Luminescence of bound excitons in NaI and RbI crystals

X-ray induced luminescence spectra were for the first time investigated at 4.2–100 K in the vicinity of fundamental absorption of NaI crystals cleaved in liquid helium. Besides the luminescence of the free exciton and its LO replicas an additional band due to radiative annihilation of bound excitons appears in NaI crystals, as was earlier observed in RbI crystals. The obtained value of the binding energy of excitons is 50 and 60 meV for NaI and RbI, respectively. The nature of the shallow trap in the crystals of wide-gap insulators is discussed.     

Structured emission of relaxing systems: molecular excitons in rare-gas crystals

We have computed the emission spectra of relaxing excitated-state systems in solids as a function of the rates of radiative decay (γ) from excited to ground state and non-radiative decay (γ) within the excited state. When these are comparable a pronounced dip is obtained in the central region of the spectrum, which then may have the appearance of two distinct, unrelated emission bands. The computations well reproduce the observed W and M₂ bands in Ar crystals. Accordingly, these arise from the initially excited and near repulsive regions of the A¹Σ⁺ emitting state, respectively.     

Interaction of nickel donor and acceptor excitons with defect vibrations in ZnSe:Ni crystals

Defect vibrations in ZnSe:Ni crystals induced by the Ni impurity with charge of ±1 relative to the neutral state are calculated by a recursive method in the shell model. This leads to an interpretation of vibronic structures in electroabsorption spectra for donor and acceptor Ni excitons in ZnSe:Ni crystals. Fiz. Tverd. Tela (St. Petersburg) 39, 2147–2151 (December 1997)     

Bound-exciton emission bands in ZnSe single crystals and mixed plasmon-phonon modes

The paper reports a cathodoluminescence study of ZnSe single crystals annealed in a Bi melt at 1200 K for 120 h. It is found that the distance between the phonon structure satellites in the bound-exciton series I ₁ ^s -nLO and I ₁ ^d -nLO and the relative satellite intensity are different in samples with different conduction electron concentrations. It is shown that this difference is due to the mixing of the plasmon and phonon modes. The shape of the bound-exciton emission spectrum in ZnSe crystals in the 450–470 nm region is calculated, and a satisfactory agreement with the experiment calculations is obtained.     

Role of localized excitons in the stimulated emission in ultra-thin CdSe/ZnSe/ZnSSe single quantum-well structures

Photo-pumped lasing properties have been investigated in a CdSe/ZnSe/ZnSSE single quantum wells (SQWs) with the well-layer thickness (L_W) of 1, 2 and 3 monolayer (ML). At 20 K, the laser threshold for the SQW withL_W = 1 ML was the lowest in spite of the smallest active layer thickness. The carrier (exciton) sheet density at the threshold (n)_thwas estimated to be as low as 7 × 10¹⁰cm⁻², which is well below Mott's screening density. Time-resolved photoluminescence has revealed that the localized biexciton band, whose peak energy agrees with the lasing peak, appeared on the low-energy side of the main PL peak at this level of carrier concentrations. Theoretical calculation has also shown that the localized biexciton recombination has to be taken into account for the lasing process. On the contrary, then_thvalues of the SQWs with 2 and 3 ML are 1 order of magnitude larger than that of the SQW with 1 ML. This may be due to the smaller oscillator strength of both localized excitons and localized biexcitons because of the larger inhomogeneous broadening, resulting in an increased carrier density for achieving optical gain sufficient to overcome the reflection losses.     

A new bound-exciton emission band in ZnSe crystals and multiplasmon optical transitions

We have investigated cathodoluminescence both in unannealed ZnSe crystals and in crystals annealed in a Bi melt at a temperature of 1200K for 120 h with subsequent quenching. In the wavelength range 450–480 nm we have detected a new line series I _i ^s -nLO-mPl consisting of the bound-exciton emission line I _i ^s with wavelength λ=455.9 nm and its plasmon and LO-phonon echoes I _i ^s -LO (λ ₁=461.3 nm), I _i ^s -2LO (λ ₂=466.8 nm), I _i ^s -3LO (λ ₃=472.4 nm), and I _i ^s -4LO (λ ₄=478.3 nm). We have determined the mean number of emitted LO phonons N _LO=2.2±0.1 per photon. It is shown that the observed finer structure of the band may be due to multiphonon optical transitions. At low plasma densities (ω _p ≪ω _LO ) the Coulomb interaction causes broadening of the I _i ^s -nLO series. In samples with denser plasma, in which the condition ω _p ⩽ω _LO is met, multiplasmon satellites of the series I _i ^s -nLO-mPl are observed. Theoretical calculations of the shape of the emission band agree with experiment. Fiz. Tverd. Tela (St. Petersburg) 41, 1176–1180 (July 1999)     

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.     

