Fast spin relaxation of Mn^{2+} ions in a magnetic quantum well of CdMnTe with 1% Mn fraction is related to a very efficient spin-flip interaction between Mn ions and free carriers. This mechanism of spin relaxation becomes dominant at increased excitation densities. The observed response of the photoluminescence bands to the Mn^{2+} magnetic resonance indicates that free carriers are heated at the magnetic resonance conditions. A decrease in formation/recombination rates of free and trion excitons is observed. Donor bound exciton photoluminescence is enhanced, which we relate to delocalization of free excitons, caused by interaction with microwave heated free carriers.
First experimental investigations on absorption and photoluminescence of the novel Hg_{3}TeCl_{4} monocrystals grown by the Bridgman method are reported. A comparison of the measurement results with theoretical band structure calculations of the Hg_{3}TeCl_{4} crystal confirmed that Hg_{3}TeCl_{4} is a wide-band-gap photoconductor (E_{g}= 3.64 eV at 24 K) with the effective masses of charge carriers characteristic for semiconductors. Energetic position of the main photoluminescence peak and its temperature dependence indicates the presence of an additional energy level in the energy gap which takes part in the radiative recombination process and whose origin was discussed.
We have studied a series of polar InGaN/GaN light emitting diodes, consisting of either a blue (440-450 nm) quantum well, or combination of blue and violet (410 nm) quantum wells (with indium content 18% and 10%, respectively). The blue quantum well was always placed close to p-type region of the particular LED. We found that the electroluminescence induced by low current is characterized by light emission from the blue quantum well only. In comparison, optical excitation of our LEDs leads to light emission with energies characteristic either for blue and/or violet quantum wells. The corresponding microphotoluminescence spectra evolve depending on external polarization and variable light intensity of excitation supplied by He-Cd laser. Interplay between built-in electric field and externally applied polarization/screening decides about the band structure profiles and thus radiative recombination mechanisms.
Origin of a fast component of the photoluminescence decay of Mn^{2+} intra-shell ^4T_1 → ^6A_1 transition is discussed based on the results of photoluminescence, photoluminescence kinetics and optically detected magnetic resonance experiments performed for bulk ZnMnS samples with about 1% Mn fraction. It is demonstrated that a fast component of the photoluminescence decay, reported previously for quantum dot structure and related to quantum confinement effects, is also observed in bulk samples and is related by us to very efficient spin cross-relaxation effects.
Bulk samples, layers, quantum well, and quantum dot structures of II-Mn-VI samples all show coexistence of slow and fast components of Mn^{2+} photoluminescence decay. Thus, fast photoluminescence decay cannot be related to low dimensionality of a host material. This also means that the model of the so-called quantum confined atom is incorrect. Based on the results of time-resolved photoluminescence and optically detected magnetic resonance investigations we relate the observed lifetime decrease in Mn^{2+} intra-shell transition to spin dependent magnetic interactions between localized spins of Mn^{2+} ions and between Mn^{2+} ions and spins/magnetic moments of free carriers. The latter mechanism is enhanced in nanostructures.
In this work we employ technique of optically detected cyclotron resonance for evaluation of the role of localization processes in CdTe/CdMnTe and CdMnTe/CdMgTe quantum well structures. From microwave-induced changes of excitonic emissions we evaluate magnitude of potential fluctuations (Stokes shift), correlate optically detected cyclotron resonance results with the results of time-resolved experiments and discuss nature of recombination processes in the limit of a strong localization.
The intention of this work is to discuss and report on our research on nonpolar laser structures grown on bulk GaN crystal substrates along the (11¯20) nonpolar direction. The main advantages of such nonpolar structures are related to the elimination of the built-in electric fields present in commonly used systems grown along the polar (0001) axis of nitride crystals. We demonstrated the optically pumped laser action on separate confinement heterostructures. Laser action is clearly shown by spontaneous emission saturation, abrupt line narrowing, and strong transversal electric polarization of output light. The lasing threshold was reached at an excitation power density of 260 kW/cm^2 for a 700μm long cavity (at room temperature).
Based on the results of optically detected magnetic resonance and time-resolved investigations we relate the observed lifetime shortening of intra-shell Mn^{2+} emission to spin dependent magnetic interactions between localized spins of Mn^{2+} ions and spins/magnetic moments of free carriers. We show that this mechanism is active in both bulk and in low dimensional structures, such as quantum wells, quantum dots, and nanostructures.
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