Modification of the properties of porous silicon on adsorption of iodine molecules

Infrared spectroscopy and electron spin resonance measurements are used to study the properties of porous silicon layers on adsorption of the I₂ iodine molecules. The layers are formed on the p-an n-Si single-crystal wafers. It is established that, in the atmosphere of I₂ molecules, the charge-carrier concentration in the layers produced on the p-type wafers can be noticeably increased: the concentration of holes can attain values on the order of ～10¹⁸?10¹⁹ cm^?3. In porous silicon layers formed on the n-type wafers, the adsorption-induced inversion of the type of charge carriers and the partial substitution of silicon-hydrogen bonds by silicon-iodine bonds are observed. A decrease in the concentration of surface paramagnetic defects, P _b centers, is observed in the samples with adsorbed iodine. The experimental data are interpreted in the context of the model in which it is assumed that both deep and shallow acceptor states are formed at the surface of silicon nanocrystals upon the adsorption of I₂ molecules.     

On the density-of-states effective mass and charge-carrier mobility in heteroepitaxial films of bismuth telluride and Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript> solid solution

It is shown that the Seebeck coefficient α, the power factor α²σ, and the density-of-states effective mass m/m ₀ in heteroepitaxial films of Bi0.5Sb1.5Te3 solid solution are higher than the corresponding characteristics of bulk thermoelectric materials. The elevated values and weak temperature dependences of these parameters lead to a rise in the parameter proportional to the effective mass, the charge-carrier mobility, and the figure of merit. The character of change in α, α²σ, and m/m ₀ is determined by the peculiarities of the mechanism of charge-carrier scattering, the anisotropy of the constant-energy surface, and the possible influence of topological surface states of Dirac fermions in the films.     

Improvement in Thermoelectric Performance of SnS Due to Electronic Structure Modification Under Biaxial Strain

Ab-initio calculations using the full potential linearized augmented plane-wave technique and the semi-classical Boltzmann theory are used to study thermoelectric properties of unstrained SnS and at 1%, 2% and 3% applied biaxial tensile (BT) strain. The studies are carried out at 800 K for p-type and n-type carriers. For an increase in BT strain, lattice constants of SnS change causing changes in the band structure and increase in the band gap which in turn modifies thermoelectric coefficients. In the case of unstrained SnS, the maximum thermopower (S) obtained is 426 μV/K at carrier concentration 5.40?×?10¹⁸ cm^?3 for p-type carriers and 435 μV/K at carrier concentration 1.68?×?10¹⁸ cm^?3 for n-type carriers. At 3% applied BT strain, S is increased to 696 μV/K at carrier concentration 4.61?×?10¹⁷ cm^?3 for p-type carriers and 624 μV/K at carrier concentration 3.21?×?10¹⁷ cm^?3 for n-type carriers. The power factor (PF) increases?～?2 times at 3% BT strain as compared to unstrained SnS, and it is 6.20 mW K^?2 m^?1 for p-type carriers. For n-type carriers, PF at 3% applied BT is slightly less than the PF for unstrained SnS, which is 6.81 mW K^?2 m^?1. For both types of carriers, the figure of merit (ZT) is found to be?～?1.5 for unstrained SnS. For p-type carriers ZT is enhanced 1.4 times at 3% applied BT strain as compared to that of unstrained SnS. However, for n-type carriers, ZT does not change drastically with increase in BT strain.     

