On the photoemission from III–V, ternary and quaternary materials: Simplified theory and relative comparison

We study theoretically the electron energy spectrum and the photoemission from III–V, ternary and quaternary materials in the presence of light waves, whose unperturbed energy band structures are defined by the three-band model of Kane. The band gap of semiconductors increases as a consequence of incident light waves and we have suggested two new experimental methods of determining the band gap of semiconductors in the presence of photoexcitations. The solution of the Boltzmann transport equation on the basis of this newly formulated electron dispersion law will introduce new physical ideas and experimental findings in the presence of external photoexcitation. It has been found taking n-InAs, n-InSb, n-Hg_1−xCd_xTe and n-In_1−xGa_xAs_yP_1−y lattice matched to InP, as examples that the photoemission increases with the increase in electron concentration and decreases in increasing intensity, wavelength and alloy composition, respectively, in various manners. The numerical values of the photoemission in the presence of light waves is less than that of the same for unperturbed three- and two-band models of Kane together with parabolic energy bands for all types of external variables. The strong dependence of the photoemission on the light intensity reflects the direct signature of light waves on the dispersion relation of the conduction electrons, which is in contrast when compared with the corresponding bulk specimens for the unperturbed band models. The rate of change is totally band structure dependent and is significantly influenced by the presence of the different energy band constants. The well-known result of the photoemission from non-degenerate wide gap materials has been obtained as a special case of the present analysis under certain limiting conditions and this compatibility is the indirect test of our generalized formalism. Besides, we have suggested six important applications of our results in this context.     

Effect of Ti and C additions on structural and magnetic properties of (Pr,Nd)–Fe–B nanocrystalline magnetic materials

Effects of titanium carbide (TiC) addition on structural and magnetic properties of isotropic (Pr,Nd)–Fe-B nanocrystalline magnetic materials have been investigated. In this work, we investigate the effect of TiC addition on a (Pr,Nd)-poor and B-rich composition, as well as on a B-poor and (Nd,Pr)-rich composition. Rapidly solidified (Pr,Nd)–Fe–B alloys were prepared by melt-spinning. The compositions studied were (Pr_1−xNd_x)₄Fe₇₈B₁₈ (x=0, 0.5, and 1) with addition of 3 at% TiC. Unlike the (Pr_xNd_1−x)_9.5Fe_84.5B₆ materials that present excellent values for coercive field and energy product, the (Pr,Nd)-poor and B-rich composition alloys with TiC addition present lower values. Rietveld analysis of X-ray data and Mössbauer spectroscopy revealed that samples are predominantly composed of Fe₃B and -Fe. For the RE-rich compositions (Pr_xNd_1−x)_9.5Fe_84.5B₆ (x=0.1, 0.25, 0.5, 0.75, and 1) with the addition of 3 at% TiC, the highest coercive field and energy product (8.4 kOe and 14.4 MGOe, respectively) were obtained for the composition Pr_9.5Fe_84.5B₆.     

Interband Stark effects in InxGa1−xAs/InyAl1−yAs coupled step quantum wells

The effects of an electric field on the interband transitions in In_xGa_1−xAs/In_yAl_1−yAs coupled step quantum wells have been investigated both experimentally and theoretically. A In_xGa_1−xAs/In_yAl_1−yAs coupled step quantum well sample consisted of the two sets of a 50 Å In_0.53Ga_0.47As shallow quantum well and a 50 Å In_0.65Ga_0.35As deep step quantum well bounded by two thick In_0.52Al_0.48As barriers separated by a 30 Å In_0.52Al_0.48As embedded potential barrier. The Stark shift of the interband transition energy in the In_xGa_1−xAs/In_yAl_1−yAs coupled step quantum well is larger than that of the single quantum well, and the oscillator strength in the In_xGa_1−xAs/In_yAl_1−yAs coupled step quantum well is larger than that in a coupled rectangular quantum well. These results indicate that In_xGa_1−xAs/In_yAl_1−yAs coupled step quantum wells hold promise for potential applications in optoelectron devices, such as tunable lasers.     

