piezoresponse force microscopy study on ferroelectric polarization of ferroelectric polymer thin films with various structural configurations

Piezoresponse force microscopy study on ferroelectric polarization of ferroelectric polymer thin films with various structural configurations , , Zongyuan Fu, Jianchi Zhang, Junhui Weng, Weibo Chen, Yulong Jiang , and Guodong Zhu Citation: AIP Advances 5, 097211 (2015); doi: 10.1063/1.4931998 View online: http://dx.doi.org/10.1063/1.4931998 View Table of Contents: http://aip.scitation.org/toc/adv/5/9 Published by the American Institute of Physics AIP ADVANCES 5, 097211 (2015) Piezoresponse force microscopy study on ferroelectric polarization of ferroelectric polymer thin films with various structural configurations Zongyuan Fu,1 Jianchi Zhang,2 Junhui Weng,1 Weibo Chen,2 Yulong Jiang,2,a and Guodong Zhu1,a Department of Materials Science, Fudan University, Shanghai, China School of Microelectronics, Fudan University, Shanghai, China (Received September 2015; accepted 16 September 2015; published online 24 September 2015) Ferroelectric polymer-based memory devices have attracted much attention due to their potential in low-cost flexible memories However, bad retention property of recorded logic states limited their applications Though mechanisms of retention degradation in ferroelectric memories are complicated and still an open question, depolarization in ferroelectric polymer layer was regarded as the main influencing factor Here we reported our piezoresponse force microscopy (PFM) study of retention property of polarization states on various ferroelectric polymer based structures PFM results indicated that, as for ferroelectric/semiconductor structure and ferroelectric/insulator/semiconductor structure with thin insulating layer, both positive and negative polarization states could retain for a relatively long time Mechanisms of good retention of polarization states were discussed The discrepancy in bad retention of logic states and good polarization retention of ferroelectric layer was also analyzed C 2015 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License [http://dx.doi.org/10.1063/1.4931998] I INTRODUCTION Organic electronics has attracted more attention due to its potential for large-area electronic applications such as flexible displays and smart labels, in which organic nonvolatile memories are still the main bottleneck which limits the applications of all-organic flexible electronic systems.1 Among all reported organic nonvolatile memories, organic ferroelectric memories have been comprehensively studied and regarded as a promising technology for low-cost flexible memories Ferroelectric polymers, such as poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)), and organic semiconductors, such as pentacene and P3HT, are integrated to form ferroelectric field-effect transistors (FeFETs) for nonvolatile memory function.2 Even recently ferroelectric memristors were also realized in phase-separated ferroelectric/semiconductor blends.3 However, bad retention performance in such ferroelectric-based memories narrows their application in flexible electronics T P Ma et al argued that depolarization field and current leakage were two key factors which degraded the retention in ferroelectric devices.4 Depolarization field tends to negate ferroelectric polarization To stabilize the polarization and thus the stored logic state in ferroelectric memories, compensation charges are required at the surfaces of the ferroelectric in order to screen depolarization field In common ferroelectric transistors, the injection of such compensation charges is limited at ferroelectric/semiconductor or ferroelectric/insulator interfaces, thus resulting in depolarization-field-driven degradation of retention of both polarization states and stored logic states While the influence of depolarization field on polarization retention in inorganic ferroelectric devices has been extensively studied, its contribution in organic ferroelectric devices is still an open a corresponding authors, yljiang@fudan.edu.cn (Dr Prof Jiang); gdzhu@fudan.edu.cn (Dr Prof Zhu) 2158-3226/2015/5(9)/097211/9 5, 097211-1 © Author(s) 2015 097211-2 Fu et al AIP Advances 5, 097211 (2015) question Some work indicated that depolarization showed relatively weaker influence in organic ferroelectric devices than in inorganic ones T Reece et al found that depolarization was not an important factor for polarization retention in P(VDF-TrFE)/100 