GaAs Photocathode: Transfer Design Collaboration Meeting 6/10/2010.
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Transcript of GaAs Photocathode: Transfer Design Collaboration Meeting 6/10/2010.
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GaAs Photocathode: Transfer DesignCollaboration Meeting 6/10/2010
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Overview• Why GaAs?• Reflection vs Transmission : Thick Vs Thin• Stamp Transfer• Smart-Cut• Backside-Etch• GaAs Wafer Bonding• Conclusion• Next Steps
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Why GaAs?
873 nm
413.2 nm
248nm
[1],[2]
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Reflection vs Transmission : Thick vs Thin
[3],Graph Courtesy of Zeke Insepov(ANL)
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Reflection vs Transmission : Thick vs Thin
GaAs
ElectricField
+-
Vacuum
}
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GaAs Test Structure
10 nm p+ (~1e18)Variable (0-200 nm) p+ layer
10 nm p++ (~1e19)
1 um AlGaAs buffer
(100) GaAs Substrate
Growing test structure to determine appropriate thickness
• Proper Absorption Length (400 nm photon)
• Reduce Recombination
•Testing doping using - Zn• Strong built in field• Band bending at
surface for CsO activation
Status: SIMS and simulation
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GaAs Material Quality
• 300nm terrace
• ~.3nm RMS roughness
• Very Good Quality Surface
AFM Images Taken by Seon Woo Lee (ANL)
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Stamp Transfer Procedure
[4],[5]
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Stamp Transfer Analysis
Handle Wafer
A : Elastomeric Stamp
Transferrable Material
Etch Holes
B: IntermediatePolymer
• Stamp transfer is fantastic for transferring large areas of arbitrary material to arbitrary substrates
NEVERTHELESS:
• While the material and substrate are arbitrary, the stamp and intermediate layer have to have certain properties
• Intermediate layer B has to be “stickier” than stamp A
• Etch holes need to be created so release happens properly
• Polymer may not stand up to CsO treatment and etch holes may reduce QE and SNR
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SmartCut Procedure
InAs Transferred to SiO2 for Nanowires
SmartCut for SOI wafers
[6],[7]
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SmartCut Analysis
Graphs Courtesy of Zeke Insepov(ANL)
• Smartcut is a good technique for transferring heterogeneous material
• Lattice damage, re-planarization and Gaussian distribution of ion species may make transfer of materials where surface quality is very important
• Low penetration depth makes backside implantation difficult (Our wafers ~ 325 µm)
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Backside EtchEtchant Etches Selective Against
Hydrofluoric Acid AlGaAs (Higher Al content etches faster)
GaAs
Citric Acid GaAs AlGaAs
• AlGaAs and GaAs are natural etch stops depending on the etchant used
• The photocathode is relatively undisturbed
• Unfortunately the wafer is dissolved and not re-useable (2” GaAs Wafer ~ 80$)
• Process would be long; Lapping or CMP could be used to spread up process
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Bonding
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Potential Fabrication1. Grow thin layer of
sacrificial AlGaAs
2. Growth of photocathode, with layers inverted
3. Deposition of intermediate bonding layers (i.e. SiO2, SixNx)
4. Wafer bonded to a glass substrate predeposited bonding layers
5. Bulk of substrate is etched/CMP away
6. Sacrificial layer removed
7. Photocathode ready for activation
Glass substrate
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Conclusion• Grown GaAs is of very high quality
− Quality wafer manufacturer and MOCVD reactors
• Stamp transfer may not be suitable to our needs- Future developments may eliminate needs of
intermediate layer
• Smart-cut and/or Backside Etch are being considered as possible routes for photocathode transfer
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Next Steps• Evaluate lattice damage from Smart-Cut
and re-planarized Surface
• Analyze post-etch surface after sacrificial layer removal using AFM
• Determine maximum transferrable area tolerated by bonding process– Investigate methods to reduce strain in
bonding material
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References[1] W.E. Spicer and A. Herrera-Gomez, “Modern theory and applications of photocathodes,” SPIE MILESTONE SERIES MS, vol. 169, 2001, pp. 104–119.
[2] http://www.eecs.umich.edu/~singh/bk7ch03.pdf
[3] R.L. Bell, Negative electron affinity devices, Clarendon Press, Oxford, 1973.
[4] M.A. Meitl, Z.T. Zhu, V. Kumar, K.J. Lee, X. Feng, Y.Y. Huang, I. Adesida, R.G. Nuzzo, and J.A. Rogers, “Transfer printing by kinetic control of adhesion to an elastomeric stamp,” Nature Materials, vol. 5, 2006, pp. 33–38.
[5] J. Yoon, S. Jo, I.S. Chun, I. Jung, H. Kim, M. Meitl, E. Menard, X. Li, J.J. Coleman, U. Paik, and J.A. Rogers, “GaAs photovoltaics and optoelectronics using releasable multilayer epitaxial assemblies,” Nature, vol. 465, May. 2010, pp. 329-333.
[6] “Smart Cut - Wikipedia, the free encyclopedia.”
[7] S.A. Dayeh, P. Chen, Y. Jing, E.T. Yu, S.S. Lau, and D. Wang, “Integration of vertical InAs nanowire arrays on insulator-on-silicon for electrical isolation,” Applied Physics Letters, vol. 93, 2008, p. 203109.