Chemistry Reference
In-Depth Information
surface results, which is the case at the potential higher than the passivation potential.
When the formation rate of the first layer of oxide is smaller than the dissolution rate,
the coverage is zero. This is the case in silicon surface cleaning in dilute HF solutions,
where it is desirable to form a ceratin amount of oxide during the cleaning but the
surface must be free of oxide at the end of cleaning. In the case of anisotropic etching
in alkaline solutions, which has to occur on a bare silicon surface, the coverage is also
near zero because oxide formation is not involved in the dissolution process. In con-
trast, for isotropic etching or electropolishing in HF solutions the presence of an oxide
of certain thicknesses is necessary to mask the crystallographic character of the silicon
surface. In the case of porous silicon formation in HF solutions, the active silicon
surface may be covered by oxide to a varying degree depending on the specific condi-
tion. The walls of pores are free of oxide while the pore tips may or may not be free
of oxide depending on current density. In the case of current oscillation on oxide
covered surface in HF solutions, the surface is fully covered with oxide but the rates
of formation and dissolution of oxide change with time.
9.4. SENSITIVITY TO CURVATURE
A particularly important property of silicon electrodes is the sensitivity of the rate
of electrochemical reactions to the radius of curvature of the surface, i.e., the sensitiv-
