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Conduction Band (conduction + band)
Terms modified by Conduction Band Selected AbstractsConduction band filling in In-rich InGaN and InN under hydrostatic pressurePHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2008G. Franssen Abstract We demonstrate the effect of conduction band shape evolution of InGaN with increasing In content and applying hydrostatic pressure. The influence of conduction band filling on the hydrostatic pressure dependence of photoluminescence in In0.7Ga0.3N and InN is investigated. It is found that the PL pressure coefficient dEPL/dp of InN changes from ,27 meV/GPa to ,21 meV/GPa when the electron concentration increases from 3.6×1017 cm,3 to 1.1×1019 cm,3. In contrast, no significant change of dEPL/dp with electron concentration was observed for In0.7Ga0.3N. We conclude that the pressure sensitivity of the Fermi level, which is responsible for the lowering of dEPL/dp with respect to dEG/dp in InN, is much less prominent in In0.7Ga0.3N than in InN. We attribute this difference to the larger band gap of In0.7Ga0.3N, which lowers the pressure sensitivity of m*. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Conformal Nano-Sized Inorganic Coatings on Mesoporous TiO2 Films for Low-Temperature Dye-Sensitized Solar Cell FabricationADVANCED FUNCTIONAL MATERIALS, Issue 2 2010Larissa Grinis Abstract Here, a new method based on sol,gel electrophoretic deposition to produce uniform high-quality inorganic conformal coatings on mesoporous nano-particulate films is presented. This novel sol preparation method allows for very fine control of the coating properties, thus inducing new adjustable functionalities to these electrodes. It is shown that the deposition of an amorphous TiO2 and/or MgO shell onto photoanodes used in dye-sensitized solar cells (DSSCs) improves their light-to-electric-power conversion efficiency without the need for sintering. It is proposed that the amorphous TiO2 coating improves the electronic inter-particle connection and passivates the surface states. The insulating MgO coating further reduces the electron transfer from the conduction band into the electrolyte while the electron injection from the excited dye state remains unperturbed for thin coatings. Using a low-temperature method for DSSC production on plastic substrates, a maximum efficiency of 6.2% applying pressure together with an optimized TiO2 coating is achieved. For systems that cannot be pressed a conversion efficiency of 5.1% is achieved using a double shell TiO2/MgO coating. [source] Molecular Design of Unsymmetrical Squaraine Dyes for High Efficiency Conversion of Low Energy Photons into Electrons Using TiO2 Nanocrystalline FilmsADVANCED FUNCTIONAL MATERIALS, Issue 17 2009Thomas Geiger Abstract An optimized unsymmetrical squaraine dye 5-carboxy-2-[[3-[(2,3-dihydro-1, 1-dimethyl-3-ethyl-1H -benzo[e]indol-2-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene]methyl]-3,3-dimethyl-1-octyl-3H -indolium (SQ02) with carboxylic acid as anchoring group is synthesized for dye-sensitized solar cells (DSCs). Although the , -framework of SQ02 is insignificantly extended compared to its antecessor squaraine dye SQ01, photophysical measurements show that the new sensitizer has a much higher overall conversion efficiency , of 5.40% which is improved by 20% when compared to SQ01. UV-vis spectroscopy, cyclic voltammetry and time dependent density functional theory calculations are accomplished to rationalize the higher conversion efficiency of SQ02. A smaller optical band gap including a higher molar absorption coefficient leads to improved light harvesting of the solar cell and a broadened photocurrent spectrum. Furthermore, all excited state orbitals relevant for the ,,,* transition in SQ02 are delocalized over the carboxylic acid anchoring group, ensuring a strong electronic coupling to the conduction band of TiO2 and hence a fast electron transfer. [source] Femtosecond dynamics of electron transfer, localization, and solvation processes at the ice,metal interfaceISRAEL JOURNAL OF CHEMISTRY, Issue 1-2 2005Uwe Bovensiepen The ultrafast dynamics of excess electrons in amorphous ice layers on single-crystal metal surfaces are investigated by femtosecond time- and angle-resolved two-photon-photoemission spectroscopy. Photoexcited electrons are injected from the metal substrate into delocalized states of the conduction band of ice and localize in the ice layer within 100 fs. Subsequently, energetic stabilization of this localized species is observed on a time scale of ,1 ps, which is attributed to electron solvation by nonadiabatic coupling to nuclear degrees of freedom of the surrounding polar molecular environment. Concomitant with this stabilization process, residual wave function overlap of the solvated electron with the metal substrate results in back-transfer by tunneling through the solvation shell. At such interfaces the correlation of electronic and molecular structure with the resulting solvation dynamics