Silicon/Electrolyte Interface - Electrochemistry of Silicon and Its Oxide - page 77

in different solutions. Figure 2.28 shows the flatband potential of n- and p -type silicon
as a function of pH in aqueous solutions. 716 The slope of both curves is about 30 mV/pH
unit, which is much lower than the 60mV/pH unit expected from Eq. 1.33.
Figure 2.29 690 shows the pH dependence of flatband potential on a silicon surface
covered with a thermal oxide layer in 1 M NaCl solutions. The slope is about
40 mV/decade for pH values far from pzc and becomes smaller approaching pzc. It has
been suggested that the effect of pH on flatband potential is mainly at the oxide/elec-
trolyte interface rather than at the silicon/oxide interface. 690,902 The less than 60mV/pH
unit on an oxide-covered electrode is attributed to the formation of silanol groups and
adsorption of ionic species on both charged and noncharged surface sites. 853 According
to Madou et al ., 716 the low dependence of the flatband potential on pH is probably due
to a voltage across a very thin oxide that exists in non-HF solutions. The flatband poten-
tial of oxide-free silicon surface in non-HF solution could be obtained by using a