1. Study of negative electron affinity photocathodes
Student: Brian Henderson
Advisors: Ivan Bazarov and Yulin Li
November 10, 2018November 10, 2018
REU 2009 CLASSE

2. Beyond Einstein… GaAs He told us that for photoelectrons KE = hn - f
True for metals, but generally not true for semiconductors
Took over 50 years before a comprehensive understanding of photoemission process was obtained
Still, no one can predict quantitatively what energy distributions will come out from a given photocathode
KE << hn - f
November 10, 2018November 10, 2018
REU 2009 CLASSE

3. Why we care? Beam Brightness: Flux/((source area)*(solid angle))
‘Cold beams’ – ones with negligible transverse momentum spread
A number of large scale accelerator projects depend on photoinjectors, in which the electrons are produced from photocathodes
The transverse energy distribution of the photoelectrons sets the upper limit on beam brightness achievable from these devices
The longitudinal energy distribution provides insights into the physics of photoemission
November 10, 2018November 10, 2018
REU 2009 CLASSE

4. 1) Design 2D spectrometer
APL 78, 2721 (2001)
E/B = const a
Provides simultaneous measurement of the transverse and longitudinal energy spectra of photoelectrons
Brian will come up with a practical design & specifications and understand limitations to the resolution of the device
November 10, 2018November 10, 2018
REU 2009 CLASSE

5. Negative Electron Affinity
after Cs
before Cs
Monolayer (or few) of Cesium atoms deposited on GaAs crystal: strong dipole field allows photoelectrons to escape
Cs alone is not stable on the surface, instead use CsF or Cs2O
November 10, 2018November 10, 2018
REU 2009 CLASSE

6. 2) Making a GaAs Cathode Get a GaAs cathode atomically clean
Grow an active electron emitting layer by dosing GaAs cathode w/Cs and NF3 (thus Yo-Yo method)
Achieved QE > 10%, but dark lifetime not good enough.
Poor lifetime seems to be due to some sort of surface reaction
Yo-Yo Activation
November 10, 2018November 10, 2018
REU 2009 CLASSE

7. What’s on the Cathode Surface ?
NH3
Thermal Desorption Spectroscopy
SR Photoelectron Spectroscopy
APL 92, (2008)
It is commonly believed that the Cs-NF3 activated layer consists CsF, but evidences indicated unstable bonded nitrogen on the surface
Is it better to use other F-carrying molecules : XeF2 to substitute NF3
November 10, 2018November 10, 2018
REU 2009 CLASSE