Electron Concentration In Conduction Band Research Articles

Theory of Sensitivity of Nanoscale-Structured Layers of Metal Oxides to Reducing Gases

A theory of sensor response on reducing gases of nanoscale-structured semiconducting oxides with the high concentration of conduction-band electrons has been developed (modeled on In2O3). The distribution of charges in nanoparticles is determined by the functional relationship of the density of negative and positive charges in nanoparticles and electrons on their surface. The capture of conduction electrons by adsorbed oxygen atoms causes electron redistribution in nanoparticles, such that the near-surface density of electrons and the conductivity of the system decrease. In this case, the conditions of the association and dissociation of oxygen molecules on the surface also change. During the adsorption of reducing gases (H2, CO), atomic oxygen ions react with them and electrons are released that enter bulk nanoparticles. The conductivity of the system increases, which corresponds to the sensor effect. A kinetic scheme of chemical reactions, which corresponds to the that described above, has been plotted and corresponding equations were solved. As a result, theoretical dependences of the sensitivity of sensor on the temperature and pressure of hydrogen were found, which agree well with experimental curves at qualitative and quantitative level.

Influence of Nanostructure Geometry on Electronic Properties

Recently, new quantum features have been studied in the area of nanostructured layers. It emerges that properties of nanostructures depend not only on their size but also on their geometry. Particularly, a nanograting (NG) on the surface of the thin layer imposes additional boundary conditions on electron wave function and forbids some quantum states. Density of quantum states reduces. Unlike conventional quantum well, state density per volume, is reduced in the case of NG layer. This leads to changes in electronic properties. Electrons, rejected from forbidden quantum states, have to occupy states with higher energy. In the case of semiconductor layers, electrons rejected from the valence band have to occupy empty quantum states in the conduction band. Such increase in conduction band electron concentration can be termed as geometry-induced doping or G-doping. G-doping is equivalent to donor doping from the point of view of the increase in electron concentration. However, there are no ionized impurities. This preserves charge carrier scattering to the intrinsic semiconductor level and increases carrier mobility with respect to the donor-doped layer. As rejected electrons occupy quantum states with the higher energy, the chemical potential of NG layer increases and becomes NG size dependent. We regard a system composed of NG layer and an additional layer on the top of the NG forming periodic series of p-n junctions. In such system, charge depletion region develops inside the top of NG and its effective height reduces, becoming a rather strong function of temperature T. Consequently, T-dependence of chemical potential magnifies and Seebeck coefficient S increases. Calculations show one order of magnitude increase in the thermoelectric figure of merit ZT relative to bulk material. In the case of metal layers, electrons rejected from forbidden quantum states below Fermi energy, occupy quantum states above Fermi energy. Fermi energy moves up on energy scale and work function (WF) reduces. WF reduction was observed in thin amorphous Au films grown on thermally oxidized Si substrates. WF was measured using Kelvin Probe and PEEM microscope. WF reduction depended on film internal structure and NG sizes. Maximum reduction in WF of 0.56 eV has been obtained.

Electronic and thermoelectric properties of nanograting layers

Recently, new quantum features have been observed and studied in the area of nanostructured layers. Nanograting (NG) on the surface of the thin layer imposes additional boundary conditions on electron wave function and forbids some quantum states. Electrons, rejected from the forbidden quantum states, have to occupy states with higher energy. In the case of semiconductor materials, electrons rejected from the valence band have to occupy empty quantum states in the conduction band. Such increase in conduction band electron concentration n can be termed as geometry-induced doping or G-doping. G-doping is equivalent to donor doping from the point of view of the increase in n. However, there are no ionized impurities. This preserves charge carrier scattering to the intrinsic semiconductor level and increases carrier mobility. G-doping involves electron confinement to NG layer. Here, we investigate the system of multiple NG layers forming a series of homo junctions. Si and GaAs homojunctions were studied and G-doping levels of 1018-1019 cm−3 were obtained. We also study a system composed of NG layer and an additional top layer forming periodic series of p-n junctions. In such system, charge depletion region develops inside the NG and its effective depth reduces, becoming a rather strong function of temperature T. Consequently, T-dependence of chemical potential magnifies and Seebeck coefficient S increases. We investigate S in the system of semiconductor NG layer having abrupt p-n junctions on the top of the grating. Analysis made on the basis of Boltzmann transport equations shows dramatic increase in S. At the same time, other transport coefficients remain unaffected by the junctions. Calculations show one order of magnitude increase in the thermoelectric figure of merit ZT relative to bulk material.