Band structure and lattice vibration properties of III-P ternary alloys

Electronic band structure, optical and vibrational properties of zinc-blende GaP_xSb_1−x and GaAs_1−xP_x ternary alloys are obtained from pseudo-potential calculations. Comparisons are made with the available experimental values and with data obtained in previous theoretical studies. These comparisons show generally good agreement between the present results and experiment. The direct and indirect band-gap energies, the transverse effective charge, and the longitudinal and transversal optical phonon energies show a non-linear behavior with varying the composition x. The ionicity of the materials of interest is discussed in terms of the antisymmetric gap.     

Dependence of the electronic parameters on the InyGa1−yAs quantum well width in modulation-doped AlxGa1−xAs/InyGa1−yAs/GaAs strained single quantum wells

The variation of the electronic parameters in the subband as a function of the In_yGa_1−yAs quantum well width in modulation-doped strained Al_xGa_1−xAs/In_yGa_1−yAs/GaAs single quantum wells were investigated by means of Shubnikov-de Haas (S-dH) and Van der Pauw Hall-effect measurements. The fast Fourier transform (FFT) of the S-dH oscillations and the Hall-effect data showed that the carrier density and the mobility of the two-dimensional electron gas (2DEG) occupied in the subband increased as the quantum well width increased. The increase in the 2DEG density with increasing the In_yGa_1−yAs well width originated from an increase in the energy difference between the energy level of the electronic subband and Fermi energy, and the increase in the 2DEG mobility is attributed to a decrease of the scattering source. The electronic subband energies, the corresponding wavefunctions, and the Fermi energies in the Al_xGa_1−xAs/In_yGa_1−yAs/GaAs single quantum wells were calculated by a self-consistent method taking into account the exchange-correlation effect together with the strain and nonparabolicity effects. These results indicate that the electronic parameters in Al_xGa_1−xAs/In_yGa_1−yAs/GaAs strained single quantum wells are significantly dependent on the quantum well width.     

W doping-dependent structural and ferroelectric properties of SrBi2Nb2O9 ferroelectric ceramics

The structural and ferroelectric characteristics of SrBi₂(Nb_1−xW_x)₂O₉ (x=0–0.12) ferroelectric ceramics were investigated. SrBi₂(Nb_1−xW_x)₂O₉ ceramics consisted of a single-phase layered perovskite structure when x was less than 0.06. Uniform microstructure and grain size reduction were observed after the introduction of W. The maximum remanent polarization of 16 μC/cm² appeared at x=0.03, and the coercive field decreased with increasing concentration of W. The ferroelectric behavior of SrBi₂(Nb_1−xW_x)₂O₉ ceramics is interpreted based on the Raman measurement.     

Optical characterization of InGaN/AlGaN/GaN diode grown on silicon carbide

Electroluminescence (EL) properties of In_xGa_1−xN/Al_yGa_1−yN/GaN/SiC diode were studied. The spectral range for which EL spectra were recorded is 1–3.5 eV. Room temperature EL was obtained for forward bias (3.18 V, 220 μA) at 446.067 nm (blue luminescence band), 606.98 nm (yellow luminescence band) and 893.84 nm (Infrared luminescence band). The EL temperature dependence shows that, BL band is mostly given by e–h recombination corresponding to indium composition equal to 0.17 ± 0.01 and 0.14 ± 0.02 obtained theoretically and experimentally, respectively. The yellow band is generally weak and absent at low temperature. The IRL band is more consistent with the DAP recombination and could be explained by the thermal activation of Mg states. The luminescence bands shift to lower energies is due probably to the larger potential fluctuations effect.     

Phase composition-dependent physical and mechanical properties of YbxZr1−xO2−x/2 solid solutions

The paper represents a detailed insight into the correlation between changes of the phase composition of crystalline Yb_xZr_1−xO_2−x/2 solid solutions and their structural, electrical, mechanical and optical properties. Particularly, the effect of the crystal growth conditions and stabilizer amount in the range of 1.5–13.8 mol% of Yb₂O₃ are studied in terms of Rietveld analysis of powder X-ray diffraction data, electrical conductivity measured by impedance spectroscopy, absorption coefficient and refractive index measurements, Vickers microhardness (classical technique) as well as the plastic microhardness and effective elastic modulus (DSI—depth-sensing indentation technique). Potential applications of the investigated systems are discussed in view of the results obtained.     

