TU Darmstadt / ULB / TUbiblio

Impact of doping conditions on the Fermi level in lead-free antiferroelectrics

Villa, Lorenzo (2023)
Impact of doping conditions on the Fermi level in lead-free antiferroelectrics.
Technische Universität Darmstadt
doi: 10.26083/tuprints-00024481
Ph.D. Thesis, Primary publication, Publisher's Version

Abstract

Extended research, aimed at improving the properties of dielectric capacitors for energy storage applications, has fostered the interest in antiferroelectric materials. In this class of materials, sodium niobate (NN) and silver niobate (AN) are amongst the most promising alternatives to the toxic lead-containing compounds, such as lead zirconate-based ceramics, due to the possibility to obtain double P-E loops at room temperature. Chemical modification has been extensively used in order to improve their antiferroelectric properties and energy efficiency. However, the defect chemistry and its connection with the antiferroelectric properties are still unknown, and a systematic approach for doping these systems is still missing. Moreover, while processing conditions and kinetics are known to play a role in the final properties of the material, their impact has not yet been systematically studied. Lastly, when impurities are introduced into a system, they will interact with the already present intrinsic defects and can lead to formation of defect dipoles, which will affect the switching behaviour of the electric dipoles and therefore the P-E loops. Nonetheless, their presence induced by doping has not yet been investigated. The scope of this doctoral thesis is therefore to study with first-principles calculations the thermodynamics of point defects in NN and AN. We investigate with density functional theory (DFT) how doping and synthesis conditions modify the electronic properties of NN and AN, with focus on the Fermi level. In particular, we determine the thermodynamic defect equilibrium by solving the charge neutrality condition, accounting for the impact of extrisic defects on the compensation mechanisms. Moreover, we develop a novel scheme to account for quenching of defects within the established point defect thermodynamics.

In pure NN, the acceptor Na vacancies and donor O vacancies are present in large concentrations, thus dictating the position of the Fermi level. When doped with Sr and Sn, at high temperature and low oxygen partial pressure NN is an n-type semiconductor, while for high oxygen partial pressure the system becomes p-type. At room temperature the material is p-type. Quenching from high temperature all defects, or just O vacancies, shifts the Fermi level towards the conduction band minimum, while quenching Na and Nb vacancies produces a less pronounced shift towards the valence band maximum. Four defect complexes are found to be stable and present in high concentrations. This suggests they play an important role in the field switching mechanism, ultimately influencing the antiferroelectric P-E loops. In AN, the formation energies of the acceptor Ag vacancies are so low that the semiconductor is always p-type. For extremely low oxygen partial pressure, the material is found to be unstable due to the unphysically high concentration of Ag vacancies, which matches the experimental results reported in the literature. Doping with Mn and/or quenching does not produce a significant shift of the Fermi level, and the system remains p-type in all conditions. In summary, our approach allows to systematically study defect thermodynamics and electronic properties in semiconductors, accounting for multiple compensation mechanisms as well as quenched defects that are motivated by the synthesis conditions.

Item Type: Ph.D. Thesis
Erschienen: 2023
Creators: Villa, Lorenzo
Type of entry: Primary publication
Title: Impact of doping conditions on the Fermi level in lead-free antiferroelectrics
Language: English
Referees: Albe, Prof. Dr. Karsten ; Spitaler, Dr. Jürgen
Date: 2023
Place of Publication: Darmstadt
Collation: VIII, 144 Seiten
Refereed: 26 July 2023
DOI: 10.26083/tuprints-00024481
URL / URN: https://tuprints.ulb.tu-darmstadt.de/24481
Abstract:

Extended research, aimed at improving the properties of dielectric capacitors for energy storage applications, has fostered the interest in antiferroelectric materials. In this class of materials, sodium niobate (NN) and silver niobate (AN) are amongst the most promising alternatives to the toxic lead-containing compounds, such as lead zirconate-based ceramics, due to the possibility to obtain double P-E loops at room temperature. Chemical modification has been extensively used in order to improve their antiferroelectric properties and energy efficiency. However, the defect chemistry and its connection with the antiferroelectric properties are still unknown, and a systematic approach for doping these systems is still missing. Moreover, while processing conditions and kinetics are known to play a role in the final properties of the material, their impact has not yet been systematically studied. Lastly, when impurities are introduced into a system, they will interact with the already present intrinsic defects and can lead to formation of defect dipoles, which will affect the switching behaviour of the electric dipoles and therefore the P-E loops. Nonetheless, their presence induced by doping has not yet been investigated. The scope of this doctoral thesis is therefore to study with first-principles calculations the thermodynamics of point defects in NN and AN. We investigate with density functional theory (DFT) how doping and synthesis conditions modify the electronic properties of NN and AN, with focus on the Fermi level. In particular, we determine the thermodynamic defect equilibrium by solving the charge neutrality condition, accounting for the impact of extrisic defects on the compensation mechanisms. Moreover, we develop a novel scheme to account for quenching of defects within the established point defect thermodynamics.

In pure NN, the acceptor Na vacancies and donor O vacancies are present in large concentrations, thus dictating the position of the Fermi level. When doped with Sr and Sn, at high temperature and low oxygen partial pressure NN is an n-type semiconductor, while for high oxygen partial pressure the system becomes p-type. At room temperature the material is p-type. Quenching from high temperature all defects, or just O vacancies, shifts the Fermi level towards the conduction band minimum, while quenching Na and Nb vacancies produces a less pronounced shift towards the valence band maximum. Four defect complexes are found to be stable and present in high concentrations. This suggests they play an important role in the field switching mechanism, ultimately influencing the antiferroelectric P-E loops. In AN, the formation energies of the acceptor Ag vacancies are so low that the semiconductor is always p-type. For extremely low oxygen partial pressure, the material is found to be unstable due to the unphysically high concentration of Ag vacancies, which matches the experimental results reported in the literature. Doping with Mn and/or quenching does not produce a significant shift of the Fermi level, and the system remains p-type in all conditions. In summary, our approach allows to systematically study defect thermodynamics and electronic properties in semiconductors, accounting for multiple compensation mechanisms as well as quenched defects that are motivated by the synthesis conditions.

