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Surface States (surface + states)
Selected AbstractsElectrical and optical properties of CdS nanocrystalline semiconductorsCRYSTAL RESEARCH AND TECHNOLOGY, Issue 1 2006Shikha Tiwari Abstract II-VI semiconductor nanoparticles have recently attracted a lot of attention due to the possibility of their application in various devices. In the present study, chemical method has been used in synthesis of CdS nanoparticles and thiophenol was used as capping agent. X-ray diffraction studies of both samples were done. The dc conductivity of CdS increases at a lower rate or is approximately constant upto 500K and thereafter the conductivity increases at a rapid rate. Beyond Tc it is seen that the portion of the ,dc versus 1000/T is almost a straight line showing an Arrehenious behaviour. The dielectric constant of nanoparticles of CdS is found to be larger than the corresponding values of CdS crystals. It is clearly observed that at lower wavelengths nanocrystalline samples show a blue-shift. The three peaks of sample (S2), A, Band C can be ascribed to the transition from Cd-O complex donor formed by adsorbed oxygen to the valance band, Cd - excess acceptor and the surface states, respectively. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Functionalized Self-Assembled InAs/GaAs Quantum-Dot Structures Hybridized with Organic MoleculesADVANCED FUNCTIONAL MATERIALS, Issue 3 2010Miaoxiang Chen Abstract Low-dimensional III,V semiconductors have many advantages over other semiconductors; however, they are not particularly stable under physiological conditions. Hybridizing biocompatible organic molecules with advanced optical and electronic semiconductor devices based on quantum dots (QDs) and quantum wires could provide an efficient solution to realize stress-free and nontoxic interfaces to attach larger functional biomolecules. Monitoring the modifications of the optical properties of the hybrid molecule,QD systems by grafting various types of air-stable diazonium salts onto the QD structures surfaces provides a direct approach to prove the above concepts. The InAs/GaAs QD structures used in this work consist of a layer of surface InAs QDs and a layer of buried InAs QDs embedded in a wider-bandgap GaAs matrix. An enhancement in photoluminescence intensity by a factor of 3.3 from the buried QDs is achieved owing to the efficient elimination of the dangling bonds on the surface of the structures and to the decrease in non-radiative recombination caused by their surface states. Furthermore, a narrow photoluminescence band peaking at 1620,nm with a linewidth of 49 meV corresponding to the eigenstates interband transition of the surface InAs QDs is for the first time clearly observed at room temperature, which is something that has rarely been achieved without the use of such engineered surfaces. The experimental results demonstrate that the hybrid molecule,QD systems possess a high stability, and both the surface and buried QDs are very sensitive to changes in their surficial conditions, indicating that they are excellent candidates as basic sensing elements for novel biosensor applications. [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] Efficient Luminescence from Rare-Earth Fluoride Nanoparticles with Optically Functional Shells,ADVANCED FUNCTIONAL MATERIALS, Issue 7 2006M. Lezhnina Abstract Rare-earth fluorides are a class of materials with considerable potential in optical applications. Fluoride lattices typically permit high coordination numbers for the hosted rare-earth ions, and the high ionicity of the rare-earth-to-fluorine bond leads to a wide bandgap and very low vibrational energies. These factors make rare-earth fluorides very useful in optical applications employing vacuum ultraviolet and near-infrared excitation. The preparation of nanometer-sized particles has opened the door for new properties and devices if the performance of their macroscopic counterparts can be conserved in the nanometer regime. However, at small particle sizes, defect surface states and adhering water reduce the optical efficiency. These shortcomings can be reduced by applying protective shells around the luminescent cores, which can also be involved in the luminescent process. [source] Synthesis and Optical Properties of Europium-Doped ZnS: Long-Lasting Phosphorescence from Aligned Nanowires,ADVANCED FUNCTIONAL MATERIALS, Issue 11 2005C. Cheng Abstract Quasi-aligned Eu2+ -doped wurtzite ZnS nanowires on Au-coated Si wafers have been successfully synthesized by a vapor deposition method under a weakly reducing atmosphere. Compared with the undoped counterpart, incorporation of the dopant gives a modulated composition and