Phase formation and magnetic properties of Nd2Fe14B-type Nd16Co76B8−xCx alloys and their hydrides

An attempt is made to synthesize Nd₂Co₁₄C compound by mechanical alloying Nd₁₆Co₇₆B_8−xC_x (0x8) alloys and subsequent annealing. Phase formation and magnetic properties of Nd₂Fe₁₄B-type Nd₁₆Co₇₆B_8−xC_x alloys and their hydrides are investigated. The Nd₂Co₁₄(B,C) phase with Nd₂Fe₁₄B-type structure is formed for Nd₁₆Co₇₆B_8−xC_x (0x7) alloys, while NdCo₇C_δ phase with TbCu₇-type structure is observed in Nd₁₆Co₇₆C₈ alloy. The lattice parameter c of the Nd₂Co₁₄(B,C) phase decreases with increasing the carbon content. A limit volume of the unit cell to form the Nd₂Fe₁₄B-type structure is estimated to be 0.870 nm³. The spin-reorientation temperature T_SR increases with increasing the carbon content, due to an enhancement of magnetocrystalline anisotropy caused by carbon substitution for boron. After hydrogenation, the lattice expansion is observed for Nd₁₆Co₇₆B_8−xC_x (0x7) alloys. The spin-reorientation temperature of Nd₁₆Co₇₆B_8−xC_xH_y (0x7) is much lower than that of the host alloys. Some structural and magnetic properties of hypothetic Nd₂Co₁₄C and Nd₂Co₁₄CH_y compounds are estimated by extrapolation.     

Composition and thermal-induced effects on the optical constants of Ge20Se80−xBix thin films

 The effects of composition and thermal annealing near crystallization temperature, T_c on the optical and structural properties of Ge₂₀Se_80−xBi_x (x=0, 2.5, 5 and 7.5 at%) was investigated. The influence of incorporation Bi content in Ge₂₀Se_80−xBi_x system results in a gradual decrease in the indirect optical gap from 1.89 to 1.44 eV, this behavior can be explained as increased tailing. On annealing, the optical band gap E_g decreases gradually for the crystallized films while the refractive index increases, this behavior can be attributed to transformation from amorphous to crystalline and was explained in the light of dangling bond model. The refractive index n of as-prepared and annealed films has been analyzed according to the Wwmple–DiDominico single oscillator model and the values of E_o and E_d were determined. The effect of annealing on the nature and degree of crystallization has been investigated by studying the structure using transmission electron microscope, X-ray diffraction and scanning electron microscope.     

Electro-optical characteristics and field-emission properties of reactive DC-sputtered p-CuAlO2+x thin films

P-type transparent-conducting CuAlO_2+x thin films were deposited on silicon and glass substrates by reactive direct current sputtering of a prefabricated metal powder target having 1:1 atomic ratio of Cu and Al in oxygen-diluted argon atmosphere. XRD spectrum confirmed the proper phase formation of the material. UV-Vis-NIR spectrophotometric measurements showed high transparency of the films in the visible region with direct and indirect band gap values around 3.90 and 1.89 eV, respectively. The room temperature conductivity of the film was of the order of 0.22 S cm⁻¹ and the activation energy was 0.25 eV. Seebeck coefficient at room temperature showed a value of +115 μV/K confirming the p-type nature of the film. Room temperature Hall effect measurement also indicated positive value of Hall coefficient with a carrier concentration 4.4×10¹⁷ cm⁻³. We have also observed the low macroscopic field emission, from the wide band gap p-CuAlO_2+x thin film deposited on glass substrate. The emission properties have been studied for different anode-sample spacing. The threshold field was found to be as low as around 0.5–1.1 V/μm. This low threshold is attributed primarily to the internal nanostructure of the thin film, which causes considerable geometrical field enhancement inside the film as well as at the film/vacuum interface.     

Magnetic properties of Lu2Co17−xSix single crystals

The Co-sublattice anisotropy in Lu₂Co₁₇ consists of four competitive contributions from Co atoms at crystallographically different sites in the Th₂Ni₁₇-type of crystal structure, which result in the appearance of a spontaneous spin-reorientation transition (SRT) from the easy plane to the easy axis at elevated temperatures. In order to investigate this SRT in detail and to study the influence of Si substitution for Co on the magnetic anisotropy, magnetization measurements were performed on single crystals of Lu₂Co_17−xSi_x (x=0−3.4) grown by the Czochralski method. The SRT in Lu₂Co₁₇ was found to consist of two second-order spin reorientations, “easy-plane”–“easy-cone” at T_SR1≈680 K and “easy-cone”–“easy-axis” at T_SR2≈730 K. Upon Si substitution for Co, both SRTs shift toward the lower temperatures in Lu₂Co₁₆Si (T_SR1≈75 K and T_SR2≈130 K) with the further onset of the uniaxial type of magnetic anisotropy in the whole range of magnetic ordering for Lu₂Co_17−xSi_x compounds with x>1 due to a weakening of the easy-plane contribution from the Co atoms at the 6g and 12k sites to the total anisotropy.     

