Conduction Band Of Semiconductor Research Articles

Field emission from nanocomposites

We study field emission from wide band gap metal–semiconductor nanocomposites. The grains of the wide band gap semiconductor are embedded into the layers of metal. Interfacial charge transfer gives rise to the metal-induced gap states (MIGS) in the vicinity of the grains. If the density of the semiconductor grains is sufficiently large, MIGS hybridize with conduction band states of the semiconductor forming a quasi-band, which can be populated by the electrons from the metallic matrix through the scattering in the metal–semiconductor composite. The location of the high-lying MIGS in the vicinity of the conduction band of the wide band gap semiconductor (e.g., diamond) significantly reduces the barrier between those states and the vacuum level compared to the work function of a metal. Thus low-threshold field emission from a nanocomposite becomes possible. This hypothesis is supported by Monte Carlo simulation of transport through a nanocomposite.

Analysis of reverse current–voltage characteristics of Schottky diodes based on phonon-assisted tunneling including Frenkel emission mechanism

Reverse current–voltage characteristics of Al–GaP Schottky diodes are studied in the temperature range from 213 to 373 K. The results are explained on the basis of phonon-assisted electron tunneling from interface states to the semiconductor conduction band. At low bias and higher temperatures the Frenkel emission mechanism is taken into account. Electric field strength and the density of generating centres in the junction is estimated from the fit of experimental results with theory. The temperature and field dependence of the leakage current in thin film transistors extracted from Ref. [Solid-State Electron 38 (12) (1995) 2075] is also explained on the basis of our model.

Electronic excitations in a nonparabolic conduction band of ann-type narrow-gap semiconductor

Taking full account of nonparabolicity of a conduction-band dispersion, we investigate those electronic excitations in the conduction band of an n-type narrow-gap semiconductor which are coupled with polar phonons. By incorporating the nonparabolic dispersion obtained by a $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ method in a complete manner into the random-phase approximation, we calculate the energy dispersion and the energy-loss intensity of two coupled plasmon-phonon modes and a longitudinal optical-phonon mode partially screened by carriers. The results are compared with those of two simplified treatments, namely, one with the spin-orbit splitting neglected and the other assuming a parabolic dispersion of the conduction band with its effective mass modified. This comparison asserts that complete treatment of the nonparabolic dispersion is indispensible to quantitative analysis of the plasmonlike mode at higher carrier concentrations. Simultaneously, it elucidates limitations and shortcomings of the simplified schemes. The complete treatment of the nonparabolicity leads to an excellent agreement with the experiment on the carrier-concentration dependence of an intensity-vanishing point in the infrared-reflection spectrum.

Sensing Alkali and Alkaline Earth Metal Cations by Conduction Band Quenching of Dye Photoluminescence

A new luminescence approach for sensing alkali and alkaline earth metal cations in fluid solution at room temperature is described. The approach is to utilize surface-adsorption/desorption–induced energetic shifts of a semiconductor conduction band to alter the electron transfer quenching efficiency of a photoluminescent dye. A proof-of-concept example is described based on TiO2 nanoparticles with a surface bound Ru(II) coordination compound as the dye, Ru(deeb)(bpy)2(PF6)2 where deeb is 4,4′-(CO2CH2CH3)2-2,2′-bipyridine and bpy is 2,2′-bipyridine, sensing alkali and alkaline earth metal cations in acetonitrile solution. The Ru(II) compound is highly luminescent with long excited state lifetimes in the absence of the cations but is quenched in their presence. The quenching is found to be reversible. The data demonstrate that this approach yields intensity, lifetime, and wavelength-ratiometric calcium ion sensors that are sensitive to 5 7 × 10−4 M concentrations.