nm SiO2/n-Si transistors and bad retention was due to trapped charge-induced shift of threshold voltage.5 In P(VDF-TrFE)/P3HT FeFETs, R Kalbitz et al observed the bi-stable polarization states even after poling process and found that the trapping of negative charges at semiconductor/ferroelectric interface resulted in the shift of flatband voltage and thus the degradation of retention.6,7 Furthermore, B Kam et al reported so-called triple logic states based on positive and negative polarization and also depolarization states of ferroelectric layer in P(VDF-TrFE)/pentacene FeFETs and claimed that all three states retained for at least one year.8 While there are still many researchers who argued that depolarization in ferroelectric polymer layer greatly degraded the retention of recorded logic states in both FeFETs and memristors and, due to lack of compensation charges, polarization state did not exist even during the application of depletion voltage or immediately depolarized once the depletion voltage was removed K Asadi et al reported the retention property in P3HT/P(VDF-TrFE) blend films and found that, if another p-type semiconducting layer was inserted between such blend film and negatively-polarized electrode, ferroelectric phase in such blend films immediately and completely depolarized once the poling voltage was removed.9 In P(VDF-TrFE)/P3HT FeFETs, R Naber et al demonstrated that the bistability originated from switching between either polarized or depolarized state of the ferroelectric dielectric.10 In P(VDF-TrFE)/PTAA FeFETs, polarization state in ferroelectric layer depolarized immediately after the removal of depletion voltage.11 Obviously an effective evaluation of depolarization-driven degradation of retention performance is still absent in ferroelectric polymer-based devices Here we did not try to give a whole and clear picture on the degradation of retention property in ferroelectric memory devices, but we reported our piezoresponse force microscope (PFM) study of ferroelectric polymer thin films with various structural configurations aiming to discuss two questions: 1) whether both polarization states exist during and after poling process, and 2) how long the polarization states can retain after the removal of poling voltage II EXPERIMENTAL To study the polarization retention of ferroelectric polymer thin films, we chose three kinds of substrates to construct various ferroelectric structures: 1) Metal substrate: Al bottom electrode was vacuum thermally evaporated onto highly doped p-type Si wafer; 2) p-Si substrate: device level p-type Si(100) with resistivity of 8-12 Ω cm was used, and before deposition of ferroelectric film, the native oxide was removed by HF solution; and 3) Al2O3/p-Si substrate: 5.0 nm thick Al2O3 layer was deposited by atomic layer deposition onto the same p-type Si substrate as mentioned above Ohmic contact was made to the back side of silicon substrates (2) and (3) with 100 nm thick vacuum-evaporated Al film Ferroelectric P(VDF-TrFE) thin films were then deposited by spin-coating method from a 1.0 % by weight solution of 70/30 P(VDF-TrFE) (Kunshan Hisense Electronics Co., China) in butanone onto these substrates, resulting in P(VDF-TrFE)/Al, P(VDF-TrFE)/p-Si and P(VDF-TrFE)/Al2O3/p-Si structure s Then ferroelectric films were annealed at 135 oC for h to increase their crystallinity The thickness of annealed ferroelectric film was ∼90 nm, determined by atomic force microscope (AFM, Ultraobjective, Bruker) PFM (Nanoscope V, Bruker) was operated on these ferroelectric structures During all PFM measurements, the conductive probe was electrically grounded and the external voltage was applied to the bottom Al electrodes PFM was operated in contact mode with driving amplitude of V and frequency of 200 kHz and with scanning rate of 20µm/s Before PFM measurements, polarization states were realized on preselected local areas by applying preset sample bias and simultaneously scanning the grounded probe with preset rate of 12 µm/s To determine the capacitance-voltage (C-V) characteristic of P(VDF-TrFE)/Al2O3/p-Si structure, circular top Al gate electrodes with thickness of 100 nm and diameter of 0.6 mm were deposited onto the annealed P(VDF-TrFE) films by vacuum evaporation through a shadow mask C-V measurements were performed by an Agilent 4294A impedance analyzer with bottom Al electrode electrically grounded 097211-3 Fu et al AIP Advances 5, 097211 (2015) III RESULTS AND DISCUSSION The bi-stable