Crystal Orientation Dependence and Anisotropic Properties of Macropore Formation of p- and n-Type Silicon

The dependence of macropore morphology on the orientation of p- and n-type silicon samples was studied for various organic and aqueous electrolytes containing hydrofluoric acid. Scanning electron microscopy was used studying the morphology of the macropores. The results show that the macropore formation in p- and n-type silicon is a strongly anisotropic process. Depending on the substrate orientation 〈100〉 and 〈113〉 preferred growth directions could be observed. The microstructure of macropores was studied by analytical and high-resolution transmission electron microscopy. The surface of macropores in n- and p-type Si(001) shows {111} facets indicating that {111} planes are stabilized against further dissolution. Breakthrough pores show very specific anisotropic properties independent of the electrolyte. These pores consist of periodic arrangements of truncated octahedral voids with {111} walls strung up in 〈100〉 directions. The crystal orientation dependence of pore formation reflects specific properties of the pore formation mechanism and one of the important electrolyte parameters is the ability to form an anodic oxide. Macropores formed in more strongly oxidizing electrolytes tend to have smoother macropore walls. © 2001 The Electrochemical Society. All rights reserved.