Reevaluation of bandgap and free exciton binding energy of GaP

Very sensitive measurements on the spectral behaviour of the free-to-bound excitation σ_pl^°(hν) from the valence band to the deep 0 donor in GaP at low temperatures are presented. Evaluation of the threshold energy for the electronic transition, together with the known value of the 0 binding energy, provides a simple and accurate way to determine the indirect bandgap E_g of GaP. Our new value is E_g = 2.3525 ± 0.003 eV at 1.5 K, which gives an exciton binding energy E_x = 24 ± 3 meV, considerably larger than previously used values. These data also imply an upward revision of acceptor binding energies in GaP with 10 ± 2 meV.     

Anomalous phonon intensities in the Raman spectrum of disordered CsMg1−xCoxCl3

The five Raman-active $\text{k}\text{= 0}$ phonons have been measured at low temperatures and for a range of x in the disordered lattice CsMg_1-xCo_xCl₃. While the E_2g^a, E_2g^b, E_2g^c and A_1g modes at 55.0, 132.0, 189.0 and 255.0 cm^?1 for x = 0 exhibit normal one-mode behaviour, the intensity of the E_1g phonon at 127.5 cm^?1 has a most unusual concentration dependence which requires a new theory.     

Determination of the indirect band edge of GaAs by quantum-well bandfilling (Lz∼100Å)

Photoexcitation-bandfilling of a quantum-well Al_xGa_1?xAs-GaAs-Al_xGa_1?xAs (x～0.6, L_z～100Å) heterostructure is used to generate visible-red recombination radiation that cuts-off at high energy (1.864 eV, 4.2°K) due to electron transfer to indirect minima. This leads to an estimate of ΔE = E_L?E_Γ～310 meV.     

Atomic vibrational dynamics of amorphous Fe-Mg alloy thin films

The atomic vibrational dynamics of ⁵⁷Fe in 800-Å thick amorphous Fe_xMg_1−x alloy thin films (0.3≤x≤0.7) has been investigated at room temperature by nuclear resonant inelastic X-ray scattering (NRIXS) of 14.4125 keV synchrotron radiation. The amorphous phase has been successfully stabilized by codeposition of Fe and Mg in ultrahigh vacuum onto a substrate held at −140 °C during deposition. The amorphous structure of the samples was confirmed by X-ray diffraction and conversion electron Mössbauer spectroscopy. The ⁵⁷Fe-projected partial vibrational density of states, g(E), has been obtained from the measured NRIXS vibrational excitation probability, together with thermodynamic quantities such as the probability of recoilless absorption (f-factor), the average kinetic energy per Fe atom, the average force constant, and the vibrational entropy per Fe atom. A plot of g(E)/E² versus E proves the existence of non-Debye-like vibrational excitations with a peak at E_bp∼3-5 meV (boson peak). Both the boson peak height and E_bp were found to depend linearly on the composition x. Above the boson peak, g(E)/E² exhibits an exponential decrease.     

Studies on the complex behavior of optical phonon modes in wurtzite (ZnO)1−x (Cr2O3) x

This paper reports on the use of phonon spectra obtained with laser Raman spectroscopy for the uncertainty concerned to the optical phonon modes in pure and composite ZnO_1?x (Cr₂O₃) _x . Particularly, in previous literature, the two modes at 514 and 640 cm^?1 have been assigned to ZnO are not found for pure ZnO in our present study. The systems investigated for the typical behavior of phonon modes with 442 nm as excitation wavelength are the representative semiconductor (ZnO)_1?x (Cr₂O₃) _x (x = 0, 5, 10 and 15 %). Room temperature Raman spectroscopy has been demonstrated polycrystalline wurtzite structure of ZnO with no structural transition from wurtzite to cubic with Cr₂O₃. The incorporation of Cr³⁺ at most likely on the Zn sub-lattice sites is confirmed. The uncertainty of complex phonon bands is explained by disorder-activated Raman scattering due to the relaxation of Raman selection rules produced by the breakdown of translational symmetry of the crystal lattice and dopant material. The energy of the E ₂ (high) peak located at energy 53.90 meV (435 cm^?1) due to phonon–phonon anharmonic interaction increases to 54.55 meV (441 cm^?1). A clear picture of the dopant-induced phonon modes along with the B ₁ silent mode of ZnO is presented and has been explained explicitly. Moreover, anharmonic line width and effect of dislocation density on these phonon modes have also been illustrated for the system. The study will have a significant impact on the application where thermal conductivity and electrical properties of the materials are more pronounced.     