operation of such ferroelectric/semiconductor structure s is obviously determined by the hysteresis of the C-V loops in an Al/P(VDF-TrFE)/Al2O3/p-Si/Al capacitor shown in Fig 1(a) We designate as “ON” the state obtained after poling by large negative gate voltage, and as “OFF” the state after poling by large positive voltage Memory window width of 1.4 V and maximum ON/OFF ratio of 2.65 at 1.65 V are obtained from such a ferroelectric capacitor However, the cause of such hysteresis is not well understood just as we discussed above Some researchers argued that the hysteresis was due to bistable positive and negative polarization states of the P(VDF-TrFE) film,5–7 while there were still some consideration that the cause of hysteresis was due to one polarization state and one depolarization state.10 Furthermore, the whole C-V curves shift toward positive voltage direction, which should be due to the existence of space charges in Al2O3 layer and/or ferroelectric films injected during the electrical application These space charges produce an internal electric field resulting in the shift of the transition voltage from the accumulation to the inversion states and vice versa Retention property of such a ferroelectric capacitor was shown in Fig 1(b) In the whole duration of 4000 s, both ON- and OFF-state capacitances degrade with time Again, as we have discussed above, the mechanisms of degraded retention performance still attract much controversy To evaluate the retention performance of polarization states in various ferroelectric structures, we conducted PFM measurements Note that in PFM measurements conductive tip was electrically grounded and external voltage was applied through the bottom electrode, different from that in C-V measurements in which external voltage was applied to the top electrode First we investigated the polarization states and polarization retention of P(VDF-TrFE)/Al structure Typical PFM results of such a structure are shown in Fig The central area of àm ì µm was poled by sample bias of +10V (labeled as positive polarization), and then the smaller central area of µm × µm was poled with bias of -10V (as negative polarization) To observe the degradation of both polarization states with time, PFM was continuously conducted in 60 minutes At the beginning of PFM measurement (0 min), PFM amplitude image shows much larger amplitude signals in both positively and negatively polarized regions, both of which are well distinguished from the surrounding unpolarized region, indicating the existence of electrical field-driven vibrations in pre-polarized local areas, i.e piezoelectricity In PFM phase image, all positively and negatively polarized and unpolarized regions are well distinguished from each other, indicating the existence of both positive and negative polarization states The averaged phase difference between both polarization states is about 170o, well consistent with the expected value of 180o.12 With the increase of retention time, both PFM amplitude and phase images show no obvious degradation (see PFM results after 30 and 60 minutes in Fig 2) These results indicate that in such a P(VDF-TrFE)/Al structure, both negative and positive polarization states can exist FIG C-V (a) and retention characteristic (b) of an Al/P(VDF-TrFE)/Al2O3/p-Si/Al structure During electrical measurement, external voltage was applied to top Al electrode while bottom Al electrode was grounded Retention property was recorded at bias of 1.6 V Prior to retention measurement, a pulse with duration of 10 s and amplitude of +15 V (-15 V) was applied to switch the ferroelectric capacitor to OFF- (ON-) state 097211-4 Fu et al AIP Advances 5, 097211 (2015) FIG PFM amplitude (a) and PFM phase (b) images of a P(VDF-TrFE)/Al structure Before PFM measurement, the central area of àm ì àm was first scanned with sample bias of +10V, and then the smaller central area of àm ì àm was scanned with bias of -10V Poling voltage was labeled in figures After these poling process, PFM was continuously conducted at central area of àm ì àm in 60 minutes to evaluate the degradation of polarization states with time The retention time for each image was labeled at the top of figures for a relatively long time, at least one hour in our microscopic observation Furthermore, for such a P(VDF-TrFE)/Al structure, sufficient charge compensation well screens the depolarization field and thus poling process results in large and continuous ferroelectric domains Next we conducted PFM