Optimisation of Activated GaAs Photocathode Surface for
Application as an Electron Source in Particle Accelerators
Narong Chanlek
School of Physics and Astronomy, The University of Manchester, Manchester, M13 9PL
The Cockcroft Institute of Accelerator Science and Technology, Warrington, WA4 4AD
Anode Plate
Introduction
Current and proposed linear colliders, energy recovery linacs and light sources
require high quality electron sources. In particular, low-emittance and polarised
electron beams are desirable. Conventional thermionic cathodes are robust but
are not able to achieve the requisite emittance. With low emittance operating with
short pulse length photocathode, in which the cathode is illuminated with a drive
laser to excite electron from the semiconductor source (see Fig. 1), can supply
such beam requirements. Photocathodes are being used as electron sources in
several modern accelerators such as the Accelerators and Lasers in Combined
Experiments (ALICE) at Daresbury and the Infrared Free Electron Laser (IR-FEL)
at Thomas Jefferson Laboratory, and are under development for the International
Linear Collider (ILC).
Stem
XHV
Laser
Electrons
Ceramic
Cathode ball
Photocathode
Figure 1. Schematic diagram of the DC photocathode gun and picture of ALICE Photocathode gun
during assembly.
Photocathode Characteristics
NEA GaAs Photocathode
● Quantum efficiency (QE), the ratio of the number of electrons emitted by
the photocathode to the number of incident photons,
hcI (  A)
QE (%) 
100%
elaser (nm) Plaser (mW )
Galium Asenide (GaAs) is becoming widely used and is a focus for accelerator
applications. Once activated to a Negative Electron Affinity (NEA) state, the GaAs
photocathode can be used as a high-brightness, low emmitance electron source [1].
This state is prepared by depositing caesium and an oxidant (either O2 or NF3) onto
the atomically-clean GaAs surface.
e-
where I is the measured photocurrent, laser is the laser wavelength and Plaser is
the drive laser power.
Eaffinity
e-
Eaffinity
● Thermal emittance, the source emittance of a photocathode can be
Eg
calculated from
h
h
Eg
Efermi
r Ethermal 1
 th 
2
2 m0c
3
GaAs
where r is the hard-wall radius of the laser beam and Ethermal is the electron
energy at the cathode surface.
● Cathode lifetime, the time taken for the QE to fall to 1/e of its initial value.
Efermi
Vacuum
GaAs
Cs-O Vacuum
Figure 2. Energy band diagram of (a) p-type GaAs and (b) NEA state (vacuum level lower than the
conduction band minimum). Electrons excited across the band gap Eg by photons of the energy h
thermalise to the conduction band minimum, then diffuse to the surface before escaping into the
vacuum [2].
Experimental Setup
Results and discussion
An experimental chamber consists of three sections, a loading chamber, cathode
preparation chamber and surface analysis chamber, which allows activation of GaAs
to the NEA state and surface characterisation of the photocathode within the same
vacuum system.
Cleaning: The effectiveness of heat-cleaning procedure for GaAs photocathodes
Surface Analysis
Chamber
Loading Chamber
Ga2p3/2
INTENSITY (a.u.)
As2p3/2
Preparation Chamber
Loading Chamber
Hemispherical
analyser for XPS
Inert sputter
ion source
O1s
a
a
a
b
b
c
c
c
d
d
d
e
e
f
f
e
f
FIGURE 3. Picture of Photocathode experimental chamber which has been set up at the Cockcroft
Institute, STFC Daresbury Laboratory.
Cs dispenser
were studied. Bulk VGF (Vertical Gradient Freeze) GaAs samples with p-type
doping (Zn) without any chemical cleaning were heated to five different
temperatures; 450, 500, 550, 600 and 625 C for 60 minutes. The XPS spectra of
the GaAs surface excited by Al K (1486.6 eV) radiation were taken before and
after the heat-cleaning process. The removal by heat-cleaning of oxides which are
the main coverage contaminants on the GaAs surface was studied, with
representative results as shown in Fig. 6.
1318
1323
1328
BINDING ENERGY (eV)
1115
b
1117
1119
1121 529
BINDING ENERGY (eV)
531
533
535
537
BINDING ENERGY (eV)
FIGURE 6. XPS spectra of the As2p3/2 , Ga2p3/2 and O1s for (a) the un-cleaned sample, (b) after
heating to 450C, (c) 500C, (d) 550C, (e) 600C and (f) 625C for 60 min.
Ion pump
Activation:
Caesium and oxygen
were deposited onto the sample by
using the standard “Yo-Yo” method [1] .
The QE were measured using a HeNe
laser at 632.8 nm for illumination with
the results shown in Fig. 7.
Hydrogen cracker
source
NEG pump
Penning Gauge
FIGURE 4. Picture of the preparation
chamber and loading chamber
RGA
FIGURE 5. The surface analysis chamber contains
with the equipments for X-ray Photoelectron
Spectroscopy (XPS) technique and Low Energy
Electron Diffraction (LEED) technique .
Summary and Further Study
The vacuum system has been used to prepare and study the surface properties
of a GaAs photocathode with XPS and LEED diagnostics at the Cockcroft Institute,
STFC Daresbury Laboratory. A heat cleaning-procedure for the NEA GaAs
photocathode has been studied. It was found that oxides from the GaAs surface
can be removed after heating to a temperature higher than 550C for 60 minutes.
However, the temperature needs to be optimised, as excessive temperatures can
result in decreasing the QE . Additional study is in progress on QE lifetime under
the influence of various gases.
FIGURE 7. QE as a function of heat-cleaning temperature.
References
1. D.T. Pierce and F. Meier, Phys. Rev. B 13, 5484 (1980).
2. D.T. Pierce, et.all., Rev. Sci. Instrum. 51(4), 478 (1980).
Acknowledgments
The author is grateful for support from the ASTeC, the STFC Daresbury Laboratory,
the Cockcroft Institute and the Royal Thai Government .