Basal plane dislocations (BPDs) in 4H-SiC epilayers lead to degradation of bipolar devices performance through the formation of Shockley stacking faults. Defect selective etching of substrates prior to growth is performed to induce the conversion of substrate BPDs to electrically benign threading edge dislocations (TEDs) at the substrate/ epilayer interface.1 It was recently reported that the locations of TEDs were shifted compared to the BPDs.2 The shift was along the BPD line directions toward the up-step direction. In this work, transmission electron microscopy (TEM) is used to analyze the conversion process and explain the dislocation shifts.
A 4H-SiC epilayer is grown on a 4° off-cut 4H-SiC substrate. The substrate was etched by KOH-NaOH-MgO prior to growth and the epilayer was etched by KOH after growth. Locations of dislocation etch pits were recorded before and after growth. Cross-sectional TEM specimens were prepared at several converted BPDs.
TEM analysis showed that the conversion point is below the substrate/epilayer interface for converted screw BPDs. However, the conversion point is exactly at the interface for converted mixed (i.e. non-screw) BPDs. A linear relationship is also observed between the conversion depths below the epilayer surface and the shift distances along the step-flow direction.
Based on the experimental observations, the dislocation shift is attributed to a combined effect of two different mechanisms. The 1st mechanism is the in situ H2etching prior to growth, which leads to removal of a certain thickness of the substrate causing a change in the location where BPDs intersect the substrate surface. The H2 etching leads to a shift in the TED locations along the BPD lines towards the up-step direction. The 2nd mechanism is the glide of TEDs after dislocation conversion. TED glide leads to a shift in the TED locations along the dislocation Burgers vector direction towards the up-step direction. The TED glide is in agreement with the observation of conversion points below the substrate/ epilayer interface, as well as the linear relationship between conversion depths and shift distances. The glide of TEDs after conversion is confirmed by analysis of BPD conversion in another epilayer grown with no H2 etching prior to growth.
Glide of TEDs after conversion was previously observed during annealing of SiC epilayers3 however, it has not been reported during epitaxial growth. TED glide is energetically favorable for all converted screw BPDs, as it decreases the total dislocation length. TED glide moves the conversion point deeper in the epilayer even below the substrate epilayer interface. Hence, it could present a viable means for removing BPDs from the epilayer during growth.
1 Zhang et al., Appl. Phys. Lett. 89, 081910 (2006).
2 Song et al., J. Cryst. Growth 371, 94 (2013).
3 Zhang et al., J. Appl. Phys. 111, 123512 (2012).