High-temperature calcination and hydrogen reduction of rutile TiO2: A method to improve the photocatalytic activity for water oxidation

Rutile titania (TiO2) is an efficient photocatalyst for oxidizing water to O2. The photocatalytic activity of particulate rutile for water oxidation was significantly improved by H2 reduction at 700°C, after calcination at 1100°C. The improved activity was due to an increase in crystalline size during calcination, and an increase in conduction band electron concentration by the creation of oxygen vacancies. In contrast to the consideration that oxygen vacancy increases the recombination of electron and holes, the hydrogenated TiO2 exhibited high apparent quantum efficiency for O2 evolution, 41% under irradiation at 365nm. It was found that H2 treatment improved the photocatalytic activity per unit of surface area not only for O2 evolution but also for H2 evolution and acetic acid decomposition. The effect of H2 reduction treatment was obtained only if the rutile particle was previously calcined at temperatures higher than 1000°C. This suggests that space charge layer in large crystalline particles is involved in the activation mechanism of hydrogenated rutile TiO2 particles.

Comparison of Trap‐state Distribution and Carrier Transport in Nanotubular and Nanoparticulate TiO2 Electrodes for Dye‐Sensitized Solar Cells

Dye-sensitized solar cells (DSCs) with nanotubular TiO(2) electrodes of varying thicknesses are compared to DSCs based on conventional nanoparticulate electrodes. Despite the higher degree of order in one-dimensional nanotubular electrodes, electron transport times and diffusion coefficients, determined under short-circuit conditions, are comparable to those of nanoparticulate electrodes. The quasi-Fermi level, however, is much lower in the nanotubes, suggesting a lower concentration of conduction band electrons. This provides evidence for a much higher diffusion coefficient for conduction band electrons in nanotubes than in nanoparticulate films. The electron lifetime and the diffusion length are significantly longer in nanotubular TiO(2) electrodes than in nanoparticulate films. Nanotubular electrodes have a trap distribution that differs significantly from nanoparticulate electrodes; they possess relatively deeper traps and have a characteristic energy of the exponential distribution that is more than two times that of nanoparticulate electrodes.

Electron–phonon interaction in zinc oxide. Plasmon–optical phonon coupled modes

AbstractThe effects of the electron–phonon interaction in zinc oxide depending on the free conduction band electron concentration are investigated. In n‐doped polar semiconductors, as the free carrier concentration is increased, the plasmonic mode associated to charge density fluctuations gradually approaches its frequency to the optical phonon ones. As a consequence, their interaction is materialized in coupled mode oscillations. Using a dielectric continuum model, applying the equation of motion technique for the operator associated to charge density fluctuations in ZnO, and taking into account the supplementary constraints introduced by its uniaxial symmetry, the response function of the system is obtained. In polar semiconductors with cubic crystalline structure there are two plasmon‐optical phonon coupled modes. It is shown that in ZnO, three such coupled modes appear. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The influence of the electron structure into the stacking fault energy of noble metals

The study of the electron structure of Ag, Au and Cu transition metals by EELS shows that the greatest electron concentration in conduction band observes in Ag and decreases through Au to Cu. At the same time the fraction of free-electron in conduction band of Ag is less than 0.2 while this value is about of 0.5 for Au and more than 0.6 for Cu. Thus the electron system of these elements consists from high-energy free-electron and low-energy nearly free-electron subsystems. The SFE of Ag, Au and Cu increases with increasing free-electron fraction in electron system of these metals. The explanation of this phenomenon was discussed in the frame of electron theory of metals.

Depletion of conduction band electrons in TiO 2 by water chemisorption – IR spectroscopic studies of the independence of Ti–OH frequencies on electron concentration

The chemisorption of water at 300 K onto reduced TiO 2 produces both adsorbed Ti–OH groups and chemisorbed water. During the adsorption of H 2O, the concentration of conduction band electrons is found to decrease as measured by transmission infrared spectroscopy. The TiO 2 electron depletion process occurs predominantly during the first stages of water adsorption, probably due to Ti–OH species formation. It is also found that the infrared spectrum of Ti–OH groups in the ν(OH) region is not influenced by the electronic state of the TiO 2 surface, comparing TiO 2 containing different concentrations of conduction band electrons.