Energy gap of the superconducting semiconductor SrTiO3−x determined by tunneling

From measurements of the differential resistance for tunneling junctions prepared from SrTiO_3?x and In(Bi) electrodes with Schottky tunneling barriers the superconducting energy gap of SrTi O_3?x(n = 3 × 10¹⁹ cm^?3) is derived. We find indications for the fact that the 1.9 meV soft phonon mode Γ₅₊ in tetragonal SrTiO₃ is important for the effective electron-electron interaction in superconducting SrTiO_3?x.     

Temperature dependance of the fundamental absorption edge in CdTe

Optical measurements made on CdTe put in light an extrinsic transition previously used to determine the interband edge of this compound. The donor level involved has a depth of 30 meV inside the bandgap at 77°K and of 60meV at 300°K. The value obtained for its temperature coefficient, suggests an association of this level with the L minimum of the conduction band.Also, determined is the true optical bandgap of CdTe between many conflicting results. One obtains: E_g = 1.529 meV with a temperature coefficient of ?3.10^?4eV/°K.     

Ionic conductivity of Zr1−xIn2xO2−x

As x in Zr(In)O_2?x is increased from 0.08 to 0.16 (9–19 mole per cent In₂O₃) the activation energy E(x) for ionic conduction increases from 1.05 to 1.51 eV; the concuctivity decreases from 2 × 10^?5 to 3 × 10^?6Ω^?1cm^?1at 400°C, is composition-independent at about 580°C, and increases from 1 × 10^?2 to 4 × 10^?2Ω^?1cm^?1 at 800°C. The pre-exponential term of the Boltzmann-type conductivity equation depends exponentially on E(x), a much stronger dependence on x than theoretically expected with a model for ionic conductivity that includes nearest-neighbor defect interactions. Analysis of reported conductivity data for Zr(M)O_2?x (M = Sc, Y, Ca and rare earth metals) and other doped oxide electrolytes with fluorite-type structure reveals that the same relationship is observed with these materials when x γ0.08. It is shown that ionic conduction in these oxides is consistent with nearest neighbor vacancy-cation defect interaction forx < 0.08 but that an additional complex interaction with composition-dependent free energy ΔG(x) occurs when xγ 0.08.The lattice constant of Zr(In)O_2?x with the cubic fluorite-type structure is independent of composition, 5.114 ± 0.002 Å, in agreement with ionic size considerations.     

Role of gas- and condensed-phase kinetics in burning rate control of energetic solids

A simplified two-step kinetics model for the combustion of energetic solids has been used to investigate the effect of gas-phase activation energy on flame structure and burning rate and the role of gas- versus condensed-phase kinetics in determining burning rate. The following assumptions are made: a single-step, unimolecular, high activation energy decomposition process which is overall relatively energetically neutral, is followed by a highly exothermic single-step, bimolecular, gas-phase reaction with arbitrary activation energy, E? _g. The results show that at extremely low (<10⁴ Pa) or high (>10¹² Pa) pressures the burning rate is controlled by the condensed-phase reaction kinetics for any E?_g. At intermediate pressures (10⁵-10¹⁰ Pa) gas reaction kinetics contribute strongly to the burning rate. In this pressure range the value of E?_g plays an important function in determining the role of gas- and condensed-phase reactions: for high E?_g a gas-phase kinetically controlled regime exists; for low E?_g both condensed and gas-phase kinetics are important. The limiting behaviour of asymptotically large E?_g (gas kinetically controlled burning rate) occurs at about E?_g=20 kcal mol^?1 for parameters representative of HMX, while the vanishingly small E?_g behaviour occurs near E?_g. Previous comparison with burning rate and temperature profile data has suggested that the small-E?_g limit is the more accurate of the two extremes. This may imply that the important (burning rate influencing) primary gas reaction zone near the surface has more the character of a chain reaction mechanism than the classical high activation energy thermal decomposition mechanism. To the degree that the low-E?_g chain reaction model is a better approximation than the high-E?_g thermal decomposition model, the possibility exists that the chemistry of either reaction zone, including the molecular structure of the material, might be exploited for favourable tailoring of burning rate. The low-E?_g model also provides a rational mechanistic explanation of observed trends in burning rate temperature sensitivity with pressure and temperature for materials like HMX in terms of a gradual transition from mixed gas- and condensed-phase kinetic control to condensed-phase only kinetic control as the pressure decreases.     

