Том 52, № 13 (2018)

The influence of phonon focusing on phonon transport in semiconductor and dielectric crystals with different types of elastic-energy anisotropy at low temperatures is investigated. McCurdy’s effects in the thermal conductivity of elastically anisotropic crystals in the mode of Knudsen phonon gas flow are calculated. The influence of phonon focusing on the propagation and boundary scattering of phonons in samples with square and rectangular cross sections is analyzed, and a physical explanation of McCurdy’s effects is given.

The dependences of the Hall electron mobility of n-InSe single crystals grown by the Bridgman method on a sample’s technological history, temperature, electric field, doping, and illumination are experimentally investigated. It is established that at temperatures below room temperature, the dependences of the electron mobility on external factors, initial resistivity, and doping are anomalous, i.e., do not obey the theory of free carrier mobility in quasi-ordered crystalline semiconductors. The observed anomalies are attributed to partial disordering and fluctuation of the potential of free energy bands of the n-InSe single crystals and can be controlled by temperature, electric field, doping, and illumination.

Samples of a gapless HgSe semiconductor with different iron impurity concentrations are investigated. HgSe:Fe samples are examined by the electron-spin-resonance technique. Multiple resonance lines caused by unpaired spins of different origins are analyzed. The properties of electrons localized at shallow impurities are described using a hydrogen-like model. The effect of an internal field on the resonance lines is established. It is found that the conduction of HgSe is not only nonparabolic, but also nonspherical.

Luminescence spectra of Ta impurity centers in CdTe are studied for the first time. It is found that, as the 3d (Ta) electron system of centers is considered instead of the 3d (V) system, a substantial change in the characteristics of impurity-related emission is observed. Electron transitions are identified in accordance with the Tanabe–Sugano diagrams of crystal-field theory. It is established that radiative transitions occur within isolated \({\text{Ta}}_{{{\text{Cd}}}}^{{3 + }}\) centers, between levels with different spins. Temperature broadening of the zero-phonon line of tantalum is induced by the interaction of d electrons of the center with TA phonons of the crystal lattice. However, in interpreting the data on the temperature shift of the line, it is essential to take into account the hybridization of local impurity states and band states. Temperature quenching of the luminescence signal occurs with activation energies of 60 and 160 meV. The radiative lifetime of centers in CdTe:Ta is 1.5 μs.

Electrical and thermal properties of lead-telluride films from 0.3 to 2.4 μm in thickness doped with antimony and bismuth, which were synthesized by thermal evaporation in vacuum, are investigated. Contact-free measurement of the in-plane thermal diffusivity is performed by the excitation of surface transient gratings and it is shown that the developed doping technology leads to a noticeable decrease in this heat-transfer parameter when compared with undoped PbTe.

The processes of the diffusion blurring of a periodic system of GaAs quantum wells separated by AlGaAs barriers are studied by photoluminescence spectroscopy. The system is grown by molecular-beam epitaxy at a low temperature (200°C) and additionally doped with Sb and P isovalent impurities. Postgrowth annealing at the temperature 750°C for 30 min induces an increase in the energy corresponding to the photoluminescence peak of the e1–hh1 exciton state in quantum wells because of blurring of the epitaxial GaAs/AlGaAs interfaces due to enhanced Al–Ga interdiffusion in the cation sublattice. For the Al concentration profile defined by linear diffusion into quantum wells, the Schrödinger equation for electrons and holes is solved. It is found that the experimentally observed energy position of the photoluminescence peak corresponds to the Al–Ga interdiffusion length 3.4 nm and to the effective diffusion coefficient 6.3 × 10^–17 cm² s^–1 at the temperature 750°C. This value is found to be close to the corresponding value for GaAs quantum wells grown at low temperatures without additional doping with Sb and P impurities. From the results obtained in the study, it is possible to conclude that enhanced As–Sb and As–P interdiffusion in the anion sublattice only slightly influences the processes of Al–Ga interdiffusion in the cation sublattice.

