Chemistry Reference
In-Depth Information
anodic potentials below Path results in formation of the Si-O-Si bond and is
responsible for the oxide formation and passivation at potentials above
Except for reaction path which is purely chemical in nature, all the other
reaction paths are electrochemical in nature, at least partially. These electrochemical
reactions depend on the carrier transfer between the states at the interface and those
in the semiconductor and thus their rates increase with increasing potential or illumi-
nation. While reaction paths result in the direct dissolution of silicon,
reaction paths and result in the formation of Si-O-Si bonds. The rate of
reaction paths and also increases with potential. As the coverage of the surface
by Si-O-Si bonds increases with increasing potential, the surface becomes increasingly
less active and becomes passivated when these bonds fully cover the surface as
shown in Fig. 5.74. Further reaction has to proceed via the dissolution of the Si-O-Si
bonds which is fast in HF solutions but is very slow in KOH solutions. The surface of
the oxidized silicon layer is terminated by OH in KOH and may be terminated by both
OH and F in HF. In HF on the surface covered by an anodic oxide layer the adsorp-
tion of OH is required for the growth of the oxide whereas the adsorption of F may be
required for the dissolution of the oxide. The Si-O-Si bonds are rather stable in KOH
such that the dissolution rate in the passive region is very low. On the other hand, the
Si-O-Si bonds are not stable in HF due to the attack by the fluoride species and the
dissolution rate is high in the passive region (at potential higher than that of first current
peak,
For the anodic dissolution of silicon in HF solutions, the first step involving
replacement of hydrogen by fluoride and evolution of hydrogen, the rate is proportional
to the product of according to reaction The following step
involving breaking the Si-Si back bond is proportional to On the other
hand, for the chemical dissolution of silicon in KOH solutions, the rate of hydrogen
replacement is proportional to and is proportional to
in breaking the Si-Si back bond according to reaction A major difference between
the reactions in the two solutions is that the hydrogen replacing step in HF requires
holes whereas in KOH it does not. Thus, it can be seen that in HF solutions the reac-
tion rate is low at OCP but rapidly increases with potential. On the other hand, in KOH
