Number Of Surface States Research Articles

Sufficient energy band utilization profited from spatially discrete heterogeneous interfaces to induce efficient photoelectrochemical water splitting for ZnIn2S4 photoanode

The applied possibility of ZnIn2S4 is improved in the photoelectrochemical (PEC) energy transfer filed through elevated light absorption, accelerating charges transfer and suppressed carrier recombination. Consequently, Sb2S3/ZnIn2S4/Cu2S ternary composite photoanode is obtained through the induction of Sb2S3/ZnIn2S4 and ZnIn2S4/Cu2S heterogeneous interfaces at different spatial locations in ZnIn2S4, which enhances photocurrent density of ZnIn2S4-based photoanode to 2.81 mA/cm2 from 0.15 mA/cm2 with the nearly full visible spectrum absorption range of ∼ 776 nm. The deposition of Sb2S3 leads to enhanced quantum efficiencies, carrier diffusion time, and number of surface states. The loading of Cu2S top layer predominantly results in optimized surface states style and holes injection efficiency of SS/ZIS/CS. The comprehensive measurements and density functional theory (DFT) calculation demonstrate Sb2S3 and Cu2S are together employed to enhance the hybridization efficiencies of energy bands, accelerate charge transfer, revise the reaction sites, reduce free energy barrier of rate-determining steps and reaction over potential of ZnIn2S4 to enhance PEC water spitting. This work pioneers the construction of ZnIn2S4-based multijunction photoanodes with sufficient energy band utilization and confirms the conspicuous prospect for remaining photoanodes.

Engineering Porous Silicon Nanowires with Tuneable Electronic Properties

Structural and electronic properties of silicon nanowires with pre-designed structures are investigated. Wires with distinct structure were investigated via advanced spectroscopic techniques such as X-ray absorption spectroscopy and Raman scattering as well as transport measurements. We show that wire structures can be engineered with metal assisted etching fabrication process via the catalytic solution ratios as well as changing doping type and level. In this way unique well-defined electronic configurations and density of states are obtained in the synthesized wires leading to different charge carrier and phonon dynamics in addition to photoluminescence modulations. We demonstrate that the electronic properties of these structures depend by the final geometry of these systems as determined by the synthesis process. These wires are characterized by a large internal surface and a modulated DOS with a significantly high number of surface states within the band structure. The results improve the understanding of the different electronic structures of these semiconducting nanowires opening new possibilities of future advanced device designs.

Complex dielectric, complex electric modulus, and electrical conductivity in Al/(Graphene-PVA)/p-Si (metal-polymer-semiconductor) structures

In this study, the real and imaginary components of the complex dielectric (ε* = ε′ - jε'′), complex electric modulus (M* = M′ + jM'′), and electrical conductivity (σac) were investigated for fabricated Al/(5% graphene (Gr)-PVA)/p-Si (metal-polymer-semiconductor (MPS)) type structures in wide frequency (5 kHz–5 MHz) and voltage (±4 V) ranges by using impedance (Y = 1/Z = G + jωC) measurements. To determine more charges/energy in a capacitor, a high dielectric (5% Gr-PVA) organic interface layer was used for growth by using the electrospinning method. We observed that the capacitance of the MPS was increased considerably by using this interlayer. As the frequency increased from 5 kHz to 5 MHz, the ε′ values changed from 32.586 to 1.132 due to the presence of surface/dipole polarization and the number of surface states (Nss). The lower M′ values observed at low frequencies were related to the long-range mobility of charge carriers. The magnitudes and positions of the peaks observed in the tanδ-V and M″-V plots varied with the frequency. Furthermore, the observed magnitudes and positions of the peaks in the tanδ-V and M″-V plots changed with the frequency. Thus, we showed that a (5% Gr-PVA) organic interlayer should be preferred to an insulator obtained with the traditional method because of its flexibility, low energy consumption, and simple grown process.

