Porous Silicon - Electrochemistry of Silicon and Its Oxide - page 393

ing light intensity at low light intensity up to but changes little at higher

intensities up to
Two-layer PS on back-illuminated n -Si can also occur at low potentials (near
the OCP) at a current density much lower than (the current peak of an i-V curve). 12
The top micro PS layer disappears with increasing potential (at a constant photocurrent
density) and only macropores are formed with their surface covered with micro PS.
The PS formed on p -Si generally has micropores, but when the resistivity exceeds
a certain value macropores can form underneath a layer of micropores. 177,952,1027 The
resistivity at which this occurs depends on the solvent, HF concentration, and current
density. In aqueous HF solutions macropores are found to occur on substrates of resis-
tivity higher than

1027

On the other hand, macropores have been found to form
on substrates with a lower resistivity of in 2M HF + DMF electrolyte. 177 The
pore size of the top layer is similar to the single-layer PS formed on p -Si while that of
the macropores is on the order of micrometers. The diameter of the macropores and the
pore spacing increase with increasing doping concentration. Pore diameter may increase
or decrease with increasing current depending on the solution composition, while inter-
pore spacing decreases with increasing current. 177,1027 Presence of water in organic
solvents tends to reduce the thickness of the micro PS layer. It has been found that a
two-layer PS is formed on lowly doped p -Si in anhydrous HF-MeCN solutions, but
addition of water results in the formation of only macropores without a top layer of
micropores. 1085
Fill of Pores. The macropores of two-layer PS may be completely filled with
micropores or only partially filled depending on the solution composition. 1027,1085 When
other conditions are the same, the macropores formed in organic solvents are more filled
than those formed in aqueous solutions or mixtures of organic solvent and water. Addi-
tion of water to organic solutions reduces the extent of filling. Among organic solu-
tions, according to Jäger et al ., 1126 the degree of filling of the macropores formed on
p -Si depends on the oxidizing nature of the solutions: Macropores are filled with micro