The present disclosure provides an apparatus, including: a substrate; a bottom electrode formed on the substrate; a first base oxide layer formed on the bottom electrode; a first geometric confining layer formed on the first base oxide layer, wherein the first geometric confining layer comprises a first plurality of pin-holes; a second base oxide layer formed on the first geometric confining layer and connected to a first top surface of the first base oxide layer via the first plurality of pin-holes; and a top electrode formed on the second base oxide layer. The first base oxide layer includes TaOx, HfOx, TiOx, ZrOx, or a combination thereof. The first geometric confining layer comprises Al2O3, SiO2, Si3N4, Y2O3, Gd2O3, Sm2O3, CeO2, Er2O3, or a combination thereof.