Photoluminescence and multiphonon resonant Raman scattering in Ni-and Co-doped Zn1−x MnxTe crystals

Low-temperature photoluminescence, exciton reflection, and multiphonon resonant Raman scattering spectra of Ni-and Co-doped Zn_1−x Mn_xTe crystals were investigated. Intense emission occurs in a broad spectral region (1100–17 000 cm⁻¹) in the crystals containing Ni atoms. It is caused by intracenter transitions involving Mn²⁺ ions and transitions between the conduction band and a level of the doubly charged acceptor. The features of the exciton photoluminescence and multiphonon resonant Raman scattering involving longitudinal-optical (LO) phonons at various temperatures are investigated. The insignificant efficiency of the localization of excitons on potential fluctuations in the Zn_1−x Mn_xTe:Co crystals is established. A temperature-induced increase in the intensity of the 5LO multiphonon resonant Raman scattering line due to the approach of the conditions for resonance between this line and the ground exciton state is observed in these crystals. Fiz. Tverd. Tela (St. Petersburg) 40, 616–621 (April 1998)     

Thermoelectric power in quasi-two-dimensional systems with scattering of current carriers by phonons

The thermopower α in electron systems with a quasi-two-dimensional energy spectrum is investigated in the relaxation-time tensor approximation. The longitudinal and transverse components of the thermopower are calculated for scattering of the current carriers by different types of phonons. It is shown that the anisotropy of the thermopower in such systems is substantial. The dependence of a on the ratio of the Fermi level ɛ _F to the half-width ɛ ₀ of the one-dimensional conduction band is considered. For scattering by acoustical and nonpolar optical phonons, the thermopower changes sign: α becomes positive for ɛ _F<ɛ ₀. Comparison of the theory with published experimental data demonstrates good qualitative agreement. Fiz. Tverd. Tela (St. Petersburg) 39, 1857–1858 (October 1997)     

Fluorescence of N,N-di(2-carboxyethyl)-<Emphasis Type="Italic">p</Emphasis>-anisidine in solution and crystalline states

The fluorescence properties of N,N-di(2-carboxyethyl)-p-anisidine (I) in solvents of various nature and in the crystalline state have been studied at room temperature (273 K) and at the boiling point of liquid nitrogen (77 K). Fluorescence in aqueous solutions of I with protonated (λ _ex /λ _fl ^max = 225/290 nm) and unprotonated (λ _ex /λ _fl ^max = 270/380 nm) amino nitrogen has been detected. On going from aqueous solutions to nonaqueous, the fluorescence band of unprotonated I experiences a blue shift and its intensity rises. The fluorescence intensity of the band in aprotic polar solvents is higher than that in protic solvents. A linear dependence of the fluorescence intensity of deprotonated I on Cu(II) concentration (ranging from 1.0 to 5.0 mg/dm³) in aqueous solution has been found. The fluorescence intensity of I in aqueous solutions at 77 K and pH 1–6 has been shown to increase in the presence of Zn(II) (1–170 mg/dm³) and Cd(II) (2–330 mg/dm³) although a similar dependence is not observed at 293 K.     

Slow components of the emission decay in fluoride crystals doped with Ce3+

Spatially separated defects created by photons with energies 6–8 eV in alkali-earth fluoride crystals doped with cerium are investigated with the help of thermoluminescence. Measuring the spectra of creation of V_k and H peaks of thermostimulated luminescence inBaF ₂:Ce³⁺. we demonstrate that photons with energies higher than 6eV induce H centers (self-trapped holes captured by interstitialF ions), whereas the formation of self-trapped holes begins on exposure to photons with energies greater than 7 eV. The influence of photoionization on theCe ³⁺ luminescence inBaF ₂, SrF₂, CaF₂, andCeF ₃ crystals is investigated in the range of photon energies 4–8 eV. An exponentialCe ³⁺-emisson decay was observed for excitation energy lying in the range 4–6 eV. Slow and fast decay components were observed under excitation by photons with energies higher than 6 eV. We believe that the slow and fast components are due to the tunnel recombination of trapped electrons with hole centers. A. P. Vinogradov Institute of Geochemistry of the Siberian Branch of the Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 43–49, March, 2000.     

