Materials Science Forum Vols. 645-648

Abstract: Synchrotron White Beam X-ray Topography (SWBXT) studies are presented of basal plane dislocation (BPD) configurations and behavior in a new generation of 100mm diameter, 4H-SiC wafers with extremely low BPD densities (3-4 x 102 cm-2). The conversion of non-screw oriented, glissile BPDs into sessile threading edge dislocations (TEDs) is observed to provide pinning points for the operation of single ended Frank-Read sources. In some regions, once converted TEDs are observed to re-convert back into BPDs in a repetitive process which provides multiple BPD pinning points.

Abstract: Observations of dislocation nucleation occurring at substrate surface scratches during 4H-SiC CVD homoepitaxial growth are reported. Sub-surface residual damage associated with the scratches is observed to act as nucleation sites for basal plane dislocations (BPDs), threading edge dislocations (TEDs) and threading screw dislocations (TSDs) in the epilayer. TEDs and BPDs replicate from the surface intersections of basal plane dislocation half-loops injected into the substrate surface. A model for the nucleation mechanism of TSDs, which nucleate in opposite sign pairs, is presented which involves overgrowth of surface indentations associated with the scratch during step flow growth. Atomic steps which approach these local surface indentations can collapse creating pairs of opposite sign screw dislocations which have Burgers vector magnitude equal to the magnitude of the step disregistry created during the collapse.

Abstract: 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.

Abstract: Burgers vector directions of threading edge dislocations (TEDs) in 4H-SiC epitaxial layer are distinguished by grazing incidence high resolution topography. Based on comparison between appearance of KOH etch pits and direction of TED Burgers vector, the size difference of the TED etch pits is found to be dependent on their Burgers vector directions. Examining TEDs in the epilayer by topography, the Burgers vector direction of basal plane dislocations (BPDs) in the substrate is identified. Correspondence between the topography contrast and the sense of a BPD is also investigated.

Abstract: Two different and novel in-grown triangular stacking faults have been observed and characterized in 4H-SiC epitaxial layers grown on 4o off-cut substrates. The faults were formed at the beginning of the growth and extended continuously in size during the growth. Their structural and optical properties were however different as seen from both synchrotron white beam topography and low temperature photoluminescence. The luminescence spectra were similar but appeared in different energy regions 2.85 – 2.95 eV and 2.48 – 2.64 eV, respectively. BPDs present in the epilayer are found to be transformed into SFs under laser excitation during high resolution optically detected lifetime mapping. The faults are found to expand from the epilayer surface towards the epi-substrate interface. The optical spectrum from this fault is identical to the emission from the single layered Shockley stacking faults with excitonic bandgap of 3.034 eV previously only observed and formed in the bipolar diodes during forward voltage operation.

Abstract: Dislocations in highly nitrogen-doped (N > 1×1019 cm-3) low-resistivity ( < 10 mcm) 4H-SiC substrates were investigated by photoluminescence imaging, synchrotron X-ray topography, and defect selective etching using molten KOH. The behavior of dislocations is discussed particularly in terms of their glide motion in the presence of a high concentration of nitrogen. The results indicate that nitrogen impurities up to mid 1019 cm-3 concentration do not show any discernible influence on the glide behavior of basal plane dislocations (BPDs) in 4H-SiC crystals grown by physical vapor transport (PVT) method.

Abstract: The structure of various inclusions in SiC epitaxial layers grown on 4o offcut substrates was investigated using three advanced techniques. Using micro-Raman spectroscopy, the observed inclusions exhibited a complex structure having either different SiC polytypes like 3C or 6H or they were misoriented 4H-SiC inclusions. The UVPL images showed dislocations and other extended defects around the inclusion-related defects, and strain fields were observed in the x-ray topographs near the defect sites. Spectral UVPL imaging shows features with varying luminescence inside the inclusion related defects which propagate and may cause deformation in the crystalline structure and lead to non-radiative recombination centers within the defect.

Abstract: Time-dependent evolutions of single and quadruple Shockley stacking faults (sSSF and 4SSF) in 4° off 4H-SiC epitaxial layers have been investigated. UV illuminations using an Hg-Xe lamp light source generate dissociations of basal plane dislocations (BPDs) into sSSFs whereas for 4SSFs no significant changes in shape occur. Detailed analyses of Photo-luminescence (PL) signals suggest that Si- and C-core partials have different PL spectrum distributions in the wavelength range larger than 750 nm, giving rise to images with different contrasts in PL mappings.

Abstract: Condition dependences of defect formation in 4H-SiC epilayer induced by the implantation/annealing process were investigated using synchrotron reflection X-ray topography and transmission electron microscopy. Nitrogen, phosphorus or aluminum ions were implanted in the 4H-SiC epilayers and then activation annealing was performed. To compare the implantation/annealing process, a sample receiving only the annealing treatment without the implantation was also performed. Two different crucibles (conventional and improved) were used in the annealing process. The formation of single layer Shockley-type stacking faults near the surface was found to have no ion-implantation condition or crucible dependence. The formation of BPD half-loops and the glide of pre-existing BPDs showed clear dependence on the crucibles.

Abstract: An extended structural defects which locally drastically reduces the carrier lifetime, has been observed in as-grown epilayers. A combination of back polishing, etching in molten KOH and optical microscopy revealed the geometrical structure of the stacking fault inside the epilayer. The fault started close to the epi-substrate interface, expanded initially rapidly but changed geometry after some time and reduced in size during further growth. The optical spectrum as well as the temperature dependence from this fault is identical to the emission from the single Shockley stacking faults previously only observed and formed in the bipolar diodes during forward voltage operation.