A phase field matching model to compute the localized electronic bands of a 3D monatomic chain of body-centered cube structure: Investigating the (100), (110) and (111) cutting directions

In this study, we compute the surface electronic bands in different cutting directions of a body-centered cubic structure. The systems under study are given as 3D leads that end up with an open cut layer in the [100], [110] and [111] directions. To build up the Hamiltonian matrix around the cutting directions, we integrate into our theoretical model the phase field matching method (PFMM) and the tight-binding approximation. In fact, a direct solution of the built-up surface eigenvalue problem is not possible since the cuts break the periodicity in Hilbert space. For this reason, we integrate the scattering reflection probabilities of the Landauer-Büttiker formalism that lead to obtaining the core surface wave vector and then the calculations of the electronic localized states on the cutting edges are obtained.Additionally, we have taken into consideration the non-ordered surfaces which describe real systems since the cutting direction is always influenced by the relaxation effect due to changes of the total energy on the surface. To illustrate these changes, we have computed the electronic branches by including relaxation effects. The equilibrium positions of the atoms caused by the relaxation on the surface could be determined by the Molecular dynamic algorithm.Our approach of calculation has been applied to evaluate the surface bands in different edges of the following elements: (i) Chrome (Cr) and Tungsten (W) described by s-like orbital. Molybdenum (Mo) defined as s-d-coupling orbitals, Iron (Fe) given as d-type orbital and the Bismuth (Bi) described as f-type orbital. The obtained results illustrate the impact of cutting direction on the number of surface states as well as the scale of the band values.

An Investigation of Surface States Energy Distribution and Band Edge Shifts in Solar Cells Based on TiO2 Submicrospheres and Nanoparticles

TiO2 submicrospheres were often used as photoanodes in dye-sensitized solar cells (DSSCs) due to the high internal surface area and great scattering properties. However, surface states in TiO2 submicrospheres limit the electron transport process. In this paper, an investigation of surface states in TiO2 submicrospheres and traditional TiO2 nanoparticles was conducted by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The chemical capacitance, electron transport resistance and recombination resistance were interpreted in a consistent framework. The electron transport and recombination behavior were also studied by open-circuit voltage decay measurements (OCVD) and intensity-modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS). The results show that the TiO2 submicrospheres based films possess a larger number of surface states and a shallow energy distribution than TiO2 nanoparticles under the same film thickness. Furthermore, the fitted data indicate that the electron transport in TiO2 submicrospheres film was faster than in traditional nanoparticles film owing to their high electron concentration and shallow surface states energy distribution. The discussion highlights the location of surface states in the band gap region of TiO2 film, which plays an important role in electron transport and charge recombination in DSSCs.

Electrical characterization of Au/poly (linoleic acid)-g-poly(methyl methacrylate) (PLiMMA)/n-Si diode in dark and under illumination

In this study, poly (linoleic acid)-g-poly(methyl methacrylate) (PLiMMA) graft copolymer is used as an interfacial layer in a Schottky diode for the first time. Au/poly (linoleic acid)-g-poly(methyl methacrylate) (PLiMMA)/n-Si diode was fabricated and main electrical characteristics of the diode were investigated using I–V measurements in dark and under illumination at room temperature. Ideality factor (n), barrier height (ΦB0) and serial resistance (Rs) values of the diode were found as 2.8, 0.87 eV and 8096 Ω for dark, and 6.3, 0.71 eV and 676 Ω for 100 mW/cm2 illumination intensity. Also, the reasons of deviation from ideal thermionic emission theory were investigated using Cheung&Cheung method and Card&Rhoderick's function which are used for calculating voltage dependence of barrier height (ΦB(V)), series resistance (Rs) and number of surface states (Nss) of the Au/PLiMMA/n-Si diode.

Anodic fluoride passivation of type II InAs/GaSb superlattice for short-wavelength infrared detector

One of the major challenges of antimonide-based devices arises owing to the large number of surface states generated during fabrication processes. Surface passivation and subsequent capping of the surfaces are absolutely essential for any practical applicability of this material system. In this paper, we proposed a new passivation method (zinc sulfide coating after anodic fluoride) for InAs/GaSb superlattice infrared detectors. InAs/GaSb superlattice short-wavelength infrared materials were grown by molecular beam epitaxy on GaSb (100) substrates. A GaSb buffer layer, which can decrease the occurrence of defects with similar pyramidal structure, was grown for optimized superlattice growth condition. High resolution X-ray diffraction indicated that the period of the superlattice corresponding to fourth satellite peak was 39.77 A. The atomic force microscopy images show the roughness was below 1.7 nm. The result of photoresponse spectra shows that the cutoff wavelength was 3.05 μm at 300 K.

