Том 50, № 7 (2016)

The specific features of oxygen dissolution in CdS using the example of single crystals grown by the gas-transport method with deviations from stoichiometry at 1100°C are considered. The effect of various types of intrinsic point defects in crystals of different composition on the form in the presence of oxygen is analyzed. It is shown that the most stable composition thermodynamically is that corresponding to nonstoichiometric “self-activated cadmium sulphide” stabilized with oxygen.

The crystal and electronic structure and also the energy and kinetic properties of n-VFeSb semiconductor heavily doped with the Ti acceptor impurity are investigated in the temperature and Ti concentration ranges of T = 4.2–400 K and N_A^Ti ≈ 9.5 × 10¹⁹–3.6 × 10²¹ cm^–3 (x = 0.005–0.20), respectively. The complex mechanism of the generation of acceptor and donor structural defects is established. It is demonstrated that the presence of vacancies at Sb atomic sites in n-VFeSb gives rise to donor structural defects (“a priori doping”). Substitution of the Ti dopant for V in VFeSb leads simultaneously to the generation of acceptortype structural defects, a decrease in the number of donor defects, and their removal in the concentration range of 0 ≤ x ≤ 0.03 via the occupation of vacancies by Sb atoms, and the generation of donor defects due to the occurrence of vacancies and an increase in their number. The result obtained underlies the technique for fabricating new n-VFeSb-based thermoelectric materials. The results are discussed in the context of the Shklovsky–Efros model for a heavily doped compensated semiconductor.

Using emission ^119mmSn(^119mSn) and ¹¹⁹Sb(^119mSn) Mössbauer spectroscopy, it is shown that impurity tin atoms in PbS_zSe_1–z alloys substitute lead atoms and are two-electron donors with negative correlation energy (U^– centers). It is found that the energy levels related to impurity tin atoms are in the lower half of the band gap at z ≥ 0.5 against the background of allowed valence-band states at z ≤ 0.4. The electron exchange between neutral and doubly ionized tin U^– centers in partially compensated Pb_0.99Sn_0.005Na_0.005S_zSe_1–z alloys is studied. The activation energy of this process decreases from 0.111(5) eV for a composition with z = 1 to 0.049(5) eV for compositions with c ≤ 0. For all z, the exchange is implemented via the simultaneous transfer of two electrons using delocalized valence-band states.

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 density-of-states spectra and the parameters of levels of electron states in locally chosen surface micrograins of indium antimonide and arsenide and gallium arsenide are studied with a tunneling electron microscope in the field-emission mode of measurements. By correlating the current–voltage characteristics with the formula for the probability of emission via levels, the activation energies of the levels (ψ) and the lifetimes of electrons at the levels (τ) are determined. Two types of levels for electron localization are identified. These are levels in the micrograin bulk (ψ ≈ 0.75, 1.15, and 1.59 eV for n-InSb, n-InAs, and n-GaAs, respectively; τ ~ 10^–8–10^–7 s) and in the surface region of an i-InSb micrograin (ψ ~ 0.73, 1.33, 1.85, 2.15, 5.1 eV; τ ≈ 5 × 10^–8–3 × 10^–7 s). A physical model involving the Coulomb-interaction-induced localization of light electrons and their size quantization determined by the electron effective mass, energy, and concentration and by the surface curvature of the micrograin is proposed.

The specific features of the cathodoluminescence (CL) spectra in AlInGaN heterostructures, caused by the influence of phase separation and internal electric fields, observed at varied CL excitation density, are studied. It is shown that the evolution of the CL spectrum and the variation in the spectral position of emission lines of nanoscale layers with current density in the primary electron beam makes it possible to identify the occurrence of phase separation in the layer and, in the absence of this separation, to estimate the electric-field strength in the active region of the structure.

The terahertz electroluminescence from Cd_0.7Hg_0.3Te/HgTe quantum wells with an inverted band structure in lateral electric fields is experimentally detected and studied. The emission-spectrum maximum for wells 6.5 and 7 nm wide is near 6 meV which corresponds to interband optical transitions. The emission is explained by state depletion in the valence band and conduction band filling due to Zener tunneling, which is confirmed by power-law current–voltage characteristics.

An experimental study of the capacitance–voltage (C–V) characteristics and deep-level transient spectroscopy (DLTS) of p⁺–p⁰–i–n⁰ structures based on undoped GaAs, grown by liquid-phase epitaxy at two crystallization-onset temperatures T_o (950 and 850°C), with optical illumination switched off and on, are performed. It is shown that the p⁰, i, and n⁰ layers of epitaxial structures are characterized by the presence of defects with deep donor- and acceptor-type levels in concentrations comparable with those of shallow donors and acceptors. Interface states are found, which manifest themselves in the C–V characteristics at different measurement temperatures and optical illumination; these states form an additive constant. A distinct temperature dependence of the steady-state capacitance of the structures is revealed. It is found that the injection of minority carriers under an applied positive filling pulse and optical recharging lead to modification of the structure and, correspondingly, the DLTS spectra of the p⁺–p⁰–i–n⁰ structures. It is revealed that the p⁺–p⁰–i–n⁰ GaAs structures grown at T_o = 850°C are characterized by a lack of interface states and that the recharging of acceptor-type deep traps under illumination does not change the C–V characteristics. The conventionally measured DLTS spectra reveal the presence of two hole traps: HL5 and HL2, which are typical of GaAs layers.