Application of the final state Auger parameter imperfect screening model for the Si L 23VV spectrum of porous silicon

In this paper we have applied the final state Auger parameter imperfect screening model for the Si L 23VV spectrum to study the local electronic structure close to the surface layer of porous silicon prepared by anodic etching. This application has allowed to reveal the lower local high-frequency dielectric constant, conduction band electron concentration, higher band gap for porous silicon in comparison with these values for the intrinsic single crystal silicon and to evaluate the size of the silicon particles. Comparison of calculated band gap values with known one at the same silicon particle sizes have allowed us to propose the higher effective density of states in conduction band and/or in valence band for porous silicon in comparison with these values for the single crystal silicon.

An approximate solution of conduction electron concentration in an insulator exposed to radiation

Abstract This paper is concerned with the conduction electron concentration in an insulator exposed to high energy radiation. We find that the steady state conductivity [sgrave] (∞) of insulators is proportional to the dose rate of irraliation if the radiation source is weak and the temperature is low; the [sgrave] (∞) could be proportional to square root of the dose rate if temperature is very high. Before the steady state is reached, The approximate solution of electron concentration in conduction band of an insulator is calculated to change exponentially with time.

Electrical characterization of arsenic-ion-implanted semi-insulating GaAs by current-voltage measurement

The characteristics of an amorphous GaAs film by implanting dense arsenic ions into semi-insulating GaAs substrate have been studied. The specific contact resistance of the sample at room temperature is found to be 1.1×10−1 Ω cm2. This indicates that the conduction-band electron concentration is less than 1013 cm−3. The activation energy at temperature above 360 K and the density of states are, respectively, 0.6 eV and 1027 m−3 eV−1 as estimated from the slope of Arrhenius plot. Fitting of the Arrhenius plot also suggests that the carrier transport at metal/semiconductor junction of this material below 360 K is dominated by the variable-range hopping conduction mechanism which depends on deep level defects.

Nonalloyed ohmic contacts on low-temperature molecular beam epitaxial GaAs: Influence of deep donor band

The Ohmic nature of the nonalloyed metal contact on molecular beam epitaxial GaAs grown at 200 °C was studied. The specific contact resistances at room temperature and 120 K were 1.5×10−3 and 7.0×10−1 Ω cm2, respectively. These values are anomalously low considering that the conduction-band electron concentration in this material is less than 1011 cm−3 at room temperature. The experimental results indicate that the carrier transport at the metal/semiconductor interface is dominated by a dense (∼3×1019 cm−3) EL2-like deep donor band, rather than the usual conduction band.<squeeze;1.6p>

Bleaching of F-type centres in alkali halides—II: Laser-induced F centre bleaching in KC1

Bleaching of the F band in KCl occurs on pulsing with 532 nm laser light. The bleaching is accompanied by the appearance of an absorption on the red side of the F band. Both bleaching and absorption are transient phenomena, and no permanent bleaching or absorption is observed. The same exponential lifetime of about 5μ S was obtained for both bleaching and absorption, and all the features were found to be independent of both temperature (between 200 and 300 K) and the intensity of the analysing light. This eliminutes the possibility of a trapped species, such as F -, being responsible. The phenomena are qualitatively different to the bleaching observed at low light intensity, where bleaching of the F band is accompanied by formation of F -, which is thermally stable below 250K. The most likely explanation of the results is that the high concentration of conduction band electrons following the laser pulse allows formation of free or quasi-free electron pairs, e = 2, which decay back into F centres on a time scale which is long compared with that for individual electrons.

Relationship between stabilization and photocurrent at n-type semiconductor electrodes on the basis of a model including surface recombination through decomposition intermediates

Following the recent publication of preliminary experimental results, a general kinetic discussion is presented for competing anodic decomposition, stabilization and surface recombination processes at illuminated n-type semiconductor electrodes, assuming that surface recombination occurs through reaction intermediates of the decomposition process and that the stabilization reaction either also occurs through such intermediates or involves free holes. The relationship between the stabilization ratio and the total photocurrent is investigated theoretically under varying light intensity, surface conduction-band electron concentration or stabilizing agent concentration, with special attention to the limiting cases. The results are discussed in terms of their mechanistic background and from the viewpoint of their implications to practical photoelectrochemical cells.

