Monte Carlo charge transport and photoemission from negative electron affinity GaAs photocathodes

Abstract

High quantum yield, low transverse energy spread, and prompt response time make GaAs activated to negative electron affinity an ideal candidate for a photocathode in high brightness photoinjectors. Even after decades of investigation, the exact mechanism of electron emission from GaAs is not well understood. Here, photoemission from such photocathodes is modeled using detailed Monte Carlo electron transport simulations. Simulations show a quantitative agreement with the experimental results for quantum efficiency, energy distributions of emitted electrons, and response time without the assumption of any ad hoc parameters. This agreement between simulation and experiment sheds light on the mechanism of electron emission and provides an opportunity to design novel semiconductor photocathodes with optimized performance.

Review and demonstration of ultra-low-emittance photocathode measurements

“…The MTE has been characterized experimentally 3,7-9 and theoretically 10,11 for various photocathodes. However, photocathodes with MTE below 100 meV have not been routinely studied and the corresponding physics mechanisms oftentimes remain poorly understood.…”

Ab initiostudies of Cs on GaAs (100) and (110) surfaces

“…While the steps of excitation and transport are well understood, the emission of electrons into vacuum uses several ad-hoc assumptions to explain experimental results. 5 Assuming conservation of transverse momentum at the surface due to translational invariance and the small electron effective mass in the Γ-valley of the first conduction band in GaAs, the emitted electrons should exhibit very low MTE of less than 5 meV. 5,6 However, even for the best quality GaAs(100) surfaces grown using molecular beam epitaxy, experimental observations indicate MTE values of 25-100meV.…”

“…5 Assuming conservation of transverse momentum at the surface due to translational invariance and the small electron effective mass in the Γ-valley of the first conduction band in GaAs, the emitted electrons should exhibit very low MTE of less than 5 meV. 5,6 However, even for the best quality GaAs(100) surfaces grown using molecular beam epitaxy, experimental observations indicate MTE values of 25-100meV. 7 The larger MTE values have been explained by introducing an ad-hoc scattering at the surface due to the non-conservation of transverse momentum.…”

“…4 The process of photoemission from activated GaAs has been best explained using MonteCarlo electron transport simulations within the framework of Spicer's three step photoemission model. 5 This model divides the process of photoemission into three steps: (i) the excitation of electrons by photon absorption, (ii) the transport of excited electrons to the surface, and (iii) the emission of electrons reaching the surface into vacuum. While the steps of excitation and transport are well understood, the emission of electrons into vacuum uses several ad-hoc assumptions to explain experimental results.…”

Delayed photo-emission model for beam optics codes

“…As a consequence, a sizable proportion of electrons that have scattered or thermalized can under favorable circumstances contribute to the yield. 7,8 Additionally, semiconductors can have comparatively long laser penetration depths, so that photoexcited electrons have a greater distance to travel to the surface. Both affect the response time of the photocathode.…”