  High quality micro- and nano-mechanical resonators are widely used in
sensing, communications and timing, and have future applications in quantum
technologies and fundamental studies of quantum physics. Crystalline thin-films
are particularly attractive for such resonators due to their prospects for high
quality, intrinsic stress and yield strength, and low dissipation. However,
when grown on a silicon substrate, interfacial defects arising from lattice
mismatch with the substrate have been postulated to introduce additional
dissipation. Here, we develop a new backside etching process for single crystal
silicon carbide microresonators that allows us to quantitatively verify this
prediction. By engineering the geometry of the resonators and removing the
defective interfacial layer, we achieve quality factors exceeding a million in
silicon carbide trampoline resonators at room temperature, a factor of five
higher than without the removal of the interfacial defect layer. We predict
that similar devices fabricated from ultrahigh purity silicon carbide and
leveraging its high yield strength, could enable room temperature quality
factors as high as $6\times10^9$
