Band Structure Model Research Articles

The angular electronic band structure and free particle model of aromatic molecules: High-frequency photon-induced ring current

This work introduces an analysis of the relationship of first-principles calculations based on DFT method with the results of free particle model for ring-shaped aromatic molecules. However, the main aim of the study is to reveal the angular electronic band structure of the ring-shaped molecules. As in the case of spherical molecules such as fullerene, it is possible to observe a parabolic dispersion of electronic states with the variation of angular quantum number in the planar ring-shaped molecules. This work also discusses the transition probabilities between the occupied and virtual states by analyzing the angular electronic band structure and the possibility of ring currents in the case of spin angular momentum (SAM) or orbital angular momentum (OAM) carrying light. Current study focuses on the benzene molecule to obtain its angular electronic band structure. The obtained electronic band structure can be considered as a useful tool to see the transition probabilities between the electronic states and possible contribution of the states to the ring currents. The photoinduced current due to the transfer of SAM into the benzene molecule has been investigated by using analytical calculations within the frame of time-dependent perturbation theory.

An origin behind Rashba spin splitting within inverted doped sGe heterostructures

In this paper, we demonstrate why cubic Rashba spin splitting is observed within inverted doped strained germanium (sGe) hetrostructures. Magnetotransport measurements showed beating within the SdH oscillation, with fast Fourier analysis revealing cubic Rashba spin splitting to be present. A cubic Rashba coefficient of β=7.97×10−29 eVm3 and a spin-splitting energy of Δ=1.17 meV were determined. The source of the cubic Rashba spin splitting was identified from a combination of ultra low energy secondary ion mass spectrometry analysis and subsequent band structure modelling using Nextnano3. Ultra-low energy secondary ion mass spectrometry revealed an unintentional, highly B doped near surface region accounted for structure inversion asymmetry, whereas subsequent band structure modelling using Nextnano3 demonstrates how this asymmetric B doping generates structural inversion asymmetry within the sGe QW structure and cubic Rashba spin splitting realized.

Electronic Structure and Redox Behavior of Tin Sulfide Films Potentiostatically Formed on Tin

The tin sulfide film, formed by potentiostatic anodic polarization of tin in aqueous electrolyte containing sulfide ions, was investigated using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), X–ray photoelectron spectroscopy (XPS) and diffuse reflectance spectroscopy (DRS). According to XPS and Mott-Schottky (MS) tests, the surface film exhibits semiconducting properties and bilayer structure with the inner of Sn(II) sulfide layer of p–type conductivity and the outer layer of Sn(IV) sulfide layer of n–type conductivity. The film's electronic structure was discussed on the basis of band structure models, potential distribution at the interface and the availability of free charge carriers on the electrode|solution interface. The electronic structure of the passive film on tin influences its susceptibility to the oxidative as well as to the reductive solid–state decomposition. The semiconducting parameters of the sulfide film: the flatband potential, EFB, the donor concentration, ND and the optical bandgap, Eg were determined.

Investigation of ultrathin Pt/ZrO2–Al2O3–ZrO2/TiN DRAM capacitors Schottky barrier height by internal photoemission spectroscopy

We report the Schottky barrier height (SBH) at metal–insulator interfaces in Pt/ZrO2–Al2O3–ZrO2(ZAZ)/TiN dynamic random access memory capacitors by analyzing the photoconductivity yield and internal photoemission (IPE) yield using IPE spectroscopy. The SBH values at the Pt/ZAZ and ZAZ/TiN interfaces in the Pt/ZAZ/TiN stack were found to be 2.77 and 2.18 eV, respectively. The SBH difference between the top electrode/oxide and bottom electrode/oxide interfaces is related to the work function difference between Pt and TiN, and the subgap defect state features (density and energy) of the given dielectric. By combining experimental analysis using IPE at the device level and ultraviolet photoelectron spectroscopy and spectroscopic ellipsometry at the film level, a band structure model is proposed.