Self-Passivation by Fluorine Plasma Treatment and Low-Temperature Annealing in SiGe Nanowires for Biochemical Sensors

Nanowires are widely used as highly sensitive sensors for electrical detection of biological and chemical species. Modifying the band structure of strained-Si metal-oxide-semiconductor field-effect transistors by applying the in-plane tensile strain reportedly improves electron and hole mobility. The oxidation-induced Ge condensation increases the Ge fraction in a SiGe-on-insulator (SGOI) and substantially increases hole mobility. However, oxidation increases the number of surface states, resulting in hole mobility degradation. In this work, 3-aminopropyltrimethoxysilane (APTMS) was used as a biochemical reagent. The hydroxyl molecule on the oxide surface was replaced by the methoxy groups of the APTMS molecule. We proposed a surface plasma treatment to improve the electrical properties of SiGe nanowires. Fluorine plasma treatment can result in enhanced rates of thermal oxidation and speed up the formation of a self-passivation oxide layer. Like a capping oxide layer, the self-passivation oxide layer reduces the rate of follow-up oxidation. Preoxidation treatment also improved the sensitivity of SiGe nanowires because the Si-F binding was held at a more stable interface state compared to bare nanowire on the SiGe surface. Additionally, the sensitivity can be further improved by either the N2 plasma posttreatment or the low-temperature postannealing due to the suppression of outdiffusion of Ge and F atoms from the SiGe nanowire surface.

InGaN–GaN MQW Metal–Semiconductor–Metal Photodiodes With Semi-Insulating Mg-Doped GaN Cap Layers

InGaN-GaN multiple-quantum-well metal-semiconductor-metal photodiodes (PDs) with in situ grown 40-nm-thick unactivated semi-insulating Mg-doped GaN cap layer were successfully fabricated. The dark leakage current of this PD was comparably much smaller than that of conventional PD without the semi-insulating layer, because of a thicker and higher potential barrier of semi-insulating cap layer, and also a smaller number of surface states involved. For the PDs with the semi-insulating Mg-doped GaN cap layers, the responsivity at 380nm was 0.372A/W when biasing at 5 V. In short, incorporating a semi-insulating Mg-doped GaN cap layer into the PDs beneficially leads to the suppression of dark current and a corresponding improvement in the ultraviolet-to-visible rejection ratio

Polyimide Passivated AlGaN–GaN HFETs With 7.65 W/mm at 18 GHz

Current metal-organic chemical vapor deposition-grown AlGaN-GaN heterojunction field-effect transistor devices suffer from threading dislocations and surface states that form traps, degrading RF performance. A passivation scheme utilizing a polyimide film as the passivating layer was developed to reduce the number of surface states and minimize RF dispersion. Continuous-wave power measurements were taken at 18 GHz on two-finger 0.23-/spl mu/m devices with 2/spl times/75 /spl mu/m total gate width before and after passivation yielding an increase from 2.14 W/mm to 4.02 W/mm in power density, and 12.5% to 24.47% in power added efficiency. Additionally, a 2/spl times/25 /spl mu/m device yielded a peak power density of 7.65 W/mm at 18 GHz. This data suggests that polyimide can be an effective passivation film for reducing surface states.

Tight-binding calculation of the electronic states of bulk-terminated GaAs(311)A and B surfaces

We have carried out theoretical investigations on the electronic structure of GaAs(311)A and GaAs(311)B surfaces. The bulk electronic structure of GaAs has been described by the second-neighbour tight-binding formalism and the surface electronic structure was evaluated via an analytic Green function method. First, we present the surface band structure together with the projected bulk band of both Ga-terminated and As-terminated for GaAs(311)A and GaAs(311)B surfaces, respectively. In each case, the number of surface states is determined and the localized surface features and orbital properties of these surface states along Γ-Y-S-X-Γ high symmetry lines of the surface Brillouin zone are discussed. For the Ga-terminated GaAs(311)A (1×1) surface, we have tested two possible structure models, i.e. ``the bridge and ``the hollow models. In comparison with the angle-resolved photoelectron spectroscopy studied recently, the results have shown that the surface electronic states of the hollow site model are in good agreement with the experiments, whereas those of the bridge site model are not. So we have concluded that the hollow site model is favourable for the Ga-terminated GaAs(311) (1×1) surface and the bridge site model should be excluded.

