Production of quantum dots by selective interdiffusion in CdTe/CdMgTe quantum wells

Individual quantum dots are produced by selective interdiffusion between the barriers and the quantum well layer in a CdTe/CdMgTe heterostructure. The heterostructure, with a SiO₂ mask preliminarily deposited onto the surface, was subjected to short-term annealing for 1 min at the temperature 410°C. The mask contained open apertures with diameter up to 140 nm. The annealing induces diffusion of Mg atoms into the depth of the quantum well. Diffusion is substantially enhanced under the mask. The induced lateral potential, with minimums in the regions of apertures of the mask, stimulates efficient localization of charge carriers that form quasi-zero-dimensional excitons. The study of radiative recombination suggests complete spatial confinement of the excitons. The confinement manifests itself in the observation of a substantially narrowed line of excitonic transitions, as well as in the observation of biexcitons and excited states at high levels of photoexcitation. The characteristic energies of interlevel splitting and the biexciton binding energy show that charge carriers are under the condition of weak confinement in the quantum dots.     

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.     

Electro-optical trap for dipolar excitons

An electro-optical trap for spatially indirect dipolar excitons is implemented in a GaAs/AlAs Schottky diode with a 400-?-wide GaAs single quantum well. In the presence of bias voltage applied to the gate, the trap for excitons appears upon annular illumination of the structure by a continuous-wave or pulsed laser generating hot electron-hole pairs in the quantum well. A barrier for excitons collected inside the illuminated ring appears due to screening of the applied electric field by nonequilibrium charge carriers within the excitation region. Excitons are collected inside the ring due to the ambipolar drift of carriers and the dipole-dipole repulsion of excitons in the optically pumped region. For dipolar excitons thus accumulated in the middle of the ring-shaped electro-optical trap, significant narrowing of the luminescence line is observed with increasing excitation density, which indicates the collective behavior of the excitons.     

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.     

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.     

Nonequilibrium population of charge carriers in structures with InGaN deep quantum dots

Electronic and optical properties of ensembles of quantum dots with various energies of activation from the ground-state level to the continuous-spectrum region were studied theoretically and experimentally with the InGaN quantum dots as an example. It is shown that, depending on the activation energy, both the quasi-equilibrium statistic of charge carriers at the levels of quantum dots and nonequilibrium statistic at room temperature are possible. In the latter case, the position of the maximum in the emission spectrum is governed by the value of the demarcation transition: the quantum dots with the transition energy higher than this value feature the quasi-equilibrium population of charge carriers, while the quantum dots with the transition energy lower than the demarcation-transition energy feature the nonequilibrium population. A model based on kinetic equations was used in the theoretical analysis. The key parameters determining the statistic are the parameters of thermal ejection of charge carriers; these parameters depend exponentially on the activation energy. It is shown experimentally that the use of stimulated phase decomposition makes it possible to appreciably increase the activation energy. In this case, the thermal-activation time is found to be much longer than the recombination time for an electron-hole pair, which suppresses the redistribution of charge carriers between the quantum dots and gives rise to the nonequilibrium population. The effect of nonequilibrium population on the luminescent properties of the structures with quantum dots is studied in detail.     

CuPc/C60 heterojunction thin film optoelectronic devices

The optoelectronic properties of heterojunction thin film devices with ITO/CuPc/C60/A1 structure have been investigated by analyzing their current-voltage characteristics,optical absorption and photocurrent.In this organic photovoltaic device,CuPc acts as an optically active layer,C60 as an electron-transporting layer and ITO and A1 as electrodes.It is observed that,under illumination,excitons are formed,which subsequently drift towards the interface with C60,where an internal electric field is present.The excitons that reach the interface are subsequently dissociated into free charge carriers due to the electric field present at the interface.The experimental results show that in this device the total current density is a function of injected carriers at the electrode-organic semiconductor surface,the leakage current through the organic layer and collected photogenerated current that results from the effective dissociation of excitons.     

