Optimization of stable multi-polarization states in ferroelectric heterostructures


Project Director: Dr. Lucian Dragos Filip

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

Contract no.: TE 192/2021

Contract begin date: 12/01/2021

Contract end date: 31/12/2021

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.

Work plan:

Stage 1: Theoretical study of multi-polarization state ferroelectric/insulator/ferroelectric heterostructures

Task 1.1: DFT calculations for the ferroelectric/insulator interface. The purpose of this task is to study the electronic properties of the interface as a function of the polarization direction in the ferroelectric layer, insulator layer thickness and ferroelectric layer thickness.

Task 1.2: Using the initial results obtained in T1.1, DFT calculations will be performed on the full FIF heterostructure. The purpose of this task is to study the effects of the insulator layer on the stability of the multi-polarization state and the electronic structure of the FIF.

Task 1.3: The hysteresis behaviour of the FIF heterostructure will be investigated in order to confirm the stability of the multiple polarization states. For this purpose the Landau-Ginzburg-Devonshire approach will be used to simulate the multi-polarization state hysteresis characteristics of the proposed heterostructure.

Task 1.4: Initial fabrication and characterization of FIF heterostructures. This task will focus on obtaining high quality heterostructures with epitaxial layers and minimal defects at the interface using Pulsed Layer Deposition Techniques (PLD) together with electrical and structural characterizations. This is an important first task that will set the stage for the rest of the experimental investigations.

Task 1.5: Comparison between the theoretical findings and the experimental results. This task will be focused on the fitting the theoretical results (i.e. capacitance-frequency characteristics, current-voltage characteristics and hysteresis characteristics) to the measured experimental data. The purpose is to confirm the coupling between the two ferroelectric layers separated by an insulator and how the geometry of the heterostructure is influencing this interaction.

Stage 2: Experimental investigation of ferroelectric/insulator/ferroelectric heterostructures.

Task 2.1: Initial fabrication and characterization of FIF heterostructures. This task will focus on obtaining high quality heterostructures with epitaxial layers and minimal defects at the interface using Pulsed Layer Deposition Techniques (PLD) together with electrical and structural characterizations. This is an important first task that will set the stage for the rest of the experimental investigations.

Task 2.2: Fabrication of heterostructures with different insulator and ferroelectric layer thicknesses. The purpose is to obtain a systematic study of the influence of the heterostructure geometric parameters on the electronic properties and the ferroelectric layer coupling.

Task 2.3: Electrical and structural characterization of the samples obtained in T2.2 in order to confirm the coupling between the two ferroelectric layers. For this purpose, the hysteresis characteristics will be analysed and compared with the theoretical results obtained in Stage 1.

Task 2.4: Asymmetric FIF heterostructure. Until now, it was assumed that both ferroelectric layers in the heterostructure were deposited from the same material. This task proposes the replacement of one of the two layers with a different type of ferroelectric material. For example, if the first ferroelectric layer is PbTiO3 and the insulator is SrTiO3, then the second ferroelectric layer can be BaTiO3 which is compatible with SrTiO3 for epitaxial growth. The difference is that its bulk polarization value is less than half (approx. 0.35 C/m2) than the one measured for bulk PbTiO3. It is therefore expected that the coupling between the two ferroelectric layers will be affected which in turn will introduce changes in the number or stability of the multi-polarization states of the heterostructure.

Task 2.5: Comparison between the theoretical findings and the experimental results. This task will be focused on the fitting the theoretical results (i.e. capacitance-frequency characteristics, current-voltage characteristics and hysteresis characteristics) to the measured experimental data. The purpose is to confirm the coupling between the two ferroelectric layers separated by an insulator and how the geometry of the heterostructure is influencing this interaction.

Project director: Dr. Lucian D. Filip

Team member: Dr. Georgia A. Boni

Team member: Dr. Cristina Chirila

Team member: Dr. Valeriu Moldoveanu


PROJECTS/ NATIONAL PROJECTS


Back to top

Copyright © 2022 National Institute of Materials Physics. All Rights Reserved