Presence Of Surface States Research Articles

Thickness dependence of temperature coefficient of resistivity of polycrystalline bismuth films

Results for the temperature coefficient of resistivity (TCR) of polycrystalline bismuth films deposited on to glass substrate are reported for the thickness range 30–300 nm. The film TCR is found to be negative for all thicknesses studied and its absolute value exhibits a maximum of 3.70×10−3 K−1 near 72.5 nm. The variation of charge carrier density with film thickness has been estimated from the presence of surface states. To include the thickness dependence of charge carrier density, a modified theory has been used to explain the observed behaviour of the TCR. The experimental results for the TCR of Bi films are found to be consistent with the theoretical values. The existence of the extremum is theoretically verified. From the analysis, the specularity parameter p is about 0.44 and the reflection coefficient R is 0.1.

Electrical resistivity of thin bismuth films

The thickness dependence of electrical resistivity of thin bismuth films deposited on to glass substrates has been studied in the temperature range 77 to 350 K. The structural studies show that films are polycrystalline with grain size increasing with thickness. The electrical resistivity decreases with increasing temperature. This type of temperature dependence of electrical resistivity arises due to the competition between the temperature dependences of carrier density and carrier mobility, the temperature coefficients of which are opposite in sign. The electrical properties of bismuth films are greatly affected by the presence of surface states. The size effect theory of Pichard and colleagues (PTT) has been modified to investigate the variation of charge carrier concentration with thickness. The experimental results on electrical resistivity are found to be consistent with the predictions of this modified three-dimensional model (PTT). The value of specularity parameter,p, is about 0.43 and it exhibits no temperature dependence. The experimental results on grain-boundary resistivity agree fairly well with the theoretical variations. The values of transmission coefficient,t, are determined from the experimental data.

Photothermal detection of surface states in amorphous silicon films

A new method for the determination of surface states in amorphous silicon films is proposed. The method is based on the observation of enhancement of the interference fringe amplitude in photothermal deflection spectra, due to the presence of surface states.

Numerical studies of surface-ion neutralization

A molecular-orbital approach is used to obtain exact numerical solutions to the equations of motion for the time-dependent Anderson-Newns Hamiltonian describing the surface-ion-neutralization process in one, two and three dimensions. The effect of dimensionality, and the presence of surface states, on the ion occupancy are discussed in detail.

Scanning tunnelling microscopy as a spectroscopic tool for the density of states: theory and application to semiconductors

Quantum mechanical calculations of the electronic transport using a two-step model confirm that the differential conductance (dI/dV) measured with the scanning tunnelling microscope represents the local density of unoccupied states, whether they are intrinsic states of the material or field-induced states. The formalism is applied to Si and GaAs surface where the presence of surface states as well as the charge depletion and accumulation layers makes an important difference in determining the position of surface states from experimental data.

Photoelectronic properties of the p-type layered trichalcogenophosphates FePS 3 and FePSe 3

The p-type as grown single crystals of the layered compounds FePS 3 and FePSe 3 have been studied by the photoelectrochemical method. The current-voltage and capacitance-voltage characteristics indicate the presence of surface states. The analysis of the photocurrent quantum efficiencies give two indirect optical transitions at 1.62 and 2.3 eV for FePS 3 and at 1.3 and 1.83 eV for FePSe 3. Hole mobilities are in the range 0.5–1.0 cm 2 V −1 sec −1 as found from the Hall effect measurements. These results are interpreted in terms of the existing electronic band scheme for the MP X 3 ( M = transition metal; X = S, Se) layer compounds.

STUDIES ON UPS AND XPS OF 0_2_ AND H_2_0 ABSORPTION ON SURFACES OF SrTi0_3_

The presence of surface state originated from Ti~(3+) ion on reductive SrTiO_3 surface has been observed. The action of Ti~(3+) on photodecomposition of water and the action of light irradiation on regeneration of activity are analysed.

ChemInform Abstract: Electrochemical Photovoltaic Cells with Layered Molybdenum Diselenide Photoanodes.

AbstractAstudy of the illuminated n‐MoSe2/ l M H2SO4 liquid junction has shown that, even in the presence of surface states, the photocurrent‐potential response appears ideal at very low light intensity.

