Vol 52, No 7 (2018)
- Year: 2018
- Articles: 25
- URL: https://journals.rcsi.science/1063-7826/issue/view/12670
Nonelectronic Properties of Semiconductors (Atomic Structure, Diffusion)
Matrix Calculation of the Spectral Characteristics of AII–BVI Semiconductors Doped with Iron-Group Ions
Abstract
A method for the matrix calculation of the spectral characteristics of AII–BVI semiconductors doped with iron-group ions is proposed, which takes into account all possible interactions in the ion and the effect of intramolecular surrounding fields of different symmetries. A technique for calculating the oscillator strength on the basis of eigenfunctions of the resulting states of 3d7, 3d8, and 3d6 electron configurations is developed. The spectral regularities of the Co2+, Ni2+, and Fe2+ energy structures for the ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, and CdTe materials are studied. All possible spectral bands in iron-group ions implanted in AII–BVI semiconductors in the range of 1.3–3 μm are established. The possibility of implementing the laser effect on these materials is estimated.
Electronic Properties of Semiconductors
Kinetics of the Variation in the Magnetic Impurity Ion Concentration in Pb1–x–ySnxVyTe Alloys upon Doping
Abstract
The field and temperature dependences of the magnetization (magnetic fields B ≤ 7.5 T, temperatures T = 2.0–75 K) of samples from a Pb1–x–ySnxVyTe (x = 0.08, y = 0.01) single-crystal ingot synthesized by the Bridgman–Stockbarger method. It is established that the sample magnetization contains two main contributions, notably, the paramagnetism of vanadium ions and diamagnetism of the crystal lattice. The field and temperature dependences of the magnetization are approximated by the sum of modified Brillouin functions corresponding to the paramagnetic contributions of vanadium in two different charge states and the diamagnetic contribution linear in terms of field. The concentrations of vanadium ions in two different magnetic states and the character of their variation along the ingot are determined within the scope of the alloy’s electronic-structure rearrangement because of doping. The results are compared with the data of X-ray fluorescence microanalysis and the results of studying the galvanomagnetic properties of the samples.
Radiative Recombination, Carrier Capture at Traps, and Photocurrent Relaxation in PbSnTe:In with a Composition Close to Band Inversion
Abstract
Based on the notions that PbSnTe:In is a direct-gap semiconductor, the radiative-recombination lifetime is calculated, and the photocurrent relaxation and dependences of the instantaneous electron and hole lifetime are calculated under the assumption that PbSnTe:In is a disordered structure containing capture centers. These calculations explain such experimentally observed peculiarities of PbSnTe:In as a high photosensitivity in a wide wavelength range, pinning of the Fermi level, and long-term photocurrent relaxation. Calculations are also compared with experimental data and the possible parameters of photodetectors are evaluated.
Photothreshold of an α-GeS Layered Crystal: First-Principles Calculation
Abstract
The photothreshold of an α-GeS layered crystal is calculated from first principles based on the functional density method depending on its thickness. Two neighboring crystal plates consisting of several layers are separated by vacuum 4 layers thick, which corresponds to the doublet unit cell size of a bulk crystal. It is shown that the magnitude of the photothreshold is almost invariable with a crystal thickness larger than 10 layers.
Spectroscopy, Interaction with Radiation
Effect of Boron Impurity on the Light-Emitting Properties of Dislocation Structures Formed in Silicon by Si+ Ion Implantation
Abstract
The effect of boron implantation on the light-emitting properties of dislocation structures formed in silicon by Si+ ion implantation with subsequent annealing is studied. It is shown that the implantation of B+ ions has a significant effect on the dislocation-related luminescence intensity, spectrum and the temperature dependence of the D1-band intensity. It is found that the temperature dependence is nonmonotonous and involves two regions, in which the D1-band intensity increases with increasing temperature and has two well-pronounced maxima at 20 K and 60–70 K. The maximum at 20 K is associated with the morphological features of the dislocation structure under study, whereas the maximum at 60–70 K is associated with the additional implantation of the boron impurity into the dislocation region of the samples. It is established that the intensities of the experimentally observed maxima and the position of the high-temperature maximum depend on the implanted boron concentration.
