In a method of manufacturing a semiconductor device, a recess is formed in an active region of a substrate. A gate insulation layer is formed in the first recess. A barrier layer is formed on the gate insulation layer. A preliminary nucleation layer having a first resistance is formed on the barrier layer. The preliminary nucleation layer is converted into a nucleation layer having a second resistance substantially smaller than the first resistance. A conductive layer is formed on the nucleation layer. The conductive layer, the nucleation layer, the barrier layer and the gate insulation layer are partially etched to form a buried gate structure including a gate insulation layer pattern, a barrier layer pattern, a nucleation layer pattern and a conductive layer pattern.