Effect of surface states on the barrier height in a MIS diode in the presence of inversion

ABSTRACT The non-linearity in the barrier height versus metal work-function plot is studied by considering the presence of surface states. It is shown thai the non-linearity diminishes as the surface state density Ds⩽ increases. For D⩽ 1012 states/ cm2 the observed bending in the versus (Φmc plot completely disappears. An expression for the critical metal work-function (at which such non-linearity appears is also derived. The variation of Φ mcwith surface state density is studied. It is seen that decreases as the surface-state density increases.

Size effect in the optical properties of small metallic particles: A solvable model for cubic symmetry

A solvable tight-binding model for the electronic states of cubic crystals of arbitrary size is used to compute the real and the imaginary part of the dielectric function of metallic particles. Two types of microscopic processes are discussed: first-order transitions between electronic states and second-order transitions due to collisions with vibrational states and impurities. Interband and intraband transitions are both taken into account fors andp bands. The results display very strong size effects due to the lack of translational symmetry and to the presence of surface states; the Lorentz-Drude behaviour of the bulk metal is approached as the number of atoms in the particles increases. The size effects in the optical functions of the metallic particles are shown to depend on the environment and this is made to account for the basic features in the optical properties, in particular for the resonance plasma absorption. The case of silver particles in a glassy environment is investigated, experimental trends of the optical absorption with varying particle sizes are explained and reasonable agreement with experimental data is obtained.

Influence of interface states on field effect and capacitance-voltage characteristics of metal/oxide/a-Si:H structures

Separate determination of the electronic density of states in bulk a-Si:H and at the oxide/a-Si:H interface of a MOS structure have been made by combining the field effect technique with the capacitance-voltage method. It is found that electronic states existing at the oxide/a-Si:H interface cause no serious errors in the determination of the bulk gap-state density. The discrepancy in the measured gap-state density between the present method and the DLTS or ICTS techniques does not arise from the presence of surface states, but from differences in the electronic properties of a-Si:H.

Feed forward due to barrier modulation in charge-coupled devices

A new type of signal degradation, namely feed forward, is shown to exist in charge-coupled devices. This arises because of barrier modulation due to signal charge and the presence of surface states. Feed forward results in a pulse coming out earlier than the main signal pulse. The magnitude of the feed forward pulse was found to be as high as 0.5 percent of the signal charge for a 24½ bit, CCD delay line. Computer calculations have been performed to quantify the effect for the structure. Results obtained with a special clock for different clocking parameters have been explained, based on the existing model of emission of charge from the surface states.

Optical determination of Fermi-level pinning using electroreflectance

We have investigated the low-field electrolyte electroreflectance (EER) spectra of matte, polycrystalline, electrodeposited $n$-CdSe in the vicinity of the ${E}_{0}(A,B)$ transitions (direct gap at $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}=0$. As the applied bias voltage ${V}_{\mathrm{dc}}$ is varied from near flatband to depletion, the EER amplitude for electrodeposited CdSe decreases by a factor of \ensuremath{\sim}40. We interpret this decrease in EER signal as evidence of Fermi-level pinning, which we attribute to the presence of surface states.

Characterized of glow-discharge deposited a-Si:H

The study of hydrogenated amorphous silicon, a-Si:H has become an active and large subfield of growing interest in noncrystalline semiconductors. This paper reviews recent successes as well as difficulties encountered in interpreting some fundamental physical properties of a-Si:H prepared by glow-discharge deposition. This material appears to contain structural and compositional heterogeneities which depend on the preparation conditions and affect its mechanical and electronic properties. These are related to the concentration and bonding configurations of hydrogen. Determinations of the density and distribution of localized gap states by means of field effect and capacitance measurements are discussed as well as evidence for and against the presence of surface states. Optical absorption near the absorption edge is described as are conductivity measurements. The latter reveal many interesting puzzles. Staebler and Wronski discovered that the dark conductivity and the photoconductivity can be changed significantly by strong exposure to light. Moreover, space charge layers adjacent to the substrate interface and near the free surface can strongly influence the electronic properties of a-Si:H films, especially those having low densities of gap states. The pre-exponential factor of the conductivity increases with its activation energy in accordance with the Meyer-Neldel rule regardless of whether the changes in dark conductivity are produced by doping, light exposure or space charge layers. Attempts are made to identify causes for the scatter of data on some film properties reported by various laboratories. Our present understanding of the recombination processes involved in photoluminescence and photoconductivity is presented. We conclude that a-Si:H films of superior quality can be expected when the preparation conditions become more fully understood.

