Photoemission Quantum Yield Research Articles

Prospect for high brightness III–nitride electron emitter

We describe p-type gallium nitride (GaN) as a candidate for high brightness photocathodes. Experiments utilizing photoemission spectroscopy and quantum yield measurements were performed on GaN films to characterize various cesium and oxygen activations. Quantum efficiencies of 0.1%–4% were obtained in reflection for the cesiated p-type 0.5 μm thick GaN films and 25%–50% on the 0.1 μm thick GaN films. The corresponding emission currents are 142–300 nA for 0.5 μm thick films and 0.7–1.3 μA for the 0.1 μm thick films. This results in an increase of several orders of magnitude in the emission current from the starting GaN films. Furthermore, an initial desorption measurement was performed in order to evaluate the Cs binding strength to GaN relative to GaAs. We observe Cs was bound to the GaN surface (0001_) at 700 °C and completely desorbed at 450 °C for a (100) GaAs surface. Finally, an alternate barium activation on GaN is included for preliminary comparison with the various cesium activations.

Escape length of ultraviolet induced photoelectrons in alkali iodide and CsBr evaporated films: Measurements and modeling

The escape length of electrons photoinduced from thin CsI, KI, RbI, NaI, and CsBr evaporated films was measured in the 140–180 nm photon spectral range. Theoretical model predictions of the escape length value are in fair agreement with the experimental results. They vary between 10 and 40 nm, the highest values being for CsI, RbI and CsBr. For CsI, measured and calculated ultraviolet-induced escape length values are consistent with that determined from x-ray photoemission quantum yield data. Post-evaporation annealing of the films had no major impact on the measured electron transport properties.

Emission of spin-polarized electrons from strained InGaP and InGaAsP photocathodes

Spectra of photoemission quantum yield and electron spin polarization are measured on the wide-bandgap epitaxial InGaP and InGaAsP layers, which are promising for use as high efficient and stable sources of polarized electrons. Pseudomorphic epitaxial layers with a thickness of 100–600 nm and band gaps of 1.4-1.9 eV are grown on GaAs substrates with various magnitudes of lattice mismatch and, thus, with various in-plane compressive strains. The magnitude of the strain-induced splitting of the valence band is measured by the photocurrent spectroscopy. The surface preparation technique includes removal of oxides in the solution of HCl in isopropanol and transfer to the Mott polarimeter under nitrogen atmosphere, followed by final heat cleaning in UHV. For thin ( d = 100–200 nm) highly strained pseudomorphic layers a polarization up to P = 0.66 is obtained. Due to strained-induced splitting of the valence band this value exceeds the theoretical limit P 0 = 0.5 for the unstrained material. Although the magnitudes of electron spin polarization are lower than those reported earlier for strained GaAs layers, high photoemission quantum yield Y in InGaAsP layers provide high values of the combination P 2 Y that is directly related to the figure of merit of polarized electron beam. Since March, 1995 the InGaP photocathodes are successfully explored in the polarized electron source at Mainz Microtron.

Evolution of electronic properties at the p-GaAs(Cs,O) surface during negative electron affinity state formation

The evolution of surface band bending and surface photovoltage was monitored in situ by photoreflectance spectroscopy during activation of the surface of epitaxial GaAs to the state of negative electron affinity by successive deposition of cesium and oxygen in a standard ‘‘yo-yo’’ technique. Considerable variations of the band bending (by approximately 0.3 eV) and surface photovoltage (by three orders of magnitude) were observed. It was found that the maximum of photoemission quantum yield corresponded to unexpectedly small value of the band bending φs=0.3 eV, as compared to widely accepted value of approximately 0.5 eV.