A Quasi-Classical Model of the Hubbard Gap in Lightly Compensated Semiconductors

A quasi-classical method for calculating the narrowing of the Hubbard gap between the A⁰ and A⁺ acceptor bands in a hole semiconductor or the D⁰ and D^– donor bands in an electron semiconductor is suggested. This narrowing gives rise to the phenomenon of a semiconductor transition from the insulator to metal state with an increase in doping level. The major (doping) impurity can be in one of three charge states (–1, 0, or +1), while the compensating impurity can be in states (+1) or (–1). The impurity distribution over the crystal is assumed to be random and the width of Hubbard bands (levels), to be much smaller than the gap between them. It is shown that narrowing of the Hubbard gap is due to the formation of electrically neutral acceptor (donor) states of the quasicontinuous band of allowed energies for holes (electrons) from excited states. This quasicontinuous band merges with the top of the valence band (v band) for acceptors or with the bottom of the conduction band (c band) for donors. In other words, the top of the v band for a p-type semiconductor or the bottom of the c band for an n-type semiconductor is shifted into the band gap. The value of this shift is determined by the maximum radius of the Bohr orbit of the excited state of an electrically neutral major impurity atom, which is no larger than half the average distance between nearest impurity atoms. As a result of the increasing dopant concentration, the both Hubbard energy levels become shallower and the gap between them narrows. Analytical formulas are derived to describe the thermally activated hopping transition of holes (electrons) between Hubbard bands. The calculated gap narrowing with increasing doping level, which manifests itself in a reduction in the activation energy ε₂ is consistent with available experimental data for lightly compensated p-Si crystals doped with boron and n-Ge crystals doped with antimony.     

High-voltage (900 V) 4 <Emphasis Type="Italic">H</Emphasis>-SiC Schottky diodes with a boron-implanted guard <Emphasis Type="Italic">p-n</Emphasis> junction

High-voltage (900 V) 4H-SiC Schottky diodes terminated with a guard p-n junction were fabricated and studied. The guard p-n junction was formed by room-temperature boron implantation with subsequent high-temperature annealing. Due to transient enhanced boron diffusion during annealing, the depth of the guard p-n junction was equal to about 1.7 μm, which is larger by approximately 1 μm than the projected range of 11 B ions in 4H-SiC. The maximum reverse voltage of fabricated 4H-SiC Schottky diodes is found to be limited by avalanche breakdown of the planar p-n junction; the value of the breakdown voltage (910 V) is close to theoretical estimate in the case of the impurity concentration N = 2.5 × 10¹⁵ cm^?3 in the n-type layer, thickness of the n-type layer d = 12.5 μm, and depth of the p-n junction r _j = 1.7 μm. The on-state diode resistance (3.7 mΩ cm²) is controlled by the resistance of the epitaxial n-type layer. The recovery charge of about 1.3 nC is equal to the charge of majority charge carriers that are swept out of an epitaxial n-type layer under the effect of a reverse voltage.     

Electrical activity of dislocations and point defects of deformation origin in Cd<Subscript>x</Subscript>Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Te crystals

The generation of dislocations with even a relatively low density (N_dis≤10⁷cm^?2) leads to significant variations in the kinetic coefficients of Cd_xHg_1?xTe (x=0.20–0.21) crystals. In n-type crystals, a substantial decrease in electron mobility takes place along with a marked growth in electron concentration. For p-type crystals, the transition from the activation conductivity to the metal one is observed in the low-temperature range of 4.2–40 K, as is the alternating-sign behavior of the Hall coefficient R _H depending on temperature and magnetic-field strength. A dominant role in the observed modifications is played by electronic states of point defects formed during the dislocation motion rather than the dislocations themselves. The totality of the data can be explained in terms of the formation of connected channels of an opposite-type conductivity in the form of a three-dimensional dislocation network in the matrix of the main crystal.     

The Stark shift of the hole states in separate InAs/GaAs quantum dots grown on (100) and (311)A GaAs substrates

Deep-level transient spectroscopy is used to study charge-carrier emission from the states of separate quantum dots in InAs/GaAs p-n heterostructures grown on (100)-and (311)A-oriented GaAs substrates in relation to the reverse-bias voltage U. It is established that the structures under consideration exhibit different bias-voltage dependences of the Stark shift for the energy levels of the quantum-dot states on the value of U.     