Composition and electric field effects on the transport properties of Bi doped chalcogenide glasses thin films

The temperature dependence of the DC electrical conductivity σ_DC was measured in the temperature range from 300–500 K. It was found that there are double activation energies, E_σ, for Ge₂₀Se_80−xBi_x (x=0, 2.5 and 5 at%) films, while there is single activation energy for Ge₂₀Se_72.5Bi_7.5. when incorporation of Bi=7.5 at%, the pre-exponential value σ₀ decreases by about six order of magnitude, the activation energy in the extended states E_σ decreases from 0.96 to 0.09 eV. Also the effect of applied electric field was studied and observed that, activation energy in high temperature region increases with increasing electric field; this behavior can be understood assuming that the contribution to the conductivity activation process is due to conduction in the extended states and also due to hopping in the localized states. With the increasing electric field, as former process, which is having high activation energy, becomes more predominant due to the dumping of the carriers in the extended states, the effective activation energy of the system increases, in spite of the fact that the activation energy of the extended states conduction may remain constant. Finally, the electrical data suggests that the addition of bismuth produces localized states near the conduction band edge so that the electrical transport is due to hopping of electrons after being excited into localized states at the conduction band edge.     

Note on spin–orbit interactions in nuclei and hypernuclei

A detailed comparison is made between the spin–orbit interactions in Λ hypernuclei and ordinary nuclei. We argue that there are three major contributions to the spin–orbit interaction: (1) a short-range component involving scalar and vector mean fields; (2) a “wrong-sign” spin–orbit term generated by the pion exchange tensor force in second order; and (3) a three-body term induced by two-pion exchange with excitation of virtual Δ(1232)-isobars (à la Fujita–Miyazawa). For nucleons in nuclei the long-range pieces related to the pion-exchange dynamics tend to cancel, leaving room dominantly for spin–orbit mechanisms of short-range origin (parametrized, e.g., in terms of relativistic scalar and vector mean fields terms). In contrast, the absence of an analogous 2π-exchange three-body contribution for Λ hyperons in hypernuclei leads to an almost complete cancellation between the short-range (relativistic mean-field) component and the “wrong-sign” spin–orbit interaction generated by second order π-exchange with an intermediate Σ hyperon. These different balancing mechanisms between short- and long-range components are able to explain simultaneously the very strong spin–orbit interaction in ordinary nuclei and the remarkably weak spin–orbit splitting in Λ hypernuclei.     

The stability of Na-doped Bi2(NbCr)O7−x pyrochlores: The non-existence of “(BiNa)(NbCr)O6”

We show that the pyrochlore described as “(BiNa)(NbCr)O₆” is in fact a non-stoichiometric pyrochlore with an approximate composition of (Bi_1.33Na_0.67)(Nb_1.33Cr_0.67)O_7−x. Refinement of this structure using constant wavelength powder neutron diffraction data reveals the presence of vacancies in the anion sites coupled with displacive disorder of the Bi and Na cations. This is necessary to achieve a satisfactory bonding arrangement for both the Bi and Na cations that occupy the pyrochlore A-type sites. Attempts to prepare other pyrochlores in the series Bi_2−xNa_x(NbCr)O₆ were unsuccessful and it appears that the pyrochlore is only stable only over a very narrow composition range. The structure of the pure Bi pyrochlore Bi₂(NbCr)O₇ is also described.     

Magnetocaloric and magnetoresistance properties of La2/3Sr1/3Mn1−xCoxO3 compounds

Structural, magnetic, magnetoresistance and magnetocaloric studies on La_2/3Sr_1/3Mn_1−xCo_xO₃ compounds were reported. The samples were prepared by the conventional ceramic method. X-ray analysis showed the presence of one phase only, in all studied samples. From electrical resistance measurements it was found that the samples show large negative magnetoresistance behavior. The magnetic measurements were performed in a large temperature range, 4.2–750 K and external magnetic fields up to 5 T. The adiabatic magnetic entropy changes, |ΔS|, were determined from magnetization data. Large magnetocaloric effect (MCE) has been obtained in all studied samples.     

