Effect of surface defects on radiative interband recombination in silicon nanocrystals highly doped with hydrogen-like impurities

The role of surface defects at the Si nanocrystal boundary during the process of interband radiative recombination is studied in the case in which nanocrystals are doped with hydrogen-like impurities with shallow energy levels. It is shown that, in the case of a nonpassivated surface with a large number of dangling bonds, the rate of radiative transitions in nanocrystals doped with donors can be larger than that in nondoped crystallites. On the other hand, doping with acceptors leads to a decrease in the rate of transitions. In the case of a completely passivated surface, the recombination rate remains almost unchanged irrespective of the type of dope.     

Tunability of the photoluminescence in porous silicon due to different polymer dielectric environments

In this report we demonstrate control over porous silicon (PSi) emission properties by changing the dielectric environment surrounding the silicon crystallites, as well as provide information on the effects of pore infiltration of PSi. This is achieved by making PSi–polymer nanocomposites by diffusing or polymerizing a range of varying dielectric constant polymers into the pores. The degree of modification in photoluminescence (PL) depends on the dielectric constant of the polymers. By increasing the dielectric constant of the environment surrounding the crystallites, a blue shift in PL as high as 222 meV has been observed. The blue shift is attributed to the high dielectric constant of the polymers relative to PSi, which causes a partial screening of the excitons allowing the excitonic levels to shift closer to the bandgap. The shift in excitonic levels increases when the dielectric constant of the polymer increases. PSi–polymer nanocomposites also exhibit an increase in PL intensity, which suggest that the inert infiltrated polymers are able to passivate existing nonradiative channels.     

Frequency and density dependent radiative recombination processes in III–V semiconductor quantum wells and superlattices

In this paper we review the radiative recombination processes occurring in semiconductor quantum wells and superlattices under different excitation conditions. We consider processes whose radiative efficiency depends on the photogenerated density of elementary excitations and on the frequency of the exciting field, including luminescence induced by multiphoton absorption, exciton and biexciton radiative decay, luminescence arising from inelastic excitonic scattering, and electron-hole plasma recombination.

Semiconductor quantum wells are ideal systems for the investigation of radiative recombination processes at different carrier densities owing to the peculiar wavefunction confinement which enhances the optical non-linearities and the bistable behaviour of the crystal. Radiative recombination processes induced by multi-photon absorption processes can be studied by exciting the crystal in the transparency region under an intense photon flux. The application of this non-linear spectroscopy gives direct access to the excited excitonic states in the quantum wells owing to the symmetry properties and the selection rules for artificially layered semiconductor heterostructures.

Different radiative recombination processes can be selectively tuned at exciting photon energies resonant with real states or in the continuum of the conduction band depending on the actual density of photogenerated carriers. We define three density regimes in which different quasi-particles are responsible for the dominant radiative recombination mechanisms of the crystal: (i) The dilute boson gas regime, in which exciton density is lower than 10¹⁰ cm^-2. Under this condition the decay of free and bound excitons is the main radiative recombination channel in the crystal. (ii) The intermediate density range (n < 10¹¹ cm^-2) at which excitonic molecules (biexcitons) and inelastic excitonic scattering processes contribute with additional decay mechanisms to the characteristic luminescence spectra. (iii) The high density range (n ?10¹² cm^-2) where screening of the Coulomb interaction leads to exciton ionization. The optical transitions hence originate from the radiative decay of free-carriers in a dense electron-hole plasma.

The fundamental theoretical and experimental aspects of the radiative recombination processes are discussed with special attention to the GaAs/Al _x Ga_1-x As and Ga _x In_1-x As/Al _y In_1-y As materials systems. The experimental investigations of these effects are performed in the limit of intense exciting fields by tuning the density of photogenerated quasi-particles and the frequency of the exciting photons. Under these conditions the optical response of the quantum well strongly deviates from the well-known linear excitonic behaviour. The optical properties of the crystal are then no longer controlled by the transverse dielectric constant or by the first-order dielectric susceptibility. They are strongly affected by many-body interactions between the different species of photogenerated quasi-particles, resulting in dramatic changes of the emission properties of the semiconductor.

