A. Yu. Serov

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.

Optical and electrical properties of GaN: Si-based microstructures with a wide range of doping levels

The optical and electrical properties of silicon-doped epitaxial gallium nitride layers grown on sapphire have been studied. The studies have been performed over a wide range of silicon concentrations on each side of the Mott transition. The critical concentrations of Si atoms corresponding to the formation of an impurity band in gallium nitride (∼2.5 × 1018 cm−3) and to the overlap of the impurity band with the conduction band (∼2 × 1019 cm−3) have been refined. The maximum of the photoluminescence spectrum shifts nonmonotonically with increasing doping level. This shift is determined by two factors: (1) an increase in the exchange interaction leading to a decrease in the energy gap width and (2) a change in the radiation mechanism as the donor concentration increases. The temperature dependence of the exciton luminescence with participating optical phonons has been studied. The energies of phonon-plasmon modes in GaN: Si layers with different silicon concentrations have been measured using Raman spectroscopy.

Optical investigation of the vertical diffusion of manganese in planar structures based on CdTe and Cd1 − xMgxTe with ultrathin MnTe layers

The emission spectra of planar structures based on CdTe and Cd1 − xMgxTe containing periodically built-in MnTe layers with a nominal thickness of one monolayer have been investigated. The luminescence spectra and luminescence excitation spectra of manganese ions and excitons, as well as the dependences of the spectra on the temperature and magnetic field strength, are used to determine the actual distribution of manganese ions. The full width at half-maximum of the profile describing the change in the concentration of manganese in the growth direction of the structures is estimated to be 7–8 monolayers.

Exciton Spectra and Electrical Conductivity of Epitaxial Silicon Doped GaN Layers

Optical spectra and electrical conductivity of silicon-doped epitaxial gallium nitride layers with uncompensated donor concentrations ND — NA up to 4.8 × 1019 cm−3 at T ≈ 5 K have been studied. As follows from the current-voltage characteristics, at a doping level of ∼3 × 1018 cm−3 an impurity band is formed and an increase of donor concentration by one more order of magnitude leads to the merging of the impurity band with the conduction band. The transformation of exciton reflection spectra suggests that the formation of the impurity band triggers effective exciton screening at low temperatures. In a sample with ND — NA = 3.4 × 1018 cm−3, luminescence spectra are still produced by radiation of free and bound excitons. In a sample with ND — NA = 4.8 × 1019 cm−3, Coulomb interaction is already completely suppressed, with the luminescence spectrum consisting of bands deriving from impurity-band-valence band and conduction-band-valence band radiative transitions.

Impurity absorption and luminescence of CuGaSe2 crystals

A comparison of the experimental and calculated absorption spectra of CuGaSe2 crystals revealed the existence of two acceptor levels with ionization energies of 66 and 167 meV in the samples under study. It was found that the luminescence spectra of CuGaSe2 measured at 77 K exhibit four impurity-band transitions with impurity ionization energies of 66, 99, 136, and 190 meV. An analysis of the temperature dependence of the luminescence intensity in the temperature range 11–77 K revealed a band peaking at 1.671 eV due to the radiation of donor-acceptor pairs. The calculated sum of the ionization energies of the impurities responsible for the formation of donor-acceptor pairs and the temperature dependence of the relative intensities of impurity-band emission were used to determine the ionization energies of the corresponding donor and acceptor.

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.

Rapid luminescence saturation of the Mn2+ 3d shell in the Cd1−xMnxTe dilute magnetic semiconductor with a high manganese concentration

The dependence of manganese-ion intracenter-luminescence intensity on optical excitation level has been studied in the Cd1−xMnxTe dilute magnetic semiconductor with 0.4<x<0.7. It is shown that the intracenter luminescence saturates due to effective nonlinear quenching already at low excitation levels. Mechanisms are proposed which can provide nonlinear quenching and offer a qualitative explanation for the temperature dependence of the luminescence saturation in samples with different manganese concentrations.

Manifestation of oxygen desorption in photoluminescence spectra of ZnO

The influence of oxygen desorption on the photoluminescence of zinc oxide has been investigated. Photodesorption and thermal desorption of oxygen have been controlled using mass spectrometry. It has been found that the removal of oxygen from the ZnO surface leads to a manifold increase in the exciton photoluminescence intensity. The process has a reversible character. The mechanism of increase in the exciton luminescence intensity of ZnO during the oxygen desorption has been discussed.

Photoluminescence Spectra of thin Zno films grown by ALD technology

The photoluminescence of ZnO films grown by atomic layer deposition (ALD) on silicon substrates has been investigated. A new broad photoluminescence band has been revealed in the exciton region of the spectrum. The properties of the band in the spectra of the films with different crystallographic orientations of substrates have been studied in a wide temperature range at different excitation levels. A model describing the origin of the new band has been proposed.