Silicon/Electrolyte Interface - Electrochemistry of Silicon and Its Oxide - page 57

high etching rates. 227 The relative amounts of the hydrides as well as the surface flat-
ness, however, change with pH. 446,574,621 The effect of pH can be attributed to the depen-
dence of the concentration of HF, and species on pH. 437 For the (111) surface,
dipping in HF solution results in termination by mono-, di-, and trihydride, whereas in
basic fluoride solutions (pH 9-10) the surface is terminated exclusively by monohy-
dride. 574 , 635 According to Higashi et al ., 574 in the basic fluoride solutions the surface steps
and defects on the (111) surface which are terminated by di- and trihydride are prefer-
entially attacked by water, resulting in Si-O bonds. These Si-O bonds are then removed
preferentially in the presence of HF, resulting in the formation of ideally terminated
(111) silicon surface with monohydride oriented normal to the surface. However, it has
been reported that increasing pH increases the attack of the atoms on the (111) plane
and thus results in large-scale roughness. 895

Hydrogen termination is found to occur at all potentials with or without signifi-
cant silicon dissolution and current flow, except for the range in which anodic oxide
215,227,446,602

Rao et al . 215 found that the silicon surface is covered with Si-H bonds
at all potentials and current densities at which porous silicon forms in HF solutions of
5% to 50% concentrations. Rappich et al . 227 found that the silicon surface is covered
with hydrogen in alkaline solutions at the open-circuit potential at which silicon is
etched at high rates. In alkaline solutions, applying a cathodic potential has no effect
on the hydrogen coverage, whereas hydrogen adsorption disappears at anodic poten-
tials where passivation occurs. In buffered HF (BHF) solutions, hydrogen coverage of
the silicon surface depends on potential. 446,621,1036

forms.

The relative amount of the hydrides depends on atomic surface flatness and water
temperature. 532,895 Water rinse after dipping in HF solutions tends to flatten the
262,532,641 Water tends to react with silicon di- and trihydrides and remove them
from the surface, leaving hydrogen termination and a flatter surface. For example, as
shown in Fig. 2.10 the (111) silicon surface after immersion in 1.5% HF is terminated
by mono-, di-, and trihydrides. 635 After immersion in 100 °C water for about 500s the
amount of di- and trihydrides was greatly reduced; they were replaced by monohydro-
gen, leaving the surface covered more homogeneously with monohydride.

surface.