crystal structure, which leads to a preferred growth of the nanowires along the [010] direction and a high density of defects in the nanowire hosts. The ion doping causes intense fluorescence and persistent phosphorescence in ZnS nanowires. The dopant Eu2+ ions form an isoelectronic acceptor level and yield a high density of bound excitons, which contribute to the appearance of the radiative recombination emission of the bound excitons and resonant Raman scattering at higher pumping intensity. Co-dopant Cl, ions can serve not only as donors, producing a donor,acceptor pair transition with the Eu2+ acceptor level, but can also form trap levels together with other defects, capture the photoionization electrons of Eu2+, and yield long-lasting (about 4,min), green phosphorescence. With decreasing synthesis time, the existence of more surface states in the nanowires forms a higher density of trap centers and changes the crystal-field strength around Eu2+. As a result, not only have an enhanced Eu2+ 4f65d1,4f7 intra-ion transition and a prolonged afterglow time been more effectively observed (by decreasing the nanowires' diameters), but also the Eu2+ related emissions are shifted to shorter wavelengths. [source] Use of multi-platform, multi-temporal remote-sensing data for calibration of a distributed hydrological model: an application in the Arno basin, ItalyHYDROLOGICAL PROCESSES, Issue 13 2006Lorenzo Campo Abstract Images from satellite platforms are a valid aid in order to obtain distributed information about hydrological surface states and parameters needed in calibration and validation of the water balance and flood forecasting. Remotely sensed data are easily available on large areas and with a frequency compatible with land cover changes. In this paper, remotely sensed images from different types of sensor have been utilized as a support to the calibration of the distributed hydrological model MOBIDIC, currently used in the experimental system of flood forecasting of the Arno River Basin Authority. Six radar images from ERS-2 synthetic aperture radar (SAR) sensors (three for summer 2002 and three for spring,summer 2003) have been utilized and a relationship between soil saturation indexes and backscatter coefficient from SAR images has been investigated. Analysis has been performed only on pixels with meagre or no vegetation cover, in order to legitimize the assumption that water content of the soil is the main variable that influences the backscatter coefficient. Such pixels have been obtained by considering vegetation indexes (NDVI) and land cover maps produced by optical sensors (Landsat-ETM). In order to calibrate the soil moisture model based on information provided by SAR images, an optimization algorithm has been utilized to minimize the regression error between saturation indexes from model and SAR data and error between measured and modelled discharge flows. Utilizing this procedure, model parameters that rule soil moisture fluxes have been calibrated, obtaining not only a good match with remotely sensed data, but also an enhancement of model performance in flow prediction with respect to a previous calibration with river discharge data only. Copyright © 2006 John Wiley & Sons, Ltd. [source] Theory of tip-dependent imaging of adsorbates in the STM: CO on Cu(111)INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 6 2006D. Drakova Abstract The processes of local electron injection or extraction in the scanning tunneling microscopy (STM) and spectroscopy (STS) lead to the creation of short-lived excited states localized at the electrode surfaces. The dynamic relaxation of the transient negative or positive ion resonances, due to both local and long-range interactions, is the clue to the understanding of numerous phenomena in STM/STS ranging from the "anomalously" large tip height corrugation amplitudes on clean metal surfaces to the observation of quantum mirages and features in the STS, which are not observed with the help of other surface spectroscopies. Quantum nanodynamics theory (QND) has been applied to calculate the interaction potential of a single CO molecule with the Cu(111) surface, with a transient negative ion resonance formed when an electron is injected from the tip, and the tunneling conductance on the clean and CO covered Cu(111) surface using a clean metal tip Al/Al(111) and a Pt(111) tip with an adsorbed CO molecule at the apex. Within QND and three-dimensional scattering theory, regarding the tunneling as an excited-state problem, we provide the explanation of the tip-dependent STM image of a single CO molecule on Cu(111). The appearance of the CO molecule as an indentation, using a clean metal tip and as a protrusion with