Electrical measurements of the electron irradiation induced E- and H-traps in GaAs under hydrostatic pressure

The change of resistivity of the 2.3 MeV-electron-irradiated bulk n- and p-GaAs have been measured at hydrostatic pressure up to 5 kbar at RT. Corrections for the changes in free electron and hole mobilities with pressure have been neglected. The resistivity changes are explained by a dependence on pressure of the ionisation energy of the radiation-induced E- and H-traps. The results indicate that most from these radiation- induced levels moves away from the conduction-band edge (γ_c-point) at a rate approximately (0.8?1.0)γ_G, here γ_G=11.6×10^?6 eV bar^?1 is the energy gap pressure coefficient for GaAs at RT. The high changes in ionization energies of E2 to E5-traps upon pressure are to be compared with the lower changes in ionization energies found for the deep-lying impurity levels. In accordance with the theoretical investigation it was suggested that most of the investigated radiation-induced levels in GaAs are t₂-states of Ga- and As-vacancies.     

Use of smoke samples in diamond-anvil cells to measure pressure dependence of optical spectra: Application to the ZnO exciton

We present measurements of ZnO exciton peak energies, E₀, at pressures up to 107.3 kbar. Smoke samples consisting of randomly oriented single crystal particles were prepared by oxidizing metallic zinc in air and were collected on one diamond face of a Merrill-Bassett pressure cell. Pressures were measured by the ruby fluorescence technique. In the pressure range between 5 and 90 kbar, our results indicate a consistent linear dependence with dE₀/dP = 2.33 × 10^?3 eV kbar^?1 for both increasing and decreasing pressures. A mixed phase structure is suggested by the observed irregular peak shapes and measured pressure dependence for the sample that had been taken beyond ? 90 kbar where the transformation to the NaCl structure has been reported.     

Fundamental optical properties of Cd1-xMnxSe single crystals

The absorption near the fundamental edge of Cd_1-xMn_xSe was measured in the composition range 0<x<0.3 at room and liquid nitrogen temperature with the electric field of the radiation parallel and perpendicular to the hexagonal axis. An exponential dependence of the absorption coefficient versus photon energy was found, and a linear dependence of energy gap E₀ on composition was obtained. The room temperature reflectivity measurements in the energy range 2.5–5.2eV, for two polarization of light were performed, and a linear dependence of the interband transitions energies vs. alloy composition was found.     

Metal-insulator transition in double cobaltites RBaCo2O5.5 (R = Eu, Gd): Specific features of their optical properties

The temperature dependence of the optical properties of GdBaCo₂O_5.5 in the region of the metal-insulator transition was studied in the spectral range λ = 4.0–0.258 μm (E = 0.3–4.8 eV) by the spectroscopic ellipsometry method. The electronic structure of the crystal at T = 300 K was calculated by the linearized muffin-tin orbital method in the local-density approximation with inclusion of on-site Coulomb interaction (LDA + U). The general features of the behavior of the optical properties of GdBaCo₂O_5.5 and EuBaCo₂O_5.5 are discussed. It is shown that the optical response on the metal side of the metal-insulator transition is determined by the redistribution of the optical-conductivity spectral weight in going from high to low energies and is significantly incoherent in character. The optical band gap width of the GdBaCo₂O_5.5 is experimentally determined to be E _g = 0.26 eV, which is close to a theoretical estimate. The type of interband transitions is determined from analyzing the temperature dependence of the optical functions for absorption bands.     

Pressure dependence of energy gaps within local density functional formalism

From a precise first principles evaluation of pressure within local density functional formalism a detailed study of the pressure dependence of the energy gaps has been made, for prototype semiconductor Si. It is shown that although the correct gaps for semiconductors cannot be obtained within local-density functional their pressure variation is still well accounted for. In particular $\text{dE}{}_{\mathrm{g}}\text{dP}$ for the minimum gap is very well reproduced. This suggests that the proposed correction Δ=E_g-ε_g, between the minimum gap E_g and the exact density functional Kohn-Sham gap, ε_g, determined by derivative discontinuities of the exchange correlation energy across the gap is pressure independent (assuming that local density functional gives a good approximation to ε_g).     

A model for electrical conductivity in (TMTSF)2ClO4, (TMTSF)2ReO4 and related materials

To describe quantitatively the diverse variations of the temperature-dependent electrical conductivities of tetramethyltetraselenafulvenium (TMTSF) salts, a model based on small Fermi (E_f) and small gap (E_g − E_f) energies and T⁻ⁿ dependence of scattering is presented. The difference between the conductivities of quenched and slowly cooled (TMTSF)₂ClO₄ is described by different values of E_f and E_g − E_f in the two cases. The model illustrates, in terms of variables E_f and E_g − E_f, the effect of pressure on the metal-insulator transition in (TMTSF)₂ReO₄, for example, and indicates a relation between E_f and E_g − E_f at the onset of superconductivity.     

