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Jacques Cazaux - One of the best experts on this subject based on the ideXlab platform.

  • backscattered electron imaging at low emerging angles a physical approach to contrast in lvsem
    Ultramicroscopy, 2013
    Co-Authors: Jacques Cazaux, Noriyuki Kuwano, Kaoru Sato
    Abstract:

    Abstract Due to the influence of refraction effects on the Escape Probability of the Back-Scattered Electrons (BSE), an expression of the fraction of these BSE is given as a function of the beam energy, E °, and emission angle (with respect to the normal) α . It has been shown that these effects are very sensitive to a local change of the work function in particular for low emerging angles. This sensitivity suggests a new type of contrast in Low Voltage Scanning Electron Microscopy (LVSEM for E ° ϕ with crystalline orientation, this possibility is supported by a new interpretation of a few published images. Some other correlated contrasts are also suggested. These are topographical contrasts or contrasts due to subsurface particles and cracks. Practical considerations of the detection system and its optimization are indicated.

  • calculated effects of work function changes on the dispersion of secondary electron emission data application for al and si and related elements
    Journal of Applied Physics, 2011
    Co-Authors: Jacques Cazaux
    Abstract:

    The published secondary electron yield (SEY) data, δ = f(E°), are characterized by a poor level of agreement, rarely more than 25% and lower for a common element such as Al. Some possible sources of discrepancies are related to sample preparation, leading to differences in surface composition (contamination and oxidation). This is theoretically explored by a quantitative estimate of a change of the work function, φ (or electron affinity, χ), on the Escape Probability, A, of secondary electrons (SEs) and consequently on the SEY data of clean and oxidized samples. An electron-stimulated reduction effect may also explain the change of the SEY with the incident electron dose (fluence). Deduced from analytical expressions for the SE angular and energy distributions, ∂δ/∂α and ∂δ/∂Ek, respectively, the present investigation also includes instrumental effects due to work function differences when a partial angular collection of SEs is conducted. Although it is illustrated here for Al and Si, the present methodol...

  • calculated influence of work function on se Escape Probability and secondary electron emission yield
    Applied Surface Science, 2010
    Co-Authors: Jacques Cazaux
    Abstract:

    Abstract The influence of changes of the work function, ϕ, or electron affinity, χ, on the Escape Probability, A, of Secondary Electrons, SE, is derived from their angular and energy distributions, respectively ∂δ/∂α and ∂δ/∂Ek. Based on the evaluation of the spectral distribution of inner SEs, the present approach quantifies the dominant role of potential barrier on the SE emission and its change with surface treatments or thin film deposits. For instance it is shown that a 1 eV-increase of ϕ for Au leads to a decrease of A, and then of SE emission yield, δ, of about 50% while a 0.4 eV-increase of χ for potassium chloride induces a decrease of a factor 4 for A and then for the SEE yield δ. These results are summarized by empirical expressions of form A/A° = (ϕ/ϕ°)−3 for Au and A/A° =(χ/χ°)−3/2 for KCl. Applied here to an insulating sample and to a metal, the present approach may be easily transposed to any kind of material of known Fermi energy and work function, metals, or known affinity, semiconducting and insulating samples. The large SEE yield values of inorganic insulators relative to that of metals are explained by larger values of their Escape Probability A – KCl: A° ∼ 25% for χ° = 0.6 eV; Au: A° ∼ 4% for ϕ° = 3.5 eV – combined to larger SE attenuation lengths and despite a less SE generation factor. This approach underlines the significant role of A on the large deviations between SEEY data as reported in literature and a strategy combining in situ δ and ϕ measurements is suggested to partly compensate the corresponding dispersion of experimental results. The present approach may be transposed to other energetic projectiles such as X-rays or ions and some practical consequences related to Scanning Electron Microscopy, mechanisms of contamination and crystalline contrasts, are pointed out.

Benkang Chang - One of the best experts on this subject based on the ideXlab platform.

  • Influence of the p-type doping concentration on reflection-mode GaN photocathode
    Applied Physics Letters, 2011
    Co-Authors: Xiaohui Wang, Benkang Chang
    Abstract:

    Four different p-type doping GaN photocathodes are activated by Cs/O, and the quantum efficiency (QE) curves are obtained. According to the QE equation, the curves are fitted. Both the QE curves and the fitting results show that the optimal p-type doping concentration is at 1017 cm−3. The electron diffusion length and surface-electron Escape Probability can be balanced well at 1017 cm−3. To a certain degree, thick emission layer is conducive to improving the QE, which is more obvious with the long wavelength.

