TU Darmstadt / ULB / TUbiblio

Investigating the influence of microstructure and grain boundaries on electric properties in thin film oxide RRAM devices – A component specific approach

Zintler, Alexander (2022)
Investigating the influence of microstructure and grain boundaries on electric properties in thin film oxide RRAM devices – A component specific approach.
Technische Universität Darmstadt
doi: 10.26083/tuprints-00021657
Dissertation, Erstveröffentlichung, Verlagsversion

Kurzbeschreibung (Abstract)

At the beginning of the 21st century, the quest for finding ever more power efficient, densely packed, and multi-bit-level storage for computational applications is still ongoing. Ever increasing demand in low power computing since the advent of the internet of things (IoT), scaling limitations contradicting Moore’s law, the rise of neuromorphic computing and in-memory computing turned a spotlight onto material classes that seem to tick all the boxes of these requirements, such as transition metal oxides. Predicted in 1971, now 50 years ago, Chua described a missing circuit element, that would complete the well-known list of resistor, capacitor and inductor: the memristor. Its behavior is that of a “nonlinear resistor with memory,” lending their names for the contraction naming of the postulated two-terminal circuit element.

It took 37 years until Strukov et al. exclaimed “The missing memristor [being] found” in 2008, with their realization of a two-terminal memristor being implemented as a metal-insulator-metal (MIM) stack consisting of a Pt/TiO2 x/Pt stack. By biasing the 5 nm oxide film, the resistivity could be changed between a high and low resistive state in a reversible and non-volatile process. Similar electrical behavior was experimentally observed for organic, amorphous and (single) crystalline oxide thin films as well as for Chalcogenide-metal stacks. Depending on the material class that is involved in the observed “negative resistive change,” several groups of memristive systems (also referred to as RRAM, as they are candidates for resistive random-access memory applications) can be defined. Common between “filamentary switching” material systems is the confinement of the volume of the thin film that takes part in the resistive switching process. Insight into these nanoscale events involved in the localization of these regions is a key step towards the full understanding of the fundamental physical processes involved.

The primary goal in the conjunction of research on resistive switching thin films and high-resolution microscopy is to image a working filament. Transmission Electron Microscopy (TEM) methods including the local mapping of structural and spectroscopic properties, Conductive Atomic Fore Microscopy (CAFM) as well X-ray microscopy methods were and still are at the forefront of this ongoing effort. Based on the findings of grain boundaries serving as preferential filament formation sites in oxide thin films, ab initio methods confirmed this structural feature’s unique role in VCM RRAM. Lab scale, photolithography-based device sizes range from several 100 µm² to 10 µm² and finding a predicted filament of only a few square nanometers in square micron sized areas is a “needle in a haystack” problem. One way to facilitate finding the filament is the creation of the filament at a predefined position. In the case of CAFM, this is inherently part of the experimental setup, but in the case of TEM, a whole new set of methods had to be developed to be able to apply a bias on a thin film stack inside a microscope.

In this work, parallel to the implementation of a preparation routine enabling the operation of an RRAM device inside a TEM, two components of TiN/HfO2/Pt MIM stacks have been investigated in detail: firstly, the TiN bottom electrode, which is an integral part in a working device due to its defining microstructural features. High substrate temperature growth of TiN on c-cut sapphire substrates showed exceptional room temperature as well as superconducting properties. The low surface roughness and nitrogen deficiency highlight the aptitude for highly textures TiN layers to act as bottom electrodes in resistive switching devices. Secondly, the HfO2 layer itself, whose arrangement of textured grains and resulting grain boundaries have been investigated at nanometer and sub-ångström resolution. It was possible to identify the terminating planes of monoclinic hafnia grains, which subsequently have been used as a basis for ab initio modeling the grain boundary structure. Identification of sub-stoichiometric and stoichiometric phases in 10 nm films was a second achievement obtained in the study of the oxide film. To probe the crystallographic phase, stoichiometry, and orientation of individual grains, the methods chosen in the TEM studies cover techniques fundamentally driven by electron diffraction and incoherent imaging.

