Evolution of exciton states near the percolation threshold in two-phase systems with II–VI semiconductor quantum dots

From studies of two-phase systems (borosilicate matrices containing ZnSe or CdS quantum dots), it was found that the systems exhibit a specific feature associated with the percolation phase transition of charge carriers (excitons). The transition manifests itself as radical changes in the optical spectra of both ZnSe and CdS quantum dot systems and by fluctuations of the emission band intensities near the percolation threshold. These effects are due to microscopic fluctuations of the density of quantum dots. The average spacing between quantum dots is calculated taking into account their finite dimensions and the volume fraction occupied by the quantum dots at the percolation threshold. It is shown that clustering of quantum dots occurs via tunneling of charge carriers between the dots. A physical mechanism responsible for the percolation threshold for charge carriers is suggested. In the mechanism, the permittivity mismatch of the materials of the matrix and quantum dots plays an important role in delocalization of charge carriers (excitons): due to the mismatch, “a dielectric trap” is formed at the external surface of the interface between the matrix and a quantum dot and, thus, surface exciton states are formed there. The critical concentrations of quantum dots are determined, such that the spatial overlapping of such surface states provides the percolation transition in both systems.     

Baric properties of InAs quantum dots

In the context of the deformation potential model, baric dependences of the energy structure of InAs quantum dots in a GaAs matrix are calculated. Under the assumption of the absence of interaction between the spherical quantum dots of identical sizes, the energy dependence of the baric coefficient of energy of the radiative transition in the quantum dot is determined. A similar dependence is also found experimentally in the photoluminescence spectra under uniform compression of the InAs/GaAs structures. Qualitative agreement between the theory and experiment as well as possible causes for their quantitative difference are discussed. It is concluded that such factors as the size dispersion, Coulomb interaction of charge carriers, and tunnel interaction of quantum dots contribute to this difference.     

The role of transport processes of nonequilibrium charge carriers in radiative properties of arrays of InAs/GaAs quantum dots

The results of time-resolved photoluminescence studies of heterostructures containing monolayer arrays of InAs/GaAs quantum dots are presented. A two-component time dependence of intensity of photoluminescence from the ground state of quantum dots, with characteristic times of the slow component up to hundreds of nanoseconds and those of rapid one several nanoseconds, is studied. It is shown that the slow component is determined by the transport of nonequilibrium charge carriers between the quantum dots. At low temperatures, the time of the slow component is determined by tunneling, and at high temperatures by thermal escape of nonequilibrium charge carriers. The ratio of the contributions of tunneling and thermal escape is determined by the degree of isolation of quantum dots. A theoretical model is constructed that describes the effect of the dynamics of carrier transport on the emergence and decay of the slow component of photoluminescence.     

Energy characteristics of excitons in structures based on InGaN alloys

Samples containing ultrathin InGaN layers that emit radiation in the spectral range from the ultraviolet to yellow region are studied. The samples are grown by metal-organic vapor-phase epitaxy. The Urbach energy, the localization energy of excitons, and the activation energy of charge carriers are determined to characterize radiative and nonradiative processes in the quantum dots and barriers of the structures. It is shown that these energy parameters are linearly dependent on the photon energy in the range from 3.05 to 2.12 eV. It is established that temperature variations in the emission intensity are due to the increase in the number of charge carriers thermally activated from the quantum wells into barriers as well as due to the enhancement of scattering of free excitons at defects.     

Trapping of charge carriers into InAs/AlAs quantum dots at liquid-helium temperature

The problem of how the probability of trapping of charge carriers into quantum dots via the wetting layer influences the steady-state and time-dependent luminescence of the wetting layer and quantum dots excited via the matrix is analyzed in the context of some simple models. It is shown that the increase in the integrated steady-state luminescence intensity of quantum dots with increasing area fraction occupied by the quantum dots in the structure is indicative of the suppression of trapping of charge carriers from the wetting layer into the quantum dots. The same conclusion follows from the independent decays of the time-dependent luminescence signals from the wetting layer and quantum dots. The processes of trapping of charge carriers into the InAs quantum dots in the AlAs matrix at 5 K are studied experimentally by exploring the steady-state and time-dependent photoluminescence. A series of structures with different densities of quantum dots has been grown by molecular-beam epitaxy on a semi-insulating GaAs (001) substrate. It is found that the integrated photoluminescence intensity of quantum dots almost linearly increases with increasing area occupied with the quantum dots in the structure. It is also found that, after pulsed excitation, the photoluminescence intensity of the wetting layer decays more slowly than the photoluminescence intensity of the quantum dots. According to the analysis, these experimental observations suggest that trapping of excitons from the wetting layer into the InAs/AlAs quantum dots at 5 K is suppressed.     

