Applications of transmission and grazing RAS i.r. spectroscopy to study the effect of annealing, thickness and electric field on the crystallinity, Curie transition, and dipole orientation in P(VDF/TrFE) (72/28) ultrathin films

초록

Reduction of ferroelectric polymeric film thickness to below 100 nm is of greater interest in order to fabricate non-volatile memory devices which must be operated at below 5V. Hence, in the first step towards understanding ferroelectric characteristics of VDF/TrFE(72/28) ultrathin films, an attempt has been made to study the nanostructure of films <100 nm using transmission and grazing RAS i.r. spectroscopy. The effect of annealing temperature (20~142oC), thickness (20~750 nm) and electric field on the ferroelectric characteristics of VDF/TrFE (72/28) ultrathin films have been reported. Annealing at or above Tc and then cooling to room temperature results in an increase in the ferroelectric crystalline phase with large increase in the intensity of the 1290 cm-l bands and decrease in the 1247 cm-l and 1124 cm-l bands. Using factor analysis of a series of transmission i.r spectra of 100 nm ultrathin film annealed at different temperatures, we have extracted the pure crystalline and amorphous spectra and also calculated the degree of crystallinity. From heating-cooling studies (30-150oC), the variation in ferroelectric phase with temperature is observed to be reversible, although thermal hysteresis exists, with the transition occurring at 105 and 70oC on heating and cooling, respectively. Factor analysis cannot be applied to grazing RAS i.r. spectra due to its sensitivity to dipole and chain orientation effects. The degree of crystallinity tends to decrease abruptly with decreasing film thickness below 200 nm. Transmission i.r. is not applicable to measuring the dipole switching in memory devices which are not i.r. transparent, whereas grazing RAS i.r. is more useful in monitoring the changes in dipole orientation and dipole switching upon poling and evaluating the degree of dipole orientation quantitatively and the results are reported in this study.