The linewidth of surface states of simple metals

The linewidths (inverse lifetimes) Γ_e-e of Be(0001) and Mg(0001) surface electronic states are calculated as the projection of the imaginary part of the self-energy operator of a quasiparticle onto the state. The screened Coulomb interaction is calculated using a model potential, which takes into account the energy gap in the band structure and a surface state located in this gap. The wave functions and energies of electron states are calculated by a self-consistent film pseudopotential method. It is shown that Γ_e-e essentially depends on the position of the surface state in the Brillouin zone. The difference between the calculated values of Γ_e-e and those obtained in a homogeneous electron gas model is shown to be basically due to transitions from surface bands.     

Nonradiative relaxation in ensembles of silicon nanocrystals

The results of calculating the lifetimes of direct electron and hole tunneling and of nonradiative exciton transport between Si nanocrystals are presented. It is shown that tunnel transitions dominate in the process of the energy exchange between quantum dots and can considerably affect radiation in the ensemble of Si nanocrystals.     

Theory of time-resolved two-photon photoemission from Cu(111): effect of Coulomb interactions among electrons

The effect of Coulomb interactions among electrons on the time-resolved two-photon photoemission spectra of Cu(111) is investigated by nonequilibrium perturbation theory. It is demonstrated that there are contributions to photoemission via an image state on the surface from processes involving scattering of photoexcited electrons and holes due to Coulomb interactions. As a result, the correlation trace (photoelectron intensity as a function of the pump-probe delay time) is affected by the lifetimes of the electron in the image state, the photoexcited electron and hole.     

Energy-time entanglement preservation in plasmon-assisted light transmission

We report on experimental evidence of the preservation of the energy-time entanglement of a pair of photons after a photon-plasmon-photon conversion. This preservation is observed in two different plasmon conversion experiments, namely, extraordinary optical transmission through subwavelength metallic hole arrays and long range surface plasmon propagation in metallic waveguides. Plasmons are shown to coherently exist at two different times separated by much more than their lifetimes. This kind of entanglement involving light and matter is expected to be useful for future processing and storing of quantum information.     

Exciton binding energy and excitonic absorption spectra in a parabolic quantum wire under transverse electric field

The effects of transverse electric field on the energy levels of electron and heavy hole, exciton binding energy and excitonic absorption spectra of GaAs parabolic quantum wire are theoretically investigated in detail. The results indicate that the electron and hole energy levels, exciton binding energy, excitonic absorption coefficient and absorption energy becomes smaller with the increase of electric field. That is more significant at the condition of weaker parabolic confinement potential. The phenomena can be explained by the separation of overlap integral of the electron and hole at the ground states.     

Theoretical Approach to Energy Resolution of Semiconductor Detectors

All processes occurring in semiconductor detectors during detection of primary monoenergetic particles lead to broadening of spectral lines. In this work, a general expression for the energy resolution of semiconductor detectors is obtained using the theory of branching cascade processes. It is shown that the general formula involves all contributions to the spectral line broadening described in the literature and additional contributions associated with fluctuations of electron and hole lifetimes caused by inhomogeneity of traps in the semiconductor material and fluctuations of the electronic channel gain.     

Investigation of electron lifetimes in molybdenum with dilute substitutional impurities

We have measured the electronic lifetimes of the systems Mo(W), Mo(Ru), Mo(Fe) and Mo(U). The concentrations were ≈ 370PPM, 70PPM, 20PPM, and 50PPM respectively. For the first three systems, only the hole octahedron centered atH was investigated thoroughly for all angles and the data inverted to get point lifetimes. For Mo(U) the investigation was done over the entire Fermi surface with the aid of an on-line PDP 11/20 mini-computer. Only selective points were used on other pieces of the Fermi surface in order to avoid problems with degenerate frequencies. A careful determination of the effective masses was also made at all angles under investigation. The masses were found to be in disagreement with those of Leaver and Meyers (these masses were used in our orginal investigation) by as much as 20% in some orientations. The electronic lifetimes were nearly isotropic on the hole octahedron in all the alloy systems investigated. In Mo(U) the isotropy was found to extend to all thed-like surfaces. Only thes-p-like hole ellipsoids centered atN possibly have a substantially different lifetime, but its measurement is inaccurate due to beats from degenerate and many close-lying frequencies.     

Luminescence in amorphous semiconductors

A review of the luminescence properties of amorphous semiconductors is presented. The materials covered are chalcogenide glasses, silicon and arsenic. Luminescence spectra, excitation spectra, temperature dependences and lifetimes are described.

The radiative transition in chalcogenides is the recombination of an electron in the conduction band tail and a trapped hole. A strong electron-phonon coupling distorts the lattice near the trapped hole, lowering its energy. This interaction is responsible for the broadness of the luminescence band and its position at about half the band gap energy. The recombination centre is thought to be a charged dangling bond with density 10¹⁷ cm^-3 in arsenic chalcogenides and 10¹⁶ cm^-3 in selenium. The same centre is observed in the hole drift mobility, and thermally stimulated conductivity.

