Controlling Ferroelectric Negative Capacitance in Multilayered Structures for Low Power Electronics/ Controlul capacitatii negative feroelectrice in sisteme multistrat pentru electronica de putere redusa

Project Director: Dr. Andra-Georgia Boni

Controlling Ferroelectric Negative Capacitance in Multilayered Structures for Low Power Electronics

ENG The negative capacitance (NC) field-effect transistor (NCFET) is a new contender on the list of solutions for overcoming the limitations of scalability and energy efficiency of conventional CMOS technology. Connecting an NC element to the gate of a FET transistor may lead to voltage amplification on the gate, thus reducing the power consumption and heat generation. Simple thermodynamic theory predicts that ferroelectrics (FE) can display such NC effect and many articles show evidence of it, both in fundamental research and in device implementations, however, there is still much ambiguity remaining in this subject. The objective of this proposal is to study the relationship between the stability of polarization in NC states in multilayers and the properties of the constituent layers. The first stage is dedicated to the influence of the passive elements on the properties of a high-quality FE thin film capacitor. Then, the electrostatic contribution of the passive elements will be replaced by thin-film layers with different electric properties (resistivity and polarizability). The main objective is to use a FE structure with NC characteristics to replace the oxide in a MOS structure and to analyze the differences in electric characteristics. With the same purpose, it will be analyzed, the FE structures that present multiple polarization states used by the candidate in previous studies. The possibility of passing through NC regime during this atypical switching, as well as the possibility of stabilizing some NC states associated with the intermediate polarizations and the continuous capacity states evidenced by the applicant will be investigated. Another novelty element of this project is to investigate the possibility of NC control using a soft FE or an antiferroelectric both with switching and back-switching at similar voltages, as a method for a hysteresis-free characteristics of a NCFET. The results from this project may be used for future implementation in NCFET devices

RO Tranzistorul cu efect de camp cu capacitate negativa (NCFET) este un concept nou introdus in lista solutiilor pentru depasirea limitarilor actualei tehnologii CMOS priviind scalabilitatea si eficienta energetica. Conectarea unui element NC la poartea uni tranzistor FET are ca efect amplificarea tensiunii pe poarta si astfel se reduce puterea consumata si disiparea caldurii. Modelul termodicamic prevede ca un feroelectric (FE) poate fi intr-un regim de NC si multe articole au ca subiect evidenta acestui regim atat in studii fundamentale cat si in implementarea in dispozitive. Obiectivul acestui proiect este studierea relatiei dintre stabilizarea unei stari NC in multistraturi si proprietatile starturilor constituente. Punctul de pornire il vor reprezenta studii ale influentei elementelor pasive de circuit externe asupra proprietatilor feroelectrice. Apoi se vor construi structuri multistrat in care elementele pasive vor fi inlocuite de filme subtiri cu diferite proprietati electrice. Ultima etapa a proiectului isi propune folosirea unei structuri feroelectrice ce prezinta charcteristicile unei NC stabilizate sau controlabila dinamic pentru inlocuirea oxidului din structurile clasice MOS si analizarea proprietatilor electrice. Totodata se vor analiza si structuri ferroelectrice ce prezinta multiple stari de polarizare folosite de candidat in studiile precendente. Se va analiza posibilitatea trecerii prin zone de NC in timpul acestui switching atipic, precum si  posibilitatea stabilizarii unor stari de NC asociate cu polarizarile intermediare si starile continui de capacitate. Un alt element de noutate al acestui proiect este investigarea posibilitatii controlului NC folosind un FE soft sau un antiferroelectric, cu switching si back-switching la tensiuni apropiate, ca solutie pentru reducerea histerezisului in caracteristica NCFET-ului. Rezultatele din acest proiect vor putea fi folosite in viitor pentru implementarea in dispozitive de tip NCFET.

Andra Georgia Boni- Project Leader/ Director Proiect

Cristian M. Teodorescu - Supervisor/ Mentor

The first stage (August-December 2020):

At this stage, capacitive type ferroelectric structures were made and analyzed. As Ferroelectric materials  PbZr0.2Ti0.8O3, BaSrTiO3  were chosen: deposited on a single crystal substrate of SrTiO3 or Si. The structural characterization achieved by XRD and TEM confirms the epitaxial growth of the layers component. AFM / PFM analyzes show that the samples have compact surfaces with low roughness. The electrical characterization results directly highlight the negative capacity regime during reversal polarization using a new dielectric characterization method. It was also studied how the elements of the external circuit influence the switching of the polarization, as well as the highlighting of the distribution of the voltage between the study element - the ferroelectric capacitor - and the rest of the circuit.

The second stage (January-December 2021):

In the second stage of the project, the influence of the intermediate layers on the structural and electrical characteristics of the ferroelectric-based multilayered structures was studied. The materials of the intermediate layers were SrTiO3 doped with Nb 0.7% with the role of semiconductor / high resistivity and BaSrTiO3 (Sr / Ti = 40/60) with the role of insulator with a high dielectric constant. The thickness of the intermediate layers was chosen around 10-20nm so that the STON layer would be the equivalent of a series resistor with Rs = 10-50kohmi and the BST layer would be the equivalent of a capacitor with Cs = 1-2nF. The thickness of the ferroelectric layer of PbZrTiO3 was chosen around 150-200 nm so that the capacity of the layer is around 50-100 pF. These ratios of equivalent physical thickness and size were chosen according to the results presented in the previous stage where the effect of the circuit elements Rs and Cs on the electrical properties of the ferroelectric structures was studied. In this stage, different configurations were built, keeping constant the total thickness of the ferroelectric (F) and of the intermediate layer (I): STO / SRO / F / I; STO / SRO / I / F; STO / SRO / F / I / F. The structural changes according to the configuration as well as the electrical characteristics according to the intermediate layer and configuration were analyzed. At the end of the report, the correlations between the structure and the electrical properties are presented, as well as some theoretical approaches regarding the role of the non-ferroelectric element on the polarization reversal characteristics and obtaining a negative capacitance regime.

  • Scientific publications:

Negative capacitance in epitaxial ferroelectric capacitors evidenced by dynamic dielectric characterization, Georgia-Andra Boni, Cristina Chirila, Lucian-Dragos Filip, Ioana Pintilie, Lucian Pintilie, Materials Today Communications, Volume 26, March 2021, 102076,


  • Conferences:

1.  Poster: Ferroelectric-based multilayered thin films structures for multi-bit memory and memcomputing, Materials Challenges for Memory April 11-13, 2021 | Virtual Conference sponsored by APL Materials.
2. Poster: The study of polarization switching and negative capacitance regime in epitaxial ferroelectric thin films structures, IEEE ISAF 2021, Virtual Conference 16-21 may 2021





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