The choice of carrier wafer was found to significantly influence etch rates, selectivity, and morphology in GaN micropillar etching in a Cl2-Ar high-density inductively coupled plasma. 7 × 7 mm2 GaN on sapphire chips with a plasma-enhanced chemical vapor deposition SiO2 hard mask was etched on top of 4-in. fused silica, silicon carbide, silicon, sapphire, aluminum nitride, and high purity aluminum carriers. Silicon and silicon carbide carriers reduced GaN:SiO2 selectivity because incidental SiClx and CClx etch products from the carriers attack the SiO2 mask. Aluminum nitride and high-purity aluminum carriers yielded the highest GaN:SiO2 selectivities due to the deposition of Al-based etched by-products, while the highest GaN etch rate was achieved using the sapphire carrier since it was the most inert carrier and did not sink any Cl2. Results indicate that SiO2 and Al may be used as passivation materials during GaN etching, as vertical profiles were achieved when SiO2 or Al is redeposited from the fused silica and aluminum carriers, respectively. Floor pitting, trenching, sidewall roughness, and faceting were all influenced by carrier wafer type and will be discussed.