Methods of forming phase-changeable memory devices include techniques to inhibit void formation in phase-changeable materials in order to increase device reliability. These techniques to inhibit void formation use an electrically insulating growth-inhibiting layer to guide the formation of a phase-changeable material region within a memory cell (e.g., PRAM cell). In particular, methods of forming an integrated circuit memory device include forming an interlayer insulating layer having an opening therein, on a substrate, and then lining sidewalls of the opening with a seed layer (i.e., growth-enhancing layer) that supports growth of a phase-changeable material thereon. An electrically insulating growth-inhibiting layer is then selectively formed on a portion of the interlayer insulating layer surrounding the opening. The formation of the growth-inhibiting layer is followed by a step to selectively grow a phase-changeable material region in the opening, but not on the growth-inhibiting layer.