The emission of correlated electrons from surfaces

This work offers a theoretical framework for the treatment of the simultaneous many-electron excitation in solids and surfaces. Starting from the transition operator, an expansion is presented that treats all involved interactions on equal footing. The method is applied to the simultaneous two-orbital excitation and the results are compared with recent experimental findings. It is concluded that such studies provide a useful tool for the investigation of inter-electronic coupling. Received: 23 March 1999 / Accepted: 25 June 1999 / Published online: 23 September 1999     

超强激光超热电子激发的Kα X射线发射研究

建立了无色散型X射线谱仪. 利用SILEX-I激光装置的超强激光辐照固体物质,分别在靶前、后定量测量了Cu和Mo物质在不同激光功率密度时的X射线谱和Kα光子产额,推导了不同激光强度时的Kα X射线光子转换效率. 实验发现,打靶激光能量越高,靶后出射的Kα产额越高,100μm Mo靶可获得10^-5量级转换效率. 关键词： X射线发射 激光-物质相互作用 Kα谱仪')" href="#">Kα谱仪     

Scattering of hot electrons by adatoms at metal surfaces

The scattering of electrons in image-potential states by Cu adatoms on Cu(001) surfaces has been investigated by means of time- and angle-resolved two-photon photoemission. Several interband and intraband-scattering mechanisms have been identified and their contributions to the total decay of the states determined quantitatively. The adsorbates mainly cause quasielastic scattering processes. Inelastic processes in contrast are due to interactions with electrons in the substrate and are not significantly increased by Cu adatoms. Quasielastic scattering into bulk bands contributes significantly to the depopulation of surface states.     

Anisotropic and polarized intraband light emission of hot electrons

Far infrared luminescence of hot electrons in InSb have been studied. The emission probabilities derived in the second order perturbation are obtained for different scattering mechanisms: acoustic phonons, optical phonons and impurities. The origin of anisotropy and polarization of the light emission due to anisotropy of the electron distribution function under the electric field is described theoretically and calculated using the Monte Carlo method. The theoretical conclusions are confirmed by the experiment.     

Characteristic photon emission from ion bombarded silicon and silica surfaces

Ion bombardment with 50 keV inert gas and reactive gas ions has been used to study the photon emission from excited radiating atoms and ions in the immediate vicinity of the surface of both silicon and silica glass targets (SiO₂).     

Light emission from non radiative plasmons excited by electrons on smooth surfaces

Theoretical results are reported on the possibility to observe light emitted from the non radiative surface plasmons which are excited by fast electrons traversing a smooth vacuum-metal boundary. By covering the backside of the metal film with a medium of refractive indexn>1, light can be decoupled from the non radiative plasmons. In the angular intensity distribution this light has a strong characteristic peak exceeding the ordinary transition radiation by a factor of the order of 10².     

Intervalley transfers of hot electrons in silicon below 77 K

Intervalley rate transfer along 〈100〉 directions is deduced from conductivity measurements versus electric field for different impurity concentrations N_D-N_A at lattice temperatures T ? 77 K. The maximum rate transfer plotted versus T reaches 1 at T < 25 K and does not depend on N_D-N_D for N_D-N_A ? 2 × 10¹⁴crmcm^?3.     

Microstructured silicon surfaces for field emission devices

Enhanced field emission of electrons from silicon surfaces was obtained by surface microstructuring, by means of electrochemical oxidation in organic solutions containing HF. Morphological characterisations showed the formation of cylindrical rods, randomly distributed with relative spacing of a few microns. They are originated at the top of silicon pyramids and have typical diameter in the 100 nm range. Variable length in the 1–50 μm range was obtained, by adjusting the process parameters. Electron field emission properties were characterised for several samples, prepared in different conditions: the emission threshold was found to be strongly correlated with the overall charge exchanged during electrochemical oxidation. In the most favourable conditions, the threshold field for the emission of an electron current I_th = 10⁻¹⁰ A was 11.1 V/μm.     

Elastic backscattering of electrons from surfaces

A Monte Carlo algorithm is proposed for calculating the elastic reflection coefficient, η_E, for elements. The algorithm accounts for the multiple elastic scattering of electrons in solids. The calculated values of η_E compare well with the literature data for elements with atomic number up to 47 and at primary energies above 2 keV. The proposed Monte Carlo method makes it possible to determine the functional relation between η_E and the inelastic mean free path, λ. This relation turned out to be non-linear, arid it deviates from a similar relation based on published earlier single elastic scattering model. The deviation is especially pronounced for elements with medium atomic numbers. The calculated function $\text{η}{}_{\mathrm{<mtext>E</mtext>}}\text{=}\text{f}\text{(λ)}$ offers a convenient method for determining the inelastic mean free path. The values of λ derived in the present work from published experimental values of η_E compare very well with the literature data.     