Rhodamine B surface state hole traps

In this paper, the surface photovoltage of Rhodamine B is measured and 5800 Å laser pulse (10 28 photons cm −2 s −1) incident on the film produced a 2.02 microV photovoltage signal which decayed exponentially with a 9.01 msec relaxation time. The Rhodamine B surface had an accumulation space charge layer resulting from the presence of surface states which trap holes. Similar hole traps in the bulk are thought to account for the n-type conductivity of this organic semiconductor.

Rhodamine B surface state hole traps

In this paper, the surface photovoltage of Rhodamine B is measured and 5800 Å laser pulse (10 28 photons cm −2 s −1) incident on the film produced a 2.02 μV photovoltage signal which decayed exponentially with a 9.01 msec relaxation time. The Rhodamine B surface had an accumulation space charge layer resulting from the presence of surface states which trap holes. Similar hole traps in the bulk are thought to account for the n-type conductivity of this organic semiconductor.

The Potential Distribution at the TiO2 Aqueous Electrolyte Interface

The flatband potentials of in aqueous electrolytes at different pH values were evaluated. The flatband potentials were measured by extrapolating the intercepts of Mott‐Schottky plots to the zero doping level. The capacitance values for the Mott‐Schottky plots were measured by relaxation spectrum analysis of the passive elements in the equivalent circuit of the electrolyte interface. The Mott‐Schottky plots used for evaluation of the flatband potential were linear over a potential range that exceeds 70% of the bandgap. It has been shown that when the doping distribution is homogeneous, the presence of surface states can be responsible for nonlinear Mott‐Schottky plots. The number, energy, and energy distribution of those states has been determined. The flatband potential of has been found to vary with pH at the rate of 60 mV/pH unit and is 100 mV more negative than the potential of reversible hydrogen electrode. The capacitance of the Helmholtz layer as a function of pH has also been determined.

Determination of surface states in selenium diodes by conductance and capacitance studies

Measurements of the conductance and capacitance of selenium diodes as a function of bias and frequency at room temperature are shown to indicate the presence of surface states in those diodes having Ni and Cd electrodes. These surface states are found to have a uniform, quasicontinuous distribution of energy in the region of measurement. The surface state density at zero bias in a sample having a Ni electrode is found to be 7 × 1011 states/eV cm−2. A sample having a Cd electrode has a surface state density of 2 × 1011 states/eV cm2 in the range of zero to 0.125 V forward bias. The selenium diodes having Ni and Cd electrodes have current—voltage characteristics typical of p—n junctions dominated by recombination currents. Measurements of samples having Al and In electrodes show small frequency dispersion of capacitance and conductance, and also have current—voltage characteristics that differ markedly from the usual p—n junction behavior.

Photoconduction in poly(ethylene terephthalate). I. Mechanisms of carrier generation

AbstractMechanisms of photocarrier generation in poly(ethylene terephthalate) (PET) have been investigated. In the wavelength range of λ ≦320 nm, the photocurrent spectra correspond closely with the optical absorption spectra of PET and the assignment of the absorption peaks revealed that photocarriers are generated through ππ* excitations. In the wavelength range from 320 to 400 nm, photocarriers are injected from metal electrodes. The threshold energies for Al and Cu electrodes are 2.87 and 2.94 eV, respectively, indicating the presence of surface states. The simplified model gives the density of the surface states as 1.7 × 1014 cm−2 eV−1.

A simple boundary condition at semiconductor-insulating substrate interface of a TFT

Using the Wexler, Green and Miles (WGM) procedure, a criterion is given for neglecting the electric field in insulating substrate of a Thin Film Transistor in presence of surface states. It is shown that this is fulfilled for a real TFT.