DISPERSION IN IMPEDANCE SPECTRA OF ISOTYPE CdO–ZnO THIN FILM HETEROSTRUCTURE ON APPLIED DC BIAS

Radio frequency magnetron sputtering technique has been used to prepare n–n isotype heterostructure of CdO–ZnO thin film on glass substrate. The energy band structure analysis shows the formation of charge accumulation and charge depletion region created near the interface of CdO–ZnO n–n isotype heterostrucure which leads to the formation of a junction capacitance. The impedance spectroscopic studies of CdO–ZnO isotype heterostructure show an excellent dispersion in the impedance spectra as observed in the Nyquist plot depending on applied DC bias voltage. The frequency response of impedances has been explained from band structure and equivalent electrical circuit model of the heterostructure. These studies will be highly significant in exploring the possibility of application of such isotype interfaces in optimizing the functionality of organic and inorganic electronic devices and solar cells.

Quantum Simulation Study on Performance Optimization of GaSb/InAs nanowire Tunneling FET

We report the computer aided design results for a GaSb/InAs broken-gap gate all around nanowire tunneling FET (TFET). In designing, the semi-empirical tight-binding (TB) method using sp3d5s* is used as band structure model to produce the bulk properties. The calculated band structure is cooperated with open boundary conditions (OBCs) and a three-dimensional Schrödinger-Poisson solver to execute quantum transport simulators. We find an device configuration for the operation voltage of 0.3 V which exhibit desired low sub-threshold swing ( 60 mV/dec) by adopting receded gate configuration while maintaining the high current characteristic (ISUBON/SUB 100 μA/μm) that broken-gap TFETs normally have.

Gaussian thermoluminescence in long-range disordered K-feldspar

The thermoluminescence behavior of long-range ordered crystals is usually explained by the band structure model, using first- and second-order kinetics. However, feldspars have order-disorder phenomena and twinning, and consequently these mathematical descriptions are not helpful in most cases. In this work, the thermally stimulated intrinsic blue luminescence at 440 nm from X-ray induced defects of the K-rich feldspars is used to show a progressive behavior change along the order-disorder series. It is observed a gradual conversion of the TL signal from a very asymmetric peak with exponential rise and power law decay in microcline and orthoclase, where a τ coefficient in log-log plots decreases with twin/domain size, to a more symmetric signal in a partially disordered sanidine, up to reach a completely symmetric Gaussian peak in fully disordered sanidine. These results are compatible with the Bassler’s model of disorder, which suggest that atomic disorder involves the transformation of delocalized bands first into band tails as the source of electron traps, and later in localized density of states following a Gaussian distribution.

Depth-Profiling Electronic and Structural Properties of Cu(In,Ga)(S,Se)2 Thin-Film Solar Cell.

Utilizing a scanning photoelectron microscope (SPEM) and grazing-incidence X-ray powder diffraction (GIXRD), we studied the electronic band structure and the crystalline properties of the pentanary Cu(In,Ga)(S,Se)2 (CIGSSe) thin-film solar cell as a function of sample depth on measuring the thickness-gradient sample. A novel approach is proposed for studying the depth-dependent information on thin films, which can provide a gradient thickness and a wide cross-section of the sample by polishing process. The results exhibit that the CIGSSe absorber layer possesses four distinct stoichiometries. The growth mechanism of this distinctive compositional distribution formed by a two-stage process is described according to the thermodynamic reaction and the manufacturing process. On the basis of the depth-profiling results, the gradient profiles of the conduction and valence bands were constructed to elucidate the performance of the electrical properties (in this case, Voc = 620 mV, Jsc = 34.6 mA/cm(2), and η = 14.04%); the valence-band maxima (VBM) measured with a SPEM in the spectroscopic mode coincide with this band-structure model, except for a lowering of the VBM observed in the surface region of the absorber layer due to the ordered defect compound (ODC). In addition, the depth-dependent texturing X-ray diffraction pattern presents the crystalline quality and the residual stress for each depth of a thin-film device. We find that the randomly oriented grains in the bottom region of the absorber layer and the different residual stress between the underlying Mo and the absorber interface, which can deteriorate the electrical performance due to peeling-off effect. An anion interstitial defect can be observed on comparing the anion concentration of the elemental distribution with crystalline composition; a few excess sulfur atoms insert in interstitial sites at the front side of the absorber layer, whereas the interstitial selenium atoms insert at the back side.