Spectroscopy of near-threshold electrons from surfaces

A time-of-flight technique for two-dimensional spectroscopy of near-threshold electrons from surfaces is described, and the properties of the method are illustrated using selected results from clean surfaces, and mono- and multilayer adsorbates. The analysis of two-dimensional data yields easy access to inner (bulk) and outer (surface) ionization edges and exciton series, for valence and core electron levels including satellite states, and to final state effects. Strong interaction between bulk and surface states is revealed in particular for samples with negative electron affinity. Core-level threshold electron spectra from monolayers adsorbed on metal surfaces exhibit strong substrate-mediated suppression of post-collision interaction-induced line broadening and shift, each of which can be quantitatively explained by image charge screening. Received: 27 April 1998/Accepted: 25 August 1998     

Luminescence from hot electrons relaxing by LO phonon emission in p-GaAs and GaAs doping superlattices

Laser excited hot electrons in GaAs relax by LO phonon emission within a few hundred femtoseconds, leading to a series of peaks in the distribution of hot electrons in the conduction band, which we observe in luminescence. We find that the luminescence peaks shift according to the acceptor binding energy for C?, Ge?, Zn?, and Be-p-doped GaAs layers grown by MBE and LPE. Thus we prove that recombination is between hot electrons and neutral acceptors. The series of peaks due to electrons from the heavy hole band agree well with $\text{k}\text{.}\text{p}$ band structure, while peaks due to those from light holes are about 15 meV lower than expected from the band structure. We show that the discrepancy is not due to heating or surface fields. The peak separation in the luminescence ladder is about 6% larger than the LO energy suggesting emission of renormalised LO phonons. We find thermalisation by LO emission also in GaAs nipi doping superlattices. In nipi crystals the emission is shifted to higher energies (by 12 meV for light and by 6 meV for heavy holes) due to a change in band structure caused by the space charge fields.     

Reflection of keV-energy electrons from multilayer surfaces

The energy spectra of electrons reflected from multilayer targets are studied theoretically and experimentally. A self-consistent theory of electron reflection from multilayer surfaces is constructed. Simple analytical models of electron reflection that illustrate the feasibility of the depth profiling of multilayer targets are presented. The energy spectra of electrons normally incident on Nb/Si and Nb/Al/Nb/Si targets and reflected from them at an angle of 45° to the normal are taken. A method for the depth profiling of such structures is elaborated.     

Anomalous emission of electrons from MIM systems

In this paper the emission of electrons from thin-film systems of the structure metal-dielectric-metal (Al-Al₂O₃-Au) is studied experimentally in the voltage region below the work potential of the top electrode. The temperature and voltage dependences of the angular distribution, energy distributionsN(E) andN (Einx) and energy-angular distribution of emitted both anomalous and normal electrons were obtained.This paper is based on the RNDr thesis of the first author.     

Statistical emission of electrons from nanocrystalline silicon quantum dot memory nodes in a few-electron MOSFET memory device

Operation of a short and narrow channel metal-oxide-semiconductor field-effect transistor (MOSFET) memory device with a few nanocrystalline Si (nc-Si) dots in the active region has been investigated at 300 and 30 K. The discrete shift of the threshold voltage (V_th) in the current-voltage characteristics that arises from the screening effect of the charge stored in the nc-Si dot above the FET channel, suggests memory operation. It is found that the value of V_th changes with temperature whereas the magnitude of the shift in V_th is independent of temperature. The lifetime of the electrons stored in the floating node has also been investigated at different read voltages.     

Secondary ion emission from silicon and silicon oxide

The emission of Si⁺, Si²⁺, Si³⁺, Si₂⁺, SiO⁺ and B⁺ from boron doped silicon has been studied at oxygen partial pressures between 2 × 10^?10 and 2 × 10^?5 Torr. Sputtering was done with 2 to 15 keV argon ions at current densities between 3 and $\text{40}\text{μ}\text{A}\text{cm}{}^2$. The relative importance of the different ionization processes could be deduced from a detailed study of the yield variation at varying bombardment conditions. Comparison with secondary ion emission from silicon dioxide allows a rough determination of the composition of oxygen saturated silicon surfaces.