Semiconductors

We develop the theory describing the topological electronic states on the surface of a gapless strained semiconductor arisen from the mixing of conduction and valence bands. It follows from the present theory that the strain-induced band gap in the Brillouin zone center of the semiconductor results in the surface electronic states with the Dirac linear dispersion characteristic for topologically protected states. The structure of these surface electronic states is studied analytically near their Dirac point within the formalism based on the Luttinger Hamiltonian. The results bring attention to the rich surface physics relevant for topological systems.

We theoretically analyze the frequency and damping of axisymmetric “dark” plasma mode in disk with 2D electron gas by calculating the absorption coefficient of an electromagnetic wave incident on the system. The problem is solved in a self-consistent approximation taking into account electromagnetic retardation. We use the Drude model for conductivity. The frequency of the axisymmetric plasmon depends on the electromagnetic retardation rather weakly compared with the case for infinite system since the plasma oscillations have zero dipole moment. The linewidth of the absorption resonance strongly changes depending on the ratio between the radiative and collisional decay. It is shown, that in the absence of collisions, the radiative decay rate is determined by the quadrupole moment of the axisymmetric plasma oscillations. In the lowest order by collisional and radiative decay, the linewidth is merely sum of the decays. In general case the collisional decay interplays with the radiative one that results in increase of the quality factor of the resonance.

Spontaneous emission in a vertical cavity surface emitting laser (VCSEL) have been studied. It was estimated that a significant part of the spontaneous emission (~1/3) in a conventional VCSEL goes into a waveguide mode that propagates in the plane of the Bragg mirrors. The theoretical study of a recently proposed anti-waveguide VCSEL design is presented and the damping of the waveguide mode emission is shown. Moreover we calculate that 3λ/2-cavity AVCSEL has almost the same gain value for vertical mode as conventional λ-VCSEL despite the higher cavity width due to the light redistribution effect while λ/2-cavity AVCSEL has 19% higher gain.

A new optical sensor system based on an array of dielectric aluminum oxide microspheres covered with a layer of chitosan polymer with molecules of a luminescent chemosensitive receptor was developed for determination of ammonia ion concentration in air. The photonic nanojet effect, i.e. strong electromagnetic field local enhancement, was implemented for most effective excitation of the sensitive layer. By using of chitosan swelling polymer the effect of refractive index variation on photonic nanojets excited sensor response was demonstrated. It was shown that under certain conditions such as microsphere diameter and refractive index contrast a detection limit of chemosensitive receptor increases in three times.

Atomic force microscopy is a unique technique for probing the mechanical properties of nanostructures. Using the bending-based test method, one can measure the Young’s modulus of the material of a suspended object, a nanobridge. This article presents a new, improved version of the bending-based test method. A special algorithm is elaborated to establish the nanobridge span length and to identify the boundary conditions of the nanobridge fixation. In particular, to realize this algorithm we propose a model of the beam with transformable boundary conditions (between clamped and supported beam model cases) varied by the only fitting parameter. To illustrate the main features of the developed bending-based test method application, the Young’s modulus measurements of mineral chrysotile nanoscrolls, forming the nanobridges across the pores of a track membrane, are presented.

Systematic studies of the effect of the electron concentration on the Raman spectra of single-layer graphene films have been carried out. The samples were grown by thermal destruction of the Si-face of the 4H-SiC substrate. Analysis of the results led us to the conclusion that for the correct estimation of the electron concentration and strain values in graphene using Raman spectroscopy data it is necessary to take into account the value of the Fermi velocity in the graphene layer. This conclusion is valid for graphene on any other substrate as well, since the Fermi velocity in graphene depends on the dielectric constant of the substrate.

Promising semiconductor devices, such as quantum-cascade lasers and similar to them heterostructures with multiple strongly-coupled quantum wells, may contain hundreds and even thousands of layers. Independent methods of photoluminescence (PL) and transmission electron microscopy (TEM), along with X-ray diffractometry and reflectometry, make it possible to characterize super-multiperiod (SMP—superlattices with 100 and more periods) structures and to determine, with high accuracy, the thicknesses of layers, the roughness/diffusion of interfaces and the composition of solid solutions. The difference between the expected and measured using PL and TEM thicknesses of the layers of GaAs/AlGaAs samples synthesized by molecular beam epitaxy was 5–10%. The SMP structures are characterized by stable thicknesses of the layers in depth and sharp boundaries with a width of transition interfaces of the order of several Å that is consistent with the X-ray analysis. Based on the data obtained on thicknesses of layers, by comparing the theoretical and experimental values of photoluminescence intensities it is possible to accurately determine the composition of Al_0.3Ga_0.7As which corresponds to the X-ray diffraction data.

