A microelectronic device comprises lateral contact structures overlying a source structure and comprising conductive material, a cap material overlying the lateral contact structures and comprising implant regions therein, a stack structure overlying the cap material and comprising a vertically alternating sequence of insulative structures and conductive structures arranged in tiers, and pillars vertically extending through the stack structure and into the source structure. The pillars individually comprise semiconductive channel material in physical contact with the lateral contact structures. The microelectronic device comprises filled slot structures vertically extending at least through the stack structure and the cap material. The filled slot structures are positioned within horizontal areas of the implant regions of the cap material. Related memory devices, electronic systems, and methods of forming the microelectronic devices are also described.