Volume 52, Nº 7 (2018)

A method for the matrix calculation of the spectral characteristics of AII–BVI semiconductors doped with iron-group ions is proposed, which takes into account all possible interactions in the ion and the effect of intramolecular surrounding fields of different symmetries. A technique for calculating the oscillator strength on the basis of eigenfunctions of the resulting states of 3d⁷, 3d⁸, and 3d⁶ electron configurations is developed. The spectral regularities of the Co²⁺, Ni²⁺, and Fe²⁺ energy structures for the ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, and CdTe materials are studied. All possible spectral bands in iron-group ions implanted in AII–BVI semiconductors in the range of 1.3–3 μm are established. The possibility of implementing the laser effect on these materials is estimated.

The field and temperature dependences of the magnetization (magnetic fields B ≤ 7.5 T, temperatures T = 2.0–75 K) of samples from a Pb_1–x–ySn_xV_yTe (x = 0.08, y = 0.01) single-crystal ingot synthesized by the Bridgman–Stockbarger method. It is established that the sample magnetization contains two main contributions, notably, the paramagnetism of vanadium ions and diamagnetism of the crystal lattice. The field and temperature dependences of the magnetization are approximated by the sum of modified Brillouin functions corresponding to the paramagnetic contributions of vanadium in two different charge states and the diamagnetic contribution linear in terms of field. The concentrations of vanadium ions in two different magnetic states and the character of their variation along the ingot are determined within the scope of the alloy’s electronic-structure rearrangement because of doping. The results are compared with the data of X-ray fluorescence microanalysis and the results of studying the galvanomagnetic properties of the samples.

The photothreshold of an α-GeS layered crystal is calculated from first principles based on the functional density method depending on its thickness. Two neighboring crystal plates consisting of several layers are separated by vacuum 4 layers thick, which corresponds to the doublet unit cell size of a bulk crystal. It is shown that the magnitude of the photothreshold is almost invariable with a crystal thickness larger than 10 layers.

The effect of boron implantation on the light-emitting properties of dislocation structures formed in silicon by Si⁺ ion implantation with subsequent annealing is studied. It is shown that the implantation of B⁺ ions has a significant effect on the dislocation-related luminescence intensity, spectrum and the temperature dependence of the D1-band intensity. It is found that the temperature dependence is nonmonotonous and involves two regions, in which the D1-band intensity increases with increasing temperature and has two well-pronounced maxima at 20 K and 60–70 K. The maximum at 20 K is associated with the morphological features of the dislocation structure under study, whereas the maximum at 60–70 K is associated with the additional implantation of the boron impurity into the dislocation region of the samples. It is established that the intensities of the experimentally observed maxima and the position of the high-temperature maximum depend on the implanted boron concentration.

The Nernst–Ettingshausen coefficient is calculated in superlattices with the cosine dispersion law in the case of the scattering of charge carriers at acoustic and polar optical phonons in a magnetic field in the layer plane. A significant increase in the Nernst–Ettingshausen coefficient of a degenerate quasi-three-dimensional electron gas in a weak magnetic field is shown. For polar optical-phonon scattering, the Nernst–Ettingshausen coefficient changes sign in a strong magnetic field.

The results of experimental studies of the time dynamics of photoexcited charge carriers in In_0.53Ga_0.47As/In_0.52Al_0.48As superlattices grown by molecular-beam epitaxy on a GaAs substrate with a metamorphic buffer are reported. On the basis of the results of the numerical simulation of band diagrams, the optimal thickness of the In_0.52Al_0.48As barrier layer (4 nm) is chosen. At this thickness, the electron wave functions in In_0.53Ga_0.47As substantially overlap the In_0.52Al_0.48As barriers. This makes it possible to attain a short lifetime of photoexcited charge carriers (τ ~ 3.4 ps) at the wavelength λ = 800 nm and the pumping power 50 mW without doping of the In_0.53Ga_0.47As layer with beryllium. It is shown that an increase in the wavelength to λ = 930 nm (at the same pumping power) yields a decrease in the lifetime of photoexcited charge carriers to τ ~ 2 ps. This effect is attributed to an increase in the capture cross section of trapping states for electrons with lower energies and to a decrease in the occupancy of traps at lower excitation densities.

