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Oxygen Engineered Hafnium Oxide Thin Films grown by Reactive Molecular Beam Epitaxy

Hildebrandt, Erwin Matti (2012):
Oxygen Engineered Hafnium Oxide Thin Films grown by Reactive Molecular Beam Epitaxy.
Darmstadt, Germany, TU Darmstadt, [Online-Edition: http://tuprints.ulb.tu-darmstadt.de/3338],
[Ph.D. Thesis]

Abstract

This study applies RMBE to grow thin films of hafnium oxide, a widely studied material which has found its way into commercialisation as a replacement of SiO2 in a field effect transistor. After investigating different substrates and probing various deposition conditions, RMBE-grown films of hafnium oxide yielded to epitaxial films of hafnia on c-cut sapphire. Having the ability to grow high-quality thin films of hafnium oxide allows studying the influence of defined oxygen deficiency on its physical properties, as the next step of this work. The optical properties changed dramatically from colourless and transparent for stoichiometric HfO2 to dark black and opaque for highly deficient films of HfO2-x. The optical band gap could be tuned within more than one eV, visualising the introduction of defects (oxygen vacancies) in situ during growth. In fact, Hafnia showed a metal to insulator transition as a function of the oxygen content, conductive HfO2-x exhibited electrical p-type conductivity with resistivities of 300 µWcm, charge carrier concentrations of 6 times 10 to the power of 21 cm-3 at mobilities of 2 cm²/(Vs). The observed conductivity seems to be intrinsic to oxygen deficient hafnia and not due to a percolation of a conducting phase in an insulating matrix, as evident from various characterisations. A simple defect band structure model has been developed based on the observations, covering the formation of defect bands within the band gap being responsible for electrical conductivity and absorption of radiation within the visible range. With respect to reports on high-Tc ferromagnetism, no evidence for d0-ferromagnetism and room temperature ferromagnetism in Ni-doped HfO2-x could be found.

Item Type: Ph.D. Thesis
Erschienen: 2012
Creators: Hildebrandt, Erwin Matti
Title: Oxygen Engineered Hafnium Oxide Thin Films grown by Reactive Molecular Beam Epitaxy
Language: English
Abstract:

This study applies RMBE to grow thin films of hafnium oxide, a widely studied material which has found its way into commercialisation as a replacement of SiO2 in a field effect transistor. After investigating different substrates and probing various deposition conditions, RMBE-grown films of hafnium oxide yielded to epitaxial films of hafnia on c-cut sapphire. Having the ability to grow high-quality thin films of hafnium oxide allows studying the influence of defined oxygen deficiency on its physical properties, as the next step of this work. The optical properties changed dramatically from colourless and transparent for stoichiometric HfO2 to dark black and opaque for highly deficient films of HfO2-x. The optical band gap could be tuned within more than one eV, visualising the introduction of defects (oxygen vacancies) in situ during growth. In fact, Hafnia showed a metal to insulator transition as a function of the oxygen content, conductive HfO2-x exhibited electrical p-type conductivity with resistivities of 300 µWcm, charge carrier concentrations of 6 times 10 to the power of 21 cm-3 at mobilities of 2 cm²/(Vs). The observed conductivity seems to be intrinsic to oxygen deficient hafnia and not due to a percolation of a conducting phase in an insulating matrix, as evident from various characterisations. A simple defect band structure model has been developed based on the observations, covering the formation of defect bands within the band gap being responsible for electrical conductivity and absorption of radiation within the visible range. With respect to reports on high-Tc ferromagnetism, no evidence for d0-ferromagnetism and room temperature ferromagnetism in Ni-doped HfO2-x could be found.

Place of Publication: Darmstadt, Germany
Uncontrolled Keywords: oxygen engineering, molecular beam epitaxy, hafnium oxide, HfO2, resistive switching, high-k dielectric, oxygen vacancies
Divisions: 11 Department of Materials and Earth Sciences > Material Science > Advanced Thin Film Technology
11 Department of Materials and Earth Sciences > Material Science
11 Department of Materials and Earth Sciences
Date Deposited: 26 May 2013 19:55
Official URL: http://tuprints.ulb.tu-darmstadt.de/3338
URN: urn:nbn:de:tuda-tuprints-33384
License: Creative Commons: Attribution-No Derivative Works 3.0
Referees: Alff, Prof. Dr. Lambert and Schröder, Prof. Dr. Thomas and Donner, Prof. Dr. Wolfgang and Ensinger, Prof. Dr. Wolfgang
Refereed / Verteidigung / mdl. Prüfung: 28 February 2013
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Alternative keywordsLanguage
Sauerstoffdefizienz, Defekte, Hafniumoxid, Molekularstrahlepitaxie, SauerstoffleerstellenGerman
Alternative Abstract:
Alternative abstract Language
Diese Arbeit adaptiert die Dünnschichtwachstumsmethodik der reaktiven Molekularstrahlepitaxie für das Wachstum von Hafniumoxidschichten, ein high-k Dielektrikum welches inziwschen kommerziellen Einsatz in Feldeffekttransistoren anstatt SiO2 findet. Nach einer Wachstumsparameteroptimierung und Test unterschiedlicher Substrate konnten epitaktische Hafniumoxidschichten auf c-cut Saphir Substraten mit hoher Kristallqualität gewachsen werden. Nach der Wachstumsoptimierung wurde nun gezielt der Einfluss der Sauerstoffstöchiometrie auf die physikalischen Eigenschaften von Hafniumoxidschichten untersucht. Eine dramatische Änderung der optischen Eigenschaften wurde deutlich durch eine Farb- und Transparenzänderung von farblos, transparent für stöchiometrisches Hafniumoxid hin zu schwarz, opak für defizitäre Schichten. Die optische Bandlücke konnte in einem weiten Bereich von mehr als ein eV gezielt als Funktion der Sauerstoffdefizienz eingestellt werden und verdeutlicht die in situ Defektformation in den gewachsenen Hafniumoxidschichten als Funktion der Sauerstoffdefizienz. Darüber hinaus wurden ein Metall-Isolator Übergang in defizitärem Hafniumoxid als Funktion der Defizienz mit p-Leitung, Ladungsträgerkonzentrationen von 6 x 10 hoch 21 cm-3, spezifischen Widerständen von 300 µWcm und Mobilitäten von 2 cm²/(Vs) entdeckt. Anhand zahlreicher Messergebnisse erscheint die beobachtete p-Leitung intrinsicher Natur und wird nicht durch eine Metall-Isolator Perkolation hervorgerufen. Die gewonnenen Ergebnisse wurden in ein einfaches Defektbandmodell überführt. Schlussendlich konnten keine Beweise für die Existenz von hoch-Tc Ferromagnetismus bzw. d0-Ferromagnetismus in defizitären und Ni-dotierten Hafniumoxidschichten gefunden werden.German
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