Structure and composition of secondary phase particles in cobalt-doped TiO2 films

The microstructural properties of secondary phase particles formed in epitaxial Co_xTi_1−xO₂ anatase thin films grown on (0 0 1)LaAlO₃ by a reactive RF magnetron co-sputter deposition are examined. These films exhibit ferromagnetic behavior in magnetization measurements, showing a M–H loop at room temperature with a saturation magnetization on the order of 0.7 μ_B /Co. X-ray photoemission spectrometry indicates that the Co cations are in the Co²⁺ valence state. Cross-section electron microscopy reveals that a significant fraction of the cobalt segregates into Co–Ti–O secondary phase particles. Selected area electron diffraction shows that the secondary phase particles are cobalt-rich anatase. While the cobalt is concentrated in the segregated particles, local energy dispersive spectrometry indicates some Co throughout the film.     

Low-field magnetoresistance of Ag-substituted perovskite-type manganites

A series of bulk polycrystalline Nd_1−xAg_xMnO₃ samples were prepared by conventional solid-state reaction processing, for x between 0.1 and 0.5. The structure, magnetism, and magnetoresistance (MR) of the samples are investigated. The X-ray diffraction patterns show that the x=0.1 sample is a single perovskite structure, while x0.167, samples consist of a ferromagnetic perovskite phase and two nonmagnetic phases, Ag and Mn₃O₄. The MR of Nd_1−xAg_xMnO₃ is enhanced by increasing the composition of Ag. Compared with the sample of x=0.1, the x=0.5 sample has a significantly larger value of low-field MR. The enhanced low-field MR is related to the spin-dependent scattering of spin-polarized electrons at the interfaces between the perovskite grains and silver granules.     

Effects of substitution of Ti for Fe in BiFeO3 films prepared by sol–gel process

Ti substituted BiFe_1−xTi_xO_3+δ films have been prepared on indium–tin oxide (ITO)/glass substrates by the sol–gel process. The films with x=0.00–0.20 were prepared at an annealing temperature of 600 °C. X-ray diffraction patterns indicate that all films adopt R3m structure and the films with x=0 and 0.10 show pure perovskite phase. Cross-section scanning shows the thickness of the films is about 300 nm. Through 0.05 Ti substitution, the 2P_r increases to 8.30 μC/cm² from 2.12 μC/cm² of the un-substituted BiFeO₃ film and show enhanced ferroelectricity at room temperature. The 2P_r values are 2.63 and 0.44 μC/cm² for the films with x=0.01 and 0.2, respectively. Moreover, the films with x=0.05 and 0.10 show enhanced dielectric property since the permittivity increases near 150 at the same measuring frequency. Through the substitution of Ti, the leakage conduction is reduced for the films with x=0.05–0.20.     

Study of the ferroelectric–paraelectric phase transition of NaH3(SeO3)2 single crystals by H and Na NMR

The ¹H and ²³Na spin–lattice and spin–spin relaxation times of NaH₃(SeO₃)₂ single crystals grown using the slow-evaporation method were measured as functions of temperature and frequency in the ferroelectric and paraelectric phases. The changes in the symmetry of the (SeO₃)²⁻ dimers as a result of the ferroelectric–paraelectric phase transition are associated with large changes in the spin–lattice and spin–spin relaxation times, and in the number of resonance lines. The large changes in the relaxation times at 195 K indicate that the H and Na ions are significantly affected by this transition. The change in the number of resonance lines for the ¹H and ²³Na nuclei means that the orientations of the (SeO₃)²⁻ dimers and the environments of the Na ions change at T_C. Therefore, the orientations of the (SeO₃)²⁻ dimers and the environments of the Na ions play important roles in the phase transitions. In conclusion, the ferroelectric–paraelectric phase transition of NaH₃(SeO₃)₂ is accompanied by changes in hydrogen-bond structure and distortions of the (SeO₃)²⁻ and Na⁺ ion lattices, which form a slightly distorted octahedron.