The valence state and annealing reactions of Mn/Fe in single crystalline α-Al2O3 have been determined following low fluence (<1012 cm − 2) 60 keV implantations of 57Mn + (T1/2 = 1.5 min) and emission Mossbauer spectroscopy on the 57mFe daughter nuclei in the temperature range from 110–700 K. At 110 K, most probe atoms are found in the Fe2 + state in amorphous surroundings due to the implantation damage. A fraction of the Fe is found in cubic environment, possibly nano-precipitates of η-Al2O3. This site is found to disappear from the spectra above 500 K. Annealing of the damage sites at increasing temperatures leads first to increased incorporation of the probe atoms as Fe3 + on Al sites, and, above room temperature, also as Fe4 + . The Fe3 + sub-spectrum is masked by slow paramagnetic relaxations following a T2 dependence, as expected for a two-phonon Raman process. Our data is consistent with data from 57Co and 57Fe implantations, suggesting a general increase in the average Fe valence state with lower implantation dose and negligible annealing reactions at room temperature.

Mössbauer spectroscopy of 57Fe in α-Al2O3 following implantation of 57Mn*

vii Útdráttur viii Publications in this thesis ix Acknowledgments xiii 1 Scientific background 1 2 Magnetism and semiconductors 5 2.1 Paramagnetism 5 2.2 Magnetic order 6 2.3 Dilute magnetism in semiconductors 7 3 Mössbauer spectroscopy 13 3.1 Recoil-free fraction of nuclear events (Mössbauer effect) 14 3.2 Mössbauer measurements 15 3.3 Diffusional broadening 17 3.4 Isomer shift, δ 18 3.4.1 Second order Doppler shift (SOD) 21 3.5 Quadrupole splitting, ΔEQ. 22 3.6 Magnetic hyperfine interaction, Bhf 27 3.6.1 Combined magnetic and quadrupole interactions 29 3.7 Relaxation effects on the Mössbauer spectrum 29 3.7.1 Spin-spin relaxation 32 3.7.2 Spin-lattice relaxation 33 3.7.3 Mössbauer spectrum of slow relaxing Fe 35 3.8 Angular dependence of relative line intensities 37 3.9 Spectral analysis 39 4 Ion-implantation 41 4.1 Mössbauer isotopes (ISOLDE/CERN) 42 4.1.1 Mn 43 4.1.2 Co 44 4.2 Mn fluence calculation 45 4.3 Point defects 47 4.3.1 Implantation damage 47 4.3.2 Substitutional 48 4.3.3 Interstitial 49 4.4 Application to emission Mössbauer spectroscopy 49

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Paramagnetism in ion-implanted oxides

The effects of 57Fe-implanted Al2O3 and post-implantation thermal annealing on surface properties and structure of an implanted layer have been studied by electric measurement. X-ray photoelectron spectroscopy (XPS) and Mossbauer spectroscopy. It has been found that the resistivity of the implanted layer of polycrystalline alumina is greatly reduced and that relatively high doses and suitable annealing temperatures must be used to achieve modification of the electrical properties. XPS and Mossbauer spectroscopy measurements have shown that the implanted iron ions are in three charge states of Fe4+, Fe2+ and Fe0 for an as-implanted sample. After thermal annealing, the iron is present as Fe3+, Fe2+ and Fe0. Finally, the conductivity mechanisms are discussed briefly. The electrical conductivity for Fe-implanted polycrystalline alumina may be caused not only by mobile interstitials of aluminum and electronic defects but also by the implanted metallic elements themselves.

Shipu Li

Papers

Structure and surface properties of polycrystalline 57Fe-implanted aluminas