Analysis of threading dislocations in 4H-SiC by defect selective etching and X-ray topography

Defect Selective Etching (DSE) is a very common method to detect dislocations intersecting the sample surface and to distinguish between different types of dislocations by means of shape and size of the etch pits. In case of 4H-Silicon Carbide (SiC), it is usually expected that Basal Plane Dislocations (BPDs) are decorated by oval shaped etch pits and that Threading Edge (TEDs) and Threading Screw Dislocations (TSDs) are related to smaller and larger hexagonally shaped etch pits, respectively. It will be shown in this paper that the sample's doping state (dopant and its concentration) must be taken into account for correct interpretation of hexagonally shaped etch pits with respect to the type of threading dislocations. In this study, 4H-SiC substrates and homoepitaxial layers were etched in molten potassium hydroxide (KOH) at about 500°C for several minutes. The highly doped n-type substrates together with the epilayers cover a wide range of doping, i.e. n-type samples with electron concentrations in the range from 5 x 1014 cm-3 to 5 x 1018 cm-3 as well as p-type samples with 1 x 1015 cm-3 < p < 1.4 x 1020 cm-3 were used. First, different etching regimes are identified by means of analysing the size distribution of hexagonally shaped etch pits for a large number of samples having different doping states. Then, the dislocation types are identified by Synchrotron X-Ray Topography (SXRT) for several samples representing the different etching regimes. This method was chosen as it is a direct and non-destructive method for the identification of dislocation types. Besides the well-known TEDs and TSDs, so-called TED II and TED III dislocations are identified in 4H-SiC for the first time. Finally, the SXRT samples were defect selectively etched in order to assign each etch pit to the underlying dislocation. For all etching regimes, each etch pit corresponds to a certain dislocation and vice versa, i.e. the etch pit density (EPD) fits the dislocation density at the sample surface. For p-type samples as well as for low n-type samples, small hexagonally shaped etch pits are correlated to all kinds of TEDs, larger hexagonally shaped etch pits are clearly linked to TSDs. For medium n-type samples, TED II dislocations are decorated by a specific etch pit type. For highly n-type SiC samples, the size of hexagonally shaped etch pits does not correlate to the type of threading dislocations anymore.