Local Charge Injection and Extraction on Surface-Modified Al2O3 Nanoparticles in LDPE.

We use a recently developed scanning probe technique to image with high spatial resolution the injection and extraction of charge around individual surface-modified aluminum oxide nanoparticles embedded in a low-density polyethylene (LDPE) matrix. We find that the experimental results are consistent with a simple band structure model where localized electronic states are available in the band gap (trap states) in the vicinity of the nanoparticles. This work offers experimental support to a previously proposed mechanism for enhanced insulating properties of nanocomposite LDPE and provides a powerful experimental tool to further investigate such properties.

Comparative calculations of electron transport properties in 6H–SiC using three and five valley models

Steady-state electron properties are investigated in 6H–SiC at various temperatures, using Monte Carlo simulation where the band structure model is a major part when dealing with high fields. The aim of this work is to optimize the number of valleys involved in the simulation program in order to obtain accurate results while improving the calculation efficiency. For high fields, a five valley model was found to be more accurate than a three valley model and as efficient as the full band method though much less computer time-consuming.

Behavior of Photocarriers in the Light-Induced Metastable State in the p-n Heterojunction of a Cu(In,Ga)Se2 Solar Cell with CBD-ZnS Buffer Layer.

We fabricated Cu(In,Ga)Se2 (CIGS) solar cells with a chemical bath deposition (CBD)-ZnS buffer layer grown with varying ammonia concentrations in aqueous solution. The solar cell performance was degraded with increasing ammonia concentration, due to actively dissolved Zn atoms during CBD-ZnS precipitation. These formed interfacial defect states, such as hydroxide species in the CBD-ZnS film, and interstitial and antisite Zn defects at the p-n heterojunction. After light/UV soaking, the CIGS solar cell performance drastically improved, with a rise in fill factor. With the Zn-based buffer layer, the light soaking treatment containing blue photons induced a metastable state and enhanced the CIGS solar cell performance. To interpret this effect, we suggest a band structure model of the p-n heterojunction to explain the flow of photocarriers under white light at the initial state, and then after light/UV soaking. The determining factor is a p+ defect layer, containing an amount of deep acceptor traps, located near the CIGS surface. The p+ defect layer easily captures photoexcited electrons, and then when it becomes quasi-neutral, attracts photoexcited holes. This alters the barrier height and controls the photocurrent at the p-n junction, and fill factor values, determining the solar cell performance.

Mesoporous InN/In2O3 heterojunction with improved sensitivity and selectivity for room temperature NO2 gas sensing

Establishing heterostructures is a good strategy to improve gas sensing performance, and has been studied extensively. In this work, mesoporous InN/In2O3 composite (InNOCs) heterostructures were prepared through a simple two-step strategy involving hydrothermal synthesis of In2O3 and subsequent nitriding into InN-composite In2O3 heterostructures. We found that the InN content has great influence on the resistance of InNOCs, and thus, the gas sensing performance. In particular, InNOC-36.9 (with InN content of 36.9% in the composites) shows an excellent sensing response towards different concentrations of NO2, as well as good stability after one week of exposure to 200 ppb NO2 at room temperature. The highest sensing response (ΔR/R0) is up to 1.8 for the low NO2 concentration of 5 ppb. Even more significantly, the theoretical limit of detection (LOD) of the InNOC-36.9 sensor is 31.7 ppt based on a signal-to-noise ratio of 3 (the measured LOD is 5 ppb), which is far below the US NAAQS value (NO2: 53 ppb). In addition, a rational band structure model combined with a surface reaction model is proposed to explain the sensing mechanism.