Analysis of mechanisms of carrier emission in the p-i-n structures with In(Ga)As quantum dots

With the help of the photocurrent spectroscopy, the mechanism of emission of charge carriers from energy levels of the (In,Ga)As/(Al,Ga)As quantum dots of different design are studied. Thermal activation is shown to be the main mechanism of carrier emission from the quantum dots for the isolated layer of quantum dots separated by wide (Al,Ga)As spacer layers. At a small width of the (Al,Ga)As spacer layer, when electron binding of separate layers of the quantum dots in the vertical direction takes place, the role of the tunneling mechanism of carrier emission between the vertically coupled quantum dots increases.     

Optical absorption and scattering at one-particle states of charge carriers in semiconductor quantum dots

A theory is developed for the interaction of an electromagnetic field with one-particle quantum-confined states of charge carriers in semiconductor quantum dots. It is shown that the oscillator strengths and dipole moments for the transitions involving one-particle states in quantum dots are rather large, exceeding the corresponding typical parameters of bulk semiconductor materials. In the context of dipole approximation it is established that the large optical absorption cross sections and attenuation coefficients in the quasi-zero-dimensional systems make it possible to use the systems as new efficient absorbing materials.     

Photoluminescence dynamics in InGaAsSb/AlGaAsSb quantum well nanostructures

The time dependences of the photoluminescence intensity is studied for InGaAsSb/AlGaAsSb quantum wells with different barrier widths and different barrier-material compositions. From analysis of the photoluminescence dynamics, the times of charge-carrier trapping at quantum wells, the energy dissipation times, and the lifetimes, including the lifetime with respect to resonant Auger recombination, are determined. It is shown that, for a certain structure configuration, it is possible to observe the resonant Auger recombination of nonequilibrium charge carriers.     

Electron heating by a strong longitudinal electric field in quantum wells

The electron heating by a strong longitudinal electric field and the energy losses due to the scattering of nonequilibrium electrons by polar optical phonons in rectangular GaAs/AlGaAs quantum wells are studied. A simple model is suggested to calculate the rate of energy losses due to the scattering of electrons by nonequilibrium optical phonons. Some of the experimental results on the heating of charge carriers in quantum wells are discussed, and it is shown that taking nonequilibrium optical phonons into account significantly improves the agreement between the theoretical and experimental data.     

Lateral photoconductivity of AlGaAs/InGaAs structures with quantum wells and self-organized quantum dots under interband illumination

The results of studying the special features of the dependence of lateral photoconductivity in AlGaAs/InGaAs structures with quantum dots and quantum wells on the intensity of interband light at low temperatures are reported. It is found that there is a threshold for the increase in photoconductivity. Oscillations of photoconductivity are observed at relatively high pulling fields. The effects of the pulling field and temperature on the photoconductivity are studied. The results are analyzed in terms of the theory of percolation of nonequilibrium charge carriers over localized states, taking into account the relaxation of stresses around quantum dots.     

Injection heterolaser based on an array of vertically aligned InGaAs quantum dots in a AlGaAs matrix

Arrays of vertically aligned InGaAs quantum dots in a AlGaAs matrix have been investigated. It is shown that increasing the band gap of the matrix material makes it possible to increase the localization energy of quantum dots relative to the edge of the matrix band, as well as the states of the wetting layer. The use of an injection laser as the active region makes it possible to decrease the thermal filling of higher-lying states, and thereby decrease the threshold current density to 63 A/cm² at room temperature. A model explaining the negative characteristic temperature section observed at low temperatures is proposed. The model is based on the assumption that a transition occurs from nonequilibrium to equilibrium filling of the states of the quantum dots. Fiz. Tekh. Poluprovodn. 31, 483–487 (April 1997)     

Resonances related to an array of InAs quantum dots and controlled by an external electric field

Photoluminescence of multilayer structures with InAs quantum dots grown in the p-n junction in GaAs by molecular-beam epitaxy is studied. Formation of vertical columns of quantum dots is verified by the data of transmission electron microscopy. It is shown that a natural increase in the size of quantum dots from layer to layer brings about their vertical coalescence at the upper part of a column. An unbalance of electronic levels caused by the enlargement of quantum dots was compensated by an external electric field, so that the resonance of ground electronic states in the column was attained. The onset of resonances was checked by the methods of steady-state and time-resolved photoluminescence. It is shown that, in the case of a resonance, the photoluminescence intensity and the radiative lifetime of excitons increase (up to 0.6–2 ns), while the time of tunneling of charge carriers becomes shorter (shorter than 150 ps). Outside the resonances, tunneling of electrons is appreciably enhanced owing to the involvement of longitudinal optical phonons. If only these phonons are involved, the time of nonresonance tunneling between quantum dots becomes shorter than the time of relaxation of charge carriers from the barrier (100 and 140 ps, respectively).     

Effects of local photoexcitation of high-concentration charge carriers in silicon

The phenomena occurring at the local pulsed photoexcitation of intrinsic nonequilibrium highconcentration charge carriers in silicon are experimentally investigated. The effect of a substantial increase in the lifetime of photoexcited charge carriers is found. It is shown that the effect of a substantial increase in the lifetime of charge carriers is caused by a change in the degree of degeneracy and displacement of the impurityrecombination level towards the Fermi level due to local thermoelastic deformation of the crystal and the corresponding distribution of the concentration of nonequilibrium charge carriers.     