Absorption spectrum and excitons in thin films of the solid electrolyte RbCu4Cl3I2

We have investigated the absorption spectrum of thin films of the superionic conductor RbCu₄Cl₃I₂ synthesized on NaCl crystalline substrates. It is shown that the electron and exciton excitations in the energy interval 3–6 eV are associated with optical transitions in the CuHal sublattice, and the edge of the fundamental band is controlled by optical transitions in the Cu(II)Hal sublattice. It is found that the large band gap of this compound (E _g =3.86 eV) in comparison with those of CuCl and CuI is a result of the small number of Cu ions in the second coordination sphere. The temperature dependence of the spectral position and half-width of the low-temperature exciton band reveals features associated with the phase transitions γ→β (T _c1=170 K) and β→α (T _c2=220 K) and with disordering of the cation sublattice attendant to the transition to the superionic state. Fiz. Tverd. Tela (St. Petersburg) 40, 1022–1026 (June 1998)     

Specific features and mechanisms of photoluminescence of nanostructured silicon carbide films grown on silicon in vacuum

The light-emitting properties of cubic silicon carbide films grown by vacuum vapor phase epitaxy on Si(100) and Si(111) substrates under conditions of decreased growth temperatures (T _gr ∼ 900–700°C) have been discussed. Structural investigations have revealed a nanocrystalline structure and, simultaneously, a homogeneity of the phase composition of the grown 3C-SiC films. Photoluminescence spectra of these structures under excitation of the electronic subsystem by a helium-cadmium laser (λ_excit = 325 nm) are characterized by a rather intense luminescence band with the maximum shifted toward the ultraviolet (∼3 eV) region of the spectral range. It has been found that the integral curve of photoluminescence at low temperatures of measurements is split into a set of Lorentzian components. The correlation between these components and the specific features of the crystal structure of the grown silicon carbide layers has been analyzed.     

Photoluminescence of nanostructured cubic-lattice silicon carbide films grown on a silicon wafer

The light-emitting properties of cubic-lattice silicon carbide SiC films grown on Si(100) and Si(111) substrates with VPE at low temperatures (T _gr ∼ 700°C) are discussed. Investigations of the grown films reveal a homogeneous nanocrystalline structure involving only the 3C-SiC phase. When the electron subsystem of the structure is excited by a He-Cd laser emitting at λ_exit = 325 nm, the photoluminescence (PL) spectra contain a rather strong emission band shifted by about 3 eV toward a short-wave spectral region. At low temperatures, the PL integral curve is split into a set of Lorentz components. The relation between these components and the peculiarities of the energy spectrum of electrons in the nanocrystalline grains of the silicon carbide layers is discussed.     

Theoretical model of phonon propagation in micro- and nanostructural materials

Peculiarities of the kinetics of weakly nonequilibrium thermal phonons (λ_ph = 10−50 nm) in nanodispersed ceramics are investigated. The features of the phonon spectrum in the wavelength range λ_ph ∼ R _g (where R _g is the average size of a ceramic grain) are studied. The conditions yielding the formation of a gap in the phonon spectrum of nanodispersed materials are analyzed. It is shown that the presence of inclusions with characteristic sizes smaller than the grain size of the parent ceramic material (pores, inclusions of other phases) shifts the gap toward the high-frequency range of the phonon spectrum. Peculiarities of the transport of nonequilibrium phonons during the transition from nano- to microdispersed sizes of the ceramic structure are analyzed.     

