MgO/Co/GaAs(001) - an interface analysis by means of XPS and XPD
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Date
2018
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Abstract
Diese Arbeit befasst sich mit den Oberflächen- und Grenzflächeneigenschaften
des Mehrschichtsystems MgO/Co/GaAs(001) im Bezug auf Struktur und
chemische Bindungen. Im gesamten Forschungsfeld der Spintronik findet
sich dieses Schichtsystem wieder, wobei der magnetische Tunnelwiderstand
(TMR) als fundamentales Phänomen genutzt wird. Da dieser Effekt sehr
empfindlich auf die Grenzfläche von Heterostrukturen reagiert, sind die Photoelektronenspektroskopie
(XPS) und die Photoelektronenbeugung (XPD)
ideale Untersuchungsmethoden, um die Grenzflächenverbindungen und die
lokale strukturelle Umgebung dieses Schichtsystems zu untersuchen. Photonen
führen hierbei zu einer Emission von Kernniveau-Elektronen aufgrund
des Photoeffekts. Sie liefern elementspezifische Informationen über die benachbarten
Verbindungen innerhalb der Probe. Durch Variation des Detektionswinkels,
d.h. des Polar- und Azimutwinkels, werden Intensitätsmodulationen
innerhalb des XPS-Spektrums aufgrund des Wellencharakters der
Elektronen beobachtet. Das Interferenzmuster enthält präzise Strukturinformationen
mit chemischer Auflösung. Diese Messungen werden an der
Synchrotronstrahlungsquelle DELTA der Technischen Universität Dortmund
durchgeführt. Eine Synchrotronquelle liefert nicht nur deutlich höhere Photonenintensitäten,
sondern auch eine durchstimmbare Energie. So werden
zeitaufwändige Experimente wie XPD innerhalb weniger Stunden durchgeführt
und durch Abstimmung der kinetischen Energie der Elektronen wird
eine grenzflächensensitiveMessung durchgeführt. Darüber hinaus ermöglicht
die hohe Auflösung eine genauere Bestimmung der chemischen Verbindungen
innerhalb des Mehrschichtsystems. Als Hauptergebnis dieser Arbeit
wird die Ga-reiche GaAs(001) c(8×2) Rekonstruktion zugunsten einer Co3Ga-
Legierung an der Co/GaAs-Grenzfläche aufgehoben. Diese kristalline Legierung
bildet eine seltene D03-Struktur zwischen einem As-terminierten GaAs Substrat
und dem darauf befindlichen Co(bcc)-Film. Leichte Relaxationen sind
hier in jeder Schicht zu beobachten. Da die Gitterkonstante von GaAs perfekt
mit der von Co3Ga übereinstimmt, wird der Schluss gezogen, dass dieses
Wachstum substratbedingt ist. As hingegen diffundiert in den Co-Film und
bildet amorphe Bindungen. MgO wird anschließend auf die gut geordneten
Co-Schichten aufgebracht, wobei das Mg:O-Verhältnis von 1:1 erhalten
bleibt. Für MgO-Schichtdicken unter 4 Monolagen bildet MgO keine
kristalline Struktur. Bei Schicktdicken von oberhalb 4 Monolage wächst
MgO in einer stark verzerrten Halitstruktur auf, aufgrund einer Gitterfehlanpassung
zu Co(bcc). Daher wird die minimale Schichtdicke für eine
kristallines MgO-Wachstums auf Co(bcc) bestimmt. An der Grenzfläche ist
keine Verbindungsbildung oder Co-Oxidation zu beobachten.