can be explored using different substrates as a template. Here we compare data on molecularly thin D2O ice layers grown on Cu(111) and Ru(001). On Ru(001) both the stabilization and back-transfer proceed about three times faster compared to Cu(111), which is attributed to different interfacial structures and the role of d-states, and projected band gaps in the electron transfer process. [source] Unusual Sb,Sb bonding in high temperature thermoelectric materialsJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 13 2008Jianxiao Xu Abstract The emerging families of advanced thermoelectrics are dominated by antimonides and tellurides. Because the structures of the tellurides are mostly composed of NaCl-related motifs, they do not contain any Te,Te bonds, and all of the antimonide structures exhibit Sb,Sb bonds of various lengths. Taking all Sb,Sb distances shorter than 3.2 Å into account, the Sb atom substructures are Sb24, pairs in ,-Zn4Sb3, linear Sb37, units in Yb14MnSb11, planar Sb44, rectangles in the skutterudites, for example, LaFe3CoSb12, and Sb8 cubes interconnected via short Sb,Sb bonds to a three-dimensional network in Mo3Sb5Te2. These interactions have a significant impact on the band gap size as well as on the effective mass around the Fermi level, for the bottom of the conduction band is in all cases predominated by antibonding Sb,Sb interactions, and,in some cases,the top of the valence band by bonding Sb,Sb interactions. © 2008 Wiley Periodicals, Inc. J Comput Chem 2008 [source] Highly conductive and optically transparent GZO films grown under metal-rich conditions by plasma assisted MBEPHYSICA STATUS SOLIDI - RAPID RESEARCH LETTERS, Issue 3-4 2010H. Y. Liu Abstract We demonstrate a critical effect of a metal-to-oxygen ratio on the electrical, optical, and structural properties of ZnO films heavily doped with Ga (carrier concentration in the range of 1020,1021 cm,3) grown by plasma-assisted molecular beam epitaxy. The as-grown layers prepared under the metal-rich conditions exhibited resistivities below 3 × 10,4 , cm and an optical transparency exceeding 90% in the visible spectral range as well as a large blue shift of the transmission/absorption edge attributed to the Burstein,Moss shift of the Fermi level deep into the conduction band, indicating high donor concentration. In contrast, the films grown under the oxygen-rich conditions required thermal activation and showed inferior properties. Furthermore, electrical measurements point to the nonuniform depth distribution of free carriers. An oxygen-pressure-dependent surface disordering is suggested to be responsible for the drastic effect of the metal-to-oxygen ratio on the film properties. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Dielectric modeling of transmittance and ellipsometric spectra of thin In2O3:Sn filmsPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 7 2010Zhaohui Qiao Abstract Thin ITO films with thickness between 0.05 and 0.4,µm were deposited on quartz substrates by direct-current magnetron-sputtering. The films' ellipsometric and transmittance spectra between 280 and 2500,nm were simulated simultaneously with a computer program based on dielectric modeling. The dielectric function used is the sum of three types of electronic excitations: intraband transitions of free electrons (extended Drude model), band gap transitions, and interband transitions into the upper half of the conduction band. A successful fit of the simulated to the experimental curves was obtained with a two-layer model (bulk and surface layers) and applying the Bruggeman effective-medium approach. From the simulation, film thickness, refractive index, band gap, and free carrier density can be obtained. The thickness of the surface layer is comparable with the surface roughness determined by AFM measurements. [source] Single impurity Anderson model and band anti-crossing in the Ga1,xInx Ny As1,y material systemPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 1 2008Nikolaos Vogiatzis Abstract The role of the single-N impurity in the GaInNAs system is evaluated using the single impurity Anderson model. The N impurities can act either as scattering resonances or as bound states depending on their energy position. For the former case, using self-energy calculations and Matsubara Green's functions we investigate the nature of the mixed single-N state (energy broadening, shift). The effect of this interaction on the perturbed conduction subbands is also examined. The single impurity Anderson model results in a complex band structure. The real part of the band structure can be directly related to the dispersion obtained with the band anti-crossing model and is in very good agreement. The imaginary bandstructure contains further information about the mixing of the nitrogen state and the conduction band which is not contained within the band anti-crossing model. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Energy structure and micromechanism of photo-electromotive force effect in V-doped CdTe crystalsPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 7 2007Yu. P. Gnatenko Abstract For the first time photo-electromotive force (photo-EMF) measurements were carried out for CdTe crystals doped with V atoms as a result of the photogeneration of carriers from deep impurity centers to the conduction band. Tilted geometry was applied that allowed two-dimensional monitoring of the vi- bration source. The CdTe:V crystals were excited by a He,Ne laser with a wavelength of 1.15 ,m (,, = 1.08 eV) and P = 2 mW. The mechanism of the appearance of the holographic current in the CdTe:V crystals taking into account real defect structure was proposed. The frequency dependence of the ac photo-EMF (holographic) current for the CdTe:V crystals was measured. It was shown that a low cut-off frequency for a laser intensity I = 0.2 mW/mm2 equals 6.0 kHz, which corresponds to a response time of 26 ,s. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] First principles calculation of isolated intermediate bands formation in a transition metal-doped chalcopyrite-type semiconductorPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 6 2006P. Palacios Abstract Density Functional Theory (DFT) calculations at the GGA level have been carried out for Ti-substituted chalcopyrite-type CuGaS2, as it might constitute an intermediate band material of the kind that has been proposed to lead to enhanced efficiency photovoltaic cells. According to these calculations an intermediate band appears when Ti substitutes Ga at a 25% level in this structure, resulting in a magnetic half-metallic compound. This intermediate band slightly overlaps the conduction band and, when a higher accuracy calculation approach like the introduction of a Hubbard-type empirical correction is used (GGA + U method), it splits leaving a filled narrow band, well isolated inside the band gap. Considering the nanocrystalline form in which these chalcopyrite-type compounds are used in solar cells, an assessment of the effects of a small crystal size in this system have been carried out with a slab model. In this calculation a decreased bandgap width is observed, which can be as a result of surface termination effects. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Multiphoton excitation of disc shaped quantum dot in presence of laser (THz) and magnetic field for bioimagingPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 4 2010Siddhartha Lahon Abstract Recently, multiphoton processes in nanostructures have attracted much attention for their promising applications, especially in growing field of bioimaging. Here we investigate the optical response of quantum disc (QD) in the presence of laser and a static magnetic field. Floquet theory is employed to solve the equation of motion for laser driven intraband transitions between the states of the conduction band. Several interesting features namely dynamic stark shift, power broadening, and hole burning on excited levels degeneracy breaking are observed with variation of electric and magnetic field strengths. Enhancement and power broadening observed for excited states probabilities with increase of external fields are directly linked to the emission spectra of QD and will be useful for making future bioimaging devices. [source] Optical properties of pure and transition metal-doped indium oxidePHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 5 2009H. A. Rahnamaye Aliabad Abstract The band structure, the dielectric function, the reflectivity, the refractive index and the oscillator strength sum rule were calculated for pure In2O3 and alloyed In1.5T0.5O3 (where T represents Sc, Y, La and Ac) using density functional theory (DFT). The full potential linearized augmented plane wave (FP-LAPW) method was used with the local density approximation (LDA + U). Calculations of the optical spectra were performed for the energy range 0,30 eV. The calculated results indicate that the upper valance bands of In2O3 show a small dispersion and the value of the band gap increases for Sc and Y dopants and decreases for Ac and La dopants. The calculations indicate that there are two band gaps for In2O3. The first shows a strong optical absorption, as a direct band gap occurs from a 0.81 eV energy level below the top of valence band. The second shows a much weaker absorption from the top of the valence band to the bottom of the conduction band. The refractive index for In2O3 is 1.69 nm at 800 nm, near the visible region. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Simulation of electronic density of states and optical properties of PbB4O7 by first-principles DFT methodPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2009Hui Wang Abstract First-principles calculations based on the density functional theory and the generalized gradient approximation were carried out to systematically investigate the electronic structure and linear optical properties of lead tetraborate (PbB4O7; PTB) single crystals. An indirect band gap of 4.34 eV is obtained. The valence band top is at the X-point and the conduction band is at the ,-point. The calculated total and partial densities of states indicate that the top valence band is constructed from O 2p states and the low conduction band mainly consists of Pb 6p states. The optical absorption spectra show several prominent structures but with very small directional anisotropy. PTB crystals have a higher refractive index but a small birefringence. The calculated frequency-dependent refractive index is in good agreement with experimental data. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Study of the electronic structures of CaMoO4 crystal related to oxygen vacancyPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 8 2008Chunying Pu Abstract The electronic structure of CaMoO4 crystal containing oxygen vacancy with the optimized lattice structure is studied within the framework of the fully relativistic self-consistent Dirac,Slater theory, using a numerically discrete variational (DV-X,) method. The calculated results indicate that the oxygen vacancy causes the appearance of the 4d state of molybdenum in the forbidden band. The optical transition energy from the 2p state of oxygen to the 4d state of molybdenum is 2.23 eV, corresponding to the 557 nm absorption band, which is very close to the experimental results. The F and F+ centers have donor energy levels and they occur in the forbidden band below the conduction band at 0.8 eV and 1.3 eV, respectively. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Pressure dependence of the optical properties of wurtzite and rock-salt Zn1,xCoxO thin filmsPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 1 2007J. A. Sans Abstract In this paper we investigate the electronic structure of Zn1,xCoxO by means of optical absorption measurements under pressure. Thin films of Zn1,xCoxO with different Co content (from 1 to 30%) were prepared by pulsed laser deposition on mica substrates. Absorption spectra exhibit three main features that are clearly correlated to the Co content in the films: (i) absorption peaks in the infrared associated to crystal-field-split internal transitions in the Co 3d shell, with very small pressure coefficients due to their atomic character; (ii) a broad absorption band below the fundamental edge associated to charge transfer transitions, that exhibit relatively large pressure coefficients, indicating that the Co 3d final states must be strongly hybridized to the conduction band; and (iii) a blue-shifted fundamental absorption edge associated to band to band transitions with a pressure coefficient close to that of pure ZnO. In the up-stroke the transition pressure from wurtzite to rock-salt phase decrease almost linearly as the Co increases, from 9.5 GPa in pure ZnO to about 6.5 GPa for x = 30%. In the down-stroke pressure we observe a similar behavior, yielding a metastable rock-salt phase at room pressure, after a pressure cycle up to 15 GPa. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Impurity levels in the layered semiconductor p-GaSe doped with group V elements As, Bi and SbPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 15 2005S. Shigetomi Abstract The radiative and non radiative recombination mechanisms in the As, Sb and Bi-doped GaSe have been investigated on the basis of photoluminescence (PL) and Hall effect measurements. The PL features (at 77 K) related to the impurity levels coming from the As, Sb and Bi atoms are dominated by a broad emission band at about 1.7 eV. From the temperature dependences of the peak energy and PL intensity and the dependence of excitation intensity of peak energy, it was found that the 1.7 eV emission band is due to the transition from the shallow donor level at about 0.08 eV below the conduction band to the deep acceptor. In addition it was found, from the temperature dependence of hole concentration, that a deep acceptor level at about 0.6 eV above the valence band is formed by the doping atoms. It is associated with defects or defect complexes. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Manifestation of the equilibrium hole distribution in photoluminescence of n-InNPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 4 2005A. A. Klochikhin Abstract Photoluminescence (PL) of n-InN grown by molecular beam epitaxy with Hall concentrations from 3.6 to 7.3 × 1017 cm,3 demonstrates dependences on carrier concentration, temperature, and excitation density which give evidences of a fast energy relaxation rate of photoholes and their equilibrium distribution over localized states. The structure of the PL spectra observed at 4.2 and 77 K in the energy interval from 0.50 to 0.67 eV indicates that a considerable part of holes is trapped by deep and shallow acceptors before the interband recombi- nation with degenerate electrons occurs. At room temperature, the band-to-band recombination of free holes and electrons dominates in PL. Experimental results on PL and absorption are described by model calculations under the assumptions of a band gap equal to 0.665,0.670 eV at zero temperature and zero carrier concentration and a non-parabolic conduction band with the effective mass at the G -point equal to 0.07 of the free electron mass. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Oxygen induced band-gap reduction in ZnOxSe1,x alloysPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 3 2004W. Shan Abstract The effect of alloying a small amount of ZnO with ZnSe on the electronic band structure has been studied. Optical transitions in MBE-grown ZnOxSe1,x epitaxial films (0 , x , 0.0135) were investigated using photoreflectance and photoluminescence spectroscopies. The fundamental band-gap energy of the alloys was found to decrease at a rate of about 0.1 eV per atomic percent of oxygen. The pressure dependence of the band gap was also found to be strongly affected by the O incorporation. Both effects can be quantitatively explained by an anticrossing interaction between the extended states of the conduction band of ZnSe and the highly localized oxygen states located at approximately 0.22 eV above the conduction band edge. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Model of a superconducting singular Fermi liquid with a first-order phase transitionPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2004Ryszard Gonczarek Abstract Model of s -wave and d -wave superconductivity in a singular Fermi liquid with a divergent scattering amplitude for particles with the same quasi-momenta and opposite spins is formulated and presented with regard to a narrow, nearly half-filled conduction band. The ground state and other eigenstates for the superconducting phase are found. Thermodynamic functions are obtained by the use of the Bogolubov method. The gap equation along with the equation for the chemical potential is derived in a general form and solved in a self-consistent manner for s -wave pairing. Above a certain temperature there are two solutions of the gap equation, however only for the greater one the superconducting phase remains stable. It is shown that the system undergoes a first-order phase transition between the superconducting and the normal phase. The critical temperature and the heat of the transition are found. The temperature dependence of the entropy and the specific heat of the system is also presented. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Light soaking effect on photocurrent collection in (Zn,Mg)O/Cu(In,Ga)Se2 solar cellsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 5 2009Takashi Minemoto Abstract The metastable behaviour, so-called light soaking (LS) effect, on (Zn,Mg)O (ZMO)/Cu(In,Ga)Se2 (CIGS) solar cells with controlled conduction band offset (CBO) values of the ZMO/CIGS layers was characterized. When the conduction band of ZMO is higher than that of CIGS, which creates the notch of the ZMO/CIGS interface in the conduction band, metastable current-voltage curves were observed. The metastability becomes prominent with increasing the CBO value. Quantum efficiency measurements before and after the LS revealed that the LS effect was mainly originated by the metastability in the photocurrent collection which influenced by the notch acting as a barrier for photo-generated electrons in the CIGS layer. The model for the LS effect is discussed with simulated energy band diagrams. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Effect of strain and alloy composition inhomogeneity on the electronic and optical properties of III-nitride semiconductorsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 10 2008Nikos Skoulidis Abstract Using the empirical tight binding method with a sp3s* basis for the first neighbour interactions and only p,p interactions for the second neighbours, we calculated the optical properties near the fundamental gap of cubic GaxAl1,xN alloys for x<0.33 grown on a substrate whose lattice constant varies from the lattice constant of cubic AlN to GaN. We found that the spectra of the fully relaxed alloy have a tail with onset energy at the indirect gap of AlN and an edge at the direct gap of the alloy which follows the Vegard's law within 0.1 eV. The maximum of band gap for each alloy composition is found for a substrate with the lattice constant of the fully relaxed alloy. When the substrate composition is such that induces negative tetragonal strain on the alloy (,xx<0) the energy band gap is mainly affected by the alloy composition while for ,xx>0 the alloy composition has a limited effect. The total effect of substrate and alloy composition in the calculated range on the band gap can be as much as 0.4 eV and is mainly due to the valence band shift as the conduction band is only slightly affected by the substrate or the alloy composition. This band gap change is adequate to explain the experimentally observed double photoluminescence peak of open quantum dots. The energy separation of the double peak is 0.1 eV and one peak can be attributed to the tetragonically strained wetting layer and the other to the fully relaxed quantum dots. Furthermore the present results can be used to estimate any possible stoichiometric inhomogeneity and help design structures with specific energies for the pair of peaks. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Conduction band filling in In-rich InGaN and InN under hydrostatic pressurePHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2008G. Franssen Abstract We demonstrate the effect of conduction band shape evolution of InGaN with increasing In content and applying hydrostatic pressure. The influence of conduction band filling on the hydrostatic pressure dependence of photoluminescence in In0.7Ga0.3N and InN is investigated. It is found that the PL pressure coefficient dEPL/dp of InN changes from ,27 meV/GPa to ,21 meV/GPa when the electron concentration increases from 3.6×1017 cm,3 to 1.1×1019 cm,3. In contrast, no significant change of dEPL/dp with electron concentration was observed for In0.7Ga0.3N. We conclude that the pressure sensitivity of the Fermi level, which is responsible for the lowering of dEPL/dp with respect to dEG/dp in InN, is much less prominent in In0.7Ga0.3N than in InN. We attribute this difference to the larger band gap of In0.7Ga0.3N, which lowers the pressure sensitivity of m*. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Energy relaxation processes of photo-generated carriers in Mg doped (0001)GaN and (1-101)GaNPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2008J. Saida Abstract Energy relaxation processes of photo-generated excess carriers in Mg doped GaN epitaxial layers were investigated at room temperature, with photoluminescence intensity correlation method using femto-second pulse laser as the excitation source. The decay curve was well fitted by exponential decay with two time constants. The slow process of the order of 100 ps was attributed to the energy relaxation of electrons in the conduction band, while the fast process of the order of several pico-seconds was attributed to the capture process in the impurity band near the valence band. Little difference has been found out between the results for (0001)GaN and those for (1-101)GaN. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Ionic and electronic defects in a-BaTiO3 thin films studied by transient and steady state conductivity measurementsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 3 2007F. El Kamel Abstract Conduction mechanisms in BaTiO3 films deposited at low temperatures on Cu-electrodes have been investigated in transient and steady regimes as a function of temperatures and electric field. This work aims to identify possible defects which govern the leakage current. Electrical measurements reveal that Space Charge Limited Current (SCLC) constitutes the main leakage mechanism in both the transient and the steady regimes. Based on the theory of SCLC, two types of defects can be detected. At higher temperatures, oxygen vacancies constitute the main defects which migrate across the film to generate an ionic leakage current. Diffusion of these defects is thermally activated with an activation energy around 1 eV. Moreover, at lower temperatures the J - E measurements reveals the presence of a discrete set of shallow traps at 0.45 eV below the conduction band with an effective density of 4 × 1022 m,3. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Effects of lattice-mismatch induced built-in strain on the valence band properties of wurtzite ZnO/Zn1,xMgxO quantum well heterostructuresPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 1 2007K. Zitouni Abstract We present a theoretical study of the effects of lattice-mismatch induced built-in strain on the electronic properties of valence band states in wurtzite ZnO/Zn1,xMgxO Quantum Well (QW) heterostructures. In this purpose, a 6x6 k·p method has been used to incorporate the effects of strain and nonparabolicity. The energies corresponding to the transitions between conduction band (C), heavy hole (HH), light hole (LH) and crystal-field split-off hole (CH) bands have been calculated as a function of Mg composition and strain. We have also calculated the energy dispersions and wave functions of strained wurtzite ZnO. It is found that ZnO is always under a biaxial tensile strain, in the whole Mg composition range investigated (x < 40%). As a consequence, the light hole valence subband is shifted upwards with respect to the corresponding heavy hole valence subband, resulting in a reduction of ZnO direct band gap by almost 6% when x = 35%. This is found to result in turn in a significantly reduced in-plane hole effective mass at the top of the valence band which is always LH-like. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Trap-sensitive relaxation of hot carriers in ZnO:Cu,AlPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 8 2006I. Konovalov Abstract Modulated photoconductivity measurements with x-ray excitation were performed on ZnO:Cu,Al thin films. We found a strong variation of the modulated photocurrent with temperature starting from the excitation energy approximately equal the binding energy of Cu 2p3 core electrons and above. Strong temperature sensitivity at ,280 K suggests participation of thermally activated trap states in the process of hot carrier relaxation. The shape of the temperature dependence was satisfactorily fitted by the respective calculation. No comparable deviations of the temperature sensitivity were observed when the Cu 2s electrons were excited. We explain the observations by assumption of fast local transitions from a valley in the conduction band into the trap state according to the symmetry selection rules. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Electronic structure of RCoxGe2 (R=Ce, Pr, Gd) compoundsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 1 2006B. Penc Abstract The electronic structure of the RCoxGe2 (R=Ce, Pr, Gd) compounds with the orthorhombic crystal structure of CeNiSi2 -type was studied by X-ray photoemission spectroscopy. The valence bands of CeCo0.86Ge2 and PrCo0.85Ge2 compounds have the similar character. Near the Fermi level the broad peak corresponding to the Co 3d and R 4f and 5d6s states is observed. In the case of GdCo0.64Ge2, the strong intensity peak at 8.7 eV corresponding to Gd 4f states is observed. The analysis of the XPS spectra of Ce and Pr 3d5/2 and 3d3/2 in the Gunnarson-Schönhammer model give the information on the hybridization of the f orbital with the conduction band. The hybridization energy equal 93 eV for R=Ce and 143 eV for R=Pr, respectively indicate stability of the shell in these compounds. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Deep level transient spectroscopy and TEM analysis of defects in Eu implanted GaNPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7 2005A. Colder Abstract Deep level transient spectroscopy (DLTS) and transmission electron microscopy (TEM) are used to study europium implanted gallium nitride layers. The implantation was realised at room temperature in the random and channeled geometries. From DLTS, we determine intrinsic defects with associated levels located in the band gap (below the conduction band). Besides, we point out a new electron trap named Eu2. Its associated level is located at about Ec ,0.36 eV and the defect is probably related to the europium rare-earth ion. TEM investigation shows a difference in structure caused by changing the geometry of implantation. The random implanted sample contains numerous planar defects. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] The electronic structure of chalcopyrites,bands, point defects and grain boundariesPROGRESS IN PHOTOVOLTAICS: RESEARCH & APPLICATIONS, Issue 6 2010Susanne Siebentritt Abstract We summarize the progress made recently in understanding the electronic structure of chalcopyrites. New insights into the dispersion of valence and conduction band allow conclusions on the effective masses of charge carriers and their orientation dependence, which influences the transport in solar cell absorbers of different orientation. Native point defects are responsible for the doping and thus the band bending in solar cells. Results of optoelectronic defect spectroscopy are reviewed. Native defects are also the source for a number of metastabilities, which strongly affect the efficiency of solar cells. Recent theoretical findings relate these effects to the Se vacancy and the InCu antisite defect. Experimentally determined activation energies support these models. Absorbers in chalcopyrite solar cells are polycrystalline, which is only possible because of the benign character of the grain boundaries. This can be related to an unusual electronic structure of the GB. Copyright © 2010 John Wiley & Sons, Ltd. [source] X-ray atomic orbital analysis of 4f and 5d electron configuration of SmB6 at 100, 165, 230 and 298,KACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2010Shiro Funahashi Accurate electron-density measurement of SmB6 at 100, 165, 230 and 298,K, and X-ray atomic orbital (XAO) analysis were carried out. The 4f -electron density around Sm and 5d electron density at ,,1,Å from Sm were analysed by XAO analysis. The 5d electron density is due to the electrons of the 5dJ = 5/2,8 orbitals which stem from the eg orbitals in the strong field approximation. The change in electron populations of the 5d5/2,8 orbitals with temperature is similar to that of the resistivity. Since the conduction band consists of 5d5/2,8 and B-2p orbitals according to band theory, this indicates that the larger populations of the 5d5/2,8 orbitals correspond to the larger number of localized electrons and are correlated to the resistivity of SmB6. The occupation of the bulky 5d5/2,8 orbitals may be the reason for the elongation of the lattice parameter below 150,K. The 4f7/2,6 orbitals are obviously occupied except at 100,K, which seems to be caused by the energy gap between 4f5/2 and 4f7/2 states, which begins to exist between 100 and 150,K, and may represent one of the properties of a Kondo insulator. 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