  A negative kink in excess conductivity is observed in p-type non-degenerate
(moderate dopant concentration) silicon wafers when excited by a very narrow
pulse of 532 nm laser appearing just after the complete positive decay of dark
conductivity voltage. Most of the Si samples are pristine, and 3 of them are
irradiated with gamma, proton, and chlorine ion beams respectively. These
transients were examined using a time-resolved millimeter-wave conductivity
apparatus (TRmmWC ) and the radiofrequency (RF) voltage response (after laser
cut-off) consistently reveals a positive peak with nominal decay to zero
followed by a negative kink. This negative photoconductivity (NPC) kink
develops just after the complete decay of the positive photoconductivity (PPC)
and lasts typically ~ 36 us. We present some data on general characteristics
obtained from a set of normal (pristine doped Si) wafers and the gamma- and ion
beam irradiated silicon (comparing with the parent pristine sample responses)
for establishing possible new links that might enable estimation of defect
parameters introduced in silicon.
