Preparation/Characterization of Atomically
Flat and Clean Mo (100) Surfaces and
Thermal Emittance/Response Time
Measurements of Cs3Sb Photocathodes
Matt Nichols
Advisor: Luca Cultrera
Introduction
•
•
•
•
•
Energy Recovery LINAC: DC gun
operates on laser based
photoemission of electrons
Performance of ERL related to
brightness of electron beam
Beam brightness is limited by intrinsic
properties of the photocathode in the
DC gun
Want 100 mA average current and 2
ps bunch length
To achieve this, we need cathodes
with high QE (in visible), small
response time, low thermal
emittance, and long lifetime
Introduction Cont…
•
•
•
Need QE> 5% (in visible),
response time < 1 ps, and
MTE of ~120 meV
GaAs photocathodes work
well, but have a short lifetime
(hours) and need pressure of
10-11 Torr
CsK2Sb photocathodes have
recently shown similar
success but with added effect
of longer lifetime
(a) Quantum Efficiency vs. Wavelength for the
CsK2Sb photocathode
•CsK2Sb cathodes grown on Si (100) substrates here at Cornell have shown QEs
that are a factor of ~2 lower than expected
Substrate-Cathode Interface
• The substrate may actually play a more important role in photocathode
properties than originally thought
• Mismatch of the Si and CsK2Sb crystal structures may lead to various crystal
defects: line defects, plane defects, etc…
• These defects could lower the mean free path of excited electrons by acting as
recombination centers
• Lower mean free path means lower QE
• It has been proposed that using Mo (100) (Molybdenum) substrates could
improve the crystalline lattice mismatch to as low as 3%
•The lattice
parameter of
CsK2Sb is 8.61 Å
•Mo is a body
centered cubic
crystal with a
lattice
parameter of
3.14 Å
Substrate-Cathode Interface Cont…
• Matching the lattice parameters of the substrate and
film could reduce the density of defects
• A smaller number of defects should theoretically
increase the mean free path of electrons
• A larger MFP means more electrons can escape which
leads to an improved QE
• This project aims to test this hypothesis by preparing
clean and atomically flat Mo (100) surfaces for
deposition of CsK2Sb thin films
45 deg rotation
of film with
respect to
substrate
Film-substrate
boundary
Project Summary
• Assembled UHV system for surface diagnostics and Mo
sample prep
• Wrote software to operate LEED/AES system devices
and save data
• Did preliminary system tests
• Prepared Mo substrates
• Analyzed effect of preparation process on substrate and
effect of substrate on photocathode
The vacuum system before the
rebuilding began
Experimental Setup/Preparation
•System already contained LEED/AES
optics, 300 L/S ion pump, Ar leak valve,
and RGA
•Things added to system:
-Heater/bellows and power supply
-TSP and TSP controller
-Load lock system
-Cold cathode gauge on load lock
-Gate valve
-New view ports
-Hanger assembly (thermocouple,
cradle, etc…)
-Camera and mount
-Ar ion gun
-Measurement devices (pico-ammeter,
thermocouple box, ADC/DAC)
•Performed 48 hour bake out at 150 C
LEED/AES System Software
LEED/AES Software
• Software used to monitor various parameters:
hanger temperature; chamber pressure; current
through puck; heater current, power, and voltage;
partial pressures and residual gas mass spectra
• This involves measuring analog output signals of
thermocouple box, cold cathode gauge, and picoammeter with an ADC
• Also used to operate heater power supply, RGA, and
camera
• RGA software edited to allow for total pressure
measurements and abiltiy to turn on/off CEM at will
Preliminary Tests
Thermocouple Calibration
•
•
•
•
We need to measure the temperature of the puck/substrate during annealing
Must calibrate the hanger thermocouple in order to find the temp of the puck
Attached a thermocouple to a Mo test puck
Increased heater current in small steps and measured temperature of both
thermocouples after equilibration at each step
Found an average calibration factor of about 2.7
Calibration factor may be off due to puck not fitting in hanger/thermocouple breaking
•
•
Calibrating Thermocouple Temp {C}
1000
P
u
c
k
T
e
m
p
800
600
Calibrating
Thermocouple Temp {C}
400
200
0
0
{
C
100
200
Hanger Temp {C}
300
400
}
LEED
• LEED: Low-Energy Electron
Diffraction
• Useful for determining 2-D
surface structure since
electrons do not penetrate
deeply
• Diffraction condition
depends on reciprocal
crystal lattice and is very
sensitive to condition of a

first ~4 monolayers of
b
Unit Cell
crystal
LEED will be used as a binary test
to determine whether or not the
Mo (100) surface is clean and flat