Role of Gd3+ ion on downshifting and upconversion emission properties of Pr3+, Yb3+ co-doped YNbO4 phosphor and sensitization effect of Bi3+ ion

Dual-mode luminescence (downshifting-DS and upconversion-UC) properties of Pr3+/Yb3+ co-doped Y1−xGdxNbO4 (x = 0.0, 0.5, and 1.0) phosphors synthesized by solid state reaction technique have been explored with and without Gd3+ ion. The structural characterizations (XRD, SEM, and FTIR) confirm the pure phase of YNbO4 phosphor. Further, with the Gd3+ ion co-doping, the YNbO4 phosphors having a random shape and the large particle size are found to be transformed into nearly spherical shape particles with the reduced particle size. The optical band gaps (Eg) of Y1−xGdxNbO4 (x = 0.00, 0.25, 0.50, and 1.00) calculated from UV-Vis-NIR measurements are ∼3.69, 4.00, 4.38, and 4.44 eV, respectively. Moreover, YNbO4 phosphor is a promising blue emitting material, whereas Y1−x−y−zPryYbzGdxNbO4 phosphor gives intense green, blue, and red emissions via dual-mode optical processes. The broad blue emission arises due to (NbO4)3− group of the host with λex = 264 nm, whereas Pr3+ doped YNbO4 phosphor gives dominant red and blue emissions along with comparatively weak green emission on excitation with λex = 300 nm and 491 nm. The concentration dependent variation in emission intensity at 491 nm (3P0→3H4 transition) and 612 nm (1D2→3H4 transition); at 612 nm (1D2→3H4 transition) and 658 nm (3P0→3F2 transition) of Pr3+ ion in YNbO4 phosphor with λex = 300 nm and 491 nm excitations, respectively, has been thoroughly explored and explained by the cross-relaxation process through different channels. The sensitization effect of Bi3+ ion co-doping on DS properties of the phosphor has also been studied. The observed DS results have been optimized by varying the concentration of Pr3+ and Bi3+ ions, and the results are explained by the well-known simple band structure model. The study of Gd3+ co-doping reveals noticeable differences in DS characteristics of Y1−xPrxNbO4 phosphors: the overall decrement and increment (except for 612 nm emission) in intensity of DS emission on excitation with λex = 264 nm and 491 nm, respectively. These observations have been thoroughly explained, and the 1D2→3H4 transition (612 nm) of Pr3+ ion is found to be strongly dependent on surrounding environment of the host matrix. The UC properties of Y0.95−xPrxYb0.05NbO4 phosphors have been explored using Near Infra-Red (NIR) excitation. The material gives intense green and relatively weak blue and red UC emissions with λex = 980 nm. Interestingly, the UC emission intensity is further enhanced in the case of Y0.949−xPr0.001Yb0.05GdxNbO4 phosphors. In addition, the less explored laser induced heating effect with the pump power as well as the irradiation time on the UC emission has been explored in Y0.949−xPr0.001Yb0.05GdxNbO4 (x = 0, 0.5, and 0.949) phosphor samples, and subsequently, this feature has been found to be superior for Gd0.949Pr0.001Yb0.05NbO4 phosphor. The comparative study between the two hosts, viz., YNbO4 and GdNbO4 shows that GdNbO4 is better than YNbO4 for UC emission behavior; however, a reverse is observed as for as DS behavior is concerned only for the particular excitation wavelength (λex = 264 nm).

A Nonparabolic Bandstructure Model for Computationally Efficient Quantum Transport Simulations

With the increasing focus on III–V materials as potential candidates for next-generation nanotransistors, advanced bandstructure models going beyond the parabolic band approximation are required to ensure accurate device simulations. For that purpose, we present a straightforward way to include nonparabolic (NP) bandstructure effects into quantum transport calculations based on the effective mass approach. This scheme does not only properly account for the NP effects present in the thermionic current of transistors, but also in their source-to-drain tunneling leakage. The method is validated by simulating 2-D and 3-D In0.53Ga0.47As nanoscale transistors with two variants of the proposed NP model. An excellent agreement with full-band results is demonstrated in all the cases.

