Luminescence of II–VI and III–V nanostructures

Photoluminescence of HgCdTe epitaxial films and nanostructures and electroluminescence of InAs(Sb,P) light-emitting diode (LED) nanoheterostructures were studied. For HgCdTe-based structures, the presence of compositional fluctuations, which localized charge carriers, was established. A model, which described the effect of the fluctuations on the rate of the radiative recombination, the shape of luminescence spectra and the position of their peaks, was shown to describe experimental photoluminescence data quite reasonably. For InAs(Sb,P) LED nanoheterostructures, at low temperatures (4.2–100 K) stimulated emission was observed. This effect disappeared with the temperature increasing due to the resonant ‘switch-on’ of the Auger process involving transition of a hole to the spin-orbit-splitted band. Influence of other Auger processes on the emissive properties of the nanoheterostructures was also observed. Prospects of employing II–VI and III–V nanostructures in light-emitting devices operating in the mid-infrared part of the spectrum are discussed.     

Shift of the photoemission threshold in III–V and II–VI semiconductors

The reasons why the photoemission threshold energy of semiconductors is lower than the ionization energy of constituent atoms have been investigated. It has been indicated that the previously proposed interpretation of this phenomenon based on the inclusion of an additional intra-atomic Coulomb interaction between the valence electrons is insufficient. It has been shown that the calculation of the electronic energy structure of semiconductors, in particular, of the photoemission threshold, requires taking into account a change in the localization region of the valence electrons when a free atom is embedded into a crystal. A way of taking this change into account in the tight-binding theory has been demonstrated. Corrections to the tight-binding Hamiltonian have been found. The photoemission thresholds of III–V and II–VI semiconductors have been calculated with the inclusion of these corrections. Comparison of the results with the experimental data has been performed.     

Synthesis and characterization of p-GaSe thin films and the analyses of I–V and C–V measurements of p-GaSe/p-Si heterojunction under electron irradiation

Gallium Selenide (GaSe) thin films were grown by the electrochemical deposition (ECD) technique on Indium tin oxide (ITO) and p-Si (100) substrates. The Electron paramagnetic resonance (EPR) spectrum of GaSe thin films’ growth on ITO was recorded at room temperature. According to EPR results, the g value of an EPR signal obtained for GaSe deposited on ITO is 2.0012?±?0.0005. In/GaSe/p-Si heterojunction was irradiated with high-energy (6?MeV) and low-dose (1.53?×?10¹⁰?e^??cm^?2) electrons. The ideality factor of the In/GaSe/p-Si device was calculated as 1.24 and barrier height was determined as 0.82?eV from I–V measurements before irradiation. Acceptor concentration, built-in potential and barrier height of the In/GaSe/p-Si device were also obtained as 0.72?×?10^14?cm^?3, 0.65?eV and 0.97?eV from C–V measurements, respectively. After irradiation, the ideality factor n and barrier height Φ_b values of the In/GaSe/p-Si device were calculated as 1.55 and 0.781?eV, respectively. Acceptor concentration, the built-in potential and barrier height values of the In/GaSe/p-Si device have also shown a decrease after 6?MeV electron irradiation. This change in heterojunction device parameters shows that current transport does not obey thermionic emission, and thus tunneling could be active due to the defects formed by irradiation at the In–GaSe interface.     

Synthesis and properties of MoO3–V2O5–PbO glasses

Lead vanadate glasses of the system xMoO₃–50V₂O₅–(50-x)PbO (0 ≤ x ≤ 25 mol. %) were synthesized and studied by FTIR and ultrasonic spectroscopy and differential scanning calorimetry to investigate the role of MoO₃ content on their atomic structure. The elastic properties and Debye temperatures of the glasses were investigated using sound velocity measurements at 4 MHz. The activation energy for the glass transition was derived from the dependence of the glass-transition temperature (T_g ) on the heating rate. Similarly, the activation energy of the crystallization process was also determined. According to the IR analysis, the vibrations of the vanadate structural units are shifted towards higher wavenumbers on the formation of bridging oxygens. The change of density and molar volume with MoO₃ content reveals that the molybdinate units are less dense than the lead oxide units. The observed compositional dependence of the elastic moduli is interpreted in terms of the effect of MoO₃ on the coordination number of the vanadate units. A good correlation was observed between the experimentally determined elastic moduli and those computed according to the Makishima–Mackenzie model. It is assumed that MoO₃ plays the role of a glass former by increasing the activation energy for the glass transition and the activation energy for crystallization and by increasing both the thermal stability and the glass formation range of the vanadate glasses.     

