Chemistry Reference
In-Depth Information
The morphology of PS is uniform in the thickness direction except for a tran-
sition layer near the surface and two layer PS.
Pore diameters are smaller in the transition layer near the surface than in the
bulk of PS.
Individual pores preferentially form along <100> direction, generally indepen-
dent of the substrate orientation. Large primary pores tend to be perpendicular
to the surface independent of the substrate orientation.
The pore bottom is curved and the radius of curvature varies from the tip to
the side wall with the smallest radius of curvature at the pore tip.
The PS/silicon interface is flat and is parallel to the substrate surface.
The surface of pore walls is rough at the atomic scale for all pores.
Pore shape may be round, square, starlike, or dendritelike for the PS formed
on (100) substrate.
The bulk pore morphology (e.g., pore diameter and density) is independent of
the initial surface conditions (e.g., defects, roughness) except for the surface
where initiation sites are artificially introduced under conditions that indepen-
dently control the current and potential (e.g., under back illumination)
The degree of branching and interpore connection increases with decreasing
pore diameter.
Two-layer structure forms on front-illuminated
n
-Si and on high-resistivity
p-
Si
;
the top layer has microscopic pores (nanometer scale) while the bottom
layer has macroscopic pores (micrometer scale).
Back illumination tends to generate straight and nonbranched pores.
Front illumination results in the formation of a micro PS and etching of the PS
during its growth.
The most significant factor determining pore diameter is doping type and
concentration.
Pore size increases with increasing current or potential, and decreases with
increasing HF concentration.
The pores in macro PS may be filled, or partially filled, or nonfilled by
micro PS.
Pores formed in non-aqueous HF solutions are smoother than those formed in
aqueous HF solutions.
Pores grow in the direction of carrier source, but are affected by the anisotropic
nature of the dissolution processes.
Branches on main pores are related to the anisotropic effect: Preferential dis-
solution is along the <100> direction.
The effect of anisotropic dissolution on morphology decreases with increasing
pore diameter; large pores tend to orient in a direction perpendicular to the
surface and have less branching.
The composition of macro PS is largely pure silicon; micro PS can have con-
siderable amounts of hydrides, oxides, and other compounds depending on the
conditions of anodization and posttreatment.
Macro PS has the same single-crystalline structure and orientation as the
substrate silicon. Micro PS, on the other hand, can have a wide range of
variation in crystalline structure, from amorphous to polycrystalline to single