measurements in a P(VDF-TrFE)/p-Si structure Both positively and negatively polarized regions were realized in a local area of àm ì àm as shown in Fig At the beginning of PFM measurement (0 min), though the whole polarized region are not so clearly visualized in both PFM amplitude and phase images as those observed in the P(VDF-TrFE)/Al structure shown in Fig 2, both negatively and positively polarized regions show relatively larger PFM amplitude signals than the surrounding unpolarized region does (Fig 3(a)) Furthermore, though the whole positively and negatively polarized regions show uneven piezoelectric property, both polarization states remain with no observable degradation during the retention measurement of 60 minutes Again PFM phase images show contrary contrast in the central area of àm ì àm which corresponds to reversed polarization states caused by the application of ±10 V bias The phase signals in the central àm ì àm area are obviously different from those in the surrounding, also indicating the existence of both polarization states With the increase of retention time, PFM phase images show no observable changes Furthermore noise-like dots are obtained in both PFM amplitude and phase images in the central polarized àm ì µm region These noise-like dots should be attributed to electrically inactive domains The cause of these inactive domains may be understood based on the effect of depolarization field As for such a P(VDF-TrFE)/p-Si structure, charge compensation may be not sufficiently provided by p-Si and thus depolarization field is not completely screened, which induces depolarization of homogeneous polarization state, leading to either a reduction of the remanent polarization or formation of ferroelectric domain patterns once the removal of poling voltage.13 Detailed discussion on these inactive domains is much more complicated and out of the scope of this paper Anyway, in such a P(VDF-TrFE)/p-Si structure, we observe stable positive and negative polarization states after the removal of poling voltage and both states retain unchanged for at least one hour in our microscopic measurement 097211-5 Fu et al AIP Advances 5, 097211 (2015) FIG PFM amplitude (a) and PFM phase (b) images of a P(VDF-TrFE)/p-Si structure Before PFM measurement, the central area of àm ì àm was first scanned with sample bias of +10V, and then the upper half region of àm ì àm in this positively pre-polarized area was scanned with sample bias of -10 V Poling voltage was labeled in figures After these poling processes, PFM was continuously conducted at central area of µm × µm in next 60 minutes to evaluate the degradation of polarization states with time The retention time for each image was labeled at the top of figures Finally we inserted a nm thick Al2O3 buffer layer between ferroelectric P(VDF-TrFE) film and p-Si wafer to see what happened on both polarization states in such a P(VDF-TrFE)/Al2O3/p-Si structure We first evaluated the retention property of polarization state caused by various positive poling voltages as shown in Fig We created four positively polarized regions (labeled as regions 1, 2, 3, and in Fig 4) by applying sample bias of 10, 9, and V, respectively During the application of positive sample voltage to bottom electrode, p-Si tended to be switched to accumulation and both ferroelectric and Al2O3 layers together share the whole applied poling voltage So the real voltage drop on ferroelectric layer should be lower than the apparent applied voltage In both PFM amplitude and phase images at the beginning of retention measurement (0 min), all four polarized regions are distinguished from the surrounding However, these local polarized regions are not imaged as a whole polarization domain, as shown in Fig 2, but show some smaller polarization domain patterns With the decrease of poling voltage from 10 to V, corresponding to region to 4, the region gets insufficiently polarized resulting in the decrease of the coverage of the polarization patterns in these local polarized regions However, for the poling voltage larger than V, once these domain patterns form, they remain for at least 60 minutes with no observable changes As for the local region polarized by V, ferroelectric domains display no obvious change in PFM phase images while get even lower coverage in PFM amplitude images with retention time up to 60 minutes, which may imply the depolarization-field-induced retention degradation after the application of lower poling voltage The