Gap-fill layers, methods of forming the same, and semiconductor devices manufactured by the methods of forming the same

A device including a gap-fill layer may include an upper layer that on a lower layer that defines a trench that extends from a top surface of the upper layer and towards the lower layer, and the gap filling layer may be a multi-layered structure filling the trench. The gap-filling layer may include a first dielectric layer that fills a first portion of the trench and has a top surface proximate to the top surface of the upper layer, a second dielectric layer that fills a second portion of the trench and has a top surface proximate to the top surface of the upper layer and more recessed toward the lower layer than the top surface of the first dielectric layer, and a third dielectric layer that fills a remaining portion of the trench and covers the top surface of the second dielectric layer.