Photoreflectance Spectroscopy Study of LT-GaAs Layers Grown on Si and GaAs Substrates
Abstract
The mechanical strains and densities of surface charge states in GaAs layers grown by low-temperature (LT) molecular-beam epitaxy on Si(100) and GaAs(100) substrates are investigated by photoreflectance spectroscopy. Lines corresponding to the fundamental transition (Eg) and the transition between the conduction band and spin-orbit-split valence subband (Eg + ΔSO) in GaAs are observed in the photoreflectance spectra of Si/LT-GaAs structures at 1.37 and 1.82 eV, respectively. They are shifted to lower and higher energies, respectively, relative to the corresponding lines in GaAs/LT-GaAs structures. Comparing the spectra of the Si/LT-GaAs and GaAs/LT-GaAs structures, it is possible to estimate mechanical strains in LT-GaAs layers grown on Si (by analyzing the spectral-line shifts) and the density of charge-carrier states at the GaAs/Si heterointerface (by analyzing the period of Franz–Keldysh oscillations).
Semiconductor Structures, Low-Dimensional Systems, and Quantum Phenomena
Transverse Nernst–Ettingshausen Effect in Superlattices Upon Electron-Phonon Scattering
Abstract
The Nernst–Ettingshausen coefficient is calculated in superlattices with the cosine dispersion law in the case of the scattering of charge carriers at acoustic and polar optical phonons in a magnetic field in the layer plane. A significant increase in the Nernst–Ettingshausen coefficient of a degenerate quasi-three-dimensional electron gas in a weak magnetic field is shown. For polar optical-phonon scattering, the Nernst–Ettingshausen coefficient changes sign in a strong magnetic field.
Electrical Properties of p-NiO/n-Si Heterostructures Based on Nanostructured Silicon
Abstract
Silicon nanowires are formed on n-Si substrates by chemical etching. p-NiO/n-Si heterostructures are fabricated by reactive magnetron sputtering. The energy diagram of anisotype p-NiO/n-Si heterostructures is constructed according to the Anderson model. The current–voltage and capacitance–voltage characteristics are measured and analyzed. The main current-transport mechanisms through the p-NiO/n-Si heterojunction under forward and reverse biases are established.
Ultrafast Dynamics of Photoexcited Charge Carriers in In0.53Ga0.47As/In0.52Al0.48As Superlattices under Femtosecond Laser Excitation
Abstract
The results of experimental studies of the time dynamics of photoexcited charge carriers in In0.53Ga0.47As/In0.52Al0.48As superlattices grown by molecular-beam epitaxy on a GaAs substrate with a metamorphic buffer are reported. On the basis of the results of the numerical simulation of band diagrams, the optimal thickness of the In0.52Al0.48As barrier layer (4 nm) is chosen. At this thickness, the electron wave functions in In0.53Ga0.47As substantially overlap the In0.52Al0.48As barriers. This makes it possible to attain a short lifetime of photoexcited charge carriers (τ ~ 3.4 ps) at the wavelength λ = 800 nm and the pumping power 50 mW without doping of the In0.53Ga0.47As layer with beryllium. It is shown that an increase in the wavelength to λ = 930 nm (at the same pumping power) yields a decrease in the lifetime of photoexcited charge carriers to τ ~ 2 ps. This effect is attributed to an increase in the capture cross section of trapping states for electrons with lower energies and to a decrease in the occupancy of traps at lower excitation densities.
In0.8Ga0.2As Quantum Dots for GaAs Solar Cells: Metal-Organic Vapor-Phase Epitaxy Growth Peculiarities and Properties
Abstract
The growth peculiarities of In0.8Ga0.2As quantum dots and their arrays on GaAs surface by metalorganic vapor-phase epitaxy are investigated. The bimodal size distribution of In0.8Ga0.2As quantum dots is established from the photoluminescence spectra recorded at different temperatures. The growth parameters were determined at which the stacking of 20 In0.8Ga0.2As quantum-dot layers in the active area of a GaAs solar cell makes it possible to enhance the photogenerated current by 0.97 and 0.77 mA/cm2 for space and terrestrial solar spectra, respectively, with the high quality of the p–n junction retained. The photogenerated current in a solar cell with quantum dots is higher than in the reference GaAs structure by ~1% with regard to nonradiative-recombination loss originating from stresses induced by the quantum-dot array.
Experimental Study of Spontaneous Emission in Bragg Multiple- Quantum-Well Structures with InAs Single-Layer Quantum Wells
Abstract
The time-resolved photoluminescence of a Bragg structure formed by InAs single-layer quantum wells in a GaAs matrix is investigated experimentally. The comparison of photoluminescence spectra recorded from the edge and the surface of the sample indicates that Bragg ordering of the quantum wells leads to substantial modification of the spectra, in particular, to the appearance of additional modes. The spectrum recorded at the edge of the sample features a single line corresponding to the exciton ground state. In the spectrum recorded at the surface, an additional line whose frequency and propagation angle correspond to the Bragg condition for quantum wells, appears at high excitation levels. The calculation of the modal Purcell factor explains the fact that spontaneous emission is enhanced only for specific propagation angles and frequencies, rather than for all angles and frequencies satisfying the Bragg condition.