patterned ferroelectric domains should be attributed to the influence of depolarization field Due to the existence of insulating Al2O3 layer between ferroelectric and p-Si layers, compensating charges are hard to be injected to the P(VDF-TrFE)/Al2O3 interface Thus insufficient compensation results in large depolarization field which breaks large and continuous ferroelectric domains, as shown in Fig 2, into smaller and patterned ferroelectric domains In fact, effective tuning of depolarization field has been demonstrated through insertion of ultrathin dielectric spacers between PbTiO3 and SrTiO3 layers.13 The strength of the depolarization field can be tuned according to the thickness of the insulating layer and thus used to control the formation of nanoscale ferroelectric 097211-6 Fu et al AIP Advances 5, 097211 (2015) FIG PFM amplitude (a) and PFM phase (b) images of a P(VDF-TrFE)/Al2O3/p-Si structure Before PFM measurement, four local regions, labeled as 1, 2, and in Fig 4(a), were sequentially polarized by sample bias of +10, +9, +8 and +7 V with areas of àm ì àm, àm ì µm, µm × 4.5 µm and µm × µm, respectively After these poling processes, PFM was continuously conducted in next 60 minutes to evaluate the degradation of polarization states with time The retention time for each image was labeled at the top of figures domain patterns Note that here we only chose nm thick Al2O3 layer for our PFM measurements and did not try to illuminate the tuning of depolarization field through the changes in the thickness of dielectric layer Then we consider the retention property of polarization state caused by negative poling voltages as shown in Fig Again we created four negatively polarized regions (labeled as region 1, 2, 3, and in Fig 5) by applying sample bias of -10, -9, -8 and -7 V, respectively p-Si was switched to depletion and ferroelectric, Al2O3 and depletion layers together share the whole bias voltage In both PFM amplitude and phase images at the beginning of retention measurement (0 min), all four polarized regions can be distinguished from the surrounding, though they are not imaged as a whole ferroelectric domain but some ferroelectric domain patterns Similar to those observed in Fig 4, with the decrease of poling voltage from -10 to -7 V, corresponding to region to 4, the local region gets insufficiently polarized with decreased coverage of polarization domains Furthermore, once these ferroelectric domains are created even by low poling voltage, such as -8 or -7 V, they can remain unchanged in the whole retention measurement of at least 60 minutes From these PFM results of various ferroelectric structures including ferroelectric/metal, ferroelectric/semiconductor/metal and ferroelectric/insulator/semiconductor/metal configurations, we can draw two conclusions One is that ferroelectric switching between positive and negative polarization states occurs during the C-V measurements of ferroelectric capacitor memories, as shown in Fig 1(a) The obvious hysteresis should be due to the switching between both polarization states, rather than the switching between one polarization state and one depolarization state reported in references 10 and 14 In fact, in the study of switching dynamics of P(VDF-TrFE)/organic semiconductor (such as α,ω-Dihexylsexithiophene15 and pentacene16) capacitors, ferroelectric switching between both polarization states was proved except that two-step reversal occurred during polarization reversal toward the positive side.15 Another is that both positive and negative polarization states can retain for relatively long time during retention measurement Though, in P(VDF-TrFE)/p-Si and P(VDF-TrFE)/Al2O3/p-Si 097211-7 Fu et al AIP Advances 5, 097211 (2015) FIG PFM amplitude (a) and PFM phase (b) images of a P(VDF-TrFE)/Al2O3/p-Si structure Before PFM measurement, four local regions, labeled as 1, 2, and in Fig 5(a), were sequentially polarized by sample bias of -10, -9, -8 and -7 V with areas of µm × µm, µm × µm, µm × 4.5 µm and µm × µm, respectively After these poling processes, PFM was continuously conducted in next 60 minutes to evaluate the degradation of polarization states with time The retention time for each image was labeled at the top of figures structures, ferroelectric domain patterns, rather than that whole and continuous domain obtained in P(VDF-TrFE)/Al structure, are observed due to the influence of depolarization field, however, once these patterns are created, they can retain for at least 60 minutes