Characterization of Process-related Defects in Silicon Carbide by Electron Microscopy
Abstract: Silicon carbide (SiC) is a semiconducting material, which provides advantages compared to other available semiconducting materials. Attractive properties of SiC are the wide bandgap (2.2-3.3 eV), high electric breakdown field (3x106 Vcm-1), high thermal conductivity (5 Wcm-1 K-1) and the chemical stabi!ity. Compared to silicon the electric breakdown field strength is 10 times higher, the thermal conductivity 3 times higher and twice the saturated electron drift velocity. Put this together and the result is fast electronic devices with low losses. However, the successful development of a material for device applications depends very strongly on the processing technology. Starting in 1978, there has been a rapid development in SiC crystal growth. Because SiC is gaining a more important commercial position the demand for knowledge regarding device processing is increasing. The main contribution of the present work relates to the investigation of process-induced structural crystal defects. It is known that structural crystal defects strongly affect device performance and lifetime. For that matter, knowledge about defects, in what the structure is, its nature and origin, is essential in a device optimization scheme. For the growth of SiC theSiC(001)/Si(001) interface was investigated by Molecular dynamics simulation, HREM image simulation and HREM microscopy. Simulated and real images match qualitatively and supports an interface model. Further, transmission electron microscopy have been employed to study the crystallinity and determine phase composition after heat treatment of heavily doped 4H-SiC. A solubility limit of ∼2x 1020 A1/cm3 (1900°C) is extracted, after which A1 containing precipitates are shown to form. By electrochemically etching SiC we find that pores propagate through the etched layers. It is shown that these pores propagate first nearly parallel to the basal planes to gradually change direction and propagate in towards the c-axis. For the development of ion-implantation processes for SiC transmission electron microscopy(TEM) was used to investigate B, C, N, Si, Ar and A1 ion-implanted 4H-SiC epilayers and subsequent defect formation after high temperature annealing with respect to implantation induced damage and ion mass of implanted ions. During the annealing process extrinsic, interstitial type, dislocation loops are formed on the SiC basal plane and their depth distribution can be related to the implanted ions. The investigation reveals that in samples where the implanted and electrically activated ions are substituting for a position in the Si sublattice, generating an excess of interstitial Si, the dislocation loops are more readily formed than in implanted with a specie substituting for C, which generates excess C. It is further shown that for the A1 implanted samples the loops were found to be subject to a conservative bulk ripening process where the average loop radius of the loop distribution increased while simultaneously decreasing the average loop/area density. It was also found that the total interstitial population, bound by loops, converges to an estimated 16 % of the implanted dose. Finally I investigated triangular structural defects which are occasionally generated during long term operation of 4H-SiC pin diodes. These are known to negatively affect the forward and reverse leakage characteristics of the diode. The techniques used for characterizing the structure and formation mechanisms of such defects were synchrotron white beam X-raytopography, scanning electron microscopy, in situ cathodo luminescence and transmission electron microscopy. It is shown from high-resolution images and two-beam tilting experiments that the defect results from glide slip on the (0001) basal plane. The defect is found to consist of a stacking fault bound by two partial dislocations with Burgers vectors 1/3<10 1̅ 0> and l /3<01 1̅ 0>. The fault is a means for stress relaxation in the epilayer, near the contact layer using an existing dislocation as a nucleation source.
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