surface conduction, bulk conduction and charge neutralization.
Surface conduction relates to the surface parameters of dielectric, e.g. surface
resistivity and morphology, which can lead to the expanding
of charge spot along the dielectric surface. But this result was
not observed in our experiments. Bulk conduction depends on
the bulk resistivity, thickness and some other characters of
dielectric. In order to clarify the role of this mechanism in our
paper, an additional experiment was done: two layers of PE film covered the BSO crystal, hence the thickness of dielectric
increased to 12 μm, and the surface charge decay under this
kind of electrode configuration was measured (as Figure 6). It
could be found that there was almost no difference with the
dielectric thickness changing. Based on the above analysis, it
is inferred that the charge neutralization contributed to the
surface charge decay in our paper. The higher concentration
of negative ions in a void caused the faster decay rate of
positive surface charge than that of negative charge.