Presence Of Surface States Research Articles

Electrical conduction and photoluminescence properties of solution-grown ZnO nanowires

We report on the optical and electrical properties of zinc oxide nanorods synthesized in solution using Oswald ripening of ZnO nanodots with the addition of ethylenediamene growth directing agent. This method results in high quality, single crystalline ZnO nanorods that extend up to 3μm in length and have an average diameter of 25±7nm, compared to ∼75nm diameter for similarly prepared nanorods but without the addition of the growth directing agent. Furthermore, we find that the higher aspect ratio nanorods exhibit strong size-dependent electrical characteristics, with a critical diameter of about 27nm delimiting nonconductive and conductive behaviors. Theoretical calculations indicate that the origin of this size-dependent conductivity is the presence of surface states that deplete the carriers in the smaller diameter nanorods, and an estimate of the density of these states is provided.

The Influence of Different Surface Terminations on Electrical Transport and Emission Properties for Freestanding Single Crystalline (100) CVD Diamond Samples

AbstractThe surface density of states of hydrogen and oxygen terminated diamond is an important parameter from the point of view of electrical transport properties at the surface. In addition, the presence of surface states has a detrimental influence on the electrical contact properties. Therefore it is of great importance that the influence of different species on the surface-related properties of the diamond layer is well understood.In this work (100)-oriented CVD diamond films are terminated using hydrogen plasma with and without small additions of oxygen (1 to 4%). XPS and UPS measurements are performed to look at the influence of this addition on the surface band gap of the diamond samples. Using the TOF (Time-of-Flight) technique a comparison is made between oxidised and differently hydrogenated diamond surfaces. The results clearly show that the different terminations of the diamond surface have an influence on the electrical transport properties. For the oxidised surface, it is found that defect states are created in the surface band gap, increasing trapping. Contrary to this, fully hydrogenated layers behave differently suggesting that the surface DOS is significantly reduced. This fact can be confirmed when applying an electric field on the CVD diamond samples sandwiched between two metallic electrical contacts. In case of a metal deposited on a hydrogen terminated surface we can see a clear UV light emission, related to free exciton recombination in the contact-diamond-junction region. When the surface is oxidised the UV emission is damped, an effect which is attributed to the parallel recombination channels via the surface states.

Nanocrystalline structure and nanopore formation in modified thermal TiO 2 films

The anodic TiO 2 films on Ti 0 substrates were synthesized using different electrochemical techniques including potential step, potentiodynamic, and galvanostatic technique, from a 1 M Na 2 SO 4 + 10 mM NaF solution. The AFM imaging of film surfaces revealed that thin films ( h < 20 nm ) are composed of small spherical-shaped nanocrystals, up to 20 nm in size. In thicker TiO 2 films ( h > 20 nm ) , cylindrical nanopores with diameter of ca. 30 nm, are formed. In contrast to the nanocrystalline nature of anodic TiO 2 films, thermally grown TiO 2 films at 850 °C are composed of oriented large pyramidal crystallites (200–500 nm in size), which are transformed into even larger moulds (2– 3 μ m in size) at 1050 °C. Thermal films of this type have been known to show inferior surface properties leading to large reflectance losses and the presence of surface states, which promote the electron–hole recombination. In this paper, we propose a new way to remedy these disadvantages of thermal films by special low temperature electrochemical anodic treatment. We have found that by the anodic treatment of thermally grown TiO 2 films, it is possible to control the surface properties, such as the nanocrystalline structure and nanopore formation. These features are desirable for solar energy conversion devices and solar hydrogen production, where the enhancement of electrocatalytic activity by increasing real surface area, reduction of reflectance losses, and removal of surface states created by thermal growth, are of primary importance. The tested electrochemical treatment included program waveforms inducing nanocrystallinity and nanopore formation. The pulse-voltammetric procedures are proposed to control surface non-stoichiometry of TiO 2 films and surface-states density of the photoelectrodes.

Organic p-n Type Heterostructures Based on the Hexatiopentacene

The major task of our work was to search for new organic p-n-type heterostructures photosensitive in a wide spectral region and with a small recombination rate at the interface. We used photosensitive hexatiopentacene layers as p-type components and methyl perylene pigment, C60 and V2O5 xerogel films as n-type ones. Photovoltage spectra are well described by the Van Opdorp model, which considers the presence of surface states at the free surface and the interface. The diffusion lengths of charge carriers in hexatiopentacene (200 nm) and methyl perylene pigment (40 nm) films are determined. We have shown the perspectivity to use the hexatiopentacene/methyl perylene pigment heterostructures for manufacturing plastic solar cells.

