| Summary: | 博士 === 國立交通大學 === 電子物理系所 === 99 === Type-II ZnSeTe/ZnSe multiple quantum wells (MQWs), ZnTe/ZnSe quantum dots (QDs), ZnMnTe/ZnSe QDs, and ZnTe/ZnMnSe QDs were grown by molecular beam epitaxy (MBE). The photoluminescence (PL), temperature dependent PL, time-resolved PL and magneto-PL were used to investigate the interesting physical properties.
The tunability of the emission energy, oscillator strength and PL efficiency by varying the well thickness and excitation density was demonstrated in the ZnSe0.8Te0.2/ZnSe multiple quantum wells. A significant blueshift about 260 meV of the PL peak energy was observed as the well width decreased from 5 to 1 nm. An extraordinary long lifetime (300 ns) of the recombination for the widest sample was detected. The binding energy of the indirect excitons is determined as 12 meV for the thinnest sample. The reduction of PL efficiency by thermal energy is greatly suppressed by employing a high excitation power.
The optical properties of type-II ZnTe QDs were also investigated by temperature-dependent and time-resolved PL spectroscopy. The initial decrease then increase with temperature for the full width at half maximum (FWHM) of PL is attributed to the hole thermal escape from the smaller QDs then transfer and re-capture to the neighboring-larger QDs. The non-mono-exponential decay profiles reflect the processes of carrier transfer and recapture. We show that the Kohlrausch’s stretching exponential well fits the decay profile of ZnTe/ZnSe QDs.
ZnMnTe/ZnSe QD structures have distinguished difference in magneto-optical properties from ZnTe/ZnSe QD structures due to the existence of Mn in the QDs. Different growth modes of ZnMnTe/ZnSe QDs were studied by using MBE. The ZnMnTe QD system grown by opened the Mn for a few seconds and then Zn+Te growth mode exhibit better emission efficiency. The σ+ and σ– circularly polarized time-integrated and time-resolved PL were employed to investigate the carrier spin dynamics of ZnMnTe/ZnSe QDs grown on GaAs substrates by molecular beam epitaxy. The Kohlrausch’s stretching exponential function well correlates both the σ+ and σ– decay profiles. The measured spin relaxation time is about 23 ns. In addition, we present a magneto-optical study of magnetic polarons (MP) in ZnMnTe/ZnSe QDs. The polarons are found due to the exchange coupling between the spins of the holes and those of the Mn ions, both of which are localized in the QDs. In this system, the MP was detected at temperature up to 100 K and the formation energy is roughly independent of temperature.
Finally, the ZnTe QDs were grown in ZnMnSe matrix. The magnetic field dependence of PL circular polarization degree follows the Brillouin function and evidences the Mn magnetism in ZnTe/ZnMnSe QDs. The magnetic field dependence of PL circular polarization degree shows the long spin relaxation time of about 35 ns. The magnetic polaron formation was observed in low temperature. The ZnTe/ZnMnSe QDs have better PL emission efficient than ZnMnTe/ZnSe QDs.
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