in our PFM measurements Good polarization retention of P(VDF-TrFE)/p-Si structure can be attributed to three facts First is that the coercive field of ferroelectric polymer films, about 48 MV/m, is one or two orders of magnitude larger than that of inorganic ferroelectric films Such large coercive field effectively enhances the resistance of polarization states to depolarization field Second is the shape of the polarization-electrical field (P-E) hysteresis loop of ferroelectric polymer films Square-like P-E loops are expected because, if this can be achieved, depolarization field will not affect the remanent polarization unless it exceeds the coercive field.4 P-E loops of inorganic ferroelectric films usually shows typical rounded shape, while ferroelectric polymer films display square-like P-E loops.17 Third is the charge compensation at P(VDF-TrFE)/semiconductor interface When accumulation voltage is applied, majority carriers accumulate at P(VDF-TrFE)/semiconductor interface In this situation, polarization charges can be well compensated from both top metal electrode and bottom semiconducting layer, resulting in stable polarization state in ferroelectric layer However, when depletion voltage is applied, majority carries are driven away from and depletion capacitance forms at P(VDF-TrFE)/semiconductor interface Thus polarization charges seem hard to be compensated at P(VDF-TrFE)/semiconductor interface resulting in unscreened large depolarization field From this consideration, many researchers believed that such large depolarization field did the main contribution to the much faster degradation of this depletion state.10,11 However, further study indicates that if depletion voltage is high enough, semiconducting layer can be switched to inversion and minority carriers can accumulate at ferroelectric/semiconductor interface to compensate polarization charges Thus ferroelectric polarization is expected to be stable In fact, such minority carrier trapping has been reported at insulator/semiconductor interfaces, such as SiO2/poly(alpha-methyl styrene)/pentacene18 and polyimide/P3HT,19 and was further illuminated at P(VDF-TrFE)/semiconductor interfaces, such as P(VDF-TrFE)/α,ω-Dihexylsexithiophene,15 097211-8 Fu et al AIP Advances 5, 097211 (2015) P(VDF-TrFE)/n-Si20 and P(VDF-TrFE)/P3HT.6,7 It is reasonable that these trapped minority carriers at ferroelectric/semiconductor interface compensate, at least partly, polarization charges in ferroelectric layer and thus effectively decrease the depolarization field resulting in stable depletion state As for P(VDF-TrFE)/Al2O3/p-Si structure, the existence of insulating Al2O3 layer inhibit the transport of majority and minority carriers from p-Si through Al2O3 layer to P(VDF-TrFE)/Al2O3 interface Polarization charges are hard to be well compensated at P(VDF-TrFE)/Al2O3 interface, resulting in large depolarization field and thus bad retention However, in PFM measurements shown in Figs and 5, we observe stable positive and negative polarization states, though ferroelectric domains show some patterns due to the influence of depolarization field These results indicate, though depolarization field breaks large ferroelectric domain into nanoscale ones,13 both polarization states still exist and can retain for a relatively long time The first two facts discussed above for P(VDF-TrFE)/p-Si sructure are still suitable to understand the good polarization retention of P(VDF-TrFE)/Al2O3/p-Si sructure Furthermore, though it is only a hypothesis and not proved here, nm thick Al2O3 layer is so thin that charges may tunnel from p-Si through Al2O3 layer to P(VDF-TrFE)/Al2O3 interface to partly compensate polarization charges S Rajwade et al have proposed and experimentally realized ferroelectric and charge hybrid nonvolatile memory devices in order to improve retention property of both logic states,21,22 where charges can tunnel through 5.4 nm thick SiO2 layer and then trap in 2.5 nm thick HfO2 layer If charge tunneling happens in P(VDF-TrFE)/Al2O3/p-Si capacitors, retention property is expected to be further improved We did not try to make PFM measurements on P(VDF-TrFE)/Al2O3/p-Si structure s with even thicker Al2O3 layer, because of large voltage drop on thick Al2O3 layer and also the maximum voltage limit of 10 V for the PFM module However, it can be imagined that with the increase of thickness of Al2O3 layer, depolarization field increases13 and most of polarization domains are depolarized once poling