Papers by Author: Jochen Friedrich

Abstract: In the photovoltaic industry a total of 100,000 tons of silicon is lost as waste per year. This waste is originating from several cropping and sawing steps of the high purity silicon blocks and ingots during the solar cell wafer production, resulting in a silicon containing suspension. Among different approaches to recycle the silicon from this waste is the utilization of hydrocyclones, which can be used to separate or classify particles by weight and size. In this work the use of a hydrocyclone was evaluated to upgrade the silicon fraction from a typical sawing waste. A potential field of use for the recycled silicon particles might be as anode material for next generation lithium ion batteries.

Abstract: The development of bipolar 4H-SiC devices for high blocking voltages requires the growth of high carrier lifetime epitaxial layers with low Z_1/2 concentrations. This paper shows a comprehensive investigation of the influence of epitaxial growth parameters (C/Si ratio and growth temperature) on Z_1/2 concentration and minority carrier lifetime. On the basis of a discovered exponential correlation of Z_1/2 with the C/Si ratio and growth temperature, a competitive low Z_1/2 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 Z_1/2 on the thermal process and process conditions of the epitaxial growth. Furthermore, the correlation between Z_1/2 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 Z_1/2 concentration.

Abstract: Electrical testing with regard to bipolar degradation of high voltage SiC devices cannot be done on wafer level, but only expensively after module assembly. We show that 4H-SiC material can be optically stressed by applying high UV laser intensities, i.e. bipolar degradation as in electrical stress tests can be provoked on wafer level. Therefore, optical stressing can be used for control measurements and reliability testing. Different injection (=stress) levels have been used similar to the typical doping level of the base material and similar to the established electrical stress test. The analysis of degradation is done by photoluminescence imaging which is a well-established technique for revealing structural defects such as Basal Plane Dislocations (BPDs) and stacking faults (SFs) in 4H-SiC epiwafers and partially processed devices.

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.

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.

Abstract: The influence of structural defects in the active layer of GaN-on-Si substrates on the vertical leakage current was studied. The structural defects were analyzed by analytical scanning electron microscopy by means of cathodoluminescence (CL). The leakage current was determined by vertical I-V measurements.Two possibilities were found, which give potential explanations for the variations of the vertical leakage current: i) Threading dislocations, which may partially form leakage paths, were detected by CL imaging. ii) Variations of the carbon doping, which is used to tune GaN to a semi insulating material were revealed by CL spectroscopy.

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 τ_av is between 218 ps and 220 ps. No temperature dependent effect on τ_av 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.

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 Z_1/2. For low growth rates, the Z_1/2 concentration slightly decreases with increasing HCl addition. For higher growth rates, the Z_1/2 concentration increases with increasing HCl addition.

Abstract: 4H-SiC PiN diodes for 6.5 kV were manufactured on both 4° and 8° off-cut substrates and subjected to an electrical stress test on wafer level and subsequent analysis of structural defects present in the active area of the diodes. For 8° off-cut diodes, the electrical characteristics with respect to leakage current and forward voltage drift are worse than the electrical characteristics of 4° off-cut diodes. Furthermore, a large number of stacking faults was found in 8° off-cut diodes, but little evidence for bipolar degradation was found in 4° off-cut diodes. Therefore, bipolar degradation was significantly reduced by avoiding BPDs in the active area of PiN diodes, i.e. by the use of 4° off-cut substrates. Furthermore, a strong correlation was found between the electrical screening test on wafer level and critical defects.

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.