N. N. Vasil’ev

Nonlinear properties of intraionic luminescence of Mn2+ in dilute magnetic semiconductors CdMnTe and CdMnMgTe

A peculiar mechanism of light absorption and emission related to the 3d-electron states in iron group atoms occurs in the dilute magnetic semiconductors (DMSs) along with the conventional band-to-band mechanism. The light emission from 3d-levels is important for electroluminescence applications. We have studied DMS Cd 1-x Mn x Te and Cd 1-x-y Mn x Mg y Te wherein the bright 3d-luminescence band near 2eV is observed for (x +y) > 0.4. A saturation of 3d-luminescence intensity J L is found to occur at a low excitation level J ex when only a minor portion of manganese ions is excited. The saturation of the 3d-luminescence is more readily achieved in Cd 1-x Mn x Te for x > 0.5 when the temperature rises from 4 up to 77K. The energy position of 3d-luminescence band in Cd 1-x-y Mn x Mg y Te depends weakly on the Mn and Cd relative concentrations, the value of y being fixed, but it shifts significantly towards higher energy with increasing value ofy. Thus the incorporation of Mg contributes strongly to the inhomogeneous broadening and suppresses a Frenkel exciton migration via Mn 2 + ions. This result is consistent with the decrease of saturation for Cd 1 -x-y Mn x Mg y Te as compared to Cd 1-x Mn x Te. The luminescence decay becomes faster with the increasing J ex due to the initiation of the nonradiative relaxation. The temperature and concentration dependences of J L point out the importance of energy transfer between Mn 2 + ions. The luminescence decay becomes faster with the increasing J ex due to the initiation of the nonradiative relaxation. It turns out that for Cd 0.6 Mn 0.4 Te wherein the 3d-luminescence excitation threshold coincides with band-to-band Wannier exciton energy, the dependence of the 3d-luminescence intensity upon J ex shows a peculiarity.

Intracenter luminescence of Mn2+ in Cd1−xMnxTe and Cd1−x−yMnxMgyTe under intense optical pumping

A comparative analysis of the kinetic properties of intracenter 3d luminescence of Mn2+ ions in the dilute magnetic superconductors Cd1−xMnxTe and Cd1−x−yMnxMgyTe is carried out. The influence of relative concentrations of the cation components on the position of the intracenter luminescence peak indicates that the introduction of magnesium enhances crystal field fluctuations. As a result, the processes facilitating nonlinear quenching of luminescence are suppressed. The kinetics of 3d-luminescence quenching in Cd1−xMnxTe are accelerated considerably upon elevation of optical excitation level due to the evolution of cooperative processes in the system of excited manganese ions.

Migration of Mn2+ excitations in Cd0.5Mn0.5Te crystal

The experimentally obtained intensity decay curves for the 2-eV intracenter luminescence band of Mn2+ ions in Cd0.5Mn0.5Te semiconductor solid solution at a temperature of 77 K have been simulated by the Monte Carlo method. The calculations show that the initial nonexponential behavior of the intensity decay curves at the band wings, as well as the time dependence of the band peak energy, are determined by the fast migration of excitations through the Mn2+ ion subsystem. There are more than 200 jumps per each emitted photon, and the migration rate increases by almost two orders of magnitude in comparison with the rate at 4 K. The analysis of the simulation results and the calculation based on the experimental data show the interaction between ions to be resonant. The estimate derived from the Anderson criterion suggests that the excited state is not delocalized. An increase in the migration rate with an increase in temperature significantly reduces the inhomogeneous broadening dispersion.

Concentration quenching and migration of excitations in a bulk Cd0.5Mn0.5Te crystal

The curves of intracenter luminescence decay for Mn2+ ions in the Cd0.5Mn0.5Te semiconductor solid solution, obtained in a low-temperature experiment, have been simulated by the Monte Carlo method. The features of the kinetics of the 2-eV band in the time interval where significant nonexponentiality of relaxation at different points of the emission band profile manifests itself, as well the integral kinetics and energy relaxation, have been considered. Migration of ion excitations and concentration quenching (which was previously disregarded) are considered to be the main mechanisms determining the kinetic curve formation. It was established that excitation by 2.34-eV photons leads to both selective (intracenter) and band excitation of Mn2+ ions. Comparison of the results of numerical simulation and experiment showed that the characteristic values of the migration and quenching rates (Wm and Wq, respectively) are close in magnitude and Wq, m ≈ 0.1/τ, where τ is the lifetime at the long-wavelength band wing with the exponential kinetics. The estimated quantum yield (0.56) indicates significant influence of the concentration quenching on the 2-eV luminescence quantum yield in Cd1 − xMnxTe and Zn1 − xMnxS crystals with a high concentration of Mn2+ ions.

