Table of contents

Editorial Preface

The 20^th Russian Young Conference on Physics of Semiconductors and Nanostructures, Opto- and Nanoelectronics was held in Saint Petersburg on November 26 – 30, 2018. It was organized by Peter the Great St. Petersburg Polytechnic University.

The program of the Conference included semiconductor technology, heterostructures with quantum wells and quantum dots, opto- and nanoelectronic devices and new materials. A large number of participants with more than 200 attendees from many regions of Russia provided a perfect platform for the valuable discussions between students and experienced scientists.

Conference included two invited talks given by leading scientists devoted to actual problems and major advances in physics and technology. The keynote speakers were A.T. Burkov from Ioffe Physical Technical Institute (presentation "Thermoelectric effects and energy converters") and K.V. Kavokin from Spin Optics Laboratory at St. Petersburg University (presentation "Spin physics: from semiconductors to birds"). Students, graduate and postgraduate students presented their results on plenary and poster sessions. The total number of accepted papers published in Russian (the official conference language) was 123. Here we publish 38 of them in English.

List of Editors, Winners of the E.F. Gross Prize for the best presentation in semiconductor optics and Winners of the competition for the best report are available in this pdf.

All papers published in this volume of Journal of Physics: Conference Series have been peer reviewed through processes administered by the proceedings Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

Recombination activity of grain boundaries in Fe²⁺:ZnSe was studied. The specific lines in the spectra (425-725 nm) were identified. The distributions of exciton and defective-impurity centers were observed in the bulk and grain boundaries of the sample. A hypothesis explaining the observed characteristics of luminescence from grain boundaries is proposed.

Photoluminescence (PL) polarization was experimentally studied for the samples with thick InGaAs epitaxial layers with different concentrations of donors in the transverse magnetic field (the Hanle effect). A well-pronounced W-structure observed in the set of samples under study indicate a strong electron-nuclear spin interaction. Analysis of the Hanle curves measured at different pumping powers provided important parameters of the electron-nuclear spin system, such as the Knight field and kinetic local field. We obtained characteristic values of the electron spin relaxation time by modeling the Hanle curves measured with fast modulation of the excitation polarization at different pumping powers,.

Nonlinear light absorption in variously thick films of Bi_2-xSb_xTe_3-ySe_y quaternary solid solution crystalline grown on sapphire substrates by means of MOCVD [1] was investigated. We measured the transmission coefficient of laser pulses through samples as a function of the incident laser radiation intensity. A pulsed Nd³⁺: YAG laser was used as a source of laser radiation (wavelength λ=1064 nm, duration of pulses τ≈35 ps, pulse repetition period 7 ns). The nonlinear absorption in Bi_1.25Sb_0.75Te_1.65Se_1.35 films was observed starting from the saturation intensity I_sat≈50 MW/cm². The data can be explained by a process of filling states in a quantum well [2]. We have not observed the nonlinear absorption in two of the investigated films and, moreover, these films were destroyed at a high radiation intensity. An earlier work [3] demonstrated a change in the type of conductivity of a semiconductor film to metallic at a high radiation intensity. Since metal films are known to be destroyed at high radiation intensities, this phenomenon can explain the destruction of the investigated films. It was also found that a nonlinear change of the transmission of quaternary solid solutions can occur only for compositions with a Sb content of about 2 times less Sb than Bi, and the film thickness should not exceed 12 nanometers.

A free-standing bulk gallium nitride layer with a thickness of 365 μm and a diameter of 50 mm was obtained by hydride vapor phase epitaxy on a sapphire substrate with a carbon buffer layer. The carbon buffer layer was deposited by thermal decomposition of methane in situ in the same process with the growth of a bulk GaN layer. The bulk GaN layer grown on the carbon buffer layer self-separated from the sapphire substrate during the cooling after the growth. The dislocation density was 8 · 10⁶ cm⁻². The (0002) X-Ray rocking curve full width at half maximum was 164 arcsec.

In this paper we investigate light emission in a tunnel junction between a thin gold film on a glass substrate and a gold-coated tungsten tip of a scanning-tunneling microscope probe. The experiments show that the size of grains in the gold film surface dramatically affects the intensity of emissions. We demonstrate that a decrease in the grain aspect ratio provides an increase in the quantum yield of the tunnel-junction emission.

Results of stress analysis of macroscopic growth defects (hillocks) on the surface of homoepitaxial diamond films produced by microwave plasma assisted CVD are presented. Local stress was evaluated by analyzing the position and splitting of the 1332 cm⁻¹ diamond Raman peak to build 2D stress maps for pyramidal hillocks and hillocks with a flat top.