  • influence of exponential doping structure on photoemission capability of transmission mode gaas photocathodes
    Journal of Applied Physics, 2010
    Co-Authors: Yijun Zhang, Benkang Chang, Jijun Zou, Jun Niu, Jing Zhao, Feng Shi, Hongchang Cheng
    Abstract:

    In order to verify the actual effect of an exponential-doping structure on cathode performance, an exponential-doping structure has been applied to the preparation of the transmission-mode GaAs photocathode via molecular beam epitaxy technique. Compared with the uniform-doping photocathode, the activation and spectral response results show that the exponential-doping photocathode can achieve a higher photoemission capability. In addition, based on the revised uniform-doping and exponential-doping transmission-mode quantum yield equations, the cathode performance parameters such as electron average transport length and electron Escape Probability of the exponential-doping photocathode are obtained, which are greater than those of the uniform-doping one. The improvement in the cathode performance is attributed to the built-in electric field arising from this special doping structure, which effectively increases the electron transport efficiency and Escape Probability.

  • high quantum efficiency of depth grade doping negative electron affinity gan photocathode
    Applied Physics Letters, 2010
    Co-Authors: Xiangyang Guo, Benkang Chang, Xiaohui Wang, Yijun Zhang, Pin Gao
    Abstract:

    A depth grade doping sample gallium nitride (GaN) photocathode was designed to obtain an extremely high quantum efficiency (QE). Two other uniform doping samples were prepared in the same procedure as contrast. The calibrated QE curves were achieved; by comparing theoretical calculated values with the experimental QE plots, the Escape Probability and diffusion length were fitted. The QE value of gradient doping sample is as high as 68.7% at 5.17 eV; the diffusion length of gradient doping sample is fitted to be 250 nm which is much higher than uniform doping samples. That explains why depth-grade-doping can improve the QE of GaN photocathode significantly.

  • variation of spectral response for exponential doped transmission mode gaas photocathodes in the preparation process
    Applied Optics, 2010
    Co-Authors: Yijun Zhang, Benkang Chang, Jijun Zou, Jun Niu, Yajuan Xiong
    Abstract:

    To confirm the actual effect of an exponential-doped structure on cathode performance, an exponential-doped structure was applied to the preparation of a transmission-mode GaAs photocathode, and spectral response curves after high-temperature activation, low-temperature activation, and the indium sealing process were separately measured by use of the on-line spectral response measurement system. The results show that, compared to the previously uniform-doped photocathode, the exponential-doped photocathode can obtain higher cathode performance and photoemission capability because of the built-in electric field. Nevertheless, cesium desorption and impurity of gas during the sealing process can cause the degeneration of spectral response in the entire response waveband, especially in the long-wavelength region, with the decrease in surface electron Escape Probability related to the adverse evolution of the surface potential barrier profile.

  • evolution of surface potential barrier for negative electron affinity gaas photocathodes
    Journal of Applied Physics, 2009
    Co-Authors: Benkang Chang, Yijun Zhang, Zhi Yang, Jianliang Qiao
    Abstract:

    The evolution of surface potential barrier for reflection-mode GaAs photocathodes in an ultrahigh vacuum system has been investigated by using spectral response and angle-dependent x-ray photoelectron spectroscopy (ADXPS) measurements at room temperature. The Escape probabilities of electrons emitted into vacuum are obtained as a function of the incident electron energy, surface barrier height, and thickness. Based on the new Escape Probability expressions, we obtain the surface barrier parameters of the reflection-mode negative-electron-affinity (NEA) cathodes from the fit of the spectral response curves by using quantum-efficiency equations. These parameters reveal the evolution of the NEA cathode surface during the degradation process. In addition, the surface layer structure of both the freshly activated and degraded cathodes is calculated from the ADXPS spectra. The calculated results are in fair agreement with the fitted barrier parameters.

I S Tilinin - One of the best experts on this subject based on the ideXlab platform.