Photophysical and photochemical events during the photosensitization of Hypocrellin A on a colloidal CdS semiconductor

UV–visible spectroscopy, fluorescence quenching measurements, and single-photon counting have been employed to study electron transfer from excited Hypocrellin A (HA) into the conduction band of a colloidal CdS semiconductor. It has been determined that HA dye adsorbed on colloidal CdS with an apparent association constant of 940 (mol/l) −1, and fluorescence lifetime measurements gave a rate constant of 5.16×10 9 s −1 for the electron transfer process. The transferred electrons could be used to reduce the substrate 2,2,6,6-tetramethyl-1-piperdinyloxy (TEMPO). In this regard electron paramagnetic resonance spectroscopy was used to study the photoreduction of TEMPO by electrons in the CdS semiconductor. The results suggested that CdS photoreduction efficiency was enhanced by photosensitization of HA in visible region.

Photo-induced electron transfer between hypocrellins and nano-sized semiconductor CdS

Both HA-CdS and HB-CdS (Hys-CdS, Hys represents HA, HB) complex systems were established according to the dynamics of heterogeneous electron-transfer process[Formula: see text]. In these systems, the electron transferring from(1)Hys* to conduction band of CdS is feasible. Determined from the fluorescence quenching, the apparent association constants (K(app)) between Hypocrellin A (HA), Hypocrellin B. (HB) and CdS sol. were about 940 (mol/L)(-1), 934 (mol/L)(-1), respectively. Fluorescence lifetime measurements gave the rate constant for the electron transfer process from(1)HA*,(1)HB* into conduction band of CdS semiconductor as 5.16 x 10(9) s(-1), 5.10 x 10(9) s(-1), respectively. TEMPO (2,2,6,6-tetramethy-1-piperdinyloxy), a stable nitroxide radical, was used in the kinetic study of the reduction reaction taking place on the surface of a CdS colloidal semiconductor, kinetics equation of the reaction was determined with the electron paramagnetic resonance (EPR) method, and the reaction order of TEMPO is zero. When Hys were added, the rate of EPR increased greatly. By comparing rate constants, the Hys-CdS systems were revealed to be about 350 times more efficient than CdS sol. alone in the photoreduction of TEMPO under visible light. It suggests that Hys can be used as efficient sensitizers of a colloidal semiconductor in the application of solar energy.

Density of states of a two-dimensional electron gas at semiconductor surfaces

The formation of a two-dimensional (2D) electron channel at semiconductor surfaces has been studied by high-luminosity and high energy-resolution ultraviolet photoelectron spectroscopy. A large variety of band-bending sources (alkali metals, silver and antimony adatoms, cleavage defects) on different narrow-gap III-V(110) substrates (InAs, InSb) has been used. The measured photoemission spectral density in the semiconductor conduction band shows a steplike structure, consistent with the description of a jelliumlike 2D electron gas confined in a potential well, and it is independent of the band-bending sources. A self-consistent solution of the Poisson and Schr\odinger equations gives the energy eigenvalues, the eigenstates, and the spectral density, in excellent agreement with the collection of photoemission results. Moreover, the accumulated charge density ranges between $3\ifmmode\times\else\texttimes\fi{}{10}^{11}$ and $2\ifmmode\times\else\texttimes\fi{}{10}^{12}{\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{n}\mathrm{s}/\mathrm{c}\mathrm{m}}^{2},$ consistent with previous experimental results on plasmon excitations.

Determination of Rate Constants for Dark Current Reduction at Semiconductor Electrodes Using ZnO Single-Crystal Microelectrodes

A method based on microelectrodes of ZnO single crystals was employed to determine the effective reaction velocity for dark reduction of ferrocenium acceptors by electrons from the semiconductor conduction band. Measurement techniques were explored and theoretical models for steady-state voltammetry at semiconductor electrodes developed. The microelectrode method as presented here represents an unambiguous experimental method for ascertaining ideal interfacial behavior even at high current flows found near the flatband potential. These results are compared with reaction velocities derived from extrapolation methods at millimeter-sized ZnO electrodes. It is seen that the extrapolation method is insensitive to nonideal behavior in the reduction reaction whereas the microelectrode method is not.

