Escape Probability Of Photoelectrons Research Articles

Atomic Rearrangements and Photoemission Processes at a p-GaN(Cs)–Vacuum Interface

Spontaneous changes in photoemission properties of a р-GaN(Cs)–vacuum interface with effective negative electronic affinity induced by rearrangements of its atomic structure have been studied for the first time. The optimum Сs coating that ensures both the maximum photoelectron escape probability and its stability has been found. A thermodynamic model has been proposed to escape the relation of the photoemission properties of the р-GaN(Cs)–vacuum interface to its free energy and entropy.

Theoretical optimization of the hole concentration for GaN photocathode

The theoretical relationship between the hole concentration (n), electron diffusion length (LD), and surface photoelectron escape probability (P) was studied. It is found that with increase of n, LD decreases and P increases. Based on the Spicer's three-step photoemission model and the quantum efficiency (QE) formula of negative electron affinity (NEA) GaN photocathode, the theoretical relationship between n and QE could be obtained. A three-dimension graph was drawn to show the relationship between QE and photon energy (E), which allowed us to observe the QE changing with n intuitively. In the whole wavelength range, QE tended to rise firstly and then fall down. The optimal n shifted to the lower concentration with the increase in wavelength due to the deeper attenuation length of the long-wavelength light. Based on the photo-electrical response of the effective band, the optimal doping concentration of 1.26 × 1017 cm−3 was obtained. The comparison results indicated that the experimental data were consistent with the simulated curves.

Theoretical study on electronic properties of p-type GaN nanowire surface covered with Cs

By using density functional theory, we have investigated the Cs adsorption mechanism of Mg-doped GaN nanowire photocathode. The results show that in the GaN nanowire, Mg atoms tend to replace the Ga atoms in the central position, because the position has the lowest formation energy, and the carrier concentration is highest, which shows better conductivity. Besides, Mg doping leads to the Fermi level close to the valence band, exhibiting p-type conductivity. After Cs activation, the most stable adsorption site of Mg-doped nanowires is B N site. After Cs adsorption, the conduction band minimum and the valence band maximum both move to the lower energy, which is favorable for forming the n-type surface state and improving the photoelectron escape probability. This study can be used to guide the Cs adsorption process of Mg-doped GaN nanowires, which is helpful to improve the performance of GaN nanowire-based optoelectronic devices.

P-GaAs(Cs,O)-photocathodes: Demarcation of domains of validity for practical models of the activation layer

The (Cs,O)-activation procedure for p-GaAs(Cs,O)-photocathodes was studied with the aim of demarcating the domains of validity for the two practical models of the (Cs,O)-activation layer: The dipole layer (DL) model and the heterojunction (HJ) model. To do this, the photocathode was activated far beyond the normal maximum of quantum efficiency, and several photocathode parameters were measured periodically during this process. In doing so, the data obtained enabled us to determine the domains of validity for the DL- and HJ-models, to define more precisely the characteristic parameters of the photocathode within both of these domains and thus to reveal the peculiarities of the influence of the (Cs,O)-layer on the photoelectron escape probability.

Analysis of Ultra-Thin HfO2/SiOn/Si(001): Comparison of Three Different Techniques

Composition depth profiling of HfO(2) (2.5 nm)/SiON (1.6 nm)/Si(001) was performed by three diffetent analytical techniques: high-resolution Rutherford backscattering spectroscopy (HRBS), angle-resolved X-ray photoelectron spectroscopy (AR-XPS) and high-resolution elastic recoil detection (HR-ERD). By comparing these results we found the following: (1) HRBS generally provides accurate depth profiles. However, care must be taken in backgroud subtraction for depth profiling of light elements. (2) In the standard AR-XPS analysis, a simple exponential formula is often used to calculate the photoelectron escape probability. This simple formula, however, cannot be used for the precise depth profiling. (2) Although HR-ERD is the most reliable technique for the depth profiling of light elements, it may suffer from multiple scattering, which deteriorates the depth resolution, and also may cause a large background.

Size-Dependent Angular Distributions of Low-Energy Photoelectrons Emitted from NaCl Nanoparticles

Angle-resolved threshold photoelectron spectra are recorded for size-selected beams of sodium chloride nanoparticles (radius = 25−250 nm). The photoelectron angular distributions exhibit a size-dependent asymmetry that is inversely proportional to particle radius. A model of the internal electric field amplitude and the photoelectron escape probability inside the particles reveals that the asymmetry in photoemission arises because the finite dimensions of the particle are comparable to both the photon penetration depth (14−22 nm) and electron escape length (10 nm).

