Influence of the substrate surface texture on the photon-sensitivity stability of CsI thin film photocathodes

A study on the influence of the substrate morphology on the photoemission properties of caesium iodide (CsI) thin film photocathodes, in the range 150–200 nm, has been performed. Various types of conductive substrates, patterned by colloidal lithography, have been compared to the standard printed circuit board (PCB), used for the ALICE experiment at CERN [M.A. Nitti, et al., Nucl. Instr. and Meth. A 523 (2004) 323.]. A correlation between the substrate surface texture and the photoemission stability of the films has been demonstrated.The combination of colloidal lithography and plasma etching, or physical evaporation, allows to create on substrates arrays of nanostructures whose shape and pitch can be controlled by changing some parameters during the patterning process.In order to be comparable with the CsI photoelectron escape length and to preserve the substrate morphology in the film, a layer of 20 nm has been deposited on all the samples.Scanning electron microscopy (SEM) investigations of the colloidal lithography patterned (CLP) substrates have been performed.Atomic force microscopy (AFM) topographic images of the CsI thin film evaporated on PCB and CLP substrates have also been acquired and compared, showing a clear difference in the surface texture.An ageing test, consisting of an air exposure with a relative humidity of about 45% for 24 h, resulted in a higher quantum efficiency stability of textured CsI thin film photocathodes evaporated on nanostructured substrates with respect to those grown on standard PCB ones.