Two technological revolutions are currently pushing (transmission) electron microscopy into a new era: new in situ specimen holders and 4D-STEM data acquisition. First, modern, micro-electro-mechanical-systems (MEMS) based in situ holders allow the (simultaneous) application of biasing and temperature stimuli as well as the introduction of controlled gaseous and liquid environments to investigate TEM samples as close to real world conditions as possible. Second, electron detector technology is being accelerated by introducing hybrid pixel detectors that allow the fast and high dynamic range capturing of pixelated reciprocal space and spectroscopic data.

Both technological developments are addressed as part of this work, as the implementation of preparation routines for in situ experiments and the integration of a 4D-STEM detector were undertaken as part of this work. At the same time, these developments are crucial methodologies that enable the study of functional thin films, with this work focusing on hafnia-based VCM RRAM. 4D-STEM specifically is proving to be an advanced method to investigate the local structure and response to external stimuli to an extent never achieved before. Consequently, the quantities of data created during these experiments (external stimuli add more dimensions to 4D-STEM experiments) in need fundamentally new approaches to data analysis. On the journey towards an all-encompassing experiment, this work aims to establish the necessary tools, both in hardware and software, and the fundamental material scientific understanding to create a unique and state-of-the-art toolset that allows proceeding in the vast field of oxide-based electronic thin films.

Typ des Eintrags: Dissertation
Erschienen: 2022
Autor(en): Zintler, Alexander
Art des Eintrags: Erstveröffentlichung
Titel: Investigating the influence of microstructure and grain boundaries on electric properties in thin film oxide RRAM devices – A component specific approach
Sprache: Englisch
Referenten: Molina-Luna, Prof. Dr. Leopoldo ; Alff, Prof. Dr. Lambert
Publikationsjahr: 2022
Ort: Darmstadt
Kollation: 117, LXIII Seiten
Datum der mündlichen Prüfung: 4 Juli 2022
DOI: 10.26083/tuprints-00021657
URL / URN: https://tuprints.ulb.tu-darmstadt.de/21657
Kurzbeschreibung (Abstract):

At the beginning of the 21st century, the quest for finding ever more power efficient, densely packed, and multi-bit-level storage for computational applications is still ongoing. Ever increasing demand in low power computing since the advent of the internet of things (IoT), scaling limitations contradicting Moore’s law, the rise of neuromorphic computing and in-memory computing turned a spotlight onto material classes that seem to tick all the boxes of these requirements, such as transition metal oxides. Predicted in 1971, now 50 years ago, Chua described a missing circuit element, that would complete the well-known list of resistor, capacitor and inductor: the memristor. Its behavior is that of a “nonlinear resistor with memory,” lending their names for the contraction naming of the postulated two-terminal circuit element.

It took 37 years until Strukov et al. exclaimed “The missing memristor [being] found” in 2008, with their realization of a two-terminal memristor being implemented as a metal-insulator-metal (MIM) stack consisting of a Pt/TiO2 x/Pt stack. By biasing the 5 nm oxide film, the resistivity could be changed between a high and low resistive state in a reversible and non-volatile process. Similar electrical behavior was experimentally observed for organic, amorphous and (single) crystalline oxide thin films as well as for Chalcogenide-metal stacks. Depending on the material class that is involved in the observed “negative resistive change,” several groups of memristive systems (also referred to as RRAM, as they are candidates for resistive random-access memory applications) can be defined. Common between “filamentary switching” material systems is the confinement of the volume of the thin film that takes part in the resistive switching process. Insight into these nanoscale events involved in the localization of these regions is a key step towards the full understanding of the fundamental physical processes involved.

