MBE growth of GaAs
photcathodes for the Cornell ERL
photoinjector and effect of
roughness on emittance
Ivan Bazarov
Bruce Dunham
Siddharth Karkare
Xianghong Liu
Tobey Moore
William Schaff
CLASSE - Cornell Laboratory for Accelerator-based ScienceS and Education
Contents
• Motivation
– Relate GaAs Surface Roughness to thermal emittance (TE)
• MBE growth of GaAs Photocathodes
– Arsenic protective caps protect surface from oxidation roughening
– GaAs epitaxial structures are implemented to test models of TE
• A speculative question – How much does surface roughness impact optical absorption?
• Summary
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Surface roughening of GaAs
Primary origins of Surface Roughness
Ex-situ processing of cathodes in atmosphere
Native oxidation promotes a roughening mechanism*
In-situ heating to remove oxides and other contaminants
High-temperature
* Recommended reference : “Monitoring epiready semiconductor wafers” Allwood et. al.,
Thin Solid Films 412 (2002) 76–83
After: 350°C hydrogen clean
550°C 1 hour anneal
Before:
Siddharth Karkare et al., APPLIED PHYSICS LETTERS 98, 094104 2011
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Reflection High Energy Electron Diffraction (RHEED)
Ga As
Looking down from
the top of the MBE
machine
GaAs
Side view of
Phosphor screen
Front view of
Phosphor screen
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Reflection High Energy Electron Diffraction
(RHEED) of GaAs
One goal for GaAs
photocathodes is to
create atomically flat
surfaces
Uniform intensity of
diffraction streaks
along their length
only occurs for flat
surfaces
RHEED of as-loaded GaAs wafer
G20189
Substrate package opened just
prior to loading
No surface preparation was
performed
The bands of rings are
diffraction from an
amorphous (but fairly
flat) surface
This is an oxide on
GaAs
There is a faint diffraction
line from underlying
GaAs
The oxide must be very
few monolayers (ML)
thick
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RHEED image changes with substrate temperature
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200°C
570°C
590°C
640°C
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Flattening of GaAs surface
15 sec
2.1nm
2.1 nm of GaAs growth quickly fills in and around
monolayer-scale pits and bumps to create an atomically flat
surface.
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MBE wafer mounting
3inch wafer
Arsenic deposited
through mask
Arsenic mask
As4 through mask:
4 hours at 3x10-6T
Tsub 580°C to -30°C
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Photocathode wafer images
1 3/8 inch cathode
in 3 inch wafer holder
prior to arsenic deposition
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Wafer G20222
after arsenic deposition
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Photocathode wafer images
Wafer G20222
after arsenic deposition
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Compare uniformly doped and undoped
near-surface layers
12nm
p-GaAs
5x1018cm-3
P-GaAs
substrate
surface
P-GaAs p-GaAs
substrate 5x1018cm-3
GaAs 100nm
undoped
101nm
1.5
Create more carriers in a
longer high-field region:
eV
1.0
0.5
0.0
Faster turn-off time
-0.5
-120
-80
-40
0
Hotter or colder emission?
Depth (nm)
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Thermal Emittance for GaAs Photocathodes
Bazarov, et Al., J. Appl.. Phys. 103, 054901 2008
Wavelength (nm)
800 700
600
500
450
Vertical
160
This work:
wafer G20222
Horizontal MBE grown
photocathode with
100nm undoped
region at the top
surface
140
kT^ (meV)
400
ref traditional
process
120
100
80
60
40
20
1.6
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2.0
2.4
Energy (eV)
2.8
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Thermal emittance and
520-532nm quantum efficiency
kT^ (meV)
160
140
100nm undoped
(first wafer)
substrate
traditonal
process
120
100nm undoped
G20222
100
80
0
4
8
12
Quantum Efficiency (%)
Measurements performed at different stages of activation cycles
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Quantum efficiency speculation
• Why is QE low?
– Is band bending different than expected?
• The surface was protected by an arsenic cap – is it flatband?
– Is optical absorption different for a smooth surface?
