Vertically aligned semiconductor nanowires (NWs) have many potential applications for NW-based technologies, ranging from solar cells to intracellular sensors. Aligned NWs can be fabricated by top-down etching of planar wafers or synthesized from the bottom up using the vapor–liquid–solid (VLS) mechanism to induce epitaxial growth on lattice-matched substrates. The VLS process permits the modulation of dopants along the NW growth axis, which if combined with dopant-dependent wet-chemical etching, can be used to encode precise morphology. However, the synthesis of vertical and linear NWs with complex morphology is nontrivial, requiring control over multiple interdependent aspects of the VLS process. Here, we demonstrate sub-10 nm morphology in ⟨111⟩ epitaxial silicon (Si) NWs grown by the VLS mechanism on (111) Si substrates with gold (Au) catalysts. Using silane (SiH₄), phosphine (PH₃), and hydrochloric acid (HCl) precursor gases at 480 °C, precise morphology is encoded through abrupt phosphorus (P) dopant transitions, which are found to be less than 5 nm in width. The results highlight three mechanistic attributes of the process. First, NW growth in the ⟨111⟩ direction is found to be unstable at high SiH₄ partial pressures and growth rates unless using HCl, which stabilizes NW growth through chlorination of the NW sidewall. Second, aggregated Au deposited on the NW surface by the VLS catalyst is found to be immobile on the chlorinated surface and to impede selective wet-chemical etching by potassium hydroxide (KOH) solution, preventing the design of precise morphology. Third, the aggregation of Au is found to be strongly dependent on the SiH₄ partial pressure and NW growth rate, and values exceeding ∼100 mTorr and ∼150 nm/min, respectively, are required to minimize Au and thereby enable selective wet-chemical etching. Under optimized growth conditions, we find that abrupt, complex, and arbitrary dopant profiles and morphologies can be encoded in vertical Si NWs, and we expect that a variety of electronic and photonic applications can be realized with these designed nanostructures.