Thesis presented December 10, 2007

Abstract:
This work presents a magnetooptical study of diluted magnetic semiconductors (DMS) based on ZnO and GaN, doped with manganese, iron, and cobalt. Both host materials, ZnO and GaN, are wide band gap semiconductors with a wurtzite structure, a weak spin-orbit coupling and a strong electron-hole exchange interaction within the excitons. In the presence of a magnetic field, the magnetic ions induce in such materials a giant Zeeman effect with a complex behavior: excitons anti-cross, and not only the transition energies, but also the oscillator strengths are strongly affected by the giant Zeeman effect. On thin epitaxial layers grown on (0001) sapphire, we observed the giant Zeeman splitting of the A and B excitons, which are optically active in the Faraday configuration when the propagating light is parallel to the c-axis. The Zeeman splitting decreases with the temperature and increases non-linearly with the magnetic field, demonstrating a dependence on the magnetization of the localized spins. A quantitative analysis allows us to discuss the detailed behavior of the magnetization and to estimate the p-d exchange integral beta for the studied wide bandgap DMS. For the d⁵ electronic configuration (Mn²⁺, and Fe³⁺) the magnetization follows a Brillouin B_5/2 function, whereas, for d⁷ or d⁴ of Co²⁺, and Mn³⁺, respectively, the spin orbit coupling and the trigonal crystal field lead to an anisotropic magnetization, consistent with that deduced independently from the analysis of intra-ionic optical transitions. We find a positive sign of beta for Ga_1-x,Mn_xN, and Ga_1-x,Fe_xN. In ZnO, the sign of the spin-orbit interaction, which determines the structure of the valence band, has been a matter of debate. Assuming that the valence band ordering in ZnO is Δ₂ (Γ₉, Γ₇, Γ₇) (this corresponds to usual, positive sign of the spin-orbit coupling), we find ß to be negative for Zn_1-xCoxO, and to be close to zero in Zn_1-xMn_xO. However, assuming the reversed valence band ordering, we find beta to be positive in both ZnO based DMS. The sign and the value of p-d exchange integrals determined from our magnetooptical measurements do not follow material trends in DMS and cannot be explained by models based on the virtual crystal approximation. This suggests that the p-d coupling in wide gap DMS is in the strong coupling regime, so that the nature of the observed giant Zeeman splitting is different from that in classical diluted magnetic semiconductors.

Keywords:
Diluted magnetic semiconductor, wide band gap semiconductor, spintronics, magnetooptical spectroscopy, exchange interaction, giant Zeeman effect, iron

On-line thesis.