Irreversible extinction of ferroelectric polarization in P(VDF-TrFE) thin films upon melting and recrystallization

Abstract

Tuned dielectric, pyroelectric and piezoelectric properties of ferroelectric P(VDF-TrFE) thin films by using mechanical loads J. Appl. Phys. 111, 044102 (2012) Dependence of threshold thickness of crystallization and film morphology on film processing conditions in poly(vinylidene fluoride-trifluoroethylene) copolymer thin films

Recovery of remanent polarization of poly(vinylidene fluoride-co-trifluoroethylene) thin film after high temperature annealing using topographically nanostructured aluminium bottom electrode

“…The improvement of the remanent polarization ͑P r ͒ is attributed to the enhanced crystallinity during annealing between T C and T m with the preferred orientation of crystal with the b axis parallel to electric field. 4 P r in a saturated curve with an etched Al and a flat electrode are approximately 12.2 and 10 C/cm 2 , respectively, when the applied sweep voltage is above ±30 V. Coercive voltage for both cases is similar and approximately 10 V. Slightly large value of P r in the device with the etched Al seems to be due to the increased contact area of polymer film with the electrode. 10 To see the ferroelectric properties after melt and recrystallization, P͑VDF-TrFE͒ films previously annealed at 135°C for 2 h were further annealed from 155°C, just above T m , to 230°C.…”

“…2͑c͒. As demonstrated in our previous work, 4 the orientation of P͑VDF-TrFE͒ crystal is changed with its polar b axis parallel to the substrate during recrystallization, resulting in the extinction of polarization in P͑VDF-TrFE͒ films on a flat Al electrode. In contrast, the annealing above T m with a capacitor with the etched Al bottom electrode gave rise to P r of 10 C/cm 2 , approximately 10% drop of its highest polarization at 135°C.…”

“…[1][2][3] For low voltage operation of the polymer devices, it is necessary to fabricate as thin polymer films as possible with large crystallinity and effective crystal orientation maintained over broad temperature range. [4][5][6] In our recent work, we have found that thermal annealing above the melting temperature T m ͑ϳ150°C͒ of P͑VDF-TrFE͒ thin film gives rise to irreversible extinction of ferroelectric polarization due to the permanent polymer crystal rotation to its b axis perpendicular to electric field, which suggests that the fabrication of P͑VDF-TrFE͒ capacitors is undesirable above T m . 4 It is, therefore, essential to find a way to provide sufficient thermal stability of ferroelectric properties above T m for the realization of polymer ferroelectric memory.…”

“…First of all, we characterized the ferroelectric polarization hysteresis loops of P͑VDF-TrFE͒ fabricated on both types of bottom electrodes with and without topographic pattern after annealing at 135°C for 2 h. We obtained P-E curves from 10 to 30 V with the step of 2 V. As-cast samples with both substrates did not show saturated loops due to low crystallinity, as previously reported. 4 Figure 2͑a͒ shows well-saturated polarization curves from capacitor with two different electrodes after annealing. The improvement of the remanent polarization ͑P r ͒ is attributed to the enhanced crystallinity during annealing between T C and T m with the preferred orientation of crystal with the b axis parallel to electric field.…”

“…T m and Curie temperature ͑T C ͒ of the bulk polymer are 150 and 80°C, respectively. 4 Thin films were formed by spin coating with 1%-3% solution in methyl ethyl ketone. Thickness of the film was measured by ellipsometry ͑Gaertner Scientific Co.͒, ranging from approximately 70 to 200 nm.…”