Porous Silicon - Electrochemistry of Silicon and Its Oxide - page 410

nature, Memming and Schwandt 34 proposed that the solid film that formed on the silicon
surface during anodic dissolution was a dissolution/precipitation product
resulting from a two-step disproportionation reaction:

-Si and the Barrier Breakdown Model. In the early 1970s,
several studies found that the formation of etch pits and tunnels on

Macropores on n

-Si occurs during
anodization in HF solutions in the dark. 38,39,42,58 The size and number of these pits and
tunnels change with doping density and potential.
Meek, 38,39 based on i-V curve and capacitance measurements, proposed that the
large current observed on n -Si at an anodic potential in the dark is due to barrier break-
down. The breakdown is not due to a bulk mechanism but rather to interface tunneling
from the states at the surface into the conduction band. Also, the breakdown is not
uniform but localized causing the formation of the etch pits and tunnels.
In a systematic investigation of i-V characteristics and the morphology of the
solid layer formed on n -Si, Theunissen 42 found that in the dark, deep etch tunnels can
form on the substrates at a doping concentration higher than 2

n

10 16 /cm 3 . He also found
that the structure of etch channels on n -Si is single crystalline, and thus concluded that
the solid layer formed during anodization is the remaining substrate silicon left after
anodic dissolution. It was also found that the channels have a rich texture which varies
with formation conditions. Furthermore, the formation of channels is not related to
crystal defects and the direction of the channels depends on the orientation of the sub-
strate. Theunissen postulated that local breakdown of the depletion layer inside the
semiconductor is responsible for the formation of etch channels. The breakdown of the
barrier layer occurs when the maximum field at the silicon/electrolyte interface is larger
than the critical breakdown field.
Characterization of PS and Growth Kinetics. A number of systematic studies
were carried out to characterize the morphology and growth kinetics of PS from the
mid-1970s to the early 1980s. 36,37,40,41,49 It was then established that the brown film
formed on p -Si consists of very small pores, observable only under TEM, and has the
same single-crystalline structure as the substrate. It thus is the substrate material
remaining after anodic dissolution, such as the etch channels on n -Si. The growth rate
of a PS layer for a given condition was found to be constant with increasing anodiza-
tion time and the morphology is generally uniform with increasing thickness of PS.
Thus, diffusion within the pores is not a rate-limiting process and the walls of the pores
are not active. The dissolution reactions occur only on pore bottoms and are limited by
the charge transfer process at the silicon/electrolyte interface. Decreased growth rate,
which is accompanied by an increase of porosity, occurs for thick PS layers. A differ-
ence in the condition at the bottom of the deep pores from that of the shallow ones was
suggested as the cause of this phenomenon. 49
Depletion Layer and Field Intensification Model. By the mid-1980s the
overall scope of the conditions for the formation of PS and of the various morpholog-
ical features were largely identified, although many details of PS morphology and the

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