Volume 51, Nº 5 (2017)

The implantation of sodium ions with an energy of 300 keV is carried out into high-resistivity p-Si. The annealing of defects at T_ann = 350–450°C and related activation of atoms (the latter occurs at the “tail” of atom distribution) are described by a first-order reaction. At T_ann = 450–525°C and irrespective of the ion dose, negative annealing is observed; this annealing is accompanied by an appreciable increase in the surface resistance ρ_s. According to estimations, the activation energy of this process amounts to ~2 eV. It is our opinion that the annealing is related to the precipitation of sodium donor atoms, which occurs at a depth exceeding by two–three times the projected range R_p of ions. The annealing of defects at T_ann = 525–700°C, which leads to a further decrease in ρ_s, features an activation energy of ~2.1 eV. The hypothesis that the “tail” in the profiles of sodium atoms measured by secondary-ion mass spectroscopy is due to the diffusion of these atoms from the walls of the crater to its center is verified. It is shown that this process is not implemented since the profiles of sodium atoms measured at room temperature do not differ from those measured at–140°C.

The density of phonon states and the phonon dispersion in the Brillouin zone are calculated by the density functional method of. The displacements of atoms in the elementary cell are constructed for vibrations of the A, B, and E symmetries. The calculated frequencies of optical phonons are consistent with the experimentally determined frequencies from the IR-absorption (infrared) and Raman-scattering spectra. In the xy plane, the intersection of low-frequency optical phonons with acoustic phonons is observed.

The dynamics of the long-wavelength edge of the spectrum of intrinsic stimulated picosecond emission occurring upon the picosecond pumping of GaAs is studied experimentally. The shortage of bandgap renormalization induced by the Coulomb interaction of charge carriers, compared to renormalization in the quasi-steady state, is observed. It is conceived that the shortage is caused by the fact that, under the experimental conditions of the study, the renormalization relaxation time is in the picosecond region.

The photoconductivity and photoluminescence spectra of ZnSe:Ti crystals in the visible and infrared spectral regions are studied. It is established that the high-temperature impurity-induced photoconductivity of ZnSe:Ti crystals is defined by the optical transitions of electrons from the ³A₂(F) ground state to highenergy excited states, with the subsequent thermal transitions of electrons to the conduction band. The efficient excitation of intracenter luminescence of ZnSe:Ti crystals is achieved by light from the region of the intrinsic absorption of Ti²⁺ ions.

The results of studies of the surface of GaAs in the presence of indium and phosphorus surfactants are reported. It is shown that, as a result of their diffusion (annealing) at a temperature of 650–670°C, clusters enriched with indium are formed in the GaAs surface region. The clusters can be seen as bright spots in an image obtained by a scanning electron microscope with the use of an in-lens detector. At the same time, studies of the morphology of this surface with an atomic-force microscope show a decrease in the root-meansquare roughness of the surface after annealing (diffusion), which is indicative of the incorporation of In atoms into the GaAs crystal lattice. The clusters are responsible for changes in the Raman spectra. Specifically, an increase in the signal intensity due to surface-enhanced Raman scattering and a shift of the vibration frequency in the surface region are observed. It is found that cluster formation is defined by the crystallographic orientation of the surface and by the technological conditions of surface preparation.

A possible explanation is suggested for discrepancies between experimental values obtained by different authors for the barrier height at the same metal–semiconductor junction. It is supposed that the problem is caused primarily by the structural inhomogeneity of the metal. As a result, the junction behaves as an assembly of a large number of subjunctions connected in parallel. To determine the effect of metal inhomogeneity on the properties of a junction, the dependence of the barrier height in a Schottky diode on the junction area is investigated under the assumption that, with an increase in the area of a junction between a singlecrystal semiconductor and a polycrystalline metal, the degree of inhomogeneity and, thus, the number of subjunctions, increases.

Injection-induced terahertz electroluminescence from silicon p⁺–n structures is observed at helium temperatures. Structures fabricated by the diffusion of boron into a phosphorus-doped n-Si substrate are studied. Relatively narrow luminescence lines are observed in the luminescence spectra against a broad smooth background. The spectral position of a number of emission lines corresponds to optical transitions between excited donor states and the ground state of phosphorus donors. The intracenter optical transitions of electrons at phosphorus donors are excited as a result of recombination processes occurring in the n-type region of the structure under the injection of nonequilibrium holes. A number of other lines in the terahertz emission spectra are associated with intracenter transitions at acceptor centers, which are also excited as a result of injection. The structureless background in the electroluminescence spectra may be due to terahertz emission upon the intraband energy relaxation of “hot” carriers having an effective temperature exceeding the lattice temperature. These “hot” carriers appear in the structure under injection conditions.

