Fermi level pinning on HF etched silicon surfaces investigated by photoelectron spectroscopy

A widely used approach to obtain smooth oxide-free and (partially) H-termin ated silicon (Si) surfaces is to immerse Si wafers into CP4A (a mixture of H2O, HNO3, CN3COOH and HF in a volume ratio of 22:5:3:3) and/or HF solution s of varying concentrations. It is usually assumed that such treatments res ult in a dramatic reduction of the surface density of states and that, ther efore, no surface band bending can occur. In our experiments we investigate d the electronic surface structure of a number of CP4A/HF treated n- and p- Si wafers with varying doping densities by x-ray photoelectron spectroscopy (XPS). XPS allows a straightforward detection of surface stoichiometry as well as one of band bending and surface photovoltages (SPV) on semiconducto r materials because the positions of the core level peaks directly depend o n the position of the Fermi level within the band gap at the surface. Our e xperiments show that on all surfaces investigated Fermi level pinning still exists after the samples were immersed in the CP4A/HF solutions and that t he pinning states are located close to the conduction band. Most of the sam ples also showed SPV when measured under illumination. The measurements als o show that up to 36.6% of the surfaces are covered by F atoms depending on the treatment and the doping density. From the amount of blind bending we estimated the density of surface states present on the various samples. (C) 1999 American Vacuum Society. [S0734-2101(99)02301-5].