a tip terminated by a CO molecule, is understood as a result of tunneling through two competing channels. Tunneling via adsorbate-induced ion resonances enhances the tunneling conductance. In contrast, tunneling via metal ion resonances only leads to attenuation of the conductance in the presence of the adsorbate. The current in the vicinity of the adsorbed CO molecule is reduced when a clean metal tip is used; i.e., CO appears dark in the STM image, because metal ion resonances on Cu(111) derive from the surface states with image state components coupling to plasmons and are therefore very diffuse. With a CO-terminated tip, the major current channel is, for symmetry reasons, from the 2,-derived orbital of the tip CO molecule, via the diffuse 2,-derived orbital of the CO molecule on the sample, hence adsorbed CO appears bright. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 [source] Electrical characteristics of Al/polyindole Schottky barrier diodes.JOURNAL OF APPLIED POLYMER SCIENCE, Issue 5 2009Abstract In this study, the forward and reverse bias current,voltage (I,V), capacitance,voltage (C,V), and conductance,voltage (G/,,V) characteristics of Al/polyindole (Al/PIN) Schottky barrier diodes (SBDs) were studied over a wide temperature range of 140,400 K. Zero-bias barrier height ,B0(I,V), ideality factor (n), ac electrical conductivity (,ac), and activation energy (Ea), determined by using thermionic emission (TE) theory, were shown fairly large temperature dispersion especially at lower temperatures due to surface states and series resistance of Al/PIN SBD. I,V characteristics of the Al/PIN SBDs showed an almost rectification behavior, but the reverse bias saturation current (I0) and n were observed to be high. This high value of n has been attributed to the particular distribution of barrier heights due to barrier height inhomogeneities and interface states that present at the Al/PIN interface. The conductivity data obtained from G/,V measurements over a wide temperature range were fitted to the Arrhenius and Mott equations and observed linear behaviors for ,ac vs. 1/T and ln ,ac vs. 1/T1/4 graphs, respectively. The Mott parameters of T0 and K0 values were determined from the slope and intercept of the straight line as 3.8 × 107 and 1.08 × 107 Scm,1K1/2, respectively. Assuming a value of 6 × 1012 s,1 for ,0, the decay length ,,1 and the density states at the Fermi energy level, N(EF) are estimated to be 8.74 Å and 1.27 × 1020 eV,1cm,3, respectively. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009 [source] Nonlinear Tamm states and surface effects in periodic photonic structuresLASER PHYSICS LETTERS, Issue 10 2008Yu. S. KivsharArticle first published online: 10 JUL 200 Abstract We present a brief overview of the basic concepts and important experimental observations of the effect of light localization near the surfaces of truncated periodic photonic structures. In particular, we discuss the formation of nonlinear localized modes and discrete surface solitons near the edges of nonlinear optical waveguide arrays and two-dimensional photonic lattices. We draw an analogy between the nonlinear surface optical modes and the surface Tamm states known in the electronic theory. We discuss the crossover between discrete solitons in the array and surface solitons at the edge of the array by analyzing the families of even and odd nonlinear localized modes located at finite distances from the edge of a waveguide array. We discuss various generalization of this concept including surface solitons in chirped lattices, multi-gap vector surface solitons, polychromatic surface states generated by a supercontinuum source, surface modes in two-dimensional photonic lattices, and spatiotemporal surface solitons. Finally, we discuss briefly several other related concepts including the enhanced beaming of light from subwavelength waveguides in photonic crystals. (© 2008 by Astro Ltd., Published exclusively by WILEY-VCH Verlag GmbH & Co. KGaA) [source] Grafted Polymer Chains Interacting with Substrates: Computer Simulations and ScalingMACROMOLECULAR THEORY AND SIMULATIONS, Issue 9 2008Radu Descas Abstract We review scaling methods and computer simulations used in the study of the static and dynamic properties of polymer chains tethered to adsorbing surfaces under good solvent conditions. By varying both the grafting density and the monomer/surface interactions a variety of phases can form. In particular, for attractive interactions between the chains and the surface the classical mushroom-brush transition known for repulsive substrates splits up into an overlap transition and a saturation transition which enclose a region of semidilute surface states. At high