  • Escape Probability of s photoelectrons leaving aluminium and copper oxides
    Surface and Interface Analysis, 1998
    Co-Authors: J Zemek, A Jablonski, S Hucek, I S Tilinin
    Abstract:

    The Escape Probability of O 1s and Al 2s photoelectrons from Al2O3 and CuO overlayers grown on Al and Cu substrates, respectively, has been studied as a function of photoelectron depth of origin and emission direction. Escape probabilities have been determined experimentally, analytically and by a Monte Carlo method. The analytical approach is based on solution of a kinetic equation satisfying appropriate boundary conditions. Both the Monte Carlo calculations and analytical theory account properly for multiple elastic and inelastic scattering of photoelectrons on their way out of the target. The results are compared with those of the straight-line approximation (SLA), where elastic scattering is neglected. It has been found experimentally that the Escape Probability as a function of depth of origin for the O 1s photoelectrons leaving Al2O3 surface at an emission angle of 60° with respect to the incident x-ray beam direction can be approximated by an exponential function. In contrast, the Escape Probability of s-photoelectrons in a direction close to that of x-ray propagation exhibits non-monotonic behaviour, with a maximum at a depth of 4–10 A. The experimental data agree well with the predictions of Monte Carlo simulations and analytical theory, and differ noticeably from the SLA results. © 1998 John Wiley & Sons, Ltd.

  • Escape Probability of signal photoelectrons from non crystalline solids influence of anisotropy of photoemission
    Journal of Electron Spectroscopy and Related Phenomena, 1997
    Co-Authors: I S Tilinin, A Jablonski, J Zemek, S Hucek
    Abstract:

    Abstract The Escape Probability of photoelectrons as a function of depth of orgin haa been studied experimentally, analytically and by the Monte Carlo (MS) technique. The depth distribution function (DDF) describing the Probability for an electron emitted at a certain depth to leave a surface without being scattered inelastically has been obtained by solving a kinetic equation in the transport approximation. The analytically derived DDF is a universal function of the ratio of the inealstic to the transport mean free paths and the asymmetry parameter. In the directions of minima of the angular distribution, this function is no longer exponential, but it may be essentially nonmonotonic, reaching its maximum value at the depth comparable with the inelastic mean free path. The maximum value of the DDF exceeds its surface value by about 50% for the asymmetry parameter being equal to 2 in the emission directions close to that of X-ray propagation. Under the same conditions, the mean Escape depth of electrons may be several times larger than the value predicted by the usual XPS formalism. Such behaviour of the Escape Probability is explained by elastic scattering of photoelectrons. The solution to the kinetic equation for a uniform target is generalized for a sample with an arbitrary depth profile and depth-dependent elastic and inelastic scattering cross-sections under the condition of the ratio of the inelastic to the transport mean free paths being independent of depth. Analytical formulas for the photoelectron yield from overlayer/substrate structure have been derived and studied in detail. The analytical predictions are compared with the experimental and Monte Carlo simulation data obtained for aluminium oxide/aluminium specimen. A satisfactory agreement is observed between the experimental and theoretical results.

  • Escape Probability of auger electrons from noncrystalline solids exact solution in the transport approximation
    Physical Review B, 1992
    Co-Authors: I S Tilinin
    Abstract:

    The Probability that an Auger electron, generated at a certain depth in a semi-infinite target, Escapes from the surface under a certain emission angle is described by the so-called depth distribution function. The exact solution for this depth distribution function has been found in the transport approximation, employing transport theory. The results are in good agreement with data found in the literature and emphasise that strong deviations from exponential behavior occur. These deviations are most pronounced for oblique emission. To assess the validity of the transport approximation two kinds of Monte Carlo calculations have been performed

  • Escape Probability of auger electrons from noncrystalline solids exact solution in the transport approximation
    Physical Review B, 1992
    Co-Authors: I S Tilinin
    Abstract:

    The Probability that an Auger electron, generated at a certain depth in a semi-infinite target, Escapes from the surface under a certain emission angle is described by the so-called depth distribution function. The exact solution for this depth distribution function has been found in the transport approximation, employing transport theory. The results are in good agreement with data found in the literature and emphasize that strong deviations from exponential behavior occur. These deviations are most pronounced for oblique emission. To assess the validity of the transport approximation two kinds of Monte Carlo calculations have been performed. In one case the realistic Mott cross section for elastic scattering has been used while in the other the corresponding momentum transfer cross section was used. The latter procedure exactly fits the transport approximation. A very good agreement between the two approaches has been obtained. This indicates that the transport approximation is an effective tool in transport problems provided the angular distribution of the particle flux density varies slowly with the angle.

Yijun Zhang - One of the best experts on this subject based on the ideXlab platform.

  • influence of exponential doping structure on photoemission capability of transmission mode gaas photocathodes
    Journal of Applied Physics, 2010
    Co-Authors: Yijun Zhang, Benkang Chang, Jijun Zou, Jun Niu, Jing Zhao, Feng Shi, Hongchang Cheng
    Abstract:

    In order to verify the actual effect of an exponential-doping structure on cathode performance, an exponential-doping structure has been applied to the preparation of the transmission-mode GaAs photocathode via molecular beam epitaxy technique. Compared with the uniform-doping photocathode, the activation and spectral response results show that the exponential-doping photocathode can achieve a higher photoemission capability. In addition, based on the revised uniform-doping and exponential-doping transmission-mode quantum yield equations, the cathode performance parameters such as electron average transport length and electron Escape Probability of the exponential-doping photocathode are obtained, which are greater than those of the uniform-doping one. The improvement in the cathode performance is attributed to the built-in electric field arising from this special doping structure, which effectively increases the electron transport efficiency and Escape Probability.

