Prevention of electron field emission from molybdenum substrates for photocathodes by the native oxide layer

¹ Bergische Universität Wuppertal, FB C Abteilung Physik, Gauss-Str. 20, 42097 Wuppertal, Germany
² Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, 14109 Berlin, Germany
³ Institut für Festkörperphysik, TU Dresden, 01069 Dresden, Germany

^a e-mail: s.lagotzky@uni-wuppertal.de

Received: 20 March 2015
Accepted: 21 April 2015
Published online: 25 May 2015

Abstract

Comprehensive investigations of the electron field emission (FE) properties of annealed single crystal and polycrystalline molybdenum plugs, which are used as substrates for actual alkali-based photocathodes were performed with a FE scanning microscope. Well-polished and dry-ice cleaned Mo samples with native oxide did not show parasitic FE up to a field level of 50 MV/m required for photoinjector cavities. In situ heat treatments (HT) above 400 °C, which are usual before photocathode deposition, activated field emission at lower field strength. Oxygen loading into the Mo surface, however, partially weakened these emitters. X-ray photoelectron spectroscopy of comparable Mo samples showed the dissolution of the native oxide during such heat treatments. These results reveal the suppression of field emission by native Mo oxides. Possible improvements for the photocathode preparation will be discussed.