Memory devices include a stack of interleaved conductive patterns and insulating patterns disposed on a substrate. A semiconductor pattern passes through the stack of conductive patterns and insulating patterns to contact the substrate, the semiconductor pattern having a graded grain size distribution wherein a mean grain size in a first portion of the semiconductor pattern proximate the substrate is less than a mean grain size in a second portion of the semiconductor pattern further removed from the substrate. The graded grain size distribution may be achieved, for example, by partial laser annealing.