The systematic investigation of these radiative recombination processes allows us to selectively monitor the many-body induced changes in the linear and non-linear optical transitions involving quantized states of the quantum wells. The importance of these effects, belonging to the physics of highly excited semiconductors, lies in the possibility of achieving population inversion of states associated with different radiative recombination channels and strong optical non-linearities causing laser action and bistable behaviour of two-dimensional heterostructures, respectively.     

Electromechanical properties of a textured ceramic material in the (1 − <Emphasis Type="Italic">x</Emphasis>)PMN-<Emphasis Type="Italic">x</Emphasis>PT system: Simulation based on the effective-medium method

A complete set of dielectric, piezoelectric, and elastic parameters for the textured ceramic material 0.67PMN-0.33PT is calculated by the self-consistency method with due regard for the anisotropy and piezoelectric activity of the medium. It is shown that the best piezoelectric properties corresponding to those of a single crystal are observed for the ceramic material with a texture in which all crystallites are oriented parallel to the [001] direction of the parent perovskite cubic cell. The simplest models of the polarization of an untextured ceramic material with a random initial orientation of crystallites are considered. The results obtained are compared with experimental data.     

Radiative recombination of ions and nuclei in electron cooling systems

Experimental data on rates for the radiative recombination of nuclei (from helium to uranium) and various ions in interaction with an electron beam in electron cooling systems are reviewed. An analysis of the experimental data has yielded the dependence of the radiative recombination rate on the relative electron energy appreciably differently than the theoretical models obtained earlier by H. Kramers and R. Schuch. In addition, it is shown that the radiative recombination rate of nuclei in the experiment depends on the transverse electron energy as T _?? ^?0.82 ,which is also different from the results of the calculations by the theoretical model proposed by M. Bell and J. Bell. Experimental data on the cooling of ions in intermediate charge states are analyzed and the dependence of the radiative recombination rate on the charge state of the ion (electron-shell configuration) is shown. For some ion charge states, the rate of the process is of a resonance character. Loss to radiative recombination in the electron cooling system of the NICA Booster is evaluated for the Au³²⁺, Au³³⁺, Au⁵⁰⁺, and Au⁵¹⁺ ion beams. Limitations imposed on the Au79+ beam lifetime by radiative recombination in the electron cooling system of the NICA Collider are analyzed. Possible ways to decrease the radiative recombination rate of nuclei by selecting the parameters of the electron cooling system for the NICA Collider are proposed.     

Dependence of the recombination kinetics of spatially separated electron-hole layers of the parallel magnetic field

The radiative recombination time of spatially separated electron-hole layers has been investigated as a function of the magnetic field parallel to the plane of a wide quantum well. The luminescence intensity has been found to decrease with increasing parallel magnetic field. This dependence is inconsistent with the theoretical predictions relating the decrease in the intensity to a decrease in the population of the energy levels allowed for radiative recombination. It has been shown that the intensity decrease is related to the exponential increase in the radiative recombination time with the magnetic field (due to a decrease in the overlapping of the electron and hole wavefunctions) and to the nonradiative recombination processes.     

Self-activated luminescence in zinc-sulfo-selenide crystals

The orange luminescence at 2.15eV of As-grown ZnS_0.52Se_0.48crystals has been studied. The peak energy, halfwidth and Gaussian band shape exhibit a temperature dependence which is similar to that of the self-activated (SA) luminescence of ZnS at 2.66 eVand to that of ZnSe at 2.03 eV, and which can be described satisfactorily by a configurational coordinate model. However, optically detected magnetic resonance experiments at 1.8 K show that the luminescence is due to the recombination of an electron thermally released from a donor state with a hole trapped at an A-center acceptor state. The temperature dependence of the thermoluminescence spectra above 77 K and their spectral shift under additional IR exposure are also consistent with radiative recombination involving distant donor-acceptor pairs.     