Exciton and intracenter luminescence in Cd0.6Mn0.4Te/Cd0.5Mg0.5Te quantum-well structures

Exciton luminescence and intracenter luminescence (IL) of Mn2+ ions in Cd0.6Mn0.4Te/Cd0.5Mg0.5Te structures with quantum wells (QWs) 7, 13, and 26 monolayers thick were studied. It was established that in QWs the intensity of exciton luminescence with respect to that of IL is a few orders of magnitude higher than that in bulk crystals. The spectral position of manganese IL profile changes noticeably in going from a bulk crystal to a QW of the same composition. The nonexponential parts of the IL decay curves are determined by excitation migration and the cooperative upconversion process, whose contribution is high under strong excitation and efficient migration. At 77 K, the IL decay constant τ within the exponential region increases with decreasing QW thickness. The decay constant τ in a QW, unlike in a bulk Cd0.5Mn0.5Te crystal, decreases substantially under cooling from 77 to 4 K.

Exciton and intracenter radiative recombination in ZnMnTe and CdMnTe quantum wells with optically active manganese ions

The emission spectra of Zn1−xMnxTe/Zn0.6Mg0.4Te and Cd1−xMnxTe/Cd0.5Mg0.5Te quantum-well structures with different manganese concentrations and quantum-well widths are studied at excitation power densities ranging from 105 to 107 W cm−2. Under strong optical pumping, intracenter luminescence of Mn2+ ions degrades as a result of the interaction of excited managanese ions with high-density excitons. This process is accompanied by a strong broadening of the emission band of quantum-well excitons due to the exciton-exciton interaction and saturation of the exciton ground state. Under pumping at a power density of 105 W cm−2, stimulated emission of quantum-well excitons arises in CdTe/Cd0.5Mg0.5Te. The luminescence kinetics of the quantum-well and barrier excitons is investigated with a high temporal resolution. The effect of the quantum-well width and the managanese concentration on the kinetics and band shape of the Mn2+ intracenter luminescence characterized by the contribution of the manganese interface ions is determined.

Saturation of the Mn2+ ionic subsystem in Cd0.5Mn0.5Te and Zn0.91Mn0.09Se bulk samples and in Cd0.6Mn0.4Te/Cd0.5Mg0.5Te superlattices

It is experimentally shown that the Mn2+ ionic subsystem in the three-and two-dimensional structures of diluted magnetic semiconductors of the II–VI groups is saturated upon pulsed excitation by high-power laser radiation with a wavelength of 532.1 nm at temperatures of 77 and 4 K. The direct excitation under these conditions leads to saturation of the inhomogeneously broadened 4T1 level of a part of Mn2+ ions, which is confirmed by the estimation of the fraction of excited ions. Processing of the integral kinetic curves of the intracenter luminescence band in the region of 2 eV shows that an increase in the excitation intensity gives rise to a fast component with a decreasing lifetime. Based on the data obtained, it is assumed that, along with the previously studied cooperative effect, redistribution occurs of excitation from localized to delocalized states as a result of more effective saturation of the former.

Radiative recombination in Zn{sub 1-x}Mn{sub x}Te/Zn{sub 0.59}Mg{sub 0.41}Te quantum well structures: Exciton emission and intracenter luminescence

The luminescence spectra controlled by excitons and intracenter 3d emission of Mn2+ ions are studied for a series of Zn1-xMnxTe/Zn0.59Mg0.41 Te quantum well (QW) structures that differ in manganese content and QW width. It is shown that the relative intensities of exciton emission of the QWs and barriers and the dependences of the intensities on the optical excitation level are controlled mainly by the manganese content in the QWs that affect the efficiency of excitons transfer of to the 3d shell of the Mn2+ ions. The effects of QW width and manganese content on the decay kinetics of the intracenter luminescence of the Mn2+ ions are studied.

Optical properties of Cd0.6Mn0.4Te/Cd0.5Mg0.5Te quantum-well structures

Emission spectra of three Cd0.6Mn0.4Te/Cd0.5Mg0.5Te superlattices with Cd0.6Mn0.4Te quantum-well (QW) widths of 7, 13, and 26 monolayers, respectively, and the same thickness (46 monolayers) of the Cd0.5Mg0.5Te barriers have been studied. The QW width affects the shape and spectral position of the Mn2+ intracenter luminescence (IL) band as a result of the crystal field being dependent on the position of the manganese ion with respect to the interface. Measured in identical experimental conditions, the exciton luminescence as compared to the IL is substantially higher in intensity in a QW than in a bulk CdMnTe crystal. Some samples of superlattices and bulk crystals exhibit, in addition to the conventional IL band near 2.0 eV, a weaker band at about 1.45 eV. This band apparently derives from intracenter transitions in the Mn2+ ions in the regions where the crystal lattice has the rock-salt rather than the conventional zinc blende structure.

Mn2+ 3d luminescence kinetics in Zn1−xMnxSe

Intracenter luminescence (IL) of Mn2+ in Zn1−xMnxSe (x=0.07, 0.02) was studied under pulsed excitation by the neodymium laser second harmonic. At 4 K, the IL saturation originates from the nonlinearity of the system only at the instant of excitation, the IL decay kinetics after the exciting pulse termination depending only weakly on the pumping level. At 77 K, the decay kinetics in Zn0.93Mn0.07Se depends substantially on the pumping level, because the migration of intracenter excitation over the manganese ions initiates up-conversion, which is a slow nonlinear process. As shown by IL decay measurements in Zn0.98Mn0.02Se (x=0.02), excitation migration over the Mn2+ ions is insignificant even at a high temperature and under strong optical pumping.