p-GaAs(Cs,O)-photocathodes: Demarcation of domains of validity for practical models of the activation layer

Abstract

The (Cs,O)-activation procedure for p-GaAs(Cs,O)-photocathodes was studied with the aim of demarcating the domains of validity for the two practical models of the (Cs,O)-activation layer: The dipole layer (DL) model and the heterojunction (HJ) model. To do this, the photocathode was activated far beyond the normal maximum of quantum efficiency, and several photocathode parameters were measured periodically during this process. In doing so, the data obtained enabled us to determine the domains of validity for the DL- and HJ-models, to define more precisely the characteristic parameters of the photocathode within both of these domains and thus to reveal the peculiarities of the influence of the (Cs,O)-layer on the photoelectron escape probability.

Non–monotonic behaviour in the mean transverse energy of electrons emitted from a reflection–mode p-GaAs(Cs,O) photocathode during its QE degradation through oxygen exposure

“…Figure 2 also shows that in region ε lon < ε cb the LEDCs are bell-shaped, while for ε lon > ε cb the LEDCs have an exponential character of the form N e (ε tr ) = A • exp − εtr ε0 where the value of ε 0 varies from 33 meV for the black trace to 27 meV for the orange trace. An LEDC of this shape is typical for a GaAs-photocathode [4,11]. Figure 2 also shows that as for a GaAsP photocathode [7], degradation through exposure to oxygen of a GaAs photocathode leads to a progressive shift in the position of the left-hand side of the LEDC without any significant change in the amplitude of the envelope curve.…”

“…Prior to first use the oxide layer was removed from the active top layer using a solution of HCl in isopropanol under an inert nitrogen atmosphere [2], and the photocathode puck then transferred into vacuum using a suitcase without subsequent exposure to the atmosphere. Following an in-vacuum heat clean, it was activated in our III-V Photocathode Preparation Facility [3] (PPF) following accepted procedures [2,4], achieving a quantum efficiency around 10.0% at 635 nm.…”

A test system for optimizing quantum efficiency and dark lifetime of GaAs photocathodes

“…The NEA activation is performed by either co-depositing Cs and O 2 (CO-DE) or Cs, Li and O 2 (CO-DE-Li), or alternating between the components (referred to as Nagoya activation [35]) while monitoring the photocurrent until it peaks. The two-stage (TS) activation [36] follows an activation-annealing-activation cycle. During the activation, the chamber pressure due to Cs/Li evaporation is not allowed to exceed 5 × 10 −10 mbar for Cs+O 2 deposition, and 5 × 10 −9 mbar for Cs/Li+O 2 deposition.…”