Researchers refine the characterisation of dislocations in 4H-SiC
Determining the dislocation densities in 4H-SiC by etch pit density measurements must account for the type of doping and its level.
Researchers from Fraunhofer Institute for Integrated Systems and Device Technology IISB in Erlangen, Germany, have unravelled inconsistencies surrounding a well established characterisation method for determining the level of crystalline defects in SiC.
What’s more they have developed a superior approach, which has been verified by means of a comparative study.
As part of their experimental effort, the scientists have also uncovered the existence of two types of threading dislocationsthat have never been seen before.
Their efforts are important, because SiC, especially the modified compound 4H-SiC, is a well-suited material for energy efficient and compact power electronic devices. The material quality, especially the density of extended defects like dislocations, is a limiting factor for device performance and production yield.
In order to develop SiC substrates and epitaxial layers with a reduced dislocation density, a reliable and economical characterisation method for dislocations is needed.
Defect Selective Etching (DSE) meets both requirements and is well-established for SiC and other materials. However, even prior to this study, some inconsistencies have been witnessed with regard to the influence of the doping states of samples on the defect-selective etching behaviour of 4H-SiC.
The researchers say their current study has proved,for the first time,that the conduction type, i.e. n-type or p-type material, and the doping level of the sample have to be taken into account in order to interpret etch pits.
Different types of threading dislocations, such as screw- (TSD) and edge- (TED) type dislocations, were identified by Synchrotron X-ray Topography (SXRT) for differently doped 4H-SiC materials.
In addition, defect selective etching was conducted in order to decorate the intersection point of each dislocation at the sample surface with an etch pit.
Then, dislocations seen from SXRT were compared to etch pits observed using DSE in order to verify the correlation of dislocation types with etch pit types. For example, the scientists observed TSD, with large hexagonally shaped pits.
A 1:1 correlation of etch pits and dislocations was found, in other words, each dislocation is decorated by an etch pit and vice versa. Furthermore, the existence of two unexpected types of threading dislocations, so-called TED II and TED III dislocations, was proven experimentally by SXRT investigations.
At certain doping levels of n-type 4H-SiC material, so-called TED II dislocations are decorated by TED II specific etch pits. For other doping levels and in p-type material, TED II dislocations are indistinguishable by their etch pits from TED and TED III dislocations.
At least, threading edge type (TED, TED II and TED III) dislocations are distinguishable from TSDs for all doping states except for highly n-doped substrates.
This investigation was set up as a comparative study based on complementary methods for dislocation characterisation. SXRT was chosen as this is a non-destructive method which is able to reveal the Burgers and line vectors of the dislocations.DSE is an indirect method for dislocation characterisation as it needs hypothesis for the interpretation of the etch pits, i.e., the identification of dislocation types based on the etch pit type.
The cost-effective method of DSE was verified by complex SXRT studies. From now on, the researchers say DSE can be applied as a reliable and economic characterisation method for dislocations in 4H-SiC. This is an important step for further developments with respect to material quality, i.e., dislocation density, and to power electronic devices.
Figure: Etch pits of different types of threading dislocations in 4H-SiC: roundish etch pits (TED II specific etch pits), small hexagonally shaped etch pits (corresponding to TED, TED III dislocations), large hexagonally shaped etch pits (correlated to TSDs). Copyright: Fraunhofer IISB
Further details of this work have been published in the paper “Threading dislocations in n- and p-type 4H - SiC material analyzed by etching and synchrotron X-ray topography” by B. Kallinger, S. Polster, P. Berwian, J. Friedrich, G. Müller, A. N. Danilewsky, A. Wehrhahn, A.-D. Weber in Journal of Crystal Growth 314 (2011) 21 – 29.