BN-C Hybrid Nanoribbons as Gas Sensors

The effects of carbon monoxide (CO) and ammonia (NH₃) molecules adsorption on the various composites of boron nitride and graphene BN-C hybrid nanoribbons are investigated using the non-equilibrium Green’s function (NEGF) technique based on density functional theory (DFT). The effects of adsorption with possible random configurations on the average of the density of states (DOS), transmission coefficient, and the current–voltage (I–V) characteristics are calculated. The results indicate that, by embedding armchair graphene nanoribbon (AGNR) with boron nitride nanoribbon (BNNR), the various electronic properties can be observed after gas molecule adsorption. The electronic structure and gap of hybrids system is modified due to gas adsorption, and the systems act like the n-type semiconductor by NH₃ molecule adsorption. The hybrid structures due to their tunable band gap are better candidates for gas detecting compared to the pristine BNNRs and AGNRs.     

Characterization of <Emphasis Type="Italic">n</Emphasis>-Type and <Emphasis Type="Italic">p</Emphasis>-Type Long-Wave InAs/InAsSb Superlattices

The influence of dopant concentration on both in-plane mobility and minority carrier lifetime in long-wave infrared InAs/InAsSb superlattices (SLs) was investigated. Unintentially doped (n-type) and various concentrations of Be-doped (p-type) SLs were characterized using variable-field Hall and photoconductive decay techniques. Minority carrier lifetimes in p-type InAs/InAsSb SLs are observed to decrease with increasing carrier concentration, with the longest lifetime at 77 K determined to be 437 ns, corresponding to a measured carrier concentration of p ₀ = 4.1 × 10¹⁵ cm^?3. Variable-field Hall technique enabled the extraction of in-plane hole, electron, and surface electron transport properties as a function of temperature. In-plane hole mobility is not observed to change with doping level and increases with reducing temperature, reaching a maximum at the lowest temperature measured of 30 K. An activation energy of the Be-dopant is determined to be 3.5 meV from Arrhenius analysis of hole concentration. Minority carrier electrons populations are suppressed at the highest Be-doping levels, but mobility and concentration values are resolved in lower-doped samples. An average surface electron conductivity of 3.54 × 10^?4 S at 30 K is determined from the analysis of p-type samples. Effects of passivation treatments on surface conductivity will be presented.     

Solution of the problem of charge-carrier injection into an insulating layer under self-consistent boundary conditions at contacts

The problem of charge carrier injection into a finite-length insulating layer is analytically solved in the drift-diffusion approximation, taking into account self-consistent boundary conditions. The main assumption is the neglect of intrinsic doping of the i-type layer. The solution allows calculation of the potential, electric field, and current-voltage characteristics of various structures, i.e., metal-i-n ⁺ (or p ⁺)-semiconductor, metal-i-layer-metal, and n ⁺(p ⁺)-i-n ⁺(p ⁺) structures. The solution allows generalization for structures having heterobarriers at semiconductor layer interfaces. The proposed approach considers contact phenomena and volume effects associated with the space-charge-limited current in the i-type layer. The solution is valid in both extreme cases and intermediate conditions.     

The effect of thickness fluctuations on the static electrical conductivity of a semiconductor quantum wire

Expressions for the relaxation time, electron mobility, and static electrical conductivity along a semiconductor quantum wire are derived in relation to a random field of Gaussian fluctuations in the wire’s thickness. In the case of nondegenerate statistics for charge carriers at relatively low temperatures (T), electron mobility is given by u _n ∝ T^1/2. In the limiting case of a strong magnetic field H directed along the wire, the factor H^?1/2 appears in the mobility expression. It is shown that the considered mechanism of charge-carrier relaxation is important for the electrical conductivity of a fairly thin and pure quantum wire at low temperatures.     

Fine Structure of the long-wavelength edge of exciton-phonon absorption and hyperbolic excitons in silicon carbide of 6<Emphasis Type="Italic">H</Emphasis> polytype

The fine structure of the long-wavelength edge of the polarization spectra of exciton-phonon absorption in moderate-purity n-type 6H-SiC crystals with a concentration of uncompensated donors N_D?N_A=(1.7–2.0)×10¹⁶ cm^?3 at T=1.7 K was studied. The analysis of new special features found at the absorption edge and the reliable detection of the onset of exciton-phonon steps related to the emission of phonons from acoustical and optical branches allowed highly accurate determination of a number of important parameters such as the band gap, the exciton band gap, the exciton binding energy, and the energies of spin-orbit and crystal-field splitting of an exciton. For the first time, transitions with the emission of LA phonons to the 1S exciton state with an M₁-type dispersion law were detected in E ∥ Z(C) polarization (the electric-field vector is parallel to the optical axis of the crystal). This observation supports the previously predicted “two-well” structure of the conduction band minimum in 6H-SiC.     