Nanocrystalline Mesoporous Strontium Titanate as Photoelectrode Material for Photosensitized Solar Devices: Increasing Photovoltage through Flatband Potential Engineering

Nanocrystalline SrTiO3 is synthesized by hydrothermal treatment of nanocrystalline titanium dioxide in the presence of strontium hydroxide. Working photoelectrochemical solar cells are produced using these nanometer-sized semiconductor particles as photoelectrode materials. At AM 1.5, measured open circuit voltages were roughly 100 mV higher than in solar cells produced using nanocrystalline titanium dioxide (anatase), in agreement with a simple relation between semiconductor conduction band edge and open circuit voltage for these cells. Photocurrents measured in the SrTiO3 cells were roughly 1/3 those measured with TiO2 (anatase) -based cells. On the basis of flash laser photolysis and absorptance studies, we suggest that low dye loading and possibly suboptimal dye−oxide interactions can be the cause for the relatively low photocurrents in the SrTiO3 system.

Photoelectrochemical Properties of a Porous Nb2O5 Electrode Sensitized by a Ruthenium Dye

In studying the photoelectrochemical properties of TiO2, Nb2O5, ZnO, SnO2, In2O3, WO3, Ta2O5, and ZrO2 porous semiconductor films sensitized by the ruthenium(II) cis-bis(thiocyanato)bis(2,2‘-bipyridyl-4,4‘-dicarboxylic acid) complex, it was found that the Nb2O5 semiconductor cell had the next highest incident of monochromatic photon-to-current efficiency (IPCE = 18%) compared to the TiO2 cell and showed the highest open-circuit photovoltage (Voc) among them. The Voc of the dye-sensitized cell was proved to be related to the flatband potential of the semiconductor electrode. The Ru dye adhered to the Nb2O5 surface mainly through an ester-like linkage. It is speculated that electrons are transferred mainly through the conjugated orbitals of the ester linkage and semiconductor conduction band and that the TiO2 and Nb2O5 conduction bands consisting of d-orbitals are more advantageous than those of s-orbitals in attaining the desired IPCE. The IPCE of the Nb2O5 cell was markedly improved by treating the Nb2O5 electrode with Nb alkoxide, and the maximum IPCE was 32% at 548 nm. The overall solar-to-electric energy conversion efficiency was about 2% (AM-1.5, 100 mW/cm2).

Logarithmic approximation for the energy band in nonparabolic semiconductors

We propose a semi-empirical logarithmic approximation to the conduction band of small gap semiconductors. This approximation enables us to obtain analytical expressions for the intrinsic absorption coefficient and free electron density, and these expressions are valid for a large temperature range. We show that the exponential variation of the absorption coefficient, as observed by Chu et al. [J. H. Chu et al., J. Appl. Phys. 71, 3955 (1991)] in Hg1−xCdxTe, can be fitted very well for any temperature using only a few parameters whereas the analytical expression for free electron density is useful in optical and transport simulations.

Electron-electron interactions in the nonparabolic conduction band of narrow-gap semiconductors

The Lindhard dielectric function is evaluated at nonzero temperatures with Fermi-Dirac statistics and a nonparabolic conduction band for bulk, narrow-gap semiconductors. This is used to study two problems of current interest: (i) inelastic scattering of single, energetic electrons by a system of plasmons, phonons, and quasiparticle excitations; and (ii) cooling of a hot, quasiequilibrium plasma by dynamically screened LO-phonon emission. Scattering of high-energy electrons injected into $n$-doped InAs at room temperature is analyzed and compared to the case where nonparabolicity is neglected. Plasmon losses and a larger density of states are shown to be more important for electrons in a nonparabolic conduction band with energy above the Fermi level. The energy-loss rate of a hot-electron--donor ion plasma to a cold lattice via screened LO-phonon emission in parabolic and nonparabolic bands is also obtained. It is found that even though dynamic screening of LO phonons is stronger in a nonparabolic band, the cooling rate of hot electrons in InAs can be significantly faster because of the increased density of conduction-band states.

