Methods of forming doped elements of semiconductor device structures include forming trenches having undercut portions separating stem portions of a substrate. The stem portions extend between a base portion of the substrate and overlying broader portions of the substrate material. A carrier material including a dopant is formed at least on the sides of the stems in the undercut portions of the trenches. The dopant is diffused from the carrier material into the stems. As such, the narrow stem portions of the substrate become doped with a targeted dopant-delivery method. The doped stems may form or be incorporated within buried, doped, conductive elements of semiconductor device structures, such as digit lines of memory arrays. Also disclosed are related semiconductor device structures.