Study of Negative Electron Affinity GaAs Photocathodes

Brian S. Henderson

Rice University

Advisors: Ivan Bazarov, Yulin Li, Xianghong Liu

August 5, 2009

August 5, 2009 CLASSE Summer REU 2

Introduction

• My work this summer consisted of two major projects– Study of photocathode activation methods, with focus on

achieving more stable GaAs cathodes

– Preliminary work on the design of a device to characterize the 2D energy distribution of photo-emitted electrons

• Photocathodes serve as the electron source for the ERL electron gun and other accelerators

• Several new methods of activation were attempted and characterized

• Electromagnetic field maps were created for the essential components of the energy distribution device

August 5, 2009 CLASSE Summer REU 3

Study of Photocathode Activation

• The goal of this part of the project was to study methods of

developing more stable, longer-lasting gallium arsenide

photocathodes

– Activation with oxidizing compounds that have simpler chemistry

to form more stable surface layers

– Study of the surface interactions via thermal desorption

spectroscopy (TDS)

– Maintaining conditions in the cathode chamber that favor longer

lifetimes

August 5, 2009 CLASSE Summer REU 4

Photoemission from Cathodes

• Photocathodes are activated by creating a strong dipole field at the cathode surface resulting in negative electron affinity (NEA)

• The goal is to produce cathodes with both high quantum efficiency (QE) and long operational and dark lifetimes

• The quality of the cathode depends heavily on the quality and stability of the surface layers

Energy diagram of an NEA cathode (M. Hoppe. PhD Thesis. Universität Heidelberg, 2001.)

August 5, 2009 CLASSE Summer REU 5

Setup and Activation

• Activations of GaAs cathodes were conducted by alternating deposition of cesium on the cathode surface with exposure to an oxidizing gas (“yo-yo” method)

• Activations were conducted using NF3, XeF2, and N2 as the oxidizing gas, each yielding unique results

• After ~15 cycles, the quantum efficiency peaks and activation is ceased

Cesiator

Oxidizer Channel

August 5, 2009 CLASSE Summer REU 6

Methods

• Prior to each activation the cathode was cleaned by heating it to ~650 oC for a few hours

• The time evolution of photocurrent, laser power, chamber pressure, and QE were monitored and recorded during the activation process and subsequent decay of the cathode using the EPICS archiver

• After allowing the cathode to decay, the cathode was heated at a controlled rate and the thermal desorption from the surface was monitored via mass spectrometry using a residual gas analyzer (RGA)– This data is useful in determining

what substances are present on the cathode surface and with what energy they are bonded there

Sample RGA spectrum showing the desorption of various compounds at specific temperatures

August 5, 2009 CLASSE Summer REU 7

Activation Results

• Good peak QE values were achieved for both NF3 and XeF2 activations– NF3: 16-18% (A oxidizer commonly used for cathode activation)

– XeF2: 12-15% (A new oxidizer that we hope will create a more stable cathode)

• Cathodes allowed to decay for extended periods eventually exhibit lifetimes of 50+ hours

• NF3 activations exhibited sharp initial decays before stabilizing, while XeF2 activations were more stable immediately following activation

• In the long-term, NF3 and XeF2 activations produced similar lifetimes, although XeF2 maintained higher QE longer due to the difference in initial behaviors (to be discussed further)

August 5, 2009 CLASSE Summer REU 8

Long-Term Decay Behavior

• The long-term cathode decay tests showed rapidly increasing lifetime during the first 10-20 hours followed by a more stable lifetime trend

• This behavior appeared regardless of the oxidizing gas used

• The various short-term tests confirmed the trend of linearly increasing lifetime in the first few hours

• Could be due to a stabilizing effect on the cathode surface or depletion of some desorbing compound

August 5, 2009 CLASSE Summer REU 9

Role of Nitrogen

• The results across various tests show that nitrogen is an important factor in surface interactions with the GaAs crystal

• No matter the oxidizer used, ammonia was found to desorb from the surface upon heating

• In fact, a cathode may be activated, albeit with poorer QE and lifetime, with pure N2

• Clearly, it is not correct to consider the cathode surface as consisting purely of Cs and the oxidizer unless absolutely clean conditions in the chamber are achieved

Thermal desorption after a purified XeF2 activation

August 5, 2009 CLASSE Summer REU 10

Annealing Process

• By heating the cathode to the temperature of ammonia desorption (~105 oC) and allowing it to cool, about 10% of the QE at the start of the anneal is recovered

• The increase in QE appears to be correlated with the removal of NH3 from the cathode

• The lifetime after anneal, however, is significantly poorer than the lifetime prior to the anneal

• Also, the cathode cannot be heated long enough to desorb all ammonia without significantly decreasing QE

• A way to simultaneous monitor QE and thermal desorption would be highly valuable

August 5, 2009 CLASSE Summer REU 11

Future Work

• Identify the source of nitrogen that allows ammonia to be present in the system regardless of the recent activity in the chamber

• Determine the cause of the characteristic “turning point” in long-term QE decay

• Determine to what degree the interaction of nitrogen/formation of ammonia affects cathode performance and what other factors limit cathode performance

• Develop a better method to remove helium from the chamber to consistently maintain UHV conditions when using XeF2 packed in helium

• Develop a way to simultaneously monitor surface desorption and QE

• Compare results with activations using O2

August 5, 2009 CLASSE Summer REU 12

Characterization of Photoemitted Electron Energies

• The goal of this part of the project was to begin work on

designing a device for the characterization of the 2D energy

distributions of photoemitted electrons

– Study the successes and limitations of others’ attempts

– Begin electromagnetic field mapping of key device components

– Lay groundwork for future progress (particle trajectory simulation,

design of full device, etc.)

August 5, 2009 CLASSE Summer REU 13

Motivation and Model

• Understanding the 2D energy distribution of electrons emitted from a cathode surface would provide much information about the photoemission process

• Following the lead of Pastuszka, et al, it is intended to build a device to make this measurement

• Thus far, electric field maps of the key components of the device have been developed

Device used by Pastuszka, et al for the energy analysis of photoemitted electrons (Pastuszka, et al. J. Appl. Phys., Vol. 88, No. 11, 1 December 2000.)

August 5, 2009 CLASSE Summer REU 14

Cathode and Electrodes Assembly

• The assembly, as shown at right, is cylindrically symmetric about the vertical axis

• Scale: mm

• Field solution accounts for Schottky barrier potential between the Pierce electrode and the GaAs puck

• Potentials of the intermediate cathodes may be adjusted to give a “slow” (as shown at right) or “fast” acceleration

0 V

-25 V

-75 V

-102.9 V

100 V

August 5, 2009 CLASSE Summer REU 15

Retarding Field Analyzer

• Device is cylindrically symmetric about the vertical axis, scale: mm

• Key issue with the RFA is the non-uniformity of the maximum potential across the gap of the retarding electrode (varies about 0.15 V)

Retarding Electrode

Entrance Electrode (Ground)

Collector (Ground)

Diaphragm Foil

August 5, 2009 CLASSE Summer REU 16

Acknowledgements

• Many thanks to my advisors Ivan Bazarov, Yulin Li, and Xianghong Liu for copious amounts of guidance in my work and hours of help with the experimental setup

• Thanks to John Dobbins for his assistance in setting up an automatic controller for the illumination laser

• Thanks to all those that organized the program (Rich Galik, Ernie Fontes, Lora Hine, Monica Wesley,…)

• And, of course, thanks to the National Science Foundation for funding this REU program