Photoemission from Metallic Cesium Oxide Films

Abstract

Ultraviolet photoelectron spectra of Cs evaporated onto Ag and oxidized under ultrahigh-vacuum conditions have been measured. Time variation of the spectra indicates the formation of Cs snboxides (e.g. , Cs,O, Ca&~03). The high infrared photoelectric yield of Ag/0/Cs cathodes can be explained as due to the excitattion of surface plasmons in these compounds.The infrared sensitivity of Ag/0/Cs photocathodes has been interpreted in terms of different mechanisms, such as defects in the lattice of Cs,O formed during the production process of Ag/0/Cs cathodes, " as well as photoemission from silver. "~A ll of these interpretations generally link the photoemission in the infrared region to the low work function of one of the cesium oxides. ' Cathodes with an excess of cesium (compaI'ed to CssO) show 'the highest infrared yield. Composition and thickness of the Ag/0/Cs layers as well as the electron emission process are still a matter of dispute. Bulk samples of metalrich cesium oxides (suboxides, e.g. , Cs,O, Cs»0, ) have recently been investigated by ultraviolet photoelectron spectroscopy (UPS). ' lt has been shown that the spectra reflect compositions and crystal structures of the bulk compounds. 'We report here UPS measurements of thin films of cesium deposited on a silver substrate and oxidized under ultrahigh-vacuum (UHV) conditions, simulating the a,ctive layers in Ag/0/Cs (S-l) photocathodes. The spectral response of films prepared in this way is similar to S-1 cathodes according to Heimann et al. ' The UPS data of these films are almost identical to those of the bulk cesium suboxides. ' A comparison with bulk data yields information about the composition of the films, the layer thickness, and the low-energy photoelectron emission process.The measurements were performed at room temperature in a Leybold-Heraeus LHS 10 ESCA system equipped with a preparation chamber and a diff erentially pumped cold-cathode gas-discharge lamp using the Hei resonance line (It v =21.2 eV). The residual gas pressure in the analyzer chamber was 2x 10 " Torr. Evaporated silver served as a substrate and was etched by argon-ion bombardment, The photoelectron energy distribution curve of this sample is shown in Fig. 1(a). The energy scale is relative to the Fermi edge; the resolution is 0.1 eV. The photoelectron spectrum of Cs evaporated for 3 min at 390 K onto the Ag substrate at room temperature is shown in Fig. 1(b). Oxygen was leaked into the analyzer chamber (basic pressure p = 2x 10 " Torr) at a pa, rtial pressure of 5&& 10 ' Torr for ft hy=21. 2 eV \ I t -120rnin I~x 1 CL BINDlNG ENERGY f.eV] FIG. 1. Photoelectron energy distribution curves for silver, a cesium layer on silver, and reaction products vrith oxygen.

Ionization energies of cesium and cesium oxide clusters

“…The discussion about the cesium oxygen concentration in the range of the minimal work function still continues: Is it cesium suboxide as a layer [21,22], a doped cesium oxide, Cs(Cs20 ) [23,24], or, an oxygen-rich cesium, Cs2027 Under the assumption that cluster properties transform with increasing cluster size into the bulk properties, it seems to be clear that doped cesium oxide should have the lowest work function. The discussion about the cesium oxygen concentration in the range of the minimal work function still continues: Is it cesium suboxide as a layer [21,22], a doped cesium oxide, Cs(Cs20 ) [23,24], or, an oxygen-rich cesium, Cs2027 Under the assumption that cluster properties transform with increasing cluster size into the bulk properties, it seems to be clear that doped cesium oxide should have the lowest work function.…”

The distribution of oxide species in the Cs∕O activation layer on InP(100) negative electron affinity photocathodes

“…It is discovered in our earlier studies that there are two types of Cs oxides in the activation layer after the InP is activated by Cs and oxygen [15], where the identification of Cs peroxide and superoxide is based on comparing the valence band spectra of our activated InP photocathode with the well established literature of oxidation of bulk Cs [16][17][18][19][20][21][22][23]. The QE of an activated InP photocathode decays in our vacuum system, accompanied by changes in both the O1s and VB spectra [15].…”

Stabilité de l'émission photoélectrique de couches minces d'argent recouvertes de césium oxydé

“…Pour obtenir une photocathode ayant un seuil voisin des = 1,2 03BCm, on peut réaliser à la surface d'un support métallique ou semiconducteur : , -soit une couche épaisse d'oxyde de césium riche en agrégats d'argent : c'est la cathode Sl, ou [Ag-O-Cs], -soit une couche de quelques nanomètres d'oxyde de Cs, formée essentiellement d'oxyde Cs20, avec éventuellement des inclusions de suboxyde Csl 103 [4,5,11], -soit enfin, comme nous l'avons fait, une couche très mince contenant des atomes de césium et d'oxygène, optimisée à chaque cycle du traitement pour obtenir le travail de sortie minimum W. min. Ce type de revêtement est utilisé pour l'obtention de cathodes à affinité négative sur substrat semiconducteur [6], en particulier la face (100) du silicium [15,17] et la face (110) de l'arséniure de gallium [18].…”

“…On ignore la composition exacte de la couche d'oxyde de césium, ce métal étant capable de former plusieurs types d'oxydes. Il semble [4,5] que l'oxyde dominant soit Cs20, mais la présence d'inclusions ou d'îlots de CSl103 semble vraisemblable. L'analyse Auger ne nous permettra d'atteindre que les taux moyens respectifs de Cs et 0 dans la couche, et non la composition vraie, mais l'évolution respective des hauteurs de pics caractéristiques de ces deux éléments au cours du temps devrait permettre d'élucider en partie le processus de dégradation.…”