A feed-forward artificial neural network was employed in order to predict the growth rate and composition of Al_xGa_1-xN metalorganic vapor deposition-grown layers depending on the flow of a precursor gas (trimethylaluminum, trimethylgallium, and ammonia). The results showed a good agreement with experimental data. The obtained dependencies seem to be reasonable in the context of well-known effects: the presence of parasitic reactions between trimethylaluminum and ammonia and GaN decomposition at low ammonia (high hydrogen) flow.

We demonstrated that the size and the distribution density of lead-containing inclusions in the topmost layer deposited on CdS depend on the pH of a Pb(NO₃)₂ solution, which is used to fabricate a coating for CdS by Langmuir-Blodgett technology. The formation of a nanostructured lead-containing film was controlled by Brewster angle microscopy and scanning electron microscopy. The best parameters of the photosensitivity and radiation hardness of a CdS semiconductor with a nanostructured lead-containing surface layer were achieved using a solution of pH ≈ 8 and a Pb(NO₃)₂ concentration of 10⁻³ mol/L. We performed modeling of the optimal size and location of lead-containing clusters.

In this paper, low-dimensional silicon carbide (SiC) obtained on the basis of SiC monolayers with a stoichiometric composition of 1:1 and a number of layers from 1 to 3 has been studied based on the quantum-chemical analysis of the structure and charge properties of this material. The redistribution of the electron density registered in a stack of graphene-like SiC layers allowed us to propose a method of identification of low-dimensional silicon carbide structures. The analysis of geometric and energy parameters made it possible to suggest the sustainable existence of two structural types of 2D-SiC, which differ in the way they are positioned within the layers. As a result of the analysis of the effective charge magnitude, the correlation between this parameter and the geometry of the optimized structure was established. It is proved that taking into account the charge properties of low-dimensional silicon carbide makes it possible to trace the structural changes in the system, identify a specific allotrope, and establish the order of the monolayers.

GaAs thermal smoothing at temperatures T ≤ 650°C under conditions close to equilibrium yields surfaces with atomically smooth terraces separated by steps of monatomic height. At higher temperatures T ≥ 700°C, surface smoothing changes to roughening. In the present paper, thermal roughening of a step-terraced surface caused by atomic step flow around step pinning centers is studied using Monte-Carlo simulation. It is proved that the Schwöbel barrier is necessary for step bunching in the presence of step pinning centers. The lower limit of the Schwöbel barrier E_S = 0.4 eV is estimated for the GaAs(001) surface.

The relaxational kinetics of the photoemission from the Cs/GaAs and GaAs(Cs,O) surfaces at elevated temperatures is studied. It was found that heating to moderate temperatures of about 150°C leads to substantial changes in the amplitude and shape of the Cs coverage dependences of the photoemission current, along with the changes of its relaxational kinetics after the cesium deposition. After the oxygen exposure on the GaAs(Cs,O) surface, a photoemission relaxational increase is observed due to changes in the affinity. The temperature dependences of the amplitude and decay time of the relaxational kinetics on the Cs/GaAs and GaAs(Cs,O) surfaces were obtained.

Controlled formation of metastable phases is one of the challenges of the physics of crystal growth. Gold-catalyzed nanowires are considered to be one of the most convenient systems to study this phenomenon. Studies of antimonide-based nanowires indicate that they preferentially crystallize in the zinc blende crystal structure rather than wurtzite, which is common in other III-V nanowire materials. Here we propose a new approach to the formation of antimonide nanowire segments in the metastable wurtzite phase and support it with theoretical results. The hexagonal crystal phase is stabilized due to the elastic strain. We suggest that this approach can be applied to other III-V nanowires as well.

The in-plane spatial distribution of photoluminescence (PL) intensity in two-dimensional layers of a type-II (buffer Si_1-yGe_y)/tSi/sSi_1-xGe_x/tSi/(cap Si_1-yGe_y) heterostructure is investigated at liquid-helium temperatures at high excitation levels. It is found that the PL spectra recorded at the excitation spot and the edge of the sample are different. This work examines the dependences of the PL spectra and intensity both on the distance between the observation point and a fixed excitation spot and on the distance between the excitation spot and a fixed observation point at the edge of the sample. The reasons for the observed behavior of the PL are determined.