A new approach to determine radiative recombination lifetime in quantum well solar cells

In this paper, the authors present and extend on an existing model, which has been developed to determine the radiative recombination lifetimes in quantum well solar cells. Given the fact that recombination reduces cell performance, the main future use of this new model is to aid in optimisation of cell designs and increase cell efficiency. In this work the authors introduce a coefficient defined as the delta factor which is based on material parameters into the existing model. The introduction of this factor into the existing model has shown an improvement in radiative recombination lifetime determination of approximately 11% when comparison is made with the previous model. This has lead to an overall average improvement in lifetime determination of approximately 9% when comparisons are made between the new model and experimental data. This is a significant improvement in lifetime determination, which will benefit cell designers.     

Radiative emission properties of a-SiN : H based nanometric multilayers for light emitting devices

Optical and photoluminescence characterizations were performed on nanometric multilayer structures based on amorphous silicon nitrogen alloys. Evidences are shown that the radiative efficiency of multilayers increases with respect to single-layer structures. This is ascribed to a strong electron–hole pair localization and a low heterointerfaces defect density. Time-resolved photoluminescence measurements yield fast recombination with an energy-dependent lifetime due to hopping processes. Finally, the performance of an electroluminescent device based on multilayers is presented.     

Radiative recombination of holes in delta-p-doped gallium arsenide

Photoluminescence spectra of δ-p-doped GaAs structures of different doping levels are studied experimentally. It is found that set of PL bands observed in δ-p-doped samples recently is regularly broadened with doping concentration increase due to appearance of additional low-energy bands and their subsequent red shift at higher doping levels. A red shift of the bands and a change of their relative intensities were caused also by excitation laser intensity decrease and/or temperature increase. These results confirm our previous assumption that the bands are due to radiative recombination of spatially separated photoelectrons with holes occupying size-quantization levels of δ-layer potential well.     

One-directional gain and frequency selection of electromagnetic waves in a medium with a running grating of inverse population

The effects of one-directional amplification, frequency selection and transformation of time structure of electromagnetic waves (the generation of ultra-short pulses) are considered theoretically for a medium, where the creation of a running wave of atomic states inverse population is possible under excitation by coherent light beams with different frequencies and directions. These effects are due to the possibility of synchronism of a group velocity of an amplified electromagnetic wave with the phase velocity of a wave of inverse population. The general theory is applied for describing two specific cases which are observed in CdS crystals where the gain effects are caused by: 1) radiative transitions on lines generating the P-band of recombination radiation (the visible range) and 2) radiative transitions between different exciton subbands (the far IR range).     

Determination of carrier density dependent lifetime and quantum efficiency in semiconductors with a photoluminescence method (application to InGaAsP/InP heterostructures)

The light beam of a laser is focussed near the surface of a semiconductor sample. Thereby the excitation rate can be controlled precisely assuming a Gaussian intensity distribution of the beam. Measuring the recombination light intensity yields the quantum efficiency of the sample. By sinusoidal modulation of the excitation light and measurement of the resulting phase shift of the recombination light, the carrier density dependent lifetime is obtained. By evaluation of measured internal quantum efficiency and phase shift, Auger and radiative recombination coefficients are determined. The analysis takes into account the carrier density dependence of the radiative coefficient and shows that for most experimental conditions carrier diffusion can be neglected. In this case the analysis can be performed without numerical integration. Application of the method to quaternary InGaAsP material yielded values for Auger coefficient and radiative coefficients in accordance with published results.     

介电球体和无限长圆柱体沉浸在另一介质中的极化模和色散关系

本文导出了介电球体和无限长圆柱体沉浸在另一极性或非极性介质中的极化本征模矢量和色散关系。计算表明,每一介面振动对应两个振动频率,它们分别局限在两种介质的TO频率(ωTo)和LO频率(ωLO)之间。但是,当介质球沉浸在另一非晶介质中时,每一个介面振动只对应一个振动频率,位于内球介质的ωTo和ωLO之间。我们还将Cds微晶无规地分布在另一非晶介质中的实验结果同计算值进行了比较,两者是基本符合的。     

Effect of a SrF<Subscript>2</Subscript> insulating film on the luminescent properties of <Emphasis Type="Italic">n</Emphasis>-InP

It is shown that SrF₂ can be used as an effective protective coating for indium phosphide films, since it decreases the surface recombination rate of the films due to lattice matching between SrF₂ and InP. As a result, the external quantum yield of radiative recombination increases.     