Temperature dependent photoluminescence of PECVD a-SiOx:H (x<2)

The temperature dependent visible photoluminescence (PL) property of a-SiO_x:H (x<2) samples prepared in a PECVD system by using SiH₄+CO₂ gas mixture is investigated at a temperature range of 20 K-400 K. One of the two explicitly distinguished PL bands, with varying peak photon energies between 1.70 and 2.05 eV, can be detected at only low temperatures below 200 K, which is attributed to tail-to-tail radiative recombination. Thermal quenching parameter (T_L) of the tail-to-tail PL band is calculated as varying between 120 and 280 K as the atomic oxygen concentration ([O]at.%) of the samples increases. Stokes shift (ΔE_Stokes) of the tail-to-tail PL band is found to change from 85 meV to 420 meV due to band tail widening. The other PL band emerges at 2.1 eV and can be detected at higher temperatures with thermal activation behavior. The activation energies calculated about room temperature vary in the range of 8 meV-50 meV with oxygen concentration. Thermal activation of the 2.1 eV PL band is attributed to the behavior of thermally activated incoherent hopping migration of electrons. These electrons combine with self trapped holes (STHs) to form self trapped excitons (STEs). STEs are localized at intrinsic defects of SiO₂ structure such as oxygen vacancies (E′ centers) and non-bridging oxygen hole centers (NBOHC).     

The effect of sub-band gap photon illumination on the properties of GaN layers grown on Si(111) by MBE

Nanostructured GaN layers are fabricated by laser-induced etching processes based on heterostructure of n-type GaN/AlN/Si grown on n-type Si(111) substrate. The effect of varying laser power density on the morphology of GaN nanostructure layer is observed. The formation of pores over the structure varies in size and shape. The etched samples exhibit dramatic increase in photoluminescence intensity compared to the as-grown samples. The Raman spectra also display strong band at 522 cm⁻¹ for the Si(111) substrate and a small band at 301 cm⁻¹ because of the acoustic phonons of Si. Two Raman active optical phonons are assigned h-GaN at 139 and 568 cm⁻¹ due to E2 (low) and E2 (high), respectively. Surface morphology and structural properties of nanostructures are characterized using scanning electron microscopy and X-ray diffraction. Photoluminance measurement is also taken at room temperature by using He–Cd laser (λ = 325 nm). Raman scattering is investigated using Ar⁺ Laser (λ = 514 nm).     

Momentum conservation in the luminescence of modulation-doped Ga x In1−x As−Al y In1−y As quantum wells

Summary Thek-conserving selection rule in the electron-hole recombination is investigated by intensity-dependent photoluminescence measurements inn-type modulation-doped Ga _x In_1−x As−Al _y In_1−y As single quantum wells intentionally doped with Be acceptors in the well centre. Thek-non-conserving recombination process involves electrons with momentum up to the Fermi edge and holes localized on the Be acceptors. The transition from a one-component electron plasma to a two-component electron-hole plasma is studied by comparing the experimental results with theoretical line shape models. The density-dependent band gap renormalization is determined for the one-component and the two-component electron-hole plasma. The obtained results are found to agree well with recent theoretical calculations.     

Optical and structural properties of Zn1−x Mg x O ceramic materials

This paper reports structural, optical and cathodoluminescence characterizations of sintered Zn_1?x Mg _x O composite materials. The effects of MgO composition on these film properties have been analyzed. X-ray diffraction (XRD) confirms that all composites are polycrystalline with prominent hexagonal wurtzite structure along two preferred orientations (002) and (101) for the crystallite growth. Above doping content x = 10 %, the formation of the hexagonal ZnMgO alloy phase and the segregation of the cubic MgO phase start. From reflectance and absorption measurements, we determined the band gap energy which tends to increase from 3.287 to 3.827 eV as the doping content increases. This widening of the optical band gap is explained by the Burstein–Moss effect which causes a significant increase of electron concentration (2.89 × 10¹⁸?5.19 × 10²⁰ cm^?3). The luminescent properties of the Zn_1?x Mg _x O pellets are studied by cathodoluminescence (CL) at room and liquid nitrogen temperatures under different electron beam excitations. At room temperature, the CL spectra of the Zn_1?x Mg _x O composites exhibit a dominant broad yellow-green light band at 2.38 eV and two ultraviolet emission peaks at 3.24 and 3.45 eV corresponding to the luminescence of the hexagonal ZnO and ZnMgO structures, respectively. For the doped ZnO samples, it reveals also new red peaks at 1.72 and 1.77 eV assigned to impurities’ emissions. However, the CL spectra recorded at 77 K show the presence of excitonic emission peaks related to recombination of free exciton (X _A), neutral donor-bound excitons (D⁰X) and their phonon replicas. The CL intensity and energy position of the green, red and ultraviolet emission peaks are found to depend strongly on the MgO doping content. The CL intensity of the UV and red emissions is more enhanced than the green light when the MgO content increases. CL imaging analysis shows that the repartition of the emitting centers in Zn_1?x Mg _x O composites is intimately connected to the film composition and surface morphology.     

