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