Zinc tetraphenylporphyrin (ZnTPP) films and ZnTPP-based composites are promising materials of organic photonics. Porphyrins are inclined towards self-assembly and the formation of molecular ensembles or differently structured aggregates. The energies of the formation of aggregates and ordered structures and modifications of their electron structures compared to that of a free ZnTPP molecule have not been adequately explored. In the study, the first comprehensive investigation of the structure, absorption and luminescence spectra, and photoluminescence kinetics of structurally perfect ZnTPP thin films produced under quasi-equilibrium conditions in vacuum is conducted. It is shown that changes in the absorption spectra and the red shift of the luminescence spectra of films by 0.15 eV from the spectra observed for ZnTPP solutions in toluene can be interpreted as a result of the formation of an ordered thin-film structure through the dimerization of porphyrin planar molecules under nearly equilibrium conditions. The optimal geometric structure, the energy of the ground state, and the electronic spectrum of excitations of the dimerized (ZnTPP)₂ state are calculated in the context of density functional theory and time-dependent density functional theory. The energy gain on the formation of a dimer of symmetry C_i compared to two separate molecules is 0.23 eV per dimer; the HOMO–LUMO gap for the dimer is decreased by 70 meV. The radiative emission time in the ordered solid phase is an order of magnitude shorter than that in the solution in toluene and corresponds to 277 ps, which is typical of J aggregates of porphyrins.

For the chalcopyrite-like ZnSnSb₂ crystal, the equilibrium crystal-lattice parameters a = 6.2893 Å, c = 12.5975 Å, and u = 0.2314 and the band structure involving the band gap E_g = 0.43 eV are determined by ab initio calculations based on density functional theory. The phonon vibrational frequencies, the elastic constants (C₁₁ = 89.3, C₁₂ = 41.9, C₁₃ = 41.8, C₃₃ = 90.4, C₄₄ = 43.9, and C₆₆ = 44.1), the phase velocities of elastic waves, the elasticity moduli, the microhardness (2.29 GPa), and the Grüneisen elastic parameter (1.5) are calculated. The temperature dependences of the heat capacity and thermodynamic potential are considered (in the range from 20 to 633 K).

Charge accumulation in metal—oxide—semiconductor (MOS) structures with a doped and undoped polysilicon gate with Al contacts and without them under tunnel electron injection from the gate and silicon substrate is investigated. It is shown that negative charge is accumulated near the polysilicon gate irrespective of the injection polarity, and positive charge is accumulated near the silicon substrate. Negative charge also appears near the silicon substrate at large injection charges. The results are described with the help of a numerical model, in which the formation of electron traps with the deposition of Al contacts and the generation of electron traps during the recombination of free electrons with holes captured at traps is taken into account.

The results of studying the photoelectric characteristics of an nBn structure are presented. The photodiodes based on such structures may have a cutoff wavelength as high as 5 μm. Calculations based on these results show that such photodiodes have a threshold photosensitivity comparable with that of traditional analogs. An energy band diagram of the nBn structure is proposed based on experimental results and theoretical estimations; this diagram makes it possible to estimate the effect of potential barriers in the valence band of the wide-gap layer and at its boundaries with narrow-gap layers on the nBn-based photodiode sensitivity. The experimental results of studying the dependence of the photocurrent thermal-activation energy on the photodiode bias turned out to be important for developing the model.

Electrical supply sources based on photovoltaic converters and tritium radioluminescent lamps with blue and green glow are developed and fabricated. Al_xGa_{1 –x}As/n-GaAs p–n heterostructures with various band-gap widths (E_g = 1.4–1.9 eV) and compositions of the active region (x = 0.1–0.35) are prepared by low-temperature liquid-phase epitaxy. The photovoltaic converter with an area of S = 0.12 cm² upon irradiation by a green tritium lamp (wavelength λ = 550 nm) has the short-circuit current density 180 nA/cm² and output electrical power >100 nW/cm².

The region of thermal stability of the size and phase composition of silver sulfide (Ag₂S) nanoparticles is determined. Nanopowders consisting of Ag₂S nanoparticles 475–50 nm in sizes are produced by chemical deposition from aqueous solutions. To analyze the thermal stability of Ag₂S nanoparticles, nanocrystalline powders are annealed upon heating from room temperature to 453 K. Annealing at temperatures of up to 453 K does not induce the growth of nanoparticles, nor any change in their phase composition, which allows the conclusion that this temperature range is the range of thermal stability of the nanostate of silver sulfide.

The results of studying the applicability of various etchants for the precision wet etching of structures of monolithic optoelectronic devices containing GaPNAs layers are presented. It is shown that an etchant based on potassium iodide and hydrochloric acid is best suited for this purpose. The presence of nitrogen (up to 4%) and arsenic in the semiconductor composition does not greatly affect the etchant action but requires additional calibration experiments to refine the etching rate in each particular case. Examples of the practical application of precision etching to measure the characteristics of GaPNAs-based solar cells are presented.