The primary goal in the conjunction of research on resistive switching thin films and high-resolution microscopy is to image a working filament. Transmission Electron Microscopy (TEM) methods including the local mapping of structural and spectroscopic properties, Conductive Atomic Fore Microscopy (CAFM) as well X-ray microscopy methods were and still are at the forefront of this ongoing effort. Based on the findings of grain boundaries serving as preferential filament formation sites in oxide thin films, ab initio methods confirmed this structural feature’s unique role in VCM RRAM. Lab scale, photolithography-based device sizes range from several 100 µm² to 10 µm² and finding a predicted filament of only a few square nanometers in square micron sized areas is a “needle in a haystack” problem. One way to facilitate finding the filament is the creation of the filament at a predefined position. In the case of CAFM, this is inherently part of the experimental setup, but in the case of TEM, a whole new set of methods had to be developed to be able to apply a bias on a thin film stack inside a microscope.

In this work, parallel to the implementation of a preparation routine enabling the operation of an RRAM device inside a TEM, two components of TiN/HfO2/Pt MIM stacks have been investigated in detail: firstly, the TiN bottom electrode, which is an integral part in a working device due to its defining microstructural features. High substrate temperature growth of TiN on c-cut sapphire substrates showed exceptional room temperature as well as superconducting properties. The low surface roughness and nitrogen deficiency highlight the aptitude for highly textures TiN layers to act as bottom electrodes in resistive switching devices. Secondly, the HfO2 layer itself, whose arrangement of textured grains and resulting grain boundaries have been investigated at nanometer and sub-ångström resolution. It was possible to identify the terminating planes of monoclinic hafnia grains, which subsequently have been used as a basis for ab initio modeling the grain boundary structure. Identification of sub-stoichiometric and stoichiometric phases in 10 nm films was a second achievement obtained in the study of the oxide film. To probe the crystallographic phase, stoichiometry, and orientation of individual grains, the methods chosen in the TEM studies cover techniques fundamentally driven by electron diffraction and incoherent imaging.

Two technological revolutions are currently pushing (transmission) electron microscopy into a new era: new in situ specimen holders and 4D-STEM data acquisition. First, modern, micro-electro-mechanical-systems (MEMS) based in situ holders allow the (simultaneous) application of biasing and temperature stimuli as well as the introduction of controlled gaseous and liquid environments to investigate TEM samples as close to real world conditions as possible. Second, electron detector technology is being accelerated by introducing hybrid pixel detectors that allow the fast and high dynamic range capturing of pixelated reciprocal space and spectroscopic data.

Both technological developments are addressed as part of this work, as the implementation of preparation routines for in situ experiments and the integration of a 4D-STEM detector were undertaken as part of this work. At the same time, these developments are crucial methodologies that enable the study of functional thin films, with this work focusing on hafnia-based VCM RRAM. 4D-STEM specifically is proving to be an advanced method to investigate the local structure and response to external stimuli to an extent never achieved before. Consequently, the quantities of data created during these experiments (external stimuli add more dimensions to 4D-STEM experiments) in need fundamentally new approaches to data analysis. On the journey towards an all-encompassing experiment, this work aims to establish the necessary tools, both in hardware and software, and the fundamental material scientific understanding to create a unique and state-of-the-art toolset that allows proceeding in the vast field of oxide-based electronic thin films.

Alternatives oder übersetztes Abstract:
Alternatives AbstractSprache

Zu Beginn des 21ten Jahrhunderts dauert die Suche nach energieeffizienten, multi-bit-Level Speichern mit hoher Speicherdichte für rechnergestützte Anwendungen immer noch an. Dazu lenkte die Nachfrage nach energiesparendem Computing (seit Anbruch von internet of things (IoT)) und, Herausforderungen bei der Einhaltung des Mooreschen Gesetz, den Fokus auf neuromporphes und in-memory Computing und damit auf eine Materialklasse, die allen Anforderungen gewachsen scheint: den Übergangsmetalloxiden. 1971, heute vor 50 Jahren, beschrieb Chua ein fehlendes Element unter den elektronischen Bauteilen das die bekannte Liste von Widerstand, Kondensator und Induktivität vervollständigt: der Memristor. Sein Verhalten sei das eines “nonlinear resistor with memory,” was zu der Namensgebung des postulierten zweipoligen Bauteils führte.