Theoretical study of optoelectronic properties of GaAs1−xBix alloys using valence band anticrossing model

The (12 × 12) and (14 × 14) valence band anticrossing (V-BAC) models were applied to calculate the electronic band structure of GaAs_1−xBi_x dilute alloys along Δ-, Λ- and Σ-directions at room temperature. A comparative study based on these models was performed in terms of energy levels, optical transitions, spin–orbit splitting and effective mass. We found a significant reduction of the band-gap energy E_g by roughly 81 meV/%Bi accompanied by an increase in the spin–orbit splitting Δ_so+ by about 56 meV/%Bi. Furthermore, Δ_so+ does come into resonance with E_g at ∼12%Bi for resonance energy equal to 0.73 eV. An excellent agreement has occurred between the (14 × 14) V-BAC model predictions and experimental results reported in the literature. In addition, we have investigated the Bi composition and k-directions dependence of the effective mass at Γ point. A slight increase of the holes effective mass with x can affect the holes transport properties of GaAsBi. The intrinsic carrier density increases with both x and the temperature T, but it remains below 10¹⁰ cm⁻³ for x ⩽ 5% and T ⩽ 300 K.     

Many-body effects in the first excited subband of the n-inversion layer of Si

We have calculated the self-energies of electrons in the lowest and first excited sub-bands of Si inversion layers. The self-consistent wavefunctions calculated in the Hartree approximation were used, and dynamic screening was approximated by the Lundqvist-Overhauser model. The correlation energy of an electron in the excited band is quite large: about ?10 meV at an inversion layer density of 10¹¹ cm^?2 to about ?16 meV at 3 × 10¹² cm^?2. The calculated separation between subbands is in very good agreement with available experimental measurements. An exciton is predicted with a binding energy of 0.9 meV at N_inv = 10¹² cm^?2 calculated in the static approximation.     

Effect of heat treatment on the optical and electrical transport properties of Ge15Sb10Se75 and Ge25Sb10Se65 thin films

The effect of heat treatment on the optical and electrical properties of Ge₁₅Sb₁₀Se₇₅ and Ge₂₅Sb₁₀Se₆₅ thin films in the range of annealing temperature 373-723 K has been investigated. Analysis of the optical absorption data indicates that Tauc's relation for the allowed non-direct transition successfully describes the optical processes in these films. The optical band gap (E_g^opt.) as well as the activation energy for the electrical conduction (ΔE) increase with the increase of annealing temperature (T_a) up to the glass transition temperature (T_g). Then a remarkable decrease in both the E_g^opt. and ΔE values occurred with a further increase of the annealing temperature (T_a>T_g). The obtained results were explained in terms of the Mott and Davis model for amorphous materials and amorphous to crystalline structure transformations. Furthermore, the deduced value of E_g^opt. for the Ge₂₅Sb₁₀Se₆₅ thin film is higher than that observed for the Ge₁₅Sb₁₀Se₇₅ thin film. This behavior was discussed on the basis of the chemical ordered network model (CONM) and the average value for the overall mean bond energy 〈E〉 of the amorphous system Ge_xSb₁₀Se_90−x with x=15 and 25 at%. The annealing process at T_a>T_g results in the formation of some crystalline phases GeSe, GeSe₂ and Sb₂Se₃ as revealed in XRD patterns, which confirms our discussion of the obtained results.     

Analysis of the sideband fluorescence of Eu2+:CaF2

The vibronic fluorescence of Eu²⁺:CaF₂ at 4130 Å is calculated be treating the associated Jahn-Teller problem in the classical static limit and extracting the usual oscillator shifts in a cluster calculation. A_1g and E_g contributions are separated via the pronounced one-phonon structure. The J-T stabilization energy and the 10Dq value for the 5d electron are estimated to be 600 cm^?1 and 16,000 cm^?1 respectively.     

Specific features of the optical and electrical properties of polycrystalline CdTe films grown by the thermal evaporation method

The results of studying the physical properties of thin CdTe films obtained by the thermal evaporation method have been presented. The optical constants and the band gap of the films under study have been determined (E _g = 1.46 eV). It has been established based on the investigation of optical properties and the Raman spectrum of the films that they possess high structural quality. The activation energy of the electrical conductivity of CdTe films has been determined: E _a = 0.039 eV. The measured spectral dependences of the impedance of CdTe thin films are characteristic of the inhomogeneous medium with two time constants: τ_gb = R _gb C _gb = 1/ω_gb = 1.62 × 10^?3 s and τ_g = R _g C _g = 1/ω_g = 9.1 × 10^?7 s for grain boundaries and grains, respectively.     

