2.3.3.2 Homoepitaxial Growth

Homoepitaxial growth, where the polytype of the SiC epilayer matches the polytype of the SiC substrate, is accomplished by step controlled epitaxy [52,63]. Step controlled epitaxy is based upon growing epilayers on a SiC wafer polished at an angle (called the "tilt-angle" or "off-axis angle") of typically 4$^\circ$ to 8$^\circ$ off the (0001) basal plane, resulting in a surface with atomic steps and flat terraces between the steps as schematically depicted in Fig. 2.5.

Figure 2.5: Cross-section schematic representation of "off-axis " polish SiC surface used for homoepitaxial growth [27].

When growth conditions are properly controlled and there is a sufficiently short distance between the steps, Si and C atoms impinging onto the growth surface find their way to steps where they bond and incorporate into the crystal. Thus, ordered lateral "step flow" growth takes place which enables the polytypic stacking sequence of the substrate to be exactly mirrored in the growing epilayer [32].

When growth conditions are not properly controlled or when steps are too far apart (as can occur with SiC substrate surfaces that are polished to within less than 1$^\circ$ of the basal plane), growth adatoms can nucleate and bond in the middle of the terraces instead of at the steps, leading heteroepitaxial growth of poor-quality 3C-SiC [52,63]. To help preventing spurious nucleation of 3C-SiC triangular inclusions during epitaxial growth, most commercial 4H- and 6H-SiC substrates are polished to tilt angles of 8$^\circ$ and 4$^\circ$ off the (0001) basal plane, respectively [51].

It is important to note that most present-day as-grown SiC epilayers contain densities of undesirable surface morphological features which could affect SiC device processing and performance [52]. In addition to triangular inclusions, these include growth pits as well as large macrosteps formed by coalescence of multiple SiC growth steps (i.e., step bunching) during epitaxy. Pre-growth wafer polishing as well as growth initiation procedures have been shown to strongly impact the formation of undesirable epitaxial growth features [52]. Further optimization of pre-growth treatments and epitaxial growth initiation processes are expected to reduce undesired morphological growth features. T. Ayalew: SiC Semiconductor Devices Technology, Modeling, and Simulation