A highly defective ~10-nm-thick layer was fabricated in a high vacuum by 2.5 keV Ar⁺ ion bombardment of the n-GaAs surface. Valence band photoelectron spectra showed a p-type conductivity of the layer arising due to the high concentration of mechanically created point defects (p-centers). J–V characteristics measured ex situ for the structure consisting of the irradiated p-layer on the n-type substrate revealed a diode effect. Analysis of the data attributes the effect to the formation of a specific p–n junction. Thereby, we demonstrated that Ar⁺ ion bombardment of the n-GaAs surface results in that a nanostructure with the p–n junction properties is formed. The p–n junction under consideration seems to deserve further study and possible application since it can be formed in high-vacuum clean conditions directly by exposure to a low-energy Ar⁺ ion beam without wet lithography.

Density functional theory calculations are performed for the electronic band structures and optical absorption spectra of the zigzag nanoribbons and armchair nanotubes of graphene and hexagonal boron nitride as well as hybrid tubular structures obtained by embedding two dimer lines of B and N atoms into an armchair nanotube. Linear correlation coefficient analysis is carried out to quantitatively investigate relations between energies of absorption resonances in these tube-ribbon pairs. Despite the large disparity in the energy band gaps of some of these structures, our results show a high degree of correlation (r > 0.85 with >95% confidence level) between them.

The article presents the results of an experimental study of the thermo-optical properties of a polymer composite nanomaterial based on 10 wt % CdS-LDPE. It is shown that the temperature dependences of the refractive index have a hysteresis in the heating-cooling cycle. The thermo-optical coefficients of the material during its heating and cooling, which had a negative value from –0.00026 to –0.0076 1/K were estimated. In the temperature range of 55–75°C, a strong nonlinearity of the temperature dependences of the refractive index of the width and energy gap of the material was observed. With increasing temperature up to 65°C, a decrease in the width of the forbidden zone was observed, and at temperatures above 65°C an increase in the width of the energy gap was observed. The maximum change in the band gap was about 0.2 eV. A strong change in the band gap in the temperature range 55–75°C is associated with a phase transition in the material. The detected thermo-optical properties of the polymer composite nanomaterial can be useful in the development of nanophotonic information-processing devices.

In the present study, the hybrid photovoltaic devices based on PbS quantum dots (QDs) and poly(3-hexylthiophene-2,5diyl) have been fabricated. The effect of the insertion of [6,6]-phenyl C₇₁ butyric acid methyl ester (PCBM) into the hybrid blend has been investigated and analyzed by using light and dark current density-voltage measurement, and impedance spectroscopy: capacitance-frequency and capacitance-voltage measurement. It is demonstrated that the introduction of PCBM into solar cells improves their efficiency with the optimal PCBM concentration of 3 wt %. It is shown that the main contribution in limitation of solar cells efficiency is brought by shallow traps induced by air exposure.

Lateral photovoltaic effect in metal/insulator/semiconductor hybrid structures is a significant phenomenon for spintronics, as it establishes the interplay between the optical irradiation, electronic transport and spin-dependent properties of carriers. In present work we investigated photovoltaic phenomena in Mn/SiO₂/n-Si MOS structure. The sample was prepared on a single-crystal n-Si (phosphorus-doped) substrate. The SiO₂ layer with thickness of 1.5 nm was formed on the substrate surface by a chemical method. Manganese film with thickness of 15nm was deposited by thermal evaporation in ultrahigh vacuum in the “Angara” chamber. It was observed that at T < 45 K the values of lateral and transversal photovoltage non-monotonically depend on the temperature and such dependences show complex behavior. Features of the photovoltage dependence on temperature, in the region above 20 K are explained by the change of carriers’ mobility and the competition between carriers’ drift velocity in the electric field of the space-charge region and their diffusion rate in the transverse and lateral directions. Below 20 K, the main contribution into the photovoltage is given by hot electrons injected from surface states levels to the conduction band. A strong magnetic field influence on the photovoltage below 20 K was observed. We associate it with the Lorenz force effect on the hot electrons, although we also don’t exclude the presence of mechanisms caused by spin-dependent scattering and recombination of hot electrons at occupied donor states.

This paper presents the results of the study the transport properties of the SOI-based structure. Measurements were carried out on an alternating current with an external magnetic field in a wide temperature range. The influence of the magnetic field was found. We associate this effect with the influence on the surface states located at the interface, this appears as a change of the energy of their levels. This effect is enhanced by the nanoscale of the silicon channel.

Current–voltage characteristics were studied for Pt/diamond-like C/Pt structures based on thin (20 nm) diamond-like carbon films, when the ratio between carbon phases with sp² and sp³ hybridization was controlled by the film growth conditions at the high-frequency diode sputtering method. The effects of resistive switching from the initial high-resistive state to a low-resistive state at an applied voltage of V ~ 3 V and reverse switching at V ~ 0 are detected. These effects are symmetric with respect to change in voltage polarity and are associated with the change in the hybridization type of local carbon regions, which causes switching from the high- to low-resistive state in strong (~10⁶ V/cm) fields because of sp³ → sp² transitions and reverse switching in the field absence. The high-to-low resistance ratio reaches ~50.