We report the studies on optical properties of a GaN/AlGaN heterostructure with a surface metal grating. The fabricated structures were optimized for the observation of 2D plasmon resonances in the spectral range of 2–5 THz. The spectra of the equilibrium optical transmission were experimentally investigated and the 2D plasmon resonance was found. The current-voltage characteristics of the grating sample and a reference sample without grating were measured and the dependence of the hot 2D electron temperature on electric field was established. Terahertz electroluminescence was studied in both samples in the sensitivity band of the Ge:Ga detector in electric fields of up to 400 V/cm. It has been shown that, due to the contribution of nonequilibrium 2D plasmons, the integral photoresponse signal for the sample with a surface metal grating increases 2–4 times as compared with the sample without grating, where the terahertz emission is due only to hot 2D electrons.

It has been clarified that the distribution of peak EQE values over wavelengths in the range of currents 0.1 − 1000 mA in InGaN/GaN MQW contains information on the In distribution. The correlation has been found between the EQE values of green polar InGaN/GaN LEDs and the In distribution and redistribution in InGaN/GaN MQW under the influence of growth conditions, accelerated aging, and electron irradiation.

This work demonstrates the possibility of growing Type-II InAs/GaSb superlattices by MOCVD. The Type-II InAs/GaSb superlattices consisting of 20 pairs of alternating InAs and GaSb layers of equal thickness (1 nm / 2 nm) were grown at a temperature of 500°C. The obtained structures were studied by transmission electron microscopy and electroluminescence. The electroluminescence spectra demonstrated a maximum at about 0.25 eV.

Structures based on AlInGaN solid solutions were studied. The types of defects in them and the corresponding emission bands of photoluminescence (PL) spectra were identified. It is possible to determine the concentration of defects in the structure from the ratio of the intensities of the emission line, corresponding to the width of the forbidden zone in GaN, and the emission lines associated with defects.

The ground state of an exciton in the GaAs-based double quantum well structure in an external electric field is calculated by the direct numerical solution of the three-dimensional Schrödinger equation. A formation of the indirect exciton is demonstrated by the study of the localization of the exciton wave function in the double quantum well structure for different electric field strengths. A relatively slow radiative decay rate of the indirect exciton is observed.

A new approach is proposed for calculating the electronic band structure of silicene in terms of the density functional theory using a model pseudopotential with a small number of adjustable parameters. The approach is based on the self-consistent solution of the Kohn-Sham equation, in which the true potential created by nuclei and all electrons is replaced by an effective potential acting on valence electrons only. As an initial approximation for this potential, it is proposed to use the pseudopotential of bulk silicon, which best determines its band structure. The dispersion of electrons in the most symmetrical directions of the Brillouin zone of silicene has been calculated. It is shown that the band structure of this material determined in the framework of this approach is in qualitative agreement with the results obtained using first-principle methods, as well as the tight-binding method and k·p perturbation theory.

The paper considers the effect of fullerene concentration in active layers on the spectral characteristics of organic photosensitive structures based on the ZnPc:C₆₀ system. The studied samples were created using the vacuum thermal evaporation method. Absorption, transmission, and photosensitivity spectra were studied for the created samples.

The electrical properties of unpassivated wurtzite indium phosphide nanowires were studied by conductive atomic force microscopy. I-V curves of single nanowires with different diameters were obtained. For relatively thin nanowires, surface states caused full depletion of the nanowire volume and low conductivity. With increasing diameter, the conductivity increased significantly due to formation of an undepleted core in the nanowire. A surface state density (∼6·10¹¹ cm⁻²eV⁻¹) was estimated from the radius and doping level of the nanowire. Additionally, experiments were performed to determine whether a piezoelectric current is generated while bending the nanowires. No current was detected.

Despite the great advances in synthesis of III-V ternary nanowires over the past decade, there is a lack of understanding and detailed information about Al_1-xIn_xAs nanowire growth. Recently, we have developed an analytical approach for understanding the composition of ternary nanowires nucleating from a quaternary liquid melt. Herein, we use our model to describe the formation of Al_1-xIn_xAs nanowires and tuning their composition within the nucleation-limited regime of nanowire growth via the vapor-liquid-solid mechanism. In particular, we examine the influence of growth temperature and the total concentrations of group III elements on the liquid-solid composition dependence. The obtained results may be useful for Al_1-xIn_xAs nanowire growth.

It has been found that the conductivity of a polycrystalline fullerene C₆₀ thin film increases by several orders of magnitude after lengthy exposure to an electric field of ∼ 10⁶ V/cm. This effect is called electroforming. After electroforming, the current-voltage characteristic shows high current stability and reproducibility of results. An increase in capacitance and a decrease in resistance by 4 orders is observed in a narrow voltage range of 0.1-1 V, which is the effect of reversible resistive switching. In fields with a strength of 10⁵-10⁶ V/cm, the I-V characteristic of molded films is determined by the current with a limited space charge. Trap concentrations are defined as N_t-1 ≈ 2.3· 10²¹ m⁻³ and N_t-2 ≈ 5.8·10²⁰ m⁻³ and their depth E_t ≈ 0.17 eV for polycrystalline C₆₀ films with thicknesses of 250 and 500 nm, respectively.