The role of interface phonons in the formation of polaron states in quantum wells

A theory of a large-radius polaron in a quantum well is developed with consideration of the interaction of charged particles with different branches of the phonon spectrum. It is shown that, in narrow quantum wells, the major contribution to the polaron binding energy is made by interaction with symmetric interface phonons. As a result of such interaction, the polaron binding energy is defined by the effective mass of charge carriers in the quantum well and by the polarization properties of barriers. The possibilities of the formation of a polaron exciton in a quantum well in the case of strong interaction of charged particles with optical phonons are analyzed. The conditions in which the polarization fields produced by the electron and hole do not substantially compensate each other are established.     

The exciton photoluminescence and vertical transport of photoinduced carriers in CdSe/CdMgSe superlattices

The photoluminescence and photoluminescence excitation spectra, phonon-related Raman scattering, and vertical transport of photoinduced carriers and excitons along the growth direction in type I lowstrained CdSe/CdMgSe superlattices, which are grown on InAs substrates using molecular-beam epitaxy, are studied for the first time. The studies are carried out at various temperatures and excitation intensities. The vertical transport is studied by a purely optical method involving an enlarged quantum well built in into the superlattice. This well serves as a sink for the excitons and charge carriers tunneled through the superlattice. At 2–150 K, the carriers are preferentially transported by free excitons. However, in superlattices with periods of 5.9 and 7.3 nm, this transport is not of the Bloch type. A comparison of the calculated energies of the band-to-band transitions in the superlattices with the experimental data yields the relative magnitude of the valence-band offset in the range 0.4–0.5. The Raman spectra indicate that the behavior of optical phonons in CdMgSe is bimodal.     

Theoretical analysis of the quantum photoelectric yield in Schottky diodes

A detailed analytical calculation of the photoelectric quantum yield in Schottky diodes is presented. The transport of carriers in the surface space charge region is treated explicitly, taking account of photogeneration, diffusion and drift in the non-uniform electric field. Boundary conditions at the interface are expressed in terms of surface recombination velocity and emission velocity of excess carriers into the metal.It is shown that the metal-semiconductor interface strongly affects the collection efficiency of short wavelength generated electron-hole pairs. This effect basically originates in the emission flux of majority carriers into the metal.Current, charge carriers distributions and quantum yields are computed using the data of AuCdTe Schottky barriers.     

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).     

Scattering of screened excitons by free carriers in semiconductingquantum well structures

The theoretical treatment of the scattering of excitons by free electrons and holes in a two-dimensional semiconducting quantum-well system is extended to take into account screening by the free carriers. The scattering cross sections are calculated using the Born approximation for elastic scattering of the excitons by the free carriers. For the heavy-hole exciton, the screening by the free carriers reduces the cross section for free-carrier exciton scattering for all values of the energy of relative motion of the free carriers and the excitors. For the light-hole exciton, however, screening can actually lead to an enhancement of the scattering cross section for low values of the energy of relative motion when the density of free carriers is high. This is because screening not only reduces the interaction between the free carriers and the exciton, but also decreases the binding of the exciton, leading to a larger effective radius of the exciton. The results for the scattering cross sections are then applied to calculate the contribution of the exciton linewidth due to elastic scattering of the excitons by free carriers. It is found that this contribution to the exciton linewidth is decreased below its value in the absence of screening for both the heavy- and light-hole excitons     

Charge carrier interference in one-dimensional semiconductor rings

Interference of the ballistic charge carriers in one-dimensional (1D) rings formed by two quantum wires in the self-ordered silicon quantum wells was investigated for the first time. The charge carrier transmission coefficient, which is dependent on the carrier energy, is calculated as a function of the length and modulation depth of the parallel quantum wires. The wires can be linked to the two-dimensional reservoirs either by the common source-drain system or by the quantum point contacts. It is predicted that the conductance of a 1D ring in the first case is four times higher than in the second due to the carrier interference. The calculated dependences manifest themselves in the conductance oscillations observed in the 1D silicon rings upon varying the source-drain voltage or the external magnetic field. The results obtained made it possible to design an Aharonov-Bohm interferometer based on a 1D silicon ring in the weak localization mode; its characteristics are demonstrated in the studies of the phase coherence in the tunneling of single charge carriers through the quantum point contact.     

