Magnetic Polarons in (Zn,Mn)Se/ZnTe and ZnSe/(Zn,Mn)Te quantum dots.

摘要

Quantum dots are highly tunable zero dimensional nanostructures as they give researchers the freedom of artificially controlling their size for a wide variety of applications from opto-electronics to solar cells. In dilute magnetic semiconductor (DMS) quantum dots (QD) the incorporation of magnetic ions leads to exotic magnetic effects. In this thesis, exchange interaction between magnetic ions and charge carriers in both MBE grown and colloidal QDs are explored.;We used continuous wave photoluminescence (cw-PL) and time resolved photoluminescence (TR-PL) spectroscopy to compare the properties of magnetic polarons (MP) in two related spatially indirect II-VI epitaxially grown quantum dot systems. In the ZnTe/(Zn,Mn)Se system the holes are confined in the non-magnetic ZnTe quantum dots (QDs), and the electrons reside in the magnetic (Zn,Mn)Se matrix. On the other hand, in the (Zn,Mn)Te/ZnSe system, the holes are confined in the magnetic (Zn,Mn)Te QDs, while the electrons remain in the surrounding non-magnetic ZnSe matrix. The magnetic polaron formation energies EMP in both systems were measured from the temporal red-shift of the band-edge emission. The magnetic polaron exhibits distinct characteristics depending on the location of the Mn ions.;In the ZnTe/(Zn,Mn)Se system the magnetic polaron shows conventional behavior with decreasing with increasing temperature T and increasing magnetic field B. In contrast, EMP in the (Zn,Mn)Te/ZnSe system has unconventional dependence on temperature T and magnetic field B; EMP is weakly dependent on T as well as on B. We discuss a possible origin for such a striking difference in the MP properties in two closely related QD systems. In the (Zn,Mn)Te/ZnSe QD system, we investigate time evolution of the peak energy of the and circularly-polarized photoluminescence components, and of the circular polarization of the emitted light. We find that this system exhibits unexpected characteristics, such as different time scales for the evolution of the magnetic polaron on one hand and the photoluminescence circular polarization on the other. We discuss these results within the framework of a theoretical model developed to describe the properties of magnetic polarons.;For the colloidal system, carrier spin polarization studies in narrow band gap Mn-doped PbS quantum dots, have been reported. The PbMnS quantum dots were synthesized by hot colloidal solution technique. They were single crystalline with cubic structure. The doping concentration was 3-4% as measured by energy dispersive X-ray spectroscopy. The system was paramagnetic down to 2 K, as measured by VSM. The PL spectra was recorded in the Faraday geometry for magnetic fields of up to 7 tesla in the 0-50 K temperature range. The PL was excited at 1590 meV and the emission was centered at 940 meV with a full width at half maximum of 100 meV. In the presence of magnetic field the emission became strongly sigma+ polarized (P = 35% at 4 tesla), suggesting carrier spin polarization. The polarization was temperature sensitive and decreased sharply with increasing temperature, eventually vanishing at around T = 40 K.