The semimagnetic semiconductors ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}\mathrm{\ensuremath{-}}\mathit{y}}$${\mathrm{Mn}}_{\mathit{x}}$${\mathrm{Mg}}_{\mathit{y}}$Te grown by molecular-beam epitaxy have been studied by photoluminescence and photoluminescence excitation spectroscopy. It is shown that the Zeeman splittings of exciton states depend on the Mn content only and are independent of the Mg content, i.e., the carrier--Mn spin-exchange constants are not affected by the Mg incorporation. Exciton magnetic polarons have been studied using the method of selective excitation of the exciton luminescence. The partial substitution of Cd by Mg leads to a strong increase of the magnetic polaron energy from 12 meV in ${\mathrm{Cd}}_{0.85}$${\mathrm{Mn}}_{0.15}$Te to 28 meV in ${\mathrm{Cd}}_{0.70}$${\mathrm{Mn}}_{0.14}$${\mathrm{Mg}}_{0.16}$Te. This result emphasizes the strong dependence of the polaron energy on the conditions of the primary localization of the excitons. Time-resolved photoluminescence measurements yield polaron formation times of about 100 ps without pronounced differences between the quaternary and ternary alloys. But the gain in polaron energy within a fixed time interval increases with increasing Mg content. The independently controllable variation of magnetic and nonmagnetic properties in ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}\mathrm{\ensuremath{-}}\mathit{y}}$${\mathrm{Mn}}_{\mathit{x}}$${\mathrm{Mg}}_{\mathit{y}}$Te alloys allows us to demonstrate the importance of the primary localization of excitons for the magnetic polaron formation in ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}\mathrm{\ensuremath{-}}\mathit{y}}$${\mathrm{Mn}}_{\mathit{x}}$${\mathrm{Mg}}_{\mathit{y}}$Te as well as in ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te alloys.
Exciton magnetic polarons are studied in CdTe/Cd1x Mn x Te (0.4 < x < 0.8) quantum wells.The magnetic polaron formation leads to the appearance of an additional line in the photoluminescence excitation spectra, which can be employed to determine the Zeeman splittings more exactly than by using the free exciton peak.We find an overall increase in the polaron energy with increasing x in the whole range of Mn contents studied.
In zinc-blende II–VI alloys the thermal-expansion coefficient for low temperatures is negative and becomes positive at higher temperatures. We investigated the luminescence properties of molecular-beam-epitaxy-grown (CdMnMg)Te layers in the temperature range from 2 up to 200 K and show that the anomalous temperature dependence of the lattice constant is reflected in the luminescence properties of the excitonic recombination and the internal transition of manganese (ITM). The temperature behavior of the ITM energy is nonmonotonic and the existence of a minimum in the photon energy (at the temperature TMn) can be correlated to the change of sign of the thermal-expansion coefficient. The decay constants of the ITM begins to decrease drastically at TMn, too. Considering a lattice constant dependent energy transfer rate to the infrared emitting state (1.2 eV) of the manganese ion the variation of the lifetimes can also be explained by the temperature dependence of the lattice constant. Furthermore, we have measured the ITM in bulk layers and observed a different dependence of the ITM properties on the temperature, demonstrating the influence of the growing conditions on the microscopic surrounding of the manganese ions.
We present experimental results showing a variation of the exchange parameters between carrier spins and Mn spins as a function of quantum-well width in semimagnetic heterostructures. Relative changes of the exchange parameters of about 0.1 are observed by analyzing the Zeeman splittings of the free-exciton states in epitaxially grown ${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}/{\mathrm{Cd}}_{1\ensuremath{-}x\ensuremath{-}y}{\mathrm{Mn}}_{x}{\mathrm{Mg}}_{y}\mathrm{Te}$ quantum wells with no discontinuity of the Mn content at the interfaces. We attribute the well-width dependence of the Zeeman splittings to a k-vector dependence of the exchange parameters.
We study the exciton localization in the semimagnetic semiconductor Cd1-xMnxΤe by selective excitation of the exciton photoluminescence.We show that the energy position of the effective mobility edge for excitons is subject to the competition between nonmagnetic and magnetic localization due to the magnetic polaron formation.External magnetic fields affect this competition by suppressing the polaron formation, which shifts the mobility edge.
Using the method of selective excitation of the exciton luminescence in ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te epilayers we have measured energies of localized magnetic polarons (LMP's) for alloys with manganese mole fractions x\ensuremath{\le}0.34. The suppression of the LMP energy has been studied in external magnetic fields and with temperature increase. Polaron formation times and exciton lifetimes have been measured by time-resolved photoluminescence. We have found that in alloys with x0.17 the polaron formation process is interrupted by exciton recombination and, as a result, the LMP does not reach its equilibrium energy. This dynamical effect on the polaron energy together with the strong sensitivity of the LMP formation to the conditions of primary exciton localization causes the absence of the LMP formation in layers with x0.1. Antiferromagnetic clustering of Mn ions, which leads to the spin-glass phase formation at low temperatures, affects the polaron energy and results in the increasing stability of LMP's against suppression by temperature increase and magnetic fields. In ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te with x>0.20 a considerable part of the polaron energy is controlled by the input of clusters of antiferromagnetically coupled Mn spins located in the nonuniform molecular field of localized excitons. The comparison of the exciton Zeeman splitting and the LMP magnetic-field suppression provides insight into the internal structure of LMP's.
We report on the growth and optical studies of II-VI semiconductor parabolic quantum wells made of Cd1-xMnxTe for a broad range of quantum well widths and Mn molar fractions x.Photoluminescence excitation spectra revealed several series of peaks equidistant in energy associated with interband optical transitions between harmonic oscillator levels.From the analysis of the spectra the valence band offset Qhh = 0.44 ± 0.1 was determined for the CdTe/Cd1-xMnxΤe system.
