We should note that there is an imprint for pre-poled PZT and PTO
films. Even though very small, it could be observed in Fig. 1a and c
that the hysteresis loops measured from the pre-poled parts of both
films shifted toward the negative voltage with increasing total dose.
The voltage shift indicates the buildup of the internal bias field in the
films [7,9,11]. The internal bias voltages, Vi = (Vc
+ + Vc
−) / 2, of the
PTO and PZT films were plotted as a function of total dose as shown in
Fig. 4,where, Vc
+ and Vc
− are positive and negative coercive voltages, respectively.
The internal bias voltages in non-pre-poled PTO and PZT
films are almost 0 and do not change with the total dose. The internal
bias voltage in pre-poled films increased in the negative direction rapidly
after irradiation with total dose of 50 kGy and then it becomes saturated
as the total dose increases to 300 kGy. The internal bias field
may be attributed to trapped defect charges injected from electrodes
and produced by ionizing effects during the irradiation. It is expected
that with increasing total dose, more defect charges will be generated
and be trapped so that internal bias will more build up in the films.
Under a relatively large irradiation total dose, the trapping sites will
be almost filled. Thus, it suggest that the internal bias voltages in the
pre-poled PTO and PZT films saturated as the irradiation total dose
increases