The Electrical Characteristics of a Pd/AlN/Si Thin Film Hydrogen Gas Sensor

ABSTRACTWe have extended our previous investigation of the electrical characteristics of a Pd/AlN/Si thin film sensor for varying thicknesses of AlN, from 300–2000Å. The capacitance vs. voltage, C(V), and conductance vs. voltage, G(V), measurements were utilized to investigate the presence of surface states within the Si gap at the AlN/Si interface. Our previous experiments on 500Å AlN did show the presence of interface traps, with an estimated surface density between 8×1014 and 1.5×1015 m−2eV−1 [1]. In our present work we've examined the effect of AlN thickness on the density of these interface traps. The density is dependent on AlN thickness. The thinner devices, 300Å, showed an interface trap density of 20–30×1015 m−2eV−1. The interface trap density decreased with increasing thickness up to 500Å, where the density remained relatively constant at about 1–5×1015 m−2eV−1 for thicknesses up to 2000Å. We have also shown that the interface trap density is independent of annealing.

Conventional Electronic Structure of MgB2 and ZrB2: LDA vs. de Haas-v. Alphen & ARPES Data

We compare full potential LDA band calculations of the Fermi surfaces areas and band masses of MgB2 and ZrB2 previously reported and new dHvA data. Discrepancies in areas in MgB2 can be removed by a small shift of σ bands relative to π bands. Comparison of effective masses lead to orbit averaged el-ph coupling constants λσ=1.3 and λπ=0.5, whereas for ZrB2 only weak el-ph coupling with λ<0.3 is found. The ARPES data can be also well described by the LDA showing the presence of surface states.

Structure and composition of cleaved and heat-treated tenfold surfaces of decagonal Al–Ni–Co quasicrystals

We investigated cleavage surfaces perpendicular to the tenfold direction of as-grown decagonal Al–Ni–Co quasicrystals by scanning tunneling microscopy, Auger electron spectroscopy, and scanning electron microscopy. The cleavage surface is determined by a cluster–subcluster structure. The image contrast of the smallest features, 1–2 nm in diameter, is related to the columnar atom arrangements extending perpendicular to the cleavage plane, which are predicted by current models of the decagonal quasicrystal structure. No voltage dependence of the STM images is observed. The presence of surface states and an enhanced density of states are discussed. Heat-treatments of the cleaved Al–Ni–Co quasicrystal surfaces show nearly no changes in chemical composition and structure up to about 750 °C. This is correlated with a much lower concentration of vacancies in as-grown decagonal Al–Ni–Co quasicrystals as compared to that in as-grown icosahedral Al–Pd–Mn quasicrystals.

Surface states in electrochemisorption

The recursive Green function (RGF) method is used to obtain the surface and adatom RGFs for an electrified substate. The electrochemisorption (ECS) process is described, by invoking the Anderson–Newns (AN) model, whereby a self-consistent treatment of the ECS energy and charge transfer is provided. The role of the electric field, and the presence of surface states (SS), in governing magnetic and non-magnetic ECS is discussed in the case of H–Cr.

Study of GaAs chemical etching in a mixture of hydrogen peroxide/succinic acid and ammonia. Thiourea effect on the surface roughness and on the presence of surface states after etching

The GaAs etching by the hydrogen peroxide–succinic acid mixture in an ammoniacal medium was studied, and the activation energy of the overall reaction was determined. It was shown that diffusion influences the etching sections and the roughness of the etched surface. Optimal conditions for etching (pH, temperature, agitation, thiourea concentration) were established by observing the samples' surfaces using optical microscopy and atomic force microscopy (AFM). The addition of thiourea to the etching bath leads to a very significant reduction in the surface's roughness. The increase in the intensity of the photoluminescence at 4 K of the etched GaAs samples in the bath containing thiourea could indicate a definite reduction in nonradiative emissions, and therefore a passivation of the surface.

Transients in light illuminated photorefractive sandwiches in the presence of surface states and of a d.c. voltage

The response of an insulator–photorefractive film–insulator sandwich is investigated to an input optical beam in the presence of an applied d.c. voltage. First the optical beam is switched on which generates free carriers and causes the appearance of an electric field. Having reached a steady state a voltage is then applied to the sandwich, and the subsequent behaviour of the electron density and of the electric field is analysed as a function of time and space subject to boundary conditions which take into account the dynamics of fast surface states. The mathematical solution obtained describes both the short-time limit when a quasi-steady electron distribution is formed and the long-time limit when the electric field is screened. The role of surface states and that of the attenuation of the optical beam is discussed.

