Methods of forming nonvolatile memory devices include forming a vertical stack of nonvolatile memory cells on a substrate. This is done by forming a vertical stack of spaced-apart gate electrodes on a first sidewall of a vertical silicon active layer and treating a second sidewall of the vertical silicon active layer in order to reduce crystalline defects within the active layer and/or reduce interface trap densities therein. This treating can include exposing the second sidewall with an oxidizing species that converts a surface of the second sidewall into a silicon dioxide passivation layer. A buried insulating pattern may also be formed directly on the silicon dioxide passivation layer.