Absorption optical bistability in semiconductors under the action of an external electric field

The conditions for the realization of absorption optical bistability under the simultaneous action of two (radiative and nonradiative) mechanisms of recombination of free charge carriers are investigated. It is shown that, in the diffusion mechanism of stream of free electrons from the beam axis, the temperature dependence of radiative recombination does not expand the temperature range of the thermostat in which the optical bistability occurs. A scheme is proposed with application of an external transverse electric field, which makes it possible to realize absorption optical bistability at any ambient temperature. An important advantage of this scheme is the conservation of the state of a bistable system after the action of a light pulse and a significant decrease in the energy spent on cooling of the medium.     

Optical band gap of Si nanoclusters

We discuss the nature of the optical transitions in porous silicon and in Si nanoclusters in the light of recent theoretical calculations. The accuracy of the different techniques used to calculate the band gap of Si nanoclusters is analyzed. We calculate the electronic structure of crystallites in the Si-III (BC8) crystalline phase which is known to have a direct gap and we examine the effect of quantum confinement on clusters of SiGe alloy and amorphous silicon. The comparison with the experiments for all the systems suggests the possibility of different channels for the radiative recombination.     

Theoretical analysis of semi/non-polar In GaN/GaN light-emitting diodes grown on silicon substrates

A theoretical study of polar and semi/non-polar In Ga N/Ga N light-emitting diodes(LEDs) with different internal surface polarization charges, which can be grown on Si substrates, is conducted by using APSYS software. In comparison with polar structure LEDs, the semi-polar structure exhibits a higher concentration of electrons and holes and radiative recombination rate, and its reduced built-in polarization field weakens the extent of band bending which causes the shift of peak emission wavelength. So the efficiency droop of semi-polar In Ga N/Ga N LEDs declines obviously and the optical power is significantly improved. In comparison with non-polar structure LEDs, although the concentration of holes and electrons as well as the radiative recombination rate of the semi-polar structure are better in the last two quantum wells(QWs) approaching the p-Ga N side, the uniformity of distribution of carriers and radiative recombination rate for the nonpolar structure is better. So the theoretical analysis indicates that the removal of the internal polarization field in the MQWs active regions for non-polar structure LEDs contributes to the uniform distribution of electrons and holes, and decreases the electron leakage. Thus it enhances the radiative recombination rate, and further improves the IQEs and optical powers, and shows the best photoelectric properties among these three structures.     

Free and localized excitons in the luminescence spectrum of C60 crystals at high pressure

The luminescence spectra of C₆₀ single crystals are studied at T≅10 K and pressure up to 4.0 GPa. It is observed that as the pressure increases, one fine-structure band in the spectrum intensifies sharply and dominates at pressures P≥1.7 GPa. The pressure shift of this band is much larger than the shift of other bands in the spectrum, and its magnitude correlates with the pressure dependence of the band gap. It is shown that this band could be due to radiative recombination of free Frenkel excitons. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 3, 234–238 (10 August 1998)     

Radiative cooling of tokamak plasmas due to multiply-charged Fe impurity ions

Calculations are presented for the rates of radiative energy loss from tokamak plasmas arising from radiation processes involving collisions between electrons and multiply-charged Fe impurity ions. The distribution of ionization states is determined from the steady-state corona model. The inclusion of dielectronic recombination raises the temperature at which each ion has its maximum equilibrium abundance. For certain nonhydrogenic ions, the dielectronic recombination rates obtained from previous calculations are found to be overestimated due to the neglect of autoionization into an excited state of the recombining ion. Electron impact excitation of resonance line radiation in the far ultraviolet and X-ray regions is the dominant radiative cooling mechanism at temperatures where ions with bound electrons are abundant. However, the radiation emitted during dielectronic recombination can be more important than direct recombination radiation and bremsstrahlung.