V.F. Aguekian

New observations on the luminescence decay lifetime of Mn2+ in Zns: Mn2+ nanoparticles.

A fast decay emission peaking at 645 nm with a decay lifetime within the experimental resolution of 0.14 micros is observed in ZnS:Mn2+ nanoparticles. This short-lived signal is also observed in pure ZnS and MgS: Eu3+ nanoparticles, which has nothing to do with Mn(2+)-doped ions but is from the deep trap states of the host materials. The short-lived component decreases in intensity relative to the Mn2+ emission at higher excitation powers, while it increases in intensity at low temperatures and shifts to longer wavelengths at longer time delays. Our observations demonstrated further that the emission of Mn2+ in ZnS: Mn2+ nanoparticles behaves basically the same as in bulk ZnS: Mn2+; the fast decay component is actually from the intrinsic and defect-related emission in sulfide compounds.

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.

The magnetic polaron effect in diluted magnetic semiconductor Cd1−xMnxTe with high concentration of manganese

The selectively excited luminescence in Cd1−xMnxTe(0.25 < x < 0.36) is interpreted in terms of the magnetic polaron formation. The energy parameter Δ0 which characterizes the magnetic relaxation was measured as a function of manganese concentration, temperature, and magnetic field. We found that antiferromagnetic pairs of neighbouring Mn2+ contribute to the magnetic polaron.

Optical properties of new diluted magnetic semiconductor Cd1−x−yMnxMgyTe

Abstract Results of the investigation of a new type of diluted magnetic semiconductor Cd 1− x − y Mn x Mg y Te are reported. The optical transitions originated from excitons and intraionic levels in Mn 2+ are observed in the absorption and photoluminescence spectra. Dependence of the energy gap on the relative concentrations of three cation components is measured in a wide range of x and y .

SIZE EFFECTS OF EXCITONS OF BiI3 IN LAYERED MATRICES

The size effects on the exciton absorption and luminescence spectra are studied in nanostructure of BiI3 embedded in the matrices of bulk BiI3 and CdI2 crystals. The quantum-confined exciton states are observed as the absorption lines of the series WJ (J=I, II, III, …). The results are compared with a model calculation of the confined excitons in the disks with thickness Lz and radius R. The disk-size distribution is directly observed with a high-resolution electron microscope. The relaxed luminescence from the BiI3 disks in CdI2 matrix is analyzed by the model of the confined excition interacting with acoustic phonons by taking into account the size-dependent Stokes shift and the distribution of the disk size.

Exciton Luminescence in Tin Dioxide Single Crystals

The exciton spectra of SnO 2 single crystals with the low and high concentrations of the oxygen vacancies (OV) are investigated. The series of luminescence lines adjacent to the lowest free exciton states (the excitons localized by the isolated OV and OV clusters) increases superlinearly with the optical excitation level. It is shown that the luminescence spectra of SnO 2 can be described as the zero-phonon and one LO-phonon assisted anihilation of the free and localized excitons. The shape of LO-phonon replicas of the free exciton luminescence corresponds to Maxwellian distribution of the excitons which have the effective temperature significantly higher than the lattice temperature in the case of pulse excitation level 10 6 –10 7 W/cm 2 .

Photoluminescence of free and localized excitons from CdTe/CdMnTe quantum well structures in the presence of a strong external magnetic field

The exciton photoluminescence from two CdTe quantum wells with the thicknesses 4 and 8 nm in CdTe/CdMnTe structures is studied in the magnetic field up to 41 T. The transformation of the photoluminescence spectrum depends strongly on the field direction and degree of the initial exciton localization. The criterion of strong field for the diamagnetic excitons related to the Landau levels is achieved.

Exciton magnetic polaron localization dynamics in bulk Cd1−xMnxTe

Abstract The pico-second time-resolved photoluminescence spectra at various temperatures have revealed characteristic dynamical behavior of exciton magnetic polarons in bulk-Cd 1− x Mn x Te ( x = 0.1−0.2). The results were consistent with those obtained from steady-state photoluminescence measurements. The temperature dependence of exciton magnetic polaron energies was determined. The overall behavior appears different from that reported previously for the bound - or localized-exciton magnetic polarons in the same system.

Local electronic properties and magnetism of (Cd,Mn)Te quantum wells

The electronic properties of heterostructures containing (Cd,Mn)Te quantum wells are probed by soft x-ray spectroscopies. We provide experimental evidence that Mn ions are in the Mn2+ (3d5) electronic configuration, and rule out the possibility that charge transfer and crystal field effects can reduce the magnetic moment of each Mn ion to the value (0.61) extracted from the fitting of the magnetization curve with a Brillouin function. These results confirm that the observed magnetic behavior can be correctly explained by assuming an antiferromagnetic coupling between nearest-neighbor S=5/2 Mn2+ ions, rather than a paramagnetic response from an ensemble of low-spin Mn ions.

Spatial evolution of inelastic scattering of high-density excitons in GaSe crystals

Abstract Spatial evolution of heavily photo-created excitons in GaSe single crystals has been studied by observing space- and time-resolved photoluminescence. At an early stage, the free-exciton recombination band shows a blue-shift with broadening due to the phase-space filling effect and the increment of the exciton self-energy. Recoil luminescence bands due to the exciton–exciton and exciton-carrier scattering shifts to the lower and higher energy sides with increasing exciton density and by separating a detecting position from the center of the exciting laser spot, respectively. These results are consistently explained by the expansion of the high-density exciton masses in the real space and by their k-space relaxation accompanying intra-band inelastic scattering under the energy and momentum conservation law.