Estimation of the energy of the electron-photon component of cosmic rays on the basis of data for Cherenkov light from ultrahigh-energy extensive air showers

The energy fraction E _em/E ₀ dissipated to the electron-photon component of extensive air showers (EASs) for E ₀=10¹⁵?10¹⁹ eV is estimated using data on Cherenkov radiation and charged particles from the Yakutsk EAS array. The results are compared with models with different dissipations to the electron-photon component and with calculations for various primary nuclei. In the energy range 10¹⁵?10¹⁶ eV and 10¹⁸?10¹⁹ eV, the ratio E _em/E ₀ is equal to 77 ± 2 and 88 ± 2, respectively, in agreement with the mixed and proton contents of primary cosmic rays in the former and latter energy ranges, respectively.     

A tunneling spectroscopy study of the temperature dependence of the forbidden band in Bi2Te3 and Sb2Te3

Tunneling measurements of dI/dV, d ² I/dV ², and d ³ I/dV ³ were formed along the C ₃ axis (normally to layers) for Bi₂Te₃ and Sb₂Te₃ layered semiconductors in the temperature range 4.2<T>29 5 K. Temperature dependences of the forbidden band energy E _g were obtained. The forbidden band energy in Bi₂Te₃ was 0.20 eV at room temperature and increased to 0.24 eV at T=4.2 K. The E _g value for Sb₂Te₃ was 0.25 eV at 295 K and 0.26 eV at 4.2 K. The distance between the top of the higher valence band of light holes and the top of the valence band of heavy holes situated lower was found to be ΔE _V≈19 meV in Bi₂Te₃; this distance was independent of temperature. The conduction bands of Bi₂Te₃ and Sb₂Te₃ each contain two extrema with distances between them of ΔE _c≈25 and 30 meV, respectively.     

The influence of AlN interlayers on the microstructural and electrical properties of p-type AlGaN/GaN superlattices grown on GaN/sapphire templates

AlN with different thicknesses were grown as interlayers (ILs) between GaN and p-type Al_0.15Ga_0.85N/GaN superlattices (SLs) by metal organic vapor phase epitaxy (MOVPE). It was found that the edge-type threading dislocation density (TDD) increased gradually from the minimum of 2.5×10⁹?cm^?2 without AlN IL to the maximum of 1×10¹⁰?cm^?2 at an AlN thickness of 20 nm, while the screw-type TDD remained almost unchanged due to the interface-related TD suppression and regeneration mechanism. We obtained that the edge-type dislocations acted as acceptors in p-type Al _x Ga_1?x N/GaN SLs, through the comparison of the edge-type TDD and hole concentration with different thicknesses of AlN IL. The Mg activation energy was significantly decreased from 153 to 70?meV with a?10-nm AlN IL, which was attributed to the strain modulation between AlGaN barrier and GaN well. The large activation efficiency, together with the TDs, led to the enhanced hole concentration. The variation trend of Hall mobility was also observed, which originated from the scattering at TDs.     

On the valence stability of Eu in the laves phase compounds Eu1-xPtxPt2 under high-pressure

High-pressure Mössbauer studies of the ¹⁵¹Eu resonance in Eu_1-xPt_xPt₂ (x = 0;0.143;0.25) have been performed between 4.2 and 300 K. A linear decrease in the magnitude of the interconfiguration excitation energy E_exc on the Eu²⁺-site with decreasing lattice constant was found (?|E_exc|/?lnV = 1.6 x 10⁴K). The isomer shift for EuPt₃ at 70 kbar and 4.2 K shows that the Eu atoms on the Eu²⁺-site are no longer pure divalent but have ? 5% admixture of the Eu³⁺ cofiguration.     

Temperature shift of the CdxHg1−xTe energy gap

The temperature coefficient of the Cd_xHg_1?xTe energy gap dE_g/dT as a sum of lattice dilatation and the phonon-electron interaction terms has been calculated as the function of molar composition x, for 0?x?0.3, in the temperature range 4.2–300 K. A non-linear dependence of $\text{d}\text{E}{}_{\mathrm{g}}\text{d}\text{T}$ vs x and a strong effect of temperature on $\text{d}\text{E}{}_{\mathrm{g}}\text{d}\text{T}$ values have been obtained and a comparison with experimental data is discussed.     

Photon-gluon fusion at HERA: measurement of the fractional momentum of the gluon in lowQ 2 events

We have performed a Monte Carlo study of photon-gluon interactions at theep collider HERA. We show that it is possible to determine the fractional momentum carried by the gluon (x _g) from hadronic informations alone. In particular, in the photoproduction region, where thex andQ ² are badly measured, this could be the only global physical variable of interest, together with the hadronic invariant mass. The study of these quantities would allow a direct measurement of the gluon density of the proton in the rangex _g=2.5×10^?3∶5×10^?1.