D. V. Korbutyak

Enhancement of the photoluminescence intensity in short-period GaAs/AlAs superlattices with different well and barrier thickness

The transition from an indirect to a direct energy band structure has been induced in short-period GaAs/AlAs superlattices by going from a symmetric to an asymmetric distribution of the well and the barrier thickness within the unit cell of the superlattice. Reducing the barrier thickness dB to half the well thickness dW moves the lowest state in the conduction band from the X point in the AlAs barrier to the Γ point in the GaAs well. For dW=2dB, the band structure becomes therefore direct for all values of dW. This change in the type of energy gap is accompanied by a significant enhancement of the integrated photoluminescence intensity for asymmetric superlattices.

Electrical properties and low-temperature photolumincesence of Si-doped CdTe crystals

The CdTe:Si single crystals with Si concentration in the range of CSi0=2×1018–5×1019 cm−3 are grown by the Bridgman-Stockbarger method. The samples were of the n-and p-type with electrical conductivity σ=2×10−1–8×10−9 Ω−1 cm−1. Being heated in the temperature range 300–440 K, the p-CdTe crystals were annealed, and their conductivity decreased. The shape of the low-temperature (5–20 K) photoluminescence spectra of the samples are indicative of their high structural quality. The specific feature of the emission of the CdTe:Si crystals is its decrease in the intensity of all lines induced by donors as the samples are cut progressively closer to the ingot top. The results obtained indicate that the Si impurity, in contrast with Ge, Sn, and Pb, does not exhibit the compensating and stabilizing effect in CdTe.

Observation of stimulated emission in an ultrashort-period nonsymmetric GaAs/AlAs superlattice

Nonsymmetric short-period GaAs/AlAs superlattices, for which the well thickness is at least a factor of 2 larger than the barrier thickness, have been shown to exhibit a direct band gap for any well thickness. These superlattices are characterized by an enhanced intensity of the luminescence as compared to their symmetric indirect-gap counterparts with the same well width, and, thus, may be used as light-emitting devices, in particular, as low-threshold lasers in the red visible spectrum. This conjecture is supported by the observation of stimulated emission at T=80 K for a GaAs/AlAs superlattice with six monolayers well and three monolayers barrier width.

Exciton Characteristics and Exciton Luminescence of Silicon Quantum Dot Structures

The exciton binding energy, the energies of the basic radiative exciton transition, and the zerophonon radiative lifetime of excitons in silicon quantum dots embedded in the SiOx matrix are calculated in effective mass approximation with quadratic dispersion relation. In addition, the spectra of steady-state photoluminescence and of time-resolved photoluminescence of excitons in the silicon quantum dots are calculated, and the kinetics of the photoluminescence relaxation is considered. The theory is compared with the experiment. It is shown that, for nanostructures involving silicon quantum dots with diameters smaller than 4 nm, the governing factor in the broadening of the spectral photoluminescence bands is the effect of mesoscopic quantum fluctuations. In this case, either an even one dangling bond at the interface, or one intrinsic point defect, or one foreign atom located inside the small-sized nanocrystallite or in its close surroundings produces a pronounced effect on the energy of the exciton transition.

Effect of microwave irradiation on the photoluminescence of bound excitons in CdTe:Cl single crystals

The effect of microwave radiation on the transformation of impurity-based structural complexes in CdTe:Cl single crystals is studied using low-temperature photoluminescence measurements. It is shown that microwave radiation activates ClTe centers, resulting in an increase in the intensity of photoluminescence line of excitons bound at the corresponding ClTe donor centers. A nonmonotonic dependence of the integrated photoluminescence intensity on the duration of microwave irradiation is observed. At the initial stage of microwave irradiation (t = 30 s), an increase in the integrated excitonic photoluminescence intensity is observed; as the duration of microwave irradiation is increased, the photoluminescence intensity decreases. The experimentally observed variations in the photoluminescence intensity are athermal in nature. The hypothetical mechanism of transformation of impurity-based structural complexes is described.

