Том 50, № 9 (2016)

The atomic and electronic structure of a Ga-stabilized GaAs(001) surface with the ζ(4 × 2) reconstruction and halogens in a number of symmetric sites on the surface are calculated by the plane-wave projector augmented wave method. The energy barriers of halogen-atom diffusion on this surface are calculated, which allow determination of the most preferred paths of their migration. It is shown that there is a low barrier (0.17–0.23 eV) for the diffusion of all halogens under consideration (I, Br, Cl, F) along the surface gallium dimer, whereas the barrier is significantly higher for diffusion between adjacent gallium dimers. In general, the energy barriers for halogen diffusion in both directions ([110] and [1-10]) point to their high surface mobility, despite high binding energies at a number of surface adsorption sites.

In samples of GdS_x (x = 1.475–2) of various compositions, the conductivity temperature dependences are investigated for the case of direct current in the low-temperature region (4.2–225 K). The presence of the activation and activationless hopping mechanisms of charge transport over the band gap of the samples of GdS_x phases is established. The parameters of localized states in GdS_x are determined.

For the single-crystal compounds In₂S₃ and AgIn₅S₈ produced by chemical gas-transport reactions and the Bridgman method (vertical version), the transmission spectra in the region of the fundamental absorption edge are studied in the temperature range from 20 to 300 K. From the recorded spectra, the band gaps of the In₂S₃ and AgIn₅S₈ single crystals are determined and the temperature dependences of the band gaps are constructed. It is established that, as the temperature is lowered, the band gap increases for both of the compounds. Calculation of the temperature dependences is performed. It is shown that the calculated and experimental values are in agreement with each other.

The results of studies of optical reflection in the far- and mid-infrared spectral regions are reported. The reflectance of five Bi₂Se₃ topological insulator films grown by molecular-beam epitaxy on Si(111) substrates is measured. The characteristic parameters of phonons and plasmons are determined by means of dispersion analysis for multilayer structures. It is found that the plasma frequency in a layer close to the Si–film interface is noticeably higher than that in the film bulk. Calculations of the loss function show that plasmon–phonon coupling plays an important role in Bi₂Se₃ films. The attenuated total internal reflection method is used to determine the frequency of the surface plasmon–phonon mode.

The effect of annealing in argon and oxygen plasma on the I–V characteristics and photoresponse of TiO₂–Si structures is investigated. The titanium oxide films are prepared by rf magnetron sputtering onto n-Si substrates. The observed features in the behavior of the electrical and photoelectric characteristics of the samples after annealing and treatment in oxygen plasma are attributed to a variation in the phase composition of the oxide film due to the appearance of anatase or rutile crystallites, depending on the treatment conditions.

In this paper we present the trapping of photogenerated charge carriers for 300 s resulted by their direct exchange under illumination between a few silicon nanocrystals (ncs-Si) embedded in an oxide tunnel layer (SiO_x = 1.5) and the tunnel oxide traps levels for a single electron photodetector (photo-SET or nanopixel). At first place, the presence of a photocurrent limited in the inversion zone under illumination in the I–V curves confirms the creation of a pair electron/hole (e–h) at high energy. This photogenerated charge carriers can be trapped in the oxide. Using the capacitance-voltage under illumination (the photo-CV measurements) we show a hysteresis chargement limited in the inversion area, indicating that the photo-generated charge carriers are stored at traps levels at the interface and within ncs-Si. The direct exchange of the photogenerated charge carriers between the interface traps levels and the ncs-Si contributed on the photomemory effect for 300 s for our nanopixel at room temperature.

It is shown that a three-dimensional fractal–percolation system is formed in nanomaterials of light-emitting InGaN/GaN and AlGaN/GaN structures in the presence of conducting extended defects and local inhomogeneities of the composition of the solid solutions; this system determines the electrophysical properties of light-emitting diodes fabricated on the basis of these structures. The geometry and properties of this system depend nonlinearly on the degree of disorder in the nanomaterial of the structures, on the value of the injection current, and on the rate of alloy growth.

The optical properties of elastically strained semiconductor heterostructures with InGaAs/InGaAlAs quantum wells (QWs), intended for use in the formation of the active region of lasers emitting in the spectral range 1520–1580 nm, are studied. Active regions with varied lattice mismatch between the InGaAs QWs and the InP substrate are fabricated by molecular beam epitaxy. The maximum lattice mismatch for the InGaAs QWs is +2%. The optical properties of the elastically strained InGaAlAs/InGaAs/InP heterostructures are studied by the photoluminescence method in the temperature range from 20 to 140°C at various power densities of the excitation laser. Investigation of the optical properties of InGaAlAs/InGaAs/InP experimental samples confirms the feasibility of using the developed elastically strained heterostructures for the fabrication of active regions for laser diodes with high temperature stability.

The results of studies of thin composite films of zinc and copper tetraphenylporphyrins with different fractions of fullerene C₆₀ are reported. The photoluminescence spectra are recorded, and the composition and surface morphology are analyzed by means of scanning electron microscopy. The results show a difference in the structure of films with two types of metals (Zn, Cu) entering into the complex of the porphyrin macrocycle. An additional long-wavelength photoluminescence band at 1.4 eV is detected for the first time, which is evidence of the formation of ZnTPP–C₆₀ molecular complexes from a gas-dynamic vapor flow upon condensation. In CuTPP thin films, the processes of self-assembly into nanowires 20 nm in diameter and up to 50 µm in length and the formation of nanoheterojunctions upon the addition of fullerene C₆₀ are observed. Quantum-chemical calculations in the context of density-functional theory are carried out to interpret the experimental data.

