Cathodic Behavior and Redox Couples - Electrochemistry of Silicon and Its Oxide - page 247

Thus, whether a metal can be deposited by electroless deposition onto a silicon surface
depends on the redox potential and its relative position to the band edges and on whether
the silicon can be dissolved under those conditions. On the other hand, whether the
deposition can be sustained to cover the entire surface area depends, on nucleation and
growth kinetics of the deposits, the catalytic effect of the deposits on silicon dissolu-
tion and hydrogen evolution and the evolution of the morphology of the surface. The
formation of a continuous and uniform metal film by electroless deposition is intrinsi-
cally difficult because a certain amount of bare silicon surface area is required for
silicon dissolution in order to sustain the deposition.
The electroless deposition of Ni in fluoride solutions depends on pH; it does not
occur at pH 1.2 but does occur on both p -Si and n -Si. 1092 This is attributed to the less
favorable position of the couple relative to the band edges at low pH than at
high pH. Also, the deposition is much faster (~10 times) on n-

-Si because
the deposition on n -Si proceeds in the conduction band with the majority carriers
whereas it is through hole injection on p- Si . The rate of deposition is limited by the
dissolution of silicon on n -Si and is limited by Ni reduction on

Si than on p

p -Si. Figure 6.11 shows

the i-V curve for Ni deposition on n -Si.

Electroless deposition of Au in can be controlled by both

971

the kinetic process and the diffusion process. The deposition is a two-step process,

with initial diffusion-limited deposition of the intermediate species, followed by
surface-limited reduction of this species. For electroless deposition of Pt, it has been
reported that the rate-determining step is the deposition on -Si, whereas it is the dis-

solution of silicon on p -Si. 253 Electroless copper deposition does not occur on
silicon surface due to the lack of anodic dissolution of silicon. 237 In a non-HF
solution, the deposition of copper on a bare silicon surface results in the formation of
oxide around the metal particles. In HF solutions, the deposition of copper proceeds
very slowly in the dark on both p -Si and n -Si samples due to the lack of carriers. 295 The

n