Papers by Author: Mathias Rommel

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Authors: Johannes Schoeck, Jonas Buettner, Mathias Rommel, Tobias Erlbacher, Anton J. Bauer
Abstract: High-voltage 4H-SiC Junction Barrier Schottky diodes with a reverse breakdown voltage of over 4.5 kV and a turn-on voltage below 1 V have been fabricated. They achieved a forward current of 5 A at a forward voltage drop of 1.8 V and 20 A at 4.2 V. A low reverse leakage current of 0.3 μA at 1.2 kV and 37 μA at 3.3 kV was measured. The chip size was 7.3 mm x 7.3 mm, the active area 0.25 cm2 and the diode was able to handle a repetitive pulse current density of over 300 A/cm2 without degradation. Floating field rings in combination with a field-stop ring were used as edge termination to reach 73 % of the theoretical breakdown voltage. The epitaxial layer was 32 μm thick, with a nitrogen doping concentration of 1 x 1015 cm-3. The JBS diodes have been manufactured in a 100 mm SiC prototyping line, using well established processing technology, to achieve cost-efficient devices.
Authors: Martin Herms, Matthias Wagner, Alexander Molchanov, Mathias Rommel, Markus Zschorsch, Sindy Würzner
Abstract: The axial distribution of electrical and optical properties of a 4 inch Czochralski-grown silicon single crystal were analyzed by different methods that can be applied in the scanning mode. These methods were tested with respect to the suitability to reveal growth striations. The residual stress was visualized by SIRIS (Scanning Infrared Stress Inspection System) and SIREX (Scanning Infrared Stress Explorer), the electrical resistivity by LPS (Lateral Photovoltage Scanning) and SRP (Spreading Resistance Profiling), and the lifetime of the minority charge carriers by MDP (Microwave Detected Photoconductivity) mapping. The concentration of interstitial oxygen (Oi) across the growth striations was determined by FTIR (Fourier Transform Infrared) spectroscopy. We demonstrate for the first time on the micrometer scale that the Oi scan is very well-correlated with the profile of Δσ (difference of the in-plane principal stress components). The stress field is tensile oriented in growth direction, i. e. perpendicularly to the growth striations. The stress-concentration coefficient has been estimated to be in the order of 10-13 Pa cm-3 what does agree well with previous XRD results.
Authors: Birgit Kallinger, Mathias Rommel, Louise Lilja, Jawad ul Hassan, Ian D. Booker, Erik Janzén, J. Peder Bergman
Abstract: Carrier lifetime measurements and wafer mappings have been done on several different 4H SiC wafers to compare two different measurement techniques, time-resolved photoluminescence and microwave induced photoconductivity decay. The absolute values of the decay time differ with a factor of two, as expected from recombination and measurement theory. Variations within each wafer are comparable with the two techniques. Both techniques are shown to be sensitive for substrate quality and distribution of extended defects.
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.
Authors: Jürgen Erlekampf, Daniel Kaminzky, Katharina Rosshirt, Birgit Kallinger, Mathias Rommel, Patrick Berwian, Jochen Friedrich, Lothar Frey
Abstract: The development of bipolar 4H-SiC devices for high blocking voltages requires the growth of high carrier lifetime epitaxial layers with low Z1/2 concentrations. This paper shows a comprehensive investigation of the influence of epitaxial growth parameters (C/Si ratio and growth temperature) on Z1/2 concentration and minority carrier lifetime. On the basis of a discovered exponential correlation of Z1/2 with the C/Si ratio and growth temperature, a competitive low Z1/2 concentration of 1.9∙1012 cm-3 could be achieved by lowering the growth temperature and switching to higher C/Si ratio. Thermodynamic considerations by an Arrhenius approach reveal a dependency of the formation enthalpy of Z1/2 on the thermal process and process conditions of the epitaxial growth. Furthermore, the correlation between Z1/2 and the effective minority carrier lifetime confirms the occurrence of a necessary second recombination mechanism beside the common recombination at deep levels by Shockley-Read-Hall for low Z1/2 concentration.
Authors: Matthias Kocher, Mathias Rommel, Tobias Erlbacher, Anton J. Bauer
Abstract: TLM structures on Al-implanted regions with different implanted Al concentrations and different annealing temperatures were processed and characterized by electrical measurements in order to determine the influence of these parameters on the ohmic resistivity. Based on these results, a TCAD model was developed that considers the carrier concentration of the implanted region, the effective density of states for holes and the hole tunneling mass at the Ti3SiC2-SiC interface. It could be shown that Ti3SiC2 allows to form ohmic contacts on Al implanted samples with an effective doping concentration down to 3·1017 cm-3. Furthermore the effective density of states for holes in Ti3SiC2 was determined to 8.5·1018 cm-3 and the hole tunneling mass in SiC and Ti3SiC2 in the range of 2·10-34 kg to 6·10-33 kg depending on the sample.
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.
Authors: Johannes Schoeck, Holger Schlichting, Birgit Kallinger, Tobias Erlbacher, Mathias Rommel
Abstract: Using a combination of photoluminescence and electrical characterization, defects in the epitaxial layer of unipolar 4H-SiC power devices were matched to device characteristics and statistically analyzed. In-grown triangles had no significant effect on diode and VDMOS blocking or conduction mode, while surface triangles lead to high leakage currents even below 1 V reverse bias.
Authors: Moriz Jelinek, Johannes G. Laven, Naveen Ganagona, Reinhart Job, Werner Schustereder, Hans Joachim Schulze, Mathias Rommel, Lothar Frey
Abstract: Two metastable defects with energy levels at Ec-0.28eV and Ec-0.37eV, which previously have been reported in proton implanted- and in proton implanted and annealed crystalline silicon are discussed. Recent results on the peculiar behavior of these defects upon periodical application of two different bias conditions during DLTS measurement are reviewed. Two specifically designed DLTS measurement sequences are proposed in order to further reveal the defects transformation rates and respective activation energies.
Authors: Daniel Kaminzky, Birgit Kallinger, Patrick Berwian, Mathias Rommel, Jochen Friedrich
Abstract: We present an extended model for the simulation of the effective minority carrier lifetime in 4H-SiC epiwafers after optical excitation. This multilayer model uses measured values (such as doping profile, point defect concentration and capture cross sections, epilayer thickness) as input parameters. The bulk lifetime and the diffusion constant are calculated from the actual time dependent excess carrier profiles, resulting in more realistic transients having different decay regimes than in other models. This enables a better understanding of optical lifetime measurements.
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