Low temperature recombination into shallow donors in germanium: Transient case

Recombination of electrons into shallow donors is traditionally described in terms of a single rate equation in the conduction band electron concentration n; this ignores the concentrations ni of electrons in excited states (i) of the donors. Formally it should be described in terms of a set of rate equations in n and the ni. It is proved here that if the sample purity is sufficiently high the two approaches give the same result. For this, suitable interpretation must be given to the ionization and recombination rates appearing in the simple rate equation; the latter represents the well-known cascade process; the former turns out to be dominated by an “inverse cascade”. The required sample purity is calculated as a function of temperature, applied electric field, and n. Calculations based on the linearized form of the rate equations, exact for small excursions, are used to test the theory.

Hall effect studies of carbon monoxide oxidation over doped zinc oxide catalysts

The role of the conduction band electrons in the chemisorption of oxygen and the oxidation of carbon monoxide on indium-doped polycrystalline zinc oxide was studied by a dual a.c. Hall effect method that employed a 52 c/sec a.c. sample current and a 60 c/sec a.c. magnetic field to give an 8 c/sec Hall voltage. This method seems to be particularly well suited for measuring the concentration of current carries in semiconductor catalysis studies.The results of this study support the conclusions that the chemisorption of oxygen on indium-doped zinc oxide in the temperature range 200–350°C, is associated with an electronic charge transfer from the zinc oxide to the surface oxygen species, and that O– plays a prominent role in this adsorption. The chemisorption of carbon monoxide in the temperature range 300–350°C, on indium-doped zinc oxide containing pre-chemisorbed oxygen, gives an increase in carrier concentration corresponding to one electron per carbon monoxide molecule chemisorbed. The measurements of the changes in carrier concentration during carbon monoxide oxidation at 350°C by this doped zinc oxide indicate that the conduction band electrons are intimately involved in the reaction, and that O– is an important transient surface species therein. A comparison of the results obtained from reaction studies at 460°C over lithium-doped and indium-doped zinc oxide supplies additional evidence that a strong correlation exists between the concentration of conduction band electrons in these materials and their level of catalytic activity for carbon monoxide oxidation.

Interdiffusion in Binary Ionic Semiconductors

Interdiffusion in a nondegenerate, exhaustion range, binary ionic semiconductor is investigated using the recently improved theory for the concentrations of defects in crystals and Wagner's phenomenological flow equation. The composition dependence of the interdiffusion constant is found to be determined by the ratio of the ion jump frequencies, the Schottky constant, and the intrinsic concentration of conduction band electrons. The removal of the restriction to an exhaustion range semiconductor and the replacement of the basic assumption of local electroneutrality are discussed.

Capacitance and Conductance Effects in Photoconducting Alkali Halide Crystals

Measurements of the ac and dc response of illuminated F-centered potassium bromide single crystals have been carried out. The dependence of crystal capacitance and conductance on frequency, light intensity, electrode character and separation, F-center concentration, applied voltage, temperature, and optical bleaching time have been investigated. Most of the measurements were made on crystals containing both U and F centers, but a few were also made on additively colored and plain untreated crystals for comparison. It was found that F centers in U-centered crystals bleached much slower under strong illumination than did those in additively F-centered crystals. The frequency response of the crystals depended significantly upon whether the electrodes were blocking or rectifying, but in both cases it was characterized by a single relaxation time given by the quotient of a series capacitance C0 and G∞, the normal conductance of the crystal. C0 was found to be independent of all variable parameters except dark storage and bleaching time and is thought to arise from a charge-free layer located at one or both electrodes. Space-charge capacitance effects were apparently largely hidden by C0 but did probably produce a small voltage-dependent non-linearity observed at low frequencies with high applied voltages. G∞, and thus the conduction-band electron concentration, depended linearly on light intensity over a wide range but underwent a transition from linear dependence on F-center concentration to independence thereof at high concentrations. Partly bleached crystals in total darkness exhibited appreciable conductivity, probably electronic in character. Most of the results on U-centered crystals can be tentatively explained in terms of electron trapping by negative-ion vacancies, F centers, interstitial hydrogen atoms, and hydrogen atoms in negative-ion vacancies.