Project code: PN-III-P1-1.1-TE-2019-0709

Contract no.: TE 192/2021

Contract begin date: 12/01/2021

Contract end date: 31/12/2022

Total value of the contract: 95745 Euro

Abstract: Ferroelectric based heterostructures may hold the key to increasing storage capacity in computer systems. Non-volatile ferroelectric random access memories have been shown to have better stability and speed of read/write cycles than traditional RAM technology. However, the storage requirements of future applications are ever increasing which means that FeRAM must overcome not only the production cost of traditional RAM but also the storage capacity in order to become a viable replacement. Unfortunately, miniaturization is limited by ferroelectric properties of layers and increasing the number of stable polarization states is the only viable option. By using two or more ferroelectric layers separated by insulators, one can create more than two ferroelectric states due to polarization coupling between ferroelectric layers. The proposed project will focus on understanding the nature of the polarization coupling across the insulator layer and controlling the stability of the multiple polarization states. Such a study will be performed both through theoretical investigations and experimental fabrication of devices. The theoretical aspect will be concentrated on numerical calculations using density functional methods to investigate the ferroelectric/insulator interfaces and the stability of the polarization states. These results will be combined with thermodynamic models to obtain the polarization hysteresis characteristics that can be compared directly to the experimental reality. Structural characterization of fabricated ferroelectric/insulator/ferroelectric heterostructures will provide important clues for the interface regions which can be used to optimize the numerical calculations. The research team has been assembled in order to balance the two proposed objectives through extensive accumulated experience in both theoretical modelling and experimental fabrication and characterization.

Ferroelectric materials have been the focus of intense research efforts for the better part of the last three decades due to their remarkable range of properties (spontaneous polarization, dielectric, piezoelectric, pyroelectric etc.) providing considerable advantages for many electronic applications (non-volatile memories, infrared detectors, nano-actuators, solar-cells and logic gates etc.) [1–13]. One of the most important properties of ferroelectric materials is the presence of two stable polarization states that can be modified by the application of external stimuli (e.g. electrical or mechanical). Using the two stable ferroelectric states can potentially lead to an increase in the storage density [14–19]. However, as the demand for information storage is ever-increasing, new approaches are needed. One idea to satisfy the industry requirements is to increase the density of memory modules through further miniaturization, but this reduces the efficacy of the ferroelectric layers [14,16,20]. Recently, our research group has shown that multilayered ferroelectric-insulator-ferroelectric (FIF) devices present multiple polarization states (as opposed to only two for a single ferroelectric layer) that can be addressed with specially designed voltage pulses [21,22]. It was demonstrated that a coupling of the ferroelectric states takes place across the insulator, giving rise to 2ⁿ polarization states, where n is the number of ferroelectric layers. The important aspect of these structures is the fact that “reading” the state at any given time (i.e. checking the polarization state) is non-destructive (i.e. the polarization state is not changed as it is the case with present ferroelectric memories). This property is crucial for practical applications because it decreases device fatigue considerably and prevents degradation of device operation. It was also shown that the concept of memcomputing (i.e. computing and storing of information on the same device) could be implemented with these devices. Unfortunately, it was shown that the polarization state in each ferroelectric layer can be influenced by factors ranging from the strain induced defects caused by the crystal network mismatch to depolarization fields and leakage currents. In a simple metal/ferroelectric/metal heterostructure, the polarization switching mechanism is usually governed by the interface, the ferroelectric bulk or a combination of the two. The usual characterization methods such as polarization hysteresis, leakage current characteristics or the capacitance-voltage characteristics should provide the necessary clues for the most probable switching mechanism. In a multiple ferroelectric layer heterostructure such as the ones described above, the results obtained using these methods are not readily interpreted. For this reason, obtaining information on the specific processes that take place in such complex heterostructures is crucial, yet almost impossible from experimental measurements only. This is the gap that theoretical modelling is able to fill.

For this purpose, the present project proposes a combined ab-initio and analytical theoretical approach to study the structural properties of multilayered FIF heterostructures. Both approaches have their sets of strengths and weaknesses but, if used together, one can obtain a better understanding of the interaction between the individual ferroelectric layers in such a complex structure as a multi-polarization state device. The ab-initio route is precise, the electronic structure of the heterostructure can be studied using the density functional theory and important information about the charge density, polarization, strain etc., at each interface, can be obtained. This method is implemented in several software suites (such as Quantum Espresso, VASP etc.) which can be used to perform the necessary calculations. Unfortunately, the limitations of this method are mostly on the technical side, due to the size of the studied systems. Judging from previous numerical studies performed in our group [23,24], heterostructures involving PbTiO₃ thin films will retain the ferroelectric property (i.e. two distinct polarization states) if the thickness is no less than 7 unit cells. This will be the lower thickness limit of the ferroelectric layers comprising the studied heterostructures. Considering, for example that there are 5 atoms in a PbTiO₃ unit cell (which is one of the ferroelectric materials that can be used), one can easily estimate that for the two ferroelectric layers, 70 atoms are needed. The insulator layer thickness will also have to be considerable since it was shown in [21] that there is a lower limit for the insulator thickness below which multi-polarization states cannot be observed experimentally. Overall, we estimate that the studied structures will require at least 100 atoms in order to be able to capture the physical phenomena appearing at the two interior interfaces. This means that calculation times and required RAM will be quite large. In order to mitigate this aspect, the present project will also augment the numerical calculations with analytical theoretical models since such a route does not depend on large IT infrastructure. The downside of this approach is that the results are obtained by modelling the heterostructure through sets of approximations meant to isolate the particular properties one is interested in. The advantage of this route is that processes like the hysteresis polarization cycles, current-voltage and capacitance-voltage characteristics can be studied for any number of heterostructure configurations, i.e. large number of ferroelectric-insulator layers.

