A semiconductor device includes: a gate trench extending into a Si substrate; a body region in the Si substrate, the body region including a vertical channel region adjacent a sidewall of the gate trench; a source region in the Si substrate above the body region; a contact trench extending into the Si substrate and separated from the gate trench by a portion of the source region and by a portion of the body region; an electrically conductive material in the contact trench; and a diffusion barrier structure interposed between a sidewall of the contact trench and the vertical channel region, the diffusion barrier structure including alternating layers of Si and oxygen-doped Si and configured to increase carrier mobility within the vertical channel region. Corresponding methods of manufacture are also described.