On the theory of electron transfer reactions at semiconductor electrode/liquid interfaces

Electron transfer reaction rate constants at semiconductor/liquid interfaces are calculated using the Fermi Golden Rule and a tight-binding model for the semiconductors. The slab method and a z-transform method are employed in obtaining the electronic structures of semiconductors with surfaces and are compared. The maximum electron transfer rate constants at Si/viologen2+/+ and InP/Me2Fc+/0 interfaces are computed using the tight-binding type calculations for the solid and the extended-Hückel for the coupling to the redox agent at the interface. These results for the bulk states are compared with the experimentally measured values of Lewis and co-workers, and are in reasonable agreement, without adjusting parameters. In the case of InP/liquid interface, the unusual current vs applied potential behavior is additionally interpreted, in part, by the presence of surface states.

Electrochemical Etching of Silicon in Nonaqueous Electrolytes Containing Hydrogen Fluoride or Fluoroborate

The electrochemical behavior and surface chemistry of anodic silicon etching in nonaqueous electrolytes was studied. Etching of single‐crystal p‐type and n‐type (100) silicon was carried out in acetonitrile and propylene carbonate with hydrofluoric acid (HF) or tetrafluoroborate providing fluoride to complex the oxidized silicon. Electrolytes containing HF resulted in tetravalent dissolution, and photocurrent quadrupling was observed. Electrolytes containing also resulted in tetravalent dissolution; however, calculated quantum efficiencies were lower depending upon the electrolyte. Current‐voltage behavior indicates the presence of surface states which affect both the onset potential for oxidation and the current multiplication. In situ multiple internal reflection Fourier transform infrared analysis confirms that silicon surfaces etched in electrolytes containing HF remain hydride‐terminated throughout etching; however, silicon etched in based electrolytes loses the initial hydride termination at the onset of etching. © 1999 The Electrochemical Society. All rights reserved.

Characterization of TiO 2 nanocrystalline thin film by scanning tunneling microscopy and scanning tunneling spectroscopy

TiO 2 nanocrystalline semiconductor films have been studied by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). The presence of interconnected TiO 2 nanocrystallites with mean diameters of 30 nm and different size of pores were indicated in the STM images. Surface structural features were characterized by fractal dimension analysis in terms of surface roughness. Tunneling current vs. bias and differential conductance spectra were measured for as prepared and air-stored nanocrystalline thin films. The results obtained by statistical analysis show that surface bandgap energy of TiO 2 nanocrystallites of as prepared thin film was 3.10±0.08 eV and quite different surface bandgap energy of 1.76±0.19 eV was observed for TiO 2 nanocrystallites in air-stored thin film. Negative shifts of onset potential of differential conductance in both negative and positive bias and the presence of surface states at 1.59±0.16 V for TiO 2 nanocrystallites of air-stored thin films revealed a difference in surface electronic properties for both TiO 2 nanocrystalline thin films.

Photocorrosion of Coupled CdS/CdSe Photoelectrodes Coated with ZnO: Atomic Force Microscopy and X‐Ray Diffraction Studies

The stability of photoelectrochemical cells based on chemically deposited CdS/CdSe coupled films has been examined. Changes in surface structure and composition of coated and uncoated coupled films as well as CdSe films have been examined by atomic force microscopy and X‐ray diffraction. The superior stability at short times of the coupled system, compared to CdSe, is related to the increase in the hexagonal character (stronger bonding) and the smaller recombination rate of the photogenerated carriers. At large operation times, the lower stability of the coupled system is related to band opening, which increases the oxidation rates of the passivating Se/S layer. The recrystallization illuminated CdSe photoanodes, and coupled films working in the dark can be explained by the presence of surface states and back reactions. Stable short‐circuit currents were obtained with coupled films coated with a thin layer (350 Å) of ZnO. It is likely that oxidation and redeposition of the protective ZnO film competes for hole consumption. The rough morphology of the coated photoelectrodes correlates to a substantial increase in surface area that resembles ZnO particulate film electrodes sensitized by CdSe and CdS.

Scanning Electron Microscopy of Dopants in Semiconductors

Abstract It has been demonstrated that doped regions in semiconductors yield contrast in secondary electron images taken under appropriate imaging conditions. Perovic et al. attributed this "electronic" contrast to surface potentials arising from the bending of the energy bands at the surface of the semiconductor. This band-bending results from the presence of surface states which pin the Fermi level. These effects have found application to an important problem in the electronics industry -- the profiling of dopant distributions in two-dimensions. However, such dopant distributions are rarely uniform, i.e., the dopant concentration may change over many orders of magnitude within distances of only a few tens to hundreds of nanometers. Therefore, it is necessary to understand how the observed contrast from the doped regions varies for non-uniform dopant distributions to apply the technique to real electronic devices.