Design considerations of biaxially tensile-strained germanium-on-silicon lasers

Physical models of Ge energy band structure and material loss were implemented in LASTIPTM, a 2D simulation tool for edge emitting laser diodes. This model is able to match available experimental data. Important design parameters of a Fabry–Perot Ge laser, such as the cavity length, thickness, width, polycrystalline Si cladding layer thickness were studied and optimized. The laser structure optimizations alone were shown to reduce Ith by 22-fold and increase ηd and ηi by 11 and 6 times. The simulations also showed that improving the defect limited carrier lifetime is critical for achieving an efficient and low-threshold Ge laser. With the optimized structure design (300 μm for the cavity length, 0.4 μm for the cavity width, 0.3 μm for the cavity thickness, and 0.6 μm for the polycrystalline Si cladding layer thickness) and a defect limited carrier lifetime of 100 ns, a wall-plug efficiency of 14.6% at 1 mW output is predicted, where Jth of 2.8 kA cm−2, Ith of 3.3 mA, I1mW of 9 mA, and ηd of 23.6% can be achieved. These are tremendous improvements from the available experimental values at 280 kA cm−2, 756 mA, 837 mA and 1.9%, respectively.

Semiconducting SnO2-TiO2 (S-T) composites for detection of SO2 gas

SnO2-TiO2 (S-T) composites with different molar ratios were prepared by mechanical mixing followed by sintering at 700 °C for 4 h in air. The structural and microstructural properties of the composites were investigated using powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM). S-T composites were investigated by introducing SO2 to test their chemical stability using PXRD and SEM coupled with energy dispersive X-ray (EDX) analysis. The sensing performance was measured at different temperatures using various SO2 concentrations (10–100 ppm). A composite comprising 25 mol% of SnO2 and 75 mol% TiO2 (S25-T75) exhibited the highest sensitivity comparing to other S-T composites studied under the presently investigated conditions. t 90 (90 % of response time) was found to be ∼5 min for thick pellet (~2 mm in thickness). SO2 sensing mechanism has been explained through the band structure model.

The Redox Processes in Lithium Batteries : Beyond the Oxidation State Formalism

In general, electrochemistry is based on electron exchange between different species (ions, atoms, molecules,grains…), in a liquid or solid environment, and this electron exchange is usually written in using the concept of oxidation state of the different elements constituting the species. Redox processes are thus introduced, where oxidation and reduction for a specie correspond to the loss or a gain of electrons, respectively. This concept is generally well adapted to electrochemistry because the oxidation state of an element corresponds to the number of electrons which is lost or gained in a chemical bond, supposed to be purely ionic, to form a cation or an anion, respectively. The electronic structure of an ion or a molecule is described by discrete energy levels which are occupied by a small and integer number of electrons. So, during an electrochemical process, electrons are added or removed and this electronic transfer can be explained by using quite simple concept. The problem is different in a solid, The problem is different in a solid for which the only way to describe the electronic structure is to use the band structure model: atomic orbitals of all elements are more or less mixed together to form the bands. Consequently, the number of valence electrons is very large and the ionic model is very often unsuitable. In this presentation we will show through different examples that use of the oxidation state can be misleading in order to explain the true behavior of a solid electrode material in lithium batteries. We will propose a more general description. First, we will focus on X-ray Absorption Spectroscopy (XAS), one of the most powerful tools to analyze precisely the state of charge of an element. It has been showed many times, on chalcogenides (1) and oxides (2), that this technique combines very nicely with the calculated electronic structure of a solid. It will be evidenced that in a redox process all elements of a material are modified by the electron transfer. Secondly, we will show how the oxidation state concept could lead the researchers to exclude a priori some reactions because they would put some elements in unusual low or high oxidation states (3). Finally, we will show recent results obtained on Li2MnO3 and HE-NMC materials (4-6). From them, it appears clearly that the oxidation and reduction processes in solid are complex phenomenon. They generally impact all elements which have to be considered as possible redox centers. The general model we propose is based on solid electronic band structure for solid, considered as an electron reservoir which can be emptied (oxidation) or filled (reduction). The reservoir is built a priori by the orbitals of all elements. The fact that an electrochemical reaction occurs, becomes reversible, or leads to degradation of the material, has to be discussed in terms of energetic stability of a given atomic structure. This can be related to the shape and the filling of the electronic reservoir. References 1 – Z.Y. Wu, G. Ouvrard, S. Lemaux, P. Moreau, P. Gressier, F. Lemoigno and J. ROUXEL, Phys. Rev. Lett. 77 (1996) 2101-2104. 2- F. Boucher, N. Bourgeon, K. Delbé, P. Moreau, D. Guyomard and G. Ouvrard, Journal of Physics and Chemistry of Solids, 67 (2006) 1238. 3 – N. Tran, L. Croguennec, M. Ménétrier, F. Weill, Ph. Biensan, C. Jordy, and C. Delmas, Chem. Mater. 20 (2008) 4815–4825 4 – H. Koga, L. Croguennec, M. Ménétrier, P. Mannessiez, F. Weill, C. Delmas, and S. Belin J. Phys. Chem. C 118 (2014) 5700−5709 5 – Yukinori Koyamaa, Isao Tanakaa, Miki Nagaob, Ryoji Kanno Journal of Power Sources 189 (2009) 798–801 6 – A. Pradon, C. La Fontaine, S. Belin, P. E. Petit, P. Moreau,, L. Lajaunie, E. Dumont, E. Elkaim, C. Tessier, G. Ouvrard, M.T. Caldes, Submitted to Chemistry of Materials