Luminescence in amorphous silicon also originates from recombination between the band tails and deep centres, with three separate transitions identified. In contrast to chalcogenides the electron-phonon coupling is not strong. The shape and intensity of the spectra are very sensitive to sample preparation and treatment, and correlate with other electrical and optical properties of Si.     

Multiple exciton generation in semiconductor nanocrystals: Toward efficient solar energy conversion

Within the range of photon energies illuminating the Earth's surface, absorption of a photon by a conventional photovoltaic semiconductor device results in the production of a single electron‐hole pair; energy of a photon in excess of the semiconductor's bandgap is efficiently converted to heat through interactions between the electron and hole with the crystal lattice. Recently, colloidal semiconductor nanocrystals and nanocrystal films have been shown to exhibit efficient multiple electron‐hole pair generation from a single photon with energy greater than twice the effective band gap. This multiple carrier pair process, referred to as multiple exciton generation (MEG), represents one route to reducing the thermal loss in semiconductor solar cells and may lead to the development of low cost, high efficiency solar energy devices. We review the current experimental and theoretical understanding of MEG, and provide views to the near‐term future for both fundamental research and the development of working devices which exploit MEG.     

Time resolved studies of intersubband relaxation in GaAs/AlGaAs quantum wells below the optical phonon energy using a free electron laser

The problem of intersubband relaxation in GaAs/GaAs quantum wells, where the energy separation of the two lowest subbands is smaller than the optical phonon energy, is considered. Time resolved pump-and-probe measurements are performed with a far-infrared free-electron laser on two multiquantum well samples with similar thicknesses (≈300 Å), but different doping and mobilities. The measured lifetimes are shorter than could be explained by acoustic phonon emission alone. Monte-Carlo calculations show the importance of electron–electron scattering for thermalization of the hot electron distribution function and subsequent optical phonon emission from the long thermal tail.     

Electronic transfer processes studied at different time scales by selective resonant core hole excitation of adsorbed molecules

Core hole spectroscopy has been shown to provide information on ultrafast charge transfer processes on a time scale of the core hole lifetime. In this paper we present high-resolution autoionization studies of SF₆ and thiophene molecules adsorbed on a Ru(001) surface to further illustrate the potential of the so-called core hole clock method. Resonance states with very different core hole lifetimes can be excited with these sulfur-containing molecules. The selection of a specific resonant core hole excitation enables a variation of the time scale for probing electronic as well as nuclear dynamics. PACS 32.80.Dz; 32.80.Hd; 73.20.Jc     

Investigation of electron lifetimes in molybdenum with dilute substitutional impurities

We have measured the electronic lifetimes of the systems Mo(W), Mo(Ru), Mo(Fe) and Mo(U). The concentrations were 370PPM, 70PPM, 20PPM, and 50PPM respectively. For the first three systems, only the hole octahedron centered atH was investigated thoroughly for all angles and the data inverted to get point lifetimes. For Mo(U) the investigation was done over the entire Fermi surface with the aid of an on-line PDP 11/20 mini-computer. Only selective points were used on other pieces of the Fermi surface in order to avoid problems with degenerate frequencies. A careful determination of the effective masses was also made at all angles under investigation. The masses were found to be in disagreement with those of Leaver and Meyers (these masses were used in our orginal investigation) by as much as 20% in some orientations. The electronic lifetimes were nearly isotropic on the hole octahedron in all the alloy systems investigated. In Mo(U) the isotropy was found to extend to all thed-like surfaces. Only thes-p-like hole ellipsoids centered atN possibly have a substantially different lifetime, but its measurement is inaccurate due to beats from degenerate and many close-lying frequencies.Work performed under the auspices of the U. S. Atomic Energy Commission.     

A Study on the Exciton-Ionized-Donor Complex Bound to the Surface of a Nonpolar Semiconductor

The exciton-ionized-donor complex with the donor on the surface of a semiconductor (X, D⁺)_s. is investigated for the first time. The binding energy of the complex is calculated variationally as a function of the effective mass ratio of the electron to the hole σ, and high-frequency dielectric constant ε_∞ of the host semiconductor occupying the semi-infinite space. The infinite potential barrier approximation, Levine wave function and the effective-mass approximation are used for the electron and the hole. The image potentials are also used to take into account the surface charge induced by the electron or the hole when it is near the surface. The results are presented graphically without any restrict of individual material. It is found that no unstable range of σ appears for (X, D⁺)_s.     

THE INTERACTION OF Au AND THE Si/SiO2 INTERFACE DEFECT Hit(0.494)

The defects at the Si/SiO₂ interface have been studied by the deep-level transient spectroscopy (DLTS) technique in p-type MOS structures with and without gold diffusion. The experimental results show that the interaction of gold and Si/SiO₂ interface defect,H_it(0.494), results in the formation of a new interface de-fect, Au-H_it(0.445). Just like the interface defect, H_it(0.494), the new interface defect possesses a few interesting properties, for example, when the gate voltage applied across the MOS structure reduces the energy interval between Fermi-level and Si valence band of the Si surface to values smaller than the hole ionization Gibbs free energy of the defect, a sharp DLTS peak is still observable; and the hole apparent activation energy increases with the decrease of the Si surface potential barrier height. These properties can be successfully explained with the transition energy band model of the Si/SiO₂ interface.     