Scanning Tunneling Microscopy and Atomic Force Microscopy Studies of Laser Irradiation of Amorphous WO3 Thin Films

The effect of irradiating amorphous WO3-x thin films by an excimer laser and a Nd-YAG (yttrium-aluminum-garnet) laser on the evolution of electronic states was investigated by scanning tunneling spectroscopy (STS) and X-ray photoelectron spectroscopy (XPS). The original films prepared by pulsed-laser deposition (PLD) were first irradiated by a KrF excimer laser at 248 nm in air. The surface was reduced to a quasi-metallic one with oxygen deficiency and the number of W 5d occupied states was increased. A number of surface states appeared on the films. When the films were subsequently irradiated by the Nd-YAG laser at 1.06 µm in air, the number of W 5d occupied states was reduced due to oxygen restoration. The number of surface states was also greatly reduced. A water layer on the surface, which played a key role in photolysis, was monitored by STS by trapping charges in the Schottky barrier. Atomic force microscopy (AFM) and scanning tunneling microscopy (STM) images of the films at three states: original, surface reduced, and surface restored, were compared in terms of average profile roughness and height. Thermal effects (thermochromism) dominated in the Nd-YAG laser irradiation, resulting in a maximum average roughness in the AFM images, whereas photon effects (photochromism) dominated in excimer laser irradiation, which led to the highest conductance. Slight nanocrystallization occurred in the laser-irradiated films.

Surface and interface shear horizontal acoustic waves in piezoelectric superlattices

The propagation of acoustic waves of shear horizontal polarization in infinite and semi-infinite superlattices made of two piezoelectric media is studied within a Green’s function method. Localized modes induced by a free surface of the superlattice or a superlattice/substrate interface are investigated theoretically. These modes appear as well-defined peaks of the total density of states inside the minigaps of the superlattice. The spatial localization of the different modes is studied by means of the local density of states. The surface of the superlattice and the superlattice/substrate interface are considered to be either metallized or nonmetallized. We show the possibility of the existence of interface modes, which are without analogue in the case of the interface between two homogeneous media (the so-called Maerfeld–Tournois modes). We also generalize to piezoelectric superlattices a rule about the existence and number of surface states, namely when one considers two semi-infinite superlattices together obtained by the cleavage of an infinite superlattice, one always has as many localized surface modes as minigaps, for any value of the wave vector k∥ (parallel to the interfaces). Specific applications of these results are given for CdS–ZnO superlattices with a free surface or in contact with a BeO substrate.

Role of surface states and adsorbates in time-resolved photocurrent measurements and photovoltage generation at phthalocyaninatozinc(II)-photocathodes

The role of surface states in the photoelectrochemistry of phthalocyaninatozinc(II) (PcZn) has been investigated by time-resolved photocurrent measurements in the millisecond range and by photovoltage measurements. Plots of the photovoltage against the redox potential of the electrolyte yielded slopes a with −1< a< 0 indicating partial Fermi-level pinning due to the presence of surface states. The charging and discharging of the surface states could be seen clearly in the photocurrent transients. The amount of charging and discharging was found to be determined by the recombination rate as a function of the applied electrode potential and by the charge–transfer rate, which was studied in dependence upon the nature and the concentration of the redox couple in the electrolyte (ferri/ferrocyanide, hydroquinone/benzoquinone) in comparison to oxygen- saturated and deaerated KCl electrolytes. The charge–transfer rate was found to be limited by an adsorption step of the electroactive species. The number of surface states was found to increase with exposure to air. This is interpreted as a modification of the PcZn surface due to reaction with oxygen. The surface state densities as derived from the extent of the Fermi-level pinning corresponded well with the discharges seen in the photocurrent transients and show that a considerable fraction of the PcZn surface molecules represents surface states after longer exposure to air.

The Electrochemical Impedance of One-Equivalent Electrode Processes at Dark Semiconductor/Redox Electrodes Involving Charge Transfer through Surface States. 2. The n-GaAs/Fe3+ System as an Experimental Example

In this paper, an electrochemical study of the n-GaAs/Fe3+ system is presented. Combining the results of current density vs potential, Mott−Schottky, and electrochemical impedance measurements, it is shown that charge transfer through surface states is the most plausible reaction mechanism for the Fe3+ to Fe2+ reduction at the n-GaAs electrode. On the basis of the experimental impedance spectra and of the theoretical equivalent circuit derived in part 1 (Hens, Z. J. Phys. Chem. 1998, 103, 122), the charge-transfer mechanism is analyzed in detail. The Fe3+ reduction rate shows a marked evolution as a function of time which, basing upon the electrochemical impedance results, is attributed to an increase in the number of surface states.

