The well-known basal plane dislocations (BPD) in SiC are high-voltage bipolar device killers that source Shockley-type stacking faults in the presence of an electron-hole plasma and cause forward voltage drifts in bipolar devices [1]. It has been imperative to develop ways to prevent the expansion of these extended defects where multiple research groups have been successful in mitigating their propagation from the substrate into the epitaxial layer [2-5]. Of course, BPD free substrates are the ideal solution, and while these substrates are being developed, most substrates contain ~200-1000 cm-2 BPDs. For most SiC device applications, the current mitigation processes that reduce these defects is sufficient. However, for applications which require high pulsed power current density or high surge current capability, the injected carrier concentration is significant enough to expand converted BPDs from the substrate into the epitaxial layer. In the last ICSCRM conference, we reported a new process to mitigate the BPD expansion at power densities up to 13 kWcm-2 UV excitation using a H2 etch before the buffer layer growth, followed by a H2 growth interrupt prior to the low doped epitaxial layer growth. Here, we will report results from comparisons of H2 etching to Ar annealing and the use of H2 versus Ar during growth interrupts to prevent BPD expansion with the goal of understanding the mechanism of the suppression of BPD expansion. SiC epitaxial layers were grown using a horizontal hot-wall CVD reactor with SiH4 (2% in H2) and C3H8 on 4° off-axis substrates toward the [11-20] that are known to have BPDs. A H2 etch or Ar anneal was performed before the buffer layer (BL; n-type ~ 2x1018 cm-3) growth while a growth interrupt in H2 or Ar was conducted prior to the intentionally low doped (n-type ~ 5x1015 cm-3) drift layer. Fig. 1 shows a schematic of the growth schedule. The H2 etch or Ar anneal was carried out at 1665 °C and 70 mbar for 50 min. During the growth interrupt, the sample was cooled to 1000 °C in either 80 slm H2 or 5 slm Ar at 100 mbar. Ultraviolet photoluminescence (UVPL) imaging was used to image the samples before and after UV stressing up to 13 kWcm-2. The film thickness was evaluated by Fourier transform infrared analysis, quality by X-ray diffraction and doping concentration via Hg probe CV measurements. As reported at the ICSCRM 2023 conference, the H2 etch and H2 growth interrupt prevented BPDs from expanding under UV stress of 13 kWcm-2 and it was believed that the H2 treatment specifically had inhibited this expansion. There has been a report where the role of H2 on preventing BPD faulting in implanted samples was not effective [6]. To confirm the role of H2, we performed a growth using the same conditions as the H2 etch/interrupt, however, an Ar anneal at 1665 °C was used instead of a H2 etch and the growth interrupt was conducted in an Ar atmosphere instead of H2. The sample was UV stressed up to 1000 Wcm-2 and it was found that four BPD expanded from the substrate into the epilayer, see Fig 2. For comparison, a sample grown with a double H2 etch (before the buffer layer growth and drift layer) and a sample grown with a H2 etch plus H2 growth interrupt did not produce faulting at the same power density. The primary difference in our sample is the in-situ H2 treatment vs implantation, which does not create implant damage. We will present detailed parametric results of samples grown with various etching/ annealing, growth interrupts, anneal times, buffer layer thickness, gas flow rates and interrupt temperature, both in H2 and Ar. [1] J.P. Bergman, et. al., Mater. Sci. Forum Vol. 353-356, 299 (2001). [2] N.A. Mahadik et.al., Mater Sci Forum 858, 233 (2016). [3] R. E. Stahlbush, et al., Appl. Phys. Lett. 94, 041916 (2009). [4] M. Kato, et al., Sci. Rep., 12, 18790 (2022). [5] N.A. Mahadik et. al., Appl. Phys. Lett., 100, 042102 (2012). [6] M. Kato, et. al., Japanese J. Appl. Phys., 63, 020804 (2024).