Percolation and excitonic luminescence in SiO<Subscript>2</Subscript>/ZnO two-phase structures with a high density of quantum dots randomly distributed over a spherical surface

The results of studies of structures formed of silica (SiO₂) nanospheres and ZnO quantum dots randomly distributed over the nanosphere surface to cover an ∼0.45 fraction of the surface area are given. Because of the large surface energy of the spheres, the quantum dots formed on their surface are shaped as disks, wherein charge carriers are influenced by the quantum-confinement effect despite the large disk radii. The disk height is calculated by the effective mass method. The height is found to be comparable with the diameter of excitons in bulk ZnO. Analysis of the optical spectra shows that, at the above-indicated surface area covered with quantum dots, excitons in the array of quantum dots are above the percolation level. The use of some concepts of the percolation theory and knowledge of the topological arrangement of the samples make it possible to obtain quantitative parameters that describe this phenomenon.     

Nonradiative recombination in GaN quantum dots formed in the AlN matrix

The mechanisms of temperature quenching of steady-state photoluminescence are studied for structures with hexagonal GaN quantum dots embedded in the AlN matrix. The structures are grown by molecular beam epitaxy. The study is conducted for structures with differently sized quantum dots, for which the peak of the photoluminescence band is in the range from 2.5 to 4.0 eV. It is found that the activation energy of thermal quenching of photoluminescence varies from 27 to 110 meV, as the quantum-dot height is decreased from 5 to 2 nm. A model is suggested to interpret the results. According to the model, the photo-luminescence signal is quenched because of the transfer of charge carriers from energy levels in the quantum dots to defect levels in the matrix.     

Dynamics of photoluminescence and recombination processes in Sb-containing laser nanostructures

The dynamics of interband photoluminescence has been studied at various temperatures and excitation levels in structures with quantum wells based on InGaAsSb alloys and barriers based on AlGaAsSb and AlInGaAsSb alloys. The lifetimes of optically injected charge carriers in quantum wells at various temperatures and levels of optical excitation have been experimentally determined. An increase in the recombination rate in structures with deeper InGaAsSb/AlGaAsSb quantum wells for electrons is attributed to manifestation of resonant Auger recombination. The Auger recombination brings about heating of electrons and holes in lower subbands of dimensional quantization. The temperature of charge carriers in the course of Auger recombination is estimated using the equation for balance of power with accumulation of nonequilibrium optical phonons taken into account. The studied structures were used to fabricate lasers of two types with lasing wavelength of approximately 3 μm; it is shown that the use of a quinary alloy as the material for the barrier leads to an improvement in the characteristics of the lasers.     

Stark effect in vertically coupled quantum dots in InAs-GaAs heterostructures

The results of studies of hole energy states in vertically coupled quantum dots in InAs-GaAs p-n heterostructures by deep-level transient spectroscopy are reported. Spectra were recorded at different reverse-bias voltages. Levels related to bonding and antibonding s and p states of vertically coupled quantum dots were revealed. The energies of these states significantly depend on an external electric field applied to a heterostructure. This dependence was attributed to the quantum-dimensional Stark effect for the hole states of vertically coupled quantum dots. In addition to this, it was found that the energy of thermal activation of carriers from vertically coupled quantum dots depends on the conditions of isochronous annealing that was carried out both with the reverse bias switched-on and switched-off and both in the presence and absence of illumination. These changes, as in the case of isolated quantum dots, are typical of a bistable electrostatic dipole formed by carriers, localized in a coupled quantum dot, and ionized lattice point defects. The built-in electric field of this dipole reduces the energy barrier for the carriers in the coupled quantum dot. The investigated structures with vertically coupled quantum dots were grown using molecular-beam epitaxy taking account of self-assembling effects.     

On the lifetime of charge carriers in quantum dots at low temperatures

The nonequilibrium lifetime of charge carriers in a quantum dot has been experimentally and theoretically investigated. It has been shown that, at low temperatures when the ground state is fully occupied, the lifetime is almost independent of the excitation density and determined only by radiative recombination. Such behavior is theoretically explained and it is shown that, under the condition of the fully occupied ground state, the Auger recombination process can be suppressed by spin effects. The suppression of Auger recombination in such a system is microscopically described.     

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 SiO_x 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 the electrochemical modification of a thin Ga(In)As cap layer on the energy spectrum of InAs/GaAs quantum dots

It was shown that the selective etching and anodic oxidation of a thin Ga(In)As cap layer makes it possible to decrease the ground-state transition energy in InAs/GaAs quantum dots from ～0.9 to ～0.7 eV due to the resulting partial stress relaxation. Similar processing of surface quantum dots leads to a decrease in the quantum-dot height that increases the transition energy.