Ab initiostudies of the (100), (110), and (111) surfaces ofCoSi2

Based on the ultrasoft pseudopotential technique of the Vienna ab initio simulation package, we performed ab initio calculations for the (100), (110), and (111) surfaces of ${\mathrm{CoSi}}_{2}$ within the framework of the generalized gradient approximation. Surface energies were derived from the total energies as well as estimated from simple models. Relaxed surface geometries were determined for the $(1\ifmmode\times\else\texttimes\fi{}1)$ surfaces by force minimization. For the (100) surface a proposed $\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}$ reconstruction was investigated that, however, is not stable. Energetical results as well as simulated scanning tunneling images strongly indicate that the reconstruction does not exist. The band structures show a number of surface states, in particular, in gaps of the projected bulk bands at and above Fermi energy for the (100) and (110) surface. For the Si-Co-Si terminated (111) surface, however, only two Si-like surface bands are found. Some surface states are analyzed in terms of density contours revealing covalent Co-Si bonding and coupling to deeper layers. Work functions are also provided.

Luminescence from Porous Silicon

Recent observations of photoluminescene (PL) and electroluminescence (EL) from poroussilicon (PS) have prompted many theoretical and experimental studies. Bulk crystalline Si is anindirect band gap material in which .recombination is dominated by non-radiative processes.Therefore, it cannot be used as light-emitting component in Si circuits. PS is a new material formed byanodisation ofsingle crystal Si wafers in hydro fluoric (liF) solution. Luminescence from this materialis being explored for technological applications all over the world. The mechanism of luminescence isstill not well-understood. Several models have been proposed but still the facts about the strong lightemission at room temperature are unknown. This paper presents a review of the fabrication process andstudies on luminescent properties of PS. A hybrid model based on quantum confinement of carriers inthe nanometer size Si crystallites having a large number of surface states is suggested to explain theobserved properties.

Effect of Surface States on the Electrical and Optical Characteristics of InP MIS Capacitor

The effect of surface states on the electrical and optical characteristics of an InP metal-insulator-semiconductor (MIS) capacitor has been studied theoretically. The semi-numerical model presented here examines the potential of the device for optically controlled applications. While the device shows much promise for use as a voltage variable and/or optically controlled capacitor, its performance is seriously limited by the property of the interface between InP crystal and insulator film (Al 2 O 3 , in this case), where a considerable number of surface states exist with their energy distribution widely spread throughout the crystal forbidden gap.

Accelerated life testing and failure analysis of single stage MMIC amplifiers

Single stage monolithic microwave integrated circuit (MMIC) amplifiers have been high temperature accelerated life tested under DC+RF biasing conditions. Most of the circuits failed parametrically due to a gradual decrease in RF output power. Failure analysis revealed localized reduction of the gate breakdown characteristics of the metal semiconductor field effect transistor (MESFET). This was occurring through degradation of the GaAs surface Si/sub 3/N/sub 4/ passivation layer interface in the channel region of the MESFET, resulting in a reduction in the number of surface states. The few catastrophically failed MMIC's are believed to represent a special case of this degradation process which occurred very rapidly. In contrast to the transistor, the other components of the circuits were unchanged following life test.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Gap-states distribution in amorphous-silicon films as obtained by photothermal deflection spectroscopy.

The paper reports results on intrinsic, device-quality amorphous silicon obtained with use of photothermal deflection spectroscopy. The gap-states distribution is obtained by means of a simple and reliable derivative procedure on the absorption-coefficient spectra. A comparison with other models is made. Finally, it is shown that the peak energy of the defects can be different for surface and bulk states, and so a shift of the occupied defect peak is observed if a considerable number of surface states are introduced. This explains the higher values for the dangling-bond correlation energy obtained by means of optical methods and agrees with other experimental evaluations of the distribution of surface states such as those carried out by means of total-yield spectroscopy.