Triplet bound excitons in copper-doped gallium phosphide

This review describes a particular class of neutral defect complexes in GaP, represented by the variety of Cu-related centres, which are observed to bind excitons in a spin-like singlet-triplet configuration. The reasons for this are discussed and two subgroups are defined. One of these consists of centres which bind holes in localized orbitally nondegenerate spin states, but give rise to highly structured optical spectra. The other group is characterized by featureless photoluminescence spectra in the mid-gap spectral region due to a tight binding of both the electron and hole in spin-like states. Both groups are well described with an essentially identical spin-Hamiltonian formalism, as detailed optically detected magnetic resonance measurements have revealed. The relation of this class of neutral defects in GaP to similar complexes in other semiconductors is discussed.     

Stimulated emission and E-H plasma in gallium selenide

The stimulated emission from extremely high quality GaSe crystals is investigated at very high values of excitation intensity by means of a nitrogen laser. The rising of the laser action due to electron-hole plasma recombination is reported. The unsaturated optical gain spectrum confirms the stimulated effect either from excitonic interaction processes or from electron-hole plasma. The role of the surface quality of the samples in the competition among these two amplification mechanisms is particularly discussed.     

Time resolved EPR of triplet excitons in Phenazine-TCNQ charge transfer crystal

Transient nutation EPR spectroscopy has been applied to study the dynamical properties of the excited triplets in the Phenazine-Tetracyanoquinodimenthane 1: 1 CT crystal. Measurements have been carried out with the magnetic field set along the principal axes of the ZFS tensor. Spin-spin and spin-lattice relaxation times have been determined at different temperatures together with the decay rate constants from the triplet sublevels which are found to be highly spin selective. The temperature dependence of the initial optical electron polarization carried by the triplet has been also analyzed. It is shown that the single fission and the intersystem crossing caused by spin-orbit coupling are both responsible for the generation of the triplet in this crystal, the former prevailing at room temperature. Our results are in agreement with previous investigations on the same crystal.     

Multiplasmon replicas of the emission lines of bound excitons in ZnSe crystals

This paper reports on the results of investigations into the cathodoluminescence spectra of ZnSe crystals grown from a vapor phase and annealed in a bismuth melt [ZnSe(Bi) crystals], as well as in a bismuth melt containing aluminum [ZnSe(Bi,Al) crystals], at a temperature of 1200 K for 120 h. The longitudinal optical (LO) phonon replicas of the emission lines of free excitons are dominant in the cathodoluminescence spectra of the samples subjected to annealing in the bismuth melt with subsequent quenching. The emission spectra of all the samples studied, including the initial crystals, contain series of I ₁ ^s,d -nLO lines of bound excitons. It is revealed that plasmon replicas are characteristic not only of the I ₁ ^s -nLO lines. A weak exciton-plasmon interaction manifests itself in the form of single-plasmon Stokes satellites of the I ₁ ^d -nLO spectral lines. A theory describing the multiphoton optical transitions of bound excitons with the participation of mixed plasmonphonon vibrational modes is proposed. This theory makes it possible to calculate the shape function of an emission spectrum without recourse to model concepts. The results of the theoretical calculations are in agreement with the experimental data.     

Optical detection of triplet excitons in GaS

Optically detected magnetic resonance has been used to investigate exciton recombination in the layered semiconductor GaS. Five triplet exciton resonances have been observed all with the same g-value of g_ex6 = 2.006 ± 0.002 but with different zero field splittings:

$\text{D}{}^{\mathrm{I}}\text{= + 0.013 cm}{}^{\mathrm{?1}}\text{, D}{}^{\mathrm{II}}\text{= + 0.024 cm}{}^{\mathrm{?1}}\text{, D}{}^{\mathrm{III}}\text{= + 0.025 cm}{}^{\mathrm{?1}}$

,

$\text{D}{}^{\mathrm{IV}}\text{= + 0.075 cm}{}^{\mathrm{?1}}\text{, D}{}^{\mathrm{V}}\text{= + 0.010 cm}{}^{\mathrm{?1}}$

. The resonances from the high energy wing are remarkably narrow and we believe that this may be the first observation of resonance from free indirect excitons.     

Non-radiative annihilation of singlet excitons on triplet excitons in organic molecular crystals

The per sec propability is calculated of the non-radiative annihilation of singlet excitons on triplet excitons in organic molecular cyrstals. The influence of the non-radiative annihilation of singlet excitons on triplet excitons on the quenching of crystal fluorescence caused by the radiative annihilation of singlet excitons is studied theoretically on the basis of formulae derived.Na Slovance 2, Praha 8, Czechoslovakia.     

Lifetimes of bound excitons in CdSe

Lifetime measurements have been made on bound excitons in CdSe, by using the time-correlated single photon counting and a cw mode-locked dye laser. The lifetimes of the T₂ (an exciton bound to a neutral donor) and I₁ (an exciton bound to be a neutral acceptor) bound excitons have been determined to be 0.50±0.05 and 0.80±0.05 ns, respectively. These values are in good agreement with those predicted by the model of Rashba and Gurgenishvili.     

Level-anticrossing effect on the magnetoluminescence of the triplet indirect bound exciton in GaSe

The presence of level-anticrossing in the triplet state of the indirect impurity-bound exciton in GaSe has been found for the first time in the magnetic field of 1.5 kOe applied along the c-axis. Measurements on the magnetic circular dichroism (MCD) of the photoluminescence and on the spin dependent formation rate (SDFR) show that the |S_z| = 1 level lies 0.016 meV higher than the S_z = 0 level in the absence of the magnetic field. The resonance-like behavior in these two types of polarization correlation measurements may be explained in terms of weak perturbation between |S_z| = 1 and S_z = 0 levels.     

Hanle effect in coherent backscattering

We study the shape of the coherent-backscattering (CBS) cone obtained when resonant light illuminates a thick cloud of laser-cooled rubidium atoms in the presence of a homogenous magnetic field. We observe new magnetic field-dependent anisotropies in the CBS signal. We show that the observed behavior is due to the modification of the atomic-radiation pattern by the magnetic field (Hanle effect in the excited state).     

Edge emission in zinc selenide

Three different series of edge emission bands, associated with donor—acceptor recombination, have been observed at 10 K in crystals of zinc selenide grown in a continuous flow of argon or in sealed capsules in the presence of excess zinc or selenium. It is tentatively suggested that two of these series are associated with random and preferential pairing of the same donors and acceptors.     

Optical absorption of triplet molecular excitons in alkali cyanides

The optical absorption spectra of alkali cyanides in the UV region present a set of weak absorption bands which are identified as triplet a'³Σ⁺ molecular excitons. The nature of the molecular exciton transitions in the ionic-molecular crystals is discussed and the existence of an admixture between molecular exciton and charge transfer exciton states is suggested.     

Excited states of bound excitons in ZnO

The bound exciton lines I₅, I₆, and I_{$\text{7}\text{8}$} in ZnO at 5 K are studied in the energetic region of the free excitons and below by means of excitation spectroscopy. Apart from the maxima at the free exciton energies, additional resonances are observed and interpreted as the direct creation of bound excitons in excited states which lie 6meV and 4.5 meV above the ground states.     

Evidences of p-type conductivity in iodine grown gallium selenide

Photoconductivity, photovoltage and photoelectromagnetic effect have been studied at different excitation intensities between 80 and 300°K in GaSe grown by the iodine transport method. The experimental results give evidence that the conductivity of these samples, which are n-type at room temperature, changes to p-type at low temperature.