Nanometer-Scale Investigation of Schottky Contacts and Conduction Band Structure on 4H-, 6H- and 15R-SiC Using Ballistic Electron Emission Microscopy

We have performed ballistic-electron emission microscopy (BEEM) of Pd and Pt Schottky contacts on 6Hand 4H-SiC and Pd Schottky contacts on 15R-SiC. Measured Schottky barrier heights (SBH’s) appear spatially uniform up to the fitting error due to noise (~0.03 and ~0.1 eV for 6Hand 4H-SiC, respectively). In 4H-SiC, we observed an additional conduction band minimum (CBM) ~0.14 eV above the lowest CBM, which is in good agreement with our band theoretical calculation. Preliminary results on Pd/15R-SiC indicate a higher CBM ~0.5 eV above the lowest CBM, and possibly another higher CBM ~0.3 eV above the lowest CBM. Also, in Pd/15RSiC, large variations in BEEM spectra at different locations were observed, suggesting an inhomogeneous interface. Additionally, we sometimes observed enhancement in ballistic transmittance over regions intentionally stressed by hot electron injection. We report ballistic-electron emission microscopy (BEEM) measurements of Pd and Pt Schottky contacts on 6Hand 4H-SiC, and Pd contacts on 15R-SiC. The BEEM technique is an extension of scanning tunneling microscopy (STM) and can be used to probe local electronic properties of buried metal/semiconductor interfaces with nanometer-scale spatial resolution and high energy resolution [1,2]. Figure 1 shows the schematic experimental setup with corresponding energy-level diagrams for BEEM. The STM tip is used to inject hot electrons into a thin metal film, with the hot-electron peak energy controlled by the applied tip bias VT . Provided the metal film is thin enough compared with the electron mean free path, a small fraction of these electrons can cross the metal film elastically and can enter the semiconductor conduction band if injected with sufficient energy. Hence, the local Schottky barrier height (SBH) [3] qVB can be directly determined by measuring BEEM Ic-VT curves (also called BEEM spectra) and evaluating the threshold voltage Vth for Fig. 1: (a) Physical setup of the BEEM experiment on SiC and (b) the corresponding energy level diagram (only the lowest CBM is shown). By measuring the dependence of Ic (flux of ballistic electrons from the STM tip into the SiC) on tunnel voltage VT (at constant tunnel current IT) we can probe the local Schottky barrier height and SiC band structure. A

High power laser semiconductor interactions: A Monte Carlo study for silicon

In this article, we use Monte Carlo methods to study the interaction of high power laser pulses with electrons in the conduction band of semiconductors. The laser field is represented by a sinusoidal electric field which tends to cause an oscillatory motion in the electrons. The scattering of electrons from the lattice force the electrons to lose phase coherence with the field. The approach is applied to silicon. We use the approach to examine the carrier energy distribution and material breakdown due to the transfer of energy from the laser to the electrons followed by impact ionization. The impact ionization coefficient, α, and its dependence on the laser frequency and field strength is examined and compared to the values in a dc field. In general, the ac value is smaller than the dc value, but at low frequencies and high field strengths, the ac impact ionization coefficient approaches the dc value at the same rms field value. The importance of collisions in the energy transfer process is elucidated.

The Photoionization in Anisotropic Polar Semiconductors

The photoionization has been considered as an optical transition from the impurity ground state to a continuum state belonging to a conduction band in semiconductors. The dependence of the photoionization cross-section on photon energy is calculated for a shallow donor impurity taking into account the effect of the electron mass anisotropy. The interaction between the electron and the longitudinal optical (LO) phonon is taken into account using Fröhlich's coupling and applying the Lee-Low-Pines transformation. The cross-sections involving zero or one phonon are evaluated for a shallow donor in CdS and it is shown that the cross-section is very sensitive to the electron mass anisotropy, the electron–phonon coupling, and their combined effect.

Weak localization effects on spin relaxation of excitons in quantum wells.

We have analyzed the effect of weak disorder, originated from impurities and imperfections that scatter excitonic elastically, on spin-relaxation dynamics of excitons in a quantum well, assuming a dominating exchange-induced transition between two optically active exciton states with spin +1 and -1. The quantum interference effects give rise to the characteristic quantum localization corrections to both longitudinal and transverse spin-relaxation rate. These corrections are time dependent and can change sign during a time interval of observation. Similar corrections are also expected for the spin relaxation rate of electrons in the spin splitted conduction band of zinc-blende semiconductors.