Silicon nanocrystals are formed in the i layers of p–i–n structures based on a-Si:H using pulsed laser annealing. An excimer XeCl laser with a wavelength of 308 nm and a pulse duration of 15 ns is used. The laser fluence is varied from 100 (below the melting threshold) to 250 mJ/cm² (above the threshold). The nanocrystal sizes are estimated by analyzing Raman spectra using the phonon confinement model. The average is from 2.5 to 3.5 nm, depending on the laser-annealing parameters. Current–voltage measurements show that the fabricated p–i–n structures possess diode characteristics. An electroluminescence signal in the infrared (IR) range is detected for the p–i–n structures with Si nanocrystals; the peak position (0.9–1 eV) varies with the laser-annealing parameters. Radiative transitions are presumably related to the nanocrystal–amorphous-matrix interface states. The proposed approach can be used to produce light-emitting diodes on non-refractory substrates.

The I–V characteristics obtained for layers of chalcogenide vitreous semiconductor of the Ge–Sb–Te system in the current-generator mode are investigated. The instability region, i.e., the region of conductivity oscillations, observed in the case of the switching effect under conditions of a set current is revealed. The key parameters describing these oscillations and the conditions of their occurrence are investigated in detail. Analysis of the obtained data shows that, to explain the oscillations in the instability region, it is necessary to take into account an increase in the current density and the process of heat exchange between the current filament that arises in the film upon switching and the environment.

The nonlinear optical response of a colloidal solution of planar CdSe semiconductor nanocrystals (nanoplatelets) is studied for the first time. The nonlinear optical response of these nanoparticles is compared to that of spherical CdSe nanocrystals (quantum dots). The photoinduced nonlinearity is attributed to the optical generation of long-lived charge carriers in the nanoobjects under study. It is shown that, upon the exposure of a cell with the solution of nanoparticles to focused continuous-wave (cw) laser radiation with a wavelength of 473 nm, the nonlinear optical responses of CdSe nanoplatelets and quantum dots are somewhat different at identical optical densities at the above-indicated wavelength. The differences are supposedly associated with a higher diffusion rate of spherical nanoparticles in the solution because of their smaller size compared to that of nanoplatelets.

The structure of graphene layers grown by sublimation on a 6H-SiC (000\(\bar 1\)) substrate surface is studied by electron diffraction depending on the sublimation temperature and substrate-surface pretreatment method. It is shown that the use of polishing sublimation etching of the substrate before thermal destruction at a temperature of 1350°C on the substrate surface results in the formation of single-crystal graphene domains with graphene-lattice rotation by 30° with respect to the SiC lattice and a small fraction of amorphous domains. An increase in the temperature to 1500°C leads to the partial formation of a polycrystalline graphene phase with turbostratic structure while retaining the preferred orientation of graphene crystallites as at 1350°C. The use of pregrowth annealing before thermal destruction makes it possible to grow a graphene film with a more ordered and homogeneous structure without inclusions of amorphous and polycrystalline components. The preferred orientation of graphene domains in the film remains unchanged.

The possibility of optimizing high-voltage hybrid SIT–MOP transistors (HSMTs) by means of a local reduction in lifetime near the anode emitter and/ or by decreasing its injection efficiency by three different methods is studied using two-dimensional numerical simulation. It is shown that all four optimization methods are equivalent from the physical point of view and make it possible to decrease the turn-off energy loss E_off by 30–40%, as in insulated gate bipolar transistors (IGBTs). However, all other conditions being equal, the energy E_off in the HSMT appeared 15–35% lower than in equivalent trench IGBTs.

The phenomenon of current spreading is essential for concentrator solar cells since it limits the conversion efficiency at high sunlight-concentration ratios. A model, which describes the regularities of the above phenomenon, is proposed and developed. The model uses a stylized representation of current lines and, respectively, of current tubes; it includes two resistive parameters accounting for the variable lateral (horizontal) component and the constant vertical component of the resistance of each tube. In the model the fact that the thickness of the spreading region is much less than the distance between the contact grid strips is taken into account. The calculated current—voltage (I–V) characteristics of a solar cell in the resistive and a nonresistive cases are obtained. The spreading-resistance I–V characteristic obtained by the voltage subtraction of these characteristics is nonlinear and depends on the photogenerated current. Thus, the equivalent electrical circuit of a solar cell includes a lumped nonlinear resistance, which depends parametrically on the photogenerated current. The comparison of experimental and calculated I–V characteristics by the example of Ge, GaAs, and GaInP solar cells is performed and both resistive parameters of the model are determined. The model describes correctly the regularities of spreading in single-junction solar cells and can be extended to multijunction solar cells.

The first results of the liquid-phase epitaxial growth of quantum dots in the InSb/GaSb system and atomic-force microscopy data on the structural characteristics of the quantum dots are reported. It is shown that the surface density, shape, and size of nanoislands depend on the deposition temperature and the chemical properties of the matrix surface. Arrays of InSb quantum dots on GaSb (001) substrates are produced in the temperature range T = 450–465°C. The average dimensions of the quantum dots correspond to a height of h = 3 nm and a base dimension of D = 30 nm; the surface density is 3 × 10⁹ cm^–2.

A new method for fabricating transparent conducting coatings based on indium-tin oxide (ITO) with a controlled refractive index is proposed. This method implies the successive deposition of material by electron-beam evaporation and magnetron sputtering. Sputtered coatings with different densities (and, correspondingly, different refractive indices) can be obtained by varying the ratio of the mass fractions of material deposited by different methods. As an example, films with effective refractive indices of 1.2, 1.4, and 1.7 in the wavelength range of 440–460 nm are fabricated. Two-layer ITO coatings with controlled refractive indices of the layers are also formed by the proposed method. Thus, multilayer transparent conducting coatings with desired optical parameters can be produced.