Porous Silicon - Electrochemistry of Silicon and Its Oxide - page 399

atoms. 36,1104 The PS formed on lowly doped p- Si in HF solution has a high concentra-
tion

6
of H, C, O, and F. The PS near the surface tends to be higher in O
content and lower in H indicating that it is oxidized. A high concentration of hydrogen,
between and 10 21 /cm 3 , is found to be incorporated into the walls of pores and
into the bulk Si (from the tips) during PS formation. 794 Penetration depth is determined
by the volume diffusion of hydrogen. 1108 Low pH favors hydrogen penetration. The
hydrogen can exist in clustered states such as (SiH 2 ) 2 and monomer states such as
SiH 2 . 130 For the PS formed in HF-MeCN solutions the surface of PS is completely
covered by hydride. 136,248 Newly formed mino PS is partially oxidized. It has been found
that the extent of oxidation for the PS on lightly doped p- Si is on the order of 10%
whereas it is much less on heavily doped materials, on the order of 1%. 35 Another study
found that for highly doped p- Si , there is very little oxide content whereas about 50%
of the PS is in the form of oxide for lowly doped

-Si. 176 The degree of oxidation of
the silicon atoms in PS, in the form of SiO , depends on the region (on an i-V curve)
it is formed. 770 Oxide content increases with potential approaching the electropolishing
region. At the boundary of the PS formation region the oxygen is incorporated as a
suboxide SiO 0

p

5 and SiO while in the transition region the oxygen is increased in the
oxides as SiO 1.5 and SiO 2 . It has been reported that oxygen in the form of oxide exists
at the bottom of the pores on

.

-Si. 762
The PS formed in the transition region (Fig. 8.5) is mostly composed of anodic silicon
oxide. 770

formed in 25% HF + 50% ethanol but not on p

The amount of Si-O and Si-H bonding varies from the surface to the bulk of the
794,1106 The relative amounts of oxidation states of the silicon atoms in the PS formed
on both p and n types of silicon are affected by illumination and light wavelength. 1124
Also, the amount of oxidation of PS tends to vary with anodization time. 1103

PS.

8.3.9. Crystallographic Structure

PS may be crystalline, polycrystalline, or amorphous depending on the type of silicon
and formation conditions. In general, PS has the same single rystallographic structure
as the substrate silicon except for the very fine micro PS in which the crystal structure
can significantly deviate from the perfect silicon crystal lattice PS. The crystal struc-
tures of some PS formed under various conditions are listed in Table 8.7.

The difference in the crystallinity of PS of different morphological structures is
shown in Fig. 8.54 as transmission electron diffraction (TED) patterns of PS formed
on different silicon materials. 624 The crystalline structure deviates more from the perfect
silicon crystalline structure as the PS structural dimension becomes smaller from
sample a to d. The TED pattern in sample d for the PS on lowly doped p- Si shows sig-
nificant electron scattering modification due to deviation from the perfect crystalline
structure. Similar findings were reported in other studies: PS formed on heavily doped
p- Si has the same single-crystalline structure as the substrate, whereas on lightly doped
p- Si the Bragg spots are less defined and much diffuse electron scattering is
35,36,566 It has been found that the PS formed on p- Si in 50% HF has an
amorphous layer of about on top of a polycrystalline layer of PS having a pore
size of less than 3 nm. 640 Except for the top surface layer, which may also be amor-
phous or polycrystalline structures, the PS layers formed in the stable growing phase

observed.