Papers by Author: Jochen Friedrich

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Authors: Birgit Kallinger, Bernd Thomas, Patrick Berwian, Jochen Friedrich, Gerd Trachta, Arnd Dietrich Weber
Abstract: Homoepitaxial growth on 4° off-axis substrates with different off-cut directions, i.e. [11-20] and [1-100], was investigated using a commercial CVD reactor. The characteristics of the growth process on substrates with different off-cut directions were determined with respect to applicable C/Si ratio, growth rate and n- and p-type doping range. Stable step flow growth was achieved over a broad range of C/Si ratio at growth rates ~ 15 µm/h in both cases. The n-type doping level of epilayers can be controlled at least in the range from 5  1014 cm-3 to 3  1017 cm-3 on both types of substrates. Highly p-type epilayers with p = 2  1019 cm-3 can also be grown on [1-100] off-cut substrates. Hence, the growth process for standard substrates was successfully transferred to [1-100] off-cut substrates resulting in epilayers with similar doping levels. The dislocation content of the grown epilayers was investigated by means of defect selective etching (DSE) in molten KOH. For both off-cut directions of the substrates, similar densities of threading edge dislocations (TED), threading screw dislocations (TSD) and basal plane dislocations (BPD) were found in the epilayers. Epilayers with very low BPD density can be grown on both kinds of substrates. The remaining BPDs in epilayers are inclined along the off-cut direction of the substrate. The surface morphology and roughness was investigated by atomic force microscopy (AFM). The epilayers grown on [1-100] off-cut substrates are smoother than those on standard substrates.
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Authors: Sonja Steinbach, Johannes Dagner, Marc Hainke, Jochen Friedrich, Lorenz Ratke
Abstract: A quantitative understanding of the effect of fluid flow on the microstructure of cast alloys is still lacking. The application of time dependent magnetic fields during solidification offers the possibility to create defined flow conditions in solidification processing. The effect of rotating magnetic fields (RMF) on the microstructure formation in cast Al-alloys (Al-7wt.%Si, Al-7wt.%Si- 0.6wt.Mg) is studied experimentally and numerically. The forced fluid flow conditions result in pronounced macrosegregation effects and affect microstructural parameters. With increasing fluid flow the primary dendrite spacing decreases whereas the secondary dendrite arm spacing increases. The experimental analysis is supported by a rigorous application of numerical modeling with the software package CrysVUn.
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Authors: Maral Azizi, Elke Meissner, Jochen Friedrich
Abstract: In this work laboratory scale multicrystalline silicon ingots were grown which have been intentionally contaminated with iron in the range between 10 to 400 ppmw by adding FeSi2 to the silicon feedstock. It is shown that an iron contamination at these high levels does not result in a structural breakdown of the columnar grain growth regime because constitutional supercooling could be avoided by strong mixing of the melt in the present crystal growth experiments. The minority carrier lifetime mappings are dominated by the iron contamination and show the distribution of the impurity over the ingot height. The measured values of the specific electrical resistivity show a significant drop from 40 to below 20 Ωcm for a contamination level of 10 ppmw Fe probably due to interactions of iron with thermal donors. At higher contamination levels the specific resistivity increases significantly with increasing iron concentration compared to the 10 ppmw ingot. Above 400 ppmw iron the specific resistivity drops below the initial value for nominally iron free material. These results indicate that interstitial iron shows a donor-like behavior in multicrystalline silicon and precipitated iron decreases the specific resistivity.
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Authors: Birgit Kallinger, Bernd Thomas, Sebastian Polster, Patrick Berwian, Jochen Friedrich
Abstract: Basal Plane Dislocations (BPDs) in SiC are thought to cause degradation of bipolar diodes with blocking voltages > 2kV by triggering the formation and expansion of stacking faults during device operation. Hence, low N doped, thick epitaxial layers without BPDs are urgently needed for the realization of long-term stable SiC bipolar diodes. Such epilayers can be achieved if the conversion of the BPD into another harmless dislocation type is supported by proper epitaxial growth parameters and use of vicinal (off-cut) substrates. In this work, the influence of the substrate’s off-cut angle and of the epilayer thickness on BPD density and surface morphology were investigated. The BPD densities of epilayers grown on 2° and 4° off-cut substrates were very low compared to growth on 8° off-axis substrates. X-Ray Topography has proved that all the Threading Dislocations (TD) propagate from the substrate to the epilayer and that BPDs in the substrate convert to Threading Edge Dislocations (TED) in the epilayer, i.e. the dislocation density (DD) of the substrate determines the epilayer’s DD. The conversion of BPDs is supported by the presence of bunched steps as for growth of thick layers on 2° and 4° off-cut substrates.