Origins of green band emission in high-temperature annealed N-doped ZnO

Nitrogen-doped ZnO sample has been annealed in O₂ ambient at high temperature (1000 °C) to improve its photoluminescence property. Low-temperature photoluminescence spectra of the sample are dominated by three near-band-edge emissions at 3.377, 3.362, and 3.332 eV, which are ascribed to free exciton emission (FX_A), and neutral donor-bound exciton (D⁰X), and its two-electron satellite (TES), respectively. With increasing temperature in low temperature region, the intensity of FX_A increases and the green band (GB) shows a negative thermal quenching effect resulting from thermal dissociation of D⁰X with more free excitons and neutral donors formed. The doublet structure with energy space ∼30 meV and repeated separation of longitudinal-optical phonon energy of 72 meV are observed in GB at low temperatures. The temperature independent energy position of GB indicates a typical recombination characteristic within strongly localized complexes. The doublet structures are considered to originate from the ground and exited states of shallow donors recombining with deep acceptors such as zinc vacancies.     

Constraints on Vector Unparticle Physics from Cosmic Censorship

Vector unparticle couplings to standard model fields produce repulsive corrections to gravity. From a general relativistic perspective, this leads to an effective Reissner-Nordstr?m-like metric whose “charge” is a function of the unparticle coupling constant λ, and therefore can admit naked singularities. Requiring the system to respect cosmic censorship provides a new method of constraining the value of λ. These limits are extremely loose for stellar-mass black holes, but commensurate with existing bounds for primordial black holes. In the case of theoretical low-mass black holes, the bounds on λ are much stricter than those derived from astrophysical and accelerator phenomenology. Additional constraints on the lower limit of λ are used to estimate the mass of the smallest possible black hole M_BH^minM_{\mathrm {BH}}^{\mathrm{min}} that can be formed in the unparticle framework, as a function of the unparticle parameters (L_U,M_U,d_U,d_BZ\Lambda_{{\mathcal{U}}},M_{{\mathcal{U}}},{d_{\mathcal{U}}},{d_{\mathit{BZ}}}).     

Intense blue-emitting Ca5Al8O14: Eu phosphor for mercury free lamp

The calcium aluminates doped with Eu ions, Ca₅Al₈O₁₄: Eu, phosphors are prepared by the combustion method. The formation of crystalline aluminates was confirmed by X-ray diffraction pattern. The prepared phosphors were characterized by SEM, TGA, DTA, particle size analyzer and Photoluminescence (PL) techniques. From the UV-excited luminescence spectra it was found that the Eu ions acts as a luminescent centre with luminescence at the blue (λ _max = 470 nm) region due to 4f ⁶5d ¹ → 4f ⁷ transition. The excitation spectra show the broad band at 355 nm wavelength (λ _em = 470 nm). The excitation 355 nm is a mercury free excitation and therefore Ca₅Al₈O₁₄: Eu may be useful for the solid state lighting phosphor in lamp industry.      

