Study of Negative Electron Affinity GaAs Photocathodes Brian S. Henderson Rice University Advisors: Ivan Bazarov, Yulin Li, Xianghong Liu 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 NF 3, XeF 2, and N 2 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 o C 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 NF 3 and XeF 2 activations –NF 3 : 16-18% (A oxidizer commonly used for cathode activation) –XeF 2 : 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 NF 3 activations exhibited sharp initial decays before stabilizing, while XeF 2 activations were more stable immediately following activation In the long-term, NF 3 and XeF 2 activations produced similar lifetimes, although XeF 2 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 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 N 2 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 XeF 2 activation

August 5, 2009 CLASSE Summer REU 10 Annealing Process By heating the cathode to the temperature of ammonia desorption (~105 o C) 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 NH 3 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 XeF 2 packed in helium Develop a way to simultaneously monitor surface desorption and QE Compare results with activations using O 2

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 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