voltage is removed Anyway, in our PFM measurements, we observe the stable existence of both positive and negative polarization domains in both P(VDF-TrFE)/p-Si and P(VDF-TrFE)/Al2O3/p-Si sructures However, our macroscopic retention measurement on both logic states of P(VDF-TrFE)/Al2O3/p-Si capacitors (shown in Fig 1(b)) and also those work from other groups indicate bad retention property for ferroelectric polymer-based capacitor and field-effect transistor memories To understand the discrepancy in bad retention of logic states and good polarization retention of ferroelectric layer, it is required to re-consider the factors that would degrade the retention property of ferroelectric memories Note that bad retention of logic states should be the phenomenological reflection of degraded ferroelectric and/or semiconducting and/or insulating layers and/or interfaces between each It is accepted that depolarization field and leakage current4 result in the losses of remanent polarization in ferroelectric polymer films Furthermore, shift in threshold voltage may be another factor which induces bad retention in ferroelectric polymer-based memory devices Shifts in threshold voltage are usually attributed to the injection of space charges in ferroelectric and/or insulating layers or at interfaces between adjacent layers T Ng et al found that shifts in threshold voltage contributed to ∼55% of the reduction of transistor current in PBTTT/P(VDF-TrFE) FETs.23 Charge injection in ferroelectric films is a common phenomenon which is called imprint effect.24 Charge-injection-induced shift in threshold voltage was also reported in pentacene-based FETs, where charges tunneled from semiconductor channel into the gate dielectric.18 Even in depletion state, electrons can still diffuse from the semiconductor into the insulator and trap at polyimide/P3HT (insulator/semiconductor) interface.19 Furthermore, as for P(VDF-TrFE)/P3HT FeFETs, R Kalbitz et al argued that the apparent instability of depletion state arose from the development of a negative interfacial charge which more than compensated the ferroelectric-induced shift, resulting in a permanent shift in threshold voltage to positive values.6,7 All these work indicates the existence of charge injection which shifts the threshold voltage and results in the degradation of retention property in ferroelectric memories Finally, atmosphere exposure can also influence the retention property of ferroelectric memory devices It has been reported that oxygen exposure can influence the decay rate of inversion layer in P(VDF-TrFE)/p-Si FETs.25 Atmosphere exposure, such as UV/ozone,26 water27 and oxygen,28 can also influence electrical characteristics in organic semiconductor devices 097211-9 Fu et al AIP Advances 5, 097211 (2015) IV CONCLUSION In conclusion, we conducted comprehensive PFM measurements on various ferroelectric capacitor structures to determine the influence of depolarization field on polarization retention In both P(VDF-TrFE)/p-Si and P(VDF-TrFE)/5nm Al2O3/p-Si structures, PFM phase and amplitude images visualized both positive and negative polarization states after poling process, and both polarization states could retain for at least one hour in our PFM measurements We discussed the reasons for the stable existence of both polarization states and also analyzed the possible factors that would result in bad retention of logic states in ferrolelectric polymer-based memory devices ACKNOWLEDGMENT The authors would like to thank the support from National Natural Science Foundation of China (61076076, 61076068), STCSM (13NM1400600), NSAF (U1430106) and ZhuoXue Plan in Fudan University Z Hu, M Tian, B Nysten, and A Jonas, Nat Mater 8, 62 (2009) R Naber, K Asadi, P Blom, D de Leeuw, and B de Boer, Adv Mater 22, 933 (2010) K Asadi, D de Leeuw, B De Boer, and P 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ADVANCES 5, 097211 (2015) Piezoresponse force microscopy study on ferroelectric polarization of ferroelectric polymer thin films with various structural configurations Zongyuan Fu,1 Jianchi Zhang,2... degradation of retention property in ferroelectric memory devices, but we reported our piezoresponse force microscope (PFM) study of ferroelectric polymer thin films with various structural configurations. .. depolarization-field-driven degradation of retention of both polarization states and stored logic states While the influence of depolarization field on polarization retention in inorganic ferroelectric