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Abstract

Thin oxide films (100nm) grown on silicon (100) substrates were characterized by Perturbed Angular Correlation (PAC). Three samples were analyzed: (a) prepared by electron beam evaporation; (b) prepared by atomic-layer chemical vapor deposition method, with H2O and HfCl4 as precursors; (c) prepared by metal organic chemical vapor deposition with tetrakis-diethyl-aminohafnium and O2 as precursors. The radioactive 181Ta isotopes used as PAC probes appeared in the samples after the β-decay of 181Hf. These isotopes were introduced, in the cases of samples prepared by chemical vapor deposition, by ion implantation. In the case of samples prepared by electron beam evaporation, the PAC isotopes 181Hf were produced by neutron activation of the film by the reaction 180Hf(n,γ)181Hf. PAC measurements were carried out at room temperature after annealing at different temperatures from 773K up to 1273K in air. The PAC technique allows to determine the electric field gradient at the probe sites. In this way, the crystallization of the hafnium oxide films was monitored, characterizing at a nanoscopic level the atomic surrounding of the probes.

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... Die Gitterstruktur muss dabei zwar vorgegeben werden, die genaue Lage der Atome wird dann aber wieder iterativ berechnet, bis nur noch verschwindende Kräfte auf die Atome wirken. Filmen [Quil10]. Zu letzterem gehören die in [Stef07] durchgeführten und bisher unveröffentlichten Vorarbeiten an neutronenaktivierten 100 nm-Schichten, welche identisch zu den in diesem Kapitel nach Ionenimplantation gemessenen Schichten ( Kap. 6.4) sind. ...
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Angefangen bei Kupferoxid (CuO) als einem der ersten, in größerem Maßstab verwendeten Halbleitern überhaupt, bis hin zu den Arbeiten der letzten Jahren an Elektroden aus transparenten, elektrisch leitfähigen Oxiden (TCO), spielten Oxidhalbleiter oder -isolatoren in den vergangenen Jahrzehnten eine durchgängig wichtige Rolle. Da die meisten Elemente des Periodensystems mindestens ein stabiles Oxid ausbilden, resultiert dies in einer Vielzahl von Materialien unterschiedlichster Eigenschaften und auch Phasen und damit auch einer Fülle möglicher Anwendungsgebiete. Am Beispiel der drei Oxidverbindungen des Hafniums, Galliums und Aluminiums wurde die Methode der gestörten γ-γ-Winkelkorrelation (PAC) unter anderem in Abhängigkeit der Probentemperatur angewendet. Die kernphysikalische Messmethode der PAC fasst die Hyperfeinwechselwirkung eines radioaktiven Sondenkerns mit dem Gradienten des elektrischen Feldes (EFG) am Sondenplatz im Wirtsgitter in messbare Größen. Da diese Wechselwirkung kurzreichweitig ist, ist die PAC sensitiv auf die nächste Umgebung der Sondenkerne. Hieraus erhält man Informationen über die Gitter- und Defektstruktur des Wirtsgitters. Eingesetzt wurden dabei die PAC-Sondenkerne 111Cd und 181Ta, welche durch Ionenimplantation in die Proben eingebracht oder mittels Neutronenaktivierung in den Proben produziert wurden. Den Experimenten mit den einzelnen Materialien übergeordnet ist die genauere Betrachtung dynamischer Hyperfeinwechselwirkungen, insbesondere jener, welche in Folge des Zerfalls des Mutterisotops181In zum Sondenkern 111Cd via Elektroneneinfang (EC) auftreten. In den Spinkorrelationsfunktionen R(t) äußern sich diese als ungewöhnlich starke Dämpfung der Spektren durch eine zusätzliche Wechselwirkung. Die dynamischen Wechselwirkungen an111Cd sind jedoch nicht durch die Eigenschaften des Wirtsmaterials gegeben, sondern werden durch die Sonde selbst erzeugt und es kann gezeigt werden, dass diese bei Verwendung anderer Sonden wie181Ta oder dem EC-losen Zerfall des 111mCd nicht auftreten. In der Literatur findet sich dafür der Name „electron-capture after-effect” (Elektroneneinfangsnachwirkung, ECAE). Beim HfO2 wird dabei besonders die Hyperfeinwechselwirkung dünner Schichten mit Dicken von 2,7 nm bis 17nm und 100 nm untersucht. Es zeigen sich stark uneinheitliche Feldgradienten und ein großer Einfluss der Probenoberfläche auf die Messung. Es kann gezeigt werden, dass die Wechselwirkungsfrequenz