A kinetic model for contact angle stability is introduced for the non-stationary case with time-dependent fluxes in a molecular-beam epitaxy chamber. We present numerical simulations for different regimes of the nanowire growth process.

Conditions and mechanisms of controlled variation of the magnetic anisotropy by means of annealing have been determined for GaMnSb films containing MnSb nanoparticles. For this purpose, the temperature and magnetic-field dependences of the magnetic moments of samples before and after annealing were measured using a SQUID magnetometer. It is established that the heat treatment of GaMnSb films leads to a significant values of characteristics determined by the magnetic anisotropy, including the growth of blocking temperature (from 95 to 390 K) and the magnetic anisotropy field (from 339 to 630 Oe).

The contributions of isolated Fe³⁺ ions and the ferrimagnetic subsystem to the total magnetization of Fe₃O₄ nanowires were separated. The magnetic anisotropy of nanowires was determined. The value of the magnetic anisotropy is smaller than the expected value of the shape anisotropy. The decrease in the magnetic anisotropy in nanowires can be explained by the dipole-dipole interaction between individual nanowires in the array. The Verwey transition suppression was found. A phase-inverted line corresponding to the microwave magnetoresistance was identified in the electron spin resonance spectra of nanowires.

Temperature effects of lD-dissipative tunneling for the two-well oscillator potential model within the strong dissipation limit for the case of semiconductor QDs of InAs, were analyzed using a combined atomic force and scanning tunneling microscope. The effect of thermal control for the amplitude of single peaks on the field dependence of the one-dimensional dissipative tunneling probability in the model under consideration was theoretically revealed. A qualitative comparison of the calculated field dependences of the lD-dissipative tunneling probability at a finite temperature within a strong coupling limit (with allowance for the influence of two local phonon modes) with experimental tunnel CVC was obtained. A fairly convincing agreement of theoretical and experimental curves was demonstrated. It is shown that in addition to temperature and external electric field, another important controlling parameter of dissipative tunneling is the type of thermostat matrix in which QDs have been synthesized, taking into account the number of local phonon modes involved in the tunneling process.

Femtosecond direct laser writing (DLW) in oxide glasses doped with photosensitive agents (noble metals and semiconductors) opens new routes for precise space-selective tuning of material properties and development of functional photonic devices including integrated waveguides, optical switches, and volume optical memory. In this study, we showed how DLW can be used for the spatially-selective formation of micron-sized luminescent domains in the bulk of Ag-doped and CdS-doped silicate glasses. Multiphoton ionization caused by DLW leads to local heating of glass in the focal point, which initiates the precipitation and growth of luminescent Ag nanoclusters and CdS quantum dots in the periphery of the domains. The luminescence intensity of the formed domains depends on the laser exposure parameters, such as pulse energy and number of pulses that can be used in the future for multilevel optical data recording.

The results of the experimental study of stimulated emission of a quantum-cascade laser with a wavelength of 7.6 μm are presented in this paper. The dependence of the intensity of laser radiation on the duty cycle was obtained. The linearity of this dependence was maintained up to a duty cycle of 40%, thus achieving the quasi-continuous mode of operation of the quantum cascade laser at 110 K.

A silicon (111) based solar cell with an emitter layer of an array of GaN nanowires was proposed, synthesized, and studied. Theoretical and experimental volt-ampere and spectral characteristics of the solar cell were compared and analyzed. The density of surface states and lifetime of minority carriers in the active region were obtained with the use of numerical modeling. A significant effect of recombination on efficiency was demonstrated. The modeling results correspond well to the experimental data obtained via the characterization of the SC prototype.

Temperature and optical power density dependences of the photovoltage at a p-GaN(Cs) photocathode surface were measured in the temperature range 90–295 K. The study demonstrated that band bending at the p-GaN(Cs) photocathode surface can be reduced by ∼ 0.5 eV without modifying the surface atomic structure. The surface photovoltage impact on the p-GaN(Cs) photocathode quantum efficiency and photoelectron energy distributions was analyzed.

AlGaN/GaN high electron mobility transistor (HEMT) heterostructures were investigated by means of the electrochemical capacitance-voltage technique. The concentration profiles of free charge carriers across the samples were experimentally obtained. The features of 2D electron gas (2DEG) appearing in GaN HEMT in relation to spontaneous and piezoelectric polarization were described in detail, along with the factors affecting the density of free charge carriers in the channel and the performance of GaN HEMTs. The 2DEG density for various technological parameters of the interface, such as the mutual orientation of the AlGaN/GaN layers and Al_xGai_-xN composition, was calculated accounting for piezoelectric effects.