grafting densities oversaturation effects and a transition to a brush state can occur. We emphasize the role of the critical adsorption parameters for a correct description and understanding of such polymer adsorption phenomena. [source] Electronic structure of GaN(0001)-2 × 2 thin films grown by PAMBEPHYSICA STATUS SOLIDI - RAPID RESEARCH LETTERS, Issue 5 2008R. Gutt Abstract Gallium nitride thin films were grown on silicon carbide (0001) by plasma-assisted molecular beam epitaxy (PAMBE). The samples were cooled down in nitrogen plasma and characterized in situ by reflection high energy electron diffraction (RHEED), photoelectron spectroscopy (XPS/UPS), and atomic force microscopy (AFM) revealing stoichiometric and smooth GaN films virtually free of contaminations. We present valence band data obtained by UPS with strong emission from surface states inside the fundamental band gap. These states and the observed 2 × 2 surface reconstruction are highly sensitive towards residual molecules. Once these surface states have disappeared the original state could not be recovered by surface preparation methods underlining the necessity of in situ investigations on as-grown surfaces. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Half-metallicity and magnetism at Heusler alloy surfaces: Co2MSi(001) (M = Ti, Cr)PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 8 2008Ying Jiu Jin Abstract We investigated the electronic structures, magnetism, and half-metallicity at the (001) surfaces of full-Heusler alloys, Co2MSi (M = Ti, Cr), by using the all-electron full-potential linearized augmented plane wave method within the generalized gradient approximation. Both the Co-terminated (Co-term) and the MSi-terminated (MSi-term) surfaces were considered. From the calculated atom-resolved density of states, we found that the half-metallicity was destroyed at the Co-term surfaces for both alloys. The electronic structures at the MSi-term surfaces of the two alloys showed much different behavior. The half-metallicity was retained at the TiSi-term for Co2TiSi(001) but the minority spin gap was much reduced due to surface states located just below the Fermi level. On the other hand the half-metallicity was destroyed at the CrSi-term of Co2CrSi(001) due to the surface states located at the Fermi level. The calculated magnetic moment of the surface Co atom of the Co-term for Co2CrSi(001) was increased slightly to 1.05,B with respect to that of the deep inner layers (,1.00,B), while that for Co2TiSi(001) was decreased to 0.88,B. Large enhancement of the magnetic moment was found for the surface Ti atoms at TiSi-term of Co2TiSi(001) and Cr atoms at CrSi-term of Co2CrSi(001) with values of 0.07,B and 2.91(B, respectively. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Gate-lag and drain-lag effects in (GaN)/InAlN/GaN and InAlN/AlN/GaN HEMTsPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 6 2007J. Kuzmik Abstract Gate and drain-lag effects are studied in (GaN)/InAlN/GaN and InAlN/AlN/GaN HEMTs grown on sapphire. Electron trapping on the surface states between the gate and the drain forming the net negative charge up-to ,2 × 1013 cm,2 is found to be responsible for the gate-lag effect in the (GaN)/InAlN/GaN HEMTs. If the polarization charge at the device surface is decreased by GaN capping, then density of the trapped charge is not changed, however the electron de-trapping process becomes faster. The drain-lag effect is caused by electron injection and trapping in the source-gate area reaching ,1 × 1013 cm,2 of the trapped charge in the steady state. In the studied voltage range the InAlN/AlN/GaN HEMT is shown to be gate-lag-free suggesting that this parasitic transient can be avoided if thin AlN is used in the epi-layer growth sequence. It is assumed that this breakthrough quality relates to the decreased or even reverted electric field in the InAlN layer if AlN is inserted. Surface states need not to be generated in this case. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Dynamics of photogenerated carriers in porous silicon probed by microwave absorptionPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 5 2007H. E. PorteanuArticle first published online: 27 APR 200 Abstract We report on dynamics of photogenerated carriers in porous silicon using contactless investigation tools. A comparison of two theoretical models: the simple point charge description versus degenerate gas model is presented. The temperature dependence reveals the dominant role of surface states below 70 K. Light intensity dependences evidence Auger processes. The optical and chemical properties can be indirectly improved using the microwave conductivity by studying the surface states of porous materials. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Microcircuit tailoring in ferromagnetic semiconductor (Ga,Mn)AsPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 1 2003T. Figielski Abstract In order to search for novel giant-magnetoresistance systems, we fabricated and investigated narrow constrictions in the layers of the ferromagnetic semiconductor (Ga,Mn)As. We found that constrictions a few hundred nanometers wide, tailored by means of the electron-beam lithography and wet etching, were not conducting at liquid helium temperatures unless illuminated, probably due to the trapping action of surface states appearing on an extra surface area denuded by the etching. To avoid this, we used selective implantation of oxygen ions into the ferromagnetic layer to tailor the constrictions. We have shown that such an implantation inactivates Mn acceptors in the layer and destroys ferromagnetism. We propose an application of oxygen ion implantation as a method of fabricating microcircuits in future spin electronics based on Mn-containing III,V semiconductor compounds. [source] Second harmonic generation spectroscopy on Si surfaces and interfacesPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 8 2010Kjeld Pedersen Abstract Optical second harmonic generation (SHG) spectroscopy studies of Si(111) surfaces and interfaces are reviewed for two types of systems: (1) clean 7,×,7 and -Ag reconstructed surfaces prepared under ultra-high vacuum conditions where surface states are excited and (2) interfaces in silicon-on-insulator (SOI) structures and thin metal films on Si surfaces where several interfaces contribute to the SHG. In all the systems resonances are seen at interband transitions near the bulk critical points E1 and E2. On the clean surfaces a number of resonances appear below the onset of bulk-like interband transitions that can be referred to excitations of dangling bond surface states. Adsorption of oxygen leads to formation of a new surface resonance. Such resonances appearing in the region between the bulk critical points E1 and E2 are also shown to be important for Si/oxide interfaces in SOI structures. Finally, examples of spectroscopy on layers buried below thin Ag and Au films are given. [source] Special type of surface states in Hg1,xCdxTePHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 6 2010Salim Gasan-zade Abstract Recently, it was shown that, in the Hg1,xCdxTe compound, the photoelectromagnetic effect (PME) exhibits some specific features and anomalies. The anomalies manifest themselves in the alternating sign behavior (including double sign reversal) of the photomagnetic current JPME in the magnetic field H. In this case, both the sign and shape of the magnetic field dependence of the PME current is critically dependent on the intensity of external excitation radiation and the temperature of the sample. In this work, we demonstrate that the anomalies are determined by hitherto unknown surface states. [source] Effect of Sr adsorption on stability of and epitaxial SrTiO3 growth on Si(001) surfacePHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 9 2006Y. Liang Abstract The geometric and electronic structures of the Sr adsorbed Si(001) surface were investigated using photoemission spectroscopy, electron diffraction, scanning tunneling microscopy, and Rutherford backscattering spectrometry. Results showed that the Sr adsorbed Si(001) exhibited several reconstructions depending upon the Sr coverage. Photoemission results revealed that at one half mono-layer of Sr coverage the dimer-derived surface states disappeared and the stability of Si increased accordingly against oxidation at elevated temperatures. Using this Sr covered surface as a template, epitaxial SrTiO3 films were grown on the vicinal Si(001) substrates using molecular beam epitaxy. The growth process is explained in terms of kinetically limited reaction facilitated by a local energy minimum induced by the Sr adsorption at Si(001) surface. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] One-dimensional surface states induced by segregated impurities at transition-metal surfacesPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 9 2006G. Trimarchi Abstract Using ab initio pseudopotential calculations, we have investigated the atomic and electronic structure of a c (3,2 × ,2) striped reconstruction, induced by segregated C impurities, at the Fe(001) surface. The segregated C atoms form zigzag chains, which in turn produce one-dimensional Fe surface states near the Fermi energy. We address the influence of the C chains on the local surface atomic geometry, local electrostatic potential, and local density of states, and discuss the formation mechanism of the one-dimensional surface states. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] A reflectance anisotropy spectroscopy study of underpotential deposition of copper onto Au(110)PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 13 2005C. I. Smith Abstract The underpotential deposition of Cu on Au(110) has been monitored by Reflection Anisotropy Spectroscopy (RAS). The changes in the intensity of spectral features observed at 2.6 eV and 3.4 eV in the RA spectrum of Au(110) that are induced by the deposition of Cu occur on different timescales. It is suggested that these changes arise, respectively, from the partial quenching of surface states and from changes in surface morphology. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] The effect of Gallium gallium adsorbate on SiC(0001) surface for GaN by MBEPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2003K. Jeganathan Abstract The adsorption of Gallium gallium (Ga) (Ga) on SiC(0001) reveals a well-ordered , R30° and 3 × 3 surface reconstructions with respect to different Ga adatom coverage. The GaN nucleation behaviours on R30° and 3 × 3 surfaces are significantly different. The GaN nucleates randomly in three-dimensional 3D islands on the R30° surface due to unoccupied surface states of dangling bonds. In contrast the 3 × 3 surface was found to be highly suitable for two-dimensional (2D) GaN nuclei which promotes a step-flow growth subsequently. The GaN growth kinetics is found to depend on the Ga atomistic process of SiC (0001). (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Angle-resolved photoelectron spectroscopy study of the GaN(0001)-2×2 surfacePHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7-8 2010P. Lorenz Abstract GaN(0001)-2×2 surfaces were investigated by angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) as well as X-ray photoelectron spectroscopy (XPS). Contamination- and metal-free GaN thin films with a 2×2 reconstruction and a rms roughness below 1 nm were grown on 6H-SiC(0001) by plasma assisted molecular beam epitaxy (PAMBE). The valence band structure of the surface was investigated in-situ with ARUPS along the and directions of the surface Brillouin zone. Weak dispersive surface states related to the unreconstructed GaN surface or to the 2×2 superstructure as well as the dispersion of electron states of the bulk band structure are identified and compared to available results from density functional theory (DFT) calculations [Phys. Rev. B 77, 115120 (2008)] for GaN(0001). (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Buffer-trap and surface-state effects on gate lag in AlGaN/GaN HEMTsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7-8 2010Kazushige Horio Abstract Two-dimensional simulation of turn-on characteristics of Al-GaN/GaN HEMTs is performed in which both buffer traps and surface states are considered. It is studied how the so-called gate lag is affected by these factors. It is shown that gate lag due to buffer traps can occur because in the off state where the gate voltage is negative, electrons are injected into the buffer layer and captured by the traps, leading to more negatively charged buffer layer. It is also shown that gate lag due to an electron-trap-type surface state can occur only when electron's gate tunneling is considered (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] High efficiency all-GaAs solar cellPROGRESS IN PHOTOVOLTAICS: RESEARCH & APPLICATIONS, Issue 2 2010Abderrahmane Belghachi Abstract The reduction of surface recombination in GaAs solar cells is known to be a major concern for photovoltaic cells designers. A common technique used to reduce this effect is to cover the GaAs surface with a wide band gap window layer, therefore the creation of a heterojunction. To avoid a heterojunction with its inconveniences; interface surface states, poor photon absorption in addition to the technological exigencies, one can use an all-GaAs solar cell. In this type of structure, a thin highly doped layer is created at the surface known as a front surface field (FSF). The main role of an FSF layer is to reduce the effect of front surface recombination and the enhancement of light-generated free carriers' collection. This is achieved by the drastic reduction of the effective recombination at the emitter upper boundary. In this work, a simple analytical model is used to simulate the influence of the FSF layer on GaAs solar cell parameters; photocurrent, open circuit voltage and energy conversion efficiency. The effects of the FSF layer doping density and its thickness on the cell performance are discussed by using computed results. Copyright © 2010 John Wiley & Sons, Ltd. [source] | ||||||||||||||||