Surface science analysis of GaAs photocathodes following sustained electron beam delivery

Abstract

Degradation of the photocathode materials employed in photoinjectors represents a challenge for sustained operation of nuclear physics accelerators and high power free electron lasers (FEL). Photocathode quantum efficiency degradation is due to residual gases in the electron source vacuum system being ionized and accelerated back to the photocathode. These investigations are a first attempt to characterize the nature of the photocathode degradation, and employ multiple surface and bulk analysis techniques to investigate damage mechanisms including sputtering of the Cs-oxidant surface monolayer, other surface chemistry effects, and ion implantation. Surface and bulk analysis studies were conducted on two GaAs photocathodes, which were removed from the JLab FEL DC photoemission gun after delivering electron beam, and two control samples. The analysis techniques include helium ion microscopy, Rutherford backscattering spectrometry (RBS), atomic force microscopy, and secondary ion mass spectrometry (SIMS). In addition, two high-polarization strained superlattice GaAs photocathode samples, one removed from the continuous electron beam accelerator facility (CEBAF) photoinjector and one unused, were also analyzed using transmission electron microscopy (TEM) and SIMS. It was found that heat cleaning the FEL GaAs wafer introduces surface roughness, which seems to be reduced by prolonged use. The bulk GaAs samples retained a fairly well organized crystalline structure after delivering beam but show evidence of Cs depletion on the surface. Within the precision of the SIMS and RBS measurements, the data showed no indication of hydrogen implantation or lattice damage from ion back bombardment in the bulk GaAs wafers. In contrast, SIMS and TEM measurements of the strained superlattice photocathode show clear crystal damage in the wafer from ion back bombardment.

Nb Texture Evolution and Interdiffusion in Nb/Si-Layered Systems

“…Another possible reason for a drop in the low-energy tail intensity close to the surface peak is the “shadowing” of sub-surface atoms by surface atoms, as shown in Figure a, which leads to the channeling of He neutrals through sub-surface layers into bulk layers without backscattering. This effect is normally observed in single-crystalline layers under a specific geometry, and it results in a strong reduction in the backscattering contribution from the sub-surface layers. , During the growth of the metal on Si layers in metal/Si multilayers, an abrupt crystallization of the metal layer is usually observed around an as-deposited metal thickness of 2–3 nm, resulting in typically 1 nm of the crystalline metal layer on top of an amorphous metal-Si intermixed zone . In order to verify the Nb crystallization step around 2.1 nm, a LEIS sputter profile measurement was performed on a Nb-on-Si sample after the crystallization step.…”

Increasing charge lifetime in dc polarized electron guns by offsetting the anode

“…The higher energy ions will penetrate into the crystal and will result in lattice distortion and back sputtering while low energy ions may sputter off the surface atoms of activation layers [24]. Experimental studies have been performed to understand the surface layer damage after beam delivery but there is no conclusive quantitative model to describe the damage rate as the function of ion energy [25]. Therefore for simplicity, we assume that the weight of ions energy damage effect (S) is one.…”

Toward a long-lifetime polarized photoelectron gun for the Ce+BAF positron source

“…It is degraded by a variety of damage mechanisms, many of which have been identified and reduced to negligible levels over the years [7,8]. The dominant remaining effect is ion back-bombardment, i.e., electron-impact ioniza-tion of residual gas molecules and subsequent acceleration of the ions toward the cathode, where they degrade the QE through sputtering and implantation [9,10]. The number of ions generated is proportional to the extracted charge 𝑄, giving rise to an exponential decay with a charge lifetime 𝜏 according to 𝜂(𝑄) ∝ exp(−𝑄/𝜏).…”