Recent Progress in Non-Cesiated III-Nitride Photocathodes
Douglas Bell, Shouleh NikzadJet Propulsion Laboratory
Amir DabiranSVT Associates, Inc.
Shadi Shahedipour, Neeraj TripathiSUNY-Albany
1st Workshop on Photocathodes: 300nm-500nm20 July 2009
20 July 2009 2
National Aeronautics and Space AdministrationJet Propulsion LaboratoryCalifornia Institute of Technology
Introduction
3x Directorate: Engineering and ScienceDivision 38: Instruments and Science Data Systems
Section 389: Instrument Electronics and Sensors
Group 389E: Advanced Detector & Nanoscience TechnologiesShouleh Nikzad (Supervisor)Doug BellJordana BlacksbergFrank GreerBlake JacquotTodd Jones
The group works on advanced device concepts (mostly detector), including new materials.
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National Aeronautics and Space AdministrationJet Propulsion LaboratoryCalifornia Institute of Technology
Topics
• Motivation for III-N Photocathodes
• Delta-doping
• Progress in delta-doping for non-cesiated photocathodes
• AlGaN versus GaN
• Summary
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National Aeronautics and Space AdministrationJet Propulsion LaboratoryCalifornia Institute of Technology
Why III-Nitrides for Photocathodes?
• Low electron affinities in the AlxGa1-xN system.
• High chemical stability.
• Low inherent surface state charge.
• Tailorable UV response and solar blindness.
• Current UV photocathodes suffer from low QE and instability in the response
Grabowski et al, Appl. Phys. Lett. 78, No. 17 (2001)
Ec
Ev
Vacuum
p-III-N
Electron Affinities in AlxGa1-xN
χ
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National Aeronautics and Space AdministrationJet Propulsion LaboratoryCalifornia Institute of Technology
p-GaNEG
3.4 eV
χ
3.1 eV
Cesiated Photocathode
Cs layer
(1) Machuca, et al. (2) Wu and Kahn (3) Shahedipour, et al. , (4)Siegumund
Machuca et al, JVST B
Decay in Quantum Efficiency from O2 Exposure
•Quantum efficiency degrades rapidly under minute exposure to O2 Degradation linked to oxidation of Cs.
•Cesiated AlGaN photocathodes are only appropriate for UHV environments.
Photocathode Activation with Cesiation
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National Aeronautics and Space AdministrationJet Propulsion LaboratoryCalifornia Institute of Technology
Approach
Goal: Demonstrate a non-cesiated III-N based photocathode
Cesiation alters surface band structure using low-work function metal
Our approach is based on experience with other devices and materials*
*Hoenk, et al., Nikzad et al., Blacksberg, et al., Delta doped CCDs
• Piezoelectrically Enhanced Photocathode (PEPC)
• Delta doped Enhanced Photocathode (DDEPC)
• Take advantage of lower work function of AlGaN
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National Aeronautics and Space AdministrationJet Propulsion LaboratoryCalifornia Institute of Technology
Delta-doped Photocathode
sapphire
p-GaNUndoped GaN Capn-delta layer
Band bending and QE depend on:
Cap thicknessDelta-layer n-doping densityGaN layer p-doping density
Ec
Ev
p-AlGaN
Ionized Mg Acceptors
Delta-layercharge
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National Aeronautics and Space AdministrationJet Propulsion LaboratoryCalifornia Institute of Technology
tunable light source
ohmic contact
A
biased anode
GaN pc
UHVchamber
quartzwindow
Keithley 486
PCcontrol
Schematic of Emission Measurement
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National Aeronautics and Space AdministrationJet Propulsion LaboratoryCalifornia Institute of Technology
IPE Spectrum for GaN / -Si / p-GaN
IPE spectrum for delta-doped GaN photocathode. The power-law threshold allows simple extraction of the emission threshold.
cap layer
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National Aeronautics and Space AdministrationJet Propulsion LaboratoryCalifornia Institute of Technology
Effect of Delta-layer Cap Thickness on Emission Intensity
Thinner GaN cap layers produce an increase in emission intensity, due to decreased hot-electron scattering.
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National Aeronautics and Space AdministrationJet Propulsion LaboratoryCalifornia Institute of Technology
Effect of Delta-layer Cap Thickness on Emission Intensity
Very short attenuation length for the cap layer indicates strong scattering. Improvement in cap quality is essential.
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National Aeronautics and Space AdministrationJet Propulsion LaboratoryCalifornia Institute of Technology
Effect of Delta-layer Cap Thickness on Emission Threshold
Only a weak dependence of emission threshold on GaN cap layer thickness observed.
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National Aeronautics and Space AdministrationJet Propulsion LaboratoryCalifornia Institute of Technology
Effect of Si Delta-doping Density on Emission Threshold
Bare GaN has a high emission threshold. Si delta-doping produces a rapid reduction of threshold until the conduction band edge reaches the Fermi level.
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National Aeronautics and Space AdministrationJet Propulsion LaboratoryCalifornia Institute of Technology
Effect of Delta Layer Doping Density on Emission Intensity
3 nm cap
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National Aeronautics and Space AdministrationJet Propulsion LaboratoryCalifornia Institute of Technology
Effect of Bulk p-Doping on Emission Intensity
1 nm cap1014 Si/cm2
Increased p-doping of the bulk GaN reduces the width of the surface well, thus decreasing scattering losses during well traversal.
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National Aeronautics and Space AdministrationJet Propulsion LaboratoryCalifornia Institute of Technology
IPE Spectrum for AlGaN / -Si / p-GaN
The lower electron affinity of AlGaN reduces the emission threshold, but delta-doping of the AlGaN layer is required.
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National Aeronautics and Space AdministrationJet Propulsion LaboratoryCalifornia Institute of Technology
Summary
III-N photocathode activation is investigated without use of cesiation
These techniques offer the advantage of stable response even with exposure to air
GaN samples grown on sapphire were examined as delta doped enhanced photocathodes
Effect of cap layer, dopant density in the bulk, and dopant density in the delta layer were determined and optimized
Work is underway to integrate other effects to achieve true NEA without cesiation