On the temperature and field dependences of the effective surface mobility in MIS structures

A physical model establishing a relation between the surface density of the free electronic charge in an inversion layer and the surface density of stationary (localized) electrons trapped in surface states at a semiconductor-insulator interface is constructed. It is established that at moderately low temperatures this relation is close to a direct proportionality. The presence of surface states, which localize some of the surface electronic charge, is manifested as a decrease in the effective electron mobility in the channel of a MIS transistor. The well-known decrease of the surface mobility with increasing transverse electric field is attributed to field-induced variations in the position of the percolation level that separates bound electronic states from free states.

Chemical Solution Deposited Copper-doped CdSe Quantum Dot Films

AbstractCopper doped CdSe quantum dots (QDs) and larger nanocrystals were deposited by chemical solution deposition at two different temperatures (5°C and 80°C) and their morphological, optical and surface properties compared with similarly deposited undoped CdSe. Crystal sizes and structure were investigated by X-ray diffraction and transmission electron microscopy. All the samples exhibited cubic (sphalerite) structure except for the 80°C Cu-doped ones which were substantially hexagonal (wurtzite). For the 5°C samples, the crystal sizes were similar for the doped and undoped samples (ca. 3.4 nm). For the 80°C samples, the crystal size of the doped CdSe was much larger (84 nm) than the undoped one (6.3 nm) and extensive twinning occurred. Optical measurements on the doped samples showed the presence of surface states due to the copper seen as an increase in absorbance in the long wavelength region. Xray photoemission spectroscopy similarly showed tails in the valence band spectra of the doped samples. These tails were dominant in the 5°C samples and less so for the 80°C ones. XPS also showed that the Cu was present predominantly in the univalent state. The copper was found to be located on the QD surface for the 2–3 nm QDs and distributed both on the surface and in the bulk (or at twin boundaries) for the larger nanoparticles.

Dye-Sensitizing Effect of TiOPc Thin Film on n-TiO2 (001) Surface

Dye-sensitization processes for the (001) surface of n-type titanium oxide (n-TiO2) single crystal coated with vacuum-deposited titanylphthalocyanine (TiOPc) were investigated by means of photoelectrochemical measurements and X-ray and ultraviolet photoelectron spectroscopy (XPS, UPS). The electrode of n-TiO2 coated with TiOPc exhibited two photooxidation current bands in an aqueous electrolyte including hydroquinone (H2Q), due to direct excitation of n-TiO2 below 420 nm and electron transfer from TiOPc to n-TiO2 under the Q-band absorption of TiOPc at 600−800 nm, respectively. The former photocurrent process due to n-TiO2 excitation was remarkably decreased by presence of surface states, which originated from defects of oxygen induced by Ar ion bombardment of the n-TiO2 surface. The photooxidation process sensitized by the latter was evaluated from the determined energy levels of electronic bands for n-TiO2 and TiOPc. The LUMO level of TiOPc was located 0.3 eV above that of n-TiO2, which enabled electron...

Photoconductivity spectroscopy of hydrogenated amorphous silicon

Abstract We have developed a numerical model that can give a realistic description of the photoconductivity of amorphous semiconductor films even in the case of subgap excitation and in the presence of surface states. We have compared the results of this model with the spectral photoconductivity, the mobility-lifetime product and the absorption coefficient measured on hydrogenated amorphous silicon (a-Si: H) samples of different thicknesses. We find that the widely accepted description, in which the deep defects are divided in two classes (acceptor like and donor like) and the capture cross-sections of charged defects are much larger than those of the neutral defects, is unable to reproduce a number of experimental results. For instance the well known fact that, in a-Si: H thick films, the absorption coefficients derived from spectral photoconductivity and from photothermal deflection spectroscopy are in fair agreement can be reproduced only if the capture cross-sections have not too different values. Our...

Improved analysis of spreading resistance measurements

In this article, a system is described for the analysis of spreading resistance data which includes carrier spilling through the solution of Poisson’s equation. The Poisson equation solution requires boundary conditions for the charge density on the beveled surface, which was modeled by considering the presence of surface states with levels within the band gap. Considering surface states allows the deviation from ideality of both n- and p-type calibration curves to be accounted for consistently without empirical contact resistance or variable radius corrections. In addition, the effects of finite probe penetration and change in energy gap due to probe pressure were considered to account for the reduction in peak resistance near junctions observed experimentally as compared to theoretical calculations. The software (SERAPES) can be used both to calculate simulated resistance profiles from doping profiles for process model characterization, as well as to calculate doping profiles directly from spreading resistance measurements.