Es dauerte 37 Jahre bis Strukov et al. 2008 mit „The missing memristor [being] found” den Memristor, umgesetzt als zweipoligen Metall-Isolator-Metall (MIM) Memristor, bestehend aus einer Pt/TiO2/Pt Schichtstruktur, für gefunden erklärte. Beim Anlegen eines elektrischen Feldes an den 5 nm Oxidfilm konnte der Memristor zwischen nicht volatilen hohen und niedrigen Widerstandzuständen reversibel geschaltet werden. Ein ähnliches elektrisches Verhalten wurde experimentell in Chalkogenid-Metall-Schichtstrukturen beobachtet. Abhängig von der Materialklasse, die in der „negativen Widerstandsänderung“ involviert ist, können unterschiedliche Gruppen memristiver Systeme (auch „RRAM“, da sie als Kandidaten für resisitve random-access memory in Frage kommen) definiert werden. Den „filamentär schaltenden“ Materialsystemen ist gemeinsam, dass ein räumlich streng limitiertes Volumen in der Dünnschicht am resistiven Schaltprozess beteiligt ist. Einsicht in die nanoskaligen Prozesse, die für die Lokalisierung dieser Volumina verantwortlich sind, ist ein relevanter Schritt für das Verständnis der fundamentalen physikalischen Prozesse.

Das Hauptziel in der vereinten Forschung im Bereich RRAM und hochauflösender Elektronenmikroskopie ist die Abbildung eines Filaments beim Schaltprozess. Transmissionselektronenmikroskopische (TEM) Methoden, wie ortsaufgelöste strukturelle und spektroskopische Untersuchungen, Conductive Atomic Force Microscopy (CAFM) sowie Röntgenmikroskopie waren und sind entscheidende Methoden in diesem Unterfangen. Basierend auf Untersuchungen, die Korngrenzen als präferentielle Orte für die ausbildung von Filamenten diskutieren, konnten ab inito Verfahren die Entscheidende Rolle dieses Merkmals bestätigen. Photolithographie basierte Speicherzellen im Labor spannen mit ihrer Querschnittsfläche einen Bereich zwischen mehreren 100 µm² und 10 µm², was die Suche eines postulierten Filaments mit dem Querschnitt weniger nm² zu einem „Nadel im Heuhaufen“ Problem werden lässt. Eine Möglichkeit, die Suche nach dem Filament zu vereinfachen ist die gezielte Erzeugung von Filamenten an selektierten Stellen in der Probe, was für CAFM eine inhärente Eigenschaft des Experiments ist. Dazu konnten ab initio Untersuchungen zeigen, dass Korngrenzen auch als präferierte Stellen zum Erzeugen von Filamenten sind. Zum anderen muss ein Repertoire an einen TEM Methoden entwickelt werden um die Abbildung eines Filaments bei Anlegen einer Spannung im TEM untersuchen zu können.

Parallel zu der Implementierung einer Präparationsroutine zum elektrischen Schalten eines RRAM Bauteils im TEM, werden in dieser Arbeit zwei Komponenten der TiN/HfO2/Pt MIM Schichtstruktur untersucht: Zum Ersten die TiN Bodenelektrode, die mit ihrer Mikrostruktur ein integraler Bestandteil der Speicherelemente ist. Das Wachstum von TiN1-x Schichten auf c-cut Saphir Substraten bei hohen Temperaturen zeigt hervorragende elektrischen Eigenschaften bei Raumtemperatur und unter supraleitenden Bedingungen. Die geringe Oberflächenrauigkeit und Stickstoffdefizienz der hochgradig texturierten TiN Elektroden ist ideal für die Anwendung als Bodenelektrode für RRAM Strukturen. Zum Zweiten, die Hafniumoxid Schicht, deren Arrangement von texturierten Körnern und die daraus resultierenden Korngrenzen auf Nanometer- und Ångström-Skala untersucht wurden. Die Terminierung von monoklinen HfO2 Körnern konnte ermittelt werden, was als Grundlage für ab initio Untersuchungen zur Modellierung der Korngrenze genutzt werden konnte. Zudem konnten in dieser Arbeit stöchiometrische und sub-stöchiometrische HfO2-x Phasen in 10 nm Schichten identifiziert werden. Die Identifikation der kristallographischen Phasen, Stöchiometrien und Orientierungen einzelner Körner basierte hierbei größtenteils auf TEM Methoden, die widerum auf Elektronenbeugung und inkohärenter Abbildung basieren.

