A semiconductor device includes: a gate trench extending into a Si substrate; a body region in the Si substrate adjacent the gate trench; a source region in the Si substrate above the body region; a diffusion barrier structure adjacent a sidewall of the gate trench, the diffusion barrier structure including alternating layers of Si and oxygen-doped Si and a Si capping layer on the alternating layers of Si and oxygen-doped Si; and a channel region formed in the Si capping layer and which vertically extends along the sidewall of the gate trench. Corresponding methods of manufacture are also described.