• Does less optical scattering result in lower near-surface light
intensity?
• Do rough surfaces contain plasmonic elements?
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High temperature surface roughening
1.0
30
25
0.8
20
15
0.6
10
5
0.4
rms Roughness (nm)
Relative Reflectance
Wafers heated in MBE
35
Specular opitcal scattering
measured at 514nm
Increased roughness
measured by AFM
Increased roughness also
observed by RHEED
200
400
600
Anneal Temperature (C)
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Thermal cleaning example
Hypothesis:
increased surface
roughness increases
quantum efficiency
due to increased light
absorption
Roughness?
Figure 2.16 The quantum efficiency dependent on heat cleaning temperature
Submitted to the Graduate School in partial fulfillment of the requirements for the degree of
Doctor of Philosophy in School of Physics, Peking University January, 2012
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Optical coupling increase due to roughness
Outside of
patterning
1200 1600 2000
Wavelength (nm)
Roughening increases PL
by a factor of 8
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CAD pattern
IR Image
Inside patterning
1560nm PL Intensity (AU)
PL Intensity (AU)
Photoluminescence (PL) from
Ga0.2In0.8N surface roughened by
scanning laser exposure
Line scan location
X. Chen, W. J. Schaff, and
L. F. Eastman, JVST B 25
3, 974-977 2007
Line scan location
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Optical coupling enhancment Roughness? Plasmonics? Both?
• In solar cells and light emitting
diodes surfaces are intentionally
roughened on a similar size scale to
improve light coupling efficiency
• Plasmonic coupling can increase
near-surface optical intensity by 10x
(as routinely seen in Raman
spectroscopy)
• Could Cs clusters form plasmonic
structures?
• Metal balls of a few nm in size
give rise to large plasmonic
coupling
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Summary
• 100nm undoped As-capped GaAs photocathodes
have low emittance
Questions:
Is surface roughening during heating partially
responsible for increased QE?
Is surface smoothing during operation partially
responsible for decreased QE?
How do we figure this out?
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Summary
• The end
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MBE in-situ activation
• Observe quantum efficiency during Cs/NF3activation.
• Simultaneous RHEED observations may reveal the surface
structure required for high QE
• Observe optical scattering with changing roughness
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Conduction band energies
and electric fields
1.4
1.2
800
Ec
600
0.8
400
0.6
0.4
200
Efield
0.2
kV/cm
Ec (eV)
1.0
0.0
0
-120
-80
-40
0
Doping - 5x1018cm-3
Depth (nm)
0nm vs 100nm undoped cap
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•
•
Is this cap protective enough?
• Is it thick enough to survive air exposure?
• Is it free of holes or cracks that might expose the surface to air?
The answer is YES !
Arsenic begins to desorb at 350°C and peaks out
at about 370°C during wafer heating (as seen on
the RGA facing the wafer). The desorbing flux
falls by about a factor of 10 when the surface is
completely clear
The wafer is then raised to a temperature below
oxide desorption temp (580°C) and GaAs is
grown.
If any residual contamination was present (even
though not visible in RHEED), the surface would
become rough within 1-2 monolayers. Instead,
perfect growth was observed.
G20192 after warm arsenic cap removal
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RHEED oscillations for GaAs and AlAs (previous work)
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Cross section, CAD and SEM images
5W 20kHz
1424mm/Sec
771 passes
675sec
5W 20kHz
1424mm/Sec
59 passes
backscattered
electron image
z-contrast shows25sec
bright regions
(more In) surrounding dark regions
10nm
78nm
18nm
2.5nm
In0.8Ga0.2N 540nm
GaN buffer
AlN buffer
Sapphire substrate
Lines are very sharp despite being written 771 times over 11 minutes
Individual spots can be seen - spacing is approximately 70µm
corresponding to 1424mm/Sec
SEM images
z-contrast shows bright regions (more
Lines are very sharp despite being written 771 times over 11 minutes
Individual spots can be seen - spacing is approximately 70µm
corresponding to 1424mm/Sec