Chemistry Reference
In-Depth Information
crystalline depending on the fitness of the PS structure and the degree of oxi-
dation hydration.
8.4. PS FORMED AT OCP
PS can also be formed chemically without the application of a current by simply
immersing a silicon sample in a HF solution containing some oxidation agents such as
nitric acid or chromium oxide. The oxidants can be the redox couples, which do not
deposit onto the surface after the reduction, or metal ions which deposit onto the surface
after the reduction. The PS formed under such a condition are often referred to as stain
films or chemical PS. As a characteristic feature, the OCP of the silicon during forma-
tion of chemical PS in the presence of an oxidant is several hundred millivolts more
positive than without addition of an oxidation agent. The formation of chemical PS can
further be modified by illumination.
658
According to a study by Beale
et al.,
764
chemical PS forms only below a certain
concentration of oxidation agent; too high a concentration results in polishing of the
surface. The PS formed at the OCP has a limited thickness depending on the substrate
material as shown in Table 8.8. The film growth rate tends to decrease with time and
eventually reach a limiting value. On
p-
Si the PS film is uniform and about
thickness and is composed of single-crystalline material. The pores are a few tens of
nanometers in size and oriented perpendicular to the direction of film growth. The
limited thickness implies that a dissolution of the PS occurs during its growth. The
steady-state thickness of a PS film therefore depends on the relative rates of formation
and dissolution of PS. A formation rate of about 10 Å/s has been found in 1 HF:3
HNO
3
:5 H
2
O.
746
in
Although the electrochemical nature of the processes involved in the formation
of PS at open-circuit conditions (nonbiased) should be similar to that under anodic bias,
there are several major differences in the formation conditions. The first is that at the
OCP the driving force is provided by the oxidation agents, the reduction of which pro-
vides the anodic polarization of the electrode needed for silicon dissolution. Unlike the
externally biased condition, the extent of polarization is limited by the oxidation power
of the oxidation agents. The second is that the carrier supply at the open-circuit con-
dition is localized and randomly oriented, while that at anodic potential is perpendic-
ular to the surface. The anodic and cathodic sites in the chemical etching process must
be in the vicinity of each other, and continuous alternations must occur between anodic
and cathodic reactions on the surface at the pores tips.