0
*
*
k park par  h1a  h2 b


b  nˆ
a *  2  
a b
h1 , h2     0

*
nˆ  a
b  2  
a b
Si Crystal Cleaving and LEED Test
• Tested LEED using a Si crystal tip surrounded by a BeO screen
• BeO screen used originally to line up beam, but BeO built up
charge which caused diffraction patterns to vary with time
• Removed BeO screen and cleaved Si crystal on magnetic arm
• Tested LEED on cleaved crystal and obtained tie-dye patterns
• This indicated that there may be some residual
magnetic/electric field problems
• This test warranted the need to test LEED on a different type
of crystal, GaAs, which had previously produced succesful
LEED patterns
Experimental Issues
(a) GaAs LEED pattern
(b) GaAs LEED pattern
(c) Si LEED pattern
(d) Si LEED pattern
•Tested LEED on clean, well-defined
crystals: freshly cleaved GaAs and Si
(with and without BeO screen)
•Expected typical LEED pattern
(several bright intense spots)
•Obtained tie-dye like pattern which
changed with both time and
incident electron energy
•Tie-dye phenomenon may be due
to presence of residual magnetic
fields (YAG screen test, and hall
probe test of LEED optics and puck
hanger)
•May also have charge building up
on LEED screen insulators or view
ports
GaAs Cleaving and LEED Test
• Before performing GaAs test, the LEED optics
and the hanger were degaussed, though
strong fields (5-15 gauss) were still present,
even after degaussing
• Cleaved GaAs crystal in same manner as for Si
to provide a clean, well-defined crystal surface
• This method had produced a good LEED
pattern on another system
• Obtained tie-dye pattern once again
Ar Ion Sputtering
•
•
•
•
•
Needed to test the abilities of the ion gun
Used an oxidized GaAs wafer which allowed us
to clearly see the effects of the sputtering
Sputter gun requires system be backfilled with
Ar to 5e-05 Torr (can be time consuming)
Sputtering occured at near normal incidence
(alligned by eye) for approximately 20 minutes
in one spot (also time consuming)
Sputtering parameters were 20 mA emission
current and 2 kV filament voltage
Mo Sample Preparation
•Prepared 2 Mo substrates from 99.9% pure
Mo foil
•Cut Mo foil (~2 mm thick) to disk shape
•Hand polished disk using 30 μm diamond
suspension followed by 15 μm sand paper
•4 or 5 deep scratches remained which
couldn’t be removed
•Moved to 400 grade silicon carbide sand
paper (with water) followed by 600 grade,
then 9 µm, 6 µm, 3 µm, 1 µm, 0.25 µm, and
0.1 µm diamond suspension pastes (in that
order)
•Between each polishing, the surface was
cleaned using coconut-oil soap and isopronal
•Following polishing, the two substrates were
taken to AFM to analyze RMS roughness
Mo Surface Roughness (Pre-Annealing)
(a) Surface of hand polished Mo substrate
(Luca). 1 micron scale, RMS Roughness: 2.27
nm
(d) Surface of hand polished Mo substrate
(Matt). 10 micron scale, RMS Roughness:
18.95 nm
(b) Surface of hand polished Mo substrate
(Luca). 1 micron scale, RMS Roughness: 2.64
nm
(c) Surface of hand polished Mo substrate
(Luca). 10 micron scale, RMS Roughness:
5.32 nm
(e) Surface of hand polished Mo substrate
(Matt). 1 micron scale, RMS Roughness: 3.52
nm
Thermal Annealing and Ion
Sputtering
•After AFM imaging, samples underwent a
sputtering/annealing process
•The flatter substrate underwent a 3-stage
process and the other underwent a 2-stage
process
•1 stage of the process consisted of at least
20 minutes of ion sputtering in the same
location followed by annealing for at least 30
minutes at temperatures >870 C
•Reached a peak temperature of 950-1000 C
on the flatter puck
•Found that currents through the heater of
>12 A cause enough heat to cause
accumulation of melted indium solder which
holds puck in place
Results
SEM/EDX Measurements
SEM/EDX Measurements Cont…
More SEM
•The EDX spectra indicate that carbon is in fact present on the surface: diamond from
the suspensions may have lodged itself in the Mo surface
•There are other elements which may have come from dust or polishing (soap,
suspensions, etc…)
•There are also clearly still large pits, holes, and scratches in the surface, even after
annealing
AFM Measurements After
Annealing
(a) Surface of hand polished Mo
substrate (Luca). 10 micron scale,
RMS Roughness: 7.43 nm
(b) Surface of hand polished Mo
substrate (Luca). 10 micron scale,
RMS Roughness: 7.46 nm
(c) Surface of hand polished Mo
substrate (Luca). 50 micron scale,
RMS Roughness: 4.05 um
Spectral Response of CsK2Sb
Photocathode with Mo (100) Substrate
1.00E+02
1.00E+01
1.00E+00
3.90E+02
4.40E+02
4.90E+02
5.40E+02
5.90E+02
6.40E+02
6.90E+02
Moly CsK2Sb QE (%)
QE (%)
Si(100) CsK2Sb QE (%)
Si Cs3Sb QE (%)
1.00E-01
1.00E-02
1.00E-03
Wavelength (nm)
Thermal Emittance and Response Time
Measurements
•Emittance measured using beam profile
after solenoid and solenoid transfer matrix
•Used 0.5mm, 1.0mm, and 1.5mm laser
aperature sizes and 473 nm, 532 nm, and
405 nm laser wavelengths
•Response time measured using
deflector cavity in ERL injector
•Measured <1ps for the Cs3Sb response
time (limited by resolution of
equipment)
These measurements resemble closely those of
CsK2Sb (to within error bars)
Future Work
• The improved QE obtained using the Mo substrate warrants a
more detailed investigation into the preparation of the Mo
substrates for the multi-alkali photocathodes and their overall
effect on the cathode’s performance
• A bi-alkali photocathode on a Mo substrate still needs to be
tested in the DC gun (need to fix the melted indium solder
problem)
• A more detailed investigation must be undergone to fix the
problems associated with LEED
Acknowledgements
• Thanks to Luca Cultrera, Ivan Bazarov , Tobey Moore,
Joseph Conway, Richard Merluzzi, John Dobbins, Karl
Smolenski, Siddharth Karkare, Adam Bartnik, and the
whole ERL injector team
• Thanks also to the NSF and DOE
• http://www.wellesley.edu/Chemistry/Chris/LEED.ht
ml
• http://www.lns.cornell.edu/~ib38/research.html