Response time of photoemission of ultrafine particle thin film

The response time of photoemissive materials is required for detecting ultrashort duration laser pulses. In this paper, the response time of photoemission in ultrafine particle thin film is studied, and the transit time spread (TTS) and response time of peak value (tM) are discussed. The photoelectron's response time will increase with increasing energy of photons. If the surface potential barrier of thin film declines, the photoemissive sensitivity or quantum yield will rise, however the response time will also increase, which means that response-time characteristic gets worse. As an example, the response time for Ag-O-Cs thin film is calculated for different cases when photoelectrons, excited in Ag ultrafine particles, travel through Cs2O semiconductor layer to the surface and escape into vacuum. The calculated response time is about 50 fs if this thin film is irradiated by infrared rays of wavelength 1.06 μm.

Asymmetry of photostimulated exoemission in ferroelectrics

Abstract It is shown that photostimulated exoemission in ferroelectrics is substantially asymmetric. Asymmetry is exhibited in the dependence of emission current on the direction of the crystal polar axis. Consideration is given to the nature of the phenomenon of emission asymmetry which is determined by two factors: a) bending of zones having opposite signs arising near the polar cuts as a result of the screening of the spontaneous induction field; b) existence of asymmetric periodic potential in an acentric polar crystal. The asymmetry parameters have been measured by registering the orientational and spectral dependences of the photoemission quantum yield. The possibility is shown of studying the asymmetry of elementary electronic processes in ferroelectric LiNbO3:Fe excited by light which does not produce an extrinsic photoeffect, but induces on the crystal surface a photovoltaic field with an intensity of 106 V.cm−1. The emission current proceeding from the cut perpendicular to the positive direction...

Reflection spectra and quantum yield of photoemission of nitrates and chlorinates of alkali metals in the energy range from 10 to 30 eV

The reflection spectra and quantum yield photoemission of CsNO 3, NaNO 3, NaClO 3 and KClO 3 crystals have been measured at photon energies from 10 to 30 eV in the VUV beam line at the 1.5 GeV synchrotron SYRIUS. The observed structure at 12–18 eV in the reflection spectra of CsNO 3 and KClO 3 is connected with the excitation of a cation exciton. The peculiarities in the reflection spectra of NaNO 3 and NaClO 3 are determined by interband transitions. The processes of the multiplication of electron excitation are observed in the high-energy region of the quantum yield spectrum of CsNO 3 and KClO 3.

Transport of nonequilibrium electrons in photoemission from semibounded medium

The equation is formulated and accurately solved for the problem of the transport of photoexcited electrons which undergo scattering before reaching vacuum, with the formation of an electron-hole pair, as well as multiple scattering of elastic type. The analytical expressions obtained permit calculations of the spectral composition of the emitted-electron flux and the quantum yield of photoemission. The limiting expression for the probability that secondary electrons will reach the vacuum which is obtained when the elastic-scattering mechanism is switched off is compared with literature data.

Photoelectron emission from metal into concentrated electrolyte: Part I. Photoemission threshold

Current-voltage characteristics of photoemission from mercury into concentrated solutions of LiCl (1.0–14.06 M) KCl (1.0–4.15 M), and CsCl (1.0–6.8 M) have been measured. It is shown that a “ 5 2 -law” is valid for all solutions, and quantum yield of photoemission does not depend on the nature and concentration of the electrolyte. The photoemission method is applicable for studies in high-concentration electrolytes. In order to remove the diffusional potential drop, the photoemission threshold was measured against a calomel electrode with the same electrolyte content, and then recalculated to 1 M solution of the corresponding salt by means of the Nernst equation, thus avoiding concentrational potential drop. Photoemission threshold values thus obtained are approximately constant for solutions with salt concentration up to 7 M, shifting, with further increasing concentration, towards the cathode region up to 0.25 eV at 14 M of LiCl. The shift has been attributed to displacement of the bottom of the delocalized (quasi-free) electron band from V o towards vacuum level with increasing concentration of the electrolyte, when the greater part of the water molecules is bound in hydrated ion shells. A comparison between the changes in V o values and absorption spectra of the hydrated electron indicate that the change in solution content mentioned increases the medium reorganization energy in the process of localized state formation.