Type-II Ge/Si quantum dots

The electronic structure of spatially indirect excitons, multiparticle excitonic complexes, and negative photoconductivity in arrays of Ge/Si type-II quantum dots (QDs) are considered. A comparison is made with the well-known results for type-II III-V and II-VI QD heterostructures. The following fundamental physical phenomena are observed in the structures under study: an increase in the exciton binding energy in QDs as compared with that for free excitons in homogeneous bulk materials, a blue shift in the excitonic transitions during the generation of multiparticle complexes (charged excitons, biexcitons), and the capture of equilibrium carriers to localized states induced by the electric ield of charged QDs.     

On the radiative recombination and tunneling of charge carriers in SiGe/Si heterostructures with double quantum wells

For SiGe/Si(001) epitaxial structures with two nonequivalent SiGe quantum wells separated by a thin Si barrier, the spectral and time characteristics of interband photoluminescence corresponding to the radiative recombination of excitons in quantum wells are studied. For a series of structures with two SiGe quantum wells different in width, the characteristic time of tunneling of charge carriers (holes) from the narrow quantum well, distinguished by a higher exciton recombination energy, to the wide quantum well is determined as a function of the Si barrier thickness. It is shown that the time of tunneling of holes between the Si_0.85Ge_0.15 layers with thicknesses of 3 and 9 nm steadily decreases from ~500 to <5 ns, as the Si barrier thickness is reduced from 16 to 8 nm. At intermediate Si barrier thicknesses, an increase in the photoluminescence signal from the wide quantum well is observed, with a characteristic time of the same order of magnitude as the luminescence decay time of the narrow quantum well. This supports the observation of the effect of the tunneling of holes from the narrow to the wide quantum well. A strong dependence of the tunneling time of holes on the Ge content in the SiGe layers at the same thickness of the Si barrier between quantum wells is observed, which is attributed to an increase in the effective Si barrier height.     

Optical studies of acceptor centre doped GaAs/AlGaAs quantum wells

We present a theoretical and experimental study of the optical properties of acceptor centre doped quantum wells. We have performed theoretical calculations for the dependence of the band structure with doping level. Steady state photoluminescence and photoluminescence excitation results are compared with theoretical calculations involving exchange and correlation effects for the electron-hole system and the interaction between charge carriers and acceptor ions. We have studied the intensity, energy peak position, and broadening effects for excitons at doping level between 10⁸ and 10¹³ cm⁻². Theoretical calculations that only consider band filling effects are not sufficient to describe the effect on the band structure due to the doping. A much better agreement is achieved when exchange and correlation effects for the electron-hole system are taken into account. Excitons can still be detected at high hole concentrations, above the degenerated limit. They survive due to the inefficiency of screening in the two-dimensional system.     

Charge spectroscopy of SiO<Subscript>2</Subscript> layers with embedded silicon nanocrystals modified by irradiation with high-energy ions

The method of charge deep-level transient spectroscopy (Q-DLTS) is used to study and compare the ejection of charge carriers from silicon nanocrystals (NCs) located in an ordered or random way in the SiO₂ matrix. It is shown that, in all cases, this ejection is a thermally activated process. The parameters of energy barriers characterizing the processes of ejection of charge carriers from the levels of nanocrystals in the layers NCs:SiO₂ before (random distribution) and after their modification by irradiation with high-energy ions (ordered distribution of nanocrystals) are determined. It is found that the activation energies for ejection of charge carriers from nanocrystals and the size of nanocrystals estimated from the difference between energies of two levels observed by the Q-DLTS method decrease as the ion fluence is increased. The density of nanocrystals observed by the Q-DLTS method decreases by approximately an order of magnitude as a result of irradiation with fluence of 10¹²–10¹³ cm⁻² in comparison with an initial unirradiated structure; this decrease is due to formation of conducting chains of nanocrystals in tracks.     