Analysis of microscopic parameters of surface charging in polymer caused by defocused electron beam irradiation

The relationship between microscopic parameters and polymer charging caused by defocused electron beam irradiation is investigated using a dynamic scattering-transport model. The dynamic charging process of an irradiated polymer using a defocused 30 keV electron beam is conducted. In this study, the space charge distribution with a 30 keV non-penetrating e-beam is negative and supported by some existing experimental data. The internal potential is negative, but relatively high near the surface, and it decreases to a maximum negative value at z = 6 μm and finally tend to 0 at the bottom of film. The leakage current and the surface potential behave similarly, and the secondary electron and leakage currents follow the charging equilibrium condition. The surface potential decreases with increasing beam current density, trap concentration, capture cross section, film thickness and electron–hole recombination rate, but with decreasing electron mobility and electron energy. The total charge density increases with increasing beam current density, trap concentration, capture cross section, film thickness and electron–hole recombination rate, but with decreasing electron mobility and electron energy. This study shows a comprehensive analysis of microscopic factors of surface charging characteristics in an electron-based surface microscopy and analysis.     

微孔阵列铜表面二次电子发射系数抑制研究

针对微孔阵列对铜表面二次电子发射系数(SEY)的抑制效应进行实验研究以提高电真空器件性能。首先利用Casino软件模拟了入射能量分别为0.5 keV和3 keV的电子束垂直入射到方形微孔阵列表面的SEY,分析了方孔阵列的深宽比和孔隙率对本征二次电子发射系数(ISEY)、背散射二次电子发射系数(BSEY)及总二次电子发射系数(TSEY)的影响。然后采用半导体光刻工艺在铜箔表面制备具有不同形貌参数的圆孔阵列,采用激光扫描显微镜进行形貌分析和几何结构参数提取,采用二次电子测试平台进行TSEY测试。仿真结果表明：微孔阵列的深宽比、孔隙率越大,其SEY抑制特性越明显;随着微孔阵列深宽比逐渐增大,SEY逐渐趋于饱和;入射电子束能量较低时,微孔阵列对SEY抑制效应比入射能量较高时更为明显。实验结果表明：微孔阵列能有效抑制铜表面SEY,实测结果与仿真结果规律一致,为微孔阵列结构用于铜表面SEY抑制提供了依据。     

Frequency-dependent C(V) spectroscopy of the hole system in InAs quantum dots

The hole system in InAs quantum dots was investigated by frequency-dependent capacitance–voltage spectroscopy. Up to eight distinct charging peaks could be observed and the energy difference between the individual peaks could be estimated. All charging peaks decrease with increasing measurement frequency; however, the lower the energy of the hole level the stronger the decrease. A comparison with the results of the electron system in similar quantum dots yields that for all hole levels the effective mass in the barrier is much larger than in the electron system.     

Ring of C60 polymers formed by electron or hole injection from a scanning tunneling microscope tip

Carrier (electron or hole) injection from a scanning tunneling microscope tip causes various surface modifications on the molecular scale. We report that injection into C60 close-packed layers forms a ring-shaped distribution of C60 polymers. This can be explained on the basis of the radial propagation and energy dissipation of carriers. Subsequent electron or hole injections enlarge the ring, showing that both carriers can induce both polymerization and depolymerization. Furthermore, we demonstrate visualization of carrier scattering by injecting carriers into C60 layers with grain boundaries.     

Influence of deformation on the mobility and lifetimes of charge-carriers in anthracene crystals

Direct determination of the mobilities and lifetimes of electrons and holes in undeformed and in dislocated single-crystals of ultra pure anthracene reveals fresh insights into the mechanism of carrier trapping. The deformation introduced electron traps at depths of approximately 1.0 eV but was found not to affect hole transport. Such traps are considered to be due to impurities preferentially introduced into the lattice as a result of the generation of the additional dislocations.     

A new approach to the determination of the Fano factor for semiconductor detectors

Values of the Fano factor are widely scattered in the literature, indicating the difficulties in its determination. We chose to analyze both the escape peaks and parent peaks of Ge detector spectra where the parent peak penetration depth and the escape peak escape depth are much larger than the size of the incomplete charge collection region. Hence, the escape peaks are expected to be free of low‐energy tailing as, although the actually deposited energy is low, it is deposited beyond the incomplete charge collection region. It was found that the product of electron–hole pair creation energy (ε) and the Fano factor (F) has an energy dependence at low energies, as is expected from electron transport theories. Although ε is expected to have its own energy dependence, if a reference value of 2.96 eV is assumed for ε, then the Fano factor values varied between 0.059 and 0.083. The escape peak is less advantageous for Si, hence a different method was used for Si(Li) detectors. Assuming the reference value of 3.8 eV for ε, the Fano factor was found to be 0.063 at 5.9 keV x‐ray energy. We consider the Fano factors reported here as upper limits, rather than the mean values. Copyright © 2004 John Wiley & Sons, Ltd.