Molecular beam epitaxy and properties of magnetite thin films on semiconducting substrates [Elektronische Ressource] / vorgelegt von Markus Christian Paul

Sujets

• Physik

Informations

• Publié par: julius-maximilians-universitat_wurzburg
• Publié le: 01 janvier 2010
• Nombre de lectures: 24
• Langue: English
• Poids de l'ouvrage: 9 Mo

Extrait

Molecular beam epitaxy and properties of
magnetite thin lms on semiconducting
substrates
Dissertation zur Erlangung des
naturwissenschaftlichen Doktorgrades der
Julius-Maximilians-Universitat Wurzburg
vorgelegt von
Markus Christian Paul
aus Wei enhorn
Wurzburg 2010Eingereicht am: 03. November 2010
bei der Fakult at fur Physik und Astronomie
Gutachter der Dissertation:
1. Gutachter: Prof. Dr. R. Claessen
2. Gutachter: Prof. Dr. W. Ossau
3. Gutachter: Priv.-Doz. Dr. C. W. Schneider
Prufer im Promotionskolloquium:
1. Prufer : Prof. Dr. R. Claessen
2. Prufer : Prof. Dr. W. Ossau
3. Prufer : Prof. Dr. F. Assaad
Tag des Promotionskolloquiums: 25. M arz 2011Contents
1 Introduction 1
2 Physical properties of magnetite 5
2.1 Crystal structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2 Magnetism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.3 Electronic band structure . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.4 Verwey transition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3 Methods 15
3.1 Molecular beam epitaxy . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.1.1 Principle and beam uxes . . . . . . . . . . . . . . . . . . . . . . 15
3.1.2 Atomistic processes on surfaces and epitaxy of magnetite . . . . . 17
3.1.3 Growth modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.1.4 Thermodynamics of epitaxy . . . . . . . . . . . . . . . . . . . . . 19
3.1.5 Thermodynamic stability of magnetite in contact with semicon-
ductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.2 Photoelectron spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.2.1 Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.2.2 Description of spectral features . . . . . . . . . . . . . . . . . . . 26
3.2.3 Surface sensitivity of electrons . . . . . . . . . . . . . . . . . . . . 28
3.2.4 Electron spectroscopy for chemical analysis . . . . . . . . . . . . . 30
3.2.5 Charge-transfer satellites and stoichiometry in the Fe 2p spectrum 31
3.2.6 Valence-band and oxygen spectra of di erent iron oxides . . . . . 34
3.3 Magnetic circular dichroism in x-ray absorption spectroscopy . . . . . . . 35
3.3.1 Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.3.2 Electron yield, saturation e ects, and probing depth . . . . . . . . 40
3.3.3 X-ray magnetic circular dichroism of Fe L-edge . . . . . . . . . . 42
4 Magnetite thin lms on ZnO 45
4.1 Substrate preparation and growth procedure . . . . . . . . . . . . . . . . 46
4.2 Structural characterization . . . . . . . . . . . . . . . . . . . . . . . . . . 46
4.2.1 Surface structure . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
4.2.2 morphology . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.2.3 Characterization by x-ray re ectivity and di raction . . . . . . . 53
iContents
4.3 Microstructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
4.4 Electronic and chemical structure studied by (hard) x-ray photoelectron
spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
4.5 Magnetic properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
4.5.1 X-ray magnetic circular dichroism . . . . . . . . . . . . . . . . . . 68
4.5.2 Magnetometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
4.5.3 Origin of the reduced surface magnetization . . . . . . . . . . . . 74
5 Fe O thin lms on InAs 793 4
5.1 Substrates and growth procedure . . . . . . . . . . . . . . . . . . . . . . 79
5.2 Microstructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
5.3 Electronic and chemical structure studied by (hard) x-ray photoelectron
spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
5.4 Magnetic properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
5.5 Discussion and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
6 Fe and Fe oxide thin lms on GaAs 89
6.1 Surface structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
6.1.1 Clean GaAs surface . . . . . . . . . . . . . . . . . . . . . . . . . . 89
6.1.2 Fe on GaAs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
6.1.3 Fe O /GaAs lm . . . . . . . . . . . . . . . . . . . . . . . . . . . 923 4
6.2 Microstructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
6.3 Investigation by photoelectron spectroscopy . . . . . . . . . . . . . . . . 98