Microcrystalline, Nanocrystalline, Porous, and Composite Semiconductors
Photoelectric Properties of ZnO Threadlike Crystals
Abstract
The photoelectric properties of ZnO whiskers obtained from the gas phase by the evaporation of a powder and metallic Zn under the continuous action of CO2 laser radiation are studied. On the basis of investigation of the InO–ZnO barrier structure, the mechanism of the photoconductivity of crystal ZnO caused by potential barriers is proposed.
Dielectric Properties of Nanocrystalline Tungsten Oxide in the Temperature Range of 223–293 K
Abstract
The dielectric properties of nanocrystalline tungsten oxide are studied in the temperature range of 223–293 K and in the frequency range ν = 10–2–106 Hz. Powders of WO3 with particle sizes of 110, 150, and 200 nm are prepared by the heat treatment of ammonium paratungstate at various temperatures. It is established that the frequency dependences of the conductivity for all samples increase with an increase in frequency, while the polarization characteristics ε'(ν) and ε"(ν) decrease. It is found that the frequency dependences of the conductivity are described by a function of the form νs with an index in the range of (0.83–0.90) ± 0.01, which is characteristic of the “hopping” mechanism of charged-particle motion (complexes) over localized states confined by potential barriers and structural defects.
Study of Current Flow Mechanisms in a CdS/por-Si/p-Si Heterostructure
Abstract
The temperature dependence of the forward and reverse portions of the current–voltage characteristic and the photovoltage spectrum of a CdS/por-Si/p-Si semiconductor heterostructure are studied. It is found that the current-flow mechanisms are controlled by generation–recombination processes in the spacecharge region of the por-Si/p-Si heterojunction, carrier tunneling in the por-Si film, and the model of space-charge-limited currents. A simplified version of the energy-band diagram of the heterostructure under study is proposed.
Photoluminescence Properties of ZnO Nanorods Synthesized by Different Methods
Abstract
The photoluminescence properties of ZnO nanorods synthesized by the low-temperature hydrothermal and high-temperature vapor-phase methods are studied. At room temperature, the photoluminescence of ZnO nanorods synthesized by the high-temperature vapor-phase method exhibits only one highintensity ultraviolet peak at a wavelength of 382 nm. At the same time, the luminescence spectra of ZnO nanorod samples grown by the low-temperature hydrothermal method, but with the use of different chemical reagents exhibit, apart from the ultraviolet peak, a violet band or a yellow-orange band at ~401 and ~574 nm, respectively. The violet luminescence band is attributed to defects or zinc vacancy complexes, and the yellow-orange band to defects associated with interstitial oxygen.
Modification of Photoconductivity Spectra in ZnO–CdSe Quantum- Dot Composites upon Exposure to Additional Photoexcitation
Abstract
The incorporation of CdS nanocrystals into a porous ZnO matrix results in sensitization of the composite in the visible spectral region. Studies of the photoconductivity spectra upon variable external illumination show that the spectra undergo reversible transformations. It is shown that the shape of the peak and the position of the local photoconductivity maximum corresponding to nanocrystals depend on the wavelength distribution of the incident radiation intensity. The mechanisms responsible for the process are discussed.
Carbon Systems
Analysis of the Structure and Conductivity of Kinked Carbon Chains Obtained by Pulsed Plasma Deposition on Various Metal Substrates
Abstract
Linear-chain carbon films with a thickness of order 100 nm were studied by tunneling spectroscopy. The oscillating dependence of the differential conductivity of the investigated structures is established. The results obtained are interpreted using a model of charge-density wave formation in regular structural kinks of carbon chains. The Raman spectra of the films are recorded. The simulated spectra of harmonic oscillations of polyines (–C≡C–)n and cumulenes (=C=)n of carbon films are theoretically compared.
Structure and Properties of Thin Graphite-Like Films Produced by Magnetron-Assisted Sputtering
Abstract
The structural, electrical, and optical properties of thin graphite-like films produced by magnetron- assisted sputtering onto crystalline silicon and quartz at substrate temperatures in the range from 320 to 620°C are studied. From analysis of the Raman spectra, it is established that, as the substrate temperature is elevated, the crystallite size increases and the concentration of structural defects and the content of amorphous carbon in the phase composition of the films decrease. It is found that, as the substrate temperature is elevated, the maximum of the absorption intensity in the ultraviolet spectral region of the optical absorption spectra shifts to longer wavelengths and the absorption intensity in the visible and near-infrared spectral regions increases. As the deposition temperature is elevated, the conductivity of the films increases from 0.2 Ω–1 cm–1 at 320°C to 30 Ω–1 cm–1 at 620°C.