Mechanical properties of ferrite-perlite and martensitic Fe–Mn–V–Ti–C steel processed by equal-channel angular pressing and high-temeperature annealing

Using the method of equal-channel angular pressing (ECAP), submicrocrystalline structure is formed in lowcarbon Fe–Mn–V–Ti–C steel with the average grain size 260 nm in the ferrite-perlite state and 310 nm in the martensitic state. It is established that the ECAP treatment gives rise to improved mechanical properties (H_μ = 2.9 GPa, σ₀ = 990 MPa in the ferrite-perlite and H_μ = 3.7 GPa, σ₀ = 1125 MPa in martensitic states), decreased plasticity, and results in plastic flow localization under tensile loading. The high strength properties formed by the ECAP are shown to sustain up to the annealing temperature 500°C.     

Heteroepitaxial III–V films on fianite substrates and buffer layers

GaAs, GaSb, AlGaAs, and InGaAs epitaxial films and multilayer AlGaAs/InGaAs/GaAs heterostructures for PHEMT field-effect transistors have been obtained on fianite substrates by metal-organic vapour phase epitaxy. Films of different III–V compounds, including GaN, were grown on Si and GaAs substrates with a simple single buffer layer (fianite) and double buffer layer (fianite on porous Si and GaAs). It is established that the use of a two-layer buffer improves the structural quality and homogeneity of III–V films. A possibility of controlling the phase composition of GaN films using a corresponding buffer layer is shown. It is found that the use of a two-layer buffer increases the electrical homogeneity and decreases the electrical activity of defects in GaN films.     

Temperature-Dependent Current–Voltage (I–V) and Capacitance–Voltage (C–V) Characteristics of Ni/Cu/n-InP Schottky Barrier Diodes

The current-voltage (I–V) and capacitance-voltage (C–V) characteristics of Ni/Cu/n-InP Schottky barrier diodes are studied over a wide temperature range, from 210 K to 420 K. The I–V characteristics display anomalous thermal behavior. The apparent barrier height decays, and the ideality factor grows at low temperatures, and the series resistances resulting from Cheung’s and Norde’s procedures are markedly temperature dependent. The nonlinearity of the Richardson plot and the strong temperature dependence of the Schottky-barrier parameters indicate that the interface is spatially inhomogeneous. Plots of the zero-bias barrier height as a function of 1/(2kT) points to a Gaussian distribution of barrier heights with 0.90 eV mean height and 0.014 eV standard deviation. When this distribution is accounted for, a Richardson of 6.5 A/(cm K)² results, relatively close to the 9.4/(cm K)² predicted by theory. We conclude that, combined with a Gaussian distribution of barrier heights, the thermionic-emission mechanism explains the temperature-dependent I–V and C–V characteristics of the studied Schottky-barrier diodes.     

Hyperfine Interactions and Magnetism of 3d Transition-Metal-Impurities in II–VI and III–V Compound-Based Diluted Magnetic Semiconductors

The electronic structure of II–VI and III–V compound-based diluted magnetic semiconductors is calculated based on the local density approximation (LDA) using the Korringa–Kohn–Rostoker method combined with the coherent potential approximation. The magnetism of 3d transition-metal-atom-doped ZnO, ZnS, ZnSe, ZnTe, GaN, GaAs is investigated from first-principles. It is suggested that the double exchange mechanism stabilizes the ferromagnetism in these DMSs. In order to obtain microscopic information on the electronic structure of transition-metal-impurities in semiconductors, the hyperfine field of respective impurities in each host material is calculated. It is found that the agreement with the experimental values is not good, probably because the LDA is not sufficient to describe the core states of transition metals. However, it is suggested that the hyperfine fields clearly reflect the local magnetic moments for 3d impurities. This revised version was published online in September 2006 with corrections to the Cover Date.     

