The transverse energy of electrons emitted from GaAs photocathodes

Bradley, D. J.; Allenson, M.B.; Holeman, B.R.

doi:10.1088/0022-3727/10/1/013

Cited by 38 publications

(24 citation statements)

References 10 publications

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“…(6) predicts a thermal emittance increase no more than 0.3%, which is negligible. When 2= ( 1, the electric field near the microsurface could be approximated as [30] …”

Section: Model Of Qe and Thermal Emittancementioning

confidence: 99%

Experimental investigation of thermal emittance components of copper photocathode

Qian

et al. 2012

Phys. Rev. ST Accel. Beams

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With progress of photoinjector technology, thermal emittance has become the primary limitation of electron beam brightness. Extensive efforts have been devoted to study thermal emittance, but experiment results differ between research groups and few can be well interpreted. Besides the ambiguity of photoemission mechanism, variations of cathode surface conditions during cathode preparation, such as work function, field enhancement factor, and surface roughness, will cause thermal emittance differences. In this paper, we report an experimental study of electric field dependence of copper cathode quantum efficiency (QE) and thermal emittance in a radio frequency (rf) gun, through which in situ cathode surface parameters and thermal emittance contributions from photon energy, Schottky effect, and surface roughness are extracted. It is found the QE of a copper cathode illuminated by a 266 nm UV laser increased substantially to 1:5 Â 10 À4 after cathode cleaning during rf conditioning, and a copper work function of 4.16 eV, which is much lower than nominal value (4.65 eV), was measured. Experimental results also show a thermal emittance growth as much as 0:92 mm mrad=mm at 50 MV=m due to the cathode surface roughness effect, which is consistent with cathode surface morphology measurements.

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“…(6) predicts a thermal emittance increase no more than 0.3%, which is negligible. When 2= ( 1, the electric field near the microsurface could be approximated as [30] …”

Section: Model Of Qe and Thermal Emittancementioning

confidence: 99%

Experimental investigation of thermal emittance components of copper photocathode

Qian

et al. 2012

Phys. Rev. ST Accel. Beams

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“…This nano-roughness develops as a result of thermal etching, surface faceting or the dissociation of GaAs which occurs at 580 • C. 8,16 . The effect of surface roughness on photoemission was previously examined 9 . The major effect of surface roughness is due to the electrons being emitted in a direction perpendicular to the local surface instead of the global normal to the surface.…”

Section: Fig 1: Afm Images Of Gaas Surfacesmentioning

confidence: 99%

Effect of nanoscale surface roughness on transverse energy spread from GaAs photocathodes

Karkare

Bazarov

2011

Applied Physics Letters

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High quantum yield, low transverse energy spread and prompt response time make GaAs activated to negative electron affinity (NEA), 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. We show that a nanoscale surface roughness can affect the transverse electron spread from GaAs by nearly an order of magnitude and explain the seemingly controversial experimental results obtained so far. This model can also explain the measured dependence of transverse energy spread on the wavelength of incident light.The need for a high brightness electron beam is well established 1 . GaAs activated to negative electron affinity via cesiation is a high quantum efficiency (QE) photocathode and can be effectively used for producing such beams 1,2 . Properties of GaAs as a photocathode have been studied for decades [3][4][5] . However, the mechanism of photoemission from these photocathodes is not well understood.Most models follow the Spicer 3-step theory 3 , and they all assume near full thermalization of electrons to the Γ valley minimum when excited with near band-gap energy photons. The difference arises when one considers the effects on the electron going through the surface (band bending and activation regions). To explain the experimental data, one approach argues that the electrons undergo sufficient scattering at the surface so that the transverse energies of the emitted electrons are of the order of 25 meV (thermal energy at room temperature) 2,5 . The other body of work, however, treats the emission process as a refraction of a Bloch wave at an ideal surface while largely ignoring scattering effects at the surface. It predicts the transverse energy of the electrons to be around 1 to 2 meV at room temperature 4,6 and the electrons are emitted in a cone with an half angle of 15 • , which is a result of the small effective mass of the electrons in the Γ valley of GaAs.Furthermore, the experimental measurements of the mean thermal energy (MTE) and thermal emittance are also inconsistent. Some groups report values of MTE close to the room temperature thermal energies of 25meV 2,5 . While others report values of measured MTE near 2meV and the 15 • angular distribution as predicted by the second model 4 . Additionally, measurements show that MTE depends strongly on the wavelength of light used for photoemission 2 . None of the existing models can quantitatively explain this dependence. MTE and normalized transverse rms emittance ( nx ) are related to the spot size of the laser (σ x ) by nx = σ x MTE/ (m e c 2 ) where m e c 2 is the rest mass energy of a free electron.In this paper, we attempt to resolve these discrepancies by considering the effects of nano-scale surface roughness of GaAs on the MTE. The surface roughness effect can explain measurement data 2 and the variation of the MTE with incident wavelength, as well as reconciles seemingly contradictory collection of data in the literature 4,5 .

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“…3 An ideal photoemitter should have a high quantum efficiency (QE), low mean tranverse energy (MTE) of the emitted electrons, a short response time, good lifetime and low sensitivity to non-ideal vacuum conditions. Despite its very stringent requirements on vacuum, GaAs activated using Cs remains an excellent photoemitter due to its high QE in visible and near infrared light and the low MTE of emitted electrons.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2015

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GaAs with an atomic monolayer of Cs is one of the best known photoemissive materials. The results of density functional theory calculations of Cs adsorption on the GaAs(100)-(4×2) galliumterminated reconstructed surface and the GaAs(110) surface are presented in this work. Coverage of up to 4 Cs atoms/nm 2 on GaAs surfaces has been studied to predict the work function reduction and adsorption energies accurately. The high mobility of Cs atoms on the (110) surface allows formation of ordered structures, whereas the low mobility of Cs of the (100) surface causes amorphous growth.

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The transverse energy of electrons emitted from GaAs photocathodes

Cited by 38 publications

References 10 publications

Experimental investigation of thermal emittance components of copper photocathode

Experimental investigation of thermal emittance components of copper photocathode

Effect of nanoscale surface roughness on transverse energy spread from GaAs photocathodes

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

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