E. Shefer

Electron field emission for ultrananocrystalline diamond films

Ultrananocrystalline diamond (UNCD) films 0.1–2.4 μm thick were conformally deposited on sharp single Si microtip emitters, using microwave CH4–Ar plasma-enhanced chemical vapor deposition in combination with a dielectrophoretic seeding process. Field-emission studies exhibited stable, extremely high (60–100 μA/tip) emission current, with little variation in threshold fields as a function of film thickness or Si tip radius. The electron emission properties of high aspect ratio Si microtips, coated with diamond using the hot filament chemical vapor deposition (HFCVD) process were found to be very different from those of the UNCD-coated tips. For the HFCVD process, there is a strong dependence of the emission threshold on both the diamond coating thickness and Si tip radius. Quantum photoyield measurements of the UNCD films revealed that these films have an enhanced density of states within the bulk diamond band gap that is correlated with a reduction in the threshold field for electron emission. In additio...

Optical and photoemission studies of DLC films prepared with a systematic variation of the sp3:sp2 composition

Abstract The optical and photoemission properties of hydrogen-free DLC films prepared under conditions which result in different types of carbon bonding (previously determined from electron energy loss spectroscopy) are reported. The films were prepared using a mass selected ion beam deposition system covering the C+ energy range 10 eV–2 keV, mostly on Si held at room temperature. Previous measurements have shown that the sp3 content of these films varied from 0 to >80%. The optical constants (n, k, absorption coefficient (α)) of these films were measured by ellipsometry. The energy gaps were derived from Tauc plots (Eg) and from α = 104 cm−1 values (E04). The energy gaps were found to vary with the sp3 content from E

Absolute quantum photoyield of diamond thin films: Dependence on surface preparation and stability under ambient conditions

Absolute quantum photoyield (QPY) measurements (140–210 nm) of chemical vapor deposited (CVD) diamond films are reported. The dependence of the QPY on hydrogenation by exposure to a hydrogen microwave (MW) plasma and oxidation by a mixture of acids or on exposure to air under ambient conditions have been studied. Films deposited by MWCVD display a higher QPY than those grown by hot filament (HF) CVD. The QPY values are found to depend on the state of the surface. Hydrogen-terminated films exhibit values above 12% at 140 nm, whereas even small amounts of oxygen strongly degrade the QPY. B-doping, at the level of 1500 ppm, has no apparent effect on the photoemission properties. Exposure of the hydrogenated films to ambient conditions results in oxygen adsorption, leading to degradation of the photoemission properties. Analysis of the data within the three-step model of photoemission clearly shows that the state of the surface is a dominant factor determining the QPY.

STUDY OF CSI PHOTOCATHODES : VOLUME RESISTIVITY AND AGEING

Abstract Wire-chamber gaseous electron multipliers coupled to CsI photocathodes, provide interesting means for UV-photon imaging over a very large area. We report on the measurement of the CsI resistivity, important for the understanding of rate-dependent effects in CsI-based detectors. Photon counting at high rates is also closely connected to long-term photocathode stability. We present results on ageing of CsI photocathodes under high photon flux, with one atmosphere CH 4 and under a detector gain of 10 5 , for several substrate materials. We compare the ageing results with that of CsI photocathodes coated with thin NaF protective films.

On the two-stage operation mechanism of low-pressure microstrip gas chambers

A two-stage multiplication mechanism has been observed in microstrip gas chambers (MSGC) operated in a pressure range of 10–50 Torr of isobutane. The resulting high gain (> 104) of single electrons photo-produced on a CsI photocathode is attributed to a preamplification in the gas gap followed by anode strip multiplication. The large and fast rise of the induced pulse in this mode leads to an efficient single electron detection at relatively low charge gain. The reduced positive ion feedback preserves radiation convertors coupled to such electron multipliers from sputtering damage. MSGCs operated in this mode are expected to have a subnanosecond time resolution and very high rate capability. Some potential applications are briefly discussed.

The protection of KCsSb photocathodes with CsBr films

Abstract We report on photoemission properties of visible KCsSb reflective photocathodes coated with thin CsBr protective films. We present the absolute quantum efficiency and its stability under prolonged contact with oxygen. It is found that KCsSb photocathodes coated with 280 A thick CsBr films have 5% quantum efficiency at 312 nm and can withstand an exposure to 150 Torr of oxygen for half an hour. We discuss the possible role of good lattice constants matching between the photocathodes and the protecting film.

Photoelectron transport in CsI and CsBr coating films of alkali antimonide and CsI photocathodes

The transport of low-energy (<3 eV) photoelectrons in CsI and CsBr films was investigated by direct photoionization in the film and by photoinjection of electrons from underlying K–Cs–Sb, Cs3Sb, and CsI photocathodes. Photoelectron energy distributions and the photoyield dependence on film thickness were studied, assisted by in situ x-ray photoelectron spectroscopy surface characterizations. A model describing electron transport through the coating film was used, which correlated well with experimental results from the various material combinations, coating thickness, and photon energies. The model provides valuable information on the interface potential barrier of theses systems, as well as quantum-yield attenuation length and photoelectron energy distributions.

Absolute photoyield from chemical vapor-deposited diamond and diamond-like carbon films in the UV

The absolute photoyields of chemical vapor deposited (CVD) diamond and amorphous hydrogen-free diamondlike carbon (DLC) films, in the range of 140–300 nm, are reported. CVD diamond films exhibit a large photoyield, of a few percent in the range 140–180 nm. DLC films have a 20–50 times lower yield. Post growth hydrogenation is found to substantially increase the photoyield of CVD diamond films. We discuss the applicability of these films as UV photocathodes coupled to electron multipliers based on gaseous charge multiplication.

Evidence for thin‐film protection of visible photocathodes

NaI and CsI protective coatings on visible cesium‐antimony photocathodes have been studied. With NaI protective films the photocathodes are shown to withstand exposure to considerable doses of oxygen and dry air. This opens the way to their handling and operation in gas media.

LABORATORY PRODUCTION OF EFFICIENT ALKALI-ANTIMONIDE PHOTOCATHODES

Abstract An experimental setup and procedures for the preparation of high-quality reflective and semitransparent alkali-antimonide photocathodes in laboratory conditions is described. Absolute quantum efficiency curves for KCsSb and Cs 3 Sb photocathodes are presented in the spectral range of 250–550 nm; their average quantum efficiency values at 312 nm are 32% and 15%, respectively. The photocathodes are stable in vacuum and in high-purity methane over a few days period. The system also permits photocathode coating with protective films.