Molecular level photovoltaics: the electrooptical properties of metal cyanide complexes anchored to titanium dioxide

The preparation, surface attachment, optical, and photoelectrochemical properties of metal cyanide complexes, cis-[(4,4[prime]-(CO[sub 2]H)[sub 2]bpy)[sub 2]M(CN)[sub 2]] where M = Ru(II) and Os(II), attached to sol-gel processed TiO[sub 2] electrodes are reported. The specific sol-gel process provides a route to stable, high surface area anatase TiO[sub 2] films. With surface anchored [(4,4[prime]-(CO[sub 2]H)[sub 2]bpy)[sub 2]Ru(CN)[sub 2]]RB we report incident-photon-to-current efficiencies exceeding 90%. The net photocurrent scales linearly over 5 decades of incident irradiance. The molecular level similarity of the two polypyridyl complexes allows for a meaningful comparison of the electrooptical properties of Ru(II) and Os(II) derivatized surfaces. A kinetic interpretation with different sacrificial electron donors provides insight into the rate limiting step(s) in the photosensitization process. An increased rate of electron tunneling from the semiconductor conduction band to the oxidized dye and a more sluggish halide oxidation rate are believed to be responsible for a low photocurrent efficiency observed with surface-anchored [(4,4[prime]-(CO[sub 2]H)[sub 2]-bpy)[sub 2]Os(CN)[sub 2]]. The results and interpretation suggest methods for developing more efficient photovoltaic devices. 35 refs., 8 figs., 2 tabs.

Deformation Potentials in AIIBIVC2V Ternary Chalcopyrite Semiconductors

AbstractDeformation potentials of valence and conduction bands in AIIBIVC2V chalcopyrite semiconductors are calculated from the variation of the energy gap with temperature and pressure. From a comparison with their binary analogues AIIIBV it is found that values of deformation potentials in these compounds are considerably higher than in the binaries which could explain their lower carrier mobility.

Radiation-detected optical pumping in solids

Radiation-detected optical pumping in solids has been developed to investigate the structure of unstable nuclei. Appreciable nuclear polarization of implanted or doped unstable-nuclei in a host crystal is achieved with the optical pumping in solids. The nuclear polarization achieved is enhanced/reduced by applying a radio-frequency magnetic field together with the optical pumping, radiation-detected magnetic resonance being thus observed to get information on electromagnetic properties of unstable nuclei. Two schemes have been successfully developed for the optical pumping of unstable nuclei in solids. One is to directly pump the atoms by the excitation from the ground state to a broad absorption band in visible and UV regions which shows a large magnetic circular dichroism. This scheme is applicable to many rare-earth atoms in alkaline-earth fluoride host. The other scheme is to pump the electrons in the conduction band of direct-type semiconductor and thus indirectly polarize the nuclei in the host material via a hyperfine interaction between the nuclei and the polarized conduction electrons. This scheme can be especially applied to the III to VI families of atoms in direct-type semiconductors. Principle of the methods, on-line experimental system, and a few examples of the results obtained so far are presented and discussed.

Influence of the surface treatments by multipolar plasma on the slow state tunnel kinetics at the In0.53Ga0.47As/Si3N4 interfaces

In this work a study of the capture and emission kinetics related to the slow interface states present in In0.53Ga0.47As/Si3N4 interfaces on MIS (metal-insulator-semiconductor) structures is reported. These states are found to be filled by electrons from the semiconductor conduction band by means of a tunnel mechanism. On the other hand, charge tunnel emission from the slow states takes place through the fast interface states energetically located in the semiconductor bandgap. The influence of surface plasma treatments on the slow state kinetics has been analyzed. The results indicate a different nature and electrical behaviour of the slow interface states when they are located in the native oxide layer or in the silicon nitride. The surface treatments allow us to eliminate completely the slow states related to the native oxide and to reduce significantly the density of slow states in the silicon nitride.