6.3.1 Surface composition studied by extreme ultraviolet photoelectron
spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
6.3.2 Spin polarization studied by extreme ultraviolet photoelectron spec-
troscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
6.3.3 Electronic and chemical structure studied by (hard) x-ray photo-
electron spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . 104
6.3.4 Discussion of optimal growth parameters and literature review . . 108
6.4 Magnetic properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
6.4.1 X-ray magnetic circular dichroism . . . . . . . . . . . . . . . . . . 110
6.4.2 Magnetometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
7 Summary and outlook 117
Bibliography 121
iiList of gures
1.1 Schematic DOS of a half-metallic ferromagnet . . . . . . . . . . . . . . . 3
2.1 Crystallographic unit cell of magnetite . . . . . . . . . . . . . . . . . . . 5
2.2 Electronic levels and magnetic moments in the ligand- eld model . . . . 7
2.3 DOS of magnetite from LSDA . . . . . . . . . . . . . . . . . . . . . . . . 10
2.4 Orbital ordering in magnetite . . . . . . . . . . . . . . . . . . . . . . . . 12
2.5 Magnetization anomaly at the Verwey transition . . . . . . . . . . . . . . 13
3.1 Sketch of an MBE setup . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.2 Atomistic processes on surfaces . . . . . . . . . . . . . . . . . . . . . . . 18
3.3 Growth modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.4 Interfaces in solids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3.5 Basic principle of PES . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.6 Connection between PES spectrum and DOS . . . . . . . . . . . . . . . . 27
3.7 IMFP of electrons in various materials . . . . . . . . . . . . . . . . . . . 29
3.8 Fe 2p spectra of reference compounds . . . . . . . . . . . . . . . . . . . . 33
3.9 VB and O 1s spectra of iron oxides . . . . . . . . . . . . . . . . . . . . . 35
3.10 X-ray absorption coe cient in magnetite . . . . . . . . . . . . . . . . . . 36
3.11 Two-step model of the XMCD e ect at the L-edge of a transition metal . 37
3.12 Probabilities of 2p!3d excitations by right-handed circularly polarized
light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.13 Data handling of XAS Fe L-edge spectra . . . . . . . . . . . . . . . . . . 40
3.14 XAS and XMCD spectra for sum-rule analysis . . . . . . . . . . . . . . . 41
3.15 XMCD and XAS Fe L-edge spectra of Fe O . . . . . . . . . . . . . . . 423 4
4.1 LEED pattern of an Fe O /ZnO lm . . . . . . . . . . . . . . . . . . . . 473 4
4.2 Integrated RHEED-image pro le of an Fe O growth on ZnO . . . . . . . 483 4
4.3 Relaxation behavior of an Fe O /ZnO lm as probed by RHEED . . . . 493 4
4.4 RHEED patterns of an Fe O /ZnO lm at di erent stages of growth . . . 503 4
4.5 AFM image of an Fe O /ZnO lm surface . . . . . . . . . . . . . . . . . 513 4
4.6 SEM and AFM image of an Fe O /ZnO lm surface . . . . . . . . . . . . 523 4
4.7 Sketch of rotational twinning in an fcc-based structure . . . . . . . . . . 52
4.8 XRR scans of an Fe O /ZnO lm . . . . . . . . . . . . . . . . . . . . . . 543 4
4.9 XRD scans of an Fe O /ZnO lm . . . . . . . . . . . . . . . . . . . . . . 553 4
4.10 XRD rocking curves of an Fe O /ZnO lm . . . . . . . . . . . . . . . . . 573 4
iiiList of gures
4.11 TEM image and electron di raction pattern of an Fe O /ZnO lm . . . . 593 4
4.12 STEM image and of two Fe O domains on3 4
ZnO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
4.13 STEM image of an Fe O /ZnO interface . . . . . . . . . . . . . . . . . . 603 4
4.14 of an APB and a TB in Fe O . . . . . . . . . . . . . . . . 613 4
4.15 STEM image of an APB in Fe O . . . . . . . . . . . . . . . . . . . . . . 633 4
4.16 Sketch of the atomic arrangement at an APB . . . . . . . . . . . . . . . 63
4.17 HAXPES Fe 2p and O 1s spectra of an Fe O /ZnO lm . . . . . . . . . 653 4
4.18 Zn 2p and XPS Auger emission Zn L M M spectra of an3=2 3 45 45
Fe O /ZnO lm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 663 4
4.19 Stoichiometry of Fe O /ZnO lms determined from XPS Fe 2 p spectra . 693 4
4.20 XAS and XMCD spectra of an Fe O /ZnO lm . . . . . . . . . . . . . . 693 4
4.21 XMCD magnetization curve of an Fe O /ZnO lm . . . . . . . . . . . . 713 4
4.22 SQUID characterization of an Fe O /ZnO lm . . . . . . . . . . . . . . . 733 4
5.1 STEM image and EDX pro les of an Fe O /InAs lm . . . . . . . . . . . 803 4
5.2 HAXPES Fe 2p and O 1s spectra of an Fe O /InAs lm . . . . . . . . . 823 4
5.3 As 2p and In 3d spectra of an Fe O