Formation and study of <Emphasis Type="Italic">p–i–n</Emphasis> structures based on two-phase hydrogenated silicon with a germanium layer in the <Emphasis Type="Italic">i</Emphasis>-type region

Four pairs of p–i–n structures based on polymorphous Si:H (pm-Si:H) are fabricated by the method of plasma-enhanced chemical vapor deposition. The structures in each pair are grown on the same substrate so that one of them does not contain Ge in the i-type layer while the other structure contains Ge deposited by molecular-beam epitaxy as a layer with a thickness of 10 nm. The pair differ from one another in terms of the substrate temperature during Ge deposition; these temperatures are 300, 350, 400, and 450°C. The data of electron microscopy show that the structures formed at 300°C contain Ge nanocrystals (nc-Ge) nucleated at nanocrystalline inclusions at the pm-Si:H surface. The nc-Ge concentration increases as the temperature is raised. The study of the current–voltage characteristics show that the presence of Ge in the i-type layer decreases the density of the short-circuit current in p–i–n structures when they are used as solar cells, whereas these layers give rise to an increase in current at a reverse bias under illumination. The obtained results are consistent with known data for structures with Ge clusters in Si; according to these data, Ge clusters increase the coefficient of light absorption but they also increase the rate of charge-carrier recombination.     

States of antimony and tin atoms in lead chalcogenides

It is shown by Mössbauer spectroscopy of the ¹¹⁹Sb(^119m Sn) isotope that impurity antimony atoms in PbS, PbSe, and PbTe lattices are distributed between cation and anion sublattices. In n-type samples, the greatest part of antimony is located in the anion sublattice; in hole ones, in the cation sublattice. The tin atoms formed as a result of radioactive decay of ¹¹⁹Sb (antisite state) are electrically inactive in the anion sub-lattice of PbS and PbSe, while, in the cation sublattice, they form donor U ^? centers. Electron exchange between the neutral and doubly ionized tin U ^? centers via the allowed band states is observed. The tin atoms formed after radioactive decay of ¹¹⁹Sb are electrically inactive in the anion and cation sublattices of PbTe.     

Analysis of Carrier Transport in <Emphasis Type="Italic">n</Emphasis>-Type Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te with Ultra-Low Doping Concentration

Mercury cadmium telluride (HgCdTe, or MCT) with low n-type indium doping concentration offers a means for obtaining high performance infrared detectors. Characterizing carrier transport in materials with ultra low doping (N_D?=?10¹⁴ cm^?3 and lower), and multi-layer material structures designed for infrared detector devices, is particularly challenging using traditional methods. In this work, Hall effect measurements with a swept B-field were used in conjunction with a multi-carrier fitting procedure and Fourier-domain mobility spectrum analysis to analyze multi-layered MCT samples. Low temperature measurements (77 K) were able to identify multiple carrier species, including an epitaxial layer (x?=?0.2195) with n-type carrier concentration of n?=?1?×?10¹⁴ cm^?3 and electron mobility of μ?=?280000 cm²/Vs. The extracted electron mobility matches or exceeds prior empirical models for MCT, illustrating the outstanding material quality achievable using current epitaxial growth methods, and motivating further study to revisit previously published material parameters for MCT carrier transport. The high material quality is further demonstrated via observation of the quantum Hall effect at low temperature (5 K and below).     

