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

Abstract

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 .

Cold electron beams from cryocooled, alkali antimonide photocathodes

“…Including the Schottky work function lowering scaled by the field enhancement coefficient obtained from QE measurement in the photoinjector gun electric field dependence is expected on MTEs and values as large as ∼115 meV should be observed at 300 K and at 3.4 MV=m. In addition, it is known that the electron beam intrinsic emittance can be degraded by the cathode surface roughness [21][22][23]. This whole picture looks in contrast with our smaller measured MTEs indicating that a model simply scaled on electrons excess energy cannot explain our experimental result.…”

“…We believe this discrepancy is due to an emittance growth caused by photocathode surface roughness. The scientific community is attempting to address the question of how the roughness affects the intrinsic emittance of a photocathode surface and several models have been proposed in recent years to describe this phenomenon [21][22][23]. Some of these models indicate that the intrinsic emittance contributions due to the surface roughness can be due to a simply geometrical effect due to the relative orientation of local surfaces from where the electron emission takes place and to an increase of transverse momentum due to transverse electric field components induced by surface roughness.…”

Surface science analysis of GaAs photocathodes following sustained electron beam delivery

“…The Cornell group has studied the effect of surface morphology on thermal emittance, of similarly treated GaAs photocathodes, showing that their analytical model for photoemission predicts their emittance measurements when the surface roughness is included in the model [24]. Unfortunately, at the moment there are no means to characterize the effect of the surface morphology changes on the electron beam emittance at the JLab FEL.…”

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

“…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.…”