Measurements of the photoemission spectra of oxygen-free and oxygen-doped p-terphenyl single crystals in the vacuum UV region

The energy distribution curves (EDCs) and photoemission yield curves (PYCs) of electrons photoemitted from oxygen-free and oxygen-doped p-terphenyl single crystals were measured in the photon energy range 6.0–10.3 eV. The oxygen impurities acted both to suppress (via ejection of electrons from the valence bands and impurity levels) and to enhance (via relaxation processes such as the Auger effect) photoemission. The first and second onset energies of electron ejection were estimated from the PYC spectra to be about 6.9 eV and 7.7 eV respectively. New peaks which depended on electron relaxation rather than electron ejection appeared in the low kinetic energy region of the EDC spectra. The presence of oxygen impurities leads to a reduction in the threshold energies of photoemission and in the quantum yield of photoemission from the crystal surface. The oxygen impurities also affect the electronic potential energy of the valence bands.

Electronic properties of polymers. Anisotropic light absorption and photoelectron emission of oriented polyethylene films in the vacuum ultraviolet region

Anisotropic optical absorption of oriented ultrathin films, (down to 50 nm thickness) of low density polyethylene (LDPE) was measured in the vacuum ultraviolet (VUV) region between 6 and 12 eV. Clear anisotropy was found in the observed spectra. The lowest-energy absorption feature at 9 eV was polarized perpendicular to the chain axis, and the second at about 11.5 eV parallel to the chain axis. The edge of the lowest perpendicular absorption was determined to be 8.05±0.1 eV. The usefulness of the anisotropic absorption data in testing two theories was verified; the anisotropy predicted by the independent system model disagrees with the data, but the energy band model gives some promising results. The photoemission threshold of the oriented LDPE was determined to be 8.5±0.1 eV. The spectral dependence of the anisotropy ratio of the photoemission quantum yield was found to coincide with that of the optical absorption in the energy range above 9.5 eV. This coincidence was explained in terms of the three-step model of photoemission.

Photoemission study of negative-electron-affinity GaP

The optical absorption coefficient and photoemissive quantum yield were measured for liquid-phase-epitaxial GaP layers containing 2% of indium. The transport properties of photogenerated electrons in NEA GaP was analyzed by the two-valley diffusion model of Xc1 and Lc1 conduction bands and compared with experimental results. From the results of the analysis, diffusion lengths and escape probabilities were determined for NEA GaP with net acceptor concentrations of 2.03×1016 to 3.5×1019/cm3.

Investigation of the photoelectric and photoluminescent properties of crystalline quartz and vitreous silica in the fundamental absorption region. A model for electronic structure and migration of energy in SiO2

AbstractElectronic structure and energy migration in SiO2 are studied by the experimental method. The spectral dependences of the related quantum yield of external photoemission, of photoconductivity, and of energy transfer to the luminescence centres are investigated in crystalline quartz and vitreous silica under irradiation by photons with energy from the fundamental absorption region. The direct allowed transition energy gap is found to be equal to 12 eV. The energy gap minimal value is estimated to be 9.5 eV. The fundamental absorption bands of SiO2 at 10.4 and 11.6 eV are shown to be due to excitons, capable of migrating in the „hot”︁ non‐thermalized state and transferring energy to the luminescence centres. Electron‐hole pairs transfer energy in thermalized state with lattice.

ChemInform Abstract: PHOTOEMISSION FROM POLYCYCLIC AROMATIC CRYSTALS IN THE VACUUM‐ULTRAVIOLET REGION. V. PHOTOELECTRON SPECTROSCOPY BY THE RARE GAS RESONANCE LINES AND VACUUM‐ULTRAVIOLET ABSORPTION SPECTRA

The photoelectron spectra (energy distribution curves, ECDs) of naphthacene, pentacene, perylene, coronene, and p-terphenyl by He(I), Ne(I), and Ar(I) resonance lines (photon energies, 11.62–21.22 eV) and the absorption spectra of naphthacene, pentacene, perylene, and coronene at 4–9.5 eV are measured for evaporated polycrystal films. The absorption spectra are compared with the spectral dependence of the quantum yield (SDQY) of photoemission. A similar comparison is also made for p-terphenyl and anthracene. The origin of the structures in SDQY curves is discussed. The analysis reveals that: (1) the MOs of a molecule are preserved even in crystals; (2) the large peak at a low kinetic energy of the photoelectron spectra is due to secondary electrons, and (3) at least for naphthacene, pentacene, and coronene, autoionization takes place with a considerable efficiency.

