PAC2013 - List of Authors (Karkare, S.S.)

Paper

Title

Page

Advances in Photocathode Technology at Cornell University

391

S.S. Karkare
Cornell University, Ithaca, New York, USA
I.V. Bazarov, L.E. Boulet, M. Brown, L. Cultrera, B.M. Dunham, N. Erickson, G. Gabriel, A. Kim, B. Lillard, T.P. Moore, C. Nguyen, W.J. Schaff, K.W. Smolenski, H. Wang
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

Funding: This work has been funded by NSF under grant number DMR-0807731, DOE under grant number DE-SC0003965
Beam brightness from modern day photoinjectors is limited by the photocathode. A multifaceted photocathode development program has been undertaken at Cornell University with a goal to develop the ultimate photocathode which has high quantum efficiency, low mean transverse energy, quick response time and a long lifetime. Positive affinity cathodes like CsK2Sb and NaK2Sbhave been grown using different kinds of alkali metal sources (alkali-azide and pure metal) , characterized and tested in the Cornell-ERL photoinjector. Novel layered structures of various III-V semiconductors like GaAs and AlGaAs grown using Molecular Beam Epitaxy and activated to negative electron affinity using Cs and NF3 are also being investigated. Surface and photoemission diagnostics like Auger spectroscopy, LEED, RHEED and the 2D-electron energy analyzers have been connected in vacuum to the photocathode growth and preparation chambers to fully characterize the surface and emission properties of the materials grown. A Monte Carlo based simulation has also been developed to predict photoemission from layered semiconductor structures and help design novel structures to optimize the photoemission properties.

Slides TUOAB1 [6.005 MB]

MOPBA21

Modeling Localized States and Band Bending Effects on Electron Emission Ion from GaAs

225

D.A. Dimitrov, Y. Choi, C. Nieter
Tech-X, Boulder, Colorado, USA
I.V. Bazarov, S.S. Karkare, W.J. Schaff
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
I. Ben-Zvi, T. Rao, J. Smedley
BNL, Upton, Long Island, New York, USA

Funding: The authors wish to acknowledge the U.S. Department of Energy (DOE) and the National Science Foundation for funding under grants DOE DE-SC0006246, NSF DMR-0807731, and DOE DE-SC0003965.
High acceptor doping of GaAs and (Cs, O) or (Cs, F) surface coating leads to downward band bending terminating with effective negative electron affinity surface. The periodicity breaking at the surface together with the formed potential leads to one or more localized states in the band bending region together with effective Fermi level pinning. We report results on how to calculate the band bending potential, the Fermi level pinning, and localized states as functions of GaAs p-doping density, surface density of states, and temperature. We also consider how these surface properties affect electron emission.

TUPMA15

Monte Carlo Simulations of Charge Transport and Electron Emission from GaAs Photocathodes

616

Y. Choi, D.A. Dimitrov, C. Nieter
Tech-X, Boulder, Colorado, USA
I.V. Bazarov, S.S. Karkare
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

Funding: The authors wish to acknowledge the support of the U.S. Department of Energy (DOE) under SBIR grant DE-SC0006246 and Early Career DE-SC0003965.
The need for a bright electron beam is increasing in the fields of x-ray science, electron diffraction and electron microscopy which are required for colliders. GaAs-based photocathodes have the potential to produce high-brightness, unpolarized and polarized, electron beams with performance that meets modern collider requirements. Even after decades of investigation, however, the exact mechanism of electron emission from GaAs is not well understood. Therefore, we investigate photoemission from a GaAs photocathode using detailed Monte Carlo electron transport simulations. Instead of a simple stepwise potential, we consider a triangular barrier including the effect of the image charge to take into account the effect of the applied field on the emission probability. The simulation results are compared with the experimental results for quantum eﬃciency, angular and energy distributions of emitted electrons without the assumption of any ad hoc parameters.

Paper	Title	Page
TUOAB1	Advances in Photocathode Technology at Cornell University	391
	S.S. Karkare Cornell University, Ithaca, New York, USA I.V. Bazarov, L.E. Boulet, M. Brown, L. Cultrera, B.M. Dunham, N. Erickson, G. Gabriel, A. Kim, B. Lillard, T.P. Moore, C. Nguyen, W.J. Schaff, K.W. Smolenski, H. Wang Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Funding: This work has been funded by NSF under grant number DMR-0807731, DOE under grant number DE-SC0003965 Beam brightness from modern day photoinjectors is limited by the photocathode. A multifaceted photocathode development program has been undertaken at Cornell University with a goal to develop the ultimate photocathode which has high quantum efficiency, low mean transverse energy, quick response time and a long lifetime. Positive affinity cathodes like CsK2Sb and NaK2Sbhave been grown using different kinds of alkali metal sources (alkali-azide and pure metal) , characterized and tested in the Cornell-ERL photoinjector. Novel layered structures of various III-V semiconductors like GaAs and AlGaAs grown using Molecular Beam Epitaxy and activated to negative electron affinity using Cs and NF3 are also being investigated. Surface and photoemission diagnostics like Auger spectroscopy, LEED, RHEED and the 2D-electron energy analyzers have been connected in vacuum to the photocathode growth and preparation chambers to fully characterize the surface and emission properties of the materials grown. A Monte Carlo based simulation has also been developed to predict photoemission from layered semiconductor structures and help design novel structures to optimize the photoemission properties.
	Slides TUOAB1 [6.005 MB]

MOPBA21	Modeling Localized States and Band Bending Effects on Electron Emission Ion from GaAs	225
	D.A. Dimitrov, Y. Choi, C. Nieter Tech-X, Boulder, Colorado, USA I.V. Bazarov, S.S. Karkare, W.J. Schaff Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA I. Ben-Zvi, T. Rao, J. Smedley BNL, Upton, Long Island, New York, USA
	Funding: The authors wish to acknowledge the U.S. Department of Energy (DOE) and the National Science Foundation for funding under grants DOE DE-SC0006246, NSF DMR-0807731, and DOE DE-SC0003965. High acceptor doping of GaAs and (Cs, O) or (Cs, F) surface coating leads to downward band bending terminating with effective negative electron affinity surface. The periodicity breaking at the surface together with the formed potential leads to one or more localized states in the band bending region together with effective Fermi level pinning. We report results on how to calculate the band bending potential, the Fermi level pinning, and localized states as functions of GaAs p-doping density, surface density of states, and temperature. We also consider how these surface properties affect electron emission.

TUPMA15	Monte Carlo Simulations of Charge Transport and Electron Emission from GaAs Photocathodes	616
	Y. Choi, D.A. Dimitrov, C. Nieter Tech-X, Boulder, Colorado, USA I.V. Bazarov, S.S. Karkare Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Funding: The authors wish to acknowledge the support of the U.S. Department of Energy (DOE) under SBIR grant DE-SC0006246 and Early Career DE-SC0003965. The need for a bright electron beam is increasing in the fields of x-ray science, electron diffraction and electron microscopy which are required for colliders. GaAs-based photocathodes have the potential to produce high-brightness, unpolarized and polarized, electron beams with performance that meets modern collider requirements. Even after decades of investigation, however, the exact mechanism of electron emission from GaAs is not well understood. Therefore, we investigate photoemission from a GaAs photocathode using detailed Monte Carlo electron transport simulations. Instead of a simple stepwise potential, we consider a triangular barrier including the effect of the image charge to take into account the effect of the applied field on the emission probability. The simulation results are compared with the experimental results for quantum eﬃciency, angular and energy distributions of emitted electrons without the assumption of any ad hoc parameters.