Tuning the Electronic Properties of Graphene Oxide Nanoribbons Through Different Oxygen Doping Configurations

The electronic properties of armchair graphene oxide nanoribbons (AGONRs) with different doped oxygen configurations are studied based on density functional theory using first principle calculations. The electronic properties of the AGONRs are tuned by different oxygen configurations for top edges, center, bottom edges and fifth width. The AGONRs for top-edge O doping configuration are indirect band gap semiconductors with an energy gap of 1.268 eV involving hybridization among C-2p and O-2s, 2p electrons and electrical conductivity of oxygen atoms. The center and bottom edges are direct band gap semiconductors with 1.317 eV and 1.151 eV, respectively. The valence band is contributed from C-2p, O-2p and H-1s for top-edge O doping. The electronic properties of AGONRs are changed due to localization in ?2.94 eV of O-2p states. The center O-doped AGONRs are n-type semiconductors with Fermi levels near the conduction band bottom. This is due to hybridization among C-2s, 2p and O-2p electrons. However, bottom-edge O-doped AGONRs are p-type semiconductors, due to the electrical conductivity of oxygen atoms. The fifth-width O-doped AGONRs are indirect band gap semiconductors with an energy gap of 0.375 eV. The projected density of states shows that the localization and hybridization between C-2 s, 2p, O-2p and H-1s electronic states are rising in the conduction band and valence band from the projected density of states. The localization is induced by O-2p electronic states at a Fermi level.     

Electronic Processes in CdIn<Subscript>2</Subscript>Te<Subscript>4</Subscript> Crystals

The results of investigations of electrical, optical, and photoelectric properties of CdIn₂Te⁴ crystals, which were grown by the Bridgman method are presented. It is shown that electrical conductivity is determined mainly by electrons with the effective mass m_n = 0.44m₀ and the mobility 120–140 cm²/(V s), which weakly depends on temperature. CdIn₂Te₄ behaves as a partially compensated semiconductor with the donor-center ionization energy E_d = 0.38 eV and the compensation level K = N_a/N_d = 0.36. The absorption-coefficient spectra at the energy hν < E_g = 1.27 eV are subject to the Urbach rule with a typical energy of 18–25 meV. The photoconductivity depends on the sample thickness. The diffusion length, the charge-carrier lifetime, and the surface-recombination rate are determined from the photoconductivity spectra.     

Photoluminescence Studies of Si-Doped Epitaxial GaAs Films Grown on (100)- and (111)A-Oriented GaAs Substrates at Lowered Temperatures

The electrical properties and photoluminescence features of uniformly Si-doped GaAs layers grown on GaAs substrates with the (100) and (111)A crystallographic orientations of the surface are studied. The samples are grown at the same As₄ pressure in the growth temperature range from 350 to 510°C. The samples grown on GaAs(100) substrates possess n-type conductivity in the entire growth temperature range, and the samples grown on GaAs(111)A substrates possess p-type conductivity in the growth temperature range from 430 to 510°C. The photoluminescence spectra of the samples exhibit an edge band and an impurity band. The edge photoluminescence band corresponds to the photoluminescence of degenerate GaAs with n- and p-type conductivity. The impurity photoluminescence band for samples on GaAs(100) substrates in the range 1.30–1.45 eV is attributed to V_As defects and Si_As–V_As defect complexes, whose concentration varies with sample growth temperature. Transformation of the impurity photoluminescence spectra of the samples on GaAs(111)A substrates is interpreted as being a result of changes in the V_As and V_Ga defect concentrations under variations in the growth temperature of the samples.     

CdHgTe heterostructures for new-generation IR photodetectors operating at elevated temperatures

The parameters of multilayer Cd_xHg_1–xTe heterostructures for photodetectors operating at wavelengths of up to 5 μm, grown by molecular-beam epitaxy (MBE) on silicon substrates, are studied. The passivating properties of thin CdTe layers on the surface of these structures are analyzed by measuring the C–V characteristics. The temperature dependences of the minority carrier lifetime in the photoabsorption layer after growth and thermal annealing are investigated. Samples of p ⁺–n-type photodiodes are fabricated by the implantation of arsenic ions into n-type layers, doped with In to a concentration of (1–5) × 10¹⁵ cm^–3. The temperature dependences of the reverse currents are measured at several bias voltages; these currents turn out to be almost two orders of magnitude lower than those for n ⁺–p-type diodes.