V. F. Agekyan

Intracenter transitions of iron-group ions in II–VI semiconductor matrices

A review is made of studies of intracenter optical transitions in 3d shells of iron-group divalent (magnetic) ions. Attention is focused on the emission spectra of Mn2+ ions in CdTe, ZnS, and ZnSe crystals. An analysis is made of the structure of intracenter absorption and luminescence and of the effect that the elemental matrix composition, magnetic-ion concentration, temperature, hydrostatic pressure, and structural phase transitions exert on the intracenter transitions. The mutual influence of two electronic excitation relaxation mechanisms, interband and intracenter, is considered. The specific features of the intracenter emission of magnetic ions embedded in two-dimensional systems and nanocrystals associated with a variation in sp-d exchange interaction and other factors are discussed. Data on the decay kinetics over the intracenter luminescence band profile are presented as a function of temperature, magnetic ion concentration, and excitation conditions. The saturation of the luminescence and the variation of its kinetic properties under strong optical excitation, which are caused by excitation migration and the cooperative effect, as well as the manifestation of a nonlinearity in intracenter absorption, are studied.

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.

Optical and structural properties of thallium halide microcrystals in a porous matrix

Thallium halide microcrystals were grown from water solutions in a porous matrix obtained from alkali borosilicate glass by etching, and their absorption and luminescence spectra were studied. Constraining the crystal size was found to affect the stability of some crystal modifications. The exciton radii were determined, and the dimensions of microcrystals in which size quantization effects become observable were estimated from the absorption spectra of bulk TlBr and TlCl crystals.

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.

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.

Luminescence of stepped quantum wells in GaAs/GaAlAs and InGaAs/GaAs/GaAlAs structures

Luminescence spectra of doped and undoped GaAs/GaAlAs and InGaAs/GaAs/GaAlAs structures containing several tens of stepped quantum wells (QW) are investigated. The emission bands related to free and bound excitons and impurity states are observed in QW spectra. The luminescence excitation spectra indicate that the relaxation of free excitons to the e1hh1 state proceeds via the exciton mechanism, whereas an independent relaxation of electrons and holes is specific to bound excitons and impurity states. The energy levels for electrons and holes in stepped QWs, calculated in terms of Kane’s model, are compared with the data obtained from the luminescence excitation spectra. The analysis of the relative intensities of emission bands related to e1hh1 excitons and exciton states of higher energy shows that, as the optical excitation intensity increases, the e1hh1 transition is more readily saturated at higher temperature, because the lifetime of excitons increases. Under stronger excitation, the emission band of electron-hole plasma arises and increases in intensity superlinearly. At an excitation level of ∼105 W/cm2, excitons are screened and the plasma emission band dominates in the QW emission. Nonequilibrium luminescence spectra obtained in a picosecond excitation and recording mode show that the e1hh1 and e2hh2 radiative transitions are 100% polarized in the plane of QWs.

Growth and optical properties of BiI3 and PbI2 microcrystals

A study has been made of low-temperature absorption and luminescence spectra of bismuth iodate microcrystals in various hosts (layered cadmium iodate, microporous glasses and polymers), as well as of the spectra of mesoscopic domains formed in bulk bismuth iodate by mechanical strains. The structure of the spectra and the quantum shifts of the exciton levels yielded information on the dimensions and size dispersion of the microcrystals and domains, as well as on the effect of microcrystal size on the Stokes losses. The quantum exciton-line shifts in microcrystalline lead-iodate films grown in pores of the glass host have been used to calculate the film thicknesses. A photoinduced change in the mechanism of radiative recombination was observed in PbI2 microcrystals. The spectrum of resonant Raman scattering in lead iodate microcrystals consists of broad bands corresponding to the branches of optical phonons undergoing strong angular dispersion.