Effect of helium ion irradiation on the structure and electrical resistivity of nanocrystalline Cr–N and V–N coatings

The structural stability and electrical resistivity of nanocrystalline Cr–N and V–N coatings prepared by ion beam-assisted deposition were studied. The results showed that under helium ion irradiation up to doses of 1.0^.10¹⁷ ion/cm² the fine-crystalline objects successively increase their resistance without apparent structural changes. The subsequent dose increase leads to gas-vacancy void formation and chromium nitride structure destruction. The presence of the initial closed porosity in vanadium nitride favors its structural stability at investigated maximum damage doses.     

Computation and measurement of corona current density and V–I characteristics in wires-to-plates electrostatic precipitator

This paper deals with experimental and numerical investigation of wire-duct type electrostatic precipitator under clean air conditions. A laboratory-scale model is used to provide measurements of corona current–voltage characteristics and current density. Different configurations of electrodes are tested in order to improve the performance of the electrostatic precipitator. Moreover, the corona governing equations are successfully solved using Comsol Multiphysics. The present work simulates the whole geometry including all discharge wires in order to take into account their mutual effect. The results of the numerical model are compared with the experimental measurements of current density and current–voltage characteristics and the general agreement is quite good.     

Theory of band gap bowing of disordered substitutional II–VI and III–V semiconductor alloys

For a wide class of technologically relevant compound III?CV and II?CVI semiconductor materials AC and BC mixed crystals (alloys) of the type A _x B_1?x C can be realized. As the electronic properties like the bulk band gap vary continuously with x, any band gap in between that of the pure AC and BC systems can be obtained by choosing the appropriate concentration x, granted that the respective ratio is miscible and thermodynamically stable. In most cases the band gap does not vary linearly with x, but a pronounced bowing behavior as a function of the concentration is observed. In this paper we show that the electronic properties of such A _x B_1?x C semiconductors and, in particular, the band gap bowing can well be described and understood starting from empirical tight-binding models for the pure AC and BC systems. The electronic properties of the A _x B_1?x C system can be described by choosing the tight-binding parameters of the AC or BC system with probabilities x and 1 ? x, respectively. We demonstrate this by exact diagonalization of finite but large supercells and by means of calculations within the established coherent potential approximation (CPA) We apply this treatment to the II?CVI system Cd _x Zn_1?x Se, to the III?CV system In _x Ga_1?x As and to the III-nitride system Ga _x Al_1?x N.     

Raman characterization of Ti–6Al–4V oxides and thermal history after kinetic friction

Raman spectroscopy is applied to study the surfaces of a pair of tantalum and titanium alloy samples after high-speed dry friction. The surface of titanium alloy (Ti–6Al–4V) shows titanium oxides on the rubbing surfaces. Raman spectra enable to differentiate the allotropic phases of anatase or rutile. The presence of these phases is the signature of the local thermal history during the friction tests. Moreover, Raman mapping allows localizing area the flash temperatures that may have been produced by the friction between sample asperities.     

Frequency and density dependent radiative recombination processes in III–V semiconductor quantum wells and superlattices

In this paper we review the radiative recombination processes occurring in semiconductor quantum wells and superlattices under different excitation conditions. We consider processes whose radiative efficiency depends on the photogenerated density of elementary excitations and on the frequency of the exciting field, including luminescence induced by multiphoton absorption, exciton and biexciton radiative decay, luminescence arising from inelastic excitonic scattering, and electron-hole plasma recombination.

Semiconductor quantum wells are ideal systems for the investigation of radiative recombination processes at different carrier densities owing to the peculiar wavefunction confinement which enhances the optical non-linearities and the bistable behaviour of the crystal. Radiative recombination processes induced by multi-photon absorption processes can be studied by exciting the crystal in the transparency region under an intense photon flux. The application of this non-linear spectroscopy gives direct access to the excited excitonic states in the quantum wells owing to the symmetry properties and the selection rules for artificially layered semiconductor heterostructures.

Different radiative recombination processes can be selectively tuned at exciting photon energies resonant with real states or in the continuum of the conduction band depending on the actual density of photogenerated carriers. We define three density regimes in which different quasi-particles are responsible for the dominant radiative recombination mechanisms of the crystal: (i) The dilute boson gas regime, in which exciton density is lower than 10¹⁰ cm^-2. Under this condition the decay of free and bound excitons is the main radiative recombination channel in the crystal. (ii) The intermediate density range (n < 10¹¹ cm^-2) at which excitonic molecules (biexcitons) and inelastic excitonic scattering processes contribute with additional decay mechanisms to the characteristic luminescence spectra. (iii) The high density range (n ?10¹² cm^-2) where screening of the Coulomb interaction leads to exciton ionization. The optical transitions hence originate from the radiative decay of free-carriers in a dense electron-hole plasma.

The fundamental theoretical and experimental aspects of the radiative recombination processes are discussed with special attention to the GaAs/Al _x Ga_1-x As and Ga _x In_1-x As/Al _y In_1-y As materials systems. The experimental investigations of these effects are performed in the limit of intense exciting fields by tuning the density of photogenerated quasi-particles and the frequency of the exciting photons. Under these conditions the optical response of the quantum well strongly deviates from the well-known linear excitonic behaviour. The optical properties of the crystal are then no longer controlled by the transverse dielectric constant or by the first-order dielectric susceptibility. They are strongly affected by many-body interactions between the different species of photogenerated quasi-particles, resulting in dramatic changes of the emission properties of the semiconductor.

The systematic investigation of these radiative recombination processes allows us to selectively monitor the many-body induced changes in the linear and non-linear optical transitions involving quantized states of the quantum wells. The importance of these effects, belonging to the physics of highly excited semiconductors, lies in the possibility of achieving population inversion of states associated with different radiative recombination channels and strong optical non-linearities causing laser action and bistable behaviour of two-dimensional heterostructures, respectively.     

Surface energy and crystal structure of nanowhiskers of III–V semiconductor compounds

A theoretical model has been proposed for calculating the surface energy of nanowhiskers in the nearest neighbor approximation. The surface energy has been calculated for different faces of III–V semiconductor crystals with cubic and hexagonal structures. The effect of the formation of the hexagonal wurtzite phase in nanowhiskers of III–V semiconductor compounds has been considered using the obtained data. Estimates for the critical radius of the phase transition in III–V semiconductor nanowhiskers are presented.     

III–V based-semiconductor photonic crystals

Three-dimensional (3D) and two-dimensional (2D) photonic crystals based on III–V semiconductors are described. On the 3D photonic crystals, the development of complete photonic crystals at optical wavelengths and their applications to ultrasmall optical integrated circuits including 3D sharp bend waveguide are described. On the 2D photonic crystals, two-unique device and/or phenomena are demonstrated.     

Structure and Phase Composition of Ti–6Al–4V Alloy Obtained by Electron-Beam Additive Manufacturing

Russian Physics Journal - The paper presents the fabrication of Ti–6Al–4V alloy specimens using two operating modes of the electron beam additive manufacturing (EBAM). The structure,...     

Mechanism of texture and microstructure evolution during warm rolling of Ti–6Al–4V alloy

Ti–6Al–4V (Ti64) plates were subjected to rolling at 600°C and 800°C, respectively, for reductions up to 90% reduction in thickness. The mechanism of texture and microstructure evolution during rolling was studied in the present study. Extension twins of coherent nature were observed in the samples rolled up to 50% of reduction. The deformation was relatively inhomogeneous in the samples rolled at 600°C compared to that at 800°C. Visco-plastic self-consistent (VPSC) simulation showed that relative activity of pyramidal <c+a> slip was higher during rolling at 800°C compared to that at 600°C. The average activity of slip systems per grain was less than five for the samples rolled at 600°C and this might be responsible for the strain heterogeneity in the large grains. Further, twinning activity was found to be limited to a true strain of 0.5, as supported by the microstructural observation. VPSC simulation also showed the presence of contraction twins in the samples which was further supported by X-ray texture measurement. Dominant basal texture was observed in the samples irrespective of the temperature of rolling.