Nanometric-width GaAs grating lines showing high aspect (height/width) ratio of up to 30, without roughness or shape deviation are reported. They were obtained by HVPE, capitalizing on the near-equilibrium nature of this epitaxial technique. HVPE rapid decomposition of GaCl growth precursors leads to immediate reactivity of the growth reaction to an increase or decrease of the supersaturation of the vapor phase. HVPE growth is also governed by surface kinetics: the morphologies are controlled by the intrinsic growth anisotropy of the crystals. By manipulating surface kinetics and exploiting the fast reactivity of the system, it is demonstrated that HVPE allows to benefit from an enhanced growth anisotropy favoring vertical growth of nanometer-scale grating lines. Unique high-aspect ratios are obtained in one-step short growth time (20 min), capitalizing upon the main advantage of the bottom-up approach: the structures are defined by their crystalline properties with no scattering defects as typically introduced by techniques involving dry etching. The role of surface kinetics and mass transport at the nanometer scale is discussed. (C) 2013 Elsevier B.V. All rights reserved.