1. A method for fabricating a resistive memory device, the method comprising: providing a substrate; forming a bottom electrode over the substrate; forming a switching medium over the bottom electrode, the switching medium defining a first region proximate to the bottom electrode and a second region distal from the bottom electrode, the first region having a higher defect density than the second region in order to facilitate a filament formation in the first region; and forming a top electrode over the switching medium, the top electrode configured to provide at least part of metal particles needed to form the filament in the first region.