This work focuses in the surface and interface properties of the multi-layer system MgO/Co/GaAs(001) in terms of structure and chemical bondings. This system is found throughout the research field of spintronics wherein the tunneling magnetoresistance (TMR) effect is considered to make use of the electrons’ spins. Since this effect is highly sensitive to the interface of heterostructures, x-ray photoelectron spectroscopy (XPS) and x-ray photoelectron diffraction (XPD) are ideal tools to investigate the sample’s interface bondings and its local structural environment. Photons lead to an emission of electrons originating from the core-levels of the elements. They yield element-specific information on the near neighboring bondings within the sample. By varying the recording angle, i.e. the polar and azimuth angle, intensity modulations within the XPS spectrum are observed due to the wave character of the electrons. The interference pattern contains precise structural information with chemical resolution. These measurements are performed at the synchrotron radiation source DELTA at the Technical University Dortmund. A synchrotron source yields not only significantly higher photon intensities but also provides a tunable energy. Hence, time-consuming experiments like XPD are performed within several hours only and by tuning the kinetic energy of the electrons an interface sensitive measurement is carried out. Further, the high resolution allows a more precise determination of the chemical bondings within the multi-layer system. As a main result of this work, the Ga-rich GaAs(001) c(8×2) surface reconstruction is destroyed in favor of a Co3Ga alloy at the Co/GaAs interface. This crystalline alloy forms a rare D03 structure between an As-terminated GaAs(001) substrate and the Co(bcc) film on top of it. It shows slight relaxations in every layer. Since the GaAs’ lattice constant matches perfectly to the one of Co3Ga, it is concluded that this growth is substrate induced. As diffuses into the Co film forming amorphous bondings. MgO is then deposited on the well ordered Co layers preserving the Mg:O ratio by 1:1. For MgO thicknesses below than 4 monolayers MgO forms no crystalline structure. For a thicknesses above 4 monolayers MgO grows in a highly distorted halite structure due to a lattice mismatch to Co(bcc). Therefore, the minimum thickness of crystalline MgO growth on Co(bcc) is determined. At the interface no compound formation or Co oxidation is observed.
This work focuses in the surface and interface properties of the multi-layer system MgO/Co/GaAs(001) in terms of structure and chemical bondings. This system is found throughout the research field of spintronics wherein the tunneling magnetoresistance (TMR) effect is considered to make use of the electrons’ spins. Since this effect is highly sensitive to the interface of heterostructures, x-ray photoelectron spectroscopy (XPS) and x-ray photoelectron diffraction (XPD) are ideal tools to investigate the sample’s interface bondings and its local structural environment. Photons lead to an emission of electrons originating from the core-levels of the elements. They yield element-specific information on the near neighboring bondings within the sample. By varying the recording angle, i.e. the polar and azimuth angle, intensity modulations within the XPS spectrum are observed due to the wave character of the electrons. The interference pattern contains precise structural information with chemical resolution. These measurements are performed at the synchrotron radiation source DELTA at the Technical University Dortmund. A synchrotron source yields not only significantly higher photon intensities but also provides a tunable energy. Hence, time-consuming experiments like XPD are performed within several hours only and by tuning the kinetic energy of the electrons an interface sensitive measurement is carried out. Further, the high resolution allows a more precise determination of the chemical bondings within the multi-layer system. As a main result of this work, the Ga-rich GaAs(001) c(8×2) surface reconstruction is destroyed in favor of a Co3Ga alloy at the Co/GaAs interface. This crystalline alloy forms a rare D03 structure between an As-terminated GaAs(001) substrate and the Co(bcc) film on top of it. It shows slight relaxations in every layer. Since the GaAs’ lattice constant matches perfectly to the one of Co3Ga, it is concluded that this growth is substrate induced. As diffuses into the Co film forming amorphous bondings. MgO is then deposited on the well ordered Co layers preserving the Mg:O ratio by 1:1. For MgO thicknesses below than 4 monolayers MgO forms no crystalline structure. For a thicknesses above 4 monolayers MgO grows in a highly distorted halite structure due to a lattice mismatch to Co(bcc). Therefore, the minimum thickness of crystalline MgO growth on Co(bcc) is determined. At the interface no compound formation or Co oxidation is observed.
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Keywords
Interface, Insulator, Ferromagnet, Semicondcutor