Fowler-Nordheim Tunneling, Photovoltaic Applications and New Band Structure Models of Electroconductive a-CN<sub>x</sub>:H Films Formed by Supermagnetron Plasma CVD

Hydrogenated amorphous carbon nitride (a-CNx:H) films were formed on Al films deposited on Si or glass (SiO2) substrates, using pulsed radio frequency (PRF) supermagnetron plasma (SMP) chemical vapor deposition (CVD) with N2/i-C4H10 mixed gases. a-CNx:H films were grown under the upper and lower electrode RF powers (13.56 MHz) of continuous and pulsed conditions, respectively, which showed low band gap of about 0.7 eV. a-CNx:H films deposited on the Al/Si or Al/SiO2 substrates showed same low threshold emission electric field (ETH) of 12 V/μm. Multiple layer of Al or ITO (anode)/50nm-SiO2/a-CNx:H/Al (cathode)/Si structures showed Fowler-Nordheim (FN) electron tunneling effect in both forward and reverse current directions. 12.5 nm a-CNx:H film on p-Si substrate showed a photoelectric conversion. Energy band structure and electron conduction models were proposed for the active states of both the field emission and FN tunneling devices and photovoltaic cells.

Metal-Rich Transition Metal Diborides as Electrocatalysts for Hydrogen Evolution Reactions in a Wide Range of pH

Solid solutions of HfB2-ZrB2 mixtures were prepared by high-energy ball milling of diboride and additive powders followed by spark plasma sintering (SPS). A mixture of stoichiometric 1:1 HfB2-ZrB2 borides was the base composition to which Hf, Zr, Ta, LaB6 or Gd2O3 was added. Hf, Zr and Ta were added in order to bring the boron-to-metal ratio down to 1.86, rendering the boride as MeB1.86. In the case of LaB6 and Gd2O3, 1.8 mol% was added. Electroanalytical behavior of hydrogen evolution reactions was evaluated in 1 M H2SO4 and 1 M NaOH solutions. The LaB6 additive material showed Tafel slopes of 125 and 90 mV/decade in acidic and alkaline solutions respectively. The Hf and Zr rich samples showed Tafel slopes of about 120 mV/decade in both electrolytes. The overpotentials of hydrogen evolution reactions (at 10 mA/cm2) in the alkaline solution were about 100 mV lower than those in acidic solution. The metal-rich diborides and addition of LaB6 showed better hydrogen evolution reaction (HER) activities than the base 1:1 HfB2-ZrB2 stoichiometric diboride solid solution. The higher activity of metal-rich borides could be attributed to the increased electron population at the d-orbitals of the metal shown by band structure modeling calculations using the Density Functional Theory approach.

Linking photoluminescence of α-Si3N4 to intrinsic point defects via band structure modelling

Photoluminescence properties have been connected to intrinsic point defects for Si abundant (red bar) and N plentiful (blue bars) α-Si3N4via band structure modelling using DFT calculations.