In situ scanning tunneling microscopy (STM) and ex situ atomic force microscopy (AFM) were used to examine the surface morphology of anodized p-Si(100) electrodes in F--containing solutions. In addition to the formation of a mainly pitted and rough surface, in situ STM observation of anisotropic etching of Si(100) in dilute (1 %) HF showed the formation of well-defined features, such as peninsulas, a 27 nm wide V-groove, and many protruding 5 nm wide micropyramids. High-resolution in situ STM resolved atomic features at the V-groove limiting (111) facets. Although this slightly etched Si sample contained no quantum pillars, it luminesced orange under UV irradiation, in the same way as a porous Si layer prepared by anodization in a more concentrated HF(1:1 HF:EtOH) solution. A loosely bound surface porous Si layer as thick as 100 nm was revealed by AFM and a 2 mum2 square depression cold be fabricated in this layer by exerting stronger compressive force. The chemical nature of the surface film prepared by anodic etching in 1:1 HF:EtOH was further probed by x-ray photoemission spectroscopy (XPS), transmission Fourier transform infrared spectroscopy (FTIR), and laser ionization microanalysis (LIMA) techniques. These results support the explanation that the photoluminescence from porous Si can be caused by a chemically modified (Si/H/O) layer on the surface (e.g., a siloxene-type material).