Quantized conductance in silicon quantum wires

The results of studying the quantum-mechanical staircase for the electron and hole conductance of one-dimensional channels obtained by the split-gate method inside self-assembled silicon quantum wells are reported. The characteristics of quantum wells formed spontaneously between the heavily doped δ-shaped barriers at the Si(100) surface as a result of nonequilibrium boron diffusion are analyzed first. To this end, secondary-ion mass spectrometry, and also the detection of angular dependences of the cyclotron resonance and ESR, is used; these methods make it possible to identify both the crystallographic orientation of the self-assembled quantum wells and the ferroelectric properties of heavily doped δ-shaped barriers. Since the obtained silicon quantum wells are ultrathin (～2 nm) and the confining δ-shaped barriers feature ferroelectric properties, the quantized conductance of one-dimensional channels is first observed at relatively high temperatures (T≥77 K). Further, the current-voltage characteristic of the quantum-mechanical conductance staircase is studied in relation to the kinetic energy of electrons and holes, their concentration in the quantum wells, and the crystallographic orientation and modulation depth of electrostatically induced quantum wires. The results show that the magnitude of quantum steps in electron conductance of crystallographically oriented n-type wires is governed by anisotropy of the Si conduction band and is completely consistent with the valence-valley factor for the [001] (G ₀=4e ²/h and g _v=2) and [011] (G ₀=8e ²/h and g _v=4) axes in the Si(100) plane. In turn, the quantum staircase of the hole conductance of p-Si quantum wires is caused by independent contributions of the one-dimensional (1D) subbands of the heavy and light holes; these contributions manifest themselves in the study of square-section quantum wires in the doubling of the quantum-step height (G ₀=4e ²/h), except for the first step (G ₀=2e ²/h) due to the absence of degeneracy of the lower 1D subband. An analysis of the heights of the first and second quantum steps indicates that there is a spontaneous spin polarization of the heavy and light holes, which emphasizes the very important role of exchange interaction in the processes of 1D transport of individual charge carriers. In addition, the temperature-and field-related inhibition of the quantum conductance staircase is demonstrated in the situation when kT and the energy of the field-induced heating of the carriers become comparable to the energy gap between the 1D subbands. The use of the split-gate method made it possible to detect the effect of a drastic increase in the height of the quantum conductance steps when the kinetic energy of electrons is increased; this effect is most profound for quantum wires of finite length, which are not described under conditions of a quantum point contact. It is shown in the concluding section of this paper that detection of the quantum-mechanical conductance under the conditions of sweeping the kinetic energy of the charge carriers can act as an experimental test aiding in separating the effects of quantum interference in modulated quantum wires against the background of Coulomb oscillations as a result of the formation of QDs between the delta-shaped barriers.     

The binding energy of excitons and <Emphasis Type="Italic">X</Emphasis><Superscript>+</Superscript> and <Emphasis Type="Italic">X</Emphasis><Superscript>−</Superscript> trions in one-dimensional systems

The exciton and trion states in semiconductor quantum wires are treated by the variational method. Simple trial functions provide an adequate precision of the calculation over a wide region of wire radii, with an arbitrary relation between the effective masses of charge carriers. The precision of the results obtained by the variational method is checked by numerical diagonalization of the Hamiltonian of excitons and positively and negatively charged trions. The asymptotic behavior of the binding energies of excitons and trions in narrow quantum wires is established in the analytical form. The structure of the excited X ⁺ trion states is analyzed in the context of the adiabatic approximation.     

Boundary condition model for the simulation of organic solar cells

Organic solar cells (OSCs) are promising photovoltaic devices to convert solar energy into electrical energy. Their many advantages such as lightweight, flexibility and low manufacturing costs are intrinsic to the organic/polymeric technology. However, because the performance of OSCs is still not competitive with inorganic solar cells, there is urgent need to improve the device performance using better designs, technologies and models. In this work, we focus on developing an accurate physics-based model that relates the charge carrier density at the metal-organic boundaries to the current density in OSCs. This analysis is based on our previous studies on single-carrier and bipolar diodes. The model for the boundary condition of the charge carrier density at the interfaces of OSCs follows a power-law function with the current density, both in dark and under illumination. Simulated current-voltage characteristics are verified with experimental results. The numerical simulations of the current-voltage characteristics of OSCs consider well-established models for the main physical and optical processes that take place in the device: light absorption and generation of excitons, dissociation of excitons into free charge carriers, charge transport, recombination and injection-extraction of free carriers. Our analysis provides important insights on the influence of the metal-organic interfaces on the overall performance of OSCs. The model is also used to explain the anomalous S-shape current-voltage curves found in some experimental data.