Papers by Author: Peter J. Wellmann

Abstract: In this paper, we present our new setup and technique for obtaining a real-time 3-D volume shape of the SiC crystal using X-ray computed tomography (CT). Hence, it is possible to determine in-situ the shape of the growth interface with high precision at growth temperatures above 2000 °C in a conventional 3" physical vapor transport (PVT) growth system. We show that the size and shape of a facet can be monitored at different stages during growth and furthermore the crystals face boundary can be determined with high precision throughout the whole growth process.

Abstract: High quality bulk-like 3C-SiC were grown on on-axis (0001) 6H-SiC substrate by sublimation epitaxy. The microwave photoconductivity decay mapping measurements revealed that this material shows considerable long carrier lifetimes varied from 3.519 to 7.834 μs under the injection level of 3.5×1012 cm-2, which are comparable with the best carrier lifetimes in 4H-SiC layers. The mapping of high resolution x-ray diffraction obtained from the same region shows that smaller carrier lifetimes seem to correspond to the larger FWHM values and vice versa. This shows that long carrier lifetime obtained in 3C-SiC is due to the improvement of the crystal quality.

Abstract: Polycrystalline doped SiC act as source for fluorescent SiC. We have studied the growth of individual grains with different polytypes in the source material. We show an evolution and orientation of grains of different polytypes in polycrystalline SiC ingots grown by the Physical Vapor Transport method. The grain influence on the growth rate of fluorescent SiC layers grown by a sublimation epitaxial process is discussed in respect of surface kinetics.

Abstract: Surface nanocones on 6H-SiC have been developed and demonstrated as an effective method of enhancing the light extraction efficiency from fluorescent SiC layers. The surface reflectance, measured from the opposite direction of light emission, over a broad bandwidth range is significantly suppressed from 20.5% to 1.0 % after introducing the sub-wavelength structures. An omnidirectional light harvesting enhancement (>91%), is also achieved which promotes fluorescent SiC as a good candidate of wavelength converter for white light-emitting diodes.

Abstract: Ballistic and diffusive growth regimes in the Fast Sublimation Growth Process of silicon carbide can be determined using suggested theoretical model for the mean free path calculations. The influences of temperature and inert gas pressure on the mass transport for the growth of epitaxial layers were analyzed theoretically and experimentally.

Abstract: In this work a new approach for the production of freestanding cubic silicon carbide (3C SiC) in (001) orientation is presented which is based on the combination of chemical vapor deposition (CVD) and the fast sublimation growth process (FSGP). Fast homoepitaxial growth of 3C SiC using sublimation epitaxy on a template created by CVD growth on silicon substrates allows to obtain thick freestanding material with low defect densities. Using standard silicon wafers as substrate material permits a cost efficient process and the applying of wafers with different orientations. The (001) orientation used in this work will potentially allow further heteroepitaxial growth of other cubic semiconductors, like e.g. gallium nitride (GaN).

Abstract: In all heteroepitaxial systems the interface between substrate and layer is a crucial point. In this work SEM and TEM studies on the interface between silicon substrate and cubic silicon carbide (3C-SiC) layers obtained by chemical vapor deposition (CVD) are presented. A clear connection between process parameters, like the design of substrate cleaning, and the heating ramp, and resulting defect structures at the substrate-layer interface could be found. Whereas the process step of etching in hot hydrogen for oxide removal is crucial for avoiding the generation of closed voids of type 2, the design of the temperature ramp up to growth temperature during carbonization influences the interface roughness. Here a fast ramp helps to obtain a flat interface.

Abstract: The determination of dislocation density and in particular the dislocation distribution in SiC wafers is of particular interest for SiC crystal growth development and production. We present an image recognition tool allowing the wafer analysis with specific needs for SiC. In the first stage of expansion, micropipes are selected and counted from SiC wafers that have been etched by KOH.

Abstract: A new type of void-like structure has been identified in thin 3C-SiC heteroepitaxial layers grown on silicon substrates. Similar surface structures can be found in micrographs published in the literature but have not been addressed so far. We propose a mechanism which explains the formation of these “type II voids” as result of hot-hydrogen etching. Type II voids seem to act as nucleation sites for the well-known faceted voids formed beneath the 3C-SiC layer during seeding (type I voids). Suppression of type II voids by appropriate high temperature cleaning steps therefore reduces the overall density of detrimental type I voids.

Abstract: Usually a waiting step at around 1000°C to 1100°C during the carbonization step for 3C-SiC on silicon is implemented for establishing a closed carbon layer to prevent the formation of voids. The latter, however, may lead to non-ideal nucleation conditions for high quality layers with a low density of domain boundaries. Our investigations indicate that a continuous ramp-up as fast as possible with no waiting step would be preferable. The worst layer quality, as measured by peak intensity and FWHM of the (200) reflection of 3C SiC, can be found at a temperature of about 1000°C, which indicates that here the nucleation rate would be the highest. So longer periods within this temperature range should be avoided by applying high ramping speeds during the carbonization step.