Record-level quantum efficiency from a high polarization strained GaAs/GaAsP superlattice photocathode with distributed Bragg reflector

Abstract

Photocathodes that provide high electron-spin polarization (ESP) and high quantum efficiency (QE) can significantly enhance the physics capabilities of electron accelerators. We report recordlevel QE from a high-polarization strained GaAs/GaAsP superlattice photocathode fabricated with a Distributed Bragg Reflector (DBR). The DBR photocathode technique enhances the absorption of incident laser light thereby enhancing QE, but as literature suggests, it is very challenging to optimize all of the parameters associated with the fabrication of complicated photocathode structures composed of many distinct layers. Past reports of DBR photocathodes describe high polarization but typically QE of only $1%, which is comparable to QE of high polarization photocathodes grown without a DBR structure. This work describes a strained GaAs/GaAsP superlattice DBR photocathode exhibiting a high polarization of 84% and significantly enhanced QE of 6.4%.

Enhanced photocathode performance through optimization of film thickness and substrate

“…Generally, the discussed principles underlying interference enhanced photoemission should be valid for other materials combinations as well to various degree, 16,17 specifically depending on the complex refractive index variation (which in turn is wavelength sensitive) between the film and the substrate. A general rule of thumb may be that, at a given wavelength, the r PCÀS should be large to minimize the energy transmitted into the substrate.…”

“…Consequently, most photocathode films (typically deposited on stainless steel or tungsten substrates >100 lm thick) in usage are of the order of 10-50 nm thick, [10][11][12][13] with larger thickness being used for higher absorption due to presumed reduced influence of grain boundaries, or are optimized in situ during film growth, 14 without much quantitative rationale. 10 Additionally, insufficient attention 15 has been paid to the specific geometric influences of the film and thickness as a function of a specific substrate, with respect to possible interference effects, 16,17 which may modulate significantly both the incident light absorption and the electron emission efficiency. Enhanced light absorption arises through carefully matching the impedance of the photocathode-substrate system to that of the vacuum to which electrons are emitted.…”

Long lifetime polarized electron beam production from negative electron affinity GaAs activated with Sb-Cs-O: Trade-offs between efficiency, spin polarization, and lifetime

“…We can define a figure of merit (FOM) for GaAs photocathodes for spin polarized electron beam production to include a factor that takes into account the lifetime gain induced by the activating layer [6]:…”

“…Additionally, spin polarized electron sources are critical for various forms of electron microscopy capable of probing the magnetic properties of materials [4,5]. GaAs superlattice (SL) based photocathodes are generally considered the best option for spin polarized electron sources: advanced band structure engineering has enabled photocathode structures capable of providing larger than 80% spin polarization and quantum efficiency (QE) of a few percents at the same wavelength [6]. The operation of all photocathodes based on GaAs with photon energy near the band gap energy is possible only by activation to negative electron affinity (NEA) conditions, which is achieved at the vacuum interface of the photocathode upon exposure to Cs and oxygen or NF 3 [7,8].…”

Energy Bandgap Engineering of Transmission-Mode AlGaAs/GaAs Photocathode

“…Since negative-electron-affinity (NEA) GaAs photocathode was proposed as a type of excellent photoemitter by Scheer and Laar [1], GaAs-based photocathodes have found widespread applications in photodetectors, accelerators, electron microscopes, photon-enhanced thermionic emission devices, and other fields [2][3][4][5]. In view of the high visible spectral response, good spectral extensibility to the near infrared (NIR) region and low dark current, NEA GaAs, GaAsP, and InGaAs photocathodes are important components in the vacuum photodetectors, for example, low-light-level (LLL) image intensifiers, photomultiplier tubes, and streak tubes [6].…”