The work is aimed at the study of multijunction solar cells with built-in Bragg reflectors. The possibility to use the photon structure for "locking" the secondary recombination radiation, which causes luminescent coupling between individual photoactive p-n junctions in a solar cell, is investigated. Temperature studies were carried out on the reflection spectra of a Bragg reflector, the electroluminescence spectrum of a middle wide bandgap subcell, and the spectra of the external quantum efficiency of a multijunction solar cell.

Performance of a passively Q-switched erbium fiber laser with a narrow bandgap Bi₂Te₃ semiconductor thin film used as a saturable absorber was studied experimentally. In order to obtain repetitively-pulsed lasing, a Q-switcher was fabricated as a section of the standard SMF-28 optical fiber with a short (∼4 mm in length) chemically etched out tapered section laterally covered with the bismuth telluride film. This Q-switcher was installed in a specially designed circuit of a ring-type erbium fiber laser pumped by a laser diode at 975 nm wavelength. The fiber diameter in the middle of the tapered section was 10-15 μm. Two types of taper coverage were studied: Bi₂Te₃ flakes suspended in isopropanol and a ZnTe/Bi₂Te₃ uniform polycrystalline thin-film heterostructure synthesized on a lateral surface of a tapered fiber section via MOCVD. In the first case, we observed laser oscillation at a wavelength of 1565 nm as a train of 7-26 μs pulses with a 86-50 μs repetition period at pump powers varying from 26 to 75 mW. In the second case, CW oscillation in the circuit appeared at a pump power of 30 mW while conversion to the repetitively-pulsed mode with a kilohertz repetition rate occurred at pump powers within the range of 34-40 mW. The results obtained indicate the feasibility of effective interaction of the evanescent field with a bismuth telluride film used as a saturable absorber. This interaction is sufficient to achieve a stable repetitively-pulsed lasing in the circuits of passively Q-switched fiber lasers.

We have used the finite element method to demonstrate numerically that losses in Tamm plasmon structures can be reduced, and Tamm resonance parameters can be tuned using a sub-wavelength ring-shaped structuration of the metal layer. The axisymmetric structure consists of a GaAs/Al_0.95Ga_0.05As Bragg reflector covered with a sub-wavelength ring. The results demonstrate that the quality factor of the Tamm plasmon mode with a ring of given parameters increases substantially. In addition, we have compared the properties of the structure with two different resist materials (PMMA and CSAR 62) and demonstrated a shift in the resonance frequency of the Tamm plasmon mode for each case.

GaAs-based LED and HEMT heterostructures were investigated using numerical simulation of the self-consistent Schrödinger and Poisson equations. The depth distributions of free charge carriers were obtained by simulation and analyzed in detail, then compared to measurements made using the ECV technique.

A topological semimetal PtSn4 single crystal was grown by method of crystallization from a solution in a melt. Then the electrical resistivity and galvanomagnetic properties (magnetoresistivity and the Hall effect) were studied in the temperature range from 4.2 to 80 K and in magnetic fields up to 100 kOe. The optical measurements were carried out at room temperature. The residual resistivity is shown to be low enough and amount to ∼ 0.5 μOhm·cm. The temperature dependence of the electrical resistivity has a metallic type, increasing monotonically with temperature. A sufficiently large magnetoresistance of 750% is observed. The majority carriers are supposed to be holes with a concentration of ∼ 6.8·10²¹ cm⁻³ and mobility of ∼ 1950 cm²/Vs at T = 4.2 K as a result of the Hall effect studies. The optical properties of PtSn₄ have features characteristic of "bad" metals.

A set of monocrystalline B₂Te_3-xSe_x films of various compositions were synthesized by metalorganic vapor epitaxy. The smooth films with thicknesses of about 500 nm were grown on (0001) Al₂O₃ substrates at 465 °C using trimethylbismuth, diethyltellurium and diisopropylselenium as meta-organic precursors. The epitaxial nature of the films and their rhombohedral crystal structure are confirmed by X-ray studies and Raman spectroscopy. The elemental composition of the films was determined by energy dispersive X-ray spectrometry. Optical properties of Bi₂Te_3-xSe_x films were examined in the 260-1000 nm range by multi-angle spectroscopic ellipsometry (SE). It was shown that the optical properties of Bi₂Te_3-xSe_x films vary monotonically depending on the ratio of selenium to tellurium. It is demonstrated that SE can be used for rapid assessment of the composition of Bi₂Te_3-xSe_x films.