1. 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

2. Schaff et. Al., Photocathode Physics for Photoinjectors 2012
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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Schaff et. Al., Photocathode Physics for Photoinjectors 2012

3. 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,
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4. Reflection High Energy Electron Diffraction (RHEED)
Side view of
Phosphor screen
Looking down from the top of the MBE machine Ga As
GaAs
Front view of
Phosphor screen
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5. 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

6. RHEED of as-loaded GaAs wafer
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
Substrate package opened just prior to loading
No surface preparation was performed
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7. RHEED image changes with substrate temperature
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8. 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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9. Schaff et. Al., Photocathode Physics for Photoinjectors 2012
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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10. Photocathode wafer images
1 3/8 inch cathode
in 3 inch wafer holder
prior to arsenic deposition
Wafer G20222
after arsenic deposition
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11. Photocathode wafer images
Wafer G20222
after arsenic deposition
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12. Compare uniformly doped and undoped near-surface layers
12nm
p-GaAs
5x1018cm-3
P-GaAs
substrate
surface
P-GaAs
substrate
p-GaAs
5x1018cm-3
GaAs 100nm
undoped
101nm
Create more carriers in a longer high-field region:
Faster turn-off time
Hotter or colder emission?
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13. Thermal Emittance for GaAs Photocathodes
Bazarov, et Al., J. Appl.. Phys. 103,
This work:
wafer G20222
MBE grown photocathode with 100nm undoped region at the top surface
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14. Thermal emittance and 520-532nm quantum efficiency
Measurements performed at different stages of activation cycles
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15. 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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16. High temperature surface roughening
Wafers heated in MBE
Specular opitcal scattering measured at 514nm
Increased roughness measured by AFM
Increased roughness also observed by RHEED
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17. Schaff et. Al., Photocathode Physics for Photoinjectors 2012
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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18. Optical coupling increase due to roughness
Photoluminescence (PL) from Ga0.2In0.8N surface roughened by scanning laser exposure
CAD pattern
IR Image
Inside patterning
PL Intensity (AU)
Outside of
patterning
Line scan location
1560nm PL Intensity (AU)
Roughening increases PL by a factor of 8
X. Chen, W. J. Schaff, and L. F. Eastman, JVST B 25 3,
Line scan location
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19. 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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20. Schaff et. Al., Photocathode Physics for Photoinjectors 2012
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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21. Schaff et. Al., Photocathode Physics for Photoinjectors 2012
Summary
The end
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22. 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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23. Schaff et. Al., Photocathode Physics for Photoinjectors 2012
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24. Conduction band energies and electric fields
Doping - 5x1018cm-3
0nm vs 100nm undoped cap
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25. Schaff et. Al., Photocathode Physics for Photoinjectors 2012
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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26. Schaff et. Al., Photocathode Physics for Photoinjectors 2012
RHEED oscillations for GaAs and AlAs (previous work)
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27. Cross section, CAD and SEM images
10nm
78nm
18nm
5W 20kHz
1424mm/Sec
771 passes
675sec
2.5nm
In0.8Ga0.2N 540nm
5W 20kHz
1424mm/Sec
59 passes
25sec
GaN buffer
AlN buffer
Sapphire substrate
backscattered electron image
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
z-contrast shows bright regions (more In) surrounding dark regions (more Ga)

28. SEM images
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
z-contrast shows bright regions (more In) surrounding dark regions (more Ga)