Semiconductor structures including an etch stop material between a substrate and a stack of alternating insulating materials and first conductive materials, wherein the etch stop material comprises an amorphous aluminum oxide on the substrate and a crystalline aluminum oxide on the amorphous aluminum oxide; a channel material extending through the stack; and a second conductive material between the channel material and at least one of the first conductive materials in the stack of alternating insulating materials and first conductive materials, wherein the second conductive material is not between the channel material and the etch stop material. Also disclosed are methods of fabricating such semiconductor structures.