In a reactor used for epitaxial growth the wafers are

Question

In a reactor used for epitaxial growth, the wafers are normally placed flat on the susceptor, and epi grows on the top side only.

If the same reactor were used to oxidize wafers, by introducing O₂ rather than SiH₁ (or another Si gas source), SiO₂ would grow on both sides of the wafer.

Explain why SiO₂ grows on both sides and epi grows only on the top side.