In situ reflection high-energy electron diffraction and thermal desorption spectroscopy were used to investigate the surface structure and chemistry of Si(001) surfaces after wet chemical etching in dilute HF and ammonium fluoride (NH4F) solutions. The HF-etched surfaces were found to be rough and terminated by mono-, di-, and tri-hydride species, in good agreement with previous vibrational spectroscopy measurements. The surface roughness observed after HF treatment appears to arise from surface defects such as adstructures and kinks left on the surface after the removal of the oxide layers. The Si(001) etching processes in NH4F solution were found to be highly anisotropic, and the etching time in the solution was found to be the key parameter which determined the chemistry and the morphology of the surface. We show that for a limited etching time (∼1 min), a flat and bulk-like dihydride-terminated Si(001) surface could be formed and this surface was found to transform, upon annealing in ultrahigh vacuum, into a well-developed 2×1 reconstruction at a temperature as low as ∼400 °C. Our results also suggested that (111) microfacets were formed on Si(001) surfaces only after a prolonged etching in NH4F solution and they remained stable up to an annealing temperature of ∼650 °C. © 2000 American Institute of Physics.