Photo-induced ultrathin electropolishing layers on silicon: formation, composition and structural properties

The properties of electropolishing layers formed on illuminated n-Si(111) in dilute NH4F are analysed using a combined (photo) electrochemistry/UHV surface analysis system. Evaluation of thickness d and composition dependencies on potential U and pH shows increasing d at lower pH (d ˆ 25 A Ê at 3 V, pH 3) and for U > 3 Vat pH 4.9 (d ˆ 7 AÊ , U ˆ 5 V). Fluorides, oxy¯uorides, hydroxides and oxides of Si are found in the ®lms. The surprising observation that thicker ®lms transmit higher currents is discussed. UP spectra (He II) reveal a valence band offset between Si and the oxidic layer of 5.2 eV. # 2000 Elsevier Science B.V. All rights reserved.