Lowest optical excitations in molecular crystals: bound excitons versus free electron-hole pairs in anthracene

By solving the Bethe-Salpeter equation for the electron-hole Green function for crystalline anthracene we find the lowest absorption peak generated by strongly bound excitons or by a free electron-hole pair, depending on the polarization direction being parallel to the short or the long molecular axis, respectively. Both excitations are shifted to lower energies by pressure. The physical difference of these excitations is apparent from the electron-hole wave functions. Our findings are a major contribution to solve the long-standing puzzle about the nature of the lowest optical excitations in organic materials.     

Vacuum ultraviolet and X-ray emission spectroscopy of anion and cation excitons in oxide crystals

The results of a combined experimental investigation into the radiative relaxation of anion and cation excitons during selective vacuum ultraviolet-(up edge) and soft X-ray (core) excitation of berylliumcontaining BeO, Be₂SiO₄ and La₂Be₂O₅ crystals are presented. They indicate that the relaxation of anion and cation excitons in multi-component oxides occurs in the same local fragments of the crystal lattice, due to their higher predisposition (in comparison with other fragments) to short-lived distortions. It has been found that in La₂Be₂O₅ crystals characterized by a considerable difference in the ionic radii of cations (beryllium and lanthanum), the beryllium-oxygen tetrahedra do not participate in exciton relaxation processes.     

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.     

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.     

Self-trapped excitons in low-symmetry fluoride crystals

Triplet self-trapped excitons have been detected in low-symmetry crystals of the fluorides BaFCl₄, BaFBr, and LaF₃. In BaFBr crystals two types of excitons, on-center and off-center (with a symmetric or displaced configuration). The slow initiation of exciton luminescence in BaFBr is associated with the barrier transition from an on-center to an off-center configuration. In LaF₃ crystals tunnelling population of the exciton state also appears in the initiation. In LaF₃ crystals excitons appear only in the luminescence and excitation spectra while in the absorption spectra the exciton transitions are concealed beneath band-to-band transitions. A. P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 45–56, November, 1996.     

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.     

Formation of near-defect excitons in alkali-halide crystals

Pulsed-spectrometric research shows that the presence of defects, including those that are electrically neutral with respect to the crystal lattice, has a significant influence on the distribution of the radiation-induced electron excitations. This is evident in the electron-excitation sink in the vicinity of defects and the formation of localized excitons around impurities in ionic crystals. This influence of defects on the electron-excitation distribution is probably due to deformation of the lattice in the region of the defect. It is shown that, in the region of the defect, there is oscillating potential relief, the presence of which leads to the capture of charge carriers and their localization in this region. Lattice distortion because of deformation in the region of the defect changes the mutual distribution of the ion pairs. This creates conditions for the effective conversion of electronic excitations to localized near-defect dihalide excitons, which are different from those created in an ideal lattice. Tomsk Polytechnic University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 94–108, November, 1996.     

Resonant Raman studies of bound excitons in CdS

A new electronic resonant Raman effect in CdS is reported. Resonance enhancement at the I₁ and I₂ bound excitons is observed in differently doped (Cl, J, Li) samples. The energy, intensity dependency and Zeeman splitting suggest an electronic Raman effect associated with excited states of acceptor and donor bound excitons.     

Excited states of donor bound excitons and bound multiexciton complexes in silicon

The high resolution photoluminescence spectra reported here show new lines giving the first evidence of excited states of excitons bound to P donors in Si. These excited states are of crucial importance for a recently proposed theory of the bound multiexciton complexes which is based on a shell model. In addition several other new lines associated with the decay of the bound multiexciton complexes, which were predicted by the shell model theory, have been observed and their behaviour is found to be in argeement with the theory.     

Optical birefringence in cubic ZnSe crystals

Optical birefringence of the spatial dispersion type has been observed in cubic ZnSe crystals.