Negative magnetopolarization in thermally annealed self-assembled quantum dots

Abstract

A negative magnetic dispersion of the degree of circular polarization is reported for the photoluminescence of self-assembled ZnCdSe quantum dots ͑QDs͒ after a postgrowth thermal annealing ͑TA͒ treatment. Multiband calculations in the framework of the k · p model reveal that the phenomenon is explicable considering, apart from the changes in the confinement and composition of the QDs due to TA-induced Zn-Cd interdiffusion, the presence of extra charges of one kind within the structure. Magneto-optical properties of self-assembled semiconductor quantum dots ͑QDs͒ have emerged in the recent years as a widely pursued field of research, 1,2 triggered by the possibility of using the spin degree of freedom of a threedimensionally confined exciton to realize the basic building blocks of quantum information processing. 3,4 The magnetooptical response of an individual QD and an ensemble of QDs yields much fundamental information, imperative to achieving this goal, i.e., the spin splitting of the exciton ground state 5-10 and the spin lifetime of the electron and the hole comprising the exciton. 11,12 The polarization behavior of photoluminescence ͑PL͒, emitting from QDs in the presence of a magnetic field, serves as an excellent tool to probe the exciton spin dynamics in QDs.In this work, the evolution of the degree of circular polarization ͑DCP͒ has been studied for the magneto-PL of epitaxially self-assembled ZnCdSe QDs before and after a postgrowth rapid thermal annealing ͑TA͒ treatment. While for the as-grown ͑not-annealed͒ QDs the DCP undergoes a reversal from positive to negative sign beyond a magnetic field strength of B = 3 T, we demonstrate here that the DCP corresponding to the annealed QDs exhibits a negative magnetic dispersion in the entire range of B =0-5 T. A multiband calculation in the framework of the k · p theory reveals that apart from changes in the composition and configuration of the QDs due to TA-induced interdiffusion of Zn and Cd, the presence of extra charges of one kind needs to be taken into account. Albeit spin splitting and DCP of magneto-PL have been studied for quantum wells of a few systems, 10,13 such studies for three-dimensionally confined excitons, especially their evolution with changing composition and confinement of QDs, to the best of our knowledge, has not been pursued so far.The samples under investigation were grown by molecular beam epitaxy on top of homoepitaxially grown undoped GaAs buffers on GaAs:Si ͑100͒ substrates. The CdSe QD layer was embedded between a 50 nm thick ZnSe buffer layer, grown atop a GaAs pseudosubstrate, and a 25 nm thick ZnSe cap layer. In this manner, two different samples were realized with the CdSe layer either nominally 2 or 1 ML thick. In CdSe/ ZnSe heteroepitaxy, QDs ensue due to fluctuations in Cd concentration in an alloyed Zn 1−x Cd x Se quantum-well ͑QW͒-like structure.Postgrowth rapid thermal annealing of the samples was performed in a N 2 environment at T A = 500°C. Thermal annealing induces a pronounced blueshift of the PL spectra as demonst...

Quantum oscillations of spin polarization in a GaAs/AlGaAs double quantum well

“…In turn, the magnetic field driven occupancy of the symmetric and antisymmetric conduction band states leads to an out-of-phase oscillations of the S and AS emissions between integer filling factors. This confirms that the modulation of structural parameters in nanostructures affects the coupling between heavy-and light-hole subbands [20][21][22] that, in turn, influences the DCP variation with magnetic field. Thus, we demonstrate that the valence band states play an important role in the circular polarization of the PL emissions in GaAs/AlGaAs strongly coupled DQW structures.…”

Characterization of spin-state tuning in thermally annealed semiconductor quantum dots

“…After the sample growth, rapid thermal annealing was performed with an annealing temperature of 500°C and different annealing times ranging from 10 to 80 s. Details can be found in Ref. 17. TA leads to a blueshift of the QD emission.…”

“…Finally, high resolution transmission electron microscopy ͑HRTEM͒ images for different QDs have shown that the average dot size increases during the thermal annealing process. 17 In order to shed light on the exciton nature that shows a paramagnetic shift, a variation of the excitation power of a single QD was performed. The signature of different excitonic complexes was then analyzed.…”

“…Though, thermally annealed QDs have been studied motivated by structural enhancements, e.g., better QD homogeneity, tuning of emission wavelengths, [6][7][8][9][10] and improving quantum efficiency, [11][12][13][14] the role of thermal annealing on magnetic properties of II-VI QDs has been studied just recently. [15][16][17] It is the purpose of this work to develop a characterization of QD electronic structure that would enable the analysis of magnetic field effects, shape, composition, and interband coupling within the same framework. These results are compared and tested with experiments in order to understand several general trends observed in the optical response of thermally annealed CdZnSe/ZnSe QDs under an applied magnetic field.…”

Carrier transfer in the optical recombination of quantum dots

“…[6][7][8] The local charge imbalance in QDs has implications in the magneto-photoluminescence (PL) of QDs as reported in Ref. 9, could this effect be experimentally confirmed in time resolved emissions?…”