The selective laser sintering of TiO₂-film nanoparticles on a plastic conductive substrate is considered for application in flexible dye-sensitized solar cells. It is shown that the absorbed energy of the laser radiation during laser sintering promotes electrical-contact formation between TiO₂ nanoparticles without damaging the plastic conductive substrate. The choice of a near-infrared laser radiation (wavelength of 1064 nm) provides an efficient laser-sintering process. The laser-sintering method promotes a decrease in recombination losses in the TiO₂ film and an improvement in the charge-collection efficiency, which can result in an increase in the efficiency of such solar cells. Furthermore, the efficient-laser sintering method has a great potential for application in the roll-to-roll technology of the fabrication of high-efficiency flexible solar cells.

The possibility of identifying DNA oligonucleotides deposited onto the region of the edge channels of silicon nanostructures is considered. The role of various THz (terahertz) radiation harmonics of silicon nanostructures in the resonance response of oligonucleotides is analyzed. In particular, this makes it possible to compare single-stranded 100_ and 50_mer DNA oligonucleotides. A technique for the rapid identification of different oligonucleotides by measuring changes in the conductance and transverse potential difference of silicon nanostructures with microcavities, embedded in the edge channels for selecting THz radiation characteristics, is proposed.

The possibility of determining the band gap of homogeneous p–n structures from the properties of the current–voltage characteristics at two temperatures (room temperature and temperatures 30 to 50°C higher) is shown. A working formula for the calculation is derived, and practical application of the formula in determining the band gap is illustrated by the example of p–n structures based on silicon, gallium arsenide, and gallium phosphide. The results are in agreement with commonly accepted data with an accuracy of 1%. In addition, the possibility of determining the band gap of homogeneous p–n structures from the capacitance–voltage characteristics recorded at the above-indicated temperatures is shown.

Semiconductor lasers based on MOCVD-grown AlGaInAs/InP separate-confinement heterostructures are studied. It is shown that raising only the energy-gap width of AlGaInAs-waveguides without the introduction of additional barriers results in more pronounced current leakage into the cladding layers. It is found that the introduction of additional barrier layers at the waveguide–cladding-layer interface blocks current leakage into the cladding layers, but results in an increase in the internal optical loss with increasing pump current. It is experimentally demonstrated that the introduction of blocking layers makes it possible to obtain maximum values of the internal quantum efficiency of stimulated emission (92%) and continuouswave output optical power (3.2 W) in semiconductor lasers in the eye-safe wavelength range (1400–1600 nm).

The systematic features of the formation of the low-resistivity compound Cu₃Ge by low-temperature treatment of a Cu/Ge two-layer system in an atomic hydrogen flux are studied. The Cu/Ge two-layer system is deposited onto an i-GaAs substrate. Treatment of the Cu/Ge/i-GaAs system, in which the layer thicknesses are, correspondingly, 122 and 78 nm, in atomic hydrogen with a flux density of 10¹⁵ at cm² s^–1 for 2.5–10 min at room temperature induces the interdiffusion of Cu and Ge, with the formation of a polycrystalline film containing the stoichiometric Cu₃Ge phase. The film consists of vertically oriented grains 100–150 nm in size and exhibits a minimum resistivity of 4.5 µΩ cm. Variations in the time of treatment of the Cu/Ge/i-GaAs samples in atomic hydrogen affect the Cu and Ge depth distribution, the phase composition of the films, and their resistivity. Experimental observation of the synthesis of the Cu₃Ge compound at room temperature suggests that treatment in atomic hydrogen has a stimulating effect on both the diffusion of Cu and Ge and the chemical reaction of Cu₃Ge-compound formation. These processes can be activated by the energy released upon the recombination of hydrogen atoms adsorbed at the surface of the Cu/Ge/i-GaAs sample.

The conditions of the epitaxial growth of high-quality relaxed Si_1–xGe_x layers by the combined method of the sublimation molecular-beam epitaxy and vapor-phase decomposition of monogermane on a hot wire are considered. The combined growth procedure proposed provides a means for growing Si_1–xGe_x layers with a thickness of up to 2 µm and larger. At reduced growth temperatures (T_S = 325–350°C), the procedure allows the growth of Si_1–xGe_x layers with a small surface roughness (rms ≈ 2 nm) and a low density of threading dislocations. The photoluminescence intensity of Si_1–xGe_x:Er layers is significantly (more than five times) higher than the photoluminescence intensity of layers produced under standard growth conditions (T_S ≈ 500°C) and possess an external quantum efficiency estimated at a level of ~0.4%.

Nanostructured aluminum-nitride films are formed by reactive ion-plasma sputtering onto GaAs substrates with different orientations. The properties of the films are studied via structural analysis, atomic force microscopy, and infrared and visible–ultraviolet spectroscopy. The aluminum-nitride films can have a refractive index in the range of 1.6–4.0 at a wavelength of ~250 nm and an optical band gap of ~5 eV. It is shown that the morphology, surface composition, and optical characteristics of AlN/GaAs heterophase systems can be controlled using misoriented GaAs substrates.