The ab-initio method can provide important accurate information about the boundary region only, which can be used as connection conditions for the more flexible analytical/semi-analytical route.

References:

[1] Y. Rong, M. Li, J. Chen, M. Zhou, K. Lin, L. Hu, W. Yuan, W. Duan, J. Deng, X. Xing, Phys Chem Chem Phys 18 (2016) 6247–6251.
[2] W.Y. Kim, H.-D. Kim, T.-T. Kim, H.-S. Park, K. Lee, H.J. Choi, S.H. Lee, J. Son, N. Park, B. Min, Nat. Commun. 7 (2016) 10429.
[3] P. Kumari, R. Rai, S. Sharma, M. Shandilya, A. Tiwari, Adv. Mater. Lett. 6 (2015) 453–484.
[4] B. Chen, X. Zheng, M. Yang, Y. Zhou, S. Kundu, J. Shi, K. Zhu, S. Priya, Nano Energy 13 (2015) 582–591.
[5] F. Liu, W. Wang, L. Wang, G. Yang, Appl. Phys. Lett. 104 (2014) 103907.
[6] Y. Yuan, T.J. Reece, P. Sharma, S. Poddar, S. Ducharme, A. Gruverman, Y. Yang, J. Huang, Nat. Mater. 10 (2011) 296–302.
[7] H. Huang, Nat. Photonics 4 (2010) 134–135.
[8] M.D. Glinchuk, E.V. Kirichenko, V.A. Stephanovich, B.Y. Zaulychny, J. Appl. Phys. 105 (2009) 104101.
[9] V. Garcia, S. Fusil, K. Bouzehouane, S. Enouz-Vedrenne, N.D. Mathur, A. Barthélémy, M. Bibes, Nature 460 (2009) 81–84.
[10] T. Choi, S. Lee, Y.J. Choi, V. Kiryukhin, S.-W. Cheong, Science 324 (2009) 63–66.
[11] J.F. Scott, Science 315 (2007) 954–959.
[12] T. Hidaka, Ferroelectrics 137 (1992) 291–295.
[13] J.F. Scott, C.A.P. de Araujo, Science 246 (1989) 4936.
[14] H. Ishiwara, M. Okuyama, Y. Arimoto, Ferroelectric Random Access Memories : Fundamentals and Applications, Springer, Berlin; New York, 2004.
[15] S. Horie, K. Noda, H. Yamada, K. Matsushige, K. Ishida, S. Kuwajima, Appl. Phys. Lett. 91 (2007) 193506.
[16] M.H. Park, H.J. Lee, G.H. Kim, Y.J. Kim, J.H. Kim, J.H. Lee, C.S. Hwang, Adv. Funct. Mater. 21 (2011) 4305–4313.
[17] A.K. Tripathi, A.J.J.M. van Breemen, J. Shen, Q. Gao, M.G. Ivan, K. Reimann, E.R. Meinders, G.H. Gelinck, Adv. Mater. 23 (2011) 4146–4151.
[18] B. Kam, X. Li, C. Cristoferi, E.C.P. Smits, A. Mityashin, S. Schols, J. Genoe, G. Gelinck, P. Heremans, Appl. Phys. Lett. 101 (2012) 033304.
[19] W.Y. Kim, H.C. Lee, Micro Nano Lett. 10 (2015) 700–702.
[20] J.H. Lee, K. Chu, K.-E. Kim, J. Seidel, C.-H. Yang, Phys. Rev. B 93 (2016) 115142.
[21] A.-G. Boni, L. Filip, C. Chirila, I. Pasuk, R. Negrea, I. Pintilie, L. Pintilie, Nanoscale (2017).
[22] G.A. Boni, L.D. Filip, C. Chirila, A. Iuga, I. Pasuk, L. Hrib, L. Trupina, I. Pintilie, L. Pintilie, Phys. Rev. Appl. 12 (2019) 024053.
[23] N. Plugaru, G.A. Nemnes, L. Filip, I. Pintilie, L. Pintilie, K.T. Butler, A. Manolescu, J. Phys. Chem. C 121 (2017) 9096–9109.
[24] R. Dorin, L.D. Filip, L. Pintilie, T.B. Keith, N. Plugaru, Under Review (2019).