A new bound-exciton emission band in ZnSe crystals and multiplasmon optical transitions

We have investigated cathodoluminescence both in unannealed ZnSe crystals and in crystals annealed in a Bi melt at a temperature of 1200K for 120 h with subsequent quenching. In the wavelength range 450–480 nm we have detected a new line series I _i ^s -nLO-mPl consisting of the bound-exciton emission line I _i ^s with wavelength λ=455.9 nm and its plasmon and LO-phonon echoes I _i ^s -LO (λ ₁=461.3 nm), I _i ^s -2LO (λ ₂=466.8 nm), I _i ^s -3LO (λ ₃=472.4 nm), and I _i ^s -4LO (λ ₄=478.3 nm). We have determined the mean number of emitted LO phonons N _LO=2.2±0.1 per photon. It is shown that the observed finer structure of the band may be due to multiphonon optical transitions. At low plasma densities (ω _p ≪ω _LO ) the Coulomb interaction causes broadening of the I _i ^s -nLO series. In samples with denser plasma, in which the condition ω _p ⩽ω _LO is met, multiplasmon satellites of the series I _i ^s -nLO-mPl are observed. Theoretical calculations of the shape of the emission band agree with experiment. Fiz. Tverd. Tela (St. Petersburg) 41, 1176–1180 (July 1999)     

Thermoelectric power and ac conductivity of A-type Nd2O3

Measurement of thermoelectric power Θ of pressed pellets of A-type Nd₂O₃ from 550 to 1180K and electrical conductivity (σ) at dc, 50 Hz, 1.542 kHz and 3 kHz at different temperatures is reported. It is concluded that electrical conduction at high temperature (T>600K) in this solid is due to positive large polarons in O²⁻ : 2p (valence) band and negative intermediate polarons in Nd³⁺ : 5d (conduction band). The energy band gap of the solid has been found to be 2.44 eV. At low temperatures, conduction by hopping of charge carriers from one impurity centre to another has been predicted.     

Theoretical models for high-temperature superconductivity

Summary A semi-phenomenological analysis is given of the effects of certain band structure features on the gap ratios 2Δ/k _B T _c for high-T _c superconductors, including multigap systems. In addition to phonons other intermediate bosons (IB) mediating the superconducting interaction are considered. Interesting results emerge when the IB energy exceeds the widths of possible narrow peaks in the density of states associated with subbands presumably belonging to substructures such as stacked Cu−O planes. Comparison with experiment is made. In particular, data obtained by Warrenet al. via nuclear-spin relaxation on Ba₂YCu₃O_7−δ can be interpreted within the present framework in terms of a model having an IB of energy ≳1 eV, which exceeds the predicted width (≲0.3 eV) of a peak in the density of states containing the normal-state Fermi level. This suggests that the IB is not a phonon. To speed up publication, the author of this paper has agreed to not receive the proofs for correction.     

Optical investigation of ferromagnetic Ga1−xMnxN layers grown on sapphire (0 0 0 1) substrates

We have investigated the temperature-dependent photoluminescence (PL) spectra in Ga_1−xMn_xN layers (where x≈0.1–0.8%) grown on sapphire (0 0 0 1) substrates using the plasma-enhanced molecular beam epitaxy technique. All the layers doped with manganese exhibited n-type conductivity with Curie temperature over 350 K. The efficient PL are peaked in the red (1.86 eV), yellow (2.34 eV), and blue (3.29 eV) spectral range. It was found that the blue band at 3.29 eV is mostly associated with the formation complexes between donors (e.g., N vacancy) and Mn acceptors, which results in forming donor levels at 0.23 eV below the conduction band edge. The yellow band is attributed to intrinsic gallium defects. The broad band at 1.86 eV is attributed to inner ⁵D state transition (T₂ to E) of Mn ions.     

Effect of biaxial strain on the band gap of wurtzite AlxGa1?xN

First-principles calculations are applied to investigate the effect of biaxial strain on the band gap of wurtzite Al _x Ga_1−x N. The band gap and band gap bowing parameter increase with compressive strain and decrease with tensile strain. The strain-induced changes in the band gap of Al _x Ga_1−x N are linear in the strain range of about −1% to 1% while the linearity is invalid out of the range. The linear coefficient B(x), characterizing the relationship between the band gap and the biaxial stress, with a quadratic form is obtained. The value of the band gap bowing parameter decreases from 1.0 eV for −2% strain to 0.91 eV for unstrained and to 0.67 eV for 2% strain.