Papers by Author: Rainer Hock

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.

Abstract: We have investigated thermally induced strain in the SiC crystal lattice during physical vapor transport bulk growth. Using high energy x-ray diffraction lattice plane bending was observed in-situ during growth. With increasing growth rate increasing lattice plane bending and, hence, strain was observed. A comparison with numerical modeling of the growth process shows that the latter is related to the heat of crystallization which needs to be dissipated from the crystal growth front. The related temperature gradient as driving force for the dissipation of the heat of crystallization causes lattice plane bending. Optimization of the growth process needs to consider such effects.

Abstract: This work reports on the in-situ observation of a polytype switch during physical vapor transport (PVT) growth of bulk SiC crystals by x-ray diffraction. A standard PVT reactor for 2” and 3” bulk growth was set up in a high-energy x-ray diffraction lab. Due to the high penetration depth of the high-energy x-ray beam no modification of the PVT reactor was necessary in order to measure Laue diffraction patterns of the growing crystal with good signal to noise ratio. We report for the first time upon the in-situ observation of polytype switching during SiC bulk PVT growth.

Abstract: Silicon carbide as a material for electronic devices still has substantial problems concerning its structural quality and defects. It has been shown that dopants can have a big influence on structural properties like polytype stability and dislocation statistics [1]. We will discuss the effect of an isoelectronic dopant in silicon carbide. Germanium, being a member of the 4th group in the periodic table of elements like silicon and carbon, will not influence the electrical properties of the material such as e.g. aluminum. In our experiments we reached concentrations of up to 1*1020 cm-3. We have observed an impact on the polytype stability during sublimation growth with in-situ germanium incorporation. We investigated an influence on the dislocation statistics during growth and, hence, varying germanium concentration. We found only a slight decrease in mobility during Hall measurements but no severe changes in electrical properties of the material.

Abstract: The thermal expansion of 6H Silicon Carbide with different dopant concentrations of aluminum and nitrogen was determined by lattice parameter measurements at temperatures from 300 K to 1575 K. All samples have a volume of at least 6 x 6 x 6 mm3 to ensure that bulk properties are measured. The measurements were performed with a triple axis diffractometer with high energy x-rays with a photon energy of 60 keV. The values for the thermal expansion coefficients along the a- and c-direction, α11 and α33, are in the range of 3·10-6 K-1 for 300 K and 6·10-6 K-1 for 1550 K. At high temperatures the coefficients for aluminum doped samples are approximately 0.5·10-6 K-1 lower than for the nitrogen doped crystal. α11 and α33 appear to be isotropic.

Abstract: The build-up of strain fields caused by the precipitation of oxygen in Czochralski-silicon during annealing up to 1200°C and for process times up to 70 hours has been observed in real time by high energy x-ray diffraction. Five different processes are distinguished in the temperature evolution of the intensity and of the rocking width of the silicon 220-reflection. These features are attributed to different precipitation mechanisms. A fit to part of the data with a diffusion limited precipitation model leads to an activation energy for oxygen diffusion in silicon of 2.2 eV in the temperature range from 700°C to 950°C.

Abstract: A structural characterisation of the first [01-15] grown 6H SiC crystals is presented. They show a different micro domain structure outside the facetted region as compared to conventionally [0001] grown crystals. It is imposed by the reduced rotational symmetry for this direction which favours the activation of a low number of glide systems.