Alternative Abstract:
Alternative abstract Language

Antiferroelektrische (AFE) Materialien finden Einsatz in dielektrischen Kondensatoren mit hohen Energiedichten und -effizienzen, da sie doppelte P-E-Schleifen bei Raumtemperatur aufweisen. In dieser Materialklasse stellen Natriumniobat (NN) und Silberniobat (AN) die vielversprechendsten Alternativen zu toxischen bleihaltigen Verbindungen, wie zum Beispiel PbZrO3-basierten Keramiken, dar. Chemische Modifikationen wurden umfangreich genutzt, um die antiferroelektrischen Eigenschaften und Energieeffizienz dieser Materialien zu verbessern. Die Defektchemie und ihre Verbindung zu den AFE-Eigenschaften sind jedoch weitestgehend unbekannt und ein systematischer Ansatz zur Dotierung in diesen Systemen fehlt bisher. Ebenso wurden die Auswirkungen von Prozessbedingungen und kinetischen Aspekten auf die endgültigen Eigenschaften des Materials noch nicht systematisch erfasst. Außerdem können die in das System eingeführten Verunreinigungen mit den bereits vorhandenen intrinsischen Defekten interagieren und zur Bildung von Defektdipolen führen, welche das Schaltverhalten der elektrischen Dipole und damit die P-E-Schleifen beeinflussen. Trotzdem wurden Defektdipole in bleifreien Antiferroelektrika noch nicht untersucht. Das Ziel dieser Doktorarbeit ist es, die Thermodynamik von Punktdefekten in NN und AN mithilfe von first-principles Methoden zu untersuchen. Dazu werden Berechnungen basierend auf der Dichtefunktionaltheorie angewandt, um die Auswirkungen von Dotierung und Synthesebedingungen auf die elektronischen Eigenschaften von NN und AN zu analysieren, insbesondere mit Fokus auf das Fermi-Niveau. Dabei bestimmen wir das thermodynamische Defektgleichgewicht durch Lösung der Ladungsneutralitätsbedingung und berücksichtigen die Auswirkungen von extrinsischen Defekten auf die Kompensationsmechanismen. Darüber hinaus entwickeln wir ein neuartiges Verfahren innerhalb der etablierten Punktdefekt-Thermodynamik, das das Einfrieren von Defekten berücksichtigt. Mit diesem Ansatz zeigen wir, dass in reinem NN Na-Vakanzen (Akzeptoren) und O-Vakanzen (Donatoren) in hohen Konzentrationen vorhanden sind und die Position des Fermi-Niveaus bestimmen. Bei Dotierung mit Sr und Sn ist NN bei hohen Temperaturen und niedrigem Sauerstoffpartialdruck n-leitend, während das System bei hohem Sauerstoffpartialdruck p-leitend wird. Bei Raumtemperatur ist das Material p-leitend. Das Einfrieren aller Defekte oder nur der O-Vakanzen bei hoher Temperatur verschiebt das Fermi-Niveau in Richtung des Leitungsbandmaximums (engl. conduction band minimum, CBM), während das Einfrieren der Na- und Nb-Vakanzen eine weniger ausgeprägte Verschiebung in Richtung des Valenzbandmaximums (engl. valence band maximum, VBM) bewirkt. Vier Defektkomplexe stellen sich als stabil heraus und sind demnach in hohen Konzentrationen vorhanden. Dies legt nahe, dass sie eine wichtige Rolle im Schaltverhalten der Polarisation spielen und letztendlich die AFE P-E-Schleifen beeinflussen. In AN sind die Bildungsenergien der Ag-Vakanzen (Akzeptoren) so niedrig, dass das Halbleitermaterial immer p-leitend ist. Bei extrem niedrigem Sauerstoffpartialdruck ist das Material aufgrund der unrealistisch hohen Konzentration an Ag-Vakanzen instabil, was mit den in der Literatur beschriebenen experimentellen Ergebnissen übereinstimmt. Die Dotierung mit Mn und/oder das Einfrieren führen nicht zu einer signifikanten Verschiebung des Fermi-Niveaus, und das System bleibt unter allen Bedingungen p-leitend. Zusammengefasst ermöglicht unser Ansatz die systematische Untersuchung der Defektthermodynamik sowie der elektronischen Eigenschaften von Halbleitern, unter der Berücksichtigung einer Vielzahl von Kompensationsmechanismen und des Einfrierens von Defekten, welches durch die Synthesebedingungen motiviert wird.

German
Status: Publisher's Version
URN: urn:nbn:de:tuda-tuprints-244811
Classification DDC: 500 Science and mathematics > 500 Science
500 Science and mathematics > 530 Physics
Divisions: 11 Department of Materials and Earth Sciences
11 Department of Materials and Earth Sciences > Material Science
11 Department of Materials and Earth Sciences > Material Science > Materials Modelling
Date Deposited: 07 Sep 2023 11:30
Last Modified: 11 Sep 2023 05:21
PPN:
Referees: Albe, Prof. Dr. Karsten ; Spitaler, Dr. Jürgen
Refereed / Verteidigung / mdl. Prüfung: 26 July 2023
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