We study the theoretical model of a ferromagnetic semiconductor as a system of randomly distributed Ising spins with a long-range exchange interaction. Using the density-of-states approach, we analytically obtain the magnetic susceptibility and heat capacity over a wide range of temperatures and magnetic fields. It is shown that the finite system of spins in magnetic field less than a certain critical field is in a superparamagnetic state due to thermodynamic fluctuations. The complex phase structure of a ferromagnetic semiconductor is discussed.

We consider a system of randomly distributed magnetic atoms and describe the ex-change interaction in the Ising model with a hydrogen-like dependence of the exchange energy on distance. The density of states for such system was calculated using an advanced numerical algorithm. Furthermore, the density of states was calculated analytically. We established that finding the density of states allows calculating the dependence of magnetic susceptibility of the system on temperature and magnetic field.

A model of an impurity system in semiconductors consisting of spins randomly distributed in space with a hydrogen-like distance dependence of the exchange energy in the Ising Hamiltonian has been studied. The distribution function of the exchange energy and the mean square of the magnetic moment have been calculated as functions of the concentration. It has been shown that the so-called spin-fluctuation transition, which is associated with a change in the mean square of the magnetic moment, occurs in the system of spins at the concentration close to the concentration at which the metal–insulator transition occurs in semiconductors.

This study presents the results of numerical simulation of donor-acceptor recombination spectra in compensated semiconductors taking into account electrostatic fluctuations associated with the presence of ionised impurities. The presence of Coulomb correlations in a system of partially ionised impurity centers results in distinctive features appearing in recombination spectra, depending on charge-carrier energy relaxation. The authors have considered the following limiting cases: absence of relaxation, partial relaxation of majority and minority charge carriers, as well as complete relaxation of a system of electrons and holes localized on impurities. The results obtained through numerical simulation have been interpreted using a previously developed analytical model, in which contributions to recombination from equilibrium and photoexcited charge carriers are considered separately. It has been shown that, under certain experimental conditions, emission lines corresponding to these contributions can be spectrally resolved.

The dependence of the impurity magnetic susceptibility on temperature was studied by numerical simulation. The direct exchange interaction of magnetic moments of randomly distributed impurities was considered within the Ising model. The dependence of the magnetic susceptibility in such a system deviates from the Curie law at low temperature, which is associated with the formation of a spin glass phase. It is shown that the pre-exponential factor of the direct exchange interaction significantly influences on the temperature dependence of the magnetic susceptibility only in the case of ferromagnetic exchange.

The numerical simulation was applied to study the temperature dependence of the impurity magnetic susceptibility. The direct exchange interaction of the impurity magnetic moments randomly distributed in space was considered within the Ising model. When the temperature in a system decreases, the magnetic susceptibility behavior in this system ceases to comply with the Curie law, which is associated with the formation of a spin glass phase. It is shown that the expression of the preexponential factor in the formula for the direct exchange interaction considerably influences the temperature dependence of the magnetic susceptibility only in the case of ferromagnetic exchange.

Experimental and theoretical piezospectroscopic investigation of A + centers in GaAs/AlGaAs quantum wells doped with beryllium is presented. Spectra of linearly polarized photoluminescence are studied experimentally depending on applied uniaxial pressure. A model of the A + center in the quantum well in the presence of uniaxial deformation in the plane of the quantum well has been constructed. Analytical expressions for the level energy, optical transition intensities, and polarization ratio have been obtained. In the framework of the proposed theory, the experimentally observed change in the polarization ratio depending on pressure and the shift of the line maximum towards short waves are explained.

AbstractExperimental and theoretical piezospectroscopic investigation of A^+ centers in GaAs/AlGaAs quantum wells doped with beryllium is presented. Spectra of linearly polarized photoluminescence are studied experimentally depending on applied uniaxial pressure. A model of the A^+ center in the quantum well in the presence of uniaxial deformation in the plane of the quantum well has been constructed. Analytical expressions for the level energy, optical transition intensities, and polarization ratio have been obtained. In the framework of the proposed theory, the experimentally observed change in the polarization ratio depending on pressure and the shift of the line maximum towards short waves are explained.

We report numerical and analytical studies of the donor-acceptor recombination in compensated semiconductors. Our calculations take into account random electric fields of charged impurities that are important in the nonzero compensation case. We show that the donor-acceptor optical spectrum can be described as a sum of two components: monomolecular and bimolecular. In the low compensation limit, we develop two analytical models for both types of recombination. Also our numerical simulation predicts that these two components of the photoluminescence spectra can be resolved under certain experimental conditions.