Effect of Electron–Phonon Interaction on the Conductivity and Work Function of Epitaxial Graphene
Abstract
In the context of the cluster model of epitaxial graphene, the condition for a stepwise change in the charge of carbon atoms under the action of graphene–substrate electron–phonon interaction and an external electric field is derived. Such charge transfer brings about corresponding steps of the static conductivity and work function of epitaxial graphene. Numerical estimates for the case of weak coupling of quasi-free graphene with metal and semiconductor substrates are given.
Physics of Semiconductor Devices
Study of Deep Levels in a HIT Solar Cell
Abstract
The results of studying a HIT (heterojunction with an intrinsic thin layer) Ag/ITO/a-Si:H(p)/a-Si:H(i)/c-Si(n)/a-Si:H(i)/a-Si:H(n+)/ITO/Ag solar cell by the capacitance–voltage characteristic and current deep-level relaxation transient spectroscopy methods are presented. The temperature dependence of the capacitance–voltage characteristics of the HIT structure and deep-energy-level parameters are studied. The results of comprehensive studies by the above methods are used to determine the features of the energy-band diagram of actual HIT structures.
Investigation of the Characteristics of Heterojunction Solar Cells Based on Thin Single-Crystal Silicon Wafers
Abstract
The operating characteristics of heterojunction solar cells based on single-crystal silicon wafers with a reduced thickness are investigated experimentally. It is found that a decrease in the wafer thickness by 40% as compared to the standard values leads to degradation of the photoelectric-conversion efficiency to 5%. The obtained results could be used for estimating the commercial feasibility of the production of solar cells with a reduced wafer thickness.
Effect of Deep Centers on Charge-Carrier Confinement in InGaN/GaN Quantum Wells and on LED Efficiency
Abstract
The deep-center-assisted tunneling of carriers in p–n structures of light-emitting diodes (LEDs) with InGaN/GaN quantum wells (QWs) makes smaller the effective height of the injection barrier, but leads to a dependence of the radiation efficiency on the density and energy spectrum of defects in GaN. In the case of hopping conduction across the space charge region, the forward voltage mainly drops near the QW boundary, where the density of deep states at the quasi Fermi-level is the lowest. As a result, band bending at the boundary decreases, and, with increasing current, the direction of the electric field also changes, which leads to a weaker confinement of holes, to their non-radiative recombination in the n barrier, and to an efficiency droop. The low efficiency of green GaN LEDs is associated with the dominance of deep centers and insufficient density of shallow centers in the energy spectrum of defects in barrier layers near the boundaries with the QW. The proposed model is confirmed by the stepwise experimental dependences of the current, capacitance and efficiency of green and blue LEDs in the case of forward bias, which reflect the contribution of color centers responsible for the defect photoluminescence bands in GaN.
Diversity of Properties of Device Structures Based on Group-III Nitrides, Related to Modification of the Fractal-Percolation System
Abstract
A fractal-percolation system that includes extended defects and random fluctuations in the alloy composition is formed during the growth of device structures based on Group-III nitrides. It is established that the specific features of this system are determined not only by the growth conditions. It is shown that the diversity of the electrical and optical properties of InGaN/GaN LEDs (light-emitting diodes) emitting at wavelengths of 450–460 and 519–530 nm, as well as that of the electrical properties of AlGaN/GaN HEMT (high-electron-mobility transistor) structures, is due to modification of the properties of the fractal-percolation system both during the growth process and under the action of the injection current and irradiation. The influence exerted by these specific features on the service life of light-emitting devices and on the reliability of AlGaN/GaN HEMT structures is discussed.
Fabrication, Treatment, and Testing of Materials and Structures
On the Fabrication and Study of Lattice-Matched Heterostructures for Quantum Cascade Lasers
Abstract
The fabrication and study of the characteristics of a lattice-matched quantum cascade laser structure on an indium-phosphide substrate, designed for a wavelength of ~4.8 μm corresponding to one of the atmospheric windows are described. The heterostructure grown by molecular-beam epitaxy consisted of thirty cascades. Lasing was experimentally observed at temperatures up to 200 K at a wavelength coinciding with the calculated one, which confirms the high heterointerface quality and high precision of the layer thicknesses and active-region doping levels.
Concentric Hexagonal GaN Structures for Nanophotonics, Fabricated by Selective Vapor-Phase Epitaxy with Ion-Beam Etching
Abstract
In the Si3N4 layer, coaxial and single submicrometer GaN structures of hexagonal shape with pyramidal facets are formed by selective vapor-phase epitaxy in windows produced with a focused ion beam. It is found that coaxial hexagonal structures are formed during the growth process in ring-shaped mask windows.