Photocathode R&D and Characterization of Photoemitted Electrons

Brian S. Henderson

Rice University

Advisors: Ivan Bazarov, Yulin Li, Xianghong Liu

June 19, 2009

June 19, 2009 2

Project Basics

• My project has two essential components– Photocathode R&D

– Characterization of photoemitted electron energies

• Photocathodes serve as the source of electrons used in accelerator beams, including ERL

• It is thus advantageous to understand the photoemission process and maximize the efficiency of cathodes

• Other applications of photocathodes include electron lithography and night vision

Photocathode R&D

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Photoemission

• Photoemission is the process by which a material releases electrons when stimulated by EM radiation (i.e. the photoelectric effect)

• Photoemission is understood to be a bulk process rather than a simple surface interaction

• Certain materials can be made to exhibit negative electron affinity (NEA) in which the energy level of electrons in the vacuum is lower than the conduction band of the material

• The goal is the maximize the quantum efficiency (QE), that is the number of electrons produced per incident photon

Energy diagram of an NEA cathode (M. Hoppe. PhD Thesis. Universität Heidelberg, 2001.)

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Photocathode Preparation

• Each photocathode consists of a p-doped GaAs crystal wafer

• Cathodes are very sensitive to surface contamination and kept under ultra high vacuum

• The crystal is first cleaned using cracked hydrogen gas and then heat-cleaned at ~650oC

• Cathodes are activated by creating a state of NEA by alternating the deposition of cesium and fluorine or oxygen on the crystal surface to create CsF dipoles (yo-yo method)

– Currently we use NF3 as the fluorine source

The photocathode preparation chamber (top) and a GaAs photocathode (bottom) (www.lns.cornell.edu/~ib38/ )

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Photocathode Goals and Challenges

• Currently, good QE values (>10%) have been achieved, but the lifetime of the cathodes needs to be improved

• The use of NF3 may be causing the formation of unstable nitrogen bonds on the GaAs surface

• After activation, the breaking of such bonds may degrade the cathode surface leading to fast drops in QE

• Possible solution: Deposit fluorine using XeF2 instead Thermal desorption spectrogram showing

evidence of the release of ammonia after cathode activation

NH3

Plot of QE versus number activation cycles for experiments conducted in 2005

Characterization of Photoemitted Electron Energies

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Measuring the Energy Distribution

• Electrons are given a small kick (~20 eV) after leaving cathode

• The longitudinal energy (E║) distribution is measured using a retarded field analyzer (RFA)

• At such low energy, deflections of the beam due to external fields are pronounced

• Measuring the transverse distribution (E┴) and keeping the electrons in the device takes some creativity

Device used by Pastuszka, et al for the energy analysis of photoemitted electrons (Pastuszka, et al. J. Appl. Phys., Vol. 88, No. 11, 1 December 2000.)

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Guiding the Beam and Measuring E┴

• To guide the electrons, a strong longitudinal magnetic is applied

• If B is varied slowly in space relative to the gyration of the electron, the magnetic moment is may be treated as an “adiabatic invariant”:

• Assuming non-magnetic forces are negligible between the electrode and the RFA, the total energy of each electron is conserved:

Simplified trajectory of an electron in a strong axial B-field (M. Hoppe. PhD Thesis. Universität Heidelberg, 2001.)

constant

B

E

constant|| EE

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Guiding the Beam and Measuring E┴

• Let α denote the ratio of B at the RFA to B at the cathode:

• Then by energy conservation and the adiabatic invariant:

• So the initial transverse energy may be found by differentiating with respect to α :

i

f

B

B

iif

if

EEE

EE

)1(||||

d

dEE f

i||

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Progress So Far

• Photocathode R&D– Learned established procedures for photocathode activation

using NF3 and achieved reasonable quantum yields

– Prepared the cathode activation chamber for more detailed analyses of the activation process and behavior of the cathodes after activation

• Characterization of Photoemitted Electron Energies– Started the examination of previous work to determine areas

of necessary improvement for a new device

– Developed equations for the “guiding center” trajectories of electrons in the electromagnetic fields characteristics of the energy distribution measurement device

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Project Goals

• Photocathode R&D– Work to improve the quantum yield of cathodes

– Conduct measurements of and work to improve cathode lifetimes

– Attempt and study activations using XeF2 and other methods

• Characterization of Photoemitted Electron Energies– Compute trajectories of electrons in guiding electric and

magnetic fields to determine tolerances for a device to measure energy distributions

– Begin work on device design

– Begin device construction if possible