Escape probability of photoelectrons from silver sulphide

Silver sulphide films, grown in situ under UHV conditions by exposing the clean silver to a sulphur beam, have been studied by X-ray induced photoelectron spectroscopy. The films composition and chemical state were determined by quantitative analysis, binding energy and Auger parameter analysis. The escape probability as a function of depth of origin for S 2s and S 2p photoelectrons leaving a Ag 2S surface at three emission angles α=0°, 39° and 60° with respect to the photon flux propagation direction has been measured and compared to the predictions of the Monte Carlo calculations. The theoretical model applied accounted for both, elastic as well as inelastic electron scattering in a near-surface region of the sample. The escape probability as a function of depth of origin for the S 2s photoelectrons at α=0° exhibited a non-monotonic behaviour with a maximum beneath the surface at a depth of 0.6–0.8 nm. This complex behaviour cannot be explained within models neglecting elastic scattering of photoelectrons. In contrast, the escape probability of the S 2s photoelectrons measured for α=39°, 60° and the escape probability of the S 2p photoelectrons measured for α=0°, 39° showed a monotonic dependence decreasing with the depth that can be approximated by a simple exponential function. The mean escape depth values are determined from the measured and calculated escape probabilities of S 2s and S 2p photoelectrons.

Measurements of the escape probability of photoelectrons and the inelastic mean free path in silver sulphide

The escape probabilities of S 2s and S 2p photoelectrons leaving an Ag2S surface in a direction parallel to the photon flux and the inelastic mean free path values in the range 200-1600 eV have been determined experimentally. The escape probability as a function of depth of origin for the S 2s photoelectrons exhibits non-monotonic behaviour, with a maximum at a depth of 0.6-0.8 nm. In contrast, the escape probability of the S 2p photoelectrons leaving the surface in the same direction can be approximated by an exponential function. The inelastic mean free path values determined by elastic peak electron spectroscopy agree well with the values resulting from the G1 predictive formula of Gries.

Experiments to determine the escape probability of photoelectrons

A novel experimental method for the assessment of the escape probability of photoelectrons as a function of depth and their mean escape depths is described and illustrated for photoelectrons leaving an aluminium oxide surface. The results are compared with those of the straight line approximation model calculations.

Escape probability of signal photoelectrons from non-crystalline solids: influence of anisotropy of photoemission

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 of photoelectrons: a major energy resolution degrading mechanism in thin superconducting tunnel junction X-ray detectors

The authors consider the consequences of fabricating integral X-ray detectors with thin electrodes, 0.17 mu m for Nb and 0.25 mu m for Sn, with 6-keV photons incident. A simple geometric argument demonstrates that energetic electron loss during the first 1 ps of each event will cause less than the full energy of the incident photon to be captured in a large fraction of the events. This reduces the ultimate energy resolution the devices can achieve. Use of thicker absorbers (electrodes) is thus highly desirable, both from this point of view and to increase the quantum efficiencies of the devices. Nb electrodes should be about 0.8- mu m thick to reduce the escape probability of photoelectrons of energy >or=5 eV during the first 1 ps of an event. >

38 P 05 Escape probability of photoelectrons from particles

38 P 05 Escape probability of photoelectrons from particles

Dependence of ESCA signal intensities of ionic compounds on chemical environment

Variation in ESCA signal intensities for ionic compounds is interpreted in terms of the dependence of photoelectron escape probability on the immediate environment of the emission center. A model has been developed to estimate this escape probability by the fractional solid angle not blocked by the nearest neighbors of the emission center. A linear correlation is found between the normalized intensity ratios for simple alkali halides and the fractional escape solid angle ratios. Absolute intensity measurements have been carried out for alkali perchlorates and alkali chlorides. The chlorine signal intensities were constant for perchlorates but decrease steadily from LiCl to CsCl. These observations are consistent with the chlorine atom in the perchlorate groups always being surrounded by four oxygens; therefore its fractional escape solid angle is independent of the compound.

Escape Probability of Photoelectrons into Vacuum

The spectral response of photocathodes is severely degraded toward the ir portion of the optical spectrum. This paper compares the quantum efficiency of the S-11 and the S-20 photocathodes, based on an absorption diffusion model, with their quantum efficiency as reported by the ITT spectral response curves. This comparison (ratio) shows that the irdegradation is due primarily to the poor escape probability of a low energy photoelectron over the surface potential barrier rather than degradation in absorption. This ratio is used to estimate an escape probability that is plotted as a function of photon energy.

Spectral response of antimony-caesium photocathodes

Measurements have been made of the relative photoelectric quantum efficiency η(λ) of SbCs3 cathodes of photomultipliers with Pyrex and quartz windows, from λ = 200-650 mμ. η(λ) depends on the window transmittance W(λ), the cathode absorbance A(λ, d), the absolute photoelectric quantum efficiency Q0(λ) and the photoelectron escape probability f(λ, d). Comparison with other data has allowed the separation of these factors, yielding inter alia the absorption spectrum of SbCs3 down to λ = 220 mμ, and the absolute photoelectric threshold curve Q0(λ) plotted against λ. The optimum thickness for a thin cathode is d = 200 A. Errors in other published data are noted, and methods of increasing η(λ) and of improving scintillation detectors are discussed.