1. Recent Progress in Non-Cesiated III-Nitride Photocathodes Douglas Bell, Shouleh Nikzad Jet Propulsion Laboratory Amir Dabiran SVT Associates, Inc. Shadi Shahedipour, Neeraj Tripathi SUNY-Albany 1 st Workshop on Photocathodes: 300nm-500nm 20 July 2009

2. National Aeronautics and Space Administration Jet Propulsion Laboratory California Institute of Technology 20 July 2009 2 Introduction 3x Directorate: Engineering and Science Division 38: Instruments and Science Data Systems Section 389: Instrument Electronics and Sensors Group 389E: Advanced Detector & Nanoscience Technologies Shouleh Nikzad (Supervisor) Doug Bell Jordana Blacksberg Frank Greer Blake Jacquot Todd Jones The group works on advanced device concepts (mostly detector), including new materials.

3. National Aeronautics and Space Administration Jet Propulsion Laboratory California Institute of Technology 20 July 2009 3 Topics Motivation for III-N Photocathodes Delta-doping Progress in delta-doping for non-cesiated photocathodes AlGaN versus GaN Summary

4. National Aeronautics and Space Administration Jet Propulsion Laboratory California Institute of Technology 20 July 2009 4 Why III-Nitrides for Photocathodes? Low electron affinities in the Al x Ga 1-x N 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) EcEc EvEv Vacuum p-III-N Electron Affinities in Al x Ga 1-x N χ

5. National Aeronautics and Space Administration Jet Propulsion Laboratory California Institute of Technology 20 July 2009 5 p-GaN E G 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 O 2 Exposure Quantum efficiency degrades rapidly under minute exposure to O 2 Degradation linked to oxidation of Cs. Cesiated AlGaN photocathodes are only appropriate for UHV environments. Photocathode Activation with Cesiation

6. National Aeronautics and Space Administration Jet Propulsion Laboratory California Institute of Technology 20 July 2009 6 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

7. National Aeronautics and Space Administration Jet Propulsion Laboratory California Institute of Technology 20 July 2009 7 Delta-doped Photocathode sapphire p-GaN Undoped GaN Cap n-delta layer Band bending and QE depend on: Cap thickness Delta-layer n-doping density GaN layer p-doping density EcEc EvEv p-AlGaN Ionized Mg Acceptors Delta-layer charge

8. National Aeronautics and Space Administration Jet Propulsion Laboratory California Institute of Technology 20 July 2009 8 tunable light source ohmic contact A biased anode GaN pc UHV chamber quartz window Keithley 486 PC control Schematic of Emission Measurement

9. National Aeronautics and Space Administration Jet Propulsion Laboratory California Institute of Technology 20 July 2009 9 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

10. National Aeronautics and Space Administration Jet Propulsion Laboratory California Institute of Technology 20 July 2009 10 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.

11. National Aeronautics and Space Administration Jet Propulsion Laboratory California Institute of Technology 20 July 2009 11 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.

12. National Aeronautics and Space Administration Jet Propulsion Laboratory California Institute of Technology 20 July 2009 12 Effect of Delta-layer Cap Thickness on Emission Threshold Only a weak dependence of emission threshold on GaN cap layer thickness observed.

13. National Aeronautics and Space Administration Jet Propulsion Laboratory California Institute of Technology 20 July 2009 13 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.

14. National Aeronautics and Space Administration Jet Propulsion Laboratory California Institute of Technology 20 July 2009 14 Effect of Delta Layer Doping Density on Emission Intensity 3 nm cap

15. National Aeronautics and Space Administration Jet Propulsion Laboratory California Institute of Technology 20 July 2009 15 Effect of Bulk p-Doping on Emission Intensity 1 nm cap 10 14 Si/cm 2 Increased p-doping of the bulk GaN reduces the width of the surface well, thus decreasing scattering losses during well traversal.

16. National Aeronautics and Space Administration Jet Propulsion Laboratory California Institute of Technology 20 July 2009 16 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.

17. National Aeronautics and Space Administration Jet Propulsion Laboratory California Institute of Technology 20 July 2009 17 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