTO

MOS-GTOs (GTO thyristors which are turned off by the action of a MOS-gate) represent a new generation of controllable thyristors offering considerable advantages in turn-off behavior as compared to conventional GTOs. However, MOS-GTOs generally require one control electrode for turn-on and another control electrode for turn-off, which might be regarded as a disadvantage. It is shown that in MOS-GTOs with a p-channel cathode structure it is possible to turn the thyristor on and off by controlling just one MOS gate electrode. As a triggering current for turn-on, the MOS capacitor charging current is used.

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.

2000-A/1-m Omega power MOSFETs in wafer repair technique

Power MOSFETs with a current capability of up to several thousand amperes and hence an active device area significantly exceeding the typical IC chip size can be realized only if a wafer repair technique is used. A suitable technique has been developed and used to realize circular power MOSFETs with a diameter of 3 cm. The devices are suited to control up to 2000-A drain current and exhibit an on-resistance of R/sub DS(on)/=0.9 m Omega . To contact such a device, a pressure contact system adequate to the high-current value is used. Typical switching times are about 100 ns. Such a large-area MOSFET was used in a MOSFET-GTO (gate turnoff thyristor) cascode circuit to switch anode currents up to 1200 A/1000 V. The total switching time was significantly reduced, and a smaller snubber capacitor could be employed than with the GTO in a conventional circuit. >

Physical foundations and limitations of displays including colour

The author presents short tutorial overviews on selected topics in display-relevant areas: some aspects of human vision are dealt with, the foundations and limitations of thin-film electroluminescent devices including colour are discussed as a brief example for technological and physical constraints, and basic aspects of flat-panel matrix addressing are addressed. Also discussed are the implications of human visual factors for the design of high-definition television (HDTV) displays. The author argues that the traditional direct-view CRT will not be able to meet the HDTV display screen size requirements-such a tube would be completely unpractical in terms of size, weight, and cost. The only possible solutions are flat panels of some type or projection displays.<<ETX>>