Atomically resolved structure of vacuum-heated porous silicon

We report atomic resolution in scanning tunneling microscopy studies of por ous silicon surfaces, after vacuum heat treatment at 850 degrees C and abov e. The (100) sample surfaces show some 2 x 1 reconstruction as for single-c rystal (100) surfaces, but the (111) samples do not show any clear reconstr uctions, probably because the column tops are too small for large structure s such as 7 x 7 to form. The atomic planes on the column tops are parallel to the substrate for both (100) and(111) samples, in agreement with low-ene rgy electron diffraction studies.