der Hyperfeinwechselwirkung νq mit der Schichtdicke des Oxides skalieren sollte und dass das temperaturabhängige Verhalten, welches von der thermischen Expansion des Gitters geprägt ist, auch dieser Skalierung unterliegt. Bedingt durch die Nähe zur Oberfläche kann bei hohen Temperaturen Sauerstoff aus den Proben entweichen und so das Oxid degradieren. Die untersuchten Ga2O3-Schichten werden durch Oxidation von GaN bei 1223 K an Raumluft produziert. Der Aufbau der Oxidschicht wurde dabei schrittweise mit der PAC verfolgt. Die Oxidation wird mit mehreren Proben bei gleicher absoluter Oxidationszeit aber unterschiedlicher Aufteilung in Zwischenschritte wiederholt. Insgesamt zeigten sich die Ergebnisse reproduzierbar, die auftretenden Unterschiede der Hyperfeinwechselwirkungen sind wahrscheinlich von externen, bei der Oxidation schwankenden Größen, gegeben. Bei der Vermessung derAl2O3-Proben in PAC-Ofen und Kryostat steht der Versuch im Vordergrund, das temperaturabhängige Verhalten der Hyperfeinwechselwirkung durch zusätzliche Dotierung zu manipulieren. Die Messungen mit dem Sondenkern111Cd zeigen bei den untersuchten Materialien das gleiche tendenzielle Verhalten: So liegen bei Ga2O3 und Al2O3 zwei spezifische Übergangstemperaturen vor, eine im Tieftemperaturbereich von 50 K bis 150 K und eine weitere bei hohen Temperaturen im Bereich von 500 K bis 850 K. Mit HfO2 werden nur Messungen oberhalb Raumtemperatur durchgeführt, so dass sich hier nur der Übergang bei hohen Temperaturen zeigt. Unter- bzw. oberhalb dieser Übergangstemperaturen liegen die Sonden vermehrt in ungedämpfter, statischer Umgebung vor. Im Zwischenbereich, welcher die Raumtemperatur bei den untersuchten Materialien immer einschließt, liegen die Sonden hingegen vorwiegend in einer stark gedämpften, als dynamisch angesehenen Wechselwirkung vor. Der bei Raumtemperatur hohe Sondenanteil in dynamischer Wechselwirkung bei Messungen an der Sonde 111Cd wird üblicherweise als Folge des Elektroneneinfangs (ECAE) des Mutterkerns 111In angesehen. Der Rückgang dieses Sondenanteils bei hohen Temperaturen wird in der Literatur als Rekombination von Löchern nahe oder an der Sonde mit Elektronen aufgefasst; das Hochtemperaturverhalten würde damit abhängig vom Elektronentransport. Entsprechend geht das in dieser Arbeit auf die Messungen angewandte analytische Modell zum ECAE von einem thermisch aktivierten Übergang von der dynamischen Defektwechselwirkung zur Statischen aus. Es kann gezeigt werden, dass die hieraus berechnete Übergangsrate Γr an Ga2O3 und Al2O3 nicht durch makroskopische Größen, wie z.B. der Leitfähigkeit des Materials, erklärt werden kann, sondern eher durch Sprungprozesse zwischen der Sonden und deren nächsten Nachbarn. Der Anstieg bei hohen Temperaturen wäre daher durch ein Auffüllen der Löcher mit Elektronen aus flachen Donatorzuständen (Ga2O3) oder mit Elektronen aus dem Valenzband in von der Sonde selbst ausgebildete Akzeptorzustände(Al2O3) erklärbar. Der Übergang zu kleinen Temperaturen ist nicht vollends verstanden und wurde bisher mit phononenunterstütztem Tunneln von Elektronen zur Sonde erklärt. Im Vergleich mit theoretischen Berechnungen von Varley et al. (Phys. Rev. B 85 (2012), 081109) zeigt sich aber eine auffällige Übereinstimmung von Materialien, welche kleine Polaronen in Form von gebundenen Löchern („self-trapped holes”, STH) ausbilden, zu jenen an welchen der ECAE mit der PAC beobachtet wurde. Das Tieftemperaturverhalten könnte dann mit der Annahme erklärt werden, dass nach der thermischen Aufbringung einer Aktivierungsenergie die beim EC-Zerfall des 111In generierten Löcher an die Sonde gebunden werden. Die eigentlich dazu notwendige lokale Gitterverzerrung müsste dabei nicht von den Löchern selber generiert werden, sondern wäre bereits durch die Anwesenheit der Sondenkerne gegeben. Dementsprechend wird ein geringerer Wert der Aktivierungsenergie bestimmt, als von Varley et al. vorhergesagt wird. Diese Bindung der Löcher an die Sonde wird in dieser Arbeit als Ursache der dynamischen Wechselwirkung angesehen. Durch Dotierung von Ga2O3 und Al2O3 soll dieses temperaturabhängige Verhalten beeinflusst werden.
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