Exciton states and photoluminescence of silicon and germanium nanocrystals in an Al2O3 matrix

This paper deals with the theoretical and experimental study of radiative processes in zero-dimensional Si and Ge nanostructures consisting of a system of Si or Ge nanocrystals embedded in an Al2O3 matrix. The Al2O3 films containing Si or Ge quantum dots were produced by pulsed laser-assisted deposition. The timeresolved photoluminescence spectra of the films were recorded in the energy range from 1.4 to 3.2 eV in the range of photoluminescence relaxation times between 50 ns and 20 μs. The exciton binding energy and the energy of radiative excitonic transitions are calculated, taking into account the finite barrier height and the polarization of heterointerfaces. In addition, the excitonic photoluminescence spectra are calculated, taking into account the effect of quantum mesoscopic fluctuations and the possible nonmonotonically varying dependence of the radiative zero-phonon lifetime of excitons on the dimensions of the quantum dots. The observed agreement between the calculated and recorded photoluminescence spectra confirms the excitonic nature of photoluminescence and provides a means for the determination of the model parameters of photoluminescence in the nanostructures.

Effect of microwave treatment on the luminescence properties of CdS and CdTe:Cl Single Crystals

The effect of microwave radiation on the luminescence properties of CdS and CdTe:Cl single crystals is studied. It is established that the exposure of these semiconductors to short-term (≤30 s) microwave radiation substantially modifies their impurity and defect structure. The mechanisms of transformation of the defect subsystem of II–VI single crystals upon microwave treatment are discussed. It is shown that the experimentally observed changes are defined by the nonthermal effects of microwave radiation at a power density of 7.5 W cm–2; at 90 W cm–2, nonthermal effects are prevailing.

Energy states in short-period symmetrical and asymmetrical (GaAs)N/(AlAs)M superlattices: The effect of the boundary conditions

Obtained experimental data on low-temperature photoluminescence are used to numerically simulate the energy states in symmetrical and asymmetrical short-period (GaAs)N/(AlAs)M superlattices with the (001) orientation. The matrix formalism in the envelope-function method is employed to study trends in the behavior of the miniband spectrum in models with different boundary conditions. It is shown that correct information about the type of transitions in the materials under consideration can be obtained even if the boundary conditions are diagonal. The effect of corrections on the miniband spectrum that arise when mixing of the states belonging to the Γ and X valleys is taken into account and a δ-functional potential localized at the heterointerface is present is studied.

Characterization of ultrashort-period GaAs/AlAs superlattices by exciton photoluminescence

Abstract Ultrashort-period GaAs/AlAs superlattices of type-II were investigated by low-temperature photoluminescence spectroscopy. The photoluminescence spectra consisting of many lines were carefully analyzed to obtain energy position, full width at half maximum and relative intensity for each individual line. Influence of the energy band structure, layer thickness and interface disorder on the radiative electron-hole transitions in these structures is discussed.

Excitonic photoluminescence of silicon quantum-well structures

The exciton binding energy and the energies of radiative excitonic transitions in the separate SiOx-Si-SiOx quantum wells are calculated in the effective-mass approximation with the quadratic dispersion relation. Along with the real finite offsets of the bands in such quantum well structures, the effect of dielectric enhancement of the exciton binding energy due polarization of the heterointerfaces is taken into account. In addition, the dependence of the zero-phonon radiative excitonic recombination time on the width of the SiOx-Si-SiOx quantum well is calculated. This dependence exhibits unsteady (oscillating) behavior, which is caused by the indirect band gap of the silicon material. It is shown that the theoretically calculated energies of the radiative excitonic transitions in the SiO2-Si-SiO2 quantum wells match the experimentally determined energies for the quantum wells whose widths are larger than 1.5 nm. Good agreement between the theoretically calculated and experimental spectral dependences of photoluminescence in the SiO2-Si-SiO2 quantum wells is attained.