Performance analysis of poly- and nano-crystalline diamond based photocathodes

The results of the external Quantum Efficiency (QE) measurements, in the range 150-210 nm, of Poly-, Nano- and Single-Crystalline Diamond (PCD, NCD and SCD) film photocathodes (PCs) are reported and discussed. The diamond films, grown by Microwave Plasma Assisted Chemical Vapour Deposition (MPACVD) technique, have been analyzed with the aim to define which characteristics enhance their QE. It has been found that NCD film PCs, generally showing a lower QE with respect to PCD ones, enhance their photoemission if a graphitic component is present in the film. Surface hydrogenation effects on the photoemissivity of PCs have been investigated. Comparative studies between the QE of an untreated and a hydrogenated-treated diamond film PC show that the hydrogenated sample exhibits Negative Electron Affinity (NEA) properties. An ageing test of 24 h air exposure shows a stability of the hydrogenated diamond PC higher than that of the CsI one. Moreover, the PC conditioning by means of H2 plasma allows to recover almost completely the loss of PC QE due to ageing effect caused by the air exposure of a previously hydrogenated sample. The sensitivity of PCs depends strongly on the topography of diamond films.