Photoemission from Polycyclic Aromatic Crystals in the Vacuum-Ultraviolet Region. V. Photoelectron Spectroscopy by the Rare Gas Resonance Lines and Vacuum-Ultraviolet Absorption Spectra

Abstract The photoelectron spectra (energy distribution curves, ECDs) of naphthacene, pentacene, perylene, coronene, and p-terphenyl by He(I), Ne(I), and Ar(I) resonance lines (photon energies, 11.62–21.22 eV) and the absorption spectra of naphthacene, pentacene, perylene, and coronene at 4–9.5 eV are measured for evaporated polycrystal films. The absorption spectra are compared with the spectral dependence of the quantum yield (SDQY) of photoemission. A similar comparison is also made for p-terphenyl and anthracene. The origin of the structures in SDQY curves is discussed. The analysis reveals that: (1) the MOs of a molecule are preserved even in crystals; (2) the large peak at a low kinetic energy of the photoelectron spectra is due to secondary electrons, and (3) at least for naphthacene, pentacene, and coronene, autoionization takes place with a considerable efficiency.

Band bending effect on the thermal dependence of photoemission from Li activated Si

The photoemissive properties of lithium activated silicon are studied in the temperature range 25°C to 500°C. The Li activation is obtained by surface segregation of bulk dissolved Li. The Li layer seems to be highly stable when Si is heated up to 500°C. On the other hand, the photoemission quantum yield changes with temperature. In order to get complementary information, work function measurements are performed on the same samples. Both photoemission and work function measurements allow us to describe the whole energy picture at the surface of n and p type samples and lead to the conclusion that band bending effect is responsible for the thermal dependence of the yield.

Charge storage in SiO2 under low-energy electron bombardment

Low-energy (≃ 10 eV) electron bombardment on an SiO2 layer gives rise to strong trapping at the surface and to a positive space-charge build-up near the Si–SiO2 interface. The density, the capture cross section, and the energy level of the traps have been evaluated using, respectively, the constant current charging process and the quantum yield of vacuum photoemission versus wavelength.

Energy level structure of ionized states of adenine in the solid state

The quantum yield of photoemission and the relative quantum efficiency of photoconductivity of vacuum deposited thin layers of adenine were measured at room temperature. The permittivity of pressed powders of adenine was investigated in the frequency range 25 to 10 000 Hz. With the aid of the values of photoemission threshold, Ic = (6.4 ± 0.15)eV, the quantum efficiency threshold of photoconductivity, Eg = (3.45 ± 0.2)eV, and polarization energy as calculated from permittivity, P = (1.6 ± 0.2) eV, the energy diagram of the ionized states in adenine is determined. [Russian Text Ignored]

Segregation of dissolved Li to the surface of Si: A new activation process

Photoemission measurements have been performed on Li covered Si samples. The Li layer is formed: (i) by adsorption, (ii) by segregation of dissolved Li to the Si surface. These two methods of activation give comparable results: the photoemissive quantum yield is about 10 per cent at 5 eV and the threshold is 2.5 eV for p type Si. Moreover, the surface layer obtained by segregation seems highly pure and uniform. The segregation kinetics are analyzed using a simple theoretic model.

Simple internal photoemission method for determining shape of yield curves of negative electron affinity semiconductors

Internal photoemission quantum yield curves using a water electrolyte Schottky contact have been shown to be identical to vacuum photoemission activation yield curve shapes near threshold for negative electron affinity semiconductors GaAs, Ga0.81In0.19As, and GaAs/Ga0.52In0.48P/GaP. Reflection and semitransparent cathode Schottky yield curves can be generated on the bench in a matter of minutes.