High current density GaN∕CsBr heterojunction photocathode with improved photoyield

Abstract

A four fold improvement in photoyield has been observed in GaN films coated with CsBr films (CsBr∕GaN) relative to CsBr∕Cr photocathodes reported in previous studies. A model is presented involving photoemission from intraband states in the CsBr film and direct electron injection through the CsBr film from the GaN substrate. The lifetime of the films at high current density >90A∕cm2 is limited by the temperature rise in the GaN films caused by the high photon absorption at 257nm. The lifetime can be improved by utilizing a cooled substrate or a high thermal conductivity substrate under the GaN films.

Quantum efficiency enhancement in CsI/metal photocathodes

“…F center defects) created by laser activation could possibly contribute to the QE enhancement in alkali halide/metal systems [6,8,10]. F center states are located at 3.37 eV and 1.9 eV below the bottom of conduction band of CsBr [10,18] and CsI [19,20] respectively (below the respective Fermi levels by roughly 1.0 and 1.3 eV). Since both F bands are located below the respective metal Fermi levels, we expect F center states to be 'full' for activated alkali halide/metal systems.…”

RobustCsBr/Cuphotocathodes for the linac coherent light source

“…This initial behavior is expected from the creation of color centers in the material and the transfer of Cs to the surface as described in Refs. [3][4][5][6], and differs from the UV surface cleaning shown in Fig. 1 samples was measured before coating.…”

“…of many hundreds of hours when illuminated with 257 nm (4.8 eV) radiation either in the transmission or reflection modes [4]. Operation below the band gap of CsBr ( 7:3 eV) was attributed [3][4][5][6] to (i) intraband states present in the CsBr layer with an energy about 3.7 eV below the conduction band, and (ii) the direct electron injection from the metal to CsBr layer. Electrons occupying those intraband states can perform transitions to the continuum with an incident photon energy of only 4.8 eV.…”

“…In the case of CsBr=GaN photocathodes, due to the better alignment of the conduction bands of CsBr and GaN substrate, the direct electron injections from the GaN substrate through the CsBr film are enhanced. Thus CsBr=GaN cathodes offer a large enhancement in the quantum efficiency relative to CsBr=Cr photocathodes [5,6].…”

Silver nanorod arrays for photocathode applications

“…For example, thin films of CsBr grown on flat metal photocathodes have been shown to increase QE by one to three orders of magnitude under ultraviolet (UV) laser irradiation. [5][6][7][8][9] A second strategy uses plasmonic field enhancement to increase the photoemission yield by incorporating metal nanostructures into the photocathode surface. Enhanced photoemission yields from sub-wavelength rectangular groove arrays (gold nanogratings) 10 show a dramatic photoelectron yield increase of greater than 6 orders of magnitude following 800 nm fs pulsed laser excitation over a flat gold surface.…”