Darüber hinaus bereicherten zwei technologische Entwicklungen die (Transmissions)elektronenmikroskopie entscheidend: neue in situ Halter und 4D-STEM Datenaufzeichnung. Moderne, mirco-electro-mechanical-systems (MEMS) basierte in situ Halter erlauben die (gleichzeitige) Anwendung von elektrischen und Temperaturstimuli sowie die Untersuchung von TEM Proben unter Gas oder Flüssigphasen, um experimentelle Zustände zu erreichen, die vergleichbar zu letztendlichen Anwendungsbedingungen sind. Zudem bringt die Einführung von Hybridpixeldetektoren in der Elektronenmikroskopie Vorteile in Dynamikumfang und Geschwindigkeit bei der Aufnahme von Daten im reziproken Raum oder in der Spektroskopie.

Beide technische Entwicklungen werden in dieser Arbeit adressiert, da die Implementierung einer Präparationsroutine für in situ Experimente und die Integration eines 4D-STEM Detektors Bestandteil der Arbeit sind. Die genannten Entwicklungen dienen in ihrer Anwendung als wichtige Grundlage für die Studie von funktionalen Dünnschichten, hier am Beispiel von Hafniumoxid basierten „valence change memory“ (VCM) RRAM. Gerade 4D-STEM kristallisiert sich als fortgeschrittene Methode zur Untersuchung der lokalen Struktur und der Reaktion auf externe Stimuli heraus. Als Konsequenz nimmt mit 4D-STEM die Menge an aufgezeichneten Daten erheblich zu, was die Entwicklung neuer Ansätze der Datenanalyse forderte. Auf dem Weg zu einem allumfassenden Experiment (in situ 4D-STEM), erstellt diese Arbeit die Grundlage im Bereich der notwendigen Methoden in hard- und software, sowie im Bereich des fundamentalen materialwissenschaftlichen Verständnisses von oxidbasierten Dünnschicht Elektronik.

Deutsch
Status: Verlagsversion
URN: urn:nbn:de:tuda-tuprints-216572
Zusätzliche Informationen:

also part of the ERC Proof of Concept (PoC) grant STARE "Machine learning based Software Toolkit for Automated identification in atomic-REsolution operando nanoscopy" 957521 https://cordis.europa.eu/project/id/957521

Sachgruppe der Dewey Dezimalklassifikatin (DDC): 500 Naturwissenschaften und Mathematik > 530 Physik
600 Technik, Medizin, angewandte Wissenschaften > 620 Ingenieurwissenschaften und Maschinenbau
Fachbereich(e)/-gebiet(e): 11 Fachbereich Material- und Geowissenschaften
11 Fachbereich Material- und Geowissenschaften > Materialwissenschaft
11 Fachbereich Material- und Geowissenschaften > Materialwissenschaft > Fachgebiet Elektronenmikroskopie
TU-Projekte: DFG|MO3010/3-1|In operando Unters..
EC/H2020|805359|FOXON
Hinterlegungsdatum: 25 Jul 2022 13:02
Letzte Änderung: 14 Dez 2022 18:54
PPN: 497916339
Referenten: Molina-Luna, Prof. Dr. Leopoldo ; Alff, Prof. Dr. Lambert
Datum der mündlichen Prüfung / Verteidigung / mdl. Prüfung: 4 Juli 2022
Export:
Suche nach Titel in: TUfind oder in Google
Frage zum Eintrag Frage zum Eintrag

Optionen (nur für Redakteure)
Redaktionelle Details anzeigen Redaktionelle Details anzeigen