The time-resolved photoluminescence of a Bragg structure formed by InAs single-layer quantum wells in a GaAs matrix is investigated experimentally. The comparison of photoluminescence spectra recorded from the edge and the surface of the sample indicates that Bragg ordering of the quantum wells leads to substantial modification of the spectra, in particular, to the appearance of additional modes. The spectrum recorded at the edge of the sample features a single line corresponding to the exciton ground state. In the spectrum recorded at the surface, an additional line whose frequency and propagation angle correspond to the Bragg condition for quantum wells, appears at high excitation levels. The calculation of the modal Purcell factor explains the fact that spontaneous emission is enhanced only for specific propagation angles and frequencies, rather than for all angles and frequencies satisfying the Bragg condition.

The photoelectric properties of ZnO whiskers obtained from the gas phase by the evaporation of a powder and metallic Zn under the continuous action of CO₂ laser radiation are studied. On the basis of investigation of the InO–ZnO barrier structure, the mechanism of the photoconductivity of crystal ZnO caused by potential barriers is proposed.

The temperature dependence of the forward and reverse portions of the current–voltage characteristic and the photovoltage spectrum of a CdS/por-Si/p-Si semiconductor heterostructure are studied. It is found that the current-flow mechanisms are controlled by generation–recombination processes in the spacecharge region of the por-Si/p-Si heterojunction, carrier tunneling in the por-Si film, and the model of space-charge-limited currents. A simplified version of the energy-band diagram of the heterostructure under study is proposed.

The incorporation of CdS nanocrystals into a porous ZnO matrix results in sensitization of the composite in the visible spectral region. Studies of the photoconductivity spectra upon variable external illumination show that the spectra undergo reversible transformations. It is shown that the shape of the peak and the position of the local photoconductivity maximum corresponding to nanocrystals depend on the wavelength distribution of the incident radiation intensity. The mechanisms responsible for the process are discussed.

Linear-chain carbon films with a thickness of order 100 nm were studied by tunneling spectroscopy. The oscillating dependence of the differential conductivity of the investigated structures is established. The results obtained are interpreted using a model of charge-density wave formation in regular structural kinks of carbon chains. The Raman spectra of the films are recorded. The simulated spectra of harmonic oscillations of polyines (–C≡C–)_n and cumulenes (=C=)_n of carbon films are theoretically compared.

In the context of the cluster model of epitaxial graphene, the condition for a stepwise change in the charge of carbon atoms under the action of graphene–substrate electron–phonon interaction and an external electric field is derived. Such charge transfer brings about corresponding steps of the static conductivity and work function of epitaxial graphene. Numerical estimates for the case of weak coupling of quasi-free graphene with metal and semiconductor substrates are given.

The results of studying a HIT (heterojunction with an intrinsic thin layer) Ag/ITO/a-Si:H(p)/a-Si:H(i)/c-Si(n)/a-Si:H(i)/a-Si:H(n⁺)/ITO/Ag solar cell by the capacitance–voltage characteristic and current deep-level relaxation transient spectroscopy methods are presented. The temperature dependence of the capacitance–voltage characteristics of the HIT structure and deep-energy-level parameters are studied. The results of comprehensive studies by the above methods are used to determine the features of the energy-band diagram of actual HIT structures.

The deep-center-assisted tunneling of carriers in p–n structures of light-emitting diodes (LEDs) with InGaN/GaN quantum wells (QWs) makes smaller the effective height of the injection barrier, but leads to a dependence of the radiation efficiency on the density and energy spectrum of defects in GaN. In the case of hopping conduction across the space charge region, the forward voltage mainly drops near the QW boundary, where the density of deep states at the quasi Fermi-level is the lowest. As a result, band bending at the boundary decreases, and, with increasing current, the direction of the electric field also changes, which leads to a weaker confinement of holes, to their non-radiative recombination in the n barrier, and to an efficiency droop. The low efficiency of green GaN LEDs is associated with the dominance of deep centers and insufficient density of shallow centers in the energy spectrum of defects in barrier layers near the boundaries with the QW. The proposed model is confirmed by the stepwise experimental dependences of the current, capacitance and efficiency of green and blue LEDs in the case of forward bias, which reflect the contribution of color centers responsible for the defect photoluminescence bands in GaN.

The fabrication and study of the characteristics of a lattice-matched quantum cascade laser structure on an indium-phosphide substrate, designed for a wavelength of ~4.8 μm corresponding to one of the atmospheric windows are described. The heterostructure grown by molecular-beam epitaxy consisted of thirty cascades. Lasing was experimentally observed at temperatures up to 200 K at a wavelength coinciding with the calculated one, which confirms the high heterointerface quality and high precision of the layer thicknesses and active-region doping levels.