The crystal and electronic structures of GeC, SiC, and SnC crystals and superlattices based on them, GeC/SiC, SnC/SiC, and SnC/GeC, are modeled within density functional theory. The equilibrium lattice constants are obtained, the band spectra and densities of states are calculated, and the specific features of the formation of the valence band and chemical bonding in these crystals and superlattices are studied.

The efficiency of electronic-excitation energy transfer from organic semiconductor TPD to CdSe/CdS/ZnS nanocrystals passivated with different organic ligands is investigated. It is shown that the rate of energy transfer from TPD to nanocrystals decreases with increasing thickness of the passivation coating. It is suggested that the Förster mechanism is responsible for the excitation transfer.

The current–voltage (I–V) and photocurrent–light intensity (I_pc–Φ) characteristics and the photoconductivity relaxation kinetics of TlInSe₂ single crystals are investigated. Anomalously long relaxation times (τ ≈ 10³ s) and some other specific features of the photoconductivity are observed, which are explained within the barrier theory of inhomogeneous semiconductors. The heights of the drift and recombination barriers are found to be, respectively, E_dr ≈ 0.1 eV and E_r ≈ 0.45 eV.

The effect of hydrogen desorption on the mechanical stiffness, intrinsic vibrational spectrum, and electron density of states in structurally diamond-like carbon nanothreads is studied in the context of the tight binding model. It is shown that the stiffness only slightly varies in the initial stage of desorption and starts noticeably decreasing after the desorption of ~20% of hydrogen atoms. As hydrogen is desorbed, energy levels that finally form an impurity band appear in the band gap.

Laser-power converters with a wavelength of λ = 809 nm are fabricated on the basis of single-junction AlGaAs/GaAs heterostructures grown by the method of liquid-phase epitaxy (LPE). Photovoltaic modules with an operating voltage of 4 V for converting radiation of various densities are developed and tested. Two approaches—without the use of optical concentrating systems and with the use of Fresnel lenses are investigated. A monochromatic conversion efficiency exceeding 44% is attained with a photovoltaic module based on 64 laser-radiation converters 0.04 cm² in area and a concentrating system made of a quartz-lens matrix.

The experimental results of studying the output characteristics of four-contact semiconductor position-sensitive photodetectors fabricated on the basis of photosensitive n-CdSe/mica epitaxial layers are presented. The position sensitivity of layers was theoretically analyzed based on elementary current flow theory and the electric dipole model. It is found that the theoretical characteristics of the coordinate sensitivity of the studied position-sensitive photodetectors correlate with the experimental dependences in terms of both curve shape and positions of the maxima. The results of determining the specific spectral sensitivity indicate the prospects for n-CdSe layers as position-sensitive photodetectors.

The comparative characteristics of photovoltaic converters (of laser radiation) based on gallium arsenide with a p-type emitter formed by gas-phase diffusion in the presence of surfactants (isovalent impurities) and without them are reported. It is shown that the use of indium and phosphorus in the process of the formation of a p–n junction significantly affects the characteristics of the obtained devices.

InAs quantum dots (QDs) in a metamorphic InGaAs matrix are grown by MOVPE (metal-organic vapor-phase epitaxy) on GaAs substrates. The QDs emit light in the range 1380–1400 nm at room temperature. A multilayer metamorphic buffer (MB) consisting of nine sublayers, each having a thickness of ~200 nm, is used to grow the structures. In each of the first seven sublayers, the indium content x successively increases by ~3.5% to reach a final value of 24.5%. Then a compensating sublayer with x = 28% and a final dislocation-free sublayer with x = 24.5% are grown. It is shown that the elastic strain relaxes with dislocation bending at both interfaces in the third, from the surface, sublayer, with the top layer being free of dislocations at both interfaces. The QDs are formed in the metamorphic matrix by the deposition of 2–2.5 InAs single layers at 520°C, with the subsequent overgrowth of a thin InGaAs layer (cap layer) at the same temperature. It is found that, to improve the optical and structural quality of the samples, it is necessary to raise the growth rate and reduce the concentration of indium in the cap layer with respect to the corresponding growth parameters of the last layer of the metamorphic buffer.