Origin of the Anisotropy in the Anodic Dissolution of Silicon

Abstract

We report on an in situ, real-time, electrochemical, polarized, attenuated total reflection Fourier transform infrared spectroscopic investigation of Si(100), (111), and (110) surfaces in diluted HF solution ( cnormalF=0.05M , pH 3). Prior to the IR measurements, cyclic voltammetry was carried out on the samples in a standard three-electrode arrangement, then IR spectra were recorded as a function of potential using 50-mV potential increments. The results give evidence for potential-induced absorption changes of the chemical surface species (Si–O, Si–H, Si–OH), which depend somewhat on substrate orientation. The potential-induced changes regard the shape and magnitude of all the above vibration bands and include a shift for some vibrations. In addition, the SiOH contribution appears about twice as large on (100) as on (110) and (111). The SiOH species, present just above the onset of anodic current flowing, at potentials around 0 V, appear as a key factor in the anisotropic dissolution of silicon. These observations are accounted for semiquantitatively in the framework of a kinetic model. The main factor responsible for the anisotropic effects appears to be the ability of surface silanols to couple and form siloxane bridges, a reaction governed by the surface concentration of available sites and the geometry of their neighborhood.

Anisotropy in the anodic dissolution of silicon elucidated by in situ infrared spectroscopy

“…It is plausible that the surface crystallography (Si-Si distance and bond angles) induces a different SiOH to SiOSi ratio at the (1 0 0) and the (1 1 1) surface, leading to a different dissolution rate. A more thorough analysis of these effects, including data obtained on other orientations and a kinetic model of Si dissolution in this regime, are presently in progress and will be fully developed elsewhere [26].…”

Photoelectrochemistry of silicon in HF solution

“…Moreover, SERS studies [22] showed that in addition to Si-H bonds, Si-O and Si-OH ones are also formed on the the surface of silicon during anisotropic etching [23] showed that while the surface of Si electrode is free from oxides in the cathodic range of potentials, i.e., at E < E OCP ≈ −0.97 V (Ag/AgCl in 40% NH 4 F, pH 8), a thin layer of oxides (1.0 -1.5 mono-layer) is already present on the surface within the double layer region, whereas in the range of anisotropic anodic etching (E > 0 V (Ag/AgCl)) the amount of oxygen on Si surface increases several times. Thus the above described data show that anodic oxidation of Si proceeds via formation of silicon oxygen species on the electrode surface, i.e.…”

(Allesandro Volta Award Presentation) Facts and Challenges in the Electrochemistry and Wet Surface Chemistry of Silicon

“…oxide"/hydroxide film (95), the current is slightly orientation dependent (50,96,97), and the surface recombination velocity is high (98). In this region, the surface remains rather rough on the nanometric scale (99).…”