Papers by Author: Peter J. Wellmann

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: Photoluminescence(PL)-topography is a powerful method to determine the charge carrier concentration of SiC-wafers. The following work describes the development of a PL-topography method for the determination of charge carrier distribution in p-type SiC and shows the correlation of PL-Intensity and charge carrier concentration. With this setup it is possible to characterize wafers up to a size of 2” at room- and low temperature in a non-destructive way.

Abstract: The co-doping of nitrogen and aluminum has been studied in the sublimation epitaxy growth process. It is shown that the doping may be tuned from n-type to p-type by effect of substrate doping, growth face and volume of the growth crucible. The co-doped layers show a nearly ideal I V characteristic and luminescence at room temperature.

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: We present p-type doping of bulk SiC crystals by the modified physical vapor transport (M-PVT) technique using TMA (Tri-Methyl-Aluminum). Using TMA as a dopant precursor allows a quite well defined crystal growth process control. The issue of improvement of conductivity (reduction of substrate resistivity) by reduction of unintentional acceptor compensation by nitrogen is addressed. It is shown that a decrease of compensation from approx. 3%...10% to approx. 0.5%...2.5% leads to a charge carrier mobility and, hence, conductivity increase of about factor two.

Abstract: Modeling and simulation of the SiC growth processes, Physical Vapor Transport (PVT), Chemical Vapor Deposition (CVD), are sufficiently mature to help building new process equipment or up-scaling old ones. It is possible (i) to simulate accurately temperature and deposition distributions, as well as doping (ii) to quantify the limiting phenomena, (iii) to understand the important role of different precursors in CVD and hydrogen additions in PVT. The first conclusion of this paper is the importance of the "effective" C/Si ratio during CVD epitaxy in hot-wall reactors and its capability to explain the doping concentrations. The second conclusion is the influence of the C/Si ratio in alternative bulk growth technique involving gas additions.