  • high quantum efficiency of depth grade doping negative electron affinity gan photocathode
    Applied Physics Letters, 2010
    Co-Authors: Xiangyang Guo, Benkang Chang, Xiaohui Wang, Yijun Zhang, Pin Gao
    Abstract:

    A depth grade doping sample gallium nitride (GaN) photocathode was designed to obtain an extremely high quantum efficiency (QE). Two other uniform doping samples were prepared in the same procedure as contrast. The calibrated QE curves were achieved; by comparing theoretical calculated values with the experimental QE plots, the Escape Probability and diffusion length were fitted. The QE value of gradient doping sample is as high as 68.7% at 5.17 eV; the diffusion length of gradient doping sample is fitted to be 250 nm which is much higher than uniform doping samples. That explains why depth-grade-doping can improve the QE of GaN photocathode significantly.

  • variation of spectral response for exponential doped transmission mode gaas photocathodes in the preparation process
    Applied Optics, 2010
    Co-Authors: Yijun Zhang, Benkang Chang, Jijun Zou, Jun Niu, Yajuan Xiong
    Abstract:

    To confirm the actual effect of an exponential-doped structure on cathode performance, an exponential-doped structure was applied to the preparation of a transmission-mode GaAs photocathode, and spectral response curves after high-temperature activation, low-temperature activation, and the indium sealing process were separately measured by use of the on-line spectral response measurement system. The results show that, compared to the previously uniform-doped photocathode, the exponential-doped photocathode can obtain higher cathode performance and photoemission capability because of the built-in electric field. Nevertheless, cesium desorption and impurity of gas during the sealing process can cause the degeneration of spectral response in the entire response waveband, especially in the long-wavelength region, with the decrease in surface electron Escape Probability related to the adverse evolution of the surface potential barrier profile.

  • evolution of surface potential barrier for negative electron affinity gaas photocathodes
    Journal of Applied Physics, 2009
    Co-Authors: Benkang Chang, Yijun Zhang, Zhi Yang, Jianliang Qiao
    Abstract:

    The evolution of surface potential barrier for reflection-mode GaAs photocathodes in an ultrahigh vacuum system has been investigated by using spectral response and angle-dependent x-ray photoelectron spectroscopy (ADXPS) measurements at room temperature. The Escape probabilities of electrons emitted into vacuum are obtained as a function of the incident electron energy, surface barrier height, and thickness. Based on the new Escape Probability expressions, we obtain the surface barrier parameters of the reflection-mode negative-electron-affinity (NEA) cathodes from the fit of the spectral response curves by using quantum-efficiency equations. These parameters reveal the evolution of the NEA cathode surface during the degradation process. In addition, the surface layer structure of both the freshly activated and degraded cathodes is calculated from the ADXPS spectra. The calculated results are in fair agreement with the fitted barrier parameters.

Kota Ogasawara - One of the best experts on this subject based on the ideXlab platform.

  • observability of the innermost stable circular orbit in a near extremal kerr black hole
    Physical Review D, 2020
    Co-Authors: Takahisa Igata, Keisuke Nakashi, Kota Ogasawara
    Abstract:

    We consider the Escape Probability of a photon emitted from the innermost stable circular orbit (ISCO) of a rapidly rotating black hole. As an isotropically emitting light source on a circular orbit reduces its orbital radius, the Escape Probability of a photon emitted from it decreases monotonically. The Escape Probability evaluated at the ISCO also decreases monotonically as the black hole spin increases. When the dimensionless Kerr parameter $a$ is at the Thorne limit $a=0.998$, the Escape Probability from the ISCO is 58.8%. In the extremal case $a=1$, even if the orbital radius of the light source is arbitrarily close to the ISCO radius, which coincides with the horizon radius, the Escape Probability remains at 54.6%. We also show that such photons that have Escaped from the vicinity of the horizon reach infinity with sufficient energy to be potentially observed because Doppler blueshift due to relativistic beaming can overcome the gravitational redshift. Our findings indicate that signs of the near-horizon physics of a rapidly rotating black hole will be detectable on the edge of its shadow.