Cathodic Behavior and Redox Couples - Electrochemistry of Silicon and Its Oxide - page 241

Hydrogen evolution on silicon may proceed chemically or electrochemically depend-
ing on the conditions. As discussed in Chapter 5 on anodic behavior, hydrogen evolu-
tion near the OCP and at anodic potentials can proceed completely chemically, i.e.,
without involving the carriers from the electrode. A change from a chemical process to
an electrochemical process occurs as the potential varies from anodic values to cathodic
values as schematically illustrated in Fig. 6.5. At anodic potentials, hydrogen evolution
is a result of the chemical reaction involved in the dissolution of silicon. At cathodic
potentials, silicon does not dissolve and the concentration of electrons on the surface
of n -Si or illuminated p- Si is high. Thus, hydrogen evolution at cathodic potentials is
predominantly electrochemical due to the lack of silicon dissolution and abundance of
electrons on the surface.

6.2.2. Surface Transformation
In non-fluoride-containing solutions, silicon is stable due to the presence of an
oxide film and the electrode behavior can remain constant under a continuous cathodic
polarization. 396 The surface of a silicon electrode in fluoride-containing aqueous solu-
tion at the open circuit potential is also stable due to hydrogen adsorption. However,
surface transformation can occur at cathodic potentials due to formation of hydrides.
Thermodynamically, silicon hydride can be a stable phase at certain cathodic potentials
as shown in Fig. 2.2.
Formation of a hydrogen-rich layer on the silicon surface in HF at cathodic poten-
tials has been observed. 241 Figure 6.6 shows that the limiting current for hydrogen evo-
lution on illuminated p- Si in 5% HF is less than up to but increases
with the duration at cathodic polarization. The hydrogen-rich surface layer can be
removed by maintaining the sample at the OCP or at an anodic potential for some time
during which the surface is etched. According to de Mierry et al ., 241 during the hydro-
gen evolution, hydrogen atoms enter the surface region to a depth of about
causing amorphization and strain-induced defects which can act as generation centers
responsible for the increase in dark current. The surface, which has been polarized at
under illumination, shows circular patterns due to bubbles adhering to the