The spectra of donor–acceptor light absorption and luminescence in lightly doped and lightly compensated semiconductors are calculated. In the photoluminescence calculation, two limiting cases of long and short carrier lifetimes relative to the carrier-energy relaxation time are considered. It is shown that, at long lifetimes, the photoluminescence spectrum is significantly shifted toward longer wavelengths due to the relaxation of minority charge carriers. At intermediate lifetimes, the photoluminescence spectrum consists of two peaks, which is in good agreement with the experimental data.

The paper reports optical orientation experiments performed in the narrow GaAs/AlGaAs quantum wells doped with Mn. We experimentally demonstrate a control over the spin polarization by means of the optical orientation via the impurity-to-band excitation and observe a sign inversion of the luminescence polarization depending on the pump power. The $g$ factor of a hole localized on the Mn acceptor in the quantum well was also found to be considerably modified from its bulk value due to the quantum confinement effect. This finding shows the importance of the local environment on magnetic properties of the dopants in semiconductor nanostructures.

We carry out microphotoluminescence measurements of an acceptor-bound exciton (A(0)X) recombination in the applied magnetic field with a single impurity resolution. In order to describe the obtained spectra we develop a theoretical model taking into account a quantum well (QW) confinement, an electron-hole and hole-hole exchange interaction. By means of fitting the measured data with the model we are able to study the fine structure of individual acceptors inside the QW. The good agreement between our experiments and the model indicates that we observe single acceptors in a pure two-dimensional environment whose states are unstrained in the QW plain.

We present the results of optical measurements on the structures containing GaAs/AlGaAs quantum wells with positively charged acceptors (A+ centers). The data on the spin structure and localization of the A+ centers in the 2D system were obtained using magneto-optical measurements. The energy structure of the A+ impurity band was studied using the temperature dependence of photoluminescence. Based on the analysis of the complete set of experimental data obtained by different optical methods and their comparison with the results of transport measurements reported in literature, we developed a unified physical picture of charge and spin phenomena in such systems. A new experimental method, based on the measurements of photoexcitation and photoluminescence spectra, is proposed for studies of the Coulomb gap appearing at low temperatures in the density of localized states.

Since June 2007 the Liulin-5 charged particle telescope, located in the spherical tissue-equivalent phantom of the MATROSHKA-R project onboard the International Space Station (ISS), has been making measurements of the local energetic particle radiation environment. From 27 December 2011 to 09 March 2013 measurements were conducted in and outside the phantom located in the MIM1 module of the ISS. In this paper Liulin-5 dose rates, due to galactic cosmic rays and South Atlantic Anomaly trapped protons, measured during that period are presented. Particularly, dose rates and particle fluxes for the radiation characteristics in the phantom during solar energetic particle (SEP) events occurring in March and May 2012 are discussed. Liulin-5 SEP observations are compared with other ISS data, GOES proton fluxes as well as with solar energetic particle measurements obtained onboard the Mir space station during previous solar cycles.

The theory of the optical orientation of charge carriers in compensated III-V semiconductors and quantum wells for the case where electrons are excited to the conduction band from Mn-charged acceptor states is presented. It is shown that, in GaAs/AlGaAs quantum wells, the degree of the spin orientation of conduction-band electrons in this excitation scheme can be as high as 85%. This spin-orientation enhancement results from an increase in the heavy-hole contribution to the acceptor state in the vicinity of the defect center rather than from level splitting caused by quantum confinement. It is shown that the degree of circular polarization of the photoluminescence emitted upon the recombination of electrons thermalized at the bottom of the band with holes occupying the acceptor ground state in a quantum well can exceed 70%.

The paramagnetic splitting of a light-hole level in a GaAs/AlGaAs quantum well is experimentally measured by the method of measuring the polarized photoluminescence in the magnetic field. The phenomenon of giant splitting, which was previously predicted theoretically and leads to an increase in the g factor of a light hole to 9.4, is found.

Study of the spectral noise density and its dependence on current density in as-fabricated and degraded blue light-emitting diodes (LEDs) based on InGaN/GaN quantum-well structures are reported. It is shown that defects are generated nonuniformly in the course of degradation, being concentrated along extended defects penetrating into the active region of LEDs. It is demonstrated that the decrease in the external quantum efficiency in the course of aging is due to the enhancement of charge transport uniformity, which leads to the formation of shunts and local overheating regions. Typically, in blue LEDs, these effects are responsible for the ambiguous development of the degradation process, which hinders prognostication of LED service life. The effect of noise suppression is observed in a narrow current density range (10−2 to 10−1 A cm−2) corresponding to the onset of radiative recombination.

Unpredictable fast failure of blue power InGaN/GaN LEDs is caused by redistribution of In under action of injection currents between nano-scale regions of InGaN alloy with non-equilibrium composition. Unreliable LEDs can be recognized by the increase in forward current values at U < 2 V which is not accompanied by simultaneous reversed current increase during short aging tests (less than 100 h).