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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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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.

4H-SiC homoepitaxial layers free of basal plane dislocations (BPDs) are urgently needed to overcome the so-called bipolar degradation of high-voltage devices. BPDs being present in substrates are able to either propagate to the epilayer or convert to harmless threading edge dislocations (TEDs) in the epilayer. The model by Klapper predicts the conversion of BPDs to TEDs to be more efficient for growth on vicinal substrates with low off-cut angle. This paper aims to verify the model by Klapper by an extensive variation of epitaxial growth parameters and the substrates' off-cut. It is shown that the off-cut angle is the key parameter for growth of BPD-free epilayers. Furthermore, it is shown that the model also describes adequately the behavior of different types of TEDs, i.e., TED II and TED III dislocations, during epitaxial growth. Therefore, the model by Klapper is verified successfully for 4H-SiC homoepitaxial growth on vicinal substrates. V C 2013 AIP Publishing LLC. [http://dx

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.

Aberrant methylation of the MGMT (O6-methylguanine-DNA methyltransferase) DNA-repair gene is a predictive marker for the response to chemotherapy with alkylating agents (e.g., temozolomide) in malignant gliomas. Since temozolomide is considered for the treatment of choroid plexus tumors, MGMT promoter methylation status was retrospectively assessed in 36 choroid plexus tumors using methylation specific PCR, combined bisulfite restriction analysis (COBRA), and clone sequencing. By methylation specific PCR, all samples demonstrated a signal for MGMT methylation. COBRA confirmed >10% methylation of CpGs 17 and 31 in 58% of tumors. Clone sequencing of six cases methylated by COBRA confirmed aberrant methylation including a previously recognized enhancer element. In conclusion, MGMT promoter methylation is frequent in choroid plexus tumors and can be quantified using COBRA. Determination of MGMT promoter methylation status might be useful for the stratification of patients for alkylator-based treatments in future clinical trials.