299
Authors: Ludwig Stockmeier, Mohamed Elsayed, Reinhard Krause-Rehberg, Markus Zschorsch, Lothar Lehmann, Jochen Friedrich
Abstract: To determine the electrically inactive fraction of As or P in heavily doped as-grown Czochralski Si 4-point resistivity and SIMS measurements were carried out. No clear trend for the electrical inactive fraction was found with an increasing dopant concentration, though a mean electrical inactive fraction of 11.5% for As doping could be determined.Experimental results on a dopant-vacancy complex in as-grown Si are scarce, hence temperature-dependent positron annihilation lifetime spectroscopy (PALS) was carried out on several heavily As and P doped as-grown Si samples. The measured average positron annihilation lifetime τav is between 218 ps and 220 ps. No temperature dependent effect on τav could be observed. Therefore, it can be concluded that in the studied doping range the dopant-vacancy complexes do not exist. The reason for the inactivation of the dopant has to be found elsewhere. A possible explanation can be the formation of dopant precipitates.
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Authors: Birgit Kallinger, Christian Ehlers, Patrick Berwian, Mathias Rommel, Jochen Friedrich
Abstract: The addition of hydrogen chloride (HCl) to our conventional CVD process allows for high growth rates up to 50 μm/h while maintaining the step-flow growth mode. Such epilayers exhibit quite low total concentrations of point defects less than 2 x 1013 cm-3. But, the HCl addition shows an ambivalent influence on the concentration of the lifetime killer defect Z1/2. For low growth rates, the Z1/2 concentration slightly decreases with increasing HCl addition. For higher growth rates, the Z1/2 concentration increases with increasing HCl addition.
210
Authors: Patrick Berwian, Daniel Kaminzky, Katharina Roßhirt, Birgit Kallinger, Jochen Friedrich, Steffen Oppel, Adrian Schneider, Michael Schütz
Abstract: A new tool for characterizing extended defects in Silicon Carbide (SiC) based on photoluminescence imaging is presented. In contrast to other techniques like Defect Selective Etching (DSE) or X-ray topography this technique is both fast and non-destructive. It is shown that several defect types, especially those relevant for the performance of electronic devices on SiC (i.e. Stacking Faults and Basal Plane Dislocations) can be investigated. The tool is therefore usable in research and development for a quick feedback on process related defect generation as well as in a production environment for quality control.
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Authors: Birgit Kallinger, Patrick Berwian, Jochen Friedrich, Mathias Rommel, Maral Azizi, Christian Hecht, Peter Friedrichs
Abstract: 4H-SiC homoepitaxial layers with different thicknesses from 12.5 µm up to 50 µm were investigated by microwave-detected photoconductivity decay (µ-PCD), deep level transient spectroscopy (DLTS) and defect selective etching (DSE) to shed light on the influence of the epilayer thickness and structural defects on the effective minority carrier lifetime. It is shown that the effective lifetime, resulting directly from the µ-PCD measurement, is significantly influenced by the surface recombination lifetime. Therefore, an adequate correction of the measured data is necessary to determine the bulk lifetime. The bulk lifetime of these epilayers is in the order of several microseconds. Furthermore, areas with high dislocation density are correlated to areas with locally reduced effective lifetime.
633
Authors: Bernd Zippelius, Michael Krieger, Heiko B. Weber, Gerhard Pensl, Birgit Kallinger, Jochen Friedrich, Bernd Thomas
Abstract: 4H-SiC epilayers are homoepitaxially grown on 4H-SiC substrates with different C/Si-ratios and different growth rates by the chemical vapour deposition method. DLTS investigations are applied in order to trace energetically deep states of electrically active point defects and extended defects, which may act as the source for the degradation of electronic devices. In addition, the dependence of the DLTS signal heights on the filling pulse length is studied.
393
Authors: Birgit Kallinger, Bernd Thomas, Jochen Friedrich
Abstract: Basal Plane Dislocations (BPD) in SiC are thought to cause degradation of bipolar devices as they can trigger the formation and expansion of stacking faults during device operation. Therefore, epilayers without any BPD are strongly recommended for the achievement of long-term reliable bipolar devices. Such epilayers can be achieved by supporting the conversion of BPD into Threading Dislocations (TD), which depends on the epitaxial growth mode (as described in literature). In this work, the influence of several pre-treatments of the SiC substrate prior to epitaxial growth and different epitaxial growth parameters on the reduction of the BPDs in the SiC epilayers was investigated on 4° off-axis substrates. The dislocation content in substrates and epilayers was determined by Defect Selective Etching (DSE) in molten KOH. The averaged BPD density in epitaxial layers can be reduced to < 100 cm-2 for substrate preparation techniques and to < 30 cm-2 for well-suited epitaxial growth parameters. A certain combination of epitaxial growth parameters leads to < 3 BPD/cm2 in the epitaxial layer.
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