Energy band structure and refractive properties of LiRbSO4 crystals

The energy band structure of mechanically free and compressed LiRbSO₄ single crystals is investigated. It is established that the top of the valence band is located at the D point of the Brillouin zone [k = (0.5, 0.5, 0)], the bottom of the conduction band lies at the Γ point, and the minimum direct band gap E _g is equal to 5.20 eV. The bottom of the conduction band is predominantly formed by the Li s, Li p, Rb s, and Rb p states hybridized with the S p and O p antibonding states. The pressure coefficients corresponding to the energies of the valence and conduction band states and the band gap E _g are determined, and the pressure dependences of the refractive indices n _i are analyzed.     

Polarization-modulated magnetoreflection in magnetic semiconductors: CdCr2S4 and EuSe

The polarization-modulated (PM) magnetoreflection technique has been used to study magnetic ordering effects in CdCr₂S₄ and EuSe. The energy gap for direct band-band transitions in CdCr₂S₄ has been measured to be E_G = 2.3 eV, and the exchange splitting of the valence band to be about 0.03 eV. Good agreement with thin film absorption measurements is obtained in the temperature dependence of spectral structure, observed at energies less than E_G, associated with crystal field and charge transfer transitions. No strongly blue-shifting peak is observed with magnetic order. PM magnetoreflection spectra of the E₁ peak of EuSe show a direct manifestation of the spin alignment from the ferrimagnetic to ferromagnetic state of this crystal at 2 K in external fields up to 16 kOe. An attempt is made to explain resonant Raman scattering in EuSe (observed by other workers) in terms of the field-induced shift of one of the polarized E₁ reflectivity components into coincidence with the 5145 Å argon-ion laser line.     

Reconstruction of heterogeneous single-channel neutralization kinetics by the method of fast-ion grazing scattering from metal surfaces

A method is suggested for the unambiguous reconstruction of the heterogeneous slow-ion neutralization kinetics near the surface of a conductor. The method is based on the special features of fast ion grazing scattering with above-thermal energies of translational motion along the normal. It is shown that the angular distributions of fast particles reflected from the surface are related to the slow-ion neutralization rate by a simple algebraic expression. The method allows the reconstruction of the coordinate dependence for the neutralization rate and the interaction potentials. Its possibilities are demonstrated by the example of neutralizing fast He⁺ ions (ion energies E₁≈2 keV and glancing angles θ₀≈0.5°–0.8°) scattered from the A1(111) surface.     

Edge-modified zigzag-shaped graphene nanoribbons: Structure and electronic properties

Control of the band gap of graphene nanoribbons is an important problem for the fabrication of effective radiation detectors and transducers operating in different frequency ranges. The periodic edge-modified zigzag-shaped graphene nanoribbon (GNR) provides two additional parameters for controlling the band gap of these structures, i.e., two GNR arms. The dependence of the band gap E _g on these parameters is investigated using the π-electron tight-binding method. For the considered nanoribbons, oscillations of the band gap E _g as a function of the nanoribbon width are observed not only in the case of armchair-edge graphene nanoribbons (as for conventional graphene nanoribbons) but also for zigzag GNR edges. It is shown that the change in the band gap E _g due to the variation in the length of one GNR arm is several times smaller than that due to the variation in the nanoribbon width, which provides the possibility for a smooth tuning of the band gap in the energy spectrum of the considered graphene nanoribbons.     

Pressure dependence of the absorption edge at the E0 gap of mocvd - Grown ZnTe films

Abstract

We have investigated the direct gap absorption of 1μm thick ZnTe-epilayers grown on GaAs substrates by metalorganic chemical vapour deposition (MOCVD). Free ZnTe-layers were obtained by selective etching. The absorption coefficient was measured up to about 50000 cm^?1 in a diamand anvil cell in the temperature range from 115–300 K. The spectra near the direct gap E₀ are dominated by a sharp excitonic structure. Its change with pressure is evaluated by a model which allows to determine the pressure shift of the gap energy dE₀/dP and the change of the Rydberg energies of the excitons dR*/dP.^[1]     

Optical properties of TiO2-MnO2 thin films prepared by electron-beam evaporation

The optical constants and thickness of TiO₂-MnO₂ films (with MnO₂ concentration of 0, 1, and 5%) prepared by electron-beam evaporation are determined. A considerable dependence of the optical properties of thin TiO₂ films on the manganese concentration is observed. It is found that thin films are indirect gap semiconductors with gap width E _g = 3.43 eV (TiO₂), 2.89 eV (TiO₂-MnO₂ (1%)), and 2.73 eV (TiO₂-MnO₂ (5%)).     

Photoluminescence studies of exciton-ionized donor complexes in high pusity epitaxial GaAs

We report the first observation of three radiative transitions associated with excitons bound to three different residual ionized donors in high purity undoped vapor phase epitaxial (VPE) GaAs at liquid helium temperature. The values of the localization energies (E_l) of excitons bounds to these ionized donors were measured. We also determine simultaneously, the ionization energies of these donors using excited state transitions of exciton-neutral donor complexes as reported earlier. The variation of the localization energy (E_l) as a function of the donor binding energy (E_D) is plotted and a linear dependence described by E_l = ? 2.22 + 0.72E_D is observed.     

The effect of pressure on impact ionisation and the Gunn effect in InAs

The pressure dependence of the threshold field for avalanche breakdown (E_A) and the Gunn effect (E_T) in InAs has been measured to 50 kbar. E_T increases approximately proportional to the effective mass when the effects of impact ionisation are eliminated. A brief comparison is made with the effects of pressure on E_T in other III–V semiconductors.     

Investigation of quasielastic muon-neutrino scattering on nuclei at <Emphasis Type="Italic">E</Emphasis><Subscript>v</Subscript> < 1 GeV

Quasielastic muon-neutrino scattering on nuclei of propane-Freon mixture at energies in the range E _v < 1 GeV is studied. The multiplicity, momentum, and emission-angle distributions of final protons are measured along with the dependence of the mean values for these distributions on the neutrino energy in the range 0.2 < E _v < 1 GeV.     

Thermoreflectance measurements on zinc sulphide-telluride alloys

Thermoreflectance measurements of the energies E₀, Δ₀ and E₁ have been made throughout the composition range of ZnS-Te alloys. The bowing parameters were found to be 2.8 ± 0.2 eV for the E₀ gap, 0.48 ± 0.08 eV for Δ₀ and 1.5 ± 0.2 eV for the E₁ energy-gap. These measured bowing parameters are in good agreement with the values calculated from EPM theory.     

Light emission at the E1 and E1+Δ1 gaps in heavily doped p-type Ge and GaAs

We report the observation in heavily doped p-type germanium (N_h ≥ 10¹⁸cm^?3) of two weak light emission bands centered at the energies of the E₁ and E₁+Δ₁ interband gaps (2.22 and 2.42 eV at 80 K). These bands, which are 100% polarized, are found only for excitation with laser frequencies slightly above the gaps. We attribute them to photon scattering by inter-valence-band excitations of the holes associated with the heavy doping. The fact that the emission bands do not shift with the exciting laser frequency is assigned to a strong resonance enhancement of this scattering near the E₁ and E₁+Δ₁ gaps. We have also observed the corresponding light emission at the E₁ gap (3.0 eV) in p-type GaAs.     

Studies on the opto-electronic properties of III–V and II–VI group semiconductors from optical electronegativities

New empirical relations have been proposed to evaluate bond energies (E_s) in compound semiconductors, first from the knowledge of optical electronegativities of the constituent ions and secondly from the energy gap values. The validity of the two relations has been tested in the case of certain III–V and II–VI group semiconductors by comparing the calculated values of E_s, with those in the literature. From the computed values of E_s, refractive indices have been calculated. The Penn gap (E_p), Fermi energy (E_F) and S_o-parameter for these semiconductors have also been determined. The estimated values of these parameters are utilized to evaluate the electronic polarizabilities (α). The computed values of a compare excellently with the standard data.     

The fission processes 236U(α, α′f) at Eα = 50 MeV and 235U(d,pf) at Ed = 23 MeV and the total kinetic energy release in fission of excited nuclei

The fissioning nucleus ²³⁶U was investigated by two different reactions. Total kinetic energies (TKE) as function of the excitation energy above the highest fission threshold are shown to have different behaviour for E_X below the two quasiparticle threshold 2Δ and above. A strong dependence on the mass splitting is observed.     

Study of structural and optical properties of ZnO films grown by pulsed laser deposition

Wurtzite zinc oxides films (ZnO) were deposited on silicon (0 0 1) and corning glass substrates using the pulsed laser deposition technique. The laser fluence, target-substrate distance, substrate temperature of 300 °C were fixed while varying oxygen pressures from 2 to 500 Pa were used. It is observed that the structural properties of ZnO films depend strongly on the oxygen pressure and the substrate nature. The film crystallinity improves with decreasing oxygen pressure. At high oxygen pressure, the films are randomly oriented, whereas, at low oxygen pressures they are well oriented along [0 0 1] axis for Si substrates and along [1 0 3] axis for glass substrates. A honeycomb structure is obtained at low oxygen pressures, whereas microcrystalline structures were obtained at high oxygen pressures. The effect of oxygen pressure on film transparency, band gap E_g and Urbach energies was investigated.