A semiconductor device includes a sidewall oxide layer covering an inner wall of a trench, a nitride liner on the sidewall oxide layer and a gap-fill insulating layer filling the trench on the nitride liner. A first impurity doped oxide layer is provided at edge regions of both end portions of the sidewall oxide layer so as to extend from an entry of the trench adjacent to an upper surface of the substrate to the nitride liner. A dent filling insulating layer is provided on the nitride liner in the trench to protect a surface of the first impurity doped oxide layer. Related methods are also disclosed.