Jinxi Shen

Formation and properties of self-organized II-VI quantum islands

Abstract The formation and optical properties of CdSe based, self-assembled quantum dot (QD) like nano-structures embedded in ZnSe have been studied. Self-assembling growth was achieved under both standard molecular beam epitaxy (MBE) and low temperature atomic layer epitaxy (230°C) with a subsequent annealing step. While in the case of standard MBE the competition between the relaxation via misfit dislocations and the desired dot formation leads to a low reproducibility, the latter method allows a more controlled formation of the QDs, which is clearly indicated by reflection high energy electron diffraction. In particular, a capping of the structures with ZnSe usually recovers a 2D surface thus allowing a stacking of several sheets of QDs. Small dots with a lateral diameter of 5–6 nm, which corresponds to the bulk exciton Bohr radius, and a height of 5–6 ML could be obtained as confirmed by transmission electron microscopy. The optical and structural properties of the QDs were studied by means of time resolved, resonant photoluminescence and were compared with a series of quantum wells (QW). Because of the high bandgap difference of ZnSe and CdSe, deep potential fluctuations exist within the QWs. These are caused by local interdiffusion and interface roughness and can act like low dimensional traps. However, because of their nature, they are not necessarily laterally isolated and can interact via tunneling and phonon assisted hopping. This leads to a very typical red shift of the emission peak with time in time-resolved photoluminescence (PL). In the case of self-assembled QDs, the potentials defined by the QDs are spatially well separated, as the typical dot densities observed are in the mid 1010–1011 cm−2 range. The interaction between these potentials is thus strongly suppressed, which clearly shows in the temporal evolution of both maximum position and the half width of the emission peaks. For the first time we were also able to demonstrate that CdSe can be grown with a CdS compound and additional Se flux. Sulfur seems to act as a surfactant that leads to surface smoothing and a reduced inhomogeneous broadening of the PL emission. The results are quite promising as the layers grown can be thermally activated to reorganize to coherently strained islands.

Dynamics of excitonic magnetic polarons in nanostructure diluted magnetic semiconductors

Abstract Exciton dynamics has been studied in quantum wells, quantum dots and quantum wires of diluted magnetic semiconductors by time-resolved photoluminescence spectroscopy. The transient exciton photoluminescence with resonant excitation in Cd 1− x Mn x Te/ZnTe ( x =0.1) multiple quantum wells shows the formation of magnetic polarons in the quantum well excitons. Free and localized states of the excitonic magnetic polarons are distinguished from the difference of the lifetimes and the binding energies. The magnetic polaron formation accompanied with emission or absorption of LO phonons was also observed. Quantum dots of Cd 1− x Mn x Se grown by the self-organization mode showed the luminescence of confined excitons in the dots. The Mn ions in the quantum dots induce large Zeeman shifts and the excitonic magnetic polaron formation in the dots. Quantum wires of Cd 1− x Mn x Se showed polarized exciton emissions parallel to the wires. The polarization was decreased by the external magnetic field perpendicular to the wires. The excitonic magnetic polaron states in the quantum wires are also discussed.

Magnetoluminescence in quantum dots and quantum wires of II–VI diluted magnetic semiconductors

We report optical properties of quantum dots and quantum wires of diluted magnetic semiconductors. The quantum dots of Cd1−xMnxSe (x=0.03) show the exciton luminescence at around 2.4 eV, which indicates a strong confinement effect of the exciton energy corresponding to the dot size of 4–6 nm. The Zeeman shift of the exciton luminescence was observed with an effective g value of 91, showing a significant exchange interaction of the excitons with the Mn ions in the dots. The exciton luminescence from the quantum wires of Cd1−xMnxSe (x=0.08) shifts by 5.2 meV to the higher energy side with decreasing the wire width from 126 to 26 nm. The high energy shift in the narrow wires indicates the influence of the one-dimensional quantum confinement effect for the exciton states. The effective g value of the exciton in these quantum wires is 100–150. The exciton luminescence from the wires is linearly polarized (up to 80%) parallel to the wire direction at zero field, which indicates one-dimensional properties of the...

Properties and self-organization of CdSe:S quantum islands grown with a cadmium sulfide compound source

We demonstrate a new technique to grow high-quality CdSe quantum films and islands with a very small sulfur contamination by using a cadmium sulfide compound source as Cd supply and additional Se flux. By monitoring the lattice constant with reflection high-energy electron diffraction, it is shown that the sulfur is almost completely substituted by Se and CdSe with a contamination below 5% sulfur is formed. The quantum structures obtained by the new method are generally of higher quality than those obtained by more conventional growth methods using elemental sources, even if migration enhanced methods were employed. With a brief growth interruption or post-growth annealing step the initially smooth CdSe layer can be reorganized into islands. The duration of this step as well as the initial amount of deposition allows a rather good control over the island formation. A strongly enhanced growth rate is observed for the first few monolayers of the ZnSe capping layer, which indicates a partial dissolution of the islands in the ZnSe growth front and Cd segregation.

Spin-flip rate of excitonic magnetic polarons in Cd1−xMnxTe/Cd1−yMgyTe quantum wells

The lifetime of excitons in Cd1−xMnxTe/Cd1−yMgyTe single quantum wells was investigated by time-resolved photoluminescence spectroscopy. Spin-flips between dark and bright excitons remarkably change the excitonic lifetime yielding a two-exponential decay in the photoluminescence intensity. The temperature dependence of the two decay times shows that the spin-flip rate depends strongly on the formation of excitonic magnetic polarons. By forming polarons, the spin-flip rate from dark excitons to bright excitons is reduced effectively by the spin- barriers of the polarized Mn ions, which extends significantly the luminescence decay time.

Dynamics of photoexcited carriers in molecular beam epitaxy grown semimagnetic epilayers in magnetic fields

Time-resolved photoluminescence has been measured on molecular beam epitaxy grown Cd1−xMnxTe epilayer samples. The photoluminescence intensity of magnetic polarons exhibits a very long emission tail. The observed phenomenon is attributed to the conversion of the dark excitons to dipole allowed transitions by a spin-flip process. The magnetic field dependence of the two-exponential decay property is discussed within the frame of giant Zeeman splitting, the spin-flip scattering, and the formation of the magnetic polarons.

Excitonic spin states and radiative recombinations in Cd1−xMnxTe/Cd1−yMgyTe spin superlattices

Abstract The formation of a spin superlattice structure was realized in Cd0.95Mn0.05Te/Cd0.95Mg0.05Te under a magnetic field. The experimental evidence of the spin superlattice structure was found from time-resolved polarized magneto-photoluminescence measurements, in the Zeeman splittings and the exciton magnetic polaron formation. The Zeeman splitting directly indicates the spatial separation of spin-up and spin-down electrons and heavy holes, which are localized in the Cd0.95Mg0.05Te and Cd0.95Mn0.05Te layers, respectively. The formation of exciton magnetic polarons under a magnetic field can be attributed to the spin-dependent confinement where a large polaron binding energy of 3.5 meV has been observed in 2.5 T to be enhanced by the field-induced localization of the excitons.

TIME-RESOLVED MAGNETO-OPTICAL EXPERIMENTS IN CD1-XMNXTE/ZNTE MULTIPLE QUANTUM WELLS

Optical pump-probe experiments were performed in Cd0.9Mn0.1Te/ZnTe multiple quantum wells to study the dynamics of the exciton magnetic polaron in this material. Two transient absorptions of opposite sign were observed. The first signal is negative and indicates the decrease of optical absorption, as the excitons are approaching saturation. The second signal is positive and is interpreted as the absorption of “new excitons” after the initial excitons created by the pump pulse have decayed leaving behind a magnetic polarized Mn environment. The spin relaxation time of the Mn ions has been determined to be 601 ps. Finally, we discuss the effect of the polaron–polaron interaction, relevant in our experiment because we saturate the excitons in the quantum wells.

Fabrication and optical study of quantum dots, quantum wires and quantum wells of II-VI diluted magnetic semiconductors

Nanometer-scale structures of II-VI diluted magnetic semiconductors were synthesized by the microfabrication techniques. Quantum dots of Cd 1 -x Mn x Se (x = 0.03) with diameters of 9.0-9.4 nm were grown by the self-organized mode in the molecular beam epitaxy. Quantum wires of Cd 1 -x Mn x Se (x = 0.08) were synthesized with the width of 60-125 nm from the two-dimensional quantum wells by the electron beam lithography and chemical etching. The optical properties and the exciton dynamics of these quantum structures were studied by the transient photoluminescence spectroscopy. The magneto-optical properties of the confined excitons in the quantum dots, the wires and the wells are discussed.

Asymmetric luminescence line shape and exciton energy relaxation in Zn1−x−yMgxCdySe epilayers

The asymmetric line shape of the transient photoluminescence (PL) spectra of high-quality Zn1−x−yMgxCdySe epilayers is deconvoluted to a narrow and a broad Gaussian peak at each time delay. The fitted energy difference between the two peaks corresponds to one longitudinal optical (LO) phonon energy. We assign the narrow peak to the recombination of free excitons (FXs) and the broad peak to the recombination of excitons localized in the broad band tail characteristic for this quaternary compound. The broad PL peak is found to arise from the energy relaxation of the FX at the exciton mobility edge to the localized states by LO phonon emission. Therefore, the population peak of